porous material

A material consisting of interconnected or closed holes to form a network structure

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Porous material is a kind of material with network structure composed of interconnected or closed holes, and the boundary or surface of holes is composed of pillars or plates.

Chinese name: porous material
Foreign name: Porous Material
Classification: Microporous materials, mesoporous materials, macroporous materials

Micropore: Hole diameter less than 2 nm
Mesopore: Hole diameter between 2 and 50 nm
Macropore: Hole diameter greater than 50 nm

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Material Introduction

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A solid containing a certain number of holes is called porous material, and the boundary or surface of the holes is composed of pillars or plates. typical pore structure One is composed of a large number of polygon Formed by gathering holes on the plane 2D structure , because its shape is similar to that of a honeycomb hexagon Structure is called "honeycomb" material.

Ceramic porous materials

More commonly, a large number of polyhedron Shaped holes in Spatial aggregation Formed three-dimensional structure , commonly referred to as "foam" material. Some documents porosity It is called porous material if it is sufficient, and it is called foam material if it is larger than the porosity of porous material. From a large number of domestic and foreign literature, the porosity of foam materials is not greater than that of porous materials, such as Aluminum foam Its porosity is often lower than that of porous materials. Some literatures call the materials with lower porosity foam materials, while others believe that because the materials were originally prepared by foaming method, they were once called Foaming material Later, the preparation methods such as percolation were developed, which were called aerated materials, and the more appropriate name should be porous foam materials, referred to as porous materials or foam materials. In short, there is no unified, strict and universally recognized definition. Most scholars regard porous materials and foam materials as equivalent concepts. Porous materials are ubiquitous in nature, such as wood cork 、 Sponge And coral (the word "cellose" comes from the meaning of "full of holes" Latin The small word "cellula").

For thousands of years, these natural porous materials have been widely used by people. In ancient Egypt many years ago Pyramid Wooden building materials have been used in Roman times Cork is used as The wine bottle Of Cork 。 In modern times, people began to make their own porous materials, the simplest of which is composed of a large number of similar prismatic holes Honeycomb Material, can be used as Lightweight components 。 More common is polymer foam material, which has a wide range of applications, and can be used for small to everywhere coffee cup , as big as the airplane cabin Shock pad 。 The development of modern technology makes metal, ceramics, glass and other materials similar to polymer It foams like that. These new foam materials are gradually used as insulation, buffer and absorption Impact energy So as to play its unique comprehensive properties determined by the porous structure.

classification

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The above methods are classified by pore size from International Pure Chemistry and Applied chemistry In order to promote the research of porous materials, the above terms are recommended. According to the different pore size, porous materials can be divided into microporous (pore size less than 2 nm) materials Mesopore (Aperture 2-50 nm) material and Macropore (pore size greater than 50 nm).

characteristic

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relative continuous medium In terms of materials, porous materials generally have relative density Low specific strength High Specific surface area High, light weight, sound insulation, heat insulation Permeability Good and other advantages. Specifically, porous materials generally have the following six characteristics:

porous material

Mechanical properties

The application of porous materials can improve strength and rigidity Mechanical properties At the same time, the density is reduced, so it has certain advantages in aerospace and aviation industries. It is estimated that if the aircraft is converted to porous materials, under the same performance conditions, Aircraft weight Reduce to half of the original. Another change in mechanical properties of porous materials is impact toughness Improvement of, applied to automotive industry Can effectively reduce traffic accident Creative damage to passengers.

Propagation performance

wave propagation Reflection and refraction will occur at the interface of two media. The existence of pores increases the possibility of reflection and refraction as well as the possibility of diffraction. Therefore, porous materials can act as wave barriers. With this property, porous materials can be used as Sound insulation material , damping materials and materials resistant to explosion shock.

Photoelectric performance

Porous materials have unique optical properties porous silicon Materials can emit under laser irradiation visible light , will become a new type of light electronic component Ideal material for. The special photoelectric properties of porous materials can also be made fuel cell Of Porous electrode This battery is considered to be the most promising energy device for the next generation of automobiles.

porous material

Permeability

Because people have been able to make rules pass Moreover, the porous material has regular arrangement, and the size and direction of the holes can be controlled. This property can be used to make molecular sieve , such as high efficiency Gas separation membrane Special reusable filtering device, etc.

Adsorbability

Because the diameter of each gas or liquid molecule is different, its freedom of movement is different, so the adsorption capacity of porous materials with different pore sizes for different gases or liquids is different. This property can be used to produce efficient gas or Liquid separation membrane , this Separation membrane It can even be reused.

Chemical properties

As the density of porous materials decreases, the activity of general materials will increase. Based on having molecular recognition Artificial enzymes produced by functional porous materials can greatly improve catalysis reaction rate 。

Material Science

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porous material

Although sintered porous materials mechanical property And corrosion resistance are inferior to dense metal due to the existence of pores, but some properties such as heat exchange capacity electrochemistry Activity Catalysis Eternal cause specific surface It is much better than dense metal. Porous materials also have a series of functions that dense metals do not have, such as pores can penetrate gas and liquid media, absorb energy, or Buffering effect 。 Sintered porous materials have special properties due to different uses, such as requirements for filter materials Filtering precision 、 Penetrability and Reproducibility ； Some porous materials require heat exchange efficiency, electrochemical activity, acoustic resistance electron emission Ability, etc.

The main parameters characterizing the porous structure are: porosity , average aperture, maximum aperture Pore size distribution , hole shape and specific surface. In addition to the material, the porous structure parameters of the material Mechanical properties of materials And various Performance Has a decisive influence. Because the pores are formed by the accumulation, compaction and sintering of powder particles; Therefore, the physical and Chemical properties In particular, the size, distribution and shape of powder particles are the main factors that determine the porous structure and even the final use performance. Porous structure parameters and some service properties (such as Penetrability There are many measuring principles and methods. Commonly used hole diameter Bubble method , gas permeation method, adsorption method, mercury pressure method, etc. The specific surface temperature is usually low Nitrogen adsorption method And fluid permeability method. When selecting the determination method, try to select a method that is similar to the use conditions. The movement of fluid through porous body laminar flow Conditional obedience Darcy formula , that is, the flow rate and Pressure gradient Proportional to Fluid viscosity Inversely proportional to Proportional constant That is to say, the transmission coefficient is the characteristic parameter 。 When the through porosity and pore diameter increase, or the thickness of porous body and fluid viscosity decrease, the permeability of sintered porous materials increases. 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, which is related to the "notch" effect. When the porosity is constant, the permeability of materials with small pore size is small, but the strength is high because there are many contact points between particles. Filtration precision, i.e. interception capacity, refers to the maximum particle size in the fluid passing through the porous body, which is generally related to the maximum pore size. Pore size distribution is the criterion of the uniformity of porous structure. The ratio of permeability to filtration accuracy shall be increased as much as possible on the premise that the filter material has sufficient strength. According to these principles, it is developed to use graded spherical powder as raw material to make uniform porous structure Powder rolling Fabrication and welding of porous thin strips Thin-walled tube A double layer porous material composed of a macroporous layer and a microporous layer was developed.

Porous materials can be made of various metals and alloys and Refractory metal Of carbide 、 nitrides 、 boride and

porous material

Silicide It is made of bronze stainless steel , nickel and titanium, etc. Porosity of porous materials is generally more than 15%, up to 90%, and the pore size ranges from hundreds of angstroms to millimeters. Porosity of porous materials generally falls into three categories: low porosity (<30%), medium porosity (30-60%), and high porosity (>60%). Pore size falls into three categories: coarse porosity (more than 50nm), medium porosity (2-50nm), and micropore (less than 2nm). Porous materials with low porosity are mainly Oil bearing , high porosity also includes Metal fibre Porous materials and foam metal are mainly used for battery plates, heat insulation, noise reduction, shock resistance, etc. Extensive use of filter materials and Sweating cooling material (See metal sweating materials ）Most of them are of medium porosity. Porous materials for filtration can be classified into grade series according to filtration accuracy and flow rate.

technology

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porous material

Powder raw materials for manufacturing porous materials can be spherical and irregular shaped powders or Metal fibre 。 Easy to obtain with spherical powder Fluid resistance Small, uniform structure and good regeneration of filtration and Fluid state Porous material used for control, but the mechanical properties of this powder product are not as good as those of irregular shaped powder products. Irregular shaped powders or fibers are used to produce materials with high porosity. In order to obtain the porous structure caused by the stacking of powder particles, the forming pressure and sintering temperature Generally lower than manufacturing sintered dense materials.

The pore size, strength and other properties of porous materials depend to a large extent on the Average particle size 、 Particle size distribution 、 Grain shape Etc; In order to produce materials with predetermined properties, the powder is usually pretreated, such as annealing Particle size classification , spheroidizing and ball sorting, and adding various additives（ Pore forming agent , lubricant Plasticizer ）Etc. In addition to ordinary cold die pressing- agglomeration process In addition, it can also be selected according to the shape and size of the products Loose sintering (simple profiled products), powder rolling (plates, strips and tubes with thickness of 0.1-3mm), extrusion (long profiled products) Isostatic pressing (large special-shaped products) and Slurry pouring (Complex shaped products) and other processes (see Powder metallurgy sintering ， Powder metallurgy forming ）。 If metal fiber is used as raw material, it is usually prepared by depositing in liquid uniform distribution And then pressed and sintered into metal fiber porous material. To make foam metal with powder foaming agent and curing agent Uniformly mixed with the powder to form, and heating process After foaming, curing and sintering. The porosity of this kind of foam metal can reach more than 90%. In order to improve the comprehensive performance, the powder with different particle sizes can also be used to make double-layer or Multilayer structure Or mix the powder with Metal mesh Or fiber together Fiber reinforced materials 。

prospect

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In many porous materials, the preparation angle and disordered porous materials are easy to prepare, low cost, easy to promote and use. for example Foam metal 。 There are five common methods: (1) Powder metallurgy , which can be divided into loose sintering and Reaction sintering Two; (2) Seepage method; (3) Jet deposition ; (4) Melt foaming method; (5) Eutectic Directional solidification method. Figure 2 shows the seepage method, which converts the solubility of certain particle size salt grain It is filled in the mold and compacted. It is put into the furnace together with the mold for heating Crucible furnace The required alloy is prepared in the Alloy melting After that, the molten metal is poured into the mold and penetrated into the gap between particles by applying a certain pressure on the surface of the molten metal; When the liquid metal solidifies, a composite of metal alloy and particles can be obtained, and the salt particles in the composite can be dissolved with water to prepare the foam alloy with three-dimensional connected bubbles. But this method produces Material properties Uneven, the quality is difficult to control.

The preparation process of porous materials with controllable pores is relatively complex, and technical conditions High requirements. From the characteristics analyzed above, controllable porous materials have many characteristics that disordered porous materials do not have. With the development of new technologies, the preparation methods of controllable porous materials will become more and more mature. Such methods will become the science of porous materials in the future Development trend 。

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