Materials of porous materials

Sintered porous materials, although the mechanical properties and corrosion resistance due to the presence of pores is not as good as dense metal, but some properties such as heat exchange capacity, electrochemical activity, catalytic effect, etc. but due to the increase in the specific surface than dense metal is much better. Porous materials also have a series of dense metal does not have the function, such as the pores can be through the gas, liquid media, can absorb energy, or play a buffer role. Sintered porous materials for different purposes and each with special properties, such as filtering materials require filtration accuracy, permeability and regeneration; for some porous materials require heat exchange efficiency, electrochemical activity, acoustic resistance, electron emission capabilities.

The main parameters that characterize the porous structure are: porosity, average pore size, maximum pore size, pore size distribution, pore shape and specific surface. In addition to the material, the porous structure parameters of the material have a decisive influence on the mechanical properties and various use properties of the material. Since the pores are formed by the accumulation, compression and sintering of powder particles; therefore, the physical and chemical properties of the raw material powder, especially the size, distribution and shape of the powder particles, are the main factors determining the porous structure and even the final use properties. Porous structure parameters and certain performance (such as permeability, etc.) have a variety of measurement principles and methods. Pore size is often measured by bubble method, gas permeability method, adsorption method and mercury pressure method, etc. Specific surface is often measured by low-temperature nitrogen adsorption method and fluid permeability method. When choosing a measurement method, the method should be as close as possible to the conditions of use. Fluid movement through the porous body in the laminar flow conditions obey Darcy's formula, that is, the flow rate and pressure gradient is proportional to the fluid viscosity is inversely proportional to the proportionality constant that is, the permeability coefficient for the reflection of the material through the ability of the characteristic parameters. When the through porosity, pore diameter increases, or the thickness of the porous body, fluid viscosity decreases, the sintered porous material through the ability to increase. The mechanical properties of sintered porous materials not only decrease with the increase of porosity and pore diameter, but also are very sensitive to the pore shape, i.e., related to the "gap" effect. When the porosity is unchanged, the permeability of the material with small pore size is small, but because of the contact points between the particles, so the strength is large. Filtration precision that is, the blocking capacity refers to the maximum particle size in the fluid through the porous body, generally related to the maximum pore size value. The pore size distribution is the criterion for the uniformity of the porous structure. For filtration materials it is required to increase the ratio of permeability to filtration accuracy as much as possible with sufficient strength. According to these principles, the development of graded spherical powder as raw material, made of uniform porous structure, powder rolling method to manufacture porous thin strip and welded thin-walled tube, the development of coarse pore layer and fine pore layer composite of double-layer porous materials.

Porous materials can be made from a variety of metals and alloys and refractory metal carbides, nitrides, borides and

silicides, etc., but commonly used are bronze, stainless steel, nickel and titanium. The porosity of porous materials is generally above 15%, up to 90% or more, with pore sizes ranging from a few hundred angstroms to millimeters. The porosity of the porous material is generally roughly divided into low porosity (<30%), medium porosity (30 to 60%), high porosity (>60%) three categories, the pore size is divided into coarse pore (>50 μm), medium pore (2 to 50 μm) and microporous (<2 μm) three kinds. Low porosity porous materials are mainly oil bearing, high porosity also includes metal fiber porous materials and foam metal, mainly used for battery pole plate, insulation, sound deadening, shockproof, etc.. A large number of filtering materials and sweating cooling materials (see metal sweating materials) are mostly of medium porosity. Porous materials for filtration can be divided into grade series according to the filtration accuracy and flow rate.