Transition Temperature Tc Onset Research Articles

Formation of Tl-1212 phase in Ga-substituted Tl(BaSr)Ca1−xGaxCu2O7 superconductor

The effect of Ga substitution on the superconductivity of Tl(BaSr)Ca1-xGaxCu2O7 (x = 0–0.5) is reported. The samples were prepared by solid-state reaction method. XRD patterns showed a single Tl-1212 phase for the x = 0 and 0.1 samples. Unreacted Ga2O3 and other impurity phases were observed in the higher substituted (x = 0.2 to 0.5) samples. The temperature-dependent resistance measurements showed metallic normal state behavior for all samples with onset transition temperature Tc-onset between 94 and 66 K and zero transition temperature Tc-zero between 86 and 55 K. AC susceptibility measurement showed the highest superconducting transition (95 K) and peak temperature (89 K) for non-substituted sample. The intergranular critical current density Jc at Tp for all samples was between 16 and 22 A cm−2. This work showed that the intergranular coupling and flux pinning energy were suppressed with Ga substitution. Some Ga+2 were most likely incorporated at Ca+2 for x = 0.1 and some resided at the grains for x ≥ 0.2 samples. This work showed that a single superconducting Tl-1212 phase was formed for x ≤ 0.1 samples. This work also showed that the internal lattice constrain which is the ratio of the lattice parameter (a/c) is important in improving the transition temperature of the Tl-1212 phase.

AC Susceptibility and Superconducting Properties of Graphene Added YBa2Cu3O7−d

The effect of graphene (G) addition on YBa2Cu3O7−δ(G)x (x = 0 − 0.03 wt%) has been studied using the X-ray diffraction method, scanning electron microscopy, electrical resistance versus temperature, transport critical current density Jc, and AC susceptibility measurements. XRD patterns showed single-phase YBa2Cu3O7−δ (YBCO) for all samples. SEM micrographs showed filling of the voids between YBa2Cu3O7−δ grains as graphene was added. The temperature-dependent electrical resistance curves showed metallic normal state behavior and onset transition temperature Tconset between 90 and 92 K for all samples. AC susceptibility measurement showed transition temperature $T_{\mathrm {c} \chi ^{\prime }}$ between 90 and 93 K. $T_{\mathrm {c} \chi ^{\prime }}$ was maintained or improved slightly as graphene was added. The x = 0.001 wt% showed the highest Jc, i.e., 2750 A cm−2 at 77 K and 5570 mA cm−2 at 30 K which was 13 and 40 times higher than that of the non-added YBCO, respectively. The peak temperature Tp of the imaginary part of the susceptibility χ″ was around 78–82 K for all samples indicating grain coupling was not weakened as graphene was added.

Growth of Tl-1223 superconducting thin films by rapidly heating-up sintering technology

Owing to high critical temperature (125 K) and high upper critical field, TlBa2Ca2Cu3O9 (Tl-1223) superconductor is a kind of superconducting power transmission material working at liquefied natural gas temperature, and it has a great potential application value in the strong and weak electric field. In this work, the Tl-1223 superconducting films are fabricated by rapidly heating-up sintering technology (RHST) on (00l) lanthanum aluminate substrates. The Tl-Ba-Ca-Cu-O target is used as a sputtering source to deposit the precursor films by the radio-frequency magnetron sputtering technique. The Tl-contained pellets, named annealing targets, are fabricated by the solid-state reaction of stoichiometric quantities of Tl2O3, BaO2, CaO and CuO powders with an initial cation ratio of m Tl:Ba:Ca:Cu=0.4-1.8:2:2:3. The amorphous precursors together with the annealing target providing Tl source are sealed in a silver foil and annealed at 820℃ for 5 min in argon atmosphere, then converted into Tl-1223 superconducting phase. The heating rates are set at 2.5℃/s from room temperature to 350℃, 5℃/s from 350℃ to 650℃, and 35℃/s from 650℃ to 820℃, respectively. The prepared films are characterized by X-ray diffraction and scanning electron microscope. In the conventional low heating rate process, all of the precursor films sintered together with the annealing targets containing different Tl content are first converted into Tl-2212 superconducting phase. That is because the sample residence time in the phase transition temperature range of Tl-2212 is longer, while the phase-formed temperature of Tl-2212 is lower than that of Tl-1223. In the RHST, when the metal ion molar ratio of Tl to Ba in the annealing target is 1.8:2, the main phase of the film is (00l)-oriented Tl-2212. In addition, the film also contains a small number of Tl-2223 grains. On reducing the ratio to 1:2, the film is composed of Tl-1212, Tl-2212, Tl-1223 and Tl-2223 grains. As the ratio decreases to 0.8:2, the film contains the (00l)-oriented Tl-1223 grains and traces of Tl-2223 grains. With the ratio decreasing to 0.4:2, purely c-axis oriented Tl-1223 film is obtained. The critical transition temperature Tc onset of the as-grown film is only 103 K. The film annealed again in oxygen gas has a dense crystal structure and excellent electrical properties. The Tc onset of the sample is about 116 K, and the critical current density Jc is about 1.5 MA/cm2 (77 K, 0 T). The experimental results show that the new sintering process to grow Tl-based films has several advantages such as the short processing cycles, less raw-material consumption, and low production cost.

Improvement in structure and superconductivity of YBa2Cu3O6+δ ceramics superconductors by optimizing sintering processing

Polycrystalline YBa2Cu3O6+δ bulks were synthesized by sol-gel method. Sintering processing played a vital role in the evolution of phase structure and microstructure, and thus significantly influenced their superconducting properties. The influence of calcination temperature, sintering temperature, on the bulks structure, morphology and superconducting behaviors were investigated. The results showed that the oxygen content drastically increased with calcination temperature and sintering temperature. The SEM images revealed that the grains grew up monotonously with increase of calcination temperature. With increased calcination and sintering temperature, the resistivity was reduced gradually and the superconducting properties increased. Moreover, it was found that the optimal superconducting properties (with the highest superconducting transition temperature Tconset and the narrowest transition width ΔT) were obtained at calcination temperature of 900 °C and sintering temperature of 950 °C.

MgB2 thin films grown on graphene/Ni–Mo alloy system

200nm Ni film is coated on 25μm thick Mo foil, and graphene is grown on the Ni–Mo system by CVD method. After the annealing process of CVD, the Ni/Mo bilayer transforms into Ni–Mo alloy, then we have successfully fabricated MgB2 films on graphene/Ni–Mo alloy system via the hybrid physical–chemical vapor deposition (HPCVD) technique. The transition temperature Tc onset is 38.25K with a corresponding transition width of 0.75K. The average thickness of MgB2 films is 200nm (25% concentration B2H6). The critical current density derives from the magnetization measurement at 5K is, jc (5K, 0T)=9.6×106A/cm2. We can easily deposite MgB2 on graphene/Ni–Mo alloy system with a lower B2H6 concentration and less gas flow, which lays a good foundation for depositing MgB2 thick films. The graphene in this system is multilayer and with defects, it may act like an intermediary film for the growth of MgB2, or a carbon-doping source.

Superconductivity in Bi–Pb–Sr–Ca–Cu–O ceramics with YBCO as additive

The effect of YBCO addition on the superconductivity in Bi1.6Pb0.4Sr2Ca2Cu3Oδ (BSCCO) systems was studied. The samples were prepared through solid state reaction and the properties have been investigated via resistivity measurement (R–T), Scanning electron microscope (SEM) and X-ray diffraction (XRD) analysis. The highest resistivity measurement was observed for the samples with x=0.02 of YBCO at 102K. Generally, all samples exhibit metallic behavior above transition temperature TC onset. From the XRD data, it is observed that the volume percentage of the Bi-2223 phase is initially 75% for the pure samples but decreases till 25% with the increasing Bi-2212 phase. Analysis of the temperature dependence of the AC susceptibility near the transition temperature has been done. The field effect on AC susceptibility was measured. The field increased from 0.05Oe to 2.00Oe and the peak temperature (TP) gradually decreased to lower temperature from 107.0K to 108.5K.

Preparation of c-axis oriented YBa2Cu3O7 polycrystalline ceramics by sol–gel method

YBa2Cu3O7 powders and ceramics with c-axis preferred orientation were synthesized by sol–gel method. The pure phase powders were obtained when calcined between 800 and 930°C, and the ceramics were sintered at 950°C. It was found that c-axis orientated ceramics were obtained only when the powders were calcined at 930°C. The X-ray diffraction patterns show that the relative intensities of c-axis (00l) peaks are much stronger than that of the standard pattern. The surface morphology own an obvious layered structure and the grains are platelike grown. The transition temperature TConset is 93.67K with a transition width ΔTC of 2.1K.

Characterization of low-temperature synthesized FeCr0.05Se superconductors

Polycrystalline samples of FeCr0.05Se were synthesized by solid-state reaction using the two-step sintering method to study the effects of various low temperatures on the microstructure and superconductivity in this system. These synthesis temperatures have been established by optimization. Electrical resistivity measurements confirmed that superconducting transition temperature Tconset reached a maximum of 10.2K at 450°C as β-FeSe phase content reached the maximum under this condition. As the sintering temperature increased, the partial substitution of Cr by Fe caused an enhancement of lattice parameters, and consequently Tconset was much higher at 550°C than at 400°C, though the amounts of β-FeSe were appropriately the same in these two samples.

Structural and superconducting properties in Y0.6Gd0.4Ba2(Nb)Cu3O7−y cuprates doped with niobium

Polycrystalline YBa2Cu3O7−y (YBCO) and Y0.6Gd0.4Ba2−xNbxCu3O7−y (YGBNCO) compounds with 0≤x≤0.225 were synthesized using standard solid state reaction technique. The structure for all samples was characterized by X-ray difference (XRD) and scanning electron microscopy (SEM). The transport properties were measured by the (FPP) method in the temperature range from 70 to 130 K. As the Nb content in the samples increased, a diffused phase indicating a niobium perovskite phase and a small amount of unidentified phase appeared. With the increase of Nb content, the superconducting transition temperature Tconset increased slowly with x≤0.125, and then it remained unchanged or slowly decreased with 0.125≤x≤0.225. It could be found that there was a slow decrease of zero-resistance temperature, Tcoffset, with the increase of Nb content. The larger transition width might result from the YBa2NbO6 phase, impurity and unidentified phases of the sample due to the Nb doping.

Superconductivity at 14.6 K in Fe(SeTe) single crystal and the role of excess Fe

Fe1.01Se0.38Te0.62 single crystal was successfully grown by self-flux method, and effect of post-annealing technique on structure and superconductivity was also systemically studied. The results show the as-grown sample has an excellent superconductivity with an onset transition temperature Tconset ∼ 14.6 K and a zero resistivity temperature Tczero ∼ 13.4 K. After annealing at 400 °C for 48 h, excess Fe in the post-annealing sample appears with Fe1.06Se0.34Te0.66 formula, and superconductivity was strongly suppressed with Tczero ∼ 8.0 K. The experiments found that excess Fe is located at Fe(2) site with local magnetic moment. According to the Werthamer-Helfand-Hohenberg (WHH) theory, the upper critical fields Hc2 at zero temperature were estimated to be 54.2 T for as-grown sample and 28.2 T for post-annealing sample. Two abnormal drops in DC magnetization are found near 40 K and 125 K for all samples, the one near 40 K may be related to a spin freezing of excess Fe and that near 120 K should be corresponding to the structural transition from tetragonal to monoclinic. This is confirmed by the strain measurement. On the other hand, a kink change of specific heat was also found around 10 K for a post-annealed sample.

A new Y-based HTSC with Tc above 100 K

In search of finding the dominant mechanism in high temperature superconductivity phenomena, the Y3Ba5Cu8O18 compound was synthesized through the standard solid-state reaction technique. Characteristic XRD experiment was performed on the samples and was analyzed by the MAUD software refinement program. The analysis results indicate a 358 phase structure with the initial nominal stoichiometry. The electrical resistivity and its behavior under different magnetic field were measured. The electrical resistivity indicates the transition temperature Tconset=102K with transition width ΔTc=2.4K. This is the first observation of such a high transition temperature in the Y-based compound. Application of magnetic field leads the resistivity curve to spread out below transition region, and Tc (ρ=0) shifts to lower temperatures. Also, a small broadening is observed by the application of high magnetic field in the Tconset region.

Flux pinning by Al-based nanoparticles embedded in YBCO: A transmission electron microscopic study

A series of YBa2Cu3Oy (YBCO) samples with small amounts (0–0.6wt.%) of nanosized alumina particles (50nm) are synthesized in air by solid state reaction. The microstructure has been characterized by transmission electron microscopy (TEM) and the critical current density Jc has been measured by the standard four-probe method in the applied magnetic field at 77K. TEM and energy dispersive X-ray spectroscopy (EDS) analysis have shown that alumina reacts with the YBCO matrix to form nanometric aluminium-rich inhomogeneities intergrown within the YBCO superconducting matrix. These inhomogeneities reduce the onset transition temperature Tconset and the zero resistance temperature Tc. In spite of the monotonic decrease of the superconducting temperature Tc with increasing alumina addition, the Jc(H) behaviour is remarkably improved. The characteristic behaviour of Jc can be explained in terms of the counterbalance of two effects simultaneously caused by the nanometric alumina addition in the system. One effect is the formation of the Al-rich nanometric inhomogeneities relevant for the flux pinning, and the other effect is the reduction of matrix superconducting volume, which is reflected by a decrease of the critical current density Jc at zero applied magnetic field.

Thermal history dependence of the superconducting properties in La2CuO4+δ

We report the thermal history dependence of the superconducting properties in the ceramic La2CuO4+δ with excess oxygen. In the sample, annealed in high pressure oxygen gas, the transition temperatureT c onset of 32K in rapidly cooling case changes into 37K in case of annealing at 200K. Another superconducting phase withT c around 45K was found to be enhanced in the samples annealed around 220K, both in the de susceptibility and resistivity measurements. We also studied thermal history dependence in the samples with electrochemical oxidation. And we observed theT c onset of 45K, however theT c onset of 45K did not change by thermal history.