Hydrothermal reactivity of basaltic glass was investigated under various chemical conditions using Morey-type autoclave. Basaltic glass, similar to andesitic glass, is less reactive to crystallize in hydrothermal media compared with the case of more siliceous volcanic glass, such as obsidian.
To clarify relationships among chemical compositions, hydrothermal stabilities and some electrical properties, the dielectric dispersions and the d. c. conductivities were carefully measured.
The results are as follows.
1) The discrimination between ionic charge carriers and electronic one in the glass sample could be attained by the analysis of low frequency dispersions less than 30c/sec. The rate of the electronic to the ionic conductivity is strongly depended on the iron content. Obsidian is an essentially ionic conductor at high temperatures and no electronic conduction is observable, but in Sanukite, an andesitic glass, electronic conduction is a few fraction of the total, and the result for basaltic glass shows far larger amount of electronic conduction than that of the andesitic one.
2) The d. c. conductivity of volcanic glasses are determined by the use of interfacial polarization arised introducing a barrier of known demensions purposely, in the form of thin sheet of mica. The activation energies for d. c. conduction are: 19.2kcal/mole (obsidian), 20.5kcal/mole (Sanukite) and 30.8kcal/mole (basaltic glass). These values should be ascribed to ionic transfer in the glass-structures.
3) The ratio of the number of mobile Na⁺ions derived from the d. c. conductivity to the total decreases in the following order: obsidian Sanukite, basaltic glass. The greater part of Na⁺ ions in obsidian contribute to the d. c. conduction, so that the network seems to be rather loose resulting in the low activation energy for conduction and also in the low resistivity. On the other hand, in andesitic or basaltic glass, a large number of Na⁺ ions will be situated in deep potential wells and take little part in the conduction, so that the activation energy and the net resistivity increase. This means that the mobility of Na⁺ ions is restricted or they are even imprisoned by immobile network modifiers. It is concluded that the network of volcanic glasses contains a large number of immobile Na⁺ ions bound in the glass-structures.
4) It was demonstrated that the chemical stability under hydrothermal condition was closely related to the ratio between mobile Na⁺ ions and immobile Na⁺ ions. Increase of the number of mobile Na⁺ ions give rise to a decrease in chemical stability of volcanic glasses, such as obsidian. It was observed experimentally, however, that the amout of silica content had almostly no effect on crystallization in basic hydrothermal media. This results indicate that the crystallization of volcanic glass is is mainly determined by the rate of the decondensation of the framework contracted by framework modifieres.
5) An interesting aspect of basaltic glass is that the mobility of Na⁺ ions is grately hampered by the increasing number of immobile ions, which manifests itself in an considerable increase of the activation energy for the d. c. conduction. This behavior is entirely different from that of other volcanic glasses investigated. Moreover, the chemical composition of basaltic glass are characterized by such a low content of network-forming ions that the coherence of the structure units becomes inadequate to form a spatial network and the amorphous pattern of X-ray diffraction is shift to higher angle compared with Sanukite and obsidian. This fact and the low content of silica suggest that the SiO₄-tetrahedra in basaltic glass are mainly in the form of tangled chains of different lenghs and not in three-dimensional silica-like structure.

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Since an establishment of the preparation method of the CrO₂ powder, various efforts have been made to obtain a compact body of the oxide to measure its electrical and magnetic properties. Since the oxide decomposes to nonmagnetic Cr₂O₃ at temperatures above 400°-450°C in the atmospheric pressure, a sintered body could not be obtained by the usual technique.
The present paper describes the preparation method of the highly dense sintered bodies of CrO₂ by hot-pressing at mechanical pressure of 30000kg/cm² and at 450°C. A high pressure apparatus used in this experiment is of a modified internal heating type. A starting powder of CrO₂ was obtained by the thermal decomposition of CrO₃ under an oxygen pressure of about 350kg/cm² at 400°C. The obtained powder was coldpressed into a disk having the diameter of 6mm and the thickness of 3mm. The pressed body was heated at 450°C for 2hours at various pressures ranging from 10000 to 30000kg/cm² in the pressing apparatus.
An electronmicroscopic observation of fractured surfaces of the samples hot-pressed at pressures less than 15000kg/cm² showed an intergranular fracture and the original shape of each grain remained unchanged. In the samples pressed at above 25000kg/cm² a transgranular fracture was observed and therefore the samples conclusively exhibited a strong bonding between grains. The applied pressures higher than 30000kg/cm² remarkably promoted the sintering of CrO² even at low temperature as 450°C.
The bulk density of the samples pressed at 30000kg/cm² was higher than 99% of the X-ray density. The electrical properties were quite different from the result obtained by CrO² powder. The electrical resistivity showed a metal-like behavior, i.e. it increased monotonously with increasing temperature in the range -196°C to 300°C and there existed a small change in a slope of temperature dependence of resestivity at the Curie temperature of CrO². The result is consistent with the metallic behavior of CrO² as predicted by a Goodenough model.

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Hot-pressing technique is very advantageous to form easily a body with high purity and high density without large grain growth. Thus hot-pressed alumina has outstanding mechanical strength and excellent electrical properties.
Dielectric properties of alumina depend on purity, density, and surface conditions as well as many other factors. Humidity also influences dielectric properties to a large extent.
An effort was made to examine the influence of porosity on dielectric properties of hot-pressed alumina.
Four kinds of alumina powder were used, as shown in Table 1. Hot-pressing of alumina was carried out at 1500°-1900°C and 200kg/cm² for 10minutes in graphite molds heated by high-frequency induction. Disk specimens prepared were 32mm in diameter and about 3.5mm thick, and of total porosity between 0% and 14%. Dielectric property measurements were carried out using Q-meter, at the frequency range from 5×10⁴ cps to 5×10⁷ cps, in atmospheres with the relative humidity of 0% to 60% at a room temperature.
The relations between the kind of alumina and the dielectric property were not found clearly in hot-pressed alumina specimens with low porosity. The dielectric loss of hot-pressed alumina was mainly determined by its porosity as well as by relative humidity of atmosphere and was dependent on frequency. At lower frequencies the dielectric loss increased with increasing porosity. At higher frequencies the dielectric loss increased independently of porosity. As the porosity increased, the minimum value of the energy loss occured at higher frequencies. Water adsorbed on open pores may be mainly contributing to the dielectric loss. The effect of relative humidity on the dielectric loss depended on open pore fraction. The dielectric constant of hot-pressed alumina was in the range of values 10 to 13. The effect of relative humidity on the dielectric constant was only found in alumina specimens with high porosity. Although the dielectric loss was influenced considerably by humidity, the dielectric constant was hardly affected at all.

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