An Investigation On Fabrication And Thermal Conductivity Of Carbon/Aluminum Matrix Composites

The power densities of the electronic chips increased dramatically,therefore,the new sink materials with a high thermal conductivity(TC)are urgently needed.For the well application prospect,the Diamond/Al and Graphite flakes/Al(Gf/Al)composites have drawn much attention from both scientific and industrial communities due to their good properties.In this study,these two composites were investigated.A new technology with sol-gel method and in-situ reaction was used to fabricate nano sized TiC and SiC coating to modify the surfaces of diamond particles which is an effective way to improve the wettability and interface bonding between diamond and Al.Meanwhile,the Gf/Al composites with high TC were produced by powder metallurgy technique to avoid the reaction between Gfand Al matrix.The key factors influencing the microstructure and properties of Gf/Al composites were analyzed.Moreover,for the service condition of the Gf/Al composites,the effect of thermal cycling on the microstructure and properties were investigated.The main contents and results are summaried as follows.The nano-TiC coating on diamond surface were fabricated by a new technique with sol-gel method and in-situ reaction.The effect of different TiO2 soliquids,sintering temperatures and times on the TiC coating quality were investigated.The results indicated that an integral nano sized TiC coating could be achieved by using the TiO2 soliquid with pH 7.5 and a molar concentration of 0.2 mol/L.The sintering temperature was 1450 ? and sintering time was 4 h.TiC@Diamond/Al composites were fabricated by the diamond with TiC coating,and the matrix alloy was 6061Al.The effect of TiC coating on the microstructure and properties was investigated.The results indicated that the TiC coating on the diamond particles could improve the wettability and interface combination between the diamond and Al matrix.The density of the 50vol.%TiC@diamond/6061Al composite reached 99.6%of the theoretical density,which was much higher than that of the 50vol.%Diamond/6061Al(96.7%).Meanwhile,the bending strength and interfacial thermal conductance(hc)of the 50vol.%TiC@diamond/6061 Al increased 35%and 54%compared to that of 50vol.%Diamond/6061 Al.In the 50vol.%TiC@diamond/6061Al,the TiC coating dissoluted into the Al matrix,while the diamond particles contacted with the Al matrix directly.The massive gap of the phonon velocity between diamond and Al resulted in relatively low thermal conductance of the composites.For the TiC@Diamond/Al composites,when the diamond content was lower than 50%,the relative densities and bending strength were high.The bending strength of 35vol.%TiC@Diamond/Al was 578 MPa.For the 60vol.%TiC@Diamond/Al,the diamond particles in the composite would form some network and the Al matrix was very hard into the interspace of the network during the fabrication of the composite.Some defects,such as pores,would form in the composite.Therefore,the relative density of 60vol.%TiC@Diamond/Al was only 94.2%.The bending strength and TC of the composite were 200 MPa and 300 W/m K,respectively.The nano-SiC coating on diamond surface was also fabricated by a new technique with sol-gel method and in-situ reaction method.The SiC coating was fabricated by different SiO2 soliquids and sintering time.The results indicated that an integral nano sized SiC coating with 20 nm thickness could be achieved by using the SiO2 soliquid of pH 7.5 and 0.15 mol/L molar concentration.There were no other compounds formed in the coating.SiC@Diamond/Al composites were fabricated by the diamond with SiC coating and 6061A1 matrix alloy.The effect of SiC coating on the microstructure and properties was investigated.The results indicated that the SiC coating on the diamond particles could improve the wettability and interface combination between the diamond and A1 matrix.The hot pressing temperature was one of the key factors affecting the properties of the composites,when the composite was produced by the diamond with SiC coating.When the composite was hot pressed at 620 ?,the highest values of the relative density,bending strength and TC were achieved,which were 99.6%,475 MPa and 485 W/m K,respectively.When the composite was hot pressed at 660 ?,SiC coating reacted with Al matrix.Therefore,the density of the composite became lower due to the poor wettability between the diamond and Al matrix.Meanwhile,the bending strength and TC of the composite were decreased by 24%and 9%,respectively,compared to that of the composite hot pressed at 620 ?.In the composite hot pressed at 620 ?,some SiC coating dissolved into the Al matrix.The SiC coating became discontinuous at the interface between diamond and A1 matrix.The phonon velocity of SiC was between that of diamond and A1 matrix.Therefore,the SiC coating relieved the phonon scattering at interface between diamond and Al matrix.Moreover,the SiC coating was thick and the TC of SiC coating was high enough.All of these improved the interface thermal conductance of SiC@Diamond/Al.The TC of the 50vol.%SiC@Diamond/Al increased by 27%compared to that of 50vol.%TiC@Diamond/Al and the he increased by five times.Similar to the TiC@Diamond/Al,the density,bending strength of SiC@Diamond/A1 were excellent and the he of the composites were almost the same,when the diamond content was low than 50%.For the 60vol.%SiC@Diamond/Al,however,all of the properties decreased compared to the other SiC@Diamond/Al.This indicated that for both the 60vol.%SiC@Diamond/Al and the 60vol.%TiC@Diamond/Al,defects were the main factor which determined the TC of composites.The Gf/Al composite with different size of graphite flakes and 6061Al matrix alloy were prepared.The effect of hot pressing temperature,size and content of graphite flake(Gf)on the microstructure and properties of graphite flake/Al(Gf/Al)were investigated.When the composite was hot pressed at 620 ? the density was near to the theoretical value.The properties of the composites were well and the(001)Gf basal plane of Gf was parallel to the x-y plane(the circular plane of the composite ingot)of the composites.With the increase of the Gf size,the strength of the Gf/Al composites decreased and the TC increased.The thermal conductivity of the composite was increased by 63%as the size of the Gf increased from 150 to 500 ?m.The highest thermal conductivity was 604 W/m-K.Besides the size of the Gf,the shape,distribution and defect of the Gf also have influence on the thermal conductivity of the composites.For the Gf/Al with 500 ?m Gf,as the Gf content increased from 50%to 70%,the bending strength decreased by 55%and the TC increased by 26%,which was 760 W/m K.The effect of thermal cycling parameters on the microstructure and properties of Gf/Al were examined.After 100 thermal cycles,the properties of composites decreased seriously.For the 50vol.%Gf/Al reinforced by 500 ?m Gf,the bending strength decreased by 34%and TC decreased by 13%,when the thermal cycles reached 100.After thermal cycling,some Gf were broken.Furthermore,the interface between Gf and Al matrix was also cracked.All of these resulted in the decreased properties of composites.Increasing the thermal stress during the thermal cycles,the microstructure and properties of 50vol.%Gf/Al were similar to that of the composites at low thermal stress.The Gf in the composites would be cracked by a small thermal stress due to its low strength.Meanwhile,the thermal stress would be released,when the Gf were broken.Therefore,the microstructure and properties of 50vol.%Gf/Al were not serious changed,when the thermal stress increased after thermal cycle.