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Low-temperature sintering and microstructural development of nanocrystalline Y-TZP powders

Abstract Powders of yttria-doped tetragonal zirconia (3 mol%) with a narrow pore size distribution and ultrafine particle size (~ 9 nm) have been prepared by the mixed organic + inorganic precursors coprecipitation method. The compaction behaviour of almost agglomerate-free calcined powders was studied, and their sintering behaviour using both isothermal and non-isothermal techniques was evaluated. Fully dense nanoscale ceramics with an average grain size below 95 nm were obtained after sintering at 1200 °C for 20 min or at 1150 °C for 4 h. Several stages were identified in the whole densification process: it was found that a particle rearrangement process assisted by sintering pressure is the densification mechanism in the earlier sintering step (up to 800 °C), and grain boundary diffusion was the dominant mechanism for densification up to 1180 °C. Activation energy values of 130 ± 20 and 300 ± 40 kJ mol−1, respectively, were calculated for these densification mechanisms. The almost complete absence of agglomerates in the calcined powders and the homogeneous pore structure of the green compacts are the two main factors leading to lowtemperature fully densified Y-TZP bodies.

Preparation and properties evaluation of zirconia-based/Al2O3 composites as electrolytes for solid oxide fuel cell systems

The powder morphology and particle size of both mixtures of ultrafine yttria-doped zirconia (3 to 8 mol% Y2O3) and aluminium hydroxide (0 to 20 wt% Al2O3), and zirconia/yttria/alumina coprecipitated powders were studied using measurements of surface area, X-ray diffraction, differential thermal analysis, infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy. All the powders were calcined at 800 °C for 10 minutes. Both the growth of zirconia crystallites and its crystallization temperature were strongly influenced by the presence of alumina. The crystallite zirconia growth was inhibited and the zirconia crystallization temperature was increased. This behaviour has been assumed to be due to the formation of a polycondensed chain of the type Zr-O-Al-O-Y-O-Al-O-Zr in which the diffusion distances were considerably augmented.

Low temperature phase equilibria and ordering in the ZrO2-rich region of the system ZrO2-CaO

From co-precipitated powder samples, the solid state reactions occurring between room temperature and 1500° C in the ZrO2-CaO system have been studied. At low temperatures, compositions containing < 25 mol% CaO show a complex picture of phase transformation and ordering in the system. From the obtained results the following singular reactions have been established. (i) Tetragonal zirconia solid solution decomposes eutectoidally at 7 mol% CaO and 1048 ± 4° C into monoclinic zirconia solid solution and calcium zirconate (CZ). (ii) Cubic zirconia solid solution undergoes a eutectoidal decomposition at 17.5 mol% Cao and 1080 ±20° C into tetragonal solid solution + calcium zirconate. (iii) The monoclinic ordered phase, CaZr4O9 (Φ1), ), undergoes an order-disorder transformation into cubic zirconia solid solution at 1232 ± 5° C. (iv) Cubic zirconia solid solution undergoes a eutectoidal decomposition into two ordered phases, Φ1 + Φ2 at 21 mol% CaO and 1200 ± 10°C. (v) Hexagonal ordered phase Ca6Zr19O44 (Φ2) decomposes peritectoidally into cubic zirconia solid solution + calcium zirconate at 1360 ± 10° C. The two ordered phases Φ1 and Φ2 seem to be unstable below ≈ 1100° C. By using DTA, X-ray diffraction and SEM techniques, the extent of the tetragonal and cubic zirconia solid solution fields have been established. From the above experimental results a new tentative phase diagram is given for the ZrO2-rich region of the system, ZrO2-CaO.

Mineralogy, geochemistry and uses of the mordenite–bentonite ash-tuff beds of Los Escullos, Almerı́a, Spain

Abstract The mordenite ore deposit of Los Escullos has a surface area of 106 m2 with an average thickness of 5 m and estimated reserves of 7,500,000 tons of mordenite–bentonite. It is made up of horizontal layers of interbedded epiclastic tuffs with volcanic bentonitised materials which have been subjected to hydromagmatic activity. The layers are essentially composed of bentonite and mordenite with lesser amounts of quartz, cristobalite, biotite, plagioclase, chlorite, amphiboles, titanomagnetite, ilmenite and calcite. The harder layers display a higher proportion of plagioclase crystals and are enriched in Al2O3, CaO, Fe2O3, TiO2, P2O5, Cu, Zn, Co, Cr, Ni and V, while the more altered layers contain larger contents of SiO2, K2O and Y. The amount of sodium increases (from 2% to 4%) relative to depth. Alteration processes resulted in a reduction in the contents of CaO, K2O and MnO and increase in Na2O and MgO. The beds of volcanic ash-tuffs have been devitrified by hydrothermal solutions giving rise to bentonites and sodium- and silica-rich residual fluids which have partly crystallized as mordenite and cristobalite. The raw material (mordenite–bentonite) can be improved removing biotite (magnetic separation) and plagioclase and quartz (by floating methods); however, the mordenite–bentonite mineral assemblage is practically impossible to separate due to the size of the crystals (average 0.5 μm under SEM–EDAX). In turn, this upgraded raw material has very useful properties (total area=520 m2/g and cation exchange capacity=70 meq/100 g) which may make it suitable for use in absorption processes (e.g. deodorization, cationic exchange), catalysis and molecular sieving.

The ZrO2-rich region of the ZrO2-MgO system

The solubility limits of MgO in tetragonal zirconia were studied by combining the differential thermal analysis data and X-ray disappearing phase method. From these experiments a eutectoid reaction, tetragonal ZrO2 solid solution → monoclinic ZrO2 solid solution + MgO, at 1120±10 °C and 1.6±0.2 mol% MgO was established. The solubility of MgO in tetragonal ZrO2 diminished as the temperature increased, and at 1700 °C the solubility was less than 0.5 mol% MgO. The extent of the cubic zirconia solid solution single field was determined by using precise lattice parameter measurements and SEM observations. In this way an invariant eutectoid point, cubic ZrO2 solid solution → tetragonal ZrO2 solid solution + MgO, was located at 1420±10 °C and 14.8±0.5 mol% MgO.

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.

On the metastability phenomena in the ZrO2-CaO system

A knowledge of the stable and metastable phase relationships in zirconia-based systems is very important, because such a phase relation can lead to a better understanding of the phase stability and phase transformation occurring in a particular area of the zirconia-MO (M203) system under study. Many preparation methods (solid-state reaction, sol-gel, skull melting, etc.) can be used to reach the equilibrium conditions for a particular composition in the system. Thus, several factors, such as the thermal history, grain size, homogeneity, annealing conditions and quenching rate, have to be taken into account better to explain the features occurring in a region of the system involving the formation of metastable phases. On the other hand, many techniques (X-ray diffraction (XRD), transmission electron microscopy, neutron diffraction, etc.) can help to elucidate a particular problem present in the zirconia-based system considered. If, in addition to this, the metastability phenomena are present in a short temperature range, then establishing its existence field becomes more complicated. Under such circumstances it becomes very difficult to reach a consensus among the different authors working in the same research field to explain a particular phenomenon observed in the same zirconia-based system. In the ZrO2-rich region of the ZrO2-CaO system, for a temperature range of about 50 °C (between 1050 and 1100°C), and between about 7 and 24 mol % CaO, the following reactions seem to take place. At about 7mo1% CaO and 1048+5°C (eutectoid reaction)

PROCESSING AND PROPERTIES OF MODIFIED-LEAD TITANATE CERAMICS

ABSTRACT Various ways to synthesize modified-lead titanate ceramics such as mixed oxides, coprecipitation and sol-gel are briefly reviewed. It has been demonstrated that the wet-chemical methods allow lower sintering temperatures and a better microstructure control. No strong differences in the Kt/Kp, ratio, were found. Better performances piezoelectric in homogeneous (Ca or lanthanide) modified-lead titanate ceramics have been achieved. From the point of view of an industrial application the coprecipitation processing for the bulk production of these ceramics is suggested.

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.