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New porous material - metal foam

Metal foam refers to a special metal material containing foam pores. Through its unique structural characteristics, metal foam has a series of good advantages such as low density, good thermal insulation performance, good sound insulation and electromagnetic wave absorption. It is a new type of material that is gradually developed with human science and technology and is often used in aerospace and petroleum. A series of industrial developments such as chemical industry.

Ubiquitous porous media

Porous media, a type of composite medium consisting of a solid skeleton and fluids containing interconnected, irregular pores that form the material basis of the Earth's biosphere.

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Humans have long recognized the physical processes in porous media. For example, the process of absorption, retention and migration of water, fertilizer and pollutants in soil; life processes in animals and plants, such as the transfer of water and nutrients in roots, stems and leaves of plants, metabolism in animal bodies; Drying of agricultural and sideline products, food, wood and textiles; exploitation of oil, natural gas and groundwater in underground rock formations; development of geothermal resources and use of thermal storage and storage of soil formations; thermal insulation of buildings; modern casting technology, combustion technology, Smelting technology, catalytic reaction technology and the development of textile and papermaking technologies.

So, what are the characteristics of porous media?

Researchers focus on four aspects: structural properties, flow properties, heat transfer properties and mass transfer properties.

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There are four main parameters for characterizing the structural properties of porous media: one is the pore size, which can be as small as a few microns, or even a few nanometers, such as thermal barrier coatings, catalyst coatings, and the like. It can be as large as a few meters (for example, a building), or even larger; the second is the porosity, that is, the ratio of the void volume in the porous medium to the total volume, and the value is between 0 and 1. The third is the shape of the hole, that is, the hole. Structural form. The pore shape of different kinds of porous media is quite different; the fourth is the specific surface area, that is, the size of the solid skeleton and fluid contact surface area per unit volume, which can be small or large, and can reach tens of thousands. Its value is determined by the structural properties of the porous medium.

The parameter characterizing the flow characteristics of a porous medium is mainly the pressure drop or the permeability coefficient, that is, the flow resistance loss of the fluid (liquid, gas) flowing through the porous medium.

The heat transfer characteristics of porous media, generally heat conduction, heat convection and heat radiation are encountered, and sometimes involve phase change heat.

The mass transfer characteristics of porous media, that is, the migration properties of materials in porous structures of different sizes.

Porous media commonly found in industry

The porous media in the industry can be divided into micro-nano-scale porous media and macroporous-scale porous media according to their sizes, and their characteristics are different, and the applicable research theories are also different.

There are basically four types of macroporous porous media: different shape particles, different channel shape honeycombs, different weaving meshes and different material foams.

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The pore sizes of the above four macroporous-scale porous media are on the order of millimeters, and their pore shape, porosity, specific surface area, flow characteristics and heat and mass transfer characteristics are quite different.

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Comparison of various porous structural properties

Metal foam and its application

As a foam-like macroporous porous medium, metal foam is a new type of metal material that has superior properties to other three porous structures and has been widely used in aerospace, aviation, transportation, environmental protection, energy, biology, etc. Various high-tech fields and general industrial fields.

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Foam iron chrome aluminum

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Aluminum foam

The structure of the metal foam can be seen as a metal stent composed of numerous open cell units in an irregular manner. Each cell is hollow in the middle and has a nearly spherical shape. Then there are fourteen holes in the boundary, much like fourteen windows.

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Metal foam cell

The cross-sectional shape of the metal pillar varies with the porosity of the metal foam: when the porosity is less than 0.9, the cross section of the metal pillar is circular; when the porosity is greater than 0.95, the cross section of the metal pillar becomes a concave triangle.

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Metal pillar cross section

As a structural material, it has the characteristics of light weight, high specific strength and high specific surface area, and it has great advantages in aviation and aerospace applications. For example, aluminum foam has been used in chips for aircraft wing composites and as an excellent material for heaters, heat exchangers and battery plates.

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Nickel Foam has also been applied to heating tiles for walls or floors; in addition, it is used as a plate material for alkaline batteries and fuel cells to greatly increase battery capacity.

As a functional material, it has good physical properties such as sound absorption, sound insulation, heat dissipation, flameout, vibration damping, damping, shock absorption energy, and electromagnetic shielding. For example, metal foam has been applied to the cylinders of internal combustion engines, high-speed train generating rooms, radio recordings, and highway noise reduction in the automotive industry, and has achieved good results.

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As a heat-insulating, flame-retardant material, metal foam can be used in flame arresters to allow the combustion of gases to pass freely but to prevent the spread of the flame.

As a cushioning, damping and absorbing impact energy material, foamed aluminum alloy can be applied to some mechanical fasteners, space shuttle protective casings, crash recorders and their landing gears and bumpers for automobiles.

Method for preparing metal foam

The preparation method of the metal foam can be roughly classified into a casting method and a non-casting method.

The casting method includes a melt foaming agent foaming method, a gas injection foaming method, and a percolation method. The melt foaming agent foaming method is prepared by adding a gas generating material (foaming agent) to the molten metal, thermally decomposing it to generate a gas, and uniformly distributing the generated gas in the metal liquid, cooling A foamed metal solid can then be obtained.

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Melt foaming

The principle of casting is basically the same, only the way of adding gas is somewhat different. For example, a gas injection method directly injects a gas into a molten metal.

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Gas injection method

Non-casting methods include powder metallurgy, sintering dissolution, and metal deposition. The powder metallurgy method is prepared by uniformly mixing the metal powder with the foaming agent powder, making it into a semi-finished product after cold pressing or hot pressing, and then heating the semi-finished product to a temperature close to or higher than the melting point of the mixture. During the heating process, the blowing agent is thermally decomposed to release a large amount of gas, which causes the dense compacted material to expand, thereby forming a porous metal foam material.

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Powder metallurgy

The principle of non-casting is basically the same. Unlike the casting method, the casting method first heats the metal to melt and then injects the gas. Instead of casting, the metal is first mixed with the gas-generating material and then heated to melt.

Heat transfer characteristics and research of metal foam

Metal foam has excellent heat transfer properties.

Good thermal conductivity. The thermal conductivity of copper is about 387W/(m?K), and the thermal conductivity of aluminum is about 218W/(m?K), so the thermal conductivity of metal foam is strong;

Convective heat transfer characteristics are strong. Its irregular skeleton structure makes the mixing of hot fluid and cold fluid intense;

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Fluid traversing metal foam

At high temperatures, the radiation heat transfer in the metal foam is also strong. Its high porosity makes the infrared radiation transmittance greatly enhanced;

The enhanced boiling heat transfer effect of the pool is better. At atmospheric pressure, for copper foam, when acetone is used as the working fluid, the heat flux density can reach 100 W/cm? or more.

The flow boiling heat transfer characteristics in the metal foam are also good, and the boiling heat transfer in the foam tube can reach three times that of the smooth tube.

In view of the excellent properties of metal foam, its industrial application potential is huge, but the current understanding of its internal heat transfer characteristics is not comprehensive enough.

The Institute of Engineering Thermophysics of the Chinese Academy of Sciences has conducted two basic research applications on heat and mass transfer in porous media:

First, the study of heat transfer characteristics in metal foam is mainly used for heat exchange, such as heat accumulators (regenerators, oscillating flow regenerators), heat storage, and enhanced heat transfer. Relying on a number of National Natural Science Foundations, a systematic simulation study on the thermal conductivity, gas-solid heat transfer characteristics and radiation heat transfer characteristics of metal foam has been carried out;

The second is the study of the mechanism of heat and mass transfer in coupled chemical reactions, and the reactor design for various catalytic reactions (catalytic combustion, hydrocarbon reforming, catalytic cracking, hydrocarbon hydrogenation, and partial oxidation of hydrocarbons). Used for thermochemical utilization of energy. At present, the related scientific problems involved in the improvement of high-efficiency chemical heat pumps for low-temperature thermal energy grades have been studied, and will be further extended to other energy thermochemical applications in the future.

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