【作者】 王建军；

【导师】 郝俊杰；

【作者基本信息】 北京科技大学 ， 材料科学与工程， 2015， 博士

【摘要】 射频等离子体具有温度高(～104℃)、等离子体炬体积大、能量密度高、传热和冷却速度快、无电极污染和反应气氛可控等优点,在材料的制备领域具有广阔的应用前景。本文以完成真空系统改造后的射频等离子体粉体处理系统为依托,针对难熔金属、陶瓷粉末的球化处理进行了系统研究。同时,开发了短流程制备球形TiC/Fe金属陶瓷复合颗粒、微细球形钛粉和四氧化三锰粉末的新工艺。此外,利用有限元分析软件对等离子体的流场、温度场和速度场进行了数值模拟,分析了工艺参数对等离子体炬的影响,并通过研究颗粒的运动轨迹探讨了粉末粒度分布和收粉率的问题。针对金属粉末易氧化的问题,对现有的100KW等离子体粉体处理系统进行了真空设计和改造。改造后的系统极限真空度达到1.0×10-3Pa,通过调整等离子体运行工艺参数,使等离子体炬达到连续稳定运行的工业生产要求。在运行功率为50KW、工作气流量为28L/min、边气流量为85L/min、系统压力为85KPa情况下,连续运行时间可达30h。采用射频等离子体直接球化处理技术,成功实现了对难熔金属粉末(Nb、Ta)和陶瓷粉末(SiO2、Al2O3)的球化处理。研究结果表明：制备的球形粉末表面光洁、分散性良好,球化率可达100%,球形粉末具有良好的球形度。球化处理后,粉末的松装密度和流动性得到明显改善。铌粉的松装密度由1.33g/cm3提高到4.35g/cm3,振实密度由1.95g/cm3提高到5.61g/cm3,粉末流动性提高为12.51s/(50g)。随着加料速率的增加,粉末球化率逐渐降低。一定粒度范围内,较小粒度粉末更易获得高球化率。此外,载气流量和等离子运行功率对粉末的球化处理也有重要影响。等离子体球化后的粉末比原料粉末性能更好。添加量相同时,球形非晶Si02粉制备的环氧塑封料体系黏度低,流动性更好,当球形硅微粉填充量为75%时,环氧塑封料的膨胀系数为10.1×10-6/℃。相同等离子喷涂工艺条件下,以自制球形A1203粉为原料制备的氧化铝涂层,结构最致密,微观组织缺陷少,显微硬度最高,达到HVo.3=957.6,优于以角形A1203和造粒A1203粉末为原料制备的涂层。将射频等离子体球化技术与自反应合成技术相结合制备的TiC/Fe金属陶瓷复合粉末,球形度高、致密度高、TiC尺寸细小且均匀分布、润湿性性良好,TiC体积分数高达82vo1.%,采用粉末冶金的压制烧结工艺制备的TiC/Fe复合材料性能优异,硬度达到HRA88.5,抗弯强度1360MPa。以不规则形状的大颗粒氢化钛粉末为原料,将射频等离子体球化处理技术与氢爆技术相结合,实现低氧含量微细球形粉末的短流程制备。平均粒径180.17μm的TiH2粉末经射频等离子体处理和后续脱氢处理后,可以得到球形度高、流动性好、平均粒径21.28μm的球形钛粉,氧含量控制在0.21wt.%。将射频等离子体球化技术与热分解技术相结合,以大尺寸的MnCO3为原料,成功制备了微细球形Mn304粉末,为微细Mn304粉末提供了新的途径。利用CFD流体力学计算软件,针对射频等离子体球化制粉过程建立了数值模拟模型,计算了流场、温度场和速度场,等离子炬中心区温度高达10100K,工作气流量和载气流量对等离子体炬的温度分布有显著影响,与实验结果相符。球化后粉末粒度分布的模拟结果与实验测试结果比较吻合,在制备特定尺寸球形粉时,可先根据数值模拟计算所需要的原料粉末的大致粒度范围,大大减少工作量。通过对收粉率问题的探讨,提出一种提高收粉率的方案,为设备的改进提供可行性指导。更多还原

【Abstract】 Radio-frequency (RF) plasma has the advantages of high temperature (up to10000K), large-volume plasma torch, high energy density, fast heat transfer and cooling speed, no electrodes pollution and controlled atmosphere, which has broad application prospects in the field of materials processing area. In this paper, the spheroidization of refractory metals and ceramic powders were studied by using the RF plasma processing system with vacuum system. At the same time, the new short preparaing processes of spherical TiC/Fe metal ceramic composite particle, fine titanium powder and mangano-manganic oxide powder were developed. In addition, numerical simulation of flow field, temperature field and velocity field of the plasma torch are completed by using finite element analysis software. The influence of operating parameters on the plasma torch was further analyzed. The particle size distribution and collecting rate of powders were studied through researching the trajectory of particles.Aiming at solving the oxidation of metal powders, the design and transformation of vacuum system which based on the existing100KW plasma powder processing system were carried out in this paper. The limiting vacuum of plasma system reaches1.0×10-3Pa after rebuilding. Through adjusting the process parameters of plasma operation, this system can continuous operating near30h in the case of the operation power of50KW, center gas flow rate of28L/min, sheath gas flow rate of85L/min and the system reactor pressure of85KPa.The experimental parameters of RF plasma treatment were optimized for obtaining spherical refractory metal powders (Nb, Ta) and ceramic powders (SiO2, Al2O3). The results show that the as-prepared spherical powder have good dispersity and smooth surfaces, and spheroidization ratio is almost100%. The loose density, tap density and powder flowability significantly increases after the treatment. The loose density increases from1.33g/cm3to4.35g/cm3and tap density increases from1.95g/cm3to5.61g/cm3. The powder flowbility increases to12.51s/(50g). With increasing the feeding rate, the spheroidization ratio of niobium powders drops gradually. The raw powders with smaller grain size are more likely to get higher spheroidization efficiency under the same technological conditions. In addition, the carrier gas flow rate and plasma operation power also have important influence on spheroidization efficiency of powders. The plasma prepared powders show better performance than raw powders. Compared with ordinary silica powder, the spherical silica powder has a higher filling level. Under otherwise equal conditions, epoxy molding compound prepared by spherical amorphous SiO2powder shows lowviscosity and good liquidity. The thermal expansion coefficient of spherical silica/epoxy resin composite is10.1×10-6/℃with the content of75%spherical silica. Under the same plasma spraying process conditions, the performance of alumina coating made by self-made spherical Al2O3powder is better than those made by the angular Al2O3and granulated Al2O3powders. The coating features are high density, less defect and high hardness of HV0.3=957.6.Spherical iron matrix composite powder with high volume fraction (82vol.%) of fine TiC reinforcement was produced by a novel process, which combines in situ and plasma techniques. The composite powder has good sphericity and dense structure, and the fine sub-micron TiC particles exhibit homogeneous distribution in the α-Fe matrix. Using the as-prepared spherical composite powder as raw material, the TiC/Fe cermet prepared by powder metallurgy technique at low sintering temperature shows a hardness of HRA88.5and flexural strength of1360MPa.Using large size irregular shape titanium hydride powder as raw material, the short process preparation of fine spherical powder with low oxygen content was realized by combined the RF plasma spheroidization technology with hydrogen decrepitation technique. The average particle size of TiH2powder reduced from180.17μm to21.28μm after plasma and subsequent dehydrogenation treatment. The prepared spherical titanium powder has high sphericity, good fluidity and low oxygen content of0.21wt.%.Using the large size of the MnCO3as raw materials, fine spherical Mn3O4powder was successfully prepared by combined the radio frequency plasma spheroidization technology with thermal decomposition technology, which offers a new production technique for the fine Mn3O4powder.A numerical simulation model of plasma spheroidization process was established by using CFD software. The flow field, temperature field and velocity field of plasma torch were successfully calculated. The centeral temperature of plasma torch is up to10100K. The working gas flow and gas flow rate have significant effect on the temperature distribution of plasma torch, which agree with the experimental results. The simulation results of particle size distribution of spheroidizing powder agree with the results of experiment, so the preparation of specific size spherical powder could estimate the necessary size range of raw powder by numerical simulation, which would greatly reduce the workload. Through the discussion of powder collecting rate, a new scheme is developed to improve powder collecting rate, which provids feasible guidance for the improvement of the equipment.更多还原