【作者】 李金洪；

【导师】 马鸿文；

【作者基本信息】 中国地质大学（北京） ， 矿物学、岩石学、矿床学， 2007， 博士

【摘要】 莫来石因独特的针、柱状穿插骨架结构使其具有优良的热稳定性、化学稳定性和机械性能而被广泛应用于耐高温、防腐蚀和耐磨损等领域。由于目前粉煤灰在我国排放量大、利用率低且技术含量不高，其资源化利用问题成为近年来我国政府所面临的一大难题。本文利用高铝粉煤灰的化学及物相组成优势，采用高粉煤灰掺量，研究中低温烧制高强度、高含量莫来石陶瓷的可行性。论文对制品的性能和显微结构进行了研究，对莫来石晶体化学与晶体结构作了分析，在莫来石陶瓷烧结机理的研究基础上，深入讨论了莫来石晶化反应热力学、动力学问题，并对比研究了铝硅凝胶体系中莫来石的形成机理。通过系统制备莫来石陶瓷的实验研究，确定了AS、AD两个系列陶瓷的制备工艺条件并获得了良好的性能指标。Al₂O₃／SiO₂（摩尔）=0.9（粉煤灰=70.1％），烧结温度为1550℃，保温时间为2～5h，获得AS90陶瓷的基本性能：吸水率为0.62～0.91％，显气孔率为1.72～2.45％，体积密度为2.76～2.82g／cm³，抗折强度为80～98MPa。这些指标达到了《耐酸砖》GB／T 8488-2001等标准的要求。AD系列陶瓷还具有较高的抗折强度，其中M2和CF2的抗折强度分别达到了169MPa和131MPa。AS系列陶瓷，烧结温度≤1300℃，其物相主要为玻璃相及莫来石，还有刚玉和方石英；烧结温度≥1450℃，主要是莫来石和玻璃相，在AS90陶瓷中莫来石含量约为80％。AD系列陶瓷，莫来石含量及晶粒大小受添加剂种类及其含量的影响，莫来石晶粒的长径比为4～7。AS90的抗折强度与气孔率关系符合Ryskewitsch经验公式，而与莫来石晶粒长度关系符合Orowan半经验公式。合成莫来石的氧空位系数、Al₂O₃含量与晶格常数之间的关系不明显，这与其较高的Ti⁴⁺、Fe³⁺固溶量造成八面体扭曲或畸变有关。烧结初、中期，熔体粘度随时间延长呈指数下降，烧结反应速率常数为0.08min^-1（1100℃）～3.42min^-1（1500℃），烧结反应级数在0.29～0.52之间，烧结反应表观活化能为178～295kJ／mol。烧结中、后期，莫来石晶粒生长符合正常晶体的生长规律，其晶粒生长速率常数随温度升高而急剧增大。1300℃时，晶粒生长指数n=2.13，晶粒生长受化学位梯度扩散控制；1500℃时，n=3，晶粒生长主要受气孔迁移控制。在1100～1300℃，晶粒生长的表观活化能为226kJ／mol，在1300～1500℃，晶粒生长表观活化能为69kJ／mol。莫来石晶化反应的4种主次形式为：Al₂O₃（liq）+SiO₂（liq）＞Al₂O₃（s）+SiO₂（liq）＞Al₂O₃（liq）+SiO₂（s）＞Al₂O₃（s）+SiO₂（s）。AS体系莫来石晶化反应的自由能要比红柱石、夕线石、蓝晶石等矿物分解生成莫来石的自由能低约100kJ／mol，比α-Al₂O₃或γ-Al₂O₃与石英反应生成莫来石的自由能也低。晶化反应的表观活化能较低，仅为151kJ／mol，原因是体系中种晶莫来石的存在免去了成核阶段所需的界面能。添加4w_B％Na₂O后，表观活化能下降至92kJ／mol。莫来石化过程具有鲜明的“玻璃微珠球体晶粒生长”特征：莫来石首先在粉煤灰玻璃微珠球体上析晶并生长导致球体破裂，继而按α-Al₂O₃与SiO₂（liq）反应模式生成莫来石。采用超临界CO₂一步法（形成与干燥）制备了铝硅前驱体干凝胶。经煅烧获得粒径较均一的纳米莫来石粉体。伴随着γ-Al₂O₃→δ-Al₂O₃→α-Al₂O₃的转变，干凝胶莫来石化在1200℃开始，至1400℃基本完成。本文研究表明，高粉煤灰掺量（＞70％）制备的高莫来石含量的AS系列陶瓷可以用作耐酸砖、热电厂烟囱内衬砖等。论文取得的成果为粉煤灰高效资源化利用提供了新途径，也为利用高铝煤灰低成本制备高强度莫来石陶瓷提供了实验依据和技术参考，也提供了粉煤灰物料体系烧结反应机理及莫来石晶化反应热力学和动力学的理论依据。更多还原

【Abstract】 Owing to the unique interlocking grain structure, excellent mechanical properties, favourable thermal and chemical stability, mullite and mullite based ceramics have been widely used in many fields such as refractory materials, anti-corrosion liners, sliding wear ceramics, etc. The huge amounts of fly ash from the coal combustion in China annually has put forwards the great challenge for the Chinese government to utilize it efficiently. In this work, author take the advantage of phase-mineral and chemical compositions of high-aluminum fly ash collected from North China Thermal Power Plant, and prepared mullite ceramics at lower sintering temperatures, which was aimed to apply in high strength, corrosion-resistance reinforced structure materials. This research focused on the correlations of the properties and structures of the mullite ceramics from point of mullitization thermodynamics, reaction kinetics and crystalloid chemistry. For comparison, this thesis also discussed the mechanism of the mullite formation from silicon-aluminium xerogel system.Processing parameter optimization and improved mechanical characteristics were obtained in two representative series of mullite ceramics named as AS and AD from series of experiments. At the molar ratio of Al₂O₃/SiO₂=0.9（ fly ash mass content is about 70%）, sintered temperature 1550℃, residence time 2～5h, the prepared AS90 sample exhibited the following properties of water absorption 0.62～0.91%, apparent porosity 1.72～2.45%, bulk density 2.76～2.82g/cm³, bending strength 80⁹0MPa, which is up to the National Standard of China "Acid-resisting bricks and tiles" GB/T 8488-2001. Moreover, M2 and CF2 samples in AD series ceramics displayed much higher bending strength at 169 MPa and 131 MPa respectively.The mineralogical phases analysis indicated that the sample sintered below 1300℃ consisted glass phase, mullite, corundum and cristobalite, and the products fired at above 1450℃ contained only mullite and glass phase in the batches of AS system. It is estimated by standard curve methods from XRD analysis results that mullite content is about 80% in AS90 ceramics. For AD ceramics, mullite content and grain size is greatly affected by additives and their contents, the aspect ratio of mullite grain obtained is 4～7. The correlation between bending strength and porosity obeys "Ryskewitsch" formula, and the relationship between bending strength and the grain size of crystallies agree well with "Orowan" semi-experienced functions. However, there is no obvious correlation among the oxygen hole rate, Al₂O₃ content, lattice parameters of synthesized mullite. This may be attributed to the high dopant content of Ti⁴⁺、Fe³⁺ that caused distortion or aberration of octahedron site in mullite crystals.At initial and intermediate sintering stage, the viscosity of the melt decreased along exponential gradient with sintering time, the constant of sintering rateincreased from 0.08mm^-1 at 1100℃ to 3.42 min^-1 at 1500℃; and the reaction exponent lies within 0.29～0.52, apparent active energy of sintering reaction is calculated at about 178 kJ/mol within the temperature of 1100℃～1450℃ and 295 kJ/mol within 1450℃～1500℃. At intermediate and final stage, the grain growth of mullite consists with general crystal theory, the constant of mullite growth rate climbed from 1100 to 1500℃. The grain growth exponent n is 2.13 at 1300℃, which is interpreted that grain growth is controlled by chemical potential gradients diffusion. At the temperature of 1500℃, the grain growth exponent n amounts to 3, which showed that the growth rate is dominated by pore transferring and elimination. The results indicated that the apparent active energy of grain growth decrease from 226 kJ/mol （1100～1300℃） to 69 kJ/mol （1300～1500℃）.This research proposed a model of mullitization mechanism of Fly ash and Bauxite reactants couples reaction, which was termed as cenosphere mullitization to summarize the sintering process of the reactions from high aluminum fly ash. Firstly, crystallization of mullite takes place around the pristine mullite seeds in the shell of the cenosphere of fly ash with the formation of gas. And then the hollow microspheres were broken for the swelling of gas and leading to green ceramics volume shrinkage. Lastly, the mullites crystals grows as the reaction route among α-Al₂O₃, amorphous SiO₂. It was found that mullitization reaction takes place in the order of: Al₂O₃（liq）+SiO₂（liq）>Al₂O₃（s）+SiO₂（liq）>Al₂O₃（liq）+SiO₂（s）>Al₂O₃（s）+ SiO₂（s） vis thermadynamic and kinetics calculation. In AS systems, the apparent free engery is ～100kJ/mol lower than those of decomposition reactions of andalusite, sillimanite and kyanite, and also lower than that of utilization of α-Al₂O₃+quartz or γ-Al₂O₃+quartz. The apparent mullitization active energy is around 151 kJ/mol in the rage of 1100℃～1500℃. When 4w_B%Na₂O added into the initial batches of AS90, the apparent crystallization active energy was evaluated to decrease further to 92kJ/mol in the rage of 1100℃～1300℃.In addition, nanosized mullite was successfully prepared by supercritical CO₂ and subsequent supercritical fluid extraction from the xerogel of mullite precursors. The mineralogical phases tracing experiment showed that the mullitization process underwent from 1200℃ to 1400℃ as followed by the phase transition of γ-Al₂O₃→ δ-Al₂O₃ →α-Al₂O₃.The prepared mullite ceramics of series AS （utilization rate of fly ash is above around 70%） and AD might find its application in middle-low temperature, anti-corrosion and high strength areas, such as acid-resisting brick, refractory materials and friction-resistance liners in boilers. The results provide inventive data in preparation of low-cost, high performance mullite based composites from high aluminum fly ash, and enrich the sintering mechanism of fly ash system in view of mullitization thermaldynamics and kinetics.更多还原