Marija Trontelj

Sintering and microstructural development of metal oxide varistor ceramics

Anomalous retardation of the sintering kinetics of ZnO with addition of Bi[sub 4]Ti[sub 3]O[sub 12] (BIT) and a low voltage varistor composition containing ZnO, BIT, Co[sub 3]O[sub 4] and Mn[sub 2]O[sub 3] in the temperature interval between 780 and 850 C was investigated. On the basis of transmission electron microscopic analysis of diffusion couples the retardation of sintering kinetics was ascribed to surface diffusion of Bi and Ti along ZnO grain surfaces. Liquid phase formation resulting from chemical reactions between the components triggers exaggerated grain growth. Nucleation and growth of anomalous grains is influenced by the presence of inversion boundaries in ZnO grains.

Preparation and properties of ceramic sensor elements based on MgCr2O4

Using TiO2, ZrO2 and SnO2 as additives, the intrinsic electrical resistivity-humidity characteristics of MgCr2O4 material used for humidity sensors is varied. By choosing a particle size fraction of the starting powders with a radius less than or equal to 1 μm and varying the sintering temperature from 1200 °C to 1500 °C, the sintered porosity and pore-size distribution of the sensor ceramic elements are controlled. By macerating the ceramic elements with solutions of various salts containing CO32− ions, the sensitivity of the impedance of the sensor elements to CO2 in air is diminished.

The Bi24CoO37 compound and the Bi2O3CoO system

Abstract The binary system Bi 2 O 3 CoO has been studied by X-ray diffraction, thermal analysis and metallographic techniques. A b.c.c. sillenite-type compound was detected at a Bi 2 O 3 :CoO ratio of 12:1. It melts incongruently at 790°C. The Bi 24 CoO 37 compound was synthesized from Bi 2 O 3 and CoO at temperatures above 660°C under a protective atmosphere and remained stable after prolonged heating (96 h) at 780°C. The eutectic in the Bi 2 O 3 CoO system was found to be at 720°C and at 15 mol.% CoO.

Microstructure and chemical reactions in the PFN-PFW-PZN system

Microstructure, phase compositions and reaction mechanisms were studied in the solid solution system of perovskites composed of 0.48 PFN-0.36 PFW-0.16 PZN as the function of sintering temperature and time. At temperatures above 750° C, perovskite grains were partly decomposed to zinc ferrite and a liquid phase with a composition close to Pb2WO5. Consequently, the perovskite solid solution was enriched in the PFN component. At longer sintering times large pyrochlore grains appeared, presumably due to PbO evaporation from the liquid phase.

Sintered CaSO4: Dy TL dosimeters

Abstract Hard and durable pellets were prepared by sintering CaSO4 and Dy2O3 with the addition of an inorganic binder. By this method crystallization from H2SO4 is avoided and thus introduction of impurities from the acid is eliminated. Glow curves of dosimeters show a small peak at 150°C and high peaks at 200°C and 300°C. Comparison of this glow curve with the glow curve of dosimeters made by crystallization shows that the ratio of the 200-150°C peaks is much in favour of the sintered dosimeters.

Correlation between microstructure and electrical conductivity in Pb(Fe12Nb12)O3Pb(Fe23W13)O3Pb(Zn13Nb23O3 relaxor ferroelectrics

Abstract The contribution of various phases to the electrical conductivity of dielectric materials, prepared at different firing conditions from a solid solution of Pb ( Fe 1 2 Nb 1 2 ) O 3  Pb ( Zn 1 3 Nd 2 3 ) O 3 ( PFN-PFW-PZN ) perovskites, was determined by the analysis of microstructure, d.c. conductivity and impedance spectroscopy. In monophosphase samples composed of perovskite grains, a single slope in the Arrhenius plot (d.c. measurements) which corresponds to an activation energy of around 1 eV (depending on the chemical composition) and a single semicircle in the impedance spectrum were found. In polyphase samples two slopes in the Arrhenius plot were observed, indicating two conduction mechanisms with approximate activation energies of 0.6 and 1 eV. In the impedance plane two thermally active semicircles were noticed with activation energies of 0.7 and 1 eV. It was shown that in the samples with lead-tungstenate liquid phase present, this phase was the main contribution to the conductivity at temperatures up to 300°C, while the perovskite grain contribution dominates the conductivity above this temperature. The electron-hopping conduction mechanism between Fe2+ and Fe3+ ions at the B sites of the perovskite structure was assumed to be the principal mechanism of perovskite grain conductivity. Based on the results from impedance measurements, it was concluded that the leadtungstenate phase, which was distributed between the perovskite grains, was not continuous.

TEM investigations in the PFN-PFW-PZN perovskite system

Using an analytical electron microscope, the distribution of phases and order-disorder phenomena in the Pb(Fe1/2Nb1/2)O3-Pb(Fe2/3W1/3)O3-Pb(Zn1/3Nb2/3)O3 (PFN-PFW-PZN) perovskite system were examined. It was found that after sintering the solid solution, small cubic pyrochlore particles with chemical composition close to Pb(Fe0.28Zn0.01Nb0.50W0.21)O3.4 anda0=1.05 nm were present in the 0.8 mol % Pb2WO5-0.2 mol % PbO liquid phase. In samples fired at 1000 °C, several 10 nm sized ordered areas with faint F spots in the selected-area electron diffraction pattern and (h+1/2k+1/2l+1/2) lattice fringes were found. Similar features were found in the PZN perovskite monocrystal, leading to the conclusion that local 1∶1 ordering of cations (presumably Zn2+ and Nb5+) in the PFN-PFW-PZN system caused partial decomposition of the perovskite phase at firing temperatures equal to or higher than 850 °C.

Influence of Chemical Composition on the Barrier Height in ZnO Varistors

ZnO-based varistors exhibit highly nonlinear electrical characteristics, which are due to the electrical properties of the grain boundary regions.