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Title: Synthesis and characterisation of secondary phases in zinc oxide varistor ceramics
Author: Harrington, Richard
Awarding Body: University of Durham
Current Institution: Durham University
Date of Award: 2003
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The phase diagrams of Zn7Sb2012 doped with Ni, Cr and Co were determined. In Zn7-xMxSb2012, Ni to substitutes up to x = 4; Co forms a full solid solution range. Cr substitutes up to x = 0.35 in Zn7-4xSb2-2xCr6xOI2. Substituting each metal ion for zinc has the effect of decreasing the a -+ Pphase transition temperature from 1225 °C in undoped Zn7Sb2012. Cr has the most drastic effect; only a small'amount of Cr, ~0.075, is required to stabilise the a-phase at all temperatures. For,no Cr-containing composition of was the phase pure p-polymorph observed, although a small compositional range yielded a mixture of a and Ppolymorphs. In both Ni and Co doped solid-solution series, no Ppolymorph was observed above x = 2. At high cobalt concentrations, a small amount of Co oxidises from C02+ to C03+, implied by the presence of the antimony-rich phase CoSb206. As temperature -is increased, the amount of C03+ decreases; at ~1200 DC, a single phase XRD pattern was observed, corresponding to the phase containing only the reduced form of cobalt, Co2+. The ternary subsolidus phase diagrams ZnO - Sb20s with one of NiO, CoO or Cr203 have been determined. Each consists of a number of solid solution regions, most notably in the Co case, in which five solid solution regions are observed. The rest of the diagrams consist ofa number oftwo- and three-phase compatibility triangles. Up to four Ni ions can substitute for Zn in Zn7Sb2012, each occupying octahedral sites according to the formula (Zn3)tet[Ni4Sb2]octOI2, determined by Rietveld refinement using neutron diffraction data. No evidence was found to suggest the presence of tetrahedral Ni. Cobalt was found to occupy both octahedral and tetrahedral sites. Co has a small preference for octahedral geometries, but the entropy contribution at high temperatures means some tetrahedral occupation becomes favourable. The preferential octahedral site occupancy of Co lead to a negative deviation from linearity in a Vegard's law plot oflattice parameter, a vs composition. Impedance spectroscopy measurements showed that doping Zn7Sb207 with any of the ions studied has the effect of increasing the conductivity by a small amount, although none of the samples are good semiconductors. The density of the pellets of each composition was low, leading to constriction resistance in many of the samples. Cu is observed to substitute for Zn up to x = 4. At low Cu concentrations the p-phase exists, in which Cu is thought to replace Zn on octahedral sites. At 900°C, a new phase, Zn3CU4Sb2012, forms. By use of Visser's algorithm with XRD data, this phase has a monoclinic unit cell, space group Cc and lattice parameters a = 21.01777 A, b = 8.7802 A, c = 5.58475 Aand P= 112.5912°. The structure was solved using direct methods followed by combined Rietveld refinement using XRD and time-of-flight neutron diffraction data. The detailed formula is the same as that of spinel, with Cu occupying the octahedral sites and Zn occupying the tetrahedral sites, according to the formula (Zn3)tet[CU4Sb2]octOI2. The octahedral linkage can be described as a cationdeficient rock salt structure, similar to that of the p-Zn7Sb2012. The tetrahedra share edges, forming infinitely-linked 'columns' parallel to the c-axis. Many ofthe sites are fractionally occupied, leading to more sites containing cations than in spinel.
Supervisor: Not available Sponsor: Not available
Qualification Name: University of Durham, 2003 Qualification Level: Doctoral
EThOS ID:  DOI: Not available