Campbell法研究MgB2临界电流密度

PIT(powder in tube)法制备了MgB2超导材料;用Campbell法测量了样品的外加交流磁场bac与磁通渗透深度λ′的关系,并计算得到样品在外磁场中的临界电流密度;讨论了bac-λ′,曲线误差形成的原因和选取曲线平稳部分为计算有效段原则;Campbell法测量考虑了样品的整体结构影响电流的特性,得到的结果更切合超导材料的实际临界电流密度。     

On the study of phase formation and critical current density in superconducting MgB2

Superconducting bulk MgB₂ samples have been synthesized by employing sintering technique without using any additional process steps, generally undertaken in view of the substantial loss of magnesium, during heat treatment. Starting with Mg rich powders having different atomic ratios of Mg : B, as against the nominally required Mg : B = 1:2 ratio, we have obtained superconducting MgB₂ samples of different characteristics. The effect of excess Mg in the starting mixture and processing temperature on the phase-formation, transition temperature (T _C) and critical current density (J _C) have been investigated by electrical transport and a.c. susceptibility measurements. The X-ray diffraction and X-ray photoelectron spectroscopic analyses of MgB₂ bulk samples have been carried out to understand the role of excess Mg and the effect of processing temperature. It is established that MgB₂ samples with high critical current density can be synthesized from a Mg rich powder having Mg : B in 2:2 ratio, at temperatures around 790°C. Critical current density has been found to vary systematically with processing temperature.     

Enhanced critical current density of ex situ MgB2 via control of Mg assisted sintering

Journal of Materials Science: Materials in Electronics - In this work, ex situ MgB2 was mixed with 0.5 mol of Mg and sintered. The sintering conditions were...     

Enhancement in the critical current density of C-doped MgB2 wire using a polyacrylic acid dopant

C-doped MgB2 wires were fabricated from a polyacrylic acid (PAA) using a conventional in-situ PIT technique. The effects of the PAA content on the lattice parameter, microstructure, critical temperature (Tc) and critical current density (Jc) were examined. With increasing PAA content, the amount of MgO in the sample increased but the crystallinity, a-axis lattice parameter, and Tc of MgB2 wires decreased, indicating that the C that decomposed from PAA during heat treatment had substituted for B. All doped samples exhibited a higher Jc than the undoped sample at high magnetic field, and the Jc(B) property improved with increasing PAA content: for the 7 wt% doped sample, the Jc was approximately 3-times higher than that of the pristine sample (1.28 kA/cm2 vs. 3.43 kA/cm2) at 5 K and 6.6 T. The improved Jc(B) of the doped sample was attributed to the decreased grain size, enlarged lattice distortion and increased C doping level.     

Improved critical current density in ex situ processed MgB2 tapes by the size reduction of grains and crystallites by high-energy ball milling

We have fabricated Fe-sheathed MgB₂ tapes through an ex situ process in a powder-in-tube (PIT) technique using powders ball milled under various conditions. Although the ex situ processed wires and tapes using the high-energy ball milled MgB₂ powders have been studied and the decrease of grain and crystallite sizes of MgB₂ and the critical current density (J_c) improvement of those conductors were reported so far, the use of filling powders milled at a higher rotation speed than previously reported further decreases the crystallite size and improves the J_c properties. The improved J_c values at 4.2 K and 10 T were nearly twice as large as those previously reported. Those milled powders and hence as-rolled tapes easily receive contamination in air. Thus, the transport J_c properties are easily deteriorated and scattered unless the samples are handled with care. The optimized heat treatment temperature (T_opt) of those tape samples at which best performance in the J_c property is obtained decreases by more than 100 °C, compared with that of tapes using the as-received MgB₂ powder.     

SiC掺杂对MgB2/Fe超导线材临界电流密度和显微结构的影响

利用原位粉末套管法制备出SiC微粉掺杂的MgB2-x(SiC)x2／Fe(z=0．00、0．05、0．10、0．20)超导线材。在750℃,流通高纯氩气的条件下热处理1h后,大部分SiC没有参与取代B位的反应。随着x的增大,线材中非超导相SiC和Mg的含量增加,MgB2的平均晶粒尺寸变小,从而使可作为磁通钉扎中心的晶界的面积相应增加。在外加磁场中,MgB2超导线材的临界电流密度(Jc)随x增大逐步升高,至x=0．10时Jc性能最好,其在6K,5T时的Jc达到了8480A／cm^2,比未掺杂线材的Jc高出约70％。但是,当x=0．20时,Jc却有所下降。Jc的这种变化规律与SiC掺杂引起的MgB2晶粒变小,以及非超导相物质含量之间的相互平衡有关,其中MgB2晶粒变小是Jc提高的主要原因。     

Correction to: Enhanced critical current density of ex situ MgB2 via control of Mg assisted sintering

Journal of Materials Science: Materials in Electronics -     

The effect of ball-milling treatment of original powders on the sintering process and critical current density of graphite-doped MgB2 bulks

In present study, the sintering process and superconducting properties of MgB₂ prepared with milled original powders of Mg, B, and 3 at.% Graphite were investigated. It was found that ball-milling treatment of original powders can significantly promote the C substitution for B in MgB₂ crystal lattice, resulting in more lattice distortion. Consequently, the values of J _c at high fields were improved. Besides, ball-milling also refined MgB₂ grains, which enhances grain boundary pinning and is also contributed to the improved J _c at high fields. On the other hand, with the milling time prolonging, Mg becomes more active and leads to an obvious increase of MgO impurities during the subsequent sintering process. These MgO impurities would degrade the connectivity between MgB₂ grains and finally lead to slight decrease in J _c at low fields.     

Improved Transport J c in MgB2 Tapes by Graphene Doping

Graphene is a special form of carbon which can effectively enhance the critical current density J _c of MgB₂. In this work, a systematic investigation on the impact of sintering conditions and doping level was carried out for graphene-doped MgB₂ tapes. It is found that an appropriate addition level, i.e., 8 at% in this work, is very critical to obtain a high J _c in graphene-doped samples. The critical field and pinning force are improved obviously due to the graphene doping. The magnetic J _c of samples sintered at 800 °C with 8 at% graphene doping reached 1.78 × 10⁴ A/cm², at 5 T, 20 K. At the same time, the transport J _c was up to 2.38 × 10⁴ A/cm² at 10 T, 4.2 K. The lattice distortion caused by C substitution and residual C at the grain boundaries were thought to be the major factors affecting the J _c of graphene-doped MgB₂ samples.     

Anisotropy of the critical current density in superconducting niobium-titanium wires

The critical current density in wires of an Nb-64 at % Ti alloy was determined in axial and circular directions by various methods. Owing to the cell structure due to cold deformation an anisotropy in the critical current density (j_{c, axially} <j_c, circularly) is present in the wires There is no major difference between magnetization currents and transport currents. Therefore it should be possible in principle to obtain the high critical current densities determined from circular magnetization currents by suitable defect structure with axial transport currents too.     

Surface effect on the critical current density of superconducting niobium

Experiments are described which measure the critical current density of single and polycrystal niobium samples. The density measured is related to the ac losses of the sample caused by an applied ac field in the presence of a dc magnetic field. It was found that the critical current density depends upon the surface of the sample increasing with the surface smoothness.     

复合TiO2纳米管制备与表征

首次采用温和的制备方法制得Al2O3-TiO2纳米管.TiO2粉体在700℃下熔融、110℃水热反 应,制备了管径约为数纳米、单层管壁厚约为0.2纳米,管长约为数微米的复合Al2O3-TiO2纳米管.组织形貌和特性使用TEM、DRS和XRD进行表征.由于Al2O3的沉积在TiO2纳米管上,导致纳米管对紫外光的吸收蓝移40 nm.并对其形成机理进行了讨论.     

Effect of platinum on the critical current density of fluorine-doped YBa2Cu3Ox superconductors

The critical current density, Jc, in fluorine-doped YBa2Cu3Ox bulk superconductors has been measured at several temperatures. The measured Jc value is 108 A m-2 at 77 K and 0.5 T. A sample with a larger critical current density can be produced by the addition of a small amount of platinum during the sintering process. The added platinum has the effect of evenly distributing Y2BaCuO5 particles and voids in the matrix in comparison with the undoped sample. These distributions of Y2BaCuO5 particles and voids are reflected in an increase in the critical current density. However, these distributions are not directly related to the observed peak effect in the critical current density. If the fluorine-doped YBa2Cu3Ox superconductor consists of the matrix and a phase with a Ginzburg–Landau parameter, that is slightly different from that of the matrix, then the occurrence of the peak effect can be explained.     

Superfluid density and critical current of3He in confined geometries

Superfluid³He in severely confined geometries is studied in the Ginzburg-Landau limit. Both A and B phases are analyzed in narrow slabs and in small cylinders. Physically measurable quantities include the superfluid density and the critical current, which are significantly suppressed by the boundaries. Related alterations in the phase diagram are also considered. For small lateral dimensions, the A phase can be favored even well below the polycritical pressure.     

Preparation and critical current density of hot pressed Bi2Sr2Ca2Cu3Ox/Ag tapes

Bi2Sr2Ca2Cu3Ox(Bi-2223)/Ag tapes have been prepared by hot pressing performed at 800–840 °C for 12–96 h under 6 or 12 MPa in air. The highest transport critical current density, Jc, is 3900 A cm–2 at 77 K and 8800 A cm–2 at 65 K under zero magnetic field, which is observed in tape hot pressed at 820 °C under 12 MPa for 24 h twice. The tape has undergone a cold pressing under 260 MPa between hot pressings. Jc is limited to 120 A cm–2 in tape hot pressed for 48 h continuously, in spite of total hot pressing time, temperature and pressure all being the same as for tape hot pressed for 24 h twice. It is found that alternate hot pressing and cold pressing is effective in the preparation of Bi-2223 tapes with high Jc, which is determined by the strength of grain coupling. Grain coupling is strengthened in tape hot pressed with an intermediate cold pressing. © 1998 Chapman & Hall     

Improved current density with Al2O3 coated ZnO nanorod in hybrid solar cell

Heterojunction photovoltaic devices consisting of hybrid p-type organic Cu-phthalocyanine and inorganic n-type Al2O3 nanoparticle-coated aligned ZnO nanorods were fabricated. With microwave treatment, an interaction occurred between the Al2O3 and ZnO, as evidenced from TEM image. This interaction shifts the absorption peak of the aligned nanorods from the UV region to visible light and subsequently causes more charge generation. For 5 mol% Al2O3 nanoparticle-coated aligned ZnO nanorods treated with microwaves of 600 W for 300 sec, the maximum incident photon to electron conversion and energy conversion efficiencies under simulated sunlight of AM1.5G (10 mW/cm2) are 0.036 mA and 1.32%, respectively.