【作者】 王飞；

【导师】 刘冰冰；

【作者基本信息】 吉林大学 ， 凝聚态物理， 2024， 博士

【摘要】 高压可以形成具有异于常态结构与性质的高压新相,是寻找全新功能材料和构筑发现新规律的重要手段。二氧化钛（TiO₂）是一种常见的无机多功能材料,它工艺成熟、成本低、无毒,是当前最具开发潜力的绿色环保型材料,在环境,能源,工业等诸多领域有着广泛的应用。作为一种半导体,TiO₂常压相的禁带激发区域都在紫外光范围,这意味着只有少部分太阳辐射能被TiO₂利用,不利于TiO₂应用的发展。而最近的研究表明纳米TiO₂-II高压相在可见光范围有较好的催化性质,是一种具有应用前景的兼具硬度与催化能力的多功能涂层材料。但目前,人们对纳米TiO₂高压相形成的机制仍不十分了解,对高压相变和影响因素之间的关系也不明确,影响纳米TiO₂高压相变的因素较多,研究表明样品的尺寸、形貌、界面、缺陷和形变等因素都会对其高压相变的压力点和序列产生影响,还未有理论能解释这些因素影响的高压相变。除纳米TiO₂外,VO₂、Y₂O₃和Mn₃O₄等其他纳米金属氧化物中也报道了受这些因素影响的高压相变,研究高压下纳米TiO₂相变微观机制,对理解纳米TiO₂高压相形成的原因,提高其可见光催化性质,确定金属氧化物高压相变与影响因素之间的关系具有重要意义。高压研究中常用的表征手段如原位拉曼谱、同步辐射等表征技术的空间分辨率低,无法对相变微观机制进行表征。电子显微镜可以对微结构进行表征,但受制于高压装置限制无法实现原位表征,只能通过对比表征不同压力卸压样品的方式间接实现,但目前高压研究中电子显微表征的作用比较有限,关于卸压TiO₂的电子显微报道多为形貌表征和晶格表征验证原位结果,无法体现微观结构的变化。针对以上问题,我们对现有高压实验中电镜制样和表征方法进行了完善和改进,并利用这些方法研究高压下纳米TiO₂相变和形变过程中微观结构的变化,取得了以下成果。1、文中对多种高压卸压样品制备方法进行了测试和对比,结果表明不使用传压介质硬压方式卸压后样品回收率高,利用双束电镜提取样品的薄区范围较大,可重复率好,适合微结构表征使用。实验还试验紫外光固化树脂做传压介质,卸压后固化回收样品,试验结果表明,紫外光固化树脂做传压介质在高压下有较好的静水压表现,在高压原位拉曼测量中不产生明显的干扰,其在金刚石压砧中可以被紫外光固化,固化后的样品平整不易破损可使用双束电镜或离子束减薄等手段进行制样处理,制得的样品可以较好的进行电镜表征。文中首次将扫描电镜中的透射菊池衍射技术应用到卸压样品表征中。结果表明,该技术可以对卸压样品中尺寸大于～20 nm的晶粒进行较好结构表征,获得样品的形貌,尺寸,取向,晶界,应力等信息更直观的分布图和较大区域内样品的统计信息,这些信息可以为透射电镜的进一步研究提供取向和晶界上的筛选,这对提高卸压样品的表征效率和表征质量有着重要作用。2、我们分别使用透射电镜和透射菊池衍射对高压下纳米TiO₂的形变机制进行了研究,确认了TiO₂高压下能够发生塑性形变,首次发现了TiO₂高压下出现的形变孪晶。利用透射电镜对比研究初始纳米锐钛矿样品和加压30 GPa卸压样品表明,加压后的TiO₂-II晶粒发生了明显形变,高分辨图显示晶粒中存在大量[001]方向层错和形变孪晶,其中亚微米级晶粒中形成了透镜形片层结构的形变孪晶带;纳米级晶粒中形成了扇形多重形变孪晶。结果表明高压下纳米TiO₂可以发生明显的形变,其形变的微观机制与金属类似,主要为形变孪晶和层错滑移,形变孪晶的形成存在明显的尺寸效应,这种现象在透射菊池衍射的表征中也有表现。这些结果完善了纳米TiO₂高压相变的尺寸效应中形变因素的影响,为其研究提供了一个新的切入点,同时还为制备孪晶TiO₂-II高压相提供了方法。3、我们结合使用透射菊池衍射和透射电镜对比研究了加压10、20和30GPa后卸压的纳米TiO₂样品,首次发现了纳米锐钛矿到TiO₂-II相变过程中的TiO₂-I中间相和晶粒中形变孪晶的变化,观察到较大的塑性形变发生在20—30GPa区间。我们根据实验结果提出了高压下直径为～100 nm的TiO₂纳米晶体的微观结构变化过程:首先在锐钛矿相的[110]方向形成以{112}面为TB呈60°的变形孪晶。随着压力的升高,锐钛矿转化为TiO₂-I并在[010]方向呈60°的孪晶。最后,TiO₂-I转化为TiO₂-II,在[010]方向形成90°孪晶。这些研究结果更直观的展示了TiO₂高压相变过程和尺寸效应,完善了以往研究中TiO₂高压相变的过程,为纳米TiO₂高压下的相变和形变行为提供了微观结构上的补充,有助于深入了解TiO₂高压相变和尺寸效应的微观机制。更多还原

【Abstract】 High pressure can form high-pressure new phases with structures and properties that are different from normal,which is an important means to search for new functional materials and discover new laws in construction.Titanium dioxide（TiO₂）is a common inorganic multifunctional material with mature technology,low cost,and no toxicity.It is currently the most promising green and environmentally friendly material with extensive applications in various fields such as environment,energy,and industry.As a kind of semiconductors,the band-gap excitation region of TiO₂ at atmospheric pressure is within the ultraviolet range,which means that only a small portion of solar radiation can be utilized by TiO₂,which is not conducive to the development of TiO₂ applications.Recent studies have shown that nano TiO₂-II high-pressure phase has good catalytic properties in the visible light range,making it a multifunctional coating material with both hardness and catalytic ability with promising application prospects.However,at present,the mechanism of high-pressure phase formation in nano TiO₂ is still not well understood,and the relationship between high-pressure phase transition and influencing factors is not clear.There are many factors that affect the high-pressure phase transition of nano TiO₂.Research has shown that factors such as sample size,morphology,interface,defects,and deformation can all affect the pressure point and sequence of its high-pressure phase transition.There is no theory that can uniformly explain the high-pressure phase transition affected by these factors.In addition to nano TiO₂,high-pressure phase transitions influenced by these factors have also been reported in other nano metal oxides such as VO₂,Y₂O₃,and Mn₃O₄.Studying the micro mechanism of TiO₂ phase transition under high pressure is of great significance for exploring the relationship between high-pressure phase transition of metal oxides and influencing factors,and understanding the reasons for the formation of high-pressure phase in nano TiO₂.The commonly used characterization techniques in high-pressure research,such as in-situ Raman spectroscopy and synchrotron radiation,have low spatial resolution and cannot characterize the microscopic mechanisms of phase transitions.Electron microscopy can characterize microstructures,but it is limited by high-pressure devices and cannot achieve in-situ characterization.It can only be indirectly observed by comparing and characterizing samples under different pressures.However,the role of electron microscopy characterization in high-pressure research is relatively limited.Electron microscopy reports on depressurized TiO₂ mostly rely on morphology characterization and lattice characterization to verify in-situ results,and cannot reflect changes in microstructure.In response to the above issues,we have improved and perfected the existing methods for electron microscopy preparation and characterization of high-pressure relief samples,and used these methods to study the changes in microstructure during TiO₂ phase transition and deformation under high pressure,achieving the following results.1.The experiment first analyzed the main reasons limiting the development of electron microscopy characterization in high-pressure science,and proposed targeted solutions and related work.（1）In response to the current lack of high-pressure in-situ high-pressure electron microscopy characterization technology,we plan to use a comparative method to characterize the microstructure of samples under different pressure relief.We used a diamond anvil to pressurize spherical rutile TiO₂with an average diameter of 32 nm,and after unloading to normal pressure at 30GPa,the transmission electron microscopy characterization results showed that the rutile transformed into a high-pressure phase of TiO₂-II and underwent deformation.This high-pressure phase can be stably stored under normal pressure conditions,providing a prerequisite for using comparative characterization methods to study the microstructure changes of TiO₂ phase transformation and deformation under different pressures.（2）We have tested and improved existing methods such as using methanol ethanol mixture as pressure medium when decompressed washing dispersion method,no pressure medium when decompressed crushing dispersion method,and no pressure medium when decompressed dual-beam electron microscopy method to address the issue of unstable quality in various high-pressure unloading electron microscopy sample preparation methods.The test results show that the recovery rate of the sample is high after using the non pressure medium hard pressing method for pressure relief.The use of dual-beam electron microscopy to extract samples from it has a large range of thin areas,good repeatability,and is suitable for microstructure characterization.The experiment also improved the recovery method of the pressure relief sample in the high-pressure experiment of the pressure transmission medium.The experimental results showed that the UV cured resin as the pressure transmission medium had good hydrostatic performance under high pressure,and did not produce significant interference in high-pressure in-situ Raman measurement.It can be cured by UV light in the diamond anvil,achieving the embedding of the sample in the sample cavity at the center of the gasket.The surface of the sample after embedding is flat and not easily broken when combined with the gasket,making it suitable for further sample preparation as a block.The cured sample can be observed better under transmission electron microscopy,better achieving microstructure characterization of the sample under hydrostatic pressure relief.（3）In response to the current problem of limited characterization of high-pressure depressurized samples using scanning electron microscopy,we attempt to apply the transmission Kikuchi diffraction technique from scanning electron microscopy to the characterization of depressurized samples.The transmission Kikuchi diffraction results of the depressurized sample indicate that it can effectively characterize the structure of grains larger than～20 nm in the depressurized sample,and obtain a more intuitive distribution map of the sample’s morphology,size,orientation,grain boundaries,stress,and statistical information of samples in a larger area.This information can provide orientation and grain boundary screening for further research by transmission electron microscopy,this plays an important role in improving the characterization efficiency and quality of depressurized samples.The research on the above technical methods provides a technical guarantee for the study of microstructure changes during titanium dioxide phase transformation and deformation under high pressure in the article.At the same time,these methods can also be extended to the study of other high-pressure nanomaterials,improving the role of electron microscopy characterization in high-pressure nanomaterial research.2.The deformation mechanism of TiO₂ under high pressure was studied by transmission electron microscope.The comparative study of the initial nano anatase samples and the pressurized 30 GPa pressure relief samples by transmission electron microscope showed that the pressurized TiO₂-II grains had obvious deformation.The high-resolution diagram showed that there were a large number of[001]direction stacking faults and deformation twins in the grains,and the lens shaped lamellar structure deformation twin bands were formed in the submicron grains;Fan shaped multiple deformation twins are formed in nanocrystalline grains.The results show that TiO₂ can undergo obvious deformation under high pressure,and the microscopic mechanism of deformation is similar to that of metal,mainly including deformation twins and stacking fault slip.The formation of deformation twins has obvious size effect,which is also reflected in the characterization of transmission Kikuchi diffraction.These results provide a new entry point for the study of the size effect of TiO₂ high-pressure phase transition,and also provide a method for the preparation of twins TiO₂-II high-pressure phase.3.Using transmission Kikuchi diffraction and transmission electron microscopy,the pressure relief nano anatase samples after 10,20 and 30 GPa were studied.The changes of TiO₂-I mesophase and deformation twins in grains during the phase transition from nano anatase to TiO₂-II were found for the first time.According to the experimental results,we proposed the microstructure change process of TiO₂nanocrystals with a diameter of～100 nm under high pressure:first,a 60°deformation twin with{112}plane TB was formed in the[110]direction of the anatase phase.With the increase of pressure,anatase was transformed into TiO₂-I and twinned at 60°in the[010]direction.Finally,TiO₂-I was transformed into TiO₂-II,forming 90°twins in the[010]direction.These results more intuitively show the phase transition process and size effect of TiO₂ under high pressure,provide a supplement to the microstructure of phase transition and deformation behavior of nano-TiO₂ under high pressure,and help to understand the mechanism of phase transition of TiO₂ under high pressure.更多还原