首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 46 毫秒
1.
Electric permittivity * = ′ − i″ of nickel-hexammino nitrate (NHN) has been measured within the range of temperature from 9 to 300 K at a frequency of 8.8 GHz (X-band). It has been found that the phase transitions at Tk1 = 247 K and Tk2 = 90 K are discontinuous structural transitions between centrosymmetric phases, whereas the transition at Tc = 63 K is a continuous phase transition (glass?).  相似文献   

2.
A new family of high-Tc superconducting material in the Tl-Pb-Ca-Sr-Cu-O system has been synthesized. Multiple phases with two-step superconducting transitions around 110 K and 80 K were observed resistively and magnetically. The 80 K phase has been tentatively identified to be (Tl, Pb)2Sr2CuOy by X-ray diffraction analysis. This material appears to have a relatively high flux pinning.  相似文献   

3.
Se58Ge42−x Pb x (9≤x≤20) glasses have been prepared using conventional melt quenching technique. Differential Scanning Calorimetric (DSC) measurements show single glass transition and double crystallization, which indicate the occurrence of phase separation in the samples. The phases present in the samples were identified using XRD. The kinetics of the glass transition has been studied in terms of the variation of glass transition temperature with composition and heating rate. In addition to this, activation energy of the glass transition (E t ) has also been evaluated and its composition dependence is also investigated. The thermal stability of these glasses has been investigated using various stability criteria: Deiztal first glass criterion, ΔT, Saad and Poulain weighted thermal stability, H′ and the S-parameter. The values of these parameters were obtained using various characteristic temperatures such as the glass transition temperature, T g , the onset temperature of crystallization, T c , and the peak crystallization temperature, T p . The values of stability parameters show that the phase corresponding to second crystallization is more stable than the phase corresponding to first one. The stability in terms of the lead (Pb) content has been determined considering the values of stability parameters of the phase corresponding to second peak. It was found that the stability increases with the lead content.  相似文献   

4.
The glass-forming regions of the GeSe2-CdI2-TeO2 (I), GeSe2-CdI2-Bi2O3 (II) and GeSe2-TeO2-Bi2O3 (III) systems have been determined. The obtained glassy phases have been characterized by their basic physicochemical parameters such as temperatures of glass transition, crystallization, and melting, density and microhardness. The phase T-X diagram of the GeSe2-CdI2 system, which is the basic joint line for systems I and II, has been specified. Three non-variant equilibria (two eutectic and one syntectic) have been observed at temperatures 350, 280 and 375 °C for compositions containing 15, 95 and 33.3 mol% GeSe2, respectively. A new intermediate phase with probable composition of 2CdI2·GeSe2 has been formed.  相似文献   

5.
Magnetic properties of tetragonal phases in Nb—Al system have been investigated down to 0.42°K. It has been found that the-phase (Nb2Al) and the intermetallic compound NbAl3 are superconductors with superconducting transition temperature 0.74°K and 0.64°K, respectively. Magnetic susceptibility of the investigated phases does not depend on temperature within the range 4.2°K–300°K, and is equal =(1.0±0.1) × 10–6 emu)/g for Nb2Al, and =(0.9±0.03) × 10–6 emu/g for NbAl3.On leave of absence from Institute for Solid State Physics of Bulgarian Academy of Sciences, Sofia, Bulgaria  相似文献   

6.
We have investigated the transports of micro-fabricated sample of 3-K phase superconductivity (Tc  3 K) in Sr2RuO4–Ru eutectic system in order to clarify the pairing symmetry. Up to now, pure Sr2RuO4 (Tc = 1.5 K: 1.5-K phase) is widely recognized to be a spin-triplet odd-parity superconductor. However, the enhancement mechanism of Tc up to 3 K and the pairing symmetry of the 3-K phase have not been cleared yet. By using micro fabrication technique with focused ion beam, we have succeeded to extract individual superconducting channels for the 3-K phase in which only a few pieces of Ru inclusions are contained. Multiple kink structures observed in differential resistance–current (dV/dI − I) characteristics indicate serially connected superconducting filaments in the 3-K phase. We confirm that the 3-K phase is an odd-parity superconductor similar to pure Sr2RuO4 from the monotonous temperature dependence of the critical currents. In addition, we observed a quite unusual hysteresis in dV/dI − I below 2 K, which suggests the internal degrees of freedom in the superconducting state: the most probably the chiral p-wave state.  相似文献   

7.
We report on a comparative crystal chemical study of the Tl2–xBa2Ca n Cu n+1O2n+6(n=0, 1, 2) high-T c superconductors. Superconducting properties were determined by DC-conductivity and-susceptibility measurements. An X-ray diffraction analysis reveals the nonstoichiometry of the compounds as a link between structural and superconducting behavior. A comparison of the different structures gives evidence that T1 deficiency in the TlO double layers increases with increasingn; this deficiency causes apparent Cu valences larger than 2+in the superconducting phases thus maintaining the charge balance. By extracting appropriate chemical formulas and performing a bond strength calculation we obtained the mixed-valent oxidation state of copper for the superconducting tetragonal phases as being 2.17+(n=0), 2.25+(n=1) and 2.31+(n=2) respectively; for the non-superconducting orthorhombic phase Tl2–xBa2CuO6–y a Cu valence of about 2+is suggested.  相似文献   

8.
The reaction process and the reaction behavior of each component in the Bi-Sr-Ca-Cu-O system are presented in this paper. It reveals that the reaction is carried out in three different stages: forming of an insulating interphase at 680°C–790°C, forming of the 2212 superconducting phase at 790°C–860°C and forming often semiconducting phases in the presence of the liquid phase at 860°C–970°C. It is also confirmed that the 2212 superconducting phase (T c=85 K) is formed by the reaction of a trinary interphase together with CuO, SrO and CaO. A new two-step method is presented to prepare the 2212 superconducting phase by a presynthesized interphase.  相似文献   

9.
The room temperature reflectance spectra in UV-VIS-NIR region (energy range of 0.6 to 6.2 eV) for glassy, partially crystalline and its fully crystalline superconducting ceramic phases of Bi3.9Pb0.1Sr3Ca3Cu4O x have been studied by Kramers-Kronig (KK) analysis. A comparative study of the energy loss function [− Im (1/ε)] and the absorption coefficient [α(E)] has been done. Excitions in the superconducting phase hitherto evidenced by the authors are located in the polarizable layers of the superconducting cuprate and their implications for superconductivity have been pointed out. An estimate of the optical band gap energy (E g) has also been made from the linear fit ofα 2 vs.E curve for the superconducting phase. Jezierski’s method ofR-extrapolations in the higher energy has been used to show that both methods yield results that agree quantitatively and can be relied upon.  相似文献   

10.
A new 1223-type (Tl, V)-based superconducting layered cuprate (Tl1–x V x ) Sr2Ca2Cu3O9 withx=0.25 and 0.50 has been successfully synthesized in the nearly-pure form and identified by powder X-ray diffration analyses. An excessive quantity of Tl is necessary for the preparation of the (Tl, V)-based 1223-type compound. Resistance and ac susceptibility measurements showed that the new (Tl, V)-based 1223 cuprate exhibits Tc above 110 K.  相似文献   

11.
A study of the structure change with temperature in amorphous Fe100xPx (13 x 24) alloys was carried out by measuring magnetization and thermal expansion and also by structural analysis using X-ray diffraction and differential thermal analysis (DTA). The structure of the amorphous alloys relaxes (the decrease of excess free volume) at temperatures 100–150 K below the crystallization temperatures. The alloys with x 15 transform into (α-Fe + amorphous) at about 600 K. The alloys with x15 transform into (α-Fe+amorphous+Fe3P) at about 600 K. With further heating, the alloys transform into (α-Fe+Fe3P) both of which are stable phases from the equilibrium phase diagram.  相似文献   

12.
Summary Superconducting thin films of the TlBaCaCuO family have been prepared through a combined approach of MOCVD and thallium vapour diffusion. A three-step process has been adopted involving i) the synthesis of Ba−Ca−Cu−O films by MOCVD using ?second-generation? barium and calcium sources, ii) the hydrolysis of the fluoride phases by annealing in water vapour-saturated oxygen and iii) the annealing in the presence of a thallium source. Reproducible results, in terms of phase formation, are discussed depending on the different conditions of the preannealing process. The superconducting films consist predominantly of single-phase Tl2Ba2Ca2Cu3O x (2223). Preferential orientation of the crystallitec-axes perpendicular to the substrate surface has been observed. The films exhibit superconducting onset temperatures of 115–120 K with zero resistance higher than 100 K. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.  相似文献   

13.
The crystallization kinetics of Sn10Sb20−x Bi x Se70 (x=0,2,4,6,8) chalcogenide system has been studied. Crystallization studies using differential scanning calorimetry under nonisothermal conditions with different heating rates are reported. The glass transition temperature is found to increase with the increase in heating rate as well as with the addition of bismuth. The apparent activation energy for glass transition and that for crystallization have been determined using the Kissinger equation. Thermal stability and glass forming tendency have also been studied.  相似文献   

14.
(NaCl)1–x(NaCN)x mixed crystals with CN-concentrationsx of 0.87, 0.76, 0.71 and 0.65 have been investigated by X-ray powder diffraction. Apart from the cubic room temperature phase, a rhombohedral and an orthorhombic phase have been identified. In addition a glass state has been observed which is characterized by a strong broadening of the cubic powder lines. The phase diagram shows a wide coexistence gap between the non-cubic phases and the glass state.  相似文献   

15.
《辐射效应与固体损伤》2013,168(1-6):375-381
Glass ceramics of the composition (Bi0.8Pb0.2)4Sr3Ca3Cu4O8 prepared by the melt quenching technique and the crystalline phases produced by the rapid thermal annealing have been studied by electrical resistivity and electron paramagnetic resonance (EPR) measurements in the temperature range from liquid helium up to room temperature. The concentration of the EPR active Cu2? paramagnetic centers decreases as conductivity increases for the glass ceramics and disappears after crystallization and the growth of superconducting phases, similar to bulk high-Tc superconductors. The KPR spectra of both glass and crystallized ceramics after short-time annealing indicate the coexistence of Cu2? paramagnetic ions and the exchange coupled clusters.  相似文献   

16.
We review and extend a previous electronic mean field theory of superconducting glass phases. These phases are defined by vanishing ensemble averaged BCS-order parameter and non-vanishing Edwards-Anderson type averages of the inhomogeneous superconducting order parameter. Solutions are worked out for the replica symmetric case, but the possibility of replica symmetry breaking and hence ergodicity breaking is also discussed in the field theory. The order parameter for Ising-like, anisotropic and isotropicXY-like superconducting glass phases are identified by their spontaneous symmetry breaking effect in the action of the discorder ensemble. The isotropicXY-like phase is found to allow superconductivity in arbitrary strong magnetic fields. Generally the results show that: the occurrence of superconducting glass phases is supported by strong local attractive electron-electron coupling together with a high probability of nonsuper-conducting areas, the vicinity of a metal-insulator transition or the presence of a magnetic field. We suggest that for strong coupling the theory is applicable to HighT c superconductors like Ba–La–Cu–O.A first result beyond mean field approximation displays at one loop order quantum fluctuation contributions to the density of states in the superconducting glass phases. We suggest that these phases may show an infinite nonlinear dc conductivity in higher order response.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday  相似文献   

17.
High-quality powder XRD data of the compound ErFe4Ge2 collected in the ESRF beam line BM16, are presented for the entire magnetically ordered regime (TN=44 K). The data analysis reveals the occurrence of a double symmetry breaking at the magnetic transition. This experiment has allowed us to distinguish between structural and magnetic satellites, both present in the neutron patterns, and to demonstrate the interdependence of structural and magnetic transitions. The high-temperature (HT) phase disproportionates by a first-order transition into two distinct phases: P42/mnm (Tc, TN=44 K)→Cmmm (majority LT phase)+Pnnm (minority IT Phase) which coexist in proportions varying with temperature down to 4 K. The phase diagram comprises three temperature regions: (a) the HT range with T>TN for the tetragonal P42/mnm phase; (b) the IT (intermediate temperature) range, 20 K<T<TN, where the two phases coexist in strongly variable proportions and the Pnnm phase reaches its highest concentration (≈31%) around 30 K and (c) the LT (low temperature) range, 1.5–20 K, where the Cmmm phase is dominating (up to 95%). We suggests that this phenomenon is the result of competing magneto-elastic mechanisms involving the Er crystal field anisotropy, the Er–Er, Er–Fe and the Fe–Fe exchange interactions and their coupling with the lattice strains.  相似文献   

18.
Alternating Differential Scanning Calorimetric (ADSC) studies show that Se rich As20Se80-xAgx (0 ≤ x ≤ 15) glasses exhibit two endothermic glass transitions and two exothermic crystallization peaks; the observed thermal behavior has been understood on the basis that As20Se80-xAgx glasses are microscopically phase separated, containing Ag2Se phases embedded in an As-Se backbone. With increasing silver concentration, the Ag2Se phase percolates in the As-Se matrix, with a well-defined percolation threshold at x = 8. A signature of this percolation transition is shown up in the thermal behavior, as sudden jumps in the composition dependence of non-reversing enthalpy, ΔHnr obtained at the second glass transition reaction. Scanning Electron Microscopic (SEM) studies also confirm the microscopic phase separation in these glasses. The super-ionic conduction observed earlier in these glasses at higher silver proportions, is likely to be associated with the silver phase percolation.  相似文献   

19.
The glass formation ranges of thin CoZr and CuZr films condensed on cold and hot substrates (10–800 K) are experimentally determined by X-ray diffraction. The maximum substrate temperatures allowed to condense amorphous films are only slightly below the crystallization temperatures of the amorphous phases. These experimental results are compared with predictions of the glass formation ranges from thermodynamic calculations and instability models. At low substrate temperatures, where no phase separation is possible due to kinetic restrictions, the amorphous phase is favoured in the range between theT o lines which are the stability limits of the extended solid solutions on both sides of a metastable binary phase diagram. For high substrate temperatures, where at least one species of the binary alloy is able to move, the amorphous state can be described to be in a metastable equilibrium with the extended solid solutions neglecting the intermetallic compounds. Therefore, the single-phase amorphous state is restricted to the range between theT ml lines which are the metastable liquidus temperatures.  相似文献   

20.
R Prakash  O Prakash  N S Tavare 《Pramana》1988,30(6):L597-L600
X-ray and resistivity measurements on YBa2Cu3O7−δ (1-2-3) samples show that for the same but low oxygen concentration,δ⋍0·55, no superconducting transition down to 4·2 K is observed for the tetragonal phase samples while the orthorhombic phase shows aT c ∼ 31 K. The effect of oxygen concentration onT c is isolated.T c=91±1 K has, however, been observed continuously for the normal oxygen annealed samples,δ⋍0·07. The experimental results suggest strongly the necessity of the 1-2-3 compound to be in the orthorhombic phase for the superconducting mechanism to be operative.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号