| With the rapid development in the field of aerospace,the development of supersonic vehicles has attracted more and more attentions of all countries in the world.Supersonic vehicles will suffer from many extremely environment such as ultra high temperature and pressure due to the drastic aerodynamic heating effect.Heat dissipation structures based on the radiant are mostly applied to protect vehicles from high temperature.The mechanism of this heat dissipation system is radiating heat into the surrounding environments as much as possible to guarantee the inside system normal operation.The research on high emissivity powders in overseas countries has been carried out earlier and some high purity powder system has been commercialized,but the domestic research is less.In this study,a high purity HfB2powder was prepared by an optimized carbothermal reduction process,MoSi2 andTaSi2 powders were prepared by solid state reaction.The high emissivity coatings was prepared by using three homemade powders above as high emissivity phase.The effect of kinds and contents of high emissivity pahse on the performance of the coating was studied.In this paper,HfO2,H3BO3,B4C and carbon black were used as raw materials to systematically study the effect of boron source content and calcination temperature system on the phase composition and microstructure of HfB2 powder in different boron source systems.It is found that the best composition of HfO2-H3BO3-C system is HfO2(45.0wt%)+ H3BO3(42.2wt%)+ C(12.8wt%),that is,boric acid is an excess of 60wt%on the basis of theoretical addition.The optimum temperature system for this formula is heated at 1600 ?for 30min.Spherical and evenly distributed particles with average particle size of 1-2 ?m will be obtained.The optimum formula for the synthesis of HfB2 powder in HfO2-B4C-C system is HfO2(80.4wt%)+ B4C(12.7wt%)+ C(6.9wt%),the optimum temperature for this formula is heated at 1500 ? for 30min.Resultant powders with average particle size of 1?m are basically spherical and have a uniform distribution.The high emissivity powders with MoSi2 and TaSi2 as the main crystalline phases were prepared by solid state reaction process with MoO2?Ta2O5 and Si as raw materials.It is found that the optimum proportion of MoSi2 powders is MoO2(59.2wt%)+Si(40.8wt%).The optimum temperature system is heated at 1400? for 60min.At this time,the reaction is the most complete,the particle size distribution is the most uniform ranging from 4 to5?m.The best raw material ratio to prepare TaSi2 powders is Ta2O5(69.8wt%)+ Si(30.2wt%).Particles with higher crystallinity and purity was obtained in the condition of 1450 ? for 60min.After that,powders with average particle size of 2?m will be acquired.At the same time,the high emissivity phase of MoSi2-SiO2,TaSi2-SiO2 and HfB2 was first used in this paper.High emissivity coatingS was prepared with borosilicate glass phase as high temperature binder.Effects of different types and different content of powders on microstructure and radiation property of the coatings were investigated.It is found that the total emissivity of the MoSi2-SiO2-borosilicate glass coating is up to 93.85%and 85.24%in the range of 0.75?2.5?m and 2.5?10?m respectively,when 60wt%MoSi2-SiO2 was added.For TaSi2-SiO2-borosilicate coating,The emissivity in 0.75?2.5?m and 2.5?10?m bands is 85.45%and 83.01%,respectively with ratio of TaSi2-SiO2:borosilicate is 60:40.For HfB2-borosilicate coating,when HfB2:borosilicate is 60:40,the emissivity of the coatings is the best,and the emittance in 0.75-2.5?m and 2.5-10?m bands is 91.32%and 85.23%,respectively.Further analysis showed that when the MoSi2-SiO2 and TaSi2-SiO2 composite powder are used as high emissivity phase,amorphous SiO2 in the silicate phase will dependent on the crystalline SiO2 existing in the composite powder during the high temperature treatment,and eventually develops into a columnar grain which has a certain influence on the high emissivity of the coating.But at the same time,the original coordination structure in the silicate phase will be destroyed due to the transfer of SiO2,resulting in more B2O3 volatilized at high temperature.This is the main reason for the relatively high quantity of pores in the MoSi2-SiO2-borosilicate coatings and the TaSi2-SiO2-borosilicate coatings.When HfB2 acted as high emissivity phase,it did not react with the glass phase,and the glass phase only performed as high temperature binder,so the relatively uniform and dense coatings were obtained.The high emissivity of the coatings in this paper may be the result of the intrinsic properties of high emissivity powder,the amorphous structure of borosilicate glass,the doping effect and the forming of cylindrical cristobalite surface texture. |
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