【作者】 张华；

【导师】 喇培清； 张玉武；

【作者基本信息】 兰州理工大学 ， 材料工程（专业学位）， 2022， 硕士

【摘要】 超高温材料具有高熔点、高热导、耐高温烧蚀以及良好的热化学稳定性等优异性能,其中以高熔点碳化物（Ti C、Si C、Zr C等）和难熔金属（V、W、Mo等）为代表,被广泛应用于航空航天领域。随航天技术的快速发展对超高温材料的性能及制备工艺提出了更高的要求。其中,高熔点碳化物作为高温结构材料其性能受到粒径和纯度的影响,难熔金属钒在作为高温超导及快中子反应堆包覆材料时杂质含量高,难以提纯一直是研究的热点及难点。此外,目前纳米级碳化物粉体及高纯难熔金属的规模化制备问题仍未得到很好的解决。鉴于燃烧合成技术具有能耗低、工艺设备简单、支持规模化生产等特点以及在高熔点材料制备方面的优势,本文选择燃烧合成法分别基于镁热、铝热还原规模化制备出纳米级Ti C、Si C、Zr C粉体和纯度较高的金属钒,并研究了稀释剂、原料粒度、物料配比对体系绝热温度、产物粒度及纯度的影响规律,探讨了其作用机制。主要结论如下:（1）利用燃烧合成技术,分别在Ti O₂-C-Mg和Si O₂-C-Mg体系中加入稀释剂Na Cl解决了因燃烧温度过高导致颗粒长大的问题,单次实验可实现高纯纳米级Ti C和Si C粉体的规模化（kg/炉）制备。结果表明,Na Cl的加入可有效减小产物粒径。其中,当加入Na Cl含量为总反应物（Ti O₂、C、Mg）质量的100%时,Ti C粉体平均粒径从178 nm减小至74 nm,获得比表面积为15.203 m²/g的纳米Ti C粉体。当加入Na Cl含量为总反应物（Si O₂、C、Mg）质量的80%时,Si C粉体平均粒径从155 nm减小至28 nm,获得比表面积为53.204 m²/g的纳米Si C粉体。稀释剂Na Cl通过作用于粉体的形成和生长两方面实现粉体粒径的减小。一方面,Na Cl相变吸收热量,有效降低了体系绝热温度;另一方面,Na Cl为反应提供液相环境,加快扩散速率,增加过冷度,提高成核速率的同时抑制颗粒长大。但随粉体粒度的减小,较高的比表面积引起颗粒表面氧含量、游离碳含量的增加。（2）采用Zr O₂、石墨及镁粉,通过盐助燃烧合成规模化（kg/炉）制备了纳米Zr C粉体,研究了原料Zr O₂粒度对产物粒度及纯度的影响。Zr C粉体粒径随Zr O₂粒径的减小（200→10 nm）而降低,平均粒径从约70 nm降低到50 nm以下,且近球形形貌更趋于均匀。当采用粒径为10 nm的Zr O₂时,Zr C粉体的中位径D50为48 nm,比表面积为19.532 m²/g,最细颗粒的粒径小于20 nm。此外,随Zr O₂粒径减小,产物Zr C中游离碳含量逐渐减少,较高的比表面积使颗粒表面发生氧化。在燃烧合成纳米Zr C时,Zr O₂粒度减小,碳锆体系活性增强,渗碳速度提高,推动反应充分进行,减缓了因反应时间过长、转化程度不够、燃烧波速度及反应物活性低对颗粒长大的影响。（3）采用铝热还原燃烧合成技术成功实现粗钒的规模化（kg/炉）制备,为后续电子束熔炼制备高纯钒奠定基础。通过研究Al配比对反应过程中的影响,以提高难熔金属钒的纯度。结果表明,金属的微观组织主要由基体相V和少量的夹杂相氧化铝组成。钒的纯度随体系中Al配比的减小而提高,在铝配比为90 wt.%时,金属基体相中V的质量分数最高为99.8%,氧化铝体积比为8.28%,ICP检测其微量杂质元素（Si、S、K、Mg、Cu）含量≤0.122 wt.%。铝热还原燃烧合成金属钒时,过量的Al会固溶在V基体,降低金属钒纯度,并引起氧化铝含量的增加,导致夹杂相聚集长大。随Al配比的减小金属基体相纯度得到显著改善。（4）在V₂O₅-Al-Al₂O₃体系中,Al₂O₃作为稀释剂以减小因体系热量过高而引入外部杂质（如铜坩埚会引入Cu杂质）导致金属纯度降低的影响。随稀释剂Al₂O₃的增加,绝热温度从3045 K降低至2177 K,氧化铝夹杂相与Al₂O₃含量成正比。在加入10 wt.%的Al₂O₃时,夹杂相体积占比减小至5.23%。适量稀释剂的加入使体系绝热温度保持在一定范围,部分杂质元素因达到沸点被挥发去除。此外,在高温条件下可延长金属钒与熔渣液相共存时间,促进剩余钒氧化物与铝的还原,同时提供足够的时间给予金属与熔渣沉降,提高金属钒纯度。更多还原

【Abstract】 Ultra high temperature materials have excellent properties such as high melting point,high thermal conductivity,high temperature ablation resistance and good thermochemical stability.Among them,high melting point carbides（Ti C,Si C,Zr C,etc.）and refractory metals（V,W,Mo,etc.）are widely used in the field of aerospace.With the rapid development of aerospace technology,higher requirements are put forward for the properties and preparation technology of ultra-high temperature materials.Among them,the properties of high melting point carbides as high-temperature structural materials are affected by particle size and purity.When refractory metal vanadium is used as high temperature superconducting material and fast neutron reactor coating material,it is difficult to purify due to the high content of impurities.In addition,the large-scale preparation of nano carbide powder and high-purity refractory metal has not been well solved.In view of the characteristics of combustion synthesis technology such as low energy consumption,simple process equipment,supporting large-scale production and its advantages in the preparation of high melting point materials,this paper selects combustion synthesis method to prepare nano Ti C,Si C,Zr C powder and high purity metal vanadium based on magnesium thermal and aluminothermic reduction respectively,and studies the effects of diluent,raw material particle size and material ratio on the adiabatic temperature,product particle size and purity of the system,Its mechanism was discussed.The main conclusions are as follows:（1）Using combustion synthesis technology,diluent Na Cl is added into Ti O₂-C-Mg and Si O₂-C-Mg systems respectively,which solves the problem of particle growth caused by excessive combustion temperature.Large scale（kg/furnace）preparation of high-purity nano Ti C and Si C powders can be realized in a single experiment.The results show that the addition of Na Cl can effectively reduce the particle size of the product.When the Na Cl content is 100%of the mass of total reactants（Ti O₂,C,Mg）,the average particle size of Ti C powder is reduced from 178nm to 74 nm,and nano Ti C powder with specific surface area of 15.203 m²/g is obtained.When the content of Na Cl is 80%of the mass of total reactants（Si O₂,C,Mg）,the average particle size of Si C powder is reduced from 155 nm to 28 nm,and nano Si C powder with specific surface area of 53.204 m²/g is obtained.Diluent Na Cl can reduce the particle size of powder by acting on the formation and growth of powder.On the one hand,Na Cl phase change absorbs heat and effectively reduces the adiabatic temperature of the system;On the other hand,Na Cl provides a liquid phase environment for the reaction,accelerates the diffusion rate,increases the undercooling,improves the nucleation rate and inhibits the particle growth.However,with the decrease of particle size,the higher specific surface area causes the increase of oxygen content and free carbon content on the particle surface.（2）Nano Zr C powder was prepared by large-scale salt assisted combustion synthesis（kg/furnace）with Zr O₂,graphite and magnesium powder.The effect of raw material Zr O₂particle size on product particle size and purity was studied.The particle size of Zr C powder decreases with the decrease of Zr O₂particle size（200→10 nm）,the average particle size decreases from about 70 nm to less than 50nm,and the near spherical morphology tends to be more uniform.When Zr O₂with particle size of 10 nm is used,the median diameter D50 of Zr C powder is 48 nm,the specific surface area is 19.532 m²/g,and the particle size of the smallest particle is less than 20 nm.In the combustion synthesis of nano Zr C,the reduction of Zr O₂particle size increased the activity and carburizing rate of the zirconium carbon system,promotes the full progress of the reaction,and slows down the effects of long reaction time,insufficient conversion degree,low combustion wave speed and reactant activity on particle growth.（3）The large-scale preparation of vanadium（kg/furnace）was successfully realized by thermit reduction combustion synthesis technology,which laid a foundation for the preparation of high-purity vanadium by subsequent electron beam melting.The effect of Al ratio on the reaction process was studied to improve the purity of refractory metal vanadium.The results show that the microstructure of the metal is mainly composed of matrix phase V and a small amount of inclusion phase alumina.The purity of vanadium increases with the decrease of Al ratio in the system.When the amount of aluminum is 90 wt.%,the mass fraction of V in the metal matrix phase is 99.8%and the volume ratio of alumina is 8.28%.The content of trace impurity elements（Si、S、K、Mg、Cu）detected by ICP is≤0.122 wt.%.Aluminothermic combustion synthesis of metal vanadium,excess al will be dissolved in V matrix,reduce the purity of metal vanadium,increase the content of alumina,and lead to the aggregation and growth of inclusion phase.With the decrease of Al ratio,the purity of metal matrix phase is significantly improved.（4）In V₂O₅-Al-Al₂O₃system,Al₂O₃is used as diluent to reduce the influence of the reduction of metal purity caused by the introduction of external impurities（such as Cu impurities in copper crucible）due to the high heat of the system.With the increase of diluent Al₂O₃,the adiabatic temperature decreases from 3045 K to 2177 K.With the increase of diluent Al₂O₃,the adiabatic temperature decreases from 3045 K to 2177 K.The alumina inclusion phase is directly proportional to the content of Al₂O₃.When adding 10 wt.%Al₂O₃,the volume proportion of inclusion phase is reduced to 5.23%.An appropriate amount of diluent is added to keep the adiabatic temperature of the system within a certain range,and some impurity elements are volatilized and removed due to reaching the boiling point.In addition,under high temperature conditions,the coexistence time of metal vanadium and slag liquid phase can be prolonged,the reduction of residual vanadium oxide and aluminum can be promoted,and sufficient time can be provided for the settlement and separation of metal and slag to improve the purity of metal vanadium.更多还原