| 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...
|
PDF Full Text Request