Thermodynamics of the YAl-YAl sub 2 system

• Published in: Intermetallics Vol. 8; no. 3; pp. 203 - 212
• Main Authors: Gozzi, D, Borzone, G, Ciccioli, A, Cignini, P L, Ferrini, M
• Format: Journal Article
• Language: English
• Published: 25.03.1999
• ISSN: 0966-9795

The emf method using solid-state galvanic cells based on a CaF sub 2 single crystal as electrolyte has been used to investigate the thermodynamics of the intermetallic couple YAl-YAl sub 2 . These represent two out of the five intermetallic compounds shown in the Y-Al phase diagram. Two kinds of cells were studied: (-)Pt/Y, YF sub 3 /CaF sub 2 s.c./YAl, YAl sub 2 , YF sub 3 /Pt(+) and (-)Pt /YAl, YAl sub 2 , CaAlF sub 5 , CaF sub 2 /CaF sub 2 s.c./Fe(Al, alpha sub + ), CaAlF sub 5 ; CaF sub 2 /Pt(+). The cell reactions expected are, respectively, 2YAl (s)=YAl sub 2 (s)+Y (s) and YAl (s)+Al (l,s)=YAl sub 2 (s). The standard thermo-dynamic quantities Delta G deg , Delta H deg and Delta S deg have been measured as a function of temperature for both the reactions. The Delta sub f H deg sub 298 of YAl has also been derived by selecting from the literature the best value of Delta sub f H deg sub 298 =-107plus /minus6 kJ/mol. Moreover, Delta sub f S deg sub 298 =2plus/minus5 J/K.mol and Delta sub f G deg sub T =(-100plus/minus5)+(1.8plus /minus0.5)10 exp -2 T kJ/mol have been estimated in the temperature range from 792-1007K.