TiO₂压敏陶瓷的晶界偏析与势垒结构研究

【作者】 严继康；

【导师】 孙加林； 甘国友；

【作者基本信息】 昆明理工大学 ， 材料学， 2009， 博士

【摘要】 Ti02是一种新型的具有电容性和压敏性双功能的低压压敏陶瓷,具有广阔的应用前景。为获得较好的性能,一般需要对Ti02进行掺杂,掺杂离子的半径、含量及其价态、烧结温度和氧分压对Ti02压敏陶瓷的显微结构、化学组成、势垒结构和电学性能等有明显的影响。本文主要研究不掺杂Ti02、单施主Nb掺杂、单受主La掺杂、(La、Nb)共掺和(Ce、Nb)共掺Ti02的晶界偏析规律与机理、势垒结构的主要影响因素。首先,采用传统电子陶瓷工艺制备在1300℃、1350℃、1400℃和1440℃烧结的不掺杂、单掺杂和共掺杂Ti02样品。采用SEM、EDS、XRD、XPS、AFM、TEM、SAED、HRTEM和电子探针等现代材料检测手段分析了样品的显微结构、化学组成、物相组成、离子价态、晶界热沟、晶界结构和元素偏析。测试了Ti02样品在不同温度和偏压的电学性能,采用热电子发射理论计算Ti02的势垒结构。势垒结构包括势垒高度ΦB和势垒宽度XD。然后,基于点缺陷的热力学方法,研究了不掺杂、单掺杂和共掺杂Ti02陶瓷在不同温度的晶界静电势分布与空间电荷(缺陷)浓度分布,探讨晶界偏析的机理。最后,建立了(310)晶界结构,组合使用第一原理和分子力场模拟计算了不掺杂、单施主、单受主和共掺杂Ti02(310)晶界的势垒高度,考查晶界势垒高度受掺杂离子、位置和含量的影响。研究结果表明：1、不掺杂Ti02压敏陶瓷在高温烧结后显微结构存在许多气孔,这些气孔是由于氧空位Vo的聚集、扩散和迁移逐渐形成的。从晶粒挥发出来的氧沿着晶界移动,在晶界和三叉晶界吸附,与Ca, Si等形成晶粒间相。不掺杂Ti02陶瓷在形成氧空位Vo的同时,四价钛会得到一个电子变成三价钛,TiTi’是补偿氧空位Vo的方式之一。三价钛和四价钛随氧空位的产生,形成间隙三价钛和四价钛,XPS证明了三价钛和四价钛的共存。2、不掺杂Ti02在化学计量比和还原气氛下,其晶界静电势和缺陷浓度分布是不同的。保持化学计量比情况下,钛空位VTi""、氧空位Vo和钛间隙Tii是主要的点缺陷,它们不同的缺陷形成能gvn,gvo和gTii决定了静电势Φ(χ)的正负与大小。在还原气氛情况下,有氧空位和钛间隙两种补偿机制来满足电中性条件,静电势和缺陷浓度由氧分压和温度决定。3、单施主Nb掺杂Ti02陶瓷Nb发生偏析,当Nb掺杂浓度较低时,XRD没有检测到第二相。Nb掺杂Ti02形成点缺陷NbTi,当钛空位与点缺陷NbTi满足电中性条件时,静电势由Nb的掺杂浓度、钛空位缺陷形成能和烧结温度所决定。随烧结温度增加势垒高度(静电势)降低,随掺杂含量增加势垒高度增加。4、单受主La掺杂Ti02发生明显的La偏析现象,并生成第二相La4Ti9O24。晶界静电势和缺点浓度取决于La掺杂浓度,而La的偏析驱动力来自于La在晶界形成的弹性应变能。5、(La、Nb)共掺杂Ti02中的静电势和缺陷浓度由Nb掺杂浓度、温度决定,而La的偏析由La在晶界所引起的弹性应变能来决定。(La, Nb)共掺杂Ti02压敏陶瓷中存在第二相,并且第二相随烧结温度转变。6、(Ce4+、Nb)共掺杂Ti02中的静电势和缺陷浓度由Nb掺杂浓度、温度决定,而Ce4+的偏析由弹性应变能来决定。(Ce3+、Nb)共掺杂Ti02中的静电势和缺陷浓度由Ce3+、Nb掺杂浓度、温度决定,而Ce3+的偏析由静电势和弹性应变能来决定。7、La单掺杂、(La, Nb)共掺杂和(Ce, Nb)共掺杂Ti02压敏陶瓷的晶界偏析驱动力为弹性应变能；而Nb单掺杂Ti02压敏陶瓷的晶界偏析驱动力为静电势。8、掺杂Ti02压敏陶瓷中第二相来源于掺杂离子在能量较高的晶界面或晶粒表面偏析成核,然后是晶核在能量较高的晶界面晶粒表面逐渐生长成第二相。9、Ti02压敏陶瓷的势垒结构接触界面模型为n型半导化晶粒和p型半导化晶界面的接触,即n-p-n型。第一原理计算表明：掺杂离子在晶界偏析和在晶粒固溶对势垒高度有明显的影响,这主要是施主固溶加强了Ti02晶粒的n型导电性,而金属离子在晶界的偏析则强化了晶界面的受主特征,因此增加了势垒高度。10、影响势垒结构的主要因素包括：掺杂离子大小、掺杂量、烧结温度、降温工艺和纳米Ti02改性等,势垒结构对电学性能有直接影响。因此,通过改变组成和工艺调控晶界势垒结构,而得到电学性能较好的Ti02压敏电阻器。更多还原

【Abstract】 TiO2 is a novel low-breakdown voltage varistor ceramic with double function of capacitance and varistor. Generally, doping is necessary for TiO2 ceramics to get better performances for a wider application fields. Radius, concentration and valence of doped ions, sintering temperature, oxygen partial pressure have obvious influence on microstructure, chemistry composition, barrier structure and electrical properties of TiO2 varistor ceramics. In this paper, space charge segregation at grain boundaries and grain boundaries barrier structure in (1)undoped, (2) single donor Nb, (3) single acceptor La, (4) (La,Nb) codoped and (5) (Ce,Nb) codoped Titanium dioxide ceramics were mainly investigated.Firstly, Titanium dioxide samples were prepared by conventional electronic ceramic technology at 1300℃,1350℃,1400 and 1440℃. Microstructure, chemistry composition, crystal structure, ionic valence, thermal groove at grain boundary, grain boudanry structure and elemant segregation of TiO2 ceramics were tested via SEM, EDS, XRD,XPS and EPMA, respectively. The electrical prperties of TiO2 ceramics at different temperature and bias DC voltage were measured. Based on the thermoelectronic emissionat theory, grain boundaries barrier structures (GBBS) of TiO2 samples were calculated. GBBS mainly included barriers heightΦB and barriers width XD.Secondly, under different temperature, distribution of electrostatic potential and defect concentration at TiO2 grain boundaries were studied by point defect thermodynamics method. Grain boundaries segregation driving force and mechanism of space charge in TiO2 ceramics were discussed.Finally, the atomic structure of (310) grain boundary was built. Barrier heightΦB of (310) grain boundary in (1) undoped, (2) single doped and (3) codoped TiO2 ceramics was simulated by combination of first principle and molecular force field. Effects of doped ion, doping sites and concentration on barrier height were researched.The studied results indicated that there exist much pores in undoped TiO2 ceramics sintered at high temperature. The formation of pores in undoped TiO2 is due to segregation, accumulation, diffusion and migration of oxygen vacancy Vo. Oxygen escaped from TiO2 grains transport along grain boundary, and is adsorbed in grain boundary or Triple junctions, and generate intergranular phase with Ca and Si element. Ti4+ gets an electron and is reduced into Ti3+ when oxygen vacancy formatted in undoped TiO2 ceramics. TiTi′is one of ways of compensations for Vo··. XPS showed that some Ti atom enter into interstitial sites and become interstitial titanium of Tii and TiiFor stoichiometric TiO2 and reduced TiO2, distribution of grain boundary electrostatic potential and defects concentration are different. For stoichiometric TiO2 case, titanium vacancy VTi″″, interstitial titanium Tii and oxygen vacancy Vo are primary point defects. The defect formation energy gVTi, gVo and gTi1 of point defect VTi″″, Vo and Tii dominate values of electrostatic potentialΦ(x). For reduced TiO2 case, there are two compensations mechanism of oxygen vacancy Vo and interstitial titanium Tii to satisfy electroneutrality. Grain boundary electrostatic potential and defects concentration are determined by oxygen partial pressure and temperature.For single donor Nb doped TiO2, Niobium (Nb) segregated in TiO2 ceramics, but no secondary phase appears. Point defect NbTi formatted in Nb doped TiO2. When titanium vacancy VTi″″and donor defect NbTi satisfied electroneutrality conditions, electrostatic potentialΦ(x) was decided by Nb doping concentration, titanium vacancy defect formation energy gVTi and sintering temperature. With sintering temperature increasing, electrostatic potentialΦ(x) decreased. With Nb doping concentration increasing, electrostatic potentialΦ(x) increased.For single acceptor La doped TiO2, Lanthanum (La) heavy segregated in TiO2 ceramics and produced secondary phase La4Ti9O24. Electrostatic potentialΦ(x) and point defects concentration was decided by La doping concentration. The driving force of La segregation was elastic strain energy formatted La segregated at grain boundary.For (La,Nb) codoped TiO2 ceramics, Electrostatic potentialΦ(x) and point defects concentration was decided by Nb doping concentration and sintering temperature. La segregation was dominated by elastic strain energy. There exist second phases in (La,Nb) doped TiO2 varistors ceramics. With sintering temperature increasing, second phases will transit. For (Ce4+,Nb) codoped TiO2, Electrostatic potentialΦ(x) and point defects concentration was decided by Nb doping concentration and sintering temperature. Ce4+ segregation was dominated by elastic strain energy. For (Ce3+,Nb) codoped TiO2, ceramics, Electrostatic potentialΦ(x) and point defects concentration was decided by Ce3+, Nb doping concentration and sintering temperature. Ce3+ segregation was dominated by elastic strain energy.The segregation driving force of single La. doped, (La,Nb) codoped and (Ce,Nb) codoped TiO2 ceramics is elastic strain energy while that of single Nb doped TiO2 ceramics is electrostatic potential. Secondary phase of codoped TiO2 ceramics is initiated from nucleation and segregation of doped ions in grain surface or grain boundary plane with higher energy, and then crystal nucleation grow gradually up to second phases in grain surface or grain boundary plane with higher energy.Barrier structure model of codoped TiO2 ceramics is contact between n type semiconducting grain and p type semiconducting grain boundary plane, i.e. n-p-n type. Segregation of doping ions at grain boundary and solid solution in grains has effect on barrier height. Solid solution of donor in grains enhances n type conductivity of TiO2 grains. However segregation of doping ions at grain boundary enhances acceptor feature of grain boundary result in rising of barrier height.The main factors of influencing barrier structure include doping ion radius, doping concentration, sintering temperature, cooling process and nano-TiO2 modification. Barrier structure has directly effect on electrical properties of TiO2 ceramics varistor. TiO2 varistor with better electrical properties can be obtained by controlling grain boundary barrier structure via varying composition and technology of TiO2 ceramics更多还原