Transformation of yttria partially stabilized zirconia by low temperature annealing in air

The tetragonal-to-monoclinic phase transformation of yttria partially stabilized zirconia by low temperature annealing in air was investigated in the temperature range 100 to 650° C using a sintered body of zirconia containing 2 to 4 mol% Y₂O₃. The amount of monoclinic phase formed was maximum at about 200° C. Both the decrease in grain size and increase in the yttria concentration were effective in decreasing the critical temperature below which the monoclinic phase was formed. The relationship between the critical temperature (T _c) and the grain size was experimentally determined.     

Destabilization of calcia stabilized zirconia

Calcia stabilized zirconia (CSZ) is destabilized by Al₂O₃, SiO₂ and TiO₂ at elevated temperatures. The product of destabilization, termed partially destabilized zirconia (PDZ) has been found to possess excellent resistance to thermal shock. The thermomechanical properties of PDZ depend, to a great extent, on the degree of destabilization which is the amount (in weight percent) of free ZrO₂ formed as a consequence of the destabilization reaction. The kinetics of the destabilization reaction has been studied by quantitative X-ray powder diffraction technique, and from the kinetic data, the optimum amount of the destabilizer required to get superior quality PDZ has been estimated.     

Strength characterization of MgO-partially stabilized zirconia

The flexural strength of MgO-partially stabilized zirconia was evaluated as a function of temperature (20–1300 °C in air environment), applied stress and time. The indentation-induced-flaw technique did not produce well-defined symmetrical cracks of controlled size, whose length (on the tensile surface) or depth (on the fracture face) can be measured unambiguously, and therefore it should not be used for measuring fracture toughness. The sudden decrease in fracture strength at moderately low temperatures (200–800 °C) is believed to be due to stability of the tetragonal phase and relative decrease in the extent of the stress-induced martensitic phase transformation of the tetragonal to monoclinic phase. Flexural stress rupture testing at 500–800 °C in air indicated the material's susceptibility to time-dependent failure, and outlines safe applied stress levels for a given temperature. Stress rupture testing at 1000 °C and above at low applied stress levels showed bending of specimens, indicating the onset of plasticity or viscous flow of the glassy phase and consequent degradation of material strength.     

Microwave synthesis of yttria stabilized zirconia

Yttria stabilised zirconia (YSZ) nanocrystals, with a mean size between 5 and 10 nm, were prepared by microwave flash synthesis. Flash synthesis was performed in alcoholic solutions of yttrium, zirconium chloride and sodium ethoxide (EtONa) using a microwave autoclave (RAMO system) specially designed by authors. Energy dispersive X-ray analysis (EDX), X-ray powder diffraction (XRD), BET adsorption technique, photon correlation spectroscopy (PCS) transmission and scanning electron microscopy (TEM and SEM) are used to characterized these nanoparticles. Compared with conventional synthesis, nanopowders can be produced in a short period (e.g. 10 s), both high purity and stoechiometric control are obtained. Nevertheless, this mean of production is more cheaper and much faster than the ones commonly used to produce yttria stabilized zirconia (YSZ) by conventional sol-gel techniques.     

Slip casting of partially stabilized zirconia

Casting behaviour and rheological properties are studied in order to define the appropriate conditions under which to prepare slips for the production of high-temperature ceramics. Various commercial powders have been used, which were characterized with respect to morphology, particle size distribution and specific surface area. Zirconia slips with 75 wt% solid content were prepared with distilled water and ethanol as dispersing agent, with and without deflocculant. Hydrochloric acid and tetramethylammonium hydroxide were used to control the pH. Investigations into rheology, i.e. the dependence of viscosity and shear stress on shear rate, were performed. The slip, green and sedimentation bulk densities were measured.     

Strength characterization of yttria-partially stabilized zirconia

The flexural strength of yttria-partially stabilized zirconia was evaluated as a function of temperature (20–1000 °C in air), applied stress and time. The material was susceptible to strength degradation at low temperatures (200 and 300 °C) due to the phase transformation of the tetragonal structure to monoclinic, possibly accompanied by microcracking. In this temperature range, the material was incapable of sustaining low applied stress levels of 276 MPa for any significant duration (> 100 h < 500 h). Stress rupture testing at 600 °C and above identified the onset of viscous flow of the glassy phase and consequent degradation of material strength.     

Electrical conductivity of tetragonal stabilized zirconia

The electrical conductivity change on annealing for tetragonal stabilized zirconia (TZP) was studied with the help of a.c. impedance dispersion analysis techniques. The dependences of the conductivity on annealing time at 1000 ° C and on temperature cycling between room temperature and 1000 ° C were investigated. A decrease in conductivity of about 30% at 1000 ° C of TZP with 3 mol% Y₂O₃ was observed during the first 200 h of annealing at 1000 ° C, and no change was observed during further annealing. A similar result was observed for TZP with 2.9 mol% Sc₂O₃. For TZP with 3.0mol% Yb₂O₃, the conductivity decreased gradually during an annealing time of over 2000 h. The impedance dispersion analysis at lower temperature suggested that the decrease in electrical conductivity by annealing at 1000 ° C could be attributed to the increases of both grain boundary and intragrain resistance. No monoclinic phase was observed for the samples annealed at 1000 ° C for 2000 h. On the other hand, a trace of a monoclinic phase was found for TZP with 3mol% Y₂O₃ after the 50th temperature cycling, but no significant decrease in conductivity was observed with the cycling.     

Microwave annealing of yttria stabilized zirconia ceramics

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

The influence of temperature and loading rate on flow and fracture of partially stabilized zirconia

The flow and fracture behaviour of magnesium stabilized partially stabilized zirconia subject to compressive loading is characterized for a wide range of strain rates and temperatures. It is found that the material exhibits plastic flow from 23 to 1200° C, and that the flow stress curve is serrated. Contrary to results for Al₂O₃, SiC and Si₃N₄, the strain rate dependence of compressive strength for partially stabilized zirconia (PSZ) does not correlate with the stress intensity dependence of subcritical crack growth velocity. These results, combined with the presence of an unusual type of deformation banding, are interpreted in terms of plastic strain-induced, co-operative martensitic transformation of metastable precipitates.     

Solubility of Mn stabilized cubic zirconia nanostructures

Mn-stabilized cubic zirconia nanostructures (Zr_1?xMn_xO₂, x = 0.10, x = 0.15 and x = 0.20) have been prepared using chemical precipitation method for different doping concentrations. The prepared powders have been characterized by X-ray diffractometer for structural analysis, vibrating sample magnetometer for analyzing magnetic properties, scanning electron microcopy–elemental compositional analysis using X-ray for morphological and elemental compositional analysis. Room temperature ferromagnetism has been found at around x = 0.15. For high doping effect of Mn impurity at host lattice, secondary phase has been detected at around x = 0.20 when exceeding the solubility limit of cubic zirconia. Due to ferromagnetic behavior, Mn doped ZrO₂ nanostructures can be considered as good candidates for spintronics applications within their solubility limit.     

Aqueous tape casting of yttria stabilized zirconia

Tape casting process was used to produce yttria stabilized zirconia (YSZ) substrates in an aqueous system with poly(vinylalcohol) (PVA) and glycerine as binder and plasticizer, respectively. Various compositions of YSZ slips with different amounts of PVA and glycerine and consequently different solid/liquid ratios were prepared. The influence of the slip composition on the rheological properties of the slips was studied. In addition, the effect of the slip composition on the properties of the green and sintered tapes was investigated. PVA and glycerine did not affect the dispersion properties of the YSZ powder. Glycerine additions enhanced the flexibility of the green tapes but also produced a decrease in the tensile strength. The increase in the PVA content increased the tensile strength but resulted in a markedly decrease in the green density of the tapes. A correlation between the green and sintered density was found. The anisotropic sintering shrinkage parallel and perpendicular to the casting direction increased with increasing the PVA content. The slip compositions with 5 wt% PVA produced green tapes with satisfactory tensile strength. They had the highest sintered density, the lower sintering shrinkage and the lesser shrinkage anisotropy.     

Synthesis and characterization of low density calcia stabilized zirconia ceramic for high temperature furnace application

Five weight percent calcia stabilized zirconia (CSZ) low density ceramic material was synthesized from zirconia and calcia powder by solid state reaction. Powder specimens were sintered at various temperatures for different time durations in air and argon atmosphere. Physico-chemical properties such as thermal stability, purity, crystalline phases identification, particle size, particle size distribution, specific surface area and porosity of CSZ were measured by thermogravimetry and differential thermal analysis (TG/DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Taller (BET) and particle size analyzer (PSA). Total relative shrinkage and coefficient of thermal expansion were estimated by dilatometry. The density and Vicker's microhardness were measured in order to evaluate the material performance. A combination of cubic zirconia and calcium zirconium oxide (Ca_0.15Zr_0.85O_1.85) phases were found when specimen sintered at 1500 °C in air while under argon atmosphere at 1750 °C and 1850 °C, the ZrO and Ca_0.15Zr_0.85O_1.85 phases were established. At higher sintering temperature (1950 °C) for 4 h in argon medium three phases ZrO, CaZrO₃ and Ca_0.15Zr_0.85O_1.85 were observed. It was found that with the increase of sintering time these phases convert into a single cubic phase Ca_0.15Zr_0.85O_1.85. Pellets sintered at different temperatures and duration were also studied by scanning electron microscopy showing that the grain structure becoming fine with increase of temperature and duration.     

Low temperature vapor-phase preparation of TiO2 nanopowders

The preparation of TiO₂ nanopowders by vapor-phase hydrolysis of TiCl₄ below 600°C is studied in this paper. Influences of preparation variables, such as preparative temperature, residence time, reactant concentration, and H₂O/TiCl₄ mole ratio, on TiO₂ particle size, morphology and chlorine contamination are investigated, followed by discussion. It shows that the hydrolysis temperature exerts greatest influences, while the residence time hardly have impact on product particles below 400°C, among the hydrolysis variables investigated. The chlorine contamination on nanopowders occurs during the preparation which can be greatly reduced by proper control on preparation variables. Unlike the high temperature gas-phase processes such as oxidation route and flame synthesis, low-temperature route shows the ready control on product powders, and thus obtains titania powders with small size, narrow size distribution and very weak agglomeration. In addition, the decreased energy consumption, retarded corrosion on the reactor and the reduced operation problems would be expected for the low temperature processes.     

Effect of dysprosia doping on structural and electrical property of stabilized zirconia for intermediate- temperature SOFCs

Present work deals with structural, micro-structural and electrical properties of dysprosia stabilized zirconia electrolyte, which have been evaluated by means of X-ray diffraction (XRD) and scanning (SEM), and complex impedance analysis respectively. The amount of dysprosia was varied from 2 to 12 mol% in zirconia. The addition of dysprosia (8-12 mol%) stabilized the cubic zirconia phase, which was analyzed from XRD analysis. SEM micrographs clearly showed the improvement in sinterability with increase in dysprosia concentration up to 6 mol% disprosia. Complex impedance analysis was performed in the temperature range from 250 to 600 °C. The results indicated a gradual decrease in impedance of both bulk and grain boundary and increase in conductivity with dysprosia doping up to 6 mol% and thereafter showing a reverse trend. The activation energies for oxygen ion migration were also found to decrease with increase in dysprosia doping which was in the range of 0.99 ? 1.28 eV. The average thermal expansion coefficient increases linearly.