Melt-textured YBa2Cu3O7 Research Articles

Measurement of the thermal expansion of melt-textured YBCO using optical fibre gratingsensors

In this paper we present measurements of the thermal expansion of melt-textured YBaCuO in thetemperature range 30–300 K by means of optical fibre sensors. The sample, which had a size of38 × 38 × 18 mm3, was prepared by our standard melt-texturing process using SmBaCuOseeds. One fibre containing three Bragg gratings which act as strainsensors was glued to the sample surface with two sensors parallel to theab-plane and onesensor parallel to the c-axis. The sample was cooled down to a minimum temperature of 30 K ina vacuum chamber using a closed cycle refrigerator. In the temperaturerange we used, the thermal expansion coefficients are in the range of(3–9) × 10−6 K−1 (ab-direction)and (5–13) × 10−6 K−1 (c-direction).

Preparation of melt-textured YBa2Cu3O7$minus;$delta; bulk with CeO2 additions and their superconducting properties

The c -axis oriented YBa 2 Cu 3 O 7− δ (Y123) bulk superconductors with up to 2.0 wt.% CeO 2 and 18 mm in diameter were prepared by the so-called MMTG process. The critical current density J C significantly increased with 1.0 or above wt.% CeO 2 additions. The sample with 2.0 wt.% CeO 2 additions exhibited a J C of 330,000 A/cm 2 at 35 K and 0 T, when the field was applied parallel to the c -axis of the sample. The maximum of the trapped magnetic field reached 4.2 T at 30 K at the center of the sample surface, which is more than 10 times larger than conventional Nd–Fe–B permanent magnets. Particles of a Ce–Ba compound, whose existence was identified from the observations using a scanning electron microscope, are not distributed uniformly in the Y123 matrix. This high trapped magnetic field of the sample with 2.0 wt.% CeO 2 additions can be explained in terms of a more homogeneous distribution of fine Y 2 BaCuO 5 (Y211) particles in the Y123 matrix compared with the CeO 2 -free sample.

Temperature dependence of vortex flux pinning in melt‐textured superconductors

Magnetic properties of melt-textured YBa2Cu3O7 superconductor are studied at different temperatures from 77 K up to the critical temperature, TC = 91 K, using vibrating sample magnetometer. Initial magnetization curve is measured from the demagnetized state in an applied magnetic field from 0 to 13.5 kOe, followed by measuring the full hysteresis loop. This procedure is repeated at different temperatures between 77 and 91 K. In all the measurements, the magnetic field was applied along the ab-plane of the sample which is perpendicular to the c-axis. Within the framework of the Bean critical state model, our results show that the maximum pinning forces which are represented by the magnetization remanence at which the internal field is zero are found to decrease with increasing temperature and they obey the empirical scale law ΔM(emu/cm3) = 34 × (1 − T/TC )1/n, where n ≈ 2.9. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Applicability of the power-law current–voltage characteristics to melt-textured YBa2Cu3O7−δ superconductors

A simple method is proposed for defining the parameters of current–voltage characteristics (CVCs) of hard superconductors, which is based on measuring the relaxation of magnetization. In this method, the inhomogeneity of the spatial distribution of the electric field and current density is taken into account. The limits of applicability of the power-law CVC are evaluated by comparing the experimental data with the results of numerical calculations.

Inductive measurements of third-harmonic voltage and critical current density in bulk superconductors

We propose an inductive method to measure critical current density Jc in bulk superconductors. In this method, an ac magnetic field is generated by a drive current I0 flowing in a small coil mounted just above the flat surface of superconductors, and the third-harmonic voltage V3 induced in the coil is detected. We present theoretical calculation based on the critical state model for the ac response of bulk superconductors, and we show that the third-harmonic voltage detected in the inductive measurements is expressed as V3=G3ωI02/Jc, where ω/2π is the frequency of the drive current, and G3 is a factor determined by the configuration of the coil. We measured the I0–V3 curves of a melt-textured YBa2Cu3O7−δ bulk sample, and evaluated the Jc by using the theoretical results.

Intergrain and intragrain currents in bulk melt-grownYBa2Cu3O7−δrings

A simple contactless method suitable for discerning between the intergrain (circular) current, which flows in the thin superconducting ring, and the intragrain current, which does not cross the weakest link, has been proposed. At first, we show that the intergrain current may directly be estimated from the magnetic flux density $B(\ifmmode\pm\else\textpm\fi{}{z}_{0})$ measured by the Hall sensor positioned in the special points $\ifmmode\pm\else\textpm\fi{}{z}_{0}$ above or below the ring center. The experimental and numerical techniques to determine the value ${z}_{0}$ are discussed. Being very promising for the characterization of a current flowing across the joints in welded YBaCuO rings (its dependences on the temperature and external magnetic field as well as the time dissipation), the approach has been applied to study the corresponding properties of the intragrain and intergrain currents flowing across the a-twisted grain boundaries which are frequent in bulk melt-textured (MT) YBaCuO samples. We present experimental data related to the flux penetration inside a bore of MT YBaCuO rings both in the nonmagnetized, virgin state, and during the field reversal. The shielding properties and their dependence on external magnetic fields are also studied. Besides, we consider flux creep effects and their influence on the current redistribution during a dwell.

Analysis of forced magnet oscillations above melt-textured YBa2Cu3O7: influence of excitation amplitude and superconductor temperature

The forced oscillations of a permanent magnet levitating over a superconducting sample are studied for different magnitudes of the external force. Results are discussed in terms of the one-dimensional model of a forced damped oscillator. It is found that there are two regimes, characterized by sudden and hysteretic changes in the waveform, frequency of resonance, and amplitude of oscillation. These changes are associated with the process of pinning/depinning of the vortex lattice, that produces also two different dependences of the amplitude of oscillation with temperature. On the other hand, it was demonstrated that it is possible to accurately monitor the superconducting transition by measuring the resonance peak changes with temperature.

The man-loading high-temperature superconducting maglev test vehicle

The first man-loading high-temperature superconducting (HTS) Maglev test vehicle in the world was successfully developed on Dec. 31, 2000 in the Applied Superconductivity Laboratory, Southwest Jiaotong University, China. Heretofore over 24 500 passengers took the vehicle, and it has been operating back and forth for about 400 km. The HTS Maglev vehicle provides inherent stability both in the vertical and lateral direction, so no control system is needed. The only control system is used for linear motor driving devices. The melt-textured YBaCuO bulk superconductors are fixed on the bottom of a liquid nitrogen vessel and cooled by liquid nitrogen. The bottom thickness of the rectangle liquid nitrogen vessel with its thin wall is only 3 mm. The onboard HTS Maglev module is placed over the guideway. The guideway consists of two parallel permanent magnetic tracks, whose concentrating magnetic field at 20 mm height above the surface is about 0.5 T. The levitation forces of 8 HTS Maglev modules were measured. The total levitation force of 8 onboard Maglev modules was 10431 N at the levitation gap of 10 mm, and 8486 N at the levitation gap of 15 mm, respectively. These results were measured on May 28, 2002.

Effect of mesoscopic inhomogeneities on the critical current of bulk melt-textured YBCO

The downsizing 211-inclusions and an increase of their density leads to rise in mean critical current value in Y-based melt-textured material. Very often 211-inclusion are spread in the material volume non-homogeneous, with typical scale 50–100 μm. Therefore it is difficult to find the real correlation between local critical current and the inclusions distribution. We performed a study of a local critical current using modified magneto-optic technique on a melt-textured YBaCuO ceramic, found the areas with constant current and studied the real structure of the material in the areas, inclusions distribution and their sizes, by scanning electron microscopy and X-ray microanalysis. The estimation of a pinning in these places, by taking into account the amount of inclusions and the length of their boundaries, and comparison with the value of local critical current reveals a strait correlation between the density of inclusions and the current but shows remarkable quantitative disagreement.

Melt-textured YBa2Cu3O7 based material doped with Li and Zn: Comparison of high trapped fields and pinning

The pinning forces and trapped fields obtained in bulk YBa2Cu3O7 (YBCO)-based material doped with Li are compared with results reported for melt-processed samples doped with Zn. The increase of the volume pinning force of Li doped material is twice that of Zn doped YBCO at 77 K. This leads to 20% higher magnetic fields trapped in the Li-doped sample at 77 K.

Barium Holmium Zirconate, A New Complex Perovskite Oxide: I, Synthesis, Characterization, and Potential Use as a Substrate for High‐Critical‐Temperature Superconductors

Barium holmium zirconate, a new complex perovskite ceramic oxide, has been synthesized through liquid‐phase sintering for the first time. The conventional solid‐state reaction method using constituent oxides and carbonates was found to be inadequate for the synthesis of Ba2HoZrO5.5 material. During high‐temperature annealing, the development of stable BaZrO3 and BaHoO2.5 phases prevented the formation of Ba2HoZrO5.5 as a single‐phase material, even at 1650°C. However, an addition of a small amount of CuO (1 wt%) in the reaction mixture has resulted in the formation of an ordered complex perovskite Ba2HoZrO5.5 phase during the heating process. The structure of Ba2HoZrO5.5 was studied by X‐ray diffraction and found to have a cubic perovskite structure with a lattice constant of a= 8.482 Å. Dielectric constant and loss factor values of Ba2HoZrO5.5 are also in the range suitable for use as a substrate for microwave applications. The X‐ray diffraction and resistivity measurements have shown that there is no detectable chemical reaction in YBa2Cu3O7−δ–Ba2HoZrO5.5 and Bi(2212)–Ba2HoZrO5.5 composites, even under extreme processing conditions. Dip‐coated and melt‐textured YBa2Cu3O7−δ and Bi(2212) thick films developed on polycrystalline Ba2HoZrO5.5 gave zero‐resistivity transition temperatures of Tc(0) = 92 and 85 K, respectively.

The first man-loading high temperature superconducting Maglev test vehicle in the world

The first man-loading high temperature superconducting Maglev test vehicle in the world is reported. This vehicle was first tested successfully on December 31, 2000 in the Applied Superconductivity Laboratory, Southwest Jiaotong University, China. Heretofore over 17,000 passengers took the vehicle, and it operates very well from beginning to now. The function of suspension is separated from one of propulsion. The high temperature superconducting Maglev provides inherent stable forces both in the levitation and in the guidance direction. The vehicle is 3.5 m long, 1.2 m wide, and 0.8 m high. When five people stand on vehicle and the total weight is 530 kg, the net levitation gap is more than 20 mm. The whole vehicle system includes three parts, vehicle body, guideway and controlling system. The high temperature superconducting Maglev equipment on board is the most important for the system. The onboard superconductors are melt-textured YBaCuO bulks. The superconductors are fixed on the bottom of liquid nitrogen vessels and cooled by liquid nitrogen. The guideway consists of two parallel permanent magnetic tracks, whose surface concentrating magnetic field is up to 1.2 T. The guideway is 15.5 m long.

Observation of a novel stacking-fault nucleation mechanism in melt-textured YBa2Cu3O7

The dislocation substructure of melt-textured YBa2Cu3O7-Y2BaCuO5 composites deformed under uniaxial compressive stress and a superposed gaseous confining pressure has been investigated by transmission electron microscopy. Observations reveal the generation of perfect dislocations in (001) as well as out of this plane. Screw segments of the latter lying on (001) are seen to act as Frank-Read sources for ⅙ ⟨301⟩ partials bounding YBa2Cu4O8-type stacking faults, according to a mechanism previously anticipated from theoretical considerations by one of the present authors.

Comparison of pulsed magnetization processes for HTS bulk parts

With respect to the potential application as cryo-permanent magnets, melt-textured YBaCuO bulk parts were used to investigate various pulsed magnetization processes employing pulsed copper coils. Results for different multi-pulse processes including different peak fields, pulse durations, temperatures and geometries are compared. The strong current-carrying capabilities of HTS material, even for currents above the critical current Jc, result in strong shielding effects, in particular for short pulse durations. It turns out that the inhomogeneities present in all real materials play an important role for the initial magnetic flux penetration. To fully magnetize the bulk samples, the non-stationary conditions and geometrical effects require peak fields, which substantially exceed twice the saturation magnetization measured at the sample surface. Transient magnetizations as well as shapes and absolute values of trapped flux profiles are reported.

Growth-induced inhomogeneities in bulk melt-textured YBaCuO crystals

The homogeneity of a large YBa 2Cu 3O 7− δ (YBCO) sample fabricated by top-seeded melt-growth has been studied in details. It was particularly shown how structural imperfections and superconducting properties are distributed along the growth front. These distributions are hard to explain within the flat a-growth front suggestion. These are better fitted by the growth scenario when the a-growth fronts seem concave and consist of small growth steps.

Effect of the addition of depleted UO 2 on the microstructure of melt processed Y–Ba–Cu–O superconductors

We have investigated the effect of the addition of depleted UO 2 to the microstructure of undoped large grain Y–Ba–Cu–O (YBCO) superconductors and those doped with Pt. It is found that the addition of UO 2 to YBCO promotes the formation of fine uranium containing, non-Y 2BaCuO 5 (Y-211), second phase particles in the YBa 2Cu 3O 7− δ (Y-123) matrix. These particles are of approximately spherical morphology and they are typically a few hundred nanometers in diameter. In addition, Pt has no significant effect on refining the second phase in UO 2 containing samples, in contrast with UO 2-free Y-123. Further and uniformly distributed second phase particles are observed in uranium doped melt textured YBCO for the first time by the addition of Y 2O 3, rather than Y-211 to the precursor powders.

Dislocation substructures and phase stability of melt-textured YBa2Cu3O7−Y2BaCuO5composites at high oxygen pressures

Abstract The evolution of the microstructure of melt-textured YBa2Cu3O7−Y2BaCuO5 (123–211) composites in O2 at 100 bar and 600°C has been characterized by transmission electron microscopy (TEM). Under these conditions the 123 phase is brought out of its stability field, within a region of the phase diagram where the weakly superconducting YBa2Cu4O8 (124) polytype is stable against a CuO + 123 mixture. The nucleation of 1/6 (301) stacking faults is not restricted to the 211–123 interfaces, as found in samples processed at 350 and 450°C but is strongly enhanced in the bulk matrix. Simultaneously, matrix decomposition into a Ba-Cu oxide, 211 and presumably CuO occurs at 211 interfaces, which is accompanied by an exaggerated growth of the stacking faults. After 12 h at PO2 = 100 bar and 600°C, the whole volume imaged by TEM appears to be covered by large stacking faults overlapping along the c axis. It is found that, when the 211 interfaces become saturated by stacking faults, new stacking faults have to nucl...

HighTC Superconductors Give New Impulse to Magnetic Levitation

When the new YBaCuO superconductors were developed about 18 years ago, the critical maximum current density, at a magnetic field of 1.0 T and a temperature of 77 K, was still quite low. Roughly 8 years ago, it became possible to raise the critical current density to 106 A/cm2 at 1.0 T and 77 K. This opened up possibilities for application in self-stabilizing superconducting magnetic bearings and also in maglev trains capable of movement along a track. An important criterion in the high current-carrying capacity of YBaCuO superconductors is the excellent texture of melt-textured YBaCuO moldings, which must also be free of microcracks. Narrow distributions of the a-, b-, and c-axis orientations, in the absence of isotropic fractions, allow almost the entire mirror flux in the superconductor to be induced by permanent magnets.

Mechanical and dielectric dissipation related to phase transitions

(1) First Order Phase Transition There always appear two dielectric loss and/or two mechanical dissipation peaks (P2 and P3 ) related to domain walls in many martensitic and ferroic materials. The P2 near Tc (or Ms ) originates from first-order phase transition or phase-like transition and shows static hysteresis character, but in a certain frequency range (kHz) manifests relaxation behavior, with t—t 0exp[− B/(Tc—T)]. The transient componant of P2 is also reviewed. (2) Glass Transition The peak P 3 at kHz often appears near a freezing temperature Tf as in KDP, TGS and occasionally in Mn-Cu at Hz, with T = T 0exp[U/(T- Tf)], a Vogel-Fulcher relation, showing a “Transition” type of glass-transition of domain wall. Whereas the glass-transition in C60 crystal obeys the Arrhenius relation, exibiting “Kinetic” type of glass-transition. (3) Flux Melting The peak just below Tc was measured for melt-textured YBa2Cu3O7 +δ superconductors in an applied magnetic field and various frequencies. The peak hight satisfies the power law Q −1 ∝ ω −n (1 < n < 2), being explained by the melting and viscous motion of flux.

Melt-textured YBaCuO with High Trapped Fields up to 1.3 T at 77 K

CeO2-doped YBaCuO monoliths synthesized with a top-seeded melt growth process in a conventional box furnace exhibited values of trapped magnetic field of up to 1.33 T at 77 K. To our knowledge, this is the highest value of trapped field reported for a melt-textured YBaCuO monolith. A suitable temperature profile and the use of high-density Y2BaCuO5 substrates led to reproducible single-domain crack-free samples investigated by optical and scanning electron microscopy and trapped field measurements. The zero-field-cooled levitation forces at 77 K of standard samples amounted to 70–83 N. A transport critical current density of up to 1.3 × 105 A/cm2 in self field at 77 K was obtained.