Y Q Cai

Mechanism of transition between a-axis and c-axis growth of YBa2Cu3Ox thick films grown on NdGaO3 substrate

Abrupt transition from a-axis growth to c-axis growth was found in the liquid phase epitaxy (LPE) process of YBa2Cu3Ox (YBCO or Y123) on (110) NdGaO3 substrate. Subsequently, the influence of supersaturation and liquid composition on the orientation of LPE thick films was systematically studied. Dissimilar to the thin film deposition, the a-phase growth was predominant at a low supersaturation (about 17K), especially grown by using a Cu-rich melt. The investigation of the initial stage of YBCO growth was performed, in which we found the protruding epitaxial a-axis grains and preferentially a-phase nucleation on the c-dominated thick film. The energy dispersive x-ray spectrometry attached scanning electron microscope was employed to confirm the composition of the outgrowth of long-axis grains, as well as the microstructure around the dipping boundary line in the initial growth. Based on the “nucleation and growth” mode, a comprehensive description of the competition between a-axis and c-axis growth under v...

Effect of Pr doping on the growth and superconducting properties of (Y1−xPrx)Ba2Cu3O7−δ

(Y1−xPrx)Ba2Cu3O7−δ (Pr-YBCO, x<0.2) single crystals and single domain bulks were grown by the top-seeded solution-growth (TSSG) and the top-seeding melt-textured growth (TSMTG) methods, respectively. The superconducting critical temperature (Tc) was determined by magnetic measurements and the critical current density (Jc) was estimated by applying the modified Bean model. For a low-level substitution (x<0.025), an increase in the Jc value as compared with that for unsubstituted YBCO was found while no apparent change in the Tc value was noticed. On the other hand, Pr substitution in YBCO leads to: (i) increasing liquid solubility of the total rare earth element in the Ba–Cu–O solution, (ii) increasing peritectic temperature (Tp), and (iii) a decrease in the liquidus slope near the Tp. As a result, the driving force for YBCO growth and the region of single grain growth are enhanced. Study of the compositional correlation between liquid and solid additionally indicates that the Pr distribution coefficient is larger than 1 in the Pr-YBCO system. Thus the Pr content and then the Tp value of Pr-YBCO in the bulk reduce with the growth time during TSMTG. The lower Tp value in the final crystallized part is beneficial to a slow cooling process. Using the TSMTG method, Pr-YBCO (x = 0.02) monodomains up to 42 mm in diameter were grown with a high growth rate.

Crystal growth of a high-Tc SmBCO superconductor in 1 atm oxygen pressure

Sm1+x Ba2−x Cu3Oz (SmBCOss) single crystals with Tc = 95 K and a sharp transition were grown by top-seeded solution growth in 1 atm oxygen pressure. Compositional analysis by the inductively coupled plasma (ICP) technique and Tc measurement with a superconducting quantum interference device (SQUID) indicate that a nearly stoichiometric SmBCO phase with a high Tc value can be reached in a rather wide range of liquid compositions and oxygen partial pressures. Specific features of the SmBCO system phase diagram were elucidated that explain why the SmBCO single crystals grow more easily than YBa2Cu3Oz (YBCO) o rN d 1+x Ba2−x Cu3Oz (NdBCO) ones. The surface morphology of nanostripes was investigated by atomic forc em icroscopy (AFM). The nanostripes were thought to act as an effective pinning medium, due to the comparable scale with the vortex core. Furthermore, two SmBCO samples with typical Tc values were analysed by energy-dispersive spectroscopy (EDS) and the composition fluctuations along their stripes were studied.

Intrinsic structure and thermal stability of YBCO thin film

The melting behaviours of two kinds of c-axis oriented YBa2Cu3Ox (YBCO) films with different four-symmetry in-plane orientations were observed in situ by using high temperature optical microscopy. The in-plane orientation was detected by pole figures and the atomic configuration in the vicinity of the interface of one sample was evaluated by electron diffraction and high-resolution transmission electron microscopy. It is found that the melting mode and the thermal stability of the YBCO thin films with different in-plane orientations differ completely from each other. Such a phenomenon is considerably influenced by the geometrical lattice matching between the YBCO thin films and the MgO substrate, as well as the terminal plane of the YBCO thin films.

In-plane orientation effect on the melting behaviour of YBCO thin film

By means of high-temperature optical microscopy (HTOM), a 60 °C gap in initial melting temperature between two YBa₂Cu₃O(x) (Y123) thin films was found in situ. Using these two films as seeds, liquid phase epitaxy (LPE) dipping experiments showed the same tendency in the melting behaviour. The in-plane orientation was detected by x-ray diffraction (XRD) pole figure. On the basis of results from HTOM, LPE and XRD, it was unveiled that the interface structure has a predominant influence on the melting mode. A semi-coherent interface suppresses not only the melting growth but also the melting nucleation, while an incoherent interface encourages both of them. (In this work, melting of YBCO refers to the peritectic decomposition of Y123.).

Origin of the in-plane alignment transformation on YBa2Cu3Ox liquid-phase epitaxial films affected by the atmosphere environment

The preferential growth phenomenon of the polycrystalline YBa2Cu3Ox seed film deposited on (100) MgO substrate was investigated using liquid phase epitaxy (LPE). The x-ray diffraction and the polarized Raman spectroscope were used to evaluate the in-plane and out-of-plane orientation of as-grown films. It is found that a so-called 45° oriented in-plane alignment preferentially formed under the pure oxygen atmosphere, which is dissimilar to the 0°-oriented grains presenting under the air environment. The mechanism study on the initial stage of REBa2Cu3Ox (REBCO, RE=Y, Nd) LPE growth was performed, in which the morphology evolution shows a breaking coalescence of 45° islands by entrapped fluxes in an intensive manner in the 45° LPE film. The peak around 230 cm−1 in Raman spectrums was detected only in the 45° LPE film, indicating relatively weak grain connectivity, which agrees well with broken coalescence structure. Moreover, a general comparison comprising interfacial terminal layer, oxygen content, geome...

Anisotropy of the superconducting state parameters and intrinsic pinning in low-level Pr-doped YBa 2 Cu 3 O 7δ single crystals

The influence of low-level Pr substitution in Y1−xPrxBa2Cu3O7−δ single crystals on the anisotropy of the superconducting state parameters was investigated by torque magnetometry in magnetic fields of up to 9 T. An averaged anisotropy parameter, γ, of 7.4 was found and no influence of the Pr ion concentration on γ was observed up to a Pr content of 2.4%. A pronounced maximum at angles between 0.5 ◦ and 1 ◦ out of the ab-plane was observed for all crystals in the irreversible angular dependence of the torque. This maximum is attributed to intrinsic pinning associated with kinked vortices. The variation of the irreversible torque with the substitution level indicates the influence of the Pr content on pinning within the CuO2 planes, even though the anisotropy of the superconducting state parameters is not affected. (Some figures in this article are in colour only in the electronic version)

Forty-five degree oriented YBa2Cu3O7− x /MgO interface structures studied by high-resolution electron microscopy

Films of YBa2Cu3O7− x (YBCO) grown on (001) MgO substrates by liquid phase epitaxy in an oxygen atmosphere were investigated by high-resolution electron microscopy. The in-plane orientation of the films with respect to the substrates was [110]YBCO//[100]MgO. Two kinds of interface structures were observed from high-resolution electron microscope images. It was determined that in one kind of interface structure the initially formed layer of YBCO on MgO is the CuO layer, while in the other it is the BaO layer followed by a CuO layer. In the latter case, a thin layer of Cu2O is first formed on MgO as a linkage between YBCO and MgO.

Influence of low-level Pr substitution on the superconducting properties of YBa2Cu3O7-δ single crystals

We report on measurements on Y1-xPrxBa2Cu3O7-δ single crystals, with x varying from 0 to 2.4%. The upper and the lower critical fields, Hc2 and Hc1, the Ginzburg-Landau parameter, κ, and the critical current density, Jc (B), were determined from magnetization measurements and the effective media approach scaling method. We present the influence of Pr substitution on the pinning force density as well as on the trapped field profiles analyzed by Hall probe scanning.

The phenomenon of the superheating of YBCO thin film and its application in processing REBCO bulk

During heating from room temperature to 1060 °C, a novel superheating phenomenon (up to over 45k) in YBa2Cu3Oz (YBCO or Y123) thin film was found. In order to clarify the origin of this superheating, the entire melting evolution of the film was observed in situ using a high temperature optical microscope. The semi-coherent interface between the YBCO film and the single-crystalline MgO substrate, as well as the unwettability of the Ba–Cu–O melt, was found to play a most significant role in causing the superheating. Furthermore, evident discrepancies in the degree of superheating and melting behaviours were observed between two YBCO thin films which have different microstructures, characterized by means of atomic force microscopy and Raman spectroscopy. The mechanism of the melting discrepancy was discussed in detail. Finally, as an application of the superheating phenomenon, a large single-grain YBCO superconductor was grown by using a homoseed of YBCO thin film.