共查询到20条相似文献,搜索用时 31 毫秒
1.
Devina Sharma Jagdish Kumar Arpita Vajpayee Ranjan Kumar P. K. Ahluwalia V. P. S. Awana 《Journal of Superconductivity and Novel Magnetism》2011,24(6):1925-1931
In the present study, we report an intercomparison of various physical and electronic properties of MgB2 and AlB2. In particular, the results of phase formation, resistivity ρ(T), thermoelectric power S(T), magnetization M(T), heat capacity (C P ), and electronic band structure are reported. The original stretched hexagonal lattice with a=3.083 Å, and c=3.524 Å of MgB2 shrinks in c-direction for AlB2 with a=3.006 Å, and c=3.254 Å. The resistivity ρ(T), thermoelectric power S(T) and magnetization M(T) measurements exhibited superconductivity at 39 K for MgB2. Superconductivity is not observed for AlB2. Interestingly, the sign of S(T) is +ve for MgB2 the same is ?ve for AlB2. This is consistent with our band structure plots. We fitted the experimental specific heat of MgB2 to Debye–Einstein model and estimated the value of Debye temperature (Θ D) and Sommerfeld constant (γ) for electronic specific heat. Further, from γ, the electronic density of states (DOS) at Fermi level N(E F) is calculated. From the ratio of experimental N(E F) and the one being calculated from DFT, we obtained value of λ to be 1.84, thus placing MgB2 in the strong coupling BCS category. The electronic specific heat of MgB2 is also fitted below T c using α-model and found that it is a two gap superconductor. The calculated values of two gaps are in good agreement with earlier reports. Our results clearly demonstrate that the superconductivity of MgB2 is due to very large phonon contribution from its stretched lattice. The same two effects are obviously missing in AlB2, and hence it is not superconducting. DFT calculations demonstrated that for MgB2, the majority of states come from σ and π 2p states of boron on the other hand σ band at Fermi level for AlB2 is absent. This leads to a weak electron phonon coupling and also to hole deficiency as π bands are known to be of electron type, and hence obviously the AlB2 is not superconducting. The DFT calculations are consistent with the measured physical properties of the studied borides, i.e., MgB2 and AlB2. 相似文献
2.
The present paper focuses on methods of further improving the flux pinning and critical current density of disk-shaped MgB2 bulk superconductors by adding excess Mg metal in combination with an optimum silver content and optimized processing conditions. Bulk MgB2 samples were produced by in situ solid-state reaction in Ar gas ambient using high purity commercial powders of Mg metal and 1.5 wt% carbon-coated amorphous B powders mixed in a fixed ratio of Mg/B = 1.1:2. Further, 4 wt% silver was added to improve flux pinning as well as mechanical performance of the bulk MgB2 material. The magnetization measurements confirmed a sharp superconducting transition with Tc,onset at around 37 K, which is only by 1 K lower than in bulk MgB2 material produced without carbon-coated amorphous boron. The critical current density (Jc) values significantly improved in the MgB2 material with 4 wt% of silver and 1.5 wt% of carbon-coated amorphous boron, sintered at 775 °C for 3 h. At 20 K, this sample showed Jc at around 500 and 350 kA/cm2 in the self-field and 1 T, respectively, which makes it suitable for several industrial applications. 相似文献
3.
Intikhab A. Ansari V. P. S. Awana Rajeev Rawat M. Shahabuddin M. Husain H. Kishan A. V. Narlikar 《Journal of Materials Science》2007,42(15):6306-6309
We report fluctuation-induced conductivity (FIC) of the polycrystalline MgB2 superconductor in the presence of magnetic field. The results are described in terms of the temperature derivative of the resistivity, dρ/dT. The dρ/dT peak temperature observed for H = 0 Tesla at 39 K remains very distinct under applied fields of 6 Tesla and 8 Tesla at 22 and 20 K respectively. Aslamazov and Larkin (AL) equations are used to explain the anisotropic nature of the polycrystalline MgB2. The effective coherence length, ξ p (0) determined experimentally is 55.17 Å, which roughly matches with previously reported experimental work. 相似文献
4.
Reena Goyal A. K. Srivastava Monu Mishra Govind Gupta Rajveer Jha V. P. S. Awana 《Journal of Superconductivity and Novel Magnetism》2018,31(4):943-949
In the present report, we investigate various properties of the Nb2PdS5 superconductor. Scanning electron microscopy displayed slabs like laminar growth of Nb2PdS5 while X-ray photoelectron spectroscopy exhibited the hybridisation of sulphur (2p) with both palladium (3d) and niobium (3d). High-field (140 kOe) magneto-transport measurements revealed that superconductivity (\(T_{c}^{\text {onset}} =?7\) K and T cρ=?0 = 6.2 K) of the studied Nb2PdS5 material is quite robust against magnetic field with the upper critical field (H c2) outside the Pauli paramagnetic limit. Thermally activated flux flow (TAFF) of the compound showed that resistivity curves follow Arrhenius behaviour. The activation energy for Nb2PdS5 is found to decrease from 15.15 meV at 10 kOe to 2.35 meV at 140 kOe. Seemingly, the single vortex pinning is dominant in low-field regions, while collective pinning is dominant in high-field region. The temperature dependence of AC susceptibility confirmed the T c at 6 K, further varying amplitude and frequency, showed well-coupled granular nature of superconductivity. The lower critical field (H c1) is extracted from DC magnetisation measurements at various T below T c. It is found that H c1(T) of Nb2PdS5 material seemingly follows the multiband nature of superconductivity. 相似文献
5.
Dinesh Kumar Miryala Muralidhar M. S. Ramachandra Rao Masato Murakami 《Journal of Superconductivity and Novel Magnetism》2018,31(7):2033-2038
We fabricated MgB2 samples with Ag additions using in situ solid-state reaction via a single-step sintering to study the effect of Ag on the structural, vibration, and superconducting properties of MgB2 samples. Ag addition to MgB2 resulted in a significant improvement in J c although no appreciable effect was observed in the lattice parameters and the superconducting transition temperature T c. Dramatic increase in the grain size was observed with Ag addition and topographic measurements with atomic force microscopy revealed the formation of Ag–Mg nanoparticles 5–20 nm in size at 2 and 4 wt% Ag additions. The fact that these samples showed high J c values suggests that the nanoparticles formed as a result of Ag addition are responsible for enhanced flux pinning. Raman spectroscopy measurements showed that Ag additions also increased disorder in the system and thereby affected the line width of the Raman active E 2g mode. 相似文献
6.
Jafar M. Parakkandy M. Aslam Manthrammel Fahad Saad Alghamdi Mohammed Shahabuddin Nasser S. Alzayed 《Journal of Superconductivity and Novel Magnetism》2018,31(4):989-993
In this study, we report an enhancement of critical current density of bulk MgB2 superconductors by glutaric acid (C5H8O4) doping. The effects of glutaric acid doping on MgB2 lattice resulted in a record self-field J c of the order of 106 A/cm2. A simultaneous improvement in the connectivity, pinning force, and H c2 is the major factor that determined excellent J c performance. X-ray diffraction analysis showed that samples were single-phase MgB2 with a minor trace of impurities. A dramatic change in grain morphology and homogeneity in grain distribution was found in the SEM images of doped samples. We observed that homogeneity in grain distribution played a crucial role in the connectivity and the upper critical field (H c2) of the doped samples. We were able to introduce a new dopant through a two-step mixing approach which is suitable to overcome the degradation of low field and self-field J c reported for carbon-doped MgB2 superconductor samples. 相似文献
7.
We study the magnetic field vs. temperature (H – T) and pressure vs. temperature (P – T) phase diagrams of the T c ≈ 5.5 K superconducting phase in Pd x Bi2Te3 (x ≈ 1) using electrical resistivity versus temperature measurements at various applied magnetic fields (H) and magnetic susceptibility versus temperature measurements at various applied magnetic fields (H) and pressure (P). The H – T phase diagram has an initial upward curvature as observed in some unconventional superconductors. The critical field extrapolated to T = 0 K is H c (0) ≈ 6–10 kOe. The T c is suppressed approximately linearly with pressure at a rate d T c /d P ≈ ?0.28 K/GPa. 相似文献
8.
Shuo Tao Yongbo Li Guowei Chen Xiaopeng Zhao 《Journal of Superconductivity and Novel Magnetism》2017,30(6):1405-1411
Enhancing the critical temperature (T C ) is important not only to widen the practical applications but also to expand the theories of superconductivity. Inspired by the meta-material structure, we designed a smart meta-superconductor consisting of MgB2 microparticles and Y2O3/Eu3+ nanorods. In the local electric field, Y2O3/Eu3+ nanorods generate an electroluminescence (EL) that can excite MgB2 particles, thereby improving the T C by strengthening the electron–phonon interaction. An MgB2-based superconductor doped with one of four dopants of different EL intensities was prepared by an ex situ process. Results showed that the T C of MgB2 doped with 2 wt% Y2O3, which is not an EL material, is 33.1 K. However, replacing Y2O3 with Y2O3/Eu3+II, which displays a strong EL intensity, can improve the T C by 2.8 to 35.9 K, which is even higher than that of pure MgB2. The significant increment in T C results from the EL exciting effect. Apart from EL intensity, the micromorphology and degree of dispersion of the dopants also affected the T C . This smart meta-superconductor provides a new method to increase T C . 相似文献
9.
X-ray diffraction data are presented for combustion products in the Al-W-N system. New, nonequilibrium intermetallic compounds have been identified, their diffraction patterns have been indexed, and their unit-cell parameters have been determined. The phases α-and β-WAl4 are shown to exist in three isomorphous forms, differing in unit-cell centering. The phases α′-, α″-, and α?-WAl4 are monoclinic, with a 0 = 5.272 Å, b 0 = 17.770 Å, c 0 = 5.218 Å, β = 100.10°; point groups C12/c1, A12/n1, I12/a1, respectively. The phases β′-, β″-, and β?-WAl4 are monoclinic, with a 0 = 5.465 Å, b 0 = 12.814 Å, c 0 = 5.428 Å, β = 105.92°; point groups A112/m, B112/m, I112/m, respectively. The compounds WAl2 and W3Al7, identified each in two isomorphous forms, differ in cell metrics (doubling) but possess the same point group: P222. WAl 2 ′ : orthorhombic, a 0 = 5.793 Å, b 0 = 3.740 Å, c 0 = 6.852 Å. WAl 2 ″ : orthorhombic, a 0 = 11.586 Å, b 0 = 3.740 Å, c 0 = 6.852 Å. W3Al 7 ′ : orthorhombic, Pmm2, a 0 = 6.225 Å, b 0 = 4.806 Å, c 0 = 4.437 Å. W3Al 7 ″ : orthorhombic, Pmm2, a 0 = 12.500 Å, b 0 = 4.806 Å, c 0 = 8.874 Å. The new phase WAl3: triclinic, P1, a 0 = 8.642 Å, b 0 = 10.872 Å, c 0 = 5.478 Å, α = 104.02°, β = 64.90°, γ = 107.15°. 相似文献
10.
Mohamed Baazaoui Sobhi Hcini Michel Boudard Sadok Zemni Mohamed Oumezzine 《Journal of Superconductivity and Novel Magnetism》2015,28(7):1887-1893
The critical behavior of perovskite manganite La0.67Ba0.33Mn0.95Fe0.05O3 at the ferromagnetic–paramagnetic has been analyzed. The results show that the sample exhibited the second-order magnetic phase transition. The estimated critical exponents derived from the magnetic data using various such as modified d’Arrott plot Kouvel–Fisher method and critical magnetization M(T C, H). The critical exponents values for the La0.67Ba0.33Mn0.95Fe0.05O3 are close to those expected from the mean field model β = 0.504 ± 0.01 with T C = 275661 ± 0.447 (from the temperature dependence of the spontaneous magnetization below T C ), γ = 1.013 ± 0.017 with T C = 276132 ± 0.452 (from the temperature dependence of the inverse initial susceptibility above T C ), and δ = 3.0403 ± 0.0003. Moreover, the critical exponents also obey the single scaling equation of M(H, ε) = |ε| β f ±(H/|ε| β+γ ). 相似文献
11.
Hasan Ağıl Erhan Aksu Ali Gencer 《Journal of Superconductivity and Novel Magnetism》2017,30(10):2735-2740
In this study, the structural and superconducting properties of aniline-added MgB2 superconductors were investigated by X-ray diffraction (XRD), thermal analysis techniques, and ac susceptibility measurements. The amount of aniline was changed from 0 to 1 mol%. Phase analysis and lattice parameters were determined from XRD measurements. X-ray diffraction analysis indicates that the main phase is MgB2 and that there is a small amount of Mg as the secondary phase in aniline-added samples. According to the determination of lattice parameters, it is seen that the addition of aniline does not give a proper distribution with the contribution amount of a and c lattice parameters. From DSC curves, two exothermic peaks and one endothermic peak were observed in all samples. Pure and aniline-added samples were found to be dependent on the magnetic field in the ac susceptibility measurements, and the superconducting transition temperature (T c ) was found to decrease to lower temperatures due to an increase in the amount of aniline. It has been determined that changes in the in-phase (χ ′) and out-of-phase (χ ″) components of the ac susceptibility by increasing the aniline amount have weakened the MgB2 phase structure and thus cause changes in the pinning mechanism. In addition, ac losses of all the samples were calculated under external fields ranging from 160 to 1280 A/m and at 25 K. 相似文献
12.
We report bulk superconductivity at 2.5 K in LaO0.5F0.5BiSe2 compound through the DC magnetic susceptibility and electrical resistivity measurements. The synthesized LaO0.5F0.5BiSe2 compound is crystallized in tetragonal structure with space group P4/nmm and Reitveld refined lattice parameters are a = 4.15(1) Å and c = 14.02(2) Å. The lower critical field of H c1 = 40 Oe, at temperature 2 K is estimated through the low field magnetization measurements. The LaO0.5F0.5BiSe2 compound showed metallic normal state electrical resistivity with residual resistivity value of 1.35 m Ω cm. The compound is a type-II superconductor, and the estimated H c2(0) value obtained by WHH formula is above 20 kOe for 90 % ρ n criteria. The superconducting transition temperature decreases with applied pressure till around 1.68 GPa and with further higher pressures a high- T c phase emerges with possible onset T c of above 5 K for 2.5 GPa. 相似文献
13.
C. E. Deluque Toro A. S. Mosquera Polo A. V. Gil Rebaza D. A. Landínez Téllez J. Roa-Rojas 《Journal of Low Temperature Physics》2018,192(5-6):265-285
We report first-principles calculations of the elastic properties, electronic structure and magnetic behavior performed over the Ba2NiMoO6 double perovskite. Calculations are carried out through the full-potential linear augmented plane-wave method within the framework of the Density Functional Theory (DFT) with exchange and correlation effects in the Generalized Gradient and Local Density Approximations, including spin polarization. The elastic properties calculated are bulk modulus (B), the elastic constants (C11, C12 and C44), the Zener anisotropy factor (A), the isotropic shear modulus (G), the Young modulus (Y) and the Poisson ratio (υ). Structural parameters, total energies and cohesive properties of the perovskite are studied by means of minimization of internal parameters with the Murnaghan equation, where the structural parameters are in good agreement with experimental data. Furthermore, we have explored different antiferromagnetic configurations in order to describe the magnetic ground state of this compound. The pressure and temperature dependence of specific heat, thermal expansion coefficient, Debye temperature and Grüneisen parameter were calculated by DFT from the state equation using the quasi-harmonic model of Debye. A specific heat behavior CV?≈?CP was found at temperatures below T = 400 K, with Dulong–Petit limit values, which is higher than those, reported for simple perovskites. 相似文献
14.
The composites of glass ceramic Bi-2212 and MgB2 superconductors were prepared at ambient conditions. The transmission electron microscopy images of the composite samples illustrate the presence of glass ceramic inclusions in bulk MgB2. Temperature-dependent magnetization of the composite samples shows two superconducting transitions: one at 80 K corresponding to the Bi-2212 phase and a second one at 39 K corresponding to the MgB2 phase, suggesting that the two superconducting phases are separated with clear boundaries. The critical current density (J c) and pinning force values are increased in composite systems by an order of magnitude compared to that of individual samples. The pinning mechanism in the composite sample is the same as in the matrix phase. Reduced field maxima (h max) are observed at 0.15 for composite samples. A low value of h max for composite samples indicates the random orientation of grain boundaries and repulsive pinning force in the composite samples. 相似文献
15.
Jiang Luo Shilie Pan Hongyi Li Zhongxiang Zhou Zhihua Yang 《Journal of Materials Science》2011,46(23):7443-7448
Single crystals of Rb2CaB8O26H24, a new non-centrosymmetric borate material, have been grown with sizes up to 8 × 5 × 3 mm3 by the slow evaporation of water solution at room temperature. The structure of the compound was determined by single-crystal X-ray diffraction. It crystallizes in the orthorhombic system, space group P212121 with a = 11.5288(3) Å, b = 12.6334(4) Å, c = 16.6966(4) Å, Z = 4 and R 1 = 0.0405, wR 2 = 0.1043. Ultraviolet (UV)–vis spectrum transmission is performed on the Rb2CaB8O26H24, which shows an absorption edge about 195 nm in the UV region. Thermal properties were investigated by TG–DSC analysis. The powder second-harmonic generation (SHG) intensity measured by the Kurtz-Perry method indicates that Rb2CaB8O26H24 has about one-third of KDP (KH2PO4).The influence of different molar ratios and evaporation speed of water solution on crystal quality and size was also performed on the reported material. 相似文献
16.
P. P. S. Bhadauria Anurag Gupta Hari Kishan Shivani Sharma A. V. Narlikar 《Journal of Superconductivity and Novel Magnetism》2017,30(2):353-358
Modification of σ and π bands was studied in MgB2 by doping 3, 6 and 9 wt% of C and Fe, respectively. The samples synthesized by a solid-state route were characterized by XRD, and magnetization (M) and resistivity (ρ) measurements were in the temperature range (T) 4.2–300 K and magnetic field range (B) 0–12 T, respectively. The decrease (increase) of the lattice parameter a with C (Fe) doping, consistent with B (Mg) site substitution, confirms the expected changes in σ (π) bands. This is supported by the fact that normal-state ρ(T) of all the samples can be fitted by a two-band model and the scattering rates in both the bands are found to be dependent on the dopant. The influence of C and Fe doping on various superconducting properties of the host MgB2 is also found to be significantly different. For instance, in the presence of magnetic field, Fe doping shows a much larger broadening of the superconducting transition when compared to C doping. The critical current density (J C(B)) at 4.2 K vanishes for C (Fe) doping at around T~12 (~3). It is shown that the band modification and the superconducting properties are correlated. 相似文献
17.
Bo Wu Jian Ma Wenjuan Wu Min Chen 《Journal of Materials Science: Materials in Electronics》2017,28(3):2358-2365
In this work, (Ba0.96Ca0.04)(Ti0.92Sn0.08)O3–xmol MnO (BCTS–xMn) lead-free piezoelectric ceramics were fabricated by the conventional solid-state technique. The composition dependence (0 ≤ x ≤ 3.0 %) of the microstructure, phase structure, and electrical properties was systematically investigated. An O–T phase structure was obtained in all ceramics, and the sintering behavior of the BCTS ceramics was gradually improved by doping MnO content. In addition, the relationship between poling temperature and piezoelectric activity was discussed. The ceramics with x = 1.5 % sintering at temperature of 1330 °C demonstrated an optimum electrical behavior: d 33 ~ 475 pC/N, k p ~ 50 %, ε r ~ 4060, tanδ ~ 0.4 %, P r ~ 10.3 μC/cm2, E c ~ 1.35 kV/mm, T C ~ 82 °C, strain ~0.114 % and \(d_{33}^{*}\) ~ 525 pm/V. As a result, we achieved a preferable electric performance in BaTiO3-based ceramics with lower sintering temperature, suggesting that the BCTS–xMn material system is a promising candidate for lead-free piezoelectric ceramics. 相似文献
18.
B. K. Kasenov E. S. Mustafin Sh. B. Kasenova Zh. I. Sagintaeva S. T. Edil’baeva S. Zh. Davrenbekov 《High Temperature》2009,47(1):27-32
The ceramic technology is employed for synthesizing manganites of composition Nd Mg 3 I Mg3Mn4O12(MeI-Li, Na, K). The X-ray technique is used to find that the compounds crystallize in tetragonal syngony. The parameters of their crystal lattices are determined. Their heat capacities are experimentally determined in the range from 298.15 to 673 K, which enables one to reveal second-order phase transitions. In view of these transitions, equations describing the C p ° ~ f(T) dependence are derived, and the thermodynamic functions C p ° (T), H°(T)-H°(298.15), S°(T), and Φ xx (T) are calculated. 相似文献
19.
D. Gajda A. Morawski A. J. Zaleski M. Akdoğan H. Yetiş F. Karaboğa T. Cetner İ. Belenli 《Journal of Superconductivity and Novel Magnetism》2017,30(12):3397-3402
This paper demonstrates the effects of hot isostatic pressure (HIP) on the structure and transport critical parameters of in situ MgB2 wires without a barrier. Our results show that only HIP and nano-boron allow the formation of more high-field pinning centers, which lead to the increase in critical current density (J c) at high applied magnetic fields. Nano-boron and annealing at a low pressure increase the J c in the low magnetic field. This indicates that nano-particles create more high-field pinning centers. In addition, the results show that nano-boron improves the connection between the grains. Scanning electron microscope results show that HIP increases the reaction rate between Mg and B, density, and homogeneity of the MgB2 material. Additionally, HIP allows to create a structure with small grains and voids and eliminates the significance of the number of voids. High isostatic pressure allows to obtain high J c of 10 A/mm2 (at 4.2 K) in 10 T and increases irreversible magnetic field (B irr) and upper critical field (B c2). Measurements show that these wires have high critical temperature of 37 K. 相似文献
20.
E. Yu. Anikina V. N. Verbetsky A. G. Savchenko V. P. Menushenkov I. V. Shchetinin 《Inorganic Materials》2017,53(10):1009-1012
Interaction of hydrogen with the intermetallic compound Nd2Fe17 has been studied for the first time by calorimetry using a differential heat conduction calorimeter coupled to a Sieverts apparatus. Hydrogen absorption and desorption reactions were run at 200°C, and two types of data were obtained: p–C–T and ΔH–C–T (where p is the equilibrium hydrogen pressure, C = H/Nd2Fe17, ΔH is the reaction enthalpy, and T is the measurement temperature). The p–C–T curves obtained for the hydrogen absorption and desorption processes have no plateau or two-phase region, in contrast to what is characteristic of the formation of a hydride phase. At the same time, the ΔH(C) curves have a few portions where the enthalpy of reaction between hydrogen and the intermetallic compound remains constant: 0 < C < 2.0, with ΔH abs =–85.05 ± 0.65 kJ/mol H 2; 2.0 < C < 2.7, with ΔH abs =–80.64 ± 1.00 kJ/mol H2; and 1.9 < C < 2.7, with ΔH des = 76.48 ± 0.85 kJ/mol H2. The data obtained in this study suggest that positions 9e and 18g in the intermetallic compound are occupied by hydrogen in a particular order. 相似文献