First-principles Calculations And Experimental Studies Of Highly Thermally Conductive Metal Matrix Diamond Composites

The thermal management issue of electronic devices has become increasingly severe with the continual adaptation of electronic devices to high power density and high integration.On the other hand,traditional thermal management materials are no longer able to satisfy the growing demand for thermal management,and there is an urgent need to create thermal management materials with improved performance.Because of their high thermal conductivity and adjustable coefficient of thermal expansion,metal matrix diamond composites are attractive materials for thermal management uses.However,at the interfaces of Cu/diamond and Ag/diamond composites,there are gaps or weak connections.Al/diamond composites have Al₄C₃ hydrolysis issues at the interfaces.The existence of these problems makes the thermal conductivity of metal matrix diamond composites not ideal.Therefore,this paper will focus on the interfaces of these three composites and investigate the interfacial bonding and thermal conductivity of Ti element enhanced Cu/diamond composites by first principles calculations and experimental methods.The interfacial structure of Cu（Ag,Al）/diamond,the effect of alloying elements on the interfacial properties of Cu/diamond composites and the surface doping of diamond for Cu/diamond composites are investigated by means of first principles calculations.The main conclusions were obtained as follows:1.The Cu-Ti/diamond interfacial structure and Cu/diamond（Ti）interfacial structure are constructed and analyzed by first principles calculation method,and the results show that either the addition of Ti element to the metal matrix or the diamond surface coating with Ti can improve the interfacial bonding strength of the composites,and Ti C is formed at the interface of the composites.Appearing in the Ti-reinforced Cu/diamond composites,the bond strengths of all four interfacial structures were higher than that of the Cu/diamond interfacial structure,which implies that the addition of Ti C could improve the interfacial bonding of the composites.Moreover,among the four interfacial structures,the interfacial structure with C atoms as the terminal atoms and diamond as part of the interfacial structure usually have higher interfacial bonding strength.In addition,we prepared Cu/diamond composites by spark plasma sintering and vacuum hot-pressure sintering methods.Among them,the addition of Ti can significantly improve the interfacial bonding of the composites,which is consistent with the first principles principle calculation results.In addition,the addition of Ti can also significantly improve the thermal conductivity of the composites.The composites prepared by spark plasma sintering have higher thermal conductivity than those prepared by vacuum hot-press sintering,probably because the composites prepared by spark plasma sintering have higher densities and can produce more uniform and dense Ti C on the diamond surface.2.The results of constructing and analyzing the Cu/diamond interface structure,Ag/diamond interface structure,Al/diamond interface structure,Cu-Si/diamond interface structure,Ag-Si/diamond interface structure and Al-Si/diamond interface structure by the first-nature principle calculation method show that among the three metal/diamond interface structures,the Al/diamond interface bond is the strongest while the Cu/diamond interface bond is the weakest.The main reason is that Al atoms form strong bonding with C atoms,while Cu atoms form weak bonding with C atoms.The doping of Si favors the interfacial bonding of the three interfacial structures,but the degree of improvement is different.Si atoms can form C-Si bonding at the Ag/diamond and Cu/diamond interfaces.At the Cu/diamond interface,the bonding interaction formed by C and Si atoms is stronger,so the addition of Si atoms improves the interfacial bonding strength of Cu/diamond to a large extent.However,the effect of Si atoms on the interfacial bonding of Al/diamond is not significant.Considering the electron-electron coupling and phonon-phonon coupling together,the addition of Si element can improve the thermal conductivity of the three composites at the interface.3.The results of constructing and analyzing the interfacial structures of clean and doped Cu/diamond composites with alloying elements（B,Cr,Hf,Mo,Nb,Si,Ti,V,and Zr）by first principles calculation methods show that the doping of alloying elements at the Cu/diamond interface increases the interfacial adhesion work by a factor of 3-4,which implies that the alloying elements improve the interfacial bonding of diamond and copper.The improved effect of alloying elements on interfacial bonding can be attributed to the increased charge transfer between alloying element atoms and carbon atoms,larger Mulliken population number,and more electron hybridization.These same factors imply the formation of carbides,which improve the thermal transport at the interface.Si and B are better performing alloying elements when the electron-phonon and phonon-phonon coupling at the interface is taken into account in combination.4.The surface structures of B-doped diamond,N-doped diamond,Si-doped diamond and their corresponding Cu/diamond composites were constructed and analyzed by first-principles calculations.The results show that the doping of B,N and Si on the diamond surface can change the electronic state of the diamond surface.Where B atoms form strong bonding with C atoms,while Si and N atoms form weak bonding with C atoms.Surface N-doping and Si-doping of diamond can improve the bond strength of Cu/diamond interface.The presence of strong covalent interaction at the N-doped Cu/diamond interface has the strongest effect on the enhancement of the interfacial bond strength,so N element is the best surface doping element.And the electronic state distribution of doped diamond surface first layer atoms near the Fermi energy level is favorable for electron-phonon coupling.