A metallic foam or ceramic foam is a cellular structure consisting of a solid metal or ceramic material containing a large volume fraction of gas-filled pores. The pores can be sealed (closed-cell foam) or can form an interconnected network (open-cell foam). The defining characteristic of these foams is a very high porosity, with typically 75-95% of the volume consisting of void spaces. The strength of foamed material possesses a power law relationship to its density: for example, a 20% dense material is more than twice as strong as a 10% dense material. Metallic foams typically retain some physical properties 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. The coefficient of thermal expansion also typically remains similar, while thermal conductivity is likely to be reduced.
Metals and Alloys
Open-celled metal foams are usually replicas produced using open-celled polyurethane foams as a skeleton. These foams have found a wide variety of applications in heat exchangers, energy absorption, flow diffusion and lightweight optics. Extremely fine-scale open-cell foams are used as high-temperature filters in the chemical industry. Metallic foams used in compact heat exchangers increase the heat transfer at the cost of an additional pressure drop. However, their use permits the physical size of a heat exchanger to be reduced substantially, and therefore also the fabrication costs.
Closed-cell metal foams have been developed since the 1950s, but although prototypes were available, commercial production was started only in the 1990s. Close-celled metal foams are commonly made by injecting a gas or mixing a foaming agent into molten metal. The material is then stabilized using a high temperature foaming agent (usually nano- or micrometer sized solid particles). The size of the pores, or cells, is usually 1 to 8 mm. Closed-cell metal foams are primarily used as an impact-absorbing material. Unlike many polymer foams, metal foams remain deformed after impact and can therefore only be used once.
Ceramic foams are usually manufactured by impregnating open-cell polymer foams internally with ceramic slurry and then firing in a kiln, leaving behind only ceramic material. Such foams are often used for thermal insulation, acoustic insulation, adsorption of environmental pollutants, filtration of molten metal alloys, and as substrate for catalysts requiring large internal surface area. Ceramic foams have also been used as stiff lightweight structural material, specifically for support of reflecting telescope mirrors.
Gas-releasing foaming agents
Adding foaming agents to a liquid melt is another method of releasing gas bubbles into the melt. The process is now used in small scale batches.
Solid-gas eutectic solidification (Gasar)
Eutectic systems (or multi-phase systems) formed between liquid metals and hydrogen gas can be used to create a two phase system by reducing temperature and pressures. The process leads to gas entrapment within the solidifying metal.
An indirect method of producing metal foams involves using polymer foam as a starting material in order to form a porous structure by using a reticulation method or foaming method. The polymer foam is filled with a heat resistant material followed by drying and removal of the polymer matrix. Molten metal is then cast into voids to produce a replica of the original foam structure.
Metal powders can also be foamed by compressing powders to a precursor material, then filling the metal compact with gas followed by heating the precursor material to expand the metal due to the entrapped gas. This process is commonly used to make porous titanium structures.
Electrodeposition of Ionized Foams
Ionized foams are made by galvanic electro-deposition. These foams have been tested for use as advanced battery electrode materials.
Chemical vapor deposition (CVD)
Either a ceramic powder is mixed to form a slurry with water or organic monomers are mixed with an initiator catalyst to form a slurry. The slurry is then mixed with a foaming agent to create a foam, followed by polymerization, drying and sintering.
Gas-releasing foaming agents
Adding foaming agents to a ceramic slurry or melt is another method of releasing gas bubbles into the melt. The process is now used in small scale batches.
Applications of Foamed Materials
Light, strong structural materials
Aircraft and automobile manufacturers frequently use lightweight strong components consisting of metal or ceramic foam. Lightweight structural materials allow increased fuel efficiency while preserving or even increasing the safety standards of the vehicle.
Porous foam membranes are used in filtration of gases and liquids.
Water remediation and absorption of environmental pollutants
Graphene oxide foams are commonly used in water treatment. In addition, spray foams have been used for environmental remediation applications.
Biomedical devices such as implants and orthopedics incorporate foams to allow for the use of biocompatible, but inconveniently heavy materials. For example, titanium foams are currently used in numerous load bearing orthopedic applications.
Lightweight armor solutions for various environments such as land, sea, and air often utilizes ceramics, composite materials and metallic foams. Aluminum foam is commonly used for lightweight armor production.
Catalyst supports use foamed ceramic structures to provide large surface areas for catalysis of chemical reactions.
Carbon-based graphite foams are commonly used in heat exchangers and heat sinks.
Vibrational dampening and noise reduction
The noise insulation properties of some foams are used to create sound barriers.
Energy storage - batteries and fuel cells
Advanced optical components and mirrors
Fuel injection & jet engines
Fuel injectors and jet engines use ceramic and metallic foam material components that can withstand high temperatures and pressures while retaining strength.
- Aluminum Foam
- Aluminum Oxide Foam
- Boron Carbide Foam
- Boron Nitride Foam
- Cadmium Foam
- Cobalt Foam
- Cobalt Chromium Foam
- Copper Foam
- Copper Aluminum Foam
- Carbon Foam
- Glassy Carbon Foam
- Vitreous Carbon Foam
- Gold Foam
- Hafnium Carbide Foam
- Iron Foam
- Iron Chromium Foam
- Iron Chromium Aluminum Foam
- Lanthanated Molybdenum Foam
- Lead Foam
- Molybdenum Foam
- Nickel Foam
- Nickel Chromium Aluminum Foam
- Nickel Chromium Foam
- Nickel Copper Foam
- Nickel Iron Foam
- Nickel Iron Chromium Foam
- Nickel Manganese Gallium Foam
- Niobium Foam
- Rhenium Foam
- Silicon Carbide Foam
- Silicon Foam
American Elements manufactures a wide variety of metal and ceramic foams. Ceramic foams are typically open-cell foams, while foamed metals can be produced with either open or closed cells. Tailoring of pore sizes and material density to customer specifications is available for most materials.