Vaporization behaviour and Gibbs energy of formation of Ni2Te3O8, NiTe2O5 and Ni3TeO6

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Abstract

Three compounds Ni2Te3O8(s), NiTe2O5(s) and Ni3TeO6(s) in Ni-Te-O system were prepared by the solid state reaction route and characterised by X-ray powder diffraction and thermal methods. The standard Gibbs free energy of formation (▴fG°) of Ni2Te3O8(s), NiTe2O5(s) and Ni3TeO6(s) were obtained by the vapour pressure measurement of TeO2(g) over these systems by employing the Knudsen Effusion Mass Loss (KEML) technique and which could be represented by the relations:fG°Ni2Te3O8(s)=(−1693.5+0.8677 T)±15 kJ/mole [973–1045 K]fG°NiTe2O5(s)=(−1000.7+0.5029 T)±25 kJ/mole [987–1018 K]fG° Ni3TeO6(s)=(−1386.7+0.6880 T)±15 kJ/mole [1042–1129 K]

Introduction

Tellurium is one of the highly reactive fission products, produced with moderate yield during the fission of nuclear fuel. Tellurium can interact with fissile elements and other fission products of the oxide fuels within the reactor forming binary and ternary oxides [1]. It attacks the stainless steel cladding components [2] containing Fe, Cr, Ni, Zr, Nb, Sn etc. used in the Fast Breeder Reactors (FBR) and causes embrittlement of the clad. Binary phase diagrams of M-Te systems, where M=Fe, Cr, Ni, Zr, Nb, Mo and La have been compiled and evaluated by Chatopadhyay et al. [3]. A thermodynamic data base for the tellurium bearing systems [4] on Fe, Cr and Ni did not show the formation of any ternary phase in M-Te-O systems from which relevant thermodynamic data could be evaluated.

In continuation of our earlier studies on M-Te-O systems, where M=Th, Ce and U [5], [6], [7], we report here the preparation, characterisation and vaporization behaviour of Ni2Te3O8, NiTe2O5 and Ni3TeO6 in Ni-Te-O system.

Section snippets

Preparation and characterisation of the compounds

Ni2Te3O8 and NiTe2O5 were prepared by the solid state reaction route by reacting the well-ground mixtures of NiO and TeO2 in their respective molar ratios in a furnace in an alumina boat in argon atmosphere at 975 K for 50 h. Ni3TeO6 was prepared by heating mixture of metal oxides in stoichiometric ratios in air at 1075 K for 24 h. The products were removed intermittently, ground and reheated to obtain pure phase of the compounds. The formation of the compounds was confirmed by recording their

X-ray and thermal studies

The X-ray powder diffraction data of Ni2Te3O8, NiTe2O5 and Ni3TeO6 are in good agreement with the values reported in the literature [8], [9]. The data were indexed and refined by least squares method, using a computer program [10]. The refined cell parameters of the three compounds are given in Table 1.

The TG curves of Ni2Te3O8 and NiTe2O5 in air showed continuous mass loss above 1125 K with the loss of TeO2 as shown in , . DTA showed sharp endothermic peaks at peak temperatures of 1178 K for Ni

Conclusion

Ni2Te3O8(s) and NiTe2O5(s) are found to vaporize incongruently with the loss of TeO2(g) as the vaporizing species. The pressures of TeO2(g) were measured by the Knudsen Effusion Mass Loss technique. Ni3TeO6(s) decomposes with the loss of TeO2(g) and O2(g) without melting and their vapour pressures data were obtained by the same technique. The molar Gibbs energy of formation of Ni2Te3O8(s), NiTe2O5(s) and Ni3TeO6(s) are being reported for the first time.

Acknowledgements

The authors are thankful to Shri D.S.C. Purushotham, Director Nuclear Fuels Group and Shri Rajendra Prasad, Head, Fuel Development Chemistry Section for their keen interest in this work.

References (13)

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