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

Abstract

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\sb3Al\sb{\rm 8(s)} + Al\sb{\rm(g)} and Ir\sb3Al\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