Effect of Yttria on Interfacial Reactions Between Titanium Melt and Hot-Pressed Yttria/Zirconia Composites at 1700°C

Various Y₂O₃/ZrO₂ samples were fabricated by hot pressing, whereby Y₂O₃ was mutually dissolved or reacted with ZrO₂ as a solid solution or Zr₃Y₄O₁₂. Hot-pressed samples were allowed to react with Ti melt at 1700°C for 10 min in argon. Microstructural characterization was conducted using X-ray diffraction and analytical electron microscopy. The Y₂O₃/ZrO₂ samples became more stable with increasing Y₂O₃ because Y₂O₃ was hardly reacted and dissolved with Ti melt. The incorporation of more than 30 vol% Y₂O₃ could effectively suppress the reactions in the Ti side, where only a very small amount of α-Ti and β'-Ti was found. When ZrO₂ was dissolved into Ti on the zirconia side near the original interfaces, Y₂O₃ reprecipitated in the samples containing 30%–70 vol% Y₂O₃, because the solubility of Y₂O₃ in Ti was very low. In the region far from the original interface, α-Zr, Y₂O₃, and/or residual Zr₃Y₄O₁₂ were found in the samples containing more than 50 vol% Y₂O₃ and the amount of α-Zr decreased with increasing Y₂O₃.     

Zirconia-Related Phases in the Zirconia/Titanium Diffusion Couple After Annealing at 1100°–1550°C

A diffusion couple of 3 mol% Y₂O₃–ZrO₂ and titanium was isothermally annealed in argon at temperatures between 1100° and 1550°C. The phases and microstructure in the ceramic side were investigated using scanning electron microscopy and transmission electron microscopy, both attached to an energy-dispersive spectrometer. After annealing at 1100°C/6 h, zirconia grains did not grow conspicuously and evolved only traces of oxygen, resulting in t -ZrO_{2− x} but not α-Zr. At temperatures above 1300°C, a significant amount of oxygen evolved from zirconia, reducing the O/Zr ratio, such that α-Zr was excluded from t -ZrO_{2− x} during cooling, yielding a higher O/Zr ratio (≈2). When held at 1550°C/6 h, zirconia grains grew rapidly. The α-Zr was segregated on grain boundaries during cooling by the exsolution of zirconium from ZrO_{2− x} , while twinned t '-ZrO_{2− x} or lenticular t -ZrO_{2− x} , which was embedded in ordered c- ZrO_{2− x} , was found. The ordered c -ZrO_{2− x} was identified by the     {113} superlattice reflections of its electron diffraction patterns.     

Ti2ZrO Phases Formed in the Titanium and Zirconia Interface After Reaction at 1550°C

Hot-pressed 3 mol% Y₂O₃ partially stabilized ZrO₂ was reacted with titanium at 1550°C/30 min. The interface was characterized by analytical transmission microscopy (transmission electron microscopy/energy-dispersive spectroscopy). The lamellar and the spherical Ti₂ZrO as well as the orthorhombic β'-Ti were found to exist in the titanium side after cooling down to room temperature. The crystal structures of the lamellar and the spherical Ti₂ZrO were orthorhombic and hexagonal, respectively. On heating, the dissolution of a large amount of zirconium and oxygen into titanium gave rise to the metastably supersaturated disordered α-Ti(Zr, O) solid solution where two different Ti₂ZrO phases subsequently precipitated, while the β-Ti coexisting with α-Ti at high temperatures was transformed to the orthorhombic β'-Ti during cooling. The spherical hexagonal Ti₂ZrO was an ordered structure, with Zr and O occupying substitutional and interstitial sites, respectively. The orientation relations between α-Ti and the lamellae orthorhombic Ti₂ZrO were determined to be [0001]_α-Ti//[110]_Ti2ZrO and (10 1 0)_α-Ti//(1 1 0)_Ti2ZrO; meanwhile, those between the α-Ti and the spherical hexagonal Ti₂ZrO were [0001]_α-Ti//[0001]_Ti2rO and (10 1 0)_α-Ti//(10 1 0)_Ti2ZrO.     

Effect of Additives on the Pressureless Sintering of Aluminum Nitride between 1500° and 1800°C

Combined oxide additives (Y₂O₃, CaO, La₂O₃, CeO₂, SiO₂, TiO₂, and Fe₂O₃) were investigated as AIN sintering aids. AIN can be fully sintered at 1600°C to substantial thermal conductivity (92 W/(m·K)) using a multiple sintering aid of Y₂O₃, CaO, SiO₂, La₂O₃, and CeO₂. This lowtemperature material has small grain size (1 to 3 μm).     

Slow Crack Growth in Sapphire Fibers at 800° to 1500°C

Sapphire fibers with (near) c -axis orientation were tested in tension over a range of strain rates (10⁻⁵ to 0.5 min⁻¹) at elevated temperatures (800° to 1500°C). The strength of the fibers was dependent on the strain rate. Slow crack growth was confirmed as the degradation process by direct inspection of the fracture surfaces and estimation of fracture stresses from measured flaw sizes. The slow crack growth parameter, N , decreased with increasing temperature. At 1400°C a threshold in strength was observed; the threshold stress intensity factor was estimated to be ≅ 1 MP·m^1/2 at 1400°C. Thermally activated bond rupture (e.g., lattice trapping model) is postulated as the mechanism responsible for the slow crack growth.     

Phase Equilibria in the System CaO-MgO-Iron Oxide at 1500°C

Phase relations in the system CaO-MgO-iron oxide were determined at 1500°C at two levels of oxygen pressure, 10^–9 atm and 0.2 atm (air). Particular attention was directed toward de termining the maximum amount of iron oxide which may be tolerated in CaO-MgO-iron oxide bodies before a liquid phase forms under equilib rium conditions at the two chosen oxygen pres sures. Inferences are made regarding the possible bearing of the results on the perform ance of tar-bonded dolomite brick used in steel making furnaces.     

Activity–Composition Relations in the Systems CaO–MnO and MgO–MnO at at 1500° and 1600°C

Activity–composition relations in CaO–MnO and MgO–MnO solid solutions were determined by equilibrating oxide solid solutions with platinum–manganese alloys under controlled oxygen partial pressures at 1500° and 1600°C. Both systems show considerable positive deviation from ideality.     

Unsteady-State Method of Measuring Thermal Diffusivity and Biot's Modulus for Alumina Between 1500° and 1800°C.

An unsteady-state method was employed for measuring values of Biot's modulus and the thermal diffusivity of pure dense alumina in the temperature range 1500° to 1800°C. Limits were established for the possible range of values of diffusivity, and corresponding values of thermal conductivity were calculated.     

Aqueous Degradation and Chemical Passivation of Yttria-Tetragonally-Stabilized Zirconia at 25°C

The objective of this study was to establish the mechanism(s) controlling degradation of yttria-tetragonally-stabilized zirconia (Y-TZP) powder in aqueous suspensions and determine the significance of this degradation to the aqueous physical chemistry of Y-TZP. Experiments were performed on commercially available Y-TZP powder placed in aqueous suspensions at 25°C. Experimental investigations included analysis of the aqueous chemistry of Y-TZP in water via ICP-MS, determination of the surface and bulk structure of the powder via XRD and solid-state NMR, and observation of changes in surface charges via zeta potential determinations. The goal of this study was to control the surface chemistry of Y-TZP in aqueous suspension to promote dispersion and permit aqueous processing of Y-TZP powders.     

Processing of Amorphous Calcium Aluminate Powders at < 900°C

Aluminum chelate and calcium nitrate precursors were used to synthesize amorphous calcium aluminate powders by solgel processing at < 900°C. The method of preparation and results of characterization of the gels by thermogravimetric analysis, X-ray diffraction, scanning Auger microscopy, and single-point BET analysis are presented. An optimum heat-treatment schedule consisting of heating the gel to 900°C at 5°C/min and holding for 16 h was developed to produce highly reactive, X-ray amorphous calcium aluminate powders.     

Reaction between Sodium Carbonate and Silica Sand at 874°C < T < 1022°C

This paper is concerned with the mechanism of the reaction between sodium carbonate and silica sand at 874° to 1022deg;C. A reaction path which evolves in four stages, i.e., decarbonization, dissolution of orthosilicate, dissolution of metasilicate, and final dissolution of silica, is dealt with in detail, including mass balances. Formulas for dissolution rates in the first two stages are suggested, and the effect of wetting the grains by the melt, amount and distribution of the melt, and nonuniform dissolution (e.g., segregation) on the dissolution rate of silica grains are discussed. The need for more detailed experimental studies is stressed.     

Kinetics of the Reaction of UC2 and Nitrogen from 1500° to 1700°C

The kinetics of the reaction of UC₂ spheres with nitrogen was studied from 1500° to 1700°C. A metallographic method was used to determine the time-dependent conversion of UC₂ to U(C,N) and free C. The conversion appeared to be controlled by the diffusion of solid carbon in solution to sites where it could precipitate as free carbon. These sites were the surface of the sphere and particles of free carbon that existed within the original UC₂. An increased distribution of these internal sites decreased the distance for carbon diffusion and resulted in an increased rate of reaction. The UC₂ appeared to undergo a very rapid initial reaction that resulted in the uptake of 1 to 5 at.% nitrogen in the UC₂ at these temperatures.     

Sintering of Alumina at Temperatures of 1400°C. and Below

Previous investigators have indicated that both small particles and the addition of certain oxides promote the sintering of alumina at temperatures below 1700°C. By utilizing combinations of oxides and small particle size, the sintering temperature of 96% alumina bodies was reduced in this investigation to the 1300° to 1400°C. range. It is proposed that this low-temperature sintering is aided by the formation of a liquid phase. Thin sections of the alumina sintered at low temperatures revealed bodies with small grain size whose bulk densities were above 3.80 gm. per cc.     

Direct Calorimetric Measurement of Enthalpies of Phase Transitions at 2000°–2400°C in Yttria and Zirconia

High-temperature differential thermal analysis provided data on phase transitions in zirconia and yttria. The tetragonal form of ZrO₂ transforms to the cubic fluorite structure at 2311°±15°C with an enthalpy of 3.4±3.1 kJ/mol. Cubic C-type Y₂O₃ transforms, probably to the fluorite structure, at 2308°±15°C with Δ H =47.7±3.0 kJ/mol. This high-temperature polymorph melts at 2382°±15°C with an enthalpy of fusion of 35.6±3.0 kJ/mol.     

Activity–Composition Relations in Platinum–Chromium and Platinum–Vanadium Alloys at 1500°C

Activity–composition relations in Pt–Cr and Pt–V alloys at 1500°C were determined from equilibration of these alloys with Cr₂O₃ and V₂O₃, respectively, in Co₂–H₂ atmospheres of known oxygen partial pressures, followed by quenching to room temperature and microprobe analyses of the quenched alloys. The activities of Cr and V were calculated from known standard free energies of formation of Cr₂O₃ and V₂O₃ from the respective elements, and the oxygen pressures were calculated from the C–H–O equilibrium in the gas phase. Both alloy systems show strong negative deviations from Raoultian behavior.     

Spinel Phase Formation During the 980°C Exothermic Reaction in the Kaolinite-to-Mullite Reaction Series

With the use of differential thermal analysis, X-ray diffraction, and transmission electron microscopic techniques, we showed that γ-Al₂O₃ type spinel phase is solely responsible for the 980°C exotherm in the kaolinite-to-mullite reaction series. Transmission electron microscopic characterization indicated that the spinel formation is preceded by a phase separation in the amorphous dehydroxylated kaolinite matrix. Chemical analysis of the spinel phase by energy dispersive X-ray spectroscopy revealed a nearly pure Al₃O₂ composition.     

Electric Field Assisted Sintering of Cubic Zirconia at 390°C

Using the flash sintering technique, cubic yttria‐stabilized zirconia is shown to sinter at 390°C, more than 1000°C below nominal sintering temperatures, by using a DC electric field of 2250 V/cm. Furthermore, flash sintering experiments performed with electric fields between 60 and 2250 V/cm were used to show that the relationship of the temperature at the onset of flash sintering (T_Onset) and the applied field (E) follows the power relationship T_Onset (K) = 2440 E^?1/5.85(V/cm). Using this relationship, and considering the sintering of the sample in the absence of an electric field, the critical electric field to enter the flash sintering regime is shown to be 24.5 V/cm. For electric fields between this critical electric field and 2250 V/cm, the onset of flash sintering occurs in the same range of critical volumetric power dissipation, between 1 and 10 mW/mm³, suggesting this is a material property. Despite the volumetric power dissipation being the critical value for the onset of flash sintering behavior, the current density achieved during sintering appears to be more critical for densification rather than maximizing power dissipation.     

Selection of Materials for Use at Temperatures above 1500°C in Oxidizing Atmospheres

The purpose of this work was to select materials for applications in oxidizing atmospheres at temperatures of a minimum of 1500°C. Several requirements had to be fulfilled, e.g. long-term oxidation resistance, low vapour pressure, high creep resistance and low gas permeability. Another important point was to avoid reactions and sintering in material combinations with silica forming materials. The investigation was based on a detailed literature screening, followed by compatibility tests at 1600°C in air with potential materials (Al₂O₃, ZrO₂, CeO₂, La₂O₃, Y₂O₃, HfO₂, MgO·Al₂O₃) and Mosi₂ as an example of a silica-forming material. The result of these experiments was, that only Y₂O₃ and HfO₂ did not show severe reactions in contact with MoSi₂. Since most of the materials available did not fulfill all the requirements, some new materials were investigated: CIPed MoSi₂ with different additives (SiC, ZrB₂) and recrystallized SiC (RSiC) with a polymer (polysiloxane) derived MoSi₂-filled coating. The oxidation behaviour of these materials was evaluated by continuous thermogravimetric measurements at 1500°C over a period of 100 h in air and by detailed postexperimental investigations.