PP/AlN复合材料导热机理研究

以氮化铝(AlN)粉末、聚丙烯(PP)为原料,通过共混-模压法制备了PP/AlN导热复合材料,并对其导热性能进行检测。研究发现,该复合材料的导热系数与AlN添加量之间不是线性关系而是呈幂函数关系,与介电逾渗理论基本吻合。根据检测数据并参考介电逾渗理论,提出了导热逾渗模型,并利用该模型和其他导热预测模型对PP/AlN复合材料的导热系数进行了计算。结果表明:复合材料的导热系数随AlN质量分数的增加呈幂函数增加趋势;实际检测结果与导热逾渗模型的预测结果基本相符,使该理论得到了验证。     

填充型导电高分子复合材料的逾渗理论进展

总结了粒子填充型导电体系逾渗网络的形成及逾渗理论概况，以热力学和动力学的观点对导电粒子种类、形状、浓度、分布和分散、基体树脂的种类、粒子与基体树脂的相互作用、加工条件等影响逾渗的重要因素作了系统的分析，阐明了体系的导电性、微观形貌、流变性能和物理性能的相互关系。对低逾渗值填充聚合物尤其是炭黑复合材料的成型与加工提供了理论指导。     

聚合物基导热绝缘复合材料的制备及逾渗模型的应用

采用乙烯一醋酸乙烯a共聚物和无机导热填料制备聚合物基导热绝缘复合材料,概述了以逾渗理论为基础的热导率计算模型,并应用所制备的导热绝缘复合材料讨论了逾渗模型的准确性。结果表明,SiC填充的复合材料具有较好的导热性能;填料体积分数达0.5时,复合材料的热导率可达1.86 W/(m0K)。研究表明,简单地运用逾渗理论在预测导热复合材料体系的热导率方面准确性不足,需要进一步考虑实际填料粒子分布与理论假设的差异以及界面相的存在等因素的影响。     

聚合物基复合材料导热模型及热导率方程的研究

根据导热填料在聚合物基体中的分布，提出了导热聚合物基复合材料两相体系的“海岛-网络”模型；并结合逾渗理论及其在导电复合材料中的应用，建立了导热复合材料的逾渗热导率方程。实验证明，该模型及热导率方程符合实际而且适用于高含量填充型导热聚合物基复合材料热导率的预测。     

基于逾渗理论对乳胶基质老化过程的分析

针对乳化炸药基质在储存过程中出现的析晶破乳现象,基于逾渗理论,建立了乳胶基质老化过程的逾渗模型。采用高低温循环法加速乳胶基质老化,并使用光学显微镜及激光粒度仪对乳胶基质进行观测。结果表明,所建逾渗模型符合实际,其中逾渗阈值p_c可以合理地解释乳胶基质的老化程度。将该模型用于先前的实验研究,分析了乳化炸药在老化过程中电导率出现变化的原因。该模型为研究乳胶基质的微观结构与宏观稳定性能之间的关系提供了一种新方法。     

高分子基导电复合材料非线性导电行为及其机理（Ⅰ）——导电通道理论

简述了高分子基导电复合材料非线性导电行为的概念,详细讨论了高分子基导电复合材料非线性导电行为的机理——导电通道理论,并分析比较了统计逾渗模型、界面热力学模型、有效介质理论等不同理论模型之间的适用范围及其优点与不足。     

基于逾渗理论的多孔过滤介质孔径分布估计方法

结合逾渗理论和网格模型,建立了过滤系数与过滤介质孔径分布的联系方程。利用此方程结合过滤实验数据获得过滤介质孔径分布的统计参数,同时与其他方法获得孔径分布参数以及其他文献中的数据进行比较,并利用计算机模拟过滤过程验证了获得的孔径分布参数,结果表明基于逾渗理论的过滤模型用于孔径分布参数的估计是比较准确和有效的。     

导电介质填充聚合物分散工艺的模拟及研究

模拟了导电介质填充聚合物体系的二维点阵逾渗过程 ,研究了分散工艺对导电性复合材料逾渗形成过程的影响 ,以及HDPE/CB复合体系导电粒子的分布状态对其逾渗网络及导电特性的影响。根据在低填充体积分数时逾渗的发生是粒子不均匀分散这一原理 ,阐述了PTC材料设计制作的注意事项。     

PPS复合导热材料的研究与有限元模型分析

通过有限元分析软件ABAQUS,模拟了三氧化二铝(Al2O3)填充聚苯硫醚(PPS)两相复合材料逾渗结构以及Al2O3填充PPS合金三相复合材料双逾渗结构,并将得到的理论导热率与实验结果进行对比验证,分析了填料分布情况对材料热导率的影响。模拟结果表明:只有当导热填料含量超过逾渗阈值,其导热性能才能有明显的提高,双重逾渗结构的形成,能有效降低逾渗阈值。SEM分析表明,选用不同的合金组分,由于相容性的差异和黏度的差异,填料在PPS合金中的分布情况也有不同,热导率也有较大的差异。在所述配方中PPS/UHMWPE/Al2O3(质量比为35:15:50)复合体系热导率最高,可达1.592W/(m.K),而同质量配比的PPS/PS/Al2O3复合体系的最低,只有0.639W/(m.K)。     

接触式机械密封界面泄漏机理分析

旋转机械装备的密封性能对生产过程的能耗、效率和环保具有重要影响。基于逾渗理论,探讨了接触式机械密封界面空隙状态随动、静环表面分形参数和接触压力变化的规律。研究建立了密封界面单层网格微通道结构模型,指出了密封界面在接触压力作用下表现的逾渗、逾渗点和非逾渗3种状态。根据液体毛细管力和气体Knudsen数,提出了微通道内流体流动判据,分析了密封界面流体的流动阻力和泄漏流量,阐释了接触式机械密封界面泄漏机理。研究结果为接触机械密封优化设计和泄漏控制提供依据。     

Film percolation for composite electrodes of solid oxide fuel cells

A film percolation model is proposed for composite electrodes of solid oxide fuel cells (SOFCs). The model is developed to predict the percolation properties of 2D-infinite structures which represent the structural characteristics of composite electrodes of electrochemical devices such as SOFCs. The model can be used to estimate electrode properties, such as percolation probability, active three-phase boundary length and interfacial polarization resistance. Compared with the classic percolation theory, which is particularly applicable to 3D-infinite bulks, the model can explicitly capture the effects of thinly layered nature of composite electrodes, and describes a cross-over between 2D-infinite films and 3D-infinite bulks. It also permits the prediction within whole electrode composition range, and can be easily applied in SOFC modeling.     

SiO2—C系陶瓷复合材料中的渗漏现象

用渗漏理论讨论了SiO_2-C系陶瓷复合材料的组成与宏观电导率的关系,结果表明按渗漏模型所得到的结果与实验结果是一致的。该SiO_2-C系复合陶瓷材料的渗漏阈值Pc(石墨的临界体积百分含量)为0.069,其临界指数t=1.72,即为三维渗漏模型的临界指数值。     

Electrically Conductive Fibre Composites Prepared from Polypyrrole‐Engineered Pulp Fibres

In this research work, electrically conductive paper composites were prepared from polypyrrole‐engineered pulp fibres by two methods: (a) exclusively from such engineered fibres; and (b) from a mixture of the modified fibres and unmodified ones. Both composites were investigated for their conductivity and strength properties as a function of monomer dosage or percentage of treated fibres in the mixture. It was found that, for the “mixture” method (i.e., by adding treated fibres as conductive fillers), less monomer (i.e., conductive polymer) was needed to achieve the same conductivity while a higher tensile strength in the paper was attained when comparing with paper obtained exclusively from treated fibres. The percolation model was adopted to describe such paper—conducting polymer composites, and the much lower percolation threshold achieved through the “mixture” method can be explained by a multiple‐percolation theory. The long‐term environmental aging stability of these composites was also monitored, and attempts were made to interpret the observed conductivity decay through existing kinetic models.     

A modified model of electrical conduction for carbon black–polymer composites

A modified model of electrical conduction for carbon black‐polymer composites is proposed based on the general effective media theory and the number of carbon black particles per aggregate to describe their nonlinear conductive behavior. In order to consider the interactions between carbon black particles, a simple self‐consistent approach is used to improve the model. Expressions of electrical conductivity and percolation threshold are derived from the modified model. The predicted results of electrical conductivity and percolation threshold are in good agreement with experimental data reported in previous literature. Moreover, the dependences of electrical conductivity and percolation threshold on the number of carbon black particles per aggregate are discussed. Copyright © 2010 Society of Chemical Industry     

Dielectric properties and morphology of polymer composites filled with dispersed iron

The dielectric properties and the structure of various metal–polymer composites, based on a polymer matrix of polyamide (PA), polyethylene (PE), polyoxymethylene (POM), or blend PE/POM filled with dispersed iron (Fe) particles, have been investigated in this work. In PE–Fe, PA–Fe, and POM–Fe composites the filler spatial distribution is random. In the PE/POM–Fe composites, the polymer matrix is two‐phase and the filler particles are localized only in the POM phase, resulting in an ordered distribution of the dispersed filler particles within the blend. The concentration and frequency dependence of the dielectric permittivity, ε′, and the dielectric loss tangent, tanδ, are described in terms of the percolation theory. The experimental values of the critical exponents (namely, s, r, and y) are in good agreement with those predicted by the theory for the composites with random filler distribution. The PE/POM–Fe composites demonstrate low value of the percolation threshold, P_C, and high values of the critical exponents r and y. This is attributed to the specific structure of these composites. A schematic model for the morphology of the composites studied has been proposed. This model explains the peculiar behavior of the PE/POM–Fe composites by assuming ordered distribution of the filler particles in a binary polymer matrix. The proposed model is in good agreement with the results of optical microscopy. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 3013–3020, 2003     

Two-Dimensional Whisker Percolation in Ceramic Matrix–Ceramic Whisker Composites

A computer model that treats ceramic-powder matrix–ceramic whisker composites as a percolative network of whiskers has been developed. The model calculates the critical fraction of whiskers at the percolation threshold for a two-dimensional random network of whiskers. The computed critical fraction was found to display an inverse dependence on whisker aspect ratio. In addition, the computed critical fraction (27 vol% for a whisker aspect ratio of 7) agreed well with the zero-shrinkage whisker fraction of 30 vol% in the densification of a colloid-pressed alumina–silicon carbide composite that exhibited a two-dimensional orientation of such whiskers.     

Percolation Phenomenon for Dissolution of Sodium Borosilicate Glasses in Aqueous Solutions

A percolation phenomenon has been discovered for the dissolution of sodium borosilicate glasses in aqueous solutions under low-pH conditions (pH ∼2). The glasses fall into one of two categories which are markedly different in the dissolution behavior, in terms of whether the Si/B ratio in the glass network exceeds a threshold value or not. The transition from one of the categories to the other is very sharp when the Si/B ratio is changed as the key parameter. The qualitative aspects of the experimental observation can be explained very well by a simple model combined with the site percolation theory.     

Linear displacement of a wetting fluid by an immiscible non-wetting fluid in a porous medium: A predictive algorithm

The linear displacement of a wetting fluid by an immiscible non-wetting fluid in a two-dimensional porous medium composed of a network of sites multi-connected by bonds has been simulated mathematically. The algorithm involves Monte Carlo decision making, random walks and principles of the percolation theory. The algorithm described in the present work successfully predicts the three distinct behaviours of immiscible displacement in porous media. This algorithm is tested against experiments available in the literature for two-dimensional porous media. The agreement between the numerical results and the experiments is very good.     

确定多孔介质孔结构参数的渗流网络分析法

According to the simulation of nitrogen sorption process in porous media with three-dimensional network model, and the analysis for such a process with percolation theory, a new method is proposed to determine a pore structure parameter--mean coordination number of pore network, which represents the connectivity among a great number of pores. Here the “chamber-throat“ model and the Weibull distribution are used to describe the pore geometry and the pore size distribution respectively. This method is based on the scaling law of percolation theory after both effects of sorption thermodynamics and pore size on the sorption hysteresis loops are considered. The results show that it is an effective procedure to calculate the mean coordination number for micro- and meso-porous media.