Excess current in contacts based on magnesium diboride

The excess current I _exc in In/MgB₂ and Pb/MgB₂ contacts was investigated as a function of the energy gaps Δ_σ and Δ_π in magnesium diboride MgB₂. It was shown that, even in the “dirty” limit, electrons from the σ and π bands do not undergo mixing; i.e., they remain confined to their bands over a long period of time. On this basis, the measurements of the excess current I _exc for contacts prepared in different crystallographic directions of the MgB₂ crystals made it possible to determine the energy gaps Δ_π ≈ 3.0 meV and Δ_σ = 7.5 meV.     

Electron-phonon interaction in NbB2: a comparison with MgB2

We present a comparison of electron-phonon interaction in NbB₂ and MgB₂, calculated using full-potential, density-functional-based methods in P6/mmm crystal structure. Our results, described in terms of (i) electronic structure, (ii) phonon density of states F(ω), (iii) Eliashberg function α²F(ω), and (iv) the solutions of the isotropic Eliashberg gap equation, clearly show significant differences in the electron-phonon interaction in NbB₂ and MgB₂. We find that the average electron-phonon coupling constant λ is equal to 0.59 for MgB₂ and 0.43 for NbB₂, leading to superconducting transition temperatures T_c at around 22 K for MgB₂ and 3 K for NbB₂.     

Enhanced superconducting properties of Eu2O3-doped MgB2

Bulk polycrystalline samples of Eu₂O₃-doped MgB₂ have been synthesized by a standard solid state reaction route and their structural and superconducting properties have been investigated. As a function of Eu₂O₃ content we have found a significant increase in the critical current density (J_c) and the irreversibility field (H_irr) in the magnetic field range 0–6 T. The XRD results reveal the presence of MgO and EuB₆ secondary phases along with the main hexagonal phase of MgB₂. The strain values and the lattice distortions have been found to increase almost linearly with the nominal Eu₂O₃ content. The observed significant improvement in J_c(H) and H_irr in the Eu₂O₃-doped MgB₂ samples, thus is mainly attributed to the lattice distortions introduced by Eu₂O₃ doping.     

Effect of SiC buffer layer on flux pinning property of MgB2 tapes

In this paper we aimed at investigating the flux pinning property of MgB₂ films on hastelloy tapes which are buffered on various thicknesses of SiC layers. We have observed that the increase in thickness of the SiC buffer layer is very closely related with the systematic improvement of the field dependence of the critical current densities (J_c) of MgB₂ tapes while the values of J_c decreased. According to the analysis of the pinning force density (F_p), there exist two pinning sources both in the pure MgB₂ and in the MgB₂ film with the thinnest SiC buffer layer. On the while, the pinning source observed in the MgB₂ films with thicker SiC buffer layers appears to be different from those previously mentioned. The different pinning behaviors of MgB₂ films may suggest that there be an additional pinning center working on the MgB₂ films with thick SiC buffer layers. The microstructural analyses of MgB₂ films confirmed that intra-granular defects and columnar grain boundaries may be a dominant pinning mechanism in the pure MgB₂ and the MgB₂ film with 170 nm-thick SiC buffer layer. For the MgB₂ films with thicker SiC buffer layers, carbon diffusion into the MgB₂ film, which is defined by the Auger electron spectroscopy, may be the origin of the additional pinning mechanism.     

Effects of ambient and humidity exposure on the MgB2 superconductor

Exposure effect of ambient (up to a period of 465 days) and humid atmosphere on stability and the physical properties of bulk samples of MgB₂ has been carried out based on ac-susceptibility and X-ray diffraction measurements performed at different times during the aging. The motivation of this study came from the device application potential of MgB₂ superconductor. It has been observed that on initial exposure to ambient for a period of about 2 months, the MgB₂ samples neither show any sign of phase decomposition nor any significant degradation of superconducting parameters. However, on further exposure to ambient for a period greater than 4 months duration, the samples progressively exhibited the partial conversion of MgB₂ phase into Mg-deficient phases, viz., MgB₆ and MgB₁₂ and also leading to formation of Mg(OH)₂ which is accompanied by decrease in the T_C and poor connectivity of the superconducting grains. When exposed to humid environment for a period of 30 days, a significant degradation was observed in the superconducting properties of MgB₂. In sharp contrast to the case of ambient exposure, the exposure of MgB₂ to humid atmosphere (for 30 days) did not result in any noticeable phase decomposition. A comparison of the magnetically inferred critical current density behavior before and after the humidity exposure of MgB₂ is also presented.     

Concurrent doping effect of Ti and nano-diamond on flux pinning of MgB2

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 J_c–H behavior and pinning force scaling features of MgB₂ have been investigated. Although T_c was slightly depressed, J_c of MgB₂ have been significantly improved by the nano-diamond doping, especially in the high field region. In the mean time, the J_c 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.     

B4C in ex-situ spark plasma sintered MgB2

Powder mixtures of MgB₂ and B₄C with composition ((MgB₂) + (B₄C)_x, x = 0.005, 0.01, 0.03) were consolidated by Spark Plasma Sintering at 1150 °C for 3 min. The average particle size of B₄C raw powder was relatively high of 4 μm. Despite this, it is shown that processing processes are fast and, as in the case of the in-situ routes, for our ex-situ method carbon substitutes for the boron in the crystal lattice of MgB₂. Specifics of microstructure are discussed based on electron microscopy observations. Carbon substitution and microstructure contribute to enhancement of the critical current density J_c at high magnetic fields and of the irreversibility field H_irr. Samples are shown to be in the point pinning limit with some tendency toward the grain boundary pinning depending on B₄C doping amount and temperature. An optimum composition is found for x = 0.01: for this sample, at 20 K, a J_c of 100 A/cm² is obtained at 5.35 T. This value is higher than for the pristine MgB₂ sample and for an optimum ex-situ nano-SiC-doped sample obtained for the same SPS processing conditions.     

Structural and thermodynamic properties of MgB2 from first-principles calculations

A first-principles plane wave method with the relativistic analytic pseudopotential of Hartwigsen, Goedecker and Hutter (HGH) scheme in the frame of local density approximation is performed to calculate the lattice parameters and the equation of states (EOS) of superconducting MgB₂. Our calculations show that the ratio c/a of about 1.134 is the most stable structure for MgB₂, as is consistent with experiment and other theoretical results. Also, the isothermal and isobaric properties are discussed from energy-volume curves using a quasi-harmonic Debey model.     

Interrelation of the critical magnetic field H c2 and the residual resistivity for the two-band superconductor MgB2

An expression that describes the upper critical magnetic field H _c2 and generalizes the Gor’kov relation has been derived for the two-band two-gap superconductor MgB₂. The expression relates the upper critical magnetic field H _c2 to the residual resistivity and the parameters of the band structure and holds in the range from the clean limit to the dirty limit. The ratios of the relaxation times τ_π/τ_σ and the mean free paths of π- and σ-band electrons for MgB₂ samples with a low defect level and Mg(B_{1 ? x} C _x )₂ samples with a partial substitution of carbon for boron are determined from experimental data.     

MgB2混合态热导率的反常增强

测量了MgB₂多晶样品的混合态热导率，磁场强度为0—7 T，温度范围为5—45 K .实验结果显示MgB₂热导率在低场下迅速上升，高场下趋于饱和，这与MgB₂的 二能隙电子结构有关.对实验结果的分析指出，低温强场下MgB₂多晶样品热导率的显著增强无法完全 用电子热导来解释，并对此进行了讨论. 关键词： 2')" href="#">MgB₂ 热导率 混合态     

利用电泳法在金属基底上制备MgB2超导厚膜

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     

Magnetic field dependence of vortex activation energy: A comparison between MgB2, NbSe2 and Bi2Sr2Ca2Cu3O10 superconductors

The dissipative mechanism at low current density is compared in three different classes of superconductors. This is achieved by measuring the resistance as a function of temperature and magnetic field in clean polycrystalline samples of NbSe₂, MgB₂ and Bi₂Sr₂Ca₂Cu₃O₁₀ (BSCCO) superconductors. Thermally activated flux flow behaviour is seen in all the three systems and clearly identified in bulk MgB₂. While the activation energy at low fields for MgB₂ is comparable to Bi₂Sr₂Ca₂Cu₃O₁₀, its field dependence follows a parabolic behaviour unlike a power-law dependence seen in Bi₂Sr₂Ca₂Cu₃O₁₀. We analyse our results based on Kramer’s scaling for grain boundary pinning in MgB₂ and NbSe₂.      

A simple theory of 40 K superconductivity in MgB2: first-principles calculations of Tc, its dependence on boron mass and pressure

We have studied the superconducting properties of MgB₂ from first-principles under isotropic, uniaxial, and biaxial compressions. We find that the in-plane boron phonons near the zone-center are very anharmonic and strongly coupled to the planar B σ bands near the Fermi level. This mode is found to be the key to quantitatively explain the observed high T_c, the total isotope effect and the pressure dependence of T_c. We propose that a stringent test on the hole and phonon based theories of the superconductivity in MgB₂ would be a measurement of the biaxial ab-compression dependence of T_c.     

Crystal orientation dependence of the critical current density in a-axis- and c-axis-oriented MgB2 films

We have investigated the critical current density for MgB₂ films having various crystal orientations prepared by using a hybrid physical-chemical vapor deposition system. An enhancement of the critical current density is clearly presented in MgB₂ films with an a-axis or a b-axis orientation rather than a c-axis orientation. X-ray diffraction patterns reveal a suppression of c-axis orientations while a (100) orientation becomes dominant, and the surface morphology of the a-axis-oriented film shows that the orientation of the c-axis-oriented MgB₂ grains parallel to the plane of the substrate. As the a-axis orientation becomes more dominant in the MgB₂ films, the field performance of the critical current density clearly becomes better. These results suggest that the synthesis of MgB₂ with high ab-plane orientations is one of the keys to enhancing the critical current density in MgB₂.     

Enhancement of the upper critical field of MgB2 by carbon-doping

We have measured the temperature dependence of the upper critical field, H_c2(T), of carbon-doped MgB₂. H_c2(T) does not follow the well-known Werthamer-Helfand-Hohenberg (WHH) result for a one-gap dirty superconductor but can be described well by the result of a recent theoretical calculation for a two-gap dirty superconductor. H_c2(0) of the carbon-doped material is determined to be between 29 and 38 T, substantially higher than that of pure MgB₂ (15-23 T).     

Effects of surface damage on critical current density in MgB2 thin films

We investigated the influence of surface damage on the critical current density (J_c) of MgB₂ thin films via 140-keV Co-ion irradiation. The J_c(H) of the surface-damaged MgB₂ films was remarkably improved in comparison with that of pristine films. The strong enhancement of J_c(H) caused by a surface damage in MgB₂ films can be ascribed to additional point defects along with an atomic lattice displacement introduced through low-energy Co-ion irradiation, which is consistent with the change in the pinning mechanism, from weak collective pinning to strong plastic pinning. The irreversible magnetic field (H_irr) at 5 K for surface-damaged MgB₂ films with a thickness of 850 and 1300 nm was increased by a factor of approximately 2 compared with that of a pristine film. These results show that the surface damage produced by low energy ion irradiation can serve as an effective pinning source to improve J_c(H) in a MgB₂ superconductor.     

Effect of ball milling time on the substitution of carbon in glucose doped MgB2 superconductors: Dispersion behavior of glucose

The effect of the ball milling time (BMT) on the substitution of the carbon in the glucose doped MgB₂ samples is investigated here. Using in situ solid state reaction, four different doped samples of Mg(B_.98C_.02)₂ were prepared by mixing powders of Mg, boron and glucose for 2 h, 4 h, 8 h and 12 h using planetary ball milling. A reference sample of un-doped MgB₂ was also prepared under same conditions. The particle size distribution of the un-reacted samples show a decrease in the particle size as the BMT is increased. Both the average particle size as well as the standard deviation show a substantial decrease with the increase in the milling time up to 8 h. After 8 h, the size reduction is rather insignificant. From the XRD data, the crystallite size of the doped MgB₂ computed using the Scherrer formula was found to decrease with the increasing BMT, showing a saturation level after 8 h of the milling time. TEM images also confirm the crystallite size obtained from the XRD data. The substitution of the C in the MgB₂ lattice, measured from the change in the c/a ratio, increases with increasing BMT. The maximum carbon substitution is achieved at approximately 8 h of BMT. Moreover, a systematic enhancement of the residual resistivity and a decrease in T_C with an increasing BMT further confirms a progressive substitution of the carbon in the MgB₂. These results suggest that a minimum ball milling time is necessary to disperse the glucose uniformly for a maximum substitution of nano C in the B plane of MgB₂ lattice. The optimum BMT is found to be 8 h. Thus, the decrease in the particle size due to the ball milling enhances the dispersion of the constituent materials thereby favoring a greater substitution of the dopant in the MgB₂ during the solid-state reaction.     

Electron transport,penetration depth,and the upper critical magnetic field in ZrB<Subscript>12</Subscript> and MgB<Subscript>2</Subscript>

We report on the synthesis and measurements of the temperature dependences of the resistivity ρ, the penetration depth λ, and the upper critical magnetic field H_c2, for polycrystalline samples of dodecaboride ZrB₁₂ and diboride MgB₂. We conclude that ZrB₁₂ behaves as a simple metal in the normal state with the usual Bloch-Grüneisen temperature dependence of ρ(T) and with a rather low resistive Debye temperature T_R = 280 K (to be compared to T_R = 900 K for MgB₂). The ρ(T) and λ(T) dependences for these samples reveal a superconducting transition in ZrB₁₂ at T_c = 6.0 K. Although a clear exponential λ(T) dependence in MgB₂ thin films and ceramic pellets was observed at low temperatures, this dependence was almost linear for ZrB₁₂ below T_c/2. These features indicate an s-wave pairing state in MgB₂, whereas a d-wave pairing state is possible in ZrB₁₂. In disagreement with conventional theories, we found a linear temperature dependence, of H_c2(T) for ZrB₁₂ (H_c2(0) = 0.15 T).     

Iron doping effect on superconducting properties of MgB2

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, T_c, and critical current J_c have been investigated. It is found that both T_c and J_c 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 T_c and J_c of MgB₂.     

MgB2 coated superconducting tapes with high critical current densities fabricated by hybrid physical-chemical vapor deposition

The MgB₂ coated superconducting tapes have been fabricated on textured Cu (0 0 1) and polycrystalline Hastelloy tapes using coated conductor technique, which has been developed for the second generation high temperature superconducting wires. The MgB₂/Cu tapes were fabricated over a wide temperature range of 460-520 °C by using hybrid physical-chemical vapor deposition (HPCVD) technique. The tapes exhibited the critical temperatures (T_c) ranging between 36 and 38 K with superconducting transition width (ΔT_c) of about 0.3-0.6 K. The highest critical current density (J_c) of 1.34 × 10⁵ A/cm² at 5 K under 3 T is obtained for the MgB₂/Cu tape grown at 460 °C. To further improve the flux pinning property of MgB₂ tapes, SiC is coated as an impurity layer on the Cu tape. In contrast to pure MgB₂/Cu tapes, the MgB₂ on SiC-coated Cu tapes exhibited opposite trend in the dependence of J_c with growth temperature. The improved flux pinning by the additional defects created by SiC-impurity layer along with the MgB₂ grain boundaries lead to strong improvement in J_c for the MgB₂/SiC/Cu tapes. The MgB₂/Hastelloy superconducting tapes fabricated at a temperature of 520 °C showed the critical temperatures ranging between 38.5 and 39.6 K. We obtained much higher J_c values over the wide field range for MgB₂/Hastelloy tapes than the previously reported data on other metallic substrates, such as Cu, SS, and Nb. The J_c values of J_c(20 K, 0 T) ∼5.8 × 10⁶ A/cm² and J_c(20 K, 1.5 T) ∼2.4 × 10⁵ A/cm² is obtained for the 2-μm-thick MgB₂/Hastelloy tape. This paper will review the merits of coated conductor approach along with the HPCVD technique to fabricate MgB₂ conductors with high T_c and J_c values which are useful for large scale applications.