Thermodynamic Properties of the SiO 2 -GeO 2 and Pt-rich Pt-Ge Systems at 1623 and 1723 K

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1 Materials Transactions, Vol. 45, No. 6 (4) pp. 847 to 85 #4 The Japan Institute of Metals Thermodynamic Properties of the Si -Ge and Pt-rich Pt-Ge Systems at 63 and 73 K Takeshi Yoshikawa ; *, Indra Astuti ; *, Hiroshi Takahashi and Kazuki Morita Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, Tokyo , Japan Department of Chemical System Engineering, Graduate School of Engineering, The University of Tokyo, Tokyo , Japan Thermodynamic properties of the Si -Ge melt were investigated from the measurement of the Ge activity in the Si -Ge melt by equilibrating it with solid platinum under controlled oxygen partial pressure. The excess Gibbs energy for the Si -Ge melt was evaluated as the following equation in the quasi-regular solution formalism. G M;ex ¼ð 476 þ 8:TÞX GeO X SiO Also, by equilibrating solid platinum with the Ge melt under controlled oxygen partial pressure and investigating the phase equilibrium between solid platinum and the liquid Pt-Ge alloy, thermodynamic properties of Pt-rich Pt-Ge system above the eutectic temperature of Pt 3 Ge was clarified. (Received February 9, 4; Accepted April 6, 4) Keywords: thermodynamics, activity, Si -Ge melt, platinum-germanium system. Introduction The telecommunication through optical fibers has spread widely for the past two decades. In the recent years, optical fiber cables laid in the early stage are being replaced and a considerable amount of wastes of optical fiber cables (a few thousand ton per year) are generated. Optical fibers consist of plastics and glassy parts called core and clad. Glassy parts are composed of pure Si (N) containing small content of Ge and may be considered precious material sources. While most wastes of optical fiber cables are treated in the landfill disposal after separating the metal-attachments and burning out the plastic parts, Ma et al. ) focused on the extremely high Si purity of optical fibers and are investigating the recycling process of optical fibers by the reduction to Si material for solar cell. In that process, the behavior of germanium should be grasped because of its much high price compared with silicon, so that the precise thermodynamic properties for the Si -Ge melt are needed. On the thermodynamic properties of the Si -Ge system, Baret et al. ) reported the phase diagram. In their work, however, the stable phases at a few temperatures and compositions were investigated and the solid-liquid phase boundaries were approximated, so the assessed phase diagram and the evaluated thermodynamic properties of the Si -Ge system would be lacking in reliability. In the present study, to clarify the thermodynamic properties of the Si -Ge melt, the Ge activity in the Si - Ge melt was determined. Also, thermodynamic properties of the Pt-rich Pt-Ge system above the peritectic temperature of Pt 3 Ge were clarified.. Principle To determine the Ge activity in the Si -Ge melt, the *Graduate Student, The University of Tokyo Si -Ge melt was equilibrated with solid platinum under controlled oxygen partial pressure. From the Gibbs energy of formation reaction for Ge represented as eq. (), the Ge activity can be expressed as eq. (3) with the relationship of eq. (). Ge(l) þ (g) ¼ Ge (l) G ¼ 56 þ 8T (J/mol) 3;4Þ a GeO (l) G ¼ RT ln Ge(l) inpt X Ge in Pt P O a GeO (l) ¼ Ge(l) inpt X Ge in Pt P O exp G Here, a i, i and X i are the activity, the activity coefficient and the mole fraction of component i, and parenthesis of the subscript denotes the reference state. In eq. (3), the Ge activity in the Si -Ge melt is determined with the germanium content of solid platinum and the activity coefficient of germanium in solid platinum. The latter value was preliminary measured as shown in section 4.. The reason for using platinum as reference metal was as follows; () the experiments had to be carried out under the relatively high oxygen partial pressure to avoid the significant vaporization of germanium sub-oxide GeO, () germanium content of solid platinum after the experiment was considered to be detectable due to the comparatively large solubility (6.6 at%) in solid platinum at the peritectic temperature of Pt 3 Ge (4 K) as reported by Oya et al. 5) Equilibrium between solid platinum and Si -Ge melt The mixture (.5 g) of Si (99.999%) and Ge powder (99.99%) was charged into a platinum pan made of platinum foil ( mm thickness, 99.98%) and pre-melted at 73 K for h in air. The pan was placed in a porous alumina holder and held in CO-C gas mixture (CO:C =:5) for RT ðþ ðþ ð3þ

2 848 T. Yoshikawa, I. Astuti, H. Takahashi and K. Morita more than 4 h at 73 K and 6 h at 63 K, respectively. The oxygen partial pressure is determined with the following reaction. C (g) ¼ CO(g) þ (g) G 4 ¼ 8 85:T (J/mol) 6Þ After equilibration, oxide melt was taken out from platinum pan and the composition was analyzed by using the inductively coupled plasma (ICP) emission spectroscopy. Platinum pan was subjected to electron probe microanalyzer (EPMA) measurement with the accelerating voltage of 5 kv, sample current of na and counting time of s. Germanium and platinum contents of platinum pan were determined from the relative intensity of Ge L radiation (.43 nm) for the sample to the pure germanium and that of Pt M radiation/(.65 nm) to the pure platinum, respectively, by accordance of the ZAF correction. 3. Equilibrium between solid platinum and the liquid Pt-Ge alloy Platinum foil (8 mm 8 mm, mm thickness) together with.5 g of Ge powder was charged into a silica crucible and held in an argon atmosphere for 5 h at 4373 K. Moisture, C and oxygen in argon gas were removed by passing it through magnesium perchlorate, soda-lime and magnesium turnings heated at 83 K, respectively. Ge powder was easily decomposed in an argon atmosphere and germanium dissolved into the platinum foil. The liquid phase formed at several places in the platinum foil, and accordingly the solid-liquid phase equilibrium was attained. Compositions of both solid platinum and the liquid Pt-Ge alloy were determined by EPMA measurement as described in the section Results 4. Thermodynamic properties of Pt-Ge solid solution When only Ge is charged into platinum pan in the experiment as described in the section 3., the activity coefficient of germanium in solid platinum can be determined. From eq. (), the activity coefficient of germanium in solid platinum relative to the liquid standard state is represented by eq. (5) in the condition of a GeO (l) ¼. exp G RT Ge(l) inpt ¼ ð5þ X Ge in Pt P O The profile of Ge L intensity in platinum crucible after the experiment with P O ¼ :667 Pa at 63 K for 3 h is shown in Fig.. Germanium distributed uniformly in platinum crucible, therefore equilibrium was confirmed to attain within 3 h. results for the equilibrium between solid platinum and Ge melt under controlled oxygen partial pressure are summarized in Table and the relationship between the germanium content and the germanium activity in solid platinum is plotted in Fig.. The activity of germanium in solid platinum can be considered to obey Henry s law within the experimental concentration range. The activity coefficients of germanium in solid platinum at ð4þ Intensity of PtM α 6 4 Ge Platinum Pan PtM α GeL α Background GeL α Distance, d / µm 5 Fig. Intensity profiles of Ge L and Pt M radiations around the interface between solid platinum and the Ge melt after the equilibration at P O ¼ :667 Pa and 63 K for 3 h (measurement conditions: accelerating voltage of 5 kv, sample current of na, sampling step of 3 mm). Table results for the equilibrium between solid platinum and the Ge melt under controlled oxygen partial pressure. Sample Temperature P O P CO =P CO number (K) (Pa) age X Ge in Pt a Ge(l) K γ Ge(l) in Pt =.48 63K.5. X Ge in solid Pt infinite dilution were evaluated by the least square method as.48 at 73 K and.5 at 63 K, respectively. results for the phase equilibrium between solid platinum and the liquid Pt-Ge alloy are summarized in Table and Fig. 3. Open marks in Fig. 3 represent the equilibrium compositions at the peritectic temperature reported by Oya et al. 5) Here, the thermodynamic properties 5 γ Ge(l) in Pt =.5 Fig. Activities of germanium in solid platinum at 73 K and 63 K. Intensity of GeL α

3 Thermodynamic Properties of the Si -Ge and Pt-rich Pt-Ge Systems at 63 and 73 K 849 Table results for the phase equilibrium between solid platinum and the liquid Pt-Ge alloy. Temperature Sample number (K) X Ge in L X Ge in Pt G Ge(l) in L (J/mol) ex ex G Ge(l) in L = 3 545X Pt in L 7 Temperature, T / K γ-pt Calculated After Oya et al. L Fig. 4 X Pt in L Relationship between Ge(l) inl and X Pt in L. supposed by Darken 7,8) were applied and those are expressed as eqs. () and (). Pt 3 Ge X Ge Fig. 3 Phase equilibrium between solid platinum and the liquid Pt-Ge alloy. of solid platinum and the Pt-rich liquid Pt-Ge alloy are determined. For the thermodynamic property of solid platinum, function, which has relationship with the activity coefficient expressed as eq. (6), was investigated. RT ln Ge(s) inpt ¼ XPt in Pt ð6þ At infinite dilution, right hand side of eq. (6) corresponds to, and whose values for were derived as 43 at 73 K and 48 at 63 K from the obtained activity coefficient of germanium in solid platinum. With the similarity of at different temperatures, the excess partial Gibbs energy of germanium in solid platinum was evaluated as eq. (7) in regular solution formalism with the standard Gibbs energy of fusion for germanium. Ge(l) inpt ¼ RT ln Ge(l) inpt¼ ð369 3:5TÞ 43XPt in Pt ðj/molþ ð7þ Ge(s) ¼ Ge(l) ð8þ G 7 ¼ 369 3:5T (J/mol) 6Þ Accordingly, the excess partial Gibbs energy of platinum in solid platinum was expressed as eq. (9) with the standard Gibbs energy of fusion for platinum. Pt(l) inpt ¼ RT ln Pt(l) inpt¼ ð97 9:63TÞ 43XGe in Pt ðj/molþ ð9þ Pt(s) ¼ Pt(l) ðþ G 9 ¼ 97 9:63T (J/mol) 6Þ For the excess partial Gibbs energies of germanium and platinum in the liquid Pt-Ge alloy, quadratic formalism Ge(l) inl ¼ RT ln Ge(l) inl ¼ A þ BXPt in L Pt(l) inl ¼ RT ln Pt(l) inl ¼ BXGe in L ðþ ðþ As described by Darken, 7) this formalism is generally applicable not to entire composition of binary system, but to the terminal region. With the equilibrium compositions between solid platinum and the liquid alloy, the excess partial Gibbs energy of germanium in the liquid alloy can be calculated from the equality of germanium chemical potential in both phases expressed as eq. (3) and is plotted against XPt(l) inl in Fig. 4. Ge(l) inl þ RT ln X Ge in L ¼ Ge(l) inpt þ RT ln X Ge in Pt ð3þ By the least square method, the excess partial Gibbs energy of germanium in the Pt-rich liquid Pt-Ge alloy is determined as eq. (4), and accordingly that of platinum is represented as eq. (5). Ge(l) inl ¼ 3 545X Pt in L ðj/molþ ð4þ Pt(l) inl ¼ 545X Ge in L ðj/molþ ð5þ Phase boundaries calculated with the evaluated thermodynamic properties of solid platinum and the liquid Pt-Ge alloy are drawn as broken lines in Fig. 3 and show the fairly good agreement with the equilibrium compositions at eutectic temperature of Pt 3 Ge reported by Oya et al. 5) 4. Thermodynamic properties of Si -Ge melt results for the equilibrium between solid platinum and the Si -Ge melt under controlled oxygen partial pressure are summarized in Table 3. The activity of Ge in the Si -Ge melt was calculated from eq. (3) with the activity coefficient of germanium in solid platinum obtained in section 4., and the Ge activity curves in the Si -Ge melt at 73 K and 63 K are plotted in Figs. 5 and 6, respectively. With the obtained activity data, thermodynamic properties

4 85 T. Yoshikawa, I. Astuti, H. Takahashi and K. Morita Table 3 results for the equilibrium between solid platinum and the Si -Ge melt under controlled oxygen partial pressure. Sample P O P CO =P CO number (Pa) XGe in Pt X GeO(l) a GeO(l) GeO(l) 73 K K Evaluated After Baret et al..8 Evaluated After Baret et al. a GeO.6.4 a GeO X GeO X GeO Fig. 5 Activities of Ge in the Si -Ge melt at 73 K. Fig. 6 Activities of Ge in the Si -Ge melt at 63 K. of the Si -Ge melt were evaluated. The regular solution model represented as eq. (6) was adopted to the Si -Ge melt due to similar properties of Si and Ge such as tetrahedral bonding. G M;ex ¼ X GeO X SiO ð6þ Here, is a regular solution parameter and is related with the activity coefficient as following equation. RT ln GeO ¼ ðtþ ð7þ X SiO Averaging the products represented in the left hand side of eq. (7) for the Si -Ge melt, the regular solution parameters were determined as 6 at 73 K and 8 at 63 K, respectively. Hence, the excess Gibbs energy for the Si -Ge melt was evaluated as eq. (8) in the quasi regular solution formalism. 9) G M;ex ¼ð 476 þ 8:TÞX GeO X SiO ðj/molþ ð8þ Baret et al. ) estimated roughly the excess Gibbs energy for the Si -Ge as eq. (9). G M;ex ¼ 55X GeO X SiO ðj/molþ ð9þ Activity curves from the excess Gibbs energies both in the

5 Thermodynamic Properties of the Si -Ge and Pt-rich Pt-Ge Systems at 63 and 73 K 85 present work and reported by Baret et al. are drawn in Figs. 5 and 6. The deviations from the ideal solution are smaller in the present study. Finally, behavior of germanium in the resurrection process of optical fibers is considered. Here, the composition of optical fibers is considered Si -. at%ge. It is assumed that the oxygen potential would be controlled by the equilibrium between Si and Si expressed as eq. (), and the metal phase in the reduction process would be always composed of Si because of the high reactivity between germanium and Ge. Si (l) ¼ Si(l) þ (g) ðþ G ¼ 896 7T (J/mol) 6Þ From the Gibbs energy change of the reactions expressed as eqs. () and (3), vapor pressures of both SiO and GeO can be calculated by eqs. () and (4) with thermodynamic properties for the Si -Ge melt. Si (l) ¼ SiO(g) þ (g) G ¼ 778 4T (J/mol) Þ G ¼ RT ln P SiOP = ¼ RT ln P SiOP = a SiO (l) SiO (l)x SiO Ge (l) ¼ GeO(g) þ (g) G ¼ 47 3T G ¼ RT ln P GeOP = a GeO (l) (J/mol) Þ ¼ RT ln P GeOP = GeO (l)x GeO ðþ ðþ ð3þ ð4þ Calculated vapor pressures are shown in Fig. 7. Vapor pressure of GeO is 3 4 orders of magnitude higher than that of SiO at any temperature and this is mainly due to the extremely large vapor pressure of GeO. Therefore, germanium can be recovered by selective vaporization in the optical fiber resurrection process. 5. Conclusion () The Ge activity in the Si -Ge melt was determined by equilibrating it with solid platinum under controlled oxygen partial pressure. The excess Gibbs energy for the Si -Ge melt was evaluated as the following equation in the quasi-regular solution formalism. G M;ex ¼ð 476 þ 8:TÞX GeO X SiO ðj/molþ () By equilibrating solid platinum with the Ge melt under controlled oxygen partial pressure, the excess partial Gibbs energies of germanium and platinum in solid platinum were evaluated as follows. P SiO, P GeO / MPa 4 3 P GeO P SiO Temperature, T / K Fig. 7 Temperature dependence of P SiO and P GeO in the reduction process of optical fibers. Ge(l) inpt ¼ RT ln Ge(l) inpt¼ ð369 3:5TÞ 43XPt in Pt ðj/molþ Pt(l) inpt ¼ RT ln Pt(l) inpt¼ ð97 9:63TÞ 43XGe in Pt ðj/molþ Phase equilibrium between solid platinum and the liquid Pt- Ge alloy were investigated and the excess partial Gibbs energies of germanium and platinum in the Pt-rich liquid Pt- Ge alloy were evaluated as follows. Ge(l) inl ¼ 3 545X Pt in L ðj/molþ Pt(l) inl ¼ 545X Ge in L ðj/molþ (3) In the optical fiber reduction process, it is expected that germanium can be recovered by selective vaporization in the optical fiber resurrection process. REFERENCES ) W. Ma, M. Ogura, T. Kobayashi and H. Takahashi: Solar Energy Mater. Solar Cells 8 (4) ) G. Baret, R. Madar and C. Bernard: J. Elctrochem. Soc. 38 (99) ) Thermodynamic properties of individual substances, editors, L. V. Gurvich, I. V. Veyts and C. B. Alcock, 4th ed., (New York, Hemisphere Pub. Corp., 989). 4) O. M. Sreedharan, E. Athiappan, R. Pankajavalli and J. B. Gnamoorthy: J. Less-Common Metals 68 (979) ) Y. Oya and T. Suzuki: Z. Metallkde. 78 (987) ) E. T. Turkdogan: Physical Chemistry of High Temperature Technology, (Academic Press, New York, 98). 7) L. S. Darken: Trans. Metall. Soc. AIME 39 (967) ) L. S. Darken: Trans. Metall. Soc. AIME 39 (967) ) C. H. P. Lupis and J. F. Elliott: Acta Metall. 5 (967) ) I. Barin: Thermochemical Data of Pure Substances, (Weinheim, New York, VCH, 989).