Sintering Behavior And Semiconduction Of Boron-doped BaTiO₃at Low Temperature

Boron-doped BaTiO3samples (BaTii-xB2xO3+x) were prepared via the glycol-glycerol complexes of boron, titanium (IV), and barium. It was confirmed that boron was very effective in lowering the sintering temperature of BaTiO3ceramics via liquid phase sintering. With optimized composition, ceramics sintered at900℃showed dielectric properties (room-temperature dielectric constant3960, dielectric strength7.8kV/mm) comparable to those of pristine BaTiO3ceramic sintered at1300℃. The appreciable decrease in sintering temperature was attributed to1) the addition of boron as dopant of BaTiO3, and2) the application of glycol-glycerol solvents/ligands in the synthesis of precursors, which guaranteed the composition homogenity of the final ceramics. The boron dopant permitted adequate grain growth and enhanced the densification process by reacting with BaO to form amorphous Ba-B-O glass at grain boundaries.Reduction of Ti4+to Ti3+was found in boron-doped BaTiO3ceramics when we sintered the boron-doped BaTiO3samples at relatively low temperature(>850℃) in Ar or5%H2/Ar. Such reduction did not occur in pristine BaTiO3ceramic. The methods such as UV-vis spectroscopy, luminescence spectroscopy, and X-ray photoelectron spectroscopy confirmed the reduction by showing the presence of Ti3+in the boron-doped samples. The results of Ti-K-edge X-ray absorption near-edge structure measurement (XANES) indicated that boron doping changed the geometry of Ti-oxygen in BaTiO3to some extent. It was likely that some boron ions stayed at interstitial sites of BaTiO3lattice and acted as donors, which might trigger the reduction of Ti4+to Ti3+. When doped with less amount of boron (<0.23wt%), the reduced ceramics had very low room temperature resistivity (<100Ω·m). However, different from the n-type rare-earth doped BaTiO3, the reduced boron-doped BaTiO3did not display positive temperature coefficient resistance (PTCR) behavior.When properly doped with both boron and rare-earth such as Nd or Sm, BaTiO3ceramics were transformed into semiconductors even sintered in air at900℃. Y was also able to dope the BaTiO3lattice as a donor when the sintering temperature was raised to950℃. Anomalous grain growth was not observed in these samples. As shown by the resistivity-temperature plots, those ceramics still had no PTCR effect. The boron dopant decreased the forming temperature of semiconducting BaTiO3, but at the same time led to the loss of PTCR effect of BaTiO3. Boron doping brought forward puzzling phenomena and new questions related to the semiconduction of BaTiO3.