The influence of structural defects on intragranular critical currents of bulk MgB[sub 2]. Page: 1 of 4
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> 1MM08- Preprint prroceedings of ASC 2002 - LA-UR-02-5176<
The influence of structural defects on intra-
granular critical currents of bulk MgB2
A. Serquis, X. Z. Liao, L. Civale, Y. T. Zhu, J. Y. Coulter, D. E. Peterson, and F. M. MuellerAbstract- Bulk MgB2 samples were prepared under different
synthesis conditions and analyzed by scanning and transmission
electron microscopy. The critical current densities were
determined from the magnetization versus magnetic field curves
of bulk and powder-dispersed-in-epoxy samples. Results show
that through a slow cooling process, the oxygen dissolved in bulk
MgB2 at high synthesis temperatures can segregate and form
nanometer-sized coherent precipitates of Mg(B,O)2 in the MgB2
matrix. Magnetization measurements indicate that these
precipitates act as effective flux pinning centers and therefore
significantly improve the intra-grain critical current density and
its field dependence.
Index Terms- Critical current, materials processing,
magnetic analysis, superconducting materials.
I. INTRODUCTION
F or practical applications of MgB2, it is important to
improve its critical current density J, especially at high
magnetic fields. J, is determined from both the intra-
granular pinning and the inter-granular connectivity.
Several studies indicate that a strong inter-granular current
network is established in MgB2 material, that is, the current is
not limited by weak-links at the grain boundaries [1-4]. For
example, by magnetization measurements of grain
agglomerates, Bugoslavsky et al.[1] have shown that within
Manuscript received August 6, 2002. This work was supported by US DOE
Office of Energy Efficiency and Renewable Energy, as part of its
Superconductivity for Electric Systems Program.
A. Serquis is with the Superconductivity Technology Center, MS K763,
Los Alamos National Laboratory, Los Alamos, NM 87544, USA
(corresponding author A.S.: 505-667-5641; fax: 505-665-3164; e-mail:
aserquis@lanl.gov).
X. Z. Liao is with the Superconductivity Technology Center, MS K763,
Los Alamos National Laboratory, Los Alamos, NM 87544, USA (e-mail:
xzliao@lanl.gov).
L. Civale is with the Superconductivity Technology Center, MS K763, Los
Alamos National Laboratory, Los Alamos, NM 87544, USA (e-mail:
lcivale@lanl.gov).
Y. T. Zhu is with the Superconductivity Technology Center, MS K763,
Los Alamos National Laboratory, Los Alamos, NM 87544, USA (e-mail:
yzhu@lanl.gov).
J. Y. Coulter is with the Superconductivity Technology Center, MS K763,
Los Alamos National Laboratory, Los Alamos, NM 87544, USA (e-mail:
jycoulter~lanl.gov).
D. E. Peterson is with the Superconductivity Technology Center, MS
K763, Los Alamos National Laboratory, Los Alamos, NM 87544, USA (e-
mail: dpeterson@lanl.gov).
F. M. Mueller is with the Superconductivity Technology Center, MS K763,
Los Alamos National Laboratory, Los Alamos, NM 87544, USA (e-mail:
fmm@lanl.gov).these microscopic structures, inter- and intra-grain Jj's are
quite comparable in value. In addition, other authors [3-4]
reported that high density samples have high superconducting
homogeneity and strong inter-granular current flow, as
determined by magneto-optical studies. However, a rapid drop
of J, at high fields, probably related to weak-link behavior,
can be seen in most studies [5]. Therefore, it is necessary to
enhance the grain connectivity to improve the field
dependence of J, [6].
On the other hand, in order to improve the intra-granular J,
it is necessary to identify the effective vortex pinning centers.
Many types of material defects (e.g. dislocation networks,
columnar defects, impurities, precipitates) are known to act as
effective flux pinning centers in other superconductors.
Oxygen tends to be easily incorporated into MgB2, usually
producing detrimental effects on J, due to the formation of
MgO at the grain boundaries. However, Eom et al. [7] have
reported that oxygen incorporation in MgB2 thin films
produces an increase in J, and the irreversibility field. They
also found that, as a result of the oxygen doping, the T, of
MgB2 was reduced by 4K. In a previous work [8] we have
shown that, through a slow cooling process, the oxygen in
bulk MgB2 can segregate and form nanometer-sized
precipitates of Mg(B,O)2, which are coherent with the matrix.
In this work we explore the influence of Mg(B,O)2
precipitates on the superconducting properties of MgB2. To
that end we prepared bulk samples with different densities of
precipitates, and we performed magnetization measurements
on pellets and on powders dispersed in epoxy. We found that
the T, of these MgB2 samples remains at 39K while the intra-
grain Jj's are greatly improved because the precipitates act as
effective flux pinning centers.
II. EXPERIMENTAL
A. Sample preparation
The MgB2 samples in this investigation were prepared by
solid-state reaction using three different sets of parameters. As
starting materials, we used amorphous boron powder (-325
mesh, 99.99% Alfa Aesar) and Mg turnings (99.98%
Puratronic). The boron powder was pressed to pellets (5 mm
diameter x 4 mm thickness). The pellets and the Mg turnings
were wrapped in Ta foil. Sample A was placed in an alumina
crucible inside a tube furnace under ultra-high purity (UHP)
Ar, heated for one hour at 9000C, slowly cooled down at the
speed of 0.50C/min to 500C, heated again for one hour at
9000C, slowly cooled again down to 500C, and then furnace1
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Serquis, A. (Adriana); Liao, Xiaozhou; Civale, L. (Leonardo); Zhu, Y. T. (Yuntian Theodore); Coulter, J. Y. (James Y.); Peterson, D. E. (Dean E.) et al. The influence of structural defects on intragranular critical currents of bulk MgB[sub 2]., article, January 1, 2002; United States. (https://digital.library.unt.edu/ark:/67531/metadc926685/m1/1/: accessed May 4, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.