Magnetization and Flux Behaviour of YBa 2 Cu 3 O 7-y Superconducting Rings
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MAGNETIZATION AND FLUX BEHAVIOUR OF YBa 2 Cu 3 0 7 - SUPERCONDUCTING RINGS SHOICHI YOKOYAMA , KENJI SHIMOHATA, TOSHIE USHIJIMA AND TADATOSHI YAMADA Central Research Laboratory, Mitsubishi Electric Corporation, Amagasaki, Hyogo'661
ABSTRACT The magnetization and flux creep in YBa 2 Cu.O 7 -,+xAg superconducting rings at 77K has been studied experimentally. Decay of the magnetization caused by the induced intergrain current was measured using the ring shaped specimen. The decay in proportion to logarithmic of the time was observed. The pinning potentials (Uo) of about 1.0eV at zero external field and 0.3eV at 10OG were estimated by Anderson and Kim model(I,2). In order to measure the intragrain flux creep, the rings were crushed into powder and pressed to disks without sintering. It was found that the dependence of Uo on external field for the ring was greater than that for the disk.
INTRODUCTION The intragrain flux creep of high Tc superconductors gives logarithmic decay of magnetization on time similarly as conventional superconductors(3,4]. The magnetization of the sintered bulk is expressed by the sum of magnetizations due to interand intragrain shielding currents(5). Therefore, the magnetization due to the intergrain current cannot be measured accurately by usual bulk specimen. In order to measure the decay of magnetization due to the intergrain current, we prepared ring shaped superconductors made by sintering process. The magnetic field at the center of the ring is not influenced by intragrain currents. In this paper, we report the magnetization and flux creep of superconducting rings of YBa 2 Cu 3 O7 -,+xAg compounds at 77K.
EXPERIMENT The samples were prepared by using Ag 2 O technique(6). This technique can improve critical current density(Jc) of the specimen at external field. The mixture of Ag 2 O and YBa 2 Cu3 O7 -, powder was pressed into a ring in shape (ODl9XID13X7mm). The weight ratio of Ag to YBa 2 Cu307-. is 13%. The sintering condition was 1050r for 10 hours. The final annealing condition of these rings was 950'C for 10 hours in an oxygen atmosphere. The measuring system of magnetic field at the center of the rings was shown in figure 1. The magnetic field was measured by Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society
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field by Intergraln current
In piles
920
V Radius r (mm
6
10
4
u0
C
2
-field by
Intragrain current
-20
(X5)
080o
Fig. 1 Measurement system of the magnetization of the rings.
Fig. 2 Results of the field calculation for the inter- (JOT=200 A/cm2 ) and the intragrain current (Joa=10000A/cm 2 ).
a Hall probe. The induction coil supplies the maximum external field of 2kG which is greater than the first critical field of the ring. Figure 2 shows two-dimensional calculations of the radial magnetic field distribution due to the shielding currents of intergrain (JOT=200A/cm 2 ) and intragrains (Joa= 10000A/cm2 ). The current density JOT is equal to the Jc of the rings at zero external field. The current density Jo(;, which is an apparent Jc of
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