ZrO2 nanopowders derived from zirconium n-propoxide [Zr(OC3H7)4]-acetylacetone-water-isopropanol have been investigated with respect to their tetragonal metastability on heating-cooling processes. The transformation temperature of metastable tetragonal to monoclinic (t′ → m) phase is found to be governed by ultimate firing temperature, time, and atmospheres employed. Crystallite growth is fastened with increase in calcination temperatures over 1000–1400 °C, and the t′ → m transformation temperature is correlated linearly with crystallite size in the studied range of 12–20 nm. Heating in an oxygen environment increases the size of the final crystallites and hence the rate of the t′ → m transformation. It is revealed that the t′ → m transformation temperature depends largely on the heating atmosphere, but only weakly on the cooling one. Based on the findings of this work, surface oxygen deficiencies are attributed to be responsible for low-temperature tetragonal metastability. A crystallite growth model to explain the decline of t′-ZrO2phase is proposed. Kinetic and thermodynamic factors are also discussed in connection with the existing theories of tetragonal metastability.

Aluminosilicate precursors were prepared by mechanochemical treatment of gibbsitesilica gel mixtures. The effect of grinding on their structure and thermal behavior has been examined by 27Al and 29Si MAS NMR, x-ray diffraction (XRD), differential thermal analysis-thermogravimetry (DTA-TG), and Fourier transform infrared (FTIR). After 8 h grinding, the hydrated alumina was completely changed to an amorphous phase which showed a new exothermic DTA peak at about 980 °C due to the formation of γ–Al2O3 or spinel phase. This behavior was related to changes in the Al and Si environments, as deduced from the MAS NMR spectra. With increased grinding time, some 4-coordinated Al appears, together with an Al resonance at about 30 ppm. Simultaneously, a new Si resonance appears at about −90 ppm, indicating a greater degree of homogeneity in the ground samples. Mullite crystallizes at 1200 °C from samples ground for 8–20 h, its XRD intensity increasing with increased milling times, in agreement with the NMR, DTA, and FTIR data. Changes in the Al and Si environments during heat treatment, as reflected by the NMR spectra, are also reported.

The frequency and temperature dependencies of the dielectric permittivity and of the electrical conductivity of excess ferrous ions hydrothermal NiZn ferrites were analyzed before and after sintering. A decreasing tendency with frequency of the dielectric responses was observed, but the high permittivities attained (ε′ ≈ 105) masked any relaxation in these materials. This behavior is characteristic of n-type semiconducting ferrites, where electron hopping between Fe+2 and Fe+3 ions leads to very high conductivity values. The temperature dependence of the dielectric permittivities revealed the existence of broader peaks. The electron hopping mechanism leads to a frequency dispersion of the temperature where the permittivities attain their maxima. The electrical conductivity variations with temperature exhibited Arrhenius-type behaviors, with activation energies ranging from 0.34 eV (hydrothermal powder) to 0.16 eV (for the highest sintering temperature). These results were correlated to the variations in Fe+2 concentration and microstructure.

We report on the preparation and characterization of epitaxial ACuO2 (A = Sr, Ca, Ba) thin films and multilayers with the so- called infinite layer (IL) structure, by rf magnetron sputtering. Films and multilayers without Ba have a remarkable crystal quality, whereas those containing this large ion are often multiphased and unstable. In spite of the excellent crystalline quality of these samples, obtaining thin films having both IL structure and displaying superconducting properties has not succeeded; our pure IL samples display semiconducting behavior, and the different procedures tried in order to dope them—annealings, introduction of disorder or cation vacancies, artificial layering—have failed. These results support that the pure IL structure ACuO2 (A = alkaline earth) cannot superconduct.

The densification behavior of silica-coated alumina particles was investigated during sintering using a laser scanning micrometer. Traditional dilatometric techniques require contact between a push-rod and the sample under study and thus place the sample under stress during the test. However, the utilization of a noncontact laser micrometer to measure dimensional changes during sintering enabled the densification behavior to be very accurately characterized under a stress-free condition. Thus higher temperature experiments, where densification rates are particularly temperature sensitive and the samples are especially soft, can be reliably investigated without the disturbing influence of an external force. The present paper describes an application of the technique to evaluate the densification behavior from 900–1300 °C of silica-coated alumina microcomposite particles used for the fabrication of mullite.

The microstructure and lattice defects of SrTiO3 and BaTiO3 thin films with columnar grains prepared by a chemical solution deposition technique have been investigated by means of transmission electron microscopy. The columnar grains in the SrTiO3 films exhibit a preferential orientation with a crystallographic 〈111〉 direction parallel to the normal of film, which, in turn, follows the orientation texture of the substrate Pt layer. Cubic-to-cubic relationships have been found for the two materials. For the BaTiO3 films the columnar grains are oriented in a random way without any preferential relationship to the substrate Pt layer. Pores, lattice defects, and grain boundaries occur in either type of film, however in different configurations. This reflects the individual nature of the materials and the different formation and growth mechanisms of the films under the preparation conditions.

The present paper reports data concerning magnesia samples obtained by calcination of different precursor salts at different increasing temperatures (873–1253 K). The oxides are characterized by x- ray diffraction, scanning electron microscopy, and N2 adsorption at subcritical temperatures. The samples appear to be composed, at any temperature, of pure periclase with a degree of crystallinity which increases with the temperature of calcination. Morphologically, the products have the shape either of lamellas or of cubes of variable dimensions, depending on the nature and route of preparation of the precursor salts. The variation of the specific surface area and the degree of porosity with the nature of the precursors and the temperature is discussed.

The biaxial stretching behavior of superplastic Y -TZP (yttria -tetragonal zirconia polycrystals) ceramics and Y -TZP/Al2O3 composites in air atmosphere was investigated. Stretching deformation can be made within a limited range. Larger grain size of Y -TZP led to higher applied load needed to obtain the same amount of deformation. However, if some monoclinic phase was present in the materials, which was caused by the over -large grain sizes, defects would occur and thus make deformation easier. Microstructure study of the deformed specimen showed the occurrence of crack -like defects on the outside surface of the stretched disk specimen. Limited grain growth of single phase Y -TZP was obtained while Y -TZP grains in Y -TZP/Al2O3 composites grew extensively after stretching.

This study investigated the modes of grinding-induced subsurface damage in dental glass-ceramics and the influence of microstructure on strength degradation. A series of micaceous glass-ceramics crystallized from the same glass composition were tested. The diameter of the mica platelets in these glass-ceramics was varied via heat treatment. Grinding was performed using three diamond wheels (with diamond particle size of 40, 100, and 180 µm, respectively) at depth of cut ranging from 5 µm to 100 µm. A bonded- interface technique was employed to examine the machining- induced subsurface damage. Relatively large median and lateral cracks were found in the glass-ceramic with the smallest mica platelets. In contrast, no cracks were found in the material containing large mica platelets. The ground specimens were fractured in four-point flexure to measure strength as a function of grinding conditions and mica platelet sizes. The strength of the ground specimens was reduced to approximately 30% of the strength of the polished specimens for the glass- ceramic containing the smallest mica platelets; that of the glass-ceramic with the intermediate mica platelet size was reduced to 60%. In contrast, virtually no strength loss occurred with the glass-ceramic containing large mica platelets. Microstructure was shown to determine the mode and degree of strength-controlling damage in the machining of these dental glass-ceramics. Polishing after grinding removes subsurface damage and recovers strength for the glass-ceramics containing fine mica crystals.

Molybdenum trioxide (MoO3) films, 15 µm thick, have been deposited on 50 µm thick polycrystalline titanium substrates from 250 to 500 °C via liquid spray pyrolysis. Molybdenum pentachloride (MoCl5) dissolved in methanol was used as the molybdenum source; ambient conditions provided the oxygen source. X-ray diffraction (XRD) data indicated that amorphous MoO3 films were produced at deposition temperatures below 400 °C. Randomly orientated polycrystalline MoO3 films were produced at 400 °C and higher deposition temperatures. The deposition temperature also influenced the surface area of the films and their average grain size. Subsequent conversion of the MoO3 films to high surface area (HSA) conductive films containing both γ–Mo2N and δ–MoN was accomplished via programmed reactions with anhydrous NH3 and involved the formation of MoO2 and MoOxN1−x as intermediate phases. The degree of crystallinity, surface area, and average grain size of the MoO3 films strongly influenced the average grain size and surface area of the resultant MoxN films.

Ammonium salt of poly(acrylic acid) (PAA-NH4) has been used to stabilize aqueous BaTiO3 suspensions at various pH values. Adsorption of PAA-NH4 causes the zeta potential to become more negative, although this effect becomes less dramatic as the pH increases. The concentration of PAA-NH4 required to stabilize aqueous BaTiO3 suspensions decreases with increasing pH. The critical amount of PAA-NH4 as a function of pH is plotted in a stability map, as determined by adsorption, rheology, and sedimentation studies.

An internally consistent set of data was generated for the chemical vapor deposition (CVD) of SiC from methyltrichlorosilane (MTS) and H2 at atmospheric pressure. A moving fiber tow was used as the substrate. Coating rates between 0.3 and 3.7 µm/min and deposition efficiencies between 24 and 48% were obtained for MTS and H2 flow rates in the range 30 to 200 cm3/min and 300 to 2000 cm3/min, respectively. The data were analyzed and found to be best fit under a mass transfer regime. Based on this fit, a value of the constant in the Chilton–Colburn j factor expression for a moving fiber tow was estimated to be 2.74 × 10−6 with a standard deviation of 3.2 × 10−7. The efficiency of the reaction was found to decrease with increases in the total flow rate, indicating that the effect of the decreased residence time of reagents in the reactor was larger than the increase in the mass transfer coefficient. Finally, a comparison between the efficiencies for a stationary and a moving tow revealed that the moving tow had a higher efficiency, possibly due to a disruption of the boundary layer by the tow motion or due to the decrease in the canning of the moving tow.

Ferroelectric layered-perovskite BaBi2Ta2O9 has been successfully prepared through a novel process using BiTaO4 as a precursor. Heating the mixtures of BiTaO4 and BaCO3 at 900 °C without soaking results in the complete formation of the monophasic powder. In contrast, the conventional solid-state reaction requires soaking at 900 °C for 2 h to obtain the pure compound. Such prolonged heat treatment causes unfavorable growth of particles. In the new process, the formation of BaBi2Ta2O9 is markedly accelerated due to the suppression of the formation of a stable intermediate Ba5Ta4O15. In addition, this process yields submicron BaBi2Ta2O9 powder with significantly improved sinterability. Sintering at 1000 °C affords well-densified ceramics. On the other hand, heating at temperatures greater than 1100 °C causes BaBi2Ta2O9 to thermally decompose and form Bi2O3 and rod-like BaTa2O6. The formation of these rod-like grains results in the expansion of the matrix, thereby reducing the density.

A micromechanistic model of the combustion synthesis of NbC has been developed by combining the results of an experimental study of the intrinsic, pore-level kinetic mechanism [C. He. and G. C. Stangle, J. Mater. Res. 10, 2829–2841 (1995)] and a theoretical model developed previously [Y. Zhang and G. C. Stangle, J. Mater. Res. 9, 2592–2604 (1994); 9, 2605–2619 (1994)], in order to account for the various physical and chemical processes that take place during the combustion synthesis process. Results of the present investigation are interpreted from both a macroscopic and a microscopic point of view. Moreover, the relationship between the microscopic processes and macroscopic features of the combustion synthesis process is discussed. The results show that the formation of a combustion wave in the Nb-C system corresponded to establishment of a proper balance between the rates of enthalpy redistribution within the sample. Furthermore, the pore size had a significant influence on the combustion synthesis process: smaller pores gave rise to a higher area of contact between the reactants, which in turn gave rise to a higher rate of enthalpy increase due to the enhanced rate of product formation. The influence of the pore size distribution on the process is also discussed.

Polycrystalline RuO2 thin films were grown by metal-organic chemical vapor deposition (MOCVD) on both SiO2/Si(001) and Pt/Ti/SiO2/Si(001) substrates. Films having a controllable and reproducible structural texture and phase purity were synthesized by carefully controlling deposition parameters. Moderate growth temperatures (∼350 °C) and low growth rates (<30 Å/min) produced highly (110)-textured RuO2 films. Highly (101)-textured RuO2 films were favored at slightly lower temperatures (∼300 °C) and much higher growth rates (>30 Å/min). The most conductive RuO3 films had resistivities of 34 to 40 µΩ−cm at 25 °C, an average grain size of 65 ± 15 nm, and a surface roughness (rms) of 3 to 10 nm. Both single-phase Ru and mixed Ru/RuO2 phase material were also fabricated at low temperatures (<350 °C) by using lower oxygen flow concentrations (<10%).

The kinetics of in situ crystallization of LaNiO3 thin films in sputtering deposition at temperatures ranging from 250 to 450 °C and isothermal crystallization of room-temperature (RT) sputtered LaNiO3 thin films in annealing at 350–500 °C were investigated by the x-ray diffraction method. The crystallization in both cases basically followed the Johnson–Mehl–Avrami (JMA) relation. However, different crystallization kinetics were observed. The transformation index and activation energy of crystallization in high temperature sputtering were about 1.5 and 33 kJ/mole, respectively, while in the annealing of RT-sputtered films, 1.0 and 63 kJ/mole were found. From the determined transformation index, it is suggested that the crystallization rate in high temperature sputtering was determined by a diffusion-controlled process of lateral growth with a decreasing nucleation rate of crystallites in the adsorption layer. However, the annealed films crystallized by an interface-controlled and one-dimensional growth of existing nuclei.

The half-thickness and mass absorption coefficient of 13.0 keV x-rays, 46.5 keV γ-rays, and 1.16 MeV βƟ particles have been measured for pristine, bromine intercalated, and iodine monobromide intercalated pitch-based graphite fiber composites. Since these materials have been proposed to replace aluminum structures in spacecraft, the results were compared to aluminum. Pristine graphite epoxy composites were found to have about 4.0 times the half-thickness, and 40% of the mass absorption of aluminum for ionizing radiation. Bromine intercalation improved performance to 90% of the half-thickness, and 1.7 times the mass absorption coefficient of aluminum. Iodine monobromide extended the performance to 70% of the half-thickness and 3.0 times the mass absorption of aluminum. Thus, intercalation not only makes up the deficiency conventional composites have in shielding components from ionizing radiation, but actually confers advantages in mass and thickness over aluminum. The βƟ particle shielding of all the materials tested was found to be very effective. The shielding of all of the materials was found to have nearly the same mass absorption coefficient of 17.8 ± 0.9 cm2/g. Inelastic scattering processes were found to be important in βƟ particle shielding; however, the extent of inelastic scattering and thus the distribution of energies of the transmitted electrons did not vary with material.

Transmission electron microscopy and high-resolution electron microscopy have been used to study microstructural properties of conductive SrRuO3 films grown by pulsed laser deposition on (001) LaAlO3 and (001) SrTiO3 substrates. It was found that the SrRuO3 films deposited on both substrates consist of mixed domains of [001] and [110] orientations, with orientation relationships that can be described as (i) (001)f ‖ (001)s and [110]f ‖ [100]s and (ii) (110)f ‖ (001)s and [001]f ‖ [100]s, respectively. The SrRuO3 films deposited on SrTiO3, in particular, were found to have a layered domain structure, with the [110] domain grown initially on the substate, followed by growth of the [001] oriented domain with increasing thickness. The films on SrTiO3 are strained and have a coherent interface with the substrate. The SrRuO3 films deposited on LaAlO3, on the other hand, contain a high density of structural defects such as stacking faults and microtwins on the (022) planes. Microtwins as large as 50 nm in thickness are observed in the films deposited on LaAlO3. Possible causes for the observed structural defects in the films are discussed.

The growth kinetics of SiO2 thin films obtained by low-pressure chemical vapor deposition (CVD) from SiH4/O2/N2 gas mixtures has been determined at different temperatures and flow rates. The results show that the film growth is originated by some intermediate species (e.g., SiOxHy) produced in the gas phase. At low temperatures the deposition rate is limited by some homogeneous reaction with an apparent activation energy of 1.42 eV. Furthermore, the observation of critical limits when total pressure, oxygen/silane flow ratio, and temperature are decreased gives support to a branching-chain mechanism of deposition. Finally, we have observed that the deposition rate shows a hysteresis behavior when varying the temperature within the 300–400 °C range, which has been attributed to the inhibition of silane oxidation by the Si–OH surface groups of the films grown on the reactor walls.

Ion beam deposition provides an additional control of ion beam energy over the chemical vapor deposition methods. We have used a low energy ion beam of hydrogen and carbon to deposit carbon films on Si(100) wafers. We found that graphitic films, amorphous carbon films, and oriented diamond microcrystallites could be obtained separatedly at different ion beam energies. The mechanism of the formation of the oriented diamond microcrystallites was suggested to include three components: strain release after ion bombardment, hydrogen passivation of sp3 carbon, and hydrogen etching. Such a process can be extended to the heteroepitaxial growth of diamond films.