Kinetic Analysis of Combustion Synthesis of Lead Magnesium Niobate from Metal Carboxylate Gels

A combustion synthesis route was developed for direct crystallization of perovskite lead magnesium niobate (PMN) from metal carboxylate gels. Direct PMN formation is attributed to a homogeneous cation distribution in the gel and the rapid heating during exothermic gel decomposition in oxidizing atmospheres. The maximum bed temperature and gel decomposition rate are determined by the combined influence of chemical kinetics, heat, and mass transfer. It is shown that there is a temperature window between 700° and 750°C which favors direct PMN formation, whereas pyrochlore formation is favored below 650°C and PbO volatilization occurs above 800°C. The effects of kinetic and transport parameters including oxygen partial pressure, gas flow rate, and bed geometry on PMN formation are discussed on the basis of a thermal ignition model for heterogeneous reactions.     

A cathodoluminescence study of the influence of the seed particle preparation method on the optical properties of GaAs nanowires

Cathodoluminescence at 8?K is used to compare the optical properties of AlGaAs-capped GaAs nanowires, grown by metal-organic vapour phase epitaxy and seeded by gold particles prepared by different methods. Six different methods were used to fabricate and deposit gold seed particles onto GaAs substrates: colloid particles, aerosol particles and particles defined by electron beam lithography. The nanowires were grown with and without an in?situ annealing step prior to the nanowire growth. The morphology showed no significant differences between the nanowires. The emissions from ensembles of nanowires have the same peak position, irrespective of seed particle type. Without the in?situ annealing step prior to the nanowire growth, there are significant differences in the emission intensity and emission patterns from nanowires grown from different seed particles. When an in?situ annealing step is included, all the resulting nanowires show identical optical emission intensity and emission patterns. This shows the importance of using an in?situ annealing step prior to growth. This study demonstrates that different preparation methods for gold seed particles can be used to produce GaAs nanowires with highly similar optical properties. The choice of particle preparation method to be used can therefore be based on availability and cost.     

Preparation of Alumina-Zirconia Powders by Evaporative Decomposition of Solutions

Fine-grained, homogeneously dispersed alumina-zirconia and zirconia powders were prepared by evaporative decomposition of solutions. The pure metastable tetragonal zirconia powder transformed to the monoclinic form when it was heated to 1150°C. The zirconia in the alumina-zirconia powder, which was also in the tetragonal form, did not transform when the powder was heated to 1150° C. This result is explained in terms of inhibition of coarsening of the zirconia grains by the alumina particles.     

Constrained densification in boehmite–alumina mixtures for the fabrication of porous alumina ceramics

It is demonstrated that inherent constrained densification characteristics of the bimodal size distributed powders can be utilized to fabricate porous alpha alumina. Seeded boehmite was mixed with coarse alpha alumina particles to produce various compositions of bimodal mixtures. The densification behaviour of the mixtures was correlated with shrinkage and microstructural evolution. The mixtures sintered at 1200 °C contained more than 30% porosity and the mechanical strength of the mixtures was increased by 2–4 fold relative to the coarse particles alone with similar porosity (<5 MPa). © 1998 Chapman & Hall     

Star wars and strategic defense initiatives: work activity and health symptoms of unionized bank tellers during work reorganization

Work activity and health symptoms of bank tellers whose work was undergoing reorganization were examined during a university-union study of the health effects of work in women's traditional jobs. Data were gathered through collective and individual interviews, analysis of work activity, and a questionnaire administered to 305 tellers. Employees worked in a standing posture over 80 percent of the time. More than two-thirds frequently suffered pain in back, legs, and feet. The average teller had been involved in 3.7 robberies as a direct victim and six as a witness. Work required feats of memory and concentration. In order to meet job demands, tellers engaged in supportive activities and teamwork. The introduction of individualized objectives threatened the employees' ability to collaborate and induced distress. More than twice as many tellers as other female workers in Québec experience psychological distress (Ilfeld scale), related to: robbery during the past two years (odds ratio = 1.7; confidence interval = 1.0-2.9); difficult relations with superiors (O.R. = 2.6; C.I. = 1.3-5.3); and full-time work (O.R. = 2.3; C.I. = 1.3-3.9). Diverse methods enriched the analysis, and union participation allowed the proposal of concrete correction measures.     

Mice lacking GFAP are hypersensitive to traumatic cerebrospinal injury

Glial fibrillary acidic protein (GFAP) is an intermediate filament protein expressed primarily in astrocytes. We have tested whether GFAP protects against mechanical stress by inducing percussive head injury in GFAP-null mice with a weight drop device. When mice were positioned on a foam bed which allowed head movement at impact, all 14 wild-type mice tested survived, but 12 of 15 GFAP-null mice died within a few minutes. The cause of death appeared to be upper cervical spinal cord injury resulting in respiratory arrest. When the foam bed was replaced by a firm support, both GFAP-null and wild-type mice survived. These results indicate that mice lacking GFAP are hypersensitive to cervical spinal cord injury caused by sudden acceleration of the head.     

Transformation and Densification of Nanocrystalline θ-Alumina during Sinter Forging

The sinter forging behavior of α-Al₂O₃ seeded and unseeded nanocrystalline θ-Al₂O₃ was investigated as a function of temperature, stress, and strain rate. Seeded samples exhibited the highest degree of plastic deformation during the θ- to α-AI₂O₃ phase transformation. As a result, microstructure control, increased densification, and a higher degree of transformation were obtained. A uniform microstructure of 150 nm α-Al₂O₃ grains developed, reaching 57% relative density after sintering 1.5 wt%α-Al₂O₃ seeded samples for 30 min at 1060°C. When sinter forged at 0.25 mm/min to 63 MPa and 1060°C for 30 min large deformations during the phase transformation increased the relative density to 74%. When the stress was increased to 235 MPa (1060°C, 30 min), 99.7% dense α-Al₂O₃ with a grain size of 230 nm was obtained. By increasing the sinter forging temperature to 1150°C, 99.5% relative density was achieved at 190 MPa for 30 min.     

High Strain, 〈001〉 Textured 0.675Pb(Mg1/3Nb2/3)O3–0.325PbTiO3 Ceramics: Templated Grain Growth and Piezoelectric Properties

Lead magnesium niobate–lead titanate, 0.675Pb(Mg_1/3Nb_2/3)O₃–0.325PbTiO₃ (PMN–32.5PT) ceramics were textured (grain-oriented) in the 〈001〉-crystallographic direction by the templated grain growth process. The textured PMN–32.5PT ceramics were produced by orienting {001}-SrTiO₃ (ST) platelets (∼10 μm in diameter and ∼2-μm thickness) in a submicron PMN–32.5PT matrix. The templated growth of 〈001〉-oriented PMN–32.5PT grains on the ST platelets resulted in textured ceramics with ∼70% Lotgering factor and >98% theoretical density. Unlike most lead-based ceramics, excess PbO was not needed for sintering or grain growth. Based on unipolar stain-field measurements at 0.2 Hz, the textured samples displayed >0.3% strain at 50 kV/cm. Low-field d ₃₃-coefficients of >1600 pC/N (<5 kV/cm) were measured directly from unipolar measurements. The low drive field d ₃₃-piezoelectric coefficient of the highly textured samples is two times greater than polycrystalline PMN–32.5PT.     

Synthesis of Solid, Spherical Zirconia Particles by Spray Pyrolysis

A model is used to predict solid particle formation during spray pyrolysis by correlating droplet shrinkage before salt precipitation with its relative solution saturation. For the rapid drying conditions and droplet size of ∼ 10 μm, which are characteristic of conventional spray pyrolysis, solid salt particles are formed when a droplet's initial relative solution saturation is ∼ 10⁻² and the precipitated salt is sufficiently permeable to permit evolution of the remaining solvent after precipitation. It is proposed that this concentration allows the drying droplet more time and a shorter diffusion distance in which to maintain chemical homogeneity before precipitation. Using these concepts it is demonstrated that zirconyl chloride (ZrOCI₂· 8H₂O) and zirconyl hydroxychloride (ZrO(OH)CI) are excellent zirconium salts for solid zirconia particle synthesis by spray pyrolysis.     

Kinetic Studies of Mullite Synthesis from Alumina Nanoparticles and a Preceramic Polymer

The crystallization kinetics of mullite formation in a diphasic precursor consisting of a silicone resin filled with commercial γ-alumina nanoparticles (15 nm mean particle size, specific surface area of 100 m²/g), heated in air from 1250° to 1350°C, was studied by X-ray diffraction. Transitional γ-alumina and amorphous silica from the pyrolysis of the preceramic polymer exhibited a remarkable reactivity, as demonstrated by a very low incubation time (from 500 s at 1250°C to 20 s at 1350°C), a high mullite yield (about 80 vol%, after 100 s at 1350°C), and a low activation energy for nucleation (677±60 kJ/mol). The activation energy values found were lower than those reported previously for other diphasic systems, including sol–gel precursors. Besides the high specific surface of nanosized γ-alumina particles, the low energy barrier could be attributed to the highly reactive silica deriving from the oxidation of Si–CH₃ bonds in the silicone and to the homogeneous dispersion of the nanosized filler inside the preceramic polymer. Furthermore, the possibility of applying plastic shaping processing methods to the mixture of a preceramic polymer and nanosized filler makes this approach particularly valuable, in comparison, for instance, with sol–gel based alternatives.