Jiro Fukunaga

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.

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.

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.

Crystallization behavior of non-alkali glasses in the SiO2CaOAl2O3ZnOTiO2B2O3 system*

The crystallization behavior of non-alkali glass in the SiO2 CaO Al2O3 ZnO TiO2 B2O3 system was investigated. Differential thermal analysis data of the base glass showed double exothermic peaks, one at a lower temperature with precipitation of anorthite (CaO·Al2O3·2SiO2) and gehlenite (2CaO·Al2O3·SiO2), the other at a higher temperature with precipitation of willemite (2ZnO·SiO2) and titanite (CaO·TiO2·SiO2). Phase separation of the glass into an Al2O3-rich glass phase and a TiO2 + ZnO-rich glass phase prior to crystallization was confirmed. The effect of additives and water vapor soaking on surface crystallization of the glass were tested to find a best way to promote the crystallization of the glass. Wollastonite (CaO·SiO2) addition proved most effective, shifting the double peaks of DTA curve into a single one. A similar effect was observed with water vapor exposure of the glass. It was revealed from SEM analyses that the glass-ceramics body consisted of four types of crystals of small grain size.

Variation of the gel region with heat-treatment in the B2O3-Na2O-TiO2 system compared with the melt-quenched glass region

Variation of the gel region with heat-treatment and variation of the melt-quenched glass region with cooling rates were compared for the B2O3-Na2O-TiO2 system. Gels were prepared by the sol-gel method without catalyst using boron tributoxide (B(OBu)3) sodium methoxide (NaOCH3) and titanium tetraisopropoxide (Ti(OPr)4). The gel region was in the composition range B2O3 = 0 to 90, Na2O = 0 to 60, and TiO2 = 0 to 100 mol % at 50 ° C. The crystalline phases precipitated were boric acid (H3BO3), sodium metaborate hydrate (NaBO22H2O), and sodium hydroxide (NaOH), or sodium carbonate (Na2CO3). A 500 °C, heat treatment for 2 h reduced the gel regions to a composition range of B2O3 = 50 to 90, Na2O = 10 to 30 and TiO2 = 0 to 20 mol %, but treatment at 800 ° C produced melts which became glassy upon cooling. On the other hand, a twin-roller method produced a wide range of glass region which includes a high concentration of TiO2 up to 70 mol %. The as-prepared gel region was found to correspond fairly well to the twin-roller glass region and the thermally most stable gel region was close to the air-quenched glass region. It was speculated that the B2O3- or Na2O-rich composition had not gelled because of the high water solubility of these gels.

Structure of alkali borate glasses based on the chemical equilibrium concept

Abstract The structure of alkali borate glass has been simulated based on a chemical equilibrium concept among possible structural units. The structural units assumed include boroxol ring, diborate, triborate, di-triborate, metaborate, pyroborate, orthoborate units, “loose” BO 3 triangles, and “loose” BO 4 tetrahedra. As a first step, only one equation was used, and then the number of equations was increased so as to be able to obtain a best fit to the experimental data. It was found that equilibrium constants were almost unanimously determined by a trial and error technique. Assuming ten equilibrium equations and equilibrium constants, it was shown that the variation of coordination number of boron atoms and that of bridging and non-bridging oxygens in the glass as a function of alkali concentration can be reproduced in good agreement with the NMR data and Raman spectra.

Nucleation kinetics of a Li2O·2SiO2 glass based on a liquid model

A liquid model was proposed wherein crystal embryos should exist in equilibrated melts at temperatures even above the melting point or liquidus temperature and the number density of embryos should decrease as the melting temperature increases. Crystallization behavior of a Li2O·2SiO2 glass was studied. Secondary heat-treatment (600 °C for 20–60 min) of the Li2O·2SiO2 glass induced different number of crystals depending upon the melting temperature in the range 1040°–1300 °C and duration of heat-treatment t. The number density of crystals N decreased with increasing temperature of melting. The number density of pre-existing embryos was estimated from (N – t) diagram and was found consistent with what the proposed model claims.

Solubility of Ag2O in the B2O3–Al2O3 system

Abstract The variation of Ag2O solubility under air atmosphere with the chemical potential of Ag was measured for the B2O3–Al2O3 system by equilibrating with a Ag–Au alloy with various Ag contents at 1373 K. It was found that, in this system, the regular solution model is not valid for estimating the activity coefficient over a wide composition range. The activity coefficient of Ag2O in B2O3 at infinite dilution and first order Gibbs energy interaction coefficients of Ag2O, Al2O3, and Na2O on the Ag2O were determined from Ag2O solubility data without assuming a regular solution. The solubility of Al2O3 in borate melts is also discussed.