Liquidus projection and isothermal section of the Pb–Sb–Se ternary system
Graphical abstract
Introduction
Thermoelectric materials and devices have attracted intensive research interests because of their potential applications in waste heat recovery [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Various promising thermoelectric materials have been investigated. Pb–Sb–Se and its based-alloys, such as Pb–Se–Te–Sb, Pb–In–Sb–Se, and Cu–Sb–Se–Pb alloys [6], [7], [8], [9], [10], [11], [12], are among the most commonly examined. Phase equilibria information is fundamentally important to material development, property assessment and processing route selection. Although Pb–Sb–Se is an important subsystem of several good performance thermoelectric materials systems, there is only one Pb–Sb–Se phase equilibria study of the PbSe–Sb2Se3 isoplethal section [13] and reports of the Pb–Sb–Se ternary compounds, Pb4Sb4Se10, PbSb2Se4, Pb6Sb6Se17, and Pb4Sb6Se13 [14], [15], [16], [17].
This study thus determines the liquidus projection and isothermal section at 400 °C (673.2 K) of the Pb–Sb–Se system. The isothermal section represents equilibrium phases at different compositional regions. The liquidus projection contains univariant lines and invariant reactions. Since the univariant lines delineate regions of different primary solidification phases, the data of the primary solidification phase can be used to determine the univariant lines [18], [19], [20], [21], [22]. Ternary Pb–Sb–Se alloys are prepared in this study, and the above-mentioned data are determined. The liquidus projection and isothermal section at 400 °C are proposed based on the experimental results of this study, phase equilibria results in the literature [13], [14], [15], [16], [17], and phase diagrams of its constituent binary systems, Pb–Sb, Pb–Se, and Sb–Se [23], [24], [25].
Section snippets
Experimental procedures
Proper amounts of pure Pb (99.999 wt.%, Alfa Aesar, Ward Hill, MA, U.S.A.), Sb (99.9999 wt.%, Alfa Aesar, Ward Hill, MA, U.S.A.), and Se (99.999 wt.%, Alfa Aesar, Ward Hill, MA, U.S.A.) with a total weight of 1 g were weighed and encapsulated in a quartz tube in a 2 × 10−5 bar vacuum. The alloys were prepared with extra care due to the high vapor pressure of Se. The sample capsules of alloys with Se contents lower than 50 atomic percent were placed in a furnace at room temperature, heated
Determination of univariant lines of liquidus projection
Forty-five Pb–Sb–Se ternary alloys were prepared. Their nominal compositions are listed in Table 1. Fig. 1(a) and (b) are the determined Pb–Sb–Se liquidus projection with and without superimpositions of the forty-five Pb–Sb–Se ternary alloys examined in this study.
Fig. 2(a) is the back-scattered electron image (BEI) micrograph of as-solidified alloy #6 (Pb-10.0 at%Sb-40.0 at%Se). The gray bulk phase is the primary solidification phase. The composition of the dendritic phase is
Conclusions
The liquidus projection and 400 °C isothermal section of the Pb–Sb–Se ternary system are determined. A new ternary compound PbSb5Se6 is found, and its structure requires further determination. There are eight primary solidification phase regimes in the liquidus projection which are (Pb), (Sb), (Se), PbSe, Sb2Se3, PbSb2Se4, Pb6Sb6Se17, and PbSb5Se6. There are nine ternary invariant reactions involving liquid. In the isothermal section at 400 °C, there are nine tie triangles:
Acknowledgment
This study is financially sponsored by the National Science Council of Taiwan (NSC 103-2923-E-007-002-MY2).
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