高热导率的聚偏氟乙烯/ 石墨烯复合材料

芯片散热问题限制了芯片技术的进一步发展,寻求高热导率的热界面材料成为突破该瓶颈的 重要手段之一。有机-无机复合材料由于其柔软性以及热导率可调控,有望取代常规材料——硅脂,成 为新一代热界面材料。实验上,有机-无机复合材料的制备方法包括物理混合、分相析出和原位氧化。 该文采用物理混合方法制备聚偏氟乙烯/石墨烯复合材料,并使用非稳态测量方法得到其热导率高达 83 W/(m·K)(温度 T＝360 K、体积分数 f＝76 vol%)。此外,复合材料的热导率与填料的体积分数、颗 粒大小形状以及填料与基体之间的相互作用等因素密切相关,利用改进的有效介质理论 Bruggeman 模 型和 Agari 模型来解释复合材料热导率的物理机制时发现,改进的有效介质理论 Bruggeman 模型并不 能很好地解释该复合材料的高热导率。由 Agari 模型可知,当填料含量较高时,填料之间更容易形成 导热通道,从而提高了复合材料的热导率。     

Magnetic Properties of La1-xSrxCoO3 （0 ≤ x ≤ 0. 5）

The measurements of temperature dependence of the magnetic susceptibility of La1-xSrxCoO3 perovskite oxides at different Sr doping （0 ≤x ≤0.5） and annealing temperature were presented. For the sample with x = 0.1, a shoulder was observed around 150 K, and a peak which is one feature of spin glass appeared around 50 K in the curve of susceptibility versus temperature. The high-temperature （250 - 420 K）susceptibility fits well with Curie-Weiss law for all samples. Weiss constant and effective magnetic moment were determined and their variations with Sr doping and oxygen annealing condition were obtained. The Weiss constant increases monotonously with Sr content for x 〉 0.2. The values of effective moments were interpreted with the spin state of cobalt ions. Studies on the susceptibilities of the samples with x = 0.2 under different preparation temperatures and annealing temperatures show that the rising of sintering temperature and annealing temperature will increase the para-ferromagnetic transition temperature, and reduce the effective moment to normal value. Our result shows that both Co^3＋ and Co^4＋ ions should be in IS state after annealing and the oxygen annealing causes the transition of Co^3＋ spin state from HS to IS.