Origins of conductive losses at microwave frequencies in YBa2Cu3O7?δ/LaAlO3/YBa2Cu3O7?δ trilayers deposited by pulsed laser deposition

Oriented YBa₂Cu₃O_7–/LaAlO₃/YBa₂Cu₃O_7– trilayers were deposited by pulsed laser deposition (PLD) onto 100 MgO and LaAlO₃. Film thicknesses varied from 2000–5000 Å/ layer. A comparision of structure and transport data for the bottom and top superconducting layers indicated a slight decrease in film quality for the top superconducting layer. The critical temperature was lower for the top superconducting layer (90.5 vs. 90 K) and the microwave surface resistance was higher (increasing from 2 to 18 m at 36 GHz, 20 K). The resistivity of the dielectric was estimated to be 10⁶ cm, and the loss tangent of the dielectric film at microwave frequencies had an upper limit of 0.01. Cross-sectional TEM analysis of the trilayer structure showed a high density of threading dislocations in the dielectric layer that appeared to nucleate at steps in the underlying superconducting layer. The threading dislocations may serve as conduction paths in the LaAlO₃ layer.     

Preparation and characterization of α-Fe2O3 nanorod-thin film by metal-organic chemical vapor deposition

Alpha iron oxide (α-Fe₂O₃) films were grown on catalyst-free silicon substrate using a vertical type metal-organic chemical vapor deposition process. X-ray powder diffraction and field-emission transmission electron microscopy measurements showed that these α-Fe₂O₃ films consisted of bundles of one dimensional (1D) nanorods and the nanorods in these α-Fe₂O₃ films were single crystalline with a well-ordered rhombohedral structure. The nanorods showed a preferred growth orientation in the [104] direction. Magnetic force microscopy image suggests that spin domains were formed in the α-Fe₂O₃ nanorods. Photo-catalytic property of these nanorod films was confirmed through the photo-degradation of Rhodamine B by UV irradiation. These α-Fe₂O₃ film/nanorod materials could be used as building blocks for nanodevice applications.     

Nanoindentation of multilayered epitaxial YBa2Cu3O7-δ thin films and coated conductors

Nanoindenter tests were carried out on YBa₂Cu₃O_7-δ (YBCO) coatings with different buffer layers, in order to obtain the Hardness (H) tendency, Young's modulus (E), and the fracture mechanism activated during the indentation process. Different pop-ins were observed in the load-displacement curves, and correlated with their residual nanoindentation imprints visualized by Atomic Force Microscopy. A trench was made by Focused Ion Beam in order to better understand the plastic behaviour activated under the residual imprint at 650 mN of applied load. During the first steps of nanoindentation experiments an elastic regimen takes place and the Hertzian equations can be applied to obtain the E for each YBCO coating. All YBCO coatings present similar E values and H tendencies. However, the YBCO/CeO₂/Yttrium Stabilized Zirconia system exhibits a better mechanical stability probably due to the absence of microcracks under the indentation. In addition, an experimental process using nanoindentation technique is obtained in order to isolate the kind of buffer layer employed (single crystal substrate or metallic substrate).     

Investigation of the Off-Diagonal Thermoelectric Effect on Textured YBa2Cu3O7?δ

Both the off-diagonal Seebeck effect and the off-diagonal Peltier effect were investigated on the same textured sample YBa₂Cu₃O_7– and with the use of the same experimental setup. The effectiveness of several kinds of heat-conductive media was studied for the measurement. The flatness of both the sample and the heat-conducting block, and the matching between them were found very important for the reduction of the heat resistance of the interfaces. A reasonable agreement was found between the off-diagonal Seebeck coefficient measured by the off-diagonal Seebeck effect and that by the off-diagonal Peltier effect. The steplike feature in the relation between the off-diagonal Seebeck coefficient and the annealing temperature may imply a nonmonotonous change of the Seebeck coefficient along the c axis (S _c) with oxygen content.     

RF plasma synthesis of YBa2Cu3O7?x powders

Fine powders of YBa₂Cu₃O_7–x have been synthesized by injecting mixed nitrate solutions of yttrium, barium, and copper into an argon rf thermal plasma. In general, the as-produced powders were dark brown and nonconducting. To obtain superconductivity, the as-produced powders were annealed either in a flowing oxygen tube furnace (at 900C) or in a lowpressure oxygen rf plasma. X-ray powder diffraction, scanning electron microscopy, and centrifugal sedimentation were used for powder characterization. For resistance measurements, bulk samples were prepared by isostatic pressing and tube furnace sintering of the annealed powders. The superconducting transition temperature (at 50% drop of resistivity) was 86 K.     

高温超导材料YBa2Cu3O7的摩擦学特性

用球盘式摩擦试验机及原子力显微镜在大气中常温下对YBa₂Cu₃O₇ (YBCO) 高温超导试样的宏观和微观摩擦学特性进行了研究,并与Cu基固体润滑复合材料进行比较。结果表明:对于Cu基复合材料,固体润滑剂含量不足将使摩擦系数增大,含量过高将导致磨损增大,含40％固体润滑剂的4060Cu在几种载荷和速度下都表现出很低的摩擦和磨损,而YBa₂Cu₃O₇的摩擦系数与4060Cu相似,磨损率却只有其1/2。XRD及TEM的检测结果表明:YBa₂Cu₃O₇结构中的CuO层结合较弱,在剪切应力的作用下易滑移,对外表现出较低的摩擦。基体中含Y₂O₃较多的YBa₂Cu₃O₇相阻碍位错滑移,强化基体,使其耐磨性能大幅提高。用粉末熔化法制备的样品中Y₂O₃颗粒更细小、弥散,其耐磨性更好。     

Anisotropic uniaxial pressure effects in YBa2Cu3O7?δ

We present measurements of the uniaxial pressure dependence ofT _c of untwinned YBa₂Cu₃O_7– crystals with various oxygen stoichiometries. For all samples investigated,T _c decreases for pressure alonga, increases for pressure alongb, and, in oxygen deficient samples, increases strongly for pressure alongc. These results are compared to the behavior found in the La_2–x Sr _x CuO₄ and YBa₂Cu₄O₈ systems. Neither the model of pressure-induced charge transfer nor coupling to orthorhombic distortions can explain all the data. However, the presence of singularities in the electronic density of states close to the Fermi energy is a possible origin of the observed behavior. Our preliminary data on the pressure dependence of thec-axis and in-plane resistivities in twinned crystals are consistent with this view.     

Preparation of YBa2Cu3O7 ? δ Thin Films by the Self-Template Sputtering Method

Using the self-template technique, c-axis-oriented epitaxial YBa₂Cu₃O_{7 –} thin films have been prepared in situ on LaAlO₃ substrate by the d.c. magnetron sputtering method. The properties of thin film dependence on the deposition conditions of the two-step self-template method have been systematically investigated. By optimizing the parameters, high-quality YBCO thin films with T _c0 90 K, T _c 1 K, R _s (77 K, 10 GHz)500 were reproducibly obtained. The best sample grown under optimal conditions gave a low R _s of 330 at 77 K, 10 GHz, which can be used in a microwave field.     

Sintering time and properties of YBa2Cu3O～7 superconducting ceramics

The effect of annealing duration in oxygen flow on the superconducting characteristics of YBa₂Cu₃O₇ bulk ceramics was investigated by structural, electrical, magnetic, and electronic microscopy investigations. The long-time annealing is deleterious for the superconductivity in YBa₂Cu₃O₇ ceramics. The optimum time interval for annealing is from 20 to 70 h at 950°C, and within this range the maximum value ofT _c is obtained.     

YBa2Cu3O7 ? x 240 K Phase Transition

YBa₂Cu₃O_{7 – x} ceramic samples have been investigated simultaneously by the thermal expansion and the acoustic emission methods during thermocycling through 200–300 K into liquid nitrogen steam. No jumps in the dilatation curves but change in the slope of one has been found at 228 and 234 K on heating and cooling respectively. These points are accompanied by the acoustic emission signals, a small value testifying for oxygen atoms displacement into the crystal lattice. On the basis of these measured experimental data, a thermal expansion coefficient and a specific capacity c _m have been calculated. By this c _m value an oxygen atom activation energy E = 0.15 eV has been calculated. It is shown that an isostructural oxygen-displacement second-order phase transition takes place between certain orthorhombic phases in YBa₂Cu₃O_{7 – x} near 240 K.     

Preparation of W–Sc2O3 targets and scandate cathodes with film prepared by pulsed laser deposition

Sub-micrometer Sc₂O₃–W powder with a narrow particle size distribution has been obtained by a sol–gel method combined with two-step hydrogen reduction process. Based on the obtained powder, the W–Sc₂O₃ targets have been sintered via spark plasma sintering (SPS) at 1300 °C. The W–Sc₂O₃ targets have the average grain size of about 1 μm. Both the sintering temperature and holding time are much lower than those of the targets prepared with micrometer sized powders. The obtained W–Sc₂O₃ targets have a high comparative density of 96.4% and rockwell hardness of 86.4 HRC. Using the target, the scandate cathode deposited with a film containing 5% Sc₂O₃ and 95% W has been obtained by pulsed laser deposition (PLD) method. This cathode has good emission property, i.e., the highest thermionic emission current density reaches 43.09 A/cm² of J_div at 900 °C_b after being activated for 8 h, which is much higher than that of scandate cathode without film. Scandium (Sc) supplied by the film on the surface during the activation forms a Ba–Sc–O active layer, which helps to the emission.     

YBa2Cu3O7-δ薄膜的BaCl2/BaF2-MOD法制备及超导特性研究

传统三氟乙酸金属有机化学溶液沉积法(TFA-MOD)制备YBa₂Cu₃O_7-δ (YBCO)超导层,Ba倾向于与F结合,从而避免BaCO₃的形成。本工作开展了新型基于BaCl₂/BaF₂途径的化学溶液法生长YBCO超导薄膜的研究。重点研究了添加Cl对YBCO薄膜晶粒取向、微观结构和超导性能的影响,并通过生长反应的热化学计算,分析了BaCl₂途径YBCO薄膜的物相转变机制。结果表明:添加Cl有利于抑制a轴晶粒取向,促进c轴晶粒成核。添加Cl的YBCO双层膜起始转变温度(Tc-onset)没有明显变化,约为89.6 K,其临界电流密度(Jc)显著提升, Jc达到2.07 MA/cm²(77K,自场)。此外,生长反应过程的物相转变分析表明Cl优先与Ba结合形成BaCl₂,有效避免BaCO₃的形成。本研究结果表明:添加Cl对制备YBCO超导厚膜有促进作用,这为MOD法制备YBC...     

Transport properties of oxygen-deficient YBa2Cu3O7?x thin films

We report on measurements of the in-plane resistivity and Hall coefficientR _H (Bc) of various oxygen-deficient epitaxial films of YBa₂Cu₃O_7–x in the normal state. The superconducting transition temperaturesT _c of the samples vary from 14 to 90 K. Both the resistivity and the Hall coefficient exhibit a strong dependence on the oxygen content and the temperature. Asx increases,T _c decreases continuously, while andR _H gradually increase in magnitude. Furthermore, also the characteristic linear dependences ofT andR _H T ^–1 of the highly doped compounds changes to a nonlinear behavior for the samples withT _c lower than 60 K. The unusual doping and temperature dependence ofR _H will be compared to the predictions of our calculations, based on a two-dimensional tight-binding model using the relaxation-time approximation. The model considers also the next-nearest-neighbor hopping, which strongly influences the predicted Hall coefficient. Additionally, the cotangent of the Hall angle cot( _H ) is discussed in the framework of the two-dimensional Luttinger liquid theory.We gratefully acknowledge financial support from the Bundesministerium fiir Forschung und Technologic.     

斜切基片上c-取向YBa2Cu3O7-δ薄膜的生长特性及开裂行为

用脉冲激光沉积方法在斜切SrTiO3(001)基片上制备了YBa2Cu3O7-δ薄膜,并用原子力显微镜、X射线衍射仪及透射电镜研究了薄膜表面形貌、取向特征、结晶性和显微结构.结果表明,薄膜表面呈"台阶-台面"结构,并伴有与台阶边缘垂直的裂纹产生,说明薄膜以"台阶流动"方式生长;而且薄膜具有c-轴取向,但其结晶性不好.在薄膜中没有观察到a-b孪晶的存在.薄膜的开裂和结晶性较差均是由于斜切基片台阶表面晶格四方畸变抑制了薄膜正交转变时a-b孪晶的产生,而使晶格错配应变不能释放造成的.     

Microwave properties of highly oriented YBa2Cu3O7-δ superconducting thin films

We have performed millimeter-wave frequency (94 GHz) measurements on high-quality YBa₂Cu₃O₇- superconducting films on yttrium-stabilized (100) ZrO₂ and MgO substrates. The 0.2m thin films fabricated by magnetron sputteringin situ with the YBa₂Cu₃O₇- powders as target exhibit superconducting transition temperatures up to 88 K. The critical current density of 6×10⁵ A/cm² at 77 K and the X-ray diffraction spectrum as well as scanning electron microscope photographs indicate these thin films are fullyc-axis oriented, extremely high in density, and universally homogeneous. Millimeter-wave surface resistances have been measured on a hemisphere open resonator in the temperature range of 20 K toT _c and beyond. The surface resistance at 94 GHz and 77 K for these films is found to be about 30 m, nearly 1/4 that for copper, and a drop of two orders in the surface resistance within 4 K is observed, which indicates that these films are good materials for applications in the millimeter-wave range, especially for fabricating microwave devices. We observed such low surface resistance in these thin films due to the near absence of grain and phase boundaries coupled with a high degree of crystalline orientation.     

Preparation and characterization of YBa2Cu3O7?x samples for microwave applications

We prepared several samples of YBa₂Cu₃O_7–x thick films electrophoretically deposited on metallic substrates, thin films sputtered on dielectric substrates, and bulk pellets, and we tested their microwave properties. Various preparation techniques are described in detail and the importance of appropriate thermal treatment during the fabrication process is emphasized. Experimental results show that electrophoretically deposited thick films on silver substrates reached, after heat treatment, a surface resistance at 50 K of 4×10^–2 comparable with the silver value, while the same measurement before sinterization gave a value of 0.5 . Either bulk pellets or thin films gave worse results, though a lack of sensitivity in our experimental apparatus could have influenced our data. The problem of sensitivity in the characterization of microwave properties of small samples is discussed in the appendices.     

Investigation of Oxidation Kinetics of YBa2Cu3O7?x Ceramics by the Acoustic Emission Method

The oxidation kinetics of YBa₂Cu₃O_7–x ceramics has been investigated by the acoustic emission method. It is observed that the oxidation follows an exponential law. For YBCO the exponential index is a specific parameter and is equal to 1.7 s±10%. It is confirmed that the related narrow surface layer is first oxidized, and oxygen dissolution in the material is controlled by the speed of volume diffusion.     

Determination of the Glassy Exponent of Melt Textured YBa2Cu3O7?x by Magnetic Relaxation

By magnetic relaxation method we obtained the glassy exponent (T, H) of the melt textured Yba₂Cu₃O_7–x sample. The results have a similar behavior compared with ones using the method of magnetic field sweep, and are compatible with the model of collective pinning theory.     

Improved Flux Pinning in Melt-Textured YBa2Cu3O7 ? δ and ErBa2Cu3O7 ? δ Through Chemical Additions

The effects of Ce-based additions (CeO₂ and BaCeO₃) in combination with MgO additions on the magnetic properties of melt-textured YBa₂Cu₃O_{7 –} and ErBa₂Cu₃O_{7 –} have been investigated. The additions lead to improvements in the magnetic properties of both RE-123 compounds compared to samples with either addition alone or with no additions. The Ce–Mg addition combination produces a peak effect in the magnetic hysteresis loop without significant T _c degradation. This is postulated to be due to the formation of a new type of pinning center. Both Ce and Mg ions are thought to substitute in the RE-Ba₂Cu₃O_{7 –} lattice, creating defects that produce a peak effect in the magnetic hysteresis loop. Mg additions alone lead to a reduced T _c, while Ce additions restore the T _c and enhance the magnitude of the peak.     

Improved critical current densities in thick YBa2Cu3O7 − δ multilayer films interspaced with non-superconducting YBa2Cu3Ox nanodots

We have fabricated and studied quasi-multilayered thick YBa₂Cu₃O_7 − δ (YBCO) films composed of several YBCO layers interspaced with quasi-layers of non-superconducting YBa₂Cu₃O_x nanodots, grown by Pulsed Laser Deposition on SrTiO₃ (100) substrates. Magnetization J_c(B) at 77.3 K for these thick films showed significant improvement as compared to pure YBa₂Cu₃O_7 − δ films of same or even smaller thickness. A high J_c(B) in our quite thick films (1 μm to 6 μm) provides a very high total critical current per centimetre of the film width, I_c − w. Critical current as high as 830 A per cm width in self field and 77.3 K was achieved in 5 μm thick quasi-multilayer film with non-superconducting YBa₂Cu₃O_x nanodots. Frequency-dependent susceptibility measurements showed also an increase in the pinning potential. The angular dependence of I_c − w at 86.5 K, in 3 T shows a clear indication of anisotropic pinning centres aligned along the c-direction.