The heats of solution and formation of RPO₃ glasses reported in the previous paper (Yogyo-Kyokai-Shi, 90, 8-13 (1982)) enabled us to calculate the total bond energies from which P-O, P-OR, and P-O^- bond energies could be deduced by assuming E(P=O)/E(P-O) 1.75 and 2. Letting E(P=O)/E(P-O)=1.75, E(P-O) was 90.3kcal/mol. Calculated E(OR) suggestedcoulombic interaction between non-bridging oxygen and alkali ion and was almost equivalent to that for R₂Si₂O₅ glass of same alkali which had no similarity in structure but the same atomic ratio of alkali to network former. The bond energy relations 1.75<E(P=O)/E(P-O)<2, E(P-O)<E(P-O^-) and E(Si-O)<E(Si-O^-) were discussed in terms of 3d-2pπ interaction in P-O and Si-O bonding.

View full abstract

Reaction or hydration of the compounds of the system CaO-Al₂O₃ in phosphoric acid soiutions was studied by hydrothermal techniques at 200°C under the pressure of 10MPa. The products of the system treated for 1day, 10days and 30days were C₃AH₆, AH (Boehmite), Ca₁₀(PO₄)₆(OH)₂: HAp, CaHPO₄ and 3 types of AlPO₄, Berlinite (Ber.), Tridymite form (Tri.) and Cristobalite form (Cri.). The products in low phosphoric acid solntions were C₃AH₆, AH and HAp. In the region where the reaction occurred instead of hydration, CaHPO₄ or Tri. tended to form instead of AH on HAp. The formation region of C₃AH₆ appeared to extend between the lines from CaHPO₄ and HAp to Al₂O₃ in the system CaO-Al₂O₃-H₃PO₄ (H₂O).
In the comparison of the results of 100°C under 10MPa with those of 200°C under 10MPa hydrothermal treatment, the produced phases of aluminum phosphate seemed to have a correspondence such as Lucin-type variscite→Cri., Messbach-type variscite→Tri. and Metavariscite→Ber..
CaHPO₄ and Ber. formed together in high phosphoric acid region above the line of H₃PO₄/CaO+Al₂O₃+H₃PO₄≅50mol%.

View full abstract

Sintering of α-FeOOH-derived Fe₂O₃ powders has been studied with special emphasis on the effect of particle shape. Acicular Fe₂O₃ powders with different states of aggregation were obtained by heating acicular α-FeOOH powder (0.2-0.4×0.05μm) at various temperatures below 750°C, and equiaxed Fe₂O₃ powder was obtained at 800°C. Comparison has been made of the densification characterisitcs and pore size distribution for these powders.
It is shown that the acicular powders gave densities higher than 99% theoretical even from the compacts of low green density of about 35% theoretical, and that the pore shape rather than pore size distribution in powder compact is highly responsible for the observed densification characteristics of acicular powdems. Studies on the effect of calcination temperature of α-FeOOH have revealed that the aggregation state in primary particles is also an imlportant factor in evaluating sinteming characteristics of powders.

View full abstract

In the present study, sintering of mixed compacts composed of Cr₂O₃ and each one of SiO₂, Al₂O₃, MgO, etc. was examined and discussed.
The compacts with SiO₂, ZrO₂ or ZrSiO₄ which did not react with Cr₂O₃ were sintered to densities >95% of true density over the ranges of composition above 20mol% Cr₂O₃ by heating in carbon powder at 1500°C. Densification of these compacts were seemed to be mainly caused by Cr₂O₃.
On the other hand, degree of densification of the compacts with Al₂O₃ or MgO which reacted with Cr₂O₃ to form Cr₂O₃-Al₂O₃ solid solution or picrochromite decreased rapidly with increase of the amount of Al₂O₃ or MgO. It was considered that characteristics of surface of Cr₂O₃ particles, which became unstable and promoted densification by heating in carbon powder, was lost by forming of Cr₂O₃-Al₂O₃ solid solution or picrochromite.
When the compacts containing Cr₂O₃ were heated in air, they were not densely sintered, since their sintering primarily occurred by evaporation-condensation mechanism of Cr₂O₃.

View full abstract

Growth conditions of LaB₆ and CeB₆ single crystals in molten aluminum in an argon atmosphere have been investigated.
The optimum growth conditions to obtain cubical single crystals are summarized as follows: The mixing atomic ratios of raw materials: for LaB₆, B/La-5.0 and Al/La=80, and for CeB₆, B/Ce=5.0 and Al/Ce=69.
The heating temperature and time held at the temperature: 1500°C and 10h for both LaB₆ and CeB₆.
LaB₆ of purplish red color and CeB₆ of blue color with the metallic luster were grown. The morphologies of grown single crystals are cubic with {100} faces, and needle and thick-plate with well-developed (100) faces.
The lattice constants (a₀) and densities (D) of single crystals are as follows,
LaB₆; a₀=4.1552±0.0001(Å), D=4.68±0.02g/cm³
CeB₆; a₀=4.1396±0.0001(Å), D=4.69±0.04g/cm³.
Values of Knoop-microhardness determined on (100) faces of single crystals are as follows,
LaB₆; ‹100›: 2180-2550kg/mm²
‹110›: 1530-1830kg/mm²
CeB₆; ‹100›: 1870-2170kg/mm²
‹110›: 1540-1780kg/mm².
As-grown cubical single crystals were used for the oxidation in air at temperatures between 700° and 1250°C.
The oxidation began to proceed at 740°-760°C. The oxidation products of LaB₆ and CeB₆ were La(BO₂)₃ (monoclinic system) and noncrystalline B₂O₃, and Ce(BO₂)₃ (monoclinic system) and noncrystalline B₂O₃, respectively.
The oxidation rate was found to be expressed by the general oxidation rate equation, (dw)ⁿ=kt, and the estimated apparent activation energies were 123.7±0.9kcal/mol for LaB₆ and 137.0±0.8kcal/mol for CeB₆.

View full abstract

Post-sintering of spinel ceramics, which were fabricated at 1400°-1600°C in air by normal sintering of the powder prepared by freeze-dried method, has been studied by HIP (hot isostatic processing) in argon of 100MPa at 1145°-1400°C without capsule.
The relative density was increased, for example, from 80-90% for the samples normally sintered at 1400°C to 84-93% by HIP at 1300°C for 3h. Vickers'hardness was also raised from 520-1030 to 670-1320 by the same treatment. The samples normally sintered at 1500° or 1600°C for various periods had the densities of 94-98% or 95-98% and the hardness of 1300-1670 or 1430-1740. The HIP raised these values only a little, that is, the densities to 96-99% and the hardness to 1420-1690 and 1460-1740 for the samples sintered at 1500° or 1600°C. Typical SEM photographs showed that the HIP eliminated pores in the grains and/or grain boundaries but resulted in no grain growth for samples sintered in air.

View full abstract

Grain boundary structure and the theoretical coincidence of Si-C bonds at the (1010) grain, boundary formed between typical SiC polytypes (2H, 3C, 4H, 6H, and 15R) are discussed under the condition, their c-axis being common. The conclusions obtained are as follotivs:
1) β-SiC (3C) showed good coincidence with 6H and 15R polytype under the condition, at least one layer being common between polytypes adjacent at the boundary. The highest coincidence observed at the boundary between 3C and 6H is ca. 33%.
2) The 6H polytype generally showed good coincidence of bonds with other polytypes at the grain boundary under the same condition with 1).
3) The coincidence, 50% is observed at the boundary formed between 2H and 4H polytype under the same condition with 1).
4) The average energy for (1010) boundary is expected to be as high as that for (0001) boundary.

View full abstract

The synthesis of LaCrO₃ from the mixture of chromium hydroxide and La₂O₃ was studied under hydrothermal conditions and in air. LaCrO₃ was directly prepared under hydrothermal conditions of above 400°C under 100MPa H₂O pressure in contrast to the formation of LaCrO₃ via La₂CrO₆ above 700°C in air. LaCrO₃ synthesized under hydrothermal conditions was well crystallized with homogeneous grain size of 0.7±0.2μm at temperatures of 700°-1000°C, while that synthesized in air was an aggregate with irregular grains.

View full abstract

A heat insulating firs brick made of magnesia or magnesia-chromite mixture can successfully be produced from the respective porous grains prepared by use of acetic acid droplets as described in Part 1 of this study, that is, when moistened with water, the bulk grains become semiplastic enough to be compressed into a shaped green body, which develops a fair mechanical strength after air drying and then can be fired at high temperature to obtain light porous magnesia or magnesia-chromite brick with a relatively small shrinkage.
The bulk grains used for this study had a size distribution of 30wt% between 0.42-1.7mm, 50wt% between 1.7-3.3mm and 20wt% between 3.3-4.7mm, for which a favourable amount of water in shaping under a pressure of 10kg/cm² appeared at about 5wt% A compressive strength of 27kg/cm² developed after drying at 105°C in the resultant shaped body. When heated, it released aceton and carbon dioxide at about 300°C by the decomposition of acetate constituent and sintered at 1500° with a linear shrikage of 4.5% into a light body (bulk density, 1.2g/cm³) having a compressive strength of about 50kg/cm². Thermal conductivity of this fired body was 0.95kcal/m⋅h⋅°C at 1000°C, which is about one-fourth of conventional magnesia briek. It was noted the fired body exhibited a fairly good resistivity to corrosive action. of slag from open hearth furnace or converter, though a slight permeation into the texture was observed.

View full abstract