Structural, characterization and electrical properties of AgPbmSbTem+2 compounds synthesized through a solid-state microwave technique

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Structural, characterization and electrical properties of AgPbmSbTemþ2 compounds synthesized through a solid-state microwave technique A. Hmood a,*, A. Kadhim a, M.A. Mahdi b, H. Abu Hassan c a

University of Basrah, College of Science, Department of Physics, Basrah, Iraq Basrah Nanomaterials Research Group (BNRG), Physics Department, College of Science, Basrah University, Basrah, Iraq c School of Physics, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia b

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abstract

Article history:

AgPbmSbTe2þm compounds are prepared through solid-state microwave synthesis method

Received 4 July 2015

with different elements ratio of m (0.0  m  10). The X-ray diffraction method confirmed

Received in revised form

that all of the synthesized samples are polycrystalline with a rock salt NaCl-type crystalline A to structure. The lattice constant of the prepared AgPbmSbTe2þm increases from 6.089 

30 December 2015 Accepted 31 December 2015

6.467  A with the increase in the mole fraction (m) from 0.0 to 10, respectively. Scanning

Available online 14 February 2016

electron microscopy images showed that increasing the mole fraction allows the contin-

Keywords:

electrical conductivity (s) measurements of AgPbmSbTe2þm prepared with a different m

Electrical properties

ratio exhibited a degenerate semiconductor behavior for all samples, and the sample

Thermoelectric materials

prepared with m ¼ 0 showed a higher s of 482.96 S/cm at 300 K compared by the others. All

uous growth of multigrains, and volume of these grains increased with m value. The

Seebeck coefficient

of the samples had a negative Seebeck coefficient (S), indicating that the AgPbmSbTe2þm

Power factor

compounds are of n-type conductivity. Moreover, the maximum value of S is 419.69 mV/K

Silver (Ag) and antimony (Sb) co-

at 338 K that obtained for the sample prepared with m ¼ 8 while the minimum value of S

doping

was /6.8 mV/K recorded when the composition ratio of 4. Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Lead telluride (PbTe) based compounds with a narrow band gap have a cubic NaCl-type crystalline structure belonging to the Fm3m space group, characterized by nearly isotropic physical properties and superior chemical stability. Therefore, single-crystal, polycrystalline bulk, and thin films of PbTebased compounds are promising materials for

thermoelectric (TE) devices [1e3]. The quaternary compounds of AgPbmSbTe2þm are best materials for TE applications at temperature of 700 K among PbTe-based alloys [4e7]. The performance of a thermoelectric material is determined by its dimensionless figure of merit, ZT ¼ S2s/k, where S is the Seebeck coefficient, s refers to electrical conductivity, and k denotes thermal conductivity [8,9]. Efficient methods for increasing the Seebeck coefficient and the power factor (S2s) of a homogeneous semiconductor are proposed [10e12], which

* Corresponding author. Tel.: þ964 7800199657; fax: þ964 7717976052. E-mail address: [email protected] (A. Hmood). http://dx.doi.org/10.1016/j.ijhydene.2015.12.205 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 5 0 4 8 e5 0 5 6

verifies the possibility of transforming bipolar semiconductor bulk. An alternative doping can be conducted to obtain practically monopolar semiconductors [13,14]. The effectiveness of using such semiconductors is ascertained by the possibility of obtaining high values of TE characteristics, such as S and S2s [15e17]. In addition, doping of semiconductor can improve electrical conductivity that is primarily determined by the electronic band structure [18,19]. Most compounds that utilized as TE materials are grown using different solid-state reaction methods, such as traditional melting, melt-quench technique, Bridgman technique, powder metallurgy, and a new technique standard called “solid-state microwave synthesis” [20e24]. The AgPbmSbTe2þm alloy is prepared by heating mixtures above the melting point of the constituent elements through the well-known metallurgy method that has been examined recently. Moreover, nanocrystalline of AgPbmSbTe2þm powder is successfully synthesized through the hydrothermal method [25, 26]. In the current study, ternary AgSbTe2 and quaternary AgPbmSbTe2þm semiconductor materials co-doping by the elements of Pb and Te are prepared using the solid-state microwave synthesis technique. The fundamental properties of prepared alloys are investigated by analyzing their morphological, crystalline structure, and electrical characterizations. Therefore, s and S are enhanced with an increase in the mole fraction ratio from 0 to 10. Accordingly, S2s increases with an increase in m mole fraction in all prepared samples.

Experimental procedure Ag, Pb, Sb, and Te powders of high purity (>99.999%, 100 meshes) were used to prepare six samples of AgPbmSbTe2þm alloys with different elements ratio of m (0, 2, 4, 6, 8, and 10) through solid-state microwave synthesis [9e11]. The samples are prepared under 800 W of 2.45 GHz microwave irradiation using a microwave oven (LG-MS2147C) for 25 min. Bright, whitish-blue plasma was observed from all of the ampoules immediately after exposure by microwave irradiation. The temperature of the preparation samples was 1173 Ke1183 K and it's measured using an OS524E infrared thermometer (OMEGA SCOPE). After the reaction is completed, the ampoules were then taken out from the microwave oven and emptied of their content. Subsequently, AgPbmSbTe2þm (m ¼ 0 to m ¼ 10) powders were shaped into disks (diameter: 13 mm; thickness: 0.58 mm) via cold pressing at 10 tons. The microstructure, morphology, and the composition of synthesized samples were characterized by X-ray diffraction (PANalytical X'Pert PRO MRD PW3040), the quantities of second phase percentage in the sublattices were examined using Rietveld refinement, scanning electron microscope, and energy dispersive X-ray (SEM and EDX; JSM-6460), respectively. The Seebeck coefficient (S) was determined by the slope of the linear relationship between the thermoelectromotive force and the temperature difference between the two ends of each sample. Electrical conductivity (s) was measured using the four-point probe dc method under a vacuum of 103 mbar within temperatures ranging from room temperature to approximately 575 K. The Hall coefficient (RH) was determined at room temperature with an applied magnetic field of 1T

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using a Lake Shore electromagnetic power supply (model: 637, USA).

Results and discussion Crystalline structure The crystalline structure of AgPbmSbTe2þm alloy powders prepared with various composition ratios (m) is investigated through XRD (see Fig. 1). All of the synthesized samples are formed in polycrystalline with a rock salt NaCl-type crystal structure. All the diffraction peaks in the typical XRD patterns correspond to the prepared sample phases with a dominant diffraction peak related to (200) plane compared by a standard database (JCPDS No. 12-0375 and 38-1435). The weak diffraction peak on the right-hand side of the (200) peak represents the second phase of AgSbTe2 in AgPbmSbTe2þm. The weak peak phase, reduced peak intensity to minimize in the respective XRD spectra of AgPb6SbTe8 alloy powder, while, the peaks at lower angle than the (200) peak for m ¼ 0 sample can be explained by some impurity as shown in Fig. 1. However, for m ¼ 2 to 10, the main phase of AgPbmSbTe2þm averages 57%, 69%, 77%, and 70%, and 70% respectively, that obtained from the Rietveld refinement of the XRD patterns of various m contents. As shown in Fig. 2, the XRD patterns fitted using the Rietveld refinement method for m ¼ 2, 6, and 10, and the corresponding results are listed in Table 1. At the same time, PbTe has the same NaCl-type structure (space group Fm3m) as the host AgSbTe2, with a smaller lattice parameter A, aAgSbTe2 ¼ 6.078  A) [1,24]. The lattice constant (aPbTe ¼ 6.438  (a) of the cubic structure can be calculated using the following Eq. (1) [27]: 1 h2 þ k2 þ l2 ¼ ; d2 a2

(1)

where h, k, and l are Miller indices in a unit cell, and d is the distance between atomic layers in a crystal. The lattice

Fig. 1 e XRD patterns of AgPbmSbTe2þm powders prepared via solid state microwave synthesis using various molar fraction ratio (m).

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Fig. 2 e X-ray diffraction patterns fitted using the Rietveld refinement method of AgPbmSbTe2þm powders: (a) m ¼ 2, (b) m ¼ 6, and (c) m ¼ 10.

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Table 1 e The values of Rietveld refinement factors (R factors) of AgPbmSbTe2þm (m ¼ 2e10). Results

m¼2

m¼4

m¼6

m¼8

m ¼ 10

Rprofile Rweight profile Rexpected RInitial RFinal c2 ¼ Rwp/Rexp d-statistics

0.2032 0.16542 0.26622 0.5611 0.1181 0.621 1.700

0.2397 0.1950 0.2725 0.6775 0.1186 0.716 0.6399

0.3336 0.3961 0.2888 0.7727 0.1482 1.372 0.5426

0.20070 0.12794 0.26918 0.6681 0.1280 0.4753 0.6204

0.1883 0.1192 0.2667 0.6495 0.1230 0.447 0.5662

constant of the prepared AgPbmSbTe2þm increases from 6.089  A to 6.467  A with the increase in the mole fraction (m) from 0 to 10, respectively (see Fig. 3). This increase in the lattice constant is an indicator of the insertion of Pb atoms into the crystal structure of AgSbTe2 compound; thus, the higher value of m in the LAST-m alloys indicates a high concentration of Pb metal in the AgSbTe2 matrix. The atomic radii of Ag and Sb are 1.60  A and 1.45  A, respectively, and are smaller than that of Pb (1.80  A). Therefore, when the Ag and Sb are entirely and well occupied in the Na site of the NaCl structure (i.e., the same as Pb in all of the composition ranges), then the respective lattice constant should increase with the increase in m value [28,29]. The XRD data of the synthesized AgPbmSbTe2þm (0.0  m  10) ternary and quaternary ingots are presented in Table 2.

Surface morphology SEM images and EDX spectra were obtained to determine the surface morphology and elemental composition of the prepared AgPbmSbTe2þm ingots. Fig. 4 presents the SEM images of AgPbmSbTe2þm synthesized using different mole fractions of m (0.0  m  10). The images show the effect of microwave irradiation on the formation and crystallization of the AgPbmSbTe2þm alloys. Surface morphology revealed that the growth of the compounds involved uniform compact rocks for the AgPbmSbTe2þm quaternary alloys and tended to refine the grain growth with increased m mole fraction (0.0  m  10). 6.50 6.45

Lattice constant, a (Å)

6.40 6.35 6.30 6.25 6.20 6.15 6.10 6.05 0

2

4

6

8

10

m, mole fraction

Fig. 3 e Variation in lattice constant of the AgPbmSbTe2þm ingots prepared using various molar fraction ratios (m).

Table 2 e Room temperature structural properties of AgPbmSbTe2þm: lattice constant (a), scattered beam angle (2q), inter-planer distance (d), Miller indices of lattice planes (hkl).  Sample 2q (o) hkl d ( A) a (A) AgSbTe2 AgPb2SbTe4 AgPb4SbTe6 AgPb6SbTe8 AgPb8SbTe10 AgPb10SbTe12

29.338 27.921 27.648 27.583 27.571 27.586

200 200 200 200 200 200

3.044 3.196 3.226 3.234 3.235 3.233

6.089 6.391 6.453 6.466 6.471 6.467

The surface of samples (compounds with m ¼ 8 and m ¼ 10) consisted of microcubby-shaped grains in addition to other small grains with a relatively uniform size. The elemental compositions of the pure ternary AgSbTe2 and quaternary AgPbmSbTe2þm compounds were estimated based on EDX. Table 3 shows the values of the elemental weight and atomic percent of every element detected in the EDX of the AgPbmSbTe2þm samples. The elemental ratios of Ag, Pb, Sb, and Te showed that the EDX results of all of the samples were nearly stoichiometric to the nominal results. After the grinding of the ingots, the resultant powders were shaped into disks by cold pressing. The typical SEM images of the surface morphology of the resultant disks of the AgPbmSbTe2þm are shown in Fig. 5. The prepared samples with m ¼ 0 and m ¼ 10 exhibit similar morphologies; the grains of all of the samples (m ¼ 2e8) that are placed tightly show no preferred orientations, which are in accordance with the high relative packets and the isotropic transport properties for the samples reported in the literature [30e32]. In addition, the grain size of samples apparently increases via the Pb insertion, which indicates that Pb alloying tends to have well-packed structures.

Electrical properties The electrical measurements of AgPbmSbTe2þm prepared with a different m ratio exhibited degenerate semiconductor behavior for all of the samples with a relatively high electrical conductivity (s). The sample prepared with m ¼ 0 showed high s of 482.96 S/cm at 300 K compared by the others that were synthesized with concentration ratio of m < 0. The decreasing conductivity above m ¼ 0 indicates an electron transition from intrinsic to extrinsic behavior in all AgPbmSbTe2þm samples (see Fig. 6). The temperature dependence of s of the AgPbmSbTe2þm samples ranged from 298 K to 575 K, which was expected for degenerate semiconductor behavior. The s is decreased during increases in m of 2, 4, and 8, and increased once again at m of 6 and 10. However, two competing factors of carrier concentration and mobility determine s. Generally, s considerably decreases from 482.96 S/cm to 167.41 S/cm at 300 K and from 156.06 S/cm to 86.61 S/cm at 575 K for m ¼ 0 and m ¼ 10, respectively. The decrease in s values could be attributed to the incorporation of Ag and Sb atoms into the crystal lattice of the prepared alloys, thus changing the formation energy of the lattice defects in the AgPbmSbTe2þm mixed crystals [21,28]. Electrical conductivity is dramatically increases for the sample prepared with m ¼ 8 value at 300 K. This behavior indicates that the increase in s could be

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Fig. 4 e SEM images of AgPbmSbTe2þm ingots prepared using various molar fraction ratios (m).

ascribed to the good crystallinity of the AgPbmSbTe2þm samples and dense microstructure with an irregular grain size (see Fig. 5). The nearly dense microstructure induces no pores; thus, the grains and grain boundary morphologies vary with m value and function as traps for free electrons and barriers for electron transport in the samples [10,15]. Furthermore, the absence of porosity associated with fusion in the grain size decreases electron scattering, which increases s [22,29]. However, the conductivity of s of AgPb8SbTe10 of the sample prepared using m < 0 is lower than that for the AgSbTe2 sample prepared with m ¼ 0. s can be expressed in terms of the carrier concentration (n) using the formula s ¼ nem ¼ ne2t/ m*, where e, m, t, and m* are electron charge, hall mobility, relaxation time, and effective mass of electron, respectively [33]. The electron concentration in AgPbmSbTe2þm compounds ranged from 6.44  1019 cm3 to 1.15  1019 cm3 when m ratio is increased from 0 to 8 as listed in Table 4. The n and m behaviors of the prepared samples are generally varies, that is, n and m decrease by increasing the m value. After reaching their

minimum values, n and m increase with a further increase in the m value. This behavior is attributed to the presence of excess Ag1þ and Sb3þ ions in the crystal lattice, which subsequently induces the resonant donor levels [25]. The increase in AgeSb concentration produces high density-of-states near the Fermi level, providing the electrons with high effective mass [34]. At the same time, covalence is related to electronegativity of atoms of the elements. The electronegativities are as follows: for Pb atoms (cpb ¼ 2.33), Te atoms (cte ¼ 2.10), Sb atoms (cSb ¼ 2.05), and Ag atoms (cAg ¼ 2.59) [35]. The difference in PbeAg electronegativities is larger than that of PbeSb, which may reduce the structure defects in the quaternary AgePbeSbeTe lattice [2,11]. The Seebeck coefficient of the samples was determined from the relation between the measured thermoelectromotive force (DV) and the temperature difference (DT) among the specimens (i.e., S ¼ DV/DT). Fig. 7 illustrates the influence of m mole fraction and the temperature difference on the S of the prepared samples. All of the samples exhibited negative S,

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Table 3 e EDX elemental composition results of AgPbmSbTe2þm samples with different m mole fraction. Sample

Element

Weight %

Atomic %

AgSbTe2

Ag Sb Te Ag Sb Te Pb Ag Sb Te Pb Ag Sb Te Pb Ag Sb Te Pb Ag Sb Te Pb

18.56±1.04 28.47±1.42 52.97±2.53 5.40±0.37 7.46±0.46 43.48±2.57 43.66±1.58 0.99±0.07 4.56±0.29 40.87±2.56 53.58±2.06 0.69±0.05 2.04±0.13 41.07±2.61 56.21±2.20 1.18±0.08 0.6±0.03 44.57±2.79 53.64±2.07 0.42±0.03 0.62±0.04 43.95±2.79 55.01±2.14

20.96±1.04 28.48±1.42 50.56±2.53 7.55±0.37 9.25±0.46 51.41±2.57 31.79±1.58 1.47±0.07 5.98±0.29 51.20±2.56 41.34±2.06 1.03±0.05 2.72±0.13 52.23±2.61 44.02±2.20 1.76±0.08 0.79±0.03 55.97±2.79 41.48±2.07 0.63±0.03 0.81±0.04 55.84±2.79 42.91±2.14

AgPb2SbTe4

AgPb4SbTe6

AgPb6SbTe8

AgPb8SbTe10

AgPb10SbTe12

for AgPb8SbTe10 is larger by approximately a factor of 16 than that of AgPb4SbTe8. This result confirms the Pb atom substitution for Ag, exhibits a donor action (n-type) behavior, and supplies one electron per impurity atom to the conduction band in AgPbmSbTemþ2 [24,37]. These findings imply that the substitution of Pb2þ ion for Ag1þ in AgSbTe2 alloy can possibly increase both the carrier concentration and mobility [10]. Consequently, the n dependence of S does not follow Eq. (2), and S is relatively enhanced, which is also observed in SbePbTe, YbeSnTe, and TlePbTe [25,37e38]. The S of the PbeTe doped samples may be considerably affected by the change in m* value. As n increased, m* also increased nonlinearly between 0.48 for an undoped sample to 0.89 for the sample that doped with m ¼ 10 with n of 1.15  1019 cm3 (see Table 4). The trend of increasing m* with n does not imply a change in the band structure of the compound. Instead, it is the result of the carrier effective mass being larger than the density of state effective mass as previously observed in PbTe doped with a small amount of Bi as an n-type donor [39]. The maximum Seebeck coefficient Smax(T) in terms of the temperature, which is considered to be effective, confirms the change in the electronic structure in which band gap. The value of the thermal band gap (Eg) can be obtained with the value of Smax(T), as follows [37,40e41]: Smax ðTÞ ¼

indicating that the AgPbmSbTe2þm compounds are of n-type conductivity, and that the most predominant carriers were electrons. The S increased to its maximum value at m ¼ 8, but dramatically decreased at m ¼ 4. The highest value of S obtained was 419.69 mV/K at 338 K for the sample with m ¼ 8 while the lowest value was of 6.8 mV/K for the sample that synthesis with m ¼ 4. In general, the S of degenerate semiconductor dependence of s can be obtained through Eq. (2) [36]: S¼

8p2 k2B *  p 2=3 mT ; 3n 3eh2

(2)

where kB and h are Boltzmann's and Plank's constant, respectively. The preceding relationship shows that S is highly dependent on the carrier concentration and increases with decreasing carrier concentration. The S for a given carrier concentration increases as the scattering factor increases. Therefore, the increased S should be attributed to an increased scattering factor, presumably from the potential barrier scattering effect. The controlling doping of PbTe in the presence of Ag and Sb is the possibility that substitutional Ag at Pb sites or Ag interstitials will compensate other donors [21,32]. However, the Seebeck coefficient of the prepared samples is increased above the doping level at m ¼ 4. Thus, these compositions have donor levels in the forbidden gap, indicating an increase in the effective mass (m*) of the compound. To understand the variations in m* for the samples with varying composition ratio (m), ascertaining the actual composition of the asprepared samples is necessary. As shown in Eq. (2), as the amount of the m mole friction is increased, m* should be increased to maintain S [12]. From the measured n and S, the effective mass is estimated, and found that the value of m*

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Eg ; 2eTmax

(3)

From Smax(T), the value of Eg is increased with the increasing meratio at the intrinsic excitation temperature of 575 K. The value of Eg of the alloys AgSbTe2 and AgSbPbmTemþ2 is decreased from 0.103 eV to 0.096 eV when m increases from 0 to 2, but the value of Eg is increased from 0.113 eV to 0.379 eV for the samples prepared with m ¼ 4e8. Moreover, the value of Eg is decreased to 0.223 eV at m ¼ 10, which indicates that the band gap is altered by the m-ratio (see Table 4). At the high-temperature region, the electrons are excited across the band gap leading to increase the concentration of the electrons in the conduction band of the semiconductor [26,41]. Thus, the contribution of the electrons to the total Seebeck coefficient increases, which shifts the value of S at high temperature with increasing m-ratios of 6 and 8. S2s reveals decreases at the high-temperature range because s exhibits an extrinsic conduction due to higher thermal excitation in the charge carriers compared with the low-temperature range. S2s exhibits a behavior typical of degenerate semiconductors, which significantly increases with the increase in temperature from 303 K to 343 K and further decreases at 343 Ke575 K. Fig. 8 presents the temperature dependence of power factor S2s for the AgPbmSbTe2þm samples. The value of S2s for the AgPb8SbTe10 sample reaches 1.87 mW K2 m1 at 310 K, which is significantly higher than those of the AgSbTe2 and AgPb10SbTe12 samples. The aforementioned results indicate that the AgPb8SbTe10 sample can exhibit moderate electrical conductivity without sacrificing the Seebeck coefficient. The experimental results and theoretical calculations suggest that the electronic transport properties of these samples are heavily influenced by the high density of state near the

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Fig. 5 e SEM images of AgPbmSbTe2þm disks prepared using various molar fraction ratios (m).

500

Electrical conductivity (S/cm)

450

Table 4 e Room temperature electrical properties: electrical conductivity (s), Seebeck coefficient (S), Hall coefficient (RH), carrier concentration (n), carrier mobility (m), thermal band gap (Eg) and effective mass (m*).

m=0 m=2 m=4 m=6 m=8 m=10

400 350 300

m

s S RH n  1019 m Eg m*/mo (S/cm) (mV/K) (cm3/C) (cm3) (cm2/V.s) (eV)

0 2 4 6 8 10

482.96 81.36 145.58 66.17 205.94 156.06

250 200 150 100 50

49.6 83.2 6.8 104 295.2 146.8

0.017 0.018 0.022 0.023 0.086 0.038

6.44 6.09 3.93 4.13 1.15 2.87

8.31 1.46 3.26 1.55 17.79 5.91

0.103 0.096 0.113 0.121 0.379 0.223

0.48 0.77 0.05 0.74 0.89 0.82

0 300

350

400

450

500

550

Temperature (K)

Fig. 6 e Electrical conductivities as a function of temperature of AgPbmSbTe2þm disks prepared using various molar fraction ratio (m).

600

Fermi level, thus producing electrons with light effective mass [17]. These effective mass carriers generate a large Seebeck coefficient when m ¼ 8 and m ¼ 10, while obtaining high power factors of m ¼ 8 and m ¼ 10 at 310 K. These

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Acknowledgments The authors are grateful for the Nano-Optoelectronics Research and Technology Laboratory (N.O.R.) of the School of Physics; Universiti Sains Malaysia for the help extended the research.

references

Fig. 7 e Seebeck coefficient as a function of temperature of AgPbmSbTe2þm disks prepared using various molar fraction ratios (m).

2.0 1.8

Power factor (mW/mK2)

1.6 m=0 m=2 m=4 m=6 m=8 m=10

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 300

350

400

450

500

550

600

Temperature (K)

Fig. 8 e Power factor as a function of temperature of AgPbmSbTe2þm disks prepared using various molar fraction ratios (m).

results provide a clear idea for the application of these alloy compositions for TE devices.

Conclusion For the first time, a highly pure AgSbTe2 compound and other five chemical compositions of AgPbmSbTe2þm (0.0  m  10) have been successfully produced using solid-state microwave synthesis at 1173 Ke1183 K for 25 min. The XRD characterization reveals that prepared products have a two phases with rock salt NaCl-type crystal structures. By increasing the m value in the AgePbeSbeTe system, the electron concentration decreased and subsequently increased S and decreased s (m ¼ 6). The AgPb8SbTe10 sample exhibits a similar and moderate electrical conductivity and a higher Seebeck coefficient compared with the AgSbTe2 and AgPb10SbTe12 samples. The maximum power factor of 1.87 mW K2 m1 is obtained for the AgPb8SbTe10 sample resulting from a higher Seebeck coefficient.

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