Journal of Alloys and Compounds 391 (2005) 307–312 Determination of the standard free energies of formation for tungsten silicides by EMF measurements using lithium silicate liquid electrolyte Hiroyasu Fujiwara a,∗ , Yukitomi Ueda a , Alok Awasthi b , Nagaiyar Krishnamurthy b , Sheo Parkash Garg b a Graduate School of Energy Science, Kyoto University, Yoshida-honmachi, Sakyo-ku, 606-8501 Kyoto, Japan b Materials Processing Division, Bhabha Atomic Research Centre, Mumbai 400085, India Received 17 August 2004; accepted 31 August 2004 Available online 28 November 2004 Abstract EMF measurements on silicon concentration cell were carried out using lithium silicate electrolyte in the temperature range 1300–1500 K. Using the EMF–temperature relations obtained, the standard free energies of formation for W 5 Si 3 and WSi 2 have been calculated as func- tions of temperature: G 0 (W 5 Si 3 ) (kJ mol -1 )= -128.6 - 0.0132 (T (K)) ± 0.42 and G 0 (WSi 2 ) (kJ mol -1 )= -98.5 + 0.0095 (T (K)) ± 0.15, respectively. Combining the results with thermodynamic data, the standard enthalpies of formation for silicides at 298K have been ob- tained as H 0 (W 5 Si 3 , 298) (kJ mol -1 )= -133 ± 25 and H 0 (WSi 2 , 298) (kJ mol -1 )= -82 ± 6, respectively. A new eutectoid reaction W 5 Si 3 ⇒ W + WSi 2 occurring around 700 K is suggested in W–Si binary system. © 2004 Published by Elsevier B.V. Keywords: High temperature alloy; Electrochemical reactions; Intermetallics 1. Introduction Transition metal silicides are effective oxidation-resistant protective coating on the metals. Although thermodynamic properties of the tungsten silicides have a bearing on vari- ous practical applications, direct measurements of them have been performed only to a limited extent. The experimental work on the direct determination of the standard free en- ergies of formation for tungsten silicides has been reported by Chart [1]. He measured the vapor pressure of SiO for the mixture of SiO 2 + tungsten silicides by Knudsen-effusion method. Kematick [2] reported the standard enthalpies of formation at 298 K for the silicides, combining the latest heat capacity data with the relation between silicon vapor pressure and temperature obtained by high temperature mass spectrometry. Calorimetric determination of the standard en- ∗ Corresponding author. Tel.: +81 75 753 5437; fax: +81 75 753 5437. E-mail address: fujiwara@energy.kyoto-u.ac.jp (H. Fujiwara). thalpy of formation at 298 K for W 5 Si 3 was reported by Tomaszkiewicz et al. [3], while that for WSi 2 was done by Robins and Jenkins [4] and by O’Hare [5]. A work on high temperature heat capacity by Bondarenko et al. [6] provides the data for both W 5 Si 3 and WSi 2 . However, the data for WSi 2 are not consistent with the results by Callanan et al. [7], in which experimentally determined low temperature heat ca- pacity was combined with the reported values by Mezaki et al. [8]. The EMF measurements have been widely used for ther- modynamic activity determination. Unfortunately, applica- tion to the refractory metal silicide systems was not so popular, because of high reactivity of silicon and low par- tial pressure of oxygen determined by Si/SiO 2 equilibrium. In the previous work by the authors [9], lithium silicate electrolyte was utilized for EMF measurements of the sil- icon concentration cells in order to determine the standard free energies of formation for niobium silicides. Because of the potential utility of the method adopted in the pre- 0925-8388/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.jallcom.2004.08.100