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Heat of formation of magnesium-germanium alloys
Journal qf the Less-Common Metals, 31 (1974) 307-309 16 Elsevier Sequoia S.A., Lausanne ~ Printed in The Netherlands
SHORT
COMMUNICATION
Heat of formation of magneshuwgermanium
R. FERRO,
307
R. CAPELLI,
A. BORSESE
alloys
and S. DELFINO
Istituto di Chimica Generale dell’Universita’ di Genova, Genova (Italy) (Received
February
12, 1974)
It is well known that, in the various binary systems of Mg with Si, Ge, Sn and Pb, only the Mg,X (anti CaFz-type) phases are stable at room temperature and pressure (in the Mg-Pb system another compound is formed which is stable only at high temperatures). As pointed out by Beardmore et al.‘, a careful measurement of the thermodynamic properties of these compounds is important; a particularly interesting point is the anomalous position of MgzSi in the series of MgzX compounds, for instance, the heat of formation of Mg,Si is smaller than that of Mg,Ge and is of the same order as that of Mg,Sn, whilst there is a progressive decrease in the sequence Mg,Ge-MglSn-Mg,Pb. So far as the heats of formation are concerned, that of Mg,Ge (together with those of MgzSn and Mg,Pb) has been measured by Beardmore et al.’ by solution calorimetry in liquid tin; for the reaction (in the solid state): 0.667Mg+0.333Ge=Mg0,,,7Ge0.333 they obtained AH= -9.18 kcal/g-at. but experienced several difficulties caused by the slow dissolution in the liquid tin. Subsequently, Rao and Belton’ studied (by means of a galvanic cell) the thermodynamic properties of Mg-Ge alloys and estimated as the most probable value of the heat of formation of Mg,Ge AH,,, = - 8.3 + 0.1 kcal/g-at. (solid state, 298K). For this purpose they took into consideration their own data together with those of other workers: the phase diagram by Klemm and Westlinning3, heat capacity measurements by Gerstein et d4, galvanic cell measurements by Eremenko and Lukashenko’, vapor pressure nieasurements by Smith6 and Mg-activity data in liquid alloys by Eldridge, Miller and Komarek’. It therefore seemed desirable to carry out another measurement of the heat of formation of MgzGe using a method different from solution calorimetry. The calorimeter used, described separately elsewhere’, consists essentially of a small tantalum furnace submerged in an oil bath (surrounded by a 25.OO”C thermostat). The temperature of the bath was measured by a multiple-junction copper/constantan thermocouple. A compacted mixture of the two metals was heated in the calorimeter until the reaction started and the total heat evolved was measured by following the bath temperature (the electrical energy input to the furnace, both in the reaction and calibration runs, was measured by a standard watt hour meter). This calorimeter was previously used to measure the heats of
SHORT COMMUNICATIONS
308
formation of Pt-Sn alloysg; it has also been used to examine other typical binary systems of metals with semi-metals such as the bismuthides of Y”, Nd” and La”. The elements used in the present work were of a purity higher than 99.99%: Ge was of semiconductor grade and, for Mg, the spectrographic standard rods by Johnson Matthey Metals Ltd. were used. The metals, of a total weight of about 5 g, were finely-powdered, thoroughly mixed and (in order to avoid Mg losses by evaporation during heating and reacting) pressed into a very thin iron container. This, after closing by welding with a plasma microtorch, was introduced into the furnace contained in the bomb which was submerged in the oil bath. It was to be foreseen that, owing to the short reaction time, no appreciable corrosion would result from the action of the alloy on the walls of the iron container; this however was confirmed by the negative results of calorimetric analysis for iron carried out on the samples. All the preliminary operations for the preparation of the sample, e.g., Mg filing, weighing, mixing of the metals, introduction into the container, etc., were performed in a glove box filled with argon. The calorimetric bomb was also filled with argon. As far as the composition of the samples was concerned it was thought better, in order to achieve a true equilibrium state in the reaction carried out in the calorimeter, to prepare a number of different samples containing an excess of Mg or of Ge, respectively, instead of preparing samples having the exact stoichiometric MgzGe composition. For all samples, as is customary in all techniques of direct calorimetry, it was necessary to check, after the calorimetric measurements, that a true equilibrium state had been obtained. Therefore, with all samples, a chemical analysis (dissolution of the alloy by aqua regia, determination of Mg with 8-oxiquinoline and of Ge as GeOz), a metallographic examination on the largest section (after polishing under argon or oil and etching with HN03 + Hz0 + CH&OOH +(CH20H)2 in the ratio 1: 19:20:60) and X-ray examination (powder method), were carried out. In Table I are reported the alloys for which the reaction resulted in an equilibrium state: two alloys are also reported for which only a partial reaction of the mixture was observed. Owing to the working conditions of the calorimeter (the sample inside the calorimeter is surrounded by a thermostat at 25°C and, TABLE I HEATS OF FORMATION xMg+(l
OF SOLID MAGNESIUM-GERMANIUM
ALLOYS
-x)Ge=Mg,Ge,_.
Alloy composition (XYCJ
AH measured (kcaljg-at.)
0.375, 0.4123 0.6246 0.714, 0.732,, 0.7435 0.750
- 4.59 + 0.25 - 5.20 k 0.20 -7.69kO.10 -7.15+0.10 ( - 3.8) -6.47f0.10 ( - 2.8)
AH computed for Mg2Ge (kcal/g-at.) -8.16+0.4 -8.41 kO.3 -8.21 kO.15 -8.05kO.15 sample not in equilibrium -8.05TO.15 sample not in equilibrium
AT 298 K
SHORT
309
COM~UNlCATIONS
during measurement, cools down to this temperature) the heats of formation can be considered as being measured at 298K. In accord with the shape of the phase diagram (only one intermediate compound) and, consequently, with the linear trend of the average value of AH between the components and the compound, it was possible, for each alloy prepared, to extrapolate and report in the table the value of AH,,, for MgZGe. The values obtained from the Ge-rich alloys and from the Mg-rich alloys, respectively, do not appear to be significantly different, and therefore it can be considered that the most probable value for Mg,Ge is AH;:zK A - 8.2 f0.2 kcal/g-at. (solid state) = - 24.4 kcaI/mol. The value obtained by Rao and Belton very good approximation.
(-8.3kO.l)
is therefore con~rmed to a
Financial help from the Italian “Consiglio. Nazionale delle Ricerche” and “Minister0 della Pubblica Istruzione” are acknowledged with thanks. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
P. Beardmore, B. W. Howlett, B. D. Lichter and M. 8. Bever, Trans. AIM& 236 (1966) 1161. Y. K. Rao and G. R. Belton, Mer. Trans., 2 (1971) 2215. W. Klemm and H. WestIinn~ng, Z. Anorg. AIIgem. Chem., 245 (1940) 365. B. f. Gerstein, P. L. Chung and G. C. Danielson, J. Phys. C&m. Solids, 27 (1966) 1161. V. N. Eremenko and G. M. Lukashenko, 1x0. Akad. Nauk SSSR Neorg. Mater.. 1 (1965) J. F. Smith, Iowa State Univ., quoted in ref. 7. J. M. Eldridge, E. Miller and K. L. Komarek. Trans. AIME, 236 (1966) 1094. R. Ferro. R. Capelli and A. Borsese, J. Sci.,Instr., in press. R. Ferro, R. Capelli, A. Borsese and S. DelIino, Rend. Accad. Naz. Lincei, in press. R. Ferro, A. Borsese, R. Capelli and S. Deltino. Thermochim. Acta, 8 (1974) 387. A. Borsese, R. Cap&, S. Delfino and R. Ferro, Thermochim. Acta, 8 (1974) 393. A. Borsese. R. Capelli. S. Delfino and R. Ferro. Thermochim. Acta, in press.
1296.
SHORT
COMMUNICATION
Heat of formation of magneshuwgermanium
R. FERRO,
307
R. CAPELLI,
A. BORSESE
alloys
and S. DELFINO
Istituto di Chimica Generale dell’Universita’ di Genova, Genova (Italy) (Received
February
12, 1974)
It is well known that, in the various binary systems of Mg with Si, Ge, Sn and Pb, only the Mg,X (anti CaFz-type) phases are stable at room temperature and pressure (in the Mg-Pb system another compound is formed which is stable only at high temperatures). As pointed out by Beardmore et al.‘, a careful measurement of the thermodynamic properties of these compounds is important; a particularly interesting point is the anomalous position of MgzSi in the series of MgzX compounds, for instance, the heat of formation of Mg,Si is smaller than that of Mg,Ge and is of the same order as that of Mg,Sn, whilst there is a progressive decrease in the sequence Mg,Ge-MglSn-Mg,Pb. So far as the heats of formation are concerned, that of Mg,Ge (together with those of MgzSn and Mg,Pb) has been measured by Beardmore et al.’ by solution calorimetry in liquid tin; for the reaction (in the solid state): 0.667Mg+0.333Ge=Mg0,,,7Ge0.333 they obtained AH= -9.18 kcal/g-at. but experienced several difficulties caused by the slow dissolution in the liquid tin. Subsequently, Rao and Belton’ studied (by means of a galvanic cell) the thermodynamic properties of Mg-Ge alloys and estimated as the most probable value of the heat of formation of Mg,Ge AH,,, = - 8.3 + 0.1 kcal/g-at. (solid state, 298K). For this purpose they took into consideration their own data together with those of other workers: the phase diagram by Klemm and Westlinning3, heat capacity measurements by Gerstein et d4, galvanic cell measurements by Eremenko and Lukashenko’, vapor pressure nieasurements by Smith6 and Mg-activity data in liquid alloys by Eldridge, Miller and Komarek’. It therefore seemed desirable to carry out another measurement of the heat of formation of MgzGe using a method different from solution calorimetry. The calorimeter used, described separately elsewhere’, consists essentially of a small tantalum furnace submerged in an oil bath (surrounded by a 25.OO”C thermostat). The temperature of the bath was measured by a multiple-junction copper/constantan thermocouple. A compacted mixture of the two metals was heated in the calorimeter until the reaction started and the total heat evolved was measured by following the bath temperature (the electrical energy input to the furnace, both in the reaction and calibration runs, was measured by a standard watt hour meter). This calorimeter was previously used to measure the heats of
SHORT COMMUNICATIONS
308
formation of Pt-Sn alloysg; it has also been used to examine other typical binary systems of metals with semi-metals such as the bismuthides of Y”, Nd” and La”. The elements used in the present work were of a purity higher than 99.99%: Ge was of semiconductor grade and, for Mg, the spectrographic standard rods by Johnson Matthey Metals Ltd. were used. The metals, of a total weight of about 5 g, were finely-powdered, thoroughly mixed and (in order to avoid Mg losses by evaporation during heating and reacting) pressed into a very thin iron container. This, after closing by welding with a plasma microtorch, was introduced into the furnace contained in the bomb which was submerged in the oil bath. It was to be foreseen that, owing to the short reaction time, no appreciable corrosion would result from the action of the alloy on the walls of the iron container; this however was confirmed by the negative results of calorimetric analysis for iron carried out on the samples. All the preliminary operations for the preparation of the sample, e.g., Mg filing, weighing, mixing of the metals, introduction into the container, etc., were performed in a glove box filled with argon. The calorimetric bomb was also filled with argon. As far as the composition of the samples was concerned it was thought better, in order to achieve a true equilibrium state in the reaction carried out in the calorimeter, to prepare a number of different samples containing an excess of Mg or of Ge, respectively, instead of preparing samples having the exact stoichiometric MgzGe composition. For all samples, as is customary in all techniques of direct calorimetry, it was necessary to check, after the calorimetric measurements, that a true equilibrium state had been obtained. Therefore, with all samples, a chemical analysis (dissolution of the alloy by aqua regia, determination of Mg with 8-oxiquinoline and of Ge as GeOz), a metallographic examination on the largest section (after polishing under argon or oil and etching with HN03 + Hz0 + CH&OOH +(CH20H)2 in the ratio 1: 19:20:60) and X-ray examination (powder method), were carried out. In Table I are reported the alloys for which the reaction resulted in an equilibrium state: two alloys are also reported for which only a partial reaction of the mixture was observed. Owing to the working conditions of the calorimeter (the sample inside the calorimeter is surrounded by a thermostat at 25°C and, TABLE I HEATS OF FORMATION xMg+(l
OF SOLID MAGNESIUM-GERMANIUM
ALLOYS
-x)Ge=Mg,Ge,_.
Alloy composition (XYCJ
AH measured (kcaljg-at.)
0.375, 0.4123 0.6246 0.714, 0.732,, 0.7435 0.750
- 4.59 + 0.25 - 5.20 k 0.20 -7.69kO.10 -7.15+0.10 ( - 3.8) -6.47f0.10 ( - 2.8)
AH computed for Mg2Ge (kcal/g-at.) -8.16+0.4 -8.41 kO.3 -8.21 kO.15 -8.05kO.15 sample not in equilibrium -8.05TO.15 sample not in equilibrium
AT 298 K
SHORT
309
COM~UNlCATIONS
during measurement, cools down to this temperature) the heats of formation can be considered as being measured at 298K. In accord with the shape of the phase diagram (only one intermediate compound) and, consequently, with the linear trend of the average value of AH between the components and the compound, it was possible, for each alloy prepared, to extrapolate and report in the table the value of AH,,, for MgZGe. The values obtained from the Ge-rich alloys and from the Mg-rich alloys, respectively, do not appear to be significantly different, and therefore it can be considered that the most probable value for Mg,Ge is AH;:zK A - 8.2 f0.2 kcal/g-at. (solid state) = - 24.4 kcaI/mol. The value obtained by Rao and Belton very good approximation.
(-8.3kO.l)
is therefore con~rmed to a
Financial help from the Italian “Consiglio. Nazionale delle Ricerche” and “Minister0 della Pubblica Istruzione” are acknowledged with thanks. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
P. Beardmore, B. W. Howlett, B. D. Lichter and M. 8. Bever, Trans. AIM& 236 (1966) 1161. Y. K. Rao and G. R. Belton, Mer. Trans., 2 (1971) 2215. W. Klemm and H. WestIinn~ng, Z. Anorg. AIIgem. Chem., 245 (1940) 365. B. f. Gerstein, P. L. Chung and G. C. Danielson, J. Phys. C&m. Solids, 27 (1966) 1161. V. N. Eremenko and G. M. Lukashenko, 1x0. Akad. Nauk SSSR Neorg. Mater.. 1 (1965) J. F. Smith, Iowa State Univ., quoted in ref. 7. J. M. Eldridge, E. Miller and K. L. Komarek. Trans. AIME, 236 (1966) 1094. R. Ferro. R. Capelli and A. Borsese, J. Sci.,Instr., in press. R. Ferro, R. Capelli, A. Borsese and S. DelIino, Rend. Accad. Naz. Lincei, in press. R. Ferro, A. Borsese, R. Capelli and S. Deltino. Thermochim. Acta, 8 (1974) 387. A. Borsese, R. Cap&, S. Delfino and R. Ferro, Thermochim. Acta, 8 (1974) 393. A. Borsese. R. Capelli. S. Delfino and R. Ferro. Thermochim. Acta, in press.
1296.