High-temperature mass spectrometric study of vaporization and thermodynamics of the Cs2 O-B2 O3 system: Review and experimental investigation

Abstract

Rationale: The compounds in the Cs₂ O-B₂ O₃ system are of particular interest for nuclear applications since cesium borates may be formed during accidents in nuclear reactors, affecting the rate of release of radiotoxic isotopes into the environment. Thus, information on the vaporization and thermodynamic properties of cesium borates is necessary for simulation and modeling of the isotope release processes taking place during the nuclear reactor accidents.

Methods: Compounds in the Cs₂ O-B₂ O₃ system were synthesized by the co-crystallization method with subsequent sintering. The sample characterization was carried out using XRD, TGA/DSC/DTA, IR spectroscopy, and ICP atomic emission spectroscopy. The vaporization and thermodynamics of the samples under consideration were investigated using an MS-1301 mass spectrometer and a single molybdenum effusion cell. The electron ionization method was employed for ionization of the vapor species with an electron energy of 30 eV. The scale of the ionizing voltage was calibrated according to the traditional technique by measuring the appearance energy of gold in the mass spectrum of the vapor over pure gold.

Results: The main vapor species over the samples in the Cs₂ O-B₂ O₃ system were CsBO₂ and Cs₂ B₂ O₄ in the temperature range 789-1215 K and in the concentration range 0.1-0.5 mole fraction of Cs₂ O. The temperature dependences of the CsBO₂ and Cs₂ B₂ O₄ partial pressures over cesium borate were obtained in the temperature range 767-990 K. The CsBO₂ and B₂ O₃ activities, the Gibbs energies of mixing, the excess Gibbs energies, the partial mole enthalpies of mixing, and total enthalpies of mixing were determined as functions of temperature and composition of the condensed phase.

Conclusions: As the Cs₂ O and B₂ O₃ activity values evidenced, negative deviations from the ideal behavior were observed in the Cs₂ O-B₂ O₃ system in the temperature range 800-1000 K. The total enthalpies of mixing and the Gibbs energies of mixing in the Cs₂ O-B₂ O₃ system were compared with the literature data, illustrating their mutual agreement.