Ceramurgia International最新文献

Possible relationships between Young's modulus at room temperature of (1–20) wt% CeO₂-fluxed HPSN and some microstructural features such as amount of the intergranular phase, porosity, α/β ratio and crystallographic β texture have been investigated. The dependence of Young's modulus on amount of integranular phase expressed in terms of the Cohen-Ishai model resulted in the relation: $\text{E}\text{= 98.0 [1 +}\text{V}{}_{\mathrm{N}}\text{(1.45 −}\text{V}{}_{\mathrm{N}}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{)]}\text{GN/m}{}^2$. A quadratic relationship, E = 290.0 (1 — AP + BP²), proved the most adequate to account for the effect of porosity on Young's modulus over the 0–36% porosity range. $\text{α}\text{β}$ ratio and β texture did not seem to have an appreciable effect.

A new technological procedure is outlined involving a hot pressing operation at 400–500°C; this operation, carried out using existing commercial equipment, can replace the traditional high temperature sintering of basic raw materials. Test results indicating the optimum conditions for hot pressing are given along with some characteristics of the resulting materials.

Fine powders (particle size smaller than 0.1 μm) are often used for the fabrication of modern fine-grained ceramics. During die compaction of such fine powders, special phenomena at the powder-wall boundary can be expected because the size of the particles is of the same order as the size of the wall asperities. In the present paper, the principles of powder mechanics are applied to experimental results obtained with a fine ferric oxide powder. The appearance of a dense boundary layer on the surface of compacts could be related to the occurrence of powder failure during compaction at the die wall.

Using a variety of surface analytical tools, the interfaces of several metal-ceramic composites have been characterized. Three processes that lead to the bonding of a metallizing to a ceramic substrate are illustrated. When a pure refractory metallizing is deposited onto a 94% Al₂O₃, bonding is achieved by glass migrating from the ceramic into the metallizing during firing. During cooling, the glass forms a mechanical-chemical bond between the ceramic and metallizing. In order to achieve bonding to a 99⁺% Al₂O₃ or 99⁺% BeO, the metallizing itself must contain a sufficient quantity of glass for wetting the ceramic, or be capable of forming a direct chemical bond to the ceramic.

Non-metallic nitrides and alloys based on them are used as refractory and electric insulating materials in modern high temperature technology. The properties of these materials depend on powder production techniques and on technological industrial processes. With efficient control of production conditions and chemical composition, materials of given property levels can be obtained.

Results of experiments to study the production of dolomite clinker having a density close to the theoretical density of pure sintered dolomite are discussed. Density values are reported for samples obtained by the following process: decarbonization of dolomite, hot briquetting at 300–700°C, firing the semi-product at 1500°C. A similar procedure was also developed using salt-doped dolomite. The technology used leads to especially good results when dealing with ‘hard to sinter’ dolomites. It also avoids the necessity to use extremely high temperatures for the sintering process, thus making the process more economical from the point of view of energy consumption.

Similar to pressed compacts, slip cast alumina can be sintered to near theoretical density if minor additions of MgO or NiO are made. Sintering kinetics of alumina were analyzed in terms of the Wong and Pask model which was shown to give the more realistic representation of powder compacts. The rate controlling step with and without additions is the migration from the neck along the pore surface rather than diffusion along the grain boundaries to the neck. Both MgO and NiO retard mass transport probably due to changes in surface energy. However, they allow sintering to near complete densification. Although grain growth limitation via second phase inclusions or solute segregation is expected, the final density improvement was shown to result from modifying pore kinetics such that the pores remain attached to the grain boundaries during grain growth until complete densification.

Barium metaborate was formed by the simultaneous hydrolysis of barium and boron alkoxides. The dehydrated compound of the as-prepared BaB₂O₄ is termed γ-BaB₂O₄. The IR pattern of γ-BaB₂O₄ was able to be ascribed to a chain anion made up of (-BO₂)⁻ units. The transformation of γ- into β-BaB₂O₄ and of β- into αBaB₂O₄ was observed at 590–650°C and 870–940°C, respectively. From the results of the high-temperature X-ray analysis as well as the DTA data, both transformations were found to be irreversible. Transformation isotherms of γ- into β-BaB₂O₄ were described by the first-order equation and activation energies were determined as 69 kcal/mol and 48 kcal/mol for nucleation process and propagation process, respectively. The kinetics of transformation of β- into α-BaB₂O₄ was best interpreted by the contracting cube equation. Activation energies were 126 kcal/mol and 76 kcal/mol for initial and final stages, respectively.