Properties of YBa2Cu3O7−δ-BaBiO3 composite superconductors

The effect of BaBiO₃ addition (5–30 mol.%) on the structural, thermal, and superconducting properties of YBa₂Cu₃O_7−δ(123) superconductor have been studied. BaBiO₃ addition leads to the formation of YBa₂BiO₆ and yttrium- and barium-deficient 123. Samples with up to 20 mol.% BaBiO₃ exhibit superconductivity with onset at a≈90 K. The lattice parameters of the superconductor phase decrease as more BaBiO₂ is added. Upon heating, the composites seem to have more stable oxygen content than does pure 123.     

Formation of the superconductor-semiconductor interface between Bi2Sr2CaCu2O8−x and indium-tin oxide

Planar heterostructures consisting of semiconducting highly textured indium-tin oxide (ITO) overlayers and c-axis oriented superconducting Bi₂Sr₂CaCu₂O_8−x (2212) underlayers have been formed onto MgO(100) substrates using dc magnetron sputtering. Annealing of the structures in air at 500 °C resulted in the appearance of both the interface resistivity anomaly at superconductive transition temperature—T_c and nonlinear current vs voltage behavior characteristics for a S-N junction. Meanwhile gradual change of the electrical properties towards S-I-N type behavior have been indicated with subsequent heat treatment in air at 50–100 °C. The origin of the insulating I-interlayer we associate with reversible oxygen removal in the 2212 compound at superconductor-semiconductor interface.     

Effect of Ag doping on structure andcritical temperature of Bi2Sr2CaCu2O8+δ superconductors

The effects of Ag substitution for Bi, Sr and Cu on the c-axis lattice parameter and critical temperature (T_c) were determined for the Bi_2−xSr_2−yCaCu_2−zO_8+δ (BSCCO-2212) superconducting phase. Two distinct regimes of behavior were observed: fast cooling represented the regime where the uptake of excess oxygen was the controlling factor and slow cooling represented the regime where the cation content was the controlling factor on the structure and properties. Samples showed higher T_c values and longer c-axes with increased cooling rate. A linear increase in T_c with increase in c-axis length was observed for the faster-cooled samples. For the slow-cooled samples, increased c-axis length was observed when Ag was substituted for Bi and Sr, whereas substituting Ag for Cu caused no change in c-axis length. In addition, any deviation from the ideal 2212 stoichiometry was shown to reduce the T_c values of samples that were slowly cooled and fully oxygenated. Preliminary results obtained by high-resolution transmission electron microscopy coupled with electron energy loss spectroscopy indicated that relatively small amounts of Ag (≈1 at.%) can occupy Bi sites and a larger amount (≈10 at.%) of Ag can substitute for Cu.     

Synthesis and characterization of EuBa2Ca2Cu3O9−x: The influence of temperature on dielectric properties and charge transport mechanism

High dielectric constant materials have a crucial importance for various microelectronic applications such as memory devices, supercapacitors etc. Among other insulators, perovskite structured oxide materials attract great interest not only for their high dielectric constants but also their unique electrical and magnetic properties such as superconductivity etc. From this point of view, a new Europium based copper oxide layered material with perovskite structure (EuBa₂Ca₂Cu₃O_9−x coded as Eu-1223) has been synthesized by solid state reaction method in this work. The physical and chemical properties of Eu-1223 have been determined by FTIR, SEM, XRF, XRD, TGA and DTA techniques. The influence of temperature on impedance and dielectric properties of Eu-1223 has been investigated by impedance spectroscopy measurements performed within the frequency interval of 5 Hz–13 MHz between 298 K and 408 K temperatures. It has been found that the Eu-1223 material has high dielectric constants at each temperature operated. In addition, Eu-1223 sample behaves as a colossal dielectric material up to 300 kHz for 408 K due to observation of dielectric constant values which are greater than 10³. Furthermore, it has been revealed that Eu-1223 material can be used as thermally sensitive resistors in electronic circuits due to its decreasing resistance with increasing temperature. Moreover, it has been observed that the relaxation frequency of the system shifts from 46.5 kHz (low frequency radio wave band) to 1.57 MHz (mid frequency radio wave band) as the temperature increasing from 298 K to 408 K. According to dc conductivity investigations, the variation of dc conductivity with the inverse of temperature satisfies linear relationship that indicates a thermally activated nearest neighbor hopping conduction. On the other hand, it has been determined that ac conductivity has frequency dependent relation which obeys ω^s for the high frequency region. Furthermore, the frequency exponent, s, which takes values between 0.7 and 0.4, shows a decreasing behavior with increasing temperature. In conclusion, ac charge transport mechanism has been predicted as correlated barrier hoping for Eu-1223.     

Semiconducting Transport in Pb10−XCux(PO4)6O Sintered from Pb2SO5 and Cu3P

The recent claim on the discovery of ambient-pressure room-temperature superconductivity in Cu-doped lead-apatite has attracted sensational attention. The intriguing compound has been fabricated by sintering lanarkite (Pb₂SO₅) and copper(І) phosphide (Cu₃P). To verify this exciting claim, Pb₂SO₅, Cu₃P, and finally the modified lead-apatite Pb_10−xCu_x(PO₄)₆O have been successfully synthesized. Detailed electrical transport and magnetic properties of these compounds are systematically analyzed. It turns out that Pb₂SO₅ is a highly insulating diamagnet and Cu₃P is a paramagnetic metal. The obtained nominal Pb_10−xCu_x(PO₄)₆O compound sintered from Pb₂SO₅ and Cu₃P exhibits semiconductor-like transport behavior with a large room-temperature resistivity of ≈1.94 × 10⁴ Ω·cm, although the major phase of the compound shows consistent X-ray diffraction spectrum with the previously reported structure data. In addition, when a Pb_10−xCu_x(PO₄)₆O pellet pressed from uniformly ground powder is located on top of a commercial Nd₂Fe₁₄B magnet at room temperature, no repulsion can be felt and no magnetic levitation is observed either. The large difference in electrical and magnetic properties between the compounds and the previously reported compounds might be induced by distinct fine crystallographic structures, diverse multi-phase distributions, and different concentrations of impurity phases such as Cu₂S, all of which deserve further study.     

The effect of porosity on elastic moduli of YBa2Cu3O7−δ high Tc superconductors

The ultrasonic velocity measurements of a series of YBa₂Cu₃O_7−δ samples having different porosities, have been undertaken by the Pulse Transmission Technique. Using the experimental values of Young's moduli, the void-free elastic moduli of all the samples have been arrived at theoretically by assuming that the shape of the pores is spherical. For this purpose, different types of equations viz. linear, non-linear, exponential and power types have been used. Later the computed values have been compared with the polycrystalline Young's modulus value obtained by Voigt-Reuss-Hill (VRH) scheme using the single crystalline elastic stiffness constants available in the literature. It has been concluded that the Young's modulus values obtained by the power equations are more accurate in predicting the void-free state of moduli of Y-Ba-Cu-O samples of present investigation, when compared with those obtained by other types of equations.     

(Ba0.5Sr0.5)TiO3/LaNiO3异质薄膜的介电调谐特性

用脉冲激光沉积工艺制备Ba0.5Sr0.5TiO3(简称BST)薄膜和Ba0.5Sr0.5TiO3/LaNiO3(简称BST/LNO)薄膜。在650℃原位退火10 min,获得了(100)和(110)择优取向生长的BST和BST/LNO薄膜,薄膜晶粒呈柱状结构,BST薄膜和BST/LNO异质结构薄膜的晶粒尺寸分别为150~200 nm和50~80 nm。在室温和1 MHz条件下,BST薄膜和BST/LNO异质结构薄膜的相对介电常数和介电调谐率分别达811和58.9%、986和60.1%;用LNO作底电极,可增益介电常数和介电调谐率。     

（Hg0.69Pb0.31）Ba2Ca2Cu3O8＋δ的表面分析研究

本文采用溅射中性粒卫质谱仪，俄歇电子谱仪和Ｘ光电子能谱仪研究分析了汞系超导材料的表面状况，其中Ｈｇ元素在超导体表面的ＨｇＯ和游离态Ｈｇ并存，Ｐｂ元素在超导体表面的富集等分析结果表明，汞系超导材料存在着表面不稳定性和表面钉扎效应的表面特性。     

Enhanced Thermoelectric Properties of Hole-Doped Bi2−x Ba x Sr2Co2O y Ceramics

The influence of Ba doping on the thermoelectric properties of Bi_2?x Ba _x Sr₂ Co₂O _y (x = 0.00, 0.025, 0.05, 0.075, 0.10, 0.125, and 0.15) samples prepared by the solid-state reaction method was investigated from 333 K to 973 K. For the samples with x ≤ 0.075, the electrical resistivity decreased with increase of the Ba doping amount due to p-type doping and they exhibited metallic electrical conductivity behavior, whereas the samples with x ≥ 0.10 exhibited semiconductor-like electrical conductivity behavior. The Seebeck coefficients of all the samples decreased with increase of the Ba doping amount. The thermal conductivity first decreased for x ≤ 0.075, then increased with higher Ba doping amounts. As an overall result, the dimensionless figure of merit (ZT) of Bi_1.925Ba_0.075Sr₂Co₂O _y reached the maximum value of 0.245 at 973 K, being 41% higher than that of the undoped sample.     

Room temperature oxidation of Cu3Ge and Cu3(Si1−xGex) on Si1−xGex

Room temperature oxidation of Cu₃(Si_1−xGe_x) and Cu₃Ge films grown on Si_1−xGe_x at a temperature of 200–400°C was studied using transmission electron microscopy (TEM) in conjunction with energy dispersive spectrometry (EDS). For Cu₃(Si_1−xGe_x) and Cu₃Ge films grown at 200°C and then exposed to air, room temperature oxidation occurred. The Cu₃Ge film was superior to the Cu₃(Si_1−xGe_x) film in reducing the oxidation rate because of its higher Ge concentration. Annealing at higher temperatures such as 400°C resulted in Ge segregation out of the Cu₃(Si_1−xGe_x) film or Si diffusion from the Si_1−xGe_x substrate into the Cu₃Ge overlayer, and hence enhanced the oxidation rate of Cu₃(Si_1−xGe_x) and Cu₃Ge films. The present study shows that upon exposure to air even the Cu₃Ge film grown on Si_1−xGe_x is subject to room-temperature oxidation, revealing that the use of Cu₃Ge contacts on Si_1−xGe_x may be limited by some strict conditions.     

Influence of film growth conditions on the transport properties of YBa2Cu3O7−δ step-edge junctions

In order to investigate the influence of film growth conditions on the transport properties of step-edge junctions, YBa₂Cu₃O_7−δ films were deposited on high-quality substrate steps by DC-sputtering at two different substrate temperatures and by off-axis laser ablation. RSJ-like junction properties and a low (1σ)-spread of the critical current density J_c (4.2 K) of 17% along one step were obtained for laserablated junctions. The transport properties of the differently prepared junctions correlated with the microstructure of the step region. This, in turn, might result from the large differences in surface diffusion during film growth.     

Dielectric dispersion and ac conduction phenomena of Li2O−Sb2O3−PbO−GeO2:Cr2O3 glass system

Lithium antimony lead germanate glasses doped with different concentrations of Cr₂O₃ (ranging from 0 to 0.5 mol %) were synthesized. The optical absorption and ESR spectral studies have indicated that there is a gradual increase in presence of octahedral chromium ions with increase of Cr₂O₃ concentration in the glass network. These studies have also revealed that a fraction of chromium ions get oxidized into Cr⁵⁺ and Cr⁶⁺ states. IR and Raman spectroscopic studies have pointed out that there is an increasing degree of depolymerization of the titled glass network with increase of Cr₂O₃ content. The dielectric parameters (viz., dielectric constant, loss and electrical modulus coefficients measured over a wide range of frequency and temperature) and also ac conductivity were found to increase with increase of Cr₂O₃ content. The reasons for such increase were discussed quantitatively in terms of various polarization mechanisms coupled with structural variations of the glass network.     

Effect of Hf and Hf-Ca substitution on the superconductivity of GdBa2Cu3O7 − δ

The structural and superconducting properties of (Gd_1−x−yCa_yHf_x)Ba₂Cu₃O_z samples are investigated using X-ray diffraction, resistivity, AC susceptibility and oxygen content measurements. The effect of increasing Hf concentration in (Gd_1−xHf_x)Ba₂Cu₃O_z lowers the oxygen content and decreases T_c which is attributed to hole filling by Hf. The substitution of Ca for Gd in (Gd_0.85−yCa_yHf_0.15)Ba₂Cu₃O_z provides proper matching between the ionic radius and valence of Gd³⁺ (0.94 Å) and the average ionic radius and valence of Hf⁴⁺ (0.78 Å) and Ca²⁺ (0.99 Å). As the Ca content increases, the T_c increases from 81 K (y = 0.05) to 86.5 K (y = 0.20, compensated oxide), closer to the value of 91 K for pure GdBa₂Cu₃O_7−δ due to the balance between the hole filling by Hf and hole doping by Ca. A comparative study of Hf doped samples of (R_1−xHf_x)Ba₂Cu₃O_z (R = Y, Er, Gd) indicates that the magnetic moment carried by R-ion plays an important role in the suppression of superconductivity and T_c.     

高温超导氧化物(Y0.6Ca0.4)(Sr2-xBax)(Cu0.5B0.5)Cu7O7-δ的电子衍射研究

在超导材料YBa2Cu3O7-δ中,用Ca、Sr、B原子分别部分取代Y、Ba、Cu,制成(Y0.6Ca0.4)(Sr.Ba)(Cu0.5B0.5)Cu2O7-δ晶体.随着B掺入量的增加,晶体的超导电性逐渐消失,但如果同时掺入Ba和Ca其超导性则可恢复[1].本文用电子衍射方法,测定了x=0.5、x=1.0、x=1.5三种样品的晶胞参数和可能空间群.     

Effects of annealing technique and TiO2 seed layer on the phase composition of 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 film

The lead magnesium niobate–lead titanate (PMN–PT) thin films with and without the TiO₂ seed layer were prepared by a pulsed laser deposition (PLD) deposited on Pt/Ti/SiO₂/Si substrates. The films were treated by two-step annealing and normal annealing with rapid thermal annealing (RTA). The effects of two-step annealing and the TiO₂ seed layer on the phase composition of PMN–PT films were studied. The results show that the PMN–PT film with TiO₂ seed layer can gain a pure perovskite phase with a high (1 0 0) preferential orientation after the two-step annealing technique.