Thermal conduction mechanism

The heat conduction process takes the form of diffusion, but the thermal conduction mechanism of various materials is different. The thermal conductivity mechanism of the material was discussed in detail by Chu Jiurong et al. There are three types of heat-conducting carriers inside the solid, namely electrons, phonons (lattice waves), and photons (electromagnetic radiation). For polymers, it is usually a saturated system, with no free electrons, the thermal conduction carrier is phonon, and heat conduction mainly depends on lattice vibration. The polymer has a large relative molecular mass and is polydisperse, and the molecular chain exists in a random entanglement manner, which is difficult to completely crystallize. In addition, the vibration of the molecular chain has a scattering effect on phonons, which makes the thermal conductivity of the polymer material. very small.

To make the polymer have better thermal conductivity, it can be modified in the following 2 ways:

(1) Synthesize polymers with high thermal conductivity;

(2) Fill the polymer with a high thermal conductivity substance to prepare a polymer-based thermally conductive composite. In production practice, the addition of high thermal conductivity fillers is usually used to improve the thermal conductivity of polymer materials to obtain thermally conductive polymer composite materials.

Thermal network chain type

1.1 Thermal network chain type The thermal conductivity of the filler and its distribution in the polymer matrix determine the thermal conductivity of the entire composite material. When the amount of filler added is small, the filler is distributed in the matrix in the form of approximately isolated islands, which is a dispersed phase, which is covered by polymerization, forming a "sea-island" structure similar to that in the polymer blend system. When the filling amount of the filler reaches a certain critical value, the fillers will contact with each other to form a heat conduction network chain. With the increase of the filling amount, the heat-conducting network chains penetrate each other, and the thermal conductivity of the composite material is significantly improved. This is like a simple circuit, the matrix and the filler are regarded as 2 thermal resistances respectively. When the filling amount is small, the heat conduction network chain cannot be formed. From the direction of heat flow, the matrix and the filler are equivalent to a series of thermal resistances. The larger the resistance value, the worse the thermal conductivity; when the filling amount is large, the fillers , forming a heat-conducting network chain, the thermal resistance of the heat-conducting network chain is small, at this time, the matrix and the filler are equivalent to parallel in the direction of heat flow, and the heat-conducting network chain plays a leading role in the heat transfer process. The Agari model is based on the mechanism of the heat-conducting network chain. . This is like a simple circuit, the matrix and the filler are regarded as 2 thermal resistances respectively. When the filling amount is small, the heat conduction network chain cannot be formed. From the direction of heat flow, the matrix and the filler are equivalent to a series thermal resistance. The larger the resistance value, the worse the thermal conductivity; when the filling amount is large, the fillers , forming a thermally conductive network chain, the thermal resistance of the thermally conductive network chain is small, at this time the matrix and the filler are equivalent to parallel in the direction of heat flow, and the thermally conductive network chain plays a leading role in the heat transfer process. The Agari model is based on the heat conduction network chain mechanism. Thermoelastic combination enhanced

1.2 Thermoelastic composite enhanced type Li Bin et al. prepared polymer matrix thermally conductive composites by melt blending method, and studied the variation law and internal reasons of thermal conductivity and electrical conductivity of composites with filler species, particle size and other factors. The research results show that the thermal conductivity of the composite system always increases gradually with the increase of filler content, and does not show a sharp change like the electrical conductivity; at the same filling amount, the thermal conductivity of the composite material decreases with the decrease of particle size. , contrary to the variation law of conductivity with particle size. This difference is mainly due to the different conduction mechanisms of the two, which are explained by the thermoelastic composite reinforcement mechanism in this paper. According to the theory of solid state physics, phonons are artificially quantified solid lattice vibrational lattice waves, which are substantially different from the movement and transmission of electrons, which are solid matter particles. The conduction process is the directional movement and conduction process of free electrons, so it is very important to form a conduction path. By analyzing the change law of thermal properties of various inorganic substances, it is found that the change of thermal conductivity of materials is very similar to the elastic modulus in classical vibration and elasticity, so the thermal conductivity of materials can be regarded as phonons (that is, thermal vibrations). ) elastic modulus of the transfer process. Similarly, the increase in thermal conductivity of polymer matrix composites filled with thermally conductive fillers can be seen as the composite of high thermal conductivity fillers to low thermal conductivity matrices (combination reinforcement).