常温、真空条件下,SiO_2气凝胶材料的导热性能决定了真空绝热板的表观导热性能,真空条件下影响其等效热导率的因素及对影响因素的规律研究可为芯材和真空绝热板的生产提供理论依据。文章基于气凝胶绝热材料的热导率计算模型,利用MATLAB软件编程计算其有效热导率,研究了压强、密度、纤维体积分数对SiO_2气凝胶绝热材料等效热导率的影响规律。结果表明:常温条件下,随着压强降低,气凝胶的热导率显著降低,压强<1 k Pa时,气凝胶的热导率随压强降低趋于平缓,材料内气相热导率降低;密度减小使得固相热导率降低,而同时孔隙率增大使气相热导率增加,二者共同作用时,气凝胶材料存在最佳密度值,导致总热导率最小;纤维体积分数增加,固相热导率增大,增加对材料内部热辐射的抑制,最佳纤维体积分数使复合了纤维的SiO_2气凝胶材料热导率最小。
Thermal conductivity measurement accuracy of sands was experimentally studied with a hot disk thermal constant analyzer and water morphologies,distribution and evolution at pore scale were observed with charge coupled device(CCD)combined with microscope.It was found that thermal conductivities of samples with a low moisture content(<25%)could not be accurately measured.For samples with a low moisture content,the analysis showed that the water in the region adjacent to the analyser sensor mainly existed as isolated liquid bridges between/among sand particles and would evaporate and diffuse to relatively far regions because of being heated by the sensor during measurement.Water evaporation and diffusion caused the sample composition in the region adjacent to the sensor vary throughout the whole measurement process,and accordingly low accuracy of the obtained thermal conductivities.Due to high water connectivity in pores,the rate of water evaporation and diffusion in porous media of high moisture content was relatively slow compared with that in porous media of low moisture content.Meanwhile,water in the relatively far region flowed back to the region adjacent to the sensor by capillary force.Therefore,sample composition of the region adjacent to the sensor maintained constant and thermal conductivities of porous media with a relatively high moisture content could be measured with high accuracy.
