Optical dilatometer
Encyclopedia
An optical dilatometer is a non-contact device able to measure thermal expansions or sintering kinetics of any kind of materials, unlike traditional push rod dilatometer, it can push up to the dilatometric softening of the specimen. It is a device for measuring changes in the dimensions of a specimen, optically, the achieved resolution can results in greater values than those of a conventional pushrod dilatometer. A monochromatic light source, such as a laser
, illuminates the specimen. Some of the light is reflected by the specimen and interferes with the incoming light, creating optical interference fringes. As the specimen contracts or expands, there is a proportional movement of the interference fringes, which can be measured using a camera system. The measurement resolution is determined by the wavelength
of the light, and is typically 0.5 μm
for blue light. Optical dilatometers are used to measure thermal expansion
.
The optical dilatometer is in fact complementary to the traditional higher resolution push rod dilatometer
when it comes to measure dimensional changes of materials, as a function of temperature, and no contact between specimen and instrument is required.
, when irradiated by a light
beam. By using a single beam of light
with short wavelength
and a very high resolution
image sensor
it is possible to achieve good resolutions, although not comparable to those of the standard dilatometers. For example, using a blue light
with a wavelength
below 1500 micrometers, it is possible to achieve images with an actual resolution
of nearly 1500 micrometers/pixel of the camera (not an interpolated resolution but an actual resolution). By using two light
beams, which illuminate two small portions of material at the extremities of the sample, set perpendicularly to the beams direction, it is then possible to achieve an absolute measurement
of longitudinal variation while heat-treating the specimen.
There are two major types of optical dilatometers which are currently in use:
in the second half of 19th century. This design has a reflected beam of monochromatic light
and the measurement
of the displacement is carried out by counting the interference fringes between the forward going beam and the reflected beam. After the Abbe invention, many improvements were achieved on the original design and there are now many models available on the market, which use modern optics
and designs.
Over the last five decades, interest has grown in the use of thermomechanical technologies for characterising materials in different fields of science and engineering. In particular, the use of optical methods has been playing a role in the field of ceramic
s and have been used to measure the coefficient of thermal expansion of bodies and glazes
in order to ensure an optimal match of their thermomechanical behaviour.
of their certified reference materials
. This method has proved to be very accurate, with a resolution of a fraction of the wavelength of the incident light, but it is limited by the reflectivity
of the surface of the specimen. If the specimen is not reflective, or it becomes non reflective during the test, it is then necessary to use a mirror, which is set in contact with the specimen by using a refractory
push rod. By doing so such method loses the advantage of being non contact and becomes substantially similar to the electronic dilatometer
.
Optical dilatometers are used along traditional dilatometers in ceramics for non-destructive testing of a sample's behavior during a thermal treatment. Optical dilatometers are used for thermal analysis
of various types of materials, such as incoherent materials (expansion and contraction of an incoherent granular frit
, as applied, for example, on raw tiles) and polymers (behaviour above the glass transition temperature, where the surface tension
starts pulling the edges and making the sample shrink), as well as for analysis of various processes in materials manufacturing, for example, sintering
kinetics, thermal expansion
and sintering
behaviour of thin glaze
layer or polymer
thin films.
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
, illuminates the specimen. Some of the light is reflected by the specimen and interferes with the incoming light, creating optical interference fringes. As the specimen contracts or expands, there is a proportional movement of the interference fringes, which can be measured using a camera system. The measurement resolution is determined by 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 light, and is typically 0.5 μm
Micrometer
A micrometer , sometimes known as a micrometer screw gauge, is a device incorporating a calibrated screw used widely for precise measurement of small distances in mechanical engineering and machining as well as most mechanical trades, along with other metrological instruments such as dial, vernier,...
for blue light. Optical dilatometers are used to measure thermal expansion
Thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
.
The optical dilatometer is in fact complementary to the traditional higher resolution push rod dilatometer
Dilatometer
A dilatometer is a scientific instrument that measures volume changes caused by a physical or chemical process. A familiar application of a dilatometer is the mercury-in-glass thermometer, in which the change in volume of the liquid column is read from a graduated scale...
when it comes to measure dimensional changes of materials, as a function of temperature, and no contact between specimen and instrument is required.
Structure and types
The most recent types of optical dilatometer use a direct-beam system so as to avoid contact-related problems by measuring the image that the specimen projects on an image sensorImage sensor
An image sensor is a device that converts an optical image into an electronic signal. It is used mostly in digital cameras and other imaging devices...
, when irradiated by a light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
beam. By using a single beam of light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
with short 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...
and a very high resolution
Optical resolution
Optical resolution describes the ability of an imaging system to resolve detail in the object that is being imaged.An imaging system may have many individual components including a lens and recording and display components...
image sensor
Image sensor
An image sensor is a device that converts an optical image into an electronic signal. It is used mostly in digital cameras and other imaging devices...
it is possible to achieve good resolutions, although not comparable to those of the standard dilatometers. For example, using a blue light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
with a 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...
below 1500 micrometers, it is possible to achieve images with an actual resolution
Optical resolution
Optical resolution describes the ability of an imaging system to resolve detail in the object that is being imaged.An imaging system may have many individual components including a lens and recording and display components...
of nearly 1500 micrometers/pixel of the camera (not an interpolated resolution but an actual resolution). By using two light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
beams, which illuminate two small portions of material at the extremities of the sample, set perpendicularly to the beams direction, it is then possible to achieve an absolute measurement
Measurement
Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...
of longitudinal variation while heat-treating the specimen.
There are two major types of optical dilatometers which are currently in use:
- Horizontal OD: the rod-specimen is laid horizontally on the sample holder and, during the thermal treatment in the oven, it is completely free to expand and contract. The displacement of the sample holder is not relevant for the result of the measurementMeasurementMeasurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...
because the optical system follows the horizontal movements of small portions of the specimen at its extremities. - Vertical OD: the rod-specimen is set vertically on the sample holder (alumina thin plate) and while one camera “watches” the top of the plate the other follows the vertical displacements of the sample’s top edge. This system has been used so far to find the characteristic temperatures (phase transitions, maximum sintering speed, swelling, etc.) of ceramic materials. In fact, phase transitions are always associated to dimension variation of the material: sinteringSinteringSintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...
is accompanied by high shrinkages, related to the decrease of porosity -swellingSwelling (medical)In medical parlance, swelling is the transient enlargement or protuberance in the body and may include tumors. According to cause, it may be congenital, traumatic, inflammatory, neoplastic or miscellaneous....
usually appears at high temperatures. After sinteringSinteringSintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...
occurs and the viscosityViscosityViscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
of the ceramics formulation strongly decreases and trapped gas is then free to exit.
History
The first optical dilatometer was invented by Abbe and FizeauHippolyte Fizeau
Armand Hippolyte Louis Fizeau was a French physicist.-Biography:Fizeau was born in Paris. His earliest work was concerned with improvements in photographic processes. Following suggestions by François Arago, Léon Foucault and Fizeau collaborated in a series of investigations on the interference of...
in the second half of 19th century. This design has a reflected beam of monochromatic light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
and the measurement
Measurement
Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...
of the displacement is carried out by counting the interference fringes between the forward going beam and the reflected beam. After the Abbe invention, many improvements were achieved on the original design and there are now many models available on the market, which use modern optics
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
and designs.
Over the last five decades, interest has grown in the use of thermomechanical technologies for characterising materials in different fields of science and engineering. In particular, the use of optical methods has been playing a role in the field of ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
s and have been used to measure the coefficient of thermal expansion of bodies and glazes
Ceramic glaze
Glaze is a layer or coating of a vitreous substance which has been fired to fuse to a ceramic object to color, decorate, strengthen or waterproof it.-Use:...
in order to ensure an optimal match of their thermomechanical behaviour.
Applications and fields of research
In order to measure thermal expansion up to high temperatures, the material to be tested has to be set on a sample holder inside an oven which has its own thermal expansion. In order to achieve a good accuracy it is necessary to measure the expansion of the sample holder and to subtract it from the actual expansion of the specimen. The best approach is to split the laser beam into two beams of light, which are reflected by the top-edge of the sample and by the top-edge of the sample holder, or by both longitudinal edges of the specimen. By detecting the longitudinal variations at both the extremities of the specimen the measurement achieved is absolute and there is no need for further corrections. This is the most accurate way of measuring thermal expansion and it may attain nanometric resolution. This is the type of instrument used by the suppliers of certified standard materials. For example, the National Institute for Standards and Technology uses a Fizeau double-beam interferometer to certify the thermal expansionThermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
of their certified reference materials
Certified reference materials
Certified Reference Materials are ‘controls’ or standards used to check the quality and traceability of products. A reference standard for a unit of measurement is an artifact that embodies the quantity of interest in a way that ties its value to the reference base for calibration.At the highest...
. This method has proved to be very accurate, with a resolution of a fraction of the wavelength of the incident light, but it is limited by the reflectivity
Reflectivity
In optics and photometry, reflectivity is the fraction of incident radiation reflected by a surface. In general it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength...
of the surface of the specimen. If the specimen is not reflective, or it becomes non reflective during the test, it is then necessary to use a mirror, which is set in contact with the specimen by using a refractory
Refractory
A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above...
push rod. By doing so such method loses the advantage of being non contact and becomes substantially similar to the electronic dilatometer
Dilatometer
A dilatometer is a scientific instrument that measures volume changes caused by a physical or chemical process. A familiar application of a dilatometer is the mercury-in-glass thermometer, in which the change in volume of the liquid column is read from a graduated scale...
.
Optical dilatometers are used along traditional dilatometers in ceramics for non-destructive testing of a sample's behavior during a thermal treatment. Optical dilatometers are used for thermal analysis
Thermal analysis
Thermal analysis is a branch of materials science where the properties of materials are studied as they change with temperature. Several methods are commonly used - these are distinguished from one another by the property which is measured:...
of various types of materials, such as incoherent materials (expansion and contraction of an incoherent granular frit
Frit
Frit is a ceramic composition that has been fused in a special fusing oven, quenched to form a glass, and granulated. Frits form an important part of the batches used in compounding enamels and ceramic glazes; the purpose of this pre-fusion is to render any soluble and/or toxic components insoluble...
, as applied, for example, on raw tiles) and polymers (behaviour above the glass transition temperature, where the surface tension
Surface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...
starts pulling the edges and making the sample shrink), as well as for analysis of various processes in materials manufacturing, for example, sintering
Sintering
Sintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...
kinetics, thermal expansion
Thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
and sintering
Sintering
Sintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...
behaviour of thin glaze
Ceramic glaze
Glaze is a layer or coating of a vitreous substance which has been fired to fuse to a ceramic object to color, decorate, strengthen or waterproof it.-Use:...
layer or polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
thin films.