石墨悬浮液的热导率与黏度分析

通过实验测量,对石墨悬浮液进行热导率和黏度系数的研究。采用不同的超声时间,从而获得不同大小颗粒的石墨悬浮液。在不同的超声时间下得到了石墨悬浮液的热导率随体积分数的变化规律,并对其热流逾渗现象进行了分析。同时,对于室温下的石墨悬浮液进行了黏度系数的测量,可以得到石墨悬浮液的逾渗点的导热性质和黏度性质的对比,从而得到了两者是截然相反的结论。     

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)。     

纳米石墨在非水介质中的分散

改变超声工艺条件、分散介质、分散剂及其用量来改善纳米石墨在非水介质中的分散稳定性,并通过测试含纳米石墨悬浮液的稳定时间、黏度及吸光度,表征纳米石墨在分散介质中的分散状态。研究结果表明,最佳超声条件:30℃,20—25min;二甲基硅油为最佳分散介质,纳米石墨质量分数为0.03%,钛酸酯偶联剂NDZ-105为最佳分散剂,其最佳添加质量分数为2%。     

闪光法测试压缩膨胀石墨热导率探讨

闪光法是广泛使用的测量材料的热扩散系数的方法。压缩膨胀石墨是具有各向异性的一种高导热复合材料。理论上闪光法并不适用于测试复合材料的热导率,但在实际的研究工作中该方法被广泛采用。本研究用闪光法测试了不同厚度和密度的压缩膨胀石墨,使用单因素方差分析,发现在实验的范围内样品厚度对其测量数据并无显著影响。结合复合材料的结构特点,认为闪光法可用于压缩膨胀石墨的热导率测试。同时也发现密度对压缩膨胀石墨导热性影响显著。     

聚合物熔体黏度对炭黑/PP/HDPE复合材料导电性能的影响

采用转矩流变仪将质量分数为3%的导电炭黑(CB)分别添加到由熔体黏度不同的3种聚丙烯(PP)和3种高密度聚乙烯(HDPE)两两混合组成的共混物中,制备出9种不同的CB/PP/HDPE导电复合材料。在双逾渗理论的基础上,研究了两种聚合物的黏度对复合材料双逾渗过程及其导电性的影响。结果表明,CB/PP/HDPE体系中聚合物的熔体黏度主要是通过影响CB在两种基体中的分布以及CB富集相的微观形貌而分别影响复合材料的两次逾渗过程,从而改变材料的导电性。另外,研究发现在CB/PP/HDPE体系中,PP相对于HDPE的熔体黏度越高,则形成的复合材料导电性越好。     

石墨/聚酰亚胺复合材料的热性能研究

以石墨填充聚酰亚胺(PI)复合材料为研究对象,考察了温度和石墨用量对石墨/PI复合材料的比热容、热导率和热扩散系数的影响。实验表明,升温速率、载气N2流速和样品质量是影响PI复合材料比热容的主要因素;PI复合材料的比热容随石墨用量的增加而降低,PI复合材料的热导率和热扩散系数都随石墨用量的增加而增大;随着温度的升高,PI复合材料与无机复合材料的比热容和热导率变化规律相同;而PI复合材料与无机复合材料的热扩散率变化规律却不同。     

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。     

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。     

TiC增强膨胀石墨/铜复合材料导热性研究

将压缩膨胀石墨(CEG)在高温下通入四氯化钛(TiCl4),得到了生长在石墨片边缘的碳化钛(TiC),利用TiC改性的CEG与铜通过液相浸渍后热压的方法制备膨胀石墨/铜复合材料,研究TiC对CEG自身热导率的影响及其加入前后复合材料的微观形貌和热导率的变化情况。结果表明,适量TiC在石墨片层之间的连接作用能有效改善压缩膨胀石墨的导热网络,提高其热导率。膨胀石墨/铜复合材料热导率随着界面TiC量的增加而先升高后降低,原因在于TiC与铜之间的界面相容性比CEG与铜的好,然而当TiC含量进一步升高,其自身热导率低产生了更大的界面热阻导致复合材料的热导率下降。因此适量的TiC能有效改善膨胀石墨导热网络,且有利于改善膨胀石墨和铜的界面相容性,从而提高复合材料的热导率。     

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。     

Reduced percolation concentration in polypropylene/expanded graphite composites: Effect of viscosity and polypyrrole

With the goal to obtain material combining electrical and thermal conductivity at low filler loadings, composites based on polypropylenes (PP) and expanded graphite (EG) have been prepared. The effects of matrix viscosity and of coating the EG particles with polypyrrole (PPy, EG/PPy = 37.5/62.5 by weight) on the EG dispersibility and formation of percolation structures have been analyzed. When increasing the EG amount from 6 to 8 wt %, the electrical conductivity of PP/EG composites increased by 7–9 orders of magnitude, independent of matrix viscosity. When EG‐PPy is added, percolation was observed between 8 and 12 wt % EG‐PPy (3 and 4.5 wt % EG) in case of PP with higher viscosity and 6 wt % EG (2.25 wt % EG) in case of PP with lower viscosity, exhibiting a strong synergistic effect of EG and PPy in the latter case. In contrast, PPy does not contribute to reduction of thermal percolation concentration. Thermal percolation is observed at 8 wt % EG in PP/EG composites, but no percolation was found in PP/EG‐PPy composites with EG‐PPy contents of up to 20 wt %, corresponding to 7.5 wt % EG. Analyzing the melt rheology it becomes clear that the contribution of PPy to the formation of a filler network is strongly dependent on the matrix viscosity. The comparison of thermal, electrical and rheological percolation reveals that PPy participates in electron transport reducing the electrical percolation but not to heat transport. Overall, we found a good correlation between electrical, thermal, and melt rheological percolation concentrations. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41994.     

Preparation of highly thermally conducting polyamide 6/graphite composites via low‐temperature in situ expansion

Highly thermally conducting polyamide 6 (PA6) composites with high loadings of low‐temperature expandable graphite (LTEG) were prepared by an in situ exfoliation melting process, and the thermal conductivity of the composites was measured by a hot‐disk method. A two‐point method was applied to evaluate the electrical conductivity of the composites with various graphite loadings, and the thermal percolation was observed in the vicinity of the electrical percolation threshold concentration. Dynamic rheology analysis was used to define the geometric change caused by the interconnection of the in situ exfoliated graphite flakes. X‐ray diffraction measurement confirmed that the exfoliation of LTEG was crucial to the overall thermal conductivity of the composites. Dynamic mechanical analysis revealed that the incorporation of LTEG significantly improved the damping properties of PA6. Thermogravimetric analysis and differential scanning calorimetry measurements were applied to study the thermal properties of the investigated PA6/LTEG composites. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39596.     

Effect of filler size on thermophysical and electrical behavior of nanocomposites based on expanded graphite nanoparticles filled in low‐density polyethylene matrix

Various aspects of electrical and thermophysical properties of nanocomposites based on low‐density polyethylene matrix filled with nanostructuralized expanded graphite (EG) and standard, microsized graphite are presented in this article. A periodical method developed in the laboratory was used to measure simultaneously thermal conductivity, specific heat, and diffusivity of composites at room temperature. The effect of micro‐ and nanosized fillers on the final thermophysical and electrical behavior is investigated. It was found that the electrical conductivity of composites strongly depends not only on the filler content but also on the filler size. When the microsized graphite was used, the percolation concentration of the filler was found to be 15 vol%, whereas the percolation concentration of the filler in nanocomposites filled with EG of large sizes was significantly lower. Similarly, it was shown that the graphite significantly improves the thermophysical behavior of composites filled with micro‐ and nanofiller sizes. The thermal conductivity measured values were also compared with some theoretical models for the prediction of the thermal conductivity. POLYM. COMPOS., 2012. © 2013 Society of Plastics Engineers     

Electrical conductivity and rheology of carbon‐filled liquid crystal polymer composites

One emerging market for electrically conductive resins is for bipolar plates for use in fuel cells. Adding carbon fillers to thermoplastic resins increases composite electrical conductivity and viscosity. Current technology often adds as much of a single type of carbon filler as possible to achieve the desired conductivity, while still allowing the carbon‐filled thermoplastic matrix material to be extruded and molded into a bipolar plate. In this study, varying amounts of two different types of carbon, one carbon black and one synthetic graphite, were added to Vectra A950RX liquid crystal polymer. The resulting single filler composites were then tested for electrical conductivity and rheological properties. The electrical conductivity followed that typically seen in polymer composites with a percolation threshold at 4 vol % for carbon black and at 15 vol % for synthetic graphite. Over the range of shear rates studied, the viscosity followed a shear‐thinning power law model with power‐law exponent (n ? 1) = ?0.5 for neat Vectra A950RX and (n ? 1) = ?0.7 for highly filled composite materials. Viscosity increased with increasing filler volume fraction for all shear rates. The viscosity–enhancement effect was more rapid for the composites containing carbon black when compared with those containing synthetic graphite. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2680–2688, 2006     

Economical conductive graphite-filled polymer composites via adjustable segregated structures: Construction,low percolation threshold,and positive temperature coefficient effect

The economical graphite-filled thermoplastic urethane/ultra-high molecular weight polyethylene (TPU/UHMWPE) composites with the segregated structure were constructed by the combination of mechanical crushing and melt blending method. The low percolation threshold of 1.89 wt% graphite in the adjustable segregated composites was obtained and high electrical conductivity was about 10⁻¹ S m⁻¹ at 10 wt% graphite loadings owing to the formation of three-dimensional conductive networks. Moreover, when the graphite loadings were over the percolation threshold, the remarkable positive temperature coefficient (PTC) effect of electrical resistivity for TPU/UHMWPE-Graphite composites were achieved, originating from the combined thermal motion of TPU and UHMWPE. Meanwhile, the outstanding repeatability of PTC effects was obtained after 5-time cycles. Therefore, economical conductive polymer composites were still the promising field in the practical application of PTC materials.     

Comparative study on the exfoliated expanded graphite nanosheet‐PES composites prepared via different compounding method

Composites made of polyethersulfone (PES) reinforced with exfoliated graphite nanoplatelets are fabricated by melt mixing, polymer solution, and coating. Coating is an efficient compounding method emphasized in this research, where expanded graphite (EG) and PES powder are premixed in isopropyl alcohol using sonication to disperse the EG by coating individual PES powder particles. The microstructure and property of EG/PES composites were investigated by X‐ray diffraction, scanning electron microscope, thermal gravimetric analysis, differential scanning calorimetric, and electronic tensile tester. The electrical conductivity was confirmed using electrochemical tester. It is found that the coating method is more effective than the polymer solution and directly melt mixing methods widely used, in terms of increasing the electrical conductivity and lowering the percolation threshold of thermoplastic composites, and enhancing the probability that the large platelet morphology of EG can be preserved in the final composite. The research reported here provides an understanding on how the compounding method used during the fabrication of composites is important to achieving the optimal mechanical properties, thermal properties, electrical conductivity, and percolation threshold. This method should have wide applicability to all thermoplastic matrix composite systems. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Synergistic effects of multiple carbon fillers on the rheology of liquid crystal polymer based resins

Adding conductive carbon fillers to insulating thermoplastic resin increases composite electrical and thermal conductivity. Often, as much of a single type of carbon filler is added to achieve the desired conductivity, while still allowing the material to be molded into a bipolar plate for a fuel cell. In this study, varying amounts of three different carbons (carbon black, synthetic graphite particles, and carbon fiber) were added to Vectra A950RX liquid crystal polymer. The rheology of resulting single filler composites was tested. In addition, the rheological properties of composites containing combinations of two different carbon fillers were studied via a factorial design. In all cases, viscosity increased with increasing filler volume fraction for all shear rates. Over the range of shear rates studied, the viscosity followed a shear‐thinning power law model. The factorial design results indicated that each of the single fillers and all of the two filler combinations caused a statistically significant increase in composite viscosity at a shear rate of 1,000 s⁻¹. The composites containing carbon black and synthetic graphite caused the largest increase in viscosity. It is possible that the highly branched, high surface area structure of carbon black ‘links’ with the synthetic graphite particles, which results in increased composite viscosity. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers.     

阻燃型鳞片石墨/尼龙6导热复合材料

为了使树脂基导热复合材料兼具优异导热性能和阻燃性能,分别选用RPM/Mg(OH)2、BPS/Sb2O3、HS-PNPAZ/Mg(OH)2作为阻燃剂,鳞片状石墨作为导热填料,PA6作为基体树脂,采用双螺杆挤出机制备了阻燃型鳞片状石墨/PA6导热复合材料。对比了不同阻燃剂体系对复合材料导热和阻燃性能的影响。结果表明,不同阻燃剂对复合材料导热系数的影响较小,复合材料的导热系数主要受鳞片状石墨的影响。当鳞片状石墨含量为40份时,复合材料的导热系数均大于2.0 W/(m·K)。BPS/Sb2O3和HS-PNP-AZ/Mg(OH)2在鳞片状石墨/PA6复合材料中无阻燃效果,不适合用于鳞片状石墨/PA6复合材料体系的阻燃。RPM/Mg(OH)2在鳞片状石墨/PA6导热复合材料中具有优异的阻燃效果,可使复合材料的阻燃性能达到UL 94~1.6 mm V-0级别。     

基于聚偏氟乙烯/石墨的换热板片材料的制备及导热性能研究

以聚偏氟乙烯（PVDF）为基体,石墨为填料,使用溶液共混法制备导热复合材料,研究了石墨填充量、混炼工艺等对复合材料导热性能的影响。结果表明,石墨填充量超过40 %（质量分数,下同）后,复合材料的热导率呈明显增大的趋势;当石墨填充量达到70 %时,PVDF/石墨复合材料的热导率达到2092 W/（m·K）,是纯PVDF热导率的110倍;270 μm：106 μm石墨含量比为1∶9时,热导率达到最大值;采用叠片式转子比腰鼓形转子可以制备出更高热导率的复合材料;转子转速和混炼时间对复合材料的热导率影响不大;设计并制备了板式换热器换热板片,实验验证了基于PVDF/石墨复合材料加工生产换热板片的可行性。