Study On The Growth Of Large-sized YBCO Single Crystals With And Without Chemical Doping

Compared with the low-temperature superconductor, the high-temperature superconductor(HTSC), e.g., REBa2Cu3O7(REBCO,RE123, RE=rare earth, i.e., Y,Gd,Sm,Nd et al.), possesses higher critical transition temperature(Tc) and stronger critical current density(Jc). Thus, the REBCO material has been widely investigated since its discovery. Particularly, large-sized and high-quality YBCO single crystals are extremely necessary for both practical applications and fundamental studies. In the latter case, for the investigation on magnetic order and related phenomena by neutron scattering experiments, sizable single crystals are essential. The conventional growth methods, however, have some disadvantages.In the present work, a modified melt growth(MMG) approach used to grow YBCO or Y1-xCaxBa2Cu3 Oy crystals was proposed and successfully applied. The main results are listed below. 1.Effective approach to prepare well c-axis-oriented YBCO single crystal by MMG.Using the modi?ed melt-growth method, a well c-axis-oriented YBCO single crystal with a size of 34 mm in diameter was successfully fabricated. First, our ?ndings show that the thermal stability of the ?lm-seed could be enhanced by introducing an intermediate mini-pellet between ?lm and bulk. Furthermore, we present evidence to prove that the serious liquid loss could be suppressed by the CeO2 addition. Finally, the interesting question we addressed is the nature of the formation of distinctive growing macrostructures, such as ambiguous growth facet lines, and the corner-concaved a-b plane. Most importantly, apart from YBCO, our new approach strongly suggests the possibility of synthesizing large crystals for more attractive members in its family, i.e., chemically doped YBCO materials, being unsuccessful by the prior art crystal growth methods. 2. Modified top-seeded melt growth for large sized Y1-xCaxBa2Cu3 Oy crystals.By employing modified top-seeded melt growth, a sizable Y1-xCaxBa2Cu3Oy(Ca–YBCO) crystal(16 mm in diameter) with a high doping level was grown for the first time. The crystal is a substance of remarkable importance in the study of superconductivity, distinctively by neutron scattering experiments. Firstly, our findings show that the addition of CaCO3 to the precursors significantly changes the liquid properties, leading to serious liquid loss. In consequence, as-grown samples are unexpectedly characterized by an incomplete growth and a high density of the Y211 phase. Furthermore, a series of experiments have been conducted to improve growth conditions. Our results prove that by using a novel sample construction, setting thin pellets underneath and on top of the main pellet as a liquid reservoir, the aforementioned problems can be effectively solved. As a result, a fully-grown Ca–YBCO crystal with less Y211 particles was achieved.The research work presented here provided a novel method for the growth of YBCO and Y1-xCaxBa2Cu3 Oy single crystals. Meanwhile, it can be expected that the crystals processed by this method could be of help for understanding its superconductivity.