Preparation and characterization of coprecipitates and mechanical mixtures of calcium-strontium oxalates using XRD, SEM-EDX and TG

Article abstract of DOI:10.1016/S0040-6031(98)00339-6

In view of the study of the chemistry of the oxalate coprecipitation process for preparing BiSrCaCuO superconductors, the characteristics of oxalates of calcium and strontium in the Ca(NO₃)₂-Sr(NO₃)₂-HNO ₃-H₂O-(NH₄)₂C₂O ₄ system are investigated. Based on XRD, SEM and TG a comparison between the crystal structure, morphology and thermal decomposition behavior of coprecipitated and physically mixed calcium and strontium oxalates with the same Ca/Sr stoichiometry is made. It is shown that the coprecipitated powders form homogeneous solid solutions of which the crystal structure, morphology and thermal behaviour depend on the Ca/Sr stoichiometry.

Full text of DOI:10.1016/S0040-6031(98)00339-6

Thermochimica Acta 318 (1998) 143±153
Preparation and characterization of coprecipitates and mechanical mixtures
of calcium-strontium oxalates using XRD, SEM-EDX and TG
E. Knaepena, M.K. Van Baela, I. Schildermansa, R. Nouwena, J. D'Haenb, M. D'Olieslaegerb,
C. Quaeyhaegensb, D. Francoa, J. Ypermana, J. Mullensa,*, L.C. Van Pouckea
a IMO, Laboratory of Inorganic and Physical Chemistry, Limburgs Universitair Centrum, Building D, B-3590 Diepenbeek, Belgium
b IMO, Materials Physics Division, Limburgs Universitair Centrum, Building D, B-3590 Diepenbeek, Belgium
Received 29 September 1997; accepted 18 December 1997
Abstract
In view of the study of the chemistry of the oxalate coprecipitation process for preparing BiSrCaCuO superconductors, the
characteristics of oxalates of calcium and strontium in the Ca(NO3)2±Sr(NO3)2±HNO3±H2O±(NH4)2C2O4 system are
investigated. Based on XRD, SEM and TG a comparison between the crystal structure, morphology and thermal
decomposition behavior of coprecipitated and physically mixed calcium and strontium oxalates with the same Ca/Sr
stoichiometry is made. It is shown that the coprecipitated powders form homogeneous solid solutions of which the crystal
structure, morphology and thermal behaviour depend on the Ca/Sr stoichiometry. # 1998 Elsevier Science B.V.
1. Introduction
the homogeneity of the precursors and, consequently,
the ultimate properties of the superconductive materi-
als, various chemical routes (mainly sol±gel [4,5],
spray-drying [6] and coprecipitation [1,2]) have
received special attention and interest during recent
years.
The preparation of coprecipitated Ca±Sr oxalates is
part of the study of the oxalate precursor synthesis
of bulk Bi2Sr2CaCu2O8‡ꢀ and Bi2Sr2Ca2Cu3O10‡ꢀ
superconductors by the coprecipitation±®ltration tech-
nique [1,2].
This `wet chemical method' provides an intimate
mixing of the component elements in solution and,
consequently, the production of more homogeneous
powders compared to powders obtained by the gen-
erally used `ceramic technique' [3]. The main dis-
advantages of the conventional `mechanically mixed-
oxide/carbonate' synthesis are microscopic composi-
tional inhomogeneities resulting in long calcination
and sintering times, nonuniformity of particle size and
shape, and lack of reproducibility. In order to improve
Among the precipitation±®ltration techniques, the
most used precipitating agent is the oxalate ion [7,8].
Metal oxalates are very suitable precursor powders for
superconductive oxides, since they can easily be
decomposed into their corresponding oxides [9].
Moreover, some oxalates can form solid solutions.
The formation of solid solutions of some multi-metal
oxalates has been extensively discussed by Scheule
[10] and Fischer et al. [11].
In this paper, we report the preparation, character-
ization and thermal analysis of coprecipitates and
mechanical mixtures of calcium±strontium oxalates
with different Ca/Sr ratios, including almost the same
*Corresponding author. E-mail: jmullens@luc.ac.be
0040-6031/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved
P I I S 0 0 4 0 - 6 0 3 1 ( 9 8 ) 0 0 3 3 9 - 6

144
E. Knaepen et al. / Thermochimica Acta 318 (1998) 143±153
ratios as in the superconductive phases. The aim of this
research is to compare the characteristics of the
coprecipitated oxalates with the physically mixed
oxalates and to investigate the in¯uence of the Ca/
Sr ratio on the morphology of the coprecipitated
powders. Comparison between coprecipitates and
mechanical mixtures having the same Ca/Sr ratio
and between mutual coprecipitates with different
Ca/Sr ratios are based on XRD, SEM-EDX and
TG.
purity (>99.9%). Since Ca(NO3)2Á4H2O is very hygro-
scopic, a standard solution was prepared, of which the
concentration was determined by an EDTA-titration
using eriochrome-T as indicator.
2.3. Preparation of oxalate powders
The Ca±Sr oxalate coprecipitates were prepared in a
thermostatted case (25.0Æ0.28C) by adding a mixture
of nitric acid and an aqueous solution of Ca(NO3)2 and
Sr(NO3)2 to an aqueous solution of 0.3 M (NH4)2C2O4
È
by two motor driven burettes (Schott Gerate T100) at
2. Experimental
the same rate and under continuous stirring. A series
of samples was prepared using different concentration
ratios Ca2‡/Sr2‡ in the metal nitrate solution which
contains 0.67 M nitric acid. The coprecipitates
obtained were ®ltered through an acid resistant 0.45
micron Millipore ®lter and washed with doubly dis-
tilled water. Finally, the powders were dried in air. The
individual calcium and strontium oxalates were pre-
pared in the same way as described above. Mechanical
mixtures of calcium and strontium oxalate were
obtained by mixing the individual precipitated oxa-
lates in a ball-mill for a period of 3 h.
2.1. Methods and apparatus
The Ca±Sr stoichiometry in the coprecipitated and
mechanically mixed powders, which were dissolved in
10% nitric acid, was determined by inductively
coupled plasma±atomic emission spectroscopy
(ICP-AES) on a Perkin±Elmer Optima 3000DV.
The elemental analysis was carried out with emission
lines of Ca ± 317.933, Ca ± 393.366, Ca ± 396.847, Sr
± 232.235, Sr ± 421.552 and Sr ± 460.733 nm. The
structure and the phase composition of all samples
were determined by a Siemens D-5000 powder dif-
fractometer using a Cu X-ray source. The morphology
and elemental composition of the individual and
binary oxalates were examined by scanning electron
microscopy (SEM) measurements and energy disper-
sive X-ray analysis (EDX) on a Philips XL-30 FEG
equipped with an EDX detector with a super-ultrathin
window. The acceleration voltage used for the SE
(secondary electrons)-images was 5 kV. EDX analysis
was carried out with higher acceleration voltage.
The thermal behaviour of the precipitates was
investigated by means of a TA Instruments Model
2950 (temperature range: ambient to 10008C) thermo-
gravimetric analyser in inert Ar atmosphere (50 ml/
min) using the following heating-rate scheme: 58C/
min until 3008C, 18C/min until 5008C and 58C/min
until 9008C.
3. Results and discussion
3.1. XRD analysis
XRD measurements were performed to identify the
prepared solids. The composition of the samples is
given in Table 1.
3.1.1. Coprecipitates
Among the individual precipitated strontium oxa-
late powders (samples 1, 2), only sample 1 (Table 1) is
identi®ed as a single phase, corresponding to the
tetragonal SrC2O4Á2.5H2O phase [12], as shown in
Fig. 1(a). Adding Ca(NO3)2 to the Sr(NO3)2 starting
solution, results in the formation of coprecipitates of
composition Cax
Sr xC2O4ÁyH2O (Table 1, samples 5
1
to 13). The x-values, derived from ICP-analysis, are
shown in Table 1. The ®rst additions of Ca2‡ (Table 1,
samples 5, 6, 7, 8 and 9) causes a shift in the diffraction
lines to higher 2ꢁ angles (Fig. 1(b±f)) compared to the
lines of SrC2O4Á2.5H2O (Fig. 1(a)). This decrease in
cell volume can be explained by the partial substitu-
2.2. Materials
All the chemicals used, Sr(NO3)2 (Merck p.a.),
Ca(NO3)2Á4H2O (Merck p.a.), (NH4)2C2O4ÁH2O
(Merck p.a.) and HNO3 (J.T. Baker) were of high

E. Knaepen et al. / Thermochimica Acta 318 (1998) 143±153
145
Table 1
Composition of single, coprecipitated and mechanically mixed oxalates: ICP and XRD results
Sample
Metal nitrate solution
ICP-analysis: Ca±Sr ratio
XRD-analysis: phase composition
[Sr(NO3)2]
[Ca(NO3)2]
Strontium oxalate
1
2
0.03726 M
0.27160 M
SrC2O4Á2.5H2O
SrC2O4ÁH2O (main fraction)‡SrC2O4Á2.5H2O
Calcium oxalate
3
4
0.01490 M
0.1500 M
CaC2O4ÁH2O
CaC2O4ÁH2O (main fraction)‡CaC2O4Á3H2O
Coprecipitates
5
6
7
8
0.03737 M
0.03725 M
0.03806 M
0.03728 M
0.00745 M
0.01490 M
0.02235 M
0.02979 M
Ca0.35Sr0.65C2O4Á2H2O
Ca0.49Sr0.51C2O4Á2H2O
Ca0.56Sr0.44C2O4Á2H2O
Ca0.62Sr0.38C2O4Á2H2O
CaxSr1 xC2O4.yH2O solid solution
substitution of Sr by Ca in the lattice of
SrC2O4Á2.5H2O
9
0.03728 M
0.03725 M
0.03816 M
0.03517 M
0.05960 M
0.10346 M
Ca0.66Sr0.34C2O4Á2H2O
Ca0.74Sr0.26C2O4Á1.7H2O
Ca0.81Sr0.19C2O4Á1.5H2O
10
11
two-phase region mixture of two solid solutions
substitution of Ca by Sr in the lattice of
CaC2O4Á1H2O
12
13
0.03727 M
0.03728 M
0.12758 M
0.16554 M
Ca0.85Sr0.15C2O4ÁH2O
Ca0.89Sr0.11C2O4ÁH2O
Mechanical mixtures
14
15
0.2 g sample 2‡0.0833 g sample 4
0.2 g sample 2‡0.1450 g sample 4
Ca/Srˆ0.35/0.65
Ca/Srˆ0.49/0.51
XRD pattern of sample 2‡sample 4
XRD pattern of sample 2‡sample 4
2‡
tion of Sr2‡ (1.12 A) by Ca (0.99 A) in the lattice of
SrC2O4Á2.5H2O. For samples 10 and 11 (Fig. 1(g) and
(h)), an additional phase is detected of which the
diffraction lines are shifted to lower 2ꢁ angles com-
pared to the lattice of CaC2O4ÁH2O (Fig. 1(k)) [13].
Ultimately, the solubility limit of Ca2‡ in the
SrC2O4Á2.5H2O lattice is reached and further addi-
tions of Ca2‡ (Table 1, samples 12 and 13) leads to the
formation of the CaC2O4ÁH2O single phase, in which
some Ca2‡ is substituted by Sr2‡, thus causing an
increase in cell volume (Fig. 1(i) and (j)). Fig. 1(k)
shows the XRD pattern of individual precipitated
calcium oxalate (Table 1, sample 3), identi®ed as
monoclinic CaC2O4ÁH2O.
precipitated, starting from a higher concentration of
the metal nitrate solution (Table 1, samples 2 and 4).
From XRD results (Fig. 2(a) and (b)) it can be con-
cluded that strontium oxalate and calcium oxalate
consists of two phases: sample 2 being a mixture of
SrC2O4ÁH2O [13] and SrC2O4Á2.5H2O, sample 4 being
a mixture of CaC2O4ÁH2O [13] and CaC2O4Á3H2O
[13]. Appropriate amounts of both solids were
weighed out in order to form mechanical mixtures
(Table 1: samples 14 and 15) of the same stoichio-
metry as in the coprecipitated samples 5 and 6.
Comparison of the XRD patterns of samples 14
(Fig. 2(c)) and 15 (Fig. 2(d)) with those of samples
2 and 4, demonstrates that all the lines of the two
individual oxalate patterns (Fig. 2(a) and (b)) are
present: the mixtures with Ca/Sr ratio equal to 0.35/
0.65 and 0.49/0.51 contain the same diffraction lines at
the same positions, but the relative intensities of the
two patterns is changed, due to a change in total
composition.
Ê
Ê
3.1.2. Mechanical mixtures
The physical mixtures were prepared by grinding
together individually precipitated strontium oxalate
and calcium oxalate. In order to mill a suf®cient
amount of these mixtures, the single oxalates were

146
E. Knaepen et al. / Thermochimica Acta 318 (1998) 143±153
Fig. 1. XRD patterns of the (co)precipitated samples: (a), sample 1; (b), sample 5; (c), sample 6; (d), sample 7; (e), sample 8; (f), sample 9;
(g), sample 10; (h), sample 11; (i), sample 12; (j), sample 13; and (k) sample 3.

E. Knaepen et al. / Thermochimica Acta 318 (1998) 143±153
147
Fig. 2. XRD patterns of individual oxalates and their mechanical mixtures: (a) sample 2; (b) sample 4; (c) sample 14; and (d) sample 15.

148
E. Knaepen et al. / Thermochimica Acta 318 (1998) 143±153
Fig. 3. SEM micrographs of the (co)precipitated samples: (a) sample 1; (b) sample 5; (c) sample 6; (d) sample 7; (e) sample 8; (f) sample 9;
(g) sample 10; (h) sample 11; (i) sample 12; (j) sample 13; and (k) sample 3.
3.2. SEM-EDX analysis
and 11 (Fig. 3(g) and (h)) show the presence of prism-
like crystals, in addition to the still present octahedral
crystals, con®rming the presence of two phases, as
already seen in the XRD patterns (Fig. 1(g) and (h)) of
these samples. Finally, samples 12 and 13 show the
same morphology (Fig. 3(i) and (j)) as CaC2O4ÁH2O
(sample 3, Fig. 3(k)), consisting of prism-like crystals.
Several EDX spot analyses on all the coprecipitated
samples show that every crystal consists of both Sr and
Ca, thereby con®rming that homogeneous, intimately
mixed oxalate powders are formed after synthesis.
3.2.1. Coprecipitates
Fig. 3 presents the morphology of the coprecipi-
tates in relation to their stoichiometry. Since these
coprecipitates, depending on their stoichiometry, have
a crystal structure similar to SrC2O4Á2.5H2O and/or
CaC2O4ÁH2O, the SE pictures of these single oxalates
are also presented. SrC2O4Á2.5H2O (sample 1) is
composed of octahedral crystals, as shown in
Fig. 3(a). The SE pictures of samples 5, 6, 7, 8 and
9 (Fig. 3(b±f)) con®rm that these samples are copre-
cipitates with Ca substitution in the lattice of
SrC2O4Á2.5H2O, since the octahedral crystals are still
present. Secondary electron micrographs of sample 10
3.2.2. Mechanical mixtures
Fig. 4 presents the SE micrographs of the single
precipitated strontium oxalate (sample 2, Fig. 4(a)).

E. Knaepen et al. / Thermochimica Acta 318 (1998) 143±153
149
Fig. 4. SEM micrographs of individual oxalates and their mechanical mixtures: (a) sample 2; (b) sample 4; (c) sample 14; and (d) sample 15.
Comparison between Fig. 4(a) and Fig. 3(a) demon-
strates clearly that sample 2 consists of two types of
crystals: octahedral-like particles (comparable with
the crystals in Fig. 3(a) and, therefore, most likely
to correspond with the SrC2O4Á2.5H2O phase) and
rounded particles. Calcium oxalate (sample 4,
Fig. 4(b)) also contains two phases. Fig. 4(c) and
(d) demonstrate the morphology of the two mechan-
ical mixtures (samples 14 and 15). From EDX mea-
surements it is con®rmed that both Ca- and Sr-rich
parts are present, con®rming that the mixtures consist
of separate grains of the starting materials. The mor-
phology of the mixtures and the coprecipitates
(Fig. 3(b), Fig. 4(c), Fig. 3(c) and Fig. 4(d)) con®rm
that the coprecipitates form highly homogenous
powders.
1. endothermic dehydration between 1308C and
2508C;
2. endothermic formation of strontium carbonate
between 4208C and 5908C with evolution of CO
and CO2 (formed by an exothermic disproportio-
nation reaction); and
3. release of CO2 during the endothermic formation
of the oxide between 770 and 10208C.
In the present study, we investigate the thermal
decomposition behaviour of the individual precipi-
tated oxalates, coprecipitates and mechanical mix-
tures. Table 2 presents a comparison between the
experimental and theoretical percentage mass loss
during each decomposition step of the single oxalates,
coprecipitates and mechanical mixtures.
3.3. Thermal analysis
3.3.1. Coprecipitates
Fig. 5 presents the DTG curves of all the copreci-
pitated powders (samples 5, 6, 7, 8, 9, 10, 11, 12 and
13, Fig. 5(b±j), respectively) and the individual oxa-
lates (samples 1 and 3, Fig. 5(a) and (k), respectively).
The DTG curves of the coprecipitated oxalates are
compared with those of the single oxalates in order to
In a previous report, the thermal decomposition of
different forms of strontium oxalate in an inert atmo-
sphere was studied [14]. Based on TG±EGA and DSC
results, it was shown that SrC2O4ÁH2O decomposes as
follows:

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E. Knaepen et al. / Thermochimica Acta 318 (1998) 143±153
Table 2
Comparison of experimental and theoretical % mass loss of single oxalates, coprecipitates and mechanical mixtures during their thermal
decomposition
Sample
Dehydration/ %
exp
Formation of carbonates/ %
Formation of oxides/ %
theor
exp
theor
exp
theor
Strontium oxalate
1
2
20.4
10.9
20.4
10.9
12.8
14.2
12.7
14.4
19.8
22.3
19.9
22.1
Calcium oxalate
3
4
12.6
15.3
12.3
15.3
19.1
18.5
19.2
19.0
29.6
28.5
30.1
29.1
Coprecipitates
18.5
19.1
20.0
20.1
19.5
17.5
16.4
13.0
12.7
18.3
19.0
20.2
20.3
19.6
17.1
16.8
13.2
12.7
14.4
14.9
15.4
15.5
15.7
16.6
17.5
18.2
18.5
14.4
14.8
15.0
15.3
15.6
16.5
17.1
18.0
18.3
22.4
23.2
23.4
24.0
24.5
25.2
26.5
28.2
28.9
22.6
23.4
23.6
24.0
24.5
25.4
26.4
28.2
28.9
6
7
8
9
10
11
12
13
Mechanical mixtures
14
15
12.2
12.7
12.2
12.7
15.8
16.2
15.7
16.3
24.0
24.7
24.1
25.0
explain the decomposition behaviour of the copreci-
pitates.
of two CaxSr1 xC2O4ÁyH2O coprecipitates with
xˆ0.35 (sample 5), xˆ0.49 (sample 6) and yˆ2 for
both samples. The dehydration of both coprecipitates
also takes place in four steps, but occurs at higher
temperatures than the dehydration of SrC2O4Á2.5H2O
(sample 1, Fig. 5(a)). The decomposition of the
dehydrated oxalate powders takes place in one step,
as demonstrated by the single DTG peak between 400
and 4708C in Fig. 5(b) and (c). This peak represent the
formation of an intimately mixed CaxSr1 xCO3
intermediate, as described by reaction (1).
3.3.1.1. DTG results of the single oxalates: sample 1
and sample 3. The thermal decomposition of
SrC2O4Á2.5H2O (sample 1) is illustrated in
Fig. 5(a). The shape of the DTG curve between 50
and 2008C shows that the dehydration takes place in
four steps. In the temperature domain between 400 and
4908C, the DTG curve shows one peak due to the
decomposition of SrC2O4 into SrCO3. The last DTG
peak, between 680 and 9008C, corresponds to the
formation of SrO.
CaxSr1 xC2O4 ! CaxSr1 xCO3 ‡ CO
(1)
The last DTG peak in Fig. 5(b) and (c) corresponds
to the formation of CaO and SrO, respectively, as
derived from the experimental percentage mass loss
between 600 and 8408C (see Table 2, samples 5 and
6). The oxide formation thus takes place in one step
according to reaction (2).
The DTG curve of CaC2O4ÁH2O (sample 3,
Fig. 5(k)) reveals three peaks in the 100±1608C,
370±4808C and 520±6908C ranges which correspond,
respectively, to water loss, formation of CaCO3 and
formation of CaO.
3.3.1.2. DTG results of the coprecipitated samples 5
and 6. Fig. 5(b) and (c) demonstrate the DTG curves
CaxSr1 xCO3 !xCaO‡ꢀ1 x†SrO‡CO2 (2)

E. Knaepen et al. / Thermochimica Acta 318 (1998) 143±153
151
Fig. 5. DTG curves of the (co)precipitated samples: (a) sample 1; (b) sample 5; (c) sample 6; (d) sample 7; (e) sample 8; (f) sample 9; (g)
sample 10; (h) sample 11; (i) sample 12; (j) sample 13; and (k) sample 3.
3.3.1.3. DTG results of the coprecipitated samples 7,
8, 9 and 10. Fig. 5(d±g) present the DTG curves of the
CaxSr1 xC2O4ÁyH2O coprecipitates with xˆ0.56 and
yˆ2 (sample 7), xˆ0.62 and yˆ2 (sample 8), xˆ0.66
and yˆ2 (sample 9) and xˆ0.74 and yˆ1.7 (sample
10), respectively. It is seen from these figures that the
temperature region, in which the dehydration reaction
takes place, is shifted to lower temperatures with
increasing x values. Samples 7 to 9 have the same
water content as samples 5 and 6. Presumably some
vacancies occur in the predominantly SrC2O4Á2.5H2O
lattice found for samples 5 to 9, equivalent to partial
dehydration. The water content of sample 10 is
situated between 2.5 and 1, which can be explained
by the fact that a two-phase mixture of the
SrC2O4Á2.5H2O and CaC2O4ÁH2O lattices is found
for this sample. The DTG curves of samples 7 to
10 (Fig. 5(d±g)) show two peaks, between 380 and
4608C, indicating that the decomposition of the
anhydrous oxalate coprecipitates into carbonates
takes place in two steps. The first step, between
380 and 4108C, can be attributed to the partial
decomposition of the Ca±Sr oxalate into calcium
carbonate (see reaction 3), followed by the
decomposition of the remaining mixed oxalate into
a homogeneous mixed Ca±Sr carbonate, between 410
and 4608C (see reaction 4). It can be concluded from
these results that partial segregation during the
decomposition process takes place. The following
reactions describe the two-step decomposition of
the anhydrous oxalate.
CaxSr1 xC2O4 ! zCaCO3
‡ Cax zSr1 xꢀC2O4† ‡ zCO
(3)
1
z
Cax zSr1 xꢀC2O4† ! Cax zSr1 xꢀCO3†
1
z
1
z
‡ ꢀ1 z†CO
(4)

152
E. Knaepen et al. / Thermochimica Acta 318 (1998) 143±153
The DTG peak, between 550 and 6908C (Fig. 5(d±
intermediary water content, as also seen for sample
10. Finally, samples 12 and 13 have the same water
content as the CaC2O4ÁH2O lattice. The dehydrated
coprecipitates are decomposed into a mixture of two
carbonates, between 370 and 4508C (see reaction 7).
The complete segregation is confirmed by the good
agreement between experimental and theoretical
percentage mass loss for the separate formation of
CaO and SrO (see reactions 8 and 9).
g)), corresponds to the formation of CaO from CaCO3
(see reaction 5). Finally, the mixed Ca±Sr carbonate is
decomposed into CaO and SrO (see reaction 6) between
710 and 8358C as shown by the last DTG peak.
CaCO3 ! CaO ‡ CO2
(5)
Cax zSr1 xꢀCO3† ! ꢀx z†CaO
1
z
‡ ꢀ1 x†SrO ‡ ꢀ1 z†CO2
(6)
CaxSr1 xC2O4 !xCaCO3 ‡ꢀ1 x†SrCO3 ‡CO
Table 2(samples7,8,9and10)showsagoodagreement
between the theoretical and experimental percentage
mass losses for the complete decomposition of the
oxalates into carbonates (reactions 3 and 4) and the
formation of the oxides (reactions 5 and 6).
(7)
CaCO3 ! CaO ‡ CO2
SrCO3 ! SrO ‡ CO2
(8)
(9)
3.3.1.4. DTG results of the coprecipitated samples
11, 12, 13. Fig. 5(h±j) show the DTG curves of
CaxSr1 xC2O4ÁyH2O coprecipitates with xˆ0.81 and
yˆ1.5 (sample 11), xˆ0.85 and yˆ1 (sample 12) and
xˆ0.89 and yˆ1 (sample 13). The water content of the
three samples is removed at lower temperatures than
for CaC2O4ÁH2O (Fig. 5(k)). Sample 11 has an
3.3.2. Mechanical mixtures
Fig. 6 shows the DTG curves of the strontium
oxalate sample 2, the calcium oxalate sample 4 and
two mechanical mixtures of both oxalates (samples 14
and 15). In the 50±9008C range, the curves of the two
mixtures (Fig. 6(c) and 6(d)) show seven peaks, which
can all be assigned to the DTG peaks of the single
Fig. 6. DTG curves of individual oxalates and their mechanical mixtures: (a) sample 2; (b) sample 4; (c) sample 14; and (d) sample 15.

E. Knaepen et al. / Thermochimica Acta 318 (1998) 143±153
153
oxalates (Fig. 6(a) and (b)). The thermal behaviour of
the mechanical mixtures is, hence, similar to that of
the individual components.
technologisch onderzoek in de industrie (I.W.T)'',
Belgium, for financial support. M.K. Van Bael is a
research fellow of the Fund for Scientific Research of
Flanders (F.W.O Vlaanderen). The scientific respon-
sibility of the contents of this paper is assumed by its
authors. The authors wish to thank Christel Willems
for the XRD measurements.
The dehydration of the two mixtures takes place in
three steps between 50 and 2008C. The decomposition
of the anhydrous mixtures occurs in two steps between
350 and 5008C: DTG peaks 4 and 5 correspond,
respectively, to the formation of CaCO3 and SrCO3.
From a comparison with the pure oxalates (Fig. 6(a)
and (b)), one can conclude that the DTG peak 6
between 550 and 7008C is due to the formation of
CaO and that the ®nal mass loss between 700 and
8608C (DTG peak 7) represents the formation of SrO.
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4. Conclusions
It is concluded from XRD and SEM-EDX results
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Depending on the Ca/Sr stoichiometry, the copreci-
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E. Knaepen is indebted to the ``Vlaams Instituut
voor de bevordering van het wetenschappelijk-