The crystal structure of monoclinic HfO2 was determined using Weissenberg techniques. Data were obtained from a needlelike single crystal which was grown from a lithium molybdate melt. The crystal was rotated about the α axis, and reflection data were obtained for nine levels. A three‐dimensional least‐squares refinement confirms that monoclinic hafnia is isomorphous with monoclinic zirconia. The atomic coordinates in the structures agree within one standard deviation. Thus, the fractional coordinates for the metal atom differ by less than 0.0005 and those for the oxygen atoms by no more than 0.01. Whereas three of the seven Hf–O distances are larger than the corresponding Zr–O distances, the average value is approximately 0.01 Å smaller. The average metal‐metal distance is approximately 0.02 Å less in HfO2 than in ZrO2.

Phase relationships in the SIC-AIN system were determined by analytical electron microscopy of local equilibria among adjacent phases in hot-pressed samples and in diffusion couples. At 2100" to 2300"C, a 4H-2H equilibrium exists, the 4H field extending from =2 mol% AlN to an upper limit of 11 to 14 mol% AIN. The wurtzite-type 2H (6) solid solution extends from an impurity-sensitive lower limit of 17 to 24 mol% AIN up to 100 mol% AIN. Semiquantitative Auger electron spectroscopy and energy dispersive X-ray spectroscopy ultrathin-window detector measurements confirmed the assumption that the solid solutions have the general formula (Sic), --x (AIN), and belong to the Sic-AIN pseudobinary system. The existence of a miscibility gap below 1950°C was confirmed, but its limits were not determined accurately. A faulted metastable cubic phase, p', exists below =2000"C and contains up to =4 mol% AIN. The transformations into the stable (Y structures occur through diffusion-controlled stacking rearrangements.

the mixture was hydrolyzed. The chemical route for powder The flexural strength and elastic modulus of cubic zirconia preparation, 1-3 scaleup to powder quantities of 400 kg, 4 and the that was stabilized with 6.5 mol% yttria was determined in specifics of billet fabrication 5 have been published previously. the temperature range of 25؇-1500؇C in air. Specimens wereThe calcined powders were used to fabricate dense ceramics diamond machined from both hot-pressed and sintered either by cold pressing and sintering or by hot pressing. billets that were prepared from alkoxy-derived powders.Two different types of Zyttrite powder (denoted A and B) The flexural strength of the hot-pressed material decreased, were developed. Type A powder was produced by washing the from }300 MPa at 25؇C to 50 MPa at 1000؇C, and thenZyttrite hydrosol with water of 0.05 ppm impurity level, drying increased slightly as the temperature increased to 1500؇C.at 110ЊC for 24 h, and calcining at 550ЊC for 0.5-1.0 h. Type B The flexural strength of the sintered material decreased, powder was made by washing the hydrosol at least twice with from }150 MPa at 25؇C to 25 MPa at 750؇C, and then dry isopropanol prior to drying and calcining. Both powders appeared to increase slightly to 1500؇C. Flexural strengths were 99.98% pure, with major impurities of 100 ppm calcium, were comparable to other fully stabilized zirconia materials.35 ppm iron, 35 ppm silicon, and 20 ppm magnesium (all The overall fracture mode was transgranular at low temvalues are Ϯ5 ppm). The average grain size of each powder was peratures, mixed mode at }500؇-1000؇C, and intergranular Ͻ10 nm, and they had similar surface areas of ϳ100 m 2 /g. at higher temperatures. Pores or pore agglomerates along However, the powder that was washed with alcohol had a dengrain boundaries and at triple points were fracture origins.sity () that was half of that of the powder washed with water The value of the porosity-corrected Young's moduli was 222 ( Յ 0.4 g/cm 3 versus Ն 0.8 g/cm 3 ). The higher density of the GPa at 25؇C, decreased to }180 GPa at 400؇C, and then was powder is attributed to the presence of harder agglomerates and relatively constant with increasing temperature to 1350؇C.their size distribution. Calcined powders were dry milled for 2-4 h in polyethylene containers using rubber balls. Analysis of the milled powders by emission spectroscopy indicated that I. Introduction high purity was maintained.Z IRCONIA (ZrO 2 ) is a widely used ceramic that exists in mono-Hoch and Nair 6 and Haberko 7 demonstrated that calcination clinic, tetragonal, or cubic crystal structures, depending on of gels washed with alcohol consistently resulted in a powder temperature and chemistry. Varying amounts of oxide additives, that was composed of mechanically weaker agglomerates of a such as calcia (CaO), magnesia (MgO), yttria (Y 2 O 3 ), and most more porous nature and lower density than agglomerates that rare-earth oxides, can stabilize ZrO 2 in the cubic phase. Despite were obtained from gels that were ...

The system zirconia-scandia was investigated using X-ray diffraction analysis, differential thermal analysis, metallographic analysis, and melting point studies. Results reveal the monoclinic a1 phase (0 to2 mol% sCzO3), the tetragonal a,' phase (5 to 8% Sc2O3), the rhombohedral p phase (9 to 13% ScZO3), the rhombohedral y phase (15 to 23% SCZO~), the rhombohedral 6 phase (24 to 40% SCZO~), and the cubic E phase (77.5 to 100% SczO3). The monoclinic a1 phase and the tetragonal a,' phase were found to transform to the tetragonal a, phase over a wide temperature range depending on composition. The p, y, and a phases transformed to a cubic phase at temperatures of =600", 1100", and 1300"C, respectively. A maximum melting point of =2870"C was found at -10% ScZO3 and a eutectic at ~2400°C at 55% Scz03.