A variety of spray pyrolysis (SP) techniques have been developed to directly produce ceramic powders from solutions. This paper reviews the current status of these processes in terms of the process parameters that enable the formation of particles with controlled morphology and composition. A model incorporating solute diffusion in the droplet and solvent evaporation from the droplet surface is presented to establish the critical parameters leading to solid particle formation. The model illustrates that solid particles can be obtained if solutes with high solubility and a large difference between the critical supersaturation and equilibrium concentration are used and if the process is designed to avoid solvent boiling. It is demonstrated that mixed metal oxide, non-oxide, and composite particles that are solid, hollow, porous, or fibrous can be produced by modifying the precursor characteristics, solution properties, and process parameters. The physical and chemical flexibility of SP processes offers numerous opportunities for the controlled synthesis of advanced ceramic powders and films. However, production rates are limited by the need to produce <5-[mu]m-diameter droplets and to avoid subsequent droplet coagulation. Developments in process controls, atomization, and system design are required for wider commercialization of SP-type processes.

Ceramic Powder Synthesis by Spray Pyrolysis

Boron carbide is characterized by a unique combination of properties that make it a material of choice for a wide range of engineering applications. Boron carbide is used in refractory applications due to its high melting point and thermal stability; it is used as abrasive powders and coatings due to its extreme abrasion resistance; it excels in ballistic performance due to its high hardness and low density; and it is commonly used in nuclear applications as neutron radiation absorbent. In addition, boron carbide is a high temperature semiconductor that can potentially be used for novel electronic applications. This paper provides a comprehensive review of the recent advances in understanding of structural and chemical variations in boron carbide and their influence on electronic, optical, vibrational, mechanical, and ballistic properties. Structural instability of boron carbide under high stresses associated with external loading and the nature of the resulting disordered phase are also discussed.

Boron Carbide: Structure, Properties, and Stability under Stress

Exploration of new superconductors and functional materials, and fabrication of superconducting tapes and wires of iron pnictides

The effect of ambient oxygen pressure on the synthesis of epitaxial YBa2Cu3O7−x films on (100) SrTiO3 substrates by post‐deposition annealing of amorphous precursor films was studied for oxygen partial pressures pO2 between 1.0 and 8.0×10−5 atm and annealing temperatures between 890 and 650 °C. A pO2−1/T diagram containing recent literature data regarding YBa2Cu3O7−x oxygen stoichiometry, phase stability, and liquid‐phase formation was compiled to provide guidance for the selection and interpretation of annealing conditions. The results evidence a strong dependency of growth properties on the oxygen pressure with enhanced c‐oriented epitaxy at lower pO2 values. A particularly interesting result is the formation of predominantly c‐oriented films at 740 °C and pO2=2.6×10−4 atm (0.2 Torr). Similar to YBa2Cu3O7−x films produced by in situ laser ablation at the same temperature and oxygen pressure, the films exhibited low ion channeling yields (χmin<0.1) and a dense (smooth) surface morphology, while critical ...

Effect of oxygen pressure on the synthesis of YBa2Cu3O7-x thin films by post-deposition annealing

Experiments are reported on two techniques for melt‐texture processing Ba2YCu3O6.5 by directional solidification from a semisolid melt containing particles of BaY2CuO5 and a copper‐rich liquid. One of these employs an electric resistance furnace with ambient or oxygen enriched atmosphere; the other is a laser‐heated furnace operating at 1.3 atm oxygen. Solidification interface morphologies and other structural features were examined in quenched specimens. Depending on growth rate and temperature gradient, three different types of growth morphologies of the growing 123 phase were observed: ‘‘faceted plane front,’’ ‘‘cellular dendritic’’ or ‘‘equiaxed blocky.’’ The interface temperature decreased markedly with increasing growth rate for the faceted plane front specimens. In the remaining specimens, solidification took place over a range of temperatures. The temperature of the ‘‘root’’ of the solidification front dropped, but temperature of the solidification front ‘‘tip’’ did not. A solidification model is ...

Semisolid solidification of high temperature superconducting oxides

The liquidus relations in the system YO1.5–BaO–CuOx in air in the compositional region near the superconducting oxide YBa2Cu3Ox were studied by differential thermal analysis, x-ray diffraction, electron microprobe analysis, and visual observation. The temperatures of 11 invariant points and the corresponding reactions were determined. YBa2Cu3Ox was found to melt incogruently at 1015 °C to Y2BaCuO5, which in turn melts incongruently to Y2O3 at 1270 °C. These reactions mean that preparing the superconducting phase by melting and rapid cooling will result in the presence of these two phases as well. The peritectic reaction YBa2Cu3Ox + CuO⇉Y2BaCuO5 + liquid at 940 °C accounts for the observation of partial melting, improved synthesis purity, and grain growth at temperatures of 950 °C. The determination of these invariant temperatures and reactions provide insight into optimal processing conditions.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400003769

Liquidus relations in Y–Ba–Cu oxides

In this paper, the lattice constants of boron carbides are determined by powder X-ray diffraction for samples with compositions between about 7.7 and 20.5 at.% carbon. The boundaries of the single-phase region are at about 9 at.% carbon and near, but likely somewhat less than 20 at.% carbon. The composition dependence of the lattice constants thus established provides a method of assessing the carbon concentration of unknown materials. In particular, assignment of the approximate composition of single crystals used in previous studies allows for a systematic examination of changes in interatomic separation as a function of composition. These changes are discussed in terms of a structural model of the boron carbide solid solution.

Lattice Constants of Boron Carbides

Knowledge of the equilibrium conditions under which the superconducting phases in the Tl-Ca-Ba-Cu-O system are stable would enable the controlled preparation of these materials. Isothermal equilibrium between samples in this quaternary system and Tl2O vapor from a condensed thallium oxide source has been studied. None of the superconductor phases coexisted in equilibrium with thallium oxide under the conditions employed. Instead, two previously unknown ternary thallium-calcium-barium oxides and one previously unknown thallium-calcium oxide were observed. The equilibrium phase assemblage at temperatures between 825° and 925°C consisted of the most thallium-rich ternary oxide plus CuO, although small amounts of the other thallium-rich phases may be required to balance stoichiometry. The vapor pressure of Tl2O established by pure, condensed thallium oxide clearly exceeds the stability limit for the superconductor phases. If the thallium-containing superconductors are stable phases at high temperature, they will exist over some range of lower Tl2O partial pressure.

Instability of Thallium‐Containing Superconductor Phases in Isothermal Equilibrium with Thallium Oxide

Boron NMR and NQR studies have been performed on three icosahedral borides: - and -rhombohedral boron and boron carbide (B12C3). Two 11B NMR peaks, separated by significant chemical shifts in the range from 130 ppm to 280 ppm, were clearly observed for all of the icosahedral borides that were not enriched in the 10B isotope. A single peak, however, was found for the 10B enriched boron carbide powder (90.6 at.% enrichment.) Moreover, the peak separation in the 11B NMR spectrum for the unenriched -boron was reduced when the sample was crushed into a fine powder. In addition to NMR responses, four strong NQR responses were observed for boron carbides from different manufacturers. Two resonance signals, observed at 513 kHz and 2769 kHz, correspond to one of the icosahedral boron sites and the boron in the CBC chain, respectively. The other two NQR responses, having frequencies of 361 and 380 kHz, are either 10B responses for the chain site or 11B responses for the other boron sites in the icosahedra. The NQR responses are not only consistent with the previous NMR studies performed independently by Silver and Bray (1959 J. Chem. Phys. 31 247) and by Hynes and Alexander (1971 J. Chem. Phys. 54 5296, 1972 J. Chem. Phys. Erratum 56), but also provide much higher accuracy for the values of the quadrupolar parameters.

Terry L. Aselage

Papers

Liquidus relations in Y–Ba–Cu oxides

Lattice Constants of Boron Carbides

Instability of Thallium‐Containing Superconductor Phases in Isothermal Equilibrium with Thallium Oxide

The NQR and NMR studies of icosahedral borides

Powder X-Ray Diffraction Data for the Superconducting Phase TlCaBa2Cu2O7-δ