Thermoelectric Properties of Alumina-Doped Bi 0 . 4 Sb 1 . 6 Te 3 Nanocomposites Prepared through Mechanical Alloying and Vacuum Hot Pressing

In this study, γ-Al2O3 particles were dispersed in p-type Bi0.4Sb1.6Te3 through mechanical alloying to form γ-Al2O3/Bi0.4Sb1.6Te3 composite powders. The composite powders were consolidated using vacuum hot pressing to produce nanoand 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 Bi0.4Sb1.6Te3 samples. The ZT values of the fabricated samples at 298–423 K are 1.0–1.22; these ZT characteristics make γ-Al2O3/Bi0.4Sb1.6Te3 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 nanoand 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 nanoand microstructured γ-Al2O3/Bi0.4Sb1.6Te3 alloy can serve as a high-performance material for application in TE devices.


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 " pα 2 σ{κqT (1) Energies 2015, 8, 12573-12583 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 ˆ6 ˆ6 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).

Results and Discussion
Figure 1 shows the XRD patterns of the Bi 0.4 Sb 1.6 Te 3 composite samples with 1 and 3 wt % γ-Al 2 O 3 additions after 2 h of milling.The diffraction peaks cited from the database of the (Bi 0.2 Sb 0.8 ) 2 Te 3 (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 0.2 Sb 0.8 ) 2 Te 3 phase (JCPDS 072-1836), implying that the (Bi 0.2 Sb 0.8 ) 2 Te 3 phase can be successfully prepared through high-energy ball milling of γ-Al 2 O 3 /Bi 0.4 Sb 1.6 Te 3 composite powders.However, as seen in Figure 1, the Bragg peaks of γ-Al 2 O 3 are barely detectable in the XRD patterns of the composite powders of the alloy mixed with γ-Al 2 O 3 particles after 2 h of milling, which may be attributable to the low volume fraction of γ-Al 2 O 3 particles and their small crystalline size.Similar to the observations regarding the preparation of Al 2 O 3 /NiAl intermetallic-matrix composite in this study, Lin et al. [21] reported that for 5 vol % Al 2 O 3 additions in mechanically alloyed NiAl alloys, no Al 2 O 3 phase could be detected using XRD after 10 h of milling.
Energies 2015, 8 4 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.The Bi0.4Sb1.6Te3composite 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 (Bi0.2Sb0.8)2Te3phase.Compared with the as-milled composite powders, the peaks of the consolidated samples are narrow because of strain relaxation and grain growth in the Bi0.4Sb1.6Te3The Bi 0.4 Sb 1.6 Te 3 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 0.2 Sb 0.8 ) 2 Te 3 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 0.4 Sb 1.6 Te 3 nanograin powders.SEM was used to examine the cross-sectional view of γ-Al 2 O 3 /Bi 0.4 Sb 1.6 Te 3 disks after vacuum hot pressing (Figure 3).Although several γ-Al 2 O 3 nanoparticles tend to agglomerate each other, most fine γ-Al 2 O 3 particles were distributed uniformly within the Bi 0.4 Sb 1.6 Te 3 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-Al 2 O 3 through energy-dispersion X-ray spectrometry analysis.Significant pores were not observed in the cross-sectional view (Figure 3) at 20,000ˆmagnification, indicating that highly dense Bi 0.4 Sb 1.6 Te 3 bulk samples can be successfully fabricated using vacuum hot pressing.The densities of the Bi 0.4 Sb 1.6 Te 3 bulk sample measured using the Archimedean method were 6.70 and 6.71 g/cm 3 for 1 and 3 wt % γ-Al 2 O 3 /Bi 0.4 Sb 1.6 Te 3 samples, respectively, yielding corresponding relative densities of 93.2% and 93.6%.To observe the microstructure within the γ-Al 2 O 3 /Bi 0.4 Sb 1.6 Te 3 composites, Bi 0.4 Sb 1.6 Te 3 with 1 wt % γ-Al 2 O 3 additions (Figure 3) was examined using TEM; a TEM bright-field image is shown in Figure 4. Two types of γ-Al 2 O 3 distributions were observed in the composites; most γ-Al 2 O 3 nanoparticles smaller than 10 nm in size were homogeneously dispersed along the grain boundary.A small quantity of the γ-Al 2 O 3 nanoparticles with irregular shapes and sizes ranging from 60 to 400 nm were embedded within the Bi 0.4 Sb 1.6 Te 3 matrix.A similar microstructure was reported for the nanocomposites of CoSb 3 /TiO 2 [13] and ZrNiSn/ZrO 2 [14].
Energies 2015, 8 5 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 Bi0.4Sb1.6Te3matrix.A similar microstructure was reported for the nanocomposites of CoSb3/TiO2 [13] and ZrNiSn/ZrO2 [14].Figure 5 shows the TE properties of the γ-Al2O3/Bi0.4Sb1.6Te3composite 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/Bi0.4Sb1.6Te3bulk 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/Bi0.4Sb1.6Te3samples, 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/Bi0.4Sb1.6Te3composite decreases as γ-Al2O3 particles increases.The highest electrical conductivities at 300 K were observed for 1 and 3 wt % γ-Al2O3/Bi0.4Sb1.6Te3samples, with values of 1080 and 895  −1 cm −1 , respectively.The power factor (PF) of TE materials is usually calculated as PF = α 2 σ; Figure 5c is a graph of the PF of Bi0.4Sb1.6Te3bulk 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/Bi0.4Sb1.6Te3samples 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/Bi0.4Sb1.6Te3samples have significantly lower thermal conductivity than the 1 wt % γ-Al2O3/Bi0.4Sb1.6Te3samples 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.The variation of ZT as a function of temperature for the γ-Al2O3/Bi0.4Sb1.6Te3bulk specimens is shown in Figure 6.The variation of ZT as a function of temperature for the γ-Al2O3/Bi0.4Sb1.6Te3bulk specimens is shown in Figure 6.For the 1 wt % γ-Al 2 O 3 /Bi 0.4 Sb 1.6 Te 3 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-Al 2 O 3 /Bi 0.4 Sb 1.6 Te 3 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 0.4 Sb 1.6 Te 3 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 0.4 Sb 1.6 Te 3 alloys as a function of temperature are plotted in Figure 7, and the results of this study are included for comparison.
Energies 2015, 8 8 For the 1 wt % γ-Al2O3/Bi0.4Sb1.6Te3sample, 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/Bi0.4Sb1.6Te3composite 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 Bi0.4Sb1.6Te3bulk 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 Bi0.4Sb1.6Te3alloys as a function of temperature are plotted in Figure 7, and the results of this study are included for comparison.In this study, ZT of the γ-Al2O3/Bi0.4Sb1.6Te3sample 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 (κl) and electronic thermal conductivity (κe) of present γ-Al2O3/Bi0.4Sb1.6Te3samples are shown in Figure 8. κl was calculated by subtracting the electronic thermal conductivity κe from κ, and κe is calculated by the Wiedemann-Franz relation, κe = LσT (where L = 2.0 × 10 −8 V 2 /K 2 is Lorenz number, σ is electrical conductivity, and T is absolute temperature) [30].In this study, ZT of the γ-Al 2 O 3 /Bi 0.4 Sb 1.6 Te 3 sample at 298-473 K were 0.86-1.22(1 wt % γ-Al 2 O 3 ) and 0.93-1.21(3 wt % γ-Al 2 O 3 ), 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-Al 2 O 3 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-Al 2 O 3 /Bi 0.4 Sb 1.6 Te 3 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 ˆ10 ´8 V 2 /K 2 is Lorenz number, σ is electrical conductivity, and T is absolute temperature) [30].Accordingly, the lattice thermal conductivity κl decreased with the addition of γ-Al2O3 particles, while the electronic thermal conductivity κe 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, Bi0.4Sb1.6Te3bulk 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.

Conclusions
Through MA and vacuum hot pressing, p-type γ-Al2O3/Bi0.4Sb1.6Te3composites were fabricated.No significant pores were observed in the hot-pressed samples, indicating that highly dense Bi0.4Sb1.6Te3bulk 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/Bi0.4Sb1.6Te3composites.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 Bi0.4Sb1.6Te3samples 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 Accordingly, the lattice thermal conductivity κ l decreased with the addition of γ-Al 2 O 3 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-Al 2 O 3 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 0.4 Sb 1.6 Te 3 bulk samples containing γ-Al 2 O 3 particles have considerable potential as a high-performance material for application in TE devices in the temperature range 348-473 K.

Conclusions
Through MA and vacuum hot pressing, p-type γ-Al 2 O 3 /Bi 0.4 Sb 1.6 Te 3 composites were fabricated.No significant pores were observed in the hot-pressed samples, indicating that highly dense Bi 0.4 Sb 1.6 Te 3 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-Al 2 O 3 particles improves the TE performance of the γ-Al 2 O 3 /Bi 0.4 Sb 1.6 Te 3 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-Al 2 O 3 particles.These ZT values are higher than those of several reported monolithic Bi 0.4 Sb 1.6 Te 3 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-Al 2 O 3 nanoparticles were dispersed along the grain boundary or inside the matrix grain, as revealed through high-resolution TEM.The dispersed γ-Al 2 O 3 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-Al 2 O 3 /Bi 0.4 Sb 1.6 Te 3 suitable for power generation applications because other materials with similarly high ZT are yet to be reported in this temperature range.

Figure 5 Figure 5 .
Figure 5 shows the TE properties of the γ-Al 2 O 3 /Bi 0.4 Sb 1.6 Te 3 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-Al 2 O 3 /Bi 0.4 Sb 1.6 Te 3 bulk composite samples decreased with increasing γ-Al 2 O 3 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-Al 2 O 3 /Bi 0.4 Sb 1.6 Te 3 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-Al 2 O 3 /Bi 0.4 Sb 1.6 Te 3 composite decreases as γ-Al 2 O 3 particles increases.The highest electrical conductivities at 300 K were observed for 1 and 3 wt % γ-Al 2 O 3 /Bi 0.4 Sb 1.6 Te 3 samples, with values of 1080 and 895 Ω ´1 cm ´1, respectively.The power factor (PF) of TE materials is usually calculated as PF = α 2 σ; Figure 5c is a graph of the PF of Bi 0.4 Sb 1.6 Te 3 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 %

Figure 6 .
Figure 6.Variation of ZT as a function of temperature for γ-Al 2 O 3 /Bi 0.4 Sb 1.6 Te 3 bulk samples.

Figure 7 .
Figure 7. Temperature dependence of ZT of the Bi 0.4 Sb 1.6 Te 3 -based bulk specimens prepared through various methods.