Research area 1 Superconductivity and superconductors

Reports from Research areas
Carla Vogt,
Wolfgang Gruner,
Rolf Kucharkowski,
Alexei Plotnikov,
Günter Behr
Fig. 1
a) Microscopic view of
the laser ablation trace
on YNi 2.02 B 2.00 C system;
length 20 mm, width
60 µm, ablation rate
20 µm/s.
b) ICP-MS signal of the
ablated material. In
heterogenic samples the
composition of phases
could be calculated from
isotopic ratios (shown
here 11 B/ 58 Ni) of all
elements.
Funded by:
DFG
Stoichiometry, purity and homogeneity of superconducting materials
Precise analysis was performed for Ca-Mg-Cu-O, Y(RE)-Ni-B-C, Bi-Pb-Sr-Ca-Cu-
O and some other material systems by ICP-OES and carrier gas hot extraction
methods for accurate oxygen determinations. In ICP-OES measurements optimised
multicomponent determination was realised by multi-line measurements
complemented by specially adapted data evaluation routines for precursors and
multi-filament super conductors. For a typical Bi-Pb-Sr-Ca-Cu-O 2223-phase precursor
with lead in the oxidation state 4 the cationic stoichiometry was determined
with optimum accuracy, e.g. Bi 1.734±0.006 Pb 0.3864±0.0003 Sr 1.935±0.006 Ca 1.74±0.02 Cu 3.000±0.003 .
The development of a special method for precise oxygen determination in oxides
was the prerequisite to determine the total composition of Ca-Mg-Cu-O compounds
experimentally for the first time. Thus, the exact stoichiometry of such complex
composed cuprates, like Ca 2.42±0.05 Mg 0.446±0.012 Cu 6.979±0.022 O 10.000±0.027 , could be
determined without any assumptions.
Laser ablation ICP-MS has been optimised
for the fast control of materials
homogeneity in cm-regions (Fig.1)
with lateral resolution down to 20 µm.
This makes the identification of phases
with dimensions below 200 µm
possible. The quantification of YNiBC systems was successful within 2 % RSD
using binary, ternary and quaternary synthetical standards.
Gernot Krabbes,
Günter Fuchs,
Stefan Gruß,
Gudrun Stöver,
Peter Verges,
Roland Hayn,
Larisa Shlyk,
Karl-Hartmut Müller,
Ludwig Schultz,
Jörg Fink
Fig. top:
Bulk cylinders, tiles and
rings
Fig. right:
Contours of the trapped
magnetic field of a
YBCO multiseeded ring
(diameter 80 mm)
Funded by:
BMBF
High performance HTSC bulk materials
High performance bulk materials are one of the preconditions for a new type of permanent
magnets and its application in newly designed electromotors, frictionless
bearings and physical technologies. Remanent induction („trapped field“) and
appearing forces between the superconductor and an applied field depend on both
the critical current density j c and diameter of the pseudo single crystalline material.
With respect to applications materials of appropriate size and shape are needed.
The properties of YBa 2 Cu 3 O 7 based melt grown material (YBCO) have been
remarkably altered by chemical modifications. Substitutions by cations of differing
valence, i.e. Y 3+ by Ca 2+ , influence the hole density in (CuO 2 ) x -planes, thus affecting
the critical temperature T c . On the other hand, substitution of in plane Cu 2+
especially by Zn 2+ was found to result in remarkably improved critical current densities
in applied fields of more than 2 T already at 77 K. An explanation can be
based on local magnetic moments which appear if Cu 2+ (S = 1/2) is substituted by
cations of differing spins S (Ni 2+ , S = 1; Zn 2+ , S = 0), thus causing alternative pinning
centres. An optimum dopand level was observed since the appearing
moments decrease T c . The trapped field on the top surface of a YBCO cylinder at
77 K achieves 1.2 T. The trapped field increases to nearly 2 T, if additional columnar
defects have been generated by neutron irradiation (co-operation H. Weber,
Vienna).
YBCO materials grown by the modified
melt crystallisation process (MMCP,
developed in IFW) distinguish by a
remarkable mechanical strength. Thus it
was possible to generate a mini-magnet
from YBCO : Zn material – reinforced by
an applied ring bandage from stainless
steel – with a trapped field of 11.2 T
already at T = 47 K. This is the highest
field ever achieved in bulk material at T >
40 K.
A multiseeding technique was developed to produce large tiles or rings consisting
of uniformly and well aligned and coherently bounded single grains. At 77 K,
the intergrain j c amounts to 10 % of the intragrain critical current density. Size and
shape are appropriate to its use in bearings and rotors of electrical motors.
Cooperation: Oswald Elektromotoren GmbH Miltenberg, Solvay Barium Strontium
GmbH, Bad Hönningen, IPHT Jena, Atominstitut der Österreichischen Universitäten
Wien, ZFW Göttingen, Univ. Stuttgart, MAI Moskau
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