共查询到20条相似文献,搜索用时 15 毫秒
1.
T. Nojima K. Takahashi M. Chotoku A. Ochiai H. Aoki H.-G. Lee S.-I. Lee 《Physica C: Superconductivity and its Applications》2009,469(15-20):1055-1058
We have studied the order–disorder transition in high quality MgB2 single crystals, using a torque magnetometry combined with a ‘vortex shaking’ technique. In the wide range of temperature T, field H and the H direction, we succeed in obtaining reversible magnetization curves Mrev(T, H) by shaking the pinned vortices. Especially at low temperatures below 25 K and high fields, where the irreversible magnetization curve exhibits the peak effect due to the order–disorder transition, it is found that the peak is transformed into the clear step in Mrev(H). Similar step-like behavior is also observed in the temperature dependence of magnetization Mrev(T). These results give direct evidence that the order–disorder transition, which is hidden by the large hysteresis of magnetization, has the nature of first-order transition. 相似文献
2.
D.H. Kim T.J. Hwang Y.J. Cha W.K. Seong W.N. Kang 《Physica C: Superconductivity and its Applications》2009,469(15-20):1059-1062
We studied the flux pinning properties by grain boundaries in MgB2 films prepared by using a hybrid physical chemical vapor deposition method on the c-axis oriented sapphire substrates. All the films we report here had the columnar grains with the growth direction perpendicular to the substrates and the grain sizes in the range of a few hundred nanometers. At very low magnetic fields, no discernable grain-boundary (GB) pinning effect was observed in all measuring temperatures, but above those fields, the effect of GB flux pinning was observed as enhanced critical current densities (Jcs) and reduced resistances when an external magnetic field (B) was aligned parallel to the c-axis. We interpret the B dependence of Jc in the terms of flux line lattice shear inside the columnar grains activated by dislocations of Frank–Read source while the flux lines pinned by GB act as anchors for dislocations. Magnetic field dependence of flux pinning force density for B parallel to the c-axis was reasonably explained by the above model. 相似文献
3.
4.
The superconductivity of a sintered pellet of the newly discovered MgB2 superconductor has been studied with magnetic measurements and its crystal structure was analysed using the Rietveld method of powder X-ray patterns. It has hexagonal symmetry (S.G. P6/mmm) with unit cell lattice parameters a=0.308136(14)nm and c=0.351782(17)nm. 相似文献
5.
Y. Zhao C. Ke C.H. Cheng Y. Feng Y. Yang P. Munroe 《Physica C: Superconductivity and its Applications》2010,470(20):1096-1099
Nano-diamond and titanium concurrently doped MgB2 nanocomposites have been prepared by solid state reaction method. The effects of carbon and Ti concurrent doping on Jc–H behavior and pinning force scaling features of MgB2 have been investigated. Although Tc was slightly depressed, Jc of MgB2 have been significantly improved by the nano-diamond doping, especially in the high field region. In the mean time, the Jc value in low field region is sustained though concurrent Ti doping. Microstructure analysis reveals that when nano-diamond was concurrently doped with titanium in MgB2, a unique nanocomposite in which TiB2 forms a thin layer surrounding MgB2 grains whereas nano-diamond particles were wrapped inside the MgB2 grains. Besides, nano-diamond doping results in a high density stress field in the MgB2 samples, which may take responsibility for the Δκ pinning behavior in the carbon-doped MgB2 system. 相似文献
6.
Fast photoelectric effects have been observed in MgB2 thin film
fabricated by chemical vapour deposition. The rise time was $\sim $10
ns and the full width at half-maximum was \sim185\,ns for the photovoltaic
pulse when the film was irradiated by a 308\,nm laser pulse of 25\,ns in
duration. X-ray diffraction and the scanning electron microscope revealed
that the film was polycrystalline with preferred c-axis orientation. We
propose that nonequilibrium electron--hole pairs are excited in the grains
and grain boundary regions for MgB2 film under ultraviolet laser and
then the built-in electric field near the grain boundaries separates
carriers, which lead to the appearance of an instant photovoltage. 相似文献
7.
We report on the high-pressure synthesis and superconductivity of MgB2 intermetallic compounds. The compounds have been obtained through high-pressure sintering of the mixtures of magnesium and boron fine powders under 5.0 GPa and at ~1000℃ for 30 min. Magnetic measurements using a SQUID magnetometer show the sharp bulk superconducting transition above 39 K; the four-probe dc resistivity measurements indicate the highly-conductive normal state and sharp superconducting transition. The results highlight that high-pressure synthesis would be a promising way to promote the studies of this new kind of intermetallic superconductors. 相似文献
8.
Microwave characteristics of MgB2/Al2O3 superconducting thin films were investigated by coplanar resonator technique. The thin films studied have different grain sizes resulting from different growth techniques. The experimental results can be described very well by a grain-size model which combines coplanar resonator theory and Josephson junction network model. It was found that the penetration depth and surface resistance of thin films with smaller grain sizes are larger than those of thin films with larger grain sizes. 相似文献
9.
报道了利用蓝宝石介质谐振器技术测量MgB2超导薄膜的微波表面电阻Rs sub>、0K时的穿透深度λ(0)和超导能隙Δ(0).λ(0)和Δ(0)的值是通过先测量样品穿透深度λ(T)的变化量Δλ(T),然后由BCS理论模型拟合Δλ(T)的实验数据得到的.测试样 品是利用化学气相沉积技术在MgO(111)基片上制备的c轴织构的MgB2超导薄膜, 薄膜的超导转变温度和转变宽度分别为38K和01K.微波测试结果表明在10K,18GHz下M gB2薄膜的Rs约为100μΩ,可以和高质量的YBCO薄膜的Rs值相比拟;BCS理论拟合得到的MgB2超导薄膜的λ(0)=102nm,Δ(0)=113k Tc. 相似文献
10.
利用混合物理化学气相沉积法(hybrid physical-chemical vapor deposition, HPCVD)可以制备出高性能的MgB2超导薄膜, 再对薄膜进行钛(Ti)离子辐照处理.经过辐照处理后的样品被掺入了Ti元素, 与未处理的干净MgB2样品相比,其超导转变温度没有出现大幅度的下降, 而在外加磁场下的临界电流密度得到了明显的提高,同时样品的上临界磁场也得到了提高. 在温度5 K, 外加垂直磁场为4 T的情况下, Ti离子辐照剂量为1× 1013/cm2的样品的临界电流密度达到了1.72× 105 A/cm2, 比干净的MgB2要高出许多,而其超导转变温度仍能维持在39.9 K的较高水平. 相似文献
11.
C.H. Cheng Y. Yang C. Ke H.T. Lin 《Physica C: Superconductivity and its Applications》2010,470(20):1092-1095
Iron-doped MgB2 bulks are prepared by hybridized diffusion method using nano-powder and macro-powder of pure iron as iron source. The doping effect on superconductivity transition temperature, Tc, and critical current Jc have been investigated. It is found that both Tc and Jc of MgB2 show quite different features depending on the particle size of the dopant powders. It is demonstrated that different from iron bulk or large size powders, iron nano-powders are active dopant for MgB2 which suppresses both Tc and Jc of MgB2. 相似文献
12.
Y. Yang C.H. Cheng L. Wang H.H. Sun Y. Zhao 《Physica C: Superconductivity and its Applications》2010,470(20):1100-1102
In this paper, we study the doping effect of sorbic acid (C6H8O2), from 0 to 20 wt.% of the total MgB2, on critical temperature (Tc), critical current density (Jc), irreversibility field (Hirr) and crystalline structure. The XRD patterns of samples show a slightly decrease in a-axis lattice parameter for doped samples, due to the partial substitution of carbon at boron site. On the other hand, we investigate the influence of doping on the behavior of flux pinning and Jc(B) in the framework of percolation theory and it is found that the Jc(B) behavior could be well fitted in high field region. The two key parameters, anisotropy and percolation threshold, play very important roles. It is believed that the enhancement of Jc is due to the reduction of anisotropy in high field region. 相似文献
13.
Micron-scale mixing of magnesium and boron powders by mechanical alloying allows the MgB2 formation reaction to proceed as low as 450 °C, with high-quality material formed in an hour at 600 °C. At these low reaction temperatures the reaction can be performed in situ in a copper sheath, and it is found that the presence of copper enhances the superconducting properties of the MgB2. Self-field critical current densities, calculated from full critical-state simulation of magnetization hysteresis, are up to 7 × 105 A/cm2 at 13.6 K and 1 × 105 A/cm2 at 30 K. 相似文献
14.
Devang A. Joshi A.K. Nigam S.K. Dhar A. Thamizhavel 《Journal of magnetism and magnetic materials》2010,322(21):3363-543
Anisotropic magnetic properties of single crystalline RAu2Ge2 (R=La, Ce and Pr) compounds are reported. LaAu2Ge2 exhibits a Pauli-paramagnetic behaviour whereas CeAu2Ge2 and PrAu2Ge2 show an antiferromagnetic ordering with Nèel temperatures TN = 13.5 and 9 K, respectively. The anisotropic magnetic response of Ce and Pr compounds establishes [0 0 1] as the easy axis of magnetization and a sharp spin-flip type metamagnetic transition is observed in the magnetic isotherms with H // [0 0 1]. The transport and magnetotransport behaviour of these compounds, in particular LaAu2Ge2, indicate an anisotropic Fermi surface. The magnetoresistivity of CeAu2Ge2 apparently reveals the presence of a residual Kondo interaction. A crystal electric field analysis of the anisotropic susceptibility in conjunction with the experimentally inferred Schottky heat capacity enables us to propose a crystal electric field level scheme for Ce and Pr compounds. For CeAu2Ge2 our values are in excellent agreement with the previous reports on neutron diffraction. The heat capacity data in LaAu2Ge2 show clearly the existence of Einstein contribution to the heat capacity. 相似文献
15.
利用电泳技术在高熔点金属基底Ta,Mo和W上制备MgB2超导厚膜.厚膜中的MgB2晶粒结合紧密,粒度小于1μm,呈随机取向生长.电阻测量表明沉积在Ta,Mo,W上的MgB2厚膜的超导起始转变温度分别为36.5K,34.8K,33.4K,对应的转变宽度为0.3K,1.5K和2.0K.三种基底上制备的MgB2厚膜的临界电流密度在不同温度下随外磁场的变化情况 基本相同,MgB2/Mo厚膜的临界电流密
关键词:
2超导厚膜')" href="#">MgB2超导厚膜
电泳
金属基底 相似文献
16.
Meral G. Babaoğlu Serap Safran Özlem Çiçek Hasan Ağıl Ercan Ertekin Md.Shahriar A. Hossain Ekrem Yanmaz Ali Gencer 《Journal of magnetism and magnetic materials》2012
The effect of aromatic hydrocarbon (benzene, C6H6) addition on lattice parameters, microstructure, critical temperature (Tc), critical current density (Jc) of bulk MgB2 has been studied. In this work only 2 mol% C6H6 addition was found to be very effective in increasing the Jc values, while resulting in slight reduction of the Tc. Jc values of 2 mol% C6H6 added MgB2 bulks reached to 1.83×106 A/cm2 at 15 K and 0 T. Microstructural analyses suggest that Jc enhancement is associated with the substitution of carbon with boron and which also results in the smaller MgB2 grain size. The change in the lattice parameters or the lattice disorder is claimed as a cause of the slight reduction in the Tc by carbon addition. We note that our results show the advantages of C6H6 addition include homogeneous mixing of precursor powders, avoidance of expansive nanoadditives, production of highly reactive C, and significant enhancement in Jc of MgB2, compared to un-doped samples. 相似文献
17.
Ultrafast photoelectric effects have been observed in MgB2 thin films fabricated by chemical vapor deposition on MgO (1 1 1) substrates. The rise time and full width at half-maximum of the photoresponse pulse signals were about 2.4 and 4 ns under the irradiation of a 248 nm laser pulse of 20 ns in duration through the MgO substrate at ambient temperature without any bias. Furthermore, the signal polarity is directly bound up with the laser illumination positions, while no photovoltage was observed when the MgO (1 1 1) single crystal was irradiated. The inner origin mechanism of the present positions-dependent photovoltaic response was discussed. 相似文献
18.
The newly discovered superconductor MgB2 has a transition temperature Tc of about 40 K, which is touching the upper line of that predicted by the phonon-mediated Bardeen--Cooper--Schrieffer (BCS) theory. It is interesting to investigate the flux creep in MgB2 and compare it with other superconductors. We have measured the magnetization relaxation of MgB2 sintered at high temperature and high pressure. It is found that the quantum tunnelling and the thermally activated flux creep are very weak, implying a strong pinning in this material. 相似文献
19.
J.H. Yi K.T. Kim B.-H. Jun J.M. Sohn B.G. Kim J. Joo C.-J. Kim 《Physica C: Superconductivity and its Applications》2009,469(15-20):1192-1195
MgB2 bulks were prepared by an in situ process which utilizes the reaction between boron and magnesium powder. The reaction time was fixed at 0.5 h and the temperature was changed from 600 °C to 1000 °C. The density decrease due to pore formation and mass (mainly magnesium) loss during the formation reaction of MgB2 was observed in all samples. In addition to the pore formation, a pellet expansion which can be explained by the outgrowth of MgB2 grains was also observed. Two different mechanisms were adopted to explain the pore formation; Kirkendall pores formed at a temperature below the melting point (m.p.) of magnesium by a difference in the diffusivity between magnesium and boron, and the pores formed at a temperature above the m.p. by melting of magnesium and a capillary movement. The density, Tc and Jc results suggest that the current carrying capacity can be improved by a careful control of the process parameters regarding a pore evolution. 相似文献
20.
Ying Tang Xiangjie Kong Jianlian Zang 《Physica C: Superconductivity and its Applications》2009,469(21):1902-1905
The Cu-doped MgB2 bulks were prepared by a high-energy milling and subsequent sintering method. Compared to the pure and Cu-doped bulks prepared only by sintering, the critical current density (Jc) of the milled Cu-doped samples was improved with a slight decrease in critical transition temperature (Tc). Using the phase analysis and microstructure observation, it has been found that the MgB2 grains in the milled Cu-doped sample was refined with the high-energy milling and thus provided more grain boundary pinning, which was contributed to the improvement of Jc at high field. 相似文献