Ceramurgia International最新文献

Reaction-bonded silicon nitride, RBSN, may be used as the starting material for a post-densification process carried out at about 1800°C in nitrogen at atmospheric pressure. The sintering aids used are similar to those employed in hot pressing and pressureless sintering (e.g. MgO, Y₂O₃). They may be added to the silicon powder before nitridation or, in the case of MgO, by diffusing the vapour into preformed RBSN. High densities (98.5% theoretical) have been achieved accompanied by a shrinkage of only 6.2%. The strength at R.T. was about 1000 MN/m²; the strength at H.T. depends on devitrification. The process thus offers complex components to be formed by injection moulding, with strength values after sintering similar to those of hot-pressed silicon nitride.

The effect of small additions of magnesium titanate, calcium titanate, strontium titanate, and barium titanate on the sintering temperature of aluminum oxide was investigated. The dependence of linear shrinkage and density on sintering temperature and dopant concentration and type was studied. X-ray analysis, microscopic examination, and electron microprobe analysis indicate that sintering occurred through a liquid phase and that Ti⁴⁺ diffused into the Al₂O₃ grains. When CaTiO₃, SrTiO₃, and BaTiO₃ were added, the compounds 3CaO · Al₂O₃, SrO · 6Al₂O₃, and BaO · 6Al₂O₂ were formed. The sintering temperature was reduced to 1500°C by adding less than 2 mol% of magnesium, calcium, strontium, or barium titanate.

Estimation of the rate controlling mechanisms of mass transport during the initial, Intermediate and a portion of the final stage of hot pressing of powder compacts by an adequate application of the theory of creep of non-porous polycrystals at elevated temperature is first discussed and their significance for understanding of the relations between powder characteristics and their densification during hot pressing stressed. Review of recent developments of the creep theory indicates that if intraparticle (intracrystallite) i.e. dislocation mechanisms are acting to a negligible extent only, the steady-state strain rate ϵ, should be controlled entirely either by boundary reactions (emission/absorption of point defects at sources and sinks, e.g. at boundary line-defects) or by diffusion between sources and sinks (Nabarro-Herring and/or Coble creep). An analysis has been made in these terms of ϵ observed with well characterised MgO powders, having a rather uniform crystallite size, during hot pressing at 775–975K under loads of P_A = 60–295 MPa. Two types of powders, obtained by thermal decomposition of Mg(OH)₂ at different temperatures, have been studied, namely i. powders constituted by well-annealed fine crystallites (d = 28–56 nm) showing no lattice microstrains, and ii. powders constituted by fine crystallites (25–45 nm) showing appreciable microstrains of the lattice and, consequently, a high density of the line defects. Experimental determination of the stress sensitivity, n, particle sensitivity, m, and true activation energy for creep, Q_c, has shown that the data correspond very closely to $\text{ϵ}{}_{\mathrm{<mtext>s</mtext>}}\text{∝}\text{P}{}_{\mathrm{A}}{}^2\text{d}$ in the first, and to $\text{ϵ}{}_{\mathrm{<mtext>s</mtext>}}\text{∝}\text{P}{}_{\mathrm{A}}\text{d}{}^3$ in the second case, respectively. The first situation is expected for strain rates controlled by boundary reactions while the second for the ones controlled by Coble creep, respectively. The different rate-controlling mechanisms enable to explain rationally the different densifications obtained under comparable conditions of hot pressing with the two types of MgO powders.

The grain-boundary thickness of alumina bicrystals, fabricated without the aid of sintering additives and pressure, was measured with an optical microscope. The thickness was 500 to 550 Å and independent of the tilt angle. Impurities segregated at the grain-boundary and the surface of the same sample was studied with an ion microanalyser. The segregation of Ca and Si at both interfaces was noticeable but that of Mg was not. The thickness of the segregation region for both interfaces was 600 to 800 Å. The spheroidization of tubular voids into pores at the grain-boundary was controlled by surface diffusion. Discussion of the fabrication process of the bicrystal in terms of initial sintering kinetics concluded that the process was controlled by the surface diffusion.

Differences between the morphology of MgFe₂O₄ precipitates, which form octahedra with {111} habit planes and grow into dendritic forms propagating along 〈100〉; MgCr₂O₄ and MgAl₂O₄, which assume plate-like morphology with a {100} habit plane; and Sc₂O₃, which precipitates as randomly oriented platelets, are discussed in terms of the differences in strain energy associated with exsolution of the phases in MgO. The strain energy effects, which occur both during nucleation and subsequent growth, provide a rational explanation of the observed behavior.

A series of MgO- and CaO- partially and fully-stabilized ZrO₂ bodies were produced from sulfate and acetate powders using a wet chemical drying method (“ hot petroleum drying method ”). The optimum sintering conditions for powders produced from sulfates and acetates were found: calcining at 1200°C for 2–5 h, followed by sintering for 5 h at 1600°C. This drying process proved to be a very effective method for preparing stabilized ZrO₂ without conventional ceramic process of mixing, milling, and granulating.

The densification of alumina to nearly full density continuous hot pressing is described. The influence of the atmosphere on hot pressing is discussed as well as the influence of the process parameters temperature, pressure and transit rate. The microstructure of hot-pressed alumina is examined and compared with that of normally sintered alumina. X-ray diffraction is applied to investigate a possible crystallographic texture of the grains in hot-pressed alumina. The optical properties of the material are treated in a second paper.

Submicron TiO₂ZrO₂ powders were prepared by the vapor phase reaction of the TiCl₄ZrCl₄O₂ system. The phases appeared were TiO₂ss, ZrO₂ss and ZrTiO₄ss. The major crystal forms of TiO₂ and ZrO₂ were rutile and tetragonal system, respectively. Pure TiO₂ was predominantly anatase and consisted of single crystalline particles with square and plate-like shape. As ZrCl₄ concentration increased, the particle size decreased, the content and lattice constants of rutile increased and particles became polycrystalline. The compound ZrTiO₄ was formed most abundantly as the ratio ZrCl₄/TiCl₄ approached 1 and the effective reaction temperature was raised. It was found that TiCl₄ accelerates the oxidation of ZrCl₄ and ZrCl₄ retards that of TiCl₄ in the vapor phase reaction of the TiCl₄ZrCl₄O₂ system. It was concluded that the formation process of TiO₂ZrO₂ particles consists of the preferential nucleatoin of TiO₂ and the growth by codeposition of TiO₂ and ZrO₂.