Aerogel is a high-quality and high-efficiency insulation material with high porosity, which can effectively reduce the solid phase heat conduction of materials. Its pore size is mainly distributed in the mesopores (2-50nm), which effectively inhibits gas phase heat transfer. . Aerogel materials can be mainly divided into oxide aerogel materials, carbon aerogel materials (high temperature resistance up to 3000 ° C) and carbide aerogel materials according to different components. The following three series introduce the above three high temperature resistant aerogel insulation materials.
A. Overview of aerogel materials
The high porosity of aerogel materials and the mesoporous structure formed by the interconnection of nano-mesh skeletons determine their excellent thermal insulation properties. They are used in military and civil applications such as high temperature catalyst carriers, high temperature furnaces and supersonic aircraft. The field is used as a highly efficient insulation material. Aerogel materials have thermal insulation, sound insulation, nonlinear optical properties, filtration and catalytic properties.
1, heat insulation characteristics
The aerogel material has a porosity of more than 90%, and the pore size in the aerogel is in the range of 2-50 nm mesopores. This special structure determines its excellent thermal insulation properties, while the aerogel material At high temperatures, it maintains the integrity of its network structure and is less prone to high temperature sintering.
2, sound insulation
The low sonic properties of silicone aerogels make it an ideal acoustic delay or high temperature sound barrier. The material has a wide range of acoustic impedance (103-107kg/m2·s), which is an ideal acoustic resistance coupling material for ultrasonic detectors.
3. Nonlinear optical properties
The silicon aerogel has a nano-network quantum dot structure. After the Si and C60 are mixed by chemical vapor infiltration, a strong visible light emission can be observed. Further, the structure of the silicon aerogel and the nonlinear optical effect of C60 can be further developed. New laser goggles.
4. Filtration and catalytic properties
Nanostructured aerogels can be used as a new type of gas filtration. Unlike other materials, this material has a uniform pore size distribution and high porosity, and is a highly efficient gas filtration material. Due to the particularly large proportion of the material, aerogels have broad application prospects as carriers for novel catalysts or catalysts.
B, High temperature aerogel insulation material
Aerogel materials can be mainly divided into oxide aerogel materials, carbon aerogel materials (high temperature resistance up to 3000 ° C) and carbide aerogel materials according to different components.
1, oxide aerogel material
Oxide aerogel materials are prone to crystal transformation and sintering of particles in high temperature regions (>1000 °C), and their temperature resistance is relatively poor, but they have lower thermal conductivity in the middle and high temperature regions (<1000 °C). .
The oxide aerogel materials mainly include SiO2, Al2O3, TiO2, ZrO2, CuO and the like.
(1) SiO2 aerogel material
SiO2 aerogel is a kind of high temperature resistant aerogel which is the most mature and mature in the field of heat insulation. Its porosity is as high as 80%~99.8%, the typical size of the hole is 1~100nm, and the specific surface area is 200~1000m2/g. The density can be as low as 3kg/m3, and the thermal conductivity at room temperature can be as low as 12m.W/(m·K).
SiO2 aerogel materials are usually compounded with infrared sunscreens and reinforcements to improve the thermal and mechanical properties of SiO2 aerogels, making them practically valuable nanoporous superinsulation materials, while also having good Insulation and mechanical properties, mainly used in the thermal insulation of aerospace, military, electronics, construction, home appliances and industrial pipelines.
Commonly used infrared sunscreens are silicon carbide, TiO2 (rutile and anatase), carbon black, potassium hexatitanate, etc.; commonly used reinforcing materials are ceramic fibers, alkali-free ultrafine glass fibers, polycrystalline mullite fibers, Aluminum silicate fiber, zirconia fiber, and the like.
(2) ZrO2 aerogel material
The pore diameter of the ZrO2 aerogel material is smaller than the mean free path of air molecules. There is no air convection in the aerogel, the porosity is extremely high, and the volume ratio of the solid is very low, so that the thermal conductivity of the aerogel is very low. Compared with SiO2 aerogel materials, ZrO2 aerogel has lower thermal conductivity at higher temperatures, and is more suitable for high temperature insulation applications. It has great potential as a high temperature thermal insulation material. At present, there are few reports on the application of ZrO2 aerogel in the field of thermal insulation. The researchers are mainly devoted to the research of ZrO2 aerogel preparation process.
The main preparation method of ZrO2 aerogel is obtained by a series of hydrolysis polycondensation processes of zirconium salt precursor. Its preparation mainly includes two parts: preparation and drying of wet gel. Supercritical drying and freeze drying are generally employed. Commonly used wet gel preparation methods include zirconium alkoxide hydrolysis, precipitation, alcohol-water solution heating, dropwise addition of propylene oxide, and inorganic dispersion sol-gel. It is one of the research hotspots to prepare ZrO2 aerogel with low-cost inorganic zirconium salt as precursor and how to improve the high temperature thermal stability of zirconia aerogel.
(3) Al2O3 aerogel material
The alumina aerogel material has a nanoporous structure, which makes it lighter in weight and smaller in volume to achieve equivalent thermal insulation effect, and has high porosity, high specific surface area and open textured structure in the catalyst and catalytic carrier. Aspects have potential application value. Alumina aerogels can also be used as high voltage insulating materials, substrate materials for high speed or super speed integrated circuits, isolation dielectrics for vacuum electrodes, and supercapacitors.
2, carbon aerogel material
The biggest characteristic of carbon aerogel is its temperature resistance up to 2000 °C under inert and vacuum atmosphere. The temperature resistance after graphitization can even reach 3000 °C, and the carbon nanoparticle in carbon aerogel itself has infrared Radiation has excellent absorption properties, resulting in an effect similar to an infrared sunscreen, so its high temperature thermal conductivity is low. However, under aerobic conditions, carbon aerogels oxidize above 350 ° C, which makes their application in the field of high temperature insulation greatly limited. With the development of high oxidation resistant coatings such as SiC, MoSi2, HfSi2, TaSi2, etc., coating a dense anti-oxidation coating on the surface of the carbon aerogel material to prevent further diffusion of oxygen will make the material extremely Application prospects.
3. Carbide aerogel materials
The carbide material has excellent oxidation resistance, but its own thermal conductivity is high, and it is made into an aerogel containing a three-dimensional network structure, which can greatly reduce the thermal conductivity of the material and further improve the material. Thermal insulation properties. However, there are relatively few studies on carbide aerogels at home and abroad. Especially for the study of bulky carbide aerogels with good formability, it is still in the initial stage, and its research as high-efficiency insulation materials is also scarce. It is limited to the preparation and characterization of this material.
4, multi-component aerogel material
The multi-component aerogel materials mainly include SiO2/Al2O3, TiO2/SiO2, C/SiC, C/SiO2, Si-C-O, Al-C-O and the like.
C, High temperature aerogel insulation material application
1. Aerospace field
Nanoporous aerogel superinsulation materials can achieve equivalent thermal insulation with lighter weight and smaller volume. This feature makes it an important advantage in aerospace and aerospace applications. Used as an insulating material for aero engines, it not only provides excellent insulation but also reduces the weight of the engine.
2. Industrial and architectural insulation
High temperature aerogel insulation has a wide range of potential applications in industrial and civil applications. First, thermal equipment is widespread in the power, petrochemical, chemical, metallurgical, building materials and other industrial sectors. In industrial energy conservation, nanoporous super insulation materials also play a very important role, some of which are special parts and environments, due to weight, volume or space constraints, the need for efficient super insulation materials.
3. Hydrogen storage materials
The carbon aerogel has a high specific surface area, a low density, a continuous network structure, and has a small pore size and communicates with the outside, and has excellent hydrogen absorption and desorption properties. It is understood that the US Department of Energy has set up a special agency to study the metal-filled carbon gas gel for hydrogen storage and provide financial support.
4, the field of thermal batteries
The high temperature resistant aerogel insulation material is applied to the battery insulation material to extend the working life of the heat battery and prevent the generated heat from affecting the components around the heat battery.
