A novel a/c grain boundary with well-defined facet of the YBa2Cu3O7 − δ (Y123) film featuring a single-crystalline nature was successfully grown by liquid phase epitaxy, differing from conventional ill-defined facet film possessing a polycrystalline characteristic by the prior art deposition techniques. Here the extremely low supersaturation as essential prerequisite was exploited on (110) NdGaO3 (NGO) substrates with partly-etched c-axis Y123 films. Based on the selective growth, preferred a-axis Y123 hetero-epitaxially grew on the bare NGO directly while c-axis Y123 homo-epitaxially proceeded on pre-existing Y123, consequently fabricating a distinct a/c grain boundary, which has great importance for fundamental studies and device applications.Download high-res image (218KB)Download full-size image

•High growth rate (via high cooling rate) leads to the less absorbed-oxygen in LREBCO.•Oxygen vacancy control is effective to adjust cation stoichiometry of LREBCO.•SmBCO with optimal cation stoichiometry shows high superconducting properties.•Reasonable high cooling rate helps to gain SmBCO bulks with high Tc performance.Among all functional perovskite oxides, LRE-Ba-Cu-O (LREBCO, LRE123, LRE: light rare earth = Sm, Nd, et al.) superconductors possess outstanding properties of higher critical transition temperature (Tc) and higher critical current density (Jc) than YBCO superconductors. However, cation off-stoichiometry caused by the substitution of LRE elements for Ba sites could deteriorate superconducting properties. Here, we develop a new approach for fabricating high performance superconductors crystal of nearly ideal stoichiometric Sm1+xBa2-xCu3Oy (i.e. low x value) in air through oxygen vacancy control. In the peritectic reaction of Sm211 (solid) + Ba–Cu–O (liquid) + O2 (gas) → Sm1+xBa2-xCu3Oy (solid), the higher growth rate (achieved via the high cooling rate during growth) leads to the less absorbed-oxygen in Sm1+xBa2-xCu3Oy (i. e. low y value), which could balance the whole valence of cations to gain a low x value, equivalent to a low substitution level of Sm3+/Ba2+. As a result, by using the normal precursor (Sm123 + Sm211), SmBCO bulks with the high performance (Tcmid over 94 K with the sharp transition) are successfully grown in air. Thus this path effectively realizes the control of cation stoichiometry for SmBCO by oxygen vacancy modification, which could be applicable in synthesizing other functional oxides with desirable performance.By using normal precursors (Sm123 + Sm211), We succeeded in growing SmBCO samples possessing with high performance (Tcmid over 94 K with the sharp transition), which is comparable to samples prepared by OCMG and CCMG.

Superheating of solids, an unconventional phenomenon in nature, can be achieved by suppressing the heterogeneous nucleation of melt at defect sites, such as free surfaces and internal grain boundaries. In recent years, experimental evidences have clearly proved that the YBCO (Y123) thin film with a free surface possesses a superheating capacity, which is mainly attributed to the film/substrate structures, distinctively consisting with low-energy surface and semi-coherent interface. Like most functional oxides, YBCO (denoted as α phase) is characterized by a peritectic melting: α → β + liq. Its superheating behavior certainly relates to this peritectic reaction. Furthermore, REBCO (RE123, RE: rare earth elements) thin films with high thermal stability have been successfully employed as seed materials in inducing the growth of REBCO materials, such as thick film, single crystal and single domain bulk. Therefore, this superheating property of thin films is of great importance in both scientific study and practical application. In this paper, the up-to-date researches covering on the superheating phenomenon of the α phase film, its mechanism and applications in growth of REBCO superconductors are reviewed, which is supposed to be valid for more thin films of functional oxides that have the same nature as the YBCO film/substrate.

Using liquid phase epitaxy (LPE), we developed a new approach to growing a-axis oriented YBa2Cu3O7−δ films (a-films) in air, which have been hard to gain in the prior art since an extremely low supersaturation (σ) is required for their formation. Instead of the conventional step-cooling from a high temperature (Th) above the peritectic temperature (Tp) directly to the growth temperature (Tg), a novel cooling plus heating mode (from Th cooling down to below Tg and then heating back to Tg) was adopted for the film growth. We found that slow cooling combined with fast heating was effective to achieve an extraordinarily low σ for preparing a-films, which are potentially appropriate for Josephson junction devices. Most importantly, the correlation between the cooling mode and the driving force was reviewed, which provided universal guidance for finely tuning supersaturation of the solution.

Exploiting their distinguished merits of commercial availability, large size, and extremely high thermal stability, a series of NdBCO film-seeds were employed to study their size effects on the YBCO bulks by melt-growth. First, our findings show that the nucleation range is almost the same with the variation of the seed size and that the effective contact area of the seed with the molten pellet is smaller than the seed area, which can be explained by the liquid’s wettability and its surface energy. Moreover, induced by the large-sized seed, the YBCO grain has the highest a-axis growth rate, Ra, because of a double mode of thermally driven plus seed-induced growth, leading to a larger c-GS (c-growth sector). Finally, the levitation force of bulks were proven to possess an ascending and subsequently descending tendency with increasing the seed size, which is clarified by the enlargement of c-GS in competition with enhancement of pore density of bulks. In short, the results from this work are helpful to understand the crystallization mechanism and to gain the optimal superconductivity property with a reasonable seed size.

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.

Using liquid phase epitaxy, a novel approach was developed to grow a-axis oriented YBa2Cu3O7−δ films (a-films) under an air atmosphere, which has been difficult previously since its formation requires extremely low supersaturation. In our new method, instead of conventional cooling from the saturated to supersaturated state, an extremely small driving force for film growth was generated from the unsaturated through saturated to supersaturated state. By controlling the amount of fresh solvent and the melting time before cooling down the Y–Ba–Cu–O solution, a growth region width up to 30 K was acquired for preparing a-films. Significantly, this work provides a low-cost and convenient way to produce high-quality a-films, which are potentially suitable for Josephson junction devices.

The growth of large single crystals of YBCO is a matter of significant importance in the study of superconductivity by neutron scattering experiments. For this reason, a well c-axis-oriented YBCO crystal with a size of 34 mm in diameter was successfully fabricated by modified melt-growth. First, our findings show that the thermal stability of the film-seed could be enhanced by introducing an intermediate mini-pellet between film 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.

In this work, we report a novel approach for a continuous growth of SmBa2Cu3Oy (Sm123) bulk superconductors. Under a conventional slow-cooling mode, it was found that the crystal growth slows down and terminates ahead of time due to a reduced effective supersaturation (σeff). Apart from commonly supposed two effects, RE2BaCuO5 (RE211, RE = rare earth element) coarsening in the melt and RE211 segregation at the growth front, we find that the noticeable reduction of t.c.s. (temperature coefficient of solubility) plays a significant role in the σeff declining phenomenon in the Sm–Ba–Cu–O system. In our new process, an accelerated cooling was applied to maintain a sufficient σeff so that a trapping mode controlled continuous growth is realized.

Systematic experiments were performed by in situ observation of the YBa2Cu3Oz (Y123 or YBCO) melting. Remarkably, the superheating phenomenon was identified to exist in all commonly used YBCO thin films, that is, films deposited on MgO, LaAlO3 (LAO), and SrTiO3 (STO) substrates, suggesting a universal superheating mode of the YBCO film. Distinctively, YBCO/LAO films were found to possess the highest level of superheating, over 100 K, mainly attributed to the lattice match effect of LAO substrate, that is, its superior lattice fit with Y123 delaying the Y123 dissolving and inferior lattice matching with Y2BaCuO5 (Y211) delaying the Y211 nucleation. Moreover, strong dependence of the thermal stability on the substrate material for Y123 films was also found to be associated with the substrate wettability by the liquid and the potential element doping from the substrate. Most importantly, the understanding of the superheating behavior is widely valid for more film/substrate constructions that have the same nature as the YBCO film/substrate.

Different kinds of REBa2Cu3O7−δ (RE = Sm, Sm−Y mixture) a/c grain boundaries are obtained by altering growth parameters such as temperature or oxygen partial pressure. We attribute the boundary structure variation to the a/c grain growth competition, which mainly depends on the supersaturation in the melt. A fine a/c boundary structure is obtained under certain growth conditions. The diffusion time, for which the growth atoms can stay on the growth interface, is considered as a unique kinetic limit in the growth orientation transition mechanism in the liquid-phase epitaxy (LPE) process. The growth mechanism is suggested as a combined effect of thermodynamic and kinetic factors.

The melting process of YBa2Cu3Ox (YBCO or Y123) films under an oxygen atmosphere was observed in situ by means of high-temperature optical microscopy. The films were classified by pole figure measurement as c-axis oriented, with two different in-plane orientations (denoted as 0 and 45°). In the 45°-oriented films, electron diffraction and high-resolution transmission electron microscopy (HRTEM) detected an intermediate Cu2O nanolayer in the vicinity of the interface. The melting mode and the thermal stability of the YBCO thin films with different in-plane orientations were greatly influenced by oxygen partial pressure. Notably, the thermal stability of the 45°-oriented YBCO films dramatically grew with increasing oxygen partial pressure. We attributed this effect to a change in the intermediate Cu2O nanolayer thermal stability. We conclude and suggest that the thermal stability of YBCO films can be significantly enhanced by inserting a Cu2O buffer nanolayer.

Formation of various Sm−Ba−Cu-O phases grown under thermal equilibrium on (010) surfaces of Sm2BaCuO5 single-crystalline whiskers was observed in situ by means of a high-temperature optical microscope. At 1100 °C a needle-like phase, identified by X-ray diffraction and energy-dispersive spectroscopy analyses as Sm2Ba4Cu2Oy, grew epitaxially on the whisker (010) surface. After being cooled to 1050 °C (close to the peritectic temperature, Tp), the SmBa2Cu3Oy (Sm-123) phase nucleated and grew epitaxially in two different orientations: [100]Sm-123//[001]Sm-211 and [110]Sm-123//[001]Sm-211. During a slow reheating, the former type of Sm-123 grains preferentially decomposed, indicating its lower thermal stability with respect to the latter type of grains.

The effect of two significant factors, flux composition and growth atmosphere, on the crystallographic axis orientation of the Sm1+xBa2−xCu3O7−δ “SmBCO” film was studied. We succeeded in achieving a pure a-axis SmBCO LPE film by applying Cu-rich Ba−Cu−O flux (Ba/Cu ratio 3:7) and 1 atm oxygen atmosphere. Compared to YBCO, it is more difficult to grow SmBCO along a-axis. Several possible reasons were considered, especially the mutual substitution of Sm3+ and Ba2+, which may prefer c-axis growth. Significant features of the LPE process, like nucleation, supersaturation, and growth rate, manifest tendency to prevent a-axis growth.

•An evolution of YBCO oriented structure was observed on (1 1 0) NdGaO3 substrate.•Y–Ba–Cu–O liquid presented an anisotropic wettability on (1 1 0) NdGaO3 substrate.•This work provides a unique method to achieve crack-free a-axis YBCO LPE films.Liquid phase epitaxy (LPE) of YBa2Cu3O7−δ (YBCO) films was performed by vertical dipping along both the [0 0 1] and the [1 1 0] directions of (1 1 0) NdGaO3 (NGO) substrates. Remarkably, an evolution of oriented structure from c-axis to a-axis, corresponding to the supersaturation (σ) change from high to low level, was explicitly observed on a single NGO substrate. Distinctively, creeping along the [0 0 1] direction and forming a low-σ-related a-oriented film with a crack-free macrostructure, the liquid presented a strong anisotropic wettability with the NGO substrate. Most importantly, this work provides a unique method to achieve high-quality a-axis YBCO LPE films, which are potentially appropriate for Josephson junction devices.

•Large sized Ag-free SmBCO bulk with high performance was prepared in air.•An YBCO buffered NdBCO thin film on MgO substrate was used to guarantee a proper maximum processing temperature (Tmax).•We introduced Ba-rich Sm2Ba4Cu2Oy (Sm242) phase into this work to ensure a high Tc (over 94 K).•Sm2Ba4CuBiOz (Sm2411) phase was employed to further reinforce Jc (about 68 KA/cm2).In this work, we reported that a large size SmBCO, 32 mm in diameter, was successfully grown in air by the cold-seeding method. During the preparation, a Ba-rich Sm2Ba4Cu2Oy (Sm242) was employed to refine Tc, while a nano-sized Sm2Ba4CuBiOz (Sm2411) phase was introduced to reinforce Jc. In addition, an NdBCO thin film seed was used to guarantee that a high maximum processing temperature (Tmax) could be adopted for the growth of large sized bulk.

Using liquid phase epitaxy, a novel approach was developed to grow a-axis oriented YBa2Cu3O7−δ films (a-films) under an air atmosphere, which has been difficult previously since its formation requires extremely low supersaturation. In our new method, instead of conventional cooling from the saturated to supersaturated state, an extremely small driving force for film growth was generated from the unsaturated through saturated to supersaturated state. By controlling the amount of fresh solvent and the melting time before cooling down the Y–Ba–Cu–O solution, a growth region width up to 30 K was acquired for preparing a-films. Significantly, this work provides a low-cost and convenient way to produce high-quality a-films, which are potentially suitable for Josephson junction devices.