Thermalphysical Properties And Phase Transformation Behavior Of Copper Matrix Composites Reinforced By Ferroelectric Ceramic With ABO₃ Structure

Materials with low thermal expansion have a wide range of applications in the electronic packaging,heat sink materials,and precision instruments field.It is tempting to design the low expansion copper-based composites by adding ceramic particles with negative expansion.In this paper,the perovskite structure ferroelectric ceramics with ABO3 structure is used as the reinforcement of the copper matrix composites,and the thermal expansion coefficient of the composites is reduced by the huge negative expansion effect during the ferroelectric-paraelectric phase transformation process.The barium strontium titanate(SBT)reinforced copper matrix composites with different microstructure and interface were prepared by different sintering process and ceramics coatings.Based on the test and analysis of the microstructure,thermophysical properties,the ceramic phase transformation behavior,and the stress state of the composites,the relationship between the thermophysical properties,the ceramic transformation behavior and the stress state of the composites were analyzed.The SBT/Cu composites prepared by the SPS method have a low coefficient of thermal expansion.The composites are compact,smooth,clean,and no interface reaction occurs during the SPS process.SBT particles are uniformly dispersed in Cu matrix.Thermal mismatch stress exists at the interface because of the difference in thermal expansion coefficient between the two phases of the composites.Therefore,there are a lot of plastic deformation near the interface,including dislocations,twins and so on.Annealing treatment can relax the residual stress in the composites and reduce the lattice constant of the Cu matrix and the thermal expansion coefficient of the composites.Both T phase and C phase SBT particles exist in SBT/Cu composites at room temperature.The residual stress in the composites affects the phase structure of SBT particles.The Debye-Waller theoretical analysis shows that the Curie temperature of SBT particles in the composites is reduced during the cooling process and elevated during the heating process,the phase transformation temperature range is broadened.(NiO-SBT)/Cu composites prepared by vacuum hot pressing method,NiO coating is distributed at the interface of the composites,which increases the roughness at the interface and improves the bonding state of the composites.The composites are compact and SBT particles are uniformly dispersed.Because of the thermal mismatch stress,there are a large number of dislocations and twins at the interface.Annealing treatment can relax the residual stress in the composites,and reduce the lattice constant of the Cu matrix and the thermal expansion coefficient of the composites.Both T phase and C phase SBT particles are present in the composites at room temperature,annealing and plastic deformation treatment can improve the T-phase SBT particles ratio of the(NiO-SBT)/Cu composites.Debye Waller analysis shows that T phase SBT particles are continuously transformed into C phase during the heating process,and the thermal expansion coefficient of the composites is reduced.PT particles show a negative expansion from the room temperature to Curie temperature.Sable thermal expansion coefficient of the PT/Cu composites can be obtained at room temperature to 350°C.The higher the volume fraction of PT particles is,the lower the coefficient of thermal expansion is.The results of in-situ XRD analysis showed that the length of the a-axis of the PT unit increased gradually and the c-axis length decreased gradually during the heating process,and the cell volume decreased gradually.The length of the a-axis and the c-axis of the PT unit were rapidly approaching after 400°C,and PT particles began to undergo phase transformation,the volume of PT cell decreased rapidly,and the increase of Cu lattice constant was obviously limited.The phase transformation temperature of PT particles in PT/Cu composites is broadened,and the negative expansion of PT limits the expansion of Cu matrix,thus obtaining a stable coefficient of thermal expansion.SBT particles play a role of particle enhancement,and increase the density of dislocation and twins in Cu matrix.Therefore,the addition of SBT particles improves the yield strength and reduces the plasticity of the SBT/Cu composites.Because of the large negative expansion effect of SBT ceramics during the phase change,SBT/Cu and(NiO-SBT)/Cu composites show the relatively high thermal conductivity and conductivity.Simultaneously,the thermal conductivity and electrical conductivity of the composites can be adjusted by the volume fraction of SBT particles.