The effectiveness of an alkali halide media for the processes of MgFe₂O₄, MgAl₂O₄ and MgCr₂O₄ formation is closely related to the value |a_MgO-a_AX|, where a is the cubic lattice dimension and AX an alkali halide.

View full abstract

Amadori's result of the phase diagram PbO-V₂O₅ are re-examined. Solidus and liquidus points of the various mixtures heated in a micro-furnace were determined with the naked eye through a binocular microscope.
In addition to 8PbO⋅V₂O₅, 3PbO⋅V₂O₅ and 2PbO⋅V₂O₅, they have already been reported, a new incongruent compound having a molecular formula of 4PbO⋅V₂O₅ was identified.
Four eutectic compositions occur at 7.3% (mole) V₂O₅ (761°C), 16.5% V₂O₅ (760°C) 31.7% V₂O₅ (725°C) and 50% V₂O₅ (502°C).
Although the 8PbO⋅V₂O₅ crystals might have pseudo tetragonal symmetry having unit cell dimensions of a=5.31₆Å and c=5.69₄Å tentatively, but unit cell dimension having a=5.6₅Å, b=10.7₁Å, c=7.4₇Å and β≅90° with monoclinic symmetry are obtained by oscillation photographs of the single crystals.
The number of fomula units in this monoclinic lattice were almost unity.
Although distorted hexagonal lattice having unit cell dimensions of a=5.78Å, c=20.47Å and c/a=3.53 are postulated for 3PbO⋅V₂O₅, unit cell dimensions of 4PbO⋅V₂O₅ and 2PbO⋅V₂O₅ are indeterminate yet.

View full abstract

The bending strength of petalite ceramics fired with addition of lithia-containing glass is affected by chemical composition of the glass added and cooling schedule of the ceramics after firing. For example, the bending strength of specimens annealed after firing with addition of 10% of Li₂O-MgO-Al₂O₃-SiO₂ glass (composition: Li₂O 10.7, MgO 8.9, Al₂O₃ 8.9, SiO₂ 71.5% wt) is 650kg/cm², whereas that of the specimens annealed after firing with addition of 10% of Li₂O-SiO₂ glass (composition: Li₂O 17.6, SiO₂ 82.4% wt) is 550kg/cm², The bending strength of these specimens increases both to about 800kg/cm² by quenching into water from 1000°C after firing (J. Ceram. Assoc. Japan, 70, 8 (1962) and 71, 65 (1963)).
In order to investigate these reasons, the microstructures of the sintered specimens were examined directly or using model specimens with electronmicroscopic or X-ray diffraction techniques.
The results obtained are as follows:
(1) In the specimens annealed after firing with addition of the Li₂O-SiO₂ glass, Li₂O⋅SiO₂ crystallites were found to separate out in the glass matrix surrounding large grains of spodumene crystals. In the specimens annealed after firing with addition of the Li₂O-MgO-Al₂O₃-SiO₂ glass, the Li₂O⋅SiO₂ crystallites were not observed. Low bending strength of the specimens fired with addition of the Li₂O-SiO₂ glass was attributed to the presence of microcracks produced by big difference in thermal expansion coefficient between the Li₂O⋅SiO₂ and spodumene crystals. (2) In both of the specimens, when quenched, no crystallites were observed to separate out in the matrix surrounding large grains of spodumene crystals; the matrix remained in the glassy state. High bending strength of the quenched specimens was attributed to the compressive microstress produced in the spodumene crystals.

View full abstract

A simple technique for preparing the thin films for direct transmission electron microscopy was developed. This thinning technique is based on a chemical etching of the thin plate which has been cut from bulk material in advance and makes possible more reliable electron microscope study of the fine scale microstructure of glass.
The microstructures of the phase separated alkali silicate, alkali borosilicate glasses and the transparent glass ceramics were observed using thin films and they were compared with those on carbon replicas of fractured and etched surfaces. The results are summarized as follows:
1) The morphology observed in the micrographs by direct transmission of thin films coincided very well with that in the corresponding replica micrographs. However, thin films are better specimens than replicas to study fine structure below about 100 Å because they reveal less ambiguous structure.
2) It was shown with real examples that one could determine by selected area electron diffraction and dark field technique whether the individual phases observed in the micrographs were crystalline or not.
3) The mean size of main crystals of transparent glass ceramics was observed to be about 300 Å in the direct transmission electron micrographs. This value was not so much different from that calculated on the basis of broadness of X-ray diffraction peaks.
4) These results mean that the thin film prepared by the described method is more suitable specimen than replica in study of the structure of phase separated or crystallised glasses in spite of the brittleness and the difficulty of preparation of many specimens at one time.

View full abstract