Study On High Thermal Conductive Phase Change Materials And Thermal Control Technology

Energy storage technology has been widely used in thermal management of electronic equipment and other devices. Due to the advantages of low cost, simple operation, high storage energy density, small temperature fluctuation, small volume variation and high chemical stability, phase change energy storage techniques become one of the most effective passive thermal control methods. With the development of Electronic components integration, heat flux generated by inner chip increases more and more dramatically. As a result, the requirement of precise thermal control becomes more and more strict, which leads to higher and higher requests of phase change materials (PCMs). Because of installation difficulty and easily leakage caused by solid and liquid phase change, shape-stablized phase change materials become feasible solutions. Although, researchers have done a lot of work on PCMs, there are still many critical problems unsolved as followings. Firstly, thermal conductivity of existing PCMs is quite low at present, but there is no effective work on the mechanism of the heat conduction. It causes certain blindness of the thermal conductivity reinforcement experiment. Secondly, existing PCMs are quite difficult to contact with complex thermal control surface and to control the contact thermal resistance. Thirdly, study on PCMs films applied in limited spaces for electronics thermal control has rarely been done.According to the problems above, this paper has done systematic research on the critical thermal conduction problems of the thermal conductivity reinforcement mechanism, flexible high thermal conductivity PCMs preparation and anisotropic flexible PCM films. This work has not only important reference value for solving the problem of phase change thermal control, but also has universal significance on the other areas of phase change energy storage.Firstly, this paper established a novel model that reveals the influence of additive properties on thermal conductivity of composite PCM (CPCM) with carbon additive.The proposed model is validated by the experimental data with calculation deviations lower than 12.8% when less than 10% loading of additive. The model analysis shows that the microstructure parameters of the additive including aspect ratio, specific surface area and equivalent thickness, especially the latter two have great impacts on the conductivity of composite PCM. Based on the above theory, paraffin-based CPCM with the conductivity up to 2.10W/m-K is prepared successfully by adding 3wt.% EG-50(282?m, expansion 550 time), which is 202% as high as the existing max conductivity(1.04 W/m·K).Secondly, a novel kind of flexible phase change film (PCF) was proposed by using difunctional olefin blockcopolymer (OBC) with phase separation morphology. The prepared PCMs based on OBC not only exhibit the advantage of shape-stabilized PCMs such as high latent capacity and encapsulation of phase change matrix, but also present the superior flexibility with the melt point of phase change matrix as temperature stimulator. During temperature variation, PCF can achieve inversely transformation between rigidity and flexibility, which makes flexible contact between PCMs and device surface. In addition, based on the composite thermal conductivity-calculating model proposed previously, the conductivity enhancement on account of the flexible PCMs was carried out.Furthermore, based on all the above study, prepared thermal sensitive flexibility PCF with high thermal conductivity is applied in different heat areas of electron device as thrmal control material. When electronic device used in indoor environment, PCF based on octodecane with the melting point of 26.2? can control the chip temperature within a low level for a long time. When used in extreme environment, paraffin with the melting point of 50.2? is fit. The effect of thermal conductivity and film thickness on control device is studied. Results indicate the higher of thermal conductivity will bring the longer of effective control time. There is an optimal film thickness for different phase change matrix.At last, PCF with enhanced horizontal thermal conductivity is used to improve thermal control system in different heat areas. It is found that the less of heat area, the better of optimized effect. Taking octodecane/aluminum honeycomb composite PCM as an example, this paper investigates the effect of anisotropic thermal conductivity kx and ky of PCM and thermal expansion on controlled temperature To by using apparent heat capacity method. It is found that the effect of ky to T0 is reduced with the decrease of heating area s, on the contrary, the effect of kx is increased. The numerical simulation of two dimensional temperature field in thermal control system analyzes the effect mechanism of anisotropic conductivity on thermal properties. Results indicate that the importance of certain direction thermal conductivity is depended on the temperature gradient on this direction. Based on the above theory, guidness is put forward for the structural arrangement of the phase change thermal control system for electronic devices with finite-space heating source.