Table of contents

Sm_1+xBa_2−xCu₃O_z (SmBCO) single crystals were grown under atmospheric ambient by the top-seeded solution growth method. Inductively coupled plasma results indicate that there is negligible Sm substitution for Ba sites in the grown SmBCO crystals, although they crystallized from different Ba–Cu–O solvents with a wide composition range (Ba/Cu ratio of 0.5–0.6). As a result, these crystals show high superconducting critical transition temperature values (T_c) of over 93 K with a sharp transition width after oxygenation. A large-sized crystal with an a–b plane of 23 × 22 mm² and a c-axis of 19 mm was obtained at a high growth rate of nearly 0.13 mm h⁻¹. In short, with more controllable thermodynamic parameters, SmBCO single crystals can readily achieve both large size and high superconducting properties.

The accuracy of three distinct experimental procedures for calibrating axial SQUID gradiometers has been compared, for the same gradiometer design and experimental setup. Each procedure employs a different field source, and a nonlinear least-squares optimization is used to fit the measured voltage to the theoretical field, thus determining Tesla/Volt calibration factors. We also studied the effect of noise and gradiometer imbalance on the accuracy of each procedure.

The effect of tungsten boride (WB) addition on the microstructure and superconducting properties of Fe-sheathed MgB₂ tapes has been investigated. The microstructure was not influenced appreciably by the addition, whereas the superconducting properties changed. Even by a 5 mol% addition, the critical temperature (T_c) decreased by about 0.5 K. However, the field dependence of the inductive critical current density (J_ci) became smaller with the increase in the amount of addition, suggesting that pinning centres effective in a high-field region were possibly introduced. The transport critical current density (J_ct) at 4.2 K increased about twice by the 5 mol% addition and reached 15 and 5 kA cm⁻² at 8 and 10 T, respectively.

An analysis expression for the stationary probability distribution of the symmetric superconducting quantum interference device with two Josephson junctions (dc SQUID) driven by thermal noise is derived from two-dimensional Fokker–Planck equations where the potential condition is satisfied. The analytical formulae for SQUID characteristics, e.g. the circulating current, the current–voltage relationship and the transfer function, are obtained by using the probability distribution. The 'ripple' phenomenon of the circulating current versus the applied flux, the effects of thermal fluctuations on current–voltage relationship and the optimum operating condition of dc SQUID system are represented.

Bars and pellets of the (Sm_0.33Eu_0.33Gd_0.33)Ba₂Cu₃O_x superconductor were processed in air, using the floating zone method and the top-seeded melt-textured growth method, respectively. The samples were prepared using different experimental conditions, i.e. maximal processing temperature, translation rate or cooling rate. Their physical properties and their microstructure were studied. All the samples exhibit a satisfying superconducting transition whereas the critical current density greatly depends on the processing parameters. The sample prepared by the floating zone method at 1070 °C with a translation rate of 2 mm h⁻¹ exhibits a very high J_c value of 70000 A cm⁻² in the self-field and more than 30000 A cm⁻² at 1.7 T. The pellet processed at 1080 °C with a cooling rate of 2 °C h⁻¹ has a high J_c reaching about 56000 A cm⁻² in the self-field and more than 32000 A cm⁻² under 1.5 T and still 20000 A cm⁻² at 2.5 T. This rare-earth mixture is promising for the application of this material under a high magnetic field.

Two well-textured single-domain YBa₂Cu₃O_7−x (YBCO) thin disc samples (ϕ30 × 7 mm), SI₁ and SI₂, were used to investigate the relationship of levitation force between individual YBCO discs and a double-layer disc sample, SD (formed by putting samples SI₁ and SI₂ coaxially together). The levitation forces for samples SI₁ and SI₂ are 18.1 and 17.8 N, respectively. The levitation force of the sample SD is 31.2 N, slightly less than the levitation force summation of the two individual samples; the difference factor is about 15.1%. The real difference factor is about 0.64% after we take into account the real distance between the magnet and each individual disc. The result is very attractive and useful for practical applications of levitation devices during the design and fabrication.

Ag-doped MgB₂ bulk superconductors have been prepared using a standard solid state processing. The addition of Ag to MgB₂ powders during the sintering process has been found to result in an important advantage, namely, the prevention/reduction of loss of Mg, a problem most commonly observed in the sintering of MgB₂ bulk samples at elevated temperature and ambient pressures. The Ag-doped MgB₂ sample has a distinct superconducting transition temperature around 39 K, while the undoped MgB₂ undergoes only a very feeble transition to a diamagnetic superconducting state at around 39 K. The normal conducting silver regions in the MgB₂ matrix act as pinning centres resulting in the realization of high critical currents in the presence of magnetic fields.

The measurement of the current–voltage (I–V) characteristics of BSCCO-2223/Ag multifilamentary tapes in a silver matrix has been performed on short samples (of several centimetres) as well as on long tape (1 m), wound in the form of a helical one-layer coil. Measurements at 77 K and in zero external magnetic field have revealed good reproducibility of the I–V hysteresis in most runs. Nevertheless, strange irregularities have sometimes been observed in the I–V curve behaviour during current ramping up and down. Quasi-reproducible drops from the ascending hysteretic branch in the direction of the descending one have been measured at higher voltage levels (∼1 mV cm⁻¹) on the curve measured on the helical coil. These have recently been explained by a sudden change in the heat transfer coefficient [1]. Rarely and non-reproducibly we have also observed these drops on short samples at E ∼ 1 × 10⁻² V m⁻¹, (and even under 1 × 10⁻³ V m⁻¹). The accidental drops have also been sporadically measured by other experimenters. We also interpret these effects as a result of changes in heat transfer dynamics to LN2 coolant, on a local scale, in the vicinity of a small locally degraded portion of the superconducting multifilament tape. The implication of the known differences in the heat transfer dynamics between ramp-up and ramp-down phases to the thermal interpretation of the I–V curve hysteresis, observed on short samples of multifilament tapes, however, fails to explain why already a rather small external magnetic field (∼15 mT) suppresses this hysteretic effect.

Biaxially textured magnesium oxide (MgO) films deposited by inclined-substrate deposition (ISD) are desirable for rapid production of high-quality template layers for YBCO-coated conductors. High-quality YBCO films were grown on ISD MgO buffered metallic substrates by pulsed laser deposition (PLD). Columnar grains with a roof-tile surface structure were observed in the ISD MgO films. X-ray pole figure analysis revealed that the (002) planes of the ISD MgO films are tilted at an angle from the substrate normal. A small full-width at half maximum (FWHM) of ≈9° was observed in the ϕ-scan for ISD MgO films deposited at an inclination angle of 55°. In-plane texture in the ISD MgO films developed in the first ≈0.5 μm from the substrate surface, and then stabilized with further increases in film thickness. Yttria-stabilized zirconia and ceria buffer layers were deposited on the ISD MgO grown on metallic substrates prior to the deposition of YBCO by PLD. YBCO films with the c-axis parallel to the substrate normal have a unique orientation relationship with the ISD MgO films. An orientation relationship of YBCO⟨100⟩∥MgO⟨111⟩ and YBCO⟨010⟩∥MgO⟨110⟩ was measured by x-ray pole figure analyses and confirmed by transmission electron microscopy. A T_c of 91 K with a sharp transition and transport J_c of 5.5 × 10⁵ A cm⁻² at 77 K in self-field were measured on a YBCO film that was 0.46 μm thick, 4 mm wide and 10 mm long.

The high-temperature superconducting magnetic bearing flywheel energy storage system (SMB-FESS) is proposed as an efficient energy storage system. It is important to identify the dynamic behaviour and the characteristics of the SMB-FESS. First, a new method for identifying SMB characteristics has been suggested. The suggested modelling method is verified by comparing the experimental and analytical frequency response functions. In this study, the analyses of critical speed and unbalance response are performed using the analytical model. The experimental test has been carried out to verify the result of simulation. A good agreement has been observed between the experiment and the simulation result.

We have investigated the electric resistance behaviour of Y₂BaCuO_x (Y211) sintered ceramics at about 80–500 °C. The resistivity of Y211 is dependent on the temperature and the oxygen partial pressure; it increases with the decrease of both temperature and oxygen partial pressure. The isothermal electric resistivity relaxation during an oxygen-uptake process at 300–500 °C was used to investigate the diffusion behaviour of oxygen in Y211. With certain assumptions, we estimated the chemical diffusion coefficient of 10⁻¹²–10⁻¹¹ cm² s⁻¹ and the diffusion activation energy of 0.38 eV at 300–500 °C.

The effect of precursor powder assemblage on the phase formation, crystallite orientation and transport critical current density of Bi2223 textured discs has been studied. We observe that starting powder composed of Bi2212 and secondary phases results in a stronger grain alignment than if composed of nearly pure Bi2223. SEM observations and XRD analyses show that the liquid phase produced during the sinter-forging step is essential to achieve sharp orientation distributions and high critical current densities.

We have studied the tuning properties of a high-temperature superconducting (HTS) half-wavelength coplanar waveguide (CPW) resonator operating at 5 GHz. The tuning schemes are based on flip-chip bonding of an electrically tunable ferroelectric (FE) thin film and a mechanically movable low-loss single crystal on top of the resonator. Using the conformal mapping method, closed-form analytical expressions have been derived for a flip-chip bonded conductor-backed and top-shielded CPW transmission line. The obtained expressions are used to analyse the volume effect of the FE thin film and the gap between the flip-chip and the CPW resonator on the tuning properties of the device. It has been found that large frequency modulation of the resonator produces impedance mismatch, which can considerably enhance the insertion loss of high-performance HTS microwave devices. Analysis also suggests that, for electrically tunable devices, flip-chip bonded FE thin films on HTS CPW devices provide a relatively higher performance compared to bilayer HTS/FE tunable devices.

Epitaxial YBa₂Cu₃O_7−x (YBCO) films with critical current densities exceeding 1 MA cm⁻² (at 77 K in self magnetic field) were epitaxially grown on biaxially textured Ni 5 at% W (Ni–W) substrate using only a CeO₂ buffer layer. Both YBCO and CeO₂ films were deposited in situ by pulsed laser ablation. The oxidation behaviour of the Ni–W substrate enables the epitaxial growth of a CeO₂ oxide buffer layer without using the forming gas (Ar 4%H₂). SEM investigations of YBCO and CeO₂ films reveal a dense microstructure without cracks or porosity. X-ray pole figures show a cube-on-cube epitaxy with a [100]YBCO//[110]CeO₂//[100]Ni–W epitaxial relationship. The YBCO superconducting film has a good out-of-plane and in-plane texture with a full-width-half-maximum of 5.8° and 6.6°, respectively. No severe degradation of J_C with film thickness, up to 1 μm thick films, was observed. The use of a single buffer layer on Ni–W substrates represents an important simplification in the scaling-up for long tape fabrication.

We propose a new microcalorimeter in which the critical current of a Josephson junction can be varied by an electron temperature in the normal metal barrier of the superconductor–normal metal–superconductor (SNS) or superconductor–normal metal–insulator–superconductor (SNIS) junctions. In this detector, a Josephson junction with a radiation absorber is included in a superconductor loop and the change of its critical current is converted into a change of magnetic flux in the loop. We estimated the energy resolution of this detector by calculating a noise equivalent power (NEP) of the detector. The estimated energy resolution and dynamic range are 4.2 eV/5.8 eV and 3.1 keV/6.2 keV, respectively with an Ag absorber of 500 × 500 × 2 μm³ at 100 mK.

A heat treatment method has been developed for the fabrication of ceramic superconductors using continuous microwave irradiation. By adjusting the sizes of the samples, isothermal heat treatment can be achieved as a result of thermal equilibrium between energy absorbed from microwaves and heat dissipated to the environment. Superconducting Bi₂Sr₂CaCu₂O_8+x (Bi2212) was made in a few hours. The mechanism of the method is explained by numerical calculation based on finite element analysis.

Three transition temperatures have been found in the M(T) curve in a lightly doped La₂CuO_4+δ sample. The high temperature transition around 239 K corresponds to the Néel transition. The low temperature transition appearing at 35 K is due to the superconducting transition. A weak but apparent transition appears at a medium temperature of about 110 K, which has been clearly observed in previous results, but has attracted no attention. Based on a theoretical model of phase separation, we argue that this weak transition originates from the condensation of ferromagnetic clusters induced by holes within the antiferromagnetic background.

Quantitative data on the diffusion between Nb and Ti are important for the processing of Nb–Ti superconductor wires through the 'artificial pinning centre–diffusion process'. In the literature, most of the Nb–Ti diffusion studies are focused on the 1173–1973 K temperature range which is inappropriate for this application. The objective of this study was to evaluate the diffusion between Nb and Ti at 1023 K and 1073 K, from a Nb–Ti cylindrical composite that has been mechanically deformed by swaging. It has been found that at both temperatures most of the diffusion layer is formed through the diffusion of Nb into Ti. A plot of diffusion layer thickness versus t^1/2 (t is the time) showed a linear behaviour for both temperatures with angular coefficients of 0.0867 μm s^−1/2 (1023 K) and 0.253 μm s^−1/2 (1073 K). Longer heat treatment and higher temperature leads to a Ti-rich diffusion layer, which is not interesting in terms of superconducting properties. After heat treatment at 1023 K for 150 h, the monofilament wire (0.610 mm) presented a critical temperature of 10 K and an upper critical field close to 6 T.

The current density J(ρ, z) in a disc-shaped superconducting bulk magnet and the magnetic levitation force F^SBM_z exerted on the superconducting bulk magnet by a cylindrical permanent magnet are calculated from first principles. The effect of the superconducting parameters of the superconducting bulk is taken into account by assuming the voltage–current law E = E_c(J/J_c)ⁿ and the material law B = μ₀H. The magnetic levitation force F^SBM_z is dominated by the remnant current density J'₂(ρ, z), which is induced by switching off the applied magnetizing field. High critical current density and flux creep exponent may increase the magnetic levitation force F^SBM_z. Large volume and high aspect ratio of the superconducting bulk can further enhance the magnetic levitation force F^SBM_z.

The third harmonic components of the ac susceptibility of MgB₂ bulk samples have been measured as a function of applied magnetic fields, together with standard magnetization cycles. The irreversibility line (IL) of the magnetic field has been extracted from the onset of the third harmonic components. Using a (1 − t)^α glass/liquid best fit where α = 1.27 IL shows a coherent length ξ divergence with exponent ν = 0.63, which indicates a 3D behaviour. Moreover, using the numerical solution of the non-linear magnetic diffusion equation, considering the creep model in a 3D vortex glass, a good description of the vortex dynamics has been obtained. The behaviour of the magnetization amplitude (∼Hz) and the ac susceptibility signals (kHz), at different applied magnetic fields, 3.5 T < H_dc < 4.5 T, and at the reduced temperature 0.86 < t < 0.93 (T = 22 K), shows that the superconducting dynamic response of vortices in the MgB₂ samples is not evidently dependent on the frequency.

The magnetoresistance of single-phase polycrystalline Gd(Ba_2−xPr_x)Cu₃O_7+δ samples has been studied within the thermally activated flux creep model. The decrease of pinning energy with applied magnetic field can be scaled to two power-law relations for magnetic fields, smaller and also larger than about 1 kOe. The derived H_c2(T), H_c2(0), and superconducting coherence length ξ show that the Pr doping, similar to weak links, decreases the vortex flux pinning energy. It is also concluded that the substitution of Pr at the Ba site has a more destructive effect on the flux dynamics than Pr at the rare-earth site.