High critical current density YBa2Cu3O7−δ tapes prepared by the surface-oxidation epitaxy method

YBa₂Cu₃O_7−δ (YBCO) films with high critical current density (J_c) were successfully fabricated on nickel tapes buffered with epitaxial NiO. NiO was prepared on the textured nickel tape by the surface-oxidation epitaxy (SOE) method. We have reported so far a critical temperature (T_c) of 87 K and J_c=4–6×10⁴ A/cm² (77 K, 0 T) for the YBCO films on NiO/Ni tapes. To enhance the superconducting properties of the YBCO films on the SOE-grown NiO, depositions of thin oxide cap layers such as YSZ, CeO₂, and MgO on NiO were investigated. These oxide cap layers were epitaxially grown on NiO and provided the template for the epitaxial growth of YBCO films. Substantially improved data of T_c=88 K and J_c=3×10⁵ A/cm² (77 K, 0 T) and 1×10⁴ A/cm² (77 K, Hc, 4 T) were obtained for YBCO film on NiO, by using a MgO cap layer with a thickness of 50 nm. The method described in this paper is a simple way to produce long YBCO tape conductors with high-J_c values.     

The effects of substrate temperature on the superconducting properties of Tl2Ca2Ba2Cu3O10 films sputter-deposited from stoichiometric oxide targets

The annealing characteristics and the superconducting properties of Tl₂Ca₂Ba₂Cu₃O₁₀ thin films sputter-deposited onto yttrium- stabilized ZrO₂ substrate at up to 500°C from two stoichiometric oxide targets are reported. The films deposited at 400–500°C were found to require a lower post-annealing temperature than the films deposited at lower temperatures to attain the highest T_c superconducting state, due to a more pronounced Ba diffusion toward the substrate as indicated by their secondary ion mass spectrometry depth profiles. The highest T_c achieved tends to degrade with increasing substrate temperatures, a zero resistance T_c of 121 and ≈90 K, respectively, being observed for the films deposited at -ambient temperature and at 500°C. The formation of the highest T_c phase (Tl₂Ca₂Ba₂Cu₃O₁₀) generally is associated with a sheet type of crystal growth morphology with smooth and aligned surfaces which can be obtained only from the films capable of sustaining prolonged annealing at 900°C. Annealing at lower temperatures (≈860°C) results in the formation of rod or sphere type of morphologies with rough and randomly oriented crystals and the lower T_c phases such as Tl₂Ca₁Ba₂Cu₂O₈.     

Laser ablated bismuth cuprate thin films preparation and effect of oxygen nonstoichiometry upon superconductivity

Thin films of Bi₂Sr₂CaCu₂O₈ and (Bi, Pb)₂Sr₂Ca₂Cu₃O₁₀ have been prepared on monocrystalline (100) MgO substrates, using a laser ablation method with post annealing treatment. The influence of substrate temperature and oxygen pressure during deposition were investigated. SEM observations, EDS analysis, electric and magnetic measurements have been used to characterize the films. Superconducting “2212” films, with T_c(R = 0) at 80–83 K and J_c (50 K) up to 5 × 10⁵ A/cm², have been currently achieved, while Pb-doped “2223” films exhibit T_c as high as 110 K with J_c = 5 × 10⁴ A/cm² at 77 K. The effect of annealing at low temperature (350°C) in an argon flow has been studied for the 2212 phase, it shows the influence of the oxygen non-stoichiometry, i.e. of the hole carrier density upon T_c's which can be measured up to 89 K (zero resistance).     

Induction of superconductivity in pulsed laser ablated La2CuO4 thin films by post-deposition fluorination

Pulsed laser ablation has been used to fabricate La₂CuO₄ thin films. Superconducting properties have been successfully induced in the films by an ex-situ, post-ablation annealing process in F₂ gas resulting in a T_c (onset) of 36 K. The presence of two slightly different c-axis expanded phases in the X-ray diffraction data of the fluorinated films implies a degree of inhomogeneity in F₂ uptake. Critical current densities (J_c) and the irreversibility line have been established from hysteresis cycles. A J_c of 10⁶ A cm⁻² for a typical film was observed at 10 K in zero field.     

Buffers for high temperature superconductor coatings. Low temperature growth of CeO2 films by metal–organic chemical vapor deposition and their implementation as buffers

Smooth, epitaxial cerium dioxide thin films have been grown in-situ in the 450–650°C temperature range on (001) yttria-stabilized zirconia (YSZ) substrates by metal–organic chemical vapor deposition (MOCVD) using a new fluorine-free liquid Ce precursor. As assessed by X-ray diffraction, transmission electron microscopy (TEM), and high-resolution electron microscopy (HREM), the epitaxial films exhibit a columnar microstructure with atomically abrupt film-substrate interfaces and with only minor bending of the crystal plane parallel to the substrate surface near the interface and at the column boundaries. With fixed precursor temperature and gas flow rate, the CeO₂ growth rate decreases from 10 Å/min at 450°C to 6.5 Å/min at 540°C. The root-mean-square roughness of the films also decreases from 15.5 Å at 450°C to 4.3 Å at 540°C. High-quality, epitaxial YBa₂C₃O_7−x films have been successfully deposited on these MOCVD-derived CeO₂ films grown at temperatures as low as 540°C. They exhibit T_c=86.5 K and J_c=1.08×10⁶ A/cm² at 77.4 K.     

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.     

Fabrication and characteristics of all YBCO sandwich-type junctions using highly crystalline DC sputtered superconducting a-axis oriented Y1Ba2Cu3O7-x films

We report on the fabrication and characteristics of sandwich-type tunnel junctions with highly crystalline sputtered a-axis oriented thin film of Y₁Ba₂Cu₃O_7-x (YBC) as the base and the counter electrode. The junctions have been fabricated on SrTiO₃ (100) and MgO (100) substrates. A non-superconducting phase of YBC corresponding to a lattice constant of 4.08 Å is used as the barrier layer making this an all YBC sandwich junction. For all temperatures below T_ce (R=0) of the device, a zero voltage current was observed. The critical current density (J_c) of the device was found to be dependent on the thickness of the barrier layer and the crystallinity of the a-axis oriented YBC electrodes. At 40 K, such a junction fabricated on a SrTiO₃ (100) substrate was found to have a J_c of 1.8 X 10⁴ A/cm² and an I_cR_n product of 0.2 mV.     

Preparation of melt-textured NdBa2Cu3Oy bulk with Nd4Ba2Cu2O10 addition

The NdBa₂Cu₃O_y (Nd123) bulk superconductor, to which Nd₄Ba₂Cu₂O₁₀ (Nd422) particles were intentionally added, was prepared through the so-called MMTG process in Ar (99% purity) flowing atmosphere at an ambient pressure. The quasi-single crystal thus grown was about 1 cm × 1 cm × 1 cm in dimension. It turned out that the Nd422 particles were uniformly distributed in the Nd123-phase matrix in a fashion similar to the distribution of the intentionally added Y211 particles in the Y123 phase matrix. The superconducting transition temperature T_c for the sample subjected to post-annealing in oxygen atmosphere was 94 K. The critical current density J_c was determined to be 45 000 A/cm² at 77 K and 1 T, when the field was applied parallel to the c-axis of the sample. To the best of our knowledge, the J_c value is the highest and the size of the quasi-single crystal is the largest in the melt-textured Nd123 bulk superconductors so far reported.     

Temperature dependence of the exafs spectrum in YBa2Cu3O7−δ compounds

The temperature dependence of the extended X-ray absorption fine structure (EXAFS) is studied in the high T_c superconductors, YBa₂Cu₃O_7−δ. The measurements were done at the Cu K-edge for samples of two orthorhombic phases (T_c≈90 K and ≈58 K, respectively) and a nonsuperconducting tetragonal phase. Interatomic distances and mean square relative displacements σ² for Cu-O bonds are determined by the least squares refinement. The results indicate that values of σ² increase near T_c for both the orthorhombic samples. It is concluded that this anomalous behavior related to T_c is caused by an anomalous vibration of oxygen atoms in the Ba-O layer. Changes in the Cu-O distances from 300 to 20 K are not found.     

MgO(111)衬底MgB2 超薄膜的制备和性质研究

利用混合物理化学气相沉积法(HPCVD)在MgO(111)衬底上制备了干净的MgB₂超导超薄膜. 在背景气体压强, 载气氢气流量以及沉积时间一定的情况下, 改变B₂H₆的流量, 制备得到不同厚度系列的MgB₂超导薄膜样品, 并测量了其超导转变温度 T_c, 临界电流密度J_c等临界参量. 该系列超导薄膜沿c轴外延生长, 表面具有良好的连接性, 且有很高的超导转变温度T_c(0) ≈ 35-38 K和很小的剩余电阻率ρ(42 K) ≈ 1.8-20.3 μΩ·cm^-1. 随着膜厚的减小而减小, 临界温度变低, 而剩余电阻率变大. 其中20 nm的样品在零磁场, 5K时的临界电流密度J_c ≈ 2.3×10⁷ A/cm². 表明了利用HPCVD在MgO(111)衬底上制备的MgB₂超薄膜有很好的性能, 预示了其在超导电子器件中广阔的应用前景. 关键词： MgO(111)衬底 2超薄膜')" href="#">MgB₂超薄膜 混合物理化学气相沉积     

MgB2超导膜的厚度与其Jc(5K,0T)的关系

通过混合物理化学气相沉积法 (hybrid physical-chemical vapor deposition, HPCVD), 在(000l) SiC 衬底上制得一系列从10 nm到8 μm的MgB₂超导膜样品, 并对它们的形貌、超导转变温度T_c 和临界电流密度J_c与膜厚度的关系进行了研究. 观察到T_c随膜厚度增加上升到最大值后, 尽管膜继续增厚, 但T_c值保持近乎平稳, 而J_c则先随膜厚度增加上升到最高值后, 继而则随膜的厚度的增加而下降. MgB₂膜的T_c(0)和T_c(onset)值与膜厚的关系基本一致, T_c(0)在膜厚为230 nm处达到最大值T_c(0)=41.4 K, 而J_c(5K,0T)在膜厚为100 nm时达到最大值, J_c (5 K, 0 T)=2.3×10⁸A·cm^-2, 这也说明了我们能用HPCVD方法制备出高质量干净MgB₂超导膜. 本文研究的超导膜厚度变化跨度非常大, 从10 nm级的超薄膜到100 nm级的薄膜, 再到几微米的厚膜, 如此T_c和J_c对膜厚度变化的依赖就有了较完整、成体系的研究. 并且本文的工作对MgB₂超导薄膜制备的厚度选取具有实际应用意义. 关键词： 2超导膜')" href="#">MgB₂超导膜 混合物理化学气相沉积法 厚度 临界电流密度     

Fabrication, microstructure and critical current density of screen-printed Ag-(Bi, Pb)2Sr2Ca2Cu3Ox superconducting tapes

The influence of the sintering conditions on the microstructure and critical current density J_c has been studied on screen-printed Ag-(Bi, Pb)₂Sr₂Ca₂Cu₃O_x tapes with a ceramics mono-layer core. Three kinds of fabrication processes, which consist of a combination of cold working (rolling and/or pressing) and sintering, are applied. Four times repetition of pressing and sintering after the pre-sintering produces the highest c-axis alignment and achieves J_c= 1.5 × 10⁴ A/cm² (77 K, 0 T). The J_c versus θ data with an angle θ between B and the c-axis elucidate the relation between the anisotropy ratio γ=J_c(Bc)/J_c(B|c and the half-height angular width Δθ of a peak for Bc. This is related to both grain alignment and the J_c value. An increase in J_c, which comes from an improvement for grain alignment, enhances γ and narrows Δθ. The J_c versus θ data are fitted to the expression J_c(B, θ)=J _c(B, 90°)/[(γ−1)|cos θ|ⁿ+1] by regarding both γ and n as adjustable parameters. Fabrication of screen-printed tapes with multilayers (1≤N≤5) is presented, where the critical current increases from 8.0 A to 30.2 A at 77 K and 0 T as N increases.     

Superconducting properties of ZnO-doped (Bi, Pb)-2223 thick film on Ni and NiO substrates prepared by spray deposition technique

The superconductivity of ZnO-doped (Bi, Pb)-2223 thick film on the Ni and NiO substrates, which was prepared by the spray deposition technique with cold forging, was investigated by characterizing the critical current density (J_c), the critical temperature (T_c), the orientation factor (f), and the microstructure of the film. The thickness of the thick film prepared by the spray deposition method was approximately 10 μm. The maximum J_c value of (Bi, Pb)-2223 film on NiO substrate was approximately 2200 A/cm² (I_c = 110 mA) when the film was sintered at 865 °C for 1 h with a cooling rate of 0.5 °C/min from 865 °C to 650 °C; in the case of Ni substrate, a maximum J_c value of approximately 2000 A/cm² (I_c = 100 mA) was obtained for the (Bi, Pb)-2223 thick film when a cooling rate was 3 °C/min. Such a difference in the J_c values of (Bi, Pb)-2223 thick film on Ni and NiO substrates is attributed to the presence of reaction layer at the (Bi, Pb)-2223 and substrate interface. In addition, the variations in the orientation factor of (Bi, Pb)-2223 thick film on NiO substrate related to those of J_c values. The J_c values of (Bi, Pb)-2223 film on NiO substrate with ZnO doping extremely depended on the amount of ZnO doping and the 0.5 wt% ZnO-doped (Bi, Pb)-2223 thick film deposited on NiO substrate, which was sintered at 835 °C for 1 h in air with a cooling rate of 1 °C/min, showed a J_c value of approximately 1200 A/cm² (I_c = 60 mA). Thus, it is considered that a small amount of ZnO doping was effective in lowering the sintering temperature of (Bi, Pb)-2223 thick film, resulting the improvement in the intragranular weak bonding or Josephson junction.     

Effective activation energy Ue(T,J,H) in textured Bi1.84Pb0.4Sr2Ca2Cu3Oy silver-clamped thick films

We have measured the resistivity of textured Bi_1.84Pb_0.4Sr₂Ca₂Cu₃O_y silver-clamped thick films as a function of temperature, current density ranging from 10 to 1×10³ A/cm² and magnetic field up to 0.3 T. We find that the effective activation energy U_e follows U_e(T,J,H)=U₀(1−T/T_p)^mln(J_c0/J)H⁻ with m=1.75 for Hab-plane and 2.5 for Hc-axis and =0.76 for Hab and 0.97 for Hc, for the current density regime above 100 A/cm², where T_p is a function of applied magnetic field and current density. This result suggests the effective activation energy U_e be correlated with the temperature, current density and magnetic field. The possible dissipative mechanisms responsible for the temperature, current density and magnetic field dependence of the effective activation energy are discussed.     

Microstructure and transport properties of YBa2Cu3O7−δ films produced by laser ablation from a BaF2/Y2O3/CuO target

The effects of varying the temperature and duration of the post-deposition anneal in watersaturated oxygen were investigated for YBa₂Cu₃O_7−δ films of varying thickness. The films were produced by laser ablation from pressed powder targets consisting of BaF₂,Y₂O₃, and CuO mixtures. This technique produces superconducting films with a highly textured surface. The films were fabricated on SrTiO₃ substrates and were analyzed with X-ray diffraction, scanning electron microscopy, and temperature dependent resistivity. Critical current density (J_c) measurements were performed in magnetic fields up to 1 T. For film thickness on the order of 900 nm, completely c-axis oriented films were obtained with a 60 min anneal at 850°C. Thinner films required less annealing, either shorter times or lower temperatures, to achieve similar results, indicating that the optimal annealing conditions are dependent on film thickness.     

Properties of a large La1.92Sr0.08CuO4+δ single crystal grown by the travelling-solvent floating-zone method

A centimeter size single crystal of La_2−xSr_xCuO_4+δ (volume=1.32 cm³) with x_Sr=0.08 has been grown by the travelling-solvent floating-zone (TSFZ) method using a double ellipsoidal-type optical furnace as the heat source. The crystallised phase was checked solvent free by X-ray powder diffraction experiments, the crystal dimensions and quality being investigated by X-ray and neutron Laüe techniques. Several rocking curves of the Bragg peaks were performed by neutron diffraction giving a full width at half maximum (FWHM) of 0.200° for (006) reflection and clearly showing the presence of twin domains as expected for such an orthorhombic structure. The superconducting critical temperature of the as-grown crystal under 2 oxygen bar was determined by SQUID measurements with T_c=18–20 K. Thermal treatments at different oxygen pressures were carried out showing no significant improvement of the transition sharpness and the T_c value. Normal state susceptibility was also measured from 6 to 800 K for two different field orientations and can be interpreted as an antiferromagnetic insulating state behaviour. The resistivity measurements display an insulating behaviour perpendicular to the CuO₂ planes and a metallic behaviour in the planes, with a high anisotropy ratio R_c/R_b350 at room temperature and a zero resistivity achieved at 27 K in both directions. The specific heat measurements have revealed no anomalies in the temperature range 15–300 K.     

Ni2O3掺杂对新固相源顶部籽晶熔渗生长法制备单畴GdBCO超导块材超导性能的影响

本文通过在新固相源中添加Ni₂O₃的方法, 采用顶部籽晶熔渗生长工艺(TSIG)制备出组分为(1-x) (Gd₂O₃+1.2BaCuO₂)+x Ni₂O₃、直径为20 mm的单畴GdBCO 超导块材(其中x = 0, 0.02, 0.06, 0.10, 0.14, 0.18, 0.30, 0.50 wt%), 并研究了Ni₂O₃的掺杂量x对样品的表面生长形貌、微观结构、临界温度T_c、磁悬浮力以及俘获磁通密度的影响. 研究结果表明, 当Ni₂O₃的掺杂量x在0–0.50 wt%的范围内时, 均可制备出单畴性良好的样品, 且Ni₂O₃的掺杂对样品中Gd211粒子的分布和粒径没有明显的影响. 在Ni₂O₃的掺杂量x从0增加到0.50 wt%的过程中, 样品的临界温度T_c呈现下降的趋势, 从x=0时的92.5 K下降到x=0.50 wt%时的86.5 K, 这是由于Ni^{3 +}替代GdBCO晶体中Cu^{2 +}所致; 样品磁悬浮力和俘获磁通密度均呈现先增大后减小的变化规律, x=0.14 wt%时, 磁悬浮力达到最大值34.2 N, x=0.10 wt%时, 俘获磁通密度达到最大值0.354 T. 样品磁悬浮力和俘获磁通密度的变化规律与Ni₂O₃的掺杂量x有密切关系, 只有当掺杂量x合适时, Ni³⁺对Cu^{2 +}的替代既不会造成T_c的明显下降, 但又能产生适量的Ni^{3 +}/Cu²⁺ 晶格畸变, 从而达到提高样品磁通钉扎能力和超导性能的效果.     

Temperature dependence of the critical current in YBa2Cu3O7−δ and Bi2Sr2Ca1Cu2O8 Josephson junctions

We have studied the stationary Josephson effect on YBa₂Cu₃O_7−δ (T_c=90 K) and Bi₂Sr₂Ca₁Cu₂ O₈ (T_c=80 K and 87 K for two samples of different origin) ceramic based junctions. The temperature dependence of the critical current near T_c has been found as I_c≈(T_c-T) for the Y-Ba-Cu-O samples indicating that they should be classified as S-N-I-N-S type junctions. The I-V curves of the Bi-Sr-Ca-Cu samples show the typical behaviour of S-I-S structures. Using Ambegaokar-Baratoff's theory for Bi₂Sr₂Ca₁Cu₂O₈, the temperature dependence of the superconducting state gap Δ(T) was calculated and it was evaluated that 1.452Δ(0)/k_BT_c3.5.     

Specific heat study of (Mg0.85Zn0.15)CNi3 and MgCNi3

We have measured the specific heat capacity of MgCNi₃ and (Mg_0.85Zn_0.15)CNi₃ in the temperature range of 0.5 K < T < 10 K with magnetic fields up to 9 T. After the Zn impurity incorporation, T_c of (Mg_0.85Zn_0.15)CNi₃ decreases but the temperature dependence of specific heat remains BCS-like. We show that the data for the two samples are identical when scaled with T_c. This result excludes possible unconventional features theoretically proposed, such as impurity-induced low-energy bound state. Our data suggest that MgCNi₃ is a conventional BCS superconductor.     

Improvement in superconductivity of TlBa2CaCu2Oy system by introduction of oxygen loss

The T_c and oxygen content of TlBa₂CaCu₂O_y have been investigated by quenching experiments, in which the heat-treated samples were dropped into liquid nitrogen. The oxygen loss, v, of TlBa₂CaCu₂O_{yo − v} was determined by thermogravimetry in a nitrogen atmosphere using an oxygen-annealed specimen of TlBa₂CaCu₂O_yo and also by measurements of the weight differences before and after quenching. T_c increased from 80 K of the oxygen-annealed specimen up to about 110 K with increasing oxygen loss up to v = 0.035 by annealing at 500°C in a nitrogen atmosphere. Judging from the high T_c above 110 K achieved by a small oxygen loss about v = 0.035, the as-sintered oxygen-annealed TlBa₂CaCu₂O_yo specimen was in the over-doping state and probably has an oxygen vacancy of 7 − y_o0.