Thermal-physical Properties Of Ferroelectric(Ba_1-xSr_x)TiO₃ Ceramics Reinforced Copper Matrix Composites

Materials with low thermal expansion and high thermal conductivity have a wide range of applications in the electronics and aerospace industries.This kind of material is mainly used as heat sink or electronic packaging materials to protect and conduct heat by directly contacting the heat generating parts of the device.At the same time,this material should own a similar coefficient of thermal expansion as the substrate to reduce cracks due to huge discrepancy in thermal expansion during heating-cooling recycling.Barium titanate is a typical ABO₃ ferroelectric.It has a considerable negative expansion effect around the Curie temperature when the ferroelectric-paraelectric phase transition occurs.The cubical expansion coefficient can reach-50×10^-6?^-1,thus it's regarded as a potential ceramic reinforcement material.By doping different contents of strontium in the barium titanate ceramic,the Curie temperature of which can be adjusted as well as to change the negative expansion phase transition temperature range.In this dissertation,barium strontium titanate(Ba_1-xSr_xTiO₃,BST)ceramic with different strontium contents are used as reinforcements.BST/Cu composites with BST volume fraction of 40 vol.%were prepared by hot pressing sintering.The analysis result shows that the composition of the composite prepared by the hot pressing sintering method is accurate,the composite material is dense and the BST reinforcements are evenly dispersed,thus the existing method is feasible.Tests on their thermal expansion properties show that the 20BST/Cu composite has the lowest thermal expansion curve in the test temperature range from room temperature to 300?.As the number of thermal cycles increases,the average thermal expansion coefficient of 20BST/Cu composite has a slight decline.Meanwhile,as the temperature increases,the thermal conductivity of the BST/Cu composite decreases and the specific heat capacity increases.Compressive tests show that the 20BST/Cu composite has the highest compressive strength and compressive fracture strain.The heterogeneous precipitation method was used to coat a Cu₂O layer on the surface of BST particles,and the thermophysical properties of Cu₂O/BST/Cu composites with different Cu₂O contens were studied.The experimental results show that the introduction of Cu₂O in the composite material helps to form semi-coherent interfaces at Cu-Cu₂O and Cu₂O-BST interfaces,which could improve the interface bonding.Compared with uncoated BST/Cu composites,the compression performance of Cu₂O/BST/Cu composite is improved.The5%Cu₂O/BST/Cu composite has the best mechanical properties.It's compressive comprssive strength reaches 729.8MPa and fracture strain reaches 32.03%.Also,studies have shown that a suitable amount of Cu₂O can relax the residual stress in the composite,thus changing the phase transition behavior of BST ceramics in the composite.In-situ XRD analysis showes that the reduction of residual stress in the composites contributes to the increase of the content of T phase BST in the composite,thereby reducing the thermal expansion coefficient of the composite.The thermal expansion tests show that the 15%Cu₂O/BST/Cu composite has the lowest average engineering thermal expansion coefficient of 11.51×10^-6?^-1,and it also has good thermal expansion cycle stability.Thermal conductivity tests show that5%Cu₂O/BST/Cu composite has the highest thermal conductivity,which may result from good interfacial bonding promoting phonon transfer.Meanwhile,since the large volume fraction of copper matrix,the electrical conductivities of composites can reach 31.9%?41.1%IACS.Annealing can relax the residual stress in the composite,which has an effect on the thermal expansion properties of the composites.Annealing treatments on BST/Cu composites with different strontium contents show that the optimal annealing conditions for achieving the lowest thermal expansion coefficient are:10BST/Cu,200?,8h;20BST/Cu,200?,4h;30BST/Cu,200?,12 h.