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1.
The effect of different doses of γ‐rays on the behavior of the critical current density, Jc in an YBa2Cu3O7 polycrystalline sample has been investigated at high temperatures. The samples were irradiated at room temperature by a 60Co γ‐ray source at a dose rate of 0.5 MR/h. Jc was found to increase significantly with after irradiation dose of 10 MR. Further irradiation dose of 50 MR produced a slight and field dependent enhancement of Jc above its values at 10 MR. The most interesting result is that the relative change in the critical current density was found to have a non‐monotonic behavior with the applied magnetic field. These results are discussed in terms of the roles of several mechanisms created by γ‐rays in the regions of the grain boundaries combined with the effect of the magnetic field on these mechanisms. (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

2.
The dependence of the critical current density, Jc, on the gamma irradiation dose in MgB2 polycrystalline superconducting specimens has been studied. The MgB2 samples were irradiated with 10 MR and 20 MR doses of γ‐rays. The critical current density, Jc was found to be enhanced almost at all temperatures after irradiation. The enhancement of Jc was found to be linear with the irradiation dose at all temperatures. We also report on the correlation between the superconducting transition temperature and the residual resistivity ratio. Most of our results agree with the Testardi and Mooij rules which suggest that the dominant pairing mechanism of MgB2 is the phonon‐mediated interaction. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

3.
Melt‐textured YBa2Cu3Ox crystals have been irradiated along the c‐axis with 208Pb56+ ions corresponding to dose matching fields, BΦ = 0.5 T and BΦ = 2.0 T. Magnetization measurements were conducted along the ab plane of the samples. The strength of pinning sites was investigated by measuring magnetization hysterisis and the saturation remanent magnetization MR at several temperatures. We have found that the pinning strength was considerably enhanced after irradiation at both doses. Interestingly, the pinning strength at a Pb‐ion irradiation which corresponds to the dose matching field BΦ = 0.5 T, was found to be significantly larger than that at the dose matching field BΦ = 2.0 T at all temperatures. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

4.
The critical temperature and critical current of Tl‐2212 superconducting sample in the form of a tape have been studied near Tc under magnetic field and gamma irradiation. Tc decreases from 109 to 94 K with increase of magnetic field up to 300 mT. In 77‐109 K range, Jc decreases rapidly in low fields up to around 50 mT followed by a very slow decrease in Jc up to 300 mT. Tc of the sample did not change up to 100 MR γ dose and then started to decrease from 109 to 102 K with increase of g dose up to 800 MR, most of the change taking place in high doses. The critical currents of the sample decreased steadily with γ irradiation up to 600 MR after which no further change was noticed.  相似文献   

5.
The absorption spectra of undoped Y2SiO5 crystals were studied before and after γ‐irradiation. After γ‐irradiation, the additional absorption peaks at 260‐270 and 320nm were observed in as‐grown and H2‐annealed Y2SiO5 crystal, but it did not occur in air‐annealed Y2SiO5 crystal. These absorption peaks were attributed to F color centers and O hole centers, respectively. Owing to more oxygen vacancies and color centers in H2‐annealed Y2SiO5 crystal than that in as‐grown Y2SiO5 crystal after γ‐irradiation, the additional absorption peaks were more intense in the former than that in the latter. With the irradiation dose increasing from 20 to 220kGy, the intensity of additional absorption peaks increased. © 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim  相似文献   

6.
The single crystals of the ternary system based on Bi2‐xTlxSe3 (nominaly x = 0.0‐0.1) were prepared using the Bridgman technique. Samples with varying content of Tl were characterized by the measurement of lattice parameters, electrical conductivity σc , Hall coefficient RH (B∥c), and Seebeck coefficient STc). The measurements indicate that by incorporating Tl in Bi2Se3 one lowers the concentration of free electrons and enhances their mobility. This effect is explained in terms of the point defects in the crystal lattice – formation of substitutional defects thallium on the site of bismuth TlBi and the decrease of concentration of selenium vacancies VSe+2. We also discuss the temperature dependence of the power factor σS2 of the samples. Upon the thallium doping we observe a significant increase of the power factor compare to the parental Bi2Se3. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

7.
MgB2 polycrystalline superconducting specimens were irradiated with several doses of γ‐rays up to 100 MR. An increase in the normal state resistivity and a broadening of the resistive transition to the superconducting state were observed with increasing γ‐irradiation dose. Although very small changes to the superconducting transition temperature were obtained after γ‐irradiation, different temperature dependence of normal‐state resistivity and different residual resistivity ratios, RRR were obtained for different doses. We have found a correlation between RRR and the power law dependence of resistivity, n as the irradiation dose increases. This correlation may be an indication that the electron‐phonon interaction is important in these samples. These results are attributed to the disorder caused by γ‐rays. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

8.
Recharging processes of chromium ions were investigated for Mg2SiO4:Mg, Cr single crystals using annealing in O2 and in air and γ‐irradiation, as compare to YAG :Ca, Cr single crystals. The formation of tetravalent Cr ions in the Mg2SiO4 :Mg, Cr is related not only to the initial Cr content in the melt, oxygen partial pressure and O2‐ vacancy existing in the crystal, but also to the external field such as γ‐irradiation. The additional absorption after γ‐irradiation shows the decrease in intensity of the absorption of Cr3+ and Cr4+ ions in some part of the spectrum and increase in the other giving evidence on recharging effects between Cr3+ and Cr4+. There arises also color centers observed between 380 nm and 570 nm that may participate in energy transfer of any excitation to Cr4+ giving rise to Cr4+ emission. Opposite to forsterite crystal, absorption spectrum of YAG:Ca, Cr crystal after γ‐irradiation reveals only increase in the absorption of the Cr bands. The observed behavior of the absorption spectrum of YAG:Ca, Cr crystal under influence of γ‐irradiation suggests that γ‐irradiation ionizes only Cr ions. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

9.
The effect of enhanced hydrostatic pressure following heat treatment on the evolution of point defects in neutron‐irradiated Czochralski‐grown silicon is investigated using infrared spectroscopy. The behavior of oxygen‐related defects, particularly of the VO and the VO2 centers, is mainly studied using samples subjected to heat treatment under hydrostatic pressure. It is observed that (1) pressure accelerates the annealing process of the VO defects and enhances the growth of the VO2 complexes and (2) the VO2 concentration is larger than expected from the corresponding decay of the VO defects. The faster decay of the VO defects is attributed to a pressure‐induced decrease of their migration energy. The larger VO2 concentration is also discussed. One possible explanation is that pressure stimulates an additional mechanism for the formation of the VO2 defects, which involves the reaction of oxygen dimers with vacancies. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

10.
Highly c‐axis textured SrTiO3 (STO) thin films have been directly grown on Si(001) substrates using ion beam sputter deposition technique without any buffer layer. The substrate temperature was varied, while other parameters were fixed in order to study effect of substrate temperature on morphology and texture evolution of STO films. X‐ray diffraction, pole figure analysis, atomic force microscope, and high‐resolution electron microscopy were used to characterize and confirm quality and texture of the STO films. The experimental results show that optimum substrate temperature to achieve highly c‐axis textured films is at 700 °C. The full width at half maximum (FWHM) of 002STO was found to be 2° and fraction of (011) orientation was as low as 1%. The surface morphology was Volmer‐Weber growth mode with a small roughness ∼1 nm. The lowest leakage current density (5.8 μA/cm2 at 2 V) and the highest dielectric constant (εSTO ∼ 98) were found for highly c‐axis textured films grown at 700 °C. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

11.
We have studied the effect of subsequent gamma (γ) irradiation on the absorption spectra and the optical energy gap of ZnO thin films doped with Li (ZnO:Li). The optical transmission (T) and optical reflection (R) in the wavelength range 190∼800 nm of films deposited at 300 °C on sapphire, MgO or quartz substrates were measured. The dependence of the absorption coefficient α on photon energy hν was determined as a function of γ‐doses. The films show direct allowed interband transition that influenced by the gamma doses. Both the optical energy gap Eoptg and the absorption coefficient (α) were found to be γ‐dose dependent. The results can be discussed on the basis of γ‐irradiation‐induced defects in the film and on the film structure. The absorption coefficient exhibits exponential dependence on photon energy obeying Urbach's rule in the absorption edge. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

12.
The effect of chemical non‐stoichiometry and γ‐irradiation on the unit cell parameters of ammonium tetrachlorozincate (NH4)2ZnCl4 (A2ZC4) has been studied. The unit cell parameters of crystal grown from solution with NH4Cl/ZnCl2 molar ratio 1:1, apparently non‐stoichiometrric, are nearly the same as those given for ammonium tetrachlorozincate in the literature. The 2:1 ratio is actually ‘pseudo‐stoichiometric’ due to the hygroscopic nature of ZnCl2. The unit cell parameters of crystal grown from solution with molar ratio 2:1 match those of the structure (NH4)3ZnCl5 (A3ZC5). The habit of the crystal grown in the former case, from solution with excess ZnCl2, was different from that of the crystal grown in the later case, from solution with excess (NH4)Cl. Between these two limits, a set of four samples were prepared from solutions with an excess of ZnCl2 of 20, 30, 60 and 80 wt% in order to detect exact stoichiometric composition to grow A2ZC4. Analysis by X‐ray diffraction shows that the first two crystals out of this set are mixed from A2ZC4 and A3ZC5 The third and fourth crystals still contain traces of A3ZC5. Analysis of the X‐ray diffraction was then confirmed by DTA study. Irradiating A2ZC4 with γ‐dose of 250 kGy slightly increased the unit cell volume due to imperfections created by irradiation. Two computer programs were used to calculate the lattice constants and the results were compared. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

13.
This paper reports on a theoretical analysis of spin‐Hamiltonian parameters and local lattice structure for Pd(I) center in γ‐irradiated Pd(II)(acac)2. Through the crystal‐ and ligand‐field theory, the microscopic spin‐Hamiltonian parameters and local molecular structure for Pd(I) center in the γ‐irradiated Pd(II)(acac)2 system have been studied by using the high‐order perturbation formulas and Newman's superposition model. Based on these calculations, it was found that the distance of the metal‐ligand bonds in the square planar complex for Pd(I) center in the γ‐irradiated Pd(II)(acac)2 system increases by 0.1Å. To understand the detailed physical and chemical properties of the [Pd(I)(acac)2]2– complex, the contributions of the spin‐orbit coupling of ligand to spin‐Hamiltonian parameters for Pd(I) ion are considered. The theoretical results are in reasonable agreement with the experimental values. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

14.
The magnetic and transport properties of polycrystalline YBa2 (Cu1‐xMx)3 O7‐δ (M = B and Mn) superconductor was investigated. Samples of YBa2(Cu1‐xBx)3O7‐δ doped with several concentrations of boron B(x = 0.05 and 0.1) were investigated using magnetization measurements. A YBa2(Cu1‐xMnx)3O7‐δ sample doped with Mn with concentration of x = 0.02 was investigated using current‐voltage (I‐V) measurements. Our results on the YBa2(Cu1‐xBx)3O7‐δ samples reveal a considerable increase in the hysterisis width of the magnetization, M versus the applied magnetic field H with increasing boron concentration. The lower critical field was also found to be enhanced by boron doping. The critical current density, Jc was found to be significantly enhanced in the Mn‐doped sample. The enhancement of Jc was found to be more significant at the lower temperatures for all applied magnetic fields used (0 Oe, 300 Oe, and 500 Oe). Thus, chemical doping is suggested to enhance the vortex pinning forces in the YBCO samples. From the resistivity (R‐T) measurements, chemical doping of the samples was found to have no significant effect on the critical temperature, Tc. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

15.
Large and high‐quality single crystals of both Pb‐free and Pb‐doped high temperature superconducting compounds (Bi1‐xPbx)2Sr2Ca2Cu3O10‐y (x = 0 and 0.3) were grown by means of a newly developed “Vapour‐Assisted Travelling Floating Zone” technique (VA‐TSFZ). This modified zone‐melting technique was realised in an image furnace and allowed for the first time to grow Pb‐doped crystals by compensating for the Pb losses occurring at high temperature. Crystals up to 3×2×0.1 mm3 were successfully grown. Post‐annealing under high pressure of O2 (up to 10 MPa at T = 500°C) was undertaken to enhance Tc and improve the homogeneity of the crystals. Structural characterisation was performed by single‐crystal X‐ray diffraction (XRD) and the structure of the 3‐layer Bi‐based superconducting compound was refined for the first time. Structure refinement showed an incommensurate superlattice in the Pb‐free crystals. The space group is orthorhombic, A2aa, with cell parameters a = 27.105(4) Å, b = 5.4133(6) Å and c = 37.009(7) Å. Superconducting studies were carried out by A.C. and D.C. magnetic measurements. Very sharp superconducting transitions were obtained in both kinds of crystals (ΔTc ≤ 1 K). In optimally doped Pb‐free crystals, critical temperatures up to 111 K were measured. Magnetic critical current densities of 2�105 A/cm2 were measured at T = 30 K and μ0H = 0 T. A weak second peak in the magnetisation loops was observed in the temperature range 40‐50 K above which the vortex lattice becomes entangled. We have measured a portion of the irreversibility line (0.1‐5 Tesla) and fitted the expression for the melting of a vortex glass in a 2D fluctuation regime to the experimental data. Measurements of the lower critical field allowed to obtain the dependence of the penetration depth on temperature: the linear dependence of λ(T) for T < 30 K is consistent with d‐wave superconductivity in Bi‐2223. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

16.
Refractive indices and effective electro‐optic coefficient γc of (1–x)Pb(Zn1/3Nb2/3)O3xPbTiO3 (PZN‐xPT, x = 0.05, 0.09 and 0.12) single crystals were measured at 532 nm wavelength. Orientation and temperature dependences of the electro‐optic coefficient were investigated. Large electro‐optic coefficient (γc = 470 pm/V) was observed in [001]‐poled PZN‐0.09PT crystal. More importantly, γc of tetragonal PZN‐0.12PT is almost unchanged in a temperature range −20 ∼ 80 °C. The γc of PZN‐xPT single crystals are much higher than that of widely used electro‐optic crystal LiNbO3 (γc = 20 pm/V). These results show that PZN‐xPT single crystals are very promising materials for electro‐optic modulators in optical communications.  相似文献   

17.
The octahedra were observed in the γ CuI crystallites synthesized by hydrothermal method using 1.6g (C2H5OO)2Cu and 2.66g KI as precursors at 200 °C for 12 hours in the de‐ionized water. The effect of additives on the morphology of γ‐CuI crystallites was investigated. Results show that the morphology of γ‐CuI crystallites prepared in the mixed solution of de‐ionized water and alcohol at 200 °C for 12 hours is the tetrahedron. In order to disclose the effect of the additive on the growth habit of γ‐CuI crystallites, the microcosmic growth mechanism of γ‐CuI crystal is investigated from the complex of I and Cu+ ions to each other. It is concluded that the effect of alcohol on the morphology of γ‐CuI crystallites is carried out through changing the relative rate of complex of anion and cation to each other at the interface. Based on the above analysis, the growth habit of γ‐CuI crystallites and the habit variation under hydrothermal conditions are explained reasonably.  相似文献   

18.
Superconducting samples of the composition Bi2-xPbxCa2Sr2Cu3Oy (x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) are prepared by co-decomposition of metal nitrates. DTA, TGA, density, and porosity studies have been performed on these samples. Characterisation techniques like XRD, SEM, EDXA, d.c. resistance, and a.c. magnetic susceptibility have been employed to study the growth of High-Tc (HTP) and Low-Tc (LTP) phases upon lead substitution for bismuth. Results have revealed the growth of HTP with increase in Pb conc. up to x = 0.3. Further increase in Pb appears to results in the deterioration of HTP resulting in the formation of LTP and Ca2PbO4 impurity phase. Transport critical current density (Jc) measurements performed on the samples indicate that Pb doping followed by densification remarkably improves the current carrying capacity of the materials.  相似文献   

19.
Plasma spraying is a potential technique for forming flexible tapes from brittle high Tc oxides. It is possible to obtain superconducting Bi(Pb) Ca Sr Cu O coating by suitable heat treatment schedule after spraying. In an effort to get maximum transport current densities (Jc) of the coating, the content of lead and sintering time have been optimised. A Jc value of 200 Amp/cm2 is obtained in Bi1.4Pb0.6 · Ca2Sr1.9Cu3Oy specimen coated on silver sprayed Fe[(Ag)/Fe] substrate. Remarkable improvement in Jc values up to 694 Amp/cm2 is obtained in the same specimen coated on Ca2Sr1.9Cu3Oy sprayed Fe[Ca2Sr1.9Cu3Oy)/Fe] substrate. The observed decrease in Jc(B) curves with increase in magnetic field shows the presence of weak coupling between the grains.  相似文献   

20.
New and high quality piezoelectric crystals La3Ga5SiO14 (LGS) grown by the Czochralski method in a Platinum or Iridium crucible are reported in this paper. The growth defects in the LGS crystals were investigated by Transmission electron microscopy (TEM). It was found that cracks, inclusions, grain boundary and thermal stress in the LGS crystals. Their formation mechanisms and the method of eliminating these defects are discussed. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

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