Unlike common aqueous foams (such as beer foams, shaving foams, whipped cream, etc.), solid foams are a porous solid material which are generally featured by high strength to weight ratio as well as excellent acoustic and thermal insulation properties.
Nonmetallic solid foams are cellular materials mainly produced by two different routes:
This method utilizes a unique three-dimensional mesh skeleton structure of a foam material to coat nonmetallic slurry on an organic foam mesh body, then the organic foam is dried at room temperature and burned to degrade it so that a nonmetallic foam can be obtained.
Its basic principle is to use a chemical reaction of a foaming agent to generate a certain amount of volatile gas to foam the slurry, and then to obtain a nonmetallic foam by drying and sintering.
Depending on the materials, the common nonmetallic foams are as follows:
(a) Foam glass, (b) Ceramic foam, (c) Polymeric foam, (d) Graphene foam
Foam glass, also referred to as cellular glass, is a heat insulating material with solid and gaseous phases which has many advantages such as a low density, thermal conductivity and expansion coefficient and a high mechanical strength.
Heating speed is crucial in the forming process, if the temperature is too high the bubbles will rise and the body will collapse and not form a foam body. Therefore, rapid heating can cause the glass foam feedstock to crack, whilst slow heating will lead to early release of the gas from the foaming agent before the viscosity of the glass is low enough to allow the glass to expand.
Ceramic foam is a kind of porous ceramics with high porosity and sometimes called as cellular ceramics and is a high-temperature-resistant, fireproof material and a convenient support material. Because of high amount of pores and high surface area, ceramic foams are especially suitable for filtering molten metals or hot gases, thermal protection systems, and heat exchangers.
The most common method for creating ceramic foams is to introduce a polymeric sponge with open pores into ceramic slurry, and after rolling to remove redundant slurry, the coated sponge is dried and pyrolyzed, leaving only the porous ceramic structure. Then, the resultant foam will be sintered for final densification to get required mechanical strength. This method is widely used because it is effective with most kinds of ceramic materials, such as silicon carbide, zirconia, silicon nitride, alumina, silica, mullite, and cordierite [1].
Polymeric foam is a foam, in liquid or solidified form, formed from polymers with the characters of low density, good heat insulation, good sound insulation effects, high specific strength, and high corrosion resistance and it is widely used in civil and industrial applications.
The foaming process of polymers can be separated into three stages: cell formation, cell growth, and cell stabilization (see figure 1)[2]. In the cell formation process, the escaped gas forms the cell nucleus through nucleation. When two cells of different sizes are close to each other, the gas will spread from small cell to a large cell, and the two cells merge together. Finally, the cells are usually stabilized by cooling or by adding surfactants.
Fig. 1 Schematic diagram of the foaming process
Graphene foam is a solid, open-cell foam made of single-layer sheets of graphene which combines the unique mechanical and electrical properties of two-dimensional graphene with the properties of a metallic foam, yielding an ultra-light, highly conductive material with excellent strength and flexibility.
Graphene foams are normally produced by chemical vapor deposition. Carbon containing gases, such as methane, and carrier gases are introduced to a heated metal foam substrate – usually nickel or copper. As the gases contact the substrate, they deposit a film of carbon atoms. Once the reaction is complete, the metal is etched away leaving behind a 3D lattice of graphene.
Alfa Chemistry is a leading manufacturer of foam materials. We offer nonmetallic foams in various of specification which can be used in civil and industrial applications. To place an order or request a quote, please don't hesitate to contact us.
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