M. Sai Baba

Thermochemistry of metal-rich chromium telluride and its role in fuel-clad chemical interactions

Abstract Vaporisation of Cr-Te alloys was studied by Knudsen-effusion mass spectrometry. The partial pressures of Te 2 (g) and Te(g) over the two-phase field, Cr + CrTe x , were determined in the temperature ranges 1015 to 1138 K and 1180 to 1285 K, respectively. The temperature dependencies of the partial pressures have indicated that nearly equimolar proportions of Te and Te 2 are present in the vapor phase and that there is a phase transformation in CrTe x at 1160 ± 20 K . The Cr-rich phase boundaries of the nonstoichiometric CrTe x were delineated at 1075 K (50.72 ± 0.7 at % Te ) as well as at 1235 K (48.25 ± 0.9 at% Te) by a continuous monitoring of the intensities of Te + and Te + 2 as a function of time, starting with samples having 55.63 and 57.22 at% Te. Enthalpies and Gibbs energy changes were derived for the equilibria. CrTe x (s) ai Cr(s) + ( x i )Te i (g) [ x = 1.029 and 0.932; i = 1 and 2] and Te 2 ( g ) ai 2 Te ( g ). Enthalpies and Gibbs energies of formation of CrTe 1.0.29 and CrTe 0.932 were arrived at. The tellurium potentials which would be required for the formation of MTe x ( M = Fe , Cr , and Ni ) in Type 316 stainless steel and those likely to exist in the fuel-cladding gap of a mixed-oxide fuel pin were computed.

VAPORISATION THERMODYNAMICS OF THE NICKEL-RICH PHASES IN THE Ni-Te BINARY SYSTEM - A HIGH TEMPERATURE MASS SPECTROMETRIC STUDY

The vaporisation of the metal-rich nickel tellurides (βh2, gb1, and β1) was studied by Knudsen effusion mass spectrometry. The vapour phase was found to consist of Te2(g) and Te(g). Partial pressure-temperature relationships over the Ni + β2 phase field were determined to be log(PTe2Pa) = (−12563 ± 223)/T(K) + 10.945 ± 0.236 and log(PTePa) = (−13350 ± 425)/T(K) + 11.109 ± 0.450, in the temperature range 893–993 K. Enthalpy and Gibbs energy of formation of the β2 phase, at the nickel-rich boundary (NiTe0.634), at 298 K were derived as −33.3 ± 5.1 and −35.0 kJ mol−1, respectively. The temperature of phase transition from Ni+β2 to Ni+ǵb1 was confirmed to be 1009 ± 10 K. P−T equations over the Ni+gb1 phase field (1020−1055 K) were deduced as log(PTe2Pa) = (−10883 ± 461)/T(K) + 9.305 ± 0.444 and log(PTePa) = (−11189 ± 510)/T(K) + 8.949 ± 0.492. Those over the Ni + β1 phase field (1090–1190 K) were log(PTe2Pa) = (−11187 ± 344)/T(K) + 9.649 ± 0.301 and log(PTePa) = (−11930±400)/T(K) + 9.671±0.349. At the nickel-rich boundary (NiTe0.587), ΔfH° and ΔfG° for the gb1 phase were determined to be −27.7 ± 4.1 and −30.1 kJ mol−1 at 1038 K and those for the β1 phase as −30.5±3.2 and −30.7 kJ mol−1 at 1140 K.

Vaporization thermodynamics of (iron + tellurium): a high-temperature mass-spectrometric study

Abstract Vaporization of (iron + tellurium) alloys was studied using Knudsen-effusion mass spectrometry. The partial pressures of Te 2 (g) and Te 3 (g) over the two-phase fields {FeTe 0.939 (s) + FeTe 1.994 (s)} and {FeTe 0.939 (s) + FeTe 1.451 (s)} were determined in the temperature ranges 659 to 759 K and 803 to 818 K, respectively. The partial pressures over the two-phase field {FeTe 0.939 (s) + FeTe 1.427 (s)} were determined at 868 K. Enthalpy or Gibbs energy changes were derived from the partial pressures for the equilibria: FeTe x (s) = FeTe y (s) + { (x−y) i } Te i (g) , where x = 1.994 (e-phase), 1.451 (δ′-phase), and 1.427 (δ-phase), and y = 0.939 (β-phase) with i = 2 or 3. The phase boundaries of the β-phase at 868 K were delineated by a continuous monitoring of the intensity of Te 2 + as a function of time, starting with a sample having 72.9 mass per cent of Te. Activities of Te were determined and those of Fe computed as a function of composition of the β-phase, using Gibbs-Duhem integration. Chemical potential differences of Te and Fe and, thus, the Gibbs free energy of formation of the β-phase, were deduced across the homogeneity range. Enthalpies or Gibbs free energies of formation of the phases β, e, δ′ and δ were derived.

Appearance potential and electron impact ionisation cross-section of C60

A Knudsen cell mass spectrometric technique was employed to determine the electron impact ionisation cross-section of C60. Ion intensities were measured as a function of time, and weight loss due to evaporation was determined. Similar experiments were carried out with silver to obtain calibration factors. From the weight loss and the ion intensity integrated over the entire duration of the experiment for both siver and C60, the ratio of the ionisation cross-section of C60 to that of silver was obtained. Taking the value for silver from the literature, the ionisation cross-section of C60 at 38 eV was calculated to be (53.5 ± 5.6) × 10−16 cm2. This experimental value (determined for the first time) is discussed in relation to those derived by using various empirical formulae generally applied for obtaining molecular cross-sections. Ion intensities were measured as a function of electron energy to obtain ionisation efficiency curves for C60, and its appearance potential was determined by linear extrapolation of these curves

Homogeneity ranges and thermodynamic properties of the Te-rich phases in the CrTe system

The Te-rich region of the CrTe system was investigated by using high temperature mass spectrometry. The composition range from ∼60 to 82.7 at.% Te was covered by preferentially evaporating off tellurium at T < 700 K from the alloys of starting compositions 66.9, 70.1, 80.0 and 82.7 at.% Te. Evidence for the existence of a new polytelluride (77.0 ± 0.3 to 78.3 ± 0.2 at.% Te) was obtained while the existing uncertainty about the homogeneity range of CrTe3, hitherto considered as the most Te-rich phase in the CrTe system, was removed. This phase exists from 70.5 ± 0.8 to 74.4 ± 0.5 at.% Te. In the case of Cr5Te8, only the Te-rich boundary (63.6 ± 0.6 at.% Te) could be deduced. The activities of tellurium across the single-phase regions were obtained from the p(Te2) measured as a function of composition and those of chromium computed by a Gibbs-Duhem integration. Subsequently Gibbs free energies of formation, ΔfGmo at T = 650 K were also deduced. The values (in kJ mol−1, given in brackets) for the formulae (given in parentheses) corresponding to the Te-rich and/or Cr-rich boundary compositions are: CrTe4−y phase: − [90.7 ± 0.4] (CrTe3.61) and − [90.5 ± 0.4] (CrTe3.35); CrTe3 phase: − [90.4 ± 1.0] (CrTe2.91) and − [89.5 ± 1.5] (CrTe2.39); Cr5Te8 phase: − [86.2 ± 2.2] (CrTe1.75, the Te-rich boundary).

Vapour Pressure and Enthalpy of Sublimation of C70

Abstract The vapour pressure of pure C70 was measured in the temperature range 650–850 K by using the Knudsen effusion mass spectrometry resulting in the equation log(p/Pa) = −10219±78 / T(K) ± 11.596±0.065. The second law enthalpy of sublimation of C70 was determined to be 195.7±1.1 kJ/mol.

Vaporisation studies on tellurium dioxide: A Knudsen effusion mass spectrometric study

Abstract The vaporisation of TeO 2 (s) was studied by Knudsen effusion mass spectrometry. The vapour phase was found to consist of (TeO 2 ) n (n = 1−3)(g), (TeO) n (g) (n = 1−3) and Te 2 (g). The p − T relations of TeO 2 (g), (TeO 2 ) 2 (g) and (TeO 2 ) 3 (g) were derived to be log( p /Pa) = (−13534 ± 78)/ T + (14.241 ± 0.09) (750–950 K), log( p /Pa) = (−14823 ± 212)/T + (14.373 ± 0.242) (825–950 K) and log( p /Pa) = (−19074 ± 540)/ T + (17.337 ± 0.606) (850–921 K) respectively. From the partial pressures, Δ r H 298.15 0 of n TeO 2 (s) = (TeO 2 ) n (g) ( n = 1–3) were evaluated by second and third law methods. Also, enthalpy of the pressure independent reaction TeO 2 (s) + TeO 2 (g) = (TeO 2 ) 2 (g) has been evaluated. Using the Δ f H 298.15 0 of TeO 2 (s), Δ f H 298.15 0 of (Te0 2 ) n (g) ( n = 1–3) were calculated. The partial pressure and enthalpy data for (TeO 2 ) 3 (g) have been obtained for the first time.

Mass spectrometric studies on irradiated (U, Pu) mixed carbide fuel of FBTR

Mass spectrometry was employed to characterise the irradiated mixed carbide fuel of fast breeder test reactor (FBTR) for assessing its performance: thermal ionisation mass spectrometry to determine the isotopic composition, concentrations of U, Pu and Nd in the dissolver solutions and to deduce the burn-up, and a quadrupole mass spectrometric system to obtain the percentage release of fission gases (Kr + Xe). A summary and analysis of the results obtained with the fuel at 25,000 and 50,000 MWd/t in these studies is given in this paper.

A mass spectrometric study of the dissociation of the gaseous diatomic tellurium molecule

Abstract The dissociation equilibrium, Te 2 (g) = 2Te(g), was evaluated from the partial pressures of Te(g) and Te 2 (g) over different metal-tellurides (where the metal = Fe, Cr, Ni or Mo). The equilibrium constants, deduced as a function of temperature (820–1285 K), correspond to a range of monomer-to-dimer ratios, i.e. p (Te)/ p (Te 2 ) varying from ~ 0.02 to 1. They are consistent with the available results corresponding to higher temperatures (1195–1467 K), and therefore, the two sets of data were combined into the following dissociation constant-temperature relation (820–1467 K): The resulting third-law dissociation enthalpy of Te 2 (g), 257.6 ± 4.1 kJ mol −1 , which we recommend, is in excellent agreement with photoionization or spectroscopie results. The pressure-independent reaction, Te(g) + Te (in condensed phase) = Te 2 (g) was also evaluated and the following relation was obtained: This evaluation confirmed the mutual consistency of the p (Te) and p (Te 2 ) values employed to deduce the dissociation constant-temperature relation as well as the dissociation energy reported here. Furthermore, a correlation between the monomer-to-dimer ratio in the vapour phase to the activity of tellurium in the condensed phase is brought out.

Appearance energy and electron impact ionisation cross-section of C70

Abstract The electron impact ionisation cross-section of the C 70 molecule as well as the appearance potential of the C + 70 ion were determined by high temperature mass spectrometry. A molecular beam of C 70 effusing from a Knudsen cell was ionised by a beam of 38 eV electrons. The appearance energy was determined by linear extrapolation of the ionisation efficiency curves. By comparing the integral of the ion intensity vs. time curve and the corresponding weight loss from the Knudsen cell for C 70 and silver, it was possible to deduce the ionisation cross-section of C 70 as (54.5 ± 1.5) × 10 −16 cm 2 . This experimental value is discussed in comparison with those derived using various empirical formulae. The ionisation cross-section of C 70 is very nearly the same as that for C 60 .