Unexpected metastable transition in high superheating state

Abstract

By high temperature microscopy, we conducted in-situ observation on the evolution of peritectic melting of YBa2Cu3O7−y thin films (denoted as the α phase). As the α phase was highly superheated above a critical temperature, we found that a non-equilibrium phase transition occurred. An unexpected metastable Y2O3 phase (denoted as the γ phase), resulted from the decomposition of the α film, nucleated preferentially to the stable phase of Y2BaCuO5 (denoted as the β phase). Both high superheating capability of the α film and low interface energy of the γ phase with substrates are responsible for the metastable phase transition in this work.

Peritectic melting of thin films, superheating and applications in growth of REBCO superconductors

“…However, most of these studies concentrated on deeply undercooled melts. From the aspect of achieving a highly superheating state, an unconventional phase transition was reported by employing HTOM [85]. Then the research interest on the unconventional phase transition and stabilization of materials was extended to more scientific and technological fields.…”

“…An illustration of the driving force for the growth of Y211 and Y200. Similar to undercooling, the superheating status can be represented by the extended parts of the liquidus curve, which equilibrates with the Y123 phase, into Y211+L region [85].…”

“…(a) The surface morphology of the obtained grains: the typical square grains are directly nucleated from YBCO matrix, while the one at the lower right corner without a regular square shape is the Y200 phase nucleated from Y211 phase; (b) the EDS mapping result of the whole region indicates those grains with gray color are of the same composition; (c) the EDS result of the region spectrum 1 shows the square grain consists of Y and O elements[85].…”

“…The optical micrographs of the unknown square grains resulted from YBCO/LAO thin film, captured after the melting process[85].…”

Melting behavior and superheating capacity of REBa2Cu3Oy films with nano‐layer buffered structures

“…Nowadays, deep investigation on the superheating mechanism, searching novel structures possessing the high superheating capability, and realizing its practical application attract great attention in the scientific world . In recent years, experimental evidences have clearly proved that the c ‐axis oriented RE123 (REBa 2 Cu 3 O y , RE = Nd, Sm and Y) films on the MgO substrate (shortly, RE123/MgO), as a quasi‐2D structure possesses a superheating capacity, i.e., the RE123 film may remain in its metastable solid phase above its peritectic melting temperature over several tens degrees . Like most functional oxide materials, RE123 features anisotropic surface energy in the crystalline plane (equivalent to anisotropic melting temperature) and the a‐b plane is characterized by the low surface energy, which is the intrinsic reason why this quasi‐2D nanocrystalline film does not easily melt.…”