Fabrication Of Ti₃(Al_1-xSi_x)C₂ Solid Solution Capillary Wicks With Composite Pore Size And Performances In Loop Heat Pipes

Loop heat pipe(LHP)is a highly efficient heat transfer device by liquid-gas phase transformation.Due to the design flexibility,anti-gravity and long-distance transfer,LHP has been widely used in space technology,electronics and other fields and demonstrated the excellent and stable control ability on temperature.The porous capillary wick in the evaporator not only provided the only force for circulation in LHP,but also acted as an important place for the liquid-gas phase transformation.Therefore,the capillary wick has become the most important factor to improve the heat transfer capacity of the LHP system.The ternary compounds MAX phase(Mn+1AXn,such as Ti3AlC2 and Ti3SiC2)exhibts both metallic and ceramic properties with lower thermal conductivity(compared with nickel and copper),excellent resistance against thermal shocks and corrosion and good machinability.Thus,MAX phase has large potential in application of porous capillary wicks used in LHP systems.However,the strength of single MAX phase in porous state decreased dramatically.The adjustment and control of pore size distribution and proportion remained to be further studied.In this work,Ti3AlC2 was chosen as the new wick material and promoted by alloying Si elements.Effects of Si addition on microstructure and properties of Ti3(Al1-xSix)C2(xsi=0～1)solid solutions in both dense and porous states were studied by hot pressing and pressureless sintering,respectively.Capillary wicks with composite pore size were fabricated by adding dissolvable pore formers and cold pressure.The LHP system with this new wick was prepared,followed by systematic heat transfer tests and analyses.The main contents are as follows:(1)In order to investigate systhesis mechanism and properties,a series of dense Ti3(AlixSix)C2 solid solutions was fabricated by hot pressing sintering.Effects of Si addition on the phase composition,microstructure and properties of Ti3(Al1-xSix)C2(xsi=0～1)were studied.Si atoms could substitute Al atoms at any proportion at A-sites,thereby forming perfect solid solutions.It was found that during sintering,the amount of Al-Si eutectic liquid mainly affected the grain growth of Ti3(Al1-xSix)C2.When xsi≤0.6,as the Si addition increased,the hardness of Ti3(Al1-xSix)C2 increased due to the solid solution hardening effect.When xSi>0.6,with the increased Si addition,the hardness was slightly reduced by the increment in grain size.The passive films with N-type semiconductor characteristics were generated on Ti3(Al1-xSix)C2 in 3.5%NaCl solution.The Si addition increased the content of SiO2 and reduced the relative dielectric constant of passive films,which enhanced the stability of passive films and improved the corrosion resistance.(2)Based on the systhesis mechanisms above,effects of Si addition on pore structure of porous Ti3(Al1-xSix)C2(xSi=0～1)were further investigated with Ti/Al/Si/TiC powder mixtures by pressureless sintering.The results showed that the pore structure of porous Ti3(Al1-xSix)C2 was mainly affected by the amount of Al-Si eutectic liquid during sintering.After systematically measuring the capillary performance,compressive strength and thermal conductivity,effects of Si addition were also clarified.The larger porosity,the higher capillary performance,the lower compressive strength and thermal conductivity.Combined with the requirements of wick materials in LHP,Ti3(Al1-xSix)C2(xSi=0.6)was chosen as the new wick material due to the most optimized comprehensive properties.(3)During pressureless sintering,pores generated from powder gaps and synthesis reactions were limited.Thus,wicks with composite pore size were further produced by adding dissolvable pore-formers NaCl,in which fine pores provided high capillary force and large pores enhanced the permeability,thereby promoting the capillary performance.According to the heat and mass transfer processes in porous wicks,the capillary performance,compressive strength and thermal conductivity were tested and effects of processing parameters(NaCl addition and cold pressure)were also analyzed.On the basis of Hagen-Poiseuille flow equations,a flow model motivated by capillary force was presented.Combined with Darcy’s law,the equation of capillary performance parameter ΔPcapK was proposed with porosity and pore size proportion.(4)As the content of pore formers increased,the increased porosity and pore size and decreased tortuosity factor enhanced ΔPcapK of wicks.For wicks with composite pore size,the increased cold pressure increased the fine pore proportion with little change in porosity,which enhanced the capillary force and accordingly promoted the capillary performance.The compressive strength and thermal conductivity of wicks were mainly affected by the porosity.The larger porosity,the lower compressive strength and thermal conductivity.However,when the porosity was similar,the compressive strength and thermal conductivity were affected mainly by fine pores proportion P0～2.The larger fine pores proportion,the larger compressive strength and thermal conductivity.When the content of pore formers was 30%and cold pressure was 20 MPa,the wick with composite pore size possessed the most optimized comprehensive properties(excellent capillary performance,higher compressive strength and relatively lower thermal conductivity).(5)Based on the conclusions above,Ti3(Al1-xSix)C2(xSi=0.6)was applied as the wick material.Ti3(Al1-xSix)C2 capillary wicks with composite pore size were prepared with Ti/Al/Si/TiC powders and pore formers NaCl by pressureless sintering.The porosity of Ti3(Al1xSix)C2 wick was 0.651.The LHP system was successfully constructed and systematically tested.The LHP system started up successfully at 50 W,and showed good stability in long-term operation at 100 W.At variable heat loads,LHP responded rapidly and reached stable states in a short time,and the maximum affordable heat load was 140 W.Heat sink temperatures directly affected the temperature of the sub-cooled working liquid,enhanced the cooling of liquid reservoir and evaporator,and accordingly reduced the temperatures of LHP.The lack in perfusion ratio had little influence on the start-up in LHP but slowed down the circulation at high heat loads,which reduced the heat transfer capacity of LHP.