The investigated alloys notations.
The demand for energy efficiency has motivated many researchers to seek for novel methods capable of enhancing the conversion of heat to electricity. Most of the recently published methods for thermoelectric (TE) efficiency enhancement discuss on the reduction of the lattice thermal conductivity, with a minor focus on improved electronic optimization. This is attributed mainly to the fact that the electronic properties are correlated and opposing each other upon increasing the carrier concentration. It has been reported that the system of PbTe-BiTe has potentially high TE performance; this chapter is focused on a detailed investigation of the co-effect of bismuth as an effective electronic dopant and at the same time, as a second phase promoter in the PbTe matrix. (PbTe)x(BiTe)1−x alloys were thermoelectrically examined and the values were analyzed analytically by the general effective media (GEM) approach.
In the past decades, global climate changes, caused by combustion of fossil fuels and greenhouse gases emission, became a major environmental concern, accompanied with the dilution of conventional energy resources, raising the need for a renewable energy alternatives. Thermoelectricity dealing with this concern, is based on a direct conversion of waste heat into usable electrical energy; even a partial conversion of this waste heat will get us one step closer toward a cleaner and greener world. This goal has been achieved by thermoelectric converters and successfully initiated by the development of various highly efficient thermoelectric material classes. Such materials require a unique combination of the electrical and lattice properties (Seebeck coefficient (
In the case of IV–VI-based chalcogenides and their alloys (known as among the most efficient thermoelectric alloys for intermediate working temperatures of up to 600°C) such nano-structuring approaches resulted in a significant increase of
Among the IV–VI-based chalcogenides, lead telluride is one of the most commonly used thermoelectric materials, but the maximal
This present work, considering our synthesis process, investigates the bismuth effects in the matrix, beneath and beyond the solubility limit, as an effective electronic dopant yet at the same time, as a second phase promoter in the PbTe matrix. Moreover, to the best of our knowledge, most of the previous researches did not investigated the individual electronic contributions of the involved secondary phases embedded in the matrix on the effective thermoelectric transport properties. The general effective media (GEM) approach  enables to estimate the transport properties based on experimentally measured properties of each of the involved phases. Since the approach is taking into consideration geometrical aspects, it can be utilized for maximizing the thermoelectric figure of merit of composite materials by artificial alignment of the embedded secondary phase in the composite.
Figure 1a shows the high potential of compositions with low amount of bismuth content, in the vicinity of the solubility limit, to attain high absolute Seebeck coefficient values. Figure 1b, demonstrates the potential to enhance the thermoelectric conversion efficiency while considering the co-effect of bismuth as an electronic donor dopant and as a secondary phase promoter in PbTe.
The thermoelectric transport properties of each alloy were measured up to 450°C as follows. The Seebeck coefficient,
The crystal structure of the alloys was analyzed by X-ray powder diffraction (Rigaku DMAX 2100 powder diffractometer). The microstructure of the alloys was observed using scanning electron microscopy (SEM, JSM-5600, JEOL, Akishima, Japan) equipped with a backscattered electron detector. The chemical composition was measured using an energy-dispersive X-ray spectroscopy (EDS).
3. Results and discussion
PbTe has a NaCl cubic crystal structure with space group Fm-3m, melts congruently at 924°C ; BiTe has a BiSe trigonal crystal structure with space group P-3m1, consists of 12 close-packed layers along the
As can be seen by the XRD diffractogram presented in Figure 2a, the reflections are all belong to the lead telluride matrix in the rock salt structure, with no evidence of other phase’s peaks. A reason for that might be that the amount of the second phase precipitants detected in the SEM analysis (Figure 3a and b) is clearly less than the detection limit of the XRD analysis.
As shown in Figure 2b, with increasing the bismuth content in the lead telluride matrix, the peaks are constantly shifted toward higher angles, reflecting a constant decrease in the lattice parameter. This may be related to the fact that Bi3+ has lower ionic radius (1.03 A ) compared to the ionic radius of Pb2+ (1.19 A ), while bismuth substitutes lead in the NaCl structure. Moreover, it is well known that introducing bismuth to the PbTe matrix creates cation vacancies, which might also contribute to the decrease in lattice parameter while releasing internal stresses/lattice strains. Although broadening of the peaks due to lattice internal stresses caused by bismuth interstitial atoms would be expected, it was not observed in the investigated samples, probably due to the fact that the amount of bismuth at alloy
Back-scattered electrons SEM micrographs of the investigated alloys are presented in Figure 3a–c. In Figure 3a of the investigated alloy
The logic behind this is that, at alloy
The Seebeck coefficient
The Bi3+ tendency to substitute Pb2+ creates one free electron, a fact which obviously affect the electronic properties of the materials. The Seebeck coefficient at room temperature for alloy
A similar trend observed in Figure 4b, from (
Since the thermal conductivity is affected both by bismuth as an effective electronic donor, but also as a phonon scatter source, both contributions were analyzed. The electronic contribution to the thermal conductivity, as shown in Figure 5a, was analyzed using Wiedemann-Franz relation,
It is noted that the dominant thermal conduction mechanism is a bit different between the alloys. While alloys
While combining both the thermal and electrical contributions to the thermoelectric efficiency (Figure 4d) it can be concluded that at 450°C, ~40% improvement compared to pristine PbTe was obtained; improvement that is attributed to an optimization between both bismuth contributions as an effective electronic dopant and as a second phase initiator.
In order to further investigate the electronic contribution of the secondary phase due to the geometrical morphology and amount, the general effective media (GEM) approach was applied . The effective thermoelectric transport properties, for composite materials consisting of two separate phases, were calculated based on the effective thermal conductivity,
Using the above data, the effective thermoelectric transport properties, shown in Figure 6a–d, were calculated. The measured room temperature experimental values for our
As been presented in the experimentally measured values (Figure 4a–c), and is in agreement with previously reports, in low amounts of bismuth in the matrix the transport properties are acting abnormally, with regard to the changing pattern of dissolution, up to a point where the dissolution pattern remains steady. Above that level, the GEM evaluation can predict the transport properties in a very reliable way, since the transport properties of the composite are expected to act as a composite with an increasing amount of a secondary metallic phase.
In our homogeneous distribution level and morphological alignment conditions, it can be seen that up to 0.1%, a slight change in bismuth concentration is accompanied with a drastic change in the electronic properties of the composite. A drastic degradation in the absolute Seebeck coefficient along with a great improvement in the electrical conductivity, sums up in an increased power factor (
The reason for the small mismatches observed in our GEM calculations, might be related to the fact that the matrix was represented by alloy
The co-effect of bismuth as an effective electronic dopant and at the same time, as a second phase promoter in the PbTe matrix was investigated and explained in details with regard to the dissolution pattern transition. These two effects on the thermoelectric properties were demonstrated and resulted in a ~40%
The work was supported by the Israel Science Foundation (ISF) Individual Research Grant No. 455/16. The authors would like to thank Mr. Yair George for the synthesis of the alloys and specimens preparation.
Conflict of interest
The authors declare no conflict of interest.
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