【作者】 李达；

【导师】 陈沙鸥；

【作者基本信息】 青岛大学 ， 材料学， 2008， 博士

【摘要】 目前,对纳米TiO₂的研究,大多集中于其功能性研究,如光催化性能、介电性能、导电性、光电转换特性、气敏、湿敏以及一些新的独特的光学性质,然而纳米氧化钛陶瓷作为结构陶瓷,具有良好的低温超塑性等力学性能,利用超塑性对纳米陶瓷进行形变加工,将会在陶瓷工业中发挥很大的作用。为了得到高致密化的纳米TiO₂陶瓷,研究其烧结理论和烧结机理,具有重要的理论价值和工程意义。本论文以纳米和亚微米锐钛矿和金红石TiO₂为原料,采用高温热膨胀仪、X射线衍射仪、扫描电镜、透射电镜、原子力显微镜和差示扫描量热仪等分析测试手段,对恒速升温无压烧结条件下的热膨胀行为、烧结致密化过程、显微结构的变化及烧结机理进行研究,并对二步烧结法制备亚微米氧化钛陶瓷的工艺过程进行研究。对比研究了纳米锐钛矿和纳米金红石、亚微米锐钛矿和纳米锐钛矿的烧结曲线、烧结行为以及显微结构的变化过程,重点研究相结构和粒径对烧结行为的影响。结果发现:亚微米锐钛矿烧结中期起始温度明显高于纳米级锐钛矿;锐钛矿在烧结过程中的相变降低了其收缩起始的温度（450℃）,最大致密化速率出现的温度（880℃）,而纳米金红石的收缩起始温度和最大致密化速率出现的温度分别为750℃和920℃。锐钛矿氧化钛的烧结起始温度,致密化速率,均与粉料粒径有关,粉料越细,越容易烧结。根据全期烧结模型,设计实验,建立TiO₂主烧结曲线。纳米TiO₂主烧结曲线对烧结路径不敏感,烧结体的相对密度仅是时间和温度的函数,利用主烧结曲线得到的相对密度和Archimedes法实测的密度吻合,证明了主烧结曲线的有效性:主烧结曲线提供了一种计算烧结激活能的新方法,根据纳米金红石的主烧结曲线,得到其在空气中的烧结激活能为105 KJ/mol。根据主烧结曲线,可以准确预测陶瓷烧结全过程的致密化行为,预测烧结体的收缩量和最终相对密度。利用高温热膨胀仪以不同的升温速率进行无压烧结,研究了纳米和亚微米氧化钛的烧结激活能。研究结果表明:随烧结温度的增加,比表面积的增加加速了致密化速率;对于纳米氧化钛,当烧结体的瞬时相对密度为70%-80%时,出现最大致密化速率,而对于亚微米氧化钛陶瓷,最大致密化速率出现在相对密度为75%-85%;纳米和亚微米氧化钛的烧结激活能分别为115±10 KJ/mol和302±15 KJ/mol,纳米氧化钛的初期烧结以界面反应为主导机制,而亚微米氧化钛的初期烧结以边界扩散为主导机制。在整个烧结过程中,在最大烧结致密化速率阶段,吸收的热量没有突然增加,比热容基本上是一个常数。比热容随温度变化的关系曲线可以拟合成直线方程,Cp=-0.7+8.79×10^-4 J/（gK）,R=0.89。热分析的结果表明,样品在烧结过程中致密化引起的显微结构的有序化,是一个熵减少的过程,它与由于温度升高引起的热振动熵增加过程是相互抵消的,烧结过程中没有出现明显的放热或吸热现象,是有序无序竞争的结果。分别采用两段式无压烧结法（Ⅰ）和粗化预处理二步烧结法（Ⅱ）制备亚微米氧化钛陶瓷。两段式无压烧结法（Ⅰ）,可以显著抑制烧结过程中的晶粒长大,得到晶粒细小的高致密度的陶瓷;粗化处理二步烧结法（Ⅱ）,第一步的粗化过程使细小的颗粒消失,试样产生更大的孔径,但得到的试样的孔径分布和粒度分布更小。在第二步烧结中,制备了显微组织更加均匀、晶粒更加细小且晶粒尺寸分布更窄的致密化陶瓷。更多还原

【Abstract】 At present,the studies on nanometer TiO₂ are largely focused on its functional properties,such as the photocatalytic properties,dielectric properties,conductivity, photoelectric conversion features,gas sensitivity,humidity sensitivity and some new unique optical properties.However,nano titanium ceramics,as structural ceramics,has good mechanical properties,such as low-temperature superplasticity.In order to prepare nano TiO₂ ceramics with high relative density,it is very important to investigate the sintering theory and mechanism.The anatase and rutile of nanometer and sub-micrometer TiO₂ were used as raw materials in this thesis.The thermal expansion behaviors,the sintering behaviors and mechanism were investigated under the pressureless sintering condition at constant heating rates using Thermal Dilatometer,X-ray Diffractometer（XRD）,Scanning Electron Microscope（SEM）,Transmission Electron Microscope（TEM）,Atom Force Microscope （AFM） and Differential Scanning Calorimeter（DSC）.The two step sintering method was also investigated to prepare sub-micrometer TiO₂ ceramics with fine size grain and high relative density.The sintering curves of nanometer anatase and rutile and submicrometer anatase titania were compared.The results show that the intermediate sintering stage of nano rutile happenes in the range of 765℃～995℃,nano anatase at 490℃～1000℃and submicrometer anatase at 975℃～1065℃.The temperature of sintering shrinkage onset and maximum densification rate appearance is 750℃and 920℃for nano rutile.However, that of nano anatase is 450℃and 880℃for the phase transformation.The SEM images of sample fracture and the AFM images of etched surface show that the grain growth before 1000℃is induced by the adhesion between small granules in the aggregate,and after 1000℃it is caused by the grain boundary motion.Based on the combined-stage sintering model,the master sintering curve（MSC） for the rutile was constructed for sintering in air with constant pressure and using a constant heating rate in a dilatometer.The MSC curve of the rutile TiO₂ samples was constructed and validated under different thermal histories.The MSC,in which the sintered density is a unique function of the integral of a temperature function over time,is insensitive to the heating path.According to the MSC of TiO₂,it is possible to predict the sintering shrinkage and final density and calculate the activation energy（105KJ/mol）.With one temperature dependent parameter determined experimentally,it becomes possible to describe accurately the densification behavior of TiO₂ from the initial to final stages during the sintering period.The nano- and micrometer rutile TiO₂ power compacts were sintered in air at 1200℃at constant heating rates of 1,3,5℃/min using a dilatometer.The shrinkage behaviors of TiO₂ were investigated to clarify the effect of specific surface area on the densification behaviors at the initial sintering stage.The apparent sintering activation energies were also investigated using the shrinkage data by the Arrhenius plot.The results show that the increase in specific surface area enhances the densification rate with increasing temperature.The activation energy values of nanometer and micrometer TiO₂ are 115±10 KJ/mol and 302±15 KJ/mol,separately.The effect of heating rates on densification rate for manometer TiO₂ is much greater than that for micrometer TiO₂.For nanometer TiO₂,a maximum vale of densification rate appears when the instantaneous relative density is in the range of 70%-80%.But for micrometer TiO₂,the maximum value of densification rate appears in the range of 75%-85%.The sintering kinetics of nanometer rutile TiO₂ was intestigeted using DSC.The whole sintering process nearly was a steady endothermic one.The specific heat capacity（Cp） had not increased abruptly at the temperature when the maximal densification rate appeared.The equation of Cp and temperature is Cp=-0.7+8.79×10^-4T.A two-step sintering process was used to prepare sub-micrometer TiO₂.The first heating step should be short at a relatively high-temperature in order to close porosity without significant grain growth.The second step at a relative low-temperature facilitates further densification with limited grain growth.Fine-grained TiO₂ with a relative density of 95%and a grain size of 0.8μm was prepared by two-step sintering.Besides,another two-step sintering method was investigated.The initial precoarsening at a low temperature with a long dwell time produced an improvement in the microstructure homogeneity during the subsequent sintering.The microstructural refinement was produced by the two-step sintering.更多还原