| Different microstructures of high temperature superconductor(HTS) possess different applications. For instance, c-axis orientated REBa2Cu3O7-δ(REBCO, RE = rare earth) films are appropriate for current transport approach, while a-axis oriented films are applied for making Josephson junction devices. Besides, grain boundaries(GBs) with a special structure are of great potential to improve superconducting performance. Thus, it is of great importance to realize artificial control of film microstructures. Among numerous preparation methods, the liquid phase epitaxy(LPE) technique has many advantages, such as high crystalline quality, low cost, high growth rate and so on. Over the years, we have been concerned with accurate control of REBCO film orientations, as well as high-quality film fabrication. On the other hand, in order to meet the growing need of HTS crystals, and to overcome the problems of large-sized crystal growth, it is necessary to provide creative and effective solutions. The main scientific significance of this paper is to thoroughly achieve tuning control of oriented microstructures, to propose several methods for gaining an ultra-low supersaturated state, to interpret the formation mechanism of the ultra-low supersaturated state based on the phase diagram, to provide a guidance for adjusting the solution states and to extend part of the methods for more applications. With respect to crystal growth, an effective method is proposed to grow a largest single crystal in air, and a universal solution for doped crystal growth is provided. The main progresses are listed below:1. tuning control of oriented microstructuresPreviously, there is a strong competition between the undercooling effect and the etch-back effect of(110) NdGaO3(NGO) substrate, resulting in a very narrow window(less than 3 K) for a-axis REBCO film growth in air. Moreover, high supersaturation promotes c-axis film growth, while a-axis film preparation requires an extremely low supersaturated state. In our work, instead of commonly used processes in a metastable zone, a-axis films were grown in an unstable zone of the Y–Ba–Cu–O solution. Under that circumstance, a homogeneous nucleation catastrophe occurred in the solution, leading to instantaneous consumption of most of the supersaturated Y solute. Thus a high supersaturated state rapidly relaxed to an ultra-low level. By controlling either the cooling rate or the growth temperature, a wide window over 35 K for the formation of the a-axis films was gained. In order to avoid the influence of particles on the liquid surface in film growth in the unstable zone, a novel approach was developed to grow a-axis films in the metastable zone. 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-axis films. Besides, liquid phase epitaxy of YBCO films was performed by vertical dipping along both the [001] and the [110] directions of(110) NGO substrates. Remarkably, an evolution of oriented structure from c-axis to a-axis, corresponding to the supersaturation change from high to low level, was explicitly observed on a single NGO substrate. Distinctively, creeping along the [001] direction and forming a low-σ-related a-axis 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.2. wide use of the ultra-low supersaturated stateUsing the ultra-low supersaturated state for realizing a small growth rate, c-axis oriented YBCO films with no crack was successfully grown on YBCO/LAO thin films, which is beneficial for device design. Employing the ultra-low supersaturated state for suppressing c-axis film growth, an a-axis SmBCO film was gained on the a/c mixed YBCO/LAO thin film, which suggests a new approach to orientation control. Utilizing the ultra-low supersaturated state realized at low temperature for restraining etch-back effect, a completely covered YBCO film could be gained on the LAO substrate, while only sparse grains was reportedly obtained on LAO. By finely adjusting the solution state, we systematically investigated the distribution of a-axis grains in the c-axis YBCO film, and obtained a GB structure that the a-axis grains uniformly distributed in the c-axis film, which is suitable for the application of flux pinning technique. Based on the achievement of the ultra-low supersaturated state, we provided a solution for fabricating Josephson junctions in the LPE process, which might give new insights into the development of quantum devices.3. Growth of large-size single crystals and doped one with high qualityWe introduced the cooling rate into the growth of YBCO single crystals, and systematically explored the effect of cooling rate on the growth rate. Eventually, a largest YBCO single crystal was obtained in air. Moreover, we proposed a universal approach to growing various doped crystals, referring to the addition of metallic oxide in the process of preparing crucibles. As a typical case, we grew a largest Mg-doped YBCO crystal with a Tc value of about 50 K from a Mg-doped crucible.Through the work presented here, we hope to provide specific guidance for orientation control of REBCO films, the preparation of special GBs, the growth of large-sized crystals, as well as supersaturated adjustment of other solution systems. On the base of various creative approaches mentioned above, we also hope that more innovative ideas could be lightened out for the application of flux pinning technique and development of Josephson junctions in the future. |
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