Variable temperature in situ X-ray diffraction study of mechanically activated synthesis of calcium titanate, CaTiO3

Article abstract of DOI:10.1016/S0025-5408(03)00113-2

The effect of mechanical activation on formation of calcium titanate (CaTiO₃) from calcined mixtures of CaCO₃ and TiO₂ was studied by monitoring the course of this solid-state reaction by variable temperature in situ X-ray diffraction (XRD) experiments, scanning electron microscopy (SEM) and dilatometry. Two equimolar mixtures of powdered CaCO₃ and TiO₂ were prepared: one was mechanically activated by grinding in a high energy vibro-mill. A total of 32 X-ray diffraction data sets for each sample, collected between 30 and 1100°C, were analyzed by multiphase Rietveld refinement. Quantitative phase analysis and microstructure analysis obtained from X-ray diffraction are correlated to results of scanning electron microscopy and dilatometry. In the non-activated sample, small quantities of the reactants remain in the product until 1100°C. In the activated sample, the reaction results in pure CaTiO₃ at 920°C.

Full text of DOI:10.1016/S0025-5408(03)00113-2

Materials Research Bulletin 38 (2003) 1203–1213
Variable temperature in situ X-ray diffraction study of
mechanically activated synthesis of
calcium titanate, CaTiO3
a,*
Ivana Radosavljevic Evans , Judith A.K. Howard ,
a
b
Tatjana Sreckovic , Momcilo M. Ristic
c
a
Department of Chemistry, University of Durham, Science Site, South Road, Durham DH1 3LE, England, UK
b
Center for Multidisciplinary Studies, University of Belgrade, Kneza Viseslava 1A, 11000 Belgrade,
Serbia and Montenegro, Yugoslavia
c
Serbian Academy of Sciences and Arts, Knez Mihailova 35, 11000 Belgrade,
Serbia and Montenegro, Yugoslavia
Received 19 December 2002; received in revised form 14 March 2003; accepted 31 March 2003
Abstract
The effect of mechanical activation on formation of calcium titanate (CaTiO ) from calcined mixtures of
3
CaCO and TiO was studied by monitoring the course of this solid-state reaction by variable temperature in situ
2
3
X-ray diffraction (XRD) experiments, scanning electron microscopy (SEM) and dilatometry. Two equimolar
mixtures of powdered CaCO and TiO were prepared: one was mechanically activated by grinding in a high
3
2
energy vibro-mill. A total of 32 X-ray diffraction data sets for each sample, collected between 30 and 1100 8C,
were analyzed by multiphase Rietveld refinement. Quantitative phase analysis and microstructure analysis
obtained from X-ray diffraction are correlated to results of scanning electron microscopy and dilatometry. In the
non-activated sample, small quantities of the reactants remain in the product until 1100 8C. In the activated
sample, the reaction results in pure CaTiO at 920 8C.
3
#
2003 Elsevier Science Ltd. All rights reserved.
Keywords: A. Ceramics; A. Oxides; C. X-ray diffraction
1
. Introduction
Calcium titanate (CaTiO ), with a perovskite related structure, has a high dielectric constant (e ), high
3
r
dielectric loss (tan d) and large positive temperature coefficient of the resonant frequency (t ) [1].
f
*
Corresponding author. Tel.: þ44-191-374-4707; fax: þ44-191-384-4737.
E-mail address: ivana.radosavljevic@durham.ac.uk (I.R. Evans).
0025-5408/03/$ – see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0025-5408(03)00113-2

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I.R. Evans et al. / Materials Research Bulletin 38 (2003) 1203–1213
CaTiO is a promising material for communications equipment operating at microwave frequencies
3
(
UHF and SHF), since it readily forms solid solutions with other compounds with negative t , yielding
f
2
þ
materials with an almost-zero t [2–4]. Furthermore, substitution of the A-site Ca ion by trivalent
Sm or Nd ions results in the formation of Ca vacancies and affects the dielectric properties; the
products obtained have high dielectric constants (e ) and low dielectric loss (tan d) [5–7]. These
f
3þ
3þ
2þ
r
properties suggest possible uses of calcium titanate in microwave dielectric applications (as resonators
and filters). An additional important property of CaTiO is its ability to serve as a host matrix for the
3
fixation of lanthanides and actinides and, as such, be used for immobilization of high-level radioactive
wastes [8,9]. The mineral perovskite, CaTiO , is a major component in Synroc [10], a synthetic
3
titanate-based rock usually also containing hollandite (BaAl Ti O ), zirconolite (CaZrTi O ) and rutile
2
2
6
2 7
(TiO ), developed for immobilizing nuclear wastes and surplus materials. Owing to a wide range of
existing and potential applications, significant research effort has been directed towards synthetic routes
2
to CaTiO and optimization of its properties.
3
The conventional method of synthesizing CaTiO is by a solid-state reaction between TiO and
2
3
CaCO or CaO at temperatures about 1350 8C [11,12]. Additionally, this phase has been prepared by
3
other techniques, including organometallic [13] and liquid mix methods [14]. Several studies of this
system so far reported [15–18] have concentrated on mechanochemical synthesis of CaTiO by
3
prolonged grinding at room temperature and in particular have addressed the dependence of reactivity
of different mixtures at ambient conditions on the reagents used (anhydrous versus hydrated calcium
oxide, rutile versus anatase).
Mechanical activation of dispersed systems, such as oxide powders, is very often accompanied by
significant changes of their physico-chemical properties [19–22]. Different processes that occur during
the mechanical activation of powders lead to the formation of specific structures that promote and
accelerate solid-state reactions, as well as densification during reaction sintering. In addition, a
reduction of both the temperature of solid-state reaction and sintering temperature can be expected.
Changes of particle size and structure during mechanical activation of the ceramic parent material are
the sources of the morphological and structural metastability of the starting powders and they can affect
the sintering process, positively or negatively. Activation must therefore be carried out under strictly
controlled conditions in order to avoid influences that might cause a deterioration of the final properties
of the ceramic materials, such as particle aggregation in unsintered compacts or uncontrolled grain
growth during sintering. Mechanical activation can be performed in various ways, but grinding is one of
the most widely used methods in powder technology, with vibro-mills used specifically for efficient fine
grinding and activation of metal and ceramic powders.
Variable temperature in situ diffraction is an ideal tool for studies of solid-state reactions. It provides
continual monitoring of the system, thus avoiding problems associated with any irreversible changes in
the material that may arise when the sample is cooled and taken out of the furnace for analysis. In situ
diffraction studies at large central facilities, using synchrotron and neutron sources, have resulted in the
accumulation of a large volume of information about the kinetics and mechanisms of solid-state
reactions, as well as the properties of materials obtained [23]. Advances in conventional X-ray
instrumentation have made possible the use of laboratory equipment for time and temperature resolved
diffraction for various dynamic studies, including thermal decomposition and phase transformations
[
24].
In the present work we describe a variable temperature in situ X-ray diffraction (XRD) study of the
solid-state formation of calcium titanate, CaTiO , conducted on a laboratory powder X-ray
3

I.R. Evans et al. / Materials Research Bulletin 38 (2003) 1203–1213
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diffractometer. The effect of mechanical activation of reactants is studied by quantitative phase
composition and microstructure analysis of two different calcined mixtures of CaCO and TiO . In
3
2
addition, the samples are characterized by scanning electron microscopy (SEM), while dilatometry is
used to study the effects of different thermal regimes on the properties of the products obtained.
2
. Experimental procedures
The two mixtures were prepared from calcite CaCO (Kemika, p.a. 99%) and rutile TiO (Alfa
3
2
Products Ventron, p.a. 99%) in a stoichiometric 1:1 ratio. Appropriate amounts of the constituent
oxides, as required by the stoichiometric ratio, were weighed, placed in a laboratory plastic ball mill
together with an appropriate amount of alumina balls, 5.0 mm diameter, and dry milled for 2 h. One
mixture was not mechanically activated and was used as a reference material, while the other was
activated by dry grinding in air, in a vibrating cup mill (type MN 954/3, KHD Humboldt Wedag AG),
2
with steel rings and a steel vessel volume of 500 cm . The amount of powders ground was 50 g.
Mechanical activation was carried out in a continual grinding regime for 60 min. A small amount of
both mixtures was pressed into briquettes, calcined at 900 8C for 2 h and then crushed. The obtained
powders were granulated, by sieving.
Samples for X-ray diffraction were prepared by pressing the polycrystalline material into pellets,
using a pressure of 245 MPa. Variable temperature powder X-ray diffraction data were collected
using a Bruker AXS D8 advance diffractometer equipped with an Anton Paar HTK1200 high
temperature stage and a Braun linear PSD. Data were collected in the 2y range between 108 and 908 in
0.0148 steps, using a counting time of 0.3 s per step, resulting in a measurement time of 28 min per
pattern. A total of 32 data sets were collected on each sample, with measurement temperatures of 30,
8
9
0, 130, 180, 230, 280, 330, 380, 400, 420, 440, 460, 480, 500, 550, 600, 650, 700, 750, 800, 850,
00, 920, 940, 960, 980, 1000, 1020, 1040, 1060, 1080 and 1100 8C with an intermittent heating rate
of 10 8C/min.
In order to obtain information about the quantitative phase content and microstructure, analyses of
the variable temperature data sets were carried out using the approach of multiphase Rietveld
refinement [25]. The starting model consisted of the room temperature crystal structures of CaCO3,
TiO and CaTiO , as given in ICSD database [26]. The only structural parameters refined were the unit
2
3
cell parameters and an isotropic temperature factor per phase. In addition, a number of instrumental
parameters were refined, including six background terms, six pseudo-Voigt profile function coefficients,
a profile asymmetry term and a sample height correction. All refinements were performed using the
Topas software suite [27].
Scanning electron microscopy was employed to probe the sample microstructure. A JEOL IC848
scanning electron microscope operating at an accelerating voltage of 20 kV and a magnification of
20,000 times was used. Prior to SEM studies, the samples had undergone Au sputtering in order to
increase their conductivity.
The relative shrinkage of samples was followed by a sensitive dilatometer (B a¨ hr Ger a¨ tebau
GmbH Type 702s) during thermal treatment in air. Three sets of measurements were carried out: at
a constant heating rate of 10 and 30 8C/min, in the range from room temperature to 1300 8C and
using the same temperature regime as in the variable temperature X-ray diffraction experiment.
Samples for dilatometry were prepared by pressing the binder-free powder into cylindrical compacts

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I.R. Evans et al. / Materials Research Bulletin 38 (2003) 1203–1213
6 mm in diameter, using a pressure of 245 MPa with initial bulk density of about 50% theoretical
density.
3
. Results and discussion
3
.1. Variable temperature X-ray diffraction
Initial inspection of the room temperature X-ray diffraction patterns of both the activated and the
non-activated samples showed that after calcination in each case there were three phases present in the
system, namely CaCO , TiO and CaTiO .
Fig. 1 shows the evolution of the X-ray diffraction pattern for the non-activated CaCO /TiO mixture
3
2
3
3
2
as a function of temperature. The main features to be noted are the increase of intensity and sharpness
of peaks at approximately 338, 478 and 608 2y, as well as the appearance of extra peaks (e.g. at about
3
28, 378 and 548 2y) in the pattern at 600 8C.
In addition to the two reactants, the starting sample at room temperature contains a small quantity
of CaTiO (as evidenced by the peaks at about 338, 478 and 608 2y). The phase composition of the
3
system remains qualitatively unchanged until the emergence of new peaks at 600 8C (328, 378 and
5
all CaCO has converted into CaO and the formation of CaTiO ensues. Fig. 2 depicts the results
48 2y, etc.), which can be assigned to CaO formed by decomposition of CaCO . By 800 8C,
3
3
3
of the Rietveld refinement of data collected at 1100 8C. At this temperature, CaTiO is the predo-
3
minant phase in the system, but small amounts of CaO and TiO are present as unreacted starting
materials.
2
3 2
Fig. 1. Evolution of the X-ray diffraction pattern for the non-activated CaCO /TiO mixture as a function of temperature (data
sets are offset by 0.18 2y per pattern for clarity).

I.R. Evans et al. / Materials Research Bulletin 38 (2003) 1203–1213
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Fig. 2. Observed (red), calculated (blue) and difference (grey) Rietveld refinement plots for the non-activated sample at
1
100 8C.
Fig. 3 shows the evolution of the X-ray diffraction pattern for the activated CaCO /TiO mixture as a
3
2
function of temperature. It can be noted that the changes in the phase composition of the system are less
dramatic than in the non-activated sample, owing to a significant amount of CaTiO already present in
3
the starting mixture (as indicated by peaks at about 338, 478 and 608 2y). Following the decomposition
of CaCO into CaO, the formation of CaTiO takes place and is accompanied by a continual increase of
3
3
3 2
Fig. 3. Evolution of the X-ray diffraction pattern for the activated CaCO /TiO mixture as a function of temperature (data sets
are offset by 0.18 2y per pattern for clarity).

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I.R. Evans et al. / Materials Research Bulletin 38 (2003) 1203–1213
Fig. 4. Observed (red), calculated (blue) and difference (grey) Rietveld refinement plots for the activated sample at 1100 8C.
observed intensity and sharpness of its diffraction peaks. At 920 8C, CaTiO is completely formed as
3
the pure product of the reaction. The final observed, calculated and difference profiles for the activated
mixture are shown in Fig. 4.
3
.2. Quantitative analysis
The quantitative aspect of the progress of the solid-state reaction of CaTiO formation can be
3
monitored readily by in situ X-ray diffraction. The amount of CaTiO present at different stages of the
3
reaction in the two samples, expressed as a weight percentage, is shown in Fig. 5.
In the non-activated sample, the calcined mixture contains about 10% CaTiO , presumably formed
3
during the 2 h calcination of the pellet. The 2 h calcination is carried out at 900 8C, above the temperature
where CaCO begins to decompose into CaO and CaTiO formation commences. Therefore, under these
specific experimental conditions, the presence of a small amount of CaTiO in the non-activated mixture
3
3
3
is not surprising. The final product at 1100 8C contains about 90% CaTiO . The calcined mixture of the
3
activated sample contains about 75% CaTiO , the extra 65% relative to the non-activated sample
3
Fig. 5. Weight percentage of CaTiO present in the activated reaction mixture as a function of temperature.
3

I.R. Evans et al. / Materials Research Bulletin 38 (2003) 1203–1213
1209
apparently formed owing to the mechanical activation during vibro-milling treatment. By 920 8C, the
reaction in the activated sample is complete and CaTiO is present as a pure phase product.
The trends illustrated by Fig. 5 show that the mechanical activation of the reactants has a key effect
on the formation of CaTiO during calcination and hence, on the composition of the mixture prior to
3
3
heating and that the subsequent rate determining factor for the formation of CaTiO is the availability
3
of CaO upon decomposition of CaCO above 600 8C.
3
3
.3. Microstructure analysis
Determination of the crystallite size based on the diffraction peak profiles of the two samples was
carried out using a direct convolution integral breadth method [27]. The necessary corrections for the
instrumental factors contributing to the peak shapes were made using annealed yttria as a standard
material. The average crystallite size in the non-activated sample ranges from about 90 nm at room
temperature and exceeds 200 nm by 1100 8C. The average crystallite size in the activated sample at
room temperature is reduced to about 70 nm and by the completion of the reaction of CaTiO formation
it reaches 120 nm.
3
SEM micrographs of fractured samples after the thermal treatment during the X-ray diffraction
measurements up to a temperature of 1100 8C are shown in Fig. 6. The changes in particle shapes and
sizes caused by mechanical activation are clearly visible. The microstructure of the activated sample is
fine grained and non-agglomerated. Particles have nearly spherical shapes and almost uniform particle
size, in contrast to the microstructure of the non-activated sample. It can also be seen that the separate
particles of the compact maintain their identity during thermal treatment up to 1100 8C, which is
characteristic of initial stage of sintering. Open porosity, another characteristic of this stage of sintering,
is evident in both cases. However, the porosity is significantly less pronounced in the mechanically
activated sample than in the non-activated one.
3
.4. Thermal analysis
The relative shrinkage of the activated (CTO60) and the non-activated (CTORM) sample heated to
300 8C at a rate of 10 8C/min is shown in Fig. 7. These results reveal that the two samples behave
1
differently. Despite the initial greater shrinkage of the non-activated sample up to 500 8C, its final
relative shrinkage is smaller than that of the activated sample. This is a consequence of a lower rate of
shrinkage during the initial and intermediate sintering stages, due to a larger amount of starting
materials present in the non-activated sample. This is supported by the results of variable temperature
X-ray diffraction. As CaCO starts to decompose, the diffusion in this sample slows down, since the
3
layer of CaO formed becomes an obstacle for further decomposition of the calcium carbonate core.
This situation is different in the activated sample in which the completion of CaCO decomposition and
3
CaTiO formation at a temperature lower than onset of sintering allows an increased shrinking rate, thus
3
yielding a more compact final product at 1300 8C.
In order to investigate the effect of heating rates during thermal treatment, two more sets of
dilatometry measurements were performed: one using an identical temperature regime to that used in
the X-ray diffraction measurements and the other using a faster heating rate of 30 8C/min. Fig. 8 (a, b)
show the relative shrinkage of the non-activated and activated sample respectively as a function of time
during heating up to 1100 8C with heating rates of 10 and 30 8C/min and during the thermal treatment

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Fig. 6. SEM micrograph of the fracture surface for the (a) non-activated and (b) activated sample (the size marker line
corresponds to 200 nm).
Fig. 7. Relative shrinkage of the activated (CTO60) and non-activated (CTORM) samples as a function of temperature during
heating to 1300 8C with a heating rate of 10 8C/min.

I.R. Evans et al. / Materials Research Bulletin 38 (2003) 1203–1213
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Fig. 8. Relative shrinkage of the (a) non-activated sample and (b) activated sample, as a function of time during heating to
100 8C with heating rates of 10 and 308C/min and a thermal treatment identical to that used in the X-ray diffraction
measurement (labelled 10, 30 and Ro, respectively).
1
identical to that used in the X-ray diffraction measurement. In the case of the non-activated samples
(Fig. 8a), the final shrinkage is very sensitive to the thermal treatment regime and significantly
decreases with decreasing heating rate or with prolonged holding time. Contrary to this, the final
relative shrinkage of the activated samples (Fig. 8b) does not vary significantly (0.5–1.0%) as a function
of the thermal regime applied.
Figs. 7 and 9 show that the non-activated samples exhibit a shrinking effect between 400 and 600 8C,
depending on the heating rate. A higher heating rate shifts this effect to higher temperatures and increases
its magnitude (as indicated by arrows in Fig. 9). This initial shrinkage at such low temperatures can be
explained as a consequence of the rearrangement of particles in the non-activated samples, which, due to
irregular particle shapes and sizes and weak cohesion bonds between them, is promoted by a larger
thermal gradient. In the case of the activated sample, particles have nearly spherical shapes, with almost
uniform particle sizes and the rearrangement process does not occur under these conditions.

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Fig. 9. Relative shrinkage of the non-activated and activated sample as a function of temperature with heating rates of 30 8C/
min and a thermal treatment identical to that used in the X-ray diffraction measurement (labelled CTORM-30, CTORM-Ro,
CTO60-30 and CTO60-Ro, respectively).
Variable temperature X-ray diffraction shows that CaCO decomposes between 600 and 800 8C
3
during the thermal treatment with a relatively low heating rate and a prolonged holding time at selected
temperatures. The corresponding dilatometry curve (CTO60-Ro) in Fig. 9 shows that this process is
accompanied by sample expansion up to about 830 8C. For a higher heating rate, the maximum on the
dilatometry curve, characteristic of the completion of chemical reactions of CaCO decomposition and
CaTiO formation, shifts to higher temperatures (as indicated by circles in Fig. 9). For the heating rate
3
3
of 30 8C/min (CTO60-30 in Fig. 9), this maximum is at about 950 8C. Similarly, the onset of sintering
shifts to higher temperatures for higher heating rates.
In the case of the non-activated sample, this characteristic maximum is observed at about 1020 8C
only for the slow thermal treatment (CTORM-Ro, indicated by a triangle in Fig. 9). For a higher
heating rate (CTORM-30, Fig. 9) it is not observed at all. This suggests that in the absence of
mechanical activation the reactions of CaCO decomposition and CaTiO formation are too slow to be
3
3
completed in the observed temperature–time regimen and to consequently allow the mass transport and
the sintering process to occur as efficiently as in the activated sample.
4
. Conclusions
The present work illustrates successful use of laboratory in situ variable temperature X-ray
diffraction to investigate various aspects of solid-state reactions, including phase evolution, quantitative
analysis and microstructure analysis. In particular, the study of a mechanically activated synthesis of
CaTiO indicates that a 60-min mechanical activation of a mixture of CaCO and TiO leads to the
3
3
2
formation of significant amounts of the target material during a 2-h calcination (75%, or a 65% excess
compared to the non-activated mixture), owing to reduced particle size, increased homogeneity of the
reactants and hence, an enhanced reactivity of the starting mixture. This results in the formation of pure
CaTiO at 920 8C. Microstructure analysis reveals that the average crystallite size in the activated
3
sample is about half that in the product obtained from the non-activated mixture. These effects promote
sintering and result in formation of dense fine-grained CaTiO at 1300 8C, as evidenced by dilatometry
3

I.R. Evans et al. / Materials Research Bulletin 38 (2003) 1203–1213
1213
measurements. Together with the differences in the behavior of different samples as a function of
different heating rates, these results suggest that the properties of the ceramic product, such as
composition, density and microstructure, depend in a complex way on the parameters of mechanical
activation and thermal treatment. Further studies of details of these relationships are underway.
Acknowledgements
We wish to acknowledge Andrew J.W. Yates of the Department of Physics, University of Durham, for
the SEM micrographs. JAKH thanks the EPSRC for the Senior Research Fellowship. This work was
funded in part by the project (‘‘Synthesis of functional materials from the viewpoint of the synthesis-
structure-properties-application relationship’’) financed by the Ministry for Science, Technology and
Development, Republic of Serbia Grant number 1832 and facilitated by EPSRC/JREI Grant number
GR/M35222, which provided high temperature powder diffraction equipment.
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