Megumi Tashiro

Rate of homogeneous nucleation in alkali disilicate glasses

Rates of crystal nucleation in alkali disilicate glasses were measured by counting the number of crystals under an optical microscope. The viscosities of these glasses were measured by the method of beam-bending and penetration. Using the data of rate of nucleation and viscosity obtained in the present study and the data of free energy measured by Takahashi and Yoshio, crystal-glass interfacial energies for alkali disilicate systems were estimated on the basis of homogeneous nucleation theory as follows: 196 erg/cm2 for Li2O·2SiO2, 126–144 erg/cm2 for Na2O·2SiO2 and 88–104 erg/cm2 for K2O·2SiO2. The effects of the viscosity of glass, the free energy difference between crystal and glass and crystal-glass interfacial energy on the rate of nucleation were discussed, and the remarkably higher rate of crystal nucleation in the Li2O·2SiO2 glass was attributed to the larger free energy difference.

Dielectric properties of fine-grained PbTiO3 crystals precipitated in a glass

Abstract The spontaneous deformation of PbTiO3 crystals precipitated in a PbOTiO2Al2O3SiO2 glass was found to decrease with decreasing grain size. This trend remains even after the glass matrix surrounding PbTiO3 crystals is removed completely by HNO3 acid immersion. Consequently, a linear electro-optic effect cannot be expected from transparent glass-ceramics which necessarily comprise very-fine-grained crystals. The dielectric constant of PbTiO3 crystals surrounded by a glassy matrix shows two maxima at grain sizes 0.15 μm and 250 A, but is still fairly high even at a grain size as small as 160 A. This suggests that a material with a large quadratic electro-optic effect may be produced from a transparent glass-ceramic. The maxima of the dielectric constant at grain sizes 0.15 μm and 250 A were interpreted in terms of the internal stress and internal electric field, respectively.

Crystallization of glasses: Science and technology

Abstract The progress in the production of commercial glass-ceramics in Japan is presented. The possibilities and limitations of the development of new and useful glass-ceramics are discussed.

Study on crystallization of glass by differential thermal analysis. Effect of added oxide on crystallization of Li2O-SiO2

Differential thermal analysis was made of 33.3 Li2O·66.7 SiO2·3 ROn and 25 Li2O·75 SiO2·3 ROn glasses, where ROn is an added oxide. The exothermic temperatures, Tc, due to the precipitation of lithium disilicate crystal, were plotted against the ionic radius of the cation of the added oxide. It was possible to classify the added oxides into a few groups on the basis of the valency of the cation. Within each group, the Tc increased as the ionic radius of the cation increased. It was shown that the temperature, Tη corresponding to the viscosity of 1010 P also increased with increasing the radius of the cation. The relation between Tc and Tη was discussed using the theory on the rates of nucleation and crystal growth. The effect of phase separation on Tc was also discussed.

Infrared transmission of (R2O OR R’O)–(TiO2, Nb2O5 OR Ta2O5)-Al2O3 glasses

Abstract The new families of aluminate glasses obtained by the present authors from their melts in the systems K2O–Ta2O5–Al2O3, Na2O–K2O–Ta2O5–Al2O3, K2O –Cs2O– Ta2O5–Al2O3, K2O–Nb2O5–Al2O3, Na2Oz.sbnd;K2O–TiO2–Al2O3, BaO–TiO2–Al2O3, BaO–ZrO2–TiO2–Al2O3 and Na2O–K2O–BaO–ZrO2–Ta2O5–TiO2 –Al2O3 showed high transmissions of visible and infrared (IR) radiation ranging from 0.4 to about 6 μm, as well as high refractive indices up to 2.0. Their physical and chemical properties such as glass-forming ability, softening temperature, hardness and hygroscopicity were comparable to conventional silicate glasses. These properties are useful for IR applications. The cause of the high IR transmission of the aluminate glasses was interpreted in terms of the masses of the constituent cations and the single bond strengths of the cations with oxygen ions.

Transparency of LiTaO3-SiO2-Al2O3 glass-ceramics in relation to their microstructure

The glasses with various compositions in the LiTaO3-SiO2-Al2O3 system were heated from room temperature to temperatures ranging from 750° to 1050° C at a rate of 5° C min−1. From the glasses in the LiTaO3-SiO2 system no transparent glass-ceramic was obtained even when their LiTaO3/SiO2 mole ratios were as high as 2.33. The diameter and number of the LiTaO3 crystal grains precipitated in the glasses were 5–15 μm and 108–1010 grains cm−3, respectively. On the contrary, transparent glass-ceramics were obtained from the glasses containing Al2O3; their compositions covered a fairly large area in the LiTaO3-SiO2 -Al2O3 system, which encompasses the compositions with the LiTaO3/SiO2+AlO1.5 mole ratio as low as 0.25. The diameter and number of the LiTaO3 crystal grains precipitated in the transparent glass-ceramics were as small as 10–20 nm and as many as 1016–1018 grains cm−3, respectively. High nucleation rates of the LiTaO3 crystals in the Al2O3-containing glasses were interpreted in terms of structural inflexibility induced in the glass-network by the addition of Al2O3 to the LiTaO3-SiO2 system.

Glass formation in the systems (K or Cs)2O(Nb or Ta)2O5Al2O3

Abstract The glass-forming ability of melts in the systems K2O(Nb and/or Ta)2O5Al2O3 as well as those in which K2O was replaced with Li2O, Na2O, Cs2O, BaO or PbO was investigated. Some melts in the systems (K or Cs)2O(Nb and/or Ta)2O5Al2O3 could be made into glasses by cooling, yielding practically useful amounts. The structures of these glasses were discussed on the basis of their infrared spectroscopic and X-ray emission spectroscopic analyses.

Alkaline durability of BaOTiO2SiO2 glasses

Abstract Glass fibers of TiO2-rich composition in the system BaOTiO2SiO2 and those modified with other components were immersed in 2N NaOH solution at 95°C. A SrOBaOTiO2SiO2 glass showed much less diameter reduction than a conventional ZrO2-containing Na2O-SiO2 glass (G20) especially when the ratio of the surface area of the glass specimens to solution volume was high. Fibers of SrOBaOTiO2-SiO2 glass and its modified version were immersed also in a Portland cement solution at 95°C. They maintained tensile strengths higher than glass G20 even after immersion for 108 h. The high alkaline durability of the TiO2-rich glasses was interpreted in terms of the low solubility of the TiO2 in the alkaline solution and preferential adsorption of the Sr2+ and Ba2+ ions on the hydrated TiO2-rich surface layer.

Effect of pressure on radiation-induced color centers in silicate glasses

Abstract Optical absorption spectra of color centers produced in silicate glasses by γ-irradiation were measured before and after application of pressure of 50 kbar to the glass. The two color center peaks, one found at about 400 nm and the other at about 600 nm, shifted toward shorter wavelengths as a result of densification of glass. The local shrinkages around the color centers estimated from the shifts of the peak wavelengths were much larger than the bulk shrinkages of the glasses obtained from the change in density of the glasses due to application of the pressure. Similar shifts of the color center peak wavelengths were observed also when the glass was γ-irradiated after densification. Besides the shift of the peaks, the application of the pressure caused a great increase of broad absorption through the visible region. A tentative explanation was given to this phenomenon on the basis of the formation of unknown defects due to the effect of shear accompanying the application of pressure. The annealing of the various pressure effects was also examined.

Effect of High Pressure on Precipitation of Silver Colloids in Glass

ブリッジマンアンビル型の加圧装置を用いて高圧を加えたガラスの光吸収を測定することにより珪酸塩ガラス中における銀コロイドの生成にたいする高圧の効果を調べた. 室温で65kbまでの高圧を加えておくと, その後250°-500℃の高温で加熱したときにおこる銀コロイドの生成が促進された. 銀コロイドの生成がおこるような高温で圧力を加えてもやはり銀コロイドの析出が促進された. ただし, この場合には, 析出した粒子の寸法はさきの場合にくらべてより小さかった. このように銀コロイドの生成が促進されるのは高圧付加によりガラス中に欠陥が生成し, そのため銀コロイドの結晶核形成が促進されるためと考えられる. なお, ガラスをいったん高温で加熱したのち室温で加圧すると大きい銀コロイドが析出し, そのためガラスは灰色または暗色になることが確かめられた.