The Research Of Spherical Tungsten Particles Prepared By Thermal Plasma Applied In W-Cu Composites

W-Cu composites are widely used in electronic packaging,electrical contact and metal sweat materials owe to combining the high temperature resistance,good mechanical strength,high density,wear resistance,low thermal expansion coefficient of tungsten and brilliant thermal and electrical properties of copper.Generally,W-Cu composites with high densification,homogeneous microstructure and controllable grain growth are significant to obtain products with good performance.At present,composite powders sintering and infiltration of porous tungsten matrix by liquid copper are two mainly adopted methods to fabricate W-Cu composites.To acquire porous skeleton with proper porosity and uniform and interlinked pores is the main point while W-Cu composites are prepared by infiltration method.Traditionally,irregular tungsten particles are usually used to fabricate porous skeleton.However,irregular particles have low stacking density and poor sintering controllability,and exist uneven contraction during sintering process,which would show adverse effect on the pore structure keeping.As a result,closed or half-connected pores form and pore distribution of porous skeleton is inhomogeneous,which would do harm to the densification and microstructure homogeneity of W-Cu composites obtained by infiltration method.Composite nanopowders are also used as starting materials which fabricated W-Cu composites by composite powders sintering.However,nanopowders prepared by conventional methods usually exhibit poor dispersity,broad particle size distribution and irregular shape,and this would worsen the sintering activity of nanopowders and result in the abnormal grain growth,which would show adverse influence on obtaining high dense and fine-grained W-Cu composites.The research starts from improving the characteristics of W(Mo)particles.Spherical and dense W(Mo)particles and well dispersed quasi-spherical W nanopowders with uniform particle size distribution prepared by thermal plasma process are used as starting materials.W(Mo)-Cu composites with high dense,homogeneous microstructure and controllable grain growth are fabricated by infiltration method and composite powders sintering,while the thermal conductivity(TC)and hardness are used as the key performance index.The results are listed as follows:(1)Spherical and dense tungsten particles are employed to fabricate porous skeleton with homogeneous pore distribution and interlinked pore structure.The influence of additives on the sintering of spherical W particles and the microstructure of porous skeleton is studied,and stearic acid and nickel are used as forming and sintering activator additives,respectively.Optimized Ni contents(0.03 wt.%)and sintering parameters are obtained in order to ensure pores open and a certain sintered neck strength together.Besides,the activated sintering mechanism of spherical tungsten particles with dense internal structure is also investigated.In addition,a comparison study used irregular powders with the same particles size as spherical powders is carried out,and the sintering kinetic mechanism is also analyzed,which indicates the more sintering stability of spherical particles.As a result,spherical powders exhibit significant advantages in fabrication porous tungsten skeleton with uniform pore distribution and open pore channel.Finally,gas flux and penetrating quality of porous skeleton are also analyzed,and copper infiltration experiments indicate spherical particles are more beneficial to obtain high dense W-Cu composites with brilliant thermal conductivity.(2)Porous tungsten skeleton with uniform pore distribution and interlinked pores is obtained using spherical particles with dense internal structure as starting materials in the first part,and porous Mo skeleton is also manufactured using the same method.The copper infiltration on the porous skeleton to fabricate Mo-Cu composites is the main research content in this part.Firstly,porous Mo skeleton is obtained at 1500?for 1 h with no sintering additive using spherical particles with dense internal structure,and the porosity is controlled by adjusting the compacting pressures.And then,Mo-Cu composites are fabricated using infiltration method.The influence of infiltration parameters on the performance of obtained materials is stressly investigated,and optimized parameters(1300? for 1 h)are obtained.Finally,the thermal conductivity of composites is analyzed using the theoretical models,and the performance of composites is promoted by controlling the sintering process.(3)The fabrication of tungsten compacts with fine grain size and their intensification are researched using quasi-spherical tungsten nanopowders with good dispersity and uniform particle size distribution prepared by thermal plasma as starting materials.The influence of sintering conditions on the microstructure and performance of obtained compacts is investigated.The densification behavior and grain growth of sintered compacts are analyzed,and sintered compacts with the relative density of 91.3%and average grain size under 2?m are obtained when sintered at 1500?for 2 h.The results indicate quasi-spherical nanoparticles with uniform particle size distribution could be well suppressed the grain growth at the early stage of sintering process(<1100?).Finally,the coupled effects of alumina reinforced nanosized tungsten matrix during nickel activated sintering process are also studied.The influence of additives on the densification and grain growth is also investigated,and the optimized proportion is obtained.(4)Based on the research on the sintering behavior of quasi-spherical W nanopowders with good dispersity and uniform particle size distribution in the third part,the fabrication of fine-grained W-Cu composites is carried out in this part.The influences of sintering temperatures and holding time on the microstructure and performance of obtained composites are investigated,and the grain growth behavior is also studied.The activation energy of grain growth at liquid phase sintering stage is calculated to be as high as 338±46 kJ/mol,and the rate of grain growth is low,which confirms the suppressed grain growth of sintering process.The densification behavior is also studied.Nanoparticles would improve the driving force of grain rearrangement(capillary force),and the small grain size because of the suppressed grain growth during the early sintering stage would enhance the grain rearrangement.In addition,tungsten particles in nanoscale would improve the solubility of W in liquid copper,and this would enhance the mass diffusion through liquid phase and improve the densification process.Finally,W-Cu composites with the relative density of 96.5%and the average grain size of 323 nm are obtained.