Preparation of Cs2ZrO3 and Cs2ThO3 through sol-gel method and their characterization

Article abstract of DOI:10.1023/A:1013192005456

Cesium zirconate and cesium thorate were prepared by sol-gel method following citrate-nitrate route. The compounds were characterized by X-ray diffraction, chemical analysis and simultaneous TG-DTA. The methods of preparation of Cs₂ZrO₃

Full text of DOI:10.1023/A:1013192005456

Journal of Thermal Analysis and Calorimetry, Vol. 66 (2001) 779–784
PREPARATION OF Cs2ZrO3 AND Cs2ThO3 THROUGH
SOL-GEL METHOD AND THEIR
CHARACTERIZATION
R. Mishra, M. Ali, S. R. Bharadwaj and D. Das*
Applied Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400 085, India
(Received April 12, 2001; in revised form June 20, 2001)
Abstract
Cesium zirconate and cesium thorate were prepared by sol-gel method following citrate-nitrate
route. The compounds were characterized by X-ray diffraction, chemical analysis and simultaneous
TG-DTA. The methods of preparation of Cs2ZrO3 and Cs2ThO3 reported in literature involve diffi-
cult steps and require stringent experimental conditions for obtaining the compounds. In this work
we report an easier method for the preparation of these compounds at relatively low temperature.
Keywords: cesium thorate, cesium zirconate, simultaneous TG-DTA, sol-gel method, X-ray pow-
der diffraction
Introduction
Thermochemistry of the compounds of cesium plays an important role in nuclear ap-
plications. The fission product iodine formed in fuel pin during burn up inside a nu-
clear reactor combines with another fission product cesium giving a significant
amount of CsI [1]. Even at a low oxygen potential of about –400 kJ mol–1, CsI reacts
with the fuel-matrix UO2 or, with ZrO2 layer on zircaloy to form ternary compounds
of cesium such as Cs2UO4, Cs2ZrO3 etc. and releases elemental iodine[2]. The migra-
tion of the released iodine to the clad surface and its subsequent reaction with clad
material (e.g. zircaloy) causes stress corrosion cracking [2, 3] which is detrimental to
the long term stability of the nuclear fuel pins. In this context the data of thermody-
namic stability of the compounds of Cs with the other fission products, fuel matrix
and the clad materials are important. Recently, Scharm et al. [4] reported the thermo-
dynamic properties of Cs2ZrO3 from the standard enthalpy of formation and heat ca-
pacity measurements. Thermodynamic investigation on Cs2ThO3 has not been re-
ported so far. Considering the importance of thermodynamic data for these com-
pounds, we initiated the program of their thermodynamic investigations and worked
*
Author for correspondence: E-mail: htschem@magnum.barc.ernet.in
1418–2874/2001/ $ 5.00
Akadémiai Kiadó, Budapest
© 2001 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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MISHRA et al.: Cs2ZrO3 AND Cs2ThO3 THROUGH SOL-GEL METHOD
out the sol-gel method for the preparation of Cs2ZrO3 and Cs2ThO3 presented in this
paper.
The reported method for the preparation of Cs2ZrO3 or Cs2ThO3 [5–7], uses ce-
sium metal or its oxide and therefore, involves stringent experimental conditions to
get the product in the pure form. In this work we report an easier method of prepara-
tion of the compounds at relatively low temperature. The compounds prepared by this
new method were subsequently used for the thermodynamic investigations [8, 9].
Experimental
Cs2ZrO3 and Cs2ThO3 were prepared by sol-gel method through citrate-nitrate route.
The chemicals used for the preparation of the compound are Cs2CO3 (99.99% purity,
Aldrich, USA), CsI (99.99% purity, Aldrich, USA), ZrOCl2·xH2O, Th(NO3)4·xH2O,
citric acid, HNO3 and NH4OH solutions (all AR Grade).
Preparation of Cs2ZrO3
The aqueous solution of cesium nitrate of known concentration was prepared from ei-
ther cesium carbonate or cesium iodide by their dissolution in nitric acid. Carbon di-
oxide or iodine evolved was removed thoroughly before use. Aqueous solution of zir-
conium nitrate was prepared from its oxychloride by precipitating zirconium
hydroxide in ammoniacal medium followed by dissolving the thoroughly washed
precipitate with nitric acid. Aqueous solutions of cesium nitrate and zirconium nitrate
were mixed in 2:1 molar ratio. Citric acid amounting to 2.5 times the number of
moles of total metal content was added to the above resulting solution. pH of the solu-
tion was adjusted by adding ammonia or nitric acid so as to maintain a homogenous
solution.
The solution was slowly heated in an open beaker at 80°C in a fume hood till it
formed a gel, which was transformed into fine powder on prolonged heating. The
solid powder was vacuum dried and transferred to a platinum boat placed in a furnace
under dry flowing oxygen gas and ignited at 400°C for 1 h. The material thus formed
during the ignition process was heated at 600°C for 6 h under similar conditions to in-
crease the crystallinity of the product. The compound was cooled to room tempera-
ture in gas flow condition and was immediately transferred to a vacuum desiccator.
Preparation of Cs2ThO3
Aqueous solutions of cesium nitrate and thorium nitrate were mixed in the molar ratio
2:1 and citric acid amounting to 2.5 times the total number of moles of total metal
content was added to this mixture. pH of the solution was adjusted so as to maintain a
homogenous solution. Further preparation of the compound Cs2ThO3 from this solu-
tion is the same as given under preparation of Cs2ZrO3.
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MISHRA et al.: Cs2ZrO3 AND Cs2ThO3 THROUGH SOL-GEL METHOD
781
Characterization of Cs2ZrO3 and Cs2ThO3
The compounds were characterized by X-ray diffraction and chemical analysis fol-
lowed by their thermogravimetry, differential thermal analysis (simultaneous
TG-DTA). Thermogravimetry and differential thermal analysis runs were taken us-
ing a SETARAM simultaneous TG-DTA instrument (Model No. 92-16.18) at a heat-
ing rate of 10°C min–1 up to 1000°C in a completely dry flowing argon atmosphere.
X-ray diffraction pattern of the compound covered with a paraffin paper, was re-
corded in a Philips X-ray Diffractometer (Model No. 1729) in the 2θ angle range
20–60° at a scan speed of 1°C min–1 using silicon powder as an internal standard and
CuKα radiation. Chemical analysis of the completely dry compound was done to de-
termine the cesium, zirconium, thorium and carbon contents.
Results and discussion
Table 1 gives the typical results of chemical analysis for these two compounds. It is
seen that the total carbon contents in the samples prepared through the citrate precur-
sor are very small. In the sample of Cs2ZrO3 the results of chemical analysis indicate
the presence of an excess zirconium over the stoichiometric compound. The excess
Zr was found to be present as ZrO2. X-ray diffraction pattern for the product (Fig. 1)
obtained after annealing at 600°C for 6 h showed the broadened lines of Cs2ZrO3 and
also, of ZrO2. XRD peaks corroborated to reported data of the compounds (JPCDS
file Nos 41-513 (Cs2ZrO3) and 37-1484 (ZrO2) respectively). The observed line
broadening corresponded to an average crystallite size of 25 nanometers. Similarly,
Cs2ThO3 was characterized by XRD. Figure 2 gives the XRD pattern of Cs2ThO3,
which matches with the reported JCPDS File No. 31-385. No thoria lines are seen in
the pattern corroborating to the result of the chemical analysis.
Table 1 Results of chemical analysis
Mass percentages of the elements analyzed/%, obs. (calc.)
Compound
Zirconium
Thorium
Cesium
Carbon
Cs2ZrO3
Cs2ThO3
25.8 (22.5)
56.4 (65.6)
48.7 (48.6)
1.04
42.7 (42.6)
0.08
The reason for the observed excess of zirconia in the preparation of cesium zir-
conate was due to the loss of Cs as Cs2O(g) and CsOH(g) during autoignition of the
dried gel. The steam coming from citrate-nitrate burning leads to the formation of the
hydroxide species. The exothermic step of ignition of gel is common to the prepara-
tion of both cesium zirconate and cesium thorate. In the ignition process, the oxides
of cesium and zirconium or thorium formed in situ combine to yield the respective
double oxides. The chemical combinations of the oxides being again exothermic re-
lease extra heats. However, it has been seen by our thermodynamic study of the com-
pounds [8, 9] that the respective values of their enthalpies of formation from the con-
J. Therm. Anal. Cal., 66, 2001

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MISHRA et al.: Cs2ZrO3 AND Cs2ThO3 THROUGH SOL-GEL METHOD
Fig. 1 X-ray diffraction pattern for the compound Cs2ZrO3. C – peaks due to the com-
pound and Z – peak due to zirconia
Fig. 2 X-ray diffraction pattern for the compound Cs2ThO3. C – peaks due to the com-
pound and P – peaks due to paraffin paper
stituent oxides are quite different for the two double oxides; cesium zirconate forma-
tion releases much more heat than cesium thorate. As a result, in the preparation of
cesium zirconate the exothermic step of autoignition of gel and formation of the dou-
ble oxide evolves comparatively larger heat and results in higher local temperature.
During the progress of the process, a portion of freshly formed cesium oxide volatil-
izes out as its oxide and hydroxide by the elevated temperature. The sacrificial aspect
of cesium in the preparation of cesium zirconate by gel ignition is an inherent limita-
tion of the technique.
Though the presence of ZrO2 phase in the prepared Cs2ZrO3 was a necessity in
defining thermodynamic univariancy of the system in volatilization study of the zir-
conate [8], attempt was made to make a stoichiometric compound by the present
method of preparation. Later findings showed that taking an excess amount of CsNO3
in the initial ingredient of the gel preparation could make up for the cesium loss in at-
J. Therm. Anal. Cal., 66, 2001

MISHRA et al.: Cs2ZrO3 AND Cs2ThO3 THROUGH SOL-GEL METHOD
783
taining stoichiometry. The sacrifice of cesium in the process of preparation was seen
to be as high as 25% of the stoichiometric requirement.
TG curve for the compound showed initial mass loss near 100°C due to the loss
of adsorbed moisture, followed by the decomposition of the compound at 830°C in a
single step. The mass loss at 830°C corresponded to the stoichiometric loss of Cs2O
from Cs2ZrO3. The simultaneous TG-DTA runs gave one small endothermic peak at
780°C, which could be due to solid-solid phase transition in the compound. Similarly,
Cs2ThO3 was characterized by simultaneous TG-DTA. Mass loss at 700°C corre-
sponding to the loss of Cs2O was observed.
With partial decompositions of Cs2ZrO3 and Cs2ThO3 the X-ray diffraction pat-
terns of the respective products showed the built up of ZrO2 and ThO2 lines. The liter-
ature data for the gaseous cesium oxides [10] indicates that at an oxygen partial pres-
sure of 10–13 Pa the predominant gaseous species is Cs2O(g). From the above observa-
tions it is concluded that Cs2ZrO3 and Cs2ThO3 vaporize incongruently to Cs2O(g)
and ZrO2(s) or ThO2(s) as the case may be. The vaporization reactions can be given
by the equations
Cs2ZrO3(s)=ZrO2(s)+Cs2O(g)
Cs2ThO3(s)=ThO2(s)+Cs2O(g)
(1)
(2)
These reactions were used to derive the thermodynamic properties of Cs2ZrO3
and Cs2ThO3 by monitoring their mass losses in a Knudsen effusion apparatus [8, 9].
Conclusions
The compounds Cs2ZrO3 and Cs2ThO3 could be prepared by sol-gel method. In this
method handling of hygroscopic and hazardous cesium metal is avoided. Pure and
crystalline samples suitable for thermodynamic measurements could be obtained.
The method, however, has an inherent limitation in the preparation of some double
oxides that have a high enthalpic stability with respect to the constituent oxides, one
of which has a significant volatility. In such cases as seen in cesium zirconate, the
method demands an excess of the volatile component in the initial ingredient for at-
taining stoichiometry in the final product, as there is a sacrificial loss of the compo-
nent at the autoignition stage of gel.
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