Determination of Thermodynamic Properties of Si-B Alloys
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, JACOB S. YOUNG, and RAMANA G. REDDY
This study establishes the thermodynamic properties of the Si-B binary alloy system. The electromotive force (emf) as a function of temperature (823 K to 923 K) was measured using solid-state electrochemical cells, represented as ðÞ Pt; Ar=fSikCaF2 k fSi BðalloyÞg=Ar; PtðþÞ Using the experimental emf data obtained from the solid-state heterogeneous phase equilibrium, the activities of Si and B in Si-B alloys were calculated. The integral Gibbs energy of mixing ðDGM Þ of alloys were calculated from the activity of Si and B. A large negative deviation from ideality was found for the integral Gibbs energy of mixing ðDGM Þ of the binary Si-B system. From the activities of Si and B, the Gibbs energies of formation DG0f of phases present were calculated. The DG0f of SiB3, the most thermodynamically stable phase, was calculated as 13.13 ± 0.19 kJ/mole-atoms at 923 K. The heterogeneous phases and microstructure after the equilibrium study of the electrode were characterized using XRD, SEM, and EDS analyses. https://doi.org/10.1007/s11663-019-01515-1 The Minerals, Metals & Materials Society and ASM International 2019
I.
INTRODUCTION
THE silicon-boron system has particular importance in the semiconducting industries as a doping element in semiconductor technologies.[1,2] Understanding the interaction between silicon and boron is essential for preparing n-type and p-type semiconductors. Furthermore, the production of solar grade Si also needs to be refined and purified to 6N (99.9999 pct). While natural impurities such as Fe, Ca, P, Ti, C, and B can be found in silicon, the removal of B is challenging for production of high purity Si. Recently, Si-B alloys and composites exhibit high-temperature thermoelectric performance, resulting in interest to understand the thermodynamic properties of Si-B alloys.[3–9] In the Si-B binary system, different intermediate compounds such as SiB3, SiB6, and SiBn (n = 14-50) were previously reported.[10–13] The reported phases in the Si-B system from the literature are summarized in Table I.
MUHAMMAD A. IMAM, JACOB S. YOUNG, and RAMANA G. REDDY are with the Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487. Contact e-mail: [email protected] Manuscript submitted August 23, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Thermodynamic studies of the Si-B system have been done in several previous studies.[11,14–20] The thermodynamic stability of the Si-B system has been primarily evaluated using high-temperature vaporization techniques. Armas and Kaufman et al. reported the thermodynamic properties at 1800 K, although with many inconsistencies.[17,20,21] Armas et al. calculated the activities of components from the partial pressure measurement.[17,21] In addition, they also reported the formation enthalpy of different borides at 1800 K.[17,21] On the other hand, Gordienko et al. obtained the thermodynamic properties using vaporization mass spectrometry in the temperature range of 2120 K to 2258
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