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Thermal insulation mechanism, influencing factors and research status of alumina thermal insulation materials

wallpapers Industry 2020-10-29
Thermal insulation refractory material refers to a material that shields heat and has the characteristics of high porosity, low thermal conductivity, and low heat capacity. Thermal insulation refractory materials are mainly used to prevent heat loss in high-temperature environments or to isolate heat inflow under low-temperature environmental conditions. Therefore, they are also called heat preservation or cold preservation materials. However, traditional heat-insulating refractory materials exhibit low corrosion resistance, strength, and wear-resistance and are often used as the working surface's outer insulating layer. The thermal insulation refractories' high-efficiency thermal insulation effect plays a vital role in the energy-saving, product quality, and efficient and safe production of thermal kilns. Therefore, they are often used as lining materials for thermal kilns. Alumina heat-insulating material has become one of the ideal choices of current thermal kiln lining materials due to its low thermal conductivity, high mechanical strength, and high operating temperature, and has attracted widespread attention.
1. Thermal insulation mechanism and influencing factors of alumina thermal insulation materials
 
Alumina thermal insulation material is a kind of porous material, and its thermal insulation behavior is a relatively complicated process. As we all know, the thermal insulation material contains many pores, so the heat transfer in it is mainly through the heat transfer between the solid phase and the gas phase. The reliable phase heat transfer method is mostly conduction, and the heat transfer through the gas phase is much more complicated than the solid phase heat transfer.
 
Convective heat transfer uses the convective air movement in the insulation material to produce an absolute displacement of heat to achieve the purpose of heat transfer. Since most of the thermal insulation materials have relatively small pores, the pores' airflow is hindered, resulting in a relatively small proportion of convective heat transfer in the thermal insulation material. In gas-phase heat transfer and convective heat transfer, there is also radiative heat transfer, and the balance of radiant heat transfer increases with the increase of temperature.
 
Thermal conductivity is an essential measure of the thermal insulation performance of materials, and its size reflects the thermal insulation performance of materials. The thermal insulation material is a combination of multi-phase materials composed of gas phase and solid phase. Based on the above-mentioned thermal insulation mechanism, the main factors affecting thermal conductivity are gas-phase content and distribution, material chemical composition, and crystalline state.
 
The gas phase's distribution and content in the insulation material have a significant effect on the heat transfer of the insulation material. Also, the gas phase's thermal conductivity is lower than that of the solid phase, so that the thermal conductivity of the material decreases with the increase of porosity under low-temperature conditions. Still, when the temperature is higher, the proportion of gas-phase conduction decreases, and radiation transmission. The effect of heat increases and its thermal conductivity does not decrease with the increase in porosity. In addition to the influence of porosity, the pores' size also has an important impact on the insulation material's heat transfer. Under the same porosity conditions, the shape of the pores in the material, the pore size's size, and the pore connectivity rate determine the thermal conductivity of the insulation material. Generally, the smaller the pore diameter of the thermal insulation material, the more significant the thermal insulation effect, which is mainly reflected in the following two aspects: the reduction of the pore diameter leads to the decrease in the movement space of gas molecules and the removal of heat flow transfer efficiency; on the other hand, the reduction of the pore diameter increases At the gas-solid interface in the material, the reliable conduction distance increases, thereby reducing the thermal conductivity of the material.
 
In the heat transfer process of thermal insulation materials, solid-phase conduction accounts for a large proportion of thermal insulation materials' heat transfer. Therefore, solid-phase materials' material properties significantly impact the thermal insulation performance of thermal insulation materials. Choose a small thermal conductivity and heat capacity. The material can improve the thermal insulation performance of the material from the source. The literature shows that silicate minerals' thermal conductivity is low, while the thermal conductivity of oxides is mostly higher than that of non-oxides. Also, from the analysis of the material phase composition, the solid phase in the thermal insulation material is mainly composed of the crystalline and glass phases. As the temperature increases, the free path of atoms or ions in the crystalline phase is shortened, thereby reducing the thermal conductivity; on the other hand, the atoms or ions in the glass phase are arranged in disorder, making the thermal conductivity of the glass phase lower than that of the crystalline phase. As the temperature rises, the glass phase's viscosity decreases, thereby reducing the resistance to the movement of the particles, increasing the thermal conductivity of the glass phase.
 
2. Research status of alumina heat insulation materials
 
Alumina heat insulation material is made of alumina as the primary raw material, using different pore-forming methods. Then the material is shaped and fired, leaving a large number of closed or interconnected pores inside. The existence of many pores makes it have a low thermal conductivity, coupled with the high-temperature resistance, corrosion resistance, and good chemical stability of alumina itself; it has broad application prospects in the field of high-temperature industrial kilns. Alumina heat insulation materials can be divided into alumina heat insulation products prepared with alumina powder as the primary raw material and alumina hollow sphere products designed using alumina hollow spheres as the primary raw material according to the different alumina raw materials used during preparation. According to the difference of pore-forming methods, the preparation methods of alumina thermal insulation materials mainly include pore former method, foam casting method, chemical reaction method, gel injection molding freeze-drying method, and in-situ decomposition method.
 
On the premise of ensuring the structural strength and heat resistance of the furnace lining, to achieve the high-efficiency heat insulation effect of the high-temperature furnace lining, the thermal conductivity of the material should be reduced as much as possible to improve the heat preservation capacity and reduce the heat storage and heat dissipation loss. As an essential part of the microstructure of thermal insulation materials, the structure and distribution of pores have the most significant impact on thermal insulation materials' performance. Alumina thermal insulation material is a complex and heterogeneous multi-phase system, and its performance cannot be characterized by the superposition behavior of one or more phases. The structure determines the properties. Therefore, the pore structure's optimization is particularly crucial for the improvement of the performance of the alumina insulation material.
 
The literature starts from the selection, type, addition amount, shape, and modification of the pore-forming agent, uses the burnout method to prepare alumina heat insulation materials, and explores the influence of the pore structure control of the alumina heat insulation materials on its performance. The alumina heat-insulating material's pore structure is adjusted by changing the added amount of rice husk and the particle size, thereby preparing an alumina heat-insulating material with a controllable pore size.
 
In short, most of the current researches focuses on the preparation methods of alumina thermal insulation materials. Simultaneously, there are few studies on adjusting and optimizing the performance of its pore structure. In particular, few reports on the use of the intrinsic texture of pore formers to control the pore structure of alumina thermal insulation materials.

Trunnano is one of the world's largest producers of Alumina powder. If you have any questions or needs, please contact Dr. Leo by email: brad@ihpa.net.

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Tag: alumina,Trunnano