Study On Preparation And Performance Of High-Performance Porous Nickel Used In Spacecraft Loop Heat Pipes

The porous nickel wick,as the "heart" of loop heat pipe,provides driving force for the spacecraft loop heat pipe.It has advantages such as good compatibility with ammonia working fluid,controllable pores,and suitable thermal conductivity.Gradient porous nickel shows both high ultimate capillary force and high permeability,and thus has great potential for application in ammonia loop heat pipes.Bidispersed porous nickel demonstrates high permeability and large evaporation area,which can significantly improve the heat transfer performance of ammonia loop heat pipes.However,the existing gradient porous nickel and bidispersed porous nickel are usually prepared with additives such as binders,stabilizers,or pore forming agents,which cannot meet the requirements of high reliability and long service life for materials used in spacecraft loop heat pipes.Gradient porous nickel usually consists of a film and a monoporous nickel support.The quantitative relationship between the permeabilities of the gradient porous nickel,the support and the film is not yet clear,as well as the evolution mechanism of the pore structure in monoporous nickel during sintering.In addition,there are currently limited researches on the impact of powder pre-sintering treatment on the structure and properties of bidispersed porous materials.To address the above issues,this paper prepared gradient porous nickel and bidispersed porous nickel that meet the requirements of high reliability and long service life for materials applied in spacecraft.In the preparation process,powder metallurgy method was used without pore forming agents,binders,and stabilizers.The structures and properties of gradient porous nickel and bidispersed porous nickel were systematically studied.The main research results obtained are summarized as follows:1.Using micron-size carbonyl nickel powder as raw material,monoporous nickel was prepared by pressing and sintering.As the sintering temperature increases from 200℃ to 600℃,the sintering process gradually progresses from the contact-formation stage before sintering to the intermediate sintering stage.The pore diameter of monoporous nickel first increases and then decreases.For the green monoporous nickel with a porosity of 45%～70%,the maximum pore size increases after sintering at 400℃ for 3 h,while the porosity and pore tortuosity factor slightly decrease,resulting in a small decrease in ultimate capillary force and a significant increase in permeability.This achieves the best comprehensive properties including an excellent combination of ultimate capillary force and permeability,pretty low thermal conductivity and pretty high radial crushing strength.By studying the structure and properties of monoporous nickel,the evolution mechanism of pore diameter during the sintering process was revealed.During the sintering process,grain boundary diffusion leads to the reduction of porosity,which results in the decrease in the area of the pore cross-section profile,tending to reduce the pore diameter,while surface diffusion makes the surface of the primary particles gradually smooth,which leads to the reduction of the perimeter of the pore cross-section profile,tending to increase the pore diameter.In the contact-formation stage before sintering and the initial sintering stage,surface diffusion is dominant,and the pore diameter continuously increases;In the intermediate sintering stage,grain boundary diffusion dominates and the pore diameter begins to decrease.2.Uniformly dispersed slurry was prepared using superfine nickel powder as raw material and anhydrous ethanol as dispersion medium,without the addition of binders and stabilizers.Subsequently,gradient porous nickel was prepared by coating and sintering a film on the surface of a monoporous nickel support,which has a maximum pore diameter of 0.37 μm and a permeability of 4.11×10-14 m2.For the uniform porous nickel with the same pore diameter,the permeability is only 1.50×10-15 m2.By analyzing the interface structure and pore structure of gradient porous nickel,and the comprehensive properties of gradient porous nickel,the quantitative relationship between the permeabilities of gradient porous nickl,the suppor ad the film(kt=k1·k2/(1-B)·K2+Bk1)was elucidated,as well as the influence of the film on the comprehensive properties of gradient porous nickel.The film significantly reduces the diameter of through pores in gradient porous nickel,thereby increasing the ultimate capillary force.For the gradient porous nickel with a thin film,the permeability is similar to that of the monoporous nickel support.Therefore,gradient porous nickel can break through the ultimate capillary force-permeability tradeoff,and exhibits both high ultimate capillary force and high permeability.3.Using micron-size carbonyl nickel powder as raw material,pre-sintered nickel clusters were prepared by pre-sintering,crushing,and screening methods,and then bidispersed porous nickel was prepared by tapping and sintering.The size of pre-sintered nickel clusters has the greatest influence on the properties of bidispersed porous nickel.When the pre-sintering temperature is 600℃ and the sintering temperature is 900℃,as the particle size increases from 0～25 μm to 74～104 μm,the tap density decreases while the proportion of sintering shrinkage within pre-sintered nickel clusters increases in the total sintering shrinkage,and due to the smaller sintering shrinkage ratio within clusters,the total sintering shrinkage ratio decreases.The tap density and total sintering shrinkage ratio gradually decrease,leading to an increase in the porosity of bidispersed porous nickel,from 47.1%to 81.3%.Due to the increase in porosity and diameter of larger pores,the permeability of bidispersed porous nickel increases from 0.40×10-12 m2 to 37.92×10-12 m2.In addition,the increases in porosity and the decrease in the number of sintered necks between pre-sintered nickel clusters result in a decrease in the thermal conductivity of bidispersed porous nickel,from 15.94 W/(m-K)to 0.98 W/(m·K).By analyzing the structure and properties of pre-sintered nickel clusters and bidispersed porous nickel,the sintering behavior of pre-sintered nickel clusters was quantified and expressed as Ktotal=)1-d/D).K1+d/D·K2.The influence of the sintering behavior of pre-sintered nickel clusters on the porosity and pore structure of bidispersed porous nickel was clarified,as well as the influence of bidispersed porous nickel structure on permeability and thermal conductivity.