EML-Thermoelectric Properties of Non-Stoichiometric MnTe Compounds

Electronic Materials Letters, Vol. 9, No. 4 (2013), pp. 477-480

Thermoelectric Properties of Non-Stoichiometric MnTe Compounds Bongseo Kim,1,* Inhye Kim,1 Bok-ki Min,1 Minwook Oh,1 Sudong Park,1 and Heewoong Lee1 1Creative and Fundamental Research Division, Korea Electrotechnology Research Institute,

Changwon 642-120, Korea

(received date: 26 October 2012 / accepted date: 9 January 2013 / published date: 10 July 2013) Non-stoichiometric Mn x Te1-x (x = 0.48-0.52) has been prepared by a melt-quench process followed by spark-

plasma-sintering to investigate its thermoelectric properties. Polycrystalline Mn x Te1-x with x > 0.51 has a nearly

single MnTe phase. The measured Seebec k c oeffic ient and elec tric al c onduc tivity show a similar trend, in

which a transition occurs near 473 K with increasing temperature. The thermal conductivity of Mn x Te1-x com-

pounds shows a tendenc y to dec rease with inc reasing Mn c ontent. Along with a low thermal c onduc tivity

and a high power fac tor, the samples with x > 0.51 have a high figure of merit, whic h reac hes 0.41 at

773 K. The results indicate that production of a homogenous MnTe single phase is an effective way to improve

the thermoelec tric properties of p-type non-stoic hiometric Mn x Te1-x c ompounds.

Keywords: MnTe, thermoelectric, figure of merit, Seebeck coefficient, electrical conductivity, thermal conduc-

tivity

1. INTRODUCTION

Chalcogenide MnTe compounds have been applied to

optical and magneto-optical devices.[1-4] MnTe has an NiAs-

type crystal structure with lattice constants of a = 4.146?

and c = 6.709?.[3] The physical properties of MnTe include

a Neel temperature (T N) of 307 K for antiferromagnetic order.[5] MnTe is a p-type semiconductor with a broad band

gap of 1.3 eV.[6,7] The electrical conductivity of MnTe shows

anomalous behavior: it acts as a metallic conductor up to

323K, and displays increased electrical conductivity with

increasing temperature beyond this temperature.[8] Previous

studies on MnTe have focused on electronic structure

calculation,[7,9-12] optical properties,[3,4,13] and neutron dif-

fraction,[5,14] among other aspects.

Researchers have recently doped MnTe to PbTe and GeTe

thermoelectric compounds, which are promising materials

for intermediate temperature applications, in efforts to

improve their thermoelectric performance.[15-18,26] Thermo-

electric materials can directly convert thermal energy into

electric energy in a solid state, and vice versa. Thermo-

electric performance depends on the dimensionless figure of

merit, ZT=α2σ/(κT), where α is the Seebeck coefficient, σthe electrical conductivity, κ the thermal conductivity, and T the absolute temperature.

I n this paper, we present the crystal structure and

thermoelectric properties of non-stoichiometric MnTe com-

pounds in order to elucidate the following thermoelectric parameters: Seebeck coefficient, electrical conductivity, power factor, thermal conductivity, and dimensionless figure of merit.

2. EXPERIMENTAL PROCEDURE

Non-stoichiometric binary MnTe compounds with com-positions of Mn x Te1-x (x = 0.48, 0.49, 0.50, 0.51, and 0.52) were prepared from 5N purity Mn and Te raw materials. The Mn and Te mixtures were sealed in a carbon-coated quartz ampoule purged with Ar gas after 10?3 Torr evacuation. The mixture of raw materials was melted in an induction melting furnace with sufficient homogenization. The melt was water-quenched and then pulverized below 325 mesh size. Pellets with a cylindrical shape were sintered at 963 K for 10 minutes at a pressure of 50 MPa through a spark-plasma-sintering process, which reach 98.5% of the theoretical density of the MnTe compound. The sintered pellets were cut to a certain size in order to evaluate their electrical and thermal properties.

The phase of the MnTe compounds was identified using an x-ray diffractometer with Cu-Kα (XRD, X’pert PRO MPD, PANalytical); the lattice constant and volume fraction of the MnTe phase were calculated by Reitveld refinement. The Seebeck coefficient and the electrical conductivity of the specimens were measured using the 4-point probe method (ZEM-3, Ulvac, Japan); the thermal conductivity was measured using the laser flash method (LFA457, Netzsch, Germany). The density of the specimens was measured using Archimedes’ principle. The figure of merit of the MnTe

DOI: 10.1007/s13391-013-0035-z

*Corresponding author: bskim@keri.re.kr

478 B. S. Kim et al.

electrical conductivity, and the thermal conductivity.

3. RESULTS AND DISCUSSION

Figure 1 shows the XRD peaks with Mn content of the Mn x Te 1-x compounds. With lower Mn content, higher MnTe 2peak intensity was observed. A considerable amount of MnTe 2 phase exists at compositions lower than 0.50 Mn.The Mn 0.51Te 0.49 compound is close to a single MnTe phase,although a minor amount of MnTe 2 exists. Mn 0.52Te 0.48 com-pound is a pure MnTe single phase. The MnTe compound (ICDD: 01-089-2886) has a hexagonal structure, P63/mmc (space group: 194), with lattice constants of a = 4.1475?and c = 6.71?. The MnTe 2 compound (I CDD: 03-065-3340) has a cubic structure, Pa-3 (space group: 205) with a lattice constant of a = 6.951?. The inset in Fig. 1 shows the volume fraction change of the MnTe and MnTe 2 phases with Mn content. The Mn x Te 1-x compounds contain 10% more

MnTe 2 phase at Mn composition below 0.50, 1.7% at 0.51Mn, and no MnTe 2 at 0.52 Mn, as determined through Rietveld refinement. The Mn 0.52Te 0.48 compound has a single MnTe phase.

Figure 2 shows the temperature dependency of the Seebeck coefficient of the Mn x Te 1-x compounds. The Mn x Te 1-x com-pounds that have a positive Seebeck coefficient throughout the temperature range are p -type semiconductors. The Seebeck coefficient is nearly constant at a temperature range below 473 K, but decrease above this temperature. Therefore,the Mn x Te 1-x compounds are degenerate semiconductors.The transition in the Seebeck coefficient in all specimens was investigated and found to be 323 K and 473 K. The Seebeck coefficients at compositions lower than 0.51 Mn at room temperature are 600-700μV/K. The Seebeck coefficient of 0.52 Mn, which is a single MnTe phase, is the lowest in the entire temperature range, 539 μV/K at room temperature.

Figure 3 shows the temperature dependency of the electrical conductivity of the Mn x Te 1-x compounds. The electrical conductivity of the Mn x Te 1-x compounds is nearly constant at temperatures lower than 473 K, but increases exponentially with temperatures above 473 K. The electrical conductivity of all samples is low, with values of 70-200 S/m at room temperature, but increases to 2500-3400 S/m at 773 K. The electrical conductivity of the Mn x Te 1-x compounds is an order of magnitude lower relative to that of other conventional thermoelectric materials such as BiTe and PbTe.[19,20] Therefore, the electrical conductivity of Mn x Te 1-x compounds should be improved to achieve a better figure of merit.

The inset in Fig. 3 shows the variation of the electrical conductivity (log σ) as a function of temperature (103/T ). We can clearly see a transition near 473 K from the relationship

of log(electrical conductivity) and temperature, delineated in

Fig. 1. XRD peaks of Mn x Te 1-x

compound.

Fig. 2. Temperature dependency of Seebeck coefficient of Mn x Te 1-x

compounds.

Fig. 3. Temperature dependency of electrical conductivity of Mn x Te 1-x compounds.

B. S. Kim et al.479

Eq. (1):

(1)

where σ is the electrical conductivity, σo a constant, T the

absolute temperature, k the Boltzmann constant, and W the activation energy.[21] A previous study reported that the transport properties of MnTe compounds show different behavior near 600 K.[22] However, in this study we confirmed that the change of the transport properties of Mn x Te 1-x compounds occurs near 473 K, not 600 K, as shown in Figs.2-3. This appears to be due to a transition from an extrinsic to an intrinsic property of the MnTe compound, because no phase transition has been reported around 473 K in the Mn-Te phase diagram.[23]

Figure 4 shows the temperature dependency of the power factor of the Mn x Te 1-x compounds. The power factor of all samples is relatively unaffected by temperatures below 473 K;

however, it increases exponentially above 473 K. The transition of the power factor near 473 K is due to the transition of the electrical conductivity with temperature.The power factor of Mn 0.51Te 0.49 is the highest, 0.414×10?3W/mK 2 at 773 K.

Figure 5 shows the temperature dependency of the thermal conductivity of the Mn x Te 1-x compounds. The thermal con-ductivity of the Mn x Te 1-x compounds increases with tem-peratures below 323 K, but decreases beyond 323 K. The thermal conductivity tends to decrease with increasing Mn content. The change of the thermal conductivity with tem-perature is similar to the variation of the Seebeck coefficient near 323 K, shown in Fig. 2. The Neel temperature of the MnTe compound is 307 K.[5] This is due to the thermal conductivity change at the Neel temperature.[24,25] The thermal conductivity of Mn 0.52Te 0.48 is 1.15 W/mK at room temperature and 0.67 W/mK at 773 K.

Figure 6 shows the temperature dependency of the dimensionless figure of merit of the Mn x Te 1-x compounds.The figure of merit has s slightly positive slope at tem-peratures below 473 K, but increases exponentially with temperatures above 473 K. This relation of the figure of merit to temperature is similar to those of the electrical conductivity and the power factor, shown in Fig. 3 and Fig.4, respectively. This similarity is due to the change in the power factor, which in turn originates from the drastic change of the Seebeck coefficient and the electrical con-ductivity near 473 K. The figure of merit of Mn 0.51Te 0.49,which is almost entirely single MnTe phase, is 0.41 at 773 K for the non-stoichiometric Mn x Te 1-x compounds.

4. CONCLUSIONS

To investigate the thermoelectric properties, sintered Mn x Te 1-x (x = 0.48, 0.49, 0.50, 0.51, and 0.52) compounds

σσo T ()exp ?W kT -----??

??

=Fig. 4. Temperature dependency of power factor of Mn x Te 1-x com-pounds.

Fig. 5. Temperature dependency of thermal conductivity of Mn x Te 1-x

compound.

Fig. 6. Temperature dependency of dimensionless figure of merit of Mn x Te 1-x compound.

480 B. S. Kim et al.

were prepared. The Seebeck coefficient of all specimens is positive in the entire temperature range, which means p-type semiconductor. The transition in the Seebeck coefficient and electrical conductivity with temperature occurs near 473 K. The electrical conductivity of all specimens is lower than 4×103S/m. The thermal conductivity of Mn x Te1-x decreases with increasing Mn content. The ZT of Mn0.51Te0.49 reaches 0.41 at 773 K.

ACKNOWLEDGEMENTS

This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. REFERENCES

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