Timing Qu

Measurement and Calculation of Residual Magnetic Field in a Bi2223/Ag Magnet

Residual magnetic field in a high temperature superconductor (HTS) magnet is a factor which should be avoided when the field of the magnet is being operated under a high uniformity and high accurate control. In this paper, the residual field in a Bi2223/Ag pancake magnet at 77 K is systematically studied. The amplitude of the residual field as a function of maximum charging current and the relaxation of the residual field are measured. For a magnet with an inner diameter of 60 mm, an outer diameter of 80 mm and a height of 30 mm, the maximum residual field is about 14.5 G after ramping the charging current up to 15 A, followed by dropping the current to zero. A simple model is proposed, from which the residual field can be estimated as along as the configurations of the magnet and the critical current (Ic) of the original short sample are given.

A rapid process of YBa2Cu3O7−δ thin film fabrication using trifluoroacetate metal–organic deposition with polyethylene glycol additive

Trifluoroacetate metal–organic deposition (TFA-MOD) is a promising technique to fabricate YBa2Cu3O7−δ (YBCO) superconducting films. However, its slow pyrolysis process, which usually takes more than 10 h, constitutes a barrier for industrial production. In this study, polyethylene glycol (PEG) was utilized to reduce the stress generation inside the coated films when the strong pyrolysis reactions happen. With the addition of 30 wt% PEG2000 to the precursor solution, a smooth film surface could be obtained through a rapid pyrolysis process of 15 min. After the optimizations of the crystallization and oxygenation processes, mass percentage and molecular weight of PEG additive, YBCO thin films with Jc of about 4.5 MA cm−2 (77 K, self-field) could be routinely fabricated using (20–30) wt% PEG(1000–2000) additive with a total treatment time of about 2 h including the 15 min pyrolysis process time. The effects of PEG additive were discussed using one of the mechanisms of buckling formation. The reduction of compressive stress by PEG additive was suggested to be the reason for preventing buckling.

Development and testing of a 2.5?kW synchronous generator with a high temperature superconducting stator and permanent magnet rotor

High temperature superconducting (HTS) armature windings have the potential for increasing the electric loading of a synchronous generator due to their high current transport capacity, which could increase the power density of an HTS rotating machine. In this work, a novel synchronous generator prototype with an HTS stator and permanent magnet rotor has been developed. It has a basic structure of four poles and six slots. The armature winding was constructed from six double-pancake race-track coils with 44 turns each. It was designed to deliver 2.5 kW at 300 rpm. A concentrated winding configuration was proposed, to prevent interference at the ends of adjacent HTS coils. The HTS stator was pressure mounted into a hollow Dewar cooled with liquid nitrogen. The whole stator could be cooled down to around 82 K by conduction cooling. In the preliminary testing, the machine worked properly and could deliver 1.8 kW power when the armature current was 14.4 A. Ic for the HTS coils was found to be suppressed due to the influence of the temperature and the leakage field.

Phase evolution of lead oxide compounds corresponding with oxygen release and absorption processes of BSCCO powders during heat treatment

The phase evolution of lead oxide compounds in (Bi,Pb)2Sr2Ca2Cu3Ox (BSCCO) precursor powder during heat treatment was studied. The BSCCO powder oxygen release and absorption processes occurring during heat treatment were investigated by using an oxygen analyser. The experimental results show that the oxygen release and absorption processes are related to evolution of the lead oxide compounds in the BSCCO powder. Oxygen absorption may result from the segregation of Pb from the (Bi,Pb)-2212 phase as well as the formation of the Pb3Sr2.5Bi0.5Ca2CuOy (3321) and Ca2PbO4 phases. Oxygen release could be related to decomposition of the 3321 and Ca2PbO4 phases as well as incorporation of Pb into the (Bi,Pb)-2212 phase. The formation and decomposition of the 3321 and Ca2PbO4 phases, which depend on temperature and oxygen partial pressure, have also been identified.

A low-fluorine solution with a 2:1 F/Ba mole ratio for the fabrication of YBCO films

In previously reported low-fluorine MOD-YBCO studies, the lowest F/Ba mole ratio of the precursor solution was 4.5. Further lowering the F/Ba ratio would bring benefits for the environment, thick film deposition, and an understanding of the heat treatment process. On the other hand, the F/Ba ratio must be at least 2 for full conversion of the Ba-precursor to BaF2 to avoid the formation of BaCO3, which is detrimental to the superconducting performance of YBCO films. In this study, a solution with a 2:1 F/Ba mole ratio was developed, and the fluorine content of this solution was approximately only 10.3% of that used in the conventional TFA-MOD method. Attenuated total reflectance-Fourier transform-infrared spectra (ATR-FT-IR) revealed that BaCO3 was remarkably suppressed in the as-pyrolyzed film?and eliminated at 700??C. Thus, YBCO films with a critical current density (Jc) of over 5?MA?cm?2 (77?K, 0?T, 200?nm thickness) could be obtained on lanthanum aluminate single-crystal substrates. In?situ FT-IR spectra showed that no obvious fluorinated gaseous by-products were detected in the pyrolysis step, which indicated that all F atoms might remain in the film as fluorides. X-ray diffraction ?/2? scans showed the presence of BaF2?but not of Y F3 or CuF2?in films quenched at 400?800??C. The formation priority of BaF2 over Y F3 and CuF2 was interpreted by examining the chemical equilibrium of the potential reactions. Our study could enlarge the synthesis window of precursor solutions for MOD-YBCO fabrication, and serve as a foundation for continuously and systematically studying the influence of fluorine content in the precursor solutions.

Simulation of AC Loss in Small HTS Coils With Iron Core

AC loss is one of the key problems in HTS applications. This study focuses on the AC loss in small Bi-2223/Ag and YBCO coils, with an emphasis on the presence of an iron core. HTS coils with an iron core can be used in a number of applications, including induction heaters, motors, fault current limiters, controllable reactors and transformers. The major concern of the study is the interaction between the iron core and the HTS coil. This study first provides a brief introduction of the resistivity-adaption algorithm (RAA) method, which is used to simulate the field penetration, based on a given flux motion theory. A general model is then proposed, with a discussion on the effect of the iron core. Results showed that a well-designed iron core is beneficial for the AC loss reduction.

AC Losses in HTS Tapes and Devices With Transport Current Solved Through the Resistivity-Adaption Algorithm

Alternating current (ac) losses in high-temperature superconductor tapes and devices with transport current are solved by using the resistivity-adaption algorithm (RAA). The most advanced feature of the RAA is that it enables the simulation of any model derived from the flux motion theory on finite-element analysis (FEA) packages that have an eddy current solver. The principle of the RAA, as well as its realization on the ANSYS FEA package, is introduced. The simulation begins with the calculation of the ac loss of an ellipse and of strips with aspect ratios ranging from 50 to 2000. The accuracy and efficiency of the calculation are verified through comparisons with the Norris theoretical curves. The possible errors and the method to overcome such errors are discussed. The most significant improvement in the proposed RAA from that discussed in a previous study is that the RAA was proven to be valid for calculating the field-dependent critical state model by using the descendant process from +Im to -Im. We then extend this method to calculate the transport ac loss of a stack of ellipses with Jc(B) characteristic from a typical Bi2223/Ag tape and the transport ac loss of a stack of strips with Jc(B) characteristic from a typical YBCO-coated conductor.

Method and Apparatus for Continuous

A new method based on the principle of magnetic circuits is proposed and realized for continuous <i>I</i>_c examination of HTS tapes. The greatest advantages of the new method are that it first eliminates all the noise caused by mechanical fluctuations, and thus makes high speed and high stability measurement possible, and second has a natural ability to measure HTS tape with a magnetic substrate. The principle of the method is introduced with the help of Finite Element Analysis. An apparatus for examination of kilometer long tapes has been constructed, by which continuous <i>I</i>_c examination for a YBa₂Cu₃O_7-<i>x</i> tape with and without a magnetic substrate and a Bi₂Si₂Ca₂Cu₃O<i>x</i> multi-filamentary tape is reported.

I_{\rm c}

A method based on the principle of the magnetic circuit is proposed and realized for contactless measurement of critical current (I(c)) of high temperature superconductor tapes. This method has two unique features: first, it eliminates noises caused by mechanical fluctuations and thus makes high speed and high stability measurement possible and second, adapts for both Bi(2)Si(2)Ca(2)Cu(3)O(x) (Bi2223) and YBa(2)Cu(3)O(7-x) (YBCO) tape, which even has a magnetic substrate. Theoretical analysis is given and an apparatus for the reel-to-reel measurement has been constructed, by which continuous inspection of I(c) uniformity of YBCO and Bi2223 tapes measured at different speeds is reported.

Examination of HTS Tape Using Magnetic Circuit

Ferromagnetic material was electroplated onto the surface of the Bi2223/Ag multi-filamentary tape and as a result changed the Ic(B) characteristic of the tape correspondingly. A numerical simulation was used to investigate the influence of ferromagnetic shielding (FS) on the performance of the tape, in particular the Ic behaviour under perpendicular external fields. Using finite element analysis, we are able to understand how FS alters the flux distribution within the superconductor region under any arbitrary shielding structure. The shielding width, thickness and nonlinear property of the ferromagnetic material were taken into account for the simulation. Finally, optimized shielding parameters in association with different operating fields were suggested and these values could be considered for the next run of experimental work.