Transparent alumina
Encyclopedia
Many ceramic materials
, both glassy and crystalline, have found use as optically transparent materials in various forms from bulk solid-state components to high surface area forms such as thin films, coatings and fibers.
Such devices have found widespread use for various applications in the electro-optical field including: optical fibers for guided lightwave transmission, optical switches, laser amplifiers and lenses
, hosts for solid-state lasers and optical window materials for gas lasers, and infrared (IR) heat seeking devices
for missile guidance
systems and IR night vision.
While single-crystalline ceramics may be largely defect-free (particularly within the spatial scale of the incident light wave), optical transparency in polycrystalline
materials is limited by the amount of light which is scattered by their microstructural features. The amount of light scattering
therefore depends on the wavelength
of the incident radiation, or light.
For example, since visible light
has a wavelength scale on the order of hundreds of nanometers, scattering centers will have dimensions on a similar spatial scale. Most ceramic materials, such as alumina and its compounds, are formed from fine powders, yielding a fine grained polycrystalline microstructure
which is filled with scattering centers comparable to the wavelength of visible light. Thus, they are generally opaque as opposed to transparent materials. Recent nanoscale technology has, however, made possible the production of (poly)crystalline transparent ceramics such as alumina Al2O3, yttria alumina garnet
(YAG), and neodymium-doped Nd:YAG.
In 1961, General Electric began selling transparent alumina Lucalox bulbs. In 1966, GE announced a ceramic "transparent as glass," called "YttraloxTM. In 2004, Anatoly Rosenflanz and colleagues at 3M
used a "flame-spray" technique to alloy aluminium oxide
(or alumina) with rare-earth metal oxides in order to produce high strength glass-ceramic
s with good optical properties. The method avoids many of the problems encountered in conventional glass forming and may be extensible to other oxides. This goal has been readily accomplished and amply demonstrated in laboratories and research facilities worldwide using the emerging chemical processing methods encompassed by the methods of sol-gel chemistry and nanotechnology
.
Many ceramic materials, both glassy and crystalline, have found use as hosts for solid-state lasers and as optical window materials for gas lasers. The first working laser was made by Theodore H. Maiman in 1960 at Hughes Research Laboratories
in Malibu, who had the edge on other research teams led by Charles H. Townes at Columbia University
, Arthur Schawlow at Bell Labs
, and Gould at TRG (Technical Research Group). Maiman used a solid-state light-pumped synthetic ruby
to produce red laser light at a wavelength of 694 nanometers (nm). Synthethic ruby lasers are still in use.
of Nd:YAG is a matured, highly reproducible and relatively simple technological procedure adds significantly to the value of the material.
Nd:YAG lasers are used in manufacturing for engraving, etching, or marking a variety of metals and plastics. They are extensively used in manufacturing for cutting and welding steel and various alloys. For automotive applications (cutting and welding steel) the power levels are typically 1–5 kW.
In addition, Nd:YAG lasers are used in ophthalmology
to correct posterior capsular opacification, a condition that may occur after cataract
surgery, and for peripheral iridotomy in patients with acute angle-closure glaucoma, where it has superseded surgical iridectomy
. Frequency-doubled
Nd:YAG lasers (wavelength 532 nm) are used for pan-retinal photocoagulation in patients with diabetic retinopathy
. In oncology
, Nd:YAG lasers can be used to remove skin cancer
s.
These lasers are also used extensively in the field of cosmetic medicine for laser hair removal
and the treatment of minor vascular
defects such as spider veins on the face and legs. Recently used for dissecting cellulitis, a rare skin disease usually occurring on the scalp. Using hysteroscopy
in the field of gynecology, the Nd:YAG laser has been used for removal of uterine septa
within the inside of the uterus.
In dentistry, Nd:YAG lasers are used for soft tissue
surgeries
in the oral cavity.
The principal differences between the behavior of glass and crystalline ceramic laser host materials are associated with the greater variation in the local environment of lasing ions in amorphous solids. This leads to a broadening of the fluorescent levels in glasses. For example, the width of the Nd3+ emission in YAG is ~ 10 angstroms as compared to ~ 300 angstroms in typical oxide glasses. The broadened fluorescent lines in glasses make it more difficult to obtain continuous wave laser operation (CW), relative to the same lasing ions in crystalline solid laser hosts.
Several glasses are used in transparent armor, such as normal plate glass (soda-lime-silica), borosilicate glass, and fused silica. Plate glass has been the most common glass used due to its low cost. But greater requirements for the optical properties and ballistic performance have necessitated the development of new materials. Chemical or thermal treatmtents can increase the strength of glasses, and the controlled crystallization of certain glass compositions can produce optical quality glass-ceramics. Alstom Grid Ltd. currently produces a lithium disilicate based glass-ceramic known as TransArmTM, for use in transparent armor systems. It has all the workability of an amorphous glass, but upon recrystallization it demonstrates properties similar to a crystalline ceramic. VycorTM is 96% fused silica glass, which is crystal clear, lightweight and high strength. One advantage of these type of materials is that they can be produced in large sheets and other curved shapes.
Primary scattering centers in polycrystalline nanomaterials—made from the sintering of high purity nanoparticles and powders—include microstructural defects such as residual porosity
and grain boundaries (see Transparent materials). Thus, opacity partly results from the incoherent scattering of light at internal surfaces and interfaces
. In addition to porosity
, most of the interfaces or internal surfaces in ceramic nanomaterials are in the form of grain boundaries
which separate nanoscale regions of crystalline order. Moreover, when the size of the scattering center (or grain boundary) is reduced well below the size of the wavelength of the light being scattered, the light scattering no longer occurs to any significant extent.
In the processing of high performance ceramic nanomaterials with superior opto-mechanical properties under adverse conditions, the size of the crystalline grains is determined largely by the size of the crystalline particles present in the raw material during the synthesis or formation of the object. Thus a reduction of the original particle size well below the wavelength of visible light (~ 0.5 µm or 500 nm) eliminates much of the light scattering, resulting in a translucent or even transparent material.
Furthermore, results indicate that microscopic pores in sintered ceramic nanomaterials, mainly trapped at the junctions of microcrystalline grains, cause light to scatter and prevented true transparency. It has been observed that the total volume fraction of these nanoscale pores (both intergranular and intragranular porosity) must be less than 1% for high-quality optical transmission, i.e. the density has to be 99.99% of the theoretical crystalline density.
Assisted by several workers from the NIF, the Livermore team has produced 15 mm diameter samples of transparent Nd:YAG from nanoscale particles and powders, and determined the most important parameters affecting their quality. In the these objects, the team largely followed the Japanese production and processing methodologies, and used an in house furnace to vacuum sinter the nanopowders. All specimens were then sent out for hot isostatic pressing (HIP). Finally, the components were returned to Livermore for coating and testing, with results indicating exceptional optical quality and properties.
One Japanese/East Indian consortium has focused specifcially on the spectroscopic and stimulated emission characteristics of Nd3+ in transparent YAG nanomaterials for laser applications. Their materials were synthesized using vacuum sintering techniques. The spectroscopic studies suggest overall improvement in absorption and emission and reduction in scattering loss. SEM and TEM observations revealed an excellent optical quality with low pore volume and narrow grain boundary width. Fluorescence and Raman measurements reveal that the Nd3+ doped YAG nanomaterial is comparable in quality to its single-crystal counterpart in both its radiative and non-radiative properties. Individual Stark levels are obtained from the absorption and fluorescence spectra and are analyzed in oredr to identify the stimulated emission channels possible in the material. Laser performance studies favor the use of high dopant concentration in the design of an efficient microchip laser. With 4 at% dopant, the group obtained a slope efficiency of 40%. High-power laser experiments yield an optical-to-optical conversion efficiency of 30% for Nd (0.6 at%) YAG nanomaterial as compared to 34% for an Nd (0.6 at%) YAG single crystal. Optical gain measurements conducted in these materials also show values comparable to single crystal, supporting the contention that these materials could be suitable substitutes to single crystals in solid-state laser applications.
In 1966, a transparent ceramic, YttraloxTM, was invented by by Dr. Richard C. Anderson at the General Electric Research Laboratory
, with further work at GE's Metallurgy and Ceramics Laboratory by Drs. Paul J. Jorgensen, Joseph H. Rosolowski, and Douglas St. Pierre. YttraloxTM is "transparent as glass," has a melting point above 4,000 degrees C, and transmits frequencies in the near infrared band as well as visible light.
Further development of yttrium ceramic nanomaterials was carried out by General Electric in the 1970s in Schenectady and Cleveland, motivated by lighting and ceramic laser applications. YttraloxTM, transparent yttrium oxide Y2O3 containing ~ 10% thorium oxide (ThO2) was fabricated by Greskovich and Woods. The additive served to control grain growth during densification, so that porosity remained on grain boundaries and not trapped inside grains where it would be quite difficult to eliminate during the initial stages of sintering. Typically, as polycrystalline ceramics densify during heat treatment, grains grow in size while the remaining porosity decreases both in volume fraction and in size. Optically transparent ceramics must be virtually pore-free.
GE's transparent YttraloxTM was folllowed by GTE's lanthana-doped yttria with similar level of additive. Both of these materials required extended firing times at temperatures above 2000 °C. La2O3 – doped Y2O3 is of interest for infrared (IR) applications because it is one of the longest wavelength transmitting oxides. It is refractory with a melting point of 2430 °C and has a moderate coefficient of thermal expansion coefficient. The thermal shock and erosion resistance is considered to be intermediate among the oxides, but outstanding compared to non-oxide IR transmitting materials. A major consideration is the low emissivity of yttria, which limits background radiation upon heating. It is also known that the phonon edge gradually moves to shorter wavelengths as a material is heated.
In addition, ytrria itself, Y2O3 has been clearly identified as a prospective solid-state laser
material. In particular, lasers with ytterbium
as dopant
allow the efficient operation both in cw
operation
and in pulsed regimes.
At high concentration of excitations (of order of 1%) and poor cooling, the quenching of emission at laser frequency and avalanche broadband emission takes place.
The Livermore team is also exploring new forming techniques (e.g. extrusion molding) which have the capacity to create more diverse, and possibly more complicated, shapes. These include shells and tubes for improved coupling to the pump light and for more efficient heat transfer. In addition, different materials can be co-extruded and then sintered into a monolithic transparent solid. An amplifier slab can formed so that part of the structure acts in guided lightwave transmission in order to focus pump light from laser diodes into regions with a high concentration of dopant ions near the slab center.
In general, nanomaterials promise to greatly expand the availability of low-cost, high-end laser components in much larger sizes than would be possible with traditional single crystalline ceramics. Many classes of laser designs could benefit from nanomaterial-based laser structures such as amplifies with built-in edge claddings. Nanomaterials could also provide more robust and compact designs for high-peak power, fusion-class lasers for stockpile stewardship, as well as high-average-power lasers for global theater ICBM missile defense systems (e.g. Strategic Defense Initiative
SDI, or more recently the Missile Defense Agency.
A night vision device (NVD) is an optical instrument
that allows images to be produced in levels of light approaching total darkness. They are most often used by the military
and law enforcement
agencies, but are available to civilian
users. Night vision devices were first used in World War II,
and came into wide use during the Vietnam War
. The technology has evolved greatly since their introduction, leading to several "generations" of night vision equipment with performance increasing and price decreasing. The United States Air Force
is experimenting with Panoramic Night Vision Goggles (PNVGs) which double the user's field of view
to around 95 degrees by using four 16 mm image intensifiers tubes, rather than the more standard two 18 mm tubes.
Thermal images are actually visual displays of the amount of infrared (IR) energy emitted, transmitted, and reflected by an object. Because there are multiple sources of the infrared energy, it is difficult to get an accurate temperature of an object using this method. A thermal imaging camera is capable of performing algorithms to interpret that data and build an image. Although the image shows the viewer an approximation of the temperature at which the object is operating, the camera is actually using multiple sources of data based on the areas surrounding the object to determine that value rather than detecting the actual temperature.
Night vision infrared devices image in the near-infrared, just beyond the visual spectrum, and can see emitted or reflected near-infrared in complete visual darkness. All objects above the absolute zero
temperature (0 K
) emit infrared radiation. Hence, an excellent way to measure thermal variations is to use an infrared vision
device, usually a focal plane array (FPA) infrared camera capable of detecting radiation
in the mid (3 to 5 µm) and long (7 to 14 µm) wave infrared bands, denoted as MWIR and LWIR, corresponding to two of the high transmittance infrared windows. Abnormal temperature profiles at the surface of an object are an indication of a potential problem.
Infrared thermography, thermal imaging, and thermal video, are examples of infrared
imaging science
. Thermal imaging camera
s detect radiation
in the infrared range of the electromagnetic spectrum
(roughly 900–14,000 nanometers or 0.9–14 µm) and produce images of that radiation, called thermograms.
Since infrared radiation is emitted by all objects near room temperature
, according to the black body
radiation law, thermography makes it possible to see one's environment with or without visible illumination. The amount of radiation emitted by an object increases with temperature. Therefore, thermography allows one to see variations in temperature. When viewed through a thermal imaging camera, warm objects stand out well against cooler backgrounds; humans and other warm-blooded
animals become easily visible against the environment, day or night. As a result, thermography is particularly useful to the military and to security services
.
Firefighter
s use thermography to see through smoke
, to find persons, and to localize the base of a fire. Maintenance technicians use thermography to locate overheating joints and sections of power line
s, which are a tell-tale
sign of impending failure. Building construction technicians can see thermal signatures that indicate heat leaks in faulty thermal insulation
and can use the results to improve the efficiency of heating and air-conditioning units. Some physiological changes in human beings and other warm-blooded animals can also be monitored with thermal imaging during clinical diagnostics.
In thermographic imaging, infrared radiation with wavelengths between 8–13 micrometers strikes the detector material, heating it, and thus changing its electrical resistance. This resistance change is measured and processed into temperatures which can be used to create an image. Unlike other types of infrared detecting equipment, microbolometers utilizing a transparent ceramic detector do not require cooling. Thus, a microbolometer is essentially an uncooled thermal sensor.
The material used in the detector must demonstrate large changes in resistance as a result of minute changes in temperature. As the material is heated, due to the incoming infrared radiation, the resistance of the material decreases. This is related to the material's temperature coefficient of resistance
(TCR) specifically its negative temperature coefficient. Industry currently manufactures microbolometers that contain materials with TCRs near −2%.
allows for the production of thin films of these crystlalline compounds with superior properties, which may be easily integrated into the fabrication process. VO2 has low resistance but undergoes a metal-insulator phase change near 67oC and also has a lower TCR value. On the other hand, V2O5 exhibits high resistance and also high TCR.
Other IR transparent ceramic materials that have been investigated include doped forms of CuO, MnO and SiO.
Thermal radiation
is electromagnetic radiation emitted from the surface of an object which is due to the object's temperature
. Infrared homing
refers to a passive missile guidance system which uses the emission from a target of electromagnetic radiation in the infrared part of the spectrum to track it. Missiles that use infrared seeking are often referred to as "heat-seekers", since infrared is just below the visible spectrum of light in frequency and is radiated strongly by hot bodies. Many objects such as people, vehicle engines and aircraft generate and retain heat, and as such, are especially visible in the infrared wavelengths of light compared to objects in the background.
Limitations to larger area sapphires are often business related, in that larger induction furnaces and costly tooling dies are necessary in order to exceed current fabrication limits. However, as an industry, sapphire producers have remained competitive in the face of coating-hardened glass and new ceramic nanomaterials, and still managed to offer high performance and an expanded market.
, offer better optical performance, but inferior mechanical durability. Future high-speed infrared-guided missiles will require new domes that are substantially more durable than those in use today, while still retaining maximum transparency across a wide wavelength range. A long standing trade-off exists between optical bandpass and mechanical durability within the current collection of single-phase infrared transmitting materials, forcing missile designers to compromise on system performance. Optical nanocomposites may present the opportunity to engineer new materials that overcome this traditional compromise.
The first full scale missile domes of transparent yttria manufactured from nanoscale ceramic powders were developed in the 1980s under Navy funding. Raytheon perfected and characterized its undoped polycrystalline yttria, while lanthana-doped yttria was similarly developed by GTE Labs. The two versions had comparable IR transmittance, fracture toughness, and thermal expansion, while the undoped version exhibited twice the value of thermal conductivity.
Renewed interest in yttria windows and domes has prompted efforts to enhance mechanical properties by using nanoscale materials with submicrometer or nanosized grains. In one study, three vendors were selected to provide nanoscale powders for testing and evaluation, and they were compared to a conventional (5 μm) yttria powder previously used to prepare transparent yttria. While all of the nanopowders evaluated had impurity levels that were too high to allow processing to full transparency, 2 of them were processed to theoretical density and moderate transparency. Samples were sintered to a closed pore state at temperatures as low as 1400 C.
After the relatively short sintering period, the component is placed in a hot isostatic press (HIP) and processed for 3 – 10 hours at ~ 30 kpsi(~200 MPa) at a temperature similar to that of the initial sintering. The applied isostatic pressure provides additional driving force for densification by substantially increasing the atomic diffusion coefficients, which promotes additional viscous flow at or near grain boundaries and intergranular pores. Using this method, transparent yttria nanomaterials were produced at lower temperatures, shorter total firing times, and without extra additives which tend to reduce the thermal conductivity.
Recently, a newer method has been devleoped by Mouzon, which relies on the methods of glass-encapsulation, combined with vacuum sintering at 1600 °C followed by hot isostatic pressing (HIP) at 1500 °C of a highly agglomerated commercial powder. The use of evacuated glass capsules to perform HIP treatment allowed samples that showed open porosity after vacuum sintering to be sintered to transparency. The sintering response of the investigated powder was studied by careful microstructural observations using scanning electron microscopy and optical microscopy both in reflection and transmission. The key to this method is to keep porosity intergranular during pre-sintering, so that it can be removed subsequently by HIP treatment. It was found that agglomerates of closely packed particles are helpful to reach that purpose, since they densify fully and leave only intergranular porosity.
In optical nanocomposites, two or more interpenetrating phases are mixed in a sub-micrometer grain sized, fully dense body. Infrared light scattering can be minimized (or even eliminated) in the material as long as the grain size of the individual phases is significantly smaller than infrared wavelengths. Experimental data suggests that limiting the grain size of the nanocomposite to approximately 1/15th of the wavelength of light is sufficient to limit scattering.
Nanocomposites of yttria and magnesia have been produced with a grain size of approximately 200 nm. These materials have yielded good transmission in the 3–5 µm range and strengths higher than that for single-phase individual constituents. Enhancement of mechanical properties in nanocomposite ceramic materials has been extensively studied. Significant increases in strength (2–5 times), toughness (1–4 times), and creep resistance have been observed in systems including SiC/Al2O3, SiC/Si3N4, SiC/MgO, and Al2O3/ZrO2.
The strengthening mechanisms observed vary depending on the material system, and there does not appear to be any general consensus regarding strengthening mechanisms, even within a given system. In the SiC/Al2O3 system, for example, it is widely known and accepted that the addition of SiC particles to the Al2O3 matrix results in a change of failure mechanism from intergranular (between grains) to intragranular (within grains) fracture. The explanations for improved strength include:
Existing transparent armor systems typically have many layers, separated by polymer (e.g. polycarbonate) interlayers. The polymer interlayer is used to mitigate the stresses from thermal expansion mismatches, as well as to stop crack propagation from ceramic to polymer. The polycarbonate is also currently used in applications such as visors, face shields and laser protection goggles. The search for lighter materials has also led to investigations into other polymeric materials such as transparent nylons, polyurethane, and acrylics. The optical properties and durability of transparent plastics limit their use in armor applications. Investigations carried out in the 1970s had shown promise for the use of polyurethane as armor material, but the optical properties were not adequate for transparent armor applications.
Several glasses are utilized in transparent armor, such as normal plate glass (soda-lime-silica), borosilicate glasses, and fused silica. Plate glass has been the most common glass used due to its low cost, but greater requirements for the optical properties and ballistic performance have generated the need for new materials. Chemical or thermal treatments can increase the strength of glasses, and the controlled crystallization of certain glass systems can produce transparent glass-ceramics. Alstom Grid Research & Technology (Stafford, UK), currently produces a lithium disilicate based glass-ceramic known as TransArm, for use in transparent armor systems. The inherent advantages of glasses and glass-ceramics include having lower cost than most other ceramic materials, the ability to be produced in curved shapes, and the ability to be formed into large sheets.
Transparent crystalline ceramics are used to defeat advanced threats. Three major transparent candidates currently exist: aluminum oxynitride (AlON), magnesium aluminate spinel
(spinel), and single crystal aluminum oxide (sapphire). Aluminum oxynitride spinel (Al23O27N5), one of the leading candidates for transparent armor, is produced by Raytheon Corporation as AlON and marketed under the trade name Raytran. The incorporation of nitrogen into an aluminum oxide stabilizes a spinel phase, which due to its cubic crystal structure, is an isotropic material that can be produced as a transparent polycrystalline material. Polycrystalline materials can be produced in complex geometries using conventional ceramic forming techniques such as pressing, (hot) isostatic pressing, and slip casting.
spinel (Al23O27N5), one of the leading candidates for tranparent armor, is produced by the Raytheon Corporation as AlONTM and marketed under the trade name RaytranTM. The incorporation of nitrogen into aluminium oxide
stabilizes a crystalline spinel
phase, which due to its cubic crystal structure and unit cell, is an isotropic material which can be produced as transparent ceramic nanomaterial. Thus, fine-grained polycrystalline nanomaterials can be produced and formed into complex geometries using conventional ceramic forming techniques such as hot pressing and slip casting.
The Surmet Corporation has acquired Raytheon's AlON business and is currently building a market for this technology in the area of point-of-sale scanner windows and as an alternative to quartz and sapphire in the semiconductor market. The high hardness of AlONTM provides a scratch resistance which exceeds even the most durable coatings for glass scanner windows, such as those used in supermarkets. Surmet has successfully produced a 15" by 18" curved AlON window and is currently attempting to scale up the technology and reduce the cost. In addition, the U.S. Army and U.S. Air Force are both seeking development into next generation applications.
As shown clearly in the field demonstration in this Online video clip a Spinel ceramic plate can survive a direct hit from a .50 BMG round fired at close range. By all indications, this is an important new technology that should find its way into military vehicles in the near future. ArmorLine's manufacturing facility, scheduled to come on-line in the first quarter of 2011, will supply transparent spinel ceramic plates to window, lens, and optical system suppliers and end-users.
Spinel offers some processing advantages over AlONTM, such as the fact that spinel powder is available from commercial manufacturers while AlON powders are proprietary to Raytheon. It is also capable of being processed at much lower temperatures than AlONTM, and has been shown to possess superior optical properties within the infrared (IR) region. The improved optical characteristics make spinel attractive in sensor applications where effective communication is impacted by the protective missile dome's absorption characteristics.
Spinel shows promise for many applications, but is currently not available in bulk form from any manufacturer, although efforts to commercialize spinel are underway. The spinel products business is being pursued by two key U.S. manufacturers: "Technology Assessment and Transfer" and the "Surmet Corporation".
An extensive NRL review of the literature has indicated clearly that attempts to make high-quality spinel have failed to date because the densification dynamics of spinel are poorly understood. They have conducted extensive research into the dynamics involved during the densification of spinel. Their research has shown that LiF, although necessary, also has extremely adverse effects during the final stages of densification. Additionally, its distribution in the presursor spinel powders is of critical importance.
Traditional bulk mixing processes used to mix LiF sintering aid into a powder leave fairly inhomogeneous distribution of Lif that must be homogenized by extended heat treatment times at elevated temperatures. The homogenizing temperature for Lif/Spinel occurs at the temperature of fast reaction between the LiF and the Al2O3. In order to avoid this detrimental reaction, they have developed a new process that uniformly coats the spinel particles with the sintering aid. This allows them to reduce the amount of Lif necessary for densification and to rapidly heat through the temperature of maximum reactivity. These developments have allowed NRL to fabricate MgAl2O4 spinel to high transparency with extremely high reproducibility that should enable military as well as commercial use of spinel.
is currently one of the most mature transparent ceramics from a production and application perspective, and is available from several manufacturers. But the cost is high due to the processing temperature involved, as well as machining costs to cut parts out of single crystal boules. It also has a very high mechanical strength – but that is dependent on the surface finish.
The high level of maturity of sapphire from a production and application standpoint can be atrributed to two areas of business: EM windows for missiles and domes, and electronic/semiconductor industries and applications.
There are current programs to scale-up sapphire
grown by the heat exchanger method or edge defined film-fed growth (EFG) processes. Its maturity stems from its use as windows and in semiconductor industry. Crystal Systems Inc. which uses single crystal growth
techniques, is currently scaling their sapphire boules to 13 inches (330.2 mm) diameter and larger. Another producer, the Saint-Gobain Group produces transparent sapphire using an edge, defined growth technique. Sapphire grown by this technique produces an optically inferior material to that which is grown via single crystal techniques, but is much less expensive, and retains much of the hardness, transmission, and scratch-resistant characteristics. Saint-Gobain is currently capable of producing 0.43" thick (as grown) sapphire, in 12" × 18.5" sheets, as well as thick, single-curved sheets.ARL is currently investigating use of this material in a laminate design for transparent armor systems. The Saint Gobain Group have commercialized the capability to meet flight requirements on the F-35 Joint Strike Fighter and F-22 Raptor next generation fighter aircraft.
For example, transparent ceramic armor consisting of a light weight composite has been formed by tilizing a face plate of transparent alumina Al2O3 (or magnesia MgO) with a back-up plate of transparent plastic. The two plates (bonded together with a transparent adhesive) afford complete ballistic protection against 0.30 AP M2 projectiles at 0° obliquity with a muzzle velocity of 2770 ft (844.3 m) per second.
Another transparent composite armor provided complete protection for small arms projectiles up to and including caliber .50 AP M2 projectiles consisting of two or more layers of transparent ceramic material.
Nanocomposites of yttria and magnesia have been produced with an average grain size of ~200 nm. These materials have exhibited near theoretical transmission in the 3 – 5 μm IR band. Additionally, such composites have yielded higher strengths than those observed for single phase solid-state components. Despite a lack of agreement regarding mechanism of failure, it is widely accepted that nanocomposite ceramic materials can and do offer improved mechanical properties over those of single phase materials or nanomaterials of uniform chemical composition.
It should also be noted here that nanocomposite ceramic materials also offer interesting mechanical properties not achievable in other materials, such as superplastic flow and metal-like machinability. It is anticipated that further development will result in high strength, high transparency nanomaterials which are suitable for application as next generation armor.
Ceramic engineering
Ceramic engineering is the science and technology of creating objects from inorganic, non-metallic materials. This is done either by the action of heat, or at lower temperatures using precipitation reactions from high purity chemical solutions...
, both glassy and crystalline, have found use as optically transparent materials in various forms from bulk solid-state components to high surface area forms such as thin films, coatings and fibers.
Such devices have found widespread use for various applications in the electro-optical field including: optical fibers for guided lightwave transmission, optical switches, laser amplifiers and lenses
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...
, hosts for solid-state lasers and optical window materials for gas lasers, and infrared (IR) heat seeking devices
Infrared homing
Infrared homing refers to a passive missile guidance system which uses the emission from a target of electromagnetic radiation in the infrared part of the spectrum to track and follow it. Missiles which use infrared seeking are often referred to as "heat-seekers", since infrared is just below the...
for missile guidance
Missile guidance
Missile guidance refers to a variety of methods of guiding a missile or a guided bomb to its intended target. The missile's target accuracy is a critical factor for its effectiveness...
systems and IR night vision.
While single-crystalline ceramics may be largely defect-free (particularly within the spatial scale of the incident light wave), optical transparency in polycrystalline
Polycrystalline
Polycrystalline materials are solids that are composed of many crystallites of varying size and orientation. The variation in direction can be random or directed, possibly due to growth and processing conditions. Fiber texture is an example of the latter.Almost all common metals, and many ceramics...
materials is limited by the amount of light which is scattered by their microstructural features. The amount of light scattering
Light scattering
Light scattering is a form of scattering in which light is the form of propagating energy which is scattered. Light scattering can be thought of as the deflection of a ray from a straight path, for example by irregularities in the propagation medium, particles, or in the interface between two media...
therefore depends on the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of the incident radiation, or light.
For example, since visible light
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
has a wavelength scale on the order of hundreds of nanometers, scattering centers will have dimensions on a similar spatial scale. Most ceramic materials, such as alumina and its compounds, are formed from fine powders, yielding a fine grained polycrystalline microstructure
Microstructure
Microstructure is defined as the structure of a prepared surface or thin foil of material as revealed by a microscope above 25× magnification...
which is filled with scattering centers comparable to the wavelength of visible light. Thus, they are generally opaque as opposed to transparent materials. Recent nanoscale technology has, however, made possible the production of (poly)crystalline transparent ceramics such as alumina Al2O3, yttria alumina garnet
Yttrium aluminium garnet
Yttrium aluminium garnet is a synthetic crystalline material of the garnet group. It is also one of three phases of the yttria-aluminium composite, the other two being yttrium aluminium monoclinic and yttrium aluminium perovskite . YAG is commonly used as a host material in various solid-state...
(YAG), and neodymium-doped Nd:YAG.
Introduction
Transparent ceramics have recently acquired a high degree of interest and notoriety. Basic applications include lasers and cutting tools, transparent armor windows, night vision devices (NVD) and nose cones for heat seeking missiles. Currently available infrared (IR) transparent materials typically exhibit a trade-off between optical performance and mechanical strength. For example, sapphire (crystalline alumina) is very strong, but lacks full transparency throughout the 3–5 micrometer mid-IR range. Yttria is fully transparent from 3–5 micrometers, but lacks sufficient strength, hardness, and thermal shock resistance for high-performance aerospace applications. Not surprisingly, a combination of these two materials in the form of the yttria-alumina garnet (YAG) has proven to be one of the top performers in the field.In 1961, General Electric began selling transparent alumina Lucalox bulbs. In 1966, GE announced a ceramic "transparent as glass," called "YttraloxTM. In 2004, Anatoly Rosenflanz and colleagues at 3M
3M
3M Company , formerly known as the Minnesota Mining and Manufacturing Company, is an American multinational conglomerate corporation based in Maplewood, Minnesota, United States....
used a "flame-spray" technique to alloy aluminium oxide
Aluminium oxide
Aluminium oxide is an amphoteric oxide with the chemical formula 23. It is commonly referred to as alumina, or corundum in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry...
(or alumina) with rare-earth metal oxides in order to produce high strength glass-ceramic
Glass-ceramic
Glass-ceramics are polycrystalline material produced through controlled crystallization of base glass. Glass-ceramic materials share many properties with both glasses and ceramics...
s with good optical properties. The method avoids many of the problems encountered in conventional glass forming and may be extensible to other oxides. This goal has been readily accomplished and amply demonstrated in laboratories and research facilities worldwide using the emerging chemical processing methods encompassed by the methods of sol-gel chemistry and nanotechnology
Nanotechnology
Nanotechnology is the study of manipulating matter on an atomic and molecular scale. Generally, nanotechnology deals with developing materials, devices, or other structures possessing at least one dimension sized from 1 to 100 nanometres...
.
Many ceramic materials, both glassy and crystalline, have found use as hosts for solid-state lasers and as optical window materials for gas lasers. The first working laser was made by Theodore H. Maiman in 1960 at Hughes Research Laboratories
Hughes Research Laboratories
HRL Laboratories , was the research arm of the Hughes Aircraft Company. Its dedicated research center was established in 1960 in Malibu...
in Malibu, who had the edge on other research teams led by Charles H. Townes at Columbia University
Columbia University
Columbia University in the City of New York is a private, Ivy League university in Manhattan, New York City. Columbia is the oldest institution of higher learning in the state of New York, the fifth oldest in the United States, and one of the country's nine Colonial Colleges founded before the...
, Arthur Schawlow at Bell Labs
Bell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...
, and Gould at TRG (Technical Research Group). Maiman used a solid-state light-pumped synthetic ruby
Ruby
A ruby is a pink to blood-red colored gemstone, a variety of the mineral corundum . The red color is caused mainly by the presence of the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires...
to produce red laser light at a wavelength of 694 nanometers (nm). Synthethic ruby lasers are still in use.
Crystals
Ruby lasers consist of single-crystal sapphire alumina (Al2O3) rods doped with a small concentration of chromium Cr, typically in the range of 0.05%. The end faces are highly polished with a planar and parallel configuration. Neodymium-doped YAG (Nd:YAG) has proven to be one of the best solid-state laser materials. Its indisputable dominance in a broad variety of laser applications is determined by a combination of high emission cross section with long spontaneous emission lifetime, high damage threshold, mechanical strength, thermal conductivity, and low thermal beam distortion. The fact that the Czochralski crystal growthCzochralski process
The Czochralski process is a method of crystal growth used to obtain single crystals of semiconductors , metals , salts, and synthetic gemstones...
of Nd:YAG is a matured, highly reproducible and relatively simple technological procedure adds significantly to the value of the material.
Nd:YAG lasers are used in manufacturing for engraving, etching, or marking a variety of metals and plastics. They are extensively used in manufacturing for cutting and welding steel and various alloys. For automotive applications (cutting and welding steel) the power levels are typically 1–5 kW.
In addition, Nd:YAG lasers are used in ophthalmology
Ophthalmology
Ophthalmology is the branch of medicine that deals with the anatomy, physiology and diseases of the eye. An ophthalmologist is a specialist in medical and surgical eye problems...
to correct posterior capsular opacification, a condition that may occur after cataract
Cataract
A cataract is a clouding that develops in the crystalline lens of the eye or in its envelope, varying in degree from slight to complete opacity and obstructing the passage of light...
surgery, and for peripheral iridotomy in patients with acute angle-closure glaucoma, where it has superseded surgical iridectomy
Iridectomy
An iridectomy, also known as a surgical iridectomy or corectomy, is the surgical removal of part of the iris. These procedures are most frequently performed in the treatment of closed-angle glaucoma and iris melanoma....
. Frequency-doubled
Second harmonic generation
An optical frequency multiplier is a nonlinear optical device, in which photons interacting with a nonlinear material are effectively "combined" to form new photons with greater energy, and thus higher frequency...
Nd:YAG lasers (wavelength 532 nm) are used for pan-retinal photocoagulation in patients with diabetic retinopathy
Diabetic retinopathy
Diabetic retinopathy is retinopathy caused by complications of diabetes mellitus, which can eventually lead to blindness....
. In oncology
Oncology
Oncology is a branch of medicine that deals with cancer...
, Nd:YAG lasers can be used to remove skin cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
s.
These lasers are also used extensively in the field of cosmetic medicine for laser hair removal
Laser hair removal
Laser hair removal was performed experimentally for about 20 years before it became commercially available in the mid 1990s. One of the first published articles describing laser hair removal was authored by the group at Massachusetts General Hospital in 1998...
and the treatment of minor vascular
Vascular
Vascular in zoology and medicine means "related to blood vessels", which are part of the circulatory system. An organ or tissue that is vascularized is heavily endowed with blood vessels and thus richly supplied with blood....
defects such as spider veins on the face and legs. Recently used for dissecting cellulitis, a rare skin disease usually occurring on the scalp. Using hysteroscopy
Hysteroscopy
Hysteroscopy is the inspection of the uterine cavity by endoscopy with access through the cervix. It allows for the diagnosis of intrauterine pathology and serves as a method for surgical intervention .-Method:...
in the field of gynecology, the Nd:YAG laser has been used for removal of uterine septa
Uterine septum
A uterine septum is a form of a congenital malformation where the uterine cavity is partitioned by a longitudinal septum; the outside of the uterus has a normal typical shape...
within the inside of the uterus.
In dentistry, Nd:YAG lasers are used for soft tissue
Soft tissue
In anatomy, the term soft tissue refers to tissues that connect, support, or surround other structures and organs of the body, not being bone. Soft tissue includes tendons, ligaments, fascia, skin, fibrous tissues, fat, and synovial membranes , and muscles, nerves and blood vessels .It is sometimes...
surgeries
Surgery
Surgery is an ancient medical specialty that uses operative manual and instrumental techniques on a patient to investigate and/or treat a pathological condition such as disease or injury, or to help improve bodily function or appearance.An act of performing surgery may be called a surgical...
in the oral cavity.
Glasses
Glasses (non-crystalline ceramics) are also widely used as host materials for lasers. Relative to crystalline lasers, they offer improved flexibility in size and shape and may be readily manufactured as large, homogeneous, isotropic solids with excellent optical properties. The indices of refraction of glass laser hosts can be varied between about 1.5 and 2.0, and both the temperature coefficient of n and the strain-optical coefficient can be tailored by altering the chemical composition. Glasses have lower thermal conductivities than the alumina or YAG, however, which imposes limitations on their use in continuous and high repetition-rate applications.The principal differences between the behavior of glass and crystalline ceramic laser host materials are associated with the greater variation in the local environment of lasing ions in amorphous solids. This leads to a broadening of the fluorescent levels in glasses. For example, the width of the Nd3+ emission in YAG is ~ 10 angstroms as compared to ~ 300 angstroms in typical oxide glasses. The broadened fluorescent lines in glasses make it more difficult to obtain continuous wave laser operation (CW), relative to the same lasing ions in crystalline solid laser hosts.
Several glasses are used in transparent armor, such as normal plate glass (soda-lime-silica), borosilicate glass, and fused silica. Plate glass has been the most common glass used due to its low cost. But greater requirements for the optical properties and ballistic performance have necessitated the development of new materials. Chemical or thermal treatmtents can increase the strength of glasses, and the controlled crystallization of certain glass compositions can produce optical quality glass-ceramics. Alstom Grid Ltd. currently produces a lithium disilicate based glass-ceramic known as TransArmTM, for use in transparent armor systems. It has all the workability of an amorphous glass, but upon recrystallization it demonstrates properties similar to a crystalline ceramic. VycorTM is 96% fused silica glass, which is crystal clear, lightweight and high strength. One advantage of these type of materials is that they can be produced in large sheets and other curved shapes.
Nanomaterials
It has been shown fairly recently that laser elements (amplifiers, switches, ion hosts, etc.) made from fine-grained ceramic nanomaterials—produced by the low temperature sintering of high purity nanoparticles and powders—can be produced at a relatively low cost. These components are free of internal stress or intrinsic birefringence, and allow relatively large doping levels or optimized custom-designed doping profiles. This highlights the use of ceramic nanomaterials as being particularly important for high-energy laser elements and applications.Primary scattering centers in polycrystalline nanomaterials—made from the sintering of high purity nanoparticles and powders—include microstructural defects such as residual porosity
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
and grain boundaries (see Transparent materials). Thus, opacity partly results from the incoherent scattering of light at internal surfaces and interfaces
Interface (chemistry)
An interface is a surface forming a common boundary among two different phases, such as an insoluble solid and a liquid, two immiscible liquids or a liquid and an insoluble gas. The importance of the interface depends on which type of system is being treated: the bigger the quotient area/volume,...
. In addition to porosity
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
, most of the interfaces or internal surfaces in ceramic nanomaterials are in the form of grain boundaries
Grain boundary
A grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are defects in the crystal structure, and tend to decrease the electrical and thermal conductivity of the material...
which separate nanoscale regions of crystalline order. Moreover, when the size of the scattering center (or grain boundary) is reduced well below the size of the wavelength of the light being scattered, the light scattering no longer occurs to any significant extent.
In the processing of high performance ceramic nanomaterials with superior opto-mechanical properties under adverse conditions, the size of the crystalline grains is determined largely by the size of the crystalline particles present in the raw material during the synthesis or formation of the object. Thus a reduction of the original particle size well below the wavelength of visible light (~ 0.5 µm or 500 nm) eliminates much of the light scattering, resulting in a translucent or even transparent material.
Furthermore, results indicate that microscopic pores in sintered ceramic nanomaterials, mainly trapped at the junctions of microcrystalline grains, cause light to scatter and prevented true transparency. It has been observed that the total volume fraction of these nanoscale pores (both intergranular and intragranular porosity) must be less than 1% for high-quality optical transmission, i.e. the density has to be 99.99% of the theoretical crystalline density.
Nd:YAG
For example, a 1.46 kW Nd:YAG laser has been demonstrated by Konoshima Chemical Co. in Japan. In addition, Livermore researchers realized that these fine-grained ceramic nanomaterials might greatly benefit high-powered lasers used in the National Ignition Facility (NIF) Programs Directorate. In particular, a Livermore research team began to acquire advanced transparent nanomaterials from Konoshima to determine if they could meet the optical requirements needed for Livermore's Solid-State Heat Capacity Laser (SSHCL). Livermore researchers have also been testing applications of these materials for applications such as advanced drivers for laser-driven fusion power plants.Assisted by several workers from the NIF, the Livermore team has produced 15 mm diameter samples of transparent Nd:YAG from nanoscale particles and powders, and determined the most important parameters affecting their quality. In the these objects, the team largely followed the Japanese production and processing methodologies, and used an in house furnace to vacuum sinter the nanopowders. All specimens were then sent out for hot isostatic pressing (HIP). Finally, the components were returned to Livermore for coating and testing, with results indicating exceptional optical quality and properties.
One Japanese/East Indian consortium has focused specifcially on the spectroscopic and stimulated emission characteristics of Nd3+ in transparent YAG nanomaterials for laser applications. Their materials were synthesized using vacuum sintering techniques. The spectroscopic studies suggest overall improvement in absorption and emission and reduction in scattering loss. SEM and TEM observations revealed an excellent optical quality with low pore volume and narrow grain boundary width. Fluorescence and Raman measurements reveal that the Nd3+ doped YAG nanomaterial is comparable in quality to its single-crystal counterpart in both its radiative and non-radiative properties. Individual Stark levels are obtained from the absorption and fluorescence spectra and are analyzed in oredr to identify the stimulated emission channels possible in the material. Laser performance studies favor the use of high dopant concentration in the design of an efficient microchip laser. With 4 at% dopant, the group obtained a slope efficiency of 40%. High-power laser experiments yield an optical-to-optical conversion efficiency of 30% for Nd (0.6 at%) YAG nanomaterial as compared to 34% for an Nd (0.6 at%) YAG single crystal. Optical gain measurements conducted in these materials also show values comparable to single crystal, supporting the contention that these materials could be suitable substitutes to single crystals in solid-state laser applications.
Yttria, Y2O3
The initial work in developing transparent yttrium oxide nanomaterials was carried out by General Electric in the 1960s.In 1966, a transparent ceramic, YttraloxTM, was invented by by Dr. Richard C. Anderson at the General Electric Research Laboratory
General Electric Research Laboratory
General Electric Research Laboratory, the first industrial research facility in the United States, was established in 1900. This lab was home to the early technological breakthroughs of General Electric and created a research and development environment that set the standard for industrial...
, with further work at GE's Metallurgy and Ceramics Laboratory by Drs. Paul J. Jorgensen, Joseph H. Rosolowski, and Douglas St. Pierre. YttraloxTM is "transparent as glass," has a melting point above 4,000 degrees C, and transmits frequencies in the near infrared band as well as visible light.
Further development of yttrium ceramic nanomaterials was carried out by General Electric in the 1970s in Schenectady and Cleveland, motivated by lighting and ceramic laser applications. YttraloxTM, transparent yttrium oxide Y2O3 containing ~ 10% thorium oxide (ThO2) was fabricated by Greskovich and Woods. The additive served to control grain growth during densification, so that porosity remained on grain boundaries and not trapped inside grains where it would be quite difficult to eliminate during the initial stages of sintering. Typically, as polycrystalline ceramics densify during heat treatment, grains grow in size while the remaining porosity decreases both in volume fraction and in size. Optically transparent ceramics must be virtually pore-free.
GE's transparent YttraloxTM was folllowed by GTE's lanthana-doped yttria with similar level of additive. Both of these materials required extended firing times at temperatures above 2000 °C. La2O3 – doped Y2O3 is of interest for infrared (IR) applications because it is one of the longest wavelength transmitting oxides. It is refractory with a melting point of 2430 °C and has a moderate coefficient of thermal expansion coefficient. The thermal shock and erosion resistance is considered to be intermediate among the oxides, but outstanding compared to non-oxide IR transmitting materials. A major consideration is the low emissivity of yttria, which limits background radiation upon heating. It is also known that the phonon edge gradually moves to shorter wavelengths as a material is heated.
In addition, ytrria itself, Y2O3 has been clearly identified as a prospective solid-state laser
Solid-state laser
A solid-state laser is a laser that uses a gain medium that is a solid, rather than a liquid such as in dye lasers or a gas as in gas lasers. Semiconductor-based lasers are also in the solid state, but are generally considered as a separate class from solid-state lasers .-Solid-state...
material. In particular, lasers with ytterbium
Ytterbium
Ytterbium is a chemical element with the symbol Yb and atomic number 70. A soft silvery metallic element, ytterbium is a rare earth element of the lanthanide series and is found in the minerals gadolinite, monazite, and xenotime. The element is sometimes associated with yttrium or other related...
as dopant
Dopant
A dopant, also called a doping agent, is a trace impurity element that is inserted into a substance in order to alter the electrical properties or the optical properties of the substance. In the case of crystalline substances, the atoms of the dopant very commonly take the place of elements that...
allow the efficient operation both in cw
Continuous wave
A continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off...
operation
and in pulsed regimes.
At high concentration of excitations (of order of 1%) and poor cooling, the quenching of emission at laser frequency and avalanche broadband emission takes place.
Future
The Livermore team is also exploring new ways to chemically synthesize the initial nanopowders. Borrowing on expertise developed in CMS over the past 5 years, the team is synthesizing nanopowders based on sol-gel processing, and then sintering them accordingly in order to obtain the solid-state laser components. Another technique being tested utilizes a combustion process in order to generate the powders by burning an organic solid containing yttrium, aluminum, and neodymium. The smoke is then collected, which consists of spherical nanoparticles.The Livermore team is also exploring new forming techniques (e.g. extrusion molding) which have the capacity to create more diverse, and possibly more complicated, shapes. These include shells and tubes for improved coupling to the pump light and for more efficient heat transfer. In addition, different materials can be co-extruded and then sintered into a monolithic transparent solid. An amplifier slab can formed so that part of the structure acts in guided lightwave transmission in order to focus pump light from laser diodes into regions with a high concentration of dopant ions near the slab center.
In general, nanomaterials promise to greatly expand the availability of low-cost, high-end laser components in much larger sizes than would be possible with traditional single crystalline ceramics. Many classes of laser designs could benefit from nanomaterial-based laser structures such as amplifies with built-in edge claddings. Nanomaterials could also provide more robust and compact designs for high-peak power, fusion-class lasers for stockpile stewardship, as well as high-average-power lasers for global theater ICBM missile defense systems (e.g. Strategic Defense Initiative
Strategic Defense Initiative
The Strategic Defense Initiative was proposed by U.S. President Ronald Reagan on March 23, 1983 to use ground and space-based systems to protect the United States from attack by strategic nuclear ballistic missiles. The initiative focused on strategic defense rather than the prior strategic...
SDI, or more recently the Missile Defense Agency.
Night vision
Optical instrument
An optical instrument either processes light waves to enhance an image for viewing, or analyzes light waves to determine one of a number of characteristic properties.-Image enhancement:...
that allows images to be produced in levels of light approaching total darkness. They are most often used by the military
Night combat
Night combat is combat that occurs during the hours of darkness. It is distinguished from daytime combat by lower visibility and its reversed relation to the Circadian cycle. Typically combat at night is favorable to the attacker, with offensive tactics being focused on exploiting the advantages...
and law enforcement
Police
The police is a personification of the state designated to put in practice the enforced law, protect property and reduce civil disorder in civilian matters. Their powers include the legitimized use of force...
agencies, but are available to civilian
Civilian
A civilian under international humanitarian law is a person who is not a member of his or her country's armed forces or other militia. Civilians are distinct from combatants. They are afforded a degree of legal protection from the effects of war and military occupation...
users. Night vision devices were first used in World War II,
and came into wide use during the Vietnam War
Vietnam War
The Vietnam War was a Cold War-era military conflict that occurred in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. This war followed the First Indochina War and was fought between North Vietnam, supported by its communist allies, and the government of...
. The technology has evolved greatly since their introduction, leading to several "generations" of night vision equipment with performance increasing and price decreasing. The United States Air Force
United States Air Force
The United States Air Force is the aerial warfare service branch of the United States Armed Forces and one of the American uniformed services. Initially part of the United States Army, the USAF was formed as a separate branch of the military on September 18, 1947 under the National Security Act of...
is experimenting with Panoramic Night Vision Goggles (PNVGs) which double the user's field of view
Field of view
The field of view is the extent of the observable world that is seen at any given moment....
to around 95 degrees by using four 16 mm image intensifiers tubes, rather than the more standard two 18 mm tubes.
Thermal images are actually visual displays of the amount of infrared (IR) energy emitted, transmitted, and reflected by an object. Because there are multiple sources of the infrared energy, it is difficult to get an accurate temperature of an object using this method. A thermal imaging camera is capable of performing algorithms to interpret that data and build an image. Although the image shows the viewer an approximation of the temperature at which the object is operating, the camera is actually using multiple sources of data based on the areas surrounding the object to determine that value rather than detecting the actual temperature.
Night vision infrared devices image in the near-infrared, just beyond the visual spectrum, and can see emitted or reflected near-infrared in complete visual darkness. All objects above the absolute zero
Absolute zero
Absolute zero is the theoretical temperature at which entropy reaches its minimum value. The laws of thermodynamics state that absolute zero cannot be reached using only thermodynamic means....
temperature (0 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
) emit infrared radiation. Hence, an excellent way to measure thermal variations is to use an infrared vision
Infrared vision
Infrared vision can be defined as the capability of biological or artificial systems to detect infrared radiation. The terms thermal vision and thermal imaging, are also commonly used in this context since infrared emissions from a body are directly related to their temperature: hotter objects...
device, usually a focal plane array (FPA) infrared camera capable of detecting radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
in the mid (3 to 5 µm) and long (7 to 14 µm) wave infrared bands, denoted as MWIR and LWIR, corresponding to two of the high transmittance infrared windows. Abnormal temperature profiles at the surface of an object are an indication of a potential problem.
Infrared thermography, thermal imaging, and thermal video, are examples of infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
imaging science
Imaging science
Imaging science is a multidisciplinary field concerned with the generation, collection, duplication, analysis, modification, and visualization of images . As an evolving field it includes research and researchers from physics, mathematics, electrical engineering, computer vision, computer science,...
. Thermal imaging camera
Thermal imaging camera
A Thermal Imaging Camera is a type of thermographic camera used in firefighting. By rendering infrared radiation as visible light, such cameras allow firefighters to see areas of heat through smoke, darkness, or heat-permeable barriers. Thermal imaging cameras are typically handheld, but may be...
s detect radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
in the infrared range of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
(roughly 900–14,000 nanometers or 0.9–14 µm) and produce images of that radiation, called thermograms.
Since infrared radiation is emitted by all objects near room temperature
Room temperature
-Comfort levels:The American Society of Heating, Refrigerating and Air-Conditioning Engineers has listings for suggested temperatures and air flow rates in different types of buildings and different environmental circumstances. For example, a single office in a building has an occupancy ratio per...
, according to the black body
Black body
A black body is an idealized physical body that absorbs all incident electromagnetic radiation. Because of this perfect absorptivity at all wavelengths, a black body is also the best possible emitter of thermal radiation, which it radiates incandescently in a characteristic, continuous spectrum...
radiation law, thermography makes it possible to see one's environment with or without visible illumination. The amount of radiation emitted by an object increases with temperature. Therefore, thermography allows one to see variations in temperature. When viewed through a thermal imaging camera, warm objects stand out well against cooler backgrounds; humans and other warm-blooded
Warm-blooded
The term warm-blooded is a colloquial term to describe animal species which have a relatively higher blood temperature, and maintain thermal homeostasis primarily through internal metabolic processes...
animals become easily visible against the environment, day or night. As a result, thermography is particularly useful to the military and to security services
Security agency
A security agency is a governmental organization which conducts intelligence activities for the internal security of a nation. They are the domestic cousins of foreign intelligence agencies...
.
Thermography
Thermography has a long history, although its use has increased dramatically with the commercial and industrial applications of the past fifty years.Firefighter
Firefighter
Firefighters are rescuers extensively trained primarily to put out hazardous fires that threaten civilian populations and property, to rescue people from car incidents, collapsed and burning buildings and other such situations...
s use thermography to see through smoke
Smoke
Smoke is a collection of airborne solid and liquid particulates and gases emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrained or otherwise mixed into the mass. It is commonly an unwanted by-product of fires , but may also be used for pest...
, to find persons, and to localize the base of a fire. Maintenance technicians use thermography to locate overheating joints and sections of power line
Electric power transmission
Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substations located near demand centers...
s, which are a tell-tale
Tell-tale
A tell-tale or telltale is an indicator, signal, or sign that conveys the status of a situation, mechanism, or system.-Automotive:...
sign of impending failure. Building construction technicians can see thermal signatures that indicate heat leaks in faulty thermal insulation
Thermal insulation
Thermal insulation is the reduction of the effects of the various processes of heat transfer between objects in thermal contact or in range of radiative influence. Heat transfer is the transfer of thermal energy between objects of differing temperature...
and can use the results to improve the efficiency of heating and air-conditioning units. Some physiological changes in human beings and other warm-blooded animals can also be monitored with thermal imaging during clinical diagnostics.
In thermographic imaging, infrared radiation with wavelengths between 8–13 micrometers strikes the detector material, heating it, and thus changing its electrical resistance. This resistance change is measured and processed into temperatures which can be used to create an image. Unlike other types of infrared detecting equipment, microbolometers utilizing a transparent ceramic detector do not require cooling. Thus, a microbolometer is essentially an uncooled thermal sensor.
The material used in the detector must demonstrate large changes in resistance as a result of minute changes in temperature. As the material is heated, due to the incoming infrared radiation, the resistance of the material decreases. This is related to the material's temperature coefficient of resistance
Temperature coefficient
The temperature coefficient is the relative change of a physical property when the temperature is changed by 1 K.In the following formula, let R be the physical property to be measured and T be the temperature at which the property is measured. T0 is the reference temperature, and ΔT is the...
(TCR) specifically its negative temperature coefficient. Industry currently manufactures microbolometers that contain materials with TCRs near −2%.
VO2 and V2O5
The most commonly used ceramic material in IR radiation microbolometers is vanadium oxide. The various crystalline forms of vanadium oxide include both VO2 and V2O5. Deposition at high temperatures and performing post-annealingAnnealing (metallurgy)
Annealing, in metallurgy and materials science, is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces conditions by heating to above the recrystallization temperature, maintaining a suitable temperature, and...
allows for the production of thin films of these crystlalline compounds with superior properties, which may be easily integrated into the fabrication process. VO2 has low resistance but undergoes a metal-insulator phase change near 67oC and also has a lower TCR value. On the other hand, V2O5 exhibits high resistance and also high TCR.
Other IR transparent ceramic materials that have been investigated include doped forms of CuO, MnO and SiO.
Missiles
Many ceramic nanomaterials of interest for transparent armor solutions are also used for electromagnetic (EM) windows. These applications include radomes, IR domes, sensor protection, and multi-spectral windows. Optical properties of the materials used for these applications are critical, as the transmission window and related cut-offs (UV – IR) control the spectral bandwidth over which the window is operational. Not only must these materials possess abrasion resistance and strength properties common of most armor applications, but due to the extreme temperatures associated with the environment of military aircraft and missiles, the must also possess excellent thermal stability.Thermal radiation
Thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....
is electromagnetic radiation emitted from the surface of an object which is due to the object's temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
. Infrared homing
Infrared homing
Infrared homing refers to a passive missile guidance system which uses the emission from a target of electromagnetic radiation in the infrared part of the spectrum to track and follow it. Missiles which use infrared seeking are often referred to as "heat-seekers", since infrared is just below the...
refers to a passive missile guidance system which uses the emission from a target of electromagnetic radiation in the infrared part of the spectrum to track it. Missiles that use infrared seeking are often referred to as "heat-seekers", since infrared is just below the visible spectrum of light in frequency and is radiated strongly by hot bodies. Many objects such as people, vehicle engines and aircraft generate and retain heat, and as such, are especially visible in the infrared wavelengths of light compared to objects in the background.
Sapphire
The current material of choice for high-speed infrared-guided missile domes is single-crystal sapphire. The optical transmission of sapphire does not actually extend to cover the entire mid-infrared range (3–5 µm), but starts to drop off at wavelengths greater than approximately 4.5 µm at room temperature. While the strength of sapphire is better than that of other available mid-range infrared dome materials at room temperature, it weakens above ~600 °C.Limitations to larger area sapphires are often business related, in that larger induction furnaces and costly tooling dies are necessary in order to exceed current fabrication limits. However, as an industry, sapphire producers have remained competitive in the face of coating-hardened glass and new ceramic nanomaterials, and still managed to offer high performance and an expanded market.
Yttria, Y2O3
Alternative materials, such as yttrium oxideYttrium(III) oxide
Yttrium oxide is Y2O3. It is an air-stable, white solid substance. Yttrium oxide is used as a common starting material for both materials science as well as inorganic compounds.-Materials science:...
, offer better optical performance, but inferior mechanical durability. Future high-speed infrared-guided missiles will require new domes that are substantially more durable than those in use today, while still retaining maximum transparency across a wide wavelength range. A long standing trade-off exists between optical bandpass and mechanical durability within the current collection of single-phase infrared transmitting materials, forcing missile designers to compromise on system performance. Optical nanocomposites may present the opportunity to engineer new materials that overcome this traditional compromise.
The first full scale missile domes of transparent yttria manufactured from nanoscale ceramic powders were developed in the 1980s under Navy funding. Raytheon perfected and characterized its undoped polycrystalline yttria, while lanthana-doped yttria was similarly developed by GTE Labs. The two versions had comparable IR transmittance, fracture toughness, and thermal expansion, while the undoped version exhibited twice the value of thermal conductivity.
Renewed interest in yttria windows and domes has prompted efforts to enhance mechanical properties by using nanoscale materials with submicrometer or nanosized grains. In one study, three vendors were selected to provide nanoscale powders for testing and evaluation, and they were compared to a conventional (5 μm) yttria powder previously used to prepare transparent yttria. While all of the nanopowders evaluated had impurity levels that were too high to allow processing to full transparency, 2 of them were processed to theoretical density and moderate transparency. Samples were sintered to a closed pore state at temperatures as low as 1400 C.
After the relatively short sintering period, the component is placed in a hot isostatic press (HIP) and processed for 3 – 10 hours at ~ 30 kpsi(~200 MPa) at a temperature similar to that of the initial sintering. The applied isostatic pressure provides additional driving force for densification by substantially increasing the atomic diffusion coefficients, which promotes additional viscous flow at or near grain boundaries and intergranular pores. Using this method, transparent yttria nanomaterials were produced at lower temperatures, shorter total firing times, and without extra additives which tend to reduce the thermal conductivity.
Recently, a newer method has been devleoped by Mouzon, which relies on the methods of glass-encapsulation, combined with vacuum sintering at 1600 °C followed by hot isostatic pressing (HIP) at 1500 °C of a highly agglomerated commercial powder. The use of evacuated glass capsules to perform HIP treatment allowed samples that showed open porosity after vacuum sintering to be sintered to transparency. The sintering response of the investigated powder was studied by careful microstructural observations using scanning electron microscopy and optical microscopy both in reflection and transmission. The key to this method is to keep porosity intergranular during pre-sintering, so that it can be removed subsequently by HIP treatment. It was found that agglomerates of closely packed particles are helpful to reach that purpose, since they densify fully and leave only intergranular porosity.
Composites
Prior to the work done at Raytheon, optical properties in nanocomposite ceramic materials had received little attention. Their studies clearly demonstrated near theoretical transmission in nanocomposite optical ceramics for the first time. The yttria/magnesia binary system is an ideal model system for nanocomposite formation. There is limited solid solubility in either one of the constituent phases, permitting a wide range of compositions to be investigated and compared to each other. According to the phase diagram, bi-phase mixtures are stable for all temperatures below ~ 2100 °C. In addition, neither yttria nor magnesia shows any absorption in the 3 – 5 μm mid-range IR portion of the EM spectrum.In optical nanocomposites, two or more interpenetrating phases are mixed in a sub-micrometer grain sized, fully dense body. Infrared light scattering can be minimized (or even eliminated) in the material as long as the grain size of the individual phases is significantly smaller than infrared wavelengths. Experimental data suggests that limiting the grain size of the nanocomposite to approximately 1/15th of the wavelength of light is sufficient to limit scattering.
Nanocomposites of yttria and magnesia have been produced with a grain size of approximately 200 nm. These materials have yielded good transmission in the 3–5 µm range and strengths higher than that for single-phase individual constituents. Enhancement of mechanical properties in nanocomposite ceramic materials has been extensively studied. Significant increases in strength (2–5 times), toughness (1–4 times), and creep resistance have been observed in systems including SiC/Al2O3, SiC/Si3N4, SiC/MgO, and Al2O3/ZrO2.
The strengthening mechanisms observed vary depending on the material system, and there does not appear to be any general consensus regarding strengthening mechanisms, even within a given system. In the SiC/Al2O3 system, for example, it is widely known and accepted that the addition of SiC particles to the Al2O3 matrix results in a change of failure mechanism from intergranular (between grains) to intragranular (within grains) fracture. The explanations for improved strength include:
- A simple reduction of processing flaw concentration during nanocomposite fabrication.
- Reduction of the critical flaw size in the material—resulting in increased strength as predicted by the Hall-Petch relation)
- Crack deflection at nanophase particels due to residual thermal stresses introduced upon cooling form processing temperatures.
- Microcracking along stress-induced dislocations in the matrix material.
Armor
There is an increasing need in the military sector for high-strength, robust materials which have the capability to transmit light around the visible (0.4–0.7 micrometers) and mid-infrared (1–5 micrometers) regions of the spectrum. These materials are needed for applications requiring transparent armor. Transparent armor is a material or system of materials designed to be optically transparent, yet protect from fragmentation or ballistic impacts. The primary requirement for a transparent armor system is to not only defeat the designated threat but also provide a multi-hit capability with minimized distortion of surrounding areas. Transparent armor windows must also be compatible with night vision equipment. New materials that are thinner, lightweight, and offer better ballistic performance are being sought.Existing transparent armor systems typically have many layers, separated by polymer (e.g. polycarbonate) interlayers. The polymer interlayer is used to mitigate the stresses from thermal expansion mismatches, as well as to stop crack propagation from ceramic to polymer. The polycarbonate is also currently used in applications such as visors, face shields and laser protection goggles. The search for lighter materials has also led to investigations into other polymeric materials such as transparent nylons, polyurethane, and acrylics. The optical properties and durability of transparent plastics limit their use in armor applications. Investigations carried out in the 1970s had shown promise for the use of polyurethane as armor material, but the optical properties were not adequate for transparent armor applications.
Several glasses are utilized in transparent armor, such as normal plate glass (soda-lime-silica), borosilicate glasses, and fused silica. Plate glass has been the most common glass used due to its low cost, but greater requirements for the optical properties and ballistic performance have generated the need for new materials. Chemical or thermal treatments can increase the strength of glasses, and the controlled crystallization of certain glass systems can produce transparent glass-ceramics. Alstom Grid Research & Technology (Stafford, UK), currently produces a lithium disilicate based glass-ceramic known as TransArm, for use in transparent armor systems. The inherent advantages of glasses and glass-ceramics include having lower cost than most other ceramic materials, the ability to be produced in curved shapes, and the ability to be formed into large sheets.
Transparent crystalline ceramics are used to defeat advanced threats. Three major transparent candidates currently exist: aluminum oxynitride (AlON), magnesium aluminate spinel
Spinel
Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4. Balas ruby is an old name for a rose-tinted variety.-Spinel group:...
(spinel), and single crystal aluminum oxide (sapphire). Aluminum oxynitride spinel (Al23O27N5), one of the leading candidates for transparent armor, is produced by Raytheon Corporation as AlON and marketed under the trade name Raytran. The incorporation of nitrogen into an aluminum oxide stabilizes a spinel phase, which due to its cubic crystal structure, is an isotropic material that can be produced as a transparent polycrystalline material. Polycrystalline materials can be produced in complex geometries using conventional ceramic forming techniques such as pressing, (hot) isostatic pressing, and slip casting.
Aluminium oxynitride spinel
Aluminium oxynitrideAluminium oxynitride
Aluminium oxynitride or AlON is a transparent polycrystalline ceramic with cubic spinel crystal structure composed of aluminium, oxygen and nitrogen. It is currently marketed under the name ALON by Surmet Corporation. ALON is optically transparent in the near ultra violet, visible and near...
spinel (Al23O27N5), one of the leading candidates for tranparent armor, is produced by the Raytheon Corporation as AlONTM and marketed under the trade name RaytranTM. The incorporation of nitrogen into aluminium oxide
Aluminium oxide
Aluminium oxide is an amphoteric oxide with the chemical formula 23. It is commonly referred to as alumina, or corundum in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry...
stabilizes a crystalline spinel
Spinel
Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4. Balas ruby is an old name for a rose-tinted variety.-Spinel group:...
phase, which due to its cubic crystal structure and unit cell, is an isotropic material which can be produced as transparent ceramic nanomaterial. Thus, fine-grained polycrystalline nanomaterials can be produced and formed into complex geometries using conventional ceramic forming techniques such as hot pressing and slip casting.
The Surmet Corporation has acquired Raytheon's AlON business and is currently building a market for this technology in the area of point-of-sale scanner windows and as an alternative to quartz and sapphire in the semiconductor market. The high hardness of AlONTM provides a scratch resistance which exceeds even the most durable coatings for glass scanner windows, such as those used in supermarkets. Surmet has successfully produced a 15" by 18" curved AlON window and is currently attempting to scale up the technology and reduce the cost. In addition, the U.S. Army and U.S. Air Force are both seeking development into next generation applications.
Spinel
Magnesium aluminate spinel (MgAl2O4) is a transparent ceramic with a cubic crystal structure with an excellent optical transmission from 0.2 to 5.5 micrometers in its polycrystlalline form. Optical quality transparent spinel has been produced by sinter/HIP, hot pressing, and hot press/HIP operaiotns, and it has been shown that the use of a hot isostatic press can improve its optical and physical properties.As shown clearly in the field demonstration in this Online video clip a Spinel ceramic plate can survive a direct hit from a .50 BMG round fired at close range. By all indications, this is an important new technology that should find its way into military vehicles in the near future. ArmorLine's manufacturing facility, scheduled to come on-line in the first quarter of 2011, will supply transparent spinel ceramic plates to window, lens, and optical system suppliers and end-users.
Spinel offers some processing advantages over AlONTM, such as the fact that spinel powder is available from commercial manufacturers while AlON powders are proprietary to Raytheon. It is also capable of being processed at much lower temperatures than AlONTM, and has been shown to possess superior optical properties within the infrared (IR) region. The improved optical characteristics make spinel attractive in sensor applications where effective communication is impacted by the protective missile dome's absorption characteristics.
Spinel shows promise for many applications, but is currently not available in bulk form from any manufacturer, although efforts to commercialize spinel are underway. The spinel products business is being pursued by two key U.S. manufacturers: "Technology Assessment and Transfer" and the "Surmet Corporation".
An extensive NRL review of the literature has indicated clearly that attempts to make high-quality spinel have failed to date because the densification dynamics of spinel are poorly understood. They have conducted extensive research into the dynamics involved during the densification of spinel. Their research has shown that LiF, although necessary, also has extremely adverse effects during the final stages of densification. Additionally, its distribution in the presursor spinel powders is of critical importance.
Traditional bulk mixing processes used to mix LiF sintering aid into a powder leave fairly inhomogeneous distribution of Lif that must be homogenized by extended heat treatment times at elevated temperatures. The homogenizing temperature for Lif/Spinel occurs at the temperature of fast reaction between the LiF and the Al2O3. In order to avoid this detrimental reaction, they have developed a new process that uniformly coats the spinel particles with the sintering aid. This allows them to reduce the amount of Lif necessary for densification and to rapidly heat through the temperature of maximum reactivity. These developments have allowed NRL to fabricate MgAl2O4 spinel to high transparency with extremely high reproducibility that should enable military as well as commercial use of spinel.
Sapphire
Single-crystal aluminum oxide (sapphire – Al2O3) is a transparent ceramic. Sapphire's crystal structure is rhombohedral and thus its properties are anisotropic vary with crystallographic orientation. Transparent aluminaTransparent alumina
Many ceramic materials, both glassy and crystalline, have found use as optically transparent materials in various forms from bulk solid-state components to high surface area forms such as thin films, coatings and fibers....
is currently one of the most mature transparent ceramics from a production and application perspective, and is available from several manufacturers. But the cost is high due to the processing temperature involved, as well as machining costs to cut parts out of single crystal boules. It also has a very high mechanical strength – but that is dependent on the surface finish.
The high level of maturity of sapphire from a production and application standpoint can be atrributed to two areas of business: EM windows for missiles and domes, and electronic/semiconductor industries and applications.
There are current programs to scale-up sapphire
Sapphire
Sapphire is a gemstone variety of the mineral corundum, an aluminium oxide , when it is a color other than red or dark pink; in which case the gem would instead be called a ruby, considered to be a different gemstone. Trace amounts of other elements such as iron, titanium, or chromium can give...
grown by the heat exchanger method or edge defined film-fed growth (EFG) processes. Its maturity stems from its use as windows and in semiconductor industry. Crystal Systems Inc. which uses single crystal growth
Crystal growth
A crystal is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. Crystal growth is a major stage of a crystallization process, and consists in the addition of new atoms, ions, or polymer strings into...
techniques, is currently scaling their sapphire boules to 13 inches (330.2 mm) diameter and larger. Another producer, the Saint-Gobain Group produces transparent sapphire using an edge, defined growth technique. Sapphire grown by this technique produces an optically inferior material to that which is grown via single crystal techniques, but is much less expensive, and retains much of the hardness, transmission, and scratch-resistant characteristics. Saint-Gobain is currently capable of producing 0.43" thick (as grown) sapphire, in 12" × 18.5" sheets, as well as thick, single-curved sheets.ARL is currently investigating use of this material in a laminate design for transparent armor systems. The Saint Gobain Group have commercialized the capability to meet flight requirements on the F-35 Joint Strike Fighter and F-22 Raptor next generation fighter aircraft.
Composites
Future high-speed infrared-guided missiles will require new dome materials that are substantially more durable than those in use today, while retaining maximum transparency across the entire operational spectrum or bandwidth. A long-standing compromise exists between optical bandpass and mechanical durability within the current group of single-phase (crystalline or glassy) IR transmitting ceramic materials, forcing missile designers to accept substandard overall system performance. Optical nanocomposites may provide the opportunity to engineer new materials that may overcome these traditional limitations.For example, transparent ceramic armor consisting of a light weight composite has been formed by tilizing a face plate of transparent alumina Al2O3 (or magnesia MgO) with a back-up plate of transparent plastic. The two plates (bonded together with a transparent adhesive) afford complete ballistic protection against 0.30 AP M2 projectiles at 0° obliquity with a muzzle velocity of 2770 ft (844.3 m) per second.
Another transparent composite armor provided complete protection for small arms projectiles up to and including caliber .50 AP M2 projectiles consisting of two or more layers of transparent ceramic material.
Nanocomposites of yttria and magnesia have been produced with an average grain size of ~200 nm. These materials have exhibited near theoretical transmission in the 3 – 5 μm IR band. Additionally, such composites have yielded higher strengths than those observed for single phase solid-state components. Despite a lack of agreement regarding mechanism of failure, it is widely accepted that nanocomposite ceramic materials can and do offer improved mechanical properties over those of single phase materials or nanomaterials of uniform chemical composition.
It should also be noted here that nanocomposite ceramic materials also offer interesting mechanical properties not achievable in other materials, such as superplastic flow and metal-like machinability. It is anticipated that further development will result in high strength, high transparency nanomaterials which are suitable for application as next generation armor.
See also
- Ceramic engineeringCeramic engineeringCeramic engineering is the science and technology of creating objects from inorganic, non-metallic materials. This is done either by the action of heat, or at lower temperatures using precipitation reactions from high purity chemical solutions...
- NanomaterialsNanomaterialsNanomaterials is a field that takes a materials science-based approach to nanotechnology. It studies materials with morphological features on the nanoscale, and especially those that have special properties stemming from their nanoscale dimensions...
- Optical fiberOptical fiberAn optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
- Transparent materials
Further reading
- Ceramic Processing Before Firing, Onoda, G.Y., Jr. and Hench, L.L. Eds., (Wiley & Sons, New York, 1979)