Here we report investigations on chemical reactions made under high
gas pressure up to 1,5 GPa of argon and the mixture of argon with up
to 30 % of oxygen with the flux containing (Hg or/and Tl), Ba, Ca, Cu,
and Y, Ba, Cu beginning from its oxides. The reactions take place in
the crucibles made of BaZrO3 and for some cases made from Al2O3.
Multiple research works done on oxide superconductors containing
mercury and thallium allowed to compose the first approach of p-T-x
conditions of synthesis.
The rich complexity of these systems is correlated with the variety of
chemical reaction not only among the melted flux but also with the
crucible and/or substrate or seed immersed in it at certain
conditions.
Therefore the DTA analysis results of the reactions running under the
high oxygen pressure and high temperature conditions are crucial for
later pure crystallization processes of bulk and thin layers.
1. DTA High Pressure System for in-situ Identification of Phase
Transformation during Hg and Tl HTS Crystallization Process
Although the high gas pressure technology for obtaining HTS
superconducting crystals is known for some time, there is a reasonable
lack of the thermodynamical data describing the crystallization
process. Sufficient information may be collected via an elementary
differential thermal analysis (DTA) run under conditions corresponding
with the crystallization process. Mercury and thallium-based HTS
crystals should be grown at high pressure and high temperature regime
with a strict time-temperature program. To fulfil these demands a
special equipment is required.
Based on results of the previous investigations, we have modified the
high pressure system used for crystallization. These modifications
allow to perform DTA experiment with following parameters: maximum
temperature 1200oC, maximum temperature rate 1000oC/min, temperature
resolution 0.1 C, time resolution 0.1 s, sample volume up to 8 cm3.
The reducing and oxygenating atmospheres within that range may be
applied as well.
The described DTA High Pressure System allows to identify phases that
appear one after another as a reaction product of the batch under high
oxygen pressure and oxides vapour. .
Here we present results obtained with that system for some mercury and
thallium-based HTS compounds during the crystallization process.
2. Hg and Tl HTS Thin Films by High Gas Pressure Treatment with DTA
Controlling Process
High gas pressure treatment of specimens initially obtained by the
laser ablation method result in very good quality thin films. On the
other hand, films made by LPE (Liquid Phase Epitaxy) method have
single crystalline mirror-like Hg-1223 layer with the highest Tc (135
K) among HTc cuprates.
Assuming the heat treatment of thin films made by laser ablation
method needs short time (less than half an hour) and requires applying
very rapid temperature rise (up to 300oC/min) and rapid cooling after
the heat treatment run in aim to avoid possibility of CaHgO[2] phase
precipitation on the film surface, it is possible to perform the DTA
control. The in-situ DTA control is very promising for tuning the
starting level of the crystallization process temperature. The
knowledge of the DTA curve from the crystallization run, performed
previously as a pattern, make possible to crystallize single-phase
films. Additionally, DTA allows to choose the proper temperature of
crystallization at various oxygen pressure for predicted oxidation
state of the thin layer in one run of high pressure high temperature
treatment applied after crystallization. Typical DTA runs for some Hg
and Tl phases are shown. The XRD, EDX and transport parameters are
presented.
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