This research was carried out on a part of study of synthesis of cubic boron nitride. I part alloy catalyst of Fe₃Al, Ag₁₈Cd₈₂ or Ag₅₈Cd₄₂ and 4 parts hexagonal boron nitride were filled up in a platinum tube approximately 4mm in diameter and 8mm long. Three filled methods were used, that is, sandwich type, mixture type of alloy catalyst and hexagonal boron nitride, and bar type of catalyst. The assembly was placed for 30 minutes at maximum about 80kbars and about 2200°C, using a modified girdle type high pressure apparatus. The cubic boron nitride thus formed was isolated from the mixture and was identified by Debye-Scherrer pattern, hardness test and etc.
In the case of Fe₃Al alloy catalyst, cubic boron nitride with tetrahedron, octahedra and etc., having a zinc-blende struture, could be synthesized over about 1550°C and 45kbar. Its crystal size was about 80μ. The hexagonal boron nitride powder showed an increase in crystal size with increasing temperatures and pressures. The equilibrium line between the hexagonal and cubic forms was located at the extensions of Wentorf's experimental line and the line calculating from thermodynamic data.
In the case of Ag₁₈Cd₈₂ and Ag₅₈Cd₄₂ alloys, cubic boron nitride could be obtained over about 1000°C and 32kbar and high yields of it did with increasing temperatures and pressures. The cubic-hexagonal boron nitride equilibrium line was in the same manner as Fe₃Al alloy catalyst. Crystal size was about 80 and 150μ, using Ag₅₈Cd₄₂ and Ag₁₈Cd₈₂ alloys, respectively.
Alloy of Ag-Cd system was diffused into hexagonal boron nitride at the rate of about 0.10mm/min.

View full abstract

The mechanism and kinetics of the reaction between CaO and 2CaO⋅SiO₂ to form 3CaO⋅SiO₂ in the presence of the melt were investigated for an example of the solid state reaction in the presence of a liquid phase.
1) When the specimen is heat-treated, the glassy phase melts with increasing temperature, and the starting materials (CaO and 2CaO⋅SiO₂) are dissolved into the melt, in which counter diffusion takes place. After the melt is supersaturated on these components, 3CaO⋅SiO₂ is crystallized out from the melt.
2) The reaction rate is found to be expressed by the Jander type equation with better accordance even in the later stage. The reaction of this type seems to be controlled by diffusion in the melt.
3) In the sintering of the specimen consisted of CaO and 2CaO⋅SiO₂ in the presence of the melt, the shrinkage is proportional to the n powre of time, and the slope representing this relation is divided into two parts. These parts seem to be due to reaction and sintering of reactants and product, and the sintering of product alone. The slope of the latter has the n value of about 1/3, which coincides with the case derived by Kingery of diffusion contolled sintering in the presence of a liquid phase.

View full abstract

The cylindrical compacts of fly ash, pulp waste liquor being used as a binder were heated at a temperature between 1000°C and 1300°C in an atmosphere of O₂, N₂, CO₂, CO₂ (+2 vol% O₂) and CO₂ (+4 vol% O₂), then subjected to chemical analysis, especially on carbon content and iron valency, and also measurement of magnetic susceptibility, apparent density, water absorption and compressive strength. As a result, there were seen remarkable influences of atmosphere on the sintering and bloating characteristics of the compacts, that is, 1) in oxygen, bloating began to occur at about 1200°C, where no carbon was found to remain, and therefore, the thermal decomposition of ferric oxide to release oxygen gas might lead to bloating, 2) carbon dioxide atmosphere facilitate sintering of the compact and bloating was effected by the reaction of ferric oxide with carbon at a temperature above 1100°C, where the favourable mole ratio of Fe₂O₃/C for bloating was likely to be from 1/1 to 5/1 and 3) there occured no bloating in N₂ and CO atmosphere and substantial amount of carbon was found to remain and thereby about half of iron content was reduced to metal.
In addition, the compressive strength (S) of the specimen after heating at a temperature (T) in sintering stage prior to bloating was examined, and In S∝ 1/T relation could virtually be verified, the discussion being made for this relation in view of the sintering mechanism based on viscous flow.

View full abstract

Some nitrides, BN, AIN and Si₃N₄, are used as electric insulators up to moderate temperature. The difficult problems occured when they were to be employed at higher temperatures than 2000°C. The reactions between those insulators and conductors of refractory metals, tungsten and tantalum, were investigated up to 2100°C in a tungsten furnace under argon atmosphere.
In all of the power reactions of W with BN with various compositions at 1100° to 2000°C for 2 hrs., W₂B was first formed, followed by WB, W₂B₅ and WB₄ stepwisely, and nitrogen gas was released. Between Ta and BN (50-50 molar composition), TaN, TaB and small amount of TaB₂ were formed. In the reactions between metals and AIN (50-50 molar composition) for 2 hrs., W did not react, but Ta formed TaN over 1600°C. Vaporisation Al and AlN were observed considerably with elevating temperature. In the reactions between metals and Si₃N₄ (50-50 molar composition) for 2 hrs., W₃Si₂ and WSi₂, and TaN and TaSi₂ were formed, but vaporisation of Si₃N₄ was observed over 1400°C.
In the reaction between sintered pellets of W and BN, reaction layers of W₂B and WB were confirmed on the original W by means of XMA. Here kinetics of formation of W₂B from W and BN were investigated at the temperatures from 1270° to 1360°C by means of the TGA. By the method of J. H. Sharp et al., the curve of a reaction yield α with a reduced time t/t_0.5 showed that the reaction rate was controlled by the phase-boundary reaction. An activation energy of this reaction was determined to be 87.0±17.0kcal/mole from the Arrhenius plots, and this value is larger than the activation energy 64.0±6.3kcal/mole of diffusion of B in W₂B given by A. P. Epik.
Insulating properties of BN for W leads dropped at higher temperature than 1500°C owing to the formation of conducting borides. Tungsten borides were easily oxidized in air in the range of 400° to 600°C. Their resistivities have a order of 10^-5 ohm-cm as metallic conductor. W₂B was weak paramagnetic, and WB and W₂B₅ were diamagnetic.

View full abstract

The energy relations in some alkali silicate glasses and crystals of the same composition at 25°C are reported. The data have been obtained by solution calorimetry in 5% HF aqueous solution and, for Na₂O-SiO₂ system, also in distilled water.
The results of these experiments in 5% HF aqueous solution are summarized as follows. Heats of solution per SiO₂ obtained for both crystals and glasses in the systems Li₂O-SiO₂, Na₂O-SiO₂ and K₂O-SiO₂ generally show the linear relationship with mole% alkali oxide in the region above 33.3mole% alkali oxide, and show the departure from linearity in the region below 33.3mole% alkali oxide. Heats of solution decrease in the order Rb>K>Na>Li in all compositions. The values of intersecting points obtained by extrapolating the straight line to the ordinate correspond to the heat of solution of Si-O-Si network of the alkali silicates. These points gather at the value ca. 11kcal/SiO₂ in crystals and in Na₂O-SiO₂ and K₂O-SiO₂ glasses; this value is ca. 1/3 of the heat of solution of quartz. The enthalpy differences between these glasses and crystals of the same composition decrease in the order Li>Na>K.
From the results of experiments of solution calorimetry in distilled water in the system Na₂O-SiO₂, it has been found that the sodium component is dissolved and weak-points in Si-O-Si bonds are broken to form poly-silicic acids. The average number of Si contained in a poly-silicic acid discrete anion formed by the dissolution of crystals are 2 to 6, slightly larger than the case formed by the dissolution of glasses. These results will be useful to elucidate the nature of bonds in glassy state.

View full abstract