Yoshihiro Nishimura

Direct observations of pseudomorphism : compositional and textural evolution at a fluid-solid interface

Abstract Solid-fluid interactions often involve the replacement of one phase by another while retaining the morphology and structural details of the parent phase, i.e, pseudomorphism. We present in situ observations of the evolution of both the solid and fluid compositions at the interface during such a replacement reaction in the model system KBr-KCl-H2O, in which a single crystal of KBr is replaced by a single crystal of KCl. The pseudomorphism is initiated by epitaxial growth at the fluid-mineral interface, when the dissolution of the parent phase results in an interfacial fluid layer that is supersaturated with respect to a different solid composition. The subsequent evolution of the coupled dissolution and growth can be related to local equilibrium defined by a Lippmann diagram. The reaction features, including the development of porosity in the new solid phase, share many characteristics of replacement reactions in nature as well as in technical applications

In-situ measurement of dissolution of anorthite in Na-Cl-OH solutions at 22 °C using phase-shift interferometry

Abstract In-situ measurements of anorthite dissolution in Na-Cl-OH solutions at an ionic strength (IS) of 0.5 mol/L (M) and in artificial seawater (IS = 0.7 M) were conducted at 22 °C using white-light, phase-shift interference microscopy (PSI-M). Nanometer-scale surface topography by PSI-M revealed three-dimensionally inhomogeneous surface dissolution, which is commonly observed as retreating steps on anorthite surfaces. Continuous dissolution of the anorthite cleavage surface (010) was successfully measured within a day. The vertical dissolution velocity was 4.3 × 10-5 to 1.4 × 10-3 nm/s. The obtained dissolution rates showed a typical dependency on pH with a reaction order of 0.191, and could be consistently extended to the previous data obtained under acidic conditions (Luttge et al. 1999). In-homogeneities in the vertical dissolution velocities at each pH condition could be interpreted by the step dynamics explained by the Burton-Cablera-Frank (BCF) theory (Burton et al. 1951). These results emphasize that the velocity of step retreat is a strong function of the step density, which has to be taken into account when describing the global dissolution phenomena on mineral surfaces

In situ observation of crystallization of YBCO via the peritectic reaction

Abstract Crystallization of YBa 2 Cu 3 O x via the peritectic reaction; Y 2 BaCuO 5 + Liquid → YBa 2 Cu 3 O x was successfully observed in situ by high temperature optical microscopy. It was found that the growth of YBa 2 Cu 3 O x crystals proceeds with the aid of the peritectic reaction, which is different from the normal peritectic reaction of the binary alloy system. The present results give us an important clue how to grow a high quality YBa 2 Cu 3 O x single crystal from the high temperature solutions.

Conversion of superconducting Bi-system single crystals from 2212 to 2223 by the annealing method

The conversion of BSCCO 2212 single crystals to 2223 was studied by annealing 2212 crystals in a Bi2Sr2Ca4Pb0.5Oy powder in the temperature range 841 to 852°C for 150 h in air. It was found that the best temperature for the conversion was 852°C. The relation between the formation of 2223 and the surface microtopographs of the annealed crystals were studied.

New growth method of oxide crystals by PO2 change applied to SmBa2Cu3Ox single crystals

Abstract Reliable liquidus lines of SmBa 2 Cu 3 O x in the SmBa 2 Cu 3 O x –Ba 7 Cu 18 O 25 system under various oxygen partial pressures were determined precisely by in situ observation. The dependence of the liquidus line on oxygen partial pressure was revealed: as the oxygen partial pressure increased, the liquidus shifted to higher temperature. Using this data, SmBa 2 Cu 3 O x crystals were grown. This process was successfully observed in situ by high-temperature optical microscopy.

Growth of Y2Cu2O5 single crystals by the traveling solvent floating zone method

Sufficiently large single crystals of Y2Cu2O5 for use in neutron diffraction experiments were successfully grown by the traveling-solvent floating zone method under an oxygen partial pressure of 40–100%. The optimum composition for growth from a high-temperature solution was determined in situ using high-temperature optical microscopy. The growth direction was parallel to the crystallographic b-axis. This crystal exhibited well-defined metamagnetic transitions.