Preparation and electrical properties of xCaRuO3/(1 - x)CaTiO3 perovskite composites

Article abstract of DOI:10.1016/j.materresbull.2009.03.019

CaRuO₃-CaTiO₃ ceramic composites were prepared by sintering for short times for potential applications in the areas of electronic ceramics. Scanning electron microscopy and energy dispersive X-ray analysis showed two separate phases,

Full text of DOI:10.1016/j.materresbull.2009.03.019

Materials Research Bulletin 44 (2009) 1738–1742
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Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Preparation and electrical properties of xCaRuO3/(1 À x)CaTiO3 perovskite
composites
*
Shuqiang Jiao , Krishnankutty-Nair P. Kumar, Kamal Tripuraneni Kilby, Derek J. Fray
Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK
A R T I C L E I N F O
A B S T R A C T
Article history:
CaRuO3–CaTiO3 ceramic composites were prepared by sintering for short times for potential applications
in the areas of electronic ceramics. Scanning electron microscopy and energy dispersive X-ray analysis
showed two separate phases, CaRuO3 and CaTiO3 in the composite. Conductivity data, measured by the
four-probe method, showed that the composites have high electrical conductivity when x ꢀ 0.19 in
xCaRuO3–(1 À x)CaTiO3 composites. Furthermore, the nanoparticle of calcium ruthenate prepared by
reverse micelle synthesis was used to be conductive agent for the composite. The result shows that the
use of nano-sized calcium ruthenate enabled higher electrical conductivity to be maintained down to
x = 0.09.
Received 27 January 2009
Received in revised form 17 March 2009
Accepted 26 March 2009
Available online 10 April 2009
Keywords:
A. Composites
A. Electronic materials
A. Nanostructure
ß 2009 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental
Mixed oxides with perovskite (CaTiO3) structure exhibit a wide
variety of electrical and magnetic properties. Calcium ruthenate
(CaRuO3), a perovskite-type compound, is well known as an
important conductive material [1–8], which shows low electrical
conductivity (10 S cmÀ1) at room temperature. Moreover, CaRuO3
has no magnetic order; therefore, it could be an effective electrode
material for ferroelectric devices [9,10] and for use in Josephson
junctionsasa metallicbarrier [11,12]. However, atpresentCaRuO3 is
unlikely to be suitable for industrial applications due to relatively
high cost. One of the commonly followed approaches to reducing
cost is to make a composite with a relatively inexpensive second
phase. However, sintering of mixtures of ruthenates and titanates
results in a solid solution which, at low concentrations of ruthenate,
have very low conductivities, 10À1 to 10À2 S cmÀ1 for x = 0.3 in
CaRuxTi1ÀxO3 [13] and 7 Â 10À4 S cmÀ1 for x = 0.1 in SrRuxTi1ÀxO3 at
room temperature [14]. These low values make the materials
unsuitable for use in electronic devices. In this work, a method is
described which allows the high conductivity to be maintained at
low concentrations of the calcium ruthenate.
2.1. Preparation of xCaRuO3/(1 À x)CaTiO3 composites
CaRuO3 powders were prepared by the standard solid reaction
technique. 99.95% pure CaCO3 (10996, Alfa Aesar) and RuO2
(40336, Alfa Aesar) were mixed using mortar and pestle in
stoichiometric proportion. The mixture was then placed into an
alumina crucible and heated to 950 8C for 2–12 h. CaTiO3 powders
were prepared using the same approach from CaCO3 and TiO2 as
starting materials with a selected reaction temperature of 1400 8C.
To prepare the desired perovskite xCaRuO3/(1 À x)CaTiO3 compo-
sites, the CaRuO3 and CaTiO3 powders were thoroughly mixed
together with arbitrarily selected ratios, where x = 0.11, 0.19, 0.28,
0.41, and 0.69. The mixtures were individually pressed into pellets
with 20 mm diameter and 3 mm thickness using a uniaxial
pressure of 1.6 tons cmÀ2. The pellets were then sintered at
1400 8C for 2–12 h.
2.2. Preparation of nano-sized calcium ruthenate
The reverse micelle system was used to prepared nano-sized
calcium ruthenate. Ruthenium(III) chloride hydrate (RuCl3 XH2O,
and calcium chloride dehydrate (CaCl2 2H2O, Alfa Awsar: 99.9%)
were used as starting materials to prepare the nano-precursor, and
sodium carbonate (Na2CO3, Fisher Scientific: 99.5%) was used as
the precipitating agent. All solutions were prepared with double
distilled water. The micelle system consisted of n-octane (Alfa
Awsar: 99%) as continuous oil phase, cetyltrimethylammonium
bromide (CTAB, Alfa Awsar: 99%) as the surfactant, 1-butanol (Alfa
* Corresponding author. Tel.: +44 1223334315; fax: +44 1223335637.
E-mail addresses: sj332@cam.ac.uk, sqjiao_ustb@yahoo.com.cn (S. Jiao),
djf25@cam.ac.uk (D.J. Fray).
0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2009.03.019

S. Jiao et al. / Materials Research Bulletin 44 (2009) 1738–1742
1739
Awsar: 99%) as the cosurfactant, and an aqueous solution as the
dispersion phase. A solution was prepared by dissolving 1.260 g of
RuCl3 XH2O and 0.736 g of CaCl2 2H2O in 20 ml of double distilled
water. This solution was dispersed in a mixture of 80 ml of n-
octane, 15 ml of 1-butanol and 16 g of CTAB to form a micelle.
Another micelle of similar composition was prepared using the
Na2CO3 solution which dissolving 1.6 g of Na2CO3 in 20 ml of
double distilled water. These two micelles were mixed using a
magnetic stirrer for 1 h. The black precipitate that formed was
extracted by centrifuging at 2000 rpm for 15 min. The precipitate
was then washed by ethanol three times by centrifuging process.
The product after washing was dried at 100 8C. The dried powders
were calcinated in separated batches at different temperature for
2–8 h.
2.3. Preparation of nano-sized CaRuO3/CaTiO3 composites
The CaRuO3 nano-precursor and CaTiO3 powders were thor-
oughly mixed together with the selected ratios. The mixtures were
individually pressed into pellets with 20 mm diameter and 3 mm
thickness using a uniaxial pressure of 1.6 tons cmÀ2. The pellets
were then sintered at 1400 8C for 2–12 h.
Fig. 2. XRD patterns of composite pellets with different ratio of x (from 0 to 1).
porosity by optimising sintering conditions. The pellets of the
xCaRuO3/(1 À x)CaTiO3 composites were sintered at 1400 8C with
different durations of 2–12 h. The composite pellets as prepared
have been characterized, and the results showed that the pellets
with low porosity would be prepared after a long sintering period
of 12 h.
2.4. Physical characterization
X-ray diffraction (XRD), field-emission SEM and TEM were used
for characterization of the products. The electric conductivity of
the composite pellets was measured by four-probe approach.
The XRD patterns of the prepared xCaRuO3/(1 À x)CaTiO3
composites are shown in Fig. 2, where x = 0.11, 0.19, 0.28, 0.41,
and 0.69. The XRD patterns of pure CaRuO3 and CaTiO3 perovskites
sintered at 1400 8C are also shown. It is noticeable from Fig. 2 that
the XRD patterns of the composite materials are similar to the pure
perovskite materials. However, upon further inspection it is
evident that as x increases the XRD pattern of the composite
progressively correspond more to the CaRuO3 diffractogram. Fig. 3
highlights this difference, when x is greater than 0.41, the peaks
relating to CaRuO3 (2 0 0) and (0 0 2) appear instead of CaTiO3
(0 2 0) and (2 0 0).
The sintered pellets including pure CaRuO3 and CaTiO3 were
analyzed by scanning electron microscopy (SEM, JEOL 5500). The
results are shown in Fig. 4. Fig. 4a and d shows the morphologies of
CaRuO3 and CaTiO3 pellets sintered at 1400 8C. It is observable
that the CaTiO3 grain size is larger and that the pellet is less porous
3. Results and discussion
For the standard solid-state reactions, the samples as prepared
have been analyzed by the X-ray diffraction technique. Fig. 1
presents the XRD patterns of as prepared CaRuO3 and CaTiO3 by
solid-state reaction CaCO3 and corresponding oxides with a long
sintering time of 12 h. All peaks visible in this figure pertain to
well-crystallized orthorhombic structures of CaRuO3 and CaTiO3.
Only negligible amounts of the raw material phase could be
observed in the diffraction traces. However, the obvious amounts
of raw materials are still remaining when the sintered period is less
than 8 h.
This work is focused on the electrical properties of the xCaRuO3/
(1 À x)CaTiO3 composites. To measure the conductivity of the
composites, it is necessary to make the composite pellets with low
Fig. 1. XRD patterns of CaRuO3 and CaTiO3 prepared.
Fig. 3. Detailed XRD patterns of composite pellets.

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S. Jiao et al. / Materials Research Bulletin 44 (2009) 1738–1742
Fig. 4. SEM images of CaRuO3 (a), CaTiO3 pellets (d), composite pellets with the ratio of x = 0.19 (b) and x = 0.11 (c).
when compared to CaRuO3. Fig. 4b and c presents the morphol-
ogies of the composite materials with a ratio of x = 0.19 and 0.11,
respectively. It is clear that the sizes of the composite grains are
smaller than that of pure CaTiO3. To evaluate the phase
constitution associated with its microstructure, energy dispersive
X-ray analysis (EDX) had been used. Generally, results show that
there are separate phases in the composite mixture. The bigger
grains are primarily comprised of elements Ca, Ti, and O,
O, indicating the possibility of the formation of a Ru rich phase
under certain conditions. At this point it is not possible to pinpoint
the exact conditions or the composition.
It was aforementioned that the specific cost of a CaRuO3/
CaTiO3 composite material will be significantly lower than pure
CaRuO3, hence ensuring a greater economic feasibility for this
material to be used in industrial applications. However, it is
important that the composites should exhibit low electrical
resistivity for the projected electrode applications. A four-probe
method was used to measure the electrical resistivity of the
composite materials. A plot of electrical resistivity versus the
CaRuO3 component composition is shown in Fig. 6. It is noticeable
that the relationship between electrical resistivity and CaRuO3
content is not linear. The electrical resistivity increases rapidly
suggesting
a CaTiO3 rich phase. Conversely, EDX analysis
conducted on smaller particles indicates the presence of a CaRuO3
rich phase.
Moreover, it is interesting to observe that the composite with
the ratio of x = 0.28 showed particles with a unique plate-like
morphology and is shown in Fig. 5. The plate-like particles are
homogenously distributed in the composite matrix. Fig. 4b also
showed few plate-like particles, but not as much as Fig. 5. EDX
analysis elucidates that this phase primarily consists of Ca, Ru, and
Fig. 6. The change in electrical resistivity for composite pellets consisting of varying
amounts of the CaRuO3 component.
Fig. 5. SEM image of composite pellets with the ratio of x = 0.28.

S. Jiao et al. / Materials Research Bulletin 44 (2009) 1738–1742
1741
Fig. 7. TEM image (a) and the selected area electron diffraction pattern (b) of the Ca–
Ru–O precursor prepared in reverse micelle.
when the composite ratio of x is less than 0.19. Therefore, despite
the high porosity of all of the tested pellets, composite materials
with x ꢀ 0.19 have an electrical conductivity that is high enough
for applications as electrode materials.
It is known that when one powder is dispersed in another, the
properties that result are influenced by the particle size ration and
volume fraction [15]. In order to maximise the conductivity and to
minimise the amount of calcium ruthenate in the composite, it was
decided to use a CaRuO3 nano-precursor. This again was prepared
by the reverse micelle synthesis. The morphology of the dried
precursor was observed through TEM analysis, with the relevant
micrographs presented in Fig. 7a. It was found in Fig. 7b that the
precursor consisted of an amorphous structure for its particles. The
average diameter of the amorphous Ca–Ru–O particles, measured
from several TEM micrographs, was shown to be around 5 nm
(within a range of Æ2 nm). The lack of contrast was associated to
their amorphous structure.
Fig. 8a shows the XRD pattern of the powder sintered the
precursor at 850 8C. It is evident that all of peaks are corresponding
to the CaRuO3. From TEM image shown in Fig. 8b, it is seen that
they were growing with one direction and formed nanorods.
Hereinbefore, the electrical resistivity would be affected by the
size of powders. We aimed to prepare the composite of 0.09nano-
CaRuO3/0.91CaTiO3. The CaRuO3 nano-precursor and CaTiO3
powders were thoroughly mixed together with the selected ratios.
The mixtures were individually pressed into pellets with 20 mm
Fig. 8. TEM image (a) and XRD spectrum (b) of the CaRuO3 prepared by sintering the
precursor at 850 8C for 2 h.
suggesting a CaTiO3 rich phase (+1). Conversely, EDX analysis
conducted on smaller particles indicates the presence of a CaRuO3
rich phase (+2).
diameter and 3 mm thickness using
a uniaxial pressure of
1.6 tons cmÀ2. The pellets were then sintered at 1400 8C for 2–
12 h. Indeed, the pellets as prepared with short sintering time
show the high porosity. The sintering time was increased to make
dense pellet. The pellets sintered in the duration of 12 h were
analyzed by scanning electron microscopy. The results are shown
in Fig. 9. It is observable that the CaTiO3 grain size is larger and that
the pellet is less porous when compared to bulk CaRuO3. To
evaluate the phase constitution associated with its microstructure,
energy dispersive X-ray analysis had been used. Generally, results
show that there are separate phases in the composite mixture. The
bigger grains are primarily comprised of elements Ca, Ti, and O,
A four-probe method was used to measure the electrical
resistivity of the composite pellet of 0.09nano-CaRuO3/
0.91CaTiO3. The corresponding result, compared to the plot of
electrical resistivity versus the bulk CaRuO3 component compo-
sition mentioned before which is shown in Fig. 6. It is noticeable
that the electrical resistivity of the pellet is much lower than that
of the same composite ratio when the bulk CaRuO3 used.
Therefore, the composite pellet with x = 0.09 nano-CaRuO3 has
an electrical resistivity that is low enough for applications as
electrode materials.

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S. Jiao et al. / Materials Research Bulletin 44 (2009) 1738–1742
room temperature. By the use of nano-particulate CaRuO3 high
electrical conductivities were obtained down to x = 0.09 in
CaRuxTi1ÀxO3. This can be compared with a solid solution, which
is obtained by sintering for long periods, of the two materials
where the conductivities were orders.
Acknowledgement
The authors are grateful to the Engineering and Physical
Sciences Research Council (EPSRC) for financial support.
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