Vaporization Thermodynamic Studies by High-Temperature Mass Spectrometry on Some Three-Phase Regions over the MnO−TeO₂ Binary Line in the Mn−Te−O Ternary System

Abstract

Knudsen effusion mass spectrometric measurements have been performed in the temperature range of 850-950 K over four three-phase mixtures, each phase mixture having at least one phase lying on the MnO-TeO2 binary line of the Mn-Te-O phase diagram, and the rest of the phases lying above this binary line. The three-phase mixtures investigated are Mn3O4 + MnO + Mn6Te5O16; Mn3O4 + Mn6Te5O16 + MnTeO3; Mn3O4 + Mn3TeO6 + MnTeO3; and Mn3TeO6 + MnTeO3 + Mn2Te3O8. The vapor pressures of the gaseous species TeO2, TeO, and Te2 over these three-phase mixtures were measured, and various heterogeneous solid-gas reactions were evaluated along with the homogeneous gas-phase reaction TeO2(g) + 0.5Te2(g) = 2 TeO(g). The enthalpy and Gibbs free energy of formation of the four ternary Mn-Te-O phases were deduced at T = 900 K. These values (in kJ.mol-1), along with the estimated uncertainties in them are Delta(f)H(o)m = 4150 +/- 19, 752 +/- 11, 1710 +/- 11, 1924 +/- 40, and Delta(f)G(o)m= 2835 +/- 28, 511 +/- 11, 1254 +/- 19, 1238 +/- 38, for Mn6Te5O16, MnTeO3, Mn3TeO6, and Mn2Te3O8, respectively. A thermochemical assessment was made to examine the conditions under which the ternary Mn-Te-O phases could be formed on a stainless steel clad of mixed-oxide-fueled (MO2; M = U + Pu) fast breeder nuclear reactors. The phase Mn3TeO6 could be formed when the fuel is even slightly hyperstoichiometric (O/M = 2.0002) and the phase Mn6Te5O16 could also be formed when O/M = 2.0004. The threshold tellurium potential for the formation of Mn3TeO6 is higher than that for MnTe0.80 and CrTe1.10, but is comparable to that for MoTe1.10, and even lower than that for FeTe0.81 or NiTe0.63.

Vaporization studies over (NiO+Ni2Te3O8) in the Ni–Te–O system by transpiration thermogravimetry and Knudsen effusion mass spectrometry

“…Because thermal functions are available for the participants of the reaction and because the second law evaluation is subject to greater errors due to the errors in pressure calibration between the runs, we consider the mean third law value as more reliable. The equilibrium constant and the mean third law enthalpy value deduced show a reasonable agreement with those obtained over TeO 2 (s) and over different phase regions in the Mn-Te-O system (using the combined equation obtained over different phase regions) [22]. For instance, the equilibrium constants deduced at 900 K over TeO 2 (s) and Mn-Te-O are 2.01 Â10 À3 and 1.30 Â 10 À3 , and the mean third law enthalpy values at 298.15 K are 116.2 AE 0.4 kJ mol À1 and 118.3 AE 0.2 kJ mol À1 .…”

“…An electron energy of 13 eV for ion intensity measurements was chosen so that fragmentation contributions were minimized. From the measured ion intensities, the partial pressures were deduced using a similar procedure followed earlier [19,22]. The measured ion intensities 'i' were converted to partial pressures 'p' using the following equations:…”

“…Fði þ Þ ¼ Iði þ ; 37:3eVÞ Iði þ ; 13:0eVÞ (7) In the case of TeO + , the ion intensity of Te + was added to that of TeO + by assuming that even at 13.0 eV, the entire I(Te + ) was due to dissociative ionization of TeO(g) [19,22]:…”

“…The oxygen potentials, Dm(O 2 ) = RT ln p(O 2 ), for the MO 2AEp [31] as well as the activity of nickel, a(Ni) in SS [32], were taken from the literature. Table 7 gives the results along with the values of tellurium potentials that are likely to be available in the Table 7 only show that this ternary phase, like the corresponding phase Mn 2 Te 3 O 8 (s) in the Mn-Te-O system [22], might not be formed on the SS clad. …”

Knudsen Effusion Mass Spectrometric Study of Some Congruently and Incongruently Vaporizing Systems

“…Sample-preparation is given in detail elsewhere (6,(8)(9)(10)(11)(12)(13)16). All samples were subjected to X-ray diffraction investigation, and often, were the vaporization residues also.…”