Ceramurgia International最新文献

Fine-grained (< 5μm), dense (3.26–3.27 g/cm³), and strong (> 200 MN/m²) polycrystalline β″-alumina (lithia-stabilized) ceramics can be fabricated by vacuum hot-pressing at temperatures ≥ 1330°C and pressures between 28 and 41 MN/m². By suitable low temperature annealing (< 1400°C), a fine-grained, high strength β″ alumina body can be fabricated with a low resistance (< 5 ohm-cm at 300°C) to sodium ion conduction. The relatively high sodium ion resistivities (6–11 ohm-cm) obtained directly after hot-pressing are caused by an incomplete phase conversion to β″-alumina during densification and not to a small grain size. Low temperature annealing can promote phase conversion and yet prevent any significant grain growth. The high strength ceramics produced by hot-pressing can serve as the standard to be achieved in conventional sintering.

A freeze-drying technique has been developed to synthesize fine-grained stoichiometric mullite (3Al₂O₃ · 2SiO₂) by doping a silica sol with an appropriate quantity of a one molar Al₂(SO₄)₃ solution followed by spray freezing in liquid N₂ and freeze drying the mixture. The freeze-dried powder when calcined at 1400°C produced a single-phase stoichiometric mullite as determined by X-ray diffraction. The final product was very friable and could be easily broken down to a mean particle size of ∼7 μm (equivalent spherical diameter).

A comparative analysis of the magnetic properties of BaFe_12−xMe_xO₁₉ (Me = Al, Ga, Sc, In) ferrites is given. The Influence of the substitution of iron ions on the magnetic ordering and on the magnetic properties is pointed out. The reported data indicate that the occupation by non magnetic ions of lattice sites inside the R-block leads to peculiar helical spin structures. Further confirmation of the relevant contribution of iron ions in the 12 K sublattice to the single ion uniaxial anisotropy has been obtained.

Scanning electron microscopy and X-ray diffraction lattice parameter measurements have been used to determine the extent of solid solubility of oxides in MgO. The solubility of Sc₂O₃ ranges between 0.005 and 0.044 mole fraction in the temperature interval from 1270°C to 1600°C, with an experimental heat of solution equalling 36.8 kcal/mole. Cr₂O₃ has a similar solubility range, between 0.007 and 0.038 mole fraction in the temperature interval from 1200°C to 1600°C, and the heat of solution was determined to be 43.6 kcal/mole. Much smaller solubility was found for Al₂O₃, between 0.004 and 0.007 mole fraction in the temperature interval from 1200°C to 1600°C, and the heat of solution was 97.2 kcal/mole. The solubility of SiO₂ is less than 0.00034 mole fraction at 1850°C and the solubility of ZrO₂ is less than 0.000075 mole fraction at 1885°C.

The kinetics and mechanism of the reaction between sodium carbonate and silica were studied using thermogravimetric analysis to monitor the percent reaction versus time. A modified form of the rate constant in the Ginstling and Brounsthein model has been shown to describe the sodium carbonate - silica reaction of dried samples. This equation is: $\text{(a)K}\text{=1−}\text{2x}\text{3}\text{−(1−}\text{x}\text{)}{}^{\mathrm{2,3}}$$\text{were}\text{K=}\text{2DC}\text{1−}\text{R}{}_{\mathrm{n}}\text{k}{}_1\text{M}\text{+}\text{Q}\text{2}\text{+}\text{1}\text{2}\text{1}\text{2}\text{−1}\text{k}{}_2\text{R}{}^2{}_{\mathrm{SiO2}}$ t is the time, D is the diffusion coefficient of the rate controlling species, C, is the concentration gradient, K₁ and K₂ are constants, R_n is the sodium carbonate particle size, R_SiO2 is the silica particle size, and M and Q are the total volumes of the sodium carbonate and silica, respectively. Surface diffusion was shown to cover all silica particles rapidly and supply Na⁺ and O⁼ ions to the reaction interface. The activation energy of the process was found to be 30 k-cal/mole. The presence of water vapor caused the diffusion rate to increase rapidly and the rate controlling step became chemical processes which occurred at the phase boundary. This process was determined to have an activation energy of 35 k-cal/mole.

The resistance of dense strontium zirconate ceramics to corrosion by Al₂O₃, CaO, MgO, SiO₂ and K₂CO₃ as well as coal ash was investigated by the ⪡ crucible method ⪢ for 2 hrs at 1700°C. The corrosion process was investigated by optical polarizing microscopy, X-ray microanalysis and scanning electron microscopy. The results showed that strontium zirconate ceramics have excellent corrosion resistance against K₂CO₃. No corrosion effect by Al₂O₃, CaO and MgO was observed as well. Considerable corrosion occurred when silica was used which formed a baddeleite dispersed phase in a glassy matrix of a variable composition. A similar effect was observed in crucibles corroded by coal ash containing silica.

The electrical conductivity of polycrystalline Y₆WO₁₂ (3Y₂O₃·WO₃) and of polycrystalline Y₆WO₁₂ doped with alkaline earth oxides or ZrO₂ were measured over the temperature range from 800 to 1400°C under oxygen partial pressures of 10⁻⁵ to 10⁰ atmospheres. Y₆WO₁₂ showed a p-type conductivity at higher oxygen pressures and an n-type conductivity at lower oxygen pressures. Additions of alkaline earth oxides increased the conductivity of Y₆WO₁₂ in the higher oxygen pressure region. The conductivity of alkaline earth oxides-doped Y₆WO₁₂ decreased monotonically with decreasing oxygen partial pressures and approached constant values at lower oxygen partial pressures. The ZrO₂ addition caused a decrease in the conductivity of Y₆WO₁₂ in the higher oxygen pressure region. The variations of the conductivity with oxygen pressure are discussed assuming various defects models.