Phase diagram study of CaBr2-CaHBr system

Article abstract of DOI:10.1007/s10973-012-2787-3

A study of binary, CaBr₂-CaHBr system was carried out by differential thermal analysis (DTA), covering the composition range from 100 % CaBr₂ to 100 % CaHBr between room temperature and 800 C. From DTA results, the contour of solidus

Full text of DOI:10.1007/s10973-012-2787-3

J Therm Anal Calorim (2013) 112:127–131
DOI 10.1007/s10973-012-2787-3
Phase diagram study of CaBr –CaHBr system
2
Chilakapati Venkata Vishnu Vardhan
Subramaniam Nagaraj Raghavachary Sridharan
Thiagarajan Gnanasekaran
•
Sajal Ghosh
•
•
•
Received: 30 May 2012 / Accepted: 15 October 2012 / Published online: 30 December 2012
Ó Akad e´ miai Kiad o´ , Budapest, Hungary 2012
Abstract A study of binary, CaBr –CaHBr system was
2
a differential scanning calorimeter with samples having a
carried out by differential thermal analysis (DTA), cover-
compositional interval of 10 mol%, a first version of the
phase diagram of this system was reported by the present
authors as having an eutectic composition of 65 mol%
CaBr –35 mol% CaH at *580 °C [5]. However, investi-
ing the composition range from 100 % CaBr to 100 %
2
CaHBr between room temperature and 800 °C. From DTA
results, the contour of solidus and liquidus temperatures
with composition is plotted and the phase diagram of
2
2
gation of the system using samples prepared with closer
composition intervals is needed to arrive at the eutectic
composition with more precision. Hence, reinvestigation of
the phase diagram of the CaBr –CaH system was carried out
CaBr –CaHBr system is constructed. The system shows an
2
eutectic reaction at 576 °C and the eutectic composition
is 79.6 mol% CaBr . Co-existing phases in different phase
2
2
2
fields are characterized by X-ray diffraction analysis.
to understand the phases that co-exist with the solid electro-
lyte, CaHBr, and to choose suitable composition for the
preparation of the electrolyte. Various compositions of CaBr2
Keywords CaHBr Á CaBr Á Phase diagram Á DTA Á
2
Alkaline earth halide–hydride Á Peritectic
and CaH were prepared in sealed capsules and differential
2
thermal analysis (DTA) experiments were carried out on
these samples. Samples with four different compositions
were equilibrated in sealed capsules at high temperatures,
quenched, and the products were analyzed by XRD. The
results of these experiments are discussed in this study.
Introduction
Hydride ion-conducting electrolytes are used in the electro-
chemical sensors for detecting hydrogen in liquid sodium
coolant of fast reactors [1, 2]. In this connection, several
combinations of alkali/alkaline earth metal halides mixed
with alkali/alkaline earth metal hydrides were investigated
for using them as hydride ion-conducting electrolytes [3, 4].
CaBr –CaH system is found to havethe potential forhydride
Experimental
Calcium bromide was prepared by reacting CaCO (Sigma-
3
Aldrich, Chelometric Standard, A.C.S reagent) with dis-
tilled AR. Grade HBr (Merk 47.8 % azeotropic) solution.
2
2
ion-conducting solid electrolyte which can be exploited for
the development of electrochemical hydrogen meter [5]. This
system is known to have a compound CaHBr which is formed
by the reaction between CaBr and CaH taken in 1:1 mol
The resultant CaBr solution was concentrated on a hot
2
plate and dried under flowing argon gas. Dry calcium
bromide obtained thus was further purified by heating it up
to is melting point in a quartz crucible placed in a quartz
vessel in a stream of dry argon gas containing HBr for 2 h
to convert any oxide impurities into calcium bromide.
Calcium hydride was prepared by hydriding calcium
shots (99.5 % pure M/s Alpha products) taken in an iron
crucible inside a stainless steel vessel. The vessel was
2
2
ratio [6]. Based on the experimental results obtained using
C. V. Vishnu Vardhan Á S. Ghosh Á S. Nagaraj Á
R. Sridharan (&) Á T. Gnanasekaran
Chemistry Group, Liquid Metals and Structural Chemistry
Division, IGCAR, Kalpakkam, India
e-mail: rsn.igcar@gmail.com
-
2
pressurized with hydrogen at 3 kg cm and maintained at
123

1
28
C. V. Vishnu Vardhan et al.
a temperature of 750 °C for 10 days. Yellow crystals of
(50 mol% CaBr –50 mol% CaH ) are shown in Fig. 1.
2
2
CaH were collected and stored inside a high purity argon
Reproducibility of the thermal events was confirmed by
duplicating the run which gave concordant results for each
sample. The liquidus and solidus temperatures are found
out from the endotherms obtained from the heating runs
only since the cooling runs showed appreciable superco-
oling effects. For sharp endothermic peaks, the onset
temperature was obtained by extrapolating the slope of the
linear portion of the peak (before it peaks) to the line
joining the base line of the peak. For endotherms which
were not sharp, the onset temperature was deduced as the
temperature at which the significant deviation from the
baseline occurred. The liquidus temperature was deter-
mined as the highest temperature at which the endotherm
reached the background signal. This was determined by
differentiating the DTA trace and noting the maximum in
the differential curve, near the tail end of the DTA trace.
As no peak was observed below 500 °C for the samples,
the DTA trace is shown from 400 to 800 °C in Fig. 1.
Table 1 gives the onset temperatures of the endotherms and
the liquidus temperatures observed for all compositions
from 100 mol% CaBr to 45 mol% CaBr . Figure 1 shows
2
atmosphere glove box.
A series of compositions ranging from 100 mol% CaBr2
to 100 mol% CaHBr were prepared at intervals of
*
CaH in required ratio in a mortar and ground thoroughly
5 mol%. The samples were prepared by mixing CaBr and
2
2
using a pestle to make *500 mg of each mixture. The
samples were stored inside the argon glove box. A small
amount of sample (about 20 mg) from each composition was
taken inside a one-end closed 15-mm long iron crucible of
outer diameter 3 mm and inner diameter 2.4 mm. After
loading the sample inside the crucible, the other end was
crimped and hermetically sealed using a pulsed arc welding
unit inside the glove box. The samples were heated at 800 °C
in reducing (hydrogen) atmosphere for 30 min and cooled.
During this process CaBr and CaH react in equal moles to
2
2
form twice the number of moles of CaHBr as given by the
reaction: CaBr ? CaH = 2CaHBr. Throughout the text,
2
2
sample compositions refer to the initial percentages of CaBr2
and CaH . Leak tightness of the crucibles was confirmed by
2
insuring no mass change in the crucibles before and after the
heating. The leak tight samples were taken for DTA studies.
Argon—4 % hydrogen mixture was used as the purge gas at
2
2
that pure CaBr2 exhibits a single endothermic peak at
738 °C corresponding to its melting point. The observed
-
1
2
0 mL min in all these experiments. DTA runs were
melting point of CaBr is in close agreement to the values
2
-
1
recorded at a heating rate of 5 K min from RT to 800 °C
using the DTA equipment, SETSYS 16-18 Evolution (M/s
Setaram, France). DTA runs were repeated several times for
each sample to obtain concordant data in two successive runs
to insure homogeneity of the sample during heating runs.
Temperature calibration of DTA was done using high purity
metal standards: gold, aluminum, silver, and zinc.
of 742 and 738 °C reported in literature [7–9]. The absence
of any other endothermic peak indicates that CaBr has no
2
phase transitions up to its melting temperature. It is shown
further in Fig. 1 that an isothermal event occurs at around
576 °C for compositions from 83.5 mol% to 52.8 mol%
CaBr . From the statistical mean value of the solidus
2
temperatures over these seven compositions listed in
Table 1, temperature corresponding to this isothermal
In order to identify the equilibrium phases that co-exist
in different phase fields, several samples were prepared in
the same way as mentioned before. The samples were
taken in iron crucibles, which in turn were placed in a leak-
tight stainless steel vessel and heated up to 750 °C initially
for an hour and then equilibrated at 50° below the solidus
temperature of the chosen composition for 10 days in
hydrogen atmosphere. The equilibrated samples along with
the stainless steel vessel were then quenched using liquid
nitrogen, collected, loaded in the Lindemann glass capil-
laries, and characterized by XRD. Works involving the
preparation of the samples and weld closing by pulsed arc
welding, loading equilibrated products in Lindemann cap-
illary tubes, and sealing them with wax were carried out
inside dry argon glove box [7].
50 mol% CaBr2
7
34 °C
5
2.8mol%CaBr2
5
81 °C
5
6
9.1mol%CaBr2
721 °C
5
77 °C
3.5mol%CaBr2
717 °C
09 °C
6
8 mol%CaBr2
570 °C
569 °C
7
72.6 mol%CaBr2
6
72 °C
5
75 °C
83 °C
682 °C
7
7.2 mol%CaBr2
5
647 °C
664 °C
8
3.5 mol%CaBr2
573 °C
9
0.7 mol%CaBr2
672 °C
705 °C
9
5.5 mol%CaBr2
695 °C
714 °C
1
00 mol%CaBr2
738 °C
Results and discussion
400
450
500
550
600
650
700
750
800
Temperature/°C
The DTA traces obtained for samples with compositions
starting from pure CaBr2 up to 100 mol% CaHBr
Fig. 1 DTA pattern of samples from 100 to 50 mol% CaBr
2
1
23

Phase diagram study
129
Table 1 DTA results obtained for CaBr
2
–CaH
2
samples
Liquidus
DTA trace obtained for 50 mol% CaBr (composition
2
corresponding to the compound CaHBr) also shows a broad
endothermic event at 718 °C followed by the liquidus. If the
compound CaHBr melts congruently, DTA trace obtained
must have shown a large single endothermic peak repre-
senting its melting. On the other hand, the DTA trace is akin
mol%
CaBr
Solidus
temperature/°C
Peritectic
temperature/°C
a
2
temperature/°C
1
9
9
8
7
7
6
6
5
5
5
4
a
00
5.5
0.7
3.5
7.2
2.6
8
738
714
708
664
651
680
684
709
714
721
734
740
695
663
574
586
575
569
570
578
574
to those obtained beyond 59.1 mol% CaBr . All these curves
2
indicate weakly the appearance of a second endothermic
peak before the tail reaches baseline. Further, the DTA trace
of the cooling run of CaHBr (50 mol% CaBr ) in Fig. 2
2
shows that instead of a single large peak expected for the
freezing of compound, a small, but broad, peak occurs
immediately following the exothermic peak for precipitation
from liquid. These observations suggest incongruent melting
of CaHBr, when it undergoes peritectic reaction with another
solid phase, ‘‘X,’’ to give rise to a liquid phase. Further it is
expected that if CaHBr melts incongruently, the liquidus
temperatures for samples with compositions beyond CaHBr
3.5
9.1
2.8
0
718
721
5
Initial percentage
(prepared by further addition of CaH to CaBr ) would
2 2
increase. This behavior can be further seen in the DTA trace
event is found to be 576 ± 6 °C. The liquidus temperatures
obtained for all the samples given in Table 1 show a
monotonic decrease from pure CaBr to 77.2 mol% CaBr
of the sample with composition of 45 mol% CaBr which is
2
richer in CaH . DTA trace of 45 and 50 mol% CaBr samples
2
2
2
2
show a single broad endothermic peak at 719 ± 2 °C which
corresponds to the pertitectic reaction between CaHBr and
and then an increase with further decrease in CaBr con-
2
tent. These observations suggest that there is an eutectic
‘‘X’’ to give a liquid phase as shown in Fig. 2. This endo-
reaction between CaBr and CaHBr at 576(± 6) °C where
2
thermic peak tails off to reach the background line at the
liquidus temperature of that composition. Table 1 shows that
the liquidus temperature indeed continues to increase with
the liquidus temperature reaches a minimum. From the
contour of the liquidus temperature in this composition
range, the eutectic composition is found to be at 79.6 mol%
further addition of CaH (the last two rows of Table 1). The
2
CaBr as shown in Fig. 3. At compositions with higher
2
peritectic temperatures could not be delineated from the
CaBr content than at the eutectic composition, Table 1
2
DTA trace of 52.8 mol% CaBr sample, mainly because of
2
shows that the onset of endothermic peaks corresponding to
the solidus temperature decreases monotonically up to
overlapping of the endothermic peaks of ‘‘peritectic reac-
tion’’ closely followed by ‘‘dissolution’’ as the actual liqui-
dus temperatures are closer to peritectic temperature.
8
3.5 mol% CaBr . This implies that this system exhibits
2
significant solid solution of CaHBr in the CaBr phase.
2
8
7
7
6
00
50
00
50
Liquid + compound ‘X’
Liquid + (CaBr2)
4
5 mol% CaBr2
7
19 ± 2
Liquid
7
40
7
21
Endo
Cooling run: 50 mol% CaBr2
Liquid + <CaHBr>
600
(CaBr2)
5
76 ± 6
80
5
5
50
00
7
34
<CaHBr> + (CaBr2)
7
18
5
0 mol% CaBr2
500
5
0
60
70
90
100
4
00
600
700
800
mol% CaBr ---------->
2
Temperature/°C
2
Fig. 3 Phase diagram of CaBr –CaHBr system established from this
Fig. 2 DTA traces obtained for 50 and 45 mol% CaBr
2
study
123

1
30
C. V. Vishnu Vardhan et al.
Table 2 XRD characterization of equilibrated samples
composition intervals, the present value is considered to be
more reliable. The peritectic reaction between CaHBr and
the co-existing solid phase ‘‘X’’ gives rise to a liquid phase
at 719 °C. In order to confirm these phase fields elucidated
from the DTA results, samples of compositions 72.5 mol%
CaBr –27.5 mol % CaH , 52.8 mol% CaBr –47.2 mol%
S. no.
Sample
composition
Products identified
by XRD after equilibration
a
1
2
3
4
a
72.5 mol% CaBr
52.8 mol% CaBr
2
2
CaBr
2
? CaHBr
CaHBr
2
2
2
45 mol% CaBr
40 mol% CaBr
2
2
CaHBr ? compound ‘X’
CaHBr ? compound ‘X’
CaH , 45 mol% CaBr –55 mol% CaH , and 40 mol%
2
2
2
CaBr –60 mol% CaH were annealed at 500 °C for
2
2
Initial percentage
10 days and the ensuing equilibrium phases were charac-
terized by XRD after quenching. The results of XRD
characterization of these equilibrated samples are given
in Table 2. The powder X-ray pattern obtained for the
presence of compound, X, in the equilibrated mixtures is
given in Table 3. Identification of the stoichiometry of the
compound X and its thermal behavior needs further
Table 3 X-ray powder pattern obtained for compound ‘X’ in the
equilibrated samples
2
h/CuKa
Relative
intensity
in the mixture
Normalized
intensity
detailed investigation of CaHBr–CaH phase diagram.
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
5
5
5
5
5
0.84
2.74
2.85
3.18
3.82
8.61
9.1
134.7
428.1
164.8
149.8
120
29.3
93.3
35.9
32.6
26.1
26.2
26.2
60.6
50.8
24.6
100.0
32.8
52.5
27.7
39.0
29.6
24.4
47.3
27.9
24.6
22.9
29.5
27.8
67.2
65.6
23.0
42.6
32.7
Conclusions
The CaBr –CaHBr system was investigated in the com-
2
120.3
120.3
278.3
233.3
112.9
459
position range of 100 % CaBr –45 mol% CaBr by DTA.
2
2
The system shows an eutectic reaction between CaBr and
2
1.37
1.47
2.16
2.59
4.13
5.04
8.1
CaHBr (a known compound in this system) at 576 °C and
the eutectic composition is 79.6 mol% CaBr . DTA results
2
further indicated that CaHBr undergoes peritectic decom-
position at 719 °C. The phase diagram of CaBr –CaHBr
system is constructed using the DTA results obtained in
2
150.4
240.8
127
the composition range of 100 % CaBr –45 mol% CaBr
2
2
and the results of XRD characterization of equilibrated
samples.
9.97
0.25
2.05
2.17
4.46
5.75
6.65
9.19
9.33
1.74
1.86
2.3
178.9
135.7
111.9
217.3
127.9
112.9
105.3
135.2
127.7
308.5
301
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7
9.6 mol% CaBr . It is to be pointed that this composition
2
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