Thermochemical investigations of selected ternary strontium oxides and iridium-aluminum alloys by high temperature mass spectrometry


A quadrupole mass spectrometer system was redesigned with pulse counting electronics and a resistively heated, high-temperature furnace. This system was employed in Knudsen effusion experiments to investigate the high temperature thermodynamics of strontium oxide, strontium metazirconate, strontium metahafnate, and three phases in the Ir-Al phase diagram. The strontium oxides were vaporized from thoria Knudsen cells and no container-sample reactions were observed. The enthalpy of formation of SrO$\sb{\rm(g)}$ and the dissociation energy of this molecule were calculated from the measured third law enthalpy of the congruent vaporization reaction. These values were $\rm\Delta\sb{f}H\sb{298}\lbrack SrO\sb{(g)}\rbrack = 1.8 \pm 0.9$ kcal/mol and D$\sb0\sp\circ = 96.2 \pm 0.9$ kcal/mol. The enthalpy of formation of SrZrO$\sb{\rm 3(s)}$ was also determined from a congruent vaporization reaction. The third law enthalpy of formation was calculated to be $\rm\Delta\sb{f}H\sb{298}\lbrack SrZrO\sb{3(s)}\rbrack\ =$ $-$405.7 $\pm$ 4.5 kcal/mol. Strontium metahafnate vaporized noncongruently in the temperature range 2280-2380 K. The third law heat of formation calculated from the measured vaporization enthalpy was $\rm\Delta\sb{f}H\sb{298}\lbrack SrHfO\sb{3(s)}\rbrack =$ $-$426.8 $\pm$ 1.6 kcal/mol. The three phases in the Ir-Al phase diagram which were investigated may be represented as IrAl$\sb3$ + IrAl$\sb{2.7},$ IrAl$\sb{2.7}$ + IrAl, and Ir$\sb{0.5}$Al$\sb{0.5}.$ Aluminum vaporized noncongruently from these phases. The Al vaporization reactions in the two-phase regions may be written 3 IrAl$\sb{\rm 3(s)} \to$ Ir$\sb3$Al$\sb{\rm 8(s)}$ + Al$\sb{\rm(g)}$ and Ir$\sb3$Al$\sb{\rm 8(s)} \to 3$ IrAl$\sb{\rm(s)} + 5$ Al$\sb{\rm(g)},$ respectively. The second law reaction enthalpies for these reactions were determined to be $\rm\Delta\sb{r}H\sb{298} = 60.7$ kcal/mol and $\rm\Delta\sb{r}H\sb{298} = 224.5$ kcal/mol, respectively. From the lines drawn through the ln a$\sb{\rm Al}$ vs 1/T plots, the activities of aluminum at 1500 K were calculated to be 0.36, 0.11, and 0.0061, respectively

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