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

[摘要] 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$sb{m(g)}$ and the dissociation energy of this molecule were calculated from the measured third law enthalpy of the congruent vaporization reaction. These values were $mDeltasb{f}Hsb{298}lbrack SrOsb{(g)}brack = 1.8 pm 0.9$ kcal/mol and D$sb0spcirc = 96.2 pm 0.9$ kcal/mol. The enthalpy of formation of SrZrO$sb{m 3(s)}$ was also determined from a congruent vaporization reaction. The third law enthalpy of formation was calculated to be $mDeltasb{f}Hsb{298}lbrack SrZrOsb{3(s)}brack =$ $-$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 $mDeltasb{f}Hsb{298}lbrack SrHfOsb{3(s)}brack =$ $-$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$sb{m 3(s)} o$ Ir$sb3$Al$sb{m 8(s)}$ + Al$sb{m(g)}$ and Ir$sb3$Al$sb{m 8(s)} o 3$ IrAl$sb{m(s)} + 5$ Al$sb{m(g)},$ respectively. The second law reaction enthalpies for these reactions were determined to be $mDeltasb{r}Hsb{298} = 60.7$ kcal/mol and $mDeltasb{r}Hsb{298} = 224.5$ kcal/mol, respectively. From the lines drawn through the ln a$sb{m Al}$ vs 1/T plots, the activities of aluminum at 1500 K were calculated to be 0.36, 0.11, and 0.0061, respectively.

[发布机构] Rice University

[效力级别] chemistry