Field induced flux pinning in Nd1+xBa2−xCu3O7−δ single crystal

The flux pinning behavior of a Nd_1+xBa_2−xCu₃O_7−δ (Nd123) single crystal, which exhibited a peak effect, has been studied by monitoring the time decay of the magnetic moment. The apparent pinning energy (U₀^*) was deduced from flux creep data on the basis of the Anderson-Kim model. The magnetic field dependence of U₀^* showed maxima at peak fields which depended on the temperatures, in a similar manner to those of critical current densities. In addition, the temperature dependence of U₀^* showed several features. To explain the increase in U₀^* with the field as well as its temperature and field dependence, we made a numerical calculation by introducing an additional pinning energy which increased with increasing field. The results are in good agreement with the experimental data, especially at temperatures above 60 K, where the contribution of field induced pinning centers is believed to be dominant.     

Transient creep of YBa2Cu3O7−x superconductor

Rapid changes of oxygen partial pressure (P_O2) between 10³ and 2.1×10⁴ Pa have been carried out during steady-state plastic deformation of polycrystalline YBa₂Cu₃O_7−x (YBCO) at temperatures between 825 and 900°C. Transient creep was observed after such P_O2 changes. The analysis of these creep transients allowed the determination of the chemical diffusion coefficient for reequilibration, which is identical to that found from thermogravimetry and electrical conductivity experiments for oxygen vacancies.     

Study on the superconducting composite material formation in the system Bi2Sr2CaCu2O8+x/Al-containing phases

Phase evolution in the Bi---Sr---Ca---Cu---Al---O system was studied. Two Al-containing phases BiSr_1.5Ca_0.5Al₂O_z and (Sr_1−xCa_x)₃Al₂O₆ (x = 0.4 − 0.45) were determined to be chemically compatible with Bi_2.18Sr₂CaCu₂O_8+x (Bi-2212) at temperatures of the samples processing. The phase equilibria in the title system were investigated above the solidus temperature. The BiSr_1.5Ca_0.5Al₂O_z was found to be in equilibrium only with the melt and the (Sr_1−xCa_x)₃Al₂O₆ phase. This latter aluminate equilibrated with Ca,Sr cuprates, CaO, the Cu-free phase, and the liquid. The melting and solidification in Bi-2212, doped with the aluminate, corresponded to the reversible reaction Bi-2212 + BiSr_1.5Ca_0.5Al₂O_z ↔ (Sr_1−xCa_x)₃Al₂O₆ + liquid. Two sets of superconducting composite materials with initial compositions Bi-2212 + nBiSr_1.5Ca_0.5Al₂O_z and Bi-2212 + m(Sr_1−xCa_x)₃Al₂O₆ were prepared by solidification from the partial melt. The former material was composed mostly of large Bi-2212 lamellas separated by the BiSr_1.5Ca_0.5Al₂O_z phase, which destroyed superconducting links between Bi-2212 grains. The latter material consisted of a Bi-2212 polycrystalline matrix with high concentration of small (ca. 3 μm) grains of (Sr_1−xCa_x)₃Al₂O₆ imbedded in Bi-2212 lamellas. The Bi-2212 + m(Sr_1−xCa_x)₃Al₂O₆ materials displayed a trend to enhance flux pinning at T = 60 K with the increase of aluminate phase content.     

Nd0.9Ca0.1Ba2Cu3O6＋x超导体系的红外光谱研究

用固相反应法制备、并通过没条件的热处理得到了一组同氧含量的Ｎｄ０．９Ｃｄ０．１ｂａ２Ｃｕ３Ｏ６＋ｘ样品。用Ｘ射线衍射确定了样品的相结构。随着氧含量的增大，结构发生从四方到正交相变。进一步用红外吸收谱研究了体系的结构变化特征。谱图上６４２ｃｍ＾－１，５７８ｃｍ＾－１，５３８ｃｍ＾－１峰的出现及位移，与氧含量引起的结构变化是一致的，并讨论结构与超导电性的关系。     

Effects of processing variables on the magnetic properties of melt-processed YBa2Cu3Ox

Samples of YBa₂Cu₃O_x (Y123) with excess Y₂BaCuO₅ (Y211) in the molar ratio of 5:1 (Y123/Y211) were processed using the SLMG (‘solid-liquid melt growth’) technique. The effects of precursor particle size, thermal pretreatment, and substrate choice were examined. Particle size was shown to accelerate degradation of the BaCuO₂ precursor. Thermal pretreatment resulted in reduction of porosity in the samples and hence improved magnetic properties. Contamination by carbon and magnesium from the substrates was also examined and eliminated by using BaZrO₃ substrates.     

Field-induced pinning effect of Nd(Ba1−xNdx)2Cu3O7−δ single crystals grown by the traveling-solvent floating-zone method

We have studied the superconducting properties of Nd(Ba_1−xNd_x)₂Cu₃O_7−δ (Nd123, x ≈ 0.1) single crystals grown by the traveling-solvent floating-zone method under 0.1% O₂ in Ar atmosphere. The enhancement of the magnetization with increasing field is observed in the hysteresis (M-H) loop in fields both parallel and perpendicular to the c-axis of the Nd123 single crystals as well as in the bulk crystals prepared by the oxygen-controlled-melt-growth (OCMG) method. The composition variation of Ba/Nd is observed in the matrix of Nd123 crystals by an analytical TEM equipped with a cold field-emission gun. It turns out that the enhancement is due to the field-induced pinning effect ascribed to the weak superconducting Nd---Ba substitution regions in the Nd123 matrix.     

Effect of sorbic acid doping on flux pinning in bulk MgB2 with the percolation model

In this paper, we study the doping effect of sorbic acid (C₆H₈O₂), from 0 to 20 wt.% of the total MgB₂, on critical temperature (T_c), critical current density (J_c), irreversibility field (H_irr) and crystalline structure. The XRD patterns of samples show a slightly decrease in a-axis lattice parameter for doped samples, due to the partial substitution of carbon at boron site. On the other hand, we investigate the influence of doping on the behavior of flux pinning and J_c(B) in the framework of percolation theory and it is found that the J_c(B) behavior could be well fitted in high field region. The two key parameters, anisotropy and percolation threshold, play very important roles. It is believed that the enhancement of J_c is due to the reduction of anisotropy in high field region.     

Increased critical current density and pinning in thick Ag/YBa2Cu3O7−x multilayers

We have investigated the superconducting properties of Ag/YBa₂Cu₃O_7−x thick multilayers grown by Pulsed Laser Deposition, and found that the artificial pinning centres induced by Ag nanodots lead to a significant increase in critical current, especially in high applied magnetic fields. Transmission Electron Microscopy showed a columnar growth of YBa₂Cu₃O_7−x induced by Ag nanodots, while angle-dependent transport measurements revealed the existence of strong, both isotropic and c-axis correlated, artificial pinning centres.     

Flux jump avalanches in torque studies of single crystal YBa2Cu3O7−δ

A series of avalanche-like jumps are observed in the mixed state of single crystal YBa₂Cu₃O_7−δ (YBCO) superconductors. Emerging as a saw-tooth pattern in torque vs. sample orientation in magnetic field, these jumps are discontinuous on our most resolute angular scale. While reminiscent of the classical flux jump instability, the present jumps are instead proposed to be associated with the layered nature of the material and twin boundary (TB) pinning, the combination of which promotes a crossover from a tilted to a kinked vortex structure.     

Oxygen overdoping in superconducting and non-superconducting Y1−xPrxBa2Cu3Oy

We present the evolution of magnetic and structural properties of Y_1−xPr_xBa₂Cu₃O_y (x0.5 and x=1) single crystals and polycrystalline materials when the oxygen concentration y is varied from under- to overdoping. We have found a monotonous evolution of the Pr Néel temperature for x=1 samples and a maximum of the superconducting critical temperature for the x0.5 samples. The structural properties as detected by X-ray diffraction and Raman spectroscopy show no instabilities when crossing the optimal doping region as was found in the x=0 material.     

Analysis of the DC and AC behaviour of Bi2Sr2Ca2Cu3Ox tapes arranged in z-stack configuration

The DC and AC behaviour of Ag-sheathed Bi₂Sr₂Ca₂Cu₃O_x(BSCCO-2223) tapes, arranged in a z-stack configuration, are investigated. The number of tapes in the stack can be varied, from one (single tape) to four, as well as the inter-tape separation. Tapes are isolated and connected in series to ensure a uniform current distribution among the tapes. The critical currents and the average AC losses per tape as a function of the transport current are measured. By increasing the number of tapes or/and reducing the separation between them, the critical current of each tape is decreased while the average losses are increased. Tapes in a stack are found to be electromagnetically coupled, the degree of this coupling being dependent on the separation among them. Theoretical models are provided to explain the experimental results.     

Concurrent doping effect of Ti and nano-diamond on flux pinning of MgB2

Nano-diamond and titanium concurrently doped MgB₂ nanocomposites have been prepared by solid state reaction method. The effects of carbon and Ti concurrent doping on J_c–H behavior and pinning force scaling features of MgB₂ have been investigated. Although T_c was slightly depressed, J_c of MgB₂ have been significantly improved by the nano-diamond doping, especially in the high field region. In the mean time, the J_c value in low field region is sustained though concurrent Ti doping. Microstructure analysis reveals that when nano-diamond was concurrently doped with titanium in MgB₂, a unique nanocomposite in which TiB₂ forms a thin layer surrounding MgB₂ grains whereas nano-diamond particles were wrapped inside the MgB₂ grains. Besides, nano-diamond doping results in a high density stress field in the MgB₂ samples, which may take responsibility for the Δκ pinning behavior in the carbon-doped MgB₂ system.     

(1-x)Sr_3Sn_2O_7+xCa_3Mn_2O_7陶瓷合成及其光电性能

为了寻求新的非常规多铁性材料,采用固相合成方法制备具有Ruddlesden-Popper结构的(1-x)Sr_3Sn_2O_7+x Ca_3Mn_2O_7(SSO+CMO)(x=0,0.05,0.10,0.15,0.20)系列陶瓷.通过X射线衍射、紫外可见光谱和磁性测量,发现SSO+CMO陶瓷为单一正交相结构,空间群为A21am.随着掺杂量x的增加,样品的晶胞参数和体积相应地减小;在室温不同频率下,样品的介电常数和介电损耗随着频率增加而减小,且在x=0.1时有弱的铁磁性.     

掺杂Y2O3和BaCeO3提高MOD-YBCO超导性能的研究

本文通过在前驱液中添加过量钇盐和铈的有机盐,采用三氟乙酸盐-金属有机沉积法(TFA-MOD) 在铝酸镧单晶基体上制备了含有纳米氧化钇和纳米铈酸钡的YBCO薄膜. 与纯YBCO薄膜相比,掺杂Y₂O₃/BaCeO₃的YBCO膜的临界转变温度几乎保持不变,为91 K左右. 而掺杂Y₂O₃/BaCeO₃的YBCO膜的临界电流密度达到5.0 MA/cm² (77 K, 0T), 是纯YBCO膜临界电流密度的1.5倍.薄膜中的Y₂O₃和BaCeO₃可能在YBCO内部起到了 有效的钉扎磁通作用. 关键词： 钇钡铜氧薄膜 2O₃和纳米BaCeO₃')" href="#">纳米Y₂O₃和纳米BaCeO₃ 磁通钉扎 三氟乙酸盐-金属有机沉积     

Investigation of the Li1+xV3O8 electrode–organic electrolyte interface by scanning photoelectron microscopy

To investigate the formation of a solid electrolyte interface (SEI) on the Li_1+xV₃O₈ electrode surface in the thermodynamic stability range of the organic electrolyte, we applied scanning photoelectron microscopy (SPEM) to a pristine electrode and to an electrode after ten cycles. The F K-edge absorption spectrum of the cycled electrode showed that LiF forms on the electrode surface during the lithium insertion–extraction process in the Li_1+xV₃O₈/Li cell. The photoelectron spectrum for the cycled electrode showed intense spectral features corresponding to Li 1s, F 2s, F 2p, and P 2p electron signals, whereas these spectral features were of negligible intensity for the pristine electrode. The above results give strong support for the formation of an SEI that consists of LiF and compounds containing phosphorus during operation of the battery. The SPEM images also revealed that the fluorine distribution on the surface of the cycled electrode was inhomogeneous.     

Critical current densities and flux creep rate in Co-doped BaFe2As2 with columnar defects introduced by heavy-Ion irradiation

We report the formation of columnar defects in Co-doped BaFe₂As₂ single crystals with different heavy-ion irradiations. The formation of columnar defects by 200 MeV Au ion irradiation is confirmed by transmission electron microscopy and their density is about 40% of the irradiation dose. Magneto-optical imaging and bulk magnetization measurements reveal that the critical current density J_c is enhanced in the 200 MeV Au and 800 MeV Xe ion irradiated samples while J_c is unchanged in the 200 MeV Ni ion irradiated sample. We also find that vortex creep rates are strongly suppressed by the columnar defects. We compare the effect of heavy-ion irradiation into Co-doped BaFe₂As₂ and cuprate superconductors.     

YBa2Cu3O7－δ/SrNb0.01Ti0.99O3 p-n结的制备及其特性

采用脉冲激光沉积技术，在n型SrNb001Ti099O3(SNTO)单晶基片上生长p型YBa2Cu3O7-δ(YBCO)薄膜，制备出YBCO/SNTO p n结.YBCO薄膜是高度c轴织构的超导薄膜，且具有良好的超导电性.YBCO/SNTO p n结具有较好的整流特性和很好的温度与磁场稳定性. 关键词： YBa2Cu3O7-δ SrNb001Ti099O3 p n结     

Magnetic and transport properties of YBa2Cu3Oy superconductor foams

The recently reported superconducting YBa₂Cu₃O_y (Y123) foams are highly interesting and promising for variety of applications. In this report we present first magneto-transport measurements of the superconducting properties of these foams. The investigations reveal the superconducting properties being similar to those of bulk melt processed materials. The 123 foams reveal a T_c of 92 K and have a magnetization J_c of 40,000 A/cm² at 77 K and 0 T. The measurements of magnetic hysteresis versus field show a high anisotropy of the critical current density up to J_c^ab/J_c^c7.     

Dy3+、Tm3+共掺杂Ca2MgSi2O7的发光特性

采用高温固相法合成了系列Ca₂MgSi₂O₇：Dy³⁺,Tm³⁺发光材料。对样品进行了XRD结构表征,测量了激发光谱、发射光谱、色温和荧光寿命。研究结果表明,Ca₂MgSi₂O₇：Tm³⁺在355 nm激发下显示出蓝色发光,在CIE1931中的色坐标为x=0.165 9,y=0.082 2,色纯度为89%。通过Dy³⁺和Tm³⁺的叠加激发谱带激发,即在349,353,365 nm激发下,Ca₂MgSi₂O₇：Dy³⁺,Tm³⁺显示出青白、冷白和暖白光,相关色温值分别为5 193,9 672,4 685 K。300~500 nm区域间可以有效地激发Ca₂MgSi₂O₇：Dy³⁺,Tm³⁺,并在400~600 nm之间产生蓝光和黄光复合产生的白光,表明该体系可用作白光LED的发光材料。     

Thermally activated flux dissipation in c-axis-oriented YBa2Cu3O7−δ ultrathin films

The resistive transitions of ultrathin YBa₂Cu₃O_7−δ (YBCO) films with thicknesses 75 and 200 Å were studied under magnetic fields. For the 75 Å film under a 5 T parallel magnetic field (Hbab-plane), no broadening of the resistive transition occurred. In the perpendicular magnetic field (H ab-plane), the broadening of the resistive transition of the 75 Å film is larger than that of the 200 Å thick film. The flux activation energy U was found to be linearly dependent on the temperature and logarithmically dependent on the magnetic field for both 75 and 200 Å films, which means the two samples have a two-dimensional vortex lattice. Furthermore, the activation energy U also increased with the film thickness, indicating that the magnetic correlation length in the c-axis direction l_c is larger than the 200 Å for bulk YBCO.