Metal foam
Encyclopedia
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
, frequently aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
, containing a large volume fraction of gas-filled pore
Pore
- Animal biology and microbiology :* Sweat pore, an anatomical structure of the skin of humans used for secretion of sweat* Canal pore, an anatomical structure that is part of the lateral line sense system of some aquatic organisms...
s. The pores can be sealed (closed-cell foam
Foam
-Definition:A foam is a substance that is formed by trapping gas in a liquid or solid in a divided form, i.e. by forming gas regions inside liquid regions, leading to different kinds of dispersed media...
), or they can form an interconnected network (open-cell foam). The defining characteristic of metal foams is a very high 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%...
: typically 75–95% of the volume consists of void spaces. The strength of foamed metal possesses a power law
Power law
A power law is a special kind of mathematical relationship between two quantities. When the frequency of an event varies as a power of some attribute of that event , the frequency is said to follow a power law. For instance, the number of cities having a certain population size is found to vary...
relationship to its density; i.e., a 20% dense material is more than twice as strong as a 10% dense material.
Metallic foams typically retain some physical properties
Physical property
A physical property is any property that is measurable whose value describes a physical system's state. The changes in the physical properties of a system can be used to describe its transformations ....
of their base material. Foam made from non-flammable metal will remain non-flammable and the foam is generally recyclable back to its base material. Coefficient of thermal expansion will also remain similar while thermal conductivity
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....
will likely be reduced.
Open-cell metal foams
Open celled metal foams are usually replicas using open-celled polyurethanePolyurethane
A polyurethane is any polymer composed of a chain of organic units joined by carbamate links. Polyurethane polymers are formed through step-growth polymerization, by reacting a monomer with another monomer in the presence of a catalyst.Polyurethanes are...
foams as a skeleton and have a wide variety of applications including heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
s (compact electronics cooling
Computer cooling
Computer cooling is required to remove the waste heat produced by computer components, to keep components within their safe operating temperature limits.Various cooling methods help to improve processor performance or reduce the noise of cooling fans....
, cryogen tanks
Cryogenics
In physics, cryogenics is the study of the production of very low temperature and the behavior of materials at those temperatures. A person who studies elements under extremely cold temperature is called a cryogenicist. Rather than the relative temperature scales of Celsius and Fahrenheit,...
, PCM
Phase Change Material
A phase-change material is a substance with a high heat of fusion which, melting and solidifying at a certain temperature, is capable of storing and releasing large amounts of energy...
heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
s), energy absorption, flow diffusion and lightweight optics. Due to the high cost of the material it is most typically used in advanced technology aerospace
Aerospace
Aerospace comprises the atmosphere of Earth and surrounding space. Typically the term is used to refer to the industry that researches, designs, manufactures, operates, and maintains vehicles moving through air and space...
and manufacturing.
Extremely fine-scale open-cell foams, with cells too small to be visible to the naked eye, are used as high-temperature filter
Filter (chemistry)
In chemistry and common usage, a filter is a device that is designed to physically block certain objects or substances while letting others through. Filters are often used to remove solid substances suspended in fluids, for example to remove air pollution, to make water drinkable, and to prepare...
s in the chemical industry.
Metallic foams are nowadays used in the field of compact heat exchangers to increase heat transfer at the cost of an additional pressure drop. However, their use permits to reduce substantially the physical size of a heat exchanger, and so fabrication costs. To model these materials, most works uses idealized and periodic structures or averaged macroscopic properties.
Closed-cell metal foams
Closed-cell metal foam was first reported in 1926 by Meller in a French patent where foaming of light metals either by inert gas injection or by blowing agent was suggested. The next two patents on sponge-like metal were issued to Benjamin Sosnik in 1948 and 1951 who applied mercuryMercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
vapor to blow liquid aluminium.
Closed-cell metal foams have been developed since about 1956 by John C. Elliott at Bjorksten Research Laboratories. Although the first prototypes were available in the 50s, commercial production was started only in the 90s by Shinko Wire company in Japan. Metal foams are commonly made by injecting a gas or mixing a foaming agent
Foaming agent
A foaming agent is a surfactant, which when present in small amounts, facilitates the formation of a foam, or enhances its colloidal stability by inhibiting the coalescence of bubbles....
(frequently TiH2) into molten
Mölten
Mölten is a comune in South Tyrol in the Italian region Trentino-Alto Adige/Südtirol, located about 60 km north of Trento and about 12 km northwest of Bolzano .-Geography:...
metal. In order to stabilize the molten metal bubbles, high temperature foaming agent (nano- or micrometer sized solid particles) is required. The size of the pore
Pore
- Animal biology and microbiology :* Sweat pore, an anatomical structure of the skin of humans used for secretion of sweat* Canal pore, an anatomical structure that is part of the lateral line sense system of some aquatic organisms...
s, or cells, is usually 1 to 8 mm.
Closed-cell metal foams are primarily used as an impact-absorbing material, similarly to the polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
foams in a bicycle helmet
Bicycle helmet
A bicycle helmet is a helmet intended to be worn while riding a bicycle. They are designed to attenuate impacts to the skull of a cyclist in falls while minimizing side effects such as interference with peripheral vision...
but for higher impact loads. Unlike many polymer foams, metal foams remain deformed after impact and can therefore only be used once. They are light (typically 10–25% of the density of an identical non-porous alloy; commonly those of aluminium) and stiff
Stiffness
Stiffness is the resistance of an elastic body to deformation by an applied force along a given degree of freedom when a set of loading points and boundary conditions are prescribed on the elastic body.-Calculations:...
, and are frequently proposed as a lightweight structural material. However, they have not yet been widely used for this purpose.
Closed-cell foams retain the fire resistant and recycling capability of other metallic foams but add an ability to float in water.
Orthopedic uses
Foam metal has also begun to be used as an experimental prostheticProsthesis
In medicine, a prosthesis, prosthetic, or prosthetic limb is an artificial device extension that replaces a missing body part. It is part of the field of biomechatronics, the science of using mechanical devices with human muscle, skeleton, and nervous systems to assist or enhance motor control...
in animals. In this application, a hole is drilled into the bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
and the metal foam inserted letting the bone grow into the metal for a permanent connection. For orthopedic uses, foams from metals such as tantalum
Tantalum
Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as tantalium, the name comes from Tantalus, a character in Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion resistant. It is part of the refractory...
or titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
are often used, as these metals exhibit high tensile strength
Tensile strength
Ultimate tensile strength , often shortened to tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract...
, corrosion resistance with excellent biocompatibility
Biocompatibility
Biocompatibility is related to the behavior of biomaterials in various contexts. The term may refer to specific properties of a material without specifying where or how the material is used , or to more empirical clinical success of a whole device in...
.
Clinical studies on mammals
A notable example of clinical use of metallic foams is a Siberian HuskySiberian Husky
The Siberian Husky is a medium-size, dense-coat working dog breed that originated in north-eastern Siberia. The breed belongs to the Spitz genetic family...
named Triumph whose both back legs received foam metal prostheses. Studies on mammals have shown that porous metals, such as titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
foam, may allow the formation of 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....
systems within the porous area.
Orthopedic use in humans
More recently, orthopedic device manufacturers have started producing devices that use either foam construction or metal foam coatings to achieve the desired levels of osseointegrationOsseointegration
Osseointegration derives from the Greek osteon, bone, and the Latin integrare, to make whole. The term refers to the direct structural and functional connection between living bone and the surface of a load-bearing artificial implant...
.
Automobile uses
Metallic foams are currently being looked at as a new material for automobiles. The main goal of the use of metallic foams in vehicles is to increase sound dampening, reduce the weight of the automobile, and increase energy absorption in case of crashes, or in military applications, to combat the concussive force of IEDsImprovised explosive device
An improvised explosive device , also known as a roadside bomb, is a homemade bomb constructed and deployed in ways other than in conventional military action...
.
The metallic foams that are being looked at currently, are aluminium and its alloys due to their low density (0.4–0.9 g/cm3). In addition these foams have a high stiffness, are fire resistant, do not give off toxic fumes, are fully recyclable, have high energy absorbance, have low thermal conductivity, have low magnetic permeability, and are efficient at sound dampening, especially in comparison to light weight hollow parts. In addition partial addition of metallic foams in hollow parts of the car will decrease weakness points usually associated with car crashes and noisy vibrations. These foams are cheap to cast by using powder metallurgy (as compared to casting of other hollow parts).
In comparison to polymer foams (for uses in automobiles), metallic foams are stiffer, stronger, and more energy absorbent. They are more fire resistant, and have better weathering properties when considering UV light, humidity, and temperature. However, they are heavier, more expensive, and non-insulating.
Metal foam technology has also been applied in the treatment of the automotive exhaust gas
Exhaust gas
Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline/petrol, diesel fuel, fuel oil or coal. According to the type of engine, it is discharged into the atmosphere through an exhaust pipe, flue gas stack or propelling nozzle.It often disperses...
. Compared to the traditional catalytic converter
Catalytic converter
A catalytic converter is a device used to convert toxic exhaust emissions from an internal combustion engine into non-toxic substances. Inside a catalytic converter, a catalyst stimulates a chemical reaction in which noxious byproducts of combustion are converted to less toxic substances by dint...
that uses cordierite
Cordierite
Cordierite or iolite is a magnesium iron aluminium cyclosilicate. Iron is almost always present and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: 2 to 2...
ceramic as substrate, the metal foam substrate can offer better heat transfer and exhibits excellent mass-transport properties (high turbulence) offering possibilities for using less platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
catalyst.