Journal of Superconductivity and Novel Magnetism
https://doi.org/10.1007/s10948-017-4551-1
ORIGINAL PAPER
The Effect of Using Different Y
2
O
3
Layers on the Activation Energy
and Irreversibility Line of MPMG YBCO Bulk at 1050
◦
C Growth
Temperature
Sedat Kurnaz
1
· Bakiye C ¸ akır
1,2
· Alev Aydıner
1
Received: 4 December 2017 / Accepted: 23 December 2017
© Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
In this study, three kinds of YBCO samples which are named Y1, Y2 and Y3 were fabricated by a melt–powder–melt–growth
(MPMG) method. The Y1 sample was placed into a platinum (Pt) crucible without Y
2
O
3
, the Y2 sample was located on a
Al
2
O
3
crucible with a freely poured Y
2
O
3
powder and the Y3 sample was located on a Al
2
O
3
crucible with a 1-mm-thick
buffer layer of Y
2
O
3
. YBCO samples were investigated by magnetoresistivity (ρ –T) measurements in dc magnetic fields
(parallel to the c-axis) up to 5 T. The effect of the Y
2
O
3
layer on the activation energy and irreversible flux of the samples
was studied. The activation energies (U) were determined using the Arrhenius activation energy law from ρ –T . The power
law relationship for U with H
−α
was investigated. α was calculated to find out which defects were dominant in the samples.
Irreversibility fields (H
irr
) and upper critical fields (H
c2
) were obtained using 10 and 90% criteria of the normal-state
resistivity value from ρ –T curves. Irreversibility lines (ILs) were estimated from the equation H
irr
∼ (1 − T
irr
(H)/T
irr
(0))
n
.
The fitting results to giant flux creep and vortex glass models were discussed.
Keywords MPMG method · Y
2
O
3
layer · Activation energy · Pinning mechanism · Irreversibility field · Vortex glass ·
Vortex liquid · Giant flux creep
1 Introduction
Melt process has been accepted as a suitable method for the
fabrication of large single-crystal YBCO high-temperature
superconductors (HTSs) having good flux pinning and
magnetic properties [1].
Since the melt–powder–melt–growth (MPMG) process is
based on the reaction: Y
2
BaCuO
5
(211) + L(3BaCuO
2
+
2CuO) → YBa
2
Cu
3
O
7−x
(123), it is possible to make the
211 remain in the final structure by changing the star-
ting composition toward the 211-rich regions. When the
distribution of 211 is not uniform, the final structure
becomes inhomogeneous and results in weak connectivity
Sedat Kurnaz
sedatkurnaz@windowslive.com
1
Department of Physics, Karadeniz Technical University,
61080 Trabzon, Turkey
2
Vocational School of Health Services, Artvin C ¸ oruh
University, 08000 Artvin, Turkey
of the superconducting phase, which leads to lower critical
current density (J
c
). In order to promote the growth of the
superconducting phase, the 211 particles must be dispersed
uniformly in the liquid [2]. Since the 211-phase nucleates
from Y
2
O
3
, it is possible to control the distribution of the
211 phase if the distribution of Y
2
O
3
is controlled [3].
The 211 particles act like flux pinning centres after the
fabrication process. Furthermore, it was observed that the
Y
2
O
3
buffer layer on crucible also prevents the liquid to
spread on the furnace plate [4] and the contamination from
Al
2
O
3
crucible to the sample during the crystal growth of
bulk YBCO.
To understand the complex pinning behaviour in high-
temperature superconductors, for simplicity, we studied
the effective activation energy (U) of the flux pinning
dependency on the magnetic field and temperature. One
accepted method for probing this dependence is to measure
the resistive transition in various applied magnetic fields and
temperature. It is commonly suggested that the resistivity of
the high-temperature superconductors in the region of low
resistivity can be described as the Arrhenius relationship [5].