【作者】 周文英；

【导师】 齐暑华；

【作者基本信息】 西北工业大学 ， 材料学， 2007， 博士

【摘要】 绝缘导热高分子材料对于高频微电子元器件散热，提高其精度、延长寿命具有愈来愈重要作用。本文以甲基乙烯基硅橡胶为基体，氧化铝和氮化铝无机粒子为主要导热填料制备出综合性能优良的导热弹性垫片。以三类聚乙烯（线性低密度聚乙烯、高度聚乙烯、超高分子量聚乙烯）为基体，氮化硼、氮化硅为主要填料，制备出高热导率电绝缘复合塑料。借助于自行研制出的高分子复合材料热导率及热阻测试仪器，以及示差扫描量热法、傅立叶红外光谱、热失重分析、扫描电镜等现代分析手段详细研究了填料种类、含量、粒径、制备工艺等因素对复合材料热导率、热阻、电绝缘性、介电、力学性能、结构及其它性能影响。实验研究发现：1．绝缘导热硅橡胶研究（1）硅橡胶热导率及热阻随氧化铝、氮化铝用量增加而分别升高和降低，填料用量达临界值后，热导率增加迅速。氮化铝加速了硅橡胶硫化，氧化铝对硫化影响不明显，两种填料均明显提高了硅橡胶热稳定性，降低了体系热膨胀系数。随填料用量增加，硅橡胶体积电阻率、表面电阻率、介电性能及介电强度性能均有所下降，但仍然保持良好的电学性能；力学性能随填料增加而下降。（2）大粒子填料形成导热通路能力强于小粒子，形成更稳定导热通路；小粒子填料更有利于提高硅橡胶力学性能。不同粒径填料粒子按照适宜比例混合组成混杂填料，所得硅橡胶具有最高热导率。对于二元混杂粒径填料，小粒子体积用量占总用量的20～35％之间时体系的热导率、拉伸强度、介电常数、热膨胀系数均能达到最佳值。氧化镁晶须和导热粒子混杂填料填充硅橡胶热导率优于等量单一粒子填充效果，力学性能有改善。（3）适宜成型压力和时间减少了材料内部空隙率，提高材料致密度，改善热导率和电学性能。偶联剂降低了材料界面处缺陷和孔洞，抑制了界面处声子散射现象，增大声子平均自由程，提高了热导率。然而，偶联剂过量降低体系热导率。偶联剂增强了两相界面粘接，提高了硅橡胶力学强度。（4）以电子级玻璃布为增强体制备出具有高热导率、良好电绝缘性、形变性能及一定力学强度的弹性热界面材料，作为一类很重要的热界面材料，弹性热垫片对于低功率芯片散热具有重要作用。（5）填充硅橡胶热导率范围介于Maxwell模型上、下限预测范围内；填料粒子形状参数愈大，体系热导率愈高；导热粒子与橡胶基体间的热阻大小决定着填料对体系热导率的影响。基于组合数学观点，推导出一新型热导率方程，经验证，该模型具有一定的适用性(k=k₁k₂／（（1-C）k₁+Ck₂）=k₁k₂／((1-V_f^1／3)k₁+V_f^1／3k₂)。2．绝缘导热聚乙烯研究（1）氮化物粒子和聚乙烯颗粒经粉末混合后，由于范德华力及静电作用力导热粒子在聚乙烯表面周围形成了包覆层；复合粒子热压成型后，在体系内部形成了以聚乙烯颗粒为中心的导热粒子环绕的“核-壳”结构的无规网状导热通路。和熔融共混相比，粉末法在较低填料含量下形成导热通路，热导率高。（2） 30wt.％氮化硼和氮化硅填充时复合塑料热导率达1.68 W／（m·K）和1.52W／（m·K），具有高的电绝缘性及低介电常数和介电损耗，但力学性能下降。随填料粒径下降，复合塑料热导率升高；随聚乙烯粒径增加，热导率升高。在氮化物填料粒子中加入少量氧化镁晶须或氧化铝短纤维，热导率增加缓慢，而力学性能提高明显。（3）适宜的成型压力和时间有助于提高热导率及电学性能。适量偶联剂提高了氮化物和聚乙烯颗粒间相容性，减少了体系内空隙，热导率升高，介电性能和力学强度和韧性得到改善。（4）导热粒子增强UHMWPE具有高热导率、一定力学强度、卓越的电学性能及高冲击性能。和普通聚乙烯相比复合UHMWPE具有优异的导热能力，是低温场合理想的电子封装及基板材料。（5）氮化物填料对聚乙烯熔融温度基本没有明显影响，但对其结晶度有影响。常规热导率模型低估粉末法制备的复合塑料热导率，以下两个方程适宜描述该类复合塑料热导率：logk=logk_p+(logk_Vm-logk_p)·（V_f／V_m）^N及k_c^a=（1-V_f）k_m^a+V_f·k_f^a。更多还原

【Abstract】 The electrically insulating and thermally conductive polymer composites areincreasingly important for high reliability and long life of the micro-electronic devicesworking under high frequency. Methyl vinyl silicone rubber filled with electricallyinsulating and thermally conductive alumina （Al₂O₃） and aluminum nitride （AlN） wereinvestigated to be used as elastomeric thermal pads, a class of thermal interfacematerials （TIM）. Moreover, the insulating heat conductive composite plastics wereobtained from three kinds of polyethylene （LLDPE, HDPE and UHMWPE） as matrixsand boron nitride （BN） and silicone nitride （Si₃N₄） as main fillers using powder mixingtechnology in this paper. The effects of fillers, content, particle sizes and processingtechnology on the thermal conductivity, thermal contact resistance, volume resistivity,surface resistivity, dielectric property, mechanical properties, structures and otherproperties of composite plastics were discussed detailedly by virtue of the apparatusmeasuring the thermal contact resistance and thermal conductivity of polymericmaterials, which were fabricated in our lab, and other modern analytical means, such asdifferential scanning calorimetry （DSC）, fourier transform infrared spectrometer （FT-IR）thermogravimetric analyzer （TGA） and scanning electron microscopy （SEM）. Thefollowing conclusions can be reached from the experiments.1. Thermal conductive silicone rubber（1） The addition of either Al₂O₃ or AlN filler particles into the silicone rubber increasesits thermal conductivity whereas decreases its thermal contact resistance. The thermalconductivity increases rapidly after the critical filler concentration. AlN filleraccelerates the vulcanization reaction of silicone rubber, whereas Al₂O₃ shows noobvious effect on the reaction. The both fillers strongly improve the thermal stabilityand remarkably reduce the coefficient of thermal expansion （CTE） of filled siliconerubber. As filler loading increases, the electrical properties of composite rubber, such asvolume and surface resistivity, dielectric properties and breakdown voltage decline,however, they still remain quite high values. The mechanical properties decrease withincreasing filler content.（2） The larger filler particles can easily form more stable conductive pathwayscompared with the smaller ones, whereas, the mechanical properties of smaller particles filled silicone rubber are superior to that filled with larger grain size. Silicone rubberincorporated with mixture of hybrid particles at a preferable mass ratio exhibits thehighest thermal conductivity compared with the cases where only filler with singleparticle size was used, furthermore, for the binary system of hybrid sizes, when thesmaller particles concentrate is 20～30 vol. % of total fillers the thermal conductivity,tensile strength, dielectric constant and CTE reach the peak values. The use of hybridMgO whisker and conductive particles provides composites with better thermalconductivity and mechanical property than the sole particles used.（3） The optimal molding pressure and time help to improving thermal conductivity andelectrical property of composites due to the improved compact structure inside materials.Optimal dosage coupling agent enhances thermal conductivity since it reduces theinterfacial voids and deflects, restrains the interracial phonon scatting and extends thephonon mean free path. In addition, coupling agent improves mechanical strength ofcomposites due to good interfacial bonding.（4） A new type of thermally conductive silicone rubber composites, possessing highthermal conductivity, good electrical insulation, mechanical properties and easydeformation under low pressure, was successfully developed using electrical glass clothas reinforcement, which serves as elastomeric thermal pads, a class of TIM, for heatdissipation purposes of low powder chip sets.（5） The thermal conductivity of Al₂O₃ or AlN filled silicone rubber lies between theupper and lower boundary of Maxwell model. The bigger particle shape factor of fillerleads to the higher thermal conductivity of composites. The thermal contact resistancebetween filler and rubber matrix poses a great impact on the thermal conductivity ofcomposite rubber. Based the combinatorics, the author proposed a new equationpredicting the thermal conductivity of filled silicone rubber. The results show theequation is in good agreement with experimental results at low filler content. (k=k₁k₂／（1-C）k₁+Ck₂=k₁k₂／(1-（V_f）^1／3k₁+（V_f）^1／3k₂).2. Thermal conductive composite polyethylene plastics（1） The powder mixing preparation condition allows the deformation of a randomdistribution of BN or Si₃N₄ particles on the surface of polyethylene （PE） matrix volumeand to create ordered shell structure in the BN/PE and Si₃N₄/PE systems due to theexisting Van der Waals and static interactions between small filler particles and larger PE particles. Thus, the reticulatal conductive channels from filler particles randomlysurrounding the larger PE particles, just like shell/kernel structure, were created after theprocess of hot pressing, which provide the composites with high thermal conductivity atlow filler concentration, compared to the composite plastics produced from the meltedmixing.（2） 30 wt. % of BN or Si₃N₄ filled PE exhibits thermal conductivity of 1.68 W/（m·K）and 1.52 W/（m·K）, excellent volume and surface resistivity, dielectric properties anddeclined mechanical property. Thermal conductivity of the composites rises as filler sizeincreases, and descends as PE size decreases. The addition of a small quantity of MgOwhisker or Al₂O₃ short fiber into BN or Si₃N₄ filler particles increases obviously themechanical property, however, thermal conductivity only gets little rise.（3） The optimal molding pressure and time enhance thermal conductivity and electricalproperty of composites. Optimal dosage coupling agent improves thermal conductivity,dielectric property, strength and toughness of composite plastics because of theenhanced interfacial compatibility between filler and PE matrix due to the reducedinterfacial deflects and voids.（4） The BN/UHMWPE demonstrates high thermal conductivity, excellent electricallyinsulating property, good impact strength, and shows eminent heat conduction propertycompared with common PE materials. So, the heat conductive BN/UHMWPE is an ideakind of materials for electric packaging and substrate maerials used at lowertemperature.（5） BN and Si₃N₄ filler have no evident effect on melting temperature of PE, but affectthe crystallinity of PE. The most of used models predicting the thermal conductivity ofbinary composites underestimate the one of the developed composite plastics in thispaper. The following two equations can predict the thermal conductivity to a certaindegree. logk=logk_p+（logk_Vm-logk_p）·（V_f/V_m）^N and k_c^a=（1-V_f）k_m^a+V_f·k_f^a·更多还原