Kazumasa Matusita

Kinetics of non-isothermal crystallization process and activation energy for crystal growth in amorphous materials

An equation expressing the volume fraction,x, of crystals precipitating in a glass heated at a constant rate, α, was derived. When crystal particles grow m-dimensionally,x is expressed as In [- ln(1 -x)] = -n (nα - 1.052mE/RT + Constant whereE is the activation energy for crystal growth andn is a numerical factor depending on the nucleation process. When the nuclei formed during the heating at the constant rate,α, are dominant,n is equal tom + 1, and when the nuclei formed in the previous heat-treatment before thermal analysis run are dominant,n is equal tom. The validity and usefulness of this equation was ascertained by applying it to a Li2O·2SiO2 glass. A method for determining the values ofn andm from DSC curves was proposed and it was concluded that the modified Ozawa-type plot is very useful and convenient to obtain the activation energy for crystal growth.

Optical properties of transparent glass-ceramics in K2ONb2O5TeO2 glasses

Abstract Transparent glass-ceramics consisting of a cubic crystalline phase with crystallites having diameters between 20 and 40 nm in the composition of 15K 2 O15Nb 2 O 5 70TeO 2 (mol%) have been fabricated. A phase with cubic structure is formed by post-heat-treatment at around 390°C for 1 h and transforms into a stable phase at temperatures above 450°C. The glass-ceramics consisting of a stable crystalline phase are opaque. The transparency of glass-ceramics is attributed to a small particle size (average radius: 10–20 nm) of the cubic crystalline phase. The optical and dielectric properties for the transparent glass-ceramics obtained by heat-treatment at 425°C for 1 h are: refractive index, n = 2.11 ± 0.02; relative permittivity (1 kHz, 300 K), ϵ r = 44 ± 1 and third-order non-linear optical susceptibility, χ (3) = 3.3 × 10 −13 esu. These values are larger than those for the original base glass, i.e. n = 2.02 ± 0.02, ϵ r = 28 ± 1 and χ (3) = 0.9 × 10 −13 esu. Second-harmonic generation is clearly observed in transparent glass-ceramics. These transparent glass-ceramics have a potential as a new type of non-linear optical material.

High-Tc Superconducting Glass Ceramics Based on the Bi-Ca-Sr-Cu-O System

Various superconducting glass ceramics based on the Bi-Ca-Sr-Cu-O system with Tc(onset)=80~95 K and Tc(zero)=63~78 K were prepared by a melt-quenching method. The melt-quenched samples of Bi1.5CaSrCu2Ox and Bi1.5Ca1.5SrCu2Ox were found to be a glass with a glass transition of about 425°C; these samples became high-Tc superconductors by annealing at 820°C.

Properties and crystallization behaviors of TeO2LiNbO3 glasses

(100−x)TeO2−xLiNbO3 (x = 10−60) glasses were prepared by using a conventional melt-quenching method, and some properties such as thermal stability, density and refractive index and crystallization behaviors of glasses were measured. The thermal stability of the glasses with x = 20 and 30 was high. The density of glasses decreased monotonically with increasing LiNbO3 content from 5.50 g/cm3 to 4.58 g/cm3. The values of the refractive index of glasses were 2.04-2.11 and showed an anomalous compositional dependence. It was found from Raman scattering spectra that the structure of glasses was composed of the structural units of TeO4 trigonal bipyramid, TeO3 trigonal pyramid and NbO6 octahedra. In the glasses with x = 10 and 20, α-TeO2 (paratellurite) crystals were precipitated by heat treatment above 450°C. In the glasses with x ≧ 40, LiNbO3 crystals were formed through the transformation of a metastable pyrochlore-type compound at temperatures above 500°C. From the present study, it is proposed that the fabrication of transparent TeO2-based glasses containing ferroelectric LiNbO3 microcrystallites may be possible.

Transparent tellurite-based glass-ceramics with second harmonic generation

Abstract Transparent glass-ceramics, in which the crystalline phase has basically a face centered cubic structure, have been prepared in the system K 2 ONb 2 O 5 TeO 2 , and a second harmonic generation (SHG) is observed. The second harmonic intensity in the glass-ceramics is comparable to that in electrically poled TeO 2 -based glasses. A very small distortion from the cubic structure might be present and an origin of SHG. Transparent glass-ceramics consisting of the cubic crystalline phase have been also fabricated in the system of Li 2 O or Na 2 ONb 2 O 5 TeO 2 , but SHG is not observed.

Crystallization of LiNbO3 in tellurite glasses

Abstract TeO 2 Li 2 ONb 2 O 5 glasses were prepared using conventional melt quenching and the precipitation behavior of LiNbO 3 crystals was examined. LiNbO 3 crystals precipitated directly from 50TeO 2 −(50− x )Li 2 O− x Nb 2 O 5 glasses with high Li + contents without passing through the metastable pyrochlore-type compound. Transparent oxide glasses containing 10 μm diameter LiNbO 3 crystals were prepared using a two-step heat-treatment process.

Preparation of High-Tc Superconducting Bi-Pb-Sr-Ca-Cu-O Ceramics by the Melt Quenching Method

Superconducting Bi-Ca-Sr-Cu-O ceramics with Tc(onset)=102 K and Tc(zero)=92 K were prepared by a melt quenching method, in which the melt of nominal composition BiCaSrCu2Ox was first quenched and then annealed at 800~850°C for 24 h in oxygen atmosphere. It was found that the rapidly cooled samples after annealing show high Tc and sharp superconducting transition, but the samples slowly cooled in the furnace after annealing show low Tc and broad transition with a tail. The critical current density at 77 K in the zero magnetic field for the rapidly cooled sample after annealing at 800°C was 102 A/cm2.

High-Tc superconducting glass-ceramics

Abstract This paper reviews glass formation and crystallization and the superconducting properties of glass-ceramics in the Bi(Pb)-Sr-Ca-Cu-O system. There has been great progress in the superconducting glass-ceramics field in the short period following the discovery of Bi-based superconductors. Many glasses, such as those with the compositions Bi 2 Sr 2 CaCu 2 O x and (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x , can be prepared using the conventional melt-quenching method. The glasses are crystallized by annealing at temperatures above 500 °C, and are converted into superconductors with a large amount of superconducting low- T c phase (critical transition temperature ( T c ) = 85 K) or high- T c phase ( T c = 105 K). It has been amply confirmed that the Bi(Pb)-based glass-ceramics (i.e. crystallized glasses) exhibit superconductivity with T c above 100 K. More extensive study will lead to the fabrication of high-performance superconductors such as fibres with high critical current densities.

Superconducting glass ceramics with Tc=100 K based on the Bi‐Pb‐Sr‐Ca‐Cu‐O system

Superconducting glass ceramics of Bi0.8Pb0.2SrCaCu1.5Oy (sample A) and BiPb0.2SrCaCu1.5Oy (sample B) have been prepared by using the melt quenching method. It was found that the volume fraction of the high Tc phases in sample A annealed at 830 or 840 °C for 250 h was much higher than that in sample B. The annealed (840 °C, 250 h) sample A exhibited superconductivity with a Tc (zero) of 100 K and a critical current density (77 K, zero magnetic field) of 120 A/cm2.

On the Reactions of Quenched BaYCuO Powders with Various Materials

The reactions of quenched BaYCuO powders (nominal composition: Ba2YCu3O7-x) with various materials (Al2O3, ZrO2, MgO, TiO2, SrTiO3, quartz, silica glass, Si, Ge and Nb) at 900°C were examined using X-ray diffraction analyses and measurements of superconducting properties and the new crystalline phases formed by the reactions were determined. It was found that the quenched BaYCuO reacted strongly with these materials and barium element was contained in the reaction phases. The relation between the crystalline phases and superconducting properties for the annealed samples has been discussed.