| Summary: | 博士 === 國立中山大學 === 材料科學研究所 === 87 === Pressureless-sintering of non-stoichiometric barium titanate (BaTiO3) powder of TiO2- and BaO-excess compositions has been investigated using both conventional furnace and C02-laser. Both donor- and acceptor-doping are studied for their effect on sintering kinetics, microstructure and the resultant semiconductivity of the sintered ceramic. Crystalline phases are analysed by X-ray diffractometry (XRD). Attention has been paid to the analysis of the corresponding sintered microstructure by adopting optical microscopy (OM), scanning, and transmission electron microscopy (SEM and TEM).
Constant-heating rate (CHR) sintering technique appears unsuitable for determining the activation enthalpy (AH) of densification in BaTiO3. A considerable span of DH = 392 - 522 kJ-mol-1 is also found from isothermal pressureless-sintering of TiO2-excess compositions. High-resolution TEM reveals that SiO2-containing glass formed due to the trace impurities associated with the initial powders at T <1332oC (lowest eutectic temperature of BaTiO3-Ba6Til7O40) is responsible for the scattering of AH values. Its formation has changed the sintering mechanism from solid-state to liquid phase.
Second-phase of Ba6Til7O40 has been identified ambiguously in sintered TiO2-excess compositions. It is formed at low temperatures (of <1332oC) by reacting the excessive TiO2 or exsolved TiO2 with BaTiO3 in the solid-state upon heating. Characteristic microstructural feature of the solid state-reacted Ba6Til7O40 has been identified. Eutectic liquid is subsequently generated at T>1332oC from BaTiO3-Ba6Til7O40- Upon cooling, the eutectic liquid solidifies again to Ba6Til7O40 which nucleating on BaTiO3 grain surface to develop characteristic lamellar structure and crystallographic orientation relationships. The lamellar microstructural characteristic disappears upon long-term annealing of laser-sintered discs when polytitanates have also homogenized to become Ba6Til7O40. It is evdient that Ba6Til7O40 is the most stable polytitanate phase at room temperature.
"Third-phases" of La2Ti2O7 and Y2Ti2O7 have also been identified by selected-area diffraction patterns (SADP) in La2O3- and Y203-doped compositions, respectively. It is indicative that for donor-concentration exceeding the grain-growth inhibition threshold (GGIT), apart from the increased densification/coarsening (dr/dG) ratio, grain growth is also hindered by a second-phase pinning mechanism. The effect of these inhibition mechanisms is largely diminished once when eutectic liquid of BaTiO3-Ba6Til7O40 is formed. De-sintering associated with eutectic liquid formation is deferred to higher temperatures of T >13320C since the eutectic compositions have been altered to BaTiO3-Ba6Til7O40-(third-phase). Electrical conductivity measurement also suggests that the substitution of A- site byy3+-cation occurs at low Y203-doping levels, but progressively changes to B-site at doping levels exceeding 0.65mol%. Charge compensation mechanism depending upon doping level and temperature can be divided into three schemes of, (1) vacancy, (2) mixed, and (3) electron regime as suggested by semiconductivity of quenched samples from laser-sintering.
Third-phases of MgTiO3, and Na2TiO4 and Na2Ti2O9 have also been identified in MgO- and Na2O-acceptor-doped compositions, respectively. Crystallographic orientation relationships Of [110]BT//[100]N4T, (11l)BT/ / (010)N4T, (112)BT/ / (001)N4T are also determined for Na2TiO4 located intragranularly in BaTiO3 grains. It is also indicative that oxygen vacancies generated by acceptor-doping has resulted to the metastable retention of high temperature hexagonal-BaTiO3 to room temperature.
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