The formation of single-phase Si-Al-O-N ceramics

Si-Al-O-N ceramics have been prepared by hot-pressing mixtures of Si₃N₄, AIN and SiO₂ (with an addition of 1% MgO) having varying ratios of AIN/SiO₂. Microstructural analysis by transmission electron microscopy and Auger electron spectroscopy has demonstrated the progressive increase in grain-boundary silicate glass in pressings prepared from compositions with excess SiO₂ to compositions given by the formula Si_6–zAl_zO_zN_8–z. This formula represents the simple substitution of Al for Si atoms and O for N atoms in the hexagonalSi₃N₄ crystal. Microstructures for this balanced composition are essentially single phase, consiting of non-faceted, sub-micron, grains with a grain-boundary segregate layer of glass-forming silicate composition, containing impurity and additive metal ions, which may be detected only by Auger spectroscopy. This microstructure is in contrast with unbalanced compositions which contain faceted grains joined by a glassy silicate phase which is easily detected by electron microscopy. Final microstructural analysis combined with observations of density and phase content with progress of hot-pressing has confirmed the important role of liquid silicate formation and a solution-reprecipitation mechanism for densification. The presence of a 1% MgO additive is shown to accelerate this process, forming a low melting point silicate by reaction with SiO₂, assisting the early solution of AIN and the reprecipitation of substituted crystals.     

Crystallization of Mg-containing phases in β′-Si-Al-O-N ceramics

The microstructures ofβ′-Si-Al-O-N ceramics of base composition Si₄Al₂O₂N₆ with large MgO additive (6 to 7 wt %) have been compared in the hot-pressed and sintered conditions. In both ceramics Mg shows only a small solubility inβ′ crystals and is mainly segregated in a “matrix” phase. In hot-pressed materials this is an Mg-Al-silicate glass whereas in sintered materials, inhomogeneities in composition and internal cavities are believed to catalyse its crystallization as the spinel phase (MgAl₂O₄ with partial Si substitution). A minor phase in sintered ceramics is the 15R “polytype” of the AlN structure.     

The microstructure of sintered Si-Al-O-N ceramics

Fine-grained ceramics, based on the substituted -Si₃N₄ crystal structure, have been prepared with near theoretical densities by pressureless sintering of compacts containing a large Y₂O₃ additive concentration. The microstructure contains two phases, and an Y/Al rich silicate glassy matrix, in the as-sintered condition. The matrix may be crystallized on heat treatment to a Si-substituted yttrium-aluminium garnet phase. The phase compositions and transformation mechanisms have been analysed by X-ray microanalysis, electron diffraction and imaging of thin sections.A preliminary survey of mechanical properties of these sintered ceramics shows that they are comparable with commercial hot-pressed silicon nitrides, particularly at high temperatures. The possibility of development of a range of ceramic alloys, dependent on their chemical environment and temperature of application, is discussed.     

Liver transplantation: the death/life paradox

Chronic encapsulated intracerebral hematoma (CEIH) is a rare disease which is believed to be caused by angiographically negative vascular malformations. CEIH has the following characteristic findings: 1. It affects all age groups 2. Clinical symptoms progress slowly after sudden onset. Often there is a latency of months or years 3. There is no correlation with arterial hypertension 4. Imaging reveals a typical fibrous capsule with enclosed blood contents and signs of recurrent bleedings 5. Cavernoma was identified histologically as the cause of bleeding in 30% of cases. 6. All patients had a primary diagnosis of intracerebral tumor. To the best of our knowledge, 27 cases have been reported in the literature. We now add two cases, one of which is the first in the available literature which was not operated and could be followed by imaging.