Modulation Of Thermal Conductivity And Compliance For Carbonbased Thermal Interface Materials

With the development of microprocessors towards miniaturization,high integration,high power density,and high frequency,while the performance of electronic devices is continuously improved,thermal challenges also come.If the waste heat cannot be removed in time,which will seriously affect the performance,reliability,and service life of electronic devices.Efficient thermal management is the key to solving the problem of heat dissipation.The thermal interface material（TIM）is the core material of thermal management,which is used between the chip and the heat spreader,and between the heat spreader and the heat sink to improve heat dissipation efficiency by enhancing thermal coupling and minimizing thermal resistance between heterogeneous components.Commercially,the traditional preparation method of TIMs is to blend polymer matrix and thermal-conductive fillers,which often need to be filled with more than 40-50 vol%of thermal-conductive fillers to form an effective heat transfer channel,due to the extremely low thermal conductivity of the polymer matrix(0.1～0.3 W m^-1K^-1).However,high filler loading not only leads to higher costs but also sacrifices the compliance of the polymer matrix.For thermal design,compliance,which can effectively reduce thermal contact resistance and relieve the warpage failure caused by stress concentration,is also important.This thesis focuses on achieving the coexistence of high thermal conductivity and excellent compliance in TIMs.We choose carbon-based materials as thermally-conductive fillers,and prepare the carbon-based TIMs with both high thermal conductivity and high compliance by constructing vertically-orientated structure,surface modification of carbon-based fillers,and modulating the compliance of the polymer matrix.The specific research contents mainly include two chapters,and the results were summarized as follows:Chapter 1:The preparation and properties of graphite/polybutadiene thermal interface materials with both high thermal conductivity and high complianceIn this chapter,we report a TIM made from vertically oriented graphite and polybutadiene（V-GP/PB）that shows high through-plane thermal conductivity of 64.90W m^-1 K^-1,excellent compliance with only 93 k Pa stress at 50%compressive strain similar to soft biological tissues,and outstanding compression resilience performance（storage modulus 220 k Pa,mechanical loss factor 0.226）.These excellent properties result from the vertical orientation of graphite films in polybutadiene and the minimized negative impact of graphite on the intrinsic mechanical properties of polybutadiene by means of cross-stacking technique.Meanwhile,by plasma bombarding and dodecyltrimethoxysilane grafting functionalization treatment on the surface of the graphite film,the interfacial thermal contact resistance between the graphite film and polybutadiene is effectively reduced and the interfacial bonding is enhanced.The optimal V-GP/PB TIM is applied in CPU microprocessor cooling and exhibits superior heat dissipation capability,by up to 158°C reduction of chip temperature compared with polybutadiene.In addition,the cross-stacking strategy and functional modification method are also applicable to the same type of thermally conductive fillers to prepare high-performance TIMs,such as boron nitride films,carbon nanotube films.Finally,the simple preparation process makes it possible to achieve mass production and is expected to achieve commercial application in the near future.Chapter 2:Preparation and properties of compliance adjustable carbon fiber/silicone rubber thermal interface materials with high thermal conductivityIn this chapter,we report a TIM made from vertically oriented carbon fiber and silicone rubber（CF/SR）that shows high through-plane thermal conductivity of 43.47W m^-1 K^-1（loading only 20 vol%）.Meanwhile,the thermal contact resistance of CF/SR TIM is further reduced via the strategy to modulate the crosslinking density of silicone rubber matrix,and the high-performance TIM for different packaging pressures is fabricated by means of modulating the compliance of CF/SR TIM.For the 20 vol%-CF/SR TIM with the best compliance,the maximum stress is only about 35 k Pa at 35%compressive strain similar to soft biological tissues,and also shows good compression resilience performance with a resilience rate of up to 85%after compression resilience cycles（10 cycles,40%strain）.In addition,the total thermal resistance of CF/SR TIM is reduced by almost half by treating the surface of CF/SR TIM with functionalized coatings.The optimal CF/SR TIM is applied in CPU microprocessor cooling exhibiting superior heat dissipation capability and stability.Compared with the commercial product of 13 W m^-1 K^-1,the chip temperature is relatively reduced by 66℃.Finally,our strategy to modulate the mechanical properties of the polymer matrix based on the vertical orientation structure opens up a new avenue for the preparation of high-performance TIM.