Study On The Preparation And Plasma Activated Sintering Of W-Cu Nanopowders

With the continous development of electronic devices toward high power and light weight,this trend requires higher performance for electronic packaging heat sink materials.W-Cu composites using as heat sink materials have a promising application because of its high strength,excellent thermal conductivity,low coefficient of thermal expansion(CTE)and controling performance.However,processing of W-Cu composites with high performance by conventional liquid phase sintering,activated sintering and hot pressing sintering is hindered by high differences between their melting points and extremely limited solubility of tungsten and copper.In order to solve these problems,it is a hopeful solution by synthesizing homogeneous nano-sized W-Cu conposite powders with good sinterability and applying advanced sintering technology.This study fabricate W-Cu composites by plasma activated sintering(PAS)from the sol-gel powders and chemical precipitated powders.The main conclusions are as follows:Nano-sized W-Cu precursor powders were synthesized by sol-gel with ammonium metatungstate(AMT)and copper nitrate Cu(NO3)2·3H2O as the raw materials and with citric acid CA as chelating sgents and with ethylene glycol as crosslinker.Following the precursor powders were calcined at 450 °C,W-Cu composites were obtained by reducing the oxide composite powders in flowing hydrogen at 750 °C.The phase and elemental distribution of the reduced composite powders were analyzed by XRD and EDS,it showed that the nano-sized WCu particles have high purty and are uniformly intermixed.Morphology of W-Cu composite powders were characterized by SEM and TEM.It was found that composite powders had excellent homogeneity and spherical shape.The average size of W grains and Cu grains was about smaller than 150 nm.Nano-sized W-Cu precursor powders were prepared by chemical precipitation method with AMT and Cu(NO3)2·3H2O as the raw materials and with ammonium NH3.H2 O as pricipitant.W-20 wt.% Cu composite powders were achieved by calcining precipitates at 450 °C and continous reduction in hydrogen gas at 750 °C.Phase composition and morphology of the reducted powders were characterized by XRD and SEM.The results showed that the homogeneity and shape of W-Cu composite powders were as same as the sol-gel produced powders.However,microstructure of powders,which was different from the sol-gel produced powders,was W coating Cu called core-shell structure.The grain size of W particles was in the range from 100 to 200 nm.W-Cu composites were rapidly consolidated by PAS under different pressure(60~120 MPa)at different temperature(850 ~ 990 °C).Research findings showed that nano-sized powders synthesized by sol-gel method and chemical precipitation method both had good sinterability.With increasing temperature,the relative density and average grain size of W particles increased.And with increasing pressure,the relative density of composites increased,but pressure had no obvious effect on average grain size of W particles.Meanwhile,the higher of temperature and pressure,the more homogeneous of microstructure of alloy.However,the melting Cu was extruded from the die when powders were sintered at 990 °C.The relative density of alloy in this case drcreased and homogeneity became worse.The W-Cu composites can reach 97.3% relative density when powders were sintered under 120 MPa at 950 °C for 10 minutes.In this case,W-30 wt.% Cu composites showed excellent thermal conductivity(235.48 W/m K),a relatively low thermal expansion(9.27×10-6 K-1),and high micro hardness(452 HV).Porosity and homogeneity of composites were the main reasons that affected thermal and mechanical performance.We also investigated the effects of nickel(0.5 wt.%)addition on the sintering behaviour of W-20 wt.% Cu nano powders and performances of the alloy.It is found that the addition of Ni promotes the relative density,thermal performance and mechanical property of W-Cu alloy.Compared to composites without Ni additive,the relative density,thermal conductivity and micro hardness increase from 96.7% to 97.83%,213.19 W/m K to 214.06 W/m k,463.61 HV to 474 HV,respectively.And the CTE decrease from 7.19×10-6 K-1 to 7.09×10-6 K-1.The reasons for excellent performances are that W-Cu alloys with Ni activator have high relative density,homogenous structure and continous Cu network structure between W particles.