Thermal evolution of ceramic powders surrounding the YBa2Cu 3O7-x composition

Article abstract of DOI:10.1007/s10973-009-0004-9

Organometallic-derived ceramic compositions surrounding YBa ₂Cu₃O_7-x (123) were evaluated via DTATG runs and dilatometric densification. The compositions were either Y, Ba or Cu deficient respect to 123. For the Yttrium de

Full text of DOI:10.1007/s10973-009-0004-9

J Therm Anal Calorim (2010) 99:369–375
DOI 10.1007/s10973-009-0004-9
Thermal evolution of ceramic powders surrounding
the YBa2Cu3O7–x composition
´
Edgardo R. Benavidez Æ Carlos J. R. Gonzalez Oliver
Received: 3 February 2009 / Accepted: 7 May 2009 / Published online: 10 June 2009
´
´
Ó Akademiai Kiado, Budapest, Hungary 2009
Abstract Organometallic-derived ceramic compositions
surrounding YBa2Cu3O7–x (123) were evaluated via DTA-
TG runs and dilatometric densification. The compositions
were either Y, Ba or Cu deficient respect to 123. For the
Yttrium deficient compact the presence of liquids con-
taining 0–1.3 mole %YO1.5—capable of dissolving the 123
grains—can promote a rapid sintering behavior. For Cop-
per deficient compact the main densification/contraction
mechanisms were delayed till 985 °C. For both Barium and
Copper deficient compacts a strong exudation of liquids
was detected at 990 °C and 1020 °C, respectively.
DL/Lo
Lo
[Lo-L(T)]/Lo: absolute shrinkage
Initial thickness of the pellet
Instantaneous thickness of the sample at
temperature T
L(T)
d(DL/Lo)/dT Rate of linear shrinkage
DHi
Enthalpy of reaction i
Introduction
As it is known, the densification degree and the micro-
structure of ceramics influence most of the properties of a
sintered material. Thus, in the YBa2Cu3O7–x (YBCO or 123)
ceramic, the residual porosity and grain boundaries can alter
significantly its superconducting properties [1]. For instance,
YBCO materials with closed porosity exhibited broad and
suppressed superconducting transitions [2]. This behavior
was attributed to the difficulty in oxygenating these materials
uniformly. However, this is not the only cause for degrada-
tion of the superconducting properties observed in high
density materials prepared from carbon containing precur-
sors: retention of C in the YBCO structure also causes a
deleterious effect on the superconducting transition [3–6].
The detected fast densification formed closed pores, thus
retaining the carbon incorporated in the system from the
organic precursors.
Keywords Dilatometry ꢀ Organometallic powder ꢀ
Sintering ꢀ Superconductors ꢀ Thermal analysis
List of symbols
123
YBa2Cu3O7
CuO
001
011
BaCuO2
211
Y2BaCuO5
Y2Cu2O5
202
Dm/mo
mo
Relative mass variation
Initial mass
E. R. Benavidez (&)
Facultad Regional San Nicolas, Universidad Tecnologica
´
Nacional, B2900LWH San Nicolas, Argentina
´
´
e-mail: ebenavidez@frsn.utn.edu.ar
In previous works YBCO organometallic solutions were
used (a) for the deposition of superconducting films and (b)
to produce -by gelation- fine powders of composition close
to that for 123 [4, 7]. The chemical composition of the
organometallic solutions was varied around 123 mainly to
study (i) compound volatilization effects, (ii) possible
reactions with particular ceramic substrates and (iii) for-
mation of other phases than 123 that may help 123 to grow
´
C. J. R. Gonzalez Oliver
Centro Atomico Bariloche (CNEA), Av. Bustillo km.9.5, 8400
´
Bariloche, Argentina
´
C. J. R. Gonzalez Oliver
C.O.N.I.C.E.T, Av. Rivadavia 1917, 1033 Buenos Aires,
Argentina
123

´
E.R. Benavidez, C.J.R. Gonzalez Oliver
370
in plate form. In such evaluations the partial pressure of O2,
the heating rate and the maximum temperature in the heat
treatment were also varied. For instance, after deposition of
123 films on SrTiO3 ceramic substrates it was found that
the final composition of the film was deficient in Y and rich
in Cu [7]. The crystal morphology consisted of needle/plate
shaped grains with a little preferred orientation. This effect
was attributed to a composition rich in BaCuO2 (or to an
oxycarbonate compound of BaCuO2), which generated a
liquid phase at around 900–920 °C (under O2 flow)
increasing significantly both the densification of the film
and the crystal growth rate of platelets. The onset super-
conducting temperature (Tc) for such film was around
89 K, and the lowering in Tc (relative to 92 K for sto-
chiometric 123) was attributed to carbon contamination of
the 123-orthorhombic phase.
[Cu(C3H7COO)2]. The Ba-methoxyethoxide was prepared
by reacting pellets of barium metal (Aldrich, 99?%) with
2-methoxyethanol (Aldrich, 99.9%). The Cu compound
was obtained by reacting Cu(OH)2 with butyric acid
(CH3CH2CH2CO2H; Riedel-de Haegen, 99%); the copper
hydroxide was obtained by reacting CuSO4 ꢀ 5H2O (Tim-
per, 99%) with NaOH (Brener, 99%). The (a, b and c)
components were dissolved in suitable organic solvents and
mixed together (according to individual calibrations after
drying and calcining steps) to give compositions C0, C1,
C2 and C3 (see Table 1). Each homogeneous liquid was
poured into a beaker which was covered with Al-paper and
let to gel and dry at 70 °C for 24 h. The dried powders had
a strong green color and were subsequently calcined under
O2 flow at 890 °C for 12 h, and subjected to further oxi-
dation at 460 °C for 6 h. After that, a black powder was
obtained.
The whole dilatometric densification data for stoichi-
ometric 123 pellets, obtained under constant heating rate
(CHR) and isothermal (ISO) conditions, could be reason-
ably well explained in terms of existing solid state diffu-
sion and liquid phase models [8]. It was also possible to
propose that the densification of 123, in the important
range between 922 °C and 970 °C in air (or 938–990 °C in
O2), is mainly controlled by the solution-precipitation
model (SP) having a characteristic activation enthalpy of
207 kJ.mole-1 (229 kJ.mole-1 under oxygen flow). In a
similar way the sintering behavior of Ag/123 laminates [9]
was analyzed. The dilatometric technique was also found
very useful to interpret the peritectic melting of 123, Ag/
123 and YBa(2–y) Sr(y) Cu3O7-x compositions [10, 11].
Furthermore, from such studies interesting information
about the properties of the different peritectic liquids
associated to these different systems was obtained. Such
liquids strongly control the growth of oriented plate-like
grains in the 123 system during cooling from the peritectic
range.
Polyvinyl butyral, (PVB, MW *150,000, Polysciences,
Inc.) was used as binder with a mass ratio PVB/pow-
der = 0.02. The powder/PVB mixtures were pressed
uniaxially at 100 MPa to form thin disks of 3.88 mm in
diameter and 1.07–1.21 mm in height.
Prior to the dilatometric densifications, the pellets were
slowly heated up to 750 °C, and maintained at that tem-
perature for 4 h (under O2 flow), attempting to eliminate
the organic components. After such pre-firing treatment no
shrinkage was detected in any compact.
Characterization techniques
The Neutron Activation Analysis (NAA) method (nuclear
´
reactor RA-6, Centro Atomico Bariloche) was employed to
measure the chemical compositions of the samples; this
method was employed owing to the small quantities of
powders available. Through this technique the c-radiation
emitted by the sample after irradiation with neutrons is
measured. The energy and half-life time (T1/2) are charac-
teristics of each isotope of a given chemical element, and, the
number of c rays is proportional to the quantity of the element
present in the sample. The analyzed isotopes were: Cu-66
(T1/2 = 5.1 min, c = 1039 keV), Ba-139 (T1/2 = 83.06 min,
c = 1421 keV) and Y-90 m (T1/2 = 3.19 h, c = 480 keV).
The aim of this work is to study the influence of
chemical composition on the sintering kinetics of particular
YBCO ceramics. The densification, the high temperature
DTA (differential thermal analysis) and TG (thermogravi-
metric analysis) data of four powders of compositions close
to and surrounding 123 are presented and analyzed. Such
powders were obtained by gelation of organometallic
solutions.
Table 1 Chemical compositions and cationic ratios of the specimens
Powder
Wt%
Cation ratio
Y:Ba:Cu
Experimental
YO1.5
BaO
CuO
Synthesis of powders and pellets
C1
C2
C3
C0
12.7
18.7
17.6
17.1
47.0
41.1
49.4
46.2
40.3
40.2
33.0
36.7
0.7:2.0:3.3
1.1:1.7:3.2
1.1:2.2:2.8
1.0:2.0:3.0
The organometallic solutions were obtained from (a)
yttrium acetate [Y(CH3COO)3 ꢀ xH2O; Aldrich, 99.9%],
(b) barium methoxyethoxide and (c) copper butyrate
123

Thermal evolution of ceramic powders
371
The standards used were: metallic copper (Reactor Experi-
ments Inc, 99.998%), BaCO3 (Johnson Matthey, 99.997%) and
Y(NO3)3.4H2O (Aldrich, 99.99%).
The crystalline phases were identified from X-rays dif-
fraction (XRD) patterns obtained with a Philips PW 1700
diffractometer using as wavelength kCu Ka = 0.154054 nm.
Simultaneous DTA and TG runs were performed using a
Netzsch STA-409 apparatus at a heating rate of 10 °C.min-1
(up to 1070 °C) under O2 flow. Sintered alumina crucibles
and kaolin powder as the reference were used. After the
DTA-TG runs up to 1,070 °C there was no indication of
sample contamination by reaction with the alumina crucible.
The linear densification data, obtained under similar
heating rates and atmosphere to those employed in the
DTA-TG runs, were collected with a differential vertical
dilatometer (Theta Dilatronics II); the push rod exerted a
10 kPa pressure on the specimen.
Results and discussion
Chemical composition and crystalline phases of the
powders
Table 1 lists the mass percentages of the constituent oxides
and the cation ratios after the NAA performed on the
powders.
The measured compositions by NAA are plotted in
Fig. 1 and it is noted that 123 composition (C0) is rea-
sonably well surrounded by the different compositions.
The XRD data for such calcined powders are shown in
Fig. 2. The crystalline phases found in these specimens are
listed in Table 2. The codes and compositions of such
Fig. 2 X-rays diffraction patterns of calcined powders
Table 2 Crystal phases found in C1, C2, C3, and C0 after treatment
at 890 °C (12 h, O2)
Powder
Crystalline phases
C1
C2
C3
C0
YBa2Cu3O7; BaCuO2; CuO
YBa2Cu3O7; Y2BaCuO5; CuO
YBa2Cu3O7; BaCuO2; Y2BaCuO5; Y2Cu2O5
YBa2Cu3O7
phases are: (123) = YBa2Cu3O7, (001) = CuO, (011) =
BaCuO2, (211) = Y2BaCuO5, (202) = Y2Cu2O5.
The main phase found for every specimen was 123, and
the secondary phases detected depended, as expected,
strongly on the individual compositions.
Reactions during heating
In Fig. 3 the DTA-TG traces of powders pre-heated at
890 °C in O2 are shown. A well-defined peritectic melting
peak (Tp) was obtained only for C0 and the composition
deficient in copper oxide C3. Besides, for C0 only one peak
at 971 °C was found in the range T \ Tp.
Fig. 1 Locations of powder chemical compositions in the YO1.5
BaO-CuO equilibrium diagram
-
123

´
E.R. Benavidez, C.J.R. Gonzalez Oliver
372
In this reaction, the composition (mole %) of L(p1) is
3.7YO1.5 ꢀ 22.3BaO ꢀ 74CuO [15]. For C1 it is difficult to
assign a definite value for Dm/mo at around 970 °C
although a small jump may be detected in Fig. 3. Follow-
ing the calculation procedure in [4] based in the DTA-TG
data for composition C2, an enthalpy of reaction (5):
DHp1 = 293 kJ/kg was obtained.
A possible third peak (C: 995–1,005 °C) may be
attributed to reaction (3):
Y2BaCuO5 þ CuO ! Y2Cu2O5 þ Lðp2Þ:
ð3Þ
From [15], the composition (mole %) of L(p2) is
4YO1.5 ꢀ 20BaO ꢀ 76CuO. This peak C is detected in C2
and may be justified by the presence of phases 211 and 001
in the powder (Fig. 2) before the DTA-TG runs. For C1
this peak is also found although the 211 phase was not
detected by XRD (Fig. 2 and Table 2). However, the
needed 211 may have come from reaction (3), which also
occurred for such composition C1 at 970 °C. Thus, process
(3) may also explain the presence of the C-peak for C1.
A fourth peak (D: 1012–1015 °C) prior to the peritectic
decomposition of 123 could be related to the reactions (4a,
4b, 4c) listed in [12].
2ðYBa2Cu3O7ꢁxÞþBaCuO2 !Y2BaCuO5 þLðp3Þ
BaCuO2 ! Lðm2Þ
Y2BaCuO5 þ BaCuO2 ! Lðe3Þ
ð4aÞ
ð4bÞ
ð4cÞ
Fig. 3 DTA and TG curves (10 °C min-1 under O2 flow) of calcined
powders
The small D-peak (or jump) for C1 and C3 corresponding
to the melting of 011 phase, according to reaction (4b),
would be expected because that phase was found in both
compositions (see Fig. 2 and Table 2).
The endothermic reactions for temperatures lower than
about 1,028 °C are analyzed following the works in ref-
erences [12–14]. The reactions were identified as eutectics
(ex), pseudo-peritectics or equilibrium between four phases
(px) and peritectics or incongruent melts (mx).The first
endothermic peak (A: 925–935 °C) is assigned to the fol-
lowing reactants and products:
According to the analysis in [14], reactions (4a) and (4c)
involve only a small quantity of yttrium and this would
mean low consumptions of 123 and 211 in these reactions.
Krabbes et al. [15] obtained the liquid L(p3) containing
0.4%YO1.5 (mole% = 0.4YO1.5 ꢀ 49.2BaO ꢀ 50.4CuO) and
the yttrium-free liquid L(m2) (mole% = 50BaO ꢀ 50CuO).
Also, from the equilibrium diagram quoted by Aselage and
Keefer [14] it may be noted that the positions for (e3) and
(p3) are very close to the position for (m2) associated to the
melting of the 011 phase. It can then be said that the three
liquids L(p3), L(e3) and L(m2) have roughly similar com-
positions and they are obtained at more or less similar
temperatures. The changes in Dm/mo for C1 and C3 for peak
D cannot be separated from that for the decomposition/
melting of 123 owing to the similar temperatures.
YBa2Cu3O7ꢁx þ BaCuO2 þ CuO ! Lðe1Þ
BaCuO2 þ CuO ! Lðe2Þ
ð1aÞ
ð1bÞ
Reaction (1a) is probably present in composition C1
since—after the treatment at 890 °C—it showed phases
001 and 011 as well as the main 123 phase (Table 2). From
the data for C1 it was obtained in [4] the enthalpy (DH) of
reaction (1a), being DHe1 = 353 kJ/kg, valid for
pO2 = 0.098 MPa. The molar composition of L(e1) is
1.25YO1.5 ꢀ 28.75BaO ꢀ 70CuO [15].
The peritectic melting reaction (E: 1,020–1,035 °C) for
the tetragonal 123 phase may be represented by reaction (5):
The second peak (B: 963–971 °C) may be assigned to
reaction (2) and it was observed in compositions C1 and C2
for which CuO was clearly detected.
2ðYBa2Cu3O7ꢁxÞ ! Y2BaCuO5 þ Lðm1Þ
ð5Þ
This reaction (5) is clearly noted for compositions C0
and C3, whereas for compositions C1 and C2 this reaction
2ðYBa2Cu3O7ꢁxÞ þ CuO ! Y2BaCuO5 þ Lðp1Þ
ð2Þ
123

Thermal evolution of ceramic powders
373
is probably masked by the preceding reactions (3) and (4a,
4b, 4c). Considering that the molar composition of L(m1)
is 4YO1.5. 36.5BaO. 59.5CuO [15], the enthalpy of reaction
(5), under pure O2 atmosphere, was calculated from the
DTA-TG data of C0 [4]: DHm1 = 143 kJ/kg.
To understand the high temperature thermodynamic
behavior of the YO1.5-BaO-CuO system the enthalpies of
reactions (1a), (2) and (5) are of great importance. As noted
above, these values were estimated from dynamic DTA-TG
measurements. To our knowledge such calorimetric esti-
mations of heat of reactions for this system are not avail-
able in the literature. Furthermore, the present value for
DHm1 of 143 kJkg-1 (pure O2) appears to be consistent
with the information in [16] and previous publications of
that group.
calculated [4] upon application of different densification
models will be presented in another work.
This agrees with the work by Hwang et. al [17] where
123 specimens, rich in CuO, exhibited a high densification
which was attributed to the presence of a liquid phase.
In the present case the existence of a liquid phase is
assigned to the eutectic reaction (1a), which is obtained at
925–935 °C generating the liquid L(e1), rich in Ba and Cu.
This reaction for C1 has a mass loss of oxygen of about
0.71% (pure O2) whereas the loss for the stoichiometric
L(e1) is 3.41% in air [15]. This allows to approximately
estimate the amount of liquid as 21% of the initial mass of
C1, and therefore, it would be expected significant densi-
fication via the rearrangement stage associated to a liquid
phase sintering mechanism.
From Fig. 3, for T [ 980 °C several endothermic reac-
tions are met in powder C1, but the contraction for compact
C1 remained constant (at DL/Lo % 24–25%) from 980 °C
up to 1,060 °C. Therefore, as no further densification is
observed it can be inferred that the liquids/solids, associ-
ated to those DTA reactions, do not present any LPS- or
SSS-mechanism acting in composition C1.
Densification behavior
In Fig. 4 are shown the dilatometric curves measured for
pre-fired (750 °C) pellets of the four powders. Compact C0
gave a sintering curve very close to that for commercial
powder SSC (Seattle Specialty Ceramics, Inc.) (see Fig. 2,
in ref. [8]). Powder C0 only differed slightly from SSC in
the DTA-TG behavior, which may be associated to the
slightly higher Cu content for C0 [4].
For compact C2 the highest shrinkage rates are found in
the following T-ranges: 950–960 °C and 1,005–1,020 °C.
The densification rate increases significantly from about
935 °C up to 960 °C, to decrease, then, to low values
between 960 °C and 1,000 °C. For the latter range, the
densification remained roughly constant with values in
between 23 and 27%.
As it is noted in Fig. 4 compacts C2 and C3 show a
strong increase in DL/Lo after they reached densifications
of around 25–28%. Such behavior corresponds to a kind of
collapse of the pellets owing to the exudation of the peri-
tectic liquids.
The next range from 1,005 °C to 1,060 °C does not
correspond to densification processes. The detected con-
traction (reaching values higher than 50%) is associated to
the peritectic melting/decomposition of C2 as judged from
the aspect of the pellet after the whole dilatometric run; the
final pellet was surrounded by the expelled peritectic
liquid.
Compact C1 starts densification under oxygen flow at
827 °C. Applying sintering models to estimate the activa-
tion enthalpy of the dominating mechanism in each
T-range it is proposed that about 85% of the total densifi-
cation of C1 is related to densification under the action of a
liquid phase. Details of the sintering activation energies
The small densification obtained for C2 from about
955 °C to 990 °C can be assigned to a weak solution-
precipitation mechanism provoked by L(p1). This fact
could be associated to either the solubility of 123 and/or the
diffusivity of the limiting species in the liquid L(p1) are
arrested for composition C2. According to the composi-
tions of the liquids that can be formed in the YO1.5-BaO-
CuO system [18], none of them accepts more than 4
mole%YO1.5, which is the YO1.5 content of L(m1). For
instance L(p1) contains three times more YO1.5 than L(e1)
and a little less YO1.5 than L(m1). Then, dissolution of 123
in L(p1) would be more difficult than in L(e1) which would
support the liquid phase mechanism is effectively acting in
composition C1.
As it was noted above the increase in DL/Lo at 1,000 °C
(Fig. 4) after C2 reached a densification of around 27% is
assigned to exudation of the peritectic liquid. Contraction
Fig. 4 Percentual absolute linear densification for compacts C0, C1,
C2 and C3
123

´
E.R. Benavidez, C.J.R. Gonzalez Oliver
374
levels higher than about 30% cannot be related to densifi-
cation processes, because the density of the pellet would
reach values higher than the theoretical density of the
ceramic. After the exudation, C2 was found to be expanded
in the radial direction and surrounded by the generated
liquid. The liquid L(p2) has 4 mole% YO1.5 and thus would
not accept more yttrium [15, 19]. As a result it could not
dissolve either the 123 particles or the expected 211 phase
[after reaction (2)]. In this way, due to the likely low vis-
cosity of L(p2) at this high temperature, it flowed easily out
of the pellet.
The adopted sol–gel method permitted the final chemi-
cal compositions to be easily varied. As it was noted in the
introduction, several compositions shifted from 123 can be
very relevant to the growth of 123 in plate form on SrTiO3
ceramic substrates.
Then, and depending strongly on composition, it is
found that, for higher temperatures, the ceramics rich in
copper oxide can sinter very strongly through the liquid
phase process. This is particularly valid when the gener-
ated liquid can dissolve 123 and this appears to be related
very strongly to the yttrium content of such liquid. From
inspection of the densification/melting curves for tem-
peratures near the peritectic point, it is clear that certain
compositions do not sustain a normal peritectic decom-
position, but that they rather collapse or melt to a large
extent. This fact makes it very difficult to consider such
ceramics for the oriented growth in plate form, useful to
attain high critical currents of 123 during solidification
cycles.
For compact C3, at temperatures lower than 1,016 °C, it
can only be detected a kind of very shallow wide endo-
thermic deviation of the base line of the DTA curve
(Fig. 3). The DTA trace showed a well-defined endother-
mic peak E between 1,016 and * 1,035 °C, associated
with a mass loss (Dm/mo) of 1.0%. The TG losses during
the peritectic reaction (5) for C0 (compositions very close
to 123) was around 0.7% under O2-flow. The increased O2
mass losses for powder C3 may indicate that other reac-
tions are acting during such endothermic process. For
instance, for reactions (4a) and (4b) Krabbes et al. [15]
found, in air atmosphere, Dm/mo losses of O2 of 2.040%
and 2.145% respectively. In [15] it is also reported reaction
(p4): 211 ? 202 ? 200 ? L (at about 1,061 °C in air)
with Dm/mo of 1.765%. Also, in [20] it has been proposed
that reactions (4a, 4b, 4c) can act as ‘‘activators’’ for the
peritectic decomposition for compositions close to 123.
As it may be noted in the shrinkage curve for C3, for the
range 1,016–1,035 °C the pellet decreased nearly 50% in
height, twice as much as that detected for C2. This indi-
cates C3 has nearly melted completely. Then, apart from
reactions (4a) and (4b) it is probable that other processes
like reactions (4c) and (p4; [15]) could have happened in
this composition C3.
Acknowledgements The authors thank to Mr. Sergio Ribeiro for
´
the NAA measurements, to Mr. Carlos Cotaro and Mrs. Silvia Dutrus
for the SEM/EDAX imaging and measurements, and, to Mr. Daniel
Quattrini for the DTA/TG measurements. It is acknowledged the
´
financial support of the Centro Atomico Bariloche Superconductivity
Group, which allowed the purchase of the main parts of the dila-
tometer. It is thank the critical reading of this work by Dr. J. M.
´
Rincon of Instituto E. Torroja (Madrid, Spain).
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compositional changes around 123 provoke very important
differences in the endothermic reactions. Such reactions
could be identified and the enthalpies of most of them
could be calculated giving reasonable values. Such values
are important in modelling the type of liquids generated in
the 123 system as function of the chemical composition
and the partial pressure of O2.
´
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