Thermoelectric Properties of Alumina-Doped Bi_0.4Sb_1.6Te₃ Nanocomposites Prepared through Mechanical Alloying and Vacuum Hot Pressing

Abstract

In this study, γ-Al₂O₃ particles were dispersed in p-type Bi_0.4Sb_1.6Te₃ through mechanical alloying to form γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ composite powders. The composite powders were consolidated using vacuum hot pressing to produce nano- and microstructured composites. Thermoelectric (TE) measurements indicated that adding an optimal amount of γ-Al₂O₃ nanoparticles improves the TE performance of the fabricated composites. High TE performances with figure of merit (ZT) values as high as 1.22 and 1.21 were achieved at 373 and 398 K for samples containing 1 and 3 wt % γ-Al₂O₃ nanoparticles, respectively. These ZT values are higher than those of monolithic Bi_0.4Sb_1.6Te₃ samples. The ZT values of the fabricated samples at 298–423 K are 1.0–1.22; these ZT characteristics make γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ composites suitable for power generation applications because no other material with a similarly high ZT value has been reported at this temperature range. The achieved high ZT value may be attributable to the unique nano- and microstructures in which γ-Al₂O₃ nanoparticles are dispersed among the grain boundary or in the matrix grain, as revealed by high-resolution transmission electron microscopy. The dispersed γ-Al₂O₃ nanoparticles thus increase phonon scattering sites and reduce thermal conductivity. The results indicated that the nano- and microstructured γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ alloy can serve as a high-performance material for application in TE devices.

1. Introduction

Thermoelectric (TE) materials directly convert thermal energy into electrical energy and vice versa and are considered clean energy converters [1]. For practical applications, the conversion efficiency of TE materials is often characterized according to a TE figure of merit, ZT, which is a dimensionless parameter and is conventionally defined as [1]:

ZT = (α²σ/κ)T

(1)

where Z, α, σ, κ, and T are figure of merit, Seebeck coefficient, electrical conductivity, thermal conductivity and T absolute temperature, respectively. A high ZT indicates high energy conversion efficiency. Clearly, an efficient TE material with high ZT requires high α, high σ, and low κ. However, satisfying these criteria in a single crystalline bulk material is difficult because these three parameters are interrelated. An increase in α normally implies a decrease in σ (because of carrier density) and an increase in σ implies an increase in the electronic contribution to κ (i.e., the Wiedemann-Franz law); hence, increasing ZT in typical TE materials is extremely difficult. Therefore, materials with ZT higher than that of conventional materials are necessary in industry, and TE conversion efficiency can and must be enhanced by increasing or maintaining the Seebeck coefficient and electrical conductivity and reducing thermal conductivity. However, material classes that contain effective TE properties are rare [2,3,4,5,6].

Over the past 30 years, alloys based on Bi₂Te₃ compounds have been extensively studied and optimized for their use as TE materials. Recently, numerous attempts have been made to increase the ZT of Bi₂Te₃-based TE materials [7,8,9,10,11,12,13,14]. An effective method is to increase the electrical conductivity and reduce the lattice thermal conductivity of TE materials by alloying, doping, or introducing complex crystal structures. Through such approaches, several Bi₂Te₃-based powders, such as CNTs/Bi_0.4Sb_1.6Te₃, C₆₀/(Bi,Sb)₂Te₃, BN/Bi_0.4Sb_1.6Te₃, WO₃/Bi_0.4Sb_1.6Te₃, and PbTe/(Bi,Sb)₂Te₃ [15,16,17,18], with various types of particles were consolidated into bulk shapes by using different consolidation methods. The results indicated that the thermal conductivity of Bi_0.4Sb_1.6Te₃ can be decreased by adding CNTs and C₆₀ particles, which eventually increases ZT [15,16]. The thermal conductivity of BN/Bi_0.4Sb_1.6Te₃ and WO₃/Bi_0.4Sb_1.6Te₃ reduces slightly from 1.5 to 1.2 W m⁻¹K⁻¹ when the volume fraction of BN and WO₃ is increased from 0 to 7 vol %. However, ZT decreases because adding BN and WO₃ considerably deteriorates the electrical conductivity [17]. Improving ZT of bulk PbTe/(Bi,Sb)₂Te₃ samples was unsuccessful because adding PbTe particles drastically reduces the Seebeck coefficient [18]. From the aforementioned results, enhancing ZT clearly strongly depends on the optimal addition of second phase particles to Bi₂Te₃-based alloys.

Recently, Kim et al. reported the high ZT value of 1.5 was obtained at 323 K for p-type Bi_0.5Sb_1.5Te₃ alloy after doping 0.3 vol % α-Al₂O₃ nanoparticles [19]. Li et al. also found that with the introduction of the 1.0 vol % γ-Al₂O₃ particles into n-type Bi₂Se_0.3Te_2.7 thermoelectric alloy, Bi₂Se_0.3Te_2.7 exhibits the highest ZT value of 0.99 at about 400 K, being 35% improvement compared to the monolithic Bi₂Se_0.3Te_2.7 alloy [20]. These results indicated the thermoelectric properties of both p-type and n-type bismuth-antimony-tellurium alloys can be improved with addition of Al₂O₃ particles. However, literature survey indicates the enhancement of p-type Bi_0.4Sb_1.6Te₃ alloy with the addition of γ-Al₂O₃ particles has never been reported. Therefore, the γ-Al₂O₃ and Bi_0.4Sb_1.6Te₃ were chosen in present study and the fabrication of γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ composites were performed by mechanical alloying (MA) and vacuum hot pressing. The detailed microstructure and TE properties of samples with varying γ-Al₂O₃ content were investigated. The results showed that the ZT values of the Bi_0.4Sb_1.6Te₃ alloy can be enhanced through the optimal addition of γ-Al₂O₃ particles.

2. Experimental Procedure

Bi_0.4Sb_1.6Te₃ materials with varying contents of γ-Al₂O₃ powder were prepared using a high-energy shaker ball mill installed inside an Ar-purified glove box in which the oxygen and moisture contents in an argon atmosphere were maintained at less than 1 ppm. To prepare Al₂O₃/Bi_0.4Sb_1.6Te₃ powders, a mixture of the elemental metallic powders Bi (99.999%), Sb (99.999%), Te (99.999%), and γ-Al₂O₃ (approximately 99%, with particle size ranging from 0.2 to 7.5 μm with a mean particle size (d(0.5)) of approximately 1.39 μm) was mechanically alloyed using an SPEX 8016 shaker ball mill. The duration of the overall milling process was 2 h. The as-milled γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ composite powders were consolidated in a vacuum hot pressing machine to prepare γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ disks with diameter and thickness of 17 and 10 mm, respectively. Vacuum hot pressing was performed at 573 K under a pressure of 0.7 GPa for 30 min. The as-milled powders and hot-pressed composite disks were examined using X-ray diffraction (XRD), differential scanning calorimetry, scanning electron microscope (SEM), and transmission electron microscopy (TEM). The TE properties were measured in the direction parallel to the hot-pressed direction. The hot-pressed bulk samples were then cut and polished into 8 × 6 × 6 mm bars. The thermoelectric properties of the hot-pressed samples were investigated using ALTEC-10001 (ITE, Ukraine). This equipment can simultaneously measure the Seebeck coefficient (α), electrical resistivity (σ), and thermal conductivity (κ) of thermoelectric materials from room temperature to 500 °C. The measurement is performed automatically, as well as the analysis of the measurements results, which excludes errors in operators work. ZT was calculated according to Equation (1).

3. Results and Discussion

Figure 1 shows the XRD patterns of the Bi_0.4Sb_1.6Te₃ composite samples with 1 and 3 wt % γ-Al₂O₃ additions after 2 h of milling. The diffraction peaks cited from the database of the (Bi_0.2Sb_0.8)₂Te₃ (JCPDS 072-1836) were also plotted with vertical lines in Figure 1 for comparison. All diffraction peak positions and (hkl) values were highly consistent with the standard diffraction data of the pure (Bi_0.2Sb_0.8)₂Te₃ phase (JCPDS 072-1836), implying that the (Bi_0.2Sb_0.8)₂Te₃ phase can be successfully prepared through high-energy ball milling of γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ composite powders. However, as seen in Figure 1, the Bragg peaks of γ-Al₂O₃ are barely detectable in the XRD patterns of the composite powders of the alloy mixed with γ-Al₂O₃ particles after 2 h of milling, which may be attributable to the low volume fraction of γ-Al₂O₃ particles and their small crystalline size. Similar to the observations regarding the preparation of Al₂O₃/NiAl intermetallic–matrix composite in this study, Lin et al. [21] reported that for 5 vol % Al₂O₃ additions in mechanically alloyed NiAl alloys, no Al₂O₃ phase could be detected using XRD after 10 h of milling.

Figure 1. XRD patterns of as-milled Bi_0.4Sb_1.6Te₃ and γ-Al₂O₃-doped Bi_0.4Sb_1.6Te₃ powders. (a) 1 wt % Al₂O₃; (b) 3 wt % Al₂O₃.

Figure 1. XRD patterns of as-milled Bi_0.4Sb_1.6Te₃ and γ-Al₂O₃-doped Bi_0.4Sb_1.6Te₃ powders. (a) 1 wt % Al₂O₃; (b) 3 wt % Al₂O₃.

The Bi_0.4Sb_1.6Te₃ composite powders were subsequently consolidated into disks using vacuum hot pressing process; the corresponding XRD patterns are shown in Figure 2. All reflection peaks are attributable to the (Bi_0.2Sb_0.8)₂Te₃ phase. Compared with the as-milled composite powders, the peaks of the consolidated samples are narrow because of strain relaxation and grain growth in the Bi_0.4Sb_1.6Te₃ nanograin powders. SEM was used to examine the cross-sectional view of γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ disks after vacuum hot pressing (Figure 3). Although several γ-Al₂O₃ nanoparticles tend to agglomerate each other, most fine γ-Al₂O₃ particles were distributed uniformly within the Bi_0.4Sb_1.6Te₃ matrix. The size distribution ranged from 0.3 μm to less than 50 nm, which is the resolution limit of the microscope. The composition of the particles was determined to be that of pure γ-Al₂O₃ through energy-dispersion X-ray spectrometry analysis. Significant pores were not observed in the cross-sectional view (Figure 3) at 20,000× magnification, indicating that highly dense Bi_0.4Sb_1.6Te₃ bulk samples can be successfully fabricated using vacuum hot pressing. The densities of the Bi_0.4Sb_1.6Te₃ bulk sample measured using the Archimedean method were 6.70 and 6.71 g/cm³ for 1 and 3 wt % γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ samples, respectively, yielding corresponding relative densities of 93.2% and 93.6%. To observe the microstructure within the γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ composites, Bi_0.4Sb_1.6Te₃ with 1 wt % γ-Al₂O₃ additions (Figure 3) was examined using TEM; a TEM bright-field image is shown in Figure 4. Two types of γ-Al₂O₃ distributions were observed in the composites; most γ-Al₂O₃ nanoparticles smaller than 10 nm in size were homogeneously dispersed along the grain boundary. A small quantity of the γ-Al₂O₃ nanoparticles with irregular shapes and sizes ranging from 60 to 400 nm were embedded within the Bi_0.4Sb_1.6Te₃ matrix. A similar microstructure was reported for the nanocomposites of CoSb₃/TiO₂ [13] and ZrNiSn/ZrO₂ [14].

Figure 2. XRD patterns of γ-Al₂O₃-doped Bi_0.4Sb_1.6Te₃ bulk specimens. (a) 1 wt % Al₂O₃; (b) 3 wt % Al₂O₃.

Figure 2. XRD patterns of γ-Al₂O₃-doped Bi_0.4Sb_1.6Te₃ bulk specimens. (a) 1 wt % Al₂O₃; (b) 3 wt % Al₂O₃.

Figure 3. Cross-sectional SEM images of γ-Al₂O₃-doped Bi_0.4Sb_1.6Te₃ disks after vacuum hot pressing at 573 K under a pressure of 0.7 GPa for 30 min (black particles: γ-Al₂O₃).

Figure 3. Cross-sectional SEM images of γ-Al₂O₃-doped Bi_0.4Sb_1.6Te₃ disks after vacuum hot pressing at 573 K under a pressure of 0.7 GPa for 30 min (black particles: γ-Al₂O₃).

Figure 4. TEM images of the consolidated 1 wt % γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ samples. (white particles: γ-Al₂O₃).

Figure 4. TEM images of the consolidated 1 wt % γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ samples. (white particles: γ-Al₂O₃).

Figure 5 shows the TE properties of the γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ composite samples characterized at temperatures ranging from 298 to 473 K. The Seebeck coefficient variations as a function of temperature are depicted in Figure 5a. All samples had positive Seebeck coefficients, suggesting that they are p-type conductive. As shown in Figure 5a, the Seebeck coefficient values of the γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ bulk composite samples decreased with increasing γ-Al₂O₃ content. For most samples, the Seebeck coefficient initially increases rapidly at 300–375 K, which is consistent with the Mott formula [22], but after peaking, it starts decreasing with rising temperatures because of the thermal excitation of extrinsic charge carriers at high temperatures. The maximum value of the Seebeck coefficient is 242, 234 and 229 μV/K at 373 K for 0, 1 and 3 wt % γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ samples, respectively. Figure 5b shows the temperature dependence of electrical conductivity. The samples exhibited a metallic dependence: conductivity gradually decreased as temperature increased from 300 to 473 K. Electrical conductivity of γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ composite decreases as γ-Al₂O₃ particles increases. The highest electrical conductivities at 300 K were observed for 1 and 3 wt % γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ samples, with values of 1080 and 895 Ω⁻¹ cm⁻¹, respectively. The power factor (PF) of TE materials is usually calculated as PF = α²σ; Figure 5c is a graph of the PF of Bi_0.4Sb_1.6Te₃ bulk composite samples versus the temperature. All samples showed positive values in the whole temperature range of measurement, indicating p-type semiconducting behavior. The 1 wt % γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ samples exhibited the highest PF (5.4 mWm⁻¹·K⁻² at 298 K). The temperature dependence of thermal conductivity is shown in Figure 5d. The 3 wt % γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ samples have significantly lower thermal conductivity than the 1 wt % γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ samples in the whole temperature range. The lowest value of thermal conductivity for this sample was 1.12 W/mK, which was obtained at 373 K.

Figure 5. Temperature dependence of the TE properties for the γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ specimens: (a) Seebeck coefficient; (b) electrical resistivity; (c) PF; and (d) thermal conductivity.

Figure 5. Temperature dependence of the TE properties for the γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ specimens: (a) Seebeck coefficient; (b) electrical resistivity; (c) PF; and (d) thermal conductivity.

The variation of ZT as a function of temperature for the γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ bulk specimens is shown in Figure 6.

Figure 6. Variation of ZT as a function of temperature for γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ bulk samples.

Figure 6. Variation of ZT as a function of temperature for γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ bulk samples.

For the 1 wt % γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ sample, a high ZT value can be obtained within the entire temperature range because of high PFs and low thermal conductivity; ZT at 300 K is 1.17 and increases with increasing temperature, peaking at 1.22 at 323 and 348 K, before subsequently decreasing to 0.86 at 473 K. For the 3 wt % γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ composite sample (Figure 6), ZT at 300 K is 1.0 and increases with increasing temperature, peaking at 1.21 at 373 and 398 K, before subsequently decreasing to 0.93 when the temperature increases to 473 K.

Several studies have reported the preparation of nanocomposite Bi_0.4Sb_1.6Te₃ bulk samples [15,16,17,22,23,24,25,26] through BM and hot pressing or spark plasma sintering (SPS). TE properties and preparation methods are listed in Table 1. The ZT values of the consolidated Bi_0.4Sb_1.6Te₃ alloys as a function of temperature are plotted in Figure 7, and the results of this study are included for comparison.

Figure 7. Temperature dependence of ZT of the Bi_0.4Sb_1.6Te₃-based bulk specimens prepared through various methods.

Figure 7. Temperature dependence of ZT of the Bi_0.4Sb_1.6Te₃-based bulk specimens prepared through various methods.

In this study, ZT of the γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ sample at 298–473 K were 0.86–1.22 (1 wt % γ-Al₂O₃) and 0.93–1.21 (3 wt % γ-Al₂O₃), with an average value of 1.10 for both samples. Compared with other studies, the ZT obtained in this study at high temperatures are higher. Advances in ZT can be achieved through considerable reductions in thermal conductivities through phonon scattering. Incorporating nanoparticles into TE materials to act as additional phonon scattering sites inside the grain boundary or matrix regions has recently been demonstrated effectively increase ZT [27,28,29]. According to this approach, for nano- and microstructured TE composite materials shown in Figure 4, the dispersed γ-Al₂O₃ nanoparticles are expected to create an additional grain boundary and interfacial area, which increases phonon scattering and decreases thermal conductivity. To further verify this argument, the temperature dependence of lattice thermal conductivity (κ_l) and electronic thermal conductivity (κ_e) of present γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ samples are shown in Figure 8. κ_l was calculated by subtracting the electronic thermal conductivity κ_e from κ, and κ_e is calculated by the Wiedemann–Franz relation, κ_e = LσT (where L = 2.0 × 10⁻⁸ V²/K² is Lorenz number, σ is electrical conductivity, and T is absolute temperature) [30].

Table 1. TE properties and preparation methods of typical Bi_0.4Sb_1.6Te₃-based nanocomposites.

Nanocomposite	Highest ZT	ZT(>400 K)	Methoda,b	Ref.
Bi0.4Sb1.6Te3	1.15 at 350 K	0.2~0.4	BM + HP (200 MPa/430 C/2 h)	[24]
Bi0.4Sb1.6Te3	1.14 at 323 K	0.2~0.74	BM (300 rpm/10 h) + SPS (50 MPa/420 C/10 min)	[25]
Bi0.4Sb1.6Te3	1.15 at 350 K	0.63~0.9	BM (1200 rpm/5 h) + SPS (60 MPa/420 C/5 min)	[23]
Bi0.4Sb1.6Te3	1.0 at 300 K	-	BM (400 rpm/2 h) + SPS (50 MPa/450 C/5 min)	[26]
Bi0.4Sb1.6Te3 + 4 wt % Te	0.98 at 343 K	0.55~0.68	BM (400 rpm/12 h) + HP (60 MPa/290 C/1 h) + ECAE (753 K)	[22]
Bi0.4Sb1.6Te3 + 1 wt % CNT	1.08 at 323 K	0.52~0.92	ZM + BM + HP (27.6 MPa/440 C/10 min)	[15]
Bi0.4Sb1.6Te3 + 1.5 wt % C60	1.15 at 375 K	0.79~1.08	BM (500~2220 rpm/30 min) + sinter (5 kbar/400 C) +annealing (300 C/2 h)	[16]
Bi0.4Sb1.6Te3 + 7 wt % BN	0.54 at 300 K	-	BM (1200 rpm/5 h) + HP (425 MPa/550 C/30 min)	[17]
Bi0.4Sb1.6Te3 + 7 wt % WO3	0.75 at 300 K	-	BM (1200 rpm/5 h) + HP (425 MPa/550 C/30 min)	[17]
Bi0.4Sb1.6Te3 + 1 wt % Al2O3	1.22 at 340 K	0.86~1.12	BM + HP (700 MPa/300 C/1 h)	This work
Bi0.4Sb1.6Te3 + 3 wt % Al2O3	1.21 at 398 K	0.93~1.21	BM + HP (700 MPa/300 C/1 h)	This work

a: BM: ball milling; SPS: spark plasma sintering; ECAE: equal channel angular extrusion; b: Experimental details are listed in parentheses.

Table 1. TE properties and preparation methods of typical Bi_0.4Sb_1.6Te₃-based nanocomposites.

Nanocomposite	Highest ZT	ZT(>400 K)	Methoda,b	Ref.
Bi0.4Sb1.6Te3	1.15 at 350 K	0.2~0.4	BM + HP (200 MPa/430 C/2 h)	[24]
Bi0.4Sb1.6Te3	1.14 at 323 K	0.2~0.74	BM (300 rpm/10 h) + SPS (50 MPa/420 C/10 min)	[25]
Bi0.4Sb1.6Te3	1.15 at 350 K	0.63~0.9	BM (1200 rpm/5 h) + SPS (60 MPa/420 C/5 min)	[23]
Bi0.4Sb1.6Te3	1.0 at 300 K	-	BM (400 rpm/2 h) + SPS (50 MPa/450 C/5 min)	[26]
Bi0.4Sb1.6Te3 + 4 wt % Te	0.98 at 343 K	0.55~0.68	BM (400 rpm/12 h) + HP (60 MPa/290 C/1 h) + ECAE (753 K)	[22]
Bi0.4Sb1.6Te3 + 1 wt % CNT	1.08 at 323 K	0.52~0.92	ZM + BM + HP (27.6 MPa/440 C/10 min)	[15]
Bi0.4Sb1.6Te3 + 1.5 wt % C60	1.15 at 375 K	0.79~1.08	BM (500~2220 rpm/30 min) + sinter (5 kbar/400 C) +annealing (300 C/2 h)	[16]
Bi0.4Sb1.6Te3 + 7 wt % BN	0.54 at 300 K	-	BM (1200 rpm/5 h) + HP (425 MPa/550 C/30 min)	[17]
Bi0.4Sb1.6Te3 + 7 wt % WO3	0.75 at 300 K	-	BM (1200 rpm/5 h) + HP (425 MPa/550 C/30 min)	[17]
Bi0.4Sb1.6Te3 + 1 wt % Al2O3	1.22 at 340 K	0.86~1.12	BM + HP (700 MPa/300 C/1 h)	This work
Bi0.4Sb1.6Te3 + 3 wt % Al2O3	1.21 at 398 K	0.93~1.21	BM + HP (700 MPa/300 C/1 h)	This work

a: BM: ball milling; SPS: spark plasma sintering; ECAE: equal channel angular extrusion; b: Experimental details are listed in parentheses.

Figure 8. Temperature dependence of lattice thermal conductivity (κ_l) and electronic thermal conductivity (κ_e) for the γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ bulk samples.

Figure 8. Temperature dependence of lattice thermal conductivity (κ_l) and electronic thermal conductivity (κ_e) for the γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ bulk samples.

Accordingly, the lattice thermal conductivity κ_l decreased with the addition of γ-Al₂O₃ particles, while the electronic thermal conductivity κ_e decreased less drastically than did the lattice thermal conductivity. It is thus concluded that the decrease in thermal conductivity with increasing the amount of γ-Al₂O₃ particles was mainly due to the reduction in lattice thermal conductivity. Bi-Sb-Te alloys are categorized as low-temperature TE materials, and their use at temperatures higher than 400 K is limited because of low TE performance. A satisfactory ZT value at high temperatures is vital for power generation. Because no other Bi-Sb-Te material with a similarly high ZT in this temperature range has been reported, Bi_0.4Sb_1.6Te₃ bulk samples containing γ-Al₂O₃ particles have considerable potential as a high-performance material for application in TE devices in the temperature range 348–473 K.

4. Conclusions

Through MA and vacuum hot pressing, p-type γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ composites were fabricated. No significant pores were observed in the hot-pressed samples, indicating that highly dense Bi_0.4Sb_1.6Te₃ bulk samples can be successfully prepared using the proposed approach. The influence of the alumina content on TE properties was measured in the temperature range 300–473 K. The measured Seebeck coefficient, electrical resistivity, and thermal conductivity indicate that adding an optimal amount of γ-Al₂O₃ particles improves the TE performance of the γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ composites. High TE performance with ZT as high as 1.22 and 1.21 were achieved at 373 and 398 K for samples containing 1 and 3 wt % γ-Al₂O₃ particles. These ZT values are higher than those of several reported monolithic Bi_0.4Sb_1.6Te₃ samples prepared through BM and hot pressing or SPS. The achieved high ZT value may be attributable to the unique nano- and microstructures in which γ-Al₂O₃ nanoparticles were dispersed along the grain boundary or inside the matrix grain, as revealed through high-resolution TEM. The dispersed γ-Al₂O₃ nanoparticles thus increase phonon scattering sites and reduce thermal conductivity. The ZT values of these samples at 298–423 K are 1.0–1.22. Such ZT characteristics render γ-Al₂O₃/Bi_0.4Sb_1.6Te₃ suitable for power generation applications because other materials with similarly high ZT are yet to be reported in this temperature range.