共查询到20条相似文献,搜索用时 15 毫秒
1.
A. Pollini A. C. Mota P. Visani R. Pittini G. Juri T. Teruzzi 《Journal of Low Temperature Physics》1993,90(1-2):15-53
Strong flux creep effects are observed in CeCu2Si2 at low magnetic inductions and temperatures in the range 6 mK<T<T
c
. Measurements of flux creep in single crystal as well as polycrystalline specimens show that the magnetization in CeCu
2
Si
2
decays following a power law time dependence of the typeM(t)t
– for 1<t<10
5
sec, where the exponent a ranges between 0.015 and 0.06. The activation energy values obtained from the relaxation rates are around 2–3 meV. The flux creep data give clear evidence of the existence of different types of pinning centers in CeCu
2
Si
2
: a large number of shallow potential barriers that characterize the vortex dynamics at short times and few strong pinning centers responsible for the long-time relaxation behavior. Quantum tunneling of flux lines through weak pinning centers probably gives rise to the non-thermally activated relaxation observed at the lowest temperatures. The physical origin of weak and strong pinning centers in CeCu
2
Si
2
is discussed. From isothermal dc magnetization measurements we estimate the lower critical fieldH
c1
as well as the critical current densityJ
c
. TheH
c1
versusT curve shows an anomalous positive curvature for temperatures close toT
c
. 相似文献
2.
True zero-field critical current densityJ
c
of a well-characterized BPSCCO/Ag tape has been determined by means of high-resolution ac susceptibility in the temperature range 77–110 K. The resultant values (30,000 A/cm2 at 77 K) agree well with the transportJ
c
of the same tape. Because of a very thin BPSCCO, the coreJ
c
determined from the imaginary part of the ac susceptibility is nearly the same as the zero field one. AllJ
c
's follow the same (1-T/T
c
)n withn=1.45 dependence.J
c
shows an approximateH
–0.5 field dependence over the explored temperature range. Accordingly, the variations ofJ
c
withT andH seem to be determined by the flux creep. 相似文献
3.
A. M. Saleh M. M. Abu-Samreh G. M. Al-Awaysa R. M.-L. Kitaneh 《Journal of Superconductivity and Novel Magnetism》2008,21(4):229-235
The ac susceptibility data was employed to extract the temperature dependence of the critical current density, J
c(T), as well as the variation of flux-creep exponent n(T,H
ac) with temperature and ac field amplitude in bulk samples of polycrystalline magnetic superconductor RuSr2GdCu2O8 (Ru-1212). The critical state models and the collective flux-creep approximation model were successfully accounted to describe
such behavior below the transition temperature. The calculated values of n(T,H) are well fitted to a power law of the following form: n(T,H)=n
0(H)T
−s(H), where s is field dependent exponent whose values varied from −2.4, −1.01 for field amplitudes ranging from 0.5 G and 3.8 G. The power
law describing the frequency dependence of χ′ is found to be consistent with the results of the current-dependent effective activation energy of the form U(J)=U
0ln (J
c/J).
Additionally, the dependence of the current density is found to scale according as: J
c(T)=J
c0(1−T/T
c)
n
, where the exponent n values varied from 1.05 to 1.25. Such dependence is an indication of intergrain coupling that could be ascribed in terms
of superconductor–insulator–superconductor junctions. The derived temperature dependence of J
c(T) is in good agreement with the data obtained from the measurements using the traditional “loss-maximum” approach. Furthermore,
the flux-creep effect increased with increasing both ac fields and temperatures except at about 15–25 K below the onset of
T
c, where a slowing down of the flux creep was observed. 相似文献
4.
Shih-Yun Chen Yung-Sheng Hsiao Chi-Liang Chen Der-Chung Yan In-Gann Chen Maw-Kuen Wu 《Materials Science and Engineering: B》2008,151(1):31
This study presented the superior superconductivity of Y–Ba–Cu–O (YBCO) bulk materials grown by using modified infiltration growth (IG) method with a small Sm–Ba–Cu–O crystal as seed. Compare to the bulks grown by melt-textured growth (MTG) method, IG bulk is denser and with finer second phase (Y2Ba1Cu1O5, Y211). In addition, remarkable peak effect was observed of the IG YBCO bulk material. Peak effect in Jc(H, T) is an distinctive property of RE–Ba–Cu–O (RE = rare earth element, e.g. Sm, Nd) superconductors. However, it is found that peak effect is unusual in Y–Ba–Cu–O material. In this study, magnetism measurements also show peak effect in Jc(H, T), especially at low temperatures. At 50 K, the peak position of Jc(H, T) was about 5 Tesla. Jc value of peak, Jc(peak, 50 K), is comparable to that of self-field, Jc(0, 50 K). On the other hand, it is noted that this effect is spatial-dependent, i.e. it only can be found in particular part of bulk. To interpret the mechanism of peak effect and the spatial-dependence, different microstructure analysis, which includes scanning electron microscopy (SEM), energy disperse spectroscopy (EDS) and X-ray absorption spectrum (XAS) were used. The results indicated that the composition is inhomogeneous throughout the bulk. Diffusion of Sm from seed to YBCO bulk was observed, which is believed to occur during high temperature, and thus changed the superconductivity. The inhomogeneity of microstructure was correlated to the spatial-dependent of peak effect. Finally, this study also demonstrates a possible way to introduce peak effect in YBCO materials and thus enhance the applications in high field regions. 相似文献
5.
Magnetic relaxation, with magnetic fields applied parallel to the c axis, was measured in a high-quality melt-textured YBa2Cu3Ox bar. Zero-field-cooled magnetization-versus-time data were obtained within a regime of field and temperatures dominated by single vortex pinning. The activation pinning energy U = U
M
was calculated from experimental data and compared to U = U
0
ln(J
c
/J), where U, is a constant, J
c
is the critical current density, and J the current density. This expression for U obtains within the individual pinning regime. A logarithmic dependence of U
M
on J was found, and the data indicated that a scaling function of U
M
, g(T/T
c
) = (1–T/T
c
)1/2, was appropriate for describing the single-vortex pinning regime. 相似文献
6.
Ana Celia Mota Elisabeth Dumont James L. Smith 《Journal of Low Temperature Physics》1999,117(5-6):1477-1481
We have found a sharp transition at T
c2
= 350 mK in the vortex creep rate of a single crystal of (U
1–x
Th
x
)Be
13
with T
c
= 523 mK (x = 0.0275). For T T
c2
, no creep of vortices is observed in a time scale of 105
s, while for T
c2
< T < T
c
, vortices creep at very high rates (30% of decay from a metastable configuration in the first 105
s at T = 400 mK). The sharp transition occurs at the same temperature at which the second jump in the specific heat appears in these samples. Similar low levels of creep rates have been reported by us in the low–T superconducting phase of UPt
3.1 相似文献
7.
D. T. Jacobs S. M. Y. Lau A. Mukherjee C. A. Williams 《International Journal of Thermophysics》1999,20(3):877-887
A ground based (1g) experiment is in progress that measures the turbidity of the density-matched, binary fluid mixture methanol–cyclohexane extremely close to its liquid–liquid critical point. By covering the range of reduced temperatures t (T–T
c)/T
c from 10–8 to 10–2, the turbidity measurements should allow the Green–Fisher critical exponent to be determined. This paper reports measurements showing ±0.1 % precision of the transmitted and reference intensities, and ±4K temperature control near the critical temperature of 320 K. Preliminary turbidity data show a nonzero consistent with theoretical predictions. No experiment has precisely determined a value of the critical exponent , yet its value is significant to theorists in critical phenomena. Relatively simple critical phenomena, as in the liquid–liquid system studied here, serve as model systems for more complex behavior near a critical point. 相似文献
8.
Hans Meissner 《Journal of Low Temperature Physics》1970,3(5):563-576
Using pulsed currents of 80-nsec duration the stability of supercurrents in cylindrical films with current return through the center has been investigated. It is found that the voltage along the film rises with timet approximately asV = a[1 – erf(t/2(t – c))
1/2] where the time constantt is an exponentially decreasing function ofi(1 + 420/d)/(1 –(T/T
c)4). Herei is the current through the sample,d the thickness of the film in angstroms, andT/T
c the reduced temperature. Comparison with older data by Mydosh and Meissner and by Hagedorn indicates that it is indeed the currenti, rather than the current densityJ, which determinest. It is suggested that the instability is of the nature of those investigated by Kramer and by Fink and Presson for currents in a superconducting surface sheath.Supported in part by the National Science Foundation and in part by the Cryogenics Center through a Grant from the DOD Themis Program. 相似文献
9.
Submicrometer epitaxial films of YBa2Cu3O7(YBCO) on (100) LaAlO3 were made by coevaporation and furnace annealing. Samples from more than a dozen runs are used in this study. The zero resistance transition temperature (T
c) is high (89 or 90 K) if the film composition is phase pure (Ba/Y=2, Cu/Y=3) or if it is enriched in Ba and Cu. For these compositions the critical current density (J
c) at 77 K has an average value of 2×105 A cm–2, with a tendency for decreasingJ
c with increasing film thickness (0.2 to 0.8m). Variations inJ
c are not correlated with deviations from ideal stoichiometry. Steeper slopes of the resistance-temperature curves above 100 K and lower values of the room-temperature resistivity are associated with high values ofJ
c. If the film composition is enriched in Y relative to Ba and Cu,T
c decreases by several degrees. 相似文献
10.
In this study, the measured curves of AC susceptibility (ACS) components, (T) and (T), of polycrystalline RuSr2GdCu2O8 (Ru-1212) superconductor were scaled onto a single curve using the peak temperature of its imaginary part (T
p) as the scaling parameter for various AC field amplitudes from 0.5 to 24 G. The dependence of the AC magnetic field amplitude on T
p is scaled as: H
ac (1–T
p/T
c)2.25. Similarly, the current density J
c, extracted from the AC field amplitude is also scaled as: J
c (1 – T
p/T
c)2.25. The dependencies of T
p on frequency and AC field amplitude are also investigated and the time parameter t
0 of the order of 10–8 s is estimated from the dependence of T
p on frequency. The dependencies of activation energy on temperature, T, and the field amplitude, H
ac, are obtained from the Arrhenius-like semilog plot of frequency () and T
p. Such dependencies on temperature and field amplitude can be described by a scaling law of the form: U(H
ac,T) = U
0[1 – T/T
p]H
ac
–0.17. 相似文献
11.
Three types of bismuth-based bulk samples were prepared through uniaxial pressing at room temperature, hot isostatic pressing (HIP) and drawing and rolling. Transport current properties were characterized in a steady field up to 1.12 T at 77 K (T/T
c=0.75). The Josephson weak-link decoupling fields have been found to be 5 mT for the cold-pressed pellet and 30 mT for the HIPed pellet and the rolled tape. At the decoupling field the transport critical current density,J
c, drops 80% from 124 (OT) to 29 A cm–2 (5 mT) for the cold-pressed pellet, 80% from 582 (OT) to 126 A cm–2 (30 mT) for the HIPed sample and 50% from6500 (0 T) to 2850 A cm–2 (30 mT) for the rolled tape. In the flux flow regime, whereB is perpendicular to thec-axis a modified Kim's modelJ
c=(/B
0)/[(1+B/B
0)]
n
can be used to describe the field dependence of the critical current density, Jc, in the field range 0.2–1.12 T. The effective upper critical fields were estimated to be 0.98, 1.54 and 1.94 T for the three types of samples, respectively. An adjustable range ofB
c2 for bismuth-based bulk highT
c superconductors is given. Flux shear may operate in these materials. The prediction of this pinning mechanism is yielded from fitting the equation qualitatively. WhenB is parallel to thec-axis, the absence of strongly intragranular flux-pinning is emphasized by the poor flux flow regime for the rolled tape sample. 相似文献
12.
F. R. Shklyar V. N. Timofeev V. S. Shvydkii Yu. G. Yaroshenko B. A. Bokovikov 《Journal of Engineering Physics and Thermophysics》1971,20(5):589-595
Results are given of an analytic investigation of transient processes inside counterflow apparatuses and heat exchangers with temperature disturbance in one of the heat carriers at the entry to the apparatus.Notation
=(t–t0)/(T0–t0),=(T–t0)/(T0 s-t0)
relative temperatures
- t, T
temperatures of material and gas respectively
- t0, T0
same for the initial state
- Z=[
Vm1/c(1–w/wg)] [–(y0–y)/wg]
dimensionless time
- m1=1/(1+Bi/)
solidity coefficient
- B1=(
FR/)
Biot number
-
F
V
heat-exchange coefficients referred to 1 m2 surface and 1 m3 layer
- R
depth of heat penetration in a portion
-
portion heat conductivity coefficient
-
shape coefficient (=0 for a plate,=1 for a cylinder,=2 for a sphere)
- c, Cg
heat capacities of material and gas respectively
-
,
g
volumetric masses
- w, Wg
flow velocities of material and gas
-
y
distance from the point of entry to the heating heat carrier
- y0
heat-exchanger length
- Y=
Vm1y/WgCg
g
dimensionless coordinate
- m=cw/Cg–
gWg
water equivalent ratio
Deceased.Translated from Inzhenerno-Fizicheskii Zhurnal, vol. 20, No. 5, pp. 832–840, May, 1971. 相似文献
13.
Tzuen Rong Yang Gheorghe Ilonca Aurel Pop G. Ştiufiuc C. Lung 《Journal of Low Temperature Physics》2003,131(3-4):539-543
The scaling behavior of the effective activation energy U(T, H) of epitaxial oriented Bi2Sr2Ca1–x
Er
x
Cu2O8+d
thin films with 0x0.10 has been studied as function of temperature and magnetic field. It has been found that the activation energy scales as U(H, T)=U
0(1–t)m
H
n
with exponent m=1.20±0.03 and n=–1/2. The field and temperature dependence of the activation energy, as well as its overall magnitude, is consistent with a model in which U(T, H) arises from plastic deformations of viscous flux liquid above the vortex-glass transition temperature. 相似文献
14.
The crystallization of pure tellurite glass during various heating rates was studied. The activation energy for crystallization was 115 × 1022 eV mol–1. The glass transformation, T
g, starting crystallization, T
x, crystallization, T
c and melting temperatures, T
m, have been reported for binary tellurite glasses of the form (1 – x) TeO2–xAnOm [AnOm = MnO2, Co3O4 and MoO3]. Among many different parameters of the glass forming potential the two-thirds rule, T
g/T
m, the glass stabilization range, T= T
x – T
g, and the glass forming tendency, K
g= (T
c– T
g)/(T
m – T
c), are reported for the first time for tellurite glasses. 相似文献
15.
Akio Kagawa Taira Okamoto Kazuo Saito Minoru Ohta 《Journal of Materials Science》1984,19(8):2546-2554
Hot hardness was measured on the primary carbides, (Fe, Cr)3C and (Fe, Cr)7C3, in unidirectionally solidified iron-carbon-chromium hypereutectic alloys with chromium more than 4.8 wt %. The hardness-temperature relation was represented by two Ito-Shishokin formulae,H
v =A(— BT), and thus was drawn by two lines on a semilogarithmic graph. The inflection temperature where the two lines intersected was found at 730 to 860 K for (Fe, Cr)3C carbide containing 0 to 14 wt % Cr, increasing with an increase in the chromium concentration in the carbide, and at about 910 K for (Fe, Cr)7C3 carbide containing 36 to 76 wt % Cr. With increasing chromium concentration in each carbide, the hardness of the carbide increased and the thermal softening coefficients decreased. The effect of chromium on the hardness, the inflection temperature and the thermal softening coefficients was more pronounced for (Fe, Cr)3C carbide than for (Fe, Cr)7C3 carbide. Each of the thermal softening coefficients,B
1(T<T
t),B
2(T>T
t), the inflection temperature,T
t, room-temperature hardness,H
v(T
RT), and the hardness atT
t,H
v(T
t), related linearly to the chromium concentration in the carbides, and hence the hot hardness of the carbides could be expressed as functions of temperature and chromium concentration in the carbides. The relationships betweenH
v(T
RT) andH
v(T
t) and between the thermal softening coefficient,B
2, and the activation energy for creep,Q
c(kJ mol–1), were represented by the following equations:H
v(T
t)0.7H
v(T
RT),B
2=1.26/Q
c. 相似文献
16.
Andreas Rydh Magnus Andersson Östen Rapp 《Journal of Low Temperature Physics》1999,117(5-6):1335-1339
We have studied the driven vortex lattice in untwinned, clean YBa
2
Cu
3
O
7–
single crystals, showing the first order (melting) transition T
m
. At high enough driving currents (j 10
3
A/cm
2
, j j
c
) and temperatures T < T
m
, a clear distinction is found between two different behaviours of the moving vortex lattice. The onset of dissipation is characterized by a noisy flux creep with a temperature independent activation energy U(j). At higher temperatures, the creep regime crosses over into a flux flow regime with linear resistivity. Apart from the dip in resistivity at T
m
, associated with the peak effect and usually assigned to a softening of the shear modulus, the resistivity of the flowing flux lattice continuously extends into the vortex-liquid. 相似文献
17.
G. R. Somayajulu 《International Journal of Thermophysics》1988,9(4):567-575
Two new equations are proposed for the enthalpy of vaporization from the triple point to the critical point. One of these equations containing four parameters is exceptionally good for fitting the data. The other equation containing three parameters is quite adequate for fitting the data but it is exceptionally suited for interpolation when the data do not cover the entire range. These equations have been tested using the enthalpy of vaporization of water from the triple point to the critical point and are compared with other equations.Nomenclature
T
c
Critical temperature, K
-
T
t
Triple point, K
-
T
x
Any particular temperature, K
-
T
r
Reduced temperature
-
P
r
Reduced pressure
-
R
Gas constant
-
P
Vapor pressure
-
X
(T
c–T)/T
c
-
Y
(T
c–T)/T
-
X
x
(T
c–T)/(T
c–T
x)
-
X
t
(T
c–T)/(T
c–T
t)
-
H
vt
Enthalpy of vaporization at the triple point, kJ · mol–1
-
H
vx
Enthalpy of vaporization at any temperature x, kJ · mol–1
-
Z
v
Compressibilty factor of the saturated vapor
-
Z
1
Compressibilty factor of the saturated liquid Relative deviation = 100[Hv(obs)–Hv(cal)]/Hv(obsd) Standard deviation = { [H
v(obs)–H
v(cal)]2/(No. points — No. parameters)}0.5 相似文献
18.
Q. Liu S. L. Wen D. S. Yan M. H. Fang Z. K. Jiao Q. R. Zhang 《Journal of Superconductivity》2000,13(1):73-76
Considerably improved flux pinning and critical current density J
c values have been achieved in Y-deficient Y-123 superconductors by directional solidification in air. In comparison with the regular Y-123 composition, Y-deficient one also has an orthorhombic structure and Y-123 main crystal phase remains in it. Whereas with the shortage of Y, Y1–x
Ba2Cu3O7–y can be regarded as (YBa2Cu3)1–x
O7–t(Ba2Cu3)
x
O
t–y
or (YBa2Cu3O7–z
)(
x
YO
z–y
), so there may develop several kinds of microstructure defects as pinning sites in the system, such as highly dense, fine-scale, and faultlike defects, as well as localized superstructure, which are able to induce the increasing in flux pinning and J
c values in higher external magnetic fields. This kind of simple nonstoichiometric route could lead to a commercial technique for flux-pinning enhancement in Y-123 bulk materials. 相似文献
19.
B. K. Roul 《Journal of Superconductivity》1993,6(4):285-290
Gd0.95Ba2Cu3.05O7–: Agx (x=0.0, 0.02, 0.04, 0.06) ceramic superconducting samples were prepared by a co-precipation technique using organic carbonates in the presence of stable polymers. The room-temperature normal-state resistivity decreases with Ag addition. However,T
cand crystallographic parameters were uneffected. Low-temperature (4.2 K) magnetic and electrical transport studies in high magnetic fields (5 T) reveal that the addition of Ag into the Gd0.95Ba2Cu3.05O7– (GBCO) enhances the magnetic critical current density (J
mc), volume pinning force (F
p), flux explusion, and transport critical current density (J
tc). Microwave-induced dc voltage studies show the reduction in the total number of weak links between superconducting GBCO grains with increase in Ag concentration. The flux creep rate was also increased with increase in Ag concentration in GBCO resulting in stronger pinning potential. The increase inJ
mc, Jtc, and, hence,F
pwith Ag addition in the GBCO suggests the creation of an SNS-type proximity junction at the intergrannular region and stronger Josephson current paths between the superconducting intergrains, which may be due to the physical densification and reduction of the total number of weak links by Ag addition into the GBCO superconducting system. 相似文献
20.
The upper critical fieldH
c2 has been measured for the superconducting amorphous alloys Zr100–x
Si
x
withx=13, 15, and 17 over a wide temperature range down to 50 mK. Measurements for related properties, such as the depinning current densityJ
p
, pinning force densityF
p
, and flux flow resistivity
f
, reveal that these systems are well characterized as typically soft and dirty type II superconductors with an extremely lowJ
p
of 1 A/cm2 andF
p
of 1.5×104 N/m3 ath (H/H
c2)=0.5 andt (T/T
c
)=0.5. The temperature dependence ofH
c2 is well fitted by a curve of the Werthamer-Helfand-Hohenberg theory with Maki parameter =1.7–1.5 and spin-orbit interaction parameter so=1–2.4. These results indicate that the Pauli spin-paramagnetic effect plays an important role in the low-temperature behavior ofH
c2 in these low-T
c
and high-H
c2 amorphous superconductors. Corrections of the electron-phonon coupling (ep=0.6) to the Pauli spin-paramagnetic limiting field lead to a reduced importance of the spin-orbit interaction effect, with so less than 1.0. A discussion is given of the structural inhomogeneities in an amorphous phase, which are sensitively reflected in the strength of the fluxoid line pinning interaction (J
p
,F
p
) and are intimately related to the nonintrinsic anomaly, observed in many amorphous alloys, thatH
c2 depends linearly on temperature even at low temperatures, exceeding the theoretically allowedH
c2 values. 相似文献