| Refractory Carbides or nitrides have often show:(1)hardness and brittleness of atomic crystals;(2)The ordered structure and high melting point of ionic crystals;(3)Conductivity and heat capacity of metals.They have the characteristics of high melting point,high hardness,good wear and corrosion resistance,as well as good thermal conductivity,electrical conductivity and chemical stability.They are widely used in making cutting tools,mechanical parts and wear-resistant materials.Among them,titanium carbonitride has become a potential material to replace WC/Co cemented carbide due to its high hardness,high red hardness,excellent wear resistance with lower density.Zirconium nitride and niobium nitride have become potential superconducting materials due to their high superconducting critical temperature.Hafnium carbide and tantalum carbide are used in rocket nozzles and supersonic aircraft due to their ultra-high melting point.Up to now,there is still a certain gap in many technical indicators between domestic enterprises and foreign enterprises for producing these ceramic materials such as purity,cost,process stability.Among them,the control of oxygen content,free carbon and particle size are hard to meet the standard.As a result,the development of these new materials has fallen into a bottleneck.Therefore,focusing on the production of high-purity refractory carbide,nitride or high entropy carbide powder at low cost,this paper puts forward a two-step reduction nitriding process for the preparation of TiN,TiCxN1-x or TiC and a carbothermic reduction-calcium treatment process for the preparation of refractory carbide or high entropy carbide.Through the analysis of thermodynamics involved in the reaction process,the reaction mechanism of the process is discussed,which has important guiding significance for future industrial production.The research results are as follows:(1)In the two-step reduction nitriding method,the traditional one-step carbothermal reduction nitriding reaction is divided into two steps.In this method,a single-phase precursor TiCnO1-n was firstly prepared from the reaction between titanium dioxide and carbon black in argon atmosphere.Then,the precursor was used as raw materials to prepare TiN,TiCxN1-x or TiC.For the production of TiN or TiCxN1-x,the precursor was further carbothermally reduced and nitrided in nitrogen gas atmosphere.For the production of TiC,the precursor was further reduced by carbon in vacuum.In the first reduction step,it was hard to determine the precise molar ratio of TiO2:C due to the uncontrollable ratio of gaseous products CO and CO2 from TiO2 to TiCnO1-n.Whereas the second step only took place in the high temperature region and the gaseous products were thermodynamically in favor of CO.Therefore,the expected products can be obtained by precisely controlling the carbon addition after analyzing the contents of carbon and oxygen in TiCnO41-n.High purity products were prepared with good dispersivity,narrow size distribution and controllable atomic ratio of C:N.The effects of reduction temperature,reaction time,ball milling and particle sizes of raw material TiO2 were studied in detail.TiN,TiCxN1-x and TiC could be obtained under the same optimum time and temperature of 1773 K for 4 h in first step and 1873 K for 8 h in second step.(2)In the carbothermic reduction-calcium treatment process,the completion of reduction is ensured by adding excess carbon,and then the extra carbon is removed by molten calcium.It is found that the low-temperature calcium treatment process shows obvious morphological genetic characteristics,and the particle size of the product is basically consistent with that of the raw material.while the hightemperature calcium treatment can increase the particle size of the product.It is also found that excess carbon can not only ensure the completion of reduction,but also limit the grain growth.(3)Generally speaking,it is difficult to prepare ultrafine or ultra coarse powder in industrial production.Although the carbothermic reduction-calcium treatment process proposed in the previous part has solved the difficulties in the preparation of ultrafine carbides,this process is used to prepare large-size carbides.In previous studies,the largest size of prepared TiC are only 1.8 ?m.The larger particle size could be obtained by increasing the temperature,which will lead to serious calcium volatilization.Therefore,in order to prepare coarse TiC with particle size over 10?m,the calcium treatment process need to be improved.Firstly,molten salt of CaCaCl2 system are used to replace calcium melt.It is found that the introduction of CaCl2 is negatice to the growth of products.Nevertheless,it can reduce the required temperature in calcium treatment,which makes it possible to prepare nano carbides.After that,aluminum melt was selected to replace calcium melt due to its higher gasification temperature.But this attempt ends due to the side reaction between aluminum and titanium carbide in the cooling process.Finally,it is decided not to change the original melt,but adopt the method of cyclic ripening.Finally,coarse TiC with 15 ?m is prepared after 5 cycles under the condition of 4:1 calcium to powder ratio and 1:4 coarse to fine ratio;(4)Compound carbide solid solutions such as Ta4HfCs,NbTaC2,TiNbTaC3,ZrNbTaC3,TiHfNbTaC4,ZrHfNbTaC4,TiZrHfNbTaCs and TiZrHfVbNbTaC6 were prepared by carbon reduction-calcium treatment process.It is found that the formation mechanism of single-phase solid solution follows the dissolution deposition mechanism.High temperature promotes the formation of solid solution,and the particle size of the product can be increased through Ostwald ripening mechanism.In the process of ball milling,high ratio of ball to powder and long milling time can also promote the formation of single-phase solid solution.In addition,a parameter TSS based on mixing enthalpy change and mixing entropy change and a parameter ?a based on lattice parameter difference are proposed to predict the possibility for formation of solid solution.When Tss<1800 K,?a<3.2%,or Tss/K<5.5×103·ln(1/?a)-19×103,the single-phase compoound carbide can be synthesized by calcium treatment. |
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