The porous material is constituted of net structure and porosity, which is interconnected or closed. We called opening pores which are interlinked, and we called closed pores which are spaced. Some are semi open¬closed pores.
The porous material is very common in nature. It shows some excellent mechanical performances and has been widely used. The most common porous materials are woods and honeycomb. The pyramids of ancient Egypt were used of porous material. Bottle stopper and human bones also belong to porous material. They possess the characteristics of low density and high mechanical strength.
People began to prepare porous material. The simplest is honeycomb material which composed of a large number of similar composition prism pores. The powder, mesh, fiber can be used to produce porous material through artificial processing. A large random or directional pores were distributed in interior. Internal pores may be designed to various shapes such as foamed type and honeycomb type, and so on.
The porous material shows many excellent performances including small specific gravity, rigidity, strength, shock absorption, sound absorption, permeability, air permeability, and so on. On the other hand, porous materials possess the characteristics of structural materials and functional materials.
Porous materials can be divided into sintered porous metal, sintered porous ceramic , nonmetallic porous , intermetallic porous materials, and so on.
The main characteristics of porous metal are listed as follows:
Permeability and adsorption
The porous material shows excellent permeability, selective permeation and adsorption. Now various shape of porous metal have been manufactured for filtration and separation.
The density is significantly reduced due to a large number of pores in the porous material body. The density of porous steel can be reduced to 34.2%comparing with dense material. The density of porous aluminum alloy or magnesium alloy may be less than l g/cm3. Powder metallurgy porous material widely used in traffic and transportation. Porous material plates are produced for cars and skinning of machine. Porous material can significantly reduce the weight in aerospace.
Energy absorption characteristics
The porous material is capable of absorbing energy. Mechanical wave will transmission in the porous material. That is to say, there will occur reflections when the wave propagates to the dense portion of the pores of the interface. The porous material can be produced to be as insulation materials, damping materials and anti¬explosion shock material according to this characteristic. To produce energy absorbers and shock-absorbing buffers according to energy absorption characteristics of the metal material, which can avoid or reduce the incidence of fatal accidents when subjected to sudden impact.
Powder metallurgy porous material shows unique optical and electrical properties. For example, Porous silicon material can emit visible light under laser irradiation, which is an ideal material for the manufacture of new optoelectronic components. The porous electrode of fuel cell can be produced according to the special optical and electrical properties of porous materials. This battery is considered to be a promising vehicle energy system.
The porous material with molecular recognition function, which will produce artificial enzymes and improve the catalytic reaction rate significantly. Heat exchange capacity, electrochemistry and catalysis of the porous material are better than the dense metal. The porous material also shows high heat exchange efficiency, acoustic resistance and anti¬corrosive.
A large amounts of gas existed in porous interior. Energy can be resolved out by gas. Powder metallurgy porous material with excellent sound absorption can isolate the noise source. Magnesium alloy porous material can prevent sonar tracking, which applied in submarine wall. Which also can significantly reduce the noise in ceiling and vehicles.
Pore size and porosity of the porous material can be controlled. Porous material is widely applied in communication, environmental and other fields due to many characteristics. The porous material possess the biological properties using simulation technology. It can be used as an ideal material of human bone.