In-situ observation and analysis of formation process of solidification structure of superconductive faceted 123 crystal

Abstract

A solidification technique has been developed for the growth of columnar or single YBa₂Cu₃O_7-x (123) crystal with finely dispersed Y₂BaCuO₅ (211) particles which work as flux pinning centers. However, there are still many unclear points on the nucleation and growth mechanism of faceted 123 phase through a peritectic reaction of liquid + 211 to 123. Therefore, using an in-situ observation apparatus with zoom type microscope, the nucleation and growth process of 123 crystals from the liquid + 211 phase was observed. YBCO paste on a MgO single crystal (100) was heated up to 1280+10K and kept for 300s in the furnace. Then the sample was cooled to an undercooling and held several hours to observe the structure formation process. In addition, the structure of specimen quenched during solidification was observed and analyzed using EPMA, SEM, and X-ray diffraction. The crystallization process of faceted 123 phase was observed directly with optical microscope and recorded by video tape. The nucleated faceted 123 crystals grew at a rate according to the undercooling. Growth rate of facet interface and incubation time for nucleation were obtained by observing the movement of faceted interface during isothermal solidification. Nucleation rate was obtained from the relationship between the number of nucleated crystals and their nucleation time, and it is revealed that nucleation rate increases exponentially with the increase in undrecooling.

Keywords:

• YBa₂Cu₃O_7-x
• Y₂BaCuO₅
• nucleation
• solidification
• peritectic reaction