Elsevier

Calphad

Volume 74, September 2021, 102310
Calphad

Phase diagram of Pb–Se–Te system I: Experimental study

https://doi.org/10.1016/j.calphad.2021.102310Get rights and content

Abstract

Pb–Se–Te ternary system is of significant importance for thermoelectric applications. However, no systematic study of its phase diagram has been carried until now. Samples of Pb–Se–Te ternary alloys were prepared, their phase equilibrium phases were determined, and the Pb–Se–Te isothermal sections at 350 °C and 500 °C were proposed based on the experimental results. No ternary compounds were found. There is one three phase field, Pb(Se,Te)+liquid(Se,Te)+(Se,Te) at 350 °C and no three phase field at 500 °C. PbSe and PbTe form a continuous solid solution at both temperatures.

Introduction

Thermoelectric modules can directly convert heat to electricity based on Seebeck effect. Energy usage efficiency can be improved when thermoelectric devices are used for waste heat recovery. The thermoelectric devices used together with other renewable energy sources e.g. solar heating devices etc., can properly extend the usability and efficiency of the green energy sources. Due to these very important energy applications, thermoelectric modules and thermoelectric materials have attracted enormous research and development interest [[1], [2], [3], [4], [5], [6], [7], [8]].

Pb–Se, Pb–Te and Pb–Se–Te are among those promising thermoelectric materials which have been intensively examined [[8], [9], [10], [11], [12], [13], [14]], and Pb–Se–Te is thus an important thermoelectric system. Phase diagrams provide phase equilibria information and are fundamentally important for material design and development [[15], [16], [17], [18]]. However, there are only limited phase equilibria studies of the Pb–Se–Te system [[19], [20], [21], [22], [23], [24], [25]] and there is no phase diagram of the entire Pb–Se–Te compositional regime.

To provide fundamental information, this study determines 350 °C and 500 °C isothermal sections of the phase diagram of the Pb–Se–Te system. Experimental measurements are carried out to determine the phase equilibria of ternary Pb–Se–Te alloys. The experimental results determined in this study and the phase diagrams of the Pb–Se, Pb–Te and Se–Te binary systems, available in the literature [[26], [27], [28], [29], [30], [31]], are used for the isothermal sections determinations of the Pb–Se–Te system.

Section snippets

Experimental procedures

Proper amounts of pure constituent elements, Pb (99.999 wt%, Alfa Aesar, U.S.A.), Se (99.999 wt%, Alfa Aesar, U.S.A.) and Te (99.99 wt%, Alfa Aesar, U.S.A.), were weighed and sealed in a quartz tube in 105bar vacuum. The sample tube was heated to 800 °C to ensure complete melting and mixing of these elements and the tube was then quenched. The quenched sample tube was placed in a furnace at 350 °C and 500 °C to equilibrate the alloys. The annealing time varied from 30 days to 245 days.

The

Pb–Se–Te isothermal section at 350 °C

Nine Pb–Se–Te alloys are prepared and equilibrated at 350 °C. Their nominal compositions are summarized in Table 1 and shown in Fig. 1(a). Note, there are two liquid phases separated by the Pb(Se,Te) phase. Both PbSe and PbTe are cubic rocksalt structures. The lattice constant PbSe is 0.612 nm, and that of PbTe is 0.646 nm [32]. They exhibit complete solubility in the temperature region of interest. The liquid phase near the Pb side is labeled Liquid (Pb) and that along the (Se,Te) constituent

Conclusions

Phase diagrams of Pb–Se–Te ternary system have been determined by experimental measurements. No ternary compounds are observed. A continuous solid solution is formed between PbSe and PbTe. There is one three-phase field (Pb(Se,Te)+liquid(Se,Te)+(Se,Te)) at 350 °C and no tie-triangle at 500 °C.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors acknowledge the financial support of Ministry of Science and Technology of Taiwan (MOST 107-2923-E-007-005-MY3 and MOST 110-2634-F-007-024) and the Czech Science Foundation No. 18-25660J.

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