High Thermal Conductivity And Electrical Insulation Polymeric Composites

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·...