1.
High pressure synthesis
Superconducting
samples in the MBa2Can-1CunOx systems
(M = Tl, C, Mg, V, Mo, Ag, other) are
synthesized under high pressure up to 3.5 Pa in an anvil type facility (RIKEN KIKI
CAP-700, TOSHIBA Tungaloy). Temperatures are around 1000oC depending on starting
composition and the phase to be obtained. Phase formation and structural investigations
of the precursors and superconducting samples are
performed using RIGAKU RINT 1000 diffractometer. Some experiments are performed,
in situ, at high temperatures with an X-ray diffractometer RIGAKU RINT
2100 (temperatures up to 1500oC). Efforts are focused on enhancement
of superconducting characteristics and reproducibility of the samples. Also, new
materials that can be synthesized exclusively or not by using this technique are
under consideration.
2.
Ambient pressure synthesis
Row
powders with low C- content are mixed and after pressing are sealed in Au, Ag,
Ni -capsules. Heat treatments are performed at 840-900oC in electric furnaces,
using different temperatures, times, heating/cooling rates and so on. Sometimes precursor powders in Ba-Ca-Cu-O system
are used.
3.
Powder in tube - Ag tapes
Are
produced through different thermo-mechanical treatments. Reacted or un-reacted
powders are loaded into Ag-tubes and rolled. Heat treatments are conducted in
order to control synthesis processes and final superconducting
characteristics.
1.
Amorphous Phase Epitaxy (APE) method
By
RF sputtering an amorphous film is deposited on a substrate as MgO, Al2O3- (saphire),
LSAT, SrTiO3, NdGaO3. Then, controlled thallionation step is applied in closed
systems (Ag/Ni -capsule) at temperatures of 830-900oC. Our films of (Cu,Tl)-1223
and (Cu,Tl)-1234 have shown Jc of 2x107 and 2x106 A/cm2
at their maximum.
2.
Self Assembling Epitaxy (SAE) method
By
using several targets artificial structures are build and designed through
layer-by-layer deposition technique. Research is conducted in order to obtain
(Cu,M)Ba(Sr)Can-1CunO structures as well as other new
ones.
The family CuBa2Can-1CunOy with capable high
Tc, Jc, Hirr and low anisotropy can
be interpreted as a combination of modulated infinite layer (IL) materials
[BaCuO2]2/[CaCuO2]n-1. We had succeeded in preparation of epitaxial
[BaCuO2]m/[(Sr,Ca)CuO2]n superlattices by alternatively depositing the charge
reservoir layer and superconducting layer on treated SrTiO3 (100). Two targets
with the compositions BaCuO2 and Sr0.7Ca0.3CuO2, which form the charge reservoir
and superconducting layers, respectively, were used. X-ray diffraction analyze
confirmed the epitexial growth of CuBa2CuOx (1201) on
(Sr,Ca)CuO2 and vice
versa, with cell parameters similar to those of the individual thin films of
each compound. The superlattice modulation of l=1.845 nm is close to that of
bulk CuBa2Ca3Cu4Oy. Ac susceptibility measurements showed a diamagnetic signal
at a temperature lower than that in the bulk material. Attempts are being made
to prepare metallic 1201 charge reservoir at different conditions and to improve
the superconducting properties.
3.
Nano-dots induced - columnar pinning center techniques
Controlled
nano-dots of different elements on the substrates to
be used for APE and SAE films have demonstrated high potential for Jc increase.
Experiments using Ag nano-dots have shown an increase of 20 times in Jc.
By very short time and very low deposition rate at a suitable substrate
temperature, 3-dimensional nano-islands with various shapes, dimensions and
surface number density are grown on substrates prior to superconducting thin
film deposition. This approach subsequently induce extended pinning centers for
the magnetic flux lines that increase significantly, with no extra cost (unlike
neutron or heavy ion irradiation) the critical current density.
