Marc Onillon

Influence of structural defects on the electrical properties of α-Fe2O3 ceramics

Abstract The electrical properties of α-Fe 2 O 3 ceramics depend strongly upon the nature of the precursor and on the sintering conditions. The ceramics obtained from fresh precursors are characterized by the presence of extended defects, which tend to order as the sintering goes on. The p -type conductivity of those ceramics is mainly due to the presence of cationic impurities. The appearance of localized defects occurs during the sintering period of aged precursors. As these defects are associated to donor states, a predominant n -type conductivity is observed in the corresponding ceramics.

Influence de la pression partielle d'oxygene sur la nature des defauts ponctuels dans Ca2LaFe3O8+x

Abstract The type of point defects in non-stoichiometric Ca2LaFe3O8+x ferrite has been studied by measuring the variation of electrical conductivity with oxygen partial pressure at various temperatures. Several ordering possibilities occur. The oxygen atoms inserted, mostly 1-charged, have a strong tendency to couple at sufficient oxygen pressure and at low temperature. Interstitial oxygen atoms in the lattice vacancies induce structural evolution as non-stoichiometry becomes significant enough.

Influence of the elaboration and dehydration conditions of Fe(III) hydrous oxides on the characteristics of resulting α-Fe2O3 powders

Abstract Fine powders of α-Fe 2 O 3 were obtained from precipitation of Fe 3+ ions in aqueous solution, drying and calcination. After characterization of the intermediate and final products, it appeared that their properties depend strongly on the ageing of the initial precipitate in the solution. For example, all the powders obtained from fresh precipitates are constituted of agglomerated fine particles, whereas those obtained after ageing of the precipitates show large particle sizes, little agglomeration and microstrains. On sintering, the powders obtained from aged precipitates are those with the highest ability to densify. We believe that these differences in morphology and properties originate in the fixation of variable amounts of crystallization water on the hydrous oxides, followed by the formation of micropores, and a subsequent increase in the specific area, when this water is released upon calcination.

Etude par mesures electriques des correlations entre defauts de structure et potentiel chimique de l'oxygene dans WO3 non-stoechiometrique

Abstract The variation of the electrical resistivity of non-stoichiometric WO 3 , as a function of oxygen pressure has been studied at 750°C and in the range of 1 atm ⩾ P O 2 ⩾ 10 −16.6 atm, corresponding to 0 ⩽ x ⩽ 0.04. The stair-like log 10 ρ vs log 10 P O 2 curve shows evidence of the existence of seven Magneli type phases in which the conduction mechanism depends on the shear planes rather than on point defects.

The influence of sintering on the electrical conductivity of the Zn0.99Co0.01O solid solution

Abstract The electrical properties of a phase of composition close to Zn 0.99 Co 0.01 O have been studied on pellets sintered at 1300°C under a pure oxygen stream for various periods of time. Plotting of electrical conductivity vs. temperature shows two domains characterized by two different activation energies. The high temperature activation energy depends on sintering time. The authors suggest that this property could be due to non-statistical distribution of cobalt and evaporation of zinc oxide.

Propriétés électriques du trioxyde de tungstène a proximité de sa composition stoechiométrique. Influence des conditions thermodynamiques de préparation et phénomènes de trempe

Abstract The experimental study of variations in electrical conductivity of WO 3− x vs oxygen partial pressure and temperature shows evidence of interstitial W 6 i · as being the major defect. Under certain conditions, point defects can order themselves in extended defects. The behavior of electrical conductivity can be interpreted in terms of thermodynamic equilibrium between point defects and extended defects, the latter leading to structural modifications beyond a certain departure from stoichiometry. Moreover, WO 3− x shows evidence of quenching phenomena allowing it to keep electrical features characteristic of high temperatures down to room temperature. These properties may explain discrepancies between previous data.

Anisotropy in the electrical properties of zirconium doped α -Fe2O3 single crystals

The electrical properties of zirconium doped α -Fe2O3 single crystals have been investigated from 470 up to 1300 K. As for the pure crystal, an anisotropy appears for the conductivity between the [001] direction and the (001) plane, although lowered by the dopant. Combining the results of electrical conductivity, thermoelectric power and complex impedance measurements, it was established that there was no significant kinetic contribution of the electrons and that their mobility was lower than a few tenths of cm2V−1s−1.

ANISOTROPY OF ELECTRICAL-PROPERTIES IN ALPHA-FE2O3 CERAMICS

Electrical conductivity and thermoelectric power measurements carried out in a heamatite ceramic showed a strong anisotropy in directions normal and parallel to the uniaxial pressing direction. This behaviour is similar to that verified in α-Fe2O3 single crystal. The results suggest that the extended structural defects, generated during sintering, disturb the magnetic order on the (001) planes of α-Fe2O3 and limit the mobility of n type carriers.

Influence of precursor processing and physical characteristics on the microstructure of sintered α-Fe2O3 ceramics

Abstract Iron sesquioxide powders obtained after calcination of aqueous precipitates of Fe(III) ions were compacted and isothermally sintered. When a common sintering process is used, the properties of the ceramics obtained depend upon the aging of the hydrous precipitate in the solution and its calcination temperature. The presence of agglomerates in the powders, together with the existence of microstrains in the individual particles, inhibits the elimination of pores on firing. In the case of fresh precipitates, the nature of the agglomeration in the powders obtained after calcination depends on the calcination temperature itself and is the major parameter controlling the final microstructure of the ceramic. Whereas powders obtained from aged precipitates calcinated above 600°C have a sintering behaviour close to that of the previous ones, those obtained from aged precipitates calcined below 600°C exhibit grain coalescence, which sets a limitation on their initial rate of densification.

Continuous evolution from point defects to shear planes in nonstoichiometric WO3

Abstract Using the results of experimental measurements of electrical conductivity and departure from stoichiometry as functions of temperature and oxygen partial pressure, a model has been derived to explain the nonstoichiometry of tungsten trioxide. According to this model, a continuous ordering of defects occurs when the departure from stoichiometry increases: initial point defects (W 6+ i ) tend to aggregate to form clusters based on a W 4 O 11 pattern. Later on, the grouping of clusters leads to shear planes. To each step corresponds a specific release of oxygen and electrons, and thus characteristic changes in the related macroscopic properties.