Donor doped barium titanate ceramics are well known for their Positive Temperature Coefficient of Resistance (PTCR) characteristic above the crystallographic transition temperature, T° '130°C, where the material changes from the ferroelectric state to paraelectric. The shape and magnitude of the PTCR characteristic are known to be dependent on the composition and preparation of the ceramic, the presence of impurities, particularly donor dopant concentration and acceptor ions, and the sintering conditions. Thirty years ago Heywang proposed a model based on the presence of two-dimensional resistive grain boundary layers consisting of discrete electron traps located in energy between the conduction and valence bands, to explain the PTCR effect. Donor doped barium titanate samples were prepared in a number of different ways: the variation of donor concentration, the addition of impurity acceptor ions, reduction of the sintering temperature and variation of the sintering atmosphere. These samples were investigated by examining their microstructure and their electric and dielectric properties, both at room temperature and above the transition. Theoretical analysis of the experimental results, based on the Heywang model, was then performed to investigate the effects of preparation on the grain boundary layer characteristics. Resistivity - temperature measurements were carried out to find the effect of composition and sintering conditions on the PTCR characteristic and capacitance - temperature measurements demonstrated the effects of donor and acceptor incorporation on the dielectric properties of barium titanate. Grain boundary and grain bulk resistance were separated by means of a. c. impedance methods at room temperature, where the effects of composition and sintering on each were observed. Finally, current - voltage measurements between TT and the resistivity maximum were made for samples containing different donor concentrations, to examine the current conduction mechanism. Detailed analysis of the electric and dielectric measurements permitted the effects of composition and sintering on the grain boundary layer characteristics to be determined. Acceptor state densities were estimated using the resistivity - temperature measurements and capacitance - temperature results, between TT and the resistivity maximum. Resistivity - temperature measurements above the maximum enabled acceptor energies to be estimated. Analysis of the dielectric properties showed that neither the composition nor sintering atmosphere affected the dielectric properties of the grain boundary layers, which were found to obey the Curie-Weiss law above the transition temperature in the same way as the grain bulk. The observed effects of the changes in the preparative conditions to the electric and dielectric properties were explained in terms of the Heywang model and microstructural development, resulting from modifications to the grain boundary layers. The conduction mechanism was examined by means of current - voltage measurements above the transition temperature and below the resistivity maximum. In contrast to the prediction of Heywang. this was found to be predominantly diffusion limited.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:291317 |
| Date | January 1990 |
| Creators | Illingsworth, J. S. |
| Publisher | Durham University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://etheses.dur.ac.uk/1118/ |
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