1 filler thermal conductivity mechanism
  The thermal conductivity of polymer materials itself is relatively small. Therefore, the thermal conductivity of filled polymer composites mainly depends on the thermal conductivity of the filler, the distribution of the filler in the matrix and the interaction with the matrix. When the amount of filler is small. Although the filler is evenly dispersed in the resin. But they did not form mutual contact and interaction with each other, and the thermal conductivity did not increase much. When the filler content increased to a certain critical value, the filler formed contact and interaction, forming a network-like or chain-like structure in the system, namely The formation of thermal network chain. When the orientation of the heat conduction net chain is in the same direction as the heat flow, the thermal conductivity of the material improves rapidly. When the heat conduction net chain is not formed in the heat flow direction in the system, heat resistance in the heat flow direction is greatly caused. Lead to poor thermal conductivity of the material. There are generally two ways to make a thermally conductive material with excellent overall performance: one is to synthesize a structural polymer with high thermal conductivity; the other is to fill a polymer with a filler of high thermal conductivity. The latter is more common. Generally, high-thermal conductivity metal or inorganic filler is used to fill the polymer material. Alumina is often used as a filler in thermally conductive polymer composites.
  2 alumina morphology and surface treatment
  2.1 alumina as a thermal insulation material characteristics
  Few thermal conductive insulation properties of the filler. Several common and its thermal conductivity are shown in Table 1. Experimental studies have shown that when the thermal conductivity of filler and matrix ratio greater than 100. Increasing the thermal conductivity of the filler has little effect. This means that the use of electrically insulating fillers such as AlzO3, MgO, BeO, AlN, etc. can be prepared with high thermal conductivity of the electrical insulation composite material. Compared with other fillers, alumina, the thermal conductivity is not high, but its lower prices, a wide range of sources, a larger amount of filling, commonly used as a filler insulation polymer. Alumina, usually used alone or in combination with other fillers.
  3 alumina in the thermal insulation materials
  Alumina is often used as a filler for insulating and thermally conductive polymers. It is widely used in thermal plastic, thermal rubber, thermal adhesive and thermal conductive coating.
  3.1 thermal plastic
  The research by McCann et al. Showed that the thermal conductivity of polypropylene (PP) increased with addition of alumina, and the thermal conductivity of AlzO3 / PP composites increased with the increase of alumina dosage. Adding the third component of Cu, ZnO, Al and graphite, to further improve the thermal conductivity of alumina composites. The thermal conductivity of the composites with grafted PP as the matrix is higher than that of PP, but the thermal conductivity of the composites with grafted PP and PP as the matrix is lower than the thermal conductivity of PP.
  3.2 thermal rubber
  Wang Qian, who studied the alumina and SiC two types of thermal conductivity filler and filler particle size distribution of room temperature vulcanized silicone rubber and silicone ester thermal conductivity and viscosity. The results show that SiC and alumina with different particle size are selected, and high thermal conductivity RTV silicone rubber and silicone ester can be obtained by filling the system with thermal conductive filler, and the process performance is good. Pan Dahai to polydimethylsiloxane-based glue, with silicon nitride, aluminum nitride and alumina, as the filler to prepare a two-component filled RTV-2 thermal conductivity of silicone rubber. The influence of the combination of filler silicon nitride / alumina or aluminum nitride / alumina on the thermal conductivity, processability and mechanical properties of RTV-2 silicone rubber was studied. The results show that the thermal conductivity of RTV-2 thermal conductive silicone rubber first and then decreases with the increase of alumina volume fraction in the silicon nitride / alumina filled system when the total filler content is 45% The strength increased first and then decreased, while the elongation at break showed a tendency of gradual increase, and the viscosity of the binder decreased first and then increased. Wang et al. Used A120 with different particle sizes in combination with SiC and filled with silicone rubber at room temperature. When the total amount of filler was 55 parts, the compounding rubber had lower viscosity and the thermal conductivity of the vulcanized silicone rubber was 1.48 W / (M · K). In addition, increase the amount of filler and control its particle size distribution. Room temperature vulcanizable silicone rubber having a thermal conductivity of 2 W / (m-K) can be obtained.
  Tang Mingming 021 In the study, it was found that the thermal conductivity of SBR increased with the increase of the filler fraction of microaluminum, but its processing and physical and mechanical properties decreased. The silane coupling agent KH-570, KH-550, A A 151 and titanate coupling agent TM-S105 after the treatment of the micron A1 0, the thermal conductivity of the filler on the thermal conductivity of rubber is not significant; the same amount of filling, the use of nano-A1203 filled with micron Al203, filled thermal conductivity Rubber has better thermal conductivity and physical and mechanical properties. The thermal conductivity of nano-alumina mixed with micro-alumina is better than that of pure rubber filled with micro-particles at the proper ratio. Zhang et al. [131] systematically studied the effects of five kinds of thermal conductive fillers, including stainless steel staple, flake graphite, short carbon fiber, aluminum powder, Al20 and powder, on the static thermal conductivity and dynamic temperature rise of NR1 matrix composites Mechanical properties.The results show that the alumina can significantly improve the static thermal conductivity of NR, and the higher the dosage, the greater the thermal conductivity.Area-filled NR dynamic temperature rise is still higher than the control rubber, and the longer the test temperature L as high as possible.
  3-3 thermal insulation coating
  Zhou Wenying l epoxy modified silicone resin as the base, silicon nitride, aluminum oxide mixed filler for the thermal conductivity of the thermal insulation coating prepared. The maximum thermal conductivity of 1.25 W / (m · K) was obtained at 40% of the total filler content and 20% of the total amount of alumina. The tensile properties and the elongation at break of this coating decreased. The adhesion at room temperature 572.2 N / cm. Coating dielectric constant 5.7. Volume and surface resistivity of 3x10 Q · cm and 4.3x10 Q, the coating can be long-term use at 200 ℃, showing good electrical insulation. Compared with the epoxy-modified silicone resin coating without heat-conductive filler, it has higher heat transfer capacity.
  3.4 Insulation thermal adhesive
  Zhang Xiaohui, respectively, with SiC, A1N, A1 0 filled epoxy adhesive. It is found that there is a critical point in the filler fraction, which is attributed to the establishment of an effective thermal conduction network inside the material. Due to the low price of SiC and the high thermal conductivity, the thermal conductivity is 4.234 W / (m · K) when the filler content is 53.9%, and the mechanical properties are better. Wang Tieru Al203, BN will be added to the epoxy resin made of thermal insulation adhesive. Zhang Wenjie, etc. studied A1 0, A1N mixed filled silicone potting material, prepared the thermal conductivity of O. 89 W / (m · K) potting material. Zhou Wen to toughening phenolic epoxy resin as the matrix resin. A new type of insulating and thermal conductive adhesive was prepared by mixing A1N, B4C and A120 mixed particles with 1: 4: 3 mass ratio. The thermal conductivity of the adhesive was found to be 0.99 W / (m · K) when the filler content was 40%. Thermal resistance is O. 7O ℃ / W, dielectric constant of 6, volume resistivity of 4.6 × 1O Q · cm, breakdown voltage of 12 kV / mm, 20 ℃, 200 ℃. The shear strengths at 250 ° C were 13.0 MPa. 10.0 MPa. 5.65 MPa. The results show that the rubber has good electrical insulation and mechanical properties, can be used for a long time at 150 oc, with the same adhesive without thermal conductivity filler, compared with good thermal conductivity. Tan Maolin with alumina, filled with silicone modified epoxy resin measured at 100 ℃ thermal conductivity of O. 64 W / (m.K)
  4 Conclusion
  Alumina is often used as a filler for insulating and thermally conductive polymers. It is widely used in thermal plastic, thermal rubber, thermal adhesive and thermal conductive coating. However, the application of nano-alumina is not reported. Tang Ming-ming found that with the same amount of filling, the use of nano-alumina, filled with aluminum oxide than the thermal conductivity of rubber filled with better thermal conductivity and physical and mechanical properties. With the development of nanocomposite technology, nano alumina can be forecasted, and the development and application of new composite technologies of nanometer alumina and polymer matrix will become the research directions in the future.