Rikuo Ota

Chemistry and inherent viscosity of glasses segregated at grain boundaries of silicon nitride and silicon carbide ceramics

Abstract Electron microscopy/microanalysis and internal friction characterizations have been concurrently used to quantitatively assess both the morphology/chemistry and the inherent viscosity of residual high-SiO 2 glasses segregated to grain boundaries of polycrystalline Si 3 N 4 and SiC ceramics. The anelastic relaxation peak of internal friction, arising from the viscous slip along grain boundaries wetted by glass, was collected in a plot of internal friction vs temperature. The peak was analyzed with respect to its shift upon changing the oscillation frequency, in order to assess the inherent viscosity of the intergranular glass film. From the internal friction analysis and the precise information of both thickness and chemistry of the grain-boundary film obtained by electron microscopy, a quantitative determination of the inter granular viscosity and the activation energy for viscous flow was attempted. The viscosity of intergranular high-SiO 2 glasses as a function of temperature is compared to the literature data for bulk SiO 2 glass and discussed in terms of the respective anion structure of the glass.

Glass formation and crystallization in Li2ONa2OK2OSiO2

Glass-forming regions in the Li2ONa2OmSiO2 (m = 2 or 1.5) and Li2ONa2OK2OmSiO2 (m = 2 or 1.5) systems were determined with various cooling rates. Viscosity and liquidus temperature were measured. Eutectics were observed in these systems. The critical cooling rate shows a minimum and the liquidus viscosity shows a maximum. It was found that Li2ONa2OmSiO2 showed more extensive glass-forming ability than Li2OmSiO2, Na2OmSiO2 or K2OmSiO2. Li2ONa2OK2OmSiO2 shows an even larger glass-forming ability than Li2ONa2OmSiO2. The significant role of mixing entropy or liquidus viscosity in the glass-forming kinetics of mixed alkali silicate systems is recognized.

Nucleation of Li2OSiO2 glass and its interpretation based on a new liquid model

Abstract A liquid model is proposed wherein crystal embryos exist at temperatures above melting point. The number density of embryos should decrease as temperature increases. Crystallization behavior of Li2OSiO2 glasses of 31, 33 and 35 mol% Li2O was studied. Heat treatments (20–60 min at 500°C and 20 min at 600°C) of the Li2OSiO2 glasses induced different numbers of crystals depending upon melting temperatures in the 1030–1300°C range. The number density of crystals, N, decreased with increasing melt temperature. The number density of pre-existing embryos was estimated from the N-t diagram and was found consistent with the proposed model.

Nucleation behavior of Na2O–SiO2 glasses with small amount of additive

Abstract Nucleation in 49Na2O · 49SiO2 · 2ZrO2 and 47Na2O · 51SiO2 · 2ZrO2 glasses were investigated by differential thermal analysis measurement after heat-treatments at 400–580°C for 0–123 h. The addition of ZrO2 increased crystallization peak temperature, TC, and shifted the temperature, Tmax, at which nucleation rate is maximum to higher temperatures. The relationship between the number density of nuclei formed during the heat-treatment and TC was calculated. The variation of the number density of nuclei was obtained from the comparison between the calculated TC and the observed TC as a function of heat-treating time. The transient time for nucleation of 47Na2O · 51SiO2 · 2ZrO2 glass was 42 h at 430°C. From these results, we found ZrO2 has an effect on the nucleation kinetics of Na2O–SiO2 glass.

Preparation of CeO2-ZrO2 Mixed Oxide Powders by the Coprecipitation Method for the Purification Catalysts of Automotive Emission

Influence of CeO2 content on the specific surface area (SA), oxygen storage capacity (OSC) and crystalline structure of the powders in the CeO2-ZrO2 system were investigated. The change of SA value by heat-treatment is almost proportional to that of OSC. The lattice parameters of the powders had a linear relationship with the OSC value. In the CeO2-ZrO2 system, powders with 20 mol% CeO2 was found to show the highest OSC value and the highest durability of SA value after calcined at high temperatures.

Evaluation of the number density of nuclei in Li2O·2SiO2 glass by DTA method

Abstract DTA measurements were performed to obtain the crystallization peak temperature, TC, for Li2O·2SiO2 glass heat-treated for nucleation. The relationship between TC and the number density of nuclei was obtained. Calculation of TC was performed by simulating the crystallization process, and the effects of the size and the number density of nuclei formed during DTA measurement on TC was evaluated. The calculated relationship between the number density of nuclei and TC agreed with the experimental one, and it was shown that the effects of the size and the number density of nuclei formed during DTA measurements is negligible in the evaluation of the number density of nuclei by TC. The steady state nucleation rate for the Li2O·2SiO2 glass evaluated by DTA agreed well with the literature.

Analysis of crystallization behavior in Li2O · 2SiO2 glass by DTA method based on a liquid model

Abstract The crystallization behavior of Li 2 O · 2SiO 2 glass samples having different melting histories were analyzed by differential thermal analysis (DTA). The glass samples were prepared by remelting a quenched glass at 1300°C or 1035°C for 1 h. The glass melted at low temperature was found to crystallize earlier and to have lower crystallization peak temperature, T C , than that melted at high temperature. The number density and sizes of crystals precipitated in glasses with the same heat treatment as the DTA run were measured and were found greater in number and larger in the glass melted at lower temperature compared with that at higher temperature. These results are in agreement with the liquid model that crystal embryos are preserved in melts even above liquids and their distribution depends on temperature.

Triboluminescence of (Sr, Ba)Al2O4 polycrystals doped with Eu3+ and Eu2+

Triboluminescence spectra have been measured for polycrystalline Sr1-xBaxAl2O4 (0x1) doped with Eu3+ and Eu2+. The polycrystalline samples of Sr1-xBaxAl2O4:Eu3+,Eu2+ with x=0-0.4 clearly exhibit triboluminescence, while the triboluminescence is hardly observed for Sr1-xBaxAl2O4:Eu3+,Eu2+ with x0.5. X-ray diffraction analysis indicates that the crystal structure changes at the composition of x=0.5, suggesting that the triboluminescence in the present materials is closely related to the crystal and/or electronic structures. Also, it has been revealed that SrAl2O4:Eu3+,Eu2+ polycrystal sintered at 1600°C exhibits intense triboluminescence, while it is hardly observed in SrAl2O4:Eu3+,Eu2+ polycrystal sintered at 1350°C.

Solubility of Ag2O into the Na2O–B2O3–Al2O3 system

Abstract The solubility of Ag 2 O was measured for the Na 2 O–B 2 O 3 and Na 2 O–B 2 O 3 –Al 2 O 3 system with the rotating crucible method and static method, respectively, under air atmosphere at temperatures ranging from 1273 to 1423 K. The contamination of melts from crucibles could be avoided by the rotating crucible method, with which it became possible to measure the solubility of Ag 2 O for the Na 2 O–B 2 O 3 system above the melting point of Ag for the first time. It was found that the addition of Na 2 O decreases the solubility of Ag 2 O while the addition of Al 2 O 3 had little effect on the solubility. The effect of Na 2 O and Al 2 O 3 on the solubility of Ag 2 O is expressed by interaction coefficients and is analyzed in terms of the basicity of melts. The solubility of Ag 2 O in Na 2 O–B 2 O 3 –Al 2 O 3 melts increased with increased temperature. This phenomena was explained by a small enthalpy change in oxidation of silver.

Raman spectroscopic study on the glasses in the M2O(M'O)-Al2O3(Ga2O3)-B2O3 system

Raman spectra were measured on the glasses in the M 2 O(M′O)-Al 2 O 3 (Ga 2 O 3 )-B 2 O 3 system, where M = Li, Na and K, and M′ = Mg and Ca. It was concluded that on the addition of Al 2 O 3 or Ga 2 O 3 into the M 2 O(M′O)-B 2 O 3 system, tetrahedra AlO 4 or GaO 4 are formed while BO 4 is converted to BO 3 . The effect of Ga 2 O 3 on the BO 4 → BO 3 conversion appears to be milder than that of Al 2 O 3 . NMR measurement confirms the structure and the scheme of reaction deduced from the Raman spectroscopic study.