I. Overview
Traditional thermal materials are mostly metals and metal oxides, as well as other non-metallic materials such as graphite, carbon black, A1N, SiC, and the like. With the development of science and technology and production, many products put forward higher requirements for thermal conductive materials, and hope that they have better comprehensive performance, light weight, strong chemical resistance, excellent electrical insulation, impact resistance, easy processing and so on. . Thermally conductive and insulating polymer composites are widely used due to their excellent comprehensive properties.
However, since the polymer material is mostly a poor conductor of heat, it limits its application in heat conduction. Therefore, the development of a new polymer material with good thermal conductivity has become an important development direction of the current heat conductive material. Especially in recent years, with the rapid development of high-power electronic and electrical products, there will inevitably be more and more problems due to product heating, resulting in reduced product efficacy and shortened service life. According to the data, the reliability of electronic components decreases by 10% for every 2 °C increase; the lifetime at 50 °C is only 1/6 of that at 25 °C.
Thermally conductive fillers are mainly divided into two types: one is a thermally conductive insulating filler, such as a metal oxide filler, a metal nitride filler, and the like. The other is a thermally conductive non-insulating filler such as a carbon-based filler and various metal fillers. The former is mainly used for occasions where electronic insulation components have high requirements for electrical insulation properties, while the latter is mainly used for heat exchangers of chemical equipment and other places where electrical insulation performance is low. The type, size and distribution of the filler, the amount of filler and the interfacial properties between the filler and the matrix have an effect on the thermal conductivity of the composite.
The base polymers used for heat conductive plastics are: PA (nylon), FEP (perfluoropolypropylene), PPS, PP, PI epoxy, POM, PS and PS and PE composites.
Research status of polymer-based thermal conductive composites at home and abroad: Polymer-based thermal conductive composites improve the thermal conductivity of polymer materials by adding thermally conductive fillers. Generally, it is based on high molecular weight polymers (such as polyolefin, epoxy resin, polyimide, polytetrafluoroethylene, etc.), and metal oxides with good thermal conductivity such as A1203, MgO, and metals with good thermal conductivity and insulation properties. Nitrides AIN, BN, and high thermal conductivity metal materials such as Cu, AI, etc. are thermally conductive fillers, which are combined in a two-phase or multi-phase system. At present, companies in Europe, Japan and the United States have reported that mature products are being used. For example: Royal DSM Engineering Plastics has introduced the first new polymer since the 21st century: Stanyl® TC series of thermally conductive plastics for LEDs; becoming the world's leading supplier of plastic thermal management solutions for LED lighting applications. Advanced Ceramics and EPIC have developed BN/Polybutylene (PB) composite engineering plastics with a thermal conductivity of 20.35W/(m•K), which can be prepared by common processes such as compression molding, and can be mainly used for electronic packaging. Integrated circuit boards, electronic control components, computer housings, and the like.
The effect of AIN content, particle size, silane coupling agent and processing technology on the thermal conductivity of the system was prepared by molding method. Studies have shown that with the increase of A1N content and particle size, the thermal conductivity of the system is continuously improved; the addition of coupling agent enhances the interfacial adhesion between AIN and epoxy resin, and reduces the thermal resistance between interfaces, which is beneficial to The thermal conductivity of the system is improved. When the AIN particle size is 5.3 μm and the content is 67 v01%, the thermal conductivity of the AIN/EP thermally conductive composite is 14 W/(m•K).
Second, the thermal mechanism
The thermal conductivity of the thermally conductive polymer material is ultimately determined by the polymer matrix, the thermally conductive filler, and the interaction between them. The polymer matrix has substantially no uniform ordered crystal structure or charge carriers required for heat transfer, and the thermal conductivity is relatively poor. As a thermally conductive filler, whether it is in the form of particles, flakes or fibers, the thermal conductivity is higher than that of the polymer matrix itself. When the filling amount of the heat conductive filler is small, the true contact and interaction between the heat conductive fillers cannot be formed, which has little meaning for improving the thermal conductivity of the polymer material; only when the polymer matrix has a filling amount of the heat conductive filler reaches a certain At the critical value, there is a real interaction between the thermally conductive fillers, and a network-like or chain-like form, ie, a thermally conductive network chain, can be formed in the system. When the orientation of the heat-conducting network chain is consistent with the direction of heat flow, the thermal conductivity is improved rapidly; when the heat-conducting mesh chain is not formed in the heat flow direction, the thermal resistance in the heat flow direction is large, and the thermal conductivity is poor. Therefore, how to form a heat conductive mesh chain in the heat flow direction in the system is the key to improve the thermal conductivity of the heat conductive polymer material.
Theoretical model of thermal conductivity: At present, the research on thermal adhesives mainly focuses on the research of filled thermal adhesives. The research on structural thermal adhesives has rarely been reported. Many researchers have proposed various models to predict the thermal conductivity of filled thermal materials, but the theoretical model discusses the filling amount generally concentrated on low or medium filling (volume fraction 10% ~ 30%), but rarely And the theoretical values ​​under high filling and ultra-high filling are consistent with the experimental results. Agari Y proposes a theoretical model for high fill and ultra-high fill. The theoretical model assumes that in a filled polymer system, if the conductive blocks formed by the aggregation of all the filler particles are parallel to the polymer conductive block in the direction of heat flow, the composite has the highest thermal conductivity; if perpendicular to the direction of the heat flow, the composite The material has the lowest thermal conductivity. The theoretical model fully considers the influence of particles on the thermal properties of the composite, and assumes that the dispersion state of the particles is uniform, thus obtaining the theoretical equation. Its expression is:
A=VfC21g), 2+ (1) lg (c1A1)
Where A is the thermal conductivity of the composite, A and A are the thermal conductivity of the polymer and filler, respectively, which is the volume fraction of the filler, c is the factor affecting the crystallinity and the crystal size of the polymer, and C is the thermal chain forming the particle. Free factor. The closer c is to 1, the easier it is for the particles to form a thermally conductive chain, which has a greater influence on the thermal conductivity of the composite. In later research, it was found that the theoretical models of MaxwELl-Eucken, Bruggeman, Cheng-Vochen and Nielsen are basically consistent with the experimental data in the low-fill to ultra-high fill range, and the other theoretical data are basically consistent with the experimental data. There is a certain deviation between the theoretical model and the experimental data.