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About Foams

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.

Foam Materials

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.

Ceramics

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.

Production Methods

Metals

Melt-based methods

Gas Injection

A liquid metallic melt can be foamed by blowing gases (air, nitrogen, argon) using rotating impellers that are responsible for producing gas bubbles in the melt.

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.

Investment casting

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.

Powder-based methods

Sintered

The Fraunhofer process consists of missing metal powders with a foaming agent, densely compacting the mixture, followed by sintering to obtain the final foam product.

Gas entrapment

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 electrodes in batteries.

Ceramics

Foamed Slurry

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.

Chemical vapor deposition (CVD)

Applying ceramic foams as a surface coating uses the process of chemical vapor deposition. Protective or anti-corrosive coatings, for example, are produced using this method.

Applications of Foamed Materials

Light, strong structural materials

Aircraft and automobiles 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.

Filtration

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

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

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

Catalyst supports use foamed ceramic structures to provide large surface areas for catalysis of chemical reactions.

Thermal management

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

Recent research has found uses for copper based foams in advanced energy storage devices and batteries.

Advanced optical components and mirrors

Lightweight mirrors and advanced optical components utilize foam components for consumer product, defense and aerospace applications.

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.

Recent Research & Development for Metal and Ceramic Foams

  • Zhenyuan Gao, Wangcheng Zhan, Yunsong Wang, Yun Guo, Li Wang, Yanglong Guo, Guanzhong Lu, Aldehyde-functionalized mesostructured cellular foams prepared by copolymerization method for immobilization of penicillin G acylase, Microporous and Mesoporous Materials, Volume 202, 15 January 2015
  • Kihun Jang, Seongil Yu, Sung-Hyeon Park, Hak-Sung Kim, Heejoon Ahn, Intense pulsed light-assisted facile and agile fabrication of cobalt oxide/nickel cobaltite nanoflakes on nickel-foam for high performance supercapacitor applications, Journal of Alloys and Compounds, Volume 618, 5 January 2015
  • A.T. Kulesa, M.J. Robinson, Analytical study of structural thermal insulating syntactic foams, Composite Structures, Volume 119, January 2015
  • Junsuk Kang, Composite and non-composite behaviors of foam-insulated concrete sandwich panels, Composites Part B: Engineering, Volume 68, January 2015
  • Van Hoa Nguyen, Jae-Jin Shim, Three-dimensional nickel foam/graphene/NiCo2O4 as high-performance electrodes for supercapacitors, Journal of Power Sources, Volume 273, 1 January 2015
  • Abdulhakeem Bello, Farshad Barzegar, Damilola Momodu, Julien Dangbegnon, Fatemeh Taghizadeh, Mopeli Fabiane, Ncholu Manyala, Asymmetric supercapacitor based on nanostructured graphene foam/polyvinyl alcohol/formaldehyde and activated carbon electrodes, Journal of Power Sources, Volume 273, 1 January 2015
  • Ye Li, Xudong Cheng, Lunlun Gong, Junjie Feng, Wei Cao, Ruifang Zhang, Heping Zhang, Fabrication and characterization of anorthite foam ceramics having low thermal conductivity, Journal of the European Ceramic Society, Volume 35, Issue 1, January 2015
  • Xun-Hui Xiong, Zhi-Xing Wang, Hua-Jun Guo, Xin-Hai Li, Facile synthesis of ultrathin nickel hydroxides nanoflakes on nickel foam for high-performance supercapacitors, Materials Letters, Volume 138, 1 January 2015
  • Zhengbin Xu, Hai Hao, Electromagnetic interference shielding effectiveness of aluminum foams with different porosity, Journal of Alloys and Compounds, Volume 617, 25 December 2014
  • Boguslaw Pierozynski, Tomasz Mikolajczyk, Ireneusz M. Kowalski, Hydrogen evolution at catalytically-modified nickel foam in alkaline solution, Journal of Power Sources, Volume 271, 20 December 2014