Properties of Ag-doped Bi-Sb alloys as thermoelectric conversion materials for solid state refrigeration

Abstract

The energy conversion properties of Bi-Sb system thermoelectric materials doped by Ag was investigated. Bi₈₅Sb_15-xAg_x (x=0, 1, 2, 3, 4) alloys with Ag substitution for Sb were synthesized by mechanical alloying and then pressed under 5 GPa at 523 K for 30 min. The phase structure of the alloys was characterized by X-ray diffraction. The electric conductivities and the seebeck coefficients were measured at the temperature range of 80-300 K. The results reveal that the electric conductivities of the Ag-doped Bi-Sb alloys are highly improved. The power factor of Bi₈₅Sb₁₄Ag₁ reaches a maximum value of 2.98×10^-3 W/(K²•m) at 255 K, which is about three times that of the un-doped sample Bi₈₅Sb₁₅ at the same temperature.

Keywords

thermoelectric conversion materials / high-pressure sintering / thermoelectric properties

Introduction

Thermoelectric refrigeration is a kind of solid-state cooling. The development of low temperature (T<200 K) thermoelectric refrigeration is of great importance in the energy industry. If thermoelectric refrigeration can provide efficient local cooling at temperatures below 200 K, it will exert great influence on the refrigeration industry, since the performances of many semiconductors and other electric devices will be dramatically enhanced below room temperature. Compared with traditional mechanical refrigeration device, a thermoelectric refrigeration device is small, durable but noise and pollution free. Therefore, much attention has been paid to the study of thermoelectric refrigeration world wide [1]. However, the low thermoelectric conversion efficiency of thermoelectric refrigeration limits its practical applications. The conversion efficiency is dependent on the properties of the thermoelectric materials which can be determined by the figure of merit Z,

(1)Z=\frac{{\alpha }^{\mathrm{2}}\sigma }{\kappa },

where α,θ and κ are the electrical conductivity, the Seebeck coefficient and the thermal conductivity of the thermoelectric material, respectively. Z has the dimension of an inverse temperature, K^-1. Multiplied by the absolute temperature T, the efficiency can also be determined by the dimensionless figure of merit ZT. The value of α²σ is defined as the power factor P=α²σ. The electrical performance of a thermoelectric material is determined by the power factor P, which is dependent on the electron effective mass, carrier concentration, electronic structure and scattering rates near Fermi surface. A good thermoelectric material must have both large power factor and low thermal conductivity.

At low temperature, single crystals of the Bi-Sb alloys exhibit the best thermoelectric performances [2]. Both Bi and Sb are semimetals that exhibit a similar rhombohedral crystal structure of point group R\overline{\mathrm{3}}m, thus Bi_100-xSb_x alloys form a solid solution over the entire composition range [3]. The Bi_100-xSb_x alloys conserve the semimetallic character when x≤7 or x≥22 but become n-type semiconductor when 7<x<22 [4]. The Z value of single crystals of the Bi₈₅Sb₁₅ alloys reaches 6.5×10^-3 K^-1 at 80 K and even higher at about 100 K under a magnetic field. However, it is very difficult to produce good homogenous single crystals of Bi_100-xSb_x alloys and the brittleness of the single crystals is another problem in practical devices. To solve these problems, many synthetic methods of fine-grained polycrystalline materials have been studied, such as mechanical alloying, spark plasma sintering, quenching and annealing and high-pressure sintering. High-pressure sintering is a novel method to synthesize high relative density materials. It is predicted that the Seebeck coefficients could be improved due to electronic topological transition under high pressure [5]. There has been a report of an estimated ZT>2 at 300 K in Bi-Sb-Te alloy under a hydrostatic pressure of 2 GPa [6]. However, the Z value of poly-crystalline Bi-Sb alloy is lower than that of a single crystalline alloy. Doping can improve the thermoelectric property by controlling the carrier concentration. Previously, improved thermoelectric properties in Bi₈₅Sb₁₅ alloys prepared by high-pressure sintering were demonstrated [7]. However, no report on doped Bi₈₅Sb₁₅ alloys prepared by high-pressure sintering has ever been found.

In this paper, the low temperature thermoelectric properties of Bi₈₅Sb_15-xAg_x (x=0, 1, 2, 3, 4) alloys are investigated after being prepared by mechanical alloying and then pressed under 5 GPa at 523 K for 30 min. The electrical conductivities and the Seebeck coefficients were measured at the temperature range of 80-300 K, respectively. The result shows that the electrical properties of Ag-doped Bi₈₅Sb₁₅ alloys prepared by high-pressure sintering are improved greatly. The thermoelectric properties are optimized and a maximum power factor of 2.99×10^-3 W/(m•K²) is obtained at 270 K for Bi₈₅Sb₁₄Ag₁ compound.

Experiment

Preparation of materials

The elemental powders of bismuth (99.999%, powder), antimony (99.999%, powder) and silver (99.9%, powder) were weighted with general formula of Bi₈₅Sb_15-xAg_x (x=0, 1, 2, 3, 4). Then the mixture was subjected to MA in a planetary ball mill (QM-BP) using carnelian jars (250 cm³) and balls. The ball-to-powder weight ratio was 20∶1. The jars were evacuated to a residual pressure below 2×10^-3 Pa and hermetically sealed under an argon atmosphere to prevent possible oxidation. The milling speed and the time were 400 r/min and 100 h, respectively. The milled powders were preformed under a pressure of 250 MPa and wrapped using Tantalum foil at room temperature. The preformed samples were finally pressed under 5 GPa at 523 K for 30 min by a cubic-anvil high-pressure apparatus. Figure 1 shows the diagram of the cubic-anvil high-pressure apparatus. The sample cell was made by pyrophyllite and cubic in shape. The pyrophyllite cube was divided into two equal parts with graphite heat pipe inside. NaCl was used as the internal pressure medium.

Measurement methods

The electric conductivities and the Seebeck coefficients were measured at the temperature range of 80-300 K. The electric conductivities were characterized using the standard four probe method [8]. The Seebeck coefficients were measured by a DC method. Long rectangular-shaped samples were used for the measurement by applying a temperature difference of 5 K between two ends. The phase structures of the samples were investigated by X-ray diffraction (XRD) at room temperature with a Rigaku D/max-RB diffractometer using CuK_α radiation (λ=0.154 056 nm). The fractured surfaces of the samples were examined by scanning electron microscope (SEM).

Results and discussion

X-ray diffractions and SEM micrographs

The XRD patterns of the ternary Bi₈₅Sb_15-xAg_x (x=0, 1, 2, 3, 4) alloys are presented in Fig. 2. The XRD patterns show that the Ag-doped alloys have the same rhombohedral lattice structure as the reference sample Bi₈₅Sb₁₅ (space group R\overline{\mathrm{3}}m). There are no visible diffraction peaks of Ag in the XRD patterns. On the other hand, several small peaks of the Ag₃Bi phases are detected in the patterns of Bi₈₅Sb_15-xAg_x (x=1, 2, 3, 4) alloys. The relative intensities of these diffraction peaks increase with the increase of Ag, indicating that the content of Ag₃Bi increases with molar fraction x.

Figure 3 shows the SEM micrographs of three bulk samples, Bi₈₅Sb₁₅, Bi₈₅Sb₁₄Ag₁ and Bi₈₅Sb₁₁Ag₄. The fracture surfaces of the samples demonstrate a compact layered structure. Some pores are observed in the Bi₈₅Sb₁₅ and Bi₈₅Sb₁₁Ag₄ samples. The high density of the samples obtained by high-pressure sintering may be helpful in decreasing the electrical resistivity of the alloys and further improve its transport properties.

Thermoelectric properties

The temperature dependence of the electric conductivities within the 80-300 K temperature range is shown in Fig. 4. All the electric conductivities increase with increasing temperature in the whole temperature range. The electric conductivities of the Ag doped Bi₈₅Sb₁₅ alloys are highly improved compared with the reference sample Bi₈₅Sb₁₅. The Bi₈₅Sb_15-xAg_x (x=3) alloy exhibits a relatively larger value, approximately 3.7 times that of the reference sample Bi₈₅Sb₁₅ at 200 K. The electric conductivities can be expressed as σ = n e μ, where n, and μ are the free-carrier concentration and the carrier mobility, respectively. As shown in Fig. 2, the crystallinity of Ag₃Bi is obviously increased with the increase of Ag content. The increased formation of Ag₃Bi in the alloys might significantly increase the carrier concentration, improving the electrical conductivities, but it may also disorder the lattice structures of the Bi-Sb based alloys, causing a decrease in carrier mobility. These two competing factors, n, and μ, influence the electrical conductivity simultaneously. Therefore, the σ values change with the increase of molar fraction x.

Figure 5 shows the temperature dependence of the Seebeck coefficients of the Bi₈₅Sb_15-xAg_x alloys (x=0, 1, 2, 3, 4). The Seebeck coefficients are all negative across the whole temperature range, indicating that all the alloys are n-type semiconductors. All the absolute values of Seebeck coefficients increase with increasing temperature at first, and then decrease with further increasing temperature. The absolute values of Seebeck coefficients of Bi₈₅Sb_15-xAg_x alloys (x=1, 2) are larger than that of the reference sample Bi₈₅Sb₁₅ in the whole measurement temperature range. As one of the most important thermoelectric parameters, the Seebeck coefficient at a given temperature can be expressed simply as [9]

(2)\alpha \approx \gamma -\ln n,

where γ is the scattering factor which may increase due to grain boundary scattering, defect scattering and charge carrier scattering, etc. The increase of the scattering factor γ can increase the Seebeck coefficient. Moreover, the increase of the carrier concentration n can decrease the Seebeck coefficient. The two factors γ and n affect the Seebeck coefficient in quite a different way. The Bi₈₅Sb_15-xAg_x (x=2) alloy has the largest absolute value of the Seebeck coefficient (128 μV/K at about 135 K) of all the samples, which is about 11% larger than the reference sample Bi₈₅Sb₁₅. The result may be attributed to the existence of Ag₃Bi, which can increase the free-carrier concentration and disorder the lattice structures, that were important in affecting the Seebeck coefficients of the Bi₈₅Sb_15-xAg_x (x=1, 2, 3, 4) alloys. The decrease of the Seebeck coefficients of the Bi₈₅Sb_15-xAg_x (x=3, 4) samples may be due to the increased carrier concentration n introduced by the increased formation of Ag₃Bi in the alloys.

With the above values of the electric conductivities and the Seebeck coefficients, the power factor was calculated according to equation P=α²σ and shown in Fig. 6. It is obvious that the Ag-doped samples show larger power factor values than that of the un-doped sample, Bi₈₅Sb₁₅. The maximum power factor is 2.98×10^-3 W/(m•K²) at 255 K for Bi₈₅Sb₁₄Ag₁ alloy, which is about three times that of the reference sample Bi₈₅Sb₁₅ at the same temperature. The improved thermoelectric power factor was attributed to the optimal combination of the electric conductivity and the Seebeck coefficient, which were due to the optimized Ag content.

Conclusions

Bi₈₅Sb_15-xAg_x (x=0, 1, 2, 3, 4) alloys were prepared by mechanical alloying and then pressed under 5 GPa at 523 K for 30 min. The electric conductivities and the Seebeck coefficients were evaluated at the temperature range of 80-300 K. The power factors for various alloys were calculated. The Ag-doped Bi-Sb based alloys synthesized by high-pressure sintering exhibited larger power factor than the reference sample Bi₈₅Sb₁₅. The power factor of Bi₈₅Sb₁₄Ag₁ alloy reached a maximum value of 2.98×10^-3 W/(m•K²) at 255 K, which was about three times that of the reference sample Bi₈₅Sb₁₅ at the same temperature.

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