Kim Critical State Model Research Articles

AC susceptibility study in the 85 K phase of the Bi-Sr-Ca-Cu-O system

The temperature variation of AC susceptibility at different AC field amplitudes has been measured on cylindrical samples of the 85 K phase of the Bi-Sr-Ca-Cu-O system. Three samples used in the present study were obtained from different starting compositions and by different preparation techniques. The susceptibility data have been analyzed in terms of Bean's and Kim's critical state models. The temperature dependence of the critical current density and the effective volume fraction of the grains were estimated. The press-sintered samples shows improvement in the critical current density and a stronger coupling between the grains. Moreover, our analysis shows that the press-sintered sample closely follows Bean's field-independent critical state model.

Ac-susceptibility study of the 110-K superconducting phase of Bi-Sr-Ca-Cu-O.

ac-susceptibility (\ensuremath{\chi}',\ensuremath{\chi}'') measurements as a function of temperature and ac field amplitude have been carried out on cylindrical samples of the 110-K phase of the Bi-Sr-Ca-Cu-O system. The two single-phase samples used for studies were prepared under different experimental conditions with an aim to understand the regimes of validity of Bean's and Kim's critical-state models. Material-dependent parameters such as critical current density as a function of temperature and effective volume fraction of grains, were estimated using these models. Improvements in the critical-current density and intergrain coupling strength were found when a sample was subjected to intermediate cold pressing between sintering. Field variation of intergranular matrix susceptibility of the sintered sample could be fitted well to Kim's model whereas the data on the press sintered sample were in agreement with Bean's model.

AC susceptibility and local intergranular magnetic field in high- Tc superconductors

AC susceptibility of the granular high- T c superconductor La 1.8Sr 0.2CuO 4− σ has been measured at 4.5 K as a function of applied DC field H DC a for different AC amplitudes H m a. Hysteresis related to thermal and magnetic history of the sample was studied and higher screening capabilities in the field-cooled virgin samples compared to the zero-field-cooled virgin samples were observed for all AC amplitudes. For both conditions the real and imaginary susceptibility qualitatively shows the DC and AC dependence predicted by Kim's critical state model. AC losses and enhanced diamagnetic response upon reducing H DC a in a field-cooled condition were studied as a function of H m a. We have used Kim's intergranular critical state model to evaluate AC susceptibility as a function of H DC a for different H m a. As the applied field differs from the local field in intergranular regions, we performed an appropriate scaling to describe measured values of AC susceptibility. Through this scaling process the local field at the junctions is determined for different magnetic conditions.

Offset susceptibility of superconductors.

The novel concept of offset susceptibility ${\mathrm{\ensuremath{\chi}}}_{0}$ is presented for superconductors in applied fields ${\mathrm{H}}_{\mathrm{dc}}$+${\mathrm{H}}_{\mathrm{ac}}$ sin\ensuremath{\omega}t. ${\mathrm{\ensuremath{\chi}}}_{0}$ represents the dc component of the time-dependent magnetization M(t). For sintered ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$, ${\mathrm{\ensuremath{\chi}}}_{0}$ was measured as a function of ${\mathrm{H}}_{\mathrm{ac}}$, ${\mathrm{H}}_{\mathrm{dc}}$, and temperature using a Hall probe magnetometer and Fourier analysis. For intragranular diamagnetism, ${\mathrm{\ensuremath{\chi}}}_{0}$ behaves as the real part of complex fundamental susceptibility ${\mathrm{\ensuremath{\chi}}}_{1}^{\ensuremath{'}}$, confirming a London-like field penetration. However, ${\mathrm{\ensuremath{\chi}}}_{0}$ arising from intergranular coupling is positive at temperatures just below the onset of the coupling transition, in good agreement with theoretical predictions of a simplified Kim critical-state model of magnetization.

The Influence of Transport Currents on the Harmonic Generation in High Tc Superconductors

ABSTRACTThis paper details the experimental results for the influence of transport currents on the ac magnetisation of high-Tc superconductors. It has been found that the variation of the third harmonic amplitude V3 with an ac field can still be described by square and cubic laws for low and high ac fields, respectively, the same relationships as observed in the absence of transport currents. The only significant difference is that the magnitude of V3 in the low field region increases linearly with transport current. Consequently, the crossover point (from square law to cubic law) moves to higher ac fields and the variation of V3 with transport current is less pronounced in the high field region. The results are explained in terms of an effective increase in Hg, which is a parameter in Kim critical state model, with increase of transport current.

AC susceptibility of grains and matrix for high-Tc superconductors

Abstract Several 123-YBCO superconducting samples cut from the same pellet have been studied by measuring AC susceptibility at 77 K as a function of the applied field amplitude Hm. Two contributions from inter- and intragranular supercurrents to the susceptibility are clearly observed with Hm up to 30 kA/m. The partial susceptibilities for the matrix, χm and χm, come from the intergranular supercurrents and show a systematic sample thickness dependence. Although Kim's critical-state model explains the essential features of these susceptibilities and can be used to determine Jc as a function of local internal field Hi, the actual Jc(Hi) is more complex, with a concave segment at low fields. The partial susceptibilities for the grains, χg and χg, do not have sample thickness dependence, and the maximum χg is much lower than predicted by the critical-state model. This is shown to be related to the bee-waist shape of the hysteresis loop, which is due to the existence of a lower critical field. The lower critical fields of the grains (Hc1g) and the matrix (Hc1m), and the effective volume fraction of the grains (⨍g) are determined on the basis of the critical-state model. The lower limit for Hc1g is found to be around 4 kA/m, but Hc1m cannot be detected for Hm as low as 40 A/m. The results have been compared with some published weak-link models. A grain-cluster model is proposed for solving the problem of over high ⨍g.

A.c. susceptibility and intergranular critical current density of high Tc superconductors

The general behaviour of a.c. susceptibility and its dependence on temperature, a.c. field amplitude and frequency and d.c. bias field in high T c superconductors is described. This behaviour is explained by supposing that these superconductors consist of high J c and high H c1 grains embedded in a normal or poorly superconducting matrix or network through which supercurrents flow. Kim's critical-state model is found to be useful for describing intergranular critical current density J c ( H i ), where H i is the local field. For a sample with rectangular cross section, useful expressions and tables are presented for a.c. susceptibility and J c ( H i ). Based on these expressions and tables, local as well as average intergranular critical current density can be determined from experimental a.c. susceptibility data.