Thermal Conductivity Of Cu/Diamond Composites Prepared By Gas Pressure Infiltration

With the development of electronic information technology,heat flow density of powered devices is continuously increasing.Traditional heat dissipating materials cannot meet the requirements of thermal conductivity.A new generation of heat dissipating material with high thermal conductivity is urgently needed.Diamond possesses superior thermal properties,including high thermal conductivity（TC of 1200-2000 W/mK）and low coefficient of thermal expansion(CTE of 2.3×10^-6/K).Diamond particles reinforced copper matrix composites（Cu/diamond composites）have high thermal conductivity and tailorable coefficient of thermal expansion and are drawing much attention.Due to non-wetting and chemical inertness characteristics between copper on diamond,weak interface bonding is derived in the Cu/diamond composites,and the excellent performance of diamond is not exploited.In this thesis,a new fabrication technique of gas pressure infiltration（GPI）is developed to produce Cu/diamond composites.Furthermore,surface metallization of diamond and alloying of metal matrix are conducted during the fabrication of the Cu/diamond composites.As a result,a transition carbide layer is found to be introduced.This is helpful to improve the interfacial bonding between diamond and copper and to decrease the interfacial thermal resistance.The Cu/diamond composites with high thermal conductivity are thus obtained.Metal elements,including molybdenum（Mo）,vanadium（V）,titanium（Ti）,chromium（Cr）and tungsten（W）,have been coated on diamond surface to produce carbide layer through different diamond surface metallization techniques.The molybdenum oxide,vanadium oxide and tungsten oxide were mixed with diamond particles,while the metal powders of titanium and chromium were mixed with diamond particles directly.The mixed powders were heated to obtain coating layer on diamond surface with different thicknesses and phase compositions.The coated diamond particles were then infiltrated with liquid copper using the GPI method to produce Cu/diamond composites.The results show that the quality and thickness of the coating layer play a determining role in enhancing thermal properties of the composites.An uniform coating layer with a suitable thickness is highly requested to acquire high thermal conductivity.The highest thermal conductivities of the composites preparared by coated diamond with V,Mo,W,Cr and Ti were 205 W/mK,221 W/mK,670 W/mK,714 W/mK and 716 W/mK,respectively.Metal matrix alloying method has been applied during the GPI process.Different transition metal elements have been added to Cu matrix with various contents to study the effect of alloying elements on thermal properties of the Cu/diamond composites.Alloying elements of Ti,Zr and Cr have been carefully studied.The results show that the alloying elements could effectively improve the thermal conductivity of the Cu/diamond composites.The thermal conductivity first increases and then drops with increasing alloying element concentration.The highest thermal conductivities were obtained to be 752 W/mK,810 W/mK and 930 W/mK with alloying of 0.5 wt.%Ti,0.5 wt.%Cr and 0.5 wt.%Zr,respectively.The thermal conductivity of 930 W/mK is almost 5 times higher than that of the non-alloyed matrix composite.According to the X-ray diffraction（XRD）analysis,the interfacial reaction between alloying elements and diamond causes the formation of interfacial carbides.Focused ion beam（FIB）and transmission electron microscopy（TEM）have been used to research the effect of Zr content in metal matrix on the thickness of the carbide layer and the thermal properties of the Cu/diamond composites.Although the interfacial carbides could enhance interfacial bonding,the low intrinsic thermal conductivity of the carbides could increase interfacial thermal resistance as well.The thickness of the carbide layer should be carefully controlled in order to attain high thermal properties.The introduction of different kinds of interfacial carbide layer prepared with different routes to the Cu/diamond composites has all improved the thermal conductivity significantly.Due to the difference in interface optimization,the thermal expansion coefficients of the Cu/diamod composites differ slightly,but they are far lower than that of pure copper and match semiconductor materials.The results show that the GPI-produced Cu/diamond composites exhibit high thermal conductivity and low thermal expansion coefficient,which can be potentially used as promising thermal management materials in the heat dissipation of high power devices like microelectronic chip,laser and semiconductor lighting.