【作者】 江垚；

【导师】 贺跃辉；

【作者基本信息】 中南大学 ， 材料学， 2008， 博士

【摘要】 当前无机多孔材料中,陶瓷材料存在严重脆性和难加工性,金属材料抗腐蚀和抗高温氧化能力不足,同时大多数材料的制备工艺复杂,生产成本高,限制了无机多孔材料的进一步发展和应用。Ti-Al合金具有优异的材料性能,然而,由于材料的难成形性和高温强度不足等难题,导致其致密材料目前仅极小规模应用于汽车行业。本论文率先提出采用Ti/Al元素反应合成工艺,利用Al元素偏扩散在材料中产生孔隙的Kirkendall效应来制备Ti-Al金属间化合物多孔体,是继陶瓷和金属多孔材料之后对无机多孔材料的新的扩展,同时制备方法具有简单可控,成本低廉等特点,对开发其它体系金属间化合物多孔材料以及工业化应用等具有重要意义。本论文通过Ti/Al元素的反应合成工艺,制备了Ti-Al合金多孔体,非对称Ti-Al合金多孔膜和Ti-Al合金多孔纸型膜等三种多孔结构,采用多种现代测试手段对Ti-Al合金多孔材料的孔结构性能,力学性能和抗环境腐蚀性能进行了深入研究。同时,通过建立模型和数学分析等手段,探讨了元素粉末反应合成Ti-Al合金多孔材料的孔隙形成机理。最后,将Ti-Al合金多孔材料成功地应用于具有强腐蚀性和高毒性的粗TiCl₄原料的固液分离过程。本研究的主要内容和获得的结论如下:提出了以固相反应及偏扩散为特征的三阶段粉末烧结工艺,并由此制备出具有良好的原坯形状相似性的无宏观缺陷的Ti-Al合金多孔体,得到α₂-Ti₃Al,γ-TiAl,和TiAl₃等三种典型物相的多孔材料,详细论述了制备过程及特点。在此基础上提出了约束烧结工艺,可以使Ti-Al坯体的径向膨胀被严格限制,从而减小多孔体的孔径并提高其过滤精度。进一步将粉末的反应合成工艺扩展至粉末/薄膜以及薄膜/薄膜之间,分别制备出了均质的非对称Ti-Al合金多孔膜和Ti-Al合金多孔纸型膜,提出采用偏扩散机制合成无机多孔膜的新方法。反应合成Ti-Al合金多孔体以大量在生成物颗粒之间或晶界处产生的连通孔隙,以及一小部分在生成物颗粒内部产生的等轴状闭合孔隙为典型特征。深入探讨了Ti-Al合金多孔材料的孔结构性能,并广泛建立了制备工艺参数与孔结构性能之间的定量关系方程。Al含量是决定Ti-Al合金多孔材料孔隙度的主要因素之一,在20wt%～60wt%的Al含量范围内,Ti-Al合金多孔材料的孔隙度与Al含量之间遵循严格的直线增加规律,其中Kirkendall孔隙度θ_K与Al含量c_Al之间满足定量方程θ_K=K_c·c_Al。粉末粒度是决定Ti-Al合金多孔材料最大孔径的主要因素之一,在18μm～125μm的粒度范围内,多孔体最大孔径d_m与粉末粒径d_p之间严格遵循d_m=K_p·d_p的直线变化规律。压制压力对孔结构性能参数的直接影响不显著,其对Ti-Al合金多孔材料孔径的影响,实质上是通过压制过程对压坯粉末颗粒的塑性变形以及对压坯间隙孔的影响来实现的。Ti-Al合金多孔材料的透气度K与开孔隙度θ和最大孔径d_m之间严格满足Hagen-Poiseuille方程。建立了Ti-Al合金多孔材料的透气性能和孔结构性能参数之间统一的普适方程:K=A·d_m²·θ,A=（2.26±0.05）×10⁷m^-1Pa^-1s^-1。定量研究了γ-TiAl合金多孔材料与应用相关的力学性能,抗氧化及抗腐蚀性能。TiAl合金多孔材料的抗拉强度σ_b与孔隙度θ之间严格遵循巴尔申方程σ_b=σ₀·（1-θ）^m。TiAl合金多孔材料在600℃空气中的循环氧化动力学方程为Δm²=1.08×10^-5·t,经过140 hours循环氧化后,其氧化增重达到0.042g·m^-2,为316L不锈钢的10.6%,同时表现出优异的孔结构高温稳定性能。TiAl合金多孔材料在PH=2,温度为90℃的盐酸溶液中的循环腐蚀动力学方程为Δm²=5.41×10^-5·t-2.08×10^-4,在50hours的循环腐蚀后,其腐蚀失重仅为0.049g·m^-2,分别是多孔Ti和多孔不锈钢腐蚀失重的14.8%和5.57%。TiAl合金多孔材料在PH=3时的腐蚀动力学方程为Δm²=2.63×10^-6·t-3.72×10^-6。建立了Ti-Al预反应模型,并推导了Ti-Al体系中抑制Self-propagationHigh-temperature Synthesis（SHS）反应的否定判据,较好的符合实验结果。从SHS反应的热力学条件进行推导,得到了预反应层厚度相关参数与体系点火温度之间满足的SHS否定判据方程。随着预反应层厚度相关参数的增大,点火温度被快速提升,表明三阶段烧结工艺对阻止有碍近净成形的SHS反应发生的高效性。在此基础上,结合物相和显微结构,详细论述了Ti-Al合金多孔材料中Kirkendall孔隙的形成过程及特点。建立了Ti-Al粉末扩散偶非对称球体模型,由此探讨了粉末体系中Kirkendall孔隙与基体之间的结构特征。以此模型为结构条件建立了Ti-Al合金中的Kirkendall孔隙与组元扩散速率、球坐标位置和扩散时间之间的动力学方程组。在此基础上,进一步建立了Kirkendall孔隙度,Ti/Al元素的物性参数以及合金成分之间的关系方程,很好的符合实验数据,同时表明Ti-Al合金体系Kirkendall孔隙度的理论值取决于Ti、Al两组元的材料密度以及混合体的成分,同时,体系最终产生的孔隙尺寸强烈依赖于成形坯体中较快扩散组元的单元尺寸。将本研究制备的Ti-Al合金多孔材料应用于工业生产中,成功的实现了粗TiCl₄原料的固液分离过程。以Ti-Al合金多孔管为过滤介质,结合过滤-反冲洗技术可以完全实现长期的密封式连续过滤粗TiCl₄原料液过程,大幅度减轻或避免原料的损失和严重的环境污染,同时,Ti-Al合金多孔管具有长期稳定的高通量和高过滤精度,过滤过程中其孔结构性能参数与液体的透过通量之间满足Hagen-Poiseuille方程。建立了Ti-Al合金多孔材料的过滤结构模型,结合鲁思方程和实验数据,建立了Ti-Al合金多孔材料在过滤粗TiCl₄原料液工业应用中的过滤方程,同时得到Ti-Al合金多孔体的介质阻力为R_m=6.12×10¹⁰m^-1。更多还原

【Abstract】 Current inorganic porous materials including ceramics and metals show some insurmountable disadvantages such as severe brittleness and unworkability for ceramics and poor corrosion and high temperature oxidation resistivities for metals,and for most inorganic porous material their complex fabrication procedures and high production cost restrict their further developments and applications.Ti-Al alloys have excellent material properties;however,due to the forming difficulty and poor strength at elevated temperature,the current dense Ti-Al alloys can only apply in automobile industry in a limited scale.It is the first time in this paper to systematically present that porous Ti-Al intermetallic compound material can be prepared through the Kirkendall effect using the diffusion rate discrepancy between Ti and Al atoms.The fabricated porous material is a novel substitute for current inorganic porous material.The mentioned fabrication method has the special characteristics of simplicity,controllability,and low cost,which shows greatly propitious to the development of other systems of porous intermetallics and the corresponding industry applications.Three typical porous structures including porous Ti-Al alloy body,asymmetric porous Ti-Al alloy membrane and porous Ti-Al alloy paper membrane were fabricated through the reactive synthesis of Ti/Al elements.The pore structure properties,mechanical properties,and environment corrosion resistivities were further investigated by various advanced testing methods.Meanwhile, the pore formation mechanism was discussed in detail by structural model building and mathematical analysis.Finally,the porous Ti-Al alloy material was successfully applied in the solid-liquid separation of suspended TiCl₄ raw liquid which has strong toxicity and corrosivity in industry production procedure,More details were given as follows.Firstly,the three stage sintering process with characteristics of solid reaction and faster diffusion of one component,which was firstly presented and further discussed in the paper,was used to fabricate sound porous Ti-Al alloys kept in good shape condition,which show three typical phases:α₂-Ti₃Al,γ-TiAl and TiAl₃ structures.Bases on the process,the confined sintering procedure was presented to prepare fine-pored and high-accuracy alloys through strictly limiting the radial expansion of compacts.A novel fabrication method based on the reactive synthesis between powders and films, or films and films was presented to prepare homogeneous asymmetric porous Ti-Al alloy membrane or porous paper membrane,respectively.The synthesized porous Ti-Al alloys have the characteristics of the combination of most interconnected pores generated between the resultant particles or along the grain boundaries and minor equiaxial closed pores generated in the resultant particles.Next,the pore structure properties of Ti-Al alloy were investigated thoroughly and detailedly,and the relation equations between the fabrication parameters and pore structure properties were established.Al content was found to be one of the main factors for the porosity,and strictly linear increment rule between the porosity and Al content was confirmed in Ti-Al alloy with the Al content ranging from 20wt%to 60wt%.The Kirkendall porosityθ_K and Al content C_Alwere proved to satisfy the proportion equation ofθ_K=K_c·c_Al.Powder size was the one of the main factors determining the maximum pore size.The maximum aperture d_m and powder size d_p were investigated to obey the equation d_m=K_p·d_p strictly.Pressing pressure did not affect the pore structures directly;however,the pore structure properties could be changed through the deformation of powders and pores in green compacts in the pressing procedure.The permeability K,the open porosityθand the maximum aperture d_m were verified to meet the Hagen-Poiseuille equation. The universal equation K=A·d_m²·θwith A=（2.26±0.05）×10⁷m^-1Pa^-1s^-1was established to reflect the relation between the permeability and pore structure parameters for porous Ti-Al alloy.The mechanical properties and the oxidation and corrosion resistivities were investigated quantitatively in porousγ-TiAl alloy.The tensile strengthσ_b and the porosityθof porous TiAl alloy obey theБалъшинequationσ_b=σ₀·（1-θ）^m strictly.The kinetic equation for the cyclical oxidation of porous Ti-Al alloy at 600℃was determined to beΔm²=1.08×10^-5·t.After total oxidation time of 140 hours,porous TiAl alloy shows high pore structure stability and the gain in weight of Ti-Al alloy is only 0.042g·m^-2,10.6%of that of 316L stainless steel.The kinetic equation for the cyclical corrosion of porous TiAl alloy at 90℃with PH=2 was determined to beΔm²=5.41×10^-5·t-2.08×10^-4.After total corrosion time of 50 hours,the lost in weight of TiAl alloy is only 0.049g·m^-2,which is 14.8%and 5.57%of porous Ti and stainless steel,respectively.The kinetic equation when PH=3 was determined to beΔm²=2.63×10^-6·t-3.72×10^-6.The denial criterion to depress the Self-propagation High-temperature Synthesis（SHS）for Ti-Al alloy was established from the pre-reaction model and the thermodynamics condition of SHS reaction.The criterion equation reflecting the relation between the solid-reaction layer thickness correlation parameter and the ignition temperature was deduced.The ignition temperature would be raised rapidly with increasing the correlation parameter,indicating the high efficiency of the three stage sintering process to avoid the possible SHS reaction which would harm the final compact shape.Based on the model and the phase and microstructure analyses,the formation procedure and characteristics of Kirkendall pores in Ti-Al alloy were investigated in details.The asymmetric diffusion couple model for Ti/Al elemental powders was built,and thus the structure characteristic of the powder system was investigated.Based on the model,the kinetic equation system was established to reflect the relationship of Kirkendall porosity,atom diffusion rate,sphere coordinate parameters and diffusion time.As a result,the relation equation reflecting Kirkendall porosity,material properties and alloy composition was built,which shows that the theoretical porosity of Ti-Al alloy depends on the density of Ti/Al components and the alloy composition,and that the final pore size depends strongly on the size of faster diffusion component in compacts.Finally,the fabricated porous Ti-Al alloy material was successfully applied to the solid-liquid separation of suspended TiCl₄ raw liquid in industry procedure.The sealed,continuous and long-term filtration procedure was realized using the filtration-backwash technology and porous Ti-Al alloy tubes as filtration elements,which can alleviate dramatically or even avoid raw material loss and severe environment pollution.The alloy tubes exhibited long-term,stable and high flux and accuracy,and the pore structure parameters and the flux were investigated to meet the Hagen-Poiseuille equation.The filtration structure model for porous Ti-Al alloy filters was built. Based on the model,the Rush equation and the experimental data,the filtration equation for porous Ti-Al alloy material in the solid-liquid separation was ultimately built,and the medium resistance R_m for porous Ti-Al alloy was then determined to be 6.12×10¹⁰m^-1.更多还原