Influence of Complexing Agent in Tl-Containing Sol-Gel Derived Precursor on Critical Temperature Enhancement of Tl0.5Pb0.5Sr1.8Yb0.2CaCu2O7 Superconductor
Superconducting oxide ceramics of composition Tl0.5Pb0.5Sr1.8Yb0.2CaCu2O7
was prepared by using a single-step sol-gel process using two different
complexing agents (tartaric acid and citric acid). The effect of the complexing
agents (0.0-11.0% (w/w %)) on superconductivity of the samples were studied.
It was found that the values of critical temperature, Tc zero,
was enhanced from 88 K (for the sample without any complexing agents)
to 95 K (for sample with 11 w/w % tartaric acid) and 97 K (for sample
with 11 w/w% citric acid). SEM analysis showed formation of ultra-fine
grains with 1-2 μm average grains size for the sample without complexing
agents. Addition of the complexing agents induced growth of elongated
grains of 5-10 μm in average length. The Tc enhancement
was discussed in terms of changes in hole concentration and sample microstructure
as a result of use of the complexing agents.
Thalium-based (Tl-based) high temperature superconductor (HTS) with
its high transition temperature is an interesting family of high-temperature
ceramic superconductors. The discovery of the thallium-based superconductor
in 1988 (Sheng and Hermann, 1988) has resulted in the findings of other
superconducting phases of the system such as Tl-2223, Tl-2234, Tl-1223
and Tl-1212 (Goretta et al., 1992; Mair et al., 1995). Among
them, more attention was given to the single Tl-layered Tl-1223 which
produces high critical temperature (Tc) and critical current
density (Jc) and has shown good performance in magnetic
fields (Lao et al., 2000). However, Tl-1212, which has similar
structure to YBCO, has been suggested to show better performance in magnetic
fields as the shorter insulating distance between the superconducting
CuO2 layers could lead to reduced anisotropy through interlayer
coupling and less severe thermally activated flux motion (Lao et al.,
Synthesis of single-phased Tl-based compounds using powder oxides is
difficult due to the constrain imposed by Tl2O3
that melts and evaporates at 717°C at atmospheric pressure whereas
formation of the high-Tc phase typically takes place above
900°C (Salleh et al., 2005). In order to reduce loss of thallium,
a modified conventional solid state synthesis which employed a two-step
method was usually used. Using this method, in the first stage non-Tl-containing
precursor was prepared before proceeding to the second stage where Tl2O3
was added to the precursor before final sintering for a short duration,
usually between 3-10 min. The short sintering period is to minimize Tl2O3
loss but it usually results in incomplete diffusion of thallium.
On the other hand, the sol gel method offers unique advantages over the
conventional solid state method as better composition control and chemical
homogeneity is achievable as in the procedure, the precursors were manipulated
at molecular level thus achieving homogeneity on a micro- or nano-meter
scale (Van Bael et al., 1998). Sol-gel synthesis of bulk superconducting
powder can be used to shorten the duration taken for the preparation process
as sol-gel procedure was suggested to reduce the diffusion path in the
solid state reaction (Van Bael et al., 1998). Use of sol-gel route
preparation of Bi-based and Y-based superconducting materials has been
widely reported (Fransaer et al., 1989; Xu et al., 1990;
Mao et al., 1997) but reported research on sol-gel route for Tl-based
superconductors is very few. Wagner and Gritzner (1994) reported on the
fabrication of bulk Tl-1234 prepared from tartaric acid gels and Yoo et
al. (1997) reported on fabrication of thick film of Tl-2223 via sol-gel
technique combined with a two-step method. Recently, Sudra and Gritzner
(2006) reported on Gd-doped (Tl,Pb)-1212 prepared using two-step approach
in which the Tl-free precursor were prepared via malic gel technique.
However, interestingly, single-step synthesis of Tl-based superconductors
using Tl-containing sol-gel derived precursors has not been reported.
In sol-gel processing, usage of complexing agent such as tartaric acid,
citric acid and oxalic acid, is an advantage as the complexing agent slows
down the pace of hydrolysis and condensation in the chemical process to
allow gel formation and thus helps in prevention of partial crystallization
of the metal salts and phase segregation during synthesis (Kareiva
et al., 1996; You et al., 1998). As a result, transparent solutions
and homogenous gels are produced. Among the complexing agents, tartaric
acid and citric acid have received the most attention. Application of
both tartrate-based gel and citrate-based gels were found to be successful
in the fabrication of Bi-based and Y-based superconductors (Varma et
al., 1990; Kareiva et al., 1996; Mao et al., 1997; Baranauskas
et al., 2001). However, to our knowledge, there is no previous
report on preparation of bulk Tl-1212 via Tl-containing tartrate gel or
Tl-containing citrate gel.
In this research, we report the effect of tartaric acid and citric acid
on preparation of bulk Tl-1212 superconductor via single-step sol-gel
process with starting composition of Tl0.5Pb0.5Sr1.8Yb0.2CaCu2O7.
In the single step process, acetate salts of thallium and lead were introduced
at the initial stage of the sol-gel route during preparation of superconducting
samples. Results of powder X-Ray Diffraction (XRD) and Scanning Electron
Microscope (SEM) investigations on the bulk samples are presented and
MATERIALS AND METHODS
Reagent-grade acetate salts of Tl, Pb, Sr, Yb, Ca and Cu with stoichiometric
ratio of 0.5:0.5:1.8:0.2:1:2 were dissolved in hot acetic acid solution
(25% (v/v)) producing blue coloured solution. The solution was stirred
for 30 min at 80°C before the addition of reagent grade tartaric acid
(or citric acid). Different amount of tartaric acid (or citric acid) were
added based on the weight to weight percent (w/w (%)) of complexing agent
to mixture of acetate salts. The tartaric acid-based samples were labeled
as SGT-1, SGT-2 and SGT-3 with addition of 1.5, 6.0 and 11.0 weight percent
(w/w (%)) of tartaric acid, respectively. The citric acid-based samples
with addition of 6.0 and 11.0 weight percent (w/w (%)) of the complexing
agent were labeled as SGC-2 and SGC-3, respectively. After addition of
tartaric acid (or citric acid), the stirring was continued for another
30 min at 80°C. The solution was kept at a constant pH of 3. Subsequently,
all solutions were then subjected to heat treatment to evaporate the solvent
which involved heating at 120°C for 5 h. During this period, the viscosities
of the blue solution increased until clear, dark blue gel were formed.
Further heating of the dark blue gel produced dried solid substance, brown
in colour, which was then ground in agate mortar. The grinding process
resulted in grayish-black powder which was then calcined at 400°C
for 6 h to produced brittle black foam. The brittle black foam was then
ground and pelletized and was sintered under flowing oxygen at 1000°C
for 5 min. All SGT and SGC samples were prepared using the same procedure
and under the same conditions mentioned earlier.
The complexing agent-free sample (SG-0) was prepared slightly differently
to avoid uncontrolled hydrolysis and condensation. After dissolving of
salts in hot acetic acid solution, the mixture was heated at ~90°C
and stirred continuously for 7 h until the volume became one-third of
the original volume and the mixture turned viscous and sticky. The continuous
stirring was to ensure that the mixture was homogenous and no precipitation
was formed during the process. The solution was then subjected to the
same calcination and sintering stages as the rest of the samples.
All thermal treatments were performed in a Lenton model PTF 12/35/500
three-zone tube furnace. Calcinations were performed in air and while
sintering of the pellets were performed in controlled oxygen flow. The
resistance versus temperature relationship of the samples were determined
using standard four-point-probe method with silver paint contacts in a
Janis model CCS 350ST cryostat combined with a closed cycle refrigerator
from CTI Cryogenics model 22. Scanning electron microscopy analysis was
carried out using JEOL model JSM-6360LA scanning electron microscope.
Structural characteristic of the samples were examined by an X-ray diffractometer,
Philips X`pert Pro model PW3040 equipped with Cu-Kα radiation.
The 1212:1201 phase ratios was calculated from the estimation of the
diffraction intensities of 1212, 1201 and other phases observed using
the equations below:
where, I is the peak intensity of the present phases. Similar equations
were used by Hamadneh et al. (2006) in the estimation of 2223:2212
phase ratio in their work of Bi(Pb)-2223 superconductor prepared via co-precipitation
RESULTS AND DISCUSSION
Powder X-ray diffraction patterns revealed that all samples consist
of dominant 1212 major phase (95-98 vol.%) accompanied with a small amount
of 1201 phase (2-5 vol.%). The XRD diffractograms also showed existence
of low intensity unidentified peaks which may be due to the presence of
small amount of unknown impurities. Figure 1 shows XRD
patterns for (a) SGT-0, (b) SGT-1, (c) SGT-2 and (d) SGC-2 samples.
Table 1 shows the weight percent (w/w%) of complexing
agent used, values of Tc zero, Tc onset
and volume ratio of 1212:1201 phases for all samples. The high
1212 phase ratio of between 97-98 vol.% for all samples showed that the
single step sol gel preparation procedure using Tl-containing precursor
is effective in producing high quality samples. The SG-0 sample prepared
without any complexing agents showed 95 vol.% 1212 hase. For SGT-1 sample
with 1.5% w/w tartaric acid, 1212 vol% was maintained at 95%. However,
there is a slight reduction of up to 3 vol.% (SGT-3 sample) in 1212 phase
with increasing tartaric acid. For the SGC-2 and SGC-3 samples, the 1212
vol.% also showed a slight reduction from 94 to 93 vol.% with increasing
citric acid of 6 and 11% w/w, respectively. In general, these results
showed that the 1212 phase formation did not deteriorate but was rather well maintained with addition of citric or tartaric acid as complexing
XRD patterns for (a) SGT-0, (b) SGT-1, (c) SGT-2 and
(d) SGC-2. The peak associated with 1201 phase is labeled with an
Normalized resistance versus temperature for SG-0 and
Normalized resistance versus temperature for SGC samples
Summary of w/w % of tartaric acid used, Tc zero,
Tc onset and volume ratio of 1212:1201 phases
for all samples
Figure 2a shows the resistance versus temperature
graph for SG-0 and SGT samples. SG-0 which was prepared with no complexing
agent showed metallic normal state behaviour with Tc zero
of 100 K.
||SEM micrographs for samples: (a) SG-0, (b) SGT-2 and
The SGT samples showed increase in Tc onset and Tc
zero with tartaric acid (Table 1). SGT samples
showed enhanced Tc zero values of 92 K (SGT-1 sample)
to 95 K (SGT-3 sample) compared the SG-0 sample which has a Tc zero
of 88 K. The normal state of all the SGT samples showed slightly more metallic
behavior with increasing tartaric acid. Figure 2b shows the
resistance versus temperature measurement graph for SGC samples. SGC samples
also showed enhanced Tc zero values of 95 K (SGC-1 sample)
and 97 K (SGC-2 sample) compared to 88 K for the SG-0 sample. The normal state
of all the SGC samples showed metallic behaviors.
SEM investigation on internal section of the SG-0 sample (Fig.
3a) shows ultra fine grains measuring between 1-2 μm.
||SEM micrographs for samples: (a) SGC-2 and (b) SGC-3
of tartaric acid in SGT samples caused formation of elongated grains which
increase in average size with tartaric acid. SGT-2 (Fig.
3b) showed porous microstructure with elongated grains with average size of 6-8
ìm. SGT-3 (Fig. 3c) also showed elongated grains
with larger average size of between 8 to 10 μm. Introduction of citric
acid in SGC-2 caused the microstructure to form irregular shaped grains
with average grains size between 5-10 μm (Fig. 4a).
Further increase in citric acid in SGC-3 sample produced slightly elongated
grains with average grain size of 10 μm (Fig. 4b).
Evidence of slight partial melting can be observed for the SGT-3 and SGC-3
samples. However, no grains alignment was observed in any of the samples
Present ressults show that addition of complexing agents in the form
of tartaric or citric acid caused increase in Tc of the samples
with increasing volume of complexing agent (Table 1).
Because the Tc enhancements were not accompanied by a similar
increase of the 1212 vol.% it is suggested that differences in 1212 phase
volume may not be the reason for the observed Tc increase.
It is possible that microstructural changes involving formation of elongated
grains observed by SEM for both SGT and SGC samples (Fig.
3, 4) can lead to improved connectivity between
grains and caused Tc to increase. However, the increase in
Tc may also be due to increase in the hole concentration in
the CuO2 sheets due to several reasons such as slight differences
in oxygen content between the samples or due to complex chemical reactions
during the sol-gel process. During gelation process using the complexing
agents, mixed-metal species which are based on citric and tartrate ligands,
were formed and these ligands may give rise to the possibility of reassembling
two or more different metals which may ultimately affect the doping state
of 1212 phase formed (Peleckis et al., 2002).
Present results also show that for the same volume of complexing agent,
the effects of tartaric (SGT samples) and citric acid (SGC samples) on
Tc are not very different. It is known that tartaric acid is
a polyprotic acid that has two carboxyl groups and two hydroxyl groups
which are able to coordinate with metal ions to form ligands, whereas
citric acid, which is also a polyprotic acid, has three carboxyl groups
and only one hydroxyl group (You et al., 1998). As such, our study
also indicates that although Tc increases with volume of the
complexing agents, it is not affected by the number of carboxyl (or hydroxyl)
group in the acids.
A simple sol-gel process based on the complexion of metal ions with
the help of chelating tartarate and citrate ligands has been successfully
employed for the preparation of Tl-containing pre-ceramic precursors for
synthesis of bulk TlPb-1212 superconductor samples. XRD confirmed formation
of dominant 1212 phase together with a small amount (<5 vol.%) of 1201
phase. The salient feature of this study is the enhancement of Tc
zero of up to 9 K and Tc onset of up to 4 K as a result
of the use of the complexing agents. Elangotated grains were observed
for samples SGT-3 and SGC-3 which ware prepared using 11 w/w % complexing
agent. The increase in Tc may be due to changes in hole concentration
or changes in sample microstructure as a result of the use of the complexing
E.S. Marsom would like to thank Universiti Tenaga Nasional for the
scholarship and study leave.
1: Baranauskas, A., D. Jasaitis, A. Kareiva, R. Haberkorn and H.P. Beck, 2001. Sol-gel preparation and characterization of manganese-substituted superconducting Yba2 (Cu1-xMnx)4O8 compounds. J. Eur. Ceramic Soc., 21: 399-408.
2: Fransaer, J., T. Eggermont, O. Arkens, O. Van Der Bies and E. Beyne et al., 1989. A new method of synthesizing high-Tc superconducting materials. Phys. C, Part 2: 881-882.
3: Goretta, K.C., N. Chen, M.T. Lanagan, S.E. Dorris, J. Hu, C.T. Wu and R.B. Poeppel, 1992. Processing TlBa2Ca2Cu3Ox powders. Supercond. Sci. Technol., 5: 534-537.
4: Hamadneh, I., S.A. Halim and C.K. Lee, 2006. Characterization of Bi1.6Pb0.4Sr2Ca2Cu3Oy ceramic superconductor prepared via coprecipitation method at different sintering time. J. Mater. Sci., 41: 5526-5530.
5: Kareiva, A., I. Bryntse, M. Karppinen and L. Niinisto, 1996. Influence of complexing agents on properties of YBa2Cu4O8 superconductors prepared by the sol-gel method. J. Solid State Chem., 121: 356-361.
Direct Link |
6: Lao, J.Y., J.H. Wang, D.Z. Wang, S.X. Yang and Y. Tu et al., 2000. Synthesis and characterization of thallium-based 1212 films with high critical current density on LaAlO3 substrates. Supercond. Sci. Technol., 13: 173-177.
Direct Link |
7: Mair, M., W.T. Konig and G. Gritzner, 1995. Fabrication, phase composition and properties of (Tl, Bi, Pb) (Sr, Ba) 2Ca2Cu3Oz superconductors. Supercond Sci. Technol., 8: 894-899.
8: Mao, C., L. Zhou and X. Sun, 1997. Optimization of the solution-sol-gel process to synthesize homogeneous BiPbSrCaCuO powder. Phys. C, 281: 27-34.
9: Peleckis, G., K. Tonsuaadu, T. Baubonyte and A. Kareiva, 2002. Sol-gel chemistry approach in the preparation of precursors for the substituted superconducting oxides. J. Non-Crystalline Solids, 311: 250-258.
10: Salleh, F., A.K. Yahya, H. Imad and M.H. Jumali, 2005. Synthesis and formation of TlSr1212 superconductors from coprecipitated oxalate precursors. Physcia C: Supercond., 426-431: 319-324.
CrossRef | Direct Link |
11: Sheng, Z.Z. and A.M. Hermann, 1988. Superconductivity in the rare-earth-free Tl-Ba-Ca-Cu-O system above liquid-nitrogen temperature. Nature, 332: 55-58.
Direct Link |
12: Sudra, H. and G. Gritzner, 2006. Under and over-doped bulk (TlaPb0.5) (Sr0.95Ba0.05)2(Ca0.8Gd0.2) Cu2Oz 1212 superconductors prepared via solution gel synthesis. Phys. C, 443: 57-60.
13: Van Bael, M.K., E. Knaepen, A. Kareiva, I. Schildermans and R. Nouwen et al., 1998. Study of different chemical methods to prepare ceramic high-temperature superconductors. Supercond. Sci. Technol., 11: 82-87.
14: Varma, H.K., K.P. Kumar, K.G.K. Warrier and A.D. Damodaran, 1990. Silver-YBCO composite derived from citrate gel. Supercond. Sci. Technol., 3: 73-75.
15: Wagner, A. and G. Gritzner, 1994. Fabrication and properties of thallium-based superconductors from tartrate gel precursors. Supercond. Sci. Technol., 7: 89-93.
16: Xu, Q., L. Bi, D. Peng, G. Meng and G. Zhou et al., 1990. Preparation of high-Tc Bi-Pb-Sr-Ca-Cu-O ceramic superconductor by using the sol-gel method. Supercond. Sci. Technol., 3: 564-567.
17: Yoo, S.H., K.W. Wong and Y. Xin, 1997. Thick films of the Tl-based superconductor fabricated via the sol-gel technique. Phys. C, 281: 55-58.
18: You, S.Y., J.T. Shy, C.M. Wang and H.C.I. Kao, 1998. Preparation of submicrometer La3CaBa3Cu7Oy superconducting powder with a polymeric precursor method using different polyprotic acids. Supercond. Sci. Technol., 11: 800-802.