In order to gain further understanding about the onset mechanisms of flash sintering, the electrical properties of cubic calcia-stabilised zirconia, CSZ (CaxZr1-xO2-x: 0.12≤x≤0.18) and Mg-doped alumina (0.05wt%MgO-Al2O3 and 0.5wt%(MgO/SiO2)- Al2O3) ceramics were investigated using impedance spectroscopy as a function of temperature, oxygen partial pressure and applied dc bias. Equivalent circuit analysis was carried out to identify the electrical microstructure of the materials. As part of this process, the relative importance of individual elements to the overall impedance response was highlighted by a new approach by the deconvolution of the impedance spectra.
The bulk, grain boundary and electrode contact impedances of CSZ ceramics were characterised over the temperature range 200-930°C. The most appropriate equivalent circuit to characterise the bulk response required inclusion of a dielectric component, represented by a series RC element, in parallel with the oxide ion conductivity represented by a parallel combination of a resistance, capacitance and constant phase element. The dielectric component may be attributed to defect complexes involving CaZr’’–VO •• pairs
whereas long range conduction involves single oxygen vacancies. The activation energy for the dielectric component increased with increasing x. This was attributed to an increase in the number of pairs or to their coalescence into larger aggregates, causing greater difficulty in reorientation of the dipoles. The sample-electrode interface response was deconvoluted into two components, the charge transfer (Rct) and diffusion resistance (Rdiff), where Rct << Rdiff.
Impedance analysis of CSZ ceramics under different atmospheres showed that the limits of the electrolytic domain vary with temperature and composition. Around 400°C, the bulk conductivity of all compositions remained independent of pO2 and the samples were pure oxide ion conductors, but near 930°C, x=0.12 showed introduction of n-type conduction and x=0.18 showed p-type whereas the intermediate composition x=0.15 did not show any pO2 dependence.
The total resistance of CSZ ceramics decreased with application of a dc bias. This was attributed to the introduction of p-type conductivity. Electron holes might be located on under-bonded oxygen ions, associated either with the Ca dopant ions, or those near the ceramic surface, resulting in a mixed ionic-electronic conduction pathway.
A flash sintering set-up was built. Flash sintering experiments on CSZ x=0.15 showed increased conductivity and emission of light, but no sample densification. It was concluded that the sintering process can be separated from the emission of light and increase in conductivity and therefore, that the origin of these phenomena, ie flash and sintering, might be different.
Impedance measurements on two Mg-doped alumina ceramics, over the temperature range 400-910°C, showed a combination of low-level oxide ion conductivity and p-type electronic conductivity, depending on temperature, oxygen partial pressure and dopant content. The oxide ion conductivity was attributed to oxygen vacancies, introduced as charge compensation for the Mg dopant. The p-type conductivity was attributed to hole location on under-bonded oxide ions and was identified as the dominant conduction pathway for measurements in air at high temperatures. The sample resistance increased on application of a dc bias. This was attributed to electron-hole recombination and the loss of p-type conductivity associated
with oxygen loss.
ORCID: https://orcid.org/0000-0002-6950-1255
(2020)
Synthesis and Electrical Characterization of Calcia-stabilised Zirconia and Mg-doped Alumina.
PhD thesis, University of Sheffield.
