Thermal conversion of acetate-derived gels to YBa2Cu3Ox (Y–123), Bi2Sr2CaCu2Ox (Bi–2212), and (Bi, Pb)2Sr2Ca2Cu3Ox (Bi-2223) has been studied by thermal analysis, x-ray diffraction, and infrared spectroscopy. Carbonates formed above 200 °C during thermal treatment of all gels. Decomposition of the carbonates proved to be more difficult for Y-123 than for Bi-2212 or Bi-2223. However, all of the gels that were heated contained significant amounts of carbon after calcination. Complete decarbonization of materials was attained by treating the intermediate phases (e.g., those formed after calcination at 600 °C) with nitric acid and then subjecting them to a final thermal treatment. Removal of carbonates from the intermediate phases strongly accelerated formation of the superconducting compounds.

Crack path stability is discussed, and it is shown that the standard continuum mechanics -force on a crack is the correct driving force for a crack. In a glass, where all cleavage planes are possible, this criterion also implies a path where the local Mode II on the crack is nullified.

The inherently weak mechanical properties associated with monolithic high-temperature superconductors (HTS) can be improved by introducing properly selected strong ceramic whiskers into the HTS materials. In this research, processing and superconducting properties of monolithic Pb-doped Bi-2223 (BPSCCO) and MgO whisker-reinforced BPSCCO HTS composite materials have been systematically studied. A solid-state processing method is successfully developed to fabricate the (MgO)w/BPSCCO composite. The HTS composite contains a dense and highly pure BPSCCO matrix phase with a preferred grain orientation, which is reinforced by MgO whiskers randomly oriented in the plane perpendicular to the hot-pressing direction. The HTS composite material is shown to exhibit excellent superconducting properties. For example, a transport Jc measured at 77 K in a zero field has been obtained to exceed 5000 A/cm2 in a (MgO)w/BPSCCO composite with 10% MgO whiskers by volume. Relationships among solid-state processing variables, HTS phase development, and superconducting properties of the monolithic BPSCCO and the HTS composite are established in the paper.

In this paper, a comprehensive study of the microstructure and the whisker/matrix interface of a (MgO)w/BPSCCOhigh-temperature superconducting composite is reported. The bulk MgO-whisker reinforced HTS BPSCCO (2223) composite was fabricated using a recently developed solid-state processing method. The (MgO)w/BPSCCO composite has been demonstrated to possess excellent combined superconducting and mechanical properties. The favorable microstructure of the HTS BPSCCO (2223) matrix and the (MgO)w/BPSCCO interfacial properties are recognized to be the critical material parameters governing electric and mechanical performance of the HTS composite. Effects of detailed microstructure variables on superconducting properties of the composite are addressed, including the aspect ratio and the orientation of MgO whiskers, structure, and texturing of the BPSCCO matrix phase, and the (MgO)w/BPSCCO interfacial microchemistry. The results obtained reveal unique characteristics of the reinforcing MgO whiskers dispersion, distribution, and orientation in the HTS composites. The evolution of the microstructure and texture of thematrix BPSCCO grains has also been studied in the process of repeated hot-pressing and annealing heat treatment. The thermodynamic compatibility and microchemistry in the MgO whisker and BPSCCO interfaceare also examined.

A two-powder process is described for the production of uniform, fine-grained Bi2Sr2Ca2Cu3Oy (Bi-2223) powders. One powder is the Bi2.1−xPbxSr1.9−yCayOz (2:2 Cu-free) phase. The other is a multi-phase powder of approximate overall composition SrCaCu3Oy. The 2:2 Cu-free is one of the first Bi-containing phases to form from a nominal Bi-2223 mixture of oxides and carbonates. This precursor route was chosen for investigation because (1) the powders have very similar particle morphologies and (2) the mixing volumes are closely matched. Both of these characteristics facilitate the milling and blending process. This precursor mix was found to be stable in that explosive grain growth of undesirable phases was not observed during sintering. Critical current densities up to 26,900 A/cm2 in self field at 75 K were obtained in tapes.

YBa2Cu3O7−δ precursor powder suitable for the fabrication of melt-processed ceramic has been prepared by spray drying and subsequent calcination. The dehydration and decomposition processes in the nitrate-based spray-dried powder and the formation processes of the YBa2Cu3O7−δ and Y2BaCuO5 phases have been studied in detail using thermal analysis and powder x-ray diffractometry. The morphology, microstructure, and size distribution of the particles in the calcined powder have been investigated using high resolution scanning electron microscopy and particle size analysis and found to contain a significant proportion of submicron sized particles of Y2BaCuO5. Melt-processed samples prepared using the spray-dried powder have been observed to exhibit a significantly finer distribution of the 211 phase than is typically obtained in similar specimens prepared via a mixed oxide route. This provides direct evidence of the suitability of spray-dried precursor powder for the fabrication of large grain, high Jc melt-processed YBCO ceramics.

Precise x-ray diffraction measurements between room temperature and ∼400 °C (Bond method) exhibit some details in the variations of strain in ∼ 1 μm thick epitaxial layers of GaAs, InP, CdTe, EuS, or SrS on Si or GaAs substrates. The lattice parameters of the cubic layers, which are deposited at high temperatures, deviate from the lattice parameters, a0, of small unconstrained single crystals by Δa/a0 = ∈0 ≲ 10−3. The layers adhere to the substrates below Tc and adopt different strains, ∈‖ and ∈⊥, parallel and perpendicular to the substrate. Frequently the Tc and ∈0 values vary on annealing at 160–400 °C. The ratio E = —(∈⊥ — ∈0)/(∈‖ — ∈0) remains constant for each sample. The change of the relative volume ΔV/V0 = ∈‖ (2—E) +∈0 (1 + E) at the variation of ∈0 can give rise to corrugations, blisters, or microcracks in the epitaxial layers. Stable epitaxial layers with constant ∈0 and Tc values can be obtained by deposition on buffer layers or stepped substrates.

The electrical activity of phosphorus in Si80Ge20 alloys prepared by two nonconventional doping processes has been investigated over the temperature range 25–1250 °C. Both a solid state (mechanical alloying) and gaseous phase doping processes were found to extend the electrical activity of phosphorus in Si80Ge20 alloys beyond the reported maximum equilibrium value (2.1 × 1020/cm3) to 2.5–2.9 × 1020/cm3 within the temperature range 900 1200 °C. It is likely that this extended electrical activity of phosphorous is associated with a high density of defects. The enhanced electrical activity of phosphorus enabled Si80Ge20 alloys to have 300 to 1000 °C integrated average electrical power factors in the range 30.1–35.7 μW/cm-°C2.

This paper describes a new application of two complementary surface characterization techniques to study solid-state bonding in an Al–Li alloy. Through the two mentioned techniques, Atomic Force Microscopy (AFM) and Secondary Ion Mass Spectrometry (SIMS), important findings about what takes place in the bond interface have been determined. These findings enclose both the formation of discontinuous mixed oxides and the evolution of Li through the bond line and into theadjacent diffusion affected zones. Homogenization of Li and Cu alloyelements has been detected even in those cases where a metallic interlayer was used to favor the union.

Fe–29Al–2Cr powders with nanoscale grain sizes were produced by mechanical milling of prealloyed intermetallic powders. A consolidation procedure employing high-pressure, low strain rate hot forging (sinter-forging) has been developed to consolidate the powders into full-density compacts. The relative density and average grain size of the compact have been studied as a function of consolidation temperature at constant pressure. Fully dense compacts (>99.5% theoretical density) were produced at a relatively low temperature of 545°C with a pressure of 1.25 GPa. Transmission electron microscopy and x-ray diffraction analysis indicate that the average grain size has been maintained to the order of 30 nm in samples consolidated under these conditions. By using protective Ar atmosphere during mechanical milling and consolidation, contamination of oxygen and carbon in consolidated samples has been controlled to below a small fraction of an atomic percent. Microhardness tests of nanocrystalline Fe–29Al–2Cr samples indicate a significant strengthening effect due to grain size refinement and a monotonic hardness increase with decreasing residual porosity. Our work demonstrates the feasibility of using mechanically milled powders as the source of nanocrystalline materials for the production of fully dense, low-impurity, nanocrystalline bulk samples needed for reliable mechanical property measurements and practical applications.

The high strength medium temperature and β metastable titanium alloy, β-CEZ, is designed for use in the compressor of a jet engine. The purpose of this research was to investigate how the thermomechanical treatments (forging or solutioning conditions and quenching) influence the microstructure. A series of various heat-treating steps were conducted in order to quantify the relationship between thermomechanical parameters and microstructural factors. The detailed characterization of microstructure was based on SEM observations of primary alpha platelets with the aid of an Image Analysis system. A pronounced influence of thermomechanical treatments on microstructure was found. Several parameters characterizing the microstructure of alpha platelets such as the area fraction, length, width, perimeter, surface, density, and aspect ratio, have been found to depend on the solutioning conditions. Furthermore, with the exception of the area fraction, these different microstructural parameters were found to be also governed by the forging conditions.

The tensile properties of the β-CEZ titanium alloy (Ti Al, 2Sn, 4Zr, 4Mo, 2Cr, 1Fe wt. %) have been investigated at intermediate temperatures as a function of the thermomechanical treatments. the strength and ductility have been correlated with the microstructural changes caused by the various heat treatments. It is found that the highest yield strength is associated with a structure containing a small area fraction, a small size, and a high density of primary α platelets (αp). Moreover, the size of the secondary α phase (αs) has been found to affect the yield strength. It is concluded that the ductility is enhanced by a high area fraction of αp platelets and is also sensitive to the density and the morphological orientation of the αp platelets. The interfaces between the αp and β phases play a dominant role in β-CEZ tensile behavior, and transgranular fracture is the main failure feature of this alloy.

Dense, crystalline, glass-ceramic coatings containing calcium metaphosphate and aluminum phosphate were prepared on aluminum oxide substrates by a three-step method. The processing involved glass (40 mol% CaO, 10 mol% Al2O3, 50 mol% P2O5) formation, deposition of a glass particle coating, and heat treatment to sinter the glass and crystallize the phosphates. Sintering and microstructure evolution were influenced by wet coating thickness, heat-treatment temperature, time, and heating rate. Heat treatment for 1 h at 725 °C using a 50 °C/min heating rate was found to give a dense, crack-free coating. The resultant coating microstructure has spherulitic morphology (0.3 μm size) with aluminum phosphate in the center of the spherulite. The hardness of the fully crystallized glass-ceramic coating was ∼5.2 GPa.

A rapidly solidified alumina-zirconia eutectic material containing nanocrystalline t-ZrO2 has been synthesized. When heated, the microstructure contained a mixture of t-ZrO2 and m-ZrO2, each of which can facilitate toughening of the composite. Dynamic shock compaction was used to accelerate densification of the material, producing crack-free specimens with high green densities. After sintering to densities measuring ∼95% of theoretical, the shock-compacted specimens fabricated with unstabilized alumina-zirconia were extensively microcracked due to an overabundance of the m-ZrO2 phase. Experiments employing Y2O3 as a chemical stabilizer have shown that the extent of the phase transformation can be controlled, and the microstructure that developed in the stabilized material contained an acceptable level of the microcrack generating m-ZrO2 phase.

Compressive creep deformation of hot-pressed silicon nitride with different amounts of grain boundary glassy phase was investigated at 1300–1400 °C under 30–100 MPa. The stress exponent of the creep rate was determined to be nearly unity. The apparent activation energy of silicon nitride with a larger amount of glassy phase was measured to be about 700 kJ/mole, and that with a smaller amount of glassy phase was found to be 400 kJ/mole. In addition, the microstructural observation found that no cavity appeared and grain boundary glass was recrystallized during creep test. Thus, the rate-limiting steps in solution/precipitation creep mechanism change from the solution-reprecipitation of Si3N4 grains to the diffusion through the grain boundary with increasing the amount of glassy phase.

The infrared complex permittivity function of pseudo-cubic, disordered, spinel-type variety of alumina, η–Al2O3, obtained by spray pyrolysis, has been determined from its IR reflectance spectra, measured at near to normal incidence on pressed powder pellets. The optical constants obtained therefrom have been verified by using them in the simulation of the corresponding absorption spectra for KBr-diluted pellets of this material, and these are in excellent agreement with the experimental spectra. All calculations are based on a procedure for the estimation of the effective dielectric function of a mixture incorporating percolation features, which has been recently developed by the authors.

Monolithic gels of phosphate ceramics were synthesized using PO(OH)3−x(OR)x and alkoxides of silicon and titanium. The PO(OH)3−x(OR)x species were synthesized from the reaction of P2O5 and ethanol or n-butanol, and the products consisted of approximately equal molar amounts of mono- and dialkyl phosphate. The phosphate gels containing titanium lost less phosphorus than from the gels of silicon/phosphorus upon firing of gels in air. At phosphorus contents above 60 mole %, the gels were completely crystallized upon firing at temperatures above 700 °C, while the gels containing zinc and alkali metals remained amorphous after firing at 850 °C. Solid state nuclear magnetic resonance spectroscopy showed that all of the silicon is hexacoordinated in the phosphate gels containing silicon and titanium upon firing at temperatures above 520 °C

Several electrorheological (ER) materials systems composed of amorphous ceramic powders dispersed in light paraffin oil were developed to determine if relationships among ER activities, dielectric properties, compositions, porosities, and oxide species could be identified. The results of the studies suggested that trends among ER activity, dielectric phenomena, and alkali metal species existed. The aluminosilicate powders developed with various alkali metals showed that the ER activity increased as the activation energy decreased. The sodium aluminosilicate appeared to have the greatest ER activity and the lowest activation energy, while the cesium aluminosilicate displayed the weakest ER response, but had the highest activation energy. The thermodielectric responses of the different oxide materials systems developed with sodium showed that the mechanisms contributing to the dielectric dispersions had similar activation energies; however, the magnitudes of the recorded ER activities varied, and thus a direct correlation was not apparent. In addition, studies conducted with ER materials composed of sodium aluminosilicate powders of varying porosities showed that ER activities increased with increasing porosity. Furthermore, the analysis of the results of the thermodielectric and rheological studies of the different amorphous materials ER systems suggested that these materials may have an optimal stimulus frequency/temperature for ER activity.

Colloidal spherical particles of SiO2, Al(OH)3, and BaTiO3 of narrow size distributions were produced in a continuous static mixer tubular reactor system. Several experimental parameters, including the flow rate, reaction temperature and time, reactant concentrations, and the dimensions of the reactor, were varied in order to establish the optimum conditions required for each material. The results were compared with those obtained in batch systems. Monodispersed colloids can be generated at a rate as high as 50 pounds per day using the described laboratory scale continuous reactor.

BaTi4O9 and PbxBa1−x Ti4O9, where x is 0.1, 0.2, 0.3, 0.4, or 0.5, have been prepared by a combustion synthesis process. The process starts with spray drying aqueous solutions of Pb(NO3)2, Ba(NO3)2, TiO(NO3)2, and β-alanine with appropriate ratios. Combustion reactions occur when heating the spray-dried products to 300 °C, which convert them to BaTi4O9 and PbxBa1−xTi4O9 directly. PbxBa1−xTi4O9 (x ≧ 0.1) are low temperature, metastable phases and have not been reported before. Pb0.5Ba0.5Ti4O9 is unstable above 800 °C and cannot be sintered. All PbxBa1−xTi4O9 compositions will decompose by 1300 °C, the temperature for solid state synthesis of BaTi4O9. Single-phase PbxBa1−xTi4O9 (x = 0.1, 0.2, 0.3, 0.4), however, have been sintered at relatively lower temperatures.