Ma Yanwei

Simple One-Step Synthesis and Superconducting Properties of SmFeAsO1–xFx

The recent discovery of superconductivity in REFeAsO (RE, rare-earth metal) has generated enormous interest because these materials are the first non-copper oxide superconductors with critical temperatures Tc exceeding 50 K as well as upper critical fields well above 100 T. However, for these new superconductors, very complicated synthesis routes, such as the complex two-step synthesis or high-pressure sintering, are required. Furthermore, there is the toxicity and volatility of arsenic to consider, sometimes a sealed quartz tube of arsenic exploded during annealing. We present a new method for producing high-temperature SmFeAsO1–xFx superconductors by using a one-step sintering process. Superconducting transition with the onset temperature of 54.6 K and high critical fields Hc2(0) ≥ 200 T were confirmed in SmFeAsO0.7F0.3. At 5 K and at self field, critical current densities Jc estimated from the magnetization hysteresis using the whole sample size and the average particle size have reached 8.5 × 103 and 1.2 × 106 A/cm2, respectively. Moreover, Jc exhibited a very weak dependence on magnetic field. This simple and safe one-step synthesis technique should be effective in other rare earth derivatives of iron-based superconductors.

Effects of Separator on the Electrochemical Performance of Electrical Double-Layer Capacitor and Hybrid Battery-Supercapacitor

Separators are important components in electrochemical energy storage devices such as electrical double layer capacitors (EDLCs) and hybrid battery-supercapacitors. We prepared activated carbon-based EDLCs using an electrolyte of 1 mol ∙L1 tetraethyl ammonium tetrafluoroborate (Et4NBF4) in propylene carbonate (PC), and (LiNi0.5Co0.2Mn0.3O2+activated carbon)/graphite hybrid battery-supercapacitors using a 1 mol∙L-1 lithium hexafluorophate (LiPF6) Li-ion electrolyte. The physicochemical properties and effect of various separators on the electrochemical properties of the EDLC and hybrid battery-supercapacitor were studied. The four separators were nonwoven polypropylene (PP) mat, porous PP membrane, Al2O3-coated PP membrane, and cellulose paper. The surface morphology, differential scanning calorimetry, electrolyte uptake, and apparent contact angle were investigated. The electrochemical characterizations of coin cells indicated that the EDLC with cellulose separator had the highest specific capacitance and rate capability. Differences in the selfdischarge of the four cells were not obvious. The specific capacities of the hybrid battery-supercapacitors with PP membrane and nonwoven PP mat separators were approximately 20% higher than the others. The capacitor with the cellulose paper separator had the highest self-discharge rate.

Ho and Ti co-doped BiFeO3 multiferroic ceramics with enhanced magnetization and ultrahigh electrical resistivity

Bi0.9Ho0.1Fe0.95O3 and Bi0.9Ho0.1Fe0.9Ti0.05O3 ceramics were prepared and compared to reveal the effects of Ho and Ti codoping in BiFeO3. X-ray diffraction indicated that both ceramics had a high rhombohedral perovskite phase content, and microstructural analyses showed that the grains of the Bi0.9Ho0.1Fe0.9Ti0.05O3 ceramics were much smaller than those of Bi0.9Ho0.1Fe0.95O3. An electrical resistivity of more than 1 × 1014 Ω cm at room temperature, and a magnetic hysteresis loop with a remnant magnetization 2Mr of ~ 0.485 emu/g were obtained for Bi0.9Ho0.1Fe0.9T0.05O3; both were much higher than those of Bi0.9Ho0.1Fe0.95O3. Changes in the defect subsystem of BiFeO3 induced by Fe-deficiency and (Ho,Ti) codoping are proposed as being responsible for the improvement in the properties.

Doping effects of ZrC and ZrB2 in the powder-in-tube processed MgB2 tapes

We have investigated the effects of ZrC and ZrB2 doping on the superconducting properties of the powder-in-tube processed MgB2/Fe tapes. Samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM/EDX), transport and magnetic measurements. We confirmed the following quite different roles of ZrC and ZrB2 in MgB2. ZrC doping was found to decrease the transport critical current density (Jc) at 4.2 K, while the critical temperature (Tc) kept constant. In contrast, the Jc values in magnetic fields were enhanced greatly by the ZrB2 addition, which resulted in a decrease in Tc by only 0.5 K. The reason for different effects of two dopants is also discussed.

Development of iron-based superconducting materials for applications

The discovery of superconductivity in iron pnictides has generated a great deal of research interest, not only in basic physics, but also in the field of applied superconductivity. It has been confirmed that the pnictides have peculiar physical properties including an unconventional pairing mechanism. This brings an opportunity to discover new kind of superconductor with high transition temperature. On the other hand, compare to the cuprate, the iron-based superconductors have a rather high upper critical field ( H c2) and low anisotropy. These properties are very attractive for practical application. An overview of the development of iron-based superconductors is presented in this article. The crystal structure of various iron-based superconductors is presented. At the same time, the fabrication techniques of iron-based tapes and films for “1111”, “122”, “11” type of pnictide are reviewed. Some promising features such as a high critical current density in tapes and thin films under high magnetic fields have been clarified. Finally, we summarize and compare the features and properties of the pnictide materials with other classical cuprate and MgB2 superconductors.

Fabrication and Properties of Aligned Sr(0.6)K(0.4)Fe(2)As(2) Superconductors by High Magnetic Field Processing

We fabricated a c—axis aligned Sr0.6K0.4Fe2As2 superconductor using a two-step magnetic field procedure. The effect of the magnetic fields on the structure and superconducting properties of Sr0.6K0.4Fe2As2 is investigated using x-ray diffraction and magnetic measurements. The degree of orientation of the samples is about 0.39 for the c axis and 0.51 for ab-plane orientation, as evaluated from the Lotgering factor of x-ray diffraction. This technology may be useful in a variety of potential applications, including preparing iron-based superconducting bulks and wires with high critical currents.

Enhancement of Critical Current Density and Flux Pinning in Acetone and La2O3 Codoped MgB2 Tapes

MgB2 tape samples with simultaneous additions of acetone and La2O3 were prepared by an in-situ processed powder-in-tube method. Compared to the pure and single doped tapes, both transport Jc and fluxing pinning are greatly improved by acetone and La2O3 codoping. Acetone supplies carbon into the MgB2 crystal lattice and increases the upper critical field, while the La2O3 reacts with B to form LaB6 nanoparticles as effective flux pining centers. The improvement of the superconducting properties in codoped tapes can be attributed to the combined effects of improvement in Hc2 and flux pinning.

Conducting polymer hydrogel materialsfor high-performance flexible solid-state supercapacitors

Different from solid electrodes, conducting polymer hydrogel electrodes swollen with water and ions, can reach contact with the electrolyte solution at the molecular level, which will result in more efficient electrochemical process of supercapacitors. Besides, the inherent soft nature of hydrogel material offers the electrode superior flexibility, which benefits to gain high flexibility for devices. Here, this perspective briefly introduces the current research progress in the field of conducting polymer hydrogel electrodes-based flexible solid-state supercapacitor and gives an outlook on the future trend of research.Different from solid electrodes, conducting polymer hydrogel electrodes swollen with water and ions, can reach contact with the electrolyte solution at the molecular level, which will result in more efficient electrochemical process of supercapacitors. Besides, the inherent soft nature of hydrogel material offers the electrode superior flexibility, which benefits to gain high flexibility for devices. Here, this perspective briefly introduces the current research progress in the field of conducting polymer hydrogel electrodes-based flexible solid-state supercapacitor and gives an outlook on the future trend of research.摘要不同于固体电极, 导电聚合物水凝胶电极因含有大量水和离子可以使得导电聚合物与电解质实现分子层次的直接接触, 这会使得电 化学反应过程更加高效. 此外, 水凝胶内在的柔软特性, 会赋予电极优越的柔性特点, 有利于制备高柔性电子器件. 本文简要介绍了基于导 电聚合物水凝胶电极材料的柔性固态超级电容器的最新研究进展, 并对未来研究方向做了展望.

Research of supercapacitors with high energy density

Compared with traditional batteries, supercapacitors exhibit the high power density, long cycle life and wide operating temperature window. However, the relatively low energy density of supercapacitor still remains a challenge. In this paper, the structure, classification and development of supercapacitors are introduced, with an emphasis on recent work in high performance supercarpacitors of our group. The strategy for enhancement of energy density is to increase the voltage window of symmetric carbon-based supercapacitors using neutral, organic, and ionic liquid electrolytes, and adopt asymmetric supercapacitors in which one electrode is pseudocapacitive while the other utilizes double layer capacitance. Briefing is also presented on our recent investigation into hybrid supercapacitor with electrode material of lithium-ion battery/activated carbon as positive electrode, and graphite as negative electrode. This paper is ended by a perspective of future development of supercapacitors.