1.The proportion of filler
When the filling amount of the thermal filler is small, the thermal contact between the thermal filler can not form a real contact and interaction, which is almost meaningless for improving the thermal conductivity of the polymer material. Only in the polymer matrix, the filling of the thermal conductive filler reaches a certain critical value, there is a real interaction between the thermal conductive filler, the system can form a mesh-like or chain-like morphology – the thermal network chain .
WANG Yu-Di et al. Polyethylene (PE) was filled with aluminum nitride and its thermal conductivity was examined. A gap between AlN and PE was observed under electron microscope, indicating that AlN did not infiltrate PE. The thermal conductivity of AlN / PE composites is almost the same when the AlN content is less than 12%. When the AlN content is 12% ~ 24%, the thermal conductivity increases rapidly. When the volume fraction is more than 24% The thermal conductivity increases slowly. When the volume fraction of AlN reaches 30.2%, the thermal conductivity of the composite tends to balance, reaching 2.44 W / (m · K).
2. Packing size
The thermal conductivity of filler-filled composites increases with the increase of particle size. When the filler content is the same, the thermal conductivity of composites filled with large-size filler is higher than that of filler with smaller filler. Hasselman studied the thermal conductivity of SiC-filled aluminum matrix composites with different particle sizes. The experimental results show that the thermal conductivity increases with the increase of SiC particle size at 20% loading at 20 ℃.
However, the ultrafine heat treatment filler can effectively improve its own thermal conductivity. Ming-Ming Tang et al. Studied the thermal conductivity of polymer materials obtained by adding nano-alumina and microaluminum oxide respectively to styrene-butadiene rubber. It was found that the thermal conductivity and physical-mechanical properties of the nano-alumina-filled styrene-butadiene rubber Both are superior to the micro-alumina-filled styrene-butadiene rubber, and the thermal conductivity of the styrene-butadiene rubber increases with the alumina loading.
3 filler shape
The filler dispersed in the resin matrix can be granular, flaky, spherical, fibrous, etc. The shape of the filler directly affects its dispersion and thermal conductivity in the polymer material. WANG Yu-Di Polyethylene / AlN composite substrate was prepared by embossing method. The influence of crystal morphology and loading of AlN on thermal conductivity of composite substrate was investigated. The results show that the thermal conductivity of the composite substrate increases with the addition of AlN, initially changes little, then increases rapidly and then decreases gradually. With the same amount of AlN added, the thermal conductivity of AlN Powder composite materials, followed by AlN fiber composites, the highest is filled with whiskers composite material.
4 matrix and filler interface
Conductive polymer composites are multiphase systems composed of a thermally conductive filler and a polymer matrix. In the process of heat transfer (ie, lattice vibration transmission), many matrix-filler interfaces inevitably have to pass, so the bonding strength between the interfaces Directly affect the thermal conductivity of the entire composite system. The bond strength between the matrix and the filler interface is strongly related to the surface treatment of the filler, depending on the degree of wettability of the particle surface. This is because the wetting degree of the filler surface affects the bonding degree between the filler and the substrate, the thermal barrier between the substrate and the filler interface, the uniform dispersion of the filler, the amount of the filler, and some other factors that directly affect the thermal conductivity of the system. Increasing the interface bonding strength can improve the thermal conductivity of the composite. Zhang Xiaohui et al found that Al2O3 particles filled with epoxy after surface treatment of the coupling agent, compared with the epoxy adhesive without direct surface treatment, the thermal conductivity increased by 10%, the maximum thermal conductivity obtained was 1.236 W / (m · K).
