火焰CVD法合成二氧化钛纳米颗粒的数值模拟
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摘要
纳米材料具有非常广泛的用途——比如日常用到的碳黑、钛白粉颜料(TiO_2)或者通讯中的光学纤维等，其制备方法的研究越来越受到重视。其中化学沉淀法(CVD)是制备纳米粉体的一种很有效的方法。由于颗粒尺寸、尺寸分布状况以及形态等特性对颗粒产品的性能都产生极大的影响，必须对生产装置的结构和操作参数要有很好的了解和控制，这也是本文研究的重点。
本文应用CFD商业软件FLUENT，对火焰CVD法合成二氧化钛纳米颗粒的过程进行了详细的数值模拟。假设燃烧反应在一步内完成，文章模拟了CVD法中的湍流扩散火焰，计算出的火焰形状与实验中所观察到的基本相符。为了模拟火焰中颗粒的聚结生长过程，采用了如下假设：当气体温度超过一定温度后，所有的先驱物分子将转化为二氧化钛单分子；不考虑先驱物TiCl4氧化反应时放热对温度场的影响；忽略颗粒相对流体的影响。在这些假设的基础上分别用Kruis等人(1993)提出的颗粒动力学模型和谢洪勇(2002)提出的颗粒动力学模型结合FLUENT软件对颗粒的特性进行了预测。在对两者的预测结果的比较中发现，前一个模型在用碰撞半径作为衡量颗粒尺寸的标准时其预测的粒径和实验数据吻合得相当好，而后一模型对颗粒尺寸的积累分数的预测却要相对理想一些。以Kruis等人的颗粒动力学模型的计算结果为例，对火焰温度，先驱物载体气体、燃料、氧化剂流量等对生成颗粒或者颗粒聚集块的尺寸的影响进行了分析，结果发现温度越高就越容易形成球形颗粒，颗粒在火焰中时间越长生成的颗粒或聚集块尺寸就越大。
Considerable interest lies in the synthesis and the use of nanosized particles for a variety of applications. Commodities such as carbon blacks, pigmentary titania or optical fibers for telecommunications are typical products of CVD. Particle characteristics like size and size distribution or the morphology mainly influence the final product quality. This emphasizes the need for a tool for optimization of the reactor geometry and the operating parameters.
Using the commercial CFD-code FLUENT, the simulation of the growth process of titania nanoparticle synthesized in a flame CVD process for nanoparticles is detailedly performed. With the suppose that the combustion reaction occurs in a single step, the turbulent diffusion flame in the flame CVD process is calculated and the configuration of the turbulent diffusion flame agrees reasonably well with that observed in experiments. To simulate the coagulation and sintering process of nanoparticle in this flame, assumptions are put up as follows: the process of all precursor molecules to free TiO2 "monomer" molecules occurs instantaneously when the gas temperature exceeds a certain given value; the effects of both the oxidation of TiCI4 on the profile of the temperature and the effects of the particle volume fraction on the fluid are negligible. Based on these assumptions, two different particle dynamic models, Kruis'(1993) and Xie's(2002) are respectively implemented into FLUENT to investigate the growth of partic
les; the results from these two models are compared with each other. It is discovered that the particles/aggregates collision radius calculated from the Kruis' model is perfectly consistent with the sizes of particles from the experiment, but the particle size distribution from the other model is better coincident with the experimental results. With the model by Kruis, the effects of flame temperature and the flow rates of fuel, oxygen and precursor on the sizes of primary particles and aggregates also are analyzed. The results indicate a flame of higher temperature more easily leads to spherical particles; the sizes of particle aggregates become bigger with the longer residence time.
本文应用CFD商业软件FLUENT，对火焰CVD法合成二氧化钛纳米颗粒的过程进行了详细的数值模拟。假设燃烧反应在一步内完成，文章模拟了CVD法中的湍流扩散火焰，计算出的火焰形状与实验中所观察到的基本相符。为了模拟火焰中颗粒的聚结生长过程，采用了如下假设：当气体温度超过一定温度后，所有的先驱物分子将转化为二氧化钛单分子；不考虑先驱物TiCl4氧化反应时放热对温度场的影响；忽略颗粒相对流体的影响。在这些假设的基础上分别用Kruis等人(1993)提出的颗粒动力学模型和谢洪勇(2002)提出的颗粒动力学模型结合FLUENT软件对颗粒的特性进行了预测。在对两者的预测结果的比较中发现，前一个模型在用碰撞半径作为衡量颗粒尺寸的标准时其预测的粒径和实验数据吻合得相当好，而后一模型对颗粒尺寸的积累分数的预测却要相对理想一些。以Kruis等人的颗粒动力学模型的计算结果为例，对火焰温度，先驱物载体气体、燃料、氧化剂流量等对生成颗粒或者颗粒聚集块的尺寸的影响进行了分析，结果发现温度越高就越容易形成球形颗粒，颗粒在火焰中时间越长生成的颗粒或聚集块尺寸就越大。
Considerable interest lies in the synthesis and the use of nanosized particles for a variety of applications. Commodities such as carbon blacks, pigmentary titania or optical fibers for telecommunications are typical products of CVD. Particle characteristics like size and size distribution or the morphology mainly influence the final product quality. This emphasizes the need for a tool for optimization of the reactor geometry and the operating parameters.
Using the commercial CFD-code FLUENT, the simulation of the growth process of titania nanoparticle synthesized in a flame CVD process for nanoparticles is detailedly performed. With the suppose that the combustion reaction occurs in a single step, the turbulent diffusion flame in the flame CVD process is calculated and the configuration of the turbulent diffusion flame agrees reasonably well with that observed in experiments. To simulate the coagulation and sintering process of nanoparticle in this flame, assumptions are put up as follows: the process of all precursor molecules to free TiO2 "monomer" molecules occurs instantaneously when the gas temperature exceeds a certain given value; the effects of both the oxidation of TiCI4 on the profile of the temperature and the effects of the particle volume fraction on the fluid are negligible. Based on these assumptions, two different particle dynamic models, Kruis'(1993) and Xie's(2002) are respectively implemented into FLUENT to investigate the growth of partic
les; the results from these two models are compared with each other. It is discovered that the particles/aggregates collision radius calculated from the Kruis' model is perfectly consistent with the sizes of particles from the experiment, but the particle size distribution from the other model is better coincident with the experimental results. With the model by Kruis, the effects of flame temperature and the flow rates of fuel, oxygen and precursor on the sizes of primary particles and aggregates also are analyzed. The results indicate a flame of higher temperature more easily leads to spherical particles; the sizes of particle aggregates become bigger with the longer residence time.
引文
(1)冯学董，微细粉体材料及其制备技术的现状与发展，广西机械，1999年第1期：19-22
(2)谢洪勇，粉体力学与工程[M]，北京，化学工业出版社，2003
(3)毛日华，孔之瑾，郭存济，纳米二氧化钛胶体和粉体的制备，上海大学学报，1999年第5卷第6期：477-480
(4)吴孟强，张其翼，陈艾，凝胶-燃烧法合成纳米晶S n O2粉体，硅酸盐学报，2002年第30卷第2期：247-253
(5)赵东林，潘正伟，周万城，激光法气相合成纳米Si／C／N复相粉体，西安工程学院学报，1998年第20卷第4期：56-58
(6)Wenhua Zhu, Sotiris E. Pratsinis, Flame Synthesis of Nanosize Powders——Effect of Flame Configuration and Oxidant Composition, Nanotechnology, 1996, P65
(7)M.R. Zachariah, D. Chin, H.G. Semerjian and J.L. Katz, Comb. Sci. Tech., 1977, 17, p119
(8)M. Choi, J. Cho, J. Lee, H.W. Kim, Journal of Nanoparticle Research 1:169-183, 1999.
(9)S. Tsantilis, S.E. Pratsinis, V. Haas, Simulation of synthesis of palladium nanoparticles in a jet aerosol flow condenser. J. Aerosol Sci. Vol. 30, No. 6, 1999: 785-803.
(10)A. Schild, A. Gutschl, H. M"uhlenweg, S.E. Pratsinis. Simulation of nanoparticle production in premixed aerosol flow reactors by interfacing fluid mechanics and particle dynamics. Journal of Nanoparticle Research 1, 1999: 305-315.
(11)Tatsuo Go, Munetaka Honda, Kunio Kato. Analysis of flame gas velocity and temperature distribution from double-pipe nozzle jet in a production unit for spherical particles by flame spray method. Heat Transfer-Japanese Reseatch, 25(4),1996:201-213.
(12)Johannessen T., S.E, Pratsinis, H. Livbjerg. Computational analysis of coagulation and coalescence in the flame synthesis of titania particles. Powder Technology 118(2001):242-250.
(13)陶文铨，数值传热学[M]西安，西安交通大学出版社2002：509
(14)Fluent Inc, Fluent:User'Guide
(15)范维澄，万跃鹏，流动及燃烧的模型与计算[M]安徽，中国科学技术大学出版社1992，69-70
(16)陈矛章，粘性流体动力学基础[M] 河北，高等教育出版社1993，363-380
(17)陆大有，工程辐射传热[M] 北京，国防工业出版社1988：294-296
(18)Tatsuo Go, Munetaka Honda, and Kunio Kato, Analysis of flame gas velocity and temperature distribution from double-pipe nozzle jet in a production unit for spherical particles by flame spray method[J], Heat Transfer-Japanese Research, 1996,25(4):201-213
(19)陶文铨，计算传热学的近代发展[M] 北京，科学出版社，2000，233-234
(20)陈义良，张孝春，孙慈，季鹤鸣 燃烧原理[M] 北京，航空工业出版社，1992，363-366
(21)朱谷君，工程传热传质学[M] 北京，航空工业出版社，1989，514-515
(22)[美]W．M．罗森诺 等，传热学基础手册(上册)[M] 北京，科学出版社，1992，173-178
(23)[美]E．L．柯斯乐，扩散：流体系统中的传质[M] 化学工业出版社，2002
(24)T. Johannessen, S.E. Pratsinis and H. Livbjerg, Computational fluid-particle dynamics for the flame synthesis of alumina particles[J] Chem. Eng. Sci., 55(2000): 177-191.
(25)F.E. Kruis, K.A. Kusters and S.E. Pratsinis A Simple Model for the Evolution of the Characteristics of Aggregate Particles Undergoing Coagulation and Sintering[J] Aerosol Science and Technology 19:514-526(1993).
(26)Kobata A., K. Kusakabe & S. Morooka, 1991. Growth and transformation of TiO2 crystallites in aerosol reactor. AIChE J. 37, 347-359.
(27)Seinfeld, J. H. (1986). Air polution: New York.
(28)Fuchs, N. A. (1964). Mechanics of aerosols. New York: Pergamon
(29)T. Matsoukas, S.K. Friedlander, Dynamics of aerosol agglomerate formation, J. Colloid Interface Sci. 146 2 1991:495-506.
(30)T. Johannessen, "Implementing User-defined Scalars/Functions in Fluent - A practical example", Department of Chemical Engineering, Technical University of Denmark (1999).
(31)孙文策(1995)，《工程流体力学》：大连理工出版社P31
(32)谢洪勇 宁桂玲 毛中强 王达望 孙雪，火焰CVD法制备TiO2纳米颗粒材料的实验与理论研究，过程工程学报第2卷增刊(2002)183-186。
(33)H.-Y. Xie, G.-L. Ning, D.-W. Zhang, A kinetic theory for the growth of nanoparticles in flame CVD process, submitted to Powder Technol.
(34)H.Y. Xie, Growth of nano- and submicron particles in aerosol processes, submitted to Chem. Eng. Sci.
(2)谢洪勇，粉体力学与工程[M]，北京，化学工业出版社，2003
(3)毛日华，孔之瑾，郭存济，纳米二氧化钛胶体和粉体的制备，上海大学学报，1999年第5卷第6期：477-480
(4)吴孟强，张其翼，陈艾，凝胶-燃烧法合成纳米晶S n O2粉体，硅酸盐学报，2002年第30卷第2期：247-253
(5)赵东林，潘正伟，周万城，激光法气相合成纳米Si／C／N复相粉体，西安工程学院学报，1998年第20卷第4期：56-58
(6)Wenhua Zhu, Sotiris E. Pratsinis, Flame Synthesis of Nanosize Powders——Effect of Flame Configuration and Oxidant Composition, Nanotechnology, 1996, P65
(7)M.R. Zachariah, D. Chin, H.G. Semerjian and J.L. Katz, Comb. Sci. Tech., 1977, 17, p119
(8)M. Choi, J. Cho, J. Lee, H.W. Kim, Journal of Nanoparticle Research 1:169-183, 1999.
(9)S. Tsantilis, S.E. Pratsinis, V. Haas, Simulation of synthesis of palladium nanoparticles in a jet aerosol flow condenser. J. Aerosol Sci. Vol. 30, No. 6, 1999: 785-803.
(10)A. Schild, A. Gutschl, H. M"uhlenweg, S.E. Pratsinis. Simulation of nanoparticle production in premixed aerosol flow reactors by interfacing fluid mechanics and particle dynamics. Journal of Nanoparticle Research 1, 1999: 305-315.
(11)Tatsuo Go, Munetaka Honda, Kunio Kato. Analysis of flame gas velocity and temperature distribution from double-pipe nozzle jet in a production unit for spherical particles by flame spray method. Heat Transfer-Japanese Reseatch, 25(4),1996:201-213.
(12)Johannessen T., S.E, Pratsinis, H. Livbjerg. Computational analysis of coagulation and coalescence in the flame synthesis of titania particles. Powder Technology 118(2001):242-250.
(13)陶文铨，数值传热学[M]西安，西安交通大学出版社2002：509
(14)Fluent Inc, Fluent:User'Guide
(15)范维澄，万跃鹏，流动及燃烧的模型与计算[M]安徽，中国科学技术大学出版社1992，69-70
(16)陈矛章，粘性流体动力学基础[M] 河北，高等教育出版社1993，363-380
(17)陆大有，工程辐射传热[M] 北京，国防工业出版社1988：294-296
(18)Tatsuo Go, Munetaka Honda, and Kunio Kato, Analysis of flame gas velocity and temperature distribution from double-pipe nozzle jet in a production unit for spherical particles by flame spray method[J], Heat Transfer-Japanese Research, 1996,25(4):201-213
(19)陶文铨，计算传热学的近代发展[M] 北京，科学出版社，2000，233-234
(20)陈义良，张孝春，孙慈，季鹤鸣 燃烧原理[M] 北京，航空工业出版社，1992，363-366
(21)朱谷君，工程传热传质学[M] 北京，航空工业出版社，1989，514-515
(22)[美]W．M．罗森诺 等，传热学基础手册(上册)[M] 北京，科学出版社，1992，173-178
(23)[美]E．L．柯斯乐，扩散：流体系统中的传质[M] 化学工业出版社，2002
(24)T. Johannessen, S.E. Pratsinis and H. Livbjerg, Computational fluid-particle dynamics for the flame synthesis of alumina particles[J] Chem. Eng. Sci., 55(2000): 177-191.
(25)F.E. Kruis, K.A. Kusters and S.E. Pratsinis A Simple Model for the Evolution of the Characteristics of Aggregate Particles Undergoing Coagulation and Sintering[J] Aerosol Science and Technology 19:514-526(1993).
(26)Kobata A., K. Kusakabe & S. Morooka, 1991. Growth and transformation of TiO2 crystallites in aerosol reactor. AIChE J. 37, 347-359.
(27)Seinfeld, J. H. (1986). Air polution: New York.
(28)Fuchs, N. A. (1964). Mechanics of aerosols. New York: Pergamon
(29)T. Matsoukas, S.K. Friedlander, Dynamics of aerosol agglomerate formation, J. Colloid Interface Sci. 146 2 1991:495-506.
(30)T. Johannessen, "Implementing User-defined Scalars/Functions in Fluent - A practical example", Department of Chemical Engineering, Technical University of Denmark (1999).
(31)孙文策(1995)，《工程流体力学》：大连理工出版社P31
(32)谢洪勇 宁桂玲 毛中强 王达望 孙雪，火焰CVD法制备TiO2纳米颗粒材料的实验与理论研究，过程工程学报第2卷增刊(2002)183-186。
(33)H.-Y. Xie, G.-L. Ning, D.-W. Zhang, A kinetic theory for the growth of nanoparticles in flame CVD process, submitted to Powder Technol.
(34)H.Y. Xie, Growth of nano- and submicron particles in aerosol processes, submitted to Chem. Eng. Sci.