Thermophysical Property Of Diamond/Al Composites Prepared By In Situ Reaction

With continuous development of the information industry and ongoing breakthrough of the chip process technology,electronic devices are gradually miniatured and integrated.The power density of the electronic devices increases constantly,which causes the generatation of a lot of heat.Heat dissipation has become a bottleneck for further advancement of the electronic devices.Traditional electronic packaging materials no longer meet the heat dissipation requirements of the high-power density electronic devices because of the low thermal conductivity(λ),and it is urgent to develop a new generation of electronic packaging materials with high λ and low coefficient of thermal expansion(CTE).Diamond particles reinforced aluminum matrix(diamond/Al)composites possess high λ,tunable CTE,reliable mechanical properties,and low density,and have become a focus of the new generation of electronic packaging materials.The diamond/Al composites have low A due to the poor interfacial bonding incurred by the non-wetting nature between diamond and Al.Improving interfacial bonding is mainly adopted by researchers to acquire high λ in the diamond/Al composites.However,the studies failed to fully optimize the "four wheels" of interfacial thermal conductance,diamond volume fraction,diamond particle size,and relative density in the diamond/Al composites,and the λ in the diamond/Al composites remains below 800 W m-1 K-1.Moreover,the nucleation and growth of Al4C3 as well as the effect of Al4C3 interlayer on thermal stability of the diamond/Al composites are still a debate,and the high temperature λ of the diamond/Al composites is rarely reported.In this thesis,the interfacial Al4C3 phase is generated through in situ reaction between the diamond particles and the Al matrix by gas pressure infiltration(GPI),and the bimodal diamond particle sizes are used as the reinforcement,both of which synthetically realize the "four-wheel drive" strategy to enhance the λ in the diamond/Al composites.The interfacial structure is thoroughly characterized by an aberration correct scanning transmission electron microscope(STEM).The nucleation and growth mechanisms of Al4C3 are revealed.The crucial role of the discrete in situ AI4C3 interlayer in improving the λ of the diamond/Al composites is clarified.The correlations between the interfacial structure,diamond particle size,diamond volume fraction,and relative density and the thermo-physical properties and thermal cycling behavior of the diamond/Al composites are established.The diamond/Al composites with excellent thermophysical properties and thermal cycling stability are obtained via synthetically optimizing the interfacial structure,diamond particle size,diamond volume fraction,and relative density.The interfacial structure in the diamond/Al composites was optimized by manipulating GPI infiltration temperature and time,which improves the interfacial thermal conductance and subsequently increases the λ in the diamond/Al composites.The nucleation and growth of Al4C3 were thoroughly characterized by advanced characterization techniques.The results reveal that Al4C3 nucleates from Al4C3(003)plane.No preferential nucleating diamond planes are observed.The nucleation and growth of Al4C3 are controlled by the diffusion of C atoms from the diamond surfaces.The growth rate of the Al4C3 along(003)plane is larger than that vertical to(003)plane,which leads to a fact that the Al4C3 presents a flake or rod shape with a large aspect ratio.According to the calculations,the thickness of the Al4C3 interlayer mainly affects the interfacial thermal conductance,and discrete in situ Al4C3 interlayer can apparently improve the interfacial thermal conductance.Since the Al4C3 nulceates and grows inhomogeneously and Al4C3 has a low λ,the formation of discrete in situ Al4C3 interlayer in the diamond/Al composites can reduce the interfacial thermal resistance.The maximum λ of 854 W m-1 K-1 was achieved in the 701μm-diamond/Al composite via optiming the discrete in situ Al4C3 interlayer.The diamond/Al composites with bimodal diamond particles were prepared via the optimized GPI parameters.The λ of the diamond/Al composites was improved by synthetically optimizing the interfacial thermal conductance,diamond particle size,diamond volume fraction,and relative density.The results show that the λ of the diamond/Al composites increases with increasing the diamond particle size.Nevertheless,the 980 μm-diamond/Al composite exhibits a lower λ due to the difficulty in densifing the composite.The diamond/Al composites have low CTE because of the well-bonded interface achieved by the discrete in situ Al4C3 interlayer.Large diamond particles tend to obtain large CTE in the diamond/Al composites,since large diamond particles have lower specific surface area.Thus,the CTE of the diamond/Al composites increases with increasing the diamond particle size.High diamond volume fraction plays a decisive role in achieving highλ and low CTE in the diamond/Al composites.The bimodal diamond particle reinforcement significantly improves both the diamond volume fraction and relative density of the diamond/Al composites.The bimodal-diamond/Al composite enjoys a diamond volume fraction of 76.0%and a relative density of 99.2%,which render a high λ of 1021 W m-1 K-1 and a low CTE of 3.72 × 10-6 K-1.The thermo-physical properties of the diamond/Al composites subjected to thermal cycling and high temperature were investigated.The results indicate that the diamond/Al composites show a small reduction of 2-6%in λ and preserve matchable CTE with semiconductors after 200 thermal cycles from 218 to 423 K,exhibiting excellent thermal cycling stability.Significantly,the bimodaldiamond/Al composite maintains a high λ over 1000 W m-1 K-1 and a low CTE of 4.21 × 10-6 K-1 after thermal cycling.The discrete in situ Al4C3 interlayer yields the strong interfacial bonding and reduces the interfacial thermal stress in the diamond/Al composites during thermal cycling,which are responsible for the highλ and low CTE after thermal cycling.The accumulated tensile plastic strain during thermal cycling decreases the λ of the Al matrix,which is the main reason for the decline of λ in the diamond/Al composites.Meanwhile,the tensile plastic stain increases the CTE of the Al matrix and then increases the CTE of the composites.In the temperature range of 298-573 K,the λ of the diamond/Al composites declines as temperature increases,and the decline of λ in diamond is the main cause.The bimodal-diamond/Al composite still has a high λ of 630 W m-1 K-1 at 573 K,which is ascribed to sound interfacial bonding maintained by the discrete in situ Al4C3 interlayer.To summarize,the discrete in situ Al4C3 interlayer is the crucial factor for the diamond/Al composites to achieve the excellent thermo-physical properties and thermal cycling stability.The diamond volume fraction is more important than both the diamond particle size and interfacial thermal conductance in acquiring the excellent thermo-physical properties.The diamond/Al composites with high λ,matchable CTE with semiconductors,excellent thermal cycling stability,and highλ at high temperature are successfully fabricated by the "four-wheel drive" strategy.It indicates that the diamond/Al composites have promising prospect in the heat dissipation applications as electronic packaging materials.The results provide guidence for fabricating the composites reinforced with high thermally conductive particles like c-BN and BAs crystals.