High-Resolution Transmission Electron Microscopy of Transformed Magnesia-Partially-Stabilized Zirconia Precipitates

Transformed Mg-PSZ precipitates consist of monoclinic bands that are twin-related across either (100)_m or (001)_mplanes in an arrangement that leads to accommodation of the shear component of the transformation shape strain. High-resolution images of these variants reveal these twin relationships quite clearly. It is shown that complete compensation of shear does not always occur. This paper also shows observations of monoclinic variants related by a rotation of 90° around the normal to (001)_m, in addition to those related by a 180° rotation for the twin-related variants. These findings conform to the predictions of martensite theory and their implications are discussed in terms of shear compensation and reduction in overall strain energy.     

Investigation of the Structure of Zirconia Nanoparticles by High-Resolution Transmission Electron Microscopy

The structure of zirconia nanoparticles is investigated by high-resolution transmission electron microscopy (HRTEM). The structurally inhomogeneous nanoparticles with coherent interfaces (centaurs) are observed. The orientation relationships of the conjugated structures are determined.     

Fe-Ta Oxides: Phase Relations at 1200°C

Subsolidus phase relations of the oxides in the system Fe-Ta-O were experimentally determined at 1200°C, 1 atm total pressure, and variable partial pressures of oxygen. Tantalum pentoxide reacts readily with either ferrous or ferric oxide at subsolidus temperatures and the following ternary compounds have been synthesized: Fe₄Ta₂0₉, Fe₃Ta₂O_8-1, FeTaO₄, FeTa₂O₆, solid solutions between the latter two compounds, and tantalian magnetites. The compositions of solid solutions between FeTaO₄ and FeTa₂O₆ were very sensitive to variation of oxygen pressure. This sensitivity explains the oxidation behavior of tapiolite. The incorporation into magnetite of up to 7 at.% Ta was demonstrated, with a resulting increase in the size of the unit cell and decrease in magnetic permeability.     

Mn-Ta Oxides: Phase Relations at 1200°C

Subsolidus phase relations among oxides in the system Mn–Ta–O were experimentally determined by the quenching method. The conditions during the heating period were 1200°C, 1 atm total pressure, and various partial pressures of oxygen between 10^–17 atm and 1 atm. At the limits of this range, the stable assemblages at f -= 10^–17 atm are: MnO, Mn₆Ta₂O₁₁, Mn₄-Ta₂O₉, Mn_1.4 TaO_3.9, MnTa₂O₆, and β-Ta₂O₅; at p _o2= 1 atm they are: Mn₃O₄, Mn_1.4TaO_4.2, MnTa₂O₆, and β-Ta₂O₅. There are five univariant assemblages of three solid phases plus vapor in the phase diagram.     

Mechanical Behavior at 20° and 1200°C of Nicalon-Silicon-Carbide-Fiber-Reinforced Alumina-Matrix Composites

Tensile and fracture tests were conducted at 20° and 1200°C on a ceramic-matrix composite that was composed of an alumina (Al₂O₃) matrix that was bidirectionally reinforced with 37 vol% silicon carbide (SiC) Nicalon fibers. The composite presented nonlinear behavior at both temperatures; however, the strength and toughness were significantly reduced at 1200°C. In accordance with this behavior, matrix cracks were usually stopped or deflected at the fiber/matrix interface, and fiber pullout was observed on the fracture surfaces at 20° and 1200°C. The interfacial sliding resistance at ambient and elevated temperatures was estimated from quantitative microscopy analyses of the saturation crack spacing in the matrix. The in situ fiber strength was determined both from the defect morphology on the fibers and from the size of the mirror region on the fiber fracture surfaces. It was shown that composite degradation at elevated temperature was due to the growth of defects on the fiber surface during high-temperature exposure.     

In Situ Tensile Measurements of Single Fibers by Scanning Electron Microscopy

A newly developed tensile module allows tensile experiments of single fibers to be carried out under visual observation in the scanning electron microscope. This allows correlation of measured data with observed changes in the microstructure, such as surface irregularities and crack formation. With point heating, the thermal behavior of the fibers may be studied up to 2500°C. The results are presented with tensile elongation recordings and micrograph sequences of the structural changes. Carbon fibers with and without an aluminum coating were selected as testing specimens.     

In Situ Electron Microscopy Studies of Catalyst Particle Behavior

Although transmission electron microscopy (TEM) has been in routine operation for over 25 years, it is somewhat surprising to find that there has only been limited use of the technique by workers in the field of catalysis. Turkevich and his co-workers [1] were among the first investigators to use the electron microscope to study catalytic substances. In general, most applications have centered around the determination of particle size distributions in supported metal catalyst systems [2]. More recently, however, it has been recognized that the technique can yield much more fundamental information on such aspects as the morphology of small particles and the nature of their interaction with a support medium. This advance is the result of the achievement of high resolution TEM (0.25 nm resolution).     

Solid-State Phase Relationships in the Calcia-Titania-Zirconia System at 1200°C

Phase relationships were investigated in the CaO–TiO₂–ZrO₂ system at 1200°C for compositions containing <50 mol% CaO using X-ray diffraction and electron probe microanalysis. The existence of two previously reported ternary phases, zirconolite (CaZrTi₂O₇) and calzirtite (Ca₂Zr₅Ti₂O₁₆), was confirmed. Each of these phases exhibited a significant range of homogeneity between TiO₂ and ZrO₂, while maintaining a nearly constant concentration of CaO. The ternary solubilities of the constituent binary phases were found to be small (typically <1 mol%), with the exceptions of the perovskites (CaTiO₃ and CaZrO₃). These latter phases displayed mutual solubilities of at least 22 mol% but exhibited significant variations in composition from grain to grain. Thermodynamic equilibrium was clearly not established in several samples, although most of the phase relationship information obtained was self-consistent.     

Transitions in Vapor-Deposited Alumina from 300° to 1200°C

The transition of amorphous alumina to α-alumina was studied by X-ray diffraction, electron diffraction, DTA, TGA, and microscopic observation. The amorphous alumina was prepared by condensing vapor from evaporating molten alumina in vacuo onto the glass envelope of the vacuum chamber. The amorphous alumina was transformed to a poorly crystalline material by heating for 16 hr between 570° and 670°C. Between 670° and 1200°C, the poorly crystalline alumina was converted to α-alumina via two parallel series of transition aluminas. The principal series was γ-alumina to δ-alumina to α-alumina. A minor amount of θ-alumina developed from the initial crystallization and persisted throughout the duration of the principal series as a parallel path. Some conversion of δ- to θ-alumina was detected above 900°C. DTA produced an unexplained exothermic peak at 320°C and a second exothermic peak at 860°C which corresponded to formation of metastable aluminas.     

Differential Thermal Analysis Above 1200°C.

Differential thermal analysis techniques and equipment for use at temperatures up to 1550°C. are described. Proper electrical shielding eliminates the spurious effects ordinarily encountered above about 1200°C. For nonvolatile materials a pellet method requiring no container and only 0.1 to 0.2 gm. of sample has been developed. For systems containing volatile components, covered crucibles are used. Examples are given of typical DTA curves obtained from pellets of CaO-Al₂O₃-Fe₂O₃ mixtures. The interpretation of DTA heating curves to give precise solidus and liquidus data is discussed briefly.     

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.     

Deposition mechanism of pyrolytic carbons at temperature between 800–1200 °C

The textures, growth features, microstructures and binding of carbon atoms of pyrolytic carbons prepared by chemical vapor deposition (CVD) at a temperature between 800–1200 °C on graphite substrate and carbon fibers were studied. The intermediate product phase of pyrolytic carbons was also investigated. Based on the present study a deposition model of viscous droplet was proposed in this paper. The viscous droplet here refers to all kinds of fine spheroids that are more or less viscous. The mechanism of the formation of three typical textures namely, smooth laminar, rough laminar and isotropic carbons can be satisfactorily explained by this model.     

Reaction Between Titanium and Zirconia Powders During Sintering at 1500°C

Zirconia–titanium (ZrO₂–Ti) composites have been considered potential thermal barrier graded materials for applications in the aerospace industry. Powder mixtures of Ti and 3 mol% Y₂O₃ partially stabilized ZrO₂ in various ratios were sintered at 1500°C for 1 h in argon. The microstructures of the as-sintered composites were characterized by X-ray diffraction and transmission electron microscopy/energy-dispersive spectroscopy. Ti reacted with and was mutually soluble in ZrO₂, resulting in the formation of α-Ti(O, Zr), Ti₂ZrO, and/or TiO. These oxygen-containing phases extracted oxygen ions from ZrO₂, whereby oxygen-deficient ZrO₂ was generated. For relatively small Ti/ZrO₂ ratios, specimens with ≤30 mol% Ti, TiO were formed as oxygen could be sufficiently supplied by excess ZrO₂. For the specimens with ≥50 mol% Ti, lamellar Ti₂ZrO was precipitated in α-Ti(Zr, O), with no TiO being found. Both m -ZrO_{2− x} and t -ZrO_{2− x} were found in specimens with ≤50 mol% Ti; however, only c -ZrO_{2− x} was formed in the specimen with 70 mol% Ti. As ZrO₂ was gradually dissolved into Ti, yttria was retained in ZrO₂ because of the very limited solubility of yttria in α-Ti(O, Zr) or TiO. The concentration of retained yttria and the degree of oxygen deficiency in ZrO₂ increased with the Ti content. The complete dissolution of ZrO₂ into Ti was followed by the precipitation of Y₂Ti₂O₇ in the specimen with 90 mol% Ti.     

Barium Carbonate Phase on the Surfaces of Barium Titanate Particles and in Situ Transmission Electron Microscopy Observation of Its Decomposition

A BaCO₃ phase is found on the surfaces of hydrothermally synthesized BaTiO₃ particles; it occurs as aggregates or small protuberances. A small proportion of the phase decomposes to BaO crystallites when heated by a convergent electron beam in a transmission electron microscope. The BaO and BaCO₃ crystallites disappear when they are irradiated successively by the convergent electron beam. The BaO crystallites and the BaCO₃ phase sublimate and/or react with BaTiO₃ crystals whose surface layers are deficient in Ba²⁺ ions.     

Effects of High Water-Vapor Pressure on Oxidation of Silicon Carbide at 1200°C

The oxidation of SiC at 1200°C in a slowly flowing gas mixture of either air or air + 15 vol% H₂O at 10 atm (1 MPa) was studied for extended times to examine the effects of elevated water-vapor pressure on oxidation rates and microstructural development. At a water-vapor pressure of 1.5 atm (150 kPa), distinct SiO₂ scale structures were observed on the SiC; thick, porous, nonprotective cristobalite scales formed above a thin, nearly dense vitreous SiO₂ layer, which remained constant in thickness with time as the crystalline SiO₂ continued to grow. The pore morphology of the cristobalite layer differed depending on the type of SiC on which it was grown. The crystallization and growth rates of the cristobalite layer were significantly accelerated in the presence of the high water-vapor pressure and resulted in rapid rates of SiC surface recession that were on the order of what is observed when SiO₂ volatility is rate controlling at high gas-flow velocities (30 m/s). The recession process can be described by a paralinear kinetic model controlled by the conversion of dense vitreous SiO₂ to porous, nonprotective SiO₂.     

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.     

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.     

Miscibility Gap in the System Sodium Oxid Silica—Sodium Sulfate at 1200°C

The two-liquid region in the system soda–silica–sodium sulfate is investigated and the results compared with the limited data available in the literature. An extensive two liquid + tridymite field, not previously reported, is also defined. The variation of the activity coefficient of sodium sulfate with soda/silica ratio is derived from the miscibility data.     

Sintering of Sol—Gel-Prepared Submicrometer Particles Studied by Transmission Electron Microscopy

We studied the sintering process of sol-gel-prepared monodisperse submicrometer CeO₂ spheres by examining the microstructural changes of single spherical particles after successive heat treatment in oxygen up to 850°C using transmission electron microscopy. Steps of organic phase removal, CeO₂ crystallization, grain growth, and particle condensation were clearly illustrated. Grain-size and sphere-diameter changes were measured quantitatively using this technique. CeO₂ particles were found to be highly porous until collapse, which occurred at~850°C.