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Title: Synthesis and electrical characterisation of lanthanum doped barium titanate
Author: Morrison, Finlay D.
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 1999
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Lanthanum is one of the most common so-called "donor-dopants" for BaTiO 3. Combined synthesis, phase diagram and electrical studies of La-doped BaTiO3 ceramics were undertaken to determine the doping mechanisms(s) and to understand the unusual resistance behaviour of air-heated ceramics. Charge compensation was found to be maintained via the creation of Ti vacancies, resulting in the solid solution Ba1-xLax Ti1-x/4O3 with x ≤ 0.25 at 1350[Special character omitted]C. No evidence for the existence of the commonly cited La-"donor-doping" or barium vacancy mechanisms was observed. ac Impedance spectroscopy on samples heated in O 2 at 1350[Special character omitted]C indicated the presence of insulating bulk and grain boundary components. The tetragonal cubic and orthorhombic tetragonal phase transition temperatures, Tc and To/t, respectively, decreased dramatically with increasing x, the former approximately linearly at a rate of dTc/dx ca. 0.24 [Special character omitted]C/x. The value of 'max at T c increased with x for 0 ≤ x ≤ 0.06 and reached a value > 25,000 at ca. -9 [Special character omitted]C. For x ≥ 0.08, Tc and To/t coalesce and, with increasing x, ' max decreased and "relaxor"-type, frequency-dependent ' behaviour was observed. The semiconductivity observed in air-heated samples was attributed to oxygen loss. Combined impedance and modulus spectroscopy of samples heated in air showed all samples to be electrically inhomogenous, however, all contained a semiconducting bulk component with an associated resistance < ca. 20O. The total dc sample resistance was dependent on the level of re-oxidation during cooling and was dominated by insulating thin layer regions. Two oxidised thin layer effects were identified. At low La concentrations, e.g. x = 0.03, re-oxidation occurred at the grain boundaries and on individual grain surfaces whereas the grain interiors remained semiconducting. At higher La concentrations, e.g. x = 0.20, re-oxidation occurred preferentially at pellet surfaces whereas the pellet interior remained semiconducting. The observed room temperature resistance anomaly for air-heated La-doped ceramics is not linked to La-"donor-doping" but is an artefact of sample processing which results from oxygen loss during heating to high temperatures, > 1200 [Special character omitted]C.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available