Structural and superconducting properties of neodymium added (Bi,Pb)2Sr2CaCu2Oy

The effects of neodymium (Nd) addition on the phase evolution, structural and superconducting properties of (Bi,Pb)₂Sr₂CaCu₂O_y [(Bi,Pb)-2212] prepared by solid state synthesis in bulk polycrystalline form were studied. The Nd content was varied from x = 0 to 0.5 on a general stoichiometry of Bi_1.7Pb_0.4Sr_2.0Ca_1.1Cu_2.1Nd_xO_y. The samples were characterized by differential thermal analysis (DTA), powder X-ray diffraction, scanning electron microscopy (SEM) equipped with energy dispersive X-ray analysis (EDX), resistance-temperature (R-T) measurements and superconductivity measurements at 64 K. It was found that the melting temperature of (Bi,Pb)-2212 slightly increases and the endotherm broadens due to the Nd-addition. The c-lattice parameter initially decreases and then increases with Nd addition. The critical temperature (T_C) and the critical current density (J_C) of the added samples are highly enhanced. The added sample shows a maximum onset critical temperature (T_C-onset) of 95.56 K (x = 0.3) and a maximum critical current density of 719 A/cm² at 64 K (x = 0.2) against 76.7 K and 100 A/cm², respectively, for the pure sample. The results show that the enhancement in superconducting properties are not due to any improvement in microstructure or grain growth, but due to a decrease in hole concentration as a result of Nd doping, which changes the system from ‘over-doped condition’ to ‘optimally doped condition’.     

Improved microstructure and flux pinning properties of Gd-substituted (Bi,Pb)-2212 superconductor sintered between 846 and 860 °C

The influence of sintering temperature on microstructural and superconducting properties of Gd-substituted (Bi,Pb)-2212 superconductor was investigated. It is found that the microstructure of the system can be tailored and refined so as to yield the best superconducting properties in terms of critical current density (J_C) and flux pinning by heat treating at a temperature in the range 846-860 °C. The samples sintered at 858 °C show the maximum self-field J_C of 31,960 kA m^{− 2} and those sintered at 846 °C show the maximum flux pinning force [F_P = 1698 kN m^{− 3}] as against 1860 kA m^{− 2} and 16 kN m^{− 3}, respectively for the undoped sample sintered at its optimum sintering temperature. The changes in microstructure followed by very high enhancement of J_C, J_C-B characteristics and F_P due to Gd-substitution within a small temperature range, is of great scientific and technological significance.     

Growth of untwinned Bi2Sr2Ca n−1Cu n O2n+4+δ thin films on Nd : YAlO3 substrates

Thin film growth of Bi₂Sr₂Ca _n?1Cu _n O_2n+4+δ by molecular beam epitaxy is performed on Nd : YAlO₃ (001) substrates. It was revealed that the orthorhombicity of the Nd:YAlO₃ substrate is quite effective to the growth of untwinned Bi₂Sr₂Ca _n?1Cu _n O_2n+4+δ thin films. In all phases withn=1～3, the incommensurate structural modulation lies parallel to the Nd : YAlO₃ [100] direction, and this relation holds even in the case of superlattices. The origin of the epitaxial relation is discussed from the viewpoint of the lattice misfit. The orthorhombicity of the substrate competes with the step edges on a vicinally polished substrate for determining the in-plane growth direction. The rotation of the modulation direction was observed when the off angles are varied.     

Influence of filament thickness, thermal process and magnetic field on misalignment angles in multifilament Bi2212 tapes

Misalignment angles in multifilament Bi2212 tapes have been studied by transport current density (J_c) measurements. Transversal misalignment angles depend on the filaments thickness due to the crystal growth along the Bi2212/silver interface. Therefore, the thinner the filaments, the higher the J_c.A dynamic thermal treatment improves grain alignment inside the tapes due to preferential crystal growth along the thermal gradient direction. Furthermore, melting in high magnetic field reduces the misalignment angle due to magnetic anisotropy effects. Therefore, good transport J_c performances can be expected for thicker multifilament Bi2212 tapes prepared using a combination of the dynamic process and melting in a high magnetic field.     

Successful synthesis of Hg0.80Re0.20Sr2Can−1CunO2n+2+δ (n = 1, 2) by the sealed quartz tube technique

Ba-free Hg,Re-1201 and -1212 were prepared by the sealed quartz tube technique. The starting materials were nominal precursor Re0.20Sr2CanCun–1Ox(n = 1, 2) and HgO. The unreacted pellets were annealed at 750 °C for 12 h for the n = 1 member and 850 °C for 12 h for the n = 2 member. Rietveld structural refinement was performed with the X-ray diffraction patterns, including impurity phases if needed. Both phases showed SG P4/mmm (nr. 123). Fitted tetragonal cell parameters were a = b = 3.7822(1) Å, c = 8.8776(5) Å and a = b = 3.8205(3) Å c = 12.0438(8) Åirc for the n = 1 and n = 2 members, respectively. DCSQUID (Superconducting Quantum Interference Device) magnetometry and AC susceptibility were used to measure the superconducting properties of the samples. We also studied the effect of post-treatments in oxygen and argon. No superconductivity was found for the n = 1 in any case. However, Tc,onset as high as 107 K was determined for the Hg,Re-1212.     

Single Crystals of HgBa2Can−1CunO2n+2+δ (n=1−5) and Layers of HgBa2CuO4+δ Grown at Gas Pressure 10 kbar

Single crystals of Hg-12(n?l) have been grown using high-pressure gas-technique. Due to the high density of Ar gas at 10 kbar the evaporation of Hg is strongly suppresed. An influence of structure defects on magnetic properties will be discussed. Superconducting parameters λ _ab ,ξ _ab and γ have been studied using torque magnetometry. Layers of Hg-1201 compound have been grown from flux.     

Electron Spectroscopy of Single-Layer (n = 1) Bi2Sr2−xLaxCuO6+δ Crystals at Optimal Doping

On single crystals of the single-layer (n = 1) high-T _c superconductor Bi₂Sr_2?x La _x CuO_6+δ at optimal doping (x = 0.4), the electron spectroscopies x-ray absorption (XAS) and high-resolution angle-resolved photoemission (ARPES) were performed. The XAS gives the intensity of the so-called prepeak of the O 1s line what is due to the unoccupied part of the Zhang–Rice (ZR) singlet band. For ARPES, the advantages of single-layer material are the absence of bilayer effects and the possibility to study the electronic properties of the normal state at a sample temperature where the thermal broadening is extremely small (<10 meV). The controlled variation of the polarization vector of the synchrotron radiation made it possible to resolve a distinct fine-structure of the occupied part of the ZR singlet band at the Fermi level. These observations have enormous consequences for line shape analyses and the determination of pseudogaps, and thus the mechanism of high-T _c superconductivity.     

Growth and characterization of Bi2Sr2Ca1Cu2O8+x single crystals

High-quality single crystals are well suited to the investigation of some intrinsic material properties. A modified Bridgman method using a sharp temperature gradient (～300°C/cm) was used to grow Bi₂Sr₂Ca₁Cu₂O_8+x single crystals. Although the samples were contained in alumina ampoules, no aluminium contamination of the samples was detected. Blade-shaped crystals up to ～7–8 mm length and 3–4 mm width could be grown by this method, although extraction from the matrix was difficult. Electron diffraction patterns of the [001] zone axis revealed a high degree of crystallinity. The narrowness of the superconducting transition temperature, as determined by ac susceptibility, also suggests the existence of well-formed crystalline domains. In order to determine the relative orientation of the crystalline domains, electron channeling patterns were recorded from several consecutive growth steps from a fracture surface. The poor contrast of these and Kikuchi patterns suggests the presence of a stacking structure. The results showed a [100] growth direction and (001) cleavage plane. Reversible oxygen loss at the peritectic decomposition temperature of 863°C was observed. Knoop indentation measurements showed that the crystals were quite soft, having a microhardness of 0.44 GPa.     

(Sr2+,Bi3+,Si4+,Ta5+)掺杂的TiO2压敏陶瓷中Ta5+的研究

研究Ta2 O5对 (Sr,Bi,Si,Ta)掺杂的TiO2 基压敏陶瓷压敏特性及电容特性的影响 ,发现按配方TiO2 0 .3%(SrCO3 Bi2 O3 SiO2 ) 0 .1 %Ta2 O5配制的样品具有最低压敏电压 (E1 0mA =1 .2V·mm- 1 )、最大相对介电常数 (εra=2 .0 0 2× 1 0 5)及较小非线性系数 (α =2 .6 )。考虑到材料的低压敏电压和大介电常数的要求 ,Ta2 O5最佳掺杂量在 0 .0 85mol%与 0 .1mol%之间。     

s -Wave Symmetry Along the c -Axis and s + d In-plane Superconductivity in Bulk YBa2Cu4O8

To clarify the order parameter symmetry of cuprates, the magnetic penetration depth λ was measured along the crystallographic directions a, b, and c in single crystals of YBa₂Cu₄O₈ via muon spin rotation. This method is direct, bulk sensitive, and unambiguous. The temperature dependences of λ _a ⁻² and λ _b ⁻² exhibit an inflection point at low temperatures as is typical for two-gap superconductivity (TGS) with s+d-wave character in the planes. Perpendicular to the planes a pure s-wave gap is observed thereby highlighting the important role of c-axis effects. We conclude that these are generic and universal features in the bulk of cuprates.     

Relationships between the chemical composition and properties of the high-temperature superconductor Bi2+x Sr2-y Ca1+y Cu2O8+d

Samples of the high-temperature superconducting solid-solution Bi_2+x Sr_2-y Ca_1+y Cu₂O_8+d (2212 phase, T _c ≤ 95 K) were prepared with different cation concentrations in order to study the influence of the cation concentration on the crystal structure, oxygen content and on several other physical properties. The measurements show that the c-axis parameter, the critical temperature, T _c, the resistivity, R, and the oxygen ion conductivity, σ_O2^- of the phase decrease with increasing calcium content. With increasing bismuth content, T _c and R decrease, whereas the c-axis parameter increases. σ_O2^- is not strongly dependent on the bismuth content of the phase. The oxygen content of the phase increases with increasing bismuth and calcium content. This revised version was published online in November 2006 with corrections to the Cover Date.     

Characterisation of YBa2Cu3O6+x films grown by the trifluoro-acetate metal organic decomposition route by infrared spectroscopy

Infrared reflectivity measurements have been performed at T = 300 K on c-axis oriented YBa₂Cu₃O_6+x films obtained by deposit of trifluoroacetate precursors on (100)-LaAlO₃ single crystalline substrates. A detailed simulation of the infrared spectra acquired on the thin films based on the use of an appropriate complex dielectric function model is proposed here. It allows us to determine with a single experiment the thickness and the residual porosity of the layers. To reinforce the statement of the infrared simulations, the results have been compared with the film porosity obtained by scanning electron microscopy imaging. Moreover the local grain orientation has been checked by polarized Raman microspectroscopy. Finally, the three complementary experimental ways give consistent results that justify the present approach.     

Ferroelectric,dielectric and piezoelectric properties of Sr0.6(BiNa)0.2Bi2Nb2O9 ceramics

Aurivillius-type ceramic, Sr_0.6(BiNa)_0.2Bi₂Nb₂O₉ (SBNBN), was synthesized by using conventional solid-state processing. Phase structure and microstructural morphology were confirmed by X-ray diffraction analyses (XRD) and the scanning electron microscopy (SEM). Dielectric, piezoelectric and electromechanical properties of the SBNBN ceramic were investigated in detail. Curie temperature (T_c), piezoelectric coefficient (d₃₃), electromechanical coupling coefficient k_p, k_t and quality factor Q_m of the SBNBN ceramic were found to be 586.5 °C, 22 pC/N, 5.0%, 8.7% and 651, respectively. In addition, the reasons for varieties of the resistivity and dielectric properties at high temperature were also discussed.     

Sr3Bi(PO4)3:Eu2+, Mn2+: Single-phase and color-tunable phosphors for white-light LEDs

Sr₃Bi(PO₄)₃:Eu²⁺, Sr₃Bi(PO₄)₃:Mn²⁺, and Sr₃Bi(PO₄)₃:Eu²⁺, Mn²⁺ phosphors were synthesized by solid state reaction. The structure and luminescent characteristics were investigated by X-ray powder diffraction and fluorescent spectrophotometer. All samples have the structural type of eulytine. The excitation and emission spectra of Sr₃Bi(PO₄)₃:0.01Eu²⁺ sample show characteristic bands of Eu²⁺ ions. Also, the excitation and emission spectra of Sr₃Bi(PO₄)₃:0.06Mn²⁺ sample show characteristic bands of Mn²⁺ ions. The emission color of Sr₃Bi(PO₄)₃:Eu²⁺, Mn²⁺ sample could be tuned through tuning the co-dopant concentration of Mn²⁺ ions. The decay times for the Eu²⁺ ions decrease with the increase of Mn²⁺ dopant concentration, but the energy transfer efficiency increases with the increase of Mn²⁺ dopant concentration. On the basis of the luminescent spectra and fluorescence decay curves, we confirm that the energy transfer process from the Eu²⁺ to Mn²⁺ ions takes place in the co-doped Sr₃Bi(PO₄)₃ phosphor. Sr₃Bi(PO₄)₃:Eu²⁺, Mn²⁺ sample shows the good thermostability. The emission intensity of the sample at 400 K is about 60% of the value at 300 K. These results show Sr₃Bi(PO₄)₃:Eu²⁺, Mn²⁺ phosphors could be anticipated for UV-pumped white-light-emitting diodes.     

BaTinO2n+1光催化性能和结构关系的研究

光催化剂BaTinP2n＋1（n=1，2和4）粉末是通过sol—gel法制得的聚合物前驱体在800℃下烧制而成．利用XRD粉末衍射和UV—Vis吸收光谱表征样品的物相和吸收光谱特性．在样品的光催化活性测试中，采取甲基橙作为染料光降解的模型，研究了BaTiO3、BaTi2O5和BaTi4O9在各种酸性条件下对甲基橙溶液的降解效率，并初步探讨了其光催化性能差异的影响机制．实验表明，在相同的酸性条件下，三种钛酸钡BaTinP2n＋1（n=1，2和4）的光催化活性都随n的增大而增强，即BaTiO3〈BaTi2O5〈BaTi4O9．引入堆积率（定义为单位晶胞内所有组成离子的体积占整个晶胞体积的百分比）的概念，堆积率越小，则表明材料晶体结构越开放．利用结构开放度的概念模型可以很好地解释光催化活性的差异，即在这三种材料中，结构越开放，光催化性能越好．     

High-Temperature Thermoelectric Power of The Metal Oxides: La2− x Sr x CuO4 and Bi1− x Sr x MnO3

We have investigated the high-temperature thermoelectric power (TEP) of La_{2? x} Sr _x CuO₄ (0.05?≤?x?≤?0.35) and Bi_{1? x} Sr _x MnO₃ (0.5?≤?x?≤?0.8) up to 700 K. Based on the TEP results we have discussed the phase transitions on each case. In the case of high-T _C cuprates, La_{2? x} Sr _x CuO₄ (0.05?≤?x?≤?0.35), the TEP shows different temperature dependences in three temperature regions. At low temperature, the positive TEP rises showing a broad peak at temperature T _P, which shifts to lower temperature upon Sr doping. Right above T _P, the TEP decreases linearly as temperature increases. At high temperature, TEP deviates from the linear-T dependence at a certain temperature, T _H, showing a saturation behavior. The systematic change of the TEP behavior is discussed in terms of the two-fluids model, which is an intrinsically inhomogeneous state, consisted of bound pairs and independent carriers in the normal state of the high-T _C superconductors. For Bi_{1? x} Sr _x MnO₃ (0.5?≤?x?≤?0.8), the negative TEP is almost temperature-independent in the high temperature regime (T _CO?<?T?<?700 K). Near the charge ordering temperature (T _CO), however, TEP suddenly decreases with decrease of temperature, indicating the suppression of carrier mobility with charge ordering transition. As Bi concentration decreases, T _CO shifts to lower temperature from T _CO?～?520 K for x?=?0.5 to T _CO?～?435 K for x?=?0.8, which suggests that charge ordering is related to the local lattice distortion due to highly polarizable 6s² character of Bi³⁺ ion. In comparison with the resistivity data, the TEP results have been discussed in terms of the carrier localization accompanied by local lattice distortion.     

Crystallization Activation Energy and Hole Concentration Properties of the Bi2Sr2Ca1 − xCdxCu2O8 + y Glass-Ceramic Superconductor Rods

The glass rods with the nominal compositions of Bi₂Sr₂Ca_{1 ? x} Cd _x Cu₂O_{8 + y} , where x = 0.0, 0.4, and 0.8, were prepared by melt-casting method. XRD studies exhibited that the Cd substitution caused a transition from Bi-2212 phase to Bi-2201 phase. The multiphase structure was obtained for higher Cd concentration cases. It was determined that the activation energy for crystallization was decreased with the increase of the Cd concentration. When the Cd concentration was increased in the system the superconducting properties decreased. The hole concentration, p, was calculated using the Presland's method. The increase in the Cd concentration increased the hole concentration and the results showed that the Cd-substituted samples were highly in the overdoped region.