The following conclusions may be drawn from the previously mentioned experiments, in which two experimental kilns were used to investigate the combustion with oxygen-enriched air and the resulting generation of NO_x.
If oxygen-enriched air is employed in place of ordinary air as combustion air, high temperatures may be attained more easily, but the concentration of NO in the combustion gas increased. Fixed temperature equilibrium calculations indicated that the concentration of NO in the combustion gas increased slightly with oxygen enrichment up to 50% oxygen, but the absolute amount of NO emitted actually declined. This decline may result in part from a decrease in the combustion gas flow rate due to the enrichment of the combustion air and in part from a decrease in the fuel gas flow rate since the fuel requirement to attain a given temperature is lower than that in the case of oxygen-enriched combustion.
However, in practice, the combustion reaction rate increase, when enriched air was used in place of ordinary air. As a result, the NO concentration increased significantly. In addition, the long residence time of the combustion gas in a high temperature region further contributed to an increase in the NO concentration.
Thus, the use of oxygen-enriched air has been shown to be a very effective means of obtaining extremely high temperatures in a combustion-type kiln, while providing significant energy savings.

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In the system Na₂O-Y₂O₃-SiO₂-P₂O₅, the following three kinds of compounds were obtained: Na_3+3x-yY_1-xP_ySi_3-yO₉, Na_5+3x-yY_1-xP_ySi_4-yO₁₂ and Na_9+3x-yY_1-xP_ySi_6-yO₁₈. The ionic conductivity and activation energy of the present materials were found to exhibit strong dependence on phase and composition: 10^-6S/cm<σ₃₀₀<10^-1S/cm and 20kJ/mol<E<88kJ/mol. The details of each NYPS are summarized as the following.
N₃YPS: C₃A-type structure with 6-membered (Si, P)O₄ rings 5×10^-5S/cm<σ₃₀₀<1×10^-2S/cm, 45kJ/mol<E<85kJ/mol single-phase region: 0.6-0.5[P]<[Y], [P]<0.8
N₅YPS: 12-membered (Si, P)O₄ rings 1×10^-2S/cm<σ₃₀₀<1×10^-1S/cm, 20kJ/mol<E<25kJ/mol single-phase region: 0.5-0.5[P]<[Y]<0.8-0.5[P], [P]<0.7
N₉YPS: probably iso-structure of N₃YPS (C₃A-type) 4×10^-4S/cm<σ₃₀₀<1×10^-3S/cm, 42kJ/mol<E<60kJ/mol single-phase region: 0.5-0.5[P]<[Y]<0.8-0.5[P], [P]<0.4

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The properties of the glasses such as density, thermal expansion, transformation temperature, Raman spectrum and infrared spectrum were measured in the binary system TeO₂-PO_5/2. The Raman peak assigned to P=O stretching vibration, which usually appears in the wave number region of 1330 to 1390cm^-1, is absent over the composition range of these glasses. Molar volume and thermal expansion coefficient decrease and transformation temperature increases with increasing PO_5/2 content. These properties show a minimum or a maximum at 40-50mol% PO_5/2. From these results the structural model of the glass is proposed that the coordination of oxygen of P=O bond changes the tetragonal-pyramidal TeO₄ to the distorted TeO₅ (or TeO₆). The infrared spectra of the glasses showed that the condensation of the PO₄ tetrahedra occurs in the compositions over 9.3mol% PO_5/2 in spite of the fact that all PO₄ tetrahedra are isolated in the crystalline Te₄O₅(PO₄)₂ (33.3mol% PO_5/2).

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Zirconium oxide sulfate (ZOS) with high crystallinity was synthesized hydrothermally from a Zr(SO₄)₂ solution. The obtained ZOS had a layer structure and was very thin hexagonal particles. The chemical composition as prepared corresponded to Zr₃O₅SO₄⋅H₂O and density was 4.12g/cm³. ZOS was dehydrated below 600°C without any change in morphology. The unit cell of anhydrous ZOS was a₀=0.631₁nm, c₀=1.025₀nm and Z=2 if assigned as hexagonal symmetry. Anhydrous ZOS was desulphurized at about 650°C to metastable cubic ZrO₂ with an orientation relationship (0001)_ZOS//(111)_c-ZrO2, [1010]_ZOS//[110]_c-ZrO2. A reasonable model was proposed for the crystal structure of anhydrous ZOS, in which the center part of a layer was composed of two zirconium planes same as the (111) planes in cubic ZrO₂ with distorted SO₄^2- tetrahedrons coordinated at both sides of the layer. The calculated intensities of the diffractions based on the model were in good agreement with those measured by X-ray powder diffractions.

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Changes in crystal structure and thermistor constant in Mn-Co-Ni oxide, a thermistor material, in cooling process below 1000°C after firing at 1400°C have been studied. The starting oxide was made of nitrates of Mn, Co and Ni with molar ratio of 3.0:1.9:1.1 by firing at 1400°C and by cooling to room temperature in the furnace. The samples used in this study were prepared by annealing the above oxide at various temperatures between 250° and 1000°C for 24 hours. Each sample was quenched rapidly in the water. X-ray diffraction study of these samples heat-treated below 900°C identified three kinds of crystal structures, such as tetragonal spinel, cubic spinel and rock salt. Among these crystal structures, no change in diffraction patterns was observed with both cubic spinel and rock salt structures. However, the axial ratio, c/a, of the tetragonal spinel, varied with increasing heat-treatment temperature, and a sharp decrease in c/a values was observed at around 600°C as shown in Fig. 4. On the other hand, the thermistor constant B values changed with increasing heat-treatment temperatures, and a remarkable depression of B values was observed between 550° and 650°C as shown in Fig. 6. This result is closely associated with the change in c/a values in the tetragonal spinal structure observed in the same temperature range.

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The effect of external conditions, such as cyclic frequency, wave form and stress amplitude, on the cyclic fatigue properties of ceramics has not been analyzed sufficiently at frequencies higher than 10Hz, because of the restriction of testing device. A cyclic fatigue phenomenon of ceramics was tested at the range from 30 to 150Hz by a Piezo-Stack type device, which can control external stress conditions accurately by adjustment of input voltage. Samples used were ZrO₂, Al₂O₃, Mg₂SiO₄ and SiC. The estimation of the change in fatigue properties with an external condition was established by following the methods. (1) The estimated methods by an experimental equation (V=AK₁ⁿ) between subcritical crack growth velocity (V) and stress intensity factor was modified with a concept of a fatigue limit. (2) The initial crack length in each sample, which caused the scattering of fatigue life, was estimated. In oxide ceramics, the cyclic fatigue properties can be estimated practically by the same crack growth equation, even though the cyclic frequency changed from 30 to 150Hz. However, when the wave form was changed, the experimental fatigue life did not always correspond to the estimated life, but the cyclic effect can be predicted with regard to crack growth.

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In order to investigate the effect of grain size on the mechanical properties of stabilized ZrO₂ ceramics, fine grained ceramics were prepared by adding carbon derived from novolak phenol resin by thermal decomposition. A stabilized ZrO₂ ceramic with an average grain size of 0.6μm and relative density of above 99% was obtained by adding 0.23wt% of carbon and sintering at 1500°C. It was speculated that the carbon at grain boundaries prevents the grain growth of ZrO₂. Hardness and Young's modulus of the fine grained ZrO₂ ceramics were nearly the same as those with large grains. However, the fracture toughness of the fine grained ZrO₂ was smaller than that of large grained one, which may have resulted from the reduced crack deflection effect of increasing fracture toughness in fine grained ZrO₂ ceramics.

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Submicron powders of SiC were synthesized at different temperatures (1700°-2000°C) by reaction of a mixed powder of carbon and silicon dioxide prepared by the spray thermal decomposition method. The structural and quantitative analyses of the synthesized SiC were carried out using the Rietveld method on the basis of the cubic and hexagonal close packed models. The calculated X-ray powder profiles, however, did not coinside with the experimental patterns. The disagreement seemed to be due to the fact that synthesized SiC includes stacking faults. A general method for calculating the X-ray powder profiles by matrices was developed for the quantitative analysis of stacking faults in the SiC structure. The profile calculated on the basis of Paterson model showed that the peak of (102) reflection moved toward the lower angle with increasing amount of stacking faults, which was not observed in the experimental patterns. The present calculation assumed that continuing probability of positive sequence was not the same as that of negative sequence in the Wilson model. The profiles calculated on this model were in good agreement with the experimental patterns, indicating that the structure of the present SiC contained stacking faults in which the proportions of continuing probabilities were different. The values of the positive and negative continuing probabilities decreased as the synthetic temperature increased. The negative probabilitiy was always larger than that of positive one under the present synthetic conditions.

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Tensile strengths of newly developed alkali-resistant glass fibers based on SrO-BaO-SiO₂-TiO₂ were investigated as a function of exposure time to a Portland cement aqueous phase (PCAP) solution at 95°C, and compared with those of conventional alkali-resistant glass fibers based on Na₂O-SiO₂-ZrO₂. Glass fibers of the compositions 15 SrO, 15 BaO, 30 SiO₂, 40 TiO₂ in mol% (S 4-15 Sr) and 7 Na₂O, 5.75 CaO, 5.75 SrO, 11.5 BaO, 30 SiO₂, 33 TiO₂, 7 ZrO₂ in mol% (S 4-134) showed twice as high tensile strengths as those of the compositions 1 Li₂O, 11 Na₂O, 1 Al₂O₃, 71 SiO₂, 16 ZrO₂ in mol% (G 20) and 17.5 Na₂O, 6 CaO, 67 SiO₂, 9.5 ZrO₂ in mol% (68 EF 88) after exposure to the PCAP solution for 108 hours, as well as before exposure (Fig. 2). Fracture toughness K_IC's of the former glasses, which were obtained by microindentation method, did not appreciably differ from those of the latter glasses (Table 2). Accordingly, higher strengths of the former glasses were attributed to smaller cracks caused at the surfaces (Fig. 7). The smaller size of cracks for the former glasses was interpreted in terms of formation of a dense and homogeneous layer (Fig. 6) rich in TiO₂ and CaO at the surfaces by chemical reaction with the cement solution (Table 3).

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The synthesis of near-zero-expansion ceramic from Na_1-xNb_xZr_2-x(PO₄)₃ was investigated. The replacement of Zr⁴⁺ and Na⁺ by Nb⁵⁺ decreased the thermal expansion of the c-axis. The thermal expansion of the a-axis was negative. Consequently the average thermal expansion exhibited zero expansion at x÷0.75. Thermal expansion coefficients of Na_0.25Nb_0.75Zr_1.25(PO₄)₃ are α_a=-3.4×10^-6/°C, α_c=+6.0×10^-6/°C, and α_avg=-0.3×10^-6/°C. Thermal expansion coefficients of Nb Zr(PO₄)₃ which does not contain Na⁺ ion are α_a=-3.4×10^-6/°C, α_c=+1.2×10^-6/°C, α_avg=-1.9×10^-6/°C with less thermal expansion anisotropy. The dense ceramics were fabricated by sintering at 1200°-1400°C for 10-60min with MgO as sintering aids. The linear thermal expansion coefficient of the sintered ceramics was near zero.

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A method of eliminating the fluoride ions was studied to complement the ferrite process for treating inorganic liquid wastes. The experiment was carried out with due attention to the fact that products from the hydrolysis of titanyl sulfate have the ability to adsorb fluoride ions. The adsorption character for fluoride ions on products from the hydrolysis of titanyl sulfate have given excellent results.

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Four kinds of silicon nitride ceramics were oxidized in flowing dry air at 1300°C for 360h and 1000h and their oxidation behavior was studied by the weight gain, thickness of oxide layer, and change of thickness of unoxidized substrate. The thickness of the substrates decreased with oxidation, but some samples expanded. No appreciable weight loss was observed during oxidation. This result is probably explained by the high oxygen partial pressure in the oxidation atmosphere and by the sintering aids.

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The abrasion wear of various ceramics were measured using a diamond grinding wheel at the load of 130 gf and the sliding velocity of 1.26m/s. The wear volume was obtained from the wear track shape on the ceramics sample, and the specific wear rate (W_s) was calculated from the linear relation between sliding distance and wear volume. W_s was highest for Al₂O₃, much lower for SiC and ZrO₂, and lowest for Si₃N₄. The grain boundary fracture caused accompanying the cutting by diamond grains may explain the high wear rate of Al₂O₃.

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An effective shrinkage ΔL_i was pointed for a green compact because of necks considerably grown during calcining process. A general sintering equation was developed with respect to ΔL according to Taylor's expansion method, where ΔL is a shrinkage of the powder compact during sintering. The resultant equations indicate that ΔL_i has influence on the apparent time dependence, that is, the 1/m-power dependence of ΔL. If ΔL_i is unknown, the reasonable 1/n-power dependence of modified shrinkage, ΔL_i+ΔL, which is determined by the mechanism of material transport, can be estimated only in the case where ΔL_i<<ΔL. If ΔL<<ΔL_i, sintering seemingly obeys a kinetic equation of m=1 irrespective of any n value, any sintering mechanism contributing to densification. The foregoing evaluation well explained data that slight pre-sintering results in appreciable decrease of the m value. The fabrication history of a powder has naturally influence on the m value in a manner that it has also influence on the degree of neck growth in hard agglomerates, that is, the ΔL_i value.

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