Thermoelectric properties of Zn4Sb3 intermetallic compound doped with Aluminum and Silver

Intermetallics 45 (2014) 60e64

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Intermetallics journal homepage: www.elsevier.com/locate/intermet

Thermoelectric properties of Zn4Sb3 intermetallic compound doped with Aluminum and Silver Riccardo Carlini a, b, *, Daniele Marré c, Ilaria Pallecchi c, Riccardo Ricciardi a, Gilda Zanicchi a, b a b c

Dipartimento di Chimica e Chimica Industriale, Università degli Studi di Genova, Via Dodecaneso 31, 16146 Genova, Italy INSTM Unità di Ricerca di Genova, Via Dodecaneso 31, 16146 Genova, Italy CNR-SPIN and Università degli Studi di Genova, Via Dodecaneso 33, 16146 Genova, Italy

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 June 2013 Received in revised form 29 August 2013 Accepted 4 October 2013 Available online 27 October 2013

The b-phase Zn4Sb3 has attracted much attention because of its high thermoelectric performance in the intermediate temperature range thanks to disorder in the Zn lattice site. In this work are presented structural, thermal, electric and thermoelectric characterization of Zn4Sb3 pure and Ag, Al doped, prepared by a simple synthesis. Structural and microstructural analyses reveal homogeneous one-phases having compositions in agreement with the nominal ones. After thermoelectric characterization, Ag doping results mostly effective in lowering the resistivity and Seebeck coefficient value, by introducing holes in the system. On the other hand, the Al substitution yields a very small decrease of the Seebeck coefficient but, at the same time, a significant decrease of the thermal conductivity mainly due to the depressed phonon contribution. The thermal conductivity behavior is the main responsible for the good thermoelectric performances of (Zn0.99Al0.01)4Sb3, whose thermoelectric figure of merit reaches the encouraging value of 0.23 at 260 K. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: A. Intermetallics B. Thermoelectric properties C. Reaction synthesis E. Physical properties G. Energy systems G. Thermoelectric power generation

1. Introduction Power-generating devices based on thermoelectric technology can play an important role in the future global requirement of energy. Thermoelectric properties of some materials can be used both in power generation and in refrigeration [1e5]. In the former case, the heat flow is converted into electrical power, while in the latter case electric power is used to draw out heat from thermoelectric materials. In this way, it is possible to increase the energy efficiency of current technological devices through waste heat capture and the consequent co-generation. These energetic approach relating to the fundamental issues of sustainability and eco-environmental compatibility can be employed in different areas: aerospace applications, automotive field, nautical freight. A good thermoelectric material is characterized by a high thermopower S combined with a low electrical resistivity r and a low thermal conductivity k [6,7], in turn constituted by an electronic

* Corresponding author. Dipartimento di Chimica e Chimica Industriale, Università degli Studi di Genova, Via Dodecaneso 31, 16146 Genova, Italy. Tel.: þ39 103536157; fax: þ39 103536163. E-mail address: [email protected] (R. Carlini). 0966-9795/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.intermet.2013.10.002

contribution ke and a lattice contribution kL. The efficiency of a thermoelectric generator is governed by a physical law that relates a dimensionless quantity, the figure of merit (ZT), with the above parameters ZT ¼ S2 T=rðke þ kL Þ, where T denote the absolute temperature. Recently, semiconducting intermetallic compounds, belonging to the Zintl’s Phases, have attracted much attention because they have unexpectedly low thermal conductivity, which leads to improved thermoelectric properties. Particularly, the glass-like thermal conductivity of Zn4Sb3, originated from its disordered structure, has made this compound one of the most studied phases in the thermoelectric field [8e13]. This disorder is expressed as a combination of defects and interstitial Zn atoms. At room-temperature, the rhombohedral unit cell of b-Zn4Sb3 contains three distinct atomic positions (36 Zn, 18  Sb1, and 12 Sb2 in space group R3 c) and can be described as consisting of channels formed by the Zn and Sb1 atoms running along the [001] direction [14]. Pair distribution function (PDF) analysis [15] of X-ray and neutron diffraction data have shown that there is local ordering of the Zn interstitials into nanoscale domains. Furthermore, Zn disorder in b-Zn4Sb3 gives rise to polymorphism at low temperatures where increasingly complex and more ordered structures are

R. Carlini et al. / Intermetallics 45 (2014) 60e64

formed. At about 263 K b-Zn4Sb3 changes reversibly to a-Zn4Sb3 [16]. The study performed by Nylén et al. [15] has shown thataZn4Sb3 is an ordered phase with triclinic unit cell (space group C1 ). Despite of low values of b-Zn4Sb3 ZT at room temperature (0.1e 0.2) [8e13], its maximum ZT ¼ 1.3, reached in the temperature range between 450 K and 670 K, makes this intermetallic compound one of the best thermoelectric materials [17]. In literature, the doping with different transition metals and p-block elements on Zn4Sb3 compound has been reported [18e28]. These studies show that doping does not always improve the thermoelectric performance of Zn4Sb3 due to difference of alloying elements or to the synthesis method. A lot of synthetic route were reported in literature: quick quenching and hot pressing process [27,29,30], ball milling and hot-pressing [31], melting in muffle under a continuous rotation [32], zone-melting process [33], solvo-chemical synthesis [34] and other ones [35]. In this research Al and Ag were selected as candidates for the substitution of Zn in the intermetallic b-Zn4Sb3 compound, considering the similarity of their properties (atomic size, electronegativity, etc.) with the Zn ones. The aims of this work are the research of a simple and cheap synthesis route and the study of the effect of the Ag and Al doping on the b-Zn4Sb3 figure of merit. Pure and doped samples were synthesized by a suitable route and investigated by microstructural, structural and thermoelectric characterization. 2. Experimental The intermetallic compounds Zn4Sb3, (Zn1xAlx)4Sb3 and (Zn1xAlx)4Sb3 (x ¼ 0.01) were synthesized through a very simple route minimizing production costs and time needed. Starting from Zinc (flakes by Alfa-Aesar: 99.9 mass%), antimony (rod by Carlo Erba: 99.99 mass%), Aluminum and Silver (pellet by NewMet: 99.99 mass%), small pieces of the stoichiometric quantities of elements were sealed in silica ampoules under Ar flow and heated at 750  C in a muffle furnace, annealed at this temperature for 10 h and spontaneously cooled in a horizontal vial. Scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDXS) and X-ray diffraction analyses were used to examine microstructures and determine phases composition. A scanning electron microscope EVO 40 (Carl Zeiss) was employed, equipped with a Pentafet Link (Oxford Instruments) detector for EDXS analysis. Smooth surfaces for microscopic observation were prepared by using SiC papers and diamond pastes with grain size down to 1 mm. For the quantitative analysis an acceleration voltage of 20 kV was applied; a cobalt standard was used for calibration. The X-ray spectra were processed by the software package Inca Energy (Oxford Instruments). X-ray diffraction analysis was used to identify the phase crystal structures and determine the lattice parameters. The measurements were performed on powdered samples mounted on a zero background Si support, by means of a vertical diffractometer (Philip X’Pert model). X-ray patterns were recorded using the Cu Ka radiation in the 19e100 2q angular range, with a step of 0.03 and a counting time per step of 3 or 4 s. A 40 kV voltage and a 30 mA current were applied to the X-ray tube. Lattice parameters were calculated and refined by a leastsquares routine, and compared with literature values. Data refinement was carried out using Rietveld method by Full Prof Suite software. Measurements of thermoelectric power, thermal conductivity and electrical were carried out in the temperature range from 10 K to 300 K using a commercial PPMS (Physical Properties Measurement System) apparatus by Quantum Design. Thermoelectric and thermal measurements were carried out in a slow temperature sweep (0.5 K/min), in ultra-high vacuum (106 Torr), applying a square-wave heat flow with adjustable period (from

61

400 s to 1450 s) and thermal gradient (from 0.1 K to few K). The electrical measurements were carried out in a temperature sweep as well, by a four-probe method, using Ag paste for electrical contacts. 3. Results and discussion The b-Zn4Sb3 compound belongs to the rombohedral space group, it has 8 Zn interstitial atoms considered as phononic scattering centers and so primarily responsible for the very low thermal conductivity. At 265 K Zn interstitial atoms are arranged according to an ordered structure, causing the reversible transition from the b-Zn4Sb3 phase to the a-Zn4Sb3 one. Rietveld refinement of X-rays patterns of undoped sample, measured at room temperature, was carried out and allowed to obtain lattice parameters, atomic positions and atomic occupancies. The plot of the experimental X-rays pattern and the corresponding Rietveld refinement of pure bZn4Sb3 is reported in Fig. 1. The low values of Bragg R-factor ¼ 6.10 and Rf-factor ¼ 4.52 indicate a good refinement of crystal parameters. In Table 1, the obtained lattice parameters are reported: a good agreement with literature data can be noted [19,29]. Atomic positions, also in good agreement with literature data [29,36], are reported in Table 2. Morphological and compositional data of the Zinc antimonides under study, obtained by EDXS, indicate that all the investigated intermetallic compounds “as cast” can be considered as one-phase. In Fig. 2 this is well evidenced by the Back Scattered Electrons (BSE) microphotograph of (Zn0.99Al0.01)4Sb3, reported as representative of all the samples. Compositional data are collected in Table 3. Good agreement between nominal and measured compositions is observed. Furthermore, a narrow solubility range (1e2 at.%) of the intermetallic phase Zn4Sb3 is consistent with the phase diagram [36]. Despite being prepared in short time and by a cheap synthesis method, all the samples result as one-phase immediately after the preparation. No further annealing is required: a significant abatement in synthesis costs and time is so reached. In Fig. 3 the results of the thermoelectric characterization of the b-Zn4Sb3 sample, as well as of the substituted (Zn0.99Al0.01)4Sb3 and (Zn0.99Ag0.01)4Sb3 ones, are presented. In all samples, the Seebeck coefficient S is positive and vanishes at low temperature because of vanishing entropy. At high temperature, S exhibits a linear temperature dependence, as predicted by the Mott’s formula [37] for the diffusive regime:

S ¼

p2

K 2T 3q b

p2 vlnðsðEÞÞ z Kb2 T vE 3q E¼EF

 vlnðnÞ  vE

E¼EF

vlnðsÞ þ vE

E¼EF

  vln meff  vE

 E¼EF

Fig. 1. Rietveld refinement plot of b-Zn4Sb3.

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Table 1 Lattice parameters of synthesized samples obtained by Rietveld refinement of X-rays patterns. *Referred to(Zn0.98Ag0.02)4Sb3. a (nm)

c (nm)

a (nm)

c (nm)

Ref.

1.2226(3) 1.2217(6) 1.2219(1)

1.2417(2) 1.2412(2) 1.2411(4)

1.2239 1.2213 1.2229*

1.2398 1.2363 1.2419*

[29] [29] [19]

Here Kb is the Boltzmann constant, q the electronic charge with the sign of the carriers, s the spectral conductivity. In the second expression, the electrical conductivity is developed within the free electron approximation for a single band metal, in terms of carrier concentration n, scattering time s and effective mass meff. The positive sign of S indicates that holes are the dominant carrier type in all the samples. From Fig. 3 it appears that the Al substitution has a small effect on the S value, while the Ag substitution significantly suppresses S, which turns out to be 62.66 mV/K at 287 K, as compared to the value 112.4 mV/K of the undoped sample at the same temperature. Around 250 K, a distinct feature is observed in the Seebeck curve of the Zn4Sb3 sample, marking the signature of the structural transition. In the doped samples, such feature is much less evident and shifted to lower temperature. These results are in agreement with literature data on Al substitution [32]. However, in our case Ag substitution seems to have a larger effect in lowering S and blurring the transition feature, as compared to literature data [30]. In Fig. 4 we present the electrical resistivity r curves of the three samples. In all cases a metallic character is observed in the whole temperature range. Similarly to the S curves, the undoped sample exhibits an abrupt jump in correspondence of the structural transition at 250 K, while doping suppresses such feature. With respect to the magnitude of resistivity, in agreement with behavior showed by thermoelectric power, it is seen that Al doping has small effect, whereas Ag doping is much more effective in lowering the resistivity value, which is 1.43$105 ohm m at room temperature, as compared to the value of 2.3$105 ohm m of the undoped sample at the same temperature. From considerations of electron valence, we assume that Ag substitution on the Zn site dopes holes into the system, while Al substitution on the Zn site dopes electrons. Since the undoped compound is dominated by hole transport, as evidenced by the positive sign of the Seebeck effect, Ag substitution adds further holes and thus lowers the resistivity. On the contrary, Al substitution tends to compensate the free charge of the system, thus being a less effective doping. The same argument based on electron valence explains the behavior of the Seebeck coefficient. If we assume that the effect of doping is mainly that of increasing the hole density n in the case of Ag, the Mott relationship actually yields a decrease of S upon Ag doping. On the other hand, Al doping possibly decreases slightly the hole density, thus having a minor effect on the magnitude of S. In the uppermost panel of Fig. 5, the thermal conductivity k of the three samples is plotted as a function of temperature. Its low value, of the order of 1 W/mK at high temperature, confirms that these compounds are promising materials for thermoelectric applications. Moreover, at temperatures below 150 K, the effect of Table 2 Atomic position of Zn4Sb3 obtained by Rietveld refinement. Atom

X

Y

Z

Occupancy

Zn1 Sb1 Sb2 Zn2 Zn3 Zn4

0.08281 0.35592 0.00000 3.77163 1.52427 3.62765

0.24523 0.00000 0.00000 0.95558 1.76389 3.51645

0.40432 0.25000 0.63887 1.63792 1.95313 3.05730

0.93123 0.50000 0.32873 0.03548 0.03561 0.02769

Fig. 2. SEM microphotograph of the (Zn0.99Al0.01)4Sb3 sample at 200 , showing a homogeneous one-phase appearance.

doping, either Ag or Al, is that of further lowering k. In the case of Al doping, the thermal conductivity is smaller than that of the undoped samples in the whole temperature range up to room temperature. Also for thermal conductivity, our data about Al substitution are in agreement with literature [32], while Ag substitution in our samples suppresses thermal conductivity more effectively than in other works of literature [30]. In order to extract additional information from these data, we try to separate the lattice contribution to the thermal conductivity, kL, from the electronic one, ke. The latter, ke, can be evaluated using the WiedemanneFranz law as ke ¼ L0T/s where L0 ¼ 2.44 108 W ohm K2 is the Lorentz number, while the former, kL, can be obtained by difference from the measured thermal conductivity as kL ¼ kke. We remark that the WiedemanneFranz law, which is based on the assumption that the electron mean free path is the same for electrical and thermal transport, is valid only at low temperatures, where the main electron scattering mechanism is elastic scattering by defects and impurities. However, our samples are characterized by a high level of disorder, associated either to Zn interstitials or to chemical substitution. This is evident from the low values of residual resistivity ratios (RRR, defined as the ratio of room Table 3 EDXS compositional data of synthesized samples. Sample

Zn (at.%)

Sb (at.%)

Al (at.%)

Zn4Sb3 (Zn0.99Al0.01)4Sb3 (Zn0.99Ag0.01)4Sb3

55.9 55.7 56.4

44.1 43.7 43.1

0.6

Ag (at.%)

0.5

150

Zn4Sb3 (Zn0.99Al0.01)4Sb3 (Zn0.99Ag0.01)4Sb3

100

S (μV/K)

Sample Zn4Sb3 (Zn0.99Al0.01)4Sb3 (Zn0.99Ag0.01)4Sb3

50

0

0

50

100

150

200

250

300

T (K) Fig. 3. Plot of Seebeck coefficient as a function of temperature of synthesized samples.

R. Carlini et al. / Intermetallics 45 (2014) 60e64

3x10

63

-5

Zn4Sb3

Zn4Sb3

(Zn0.99Al0.01)4Sb3 -5

1x10

-5

ZT

ρ (Ωm)

2x10

0 0

50

100

150

(Zn0.99Al0.01)4Sb3

0.2

(Zn0.99Ag0.01)4Sb3

200

250

300

T (K)

(Zn0.99Ag0.01)4Sb3

0.1

0.0

0

50

100

150

200

250

300

T (K)

Fig. 4. Plot of electrical resistivity as a function of temperature of synthesized samples. Fig. 6. Plot of figure of merit ZT as a function of temperature of synthesized samples.

temperature to low temperature resistivities), ranging from 2 to 4. These low values indicate that elastic scattering of electrons by impurities and defects is still significant even at room temperature. For this reason, the WiedemanneFranz law, even if not strictly applicable in the whole temperature range up to 300 K, may give a rough indication about electron and phonon contribution to the thermal conductivity. The resulting ke and kL are plotted in the middle and bottom panels of Fig. 5, respectively. Clearly, kL is much larger than ke by one order of magnitude for all the samples in the whole temperature range, indicating that all these thermal conductivity curves are dominated by the phonon contribution. Hence we conclude that doping mainly lowers the phonon contribution to the thermal conductivity. The reason for such effect of doping is likely due to

κ (W/mK)

3

Zn4Sb3 (Zn0.99Al0.01)4Sb3

2

(Zn0.99Ag0.01)4Sb3

1

4. Conclusions

0 κ e (W/mK)

0.4

0.2

κ L (W/mK)

0.0 2

1

0

0

structural distortions in substituted samples, which act as phonon scatterers. Indeed, in general, the suppression of the characteristic low temperature peak in the thermal conductivity curve is typical of disordered samples, where phonons are predominantly scattered by defects rather than by other phonons. From the above experimental data, the dimensionless figure of merit ZT can be extracted. Indeed Fig. 6 shows ZT as a function of temperature for the three samples. The ZT values increase with temperature and are of the order of 101 at room temperature, in agreement with literature. More importantly, the Al doped sample exhibits an enhanced ZT value at high temperature, which reaches 0.23 at 260 K, as compared to the undoped sample. This result is related to the lower thermal conductivity of the Al doped sample. The ZT enhancement above the structural transition suggests that room temperature is the most suitable regime for application of these thermoelectrics. The Ag doped sample, although it also has lower thermal conductivity than the undoped one, presents a much lower S, which is critically detrimental to ZT.

50

100

150

200

250

300

T (K) Fig. 5. Plot of thermal conductivity as a function of temperature of synthesized samples (top panel: total thermal conductivity; middle panel: carrier thermal conductivity; bottom panel: lattice thermal conductivity).

The potential advantages on thermoelectric performances of bZn4Sb3 Ag, Al doped are studied in this work. Zn4Sb3, (Zn1xAlx)4Sb3 and (Zn1xAgx)4Sb3 (x ¼ 0.01) intermetallic compounds were synthesized by a very cheap route in terms of energy and time, using a muffle furnace and quick thermal cycles. A comparative analysis of thermoelectric and transport measurements indicates the holes as the dominating charge carriers in b-Zn4Sb3. While Ag substitution dopes further holes into the system, increasing the overall carrier concentration, the Al substitution dopes electrons and compensates the free charge of the system, thus resulting a less effective doping. Indeed, it is found that the Al substitution has a small effect on Seebeck coefficient and resistivity, while Ag substitution significantly suppresses both of them. The measured thermal conductivity of the studied compounds is close to 1 W/mK, a pretty appealing value for thermoelectric applications. Most importantly, the effect of doping, especially Al doping, leads to a further decrease of k, probably due to the introduction of structural distortions that act as phonon scatterers. These thermal conductivity values are primary responsible for the good thermoelectric performances of these samples, whose thermoelectric figure of merit reaches the encouraging value of 0.23 at 260 K. This value, obtained by samples synthesized through a simpler route, is in good agreement with the corresponding data reported in literature for this system.

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