Research On Fabrication And Properties Of Graphite Flake/Cu Composites

With the increasing pursuit on the performance of electronic device,effective heat removal has become a crucial issue to maintain the semiconductor components running stably and reliably,which makes increasing challenges in electronic packaging materials.Graphite flake/Cu composites with high thermal conductivity,good machinability and reasonable cost have attracted extensive attentions.However,the poor wettability between Cu and graphite results in a weak interfacial bonding in the graphite flake/Cu composites.In this thesis,surface coating technique and matrix alloying method were employed to improve the interfacial bonding,and then the graphite flake/Cu composites were fabricated via vacuum hot pressing method.The effects of graphite flake volume fraction,carbide coating,as well as alloying elements content on the microstructure and properties were detailed investigated.The main results are presented as followed:Uniform and continuous carbide coating were successfully formed on the graphite flake surface through molten salt method.The Ti coating is constitutive of TiC while the Cr coating is composed of Cr7C3 and Cr3C2.Wetting mixing and repeated filling processes were adopted to guarantee the graphite flakes homogeneously distributed and preferred oriented in the Cu matrix.Graphite flake/Cu composites were fabricated via vacuum hot pressing under the temperature of 980? and pressure of 40 MPa for 30 min,possessing a relative density higher than 98.5%.After sintering,the composition of Ti coating did not change,but Cr7C3 phase in Cr coating reacted with graphite and transformed into Cr3C2.The introduction of carbide coating on graphite flake surface significantly reduces the defects such as holes in the interface and improves the interface bonding.Graphite flake/Cu composites exhibit obvious anisotropy,in which the thermal conductivity in X-Y plane is much higher than that in the Z direction.With the graphite flake content ranging from 30vol.%to 60vol.%,the thermal conductivity of Ti-coated and Cr-coated graphite flake/Cu composites is 438?571 W·m-1·K-1 and 498?625 W·m-1·K-1,respectively.The coefficient of thermal expansion slightly decreases with the increasing graphite flake content in the X-Y direction,matched well with the Terner model.However,in the Z direction,the coefficient of thermal expansion of the composites rapidly droped with the increasing graphite flake content,which is closer to the theoretical value predicted by the modified Kerner model.The introduction of carbide coating layer is beneficial to reduce the coefficient of thermal expansion of the composite,especially in Z direction.The carbide coating significantly improved the bending strength of the composite.When the graphite flake volume fraction is 60%,the bending strength of the Ti-coated and Cr-coated composites is 53.2 MPa and 55.7 MPa,57.9%and 65.3%higher than that of the uncoated one,respectively.Matrix alloying method can significantly improve the interfacial bonding by forming carbide in situ at the interface.The thermal conductivity of the composites initially increased and then declined with the increasing alloying elements content.When the graphite flake content is 50 vol.%,the highest thermal conductivity is 597 W·m-1·K-1 and 640 W·m-1·K-1 by adding 1 wt.%Ti and Zr,respectively.The coefficient of thermal expansion slightly decreased in the X-Y plane but significantly dropped in the Z direction with the increase of allloying element content.The bending strength increases with the increasing alloy element content,the maximun value is obtained to be 80 MPa and 95MPa with 2 wt.%Ti and Zr addition,respectively,which is 61%and 90%higher compared with the non-added one.