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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.

Foamed 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.

Ultra High Purity Metal Foams

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

molten metal

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

metal powder
Sintered

The Fraunhofer-Institute process consists of mixing 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 advanced battery electrode materials.

Ceramics

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.

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.

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.

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 such as batteries and fuel cells.

Advanced optical components and mirrors

Lightweight mirrors and advanced optical components utilize foam components for consumer products, 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

  • Cell Morphology and Improved Heat Resistance of Microcellular Poly(L-lactide) Foam via Introducing Stereocomplex Crystallites of PLA. Pin Jia, Jie Hu, Wentao Zhai, Yongxin Duan, Jianming Zhang, and Chang-Yu Han. Ind. Eng. Chem. Res.: February 6, 2015
  • Micromixing Efficiency Enhancement in a Rotating Packed Bed Reactor with Surface-Modified Nickel Foam Packing. Guang-Wen Chu, Yin-Jiang Song, Wen-Jie Zhang, Yong Luo, Hai-Kui Zou, Yang Xiang, and Jian-Feng Chen. Ind. Eng. Chem. Res.: January 22, 2015
  • Ni2P Nanosheets/Ni Foam Composite Electrode for Long-Lived and pH-Tolerable Electrochemical Hydrogen Generation. Yanmei Shi, You Xu, Sifei Zhuo, Jingfang Zhang, and Bin Zhang. ACS Appl. Mater. Interfaces: January 7, 2015
  • Engineered Si Sandwich Electrode: Si Nanoparticles/Graphite Sheet Hybrid on Ni Foam for Next-Generation High-Performance Lithium-Ion Batteries. Chunhui Gao, Hailei Zhao, Pengpeng Lv, Tianhou Zhang, Qing Xia, and Jie Wang. ACS Appl. Mater. Interfaces: January 5, 2015
  • CO2-in-Water Foam at Elevated Temperature and Salinity Stabilized with a Nonionic Surfactant with a High Degree of Ethoxylation. Yunshen Chen, Amro S. Elhag, Leyu Cui, Andrew J. Worthen, Prathima P. Reddy, Jose A. Noguera, Anne Marie Ou, Kun Ma, Maura Puerto, George J. Hirasaki, Quoc P. Nguyen, Sibani L. Biswal, and Keith P. Johnston. Ind. Eng. Chem. Res.: December 31, 2014
  • Sulfur Nanodots Electrodeposited on Ni Foam as High-Performance Cathode for Li–S Batteries. Qing Zhao, Xiaofei Hu, Kai Zhang, Ning Zhang, Yuxiang Hu, and Jun Chen. Nano Lett.: December 26, 2014
  • Hydrothermal Growth of Hierarchical Ni3S2 and Co3S4 on a Reduced Graphene Oxide Hydrogel-Ni Foam: A High-Energy-Density Aqueous Asymmetric Supercapacitor. Debasis Ghosh and Chapal Kumar Das. ACS Appl. Mater. Interfaces: December 24, 2014
  • Formation of Layer-by-Layer Assembled Titanate Nanotubes Filled Coating on Flexible Polyurethane Foam with Improved Flame Retardant and Smoke Suppression Properties. Haifeng Pan, Wei Wang, Ying Pan, Lei Song, Yuan Hu, and Kim Meow Liew. ACS Appl. Mater. Interfaces: December 12, 2014
  • Supercritical Carbon Dioxide Anchored Fe3O4 Nanoparticles on Graphene Foam and Lithium Battery Performance. Xuebo Hu, Minhao Ma, Mengqi Zeng, Yangyong Sun, Linfeng Chen, Yinghui Xue, Tao Zhang, Xinping Ai, Rafael G. Mendes, Mark H. Rümmeli, and Lei Fu. ACS Appl. Mater. Interfaces: December 1, 2014
  • Dielectric Change of Copper Phthalocyanine and Polyurethane Foam with High Elasticity as a Function of Pressure Discussed in Terms of Conversion from Natural Mechanical Energy to Electric Energy. Shaoyan Fan, Yuezhen Bin, Rong Zhang, Panpan Zhang, Dan Zhu, and Masaru Matsuo. Macromolecules: November 20, 2014
  • Synergistic Effects of Ionic Characteristics of Surfactants on Aqueous Foam Stability, Gel Strength, and Rheology in the Presence of Neutral Polymer. Amit Saxena, A. K. Pathak, and Keka Ojha. Ind. Eng. Chem. Res.: November 20, 2014
  • A Flexible Alkaline Rechargeable Ni/Fe Battery Based on Graphene Foam/Carbon Nanotubes Hybrid Film. Jilei Liu, Minghua Chen, Lili Zhang, Jian Jiang, Jiaxu Yan, Yizhong Huang, Jianyi Lin, Hong Jin Fan, and Ze Xiang Shen. Nano Lett.: November 17, 2014