X.N. Xu

Evidence of spin-density-wave transition and enhanced thermoelectric properties in Ca3−xCexCo4O9+δ

We have investigated the effect of Ce doping on the magnetic and low-temperature thermoelectric properties of Ca3Co4O9+δ. A small shoulder occurring at about 23 K in the dχ−1/dT curve indicates an occurrence of spin-density-wave transition, which is confirmed by resistance characteristics. With partial Ce substitution, the thermopower increases while the thermal conductivity decreases. The magnetic measurements give a support to the concentration reduction in Co4+ ions, which results in the enhancement of thermopower. The dimensionless figure of merit ZT=0.016 is achieved at 335 K for Ca2.9Ce0.1Co4O9+δ, demonstrating that heavy atoms Ce doping may promise an effective way for improving thermoelectric properties of Ca3Co4O9+δ system.

Studies of strong magnetic field produced by permanent magnet array for magnetic refrigeration

The success of a room temperature magnetic refrigerator (RTMR) depends critically on two essential parts: a high magnetic field and a magnetic refrigerant material with large magnetocaloric effect. A carefully designed hollow cylindrical permanent magnet array (HCPMA) can be used to provide strong magnetic field in the cavity, the magnitude of the resulting static field can be even greater than the remanence magnetization of the magnets comprising a HCPMA. A thorough understanding of the magnetic field distribution will provide an invaluable insight into the design and optimization of HCPMA in the reciprocating and rotary RTMR systems. Here, we show a construction of a 16 piece HCPMA with realistic dimensions and we illustrate the mechanism of generating a high magnetic field in such device. We present an effective way to calculate the field distribution of a permanent magnet array with finite size and an unsymmetrical geometry. Furthermore, detailed numerical results of the magnetic field distribution an...

HgBa2CaCu2Oy superconducting thin films prepared by laser ablation

Abstract HgBa 2 CaCu 2 O y superconducting thin films were prepared on SrTiO 3 (100) and LaAlO 3 (100) substrates using laser ablation and post-Hg-vapor annealing. X-ray diffraction and electromagnetic measurements show that these films are dominated by c -axis-oriented Hg-1212 phase. For the Hg-1212/SrTiO 3 sample, a zero-resitance superconducting transition temperature ( T c0 ) of 110 K and critical current density ( J c ) of up to 10 6 A/cm 2 at 77 K and zero field have been obtained. The Hg-1212/LaAlO 3 sample exhibits slightly lower T c0 ) ( = 100 K) and J c (= 2 × 10 5 A/cm 2 ). Our investigations suggest that by using superconducting HgBaCaCuO target mercury can be incorporated into the films easily during post annealing.

Characterization of Hg-Ba-Ca-Cu-O 1223 superconductor

Abstract The HgBaCaCuO 1223 superconductors have been characterized in this work. XRD and SEM analyses indicate that Hg-Ba2Ca2Cu3O8+δ is the major superconducting phase in the shape of a bar or plate in the samples. The transition temperature Tc of one sample is 129 K by resitivity, AC and DC susceptibility measurements. Magnetic measurements including the magnetic critical current density and magnetization relaxations indicate that a strong thermally activated flux motion also exists in the Hg-based system of high-temperature superconductors.

Improved Jc properties and microstructure in Na-doped MTG-YBCO crystals

The effect of Na2C2O4 addition on the microstructure, magnetization and dc transport properties of melted-textured-growth (MTG) YBa2?xNaxCu3Oy + 40 mol% Y2BaCuO5 crystals with x = 0, 0.1 and 0.2 has been investigated. X-ray diffraction patterns reveal that the length of the c-axis increases with increasing the Na-doped concentration and that these crystals are in single phase except for very weak peak of Y211. Composition analyses indicate the evidence of Na+ ion existing in the Na-doped samples and the part Y3+ or Ba2+ ion being replaced by Na+ ion. Magnetization curves at 77 K show an improved field dependence of the critical current density and a second peak around 0.8?1 T for Na-doped YBCO crystals. DC transport measurement gives a shift of the irreversibility line to lower temperatures and a larger anisotropy for the Na-doped YBCO crystal. The possible source of these behaviours were supposed to be connected with the certain inhomogeneous distribution of Na (different x) in MTG YB(Na)CO crystal, which leads to a distribution of different Tc clusters.

Microstructure, magnetization and dc transport properties of MTG-YBa1.8 Na0.2Cu3Oy crystal

The Na-doping MTG-YBCO crystal with the nominal composition YBa1.8Na0.2Cu3Oy + 40?mol% Y211 has been successfully synthesized by the melted-textured-growth (MTG) method with top seed. The microstructure photographs show that the crystal has a good layered structure with a number of twins. There is a second peak in the magnetization curve for the magnetic field parallel to the c-axis and at 77?K. The field of zero resistance temperature Tc0 shows a power law, H = H0 (Tc0(0)-Tc0(H))n with n = 1.52 for H?ab and n = 1.41 for H?c, respectively. The thermally activated flux creep model can scale the dissipative resistivity below the peak temperature Tp. The temperature and field dependences of the effective activation energy can be described by the formula U = U0(1-T/Tp)mH-?, where m = 1.75, ? = 0.75 for H?ab and m = 2.5, ? = 1.52 for H?c, respectively. The effect of Na-doping induced disorder on flux pinning has been discussed.

Self-modification of the remanence distribution of a hollow cylindrical permanent magnet array

A hollow cylindrical permanent magnet array (HCPMA) used for refrigeration generates a strong magnetic field in the access hole and also inside the permanent magnets. The generated field modifies the remanence distribution of the magnets. Here, we describe a self-consistent calculation method for the remanence vectors and magnetic inductions using the Biot-Savart law. The method takes into account the anisotropic magnetization loops of commercial permanent magnets. Results obtained for the modified remanence distribution of a long HCPMA show demagnetization and deflection, including the reversal of original remanence vectors, when the generated field of the HCPMA exceeds the coercivity of the permanent magnets. The reversed remanence vectors lead to the formation of remanence vortices. In comparison with the field obtained from the ideal model assuming constant remanence vectors, remanence demagnetization and deflection result in obvious decreases in both the field magnitude and field homogeneity in the a...

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.

Dependence of flux-creep activation energy upon current density in HgBa2Ca2Cu3Oy

Abstract We have investigated the magnetic relaxation on a HgBa 2 Ca 2 Cu 3 O y superconductor over the range 4.2–60 K. Using a linear relation between U ( J ) and kT ln(1 + t / t 0 ), we extracted the effective activation energy and the time-window constant t 0 . A nonlinear dependence of U eff on J , U eff ∝ J − μ , was found. μ is about 0.98 in the temperature range 4.2–60 K, close to the value of 7 9 in the regime of flux creep in large vortex bundles predicted by the three-dimensional collective-creep theory. The time scale t 0 depends exponentially on the initial conditions. The relationship between U ( J ) and the magnetic relaxation curve is discussed. With logarithmic accuracy one can construct the U ( J ) curve simply using several long-time relaxation curves at different temperatures.

Flux pinning characteristics and material power law of melt-textured YBa2Cu3O7-

Low-electric-field E-J curves of YBa2Cu3O7- were determined from 0.27 to 1.34 T at various temperatures from 52 to 83 K with the magnetic sweeping method by VSM. Using an empirical material power law E = Ec(J/Jc)n, we obtained the characteristic pinning potential U0(T,H) and real critical current density Jc(T,H) by an appropriate definition of Ec. By further investigation of the changing shape of the pinning potential well, we interpreted the peak effect of U0 with temperature in high temperature superconductors (HTSCs).