C. Pascual

A new tentative phase equilibrium diagram for the ZrO2-CeO2 system in air

The phase relationships over a wide range of temperature and compositions in the ZrO2-CeO2 system have been reinvestigated. From DTA results, thermal expansion measurements andKIC determinations it was established that additions of CeO2 to ZrO2 decreases the monoclinic to tetragonal ZrO2 transition temperature, from 990 ° C to 150 ∓ 50 ° C, and an invariant eutectoid point at approximately 15 mol% CeO2 exists. The extent of the different single- and two-phase fields were determined with precise lattice parameter measurements on quenched samples. Evidence for the existence of a binary compound Ce2Zr3O10 (ø-phase) was obtained by X-ray diffraction. The ø-phase was stable below approximately 800 ° C, above which it decomposes into tetragonal zirconia + fluorite ceria solid solutions. Taking into account the polymorphic tetragonal-cubic transition and the narrowness of the two-phase tetragonal zirconia + fluorite ceria field above 2000 ° C, the existence of a new invariant eutectoid point was assumed, in which the metastable fluorite zirconia solid solution decomposes into tetragonal zirconia + fluorite ceria solid solutions. From the results obtained, the phase diagram also incorporates a eutectic point located at approximately 2300 ° C and 24 mol % CeO2.

Phase equilibria and ordering in the system HfO2-Yb2O3

The system HfO2-Yb2O3 was investigated in the 0 to 100 mol % Yb2O3 range using X-ray diffraction analysis, linear thermal expansion measurements and melting point studies. At high temperatures, the system is dominated by wide regions of solid solutions based on HfO2 and Yb2O3 separated by a two-phase field which appears to extend to the solidus. The extent of the cubic hafnia and ytterbia C-type solid solution fields was established using the precision lattice parameter method. At low temperature (< 1800° C) two ordered phases were found in the system, one at 40 mol % ytterbia with ideal formula Yb4Hf3O12, and another at ∼ 70 mol % ytterbia with formula Yb6HfO11. Four eutectoid reactions and a peritectic reaction cubic ytterbia solid solution → cubic hafnia solid solution + liquid at ∼ 67 mol % and ∼ 2380° C have been established in the system. By incorporating the known tetragonal-cubic hafnia and C-type-hexagonal ytterbia transition temperatures, and the melting points data in the system, a tentative phase diagram is given for the system HfO2-Yb2O3.

Y(Er)-doped tetragonal zirconia polycrystalline solid electrolyte: Part 3 Electrical properties

The electrical conductivity of fully dense bodies of polycrystalline tetragonal zirconia (TZP) in the composition range 2 to 3 mol % Y2O3 (Er2O3) was measured. Throughout this work, a.c. impedance complex plane analysis was used. From this method grain-interior and grain-boundary conductivity contributions were obtained separately. Plots of conductivity against reciprocal temperature of both contributions were evaluated. An electrode-electrolyte interphase conductivity contribution was detected and considered. The influence of impurities, and ageing behaviour was also studied. The activation, migration and association energies were estimated and discussed on the basis of up-to-date theoretical and structural information.

Y(Er)-doped tetragonal zirconia polycrystalline solid electrolyte: Part 1 Powder processing

Submicronized Y(Er)-doped zirconia powders were prepared by the hydroxide-gel precipitation method. The agglomeration state and surface of both precursors and calcined oxides are studied and are related to the compaction and sintering behaviour. The compaction response was different for the two doped zirconias which was related to the different strength of aggregates present in the powders. Although a different breakpoint in the compaction curve for each of the powders was found, the sintering behaviour was quite similar which indicates that at high compaction pressures all the aggregates were broken up and uniformly distributed in a regular packing of very fine particles. Densification during sintering seems to be governed by both the high surface activity of the powders as the driving force (below 1400° C) and the grain growth above that temperature: the shrinkage behaviour was in close agreement with the isothermal experiment results.

Electrical properties of Cr2O3-containing glasses

Abstract Glass compositions of SiO 2 MgOLi 2 OCr 2 O 3 and SiO 2 CaOLi 2 OCr 2 O 3 systems have been prepared by melting in a platinum crucible and induction heating at 1500°C. The glass transition and crystallization temperatures were estimated by D.T.A. and electrical d.c. conductivity, dynamic measurements at a heating rate of 10°C/min. The electrical conductivity data have been used to describe the thermal evolution of glass structure. It can also be correlated with the conduction mechanism probably governed by the Cr 2 O 3 content of the glass compositions. Thermally stimulated polarization techniques have been applied to elucidate the nature of the charge carrier and conduction process. The influence of the modifier ion, MO, on the crystallization and electrical properties of the glasses is discussed also.