【作者】 张新铭；

【导师】 曾丹苓； 刘朝；

【作者基本信息】 重庆大学 ， 工程热物理， 2010， 博士

【摘要】 中间相沥青基石墨泡沫是一种新型的功能性多孔材料,具有低密度、高孔隙率、高导热、耐高温、耐腐蚀,以及良好的电磁波吸收及屏蔽等性能,在重量敏感的航空航天、微电子及国防等领域有着非常广阔的应用前景,因而在其问世的近十几年来,受到越来越多的关注和研究。然而对于这种高性能石墨泡沫的热物理性能,特别是对于其孔隙结构的随机性对有效导热系数所产生的影响方面,普遍缺乏研究,阻碍了其制备、性能及应用研究的进一步深入。本文工作的主要目的是希望能够为目前正在进行的高性能石墨泡沫制备研究,特别是制备工艺参数与成型样品孔隙均匀度的关系,乃至孔隙均匀度对导热性能影响的研究,提供实验、数值模拟和理论分析的研究结果,为非均质泡沫材料有效导热系数的预测式的研究提供参考依据,并为今后进一步开展高性能石墨泡沫及其他多孔泡沫材料的热物性研究积累经验与条件。本文所取得的主要成果有:(1)在中间相沥青基石墨泡沫的制备实验中,运用试验设计方法研究了工艺参数对石墨泡沫孔隙结构的影响,指出各工艺参数对成型样品孔隙均匀度的影响显著性依次为成型压力、加热功率(或升温速率)、发泡温度、发泡恒温时间、焦化温度和焦化恒温时间,其中前3个参数的影响最为显著;特别是成型压力对石墨泡沫的孔隙分布均匀度有明显的控制作用,较高的成型压力可抑制气泡过快产生和长大而使孔泡分布趋于均匀;接近发泡温度时的升温速率越高则成型压力也应随之提高;对于低压发泡工艺则应非常缓慢地升温以阻止气泡过快生长而获得孔隙结构较均匀的石墨泡沫。(2)通过对具有周期性孔隙结构的FCC和BCC泡沫模型的数值模拟结果的分析,提出对于均质泡沫介质,有效导热系数的预测式方可表达为k~*=(1–ε)1/C的形式,其中k~*为相对有效导热系数,ε为孔隙率,C为孔形因子;C=0.67时的预测式k~*=(1–ε)1.5较适用于具有“顺排”特征的均质泡沫材料,而C=0.77时的预测式k~*=(1–ε)1.3则较适用于具有“错排”特征的均质泡沫材料。(3)通过对具有随机性孔隙结构的泡沫模型的数值模拟结果的分析,提出对于非均质泡沫介质,有效导热系数的预测式应具有k~*=f(ε,U)的形式,其中U为孔隙均匀度;孔隙分布越不均匀(U值越小),则微观上导热路径越曲折,宏观上导热热阻越大,有效导热系数越小。(4)提出了一种研究非均质泡沫材料导热性能与孔隙结构关系的2D模型,该模型包含2个可调节随机参数:孔隙位置随机偏离因子a和孔隙尺寸随机偏离因子b;并将ε和U作为实验参数a和b的函数,设计了较大样本(110个模型)的FEM数值模拟实验,较系统地研究了泡沫介质孔隙结构的随机性对有效导热系数的影响;结果表明:孔隙均匀度U是一个与孔隙率ε同样重要的孔隙结构参数;k~*是ε的减函数,U的增函数;k~*=f(ε,U)的关系可表达为幂函数形式k~*=Um(1–ε)n,其中指数m≥1且n≥1。(5)在中间相沥青基石墨泡沫的制备中,可能因成型压力与温度的配合不佳而造成孔隙分布不均匀,还可能因原料颗粒的不均匀、杂质的存在以及工艺参数的扰动等因素而造成孔隙结构的随机变化;而本文的所有研究结果表明,在制备工艺尤其是发泡工艺过程中,应尽可能排除导致孔隙均匀度减小的因素,以获得高导热性能的中间相沥青基石墨泡沫材料。更多还原

【Abstract】 Graphitic foam derived from mesophase pitch is a newly developed, functional porous material which exhibits low density, high porosity and high thermal conductivity. Since its network of interconnected graphite ligaments is high thermal conductive and its open-cell structure is permeable to fluid, it can used as lightweight heat radiator material in aerospace and aviation industry and microelectronics for which the weight should be much concerned. Preparation of mesophase pitch based graphitic foam and analysis of its bulk thermal conductivity have been in this work which include the optimization of process parameters, numerical analysis of effective thermal conductivity of graphitic foam and the depiction of the randomness in graphitic foam’s structure and its impact on graphitic foam’s thermal properties.Apart form the properties of solid phase, the thermal properties of cellular material is dependent on its micro-structure, such as the size of the cellular structure and its distribution to a large certain extent can not be neglected. As for the graphitic foam produced and analyzed in this work, its micro-structure is largely decided by the process parameters (pressure, temperature and so on) during the foaming and carbonization process. Due to its’complexity and hard to visual observation, pore uniformity is proposed as a parameter to characterize the micro-structure of carbon foam, its micro-structure is analyzed by using the SEM image after the carbonization process. The influence of the parameter pressure, temperature and heating power on the micro-structure of carbon foam is obtained.Although the connection between the thermal properties of porous material and its micro-structure does exist, it is hard to be described clearly. Due to the difficulty and high cost and the lack of generality of experimental outcome in practice for the experimental method and the inaccuracy of the simplified models for the theoretical and semi-empirical methods, numerical method is widely used in the investigation of porous material with the development of computer technologies. Effective thermal conductivity is one of the most important properties of porous material. Semi-empirical analysis has been conducted for the effective thermal conductivity of porous material, but it is usually fit for low porosity material. Therefore numerical method is used by most researchers. In this work, face centered and body centered cubic models have been set up for high porosity material. Based on the models, FEM method was carried out and efficient thermal conductivity is obtained. Compared with semi-empirical results, it is found that shape factor from relevant investigation of others can be used in face centered cubic model when porosity is between 0.75 and 0.90. But the larger deviation exists when it is used in body center cubic model.The properties of homogeneous porous material such as the conductivity are generally expressed as the function of the conductivity of its solid phase and its porosity based on the homogeneous assumption. But except for a few man-made porous materials, the properties of most porous materials deviate far from the homogeneous assumption in production process, especially for graphitic foam through blow method with high temperature and high pressure. The impurity of the raw material, the non-homogeneity of the raw material granule, and the disturbance of process parameter can led to random diversification of its pore structure and result in complex nonlinear properties. So, apart form the conductivity of solid phase and its porosity, the geometrical structure of pores is also one of the influence factors of the thermal conductivity of porous material. For the expression and investigation of the relationship between thermal conductivity and geometrical structure of pores, randomness factor is used to modify the location of pores and the diameter of pores. Models with random geometrical structure were set up by using programming and CAD technologies to simulate the pore structure of non-homogeneous porous material. The influence of pore location randomness and the randomness of pore size on conductivity of porous material was investigated. Analysis was conducted regarding pore uniformity with combination of these two randomness. It is point that the effective thermal conductivity of porous material can be expressed as function of the conductivity of solid phase, macro-porosity and pore uniformity. This can be used as in manufacturing of graphitic foam and the research for non-homogeneous porous material.更多还原