Investigation On Method Of Heat Transfer Enhancement Across Contact Surfaces Based On Carbon Nanotube Array

With the rapid development of microelectronics and semiconductor,the electronic components branch out into high-packaging density and high-integration,which has put forward rigorous requirements on the heat dissipation of chip cooling.In order to ensure the chip proper functioning,the heat flux generated by operation must be spread quickly and efficiently,while interface thermal resistance is one of the key issue and appeals to excellent Thermal Interface Materials(TIMs).Recently,vertically aligned carbon nanotube array(VACNT)has been viewed as the most promising candidate for TIMs,due to its unique oriented structure,outstanding thermal properties,low expansion coefficient in radial plane and controllable morphology which can adapt to the surface roughness.Making it as thermal conductive filler or independently as TIMs both can effectively reduce the interface thermal resistance.However,the previous researches have found that the parameter of VACNT,composite process or membrane preparation process would affect the thermal performance of VACNT-based TIMs.Furthermore,its theoretical model is too complex thus restricting its application.Hence,the study of preparation technology,parameter adjustment and optimizing,and research on theoretical model of thermal resistance was significant for promoting the application of VACNT-based TIMs.In this study,the VACNT/polymer composite film was prepared with flexibility and highly thermal conductivity.Meanwhile,the influence factors of thermal performance of VACNT-based TIMs were investigated.By retrieving the weak-oxidation assisted transfer process of VACNTs,a freestanding VACNT film was obtained.Considering the thermal contact resistance of VACNT,enhanced thermal and mechanical properties were achieved after reactive ion etching(RIE)and subsequent metallization of the surfaces of the VACNTs.Furthermore,the theoretical model of thermal resistance of VACNT-based TIMs in packing was analyzed and compared with experimental results.The main work of this paper includes the following aspects:1.Research on the preparation and performance of VACNT/polymer composite TIMs Large-diameter,high-density and well-crystallized multi-walled CNT(MWCNTs)arrays were fabricated(the diameter of individual MWCNTs was approximately 80 nm,the G/D ratio of the nanotubes is-2.51,and the density of the arrays was 0.17 g/cm3).The large diameter of MWCNTs in arrays provide high density and strong structure from collapse in composing to keep the original ideal thermal paths.MWCNT arrays with large diameter avoid shrinkage in solution and thus achieve crack-free composite film.Epoxy,Sylgard 160 and TPU.three kinds of polymer mostly used in commercialized TIMs,were infiltrated into VACNT via in-situ polymerization to prepare the VACNT/polymer composite TIMs.By flexible modification of polymer and optimization of composite technology,the thermal properties and mechanical performance of the VACNT/polymer composite TIMs were improved.2.Reaech on the parameter adjustment and optimizing of VACNT-based TIMs The influence factors of thermal performance of VACNT-based TIMs were investigated,which include the fabrication process,diameter,height,annealing treatment and nano silver modified reinforcement.(1)The experimental contrast showed that the thermal conductivity and thermal tolerance of VACNT fabricated via FCCVD was superior to that fabricated via CCVD.(2)The diameter range of VACNT fabricated in our laboratory was from 20nm to 200nm.In this range,the thermal conductivity of VACNT increased with the diameter increased.Because the larger diameter CNTs has large density and robust structure,which was the key issue for keeping the original thermal paths of MWCNTs.Meanwhile,the larger diameter CNTs has more surface contact area for heat transfer.(3)With the increase of its height,the thermal conductivity of VACNT increase firstly and then decrease.The height of 500 ?m is the optimization height in our experiment,in which the thermal properties of VACNT is the best.(4)Pre-annealing treatment was used to purify CNT arrays and improve thermal conductive performance of VACNT arrays/silicone composite.The thermal conductivity of the composite was enhanced by 34.52%and the thermal interface resistance was reduced by 65.94%at a pre-annealing temperature of 490? for 5 min.(5)Nano silver was used to modify the thermal performance of polyurethane.After composing the VACNT,a three-dimensional thermal conducting network was established.When the silver content is 60wt%.the thermal resistance of VACNT/TPU film was reduced by 50%compared with no adding.3.Research on the preparation and thermal properties of freestanding VACNT film The well-controlled weak-oxidation-assisted transfer method was proposed to prepare the freestanding VACNT film.Then the VACNTs could be independently assembled onto specific substrates for its real applications without considering the high growth temperature and poor mechanical performance.No degradation in the quality was observed before and after the weak oxidation according to thermo gravimetric analysis(TGA)and Raman spectra of VACNT.After transferring the VACNT to the cooper,the test results shows that the total thermal resistance of cooper/VACNT/cooper interface was reduced by 38%.One structure,in which the top side of the VACNTs is bonded to the target substrate,can also be obtained by other transfer methods.The other one,in which the bottom side of the VACNTs is assembled onto the target substrate,can only be achieved by our method reported herein.The samples in which the bottom side of the VACNTs is bonded to the copper surface have lower thermal resistance and thus stronger interfacial bonding with the target substrates than the samples in which the bottom side is assembled onto the copper.More importantly,the growth-transfer synthesis of VACNT arrays can be realized in one batch,which saves a lot of energy.In addition,the silicon substrate and the catalysts can be recycled multiple times for the growth-transfer synthesis of VACNTs.Thus,the repeated growth-transfer synthesis of VACNT arrays in one batch may open up commercial realization of various functional devices based on VACNT-TIMs.4.Research on the preparation and thermal properties of Au-VACNT-Au film In order to further reduce the interface thermal resistance between VACNT films and packing substrate and make the VACNT film mechanically more robust,a thin layer of titanium/nickel/gold was deposited on both the top and bottom surfaces of the freestanding VACNT films to fabricate the Au-VACNT-Au film.The thermal diffusivity of Au-VACNT-Au film was increased by 38%,and the thermal contact resistance was reduced by 50%compared with that of the original VACNTs.After the metallization on the RIE-treated surfaces,the Au-VACNT-Au film would not break even under a loaded force of 641 mN,which is close to 2 orders of magnitude higher than that of the VACNT film.5.Research on the theoretical model of thermal resistance of VACNT-based TIMsThe theoretical model of thermal resistance of VACNT-based TIMs in packing was considered and analyzed with heat transfer theory.The factors influenced on the total thermal resistance and interface thermal resistance were investigate and their influence principles were explored compared with experimental results.The study found that the theoretical results trend was consistent with the experimental results trend.