The Reaction-Bonded Aluminum Oxide Process: I, The Effect of Attrition Milling on the Solid-State Oxidation of Aluminum Powder

The effect of attrition milling on the solid-state oxidation of aluminum powder is important for the reaction-bonded aluminum oxide process. Attrition milling increased the surface area to 14.4 and 20.2 m²/g versus 1.2 m²/g for unmilled powder and smeared the Al particles, and the surface was hydrolyzed to form bayerite and boehmite. Upon heating the hydroxides decompose to form an 11–13 nm thick amorphous plus γ-Al₂O₃ layer which subsequently retards oxidation kinetics. The oxidation per unit area decreases for the higher surface area powders at temperatures below the critical temperature but the total oxidation of the milled powder is ∼70% versus ∼9% for the as-received powder because of the higher surface area. The critical temperature depends on Al particle surface characteristics and is defined as the transition temperature above which the initial rate of oxidation is linear, not parabolic. Above the critical temperature the oxidation per unit area decreases significantly. In addition, linear oxidation occurs faster than parabolic oxidation and thus the initial fast oxidation kinetics (i.e., linear) can cause thermal runaway during oxidation. Therefore, oxidation below the critical temperature is essential to maximize solid-state oxidation and to prevent thermal runaway. The critical temperatures for the as-received (1.24 m²/g), the 6 h (14.4 m²/g), and 8 h (20.2 m²/g) attrition-milled Al powders were 500°, 475°, and 500°C, respectively. A model for oxidation during the parabolic and linear oxidation stages is presented.     

Comparison of Texture Analysis Techniques for Highly Oriented α-Al2O3

Texture measurements were performed on liquid-phase-sintered alumina textured by a templated grain growth process from 1250° to 1650°C. Texture distributions were measured using X-ray pole figures, rocking curves, Rietveld refinement, and stereology. The March–Dollase equation fitted the measured distributions very well and gave quantitative values of the degree of texture and the texture fraction. The fitting parameters of the X-ray diffraction measurements were comparable to those measured by stereology. Rocking curve analysis was found to be straightforward and to give accurate characterization of texture in the alumina system of this study in a relatively short time.     

Hybrid Gels for Homoepitactic Nucleation of Mullite

Hybrid gels, defined as gels from mixtures of polymerically and colloidally derived sols, offer many opportunities for crystalline microstructure development upon heating. In this study, hybrid mullite gels are formed by mixing a colloidal boehmite—silica sol with a polymeric aluminum nitrate—tetraethoxysilane-derived sol. The polymeric gel crystallizes in situ to form mullite that acts as seed crystals for homoepitactic nucleation during the subsequent transformation of the colloidal component of the hybrid gel. Compared with the entirely colloidal gel, the introduction of a 30 wt% polymeric gel fraction results in an increase in apparent nucleation frequency from ∼5x10¹¹ to ∼1x10¹⁴ nuclei / cm³ at 1375°C, a reduction in high-temperature grain size from 1.4 to 0.4 μm at 1550°C, and an increase in the degree of microstructural homogeneity, as evidenced by intragranular pore removal.     

A description of agricultural releases of anhydrous ammonia in Minnesota

Anhydrous ammonia releases were reviewed during a three-year period (1995 to 1997) by the Minnesota Hazardous Substances Emergency Events Surveillance Program. Of the 800 hazardous substances emergency events recorded, 97 (12.1%) involved anhydrous ammonia. This review found that 51% of the total quantity of anhydrous ammonia released in emergency events came from agriculture (92% excluding one train car event). Emergency events involving anhydrous ammonia were almost twice as likely to involve victims than other events. However, nonagricultural and agricultural anhydrous ammonia emergency events had similar probabilities for victims. Nonagricultural anhydrous ammonia events were more likely to result in evacuations than agricultural anhydrous ammonia events. Given the large amount of agricultural anhydrous ammonia used every year in Minnesota, an average of 690 million pounds per year, the tiny fraction (0.005%) of agricultural ammonia released in emergency events, and the small number of victims and evacuations, agricultural ammonia is being handled relatively safely. Nevertheless, most accidents involving agricultural ammonia could be prevented if operators would practice relatively simple safety precautions.     

(Reactive) Templated Grain Growth of Textured Sodium Bismuth Titanate (Na1/2Bi1/2TiO3-BaTiO3) Ceramics—II Dielectric and Piezoelectric Properties

(Na_1/2Bi_1/2)TiO₃-BaTiO₃ (<6.5% BaTiO₃) ceramics with <001>_pc orientation were fabricated by Templated Grain Growth (TGG) or Reactive Templated Grain Growth (RTGG) using tabular SrTiO₃ template particles. The maximum electrically-induced strain was 0.26% at 70 kV/cm. d₃₃ coefficients over 500 pC/N were obtained for highly textured samples (f 90%) when driven at high electric fields. Under these conditions, the materials show considerable hysteresis in the strain—field response, even after poling. Berlincourt piezoelectric coefficients for the same samples gave d₃₃ of 200 pC/N.     

Using the sewerage system main conduits for biological treatment. Greater Tel-Aviv as a conceptual model

The long-sought idea of using main sewerage conduits as a step-fed plug-flow reactor, where biological sludge is introduced at the head of the main pipelines (hence loop system) and where aeration is provided at various points along the loop, was investigated in this paper in both a bench-scale physical model and using the Greater Tel-Aviv (Dan Region) sewerage with design population of 1.3 million and flow of 300,000 m³ day⁻¹, as a conceptual model.Due to the inherent advantages of a step-fed plug-flow reactor where high substrate concentrations are maintained along most of the length of the reactor, (except for the final section which provides effluent polishing), the viability of the biomass is 3–5.2-fold higher than the one usually encountered in a conventional activated sludge plant, thus substantially increasing its rate and efficiency of organic substrate removal.It has been shown that in the case of Greater Tel-Aviv, replacing the activated sludge conventional treatment plant with the loop system (with only secondary clarifiers to provide the return biological sludge), or alternatively resorting to the loop system instead of doubling an existing overloaded treatment plant, will produce final efluents well below the requirements of 25 mg l⁻¹ BOD₅ and will provide savings of more than 50% in construction costs. An additional advantage of corrosion control along the main conduits thus increasing their longevity is also discussed.     

Liquid-Phase Sintering of Alumina Coated with Magnesium Aluminosilicate Glass

Densification of alumina coated with a MgO-Al₂O₃-SiO₂ (MAS) glass was investigated from 1400° to 1460°C. Spinel was observed to form at the liquid-alumina interface, whereas mullite crystallized uniformly in the liquid. Spinel and mullite crystallization kinetics were accelerated by smaller alumina particle size. Mullite and spinel crystallization retarded densification by forming a percolating network. Boron doping suppressed spinel formation, and thus alumina sintered to higher densities at low temperature. The concept of glass basicity is proposed as a useful guide for selecting dopants for low-temperature sintering.