Atit Bhargava

Crystallization of glasses in the system BaOTiO2B2O3

Abstract Glass formation by rapid quenching was studied for the system BaOTiO 2 B 2 O 3 . Compositions on the periphery of the glass forming region either phase separate or crystallize. Crystalline phases are identified by X-ray diffraction. Differential thermal analysis was used to study the crystallization behavior of the glasses. The glasses generally show two crystallization temperatures. The effect of composition on the first crystallization temperature is studied for increasing BaO and TiO 2 contents. Anomalous behavior, similar to that for properties of the parent glass, is observed for various series of glasses in the system. Glasses were heat treated at the onset temperature of their first crystallization. X-ray diffraction was employed to determine the phases which occur in the glass-ceramic.

The Raman and infrared spectra of the glasses in the system BaOTiO2B2O3

Infrared and Raman spectra were measured for various glasses in the system BaOTiO2B2O3, and interpreted on the basis of structure. Emphasis was placed on the spectral effects due to changes in coordination for boron along with BaO and TiO2 addition. Various vibrational bands and splitting in the spectra indicated TiO bonds and TiOB linkages. The resulting structural model was used to interpret compositional effects upon properties, such as glass transformation temperature, d.c. conductivity and coefficient of thermal expansion.

Preparation of BaTiO3 glass-ceramics in the system BaTiBO. I

Abstract On the basis of earlier crystallization and X-ray diffraction studies, specific glass compositions in the system Ba-Ti-B-O were studied for synthesis of BaTiO 3 glass-ceramics. The glass-ceramics show significant differences in their surface microstructure and their crystalline phases. These differences are explained on the basis of structure of the parent glasses.

Atmospheric effects during thermal cycling of Ba2YCu3O7±x superconductors

Abstract Thermal cycling of Ba2YCu3O7±x powder under air and nitrogen atmospheres in either a differential scanning calorimeter or thermogravimetric analyzer results in alternating oxidation/reduction reactions. Thermal cycling also leads to substantial modifications of the crystallographic unit cell and microstructure. The 440°C phase transformation disappears during this process.

Preliminary investigations of superconducting glass-ceramics in the Bi-Sr-Ca-Cu-B-O system

Abstract We have made glasses with starting batch stoichiometries (Bi-Sr-Ca-Cu) 4-3-3-4, 2-2-2-3 and 2-2-1-2 by a melt-quench process. Glasses thus formed superconduct upon suitable heat treatment due to devitrification of the Bi 2 Sr 2 Ca 1 Cu 2 O 8 phase. Upon addition of a glass-former (boric acid/silica), we were able to form stable glasses in a variety of batch compositions. Some of these compositions showed crystallization of the Bi 2 Sr 2 Ca 1 Cu 2 O 8 phase and were also superconducting in the 90 K range.

Synthesis of superconducting ceramic coated metal wires by a glass-ceramic technique

Abstract A metal wire, coated with a glass of the stoichiometry (Bi-Sr-Ca-Cu) 2-2-1-2 by a melt-quench process, is subjected to a suitable heat treatment which results in subsequent superconducting behavior due to devitrification of the Bi 2 Sr 2 Ca 1 Cu 2 O 8 phase. The composite wire has a superconducting transition around 60 K and can be flexed and wound without losing its superconducting behavior.

Effects of atmosphere on Ba2YCu3Ox during processing

Abstract Phase pure Ba2YCu3Ox was exposed to atmospheres of oxygen, air, nitrogen, carbon-dioxide, carbon monoxide and ammonia. High temperature treatments show that the maximum processing temperature must stay below the incongruent melting peak, which is shown to vary with atmosphere. It is also shown that partial pressure of oxygen affects the crystal structure of the sintered material. Low temperature treatments (555°C) reveal a change of the Ba2YCu3Ox material from a superconductor to a semiconductor, the most intriguing being a material which shows a change in sign of do/dT at 88 K, thus offering interesting applications in electrical circuits.

On the stability of superconducting Y1Ba2Cu3O6+δ in a borate glass-ceramic matrix

Abstract We have been successful in formulating a glass in the system Y 2 O 3 -BaO-CuO-B 2 O 3 which, upon subsequent suitable heat treatment, precipitates superconducting crystallites of Y 1 Ba 2 Cu 3 O 6+δ . The intent of the B 2 O 3 addition was to cause stable glass formation allowing controlled crystal growth rate during the subsequent heat treatment. This work establishes that a borate glass matrix will thermodynamically support the superconducting Y 1 Ba 2 Cu 3 O 6+δ phase. Further work to increase its concentration should allow for a host of continuous glass-forming processes to be used to make virtually pore-free, exceptionally strong bulk superconductor products.

The reproducible production of pure superconducting Ba2YCu3Ox

Abstract A systematic study involving the effects of temperature and different atmospheres on the formation of Ba 2 YCu 3 O x was performed. The gases used were oxygen, air, nitrogen, carbon-dioxide and carbon-monoxide. The solid state synthesis procedure employed gave an XRD pattern which indexed phase pure in the orthorhombic system. A narrow temperature range for calcining was found to be the most crucial parameter for obtaining reproducible and phase pure superconducting powder. The results of this work offer significant suggestions for the processing of Ba 2 YCu 3 O x .

Preparation of BaTiO3 glass-ceramic in the system BaTiBO. II

Abstract One glass composition in the system Ba-Ti-B-O was selected to develop a glass-ceramic that had BaTiO 3 as the only crystalline phase. The novel technique that was developed in this study involved crystallization on an isostructural substrate. A model explaining this process was provided on the basis of Raman microprobe, SEM and XRD data.