Plasma Sintering of the YBa2Cu3O7-δ Superconductor
A. Sarmiento Santos1, U. Fuentes Guerrero1, J. Roa Rojas3, D. Martínez2,3, E. Vera1, C. A. Parra Vargas2, 3
(1)Grupo de Superficies Electroquímica y Corrosión. UPTC-Colombia
(2)Grupo de Física de Materiales. UPTC-Colombia
(3)Grupo de Física de Nuevos Materiales, Universidad Nacional de Colombia
Abstract: The glow discharge applied to the sintering process is a technique recently used for
processing both metallic and ceramic materials. This represents a decrease in the time
required for sintering and lower energy consumption. In this paper we use the glow
discharge of low pressure as an alternative method for the sintering step of the ceramic
YBa2Cu3O7-δ. The physical properties of samples sinterized in glow discharge were
compared with the properties of samples sinterized in resistive furnace, which is commonly
used for production of such materials. Then, a structural analysis is carried out by XRD,
microstructural analysis by SEM and electrical analysis by the method of the four corners of
YBa2Cu3O7-δ samples, which were sinterized by the two methods. The experimental results
allowed stablishing a similar structure for both cases and a lower surface porosity in the
samples sintered by plasma. In addition, in the plasma sintering process, the samples show a
slight contamination of the surface. The contamination is produced due to sputtering of
cathode material of the discharge and can be easily removed by mechanical methods
(sanding). In curves of resistance versus temperature superconductor behavior was observed
for samples obtained by applying the two sintering processes.
Keywords: Perovskites, Plasma Sintering, superconductor
1. Introduction
The first superconductor system with critical
temperature above the liquid nitrogen was the YBCO
[1], because of this became very attractive
technologically, being identified as one of the most
promising candidates for applications in electrical
engineering [2]. Operated at liquid nitrogen
temperature (77 K) in the presence of magnetic fields,
suffers no significant degradation of the
superconducting current and has low loss factors.
However, these features are also extremely sensitive to
manufacturing process, which must be careful as to
produce a material with high texture and high current
carrying capacity [3]. The more known method for
producing perovskite-type samples is the method of
solid state reaction. This method generally consists of
two phases, calcined and sintered achieving high
quality results but, involve considerable time for its
production. Sintering depends on among other
variables, of the heating rate and temperature
treatment, geometry, particle size and its size
distribution and physical nature, such as purity, the
diffusion coefficient and surface tension of the
powders used [4]. The process requires heat to activate
the atomic diffusion, and conventionally this is
provided by resistive furnaces. Moreover, in order to
improve process performance, uniformity and density,
have been developed alternative techniques for
sintering of ceramics and metal powders. The main
alternative techniques are: spark sintering (SPS),
selective laser sintering (SLS), microwave sintering
(MS) and sintering in DC glow discharge [5]. These
new methods of sintering provide a faster and more
uniform heating of the sample.
In this paper we present the application of low
pressure glow discharge in direct current to produce
perovskite-like samples, as a substitute of the sintering
stage in resistive furnace [1]. It evaluates the structure
and microstructure of the samples produced by the
standard sintering method in resistance furnace and
compared with those obtained by applying the
abnormal glow discharge for the sintering step.
2. Experimental procedure
Samples of YBa2Cu3O7-δ were prepared following the
usual methods for ceramics processing. For this
purpose were used the powder precursor oxides, Y2O3
(99.99%), BaCO3
(99.9%) and CuO
(97%),
purchased from Aldrich Inc. The materials were dried,
weighed in stoichiometric amounts and mixed and
ground in an agate mortar for a period of 2 hours. This
mixture was compacted into a double effect
cylindrical matrix, made of stainless steel, with a
pressure of 1273.2 MPa. Samples obtained were 0.87
mm in diameter and about 5 mm high. After
compacted, samples were calcined in resistance
furnace at 890°C for 14 hours. To reach this
temperature, we used a heating rate of 1.9°C / min.
and after calcination, the samples were cooled at the
same rate used for heating. After the calcination, the
samples were milled for 2 hours again, obtaining a
very fine powder which was compacted again and
finally was sintered. The sintering was carried out
following the same thermal cycle used for calcination.
Subsequently, other samples were prepared following
the procedure described above until the stage of
calcined. For these samples, the step was performed at
the cathode of abnormal glow discharge in an
atmosphere of air at a pressure of 3 torr and a flow of
0.05 l /min. The sintering temperature, after tests
conducted at 850°C (at this temperature the sample
presented pyrolysis in the glow discharge), was chosen
at 510°C. To set this temperature was necessary to
adjust the discharge potential of 310 VDC with a
current of 140 mA. The time during which the
sintering was carried out was 60 min. and was
established after two testing with times of 15 and 30
min. (in these times there is no pure formation of the
perovskite structure characteristic of this material).
Structural characterization was carried out by X-ray
diffraction (XRD) on a PANalytical’s X'Pert
diffractometer with λ=1,54064 Å of Kα cooper line
and subsequent analysis by Rietveld refinement with
PCW23 software. The diffractograms were taken
using the Bragg-Brentano geometry by sweeping the
angle 2θ between 10° and 90°. The surface
microstructure of the samples was analyzed by
scanning electron microscopy (SEM). Resistivity
measurements were performed by the method of the
four corners under pressure with a current of 100 mA.
Before resistivity measurements, the samples were
oxygenated at a temperature of 700°C in controlled
atmosphere of oxygen for 24 hours (the heating and
cooling rate was 1.2°C/min.).
3. Results and discussion
Theoretical diffractogram of compound YBa2Cu3O7-δ
built through the program PCW23 served to
comparethe experimental results. Figure 1 shows the
diffractogram for samples sintered in resistive furnace
and Figure 2 shows the diffractogram of plasma
sintered samples. Through the program GSAS is found
that the structure of YBa2Cu3O7-δ, obtained in the
theoretical diffractogram, was also obtained in
samples processed in both resistance furnace and by
plasma. For this, was necessary to compare the
diffractograms theoretical with those of Figures 1 and
3 (experimental). Then, the diffractograms were
refined by the GSAS program and were obtained
crystallographic parameters a= 3.9877 Å, b = 4.0976Å
and c = 11.9877Å, corresponding to an orthorhombic
cell perovskite-like, for the two sintering types.
It is noteworthy that, although was done in less time
and temperature, the plasma sintering step allows for
the same structure obtained by the usual method for
sintering of YBa2Cu3O7-δ samples. This result is due to
the interaction of active species of the plasma with
sample surface, which establishes new conditions of
equilibrium on the sample surface and favoring the
solid state reaction [6]. It is also important to note that
this interaction of the sample surface and atmosphere
is not present in the sintering in resistance furnace.
Comparing the diffractograms of samples sintered in
resistive furnace (Figure 1) with the sample sintered in
the abnormal glow discharge (Figure 2), additional
peaks in the latter are observed in 2θ angles of 45° and
65°. These peaks correspond to iron impurities from
the cathode discharge, which it was manufactured in
1020 steel. Indeed, during the process in the abnormal
glow discharge, it is inevitable the phenomenon of
sputtering by bombardment of the plasma active
species on the discharge cathode, [7], and this is
verified by the appearance of the peaks of the
discharge cathode material. These impurities were
removed by mechanical methods, by sanding the
surface of the sample gently with sandpaper No. 600
to obtain a sample free of impurities, as shown in
Figure 3 diffractogram, where not shown the
diffraction peaks of the contaminating material from
the cathode. This fact allowed us to deduce that the
contamination of samples by the cathode material,
during sintering by plasma in the abnormal glow
discharge, is not present in the bulk sample but located
superficially.
Figure 4 shows the image obtained by SEM of the
surface of the samples sintered in resistive furnace
Figure 1. Diffractogram of the samples sintered in resistive
furnace.
Figure 2. Diffractogram of the samples plasma sintered.
Figure 3. Diffractogram of sample plasma sintered after sanding.
with magnification 5000. The micrograph shows a
uniform grain size, which indicates a satisfactory
sintering process similar to observations reported by
other authors for the synthesis of YBa2Cu3O7-δ [8].
Figure 5 shows the image obtained by SEM for
samples sintered in the plasma of the abnormal glow
discharge with magnification 5000. On the surface of
these samples can be observed greater continuity of
matter with respect to samples sintered in resistive
Figure 4. YBa2Cu3O7-δ sample sintered in resistive furnace
(5000x).
Figure 5. YBa2Cu3O7-δ sample sintered by plasma (5000x).
furnace from figure 5. Thus, there is an advanced
sintering stage; this is further reinforced by the smaller
amount of pores and its rounded structure. This does
not occur in YBa2Cu3O7-δ samples sintered in resistive
furnace (Figure 4) and reported in literature [8]. The
best sintering achieved in the abnormal glow
discharge occurs due to activation of surface diffusion
of the samples which occur by the impact of energetic
particles from the negative column of the discharge
[5]. The sputtering and also the backscattered material
from the sample surface, characteristic of glow
discharge [7], promote the formation of a surface with
low porosity.
Figure 6 shows the resistivity curves as a function of
the temperature of the samples sintered in resistive
furnace and the sintered in the glow discharge.The
curves for the two processes show a critical
temperature of the samples Tc = 92°C which is
characteristic of the YBa2Cu3O7-δ [1]. Samples
sintered in the glow discharge (Figure 6b) shows a
slight increase in resistivity at the critical temperature
4,5
4,0
ρ(mΩ xcm)
3,5
3,0
2,5
2,0
60
80
100
120
140
160
180
200
220
240
260
280
300
T(K)
(a)
5
ρ(mΩxcm)
4
3
2
a similar structure to what was obtained by the method
of sintering in resistance furnace, which is usually
used for sintering this material. By plasma sintering,
also is achieved a great reduction of time and sintering
temperature with respect to the method of sintering in
resistance furnace. The microstructure of the sintered
samples in glow discharge has a greater continuity of
matter, rounded and closed pores in the surface, which
shows a better state of superficial sintering with
respect to the sintering resistance furnace. Although
the samples obtained via plasma sintering are
contaminated by material of the discharge cathode,
these impurities can be easily removed by mechanical
methods easy to implement. Electrical measurements
have similar physical characteristics between the
samples processed by both sintering methods and
there is a slight increase in resistivity at the critical
temperature in the samples sintered in the glow
discharge.
REFERENCES
1
60
80
100
120
140
160
180
200
220
240
260
280
300
T(K)
(b)
Figure 6. Resistivity versus temperature: a) Samples sintered in
resistive furnace and b) Sintered samples in glow discharge.
and a lower slope after this, compared to samples
sintered in resistive furnace (Figure 6a). This behavior
is because of a slight decrease in the sintering in the
volume of processed samples in plasma when
compared with those processed in the resistance
furnace. This lower sintering is due to the lower
temperature and less time spent in the glow discharge
process. Finally, the differences between the final
physical properties of plasma sintered material are not
very large compared to the sintered material in the
resistance furnace. This enables the plasma sintering
as an alternative method for the production of
YBa2Cu3O7-δ superconductor material, with a
significant decrease in the time and temperature for
processing.
4. Conclusions
Using the abnormal glow discharge, as an alternative
method for sintering ceramic YBa2Cu3O7-δ, is obtained
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