Characteristics and sintering behaviour of 3 mol % Y2O3-ZrO2 powders synthesized by reaction in molten salts

Yttria-stabilized tetragonal zirconia powders have been synthesized by reaction in molten salts at 450 °C. To obtain a Y₂O₃-ZrO₂ solid solution powder, it was necessary to use yttrium salts (not yttrium oxide directly) in molten NaNO₃-KNO₃ eutectic. The agglomeration state of the powders depended on their washing conditions. An alcohol-washed powder containing soft agglomerates led to a fine-grained and high-density sintered body.     

Sintering of 3 mol % Y2O3-TZP and its fracture after ageing treatment

TZP ceramic of 99.7% theoretical relative final density was obtained by pressureless sintering a commercial co-precipitated 3 mol % Y₂O₃-ZrO₂ powder at 1400° C for 10 h. Fracture surfaces of the aged material revealed that the fracture of TZP ceramic was typified by an intergranular mode in areas where the phase was mainly tetragonal, whereas the transgranular mode was found predominantly in the area containing more monoclinic phase. Microcracks induced by the (t) (m) transformation provided short paths for water to accelerate the property degradation of TZP upon low-temperature ageing in a humid atmosphere.     

Raman Scattering and t-Phase Lattice Vibration of 3% (mole fraction) Y2O3-ZrO2

The Raman spectra of 3% (mole fraction) Y2O3-ZrO2 (3Y) are obtained at different temperature from 77 K to 853 K. The results show that 260 cm-1 Eg and 460 cm-1 Eg modes on the spectra shift toward lower wave number with the increase of temperature; meanwhile, the continuity of the Half Width at Half Maximum (HWHM) and wave number (frequency) of Raman bands are broken and t-phase only partially transforms to m-phase at 523 K. Based on the experimental results, this paper discusses the t-phase lattice vibration of 3Y and presents the images of vibration displacement of six Raman-active modes for t-phase of ZrO2. The analysis indicates that there are diversities existing in the displacement of the atomic vibration of Raman-active modes because of their different symmetries at various temperature, which leads to the different change tendencies of Raman bands. Furthermore, the abrupt changes of some Raman-active modes indicating the atomic displacement are assumed to be the condensations of phonon modes in t     

Mechanical properties of sinter-forged Al2O3-ZrO2 ceramics

Two kinds of composites, Al₂O₃-25 wt% ZrO₂(2 mol% Y₂O₃) (Y-ZTA), Al₂O₃-25 wt% ZrO₂(8 mol% CeO₂) (Ce-ZTA) were produced by the sinter-forging process. The effect of presintering temperature on the mechanical properties of the composites was examined. The sinter-forging process increased the room-temperature bending strength in comparison with pressureless sintering, owing to the smaller grain size in sinter-forged bodies than in pressureless sintered ones. It was found necessary to keep the presintering temperature considerably lower than sinter-forging temperature in order to improve the room-temperature strength. The strength of sinter-forged Ce-ZTA was higher than that of sinter-forged Y-ZTA. The residual surface compressive stress induced by the phase transition during grinding in Ce-ZTA was found to be effective to further improve the strength and fracture toughness.     

The electrical properties of some cordierite glass ceramics in the system MgO-Al2O3-SiO2-TiO2

The a.c. dielectric breakdown and electrical resistivity of glass ceramics in the system MgO-Al₂O₃-SiO₂-TiO₂ have been studied using three sets of samples having different crystallinity contents. Standard a.c. (50 Hz) breakdown tests were performed at room temperature (18 °C) using planar disc specimens and hemispherically-ended brass contact electrodes. The breakdown process caused the formation of a breakdown channel which terminated at the specimen surface in a crater. The breakdown strength was independent of the rate of voltage rise, but decreased exponentially (60 to 10 kV mm^–1) with increasing specimen thickness. A high crystallinity content, good surface finish and a homogeneous microstructure yielded high breakdown strengths whilst poor microstructural development caused a reduction in breakdown strength. The breakdown mechanism is believed to be a combination of electronic, thermal and electromechanical processes.     

Fabrication and sinterability in Y2O3-CeO2-ZrO2

CeO₂-stabilized tetragonal zirconia polycrystal (Ce-TZP) containing 1 to 6 mol % YO_1.5 have been fabricated as fine powders by a coprecipitation technique. The microstructure of the as-sintered surface and fracture surface were examined by electron microscopy. CeO₂ dopants reduced the phase transformation temperature from amorphous to tetragonal and stabilized the tetragonal phase at low temperature. The addition of Y₂O₃ to Ce-TZP inhibited the grain growth. The sintered density reached 99% theoretical for short sintering times at 1440 and 1540° C, but decreased slightly to 97 to 98% theoretical for longer sintering times. The decrease in density is attributed to the morphological development of agglomerates, which induce large pores during sintering. The average grain size decreased significantly as the yttrium content increased from 1 to 3 mol %. Specimens aged in water at low temperatures exhibited no phase transformation. This implies fairly good thermal stability in the Y₂O₃-doped Ce-TZP system.     

Low-temperature ageing map for 3mol% Y2O3-ZrO2

The ageing behaviour of 3mol% Y₂O₃-ZrO₂ at 100–500° C in a water-containing atmosphere was studied. A critical grain size of ～ 0.37 μm and a lower temperature limit of ～80°C for retaining the tetragonal symmetry were deduced from the kinetic study. An “ageing map” constructed on the grain size (G) against ageing temperature (T) plot is proposed to describe the low-temperature ageing behaviour of 3mol% Y₂O₃-ZrO₂.     

Infrared transmission of sintered 3 mol% Y2O3-ZrO2 gel

Translucent ZrO₂ film was successfully prepared by gelling hydrothermally produced nano-ZrO₂ powders. The film (300 m thick) was found to transmit light to 6.5 m (40% transmission) when sintered at 1200 °C, but transmission was totally lost after sintering at 1300 °C for 1 h. Residual organic material such as urea, which was used for preparing the powder, dominated the transmission of the film in the region between 1.3 and 4.5 m when sintered below 1000 °C. When sintered above 1000 °C, the microstructure controlled the transmission. Both organic residuals and the microstructure of the zirconia were found to determine the transmission in 4.5–6.5 m region.     

Sintering behaviour of (Y2O3-ZrO2 gels)

The sintering behaviour of yttria-stabilized-zirconia (YSZ) gels prepared by a sol-gel process has been studied. The YSZ gels in the forms of bulk and film were sintered at various temperatures up to 1300 °C and the crystalline structures and microstructures were examined by X-ray diffraction and scanning electron microscopy (SEM).     

Microstructure and grain growth in Li2O-Al2O3-SiO2 glass ceramics

Microstructure and grain growth were studied in two glass ceramics of the Li₂O-Al₂O₃-SiO₂ system, one an experimental material of basic composition Li₂O-Al₂O₃-4SiO₂ and the second a commercial material of approximately 0.7Li₂O-Al₂O₃-6SiO₂ composition with small amounts of other oxides. There was evidence from transmission electron microscopy that the commercial material contained residual glass at grain-boundary triple points and glass layers at some but not all grain boundaries. No definite evidence was found for residual glass in the experimental material. The composition of secondphase regions in the commercial material was studied by STEM microanalysis. Al-rich regions of irregular morphology were found but there was no evidence that residual glass was SiO₂-rich, as has been previously suggested for this type of glass-ceramic. Grain growth showed a fairly similar time dependence in the two materials with growth more rapid in the commercial material at a given temperature. It is suggested that grain growth is controlled by precipitate particles rather than by residual glass phase.     

The modulated structure formed by isothermal ageing in ZrO2-5.2 mol % Y2O3 alloy

The modulated structure produced by isothermal ageing of ZrO₂-5.2 mol % Y₂O₃ alloy was examined mainly by electron microscopy. It was found that the modulated structure was formed at ageing temperatures between 1400 and 1600° C, but not at 1700° C. The structure is developed by spinodal decomposition, which produces compositional fluctuation in the elastically soft 111 direction in cubic zirconia. The hardness increase caused by the development of modulated structure during ageing is larger than the hardening by precipitation of tetragonal phase in the cubic matrix.Graduate Student, Tohoku Univerisy, Sendai, Japan.