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1.
S. Deng A. Simon J. Köhler A. Bussmann-Holder 《Journal of Superconductivity and Novel Magnetism》2003,16(6):919-922
The discovery of superconductivity in MgB2 at T c = 39 K has initiated great efforts to identify its microscopic origin. A superconducting two-gap structure has been firmly established experimentally. However, the importance of the smaller gap which is essential in pushing T c above the BCS value of a one-gap superconductor has mostly been ignored. Our results based on ab initio calculations and a newly introduced k, j-dependent bonding indicator show that Mg contributes significantly to states from which the small gap originates. In addition, it is revealed that Mg—B covalent bonding is an important factor in limiting the substitution of Mg in MgB2. 相似文献
2.
A. S. Sidorenko V. I. Zdravkov V. V. Ryazanov M. Klemm S. Horn R. Tidecks C. Müller A. Wixforth 《Journal of Superconductivity》2004,17(2):211-213
The origin of the resistive transition broadening for superconducting MgB2 films is investigated experimentally. The crucial role of two-dimensional weak and critical fluctuations is demonstrated. 相似文献
3.
Shao-Min Zhou Peng Wang Sheng Li Bin Zhang He-Chun Gong Xing-Tang Zhang 《Materials Letters》2009,63(20):1680-1682
Large-scale single crystalline MgB2 tubelike nanostructures were successfully prepared by thermal evaporation of MgB2 particles precursors without involvement of template or patterned catalyst. The inner diameter, outer diameter and length of the as-fabricated single MgB2 nanotube (NT) are respectively about 30 nm, 90 nm and several tens of microns. Existence of superconductivity within the products is confirmed by AC and DC magnetic susceptibility at low temperatures. The work represents the achievement to produce the bulk superconductivity with the hollow-structured morphology. Synthesis of MgB2 NTs with bulk superconductivity may open up new possibilities for the fundamental understanding of the effect of dimensionality on superconductivity. 相似文献
4.
Dense pure and doped (Mg1 – x
A
x
)B2 samples with A = Na, Ca, Cu, Ag, Zn, and Al were synthesized at high pressure–high temperature in a multi-anvils press (3.5–6 GPa, 900–1000°C) for 0 < x 0.20. They were studied by X-ray diffraction, scanning electron microscopy and their superconducting properties were investigated by AC susceptibility, magnetization, and transport measurements. Only Al is really substituted on the Mg site. The other elements form secondary phases with B or Mg. No large effect is observed on the superconducting properties T
c, j
c critical current, H
irr, and H
c2. 相似文献
5.
6.
The superconducting properties of magnesium diboride (MgB2) films prepared by electroless deposition on various substrates including silver, gold and silicon are reported. In this study, MgB2 films were fabricated on silver, gold, and silicon using an electroless plating technique, while controlling the redox potential to improve the deposition quality. The structure, morphology, and superconducting properties of the samples were investigated using X-ray diffraction, magnetometry, scanning electron microscopy, and Raman spectroscopy. X-ray diffraction and Raman spectroscopy confirmed that the films are polycrystalline MgB2 but also contain some impurity phases. All the MgB2 films show superconducting transitions near 39 K, the value for bulk MgB2, with the superconducting volume fraction ranging from approximately 1 to 2%. We find a strong dependence of film quality with the oxidation potential of the bath. 相似文献
7.
Though low-temperature sintering of MgB2 superconductors below Mg melting point can effectively depress volatility of Mg and increase sintering density, its development was limited in recent years for the reason that it usually took very long time to form complete MgB2 phase with excellent Jc at low temperature. In present work, significantly improved Jc was surprisingly obtained in the MgB2 samples sintered at 575 °C for only 5 h after short-time ball milling, even though formation of MgB2 phase is not completed and lots of residual Mg is still present in these samples. The grain connectivity in prepared samples is obviously improved compared to referred MgB2 sample sintered at high temperature, which is responsible for the improvement of Jc. The method developed in present work seems bring a new opportunity for the development of low-cost practical MgB2 superconductors with improved Jc without using expensive nanometer-size dopants or high-temperature sintering. 相似文献
8.
Yuan-Chang Liang 《Thin solid films》2009,517(14):3874-3878
5 wt.% Zr-doped In2O3 (Zr-In2O3) films with thicknesses from 95 to 220 nm were grown on 90 nm-thick ZnO-buffered sapphire (0001) substrates by radio-frequency magnetron sputtering in an oxygen-deficient atmosphere. The dependence on thickness of the structural information and electrical properties of the Zr-In2O3 films on the ZnO-fuffered sapphire substrates was studied. The X-ray diffraction patterns show that the (002)-textured ZnO buffer-layer is a good template for the growth of the highly (222)-textured In2O3 films on the sapphire substrate. The surface of the Zr-In2O3 film becomes rougher as the film thickness increases, perhaps because of the formation of larger mounds on the film surface as the thickness of Zr-In2O3 increases. The carrier concentration increased markedly from 5.8 × 1020 to 1.83 × 1021 cm− 3 with film thickness from 95 to 220 nm, because more growth-induced defects are formed in the thick Zr-In2O3 film. The large increase in the number of charge carriers and the improvement in the crystalline quality in the film reduce the resistivity of the thicker Zr-In2O3 film. 相似文献
9.
We report a simple method for the fabrication of reproducible, clean, and stable MgB2 superconducting tips. The quality of these tips has been verified by imaging the surface of a thin Au(111) film sample, using a low temperature scanning tunneling microscopy (STM). Using the MgB2 superconducting tip, high-quality semiatomically resolved STM surface images of the thin Au(111) film sample have been observed, which unambiguously indicates that the fabrication of relatively superconducting MgB2, suitable for use as STM tips, is feasible. 相似文献
10.
E. L. Wolf A. Chang Z. Y. Rong Yu. M. Ivanchenko Farun Lu 《Journal of Superconductivity》1994,7(2):355-360
Scanning tunneling spectroscopy applied to BiO cleavage planes of 90 KT
c BSCCO 2212 at 4.2 K simultaneously provides topography and localdI/dV spectra (superconducting DOS). The spectra, which are similar to recent photoemission spectra, confirm a large gap parameter (x, y) associated with an apparently gapless DOS on the uppermost layer. Transverse spatial variations of on a 100 Å scale are attributed to variations in BiO metallicity, presumably originating in oxygen stoichiometry variations in an unannealed crystal. We identify two characteristicdI/dV spectral shapes with regions of metallic and nonmetallic BiO layers, and can relate these by the McMillan model of the superconducting proximity effect. A sharply peaked spectral shape, similar to that observed in photoemission, is predicted for the metallic BiO layer induced superconducting by its proximity to the underlying CuO2 planes. The short mean free path and short coherence length imply that both tunneling and photoemission spectra are heavily weighted toward contributions from the BiO layer in fully metallic 2212. The present results and analysis thus suggest that the superconducting proximity effect influences the lineshapes seen by both techniques. 相似文献
11.
M. E. Yaknc Y. Balc M. A. Aksan H. . Adigüzel A. Gencer 《Journal of Superconductivity》2002,15(6):607-611
Superconducting MgB2 polycrystalline samples have been fabricated under two different conditions in order to determine the effect of MgB4 phase. A series of samples was placed in an -alumina container closed with a cup and fired under high purity argon gas. The other series of samples was placed in an -alumina boot without any lid and fired under similar conditions. For the first series of samples, we have found pure MgB2 phase formation and a narrow transition width at 0.4 K. For the second series of samples, significant amount of MgB4 phase were formed and the T
zero was decreased to 27 K. For both the group of samples magnetization hysteresis loops obtained at various temperature range and applied field up to 2 T. The best J
cmag for the first series of samples was 1.9 × 105 A/cm2 at 10 K and 0 T, and for the second series of samples was 0.7 × 104 A/cm2 at 10 K and 0 T. 相似文献
12.
B. Kugler C. StahlS. Treiber S. Soltan S. HaugG. Schütz J. Albrecht 《Thin solid films》2012,520(23):6985-6988
Surface morphology and superconducting properties of MgB2 superconducting thin films prepared by ex-situ annealing of multilayer Mg/B precursors in Mg vapor are studied.Depending on the precursor structure different physical and microstructural properties of the superconductor evolve. Structure and composition of the films are analyzed by scanning electron microscopy and wavelength dispersive x-ray spectroscopy. It is found that certain precursor structures can lead to high quality superconducting films, however, in specific precursor structures mechanical stress leads to the formation of wrinkles strongly affecting the superconducting homogeneity of the films. A correlation between microstructure and superconducting properties, such as pinning or critical current density, can be provided via magneto-optical Faraday microscopy. 相似文献
13.
Resistive joints are found in many systems using superconductors. Joints are used to connect the superconductor to a normal conducting current terminal or to connect two superconductors, for example in pancake type windings. Knowing the resistance between the contacts is important in studying the heat balance of a superconducting system. We performed several experiments with MgB2–MgB2 lap joints to determine the relation between the contact resistance and solder joint length. Also, the effect of the outer sheath material on the contact resistance was studied. To support the experiments, a computational model using Finite Element Method was created. The measured and computed results showed adequate correlation. In the experiments, the soldered joint length was varied from 3 to 20 mm resulting in contact areas between 2.5 and 16.6 mm2. The results indicated that the outer sheath material has significant effect on the contact resistance. For a Monel sheathed conductor the measured contact resistances varied between 4 and 16 μΩ and if a copper sheath was used, the resistances were an order of magnitude smaller. 相似文献
14.
Cu2O/TiO2, Cu/Cu2O/TiO2 and Cu/Cu2O/CuO/TiO2 heterojunctions were prepared and studied for their potential application as photocatalysts able to induce high performance under visible light. Orange II was used as a representative dye molecule. The effect of the amount and composition of the photosensitizers toward the activation of TiO2 was studied. In each case, the global mechanism of Inter Particle Electrons Injection (IPEI) was discussed. The highest photocatalytic activity was observed for the system Cu/Cu2O/CuO (MB2 catalyst) under visible light (t1/2 = 24 min, k = 159.7 × 10−3 min−1) and for the heterojunction cascade Cu/Cu2O/CuO/TiO2 (MB2 (50%)/TiO2) under UV–vis light (t1/2 = 4 min, k = 1342 × 10−3 min−1). In the last case, the high performance was attributed firstly to the electromotive forces developed under this configuration in which CuO energy bands mediate the electrons transfer from Cu2O to TiO2. The formation of monobloc sensitizers also accounts for the decrease of the probability of the charges lost. It was demonstrated that “Cu2O/CuO” governs the capability of the heterojunction cascade and Cu does not play a significant role regardless of the heterojunction cascade efficiency. The electrical energy consumption per order of magnitude for photocatalytic degradation of Orange II was investigated for some representative catalytic systems. Visible/MB2 and UV/vis MB2 (50%)/TiO2 exhibited respectively 0.340 and 0.05 kWh m−3 demonstrating the high efficiency of the systems. 相似文献
15.
Intermetallic compound superconductor MgB2 was synthesized from spherical magnesium powder and lower purity amorphous boron powder by microwave direct heating. Powder X-ray diffraction (XRD) analysis indicates that the phases of the synthesis sample are MgB2 (major phase) and a small amount of MgO. Scanning electron microscope (SEM) observation shows that the MgB2 grain size is homogeneous and the particle size is about several hundreds of nanometers. The onset superconducting transition temperature of the MgB2 sample measured by the temperature dependence of magnetization measurement is about 37.6 K. The critical current density Jc calculated according to the Bean model are about 2.0 × 105 A/cm2 at 20 K in self-field and 1.0 × 105 A/cm2 at 20 K in 1 T applied field. 相似文献
16.
N-type nanocrystalline 3C-SiC films were prepared by hot-wire chemical vapor deposition from SiH4/CH4/H2 and N2 as a doping gas and the structural and electrical properties were investigated. The gas flow rates of SiH4, CH4 and H2 were 1, 1 and 200 sccm, respectively. As the N2 gas flow rate was increased from 0 to 10 sccm, the conductivity and the activation energy improved from 0.05 to 0.3 S/cm and from 45 to 28 meV, respectively. The Hall Effect measurement proved that the improvement of the electrical properties was caused by the increase in the carrier concentration. On the other hand, in the N2 gas flow rate between 10 and 50 sccm, the conductivity and the activation energy remained unchanged. The crystallinity deteriorated with increasing N2 gas flow rate. This gave rise to the unchanged electronic properties in spite of the increase in the intake of N atoms. 相似文献
17.
在Li2SnO3中掺杂Si,研究了Si掺杂Li2SnO3对四环素的光催化降解性能。结果表明,对Li2SnO3进行等电子Si掺杂使其光学吸收带隙减小和光吸收系数增大,提高了对四环素的光催化降解效率。等电子Si掺杂Li2SnO3为纯相不规则块状固体,随着Si掺杂量的增加其晶格参数呈减小的趋势。Si掺杂使样品的光催化性能显著提高。Si掺杂量为10%的样品,在紫外光照射25 min后光催化降解效率为75.8%,约为母体的2倍。Si掺杂Li2SnO3的光催化降解行为满足赝一级动力学模型,拟合速率常数为0.02464 min-1。在Si掺杂Li2SnO3的价带顶形成的Si-O键减少了光学吸收带隙,使其光吸收能力增强。Si掺杂Li2SnO3的光催化降解机制,属于空穴主导型。 相似文献
18.
We investigated an influence of gas pressure on low-temperature preparation of nanocrystalline cubic silicon carbide (nc-3C-SiC) films by hot-wire chemical vapour deposition (HW-CVD) using SiH4/CH4/H2 system. X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectra revealed that the films prepared below 1.5 Torr were Si-nanocrystallite-embedded hydrogenated amorphous SiC. On the other hand, nc-3C-SiC films were successfully prepared at gas pressure above 2 Torr. The high gas pressure plays two important roles in low-temperature preparation of nc-3C-SiC films: (1) leading to sufficient decomposition of CH4 molecules through a gas phase reaction and an increase in the incorporation of carbon atoms into film and (2) promoting a creation of H radicals on the heated filament, allowing the sufficient coverage of growing film surface and a selective etching of amorphous network structure and/or crystalline-Si phase. It was found that total gas pressure is a key parameter for low-temperature preparation of nc-3C-SiC films. 相似文献
19.
Glasses with the nominal compositions of xWO325La2O3(75 − x)B2O3 (mol%) with x = 15, 25, and 50 were prepared using a conventional melt quenching method, and their structure and crystallization behavior were examined from Raman scattering spectra and X-ray diffraction analyses. The glasses are colorless in the visible light region and give the optical band gap energy of 3.49-3.61 eV. The glass transition and crystallization temperatures and the thermal stability against crystallization decrease with increasing WO3 content. The strong Raman bands at 840 and 940-960 cm−1 suggest that the main coordination state of W6+ ions in the glasses is isolated (WO4)2− tetrahedral units. The formation of WO6 octahedral units is also suggested in the glasses with high WO3 contents. The main crystallization mechanism in the glasses is the surface crystallization, and the glass of 50WO325La2O325B2O3 shows the crystallization of LaBWO6 single phase. The present study proposes that WO3La2O3B2O3 glasses and crystallized glasses are very interesting as optical functional materials. 相似文献
20.
Mauro Rajteri Claudio Gandini Eugenio Monticone Chiara Portesi Aldo Masoero Chiara Boveri Piero Mazzetti 《Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment》2004,520(1-3):351-353
In this paper we present some experimental results concerning the current noise produced during the resistive transition in MgB2 thin films. Preliminary investigations evidenced the presence of electrical noise whose power spectrum has a region of the 1/fn type with n 3. We suggest that the noise may originate from abrupt rearrangement of the current distribution inside the specimen during the percolative process of a diphasic system. Experimental measurements of the spectral components of the current noise taken during the resistive transition will be given and discussed. 相似文献