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.     

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.     

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.     

Optical properties of transparent nanocrystalline yttria stabilized zirconia

The optical properties of transparent nanocrystalline zirconia produced using a current activated method were characterized over the entire visible spectrum. The resolutions of the samples were characterized using standard resolution targets. All of the samples produced were found to have as high a resolution as detectable from the test, i.e., they are transparent not translucent. Transmission, reflectance, and absorption coefficients are reported for various wavelengths. The absorption coefficients were found to be highly dependent on processing time. Annealing experiments helped determine that oxygen vacancies (with free electrons) are the primary absorption centers in the visible wavelengths. In addition it was found that grain boundary cores or their associated defects do not contribute significantly to light absorption in the visible range. The lack of an influence of the grain boundary regions is discussed in terms of low oxygen vacancy concentration in the grain boundary space charge layer.     

Fracture resistance of 8 mol% yttria stabilized zirconia

Anin situ technique for the assessment of fracture resistance employing double cantilever beam (DCB) specimens was developed in the present study. The side-grooved DCB specimens were loaded with pure bending moments in a specially designed and fabricated test fixture which went inside the specimen chamber of a scanning electron microscope. The study as conducted on a 8 mol% fully stabilized cubic phase yttria (Y₂O₃) stabilized zirconia (YSZ) ceramic. The powder processed sheets were sintered at 1600°C for 2 h in a zirconia tube furnace. The mode I applied energy release rate, G_I was determined for both pure YSZ and treated YSZ. Two sets of experiments were conducted for the complete characterization of the ceramics. Three fracture toughness values were determined for the pure and treated ceramics, viz. (i) at the onset of the crack initiation,G _ic, (ii) at the arrest of a subcritical crack, G_ia and (iii) at the onset of the fast fracture,G _if. Two analyses of the experimental data were carried out, viz. method of extrapolation and statistical analysis. In case of the pure YSZ, a transgranular mode of the stable crack growth was identified to be predominant. The porous coating treatment appeared to have positive effects as the crack initiation resistance increased due to electrode layers. The stable crack growth behaviours of the ceramics were investigated by monitoring the crack growth velocity as a function of appliedG values. The results obtained were of direct significance in designing and fabrication of SOFC stacks.     

Zr surface diffusion in tetragonal yttria stabilized zirconia

The value for surface diffusivity of Zr tetragonal ZrO₂–3mol% Y₂O₃ has been calculated from measurements of surface area reduction and pore growth in powder compacts during sintering. The surface diffusivity thereby obtained can be described by D =5.52×10⁵ exp[–531(kJ mol^-1)/RT] m²/s, which is in reasonable agreement with values calculated by prior researchers from direct TEM observation of neck growth between touching particles.     

Control of the tetragonal-to-monoclinic phase transformation of yttria partially stabilized zirconia in hot water

The tetragonal-to-monoclinic phase transformation of yttria partially stabilized zirconia caused by annealing in hot water was investigated in the temperature range 80 to 200° C using sintered bodies in zirconia containing 2, 3 and 4 mol % Y₂O₃. Three approaches, alloying ZrO₂(Y₂O₃) with 0 to 20wt% CeO₂, dispersing 0 to 40 wt % Al₂O₃ into ZrO₂(Y₂O₃) ceramics and decreasing the grain size of zirconia, were examined to inhibit the tetragonal-to-monoclinic phase transformation. The amount of monoclinic phase formed decreased with increasing concentrations of CeO₂ alloyed and Al₂O₃ dispersed, and with decreasing grain size of zirconia.     

Time-dependent tetragonal to monoclinic transition in hot-pressed zirconia stabilized with 2 mol % yttria

The kinetics of isothermal transformation from tetragonal (T) to monoclinic (M) structure in zirconia stabilized with 2 mol% yttria was systematically studied using thermal expansion analysis, electron probe microanalysis, X-ray diffractometry and transmission electron microscopy. Results showed that tetragonal phase obtained from rapid cooling transforms isothermally to monoclinic phase during holding at 200–400C for 4 h. The time-temperature-transformation (TTT) curve has a characteristic C-shape, with 300C as the nose temperature. A small amount of M phase can be detected when the holding temperature is not in the preceding temperature range. The isothermal transformation kinetics can be expressed in terms of the Johnson-Mehl-Avrami equation in which the nucleation and growth of new phase are both functions of time with the activation energy determined to be 28.67 KJ mol^–1. The metastability of T phase is associated with cooling rate, stabilizer content, as well as grain size.In situ observation of M phase propagation induced by the electron beam reveals the T M transition to be diffusion controlled.     

Plasma spraying of nanostructured partially yttria stabilized zirconia powders

Nanosized partially yttria stabilized zirconia particles, prepared using a co-precipitation method, were reprocessed into agglomerate powders using two methods for plasma spraying. The first method was to make micrometer-sized agglomerates directly following the grinding of the calcined yttria–zirconia agglomerates. The second method was to reconstitute the nanosized particles into micrometer agglomerates using spray drying. The deposition efficiency, porosity, microhardness and average grain size of the deposits made from these two reprocessed powders were studied. Distinct results related to the process parameters were obtained for the two types of powders. The second type of powder was more suitable for plasma spraying than the first one. Using the second type of powder, some unique results distinguished from those of the conventional partially yttria stabilized zirconia powders were observed and an optimized coating with a porosity of 3.8%, Hv_0.3 of 953 and mainly consisting of 1–3 μm columnar grains in the columnar direction and smaller than 100 nm in their cross-sections was achieved.     

Contact damage in an yttria stabilized zirconia: Implications

This paper presents the results of a combined experimental and computational study of contact damage in a 3 mole% yttria partially stabilized zirconia (3-YSZ) that is relevant to hip implants and dental restorations. Contact-induced loading in real applications is idealized using Hertzian contact model to explain plasticity phenomena and failure mechanisms observed under monotonic and cyclic loading. Under monotonic loading, the elastic moduli increase with increasing loading levels. Under cyclic loading, the ceramic specimens fail with progressive cone cracking. X-ray analyses reveal that stress-induced phase transformation (from tetragonal to monoclinic phases) occurs under cyclic contact loading above the critical load levels (~8.5 kN). Furthermore, when the cyclic loading level (5.0 kN) is less than a critical load levels (7.5 kN) that is required to induce surface cone cracks, significant plastic damage is observed in the subsurface zone underneath the contact area. These suggest that the cyclic contact loading induce both plastic damage and tetragonalto-monoclinic phase transformation in the 3-YSZ, leading to significant degradation in long-term strength. The implications of the results are discussed for the design of zirconia femoral heads in total hip replacements and zirconia crowns in dental restoration.     

Electron spin resonance in single crystal yttria stabilized zirconia

Electron spin resonance has been observed at 35 GHz in 8 and 12 mol % yttria stabilized zirconia single crystals before and after blackening by current passage. Measurements were made between 293 and 77K. In both as-grown and blackened crystals the spectra showed anisotropic lines (type A) characterized by g =2.003±0.002 and g =1.880 ±0.002 with respect to a [1 1 1] symmetry axis and a broader, slightly anisotropic line (type B) centred near g=1.993. In current blackened crystals a weak isotropic line (type C) was also found near g=1.986. The type A lines are attributed to a charged complex formed by an electron trapped at an oxygen vacancy and associated with an yttrium ion. Tentative models are suggested to explain the type-B and type-C lines.     

Role of oxygen phonons in the martensitic phase transformation of yttria stabilized tetragonal zirconia polycrystals

Abstracts are not published in this journal     

Transformation toughening of NiAl composites reinforced with yttria partially stabilized zirconia particles

The paper presents the results of a combined experimental and analytical study of transformation toughening in NiAl composites reinforced with 20 vol. % of 2 mole % yttria stabilized zirconia particles. The extent of stress-induced phase transformation is characterized using laser Raman spectroscopy techniques. The overall toughening increment due to stress-induced transformation is also predicted using micromechanics models that account for the role of dilatation only, or the combined effects of dilatation and shear.     

氧化钇稳定二氧化锆(YSZ)微波烧结陶瓷的复阻抗谱分析

研究了8mol%Y2O3掺杂ZrO2(8YSZ)材料微波烧结陶瓷在300～850℃温度范围内的交流复阻抗谱,获得了该材料的温度-离子电导率曲线,并与常规烧结的陶瓷体进行了比较.结果发现8YSZ的微波烧结陶瓷的晶界势垒在550℃被击穿,常规烧结陶瓷的晶界势垒在500℃被击穿.击穿后晶界电阻消失,离子电导率的变化主要由晶粒电导率的变化决定.在击穿温度点以下,陶瓷体的离子电导率随温度的升高呈波浪式上升,即曲线呈上升～下降～上升趋势.     

Microstructure evolution and electrical properties of yttria and magnesia stabilized zirconia

The paper contains the results of microstructure and chemical composition analyses of micrograins, which were formed on initial grain boundaries in ZrO₂-Y₂O₃ and ZrO₂-Y₂O₃-MgO systems. It has been found that the micrograins appear in the process of diffusion induced by grain boundary migration (DIGM). The observed processes can be described as both liquid film migration (LFM) and chemically induced grain boundary migration (CIGM). New micrograins had an increased content of Y₂O₃ and a cubic symmetry. Zirconia-yttria solid solutions with magnesia particulate addition showed an increased amount of migration nuclei and bigger size of new grains. However, no change in the chemical composition of the grains has been detected. The ionic conductivity measurements have shown that the activation energy (E_a) of conductivity at lower temperatures is connected to a DIGM-like process and to the distance of grain boundary migration. In the case of materials with dominating LFM process an increased grain boundary migration distance leads to a lowering of the activation energy of conductivity. Contrary to that, in the materials with dominating CIGM process an increase of migration zone causes increase of E_a values. The data obtained with respect to the type of DIGM process (LFM or CIGM) indicate that the grain boundary conductivity contribution increases with the CIGM distance.     

Fabrication of yttria stabilized zirconia coatings by a novel slurry method

By adopting a ‘bricks and mortar’ approach, yttria stabilized zirconia (YSZ) coatings were produced on Fecralloy substrates by a novel slurry method. The slurry contained large preformed particles of 10–60 μm, as the ‘bricks’, and concentrated nano-particle slurry, as the ‘mortar’. Green coatings were prepared by spreading the slurry on substrates. Then sintering at 1200 °C was carried out to produce coatings with grain size up to 200 nm. The presence of larger preformed particles hindered the shrinkage of the coatings during drying and sintering. It was observed that the ‘mortar’ could form inter-preformed particle ‘bridging’, an underlying mechanism for the formation of ‘cemented’ preformed particle network. By using different preformed particles, different micro-architectures were obtained and reduced thermal diffusivities were achieved. The microstructure and density of the coatings can also be modified by infiltration of the nano-slurry.     

Composite ceramics thermal barrier coatings of yttria stabilized zirconia for aero-engines

Composite ceramics thermal barrier coatings(TBCs) are widely used in the aero-engines field due to their excellent thermal insulation, which improves the service life and durability of the inherent hot components. The most typical, successful and widely used TBCs material is yttria stabilized zirconia(YSZ). In this paper, fabrication methods, coating structures, materials, failure mechanism and major challenges of YSZ TBCs are introduced and reviewed. The research tendency is put forward as well. This review provides a good understanding of the YSZ TBCs and inspires researchers to discover versatile ideas to improve the TBCs systems.     

Structure of yttria stabilized zirconia beads produced by gel supported precipitation

Yttria stabilized zirconia (YSZ) is one of the inert matrix candidates selected for investigation as host matrix for minor actinide (MA) transmutation. The structural properties of (Zr_0.84, Y_0.16)O_1.92 beads prepared by a sol–gel method for MA infiltration, are characterized as calcined (850 °C) and sintered (1,600 °C) beads. The calcined YSZ beads are fine-grained and homogenous over the entire sphere and are surrounded by a uniform outer layer of approximately 30 μm thickness. After sintering at 1,600 °C, the beads are compacted to 51% of their initial volume and exhibit a granular structure. The thermal expansion is nearly linear for the calcined material, but shows a parabolic behavior for the sintered (1,400 °C) beads. In addition, the thermal expansion of calcined material is 20–25% less than after sintering. During heating up to 1,400 °C, two processes can be distinguished. The first occurs between 900 and 1,000 °C and is related to an increase in unit cell order. The second process involves grain-growth of the less crystalline calcined material between 1,100 and 1,300 °C. These results have implications for preparation of YSZ and its use as an inert MA transmutation matix.     

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.