Wu Ming Chen

Preparation and magnetocaloric effect of self-doped La0.8?xNa0.2xMnO3+? ( = vacancies) polycrystal

Single-phase perovskite-type manganese oxides La0.8 −xNa0.2xMnO3+δ with x = 0.01–0.04 have been prepared by the modified sol–gel method and the influence of the vacancy concentration in the A site on their structures and magnetocaloric properties has systematically been investigated. The experimental results revealed that the samples calcinated at 1573 K remained single phase and were constituted by submicrometre homogeneous particles without deviation from the nominal stoichiometry upon heating. For all the compositions explored in this work, the average manganese oxidation state is practically constant, at 3.37 ± 0.02. Magnetic measurement results show that an appropriate amount of vacancy concentration in the A site is favourable to both the Curie temperature and magnetic entropy change for polycrystalline La0.8−xNa0.2xMnO3+δ compounds. An approximately linear relation between Curie temperature and magnetic entropy change was observed. For La0.771Na0.198Mn1.000 0.029O2.94, the largest value of magnetic entropy change of 3.86 J kg−1 K−1 under a field of 10 kOe and the Curie temperature of 364 K were obtained.

Mechanochemical effects of grinding on the YBa2Cu3Ox superconductor

Abstract The mechanical grinding effects on the powders of YBa 2 Cu 3 O x superconductor are examined. The grain size, the oxygen content of the powders decrease, and the content of the amorphous phase, the crystal strain increase, with the increasing of grinding time. Except that for amorphous phase, the crystallized phase can be refined by the orthorhombic and tetragonal phases. The refined results show that the diffraction peaks broaden after continuous grinding. This broadening can be explained from the crystal strain and the small grain size due to prolonged grinding. The lattice constants of the refined phases vary with the grinding time.

Iodometric titration for determining the oxygen content of samples doped with Fe and Co

Abstract Double iodometric titration is a method developed for high-Tc superconductivity. It has been widely used to determine the effective oxidation of copper and to figure out the oxygen content in a sample in the past years. If a sample is doped with some metals like iron and cobalt, these metal ions can interfere in experiment results. This is a major defect for iodometric titration. To solve this problem, we introduce ammonium hydrogen fluoride as a complex agent into the method. The agent can prevent the interference of ions of iron and cobalt. For samples LaBa2Cu3−xFexOy and YBa2Cu3−xCoxOy, we determine the oxygen content by an improved double iodometric titration in which the ammonium hydrogen fluoride is used as an interference-free agent. For samples doped with a small amount of Fe or Co, experiment results agree well with those of other researchers. As to samples doped with a large amount of Fe or Co, we determine their values of oxygen content and the results are reasonable.

A new calculating formula of iodometric titration for high Tc superconductors

Abstract In order to determine the effective oxidation state of Cu (p) and oxygen content (y) with the iodometric method, a new formula for calculating p and y of high Tc copper oxides is presented here. The formula of p involves a corrective term of 8CV1/M1, which reduces a systematic calculating error. With this approach, for YBCO, the value of p of YBCO is improved by 5% and y by 0.02, as compared with the conventional method.

Effect of various mechanical deformation techniques on critical current densities of Ag/Bi-2223 tapes

Abstract We have systematically studied the reduction rate dependence of critical current densities J c of Ag/Bi-2223 tapes using three methods, including normal rolling, pressing and “sandwich” rolling. Experimental results show that J c dependence on the reduction rate for the three processes follows the same law. When the reduction rate increases, J c initially increases, reaches a maximum and then reduces. The core density-reduction rate follows a corresponding trend. However, the maximum J c of the Ag/Bi-2223 tapes made using “sandwich” rolling is 35% higher than for those made using normal rolling even though the tapes in both cases have the same core density. This indicates that “sandwich” rolling reduces the cracks along the transverse direction in the tape in comparison with normal rolling.

Transmission electron microscopy evidence for phase transformation from Bi2Sr2CuO6 to Bi2Sr2Ca2Cu3O10

The growth mechanism of the Bi-2223 phase was studied using multifilamentary Ag/Bi-2223 tapes manufactured by the powder-in-tube method. X-ray powder diffraction was performed to analyze the developments of Bi-2201, Bi-2212, and Bi-2223 phases during sintering periods at high temperatures. Transmission electron microscopy (TEM) was used to investigate the growth mechanism of the 2223 phase. TEM images indicated that 2201 and Ca2CuO3 crystals come together to form platelets of 2223. The structural phase transformation established a structural frame of the 2223 crystal. There were some vacant defects retaining in the established 2223 crystal cells at copper and oxygen sites after the phase transformation, and the Cu/O compound at high temperatures would diffuse into 2223 cells and occupy the corresponding vacancies to complete the 2223 crystal structure. The present work provides an evidence for growing the 2223 phase from the 2201 phase and Ca2CuO3 in the samples studied.

Effects of low-energy neutron irradiation on Bi-based superconductors

Abstract Bi-based and Sn-doped Bi-based superconductors were subjected to low-energy neutron irradiation. Experimental results showed that J c of irradiated Bi-based sample was increased by 30%, and T c of irradiated Bi-based and Sn-doped Bi-based samples were increased by 2.5–5.0 K. The low-energy neutron irradiation was also found to decrease the normal state resistivity of samples. Therefore, low-energy neutron irradiation is a useful method to improve the superconductivity of Bi-based materials. The reasons for the improvement of T c , J c and normal state resistivity by low energy neutron irradiation were discussed.

A modified function of the formula of double iodometric titration

Abstract Double iodometric titration is the most useful method to determine oxygen content of high Tc superconductors. Its traditional calculating formula is not accurate enough, and a new calculating formula has just been put forward. To understand which relationship of the traditional formula depends on the new one is an important problem. On studying the new formula, we find a modified function F relating the traditional formula to the new one, and also find that the value of F depends only on molar weight of the superconductor. For Y, Bi, and Hg systems, a group of curves of the modified function are put forward here. Since many results about average copper valence and oxygen content have been published, to correct those results one can modify them by means of the corresponding F. As a sample, we modify two published data of average copper valence and oxygen content of Y and Bi systems. After comparing the unmodified with the modified data, we find that it is necessary to modify the published data for quantitative research.

Improved uniformity of microstructure and electrical properties of Bi-2223/Ag superconducting tapes

Abstract A special mechanical deformation technique, ‘sandwich’ rolling (SR), together with the conventional rolling technique, normal rolling (NR), was used to intermediately deform Bi-2223/Ag superconducting tapes. The tapes processed by these two techniques were compared in terms of phase composition, microstructure, critical temperature ( T c ) and critical current density ( J c ). It was found that while there was no difference in phase composition between the two types of tapes, density, degree of grain alignment and J c were considerably higher in the SR tapes than in the NR tapes. In particular, the uniformity of microstructure, T c , and J c between the filaments across the width of the tapes were significantly improved by the SR compared to the NR technique. The results show that in terms of mechanical effects on Bi-2223/Ag tapes SR is more like uniaxial pressing, rather than conventional rolling.

Simulation of the phase diagram of magnetic vortices in two-dimensional superconductors: evidence for vortex chain formation

We study the superconducting vortex states induced by the interplay of long-range Pearl repulsion and short-range intervortex attraction using Langevin dynamics simulations. We show that at low temperatures the vortices form an ordered Abrikosov lattice both in low and high fields. The vortices show distinctive modulated structures at intermediate fields depending on the effective intervortex attraction: ordered vortex chain and kagome-like vortex structures for weak attraction; bubble, stripe and antibubble lattices for strong attraction. Moreover, in the regime of the chain state, the vortices display structural transitions from chain to labyrinthine (or disordered chain) and/or to disordered states depending on the strength of the disorder.