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Appendix J
The annual report on research 070-589 (3rd year, 2004)
HTSC ceramic sintering by FEL Radiation
Part 2. HTSC material preparation and
characterization
By
Prof. Gideon Grader
Dr.Shter Gennady
Chemical Engineering Department
Technion- Israel Institute of Technology
Technion R&D Authority Ltd
10.01.2005
Haifa, Israel
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Part 2. HTSC material preparation and characterization
Prof. Gideon Grader, Dr. Gennady Shter
Chemical Engineering Department, Technion
Over a period from December 2003 to December 2004, according to schedule
of our part in joint project “HTSC ceramic sintering by FEL Radiation” the following
work has been done in Technion:
1. Synthesis of high quality YBCO superconducting powder via
oxalate coprecipitation method.
In general, a procedure of powder synthesis was described earlier in short annual
report for 2003 year, (2nd year of project), part 2, Fig.4. At final step of project two
main different batches of powder (batches A and B) were prepared and characterized.
Here we present the basic results of the powder synthesis and characterization.
1.1.A thermal behavior of synthesized oxalates mix (blue powder) was
investigated by TGA/DTA in flowing air in the range of 20-1100oC
(Fig.1).
1.2.Based on the TGA/DTA data the mechanism of the blue powder
pyrolysis was suggested and described (Table 1).
1.3.Precalcination stage was conducted under air at 700 oC for 7-10 hr. The
morphology of precalcined powders is presented in Figure 2 (a, b) by
SEM micrographs.
Thermogram of blue powder
5
0
TGA
DTA
-5
-10
-15
%w lost
-20
-25
-30
-35
DTA
-40
-45
TGA
-50
-55
-60
0
50
100
150
200
250
300
350
400
450
500
550
600
650
700
750
800
850
900
950
1000
1050 1100
Temp (C)
Fig.1 . Thermal analysis of synthesized oxalate mix in air (Setaram TG-92 Unit):
3
Stage
1
Table 1. The mechanism of the oxalate mix thermolysis
∆m –
∆m –
Temp.
REACTIONS
Theor exp.
Range
. (%)
(%)
(Cº)
9.66
13.7
100 0.5Y2 (C2O4 )3 *9H 2O  0.5Y2 (C2O4 )3  4.5H 2O
235
2BaC2O4 *0.5H 2O  2BaC2O4  H 2O
3CuC2O4 *0.5H 2O  3CuC2O4  1.5H 2O
235 540
3CuC2O4  1.5O2  3CuO  6CO2
540970
540970
Y2 O3  2CuO  Y2 Cu 2 O5
5
890920
YBa2Cu3O6.5  BaCuO2  CuO  L( BaCuO2  CuO)
6
920970
2
39.55
36.96
3
4
11.8
10.2
2 BaC2O4  O2  2BaCO3  2CO2
0.5Y2 (C2O4 )3  0.75O2  0.5Y2O3  3CO2
BaCO3  CuO  BaCuO2  CO2 (Secondary reaction)
0.5Y2O3  2BaCO3  3CuO  YBa2Cu3O6.5  2CO2
4BaCO3  Y2Cu2O5  4CuO  2YBa2Cu3O6.5
(Eutectic reaction)
2YBa2Cu3O6.5  BaCuO2  CuO 
Y2 BaCuO5  L(4 BaCuO2  3CuO)
(Peritectic reactions)
7
9701050
A
2YBa2Cu3O6.5  Y2 BaCuO5  L(3BaCuO2  2CuO)
(Incongruent reaction)
B
1.3 μ
1.3 μ
Fig.2. Morphology of the coprecipitated oxalates powder in batches A and B
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Morphology of precipitated oxalates powder is shown in Fig.2. The effect of synthesis
parameters on the particle shapes and size distribution, as well on aggregation is
clearly demonstrated.
1.4.A thermal behavior of pre-calcined powder (grey powder) is presented
by thermograms (TGA/DTA in flowing air in the range of 20-1100oC)
in Figure 3. The composition of this powder was studied by XRD and
included BaCO3, CuO and Y2O3 as main components with small
amounts of Y2Cu2O5, BaCuO2 and YBCO. The high temperature
chemistry of grey powder is described in Table 2.
1.5.The particles shape and size in powders after pre-calcination are
evaluated by SEM micrographs that presented in Figure 4.
pre-calcined powder thermogram
TGA
DTA
4.5
%w lost
1.5
-1.5
-4.5
-7.5
-10.5
-13.5
-16.5
0
100
200
300
400
500
600
700
800
900 1000 1100
T (C)
Fig.3. TGA/DTA of pore-calcined powder (air flow, Setaram TG-92, alumina crucible)
Table 2. Mechanism of the pre-calcined powder thermolysis.
Stage ∆m
Theo
(%)
1
11.8
∆m
Exp
(%)
11.02
Temp.
Range
(Cº)
780-960
REACTIONS
0.5Y2O3  2BaCO3  3CuO  YBa2Cu3O6.5  2CO2
4BaCO3  Y2Cu2O5  4CuO  2YBa2Cu3O6.5
2
960-1050
YBa 2 Cu3O6.5  Y2 BaCuO5  BaCuO2  CuO
1.6.Calcinations of grey powder were carried out under heating in oxygen
and vacuum in the range of 20-800oC for about 100 hrs. The black soft
powder was obtained. The TGA/DTA data is shown in Figure 5 where
significant weight loss was observed between 800 and 900 oC
associated with reaction of YBCO formation (Fig.4). The composition
of calcined powder was investigated by XRD presented in Figure 6. It
should be noted, that calcined powder includes along with YBCO
significant amount of other phases, such as BaCO3, CuO and Y2Cu2O5.
In the framework of presented work we have demonstrated a
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possibility to obtain the pure YBCO bulk product from the powder not
calcined completely too pure (single) YBCO phase. It is important
conclusion for development of simplified YBCO synthesis procedure.
Morphology of black calcined powders from 2 batches presented in
Figure 7.
13 μ (A)
1.7 µ (B)
Fig. 4. SEM micrograph of pre-calcined powders.
calcined powder thermogram
TGA
DTA
5
%w lost
0
-5
-10
-15
-20
0
100
200
300
400
500
600
700
800
900
1000 1100
T (C)
Fig. 5. TGA/DTA of calcined powder (air flow, Setaram TG-92, alumina crucible)
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After Calcination powder
1000
900
Intensity (a.u.)
800
700
600
500
400
300
200
100
0
10
15
20
25
30
35
40
45
50
2 Tetha
YBa2Cu3O7
BaCO3
Y2Cu2O5
CuO
Fig. 6. The phase composition of the powder after calcinations by XRD.
B
A
5μ
2.5 µ
Fig.7. Morphology of the calcined black powder.
1.7.The change of powder morphologies in the sequence blue powder
(oxalates)  grey powder (pre-calcined )  black powder (calcined
superconducting powder) is well corrected with data on Specific
Surface Area (SA) measured by BET at temperature of liquid nitrogen
(Fig.8). These data also represent an effect of procedure parameters on
the properties of final powders.
55
60
7
Surface area vs T
surface area (m^2/gr)
40
35
Batch A
30
Batch B
25
20
15
10
5
0
110
700
780
T (C)
Fig.8. Specific surface area of coprecipitated oxalates (110oC), pre-calcined (700 oC)
and calcined (780 oC) powders.
2. Preparation of the YBCO bulk superconducting samples.
The works in this part of project were focused on the preparation of YBCO bulk
superconductors: 1st group – the sintered under varied conditions samples; and 2nd
group – the melt-textured under special temperature profiles samples. All samples
were shaped by pressing of YBCO powders in the form of disc with dimensions of 2550 mm in diameter and 5-15 mm in thickness. The both sintered and met-textured discs
were thoroughly polished in organic media to provide the quality of the electrical and
magnetic measurements made by another project groups. Below, the main results of
sintering and melt-texturing processing and bulk samples characterization are presented
in short form.
2.1 Sintering of the YBCO bulk superconducting samples.
The principle sequence of discs preparation via sintering is presented by flow
chart in short annual report for 2003 year (2nd year of project –part 2, Fig.5). From each
synthesized batches of YBCO powder, described above, the different bulk samples
were pressed and sintered under varied processing parameters: pressure, heating and
cooling rate during sintering, atmosphere and gas pressure in sintering furnace,
sintering time and temperature, annealing time and temperature, etc.
2.1.1. A dependence of bulk density, porosity and shrinkage on the sintering
temperature was studied for samples pressed from powder batches A and B. Typical
examples of these relationships are presented in Table 3 and Fig.9. The sharp change of
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the macro parameters of sintered bodies was found in the narrow temperature range
from 880 to 890 oC, where the formation of YBCO liquid phase is occurred. In this
range the mechanism of sintering is changed from solid state to liquid phase. The strong
effect of sintering mechanism on the final dimensions of sample is clearly shown in
Fig.10, where the samples # is the same with those in Table 3.
The effect of temperature and change of sintering mechanism on the
microstructure of YBCO bulk are demonstrated by SEM at different magnifications
(Figs. 11 and 12).
In these figures the drastically change of microstructure at temperature change of only
10 oC is clearly observed: the relatively large particle size and dense morphology of
sample shown in Fig. 12 (T=890 oC) is well correlated with liquid phase sintering
mechanism in opposite to sample presented in Fig. 11 (T=880 oC, solid phase
mechanism).
Table 3.Effect of sintering temperature on the macro parameters of YBCO samples
(Powder- Batch A, sintering time – 13-14 hrs)
Samples
T sintering, oC Density, g/cc
Porosity , %
Shrinkage ,%
1
890
4.497
30.0
24.5
2
890
4.380
32.1
23.8
3
880
2.259
64.6
4.6
4
880
2.211
65.3
4.6
5
870
2.123
66.7
3.6
6
870
2.124
66.7
3.1
Batch A: density vs. sintering max temp.
density (gr/cm^3)
4.50
4.00
3.50
3.00
2.50
2.00
1.50
1.00
0.50
0.00
870
870
880
880
890
890
sintering max temp (C)
Fig. 9. Effect of sintering temperature on the density of prepared discs (Powder- Batch
A).
9
1
2
3
4
5
6
Fig. 10. The effect of sintering temperature on the final dimensions of sintered YBCO
discs.
2.5 μ
Fig.11. SEM micrographs of YBCO samples sintered at 880 oC (Solid phase sintering).
10
2.5 μ
5μ
Fig.12. SEM micrographs of YBCO samples sintered at 890 oC (Liquid phase
sintering).
2.1.2. Effect of sintering time on the density, shrinkage and porosity of final bulk was
investigated and the results are presented in Table 4. In comparison with sintering
temperature, the time is found to have only a slight effect on the macroparameters of
YBCO samples (in the sintering range of 10-50 hr).
Table 4. Effect of sintering time on the macro parameters of YBCO bulk (Powder batch
B, sintering temperature -880 oC)
Samples
1
2
3
4
5
6
Sintering time, hr Density, g/cc Porosity , %
13
2.21
64.3
13
2.238
64.9
30
2.250
63.5
30
2.262
63.7
45
2.38
62.7
45
2.386
62.5
Shrinkage ,%
3.7
3.8
3.9
4.1
4.3
4.6
2.1.3. The phase composition of all samples was tested by X-ray powder diffraction. In
spite of the fact that YBCO powder was not calcined completely to single phase
composition (Fig.6), the final superconducting samples are characterized by pure
YBCO phase (Fig. 13). These results represented the finding of optimal technology
sequence and parameters that allow to shorter and simplify the general procedure of
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YBCO
sample
preparation
via
coprecipitation.
Batch A Disc 4 after Sintering
2500
Intensity (a.u.)
2000
1500
1000
500
0
10
20
30
40
50
60
2 Tetha
Fig.13. XRD Patterns for sintered superconducting samples (Pure YBCO phase)
Part of the sintered samples was used as starting materials for next step of processing –
melt texturing while the second part was polished and passed to other group of project
for characterization of electrical and magnetic properties.
2.2. Melt-texturing of the YBCO bulk superconducting samples.
In a sintered YBCO samples the crystals are not regularly shaped and also are not
ordered in the space of bulk body (Figs. 11, 12). In this case the transport of current is
hampered by numerous contacts on grain borders and critical current is limited to small
values. One of the routes to avoid this problem is to provide the continuous growth of
the crystals in an ordered manner. As result, the large (long) crystals should be grown
with the a-b planes oriented in the desired direction. In turn, the crystals are also
oriented in parallel each with other and form the domains with textured microstructure.
In general, the method used to achieve this type of growth in superconducting materials
is Melt-Texturing. The various different sub-techniques of melt-texturing are known:
MPMG (Melt Powder Melt Grows), Zone Melting, SLMG (Solid-Liquid Melt Grows),
QMG (Quench melt Growth), TSMG (Top Seeded Melt Growth), OCMG (Oxygen
Controlled Melt Growth), etc. In all these sub-techniques the crystal growth is
achieved by sintering at high temperature in the presence of liquid phase. The main
conditions for texturing are as follows: a) the fast heating to partial incongruent melting
of material; b) fast cooling to sub-liquidus temperature; c) very slow cooling in the
critical temperature range to provide the growth crystals in desired direction.
In present work the different temperature profiles were investigated and the
materials with textured domains were obtained. The differences in these profiles were
the time and temperatures of fast and slow heating-cooling steps.
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The microstructures of the melt-textured at different conditions samples are
presented in Fig 14 A, B, C. These samples are characterized by crystal orientation and
formation of the domains with ordered structure. The sample A was melt-textured at
temperature lower than needed to form enough amount of liquid for continuous crystal
growth with preferable orientation. Results of melt-textured are clearly observed in
samples B and C with ordered large and long crystals.
A
B
50 µ
25 µ
C
50 µ
Fig. 14. SEM micrographs of the melt-textured YBCO samples: AThe macrostructure of the melt-textured samples is shown in the Fig. 15. The final
dimensions and densities of the samples are close each with other, while the cracks
were derived in some of them during processing. The samples were very dense with
low apparent porosity.
13
Batch A
Batch B
1
3
2
4
3 cm
Fig. 15. The optical photographs of the melt-textured samples.
Batch B Disc 1,Melt Texturing, without polish
800
700
600
500
400
300
200
100
0
10
20
30
40
50
YBCO
2 Tetha
BaCuO2, Y2Cu2O5, BaCO3
Fig. 16. XRD data on the phase composition of melt-textured sample before polish.
60
14
batch B Disc 1 Melt Texturing After Polish
2500
intensity (a.u.)
2000
1500
1000
500
0
10
YBCO
BaCuO2
20
30
40
50
2 tetha
Fig.17. XRD data on phase composition of the melt-textured samples after polishing.
The phase composition of the melt –textured samples was investigated by XRD
and the formation of secondary phases along with main phase of YBCO was shown.
During of incongruently melting the partial decomposition of YBCO is occurred and
secondary phases are formed preferably on the surface of discs. Therefore, on XRD of
samples before polishing the presence of secondary phases in significant amounts was
detected (Fig.16). After polishing, the surface with high concentration of secondary
phases was removed and the inner part of sample was found more pure (Fig. 17). The
as melt-textured samples will be used as prototype for comparison with samples melttextured by heating combined with FEL irradiation.
The sintered samples with low density and high porosity (solid phase sintering),
sintered samples with intermediate density and porosity (liquid phase sintering) and
melt-textured samples with high density and low porosity were passed to group from
College of Judea and Samaria for further millimeter wave measuring.
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