Preparation,Structure And Properties Of Cu-B/Diamond Composites

With the rapid development of electronic information technology,the miniaturization and high integration of electronic components are leading to a rapid increase in the heat flux density of electronic devices.Traditional thermal management materials are difficult to guarantee the safety and reliability of high-power devices such as large-scale integrated circuit,semiconductor laser,and phased array antenna.It is urgent to develop a new generation of thermal management materials.Diamond particles reinforced copper matrix(Cu/diamond)composites are becoming a focus of the next generation of thermal management materials due to their excellent thermophysical properties,good mechanical properties,and relatively low density.The interface bonding state directly determines the thermo-physical and mechanical properties of the Cu/diamond composites.Tailoring interface structure is an effective way to improve the performance of composites.At present,the focus is to introduce carbides at the Cu/diamond interface so as to improve the thermo-physical properties of composites.However,the formation mechanism of interfacial carbides is still unclear because there is a lack of basic research work on in-depth characterization and analysis of interfacial microstructure of Cu/diamond composites.Therefore,the influence mechanism of interfacial structure on thermo-physical,mechanical,and thermal cycling properties of the composites has not been established.In this thesis,the Cu-B/diamond composites with different boron contents were prepared by gas pressure infiltration.The interface structure of the Cu-B/diamond composites was studied by using focused ion beam etching system(FIB),transmission electron microscopy(TEM),and scanning transmission electron microscopy(STEM).The purpose is to clarify the formation mechanism of interfacial carbides and establish the relationship between interface structure and thermo-physical,mechanical and thermal cycling properties.As a result,excellent thermo-physical,mechanical,and thermal cycling properties could be obtained by regulating the interface of Cu-B/diamond composites.The interface structure of the Cu-B/diamond composites and its influence on the thermal conductivity was systematically studied.The theoretical thermal conductivity of the Cu-B/diamond composites with different boron contents was predicted by H-J and DEM models as well as by finite element method,and the variation of thermal conductivity with testing temperature was surveyed.The results show that the morphology and thickness of’ the interfacial carbides generated at the interface of Cu-B/diamond composites are different in the range of 0.1-1.0 wt.%B addition.The boron carbide nucleates and grows on the diamond surface,and a semi-coherent relationship exists between the boron carbide and the diamond,with a crystallographic orientation relationship of(021)B4C//(111)diamond and[112]B4C//[110]diamond.Comparing the thermal conductivity of Cu-B/diamond composites with different carbide morphologies and thicknesses,it is found that the highest value of 868 W/mK is obtained at 0.3 wt.%B addition when the interfacial carbides are thin,discontinuous triangular morphologies and the carbides are at an appropriate spacing.This is attributed to the pinning effect of the discontinuous triangular carbides to improve the interfacial bonding as well as the parallel connection of thermal resistances of the discontinuous carbides to reduce the total interfacial thermal resistance.In contrast,the thermal conductivity of the composites decreases dramatically when the interfacial carbides show a thick and continuous zig-zag morphology.The predicted thermal conductivity by H-J and DEM models is much higher than that obtained experimentally,since the interface is assumed to be a perfect one.The finite element method considering the actual interface structure is applied to simulate the heat transfer behavior and to calculate the theoretical thermal conductivity of the Cu-B/diamond composites in order to understand the relationship between the interface structure and thermal conductivity.The results show that the thermal conductivity predicted by the finite element method is closer to the experimental value.The finite element simulation provides a theoretical guidance for the improvement of thermal conductivity of the composites.Subsequently,the thermal conductivity is further increased from 868 W/mK to 912 W/mK by optimizing the interface structure of the Cu-0.3wt.%B/diamond composite.In addition,it is found that the thermal conductivity of the Cu-B/diamond composites decreases with increasing temperature in the range of 323-573 K,which is closely related to the changes of thermo-physical properties of Cu matrix and diamond particles as well as the evolution of interface structure.The influence of interface structure on coefficient of thermal expansion(CTE)of the Cu-B/diamond composites was systematically studied.The effects of thermal cycling on thermal conductivity and CTE of the composites were also investigated.The results show that the CTE of Cu-B/diamond composites first decreases and then increases with increasing boron content.The lowest CTE of 4.88×10-6/K is obtained at 0.5 wt.%B addition,which is related to the morphological evolution of the interfacial carbides.In the case of lower B addition,the discontinuous triangular carbides exhibit "pinning effect" and the increase in the number of carbides enhances the interfacial bonding/adhesion of the Cu-B/diamond composites,thereby reducing the CTE.However,in with the case of higher B addition,the continuous zig-zag carbides weaken the interfacial bonding/adhesion.thereby increasing the CTE.By measuring the thermo-physical properties before and after thermal cycling,it is found that the Cu-0.5wt.%B/diamond composite has the best thermal stability.After 100 thermal cycles from 218 to 423 K.the room-temperature thermal conductivity remains almost unchanged at～740 W/mK,and the CTE increases slightly from 4.88×10-6/K to 4.97×10-6/K.On the contrary,the pristine Cu/diamond composite has the worst thermal stability，with the thermal conductivity decreasing from 112 W/mK to 88 W/mK and the CTE increasing from 14.81×10-6/K to 16.78×10-6/K after 100 thermal cycles.The influence of interface structure on mechanical properties of the Cu-B/diamond composites was systematically studied.The results show that the mechanical properties of the Cu-B/diamond composites first increase and then decreases with increasing boron content.The maximum tensile,compressive and bending strengths of the Cu-B/diamond composites are 204 MPa,608 MPa and 513 MPa,respectively,much higher than the pristine Cu/diamond composite.The change of mechanical properties of the Cu-B/diamond composites is closely related to the interfacial adhesion/bonding strength.The discontinuous triangular carbides can strengthen the interfacial bonding and thus improve the mechanical properties of the composites.The calculation results show the interfacial bonding energy of the Cu-0.5wt.%B/diamond composites with good mechanical properties is two orders of magnitude higher than that of the pristine Cu/diamond composite.This indicates that the improvement of interfacial bonding can improve load transfer efficiency at the interface and exploit the high strength and stiffness characteristics of the diamond particles,and thus the mechanical properties of the Cu-B/diamond composites are enhanced.The measurment of thermo-physical,mechanical and thermal cycling properties shows that the Cu-0.5wt.%B/diamond composite has a high thermal conductivity of 722 W/mK,a CTE of 4.88×10-6/K matching with electronic components,a high tensile strength of 204 MPa,and stable thermal cycling performance.As a result,the Cu-0.5wt.%B/diamond composite shows a potential application in the field of electronic packaging for heat dissipation.