High Zt Enhancement In Ag Te2

Published Date: 02 Nov 2017

The rousing worldwide concern over our reliance on fossil fuels is driving the need for alternative energy sources and novel energy conversion techniques, among which the thermoelectric (TE) technique has several special features such as its all solid-state assembly without moving parts, comfort of switching between the power generation mode (based on the Seebeck effect) and the refrigeration mode (based on the Peltier effect), low cost maintenance and its capability to easily couple to other energy conversion devices. Thermoelectric effects associate with direct conversion between thermal and electrical energy by employing electrons and holes as energy carriers. Such effects are potentially useful for waste heat recovery and environmentally friendly refrigeration.1,2 The performance of TE devices is assessed by the dimensionless figure of merit (ZT), defined as ZT = α2σT/, where α, σ, T and  are the Seebeck coefficient, the electrical conductivity, the absolute temperature and the thermal conductivity, respectively. Since α, σ, and the electronic contribution to  are involve via band structures (energy gap Eg, effective mass carrier m*, etc.) and scattering mechanisms, it is difficult to control the parameters independently.1 Therefore, ZT1 has been considered as a benchmark for many thermoelectric materials for a long time. Based on the above relation, the best performance TE materials should have high electrical conductivity, large Seebeck coefficient and low thermal conductivity.1

AgSbTe2 has been known as a promising thermoelectric material over the medium temperature range from 300 K to 700 K3-11 due to its relatively low thermal conductivity (0.6 Wm-1K-1 ~ 0.7 Wm-1K-1) and large Seebeck coefficient (~ 200 µVK-1).12,13 AgSbTe2 is widely identified as the disordered NaCl type (Fm-3m) where Ag and Sb randomly occupy the Na site.14 According to the previous studies,11 the disordered lattice structure dominantly contribution to low lattice thermal conductivity through Umklapp and intrinsic phonon-phonon scattering processes without any reduction in the electrical conductivity. Recently, the AgSbTe2 compound has attracted much attention to construct the so called bulk nanostructured TE materials with excellent TE properties,15-20 such as (AgSbTe2)1−x(GeTe)x (also called TAGS) and (AgSbTe2)1−x(PbTe)x (also called LAST).

Doping is a possible approach to optimize the thermoelectric properties of p-type AgSbTe2 by reducing its thermal conductivity and adjusting its carrier concentration. In this study, trivalent In3+ ions were selected to substitute Sb3+ ions in a p-type Ag(Sb1-xInx)Te2 systems inorder to suppress lattice thermal conductivity dramatically while simultaneously contributing to the total charge-carrier concentration. Meanwhile, to the best of our knowledge, In-substitution in p-type AgSbTe2 has not been reported so far. The influence of In doping on the thermodynamic properties, microstructure and TE transport behavior is investigated systematically.

Polycrystalline AgInxSb1-xTe2 compounds with x = 0, 0.03 and 0.05 were prepared by solid state reaction. High-purity starting elements of Ag (99.995%, filament), Sb (99.9999%, shot), In (99.999%, shot) and Te (99.999%, shot), were melted in carbon coated quartz tubes with diameter of 1.6 cm under the vacuum at 800 °C for 6 h, then slowly cooled to 475 °C, followed by a rapid quenched in water. The obtained ingots were pulverized to powder, the mixed powder then sealed in quartz tubes under a vacuum of about 10-4 Torr after multiple argon gas purging cycles, pretreated at 500 °C for 18 h in a box furnace, and furnace cooled to room temperature. Crystals were grown with a vertical Bridgman furnace starting from the pretreated powder and vacuum sealed in carbon coated quartz tubes of 10 cm length and 1.6 cm in diameter. The temperature profile of the Bridgman furnace used for the whole series was maintained at 450–700 °C within 25 cm region. Initial complete melting was achieved at 700 °C for 24 h to ensure complete reaction and mixing. The temperature gradient of 1°Ccm-1 was programed around the solidification point near 555 °C, and the quartz tube was then slowly lowered into the cooling zone at a rate of ∼0.5 mmh-1. Single phase and highly dense ingots were obtained with dark silvery metallic shine. These ingots were stable in water and air. The obtained crystalline ingots were cut and polished into rectangular shapes of approximately 3x3x12 mm3, circular discs of a diameter of 12 mm and a thickness of 12 mm for later physical properties measurements. The density of the ingots was measured by the Archimedes method and varied from 7.11∼7.12 gcm-3, which means that the relative densities of the obtained samples are more than 99.9% of the theoretical density.

X-ray diffraction experiments were conducted for phase identifications using a powder X-ray diffractometer (X’Pert PRO-PANalytical, CuKα radiation) at 2 angles of 20-80°. The lattice parameters of AgInxSb1-xTe2 (x = 0, 0.03 and 0.05) were obtained from least square fit of data in the range of 2 between 10° and 80° with the aid of a Rietveld refinement program. Fractured surface morphology was characterized with a field emission scanning electron microscopy (FESEM, Hitachi, S-4800). The chemical composition of the as-prepared ingots was determined by wavelength dispersive X-ray fluorescence spectrometry (WD-XRF). The Hall Effect was measured at room temperature under 0.55T with a four probe configuration using the van der Pauw method (ECOPIA HMS-5000). The electrical conductivity σ and Seebeck coefficient α were measured simultaneously by commercial equipment (ZEM-3, ULVAC-RIKO, Japan) in He atmosphere from 300 to 700 K. Thermal conductivity κ was calculated using the equation κ = DCpd from the thermal diffusivity D obtained by a laser flash apparatus (NETZSCH, LFA 457), specific heat Cp determined by a differential scanning calorimeter (NETZSCH, STA 449), and the density d obtained by the Archimedes method. FIG. 1. (Color online) (a) Powder XRD patterns of Ag(Sb1-xInx)Te2 samples. The inset of (a) shows lattice parameter of Ag(Sb1-xInx)Te2 and (b) SEM photographs of the bulk AgIn0.03Sb0.97Te2. The inset of (b) shows its magnified portion.

The samples of Ag(Sb1-xInx)Te2 were obtained as crystalline ingots which we then cut and polished for the transport properties presented here. Fig. 1(a) represents powder X-ray diffraction (XRD) patterns of samples with composition Ag(Sb1-xInx)Te2 (x = 0, 0.03, and 0.05). All diffractions of the matrix phase can be indexed into the face centered cubic (fcc) AgSbTe2 structure (reference code: 01-089-3671). For undoped sample, weak diffraction peaks due to the Sb7Te impurity that are often reported in the literature4 are detected. This indicates that the undoped sample consists of the major phase AgSbTe2 and the precipitated Sb7Te. However, as the substitution of In for Sb increases from x = 0 to x = 0.05, the weak diffraction peaks due to the impurity become weaker and disappear altogether in the background [shown in Fig. 1(a)]. It suggests that, upon substituting Sb with In, the tendency to form impurity phases in AgSbTe2 is completely suppressed. The lattice parameter as a function of In fraction is displayed in [inset of Fig. 1(a)]. As shown in inset of Fig. 1(a), the lattice parameter increases with increasing x value, as expected based on the difference between the ionic radii of In (81 pm) and Sb (76 pm). The linear dependence of the lattice parameter versus x indicates that In is substituting Sb in the crystal lattice. Fig. 1(b) shows the FESEM photograph of the free fracture surface of the AgIn0.03Sb0.97Te2 sample. The magnified photo [inset of Fig. 1(b)] shows that nanoprecipitate with a feature size of few hundred nanometers are distributed at grain boundary.

TABLE I. Carrier concentration n, Hall mobility µH, electrical conductivity σ, Seebeck coefficient α and measured composition of all samples at room temperature

Nominal composition

Measured composition

n ( 1019cm-3)

µH

(cm2V-1s-1)

σ

104 (Sm-1)

α (µVK-1)

AgSbTe2

Ag0.98Sb1.016Te2

4.5

27.62

2

229

AgIn0.03Sb0.97Te2

AgIn0.023Sb0.98Te2

8.7

22.64

3.17

193

AgIn0.05Sb0.95Te2

AgIn0.042Sb0.93Te2

6.3

22.15

2.6

211.7

The average compositions of the AgInxSb1-xTe2 series obtained from wavelength dispersive X-ray fluorescence analysis were consistent with the nominal compositions (Table I).

Fig. 2 shows the temperature dependences of the thermoelectric properties of Ag (InxSb1-x) Te2 samples. As expected, the sample with higher electrical conductivity has a lower Seebeck coefficient, and the trend of α versus T curve is basically consistent with that of the σ versus T curve are shown in Figs. 2(a) and 2(b), respectively. The temperature dependence of the electrical conductivity can be explained by the decreasing carrier concentration with increasing In content in these compounds. The sample with x = 0.03 has the highest electrical conductivity among all the samples and the electrical conductivity has a room temperature value of about 3.17 x 104 Sm-1. Table 1 shows the properties used to describe the electron transport characteristics of In doped Ag(InxSb1-x)Te2 compounds at room temperature. Compared with undoped sample, the In-AST samples display high carrier concentration and lower mobility. The Seebeck coefficients of all the specimens are positive, as shown in Fig. 2(b), indicating the p-type conduction. At room temperature, the Seebeck coefficients (α) for In-AST samples span from (190 ~ 215) µVK-1, slightly lower than that of the undoped AgSbTe2 sample. The increased electrical conductivity and the decreased Seebeck coefficient of the sample with x = 0.03 can be ascribed to the increased carrier concentration due to the In-doping effect, which is similar to that in the Se-doped AgSbTe2 sample11. The power factor P.F = α2σ, curves are plotted in Fig. 2 (c). The power factor initially increases, reaches a maximum and then decreases. The sample with x = 0.03 has the highest power factor (P.F) among the three samples. A maximum P.F value about 1.42 x 10-3 Wm-1K-2 is attained in the sample with x = 0.03 at 540 K.

FIG. 2. (Color online) Temperature dependences of (a) Electrical conductivity, (b) Seebeck coefficient and (c) power factor for samples of Ag(Sb1-xInx)Te2 (x = 0, 0.03, and 0.05).

Fig. 3(a) shows the temperature dependence of the total thermal conductivity of the AgInxSb1-xTe2 samples. For all samples the  first decreases and then increase with the increasing temperature and the magnitude spans the range from 0.65 to 0.9 Wm-1K-1. The thermal conductivity of the sample with x = 0.03 exhibits lowest thermal conductivity than all other samples with the minimum  0.66 Wm-1K-1 over the temperature range from 550 to 600 K. In general, the thermal conductivities of In-AST samples shows lower than that of undoped AgSbTe2 sample. Near room temperature, bulk of the heat is conducted by long-wavelength acoustic phonons3 and the rapid increase in thermal conductivity at high temperature may be related to an enhancement in ambipolar thermal conductivity.21 Using the Wiedmann–Franz law, e = LσT, the lattice thermal conductivity L can be estimated by subtracting the electronic thermal conductivity e from the total thermal conductivity, L =  - LσT, where L is the Lorentz constant. We take L = 0.7L0 (L0 = π2/3(kB/e)2 = 2.45x10-8 V2K-2) for a degenerate semiconductor.21 As shown in Fig. 3(b), the kL of In-AST samples increases with increasing In substitution fraction. The sample with x = 0.03 exhibits lowest lattice thermal conductivity (~ 0.42 Wm-1K-1) than undoped AgSbTe2 (~ 0.59 Wm-1K-1) at 650 K, closing to the minimum theoretical thermal conductivity (~ 0.3 Wm-1K-1) calculated from formulas reported by cahill et al.22 The reason for the very low lattice thermal conductivity in AgBi0.05Sb0.95Te2 may be the nanoprecipitates with a feature size of few hundred nanometers [as shown in Fig. 1(b)] that are effective in scattering the phonons with mid-to-long mean free paths, as suggested in Ref.23-25 The dimensionless thermoelectric figure of merit ZT is calculated based on the measured values of σ, α and  by using the equation ZT =α2σT/. Fig. 3(c) shows the temperature dependence of ZT of all AgInxSb1-xTe2 (x = 0, 0.03 and 0.05) compounds. While at room temperatures the values of ZT of all samples are comparable, at elevated temperatures the benefit of In doping is more clearly demonstrated. The highest figure of merit is observed for AgIn0.03Sb0.97Te2 where it reaches ZT = 1.33 at 650 K because of the relative higher power factor and lower thermal conductivity among all samples. This value is 50% higher than that of the undoped AgSbTe2 at the same temperature.

In summary, the TE properties of In-doped AgInxSb1-xTe2 compounds have been investigated. XRD analysis indicates that single phase material crystallizing in a cubic NaCl- type structure in In-doped AgSbTe2 samples. The lattice thermal conductivities were greatly reduced by enhanced phonon scattering, and the power factors were enhanced due to increase in electrical conductivity and the moderate decrease in Seebeck coefficient. The best TE performance is achieved for the sample with x = 0.03, since the substitution of In for Sb leads to the increased carrier concentration and the enhanced phonon scattering. A maximum ZT value of 1.33 is obtained at 650 K for the sample with x = 0.03, this value is 50% higher than that of the undoped AgSbTe2 at the same temperature. These results indicate that slight doping with In on Sb site is an effective way to enhance the thermoelectric performance of p-type AgSbTe2.

FIG. 3. (Color online) Temperature dependences of (a) total thermal conductivity, (b) lattice thermal conductivity and (c) thermoelectric figure of merit ZT for samples of Ag(Sb1-xInx)Te2 (x = 0, 0.03, and 0.05).

This work was supported by Academia Sinica and National Science Council, Taiwan, Republic of China, under Grant No. NSC100-2112-M-001-019-MY3.