Vaporization behaviour and thermodynamic stability of zirconium tellurate ZrTe3O8

jOUllil Journal

ELSEVIER

of Nuclear

Materials

211 (1994) 181-185

Uf

nuclear malerlals

Vaporization behaviour and thermodynamic stability of zirconium tellurate ZrTe,O, * M.S. Samant, S.R. Bharadwaj, A.S. Kerkar, S.N. Tripathi, S.R. Dharwadkar Applied Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400085, India Received

18 May 1994; accepted

21 June 1994

Abstract The standard Gibbs energy of formation for zirconium tellurate ZrTe,O, was derived from its vapour pressure measured in the temperature range 1008 to 1146 K, employing the transpiration technique. ZrTe,O, vaporizes incongruently, according to the reaction (ZrTe,O,)

* (ZrO,)

+ 3(TeO,).

The standard Gibbs energy of formation (A,G”) of (ZrTe,O,) calculated from the partial pressure vapour phase above the (ZrO,) + (ZrTe,O,) mixture can be represented by the relation A,G”(ZrTe,O,)(

+14.3 kJ mol-‘)

= -2168.3

+ 0.801T(K)

of TeO,(g)

in the

(1008 I T/K I 1146).

The standard enthalpy of formation A,H”(298.15K) for (ZrTe,O,) derived from these data employing the estimated heat capacity for the compound was found to be -(2153.0 + 18.3) kJ mol-‘, in good agreement with the value of - (2131.2 + 9.64) kJ molP1 determined by isoperibol calorimetry.

1. Introduction Studies of chemical thermodynamics and phase equilibria of the systems with the components of irradiated fuels are important at various stages of the nuclear fuel cycle. Several exhaustive reviews have appeared [l-3] highlighting the need of such investigations. The present paper forms a part of our ongoing investigations on the thermochemistry of the compounds formed in the pseudo-binary systems involving the oxides of some of the major fission products. Tellurium being one of the most important chemically

hostile fission products, the thermochemistry of the compounds formed by it with the fuel, other fission products and the clad materials is of considerable interest. In the present work we report our data on the vapour pressure of zirconium tellurate ZrTe,O, and its Gibbs energy of formation derived from these measurements. This is the first thermochemical study on this only ternary compound reported in the Zr-Te-0 system [4]. The preliminary results of this study were presented at the recently held symposium [5].

2. Experimental

*Taken in part from the work of Mr. MS. Samant, to be submitted for the degree of Doctor of Philosophy in Chemsitry to the University of Bombay. 0022-3115/94/$07.00 0 1994 Elsevier SSDI 0022-3115(94)00334-K

Science

The compound ZrTe,O, was prepared by heating thoroughly ground mixture of ZrO, (nuclear grade (99.9%), containing 0.1 mol% HfO,, supplied by Nuclear Fuel Complex, Hyderabad, India) and TeO,

B.V. All rights reserved

182

MS. Samant et al. /Journal

of Nuclear Materials 211 (1994) 181-185

(99.999%,

Koch-Light, UK) in the ratio 1: 3 at 925 K till the reaction was complete. The procedure involved intermittent withdrawal of the mixture from the furnace, repeated grinding and heating. The completion of the reaction was confirmed from the X-ray powder diffraction pattern of the product [6] recorded on the Philips automatic recording X-ray diffractometer (Model PW 1729/40) using Cu Ka radiation. The stoichiometry of the compound was also established independently by chemical analysis. The compound ZrTe,O, so prepared was found to decompose in air above 975 K giving ZrO, and tellurium bearing species. The Knudsen effusion mass spectrometric measurements [7,8] have shown TeO, to be the most abundant species in the vapour phase above TeO, in the temperature range of the present investigations. The contribution of the TeO species to the total pressure of tellurium bearing species was found to be insignificant. It is assumed that TeO, is the predominant tellurium bearing species also in the vaporization of the compound. The X-ray pattern of the partially decomposed compound showed the lines of ZrOz in addition to those of the parent compound. The product obtained on condensation of the vapour on the colder part of the reaction tube gave the X-ray pattern matching with that of pure TeO,, in support of our assumption. The experiment for the determination of the vapour pressure consisted, therefore, of monitoring the mass loss due to incongruent vaporization of ZrTe,O, according to the reaction (ZrTe,O,)

ti (ZrO,)

+ 3(TeO,).

(1)

An automatic recording transpiration assembly [9] was used for following the mass loss. Zirconium tellurate was mixed thoroughly with about 5% by weight of ZrO, and spread uniformiy over two pans of tiered sample holder fabricated from fused silica. This arrangement increased the surfaceto- volume ratio and facilitated rapid equilibrium between the sample and the vapour. The mass loss of the sample was monitored on an x-t recorder (Ohkura Model DR 1132BOO) both as a function of flow rate of the carrier gas swept over the sample (maintained at a constant temperature) and as a function of temperature at a constant flow rate. High purity argon (total impurities < 35 ppm) used as carrier gas in the present study was dried by passing over a column of anhydrous magnesium perchlorate to eliminate the possibility of formation of any tellurium hydroxide species in the vapour phase [lo] due to the interaction of the sample with the water vapour which may inadvertantly enter the system. The uniform flow rate of argon over the sample was maintained employing the calibrated capillary flow meter. The details of flowmeter calibration are described elsewhere [ll]. The temperature of the

.oo FLOW

RATE

x lo’/

I m’/sI

Fig. 1. Plot of mass loss versus flow rate of the carrier gas (argon).

sample was measured with a chrome]-alumel thermocouple located near the sample in the uniform temperature zone in the reaction tube. The temperatures were maintained constant to f 1 K with the help of Indotherm temperature controller (Model MPC-500). The temperature measuring thermocouple was calibrated at the melting points of pure In, Cd, Zn, Sb, and Ag employing the drop method standardized in our laboratory [12]. The sample temperature was measured with the ice junction reference. The mass loss of the sample per unit volume of the carrier gas (argon) swept over it was monitored as a function of flow rate at 1106 K. It was found to remain virtually constant in the range of flow rates between 7.92 X lo-’ and 8.67 X lo-’ m3 s-l indicating thereby the saturation of the carrier gas by the vapour (Fig. 1). The measurements at different temperatures were carried out employing the flow rate of 8.58 X lo-’ m3 s-l. The vapour pressure of pure TeO, below and above its melting point was measured with the present apparatus before the measurements were made on ZrTe,O,. The data obtained on tellurium oxide confirmed once again the efficacy of the measurements made with this instrument. The results obtained on pure TeO, are being published separately [13].

3. Result and discussion The apparent pressure of tellurium bearing species in equilibrium with ZrTe,O, was calcuiated from the mass loss of the sample per unit volume of the carrier gas swept over it from the relation W RT, Papp = v -@ c

T

(2)

M.S. Samant et al./Journal

5 500

-

I\

-Hc

183

of Nuclear Materials 211 (1994) 181-185

The standard Gibbs energy could be expressed by

Teoz

change

for reaction

(1)

ZTTe30g

- A,G” = AtG;zrre,o,)

- ArG:zro2)

-

%G;T~o~,.

4 500 -

(4)

Thus AfG~zrTe,o,)

\

2

3.500

ArG:~rre~o,)

-'.::'--

can be written

= A,G:zro,)

+

as

%G;T~o~)

-

ArG".

(5)

4

2.500 -

But

-RT In K,

A,G”=

(6)

1.500-

K is an equilibrium

where 0.500 "ll'll"'l""I'II'I""i' 0.865 0.885 0.905 0.925

0 9‘5

0.965

"I1 0.995

Fig. 2. Plot of ln(p/Pa) versus l/T.

and (ZrTe@s)

where W is the mass transported, V, is the total volume of the carrier gas passed over the sample, T, is the ambient temperature (in the present case 296 + 0.5 K) at which volume of the carrier gas was measured and M is the molecular weight of the vapour species. The values of vapour pressure of ZrTe,O, at different temperatures are listed in Table 1. The corresponding linear least squares plot of In pCTeo,) versus l/T is shown in Fig. 2 which can be expressed by the equation In pCreoz,/Pa( (1008 I T/K<

f0.04)

= -32940.6/T+ 33.42

1146).

The enthalpy of vaporization derived for reaction (1) at the mean temperature ment is found to be 273.9 kJ molY’.

(7)

a (ZrTe,08)

x 103K

of pure UkO,)

given by

P&OZ) x a(zro,)

K= l/T

constant

(3) from this plot of the experi-

For pure stoichiometric rium with TeO, vapour, K =

condensed

phases

K can be expressed

P&O&

(8)

Therefore,

the ArGTzire,08)

AfG:zr~c+o~)

=

4G:zroz) +

3~

can be expressed

+%‘?Teoz) In P(TeO>).

(9)

AtG;zr~e,o~j + 3[-RT

= AfG;zrol) 3RT

Jn

P;Te02)

+

A@'Teo,)]

In P(TeO+

(10)

Table 1 Gibbs energy of formation AfG~ZrTe,OR) Mass loss (g mm31

~(TeW (Pa)

1008 1019 1027 1037 1047 1057 1067 1067 1077 1085 1096 1105 1127 1137 1146

0.13 0.18 0.26 0.35 0.46 0.62 0.87 0.84 1.10 1.44 1.88 2.48 4.25 5.45 6.51

2.00 2.76 3.99 5.38 7.06 9.52 13.36 12.90 16.89 22.12 28.87 38.09 65.27 83.70 99.98

a Gibbs energy of formation calculated using Eq. (9). b Gibbs energy of formation calculated using Eq. (13).

as

The value of ArGtT,ozj can be obtained from the equilibrium between (TeO,) and (TeO,) or (TeO,} and (TeO,) depending on the temperature of the experiment. The above equation can then be written as

+

Temperature (K)

in equilibas

15.66 20.64 25.13 32.01 40.59 51.23 64.39 64.39 80.57 96.12 122.00 147.76 232.96 284.87 340.37

ArG; a &J mol-‘1

oh A&r &J mol-‘)

-

-

1362.18 1354.15 1344.71 1337.02 1329.78 1321.76 1312.50 1313.42 1305.87 1298.28 1290.49 1282.40 1265.97 1258.20 1252.42

1367.61 1359.77 1350.48 1342.97 1335.90 1328.06 1318.98 1319.90 1312.52 1305.08 1297.49 1289.56 1273.53 1265.94 1260.32

M.S. Samant et al. /Journal

184

of Nuclear Materials 211 (1994) 181-185

or

values of the again ments

AfG&e,o,) = AfG:zroz)

+ 3 [ -RT

ln P&O,) + AfG+_oZj]

+ 3RT In P(Teo,). On rearranging

(11) 4. Conclusion

we get

f

3RT In-

P(Teoz) (12)

&_OZ) ’ or AfG;zr~e,o,)

= AfG;zroz)

+ 3A,G&,oz,

P(Teo,) -I- 3RT In-,

(13)

@Te021

where ppTeo,) is the partial pressure of (TeO,) bearing species over pure solid of liquid tellurium oxide [13]. In the present investigation, the temperature range of measurement is 1008 to 1146 K which is above the melting point of (TeO,). Therefore, equation (13) was used for the calculation of AfGTzrTe,O,). pTTeO,) over liquid tellurium oxide could be expressed as a function of temperature by the following equation [13]: In PFTeo,,/Pa(

f 0.05) = -25773.5/T

(14)

values were obtained by combinThe AfGTzr~e30,) ing the measurements of vapour pressure of (ZrTe,O,) in the present work and of pure {TeO,} from Ref. 1131 using Eq. (13). The AfGTzrO,) and ArGFTeozl values were taken from the recent compilation of Cordfunke and Konings [14]. The values for AfG(ozrTe,OHj are listed in Table 1 and can be expressed as a function of temperature by the following equation AfGTZrTrzox,( f 14.3 kJ mol-‘) + 0.801T(K)

(1008 I T/K<

1146). (15)

Table 1 also presents The AfGTZrTelO,) values calculated from Eq. (9) using the measured vapour pressure of the compound and the values for AfG&eOZ, and from the compilation of Cordfunke and AfG;zro,j Konings [14] and could be expressed as a function of temperature by the equation AfGTZrTe20,)( k 14.3 kJ mol-‘) = -2155.7

+ 0.783T(K)

(1008 I K/K

I 1146). (16)

It can be seen

that

The Gibbs energy of formation for zirconium tellurate derived from its vapour pressure, presented here, is the first measurement on the thermochemical property of this compound. Subsequent measurements on the standard enthalpy of formation of ZrTe,O, carried out in our laboratory [15] employing isoperibol solution calorimetry yielded the value of -(2131.2 + 9.64)kJ mol-’ for A,&,,,,,. The average second-law heat of formation obtained from Eq. (15) at the mean temperature of the experiment was found to be -(2168.3 f 14.3)kJ mol-‘. The standard enthalpy of formation derived from this value and the estimated A,H&,, heat capacity of the compound by the Neumann-Kopp rule [16] was found to be -(2153.0 f 18.3)k.I mol-‘, fairly in good agreement with our calorimetric measurements.

References

+ 28.32

(1006 < T/K < 1074).

= -2168.3

obtained in this way agreed within the precision present measurements; thus establishing once the credibility of the vapour pressure measureof TeO, in Ref. [13].

the two sets of AfG;)Zr.re20xj

111 H. Kleykamp, J. Nucl. Mater. 131 (1985) 221. Dl E.H.P. Cordfunke and R.J.M. Konings, J. Nucl. Mater.

152 (1988) 301. and [31 P.E. Potter, Proc. Int. Symp. on Thermochemistry Chemical Processing, ed. C.K. Mathews, IGCAR, Kalpakkam, India, November 20-22, 1989, p. 107. N. Takatsuka, M. Katsura and M. Miyaka, [41 S. Yamanaka, J. Nucl. Mater. 161 (1989) 210. Proc. 30th Annual [51 M.S. Samant and S.R. Dharwadkar, Convention of Chemist, Bose Institute, Calcutta, 22-24, December 1993, Abstract No. PHY-48, p. B13. [61 G. Bayer, Ber. Dtsch. Keram. Ges. 39 (1962) 535, quoted in JCPDS-ICDD X-ray diffraction file (1990), card No. 15-692. [71 D.W. Muewnow, J.W. Hastie, R. Hauge, R. Bautista and J.L. Margrave, Trans. Faraday Sot. 65 (1969) 3210. G. Bardi and R. Gigli, Rev. [81 V. Piacente, L. Malaspina, Int. Hautes TempCr. et Rtfract. 6 (1969) 91. A.S. Kerkar and M.S. Samant, Ther[91 S.R. Dharwadkar, mochimica Acta 217 (1992) 175. J. [lOI R.J.M. Konings, E.H.P. Cordfunke and V. Smit-Groen, Chem. Thermodyn. 22 (1990) 751. S.R. Dharwadkar and M.S. Chan[ill O.M. Sreedharan, drasekharaiah, Bhabha Atomic Research Centre, Internal Report No. I-239 (1973). M.S. Samant and AS. Kerkar, Proc. WI S.R. Dharwadkar, 9th National Symp. on Thermal Analysis, eds. P.V. Ravindran, S.R. Bharadwaj, M. Sudersenan and S.R. Dharwadkar, Goa University, Bambolim, Goa, India, November 8-10, 1993, p. 590.

M.S. Samant et al. /Journal

of Nuclear Materials 211 (1994) 181-185

[13] M.S. Samant, A.S. Kerkar, S.N. Tripathi and S.R. Dharwadkar, to be published. [14] E.H.P. Cordfunke and R.J.M. Konings, Eds., Thermochemical Data for Reactor Materials and Fission Products (North-Holland, Amsterdam, 1990).

185

1151 S.R. Bharadwaj, M.S. Samant, R.K. Mishra, S.R. Dharwadkar, S.S. Sawant and R. Kalyanraman, to be published. [16] 0. Kubaschewski and C.B. Aicock, Metallurgicual Thermochemistry (Pergamon, Oxford, 1983) p. 184.