共查询到10条相似文献,搜索用时 218 毫秒
1.
N. Balchev K. Nenkov B. Kunev A. Souleva Ts. Tsacheva S. Hoste 《Journal of Superconductivity》2001,14(6):713-717
The effect of Sn doping in (Pb0.6Sn
y
Cu0.4 – y
)Sr2(Y1 – x
Ca
x
)Cu2O
z
with 0 y 0.3 and 0 x 0.7 was investigated. It was established that a nearly pure 1212 phase can be obtained at 0 y 0.1 and 0 x 0.3. The obtained XRD patterns as well as the results of the EDX and ICP-AES analyses showed that Sn substitution is possible in the (Pb,Cu)-1212 phase. Superconductivity was observed at 0.4 x 0.7. The onset of the diamagnetic transitions varied from 10 to 30 K. The influence of the strong Pb deficiency on the superconducting properties of the samples was discussed. 相似文献
2.
J. Halbritter 《Journal of Superconductivity》1992,5(4):331-337
Surface impedance measurements in the normal and superconducting state are an excellent method to study conduction electron dynamics and extended defects. Electron dynamics show up most clearly in the relaxation range, i.e., for distances traveled in one rf periods=
F/ (
F Fermi velocity) being smaller or of the order of the penetration depth and mean free pathl. For materials with
F107 cm/sec the relaxation range is easily accessible forf0.1 THz. Then, in the normal state, relaxation defines the surface impedance with an intrinsic penetration depth
I approaching the London penetration depth
L andR
I
0
L/ 2 as surface resistance, allowing measurement of
L and relaxation time(T, ). In the superconducting state the photon interaction scales with
L/
L=1/(
F dimension of Cooper pairs forl) and causes at low frequencies an absorption rate growing with, which is decreasing with
F/l. The rate increase proportional to turns to a decrease above 0.1 THz, which is accompanied by a decrease ofA with frequency which is stronger for large and small
F/l. These characteristic dependences allow measurement of material parameters, anisotropy, and dynamics of electrons, especially the relaxation rate. But presently, the rf surface impedanceZ is still shrinking with material improvements, which shows, clearly, that theZ=Z
I+Z
res is still dominated by extrinsic properties summarized inZ
res. We present evidence thatZ
res is due to the large leakage currentj
bl and the smallj
cJ of weak links where the latter destroys the intrinsic shielding from a
I-thin seam
J deep into the bulk. This causes rf residual lossesR
res(
0)2
J
3
bl/2.R
res stays finite atT-0 due to
bl(T0)
bl(j
bl) being amplified by (
J/
I)3>103 as a weighting factor. The appropriate measure of weak links are the grain-boundary resistanceR
bn((0)) enhancing
JR
bn andR
resR
bn
2
. Thus,Z
res is minimal for minimal extrapolated resistivity(T0).To identify the weak links as a new entity, the H-field dependence is most helpful, because at very low fieldsH
c
1J1/
J Josephson fluxons penetrate into the weak links. These Josephson fluxons show negligible flux flow or flux creep, and enhanceZ
res by
J(H, T)
. The measuredj
cJ(H, T) andj
bl values explainZ
res quantitatively as well as in temperature (a+T
n
) (n1,T<T
c
/2) and in field (b+H
n
) (n1,H>H
c1J) dependence. The strength of the field dependencedZ
res/dHZ
res(H
c1J
)/H
c2J(T) is not only a measure ofZ
res andH
c2J(T) but is crucial for nonlinear effects and (fluxon) noise also, which limit the performance of rf devices. 相似文献
3.
S. Balamurugan Om Prakash B. D. Padalia I. K. Gopalakrishnan J. V. Yakhmi P. Selvam 《Journal of Superconductivity》2001,14(6):687-691
The nominal (Hg0.7Cr0.3)Sr2(Ca0.7Y0.3)Cu2O6 + , referred to as (Hg,Cr)-1212, were successfully synthesized at high temperature in partial vacuum. The samples were subsequently annealed in Ar or O2 annealed at 300°C under ambient pressure. No T
c was observed in any of these samples down to 12 K which is in contrast to high pressure synthesized Y-free (Hg,Cr)-1212 phase where the latter exhibits a superconductivity at 80 K. The divalent state of Cu, as shown by XANES in the as-prepared (Hg,Cr)-1212, is not affected either by Ar or O2 annealing. Depletion of holes due to trivalent Y substitution at the Ca-site, and a relatively higher 'a' lattice parameter (3.851 Å) are the apparent reasons for the non-superconducting nature of the samples. 相似文献
4.
The field dependence of real () and imaginary () component of ac susceptibility of superconductors within the critical state model can conveniently be used for evaluating the critical current when field amplitude is larger than the penetration field. A method to analyze the real () and imaginary () component of fundamental ac susceptibility with the objective of extracting the temperature dependence of critical current density J
c(T) is reported. The procedure makes use of the ac susceptibility data of two polycrystalline (Bi-Pb)-2223 samples measured with different excitation amplitudes below and close to the critical temperature. The temperature dependence of J
c is extracted using the isothermal scan over and data. Results obtained from this procedure are found to be in fair agreement with J
c(T) calculated from traditional loss-maximum data. 相似文献
5.
The dielectric properties of chemically vapour-deposited (CVD) amorphous and crystalline Si3N4 were measured in the temperature range from room temperature to 800° C. The a.c. conductivity (
a.c.) of the amorphous CVD-Si3N4 was found to be less than that of the crystalline CVD-Si3N4 below 500° C, but became greater than that of the crystalline CVD-Si3N4 over 500° C due to the contribution of d.c. conductivity (
d.c.). The measured loss factor () and dielectric constant () of the amorphous CVD-Si3N4 are smaller than those of the crystalline CVD-Si3N4 in all of the temperature and frequency ranges examined. The relationships of
n-1, (-
)
n-1 and/(-
) = cot (n/2) (were observed for the amorphous and crystalline specimens, where is angular frequency andn is a constant. The values ofn of amorphous and crystalline CVD-Si3N4 were 0.8 to 0.9 and 0.6 to 0.8, respectively. These results may indicate that the a.c. conduction observed for both of the above specimens is caused by hopping carriers. The values of loss tangent (tan) increased with increasing temperature. The relationship of log (tan) T was observed. The value of tan for the amorphous CVD-Si3N4 was smaller than that of the crystalline CVD-Si3N4. 相似文献
6.
The mechanical behaviour of Mg-5.1 wt % Zn alloy single crystals was studied in the 4.2 to 300° K temperature range. Quenched crystals have activation energies and volumes best associated with the cutting of small clusters of Zn atoms by dislocations. Fully hardened crystals contain fine
1 and occasional
2 precipitates with an average
1 interparticle spacing of 330 to 660 Å. Strengthening in these crystals is mainly ascribed to the cutting of
1 particles by dislocations. In the overaged condition
1,
2 and equilibrium particles are present and lead to a considerable temperature-dependence unusual for an overaged condition. Analysis of this temperature-dependence suggests that below 77° K the relatively easy cutting of
1 particles by dislocations takes place in addition to the cutting of
2 and particles. Above 77° K the difficult cutting of
2 and particles alone controls the deformation,
1 being more easily cut with the aid of thermal fluctuations. 相似文献
7.
Polycrystalline samples La2 – x
Sr
x
CuO4 – (0.06 x 0.5) and La1.85 – x
Sr0.15 + x
Cu1 – x
Co
x
O4 – (0 x 1) were synthesized by a conventional solid state reaction method. The structure, phonon vibration, and conduction were investigated by means of XRD, infrared (IR) spectra, and resistance. It is found that the increase of itinerant hole carriers could mask the in-plane
stretching vibration mode (689 cm–1). The softening of the
phonon vibration mode gives an index of the weakening of
hybridization. The distortion of CuO6 octahedron and the microstructural inhomogeneity induced by Co doping lead to the widening of the IR absorption peaks. 相似文献
8.
A. G. Shashkov S. Yu. Yanovskii 《Journal of Engineering Physics and Thermophysics》1977,33(5):1363-1369
Using the structural approach, the temperature stresses are examined in a semiinfinite rod, insulated on the lateral faces and rigidly fixed at the end. A comparative analysis is made for three heat-transfer models.Notation k(t)
heat flux relaxation function
-
(t)
internal energy relaxation function
- T
rod temperature
-
ambient temperature
- t
time
- x
coordinate along the rod
- xx(x, t)
stress
- u(x, t)
displacement
- (x, t)
deformation
- c0=(E/)1/2
speed of sound in the rod under isothermal conditions
- E
elasticity modulus
-
density of the material
-
t
coefficient of thermal expansion
-
thermal-conductivity coefficient
-
a
thermal-diffusivity coefficient
- b
thermal-activity coefficient
- cq=(a/r)1/2
velocity of heat propagation
- r
heat flux relaxation time
-
(t)
unique Heaviside function
Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 33, No. 5, pp. 912–921, November, 1977. 相似文献
9.
T. Herrmannsdörfer S. Rehmann M. Seibold F. Pobell 《Journal of Low Temperature Physics》1998,110(1-2):405-410
The ratio of nuclear saturation magnetization and superconducting critical field,
0
M
sat
/ B
S0*, classifies the strength of mutual influence of nuclear magnetism and superconductivity. In order to investigate the interplay of both phenomena for the three distinct cases 1, 1, and 1 we have measured the ac susceptibility of Al, of the intermetallic compound AuIn
2
, and of the metal hydride TiH
2.07
at ultralow temperatures, 17 K T 1 K, as function of static field 0 B 15 mT. For Al, the interplay enables an absolute measurement of the nuclear magnetization. For AuIn
2
, we get a steep decrease of B
S
(T) and a broadening of the superconducting transition in its nuclear ferromagnetic phase. Surprisingly, the nuclear ferromagnetic state coexists with type-I superconductivity in AuIn
2. The metal hydride TiH
2.07
, which is under present investigation, is a good candidate to show reentrant superconductivity. 相似文献
10.
Numerical method for calculating the filling of a mold and heat transfer of a melt under the action of centrifugal forces 总被引:3,自引:0,他引:3
A. P. Ogurtsov I. A. Milen'kii S. E. Samokhvalov A. V. Gress 《Journal of Engineering Physics and Thermophysics》1995,68(4):555-562
This paper presents an explicit difference method for solving the conjugate problem of pouring molten metal into a casting mold and its solidification under the action of centrifugal forces with allowance for the free surface.Notation
r, z
transverse and longitudinal coordinates
-
u, v
horizontal and vertical velocity components
- V
velocity vector
-
angular velocity of rotation
-
P
pressure
-
P
normalized pressure
-
T
temperature
-
time
-
g
free fall acceleration
-
coefficient of volumetric expansion
-
C
heat capacity
-
thermal conductivity
-
density
-
L
crystallization heat
-
v
viscosity
- Re=|V|h/v
Reynolds grid number
-
h
grid spacing
-
l
mixing length in a turbulent flow
-
relaxation parameter
-
coefficient of convective heat transfer
-
coefficient defining the boundary conditions at the solid wall
-
D
flow divergence
-
volumetric velocity of filling
-
emissivity
- 0
Stefan-Boltzmann constant
-
thickness of a layer
Indices
r, z, I, j
numbers of grid nodes
-
n
number of the integration step with respect to time
-
L, S
temperatures of the liquidus and solidus, respectively
- s
temperature on the surface
- med
temperature of the medium
- 0
initial state of the system
- m
metal
- mol
molten state of the metal
- red
reduced emissivity in the gap
- rad
radiant component of the heat transfer coefficient; g, gas-air gap
- coat
heat-insulating coating
- fil
filling
Dneprodzerzhinsk Industrial Institute. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 68, No. 4, pp. 678–686, July–August, 1995. 相似文献