The Mechanism Of Interfacial Structure Formation And Thermal Properties Of Al/diamond Composite

With the development of electronic technology,the powder density and heat generation of electronic devices are increasing rapidly,which affects the safety and effective operation of the electronic devices.Thus,a new generation of thermal management material is highly requested to ensure the efficient heat dissipation from the electronic devices.Diamond has superior thermal properties(a high thermal conductivity of 600-2000 W/mK).Therefore,diamond particles reinforced metal matrix composite possesses high thermal properties and is considered as a promising candidate for advanced thermal management materials.Diamond particles reinforced aluminum matrix composite(Al/diamond composite)has been produced by a gas pressure infiltration(GPI)technique.The Al/diamond composite combines the superior thermal properties of diamond with the low density of aluminum matrix and becomes the focus of the new generation of thermal management materials.The interface between A1 matrix and diamond particles directly determines the properties of the Al/diamond composite.Controlling of the interfacial structure is an effective strategy to improving the thermal properties of the Al/diamond composite.However,the characterization of the interfacial structure and the reaction mechanism of the Al/diamond interface are rarely reported,which impedes the further improvement of the properties of Al/diamond composite.In this thesis,Al/diamond composite was produced by the gas pressure infiltration technique.The interfacial structure of the Al/diamond composite was characterized by advanced techniques such as focused ion beam(FIB)and transmission electron microscope(TEM).The relationship among the fabrication process,interfacial structure and thermal properties of the Al/diamond composite was investigated.The results provide insight for the improvement of the properties of Al/diamond composite.By modifying the GPI fabrication process,the interfacial structure of the Al/diamond composite was changed as well,and the mechanism of the formation and growth of interfacial carbides was systematically studied.The results show that the formation of Al4C3 is an inhomogeneous nucleation process that is decided by the diamond surface condition.The steps on diamond surface act as nucleation site for Al4C3 particles.On diamond(100)surface,high density of small Al4C3 particles are formed on diamond facets with an inclination angle of 55° with diamond surface.On diamond(111)surface,low density of large Al4C3 particles are formed parallel with diamond surface.After comparing the thermal conductivities of the Al/diamond composites with different interfacial structure,the optimum morphology of Al4C3 is revealed to be high density and small size,since the formation of high density of small carbide particles improves the interfacial bonding while the interfacial thermal resistance caused by the low thermal conductivity of Al4C3 is minimum.The pre-annealing period of GPI procedure triggers the transformation of diamond surface structure to enhance the interfacial reaction between diamond and Al.High density of small Al4C3 particles are formed at the interface of the Al/diamond composite,resulting in the increase of thermal conductivity from 540 to 710 W/mK.The hydrolysis of Al4C3 limits the application of the Al/diamond composite.Thus,diamond surface metallization has been utilized to introduce new interfacial reaction layers in the Al/diamond composite to improve the interfacial bonding and to impede the formation of Al4C3.However,the introduction of new interfacial layers affects the thermal conductivity of the Al/diamond composite.Therefore,the modification of the interfacial layers is necessary to impede the Al/diamond interfacial reaction and to enhance the thermal properties of the Al/diamond composite at the same time.In this thesis,Ti and W were coated on diamond surface to produce Al/diamond composites.The structure evolution of both Ti and W coating layer through the fabrication process of the Al/diamond composite was systematically studied to modify the coating structure and the interfacial structure of the Al/diamond composite.The results show that the Ti coating layer reacts with diamond surface to form TiC which keeps stable during the infiltration period of GPI process.As the thickness of Ti coating layer increases,the thermal conductivity of the composite firstly increases and then decreases.When the thickness of Ti coating layer is 200 nm,the Al/diamond(Ti)composite obtains the highest thermal conductivity of 650 W/mK.Further decreasing the thickness of Ti coating layer,the Ti coating layer could not effectively improve the interfacial bonding in the Al/diamond composite and deteriorate the thermal conductivity of Al/diamond(Ti)composite.When annealed at low temperature,the TiC layer reacts with Al matrix to form a small quantity of Al4C3 at the interface of Al/diamond(Ti)composite.The W coating layer reacts with Al matrix to form interfacial Al5W layer during the GPI infiltration period.The results show that the rate of Al/W reaction is relatively slow,and the modification of fabrication parameters during GPI process controls the thickness of Al5W layer in the Al/diamond(W)composite,which enhances the thermal properties of the Al/diamond(W)composite.When increasing the GPI infiltration period from 10 to 60 min,the thermal conductivity of the Al/diamond(W)composite increases from 520 to 630 W/mK.Although the thennal conductivity of the Al/diamond(W)composite is relatively lower compared with the Al/diamond and the Al/diamond(Ti)composites,the formation of Al5W interfacial layer could effectively impede the formation of Al4C3,which extends the application of the Al/diamond composite.In conclusion,the mechanism of the interfacial reaction in the Al/diamond composites was systematically investigated.The relationship among the fabrication process,interfacial structure and thermal properties of the Al/diamond composite was then established to provide insight for the improvement of both interfacial structure and thermal properties of the Al/diamond composite.The results show that the gas pressure infiltration produced Al/diamond composite possesses superior thermal properties and is suitable for the application in electronic industries.