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Charge And Material Transport In Porous Materials

Jun 28, 2018

I. Introduction to the definition and application of porous materials

A porous material is a material that forms a network structure from pores that penetrate or close each other, and the boundaries or surfaces of the pores are formed by pillars or plates. Typical hole structures are: a two-dimensional structure formed by a large number of polygonal holes converging on a plane; it is called a "honeycomb" material because its shape is similar to the hexagonal structure of the honeycomb; more generally, it is a large number of The three-dimensional structure of the polyhedral holes formed in space is often called "foam" material. If the solids that make up the hole exist only at the boundary of the hole (that is, the holes are connected), they are called open holes; if the hole surface is solid, that is, each hole is completely separated from the surrounding holes, it is called closed hole. And some holes are semi-closed holes.

Porous materials are one of the most rapidly developing materials in materials science. In particular, porous materials with pore sizes in the nanometer range have many unique properties and strong applicability, which has attracted the attention of European and American scientific circles as well as industrial and commercial circles. At the 1994 MRS meeting, many companies reported their new progress in the practical application of porous materials. The U.S. Department of Energy has provided huge funding for the further study of porous materials used to select transmembrane separation technologies. Porous materials have a wide range of research. There are many types of inorganic aerogels, organic aerogel closures, porous semiconducting materials, and porous metal materials. The common characteristics of these materials are their low density, high porosity, large surface area, and selective gas permeation.

II. Charge and Material Transport in the Classification of Porous Materials

Mechanical properties

Parts made of porous materials can improve mechanical properties such as strength and stiffness while reducing density. It is estimated that the use of porous materials in the aircraft, under the same mechanical properties, the net mass will be reduced by half. In addition, porous materials have higher impact toughness and are applied to the automotive industry, which will effectively reduce the damage to passengers caused by traffic accidents.

The effect of the porous structure on the mechanical properties should be divided into direct and indirect effects. For example, accelerating (or slowing) the diffusion process, the effect of the phase change on the indirect effects of such pores is the formation of certain structures. The direct influence of stomata is the relationship between the characteristics of stomata and the mechanical properties. By using single-phase materials, the direct and indirect effects of stomata can be distinguished.

“Mechanical properties of porous materials,” the literature mentions that the results of tests on various grades of industrial iron powder show that as the concentration of contaminant impurities decreases, the minimum and maximum values of KIC correlation are toward lower porosity. The direction of the change, but can not successfully distinguish the impact of the above factors. For all of the porosity values, the fracture of the iron specimen was intragranular, and the maximum breaking rate on the fractured surface corresponded to the minimum crack resistance. The results obtained have been interpreted. According to the assumption that the pores all have a spherical shape and uniform distribution, cracks will purify and bend like elastic fibers when passing between the pores. Experiments have shown that it is possible to prevent the propagation of cracks in iron due to pores. At the same time, it is proved that in addition to the general porosity, the properties of other porous structures must also be considered, in this case for the same spherical pores.

2. Adsorption performance

The molecular diameters and the degrees of freedom of thermal movement of different gases or liquids are different. Therefore, it is possible to use the same porous materials for different gas or liquid adsorption capacity differences to prepare highly reusable porous gas for adsorption of gases or liquids. Purification material. Different pore materials have different ability to adsorb nitrogen, and their application in daily production and life is different. In other words, different applications require that hole materials have different properties. Microporous zeolites, mesoporous materials, multi-porous materials, and porous aromatic frameworks (PAF) materials have their own characteristics, and because of their different structures and properties, they are applied to various fields of daily life. For example, the microporous zeolite molecular sieve is widely used in petroleum catalysis, environmental protection, fine chemical industry and other fields with its regular pore structure and size, strong adsorption capacity and high catalytic performance. Mesoporous materials are based on their narrow pore size distribution, relatively regular pore arrangement sequence and large specific surface area, and are good catalysts and catalyst carriers in macromolecular catalytic reactions.

3. Penetration performance

In the process of material preparation, porous membranes, high-temperature gas separation membranes and other filtration devices can be prepared by controlling the structural characteristics such as pore size, direction, pore type, and arrangement rules, combining the inherent characteristics of porous materials, such as good heat resistance and high structural stability. .

The literature reports that when the porosity of porous materials is in the range of 57-95%, both the viscous permeability coefficient and the inertial permeability coefficient increase significantly with the increase of porosity; when the porous material size range is 10-40PPI, the viscous permeability coefficient and The inertial permeability coefficient also increases significantly with the increase in pore size. When the pore size decreases, the volumetric specific surface area increases, the fluid resistance increases, and the viscosity permeability coefficient decreases. When the pore size decreases, the flow rate of the fluid through the porous material at the same pressure decreases, the fluid inertial energy loss decreases, and the inertial washing decreases.

4. Optoelectronic performance

Porous silicon can emit visible light when irradiated with laser light. Based on this characteristic, it is considered to be an ideal material for new optoelectronic components. At the same time, porous materials are also considered to be the first choice for electrode materials in future hybrid fuel vehicles. Recent studies have shown that the pore size, pore structure, pore distribution, and pore wall thickness of the electrode active material can influence the electrolyte infiltration, ion transport, and ion diffusion in the active material crystal to a large extent, thereby affecting the electrode. The overall performance of porous materials in electrochemical energy conversion and storage applications has become a new topic and has attracted widespread attention.

For the types of lithium-ion battery anode materials, the types can be mainly divided into carbon anode and cathode non-carbon materials. Carbon anode materials mainly include graphite anode materials, soft carbon materials, hard carbon materials, and some carbon composite materials. Such anodes have a low cost and good cycle stability, and have been well applied in industry, but the theoretical specific capacity is relatively low. Small, not very conductive. In order to increase the specific capacity of the negative electrode material, many studies have shifted to some metal oxides, and some lithium alloys. These materials have a higher specific capacity than graphite, but in the charge-discharge process, the material itself will undergo severe volume expansion, which will cause the material structure to be destroyed, and the cycle stability of the battery is very poor, hindering its industrial production. Yu et al. prepared a novel electrode material (MTO/3D-GN) by embedding porous titania microspheres in a graphene-based porous material. This material has excellent electrochemical properties, with a reversible specific capacity of up to 124 mAh/g at a current density of 20C. The cycle performance and rate performance of this material are also superior to pure titanium dioxide materials. The reaction mechanism here can be explained as follows: The three-dimensional graphene-based porous material (3D-GN) provides a large contact area for the electrode material and the electrolyte, accelerates the transmission speed of electrons and lithium ions, and is a titanium oxide during electrochemical reaction. The volume change provides double protection, the most important being that the conductivity of the entire electrode material is increased during the electrochemical process. Cao et al. prepared a lithium ion battery electrode material that does not require a binder by depositing molybdenum disulfide on the surface of a porous graphene skeleton. This material has excellent electrochemical properties. In addition, the interpenetrating pore structure provided by the graphene-based porous material can facilitate the diffusion and transfer of electrons and lithium ions, effectively reducing the transfer path, and thus does not require the provision of other conductive agents.