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Effect of high pressure sintering and annealing on microstructure and thermoelectric properties of nanocrystalline Bi2Te2.7Se0.3 doped with Gd
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Original Research
Effect of high pressure sintering and annealing on microstructure and thermoelectric properties of nanocrystalline Bi2Te2.7Se0.3 doped with Gd Ping Zou, Gui-Ying Xun, Song Wang, Peng-Lei Chen, Feng-Zhu Huang Institution of Nuclear and New Energy Materials, University of Science and Technology Beijing, Beijing 100083, China Received 5 December 2013; accepted 20 April 2014
Abstract Bi2Te2.7Se0.3 of high performance doped with Gd bulk materials was prepared by a high pressure (6.0 GPa) sintering (HPS) method at 593 K, 633 K, 673 K and 693 K. The sample was then annealed for 36 h in a vacuum at 633 K. The phase composition, crystal structure and morphology of the sample were analyzed by X-ray diffraction and scanning electron microscopy. The electric conductivity, Seebeck coefficient, and thermal conductivity aspects of the sample were measured from 298 K to 473 K. The results show that high pressure sintering and the doping with Gd has a great effect on the crystal structure and the thermoelectric properties of the samples. The samples are consisted of nanoparticles before and after annealing, and these nanostructures have good stability at high temperature. HPS together with annealing can improve the TE properties of the sample by decreasing the thermal conductivity of the sample with nanostructures. The maximum ZT value of 0.74 was obtained at 423 K for the sample, which was sintered at 673 K and then annealed at 633 K for 36 h. Compared with the zone melting sample, it was increased by 85% at 423 K. Hence the temperature of the maximum of figure of merit was increased. The results can be applied to the field of thermoelectric power generation materials. & 2014 Chinese Materials Research Society. Production and hosting by Elsevier B.V. All rights reserved.
Keywords: Bi2Te2.7Se0.3; High pressure sintering; Annealing; Gd doped; Thermoelectric properties
1. Introduction Thermoelectric (TE) materials are the functional materials that can convert heat energy to electric energy directly due to thermoelectric effects. The performance of such materials can be represented by the dimensionless figure of merit: ZT ¼ α2 sT=κ, where α, s, κ, and T are Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature, respectively. In the recent years, most of the commercial thermoelectric devices operating near room temperature consist of Bismuth telluride (Bi2Te3) and its alloys. The crystal structure n
Corresponding author. Tel.: þ86 10 62333516. E-mail address: [email protected] (G.-Y. Xu). Peer review under responsibility of Chinese Materials Research Society.
of Bi2Te3 compounds is a hexagonal layered structure and it consists of five atomic planes in the sequence of Te(1)–Bi– Te(2)–Bi–Te(1) [1,2]. Although studied for more than 50 years, they are still the most widely used thermoelectric materials because of their remarkable TE performance [3]. The typical fabrication methods include unidirectional crystal-growth methods, such as Bridgman and zone-melting techniques [4–7], powder metallurgy techniques, such as hot-pressing (HP) [8,9], ultra high pressure [10–13], hot extrusion [14,15], spark plasma sintering (SPS) methods [16–18] and high pressure sintering (HPS) methods [19]. Compared to other methods, the samples prepared by high pressure sintering have the following advantages: efficient, low cost, suitable for large-scale production, more homogeneous nanocrystalline grain and restraining grain from coarsening during the sintering process, etc. Recently, many experimental results indicate that the refinement of Bi2Te3-based alloys can enhance the thermoelectric
http://dx.doi.org/10.1016/j.pnsc.2014.05.009 1002-0071/& 2014 Chinese Materials Research Society. Production and hosting by Elsevier B.V. All rights reserved.
Please cite this article as: P. Zou, et al., Effect of high pressure sintering and annealing on microstructure and thermoelectric properties of nanocrystalline Bi2Te2.7Se0.3 doped with Gd, Progress in Natural Science: Materials International (2014), http://dx.doi.org/10.1016/j.pnsc.2014.05.009
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performances [19–21]. Xu et al. [19] employed the HPS method to fabricate nanostructured p-type Bi2Te3-based materials. They found that the HPS technique can generate nanoparticles in the bulk materials which significantly reduce the lattice contribution to thermal conductivity, and the asprepared alloys show a highest ZT of 1.16 at room temperature. Yu et al. [20] prepared pure Bi2Te3 by mechanical alloying (MA) and high pressure sintering (HPS) methods. In their work, they synthesized Bi2Te3 nanopowder by an MA method at first and then fabricated bulk materials by the HPS technique. They investigated the effect of variation of sintering pressure on the properties of Bi2Te3. Zhang et al. [21] utilized a hydrothermal method to create nano-scale La- containing Bi–Te–Se starting powders and then hot-pressed the powders into a bulk which shows a maximum ZT of 0.5 at 450 K. But Bi2Te2.7Se0.3 doped with rare earth element Gd fabricated by the HPS method has not been examined until now. In this paper, the fabrication procedure of Bi2Te2.7Se0.3 þ 0.12 wt% SbI3 þ 0.12 wt% Gd with good thermoelectric properties using the HPS method was introduced. The effect of high pressure sintering and annealing on microstructure and thermoelectric properties of Bi2Te2.7Se0.3 doped with Gd was investigated. The formulas about the relationships between energy band gap, cell volume and sintering pressure were established. The formation mechanism of nanocrystalline was explained, and the effect of Gd was discussed. The thermoelectric properties of samples fabricated at different sintering temperatures under 6.0 GPa and then annealed at 633 K for 36 h were analyzed, and a qualitative correlation between sintering temperature and TE properties was revealed. The samples that were tested with the optimal 673 K HPS and were then annealed exhibited good thermoelectric properties. ZTmax is 0.74 occurring at 423 K, which can be applied to the field of thermoelectric power generation materials. 2. Experimental procedure Pure Bi, Te, Se, SbI3, and Gd powders with the purity of 99.99% were used as starting materials. The raw materials were weighed in appropriate atomic ratios, mixed in an agate mortar for 2 h and then synthesized for 4 h in a vacuum at 743 K. After crushing and cold compacting, the alloy was sintered for 1 min under high pressure (6.0 GPa) in cubic press (CS-IVY) at 593 K, 633 K, 673 K and 693 K respectively. Then the samples were sealed in quartz tubes under vacuum and annealed at 633 k for 36 h. After annealing, the samples were cooled to room temperature naturally. Phase identification of the samples were performed by X-ray diffraction (XRD, CuKα, λ¼ 0.15406 nm, D/max 2500), using a scan rate of 41/min to record the patterns in the 2θ range from 101 to 601. Accurate measurement of the lattice parameter was obtained through calibration with the silicon standard. The morphology was analyzed by field-emission scanning electron microscopy (JSM-7001F). The electric conductivity (s) was measured by the standard four-probe method in an Ar atmosphere. The Seebeck coefficient (α) was measured concurrently when a temperature difference (5 1C) was applied between two
ends of the sample. The power factor (P) was calculated by the formula: P ¼ α2 s. The thermal conductivity (κ) was calculated from the measured specific heat (Cp), thermal diffusivity (λ), and density (d) using the relationship of κ ¼ C p λd. Cp was measured using thermal analyzing apparatus (DuPont 1090B, America), λ was measured by a laser flash method (NETZSCH, LFA427, Germany), and d was measured by the Archimedes method. The figure of merit (ZT) was calculated by substituting the measured data of α, s and κ at given temperature into the formula: ZT ¼ α2 sT=κ. The Hall coefficient (RH) of the sample was measured at 30 1C using a physical property measurement system (PPMS-9T, Quantum Design Inc., USA) at a magnetic field of 2 T and an electrical current of 50 mA. The carrier concentration (n) was calculated from the Hall coefficient (RH) datum using n ¼ 1/RHe, where e is the electron charge. 3. Results and discussion Fig. 1 shows the XRD patterns of samples sintering at 593 K, 633 K, 673 K and 693 K under the pressure of 6.0 GPa. As shown in Fig. 1, all of the main diffraction peaks move towards large angle slightly compared with the standard diffraction data of Bi2Te2.7Se0.3 (PDF#50-0954) with a typical rhombohedral structure and no other phase exists. The broadened diffraction peaks of the HPS samples indicate a very small grain size. The average grain size of the HPS sample which sintered at 673 K is 27.72 nm (calculated using the classic Williamson–Hall method based on the measured width of diffraction peaks). The lattice parameters of samples also have been calculated, and the results are listed in Table 1. Table 1 presents the lattice constant against the cell volume of Bi2Te2.7Se0.3-based samples. From Table 1, it can be observed that the Bi2Te2.7Se0.3-based samples' properties have two obvious characteristics. The first one is that the lattice constant and the cell volume increased as the sintering temperature increased from 593 K to 673 K. But when the sintering temperature increased up to 693 K, the lattice
Fig. 1. XRD patterns of samples at different sintering temperatures.
Please cite this article as: P. Zou, et al., Effect of high pressure sintering and annealing on microstructure and thermoelectric properties of nanocrystalline Bi2Te2.7Se0.3 doped with Gd, Progress in Natural Science: Materials International (2014), http://dx.doi.org/10.1016/j.pnsc.2014.05.009
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constant decreased, and the cell volume also decreased. In addition, when the sintering temperature reached 673 K, the lattice constant a decreased but c and cell volume increased compared to that of the Bi2Te2.7Se3. This suggests that the high pressure sintering process produces obvious lattice distortion. The reason is that the ionic radius of Gd3 þ (0.94 Å) is smaller than the ionic radius of Bi3 þ (0.96 Å), Table 1 Lattice constant and cell volume of samples. Samples
Bi2Te2.7Se0.3 593 K(sintered) 633 K(sintered) 673 K(sintered) 693 K(sintered) 673 K(sintered), 633 K(annealed)
Lattice constant (nm) a¼b
c
0.4374 0.4356 0.4365 0.4368 0.4365 0.4349
3.0424 3.0368 3.0470 3.0599 3.0341 3.0513
Cell volume (nm3)
0.5041 0.4989 0.5027 0.5056 0.5007 0.4998
3
therefore the cell size decreased when the sample doped with Gd under atmospheric pressure. But after high pressure sintering, Gd3 þ is the cause of lattice distortion due to significant pressure, which further induces the lattice distortion to emerge and increase in size, resulting in cell size of HPS sample increasing. This indicates that the effect of trace Gd doping and high pressure on the crystal structure of Bi2Te2.7Se0.3 is significant. The second characteristic is that after annealing, the cell volume decreases. Fig. 2 shows the FE-SEM morphologies of the HPS samples sintered at different temperatures and annealed sample (sintered at 673 K, annealed at 633 K). The grain size of all HPS samples was smaller than 50 nm, after annealing the grain size was a little larger while still smaller than 100 nm, hence the sample still consisted of nanometer grains after annealing. These results show that the nanocrystalline prepared by the HPS method has good stability at high temperature. The formation mechanisms of nanocrystalline are discussed as the following. Firstly, the Bi2Te2.7Se0.3
Fig. 2. FE-SEM of sample (a) 6.0 GPa HPS sample (sintered at 593 K), (b) 6.0 GPa HPS sample (sintered at 633 K), (c) 6.0 GPa HPS sample (sintered at 673 K), (d) 6.0 GPa HPS sample (sintered at 693 K), (e) 6.0 GPa annealed sample(sintered at 673 K, annealed at 633 K). Please cite this article as: P. Zou, et al., Effect of high pressure sintering and annealing on microstructure and thermoelectric properties of nanocrystalline Bi2Te2.7Se0.3 doped with Gd, Progress in Natural Science: Materials International (2014), http://dx.doi.org/10.1016/j.pnsc.2014.05.009
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Fig. 3. Electrical conductivity values of samples as a function of temperature (a) HPS sample, and (b) annealed sample.
Table 2 Hall coefficient, carrier concentrations and mobility of samples. Samples
Hall coefficient (cm3/C)
Carrier concentration (n) 1019/(cm 3)
Carrier mobility μ (cm2/vs)
593 K 633 K 673 K 693 K 673 K 633 K
0.19 0.2 0.22 0.52 0.53
3.25 3.06 2.80 1.21 1.19
14.53 16.52 22.69 27.62 84.73
(sintered) (sintered) (sintered) (sintered) (sintered), (annealed)
samples are brittle materials and the pressurization rates are very high, reaching set pressure of 6.0 GPa within 2 min, which is equivalent to the application of bursting high pressure to the sample, leading to the refinement of brittle particles. Thus the raw materials have become nanoparticle before reaching the specified sintering temperature. The second is the higher supercooling (ΔT), i.e., the higher cooling rate. The higher the ΔΤ, the smaller the crystal critical radius, and the larger the nucleation rate becomes. As a result, the high degree of supercooling will promote the formation of many very small critical nuclei in a very short amount of time. In addition, the high pressure also limits the movement of the atoms or ions and the size of critical nucleation radius. Therefore, the growth rates of the grains decreased greatly, and the nanoparticles formed [19]. It is believed that nanostructure may be one of the most effective ways to improve the performance of TE materials, which can scatter phonons more effectively than electrons [22], hence decreasing the thermal conductivity (κ), and consequently enhancing the value of ZT. Fig. 3(a) and (b) shows the temperature dependence of electrical conductivities of the HPS samples and annealed samples. It can be seen that the HPS samples' properties have three obvious features. The first one is that all the samples show a decrease in electrical conductivity with increasing measuring temperature from 298 K to 473 K, indicating a metallic conducting behavior. The second feature is that the electrical conductivity increases with increasing sintering
temperature, the highest electrical conductivity of 101.68 (Ω mm) 1 is obtained at 298 K for the HPS sample sintered at 673 K as shown in Fig. 3(a). The increase in electrical conductivity can be attributed to the increase in carrier mobility resulting from the grain growth and densification. However, when the sintering temperature was further increased to 693 K, the electrical conductivity decreased to 48.97 (Ω mm) 1. While raising the HPS temperature resulted in the decrease of carrier concentration (n) as presented in Table 2. It is well known that two competing factors of carrier concentration (n) and carrier mobility (μ) determine the electrical conductivity, and the relationship of both can be described by the equation of s ¼ neμ. The values of (n) and (μ) of HPS sample sintered at 693 K are both low as presented in Table 2, and the electrical conductivity of HPS sample sintered at 693 K is the lowest as shown in Fig. 3(a). The third one is that the electrical conductivity of annealed sample is higher than that of the HPS sample as shown in Fig. 3(b). The highest electrical conductivity value of 161.35 (Ω mm) 1 was obtained at 298 K for the annealed sample (sintered at 673 K, and annealed at 633 K). The increase in electrical conductivity after annealing can be attributed to the increase in carrier mobility. The variation of carrier concentration and carrier mobility before and after annealing is listed in Table 2. As presented in Table 2 the carrier concentration of HPS sample decreases with increasing sintering temperature. In addition, the carrier concentration of HPS sample is higher than that of the annealed sample, but the carrier mobility of the HPS sample is lower than that of the annealed sample. The relationship between the energy band gap of Bi2Te3 and temperature can be given as follows [23]: Eg ¼ 0:13 0:95 10 5 ðT 293Þ
ð1Þ
The energy band gap (Eg) of Bi2Te3 decreases with increasing temperature. Assuming that ideal gas state equation is still applicable to solid. PV ¼ nRT
ð2Þ
Please cite this article as: P. Zou, et al., Effect of high pressure sintering and annealing on microstructure and thermoelectric properties of nanocrystalline Bi2Te2.7Se0.3 doped with Gd, Progress in Natural Science: Materials International (2014), http://dx.doi.org/10.1016/j.pnsc.2014.05.009
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Fig. 4. Seebeck coefficient values of samples as a function of temperature (a) HPS sample, and (b) annealed sample.
Fig. 5. Power factor values of samples as a function of temperature (a) HPS sample, and (b) annealed sample.
By substituting Eq. (2) into Eq. (1), we can get PV 293 E g ¼ 0:13 0:95 10 5 nR
ð3Þ
By assuming n is a constant during high pressure sintering and annealing process, and taking a derivative with respect to Eq. (3), we can obtain the following equation: 0:95 10 5 ðdP þ dVÞ ð4Þ nR During sintering and annealing process, P is a constant, so dP ¼ 0. By substituting dP ¼ 0 into Eq. (4), we can get dE g ¼
0:95 10 5 dV ð5Þ nR This means that the energy band gap is only related to volume. Energy band gap is inversely proportional to volume. According to the relationship between carrier concentration and energy band gap [24]
dE g ¼
np ¼ 4ð2πKT=ℏ2 Þ3=2 ðme mh Þ3=2 e Eg =KT
ð6Þ
where n is the electron concentration, p is the hole concentration, K is the Boltzmann constant, T is the absolute
temperature, ħ ¼ h/2π, h is Planck's constant, me is the electron mass, mh is the hole mass, and Eg is the energy band gap (eV). From Eq. (6), it can be seen that carrier concentration is inversely proportional to energy gap. After annealing, since the cell volume decreased (dV o 0), the energy band gap increased in width (according to Eq. (5)), and carrier concentration decreased (according to Eq. (6)). Secondly, the electronegativity of Bi and Te are 2.02 and 2.01, respectively, and hence pure Bi2Te3 should be a covalent compound. The electronegativity of Gd is 1.11, which is much lower than the electronegativity of Te, hence the bonding of Bi2Te3 changes from covalent bonding to ionic bonding after Gd doping. As a result, the electrical conductivity of HPS sample decreased. Thirdly, the HPS sample consists of nanometer grains, which means that there are numerous grain boundaries in the HPS sample. Carrier was scattered by grain boundary and the large amount of residual stress. Therefore, the carrier mobility of HPS sample was low. While after annealing the carrier mobility is significantly increased due to the lattice defects reduction and lattice integrality improvement. Fig. 4(a) and (b) shows the temperature dependence of HPS sample's Seebeck coefficient and annealed sample's Seebeck
Please cite this article as: P. Zou, et al., Effect of high pressure sintering and annealing on microstructure and thermoelectric properties of nanocrystalline Bi2Te2.7Se0.3 doped with Gd, Progress in Natural Science: Materials International (2014), http://dx.doi.org/10.1016/j.pnsc.2014.05.009
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Fig. 6. Temperature dependence of the thermal conductivities of the HPS sample and annealed sample (a) κ, (b) κp , and (c) κbip þ κph.
coefficient respectively. Four phenomena appeared for the sample's properties. The first one is that the negative values of Seebeck coefficients indicate that all the samples are n-type semiconductor. The second one is that the absolute value of Seebeck coefficient of all samples reaches the minimum at room temperature, and increases with increasing temperature. The third one is that the Seebeck coefficient increases with increasing sintering temperature from 593 K to 693 K. Consequently, the highest Seebeck coefficient value of 126.56 μV/K is obtained at 473 K for the HPS sample sintered at 693 K as shown in Fig. 4(a). The relationship between Seebeck coefficient α and carrier concentration n can be expressed as follows [25]: " # KB 2ð2πmn K B TÞ3=2 α¼ 7 ðγ þ 2Þ þ ln ð7Þ e h3 n where7represents the polarity of carrier charge, KB is the Boltzmann constant, e is the electron charge, γ is the scattering factor, mn is the effective mass, h is Planck’s constant, n is carrier concentration, and T is the absolute temperature. From Eq. (7) it can be seen that Seebeck coefficient (α) is proportional to scattering factor (γ), and is inversely proportional to the carrier concentration (n). The increased Seebeck coefficient with increasing sintering temperature probably resulted from the decreased carrier concentration. The fourth one is that the Seebeck
coefficient of annealed sample is much higher than that of HPS sample as shown in Fig. 4(b). The reason can be attributed to the decrease in carrier concentration resulting from energy gap widening after annealing. The maximum Seebeck coefficient value of 182.94 μV/K was obtained at 473 K for the annealed sample (sintered at 693 K, annealed at 633 K). Fig. 5(a) and (b) shows the temperature dependence of the power factor (P) of the HPS sample and annealed sample respectively. It can be seen that as sintering temperature increases from 593 K to 673 K, the power factor gradually increased at first and then decreased when the sintering temperature reached 693 K. From Fig. 5(b) it can be seen that the power factor of annealed sample is much higher than that of the HPS sample. This can be attributed to the electrical conductivity and Seebeck coefficient's significant increase after annealing. The highest power factor of 3.31 mW K 2 m 1 was obtained at 298 K for the sample sintered at 673 K, and then annealed at 633 K for 36 h. For thermoelectric materials, the thermal conductivity can be expressed by the sum of the electronic component (κe) and lattice component (κph), where κe consists of the polar thermal conductivity (κp) and the bipolar components (κbip) [26]. κ ¼ κe þ κph ¼ κp þ κ bip þ κph
ð8Þ
The values of κp can be estimated from Wiedemann–Franz's law as κp ¼ LsT, where L is the Lorenz number
Please cite this article as: P. Zou, et al., Effect of high pressure sintering and annealing on microstructure and thermoelectric properties of nanocrystalline Bi2Te2.7Se0.3 doped with Gd, Progress in Natural Science: Materials International (2014), http://dx.doi.org/10.1016/j.pnsc.2014.05.009
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Fig. 7. Temperature dependence of the figure of merit ZT of the HPS sample and annealed sample.
(L ¼ 2.45 10 8 W Ω K 2 for semiconductor), s is electrical conductivity, and T is absolute temperature. Consequently, κbip þ κph can be obtained from κ and κp. The temperature dependences of the thermal conductivities, including κ, κp and κbip þ κph of the HPS samples sintered at 673 K and annealed sample (sintered at 673 K,annealed at 633 K), are shown in Fig. 6(a)–(c), respectively. From Fig. 6(a) it can be seen that thermal conductivity κ of HPS sample and annealed sample decreases with increasing temperature from 298 K to 423 K, and reaches its minimum value 2.18 Wm 1 K 1, and 1.55 Wm 1 K 1 at 423 K, respectively. The thermal conductivity κ of HPS sample and annealed sample increased when temperature was higher than 423 K. The reason for this is that at low temperature, phonon scattering enhancement due to lattice vibration is intensified with increase in the temperature. Therefore the lattice thermal conductivity (κph) decreased, as a result the value of κ decreased. When temperature was higher than 423 K, the intrinsic excitation increased the carrier concentration, and the bipolar thermal conductivity increased quickly, which lead to a significant increase of κ. From Fig. 6 (b) it can be seen that the polar thermal conductivity (κp) of HPS sample almost linearly increased with the increasing temperature, but κp of annealed sample decreases with increase in the temperature, and is equal to 1.03 Wm 1 K 1 at 473 K. Because of the narrow energy band gap and the high carrier concentration before annealing, the effect of all kinds of scattering on polar thermal conductivity (κp) is relatively weak, thus κp increased with the temperature increment. While after annealing, the energy band gap became wider, carrier concentration decreased and carrier mobility increased, and therefore κp decreased with temperature increasing. From Fig. 6(c) it can be seen that κph þ κbip of HPS sample and annealed sample decreases with increasing temperature between 298 K and 423 K, reaching its minimum value of 1.23 Wm 1 K 1, and 0.60 Wm 1 K 1 at 423 K, respectively. When temperature was higher than 423 K, κph þ κbip increased with increase in the temperature. Furthermore, the bipolar thermal conductivity (κbip) of annealed sample was lower than HPS sample.
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The reason is that when temperature is higher than 423 K, the intrinsic excitation increases the carrier concentration, and the bipolar thermal conductivity increases with increase in the temperature. Before annealing, due to the abundance of lattice defect which originates from high pressure sintering, energy band gap decreased in width, intrinsic excitation increased in intensity, and the anti-site defect increased, thus the κbip was increased. This result indicates that for the HPS sample the effect of κbip on κ is very significant. After annealing, the bipolar effect is suppressed due to energy band gap's increase in width, so κbip decreased. This suggests that a wider energy band gap is useful in terms of decreasing κbip. In addition, κ of annealed sample was lower than that of κ of HPS sample. Fig. 7 shows the temperature dependence of the figure of merit, ZT of HPS sample sintered at 673 K, and annealed sample (sintered at 673 K, and annealed at 633 K) respectively. ZT of HPS sample increased with increase in the temperature. The highest ZT value of 0.23 for HPS sample was obtained at 473 K. ZT value of annealed sample increased with temperature and reached the maximum value at 423 K. For the annealed sample, the maximum ZT of 0.74 was achieved at 423 K, which was enhanced by 85% compared with that of the ZM sample (ZT was 0.4) [27] at 423 K. These results can be applied to the field of thermoelectric power generation materials. The ZT value is improved in the whole temperature range for the annealed sample because of low thermal conductivity and good electrical property. 4. Conclusions N-type Bi2Te2.7Se0.3 compounds doped with Gd fabricated by the high pressure (6.0 GPa) sintering method at 593 K, 633K, 673 K and 693 K respectively. The experimental results indicate that the electrical transport properties of HPS samples are enhanced by increasing the sintering temperature from 593 K to 673 K while the sintering temperature must be at a moderate value. The bipolar diffusion has a strong effect on the thermal conductivity for HPS sample. HPS together with annealing can improve the TE properties of the sample by decreasing the thermal conductivity of the sample with nanostructure. It should be noted that this nanostructure has a good stability at high temperature. After annealing, the electrical conductivity and Seebeck coefficient both increased, while thermal conductivity decreased and ZT increased. ZTmax is 0.74 at 423 K, thus the temperature of the maximum of figure of merit has been increased. These results can be applied to the aspect of thermoelectric power generation materials. References [1] [2] [3] [4]
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Please cite this article as: P. Zou, et al., Effect of high pressure sintering and annealing on microstructure and thermoelectric properties of nanocrystalline Bi2Te2.7Se0.3 doped with Gd, Progress in Natural Science: Materials International (2014), http://dx.doi.org/10.1016/j.pnsc.2014.05.009
Original Research
Effect of high pressure sintering and annealing on microstructure and thermoelectric properties of nanocrystalline Bi2Te2.7Se0.3 doped with Gd Ping Zou, Gui-Ying Xun, Song Wang, Peng-Lei Chen, Feng-Zhu Huang Institution of Nuclear and New Energy Materials, University of Science and Technology Beijing, Beijing 100083, China Received 5 December 2013; accepted 20 April 2014
Abstract Bi2Te2.7Se0.3 of high performance doped with Gd bulk materials was prepared by a high pressure (6.0 GPa) sintering (HPS) method at 593 K, 633 K, 673 K and 693 K. The sample was then annealed for 36 h in a vacuum at 633 K. The phase composition, crystal structure and morphology of the sample were analyzed by X-ray diffraction and scanning electron microscopy. The electric conductivity, Seebeck coefficient, and thermal conductivity aspects of the sample were measured from 298 K to 473 K. The results show that high pressure sintering and the doping with Gd has a great effect on the crystal structure and the thermoelectric properties of the samples. The samples are consisted of nanoparticles before and after annealing, and these nanostructures have good stability at high temperature. HPS together with annealing can improve the TE properties of the sample by decreasing the thermal conductivity of the sample with nanostructures. The maximum ZT value of 0.74 was obtained at 423 K for the sample, which was sintered at 673 K and then annealed at 633 K for 36 h. Compared with the zone melting sample, it was increased by 85% at 423 K. Hence the temperature of the maximum of figure of merit was increased. The results can be applied to the field of thermoelectric power generation materials. & 2014 Chinese Materials Research Society. Production and hosting by Elsevier B.V. All rights reserved.
Keywords: Bi2Te2.7Se0.3; High pressure sintering; Annealing; Gd doped; Thermoelectric properties
1. Introduction Thermoelectric (TE) materials are the functional materials that can convert heat energy to electric energy directly due to thermoelectric effects. The performance of such materials can be represented by the dimensionless figure of merit: ZT ¼ α2 sT=κ, where α, s, κ, and T are Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature, respectively. In the recent years, most of the commercial thermoelectric devices operating near room temperature consist of Bismuth telluride (Bi2Te3) and its alloys. The crystal structure n
Corresponding author. Tel.: þ86 10 62333516. E-mail address: [email protected] (G.-Y. Xu). Peer review under responsibility of Chinese Materials Research Society.
of Bi2Te3 compounds is a hexagonal layered structure and it consists of five atomic planes in the sequence of Te(1)–Bi– Te(2)–Bi–Te(1) [1,2]. Although studied for more than 50 years, they are still the most widely used thermoelectric materials because of their remarkable TE performance [3]. The typical fabrication methods include unidirectional crystal-growth methods, such as Bridgman and zone-melting techniques [4–7], powder metallurgy techniques, such as hot-pressing (HP) [8,9], ultra high pressure [10–13], hot extrusion [14,15], spark plasma sintering (SPS) methods [16–18] and high pressure sintering (HPS) methods [19]. Compared to other methods, the samples prepared by high pressure sintering have the following advantages: efficient, low cost, suitable for large-scale production, more homogeneous nanocrystalline grain and restraining grain from coarsening during the sintering process, etc. Recently, many experimental results indicate that the refinement of Bi2Te3-based alloys can enhance the thermoelectric
http://dx.doi.org/10.1016/j.pnsc.2014.05.009 1002-0071/& 2014 Chinese Materials Research Society. Production and hosting by Elsevier B.V. All rights reserved.
Please cite this article as: P. Zou, et al., Effect of high pressure sintering and annealing on microstructure and thermoelectric properties of nanocrystalline Bi2Te2.7Se0.3 doped with Gd, Progress in Natural Science: Materials International (2014), http://dx.doi.org/10.1016/j.pnsc.2014.05.009
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performances [19–21]. Xu et al. [19] employed the HPS method to fabricate nanostructured p-type Bi2Te3-based materials. They found that the HPS technique can generate nanoparticles in the bulk materials which significantly reduce the lattice contribution to thermal conductivity, and the asprepared alloys show a highest ZT of 1.16 at room temperature. Yu et al. [20] prepared pure Bi2Te3 by mechanical alloying (MA) and high pressure sintering (HPS) methods. In their work, they synthesized Bi2Te3 nanopowder by an MA method at first and then fabricated bulk materials by the HPS technique. They investigated the effect of variation of sintering pressure on the properties of Bi2Te3. Zhang et al. [21] utilized a hydrothermal method to create nano-scale La- containing Bi–Te–Se starting powders and then hot-pressed the powders into a bulk which shows a maximum ZT of 0.5 at 450 K. But Bi2Te2.7Se0.3 doped with rare earth element Gd fabricated by the HPS method has not been examined until now. In this paper, the fabrication procedure of Bi2Te2.7Se0.3 þ 0.12 wt% SbI3 þ 0.12 wt% Gd with good thermoelectric properties using the HPS method was introduced. The effect of high pressure sintering and annealing on microstructure and thermoelectric properties of Bi2Te2.7Se0.3 doped with Gd was investigated. The formulas about the relationships between energy band gap, cell volume and sintering pressure were established. The formation mechanism of nanocrystalline was explained, and the effect of Gd was discussed. The thermoelectric properties of samples fabricated at different sintering temperatures under 6.0 GPa and then annealed at 633 K for 36 h were analyzed, and a qualitative correlation between sintering temperature and TE properties was revealed. The samples that were tested with the optimal 673 K HPS and were then annealed exhibited good thermoelectric properties. ZTmax is 0.74 occurring at 423 K, which can be applied to the field of thermoelectric power generation materials. 2. Experimental procedure Pure Bi, Te, Se, SbI3, and Gd powders with the purity of 99.99% were used as starting materials. The raw materials were weighed in appropriate atomic ratios, mixed in an agate mortar for 2 h and then synthesized for 4 h in a vacuum at 743 K. After crushing and cold compacting, the alloy was sintered for 1 min under high pressure (6.0 GPa) in cubic press (CS-IVY) at 593 K, 633 K, 673 K and 693 K respectively. Then the samples were sealed in quartz tubes under vacuum and annealed at 633 k for 36 h. After annealing, the samples were cooled to room temperature naturally. Phase identification of the samples were performed by X-ray diffraction (XRD, CuKα, λ¼ 0.15406 nm, D/max 2500), using a scan rate of 41/min to record the patterns in the 2θ range from 101 to 601. Accurate measurement of the lattice parameter was obtained through calibration with the silicon standard. The morphology was analyzed by field-emission scanning electron microscopy (JSM-7001F). The electric conductivity (s) was measured by the standard four-probe method in an Ar atmosphere. The Seebeck coefficient (α) was measured concurrently when a temperature difference (5 1C) was applied between two
ends of the sample. The power factor (P) was calculated by the formula: P ¼ α2 s. The thermal conductivity (κ) was calculated from the measured specific heat (Cp), thermal diffusivity (λ), and density (d) using the relationship of κ ¼ C p λd. Cp was measured using thermal analyzing apparatus (DuPont 1090B, America), λ was measured by a laser flash method (NETZSCH, LFA427, Germany), and d was measured by the Archimedes method. The figure of merit (ZT) was calculated by substituting the measured data of α, s and κ at given temperature into the formula: ZT ¼ α2 sT=κ. The Hall coefficient (RH) of the sample was measured at 30 1C using a physical property measurement system (PPMS-9T, Quantum Design Inc., USA) at a magnetic field of 2 T and an electrical current of 50 mA. The carrier concentration (n) was calculated from the Hall coefficient (RH) datum using n ¼ 1/RHe, where e is the electron charge. 3. Results and discussion Fig. 1 shows the XRD patterns of samples sintering at 593 K, 633 K, 673 K and 693 K under the pressure of 6.0 GPa. As shown in Fig. 1, all of the main diffraction peaks move towards large angle slightly compared with the standard diffraction data of Bi2Te2.7Se0.3 (PDF#50-0954) with a typical rhombohedral structure and no other phase exists. The broadened diffraction peaks of the HPS samples indicate a very small grain size. The average grain size of the HPS sample which sintered at 673 K is 27.72 nm (calculated using the classic Williamson–Hall method based on the measured width of diffraction peaks). The lattice parameters of samples also have been calculated, and the results are listed in Table 1. Table 1 presents the lattice constant against the cell volume of Bi2Te2.7Se0.3-based samples. From Table 1, it can be observed that the Bi2Te2.7Se0.3-based samples' properties have two obvious characteristics. The first one is that the lattice constant and the cell volume increased as the sintering temperature increased from 593 K to 673 K. But when the sintering temperature increased up to 693 K, the lattice
Fig. 1. XRD patterns of samples at different sintering temperatures.
Please cite this article as: P. Zou, et al., Effect of high pressure sintering and annealing on microstructure and thermoelectric properties of nanocrystalline Bi2Te2.7Se0.3 doped with Gd, Progress in Natural Science: Materials International (2014), http://dx.doi.org/10.1016/j.pnsc.2014.05.009
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constant decreased, and the cell volume also decreased. In addition, when the sintering temperature reached 673 K, the lattice constant a decreased but c and cell volume increased compared to that of the Bi2Te2.7Se3. This suggests that the high pressure sintering process produces obvious lattice distortion. The reason is that the ionic radius of Gd3 þ (0.94 Å) is smaller than the ionic radius of Bi3 þ (0.96 Å), Table 1 Lattice constant and cell volume of samples. Samples
Bi2Te2.7Se0.3 593 K(sintered) 633 K(sintered) 673 K(sintered) 693 K(sintered) 673 K(sintered), 633 K(annealed)
Lattice constant (nm) a¼b
c
0.4374 0.4356 0.4365 0.4368 0.4365 0.4349
3.0424 3.0368 3.0470 3.0599 3.0341 3.0513
Cell volume (nm3)
0.5041 0.4989 0.5027 0.5056 0.5007 0.4998
3
therefore the cell size decreased when the sample doped with Gd under atmospheric pressure. But after high pressure sintering, Gd3 þ is the cause of lattice distortion due to significant pressure, which further induces the lattice distortion to emerge and increase in size, resulting in cell size of HPS sample increasing. This indicates that the effect of trace Gd doping and high pressure on the crystal structure of Bi2Te2.7Se0.3 is significant. The second characteristic is that after annealing, the cell volume decreases. Fig. 2 shows the FE-SEM morphologies of the HPS samples sintered at different temperatures and annealed sample (sintered at 673 K, annealed at 633 K). The grain size of all HPS samples was smaller than 50 nm, after annealing the grain size was a little larger while still smaller than 100 nm, hence the sample still consisted of nanometer grains after annealing. These results show that the nanocrystalline prepared by the HPS method has good stability at high temperature. The formation mechanisms of nanocrystalline are discussed as the following. Firstly, the Bi2Te2.7Se0.3
Fig. 2. FE-SEM of sample (a) 6.0 GPa HPS sample (sintered at 593 K), (b) 6.0 GPa HPS sample (sintered at 633 K), (c) 6.0 GPa HPS sample (sintered at 673 K), (d) 6.0 GPa HPS sample (sintered at 693 K), (e) 6.0 GPa annealed sample(sintered at 673 K, annealed at 633 K). Please cite this article as: P. Zou, et al., Effect of high pressure sintering and annealing on microstructure and thermoelectric properties of nanocrystalline Bi2Te2.7Se0.3 doped with Gd, Progress in Natural Science: Materials International (2014), http://dx.doi.org/10.1016/j.pnsc.2014.05.009
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Fig. 3. Electrical conductivity values of samples as a function of temperature (a) HPS sample, and (b) annealed sample.
Table 2 Hall coefficient, carrier concentrations and mobility of samples. Samples
Hall coefficient (cm3/C)
Carrier concentration (n) 1019/(cm 3)
Carrier mobility μ (cm2/vs)
593 K 633 K 673 K 693 K 673 K 633 K
0.19 0.2 0.22 0.52 0.53
3.25 3.06 2.80 1.21 1.19
14.53 16.52 22.69 27.62 84.73
(sintered) (sintered) (sintered) (sintered) (sintered), (annealed)
samples are brittle materials and the pressurization rates are very high, reaching set pressure of 6.0 GPa within 2 min, which is equivalent to the application of bursting high pressure to the sample, leading to the refinement of brittle particles. Thus the raw materials have become nanoparticle before reaching the specified sintering temperature. The second is the higher supercooling (ΔT), i.e., the higher cooling rate. The higher the ΔΤ, the smaller the crystal critical radius, and the larger the nucleation rate becomes. As a result, the high degree of supercooling will promote the formation of many very small critical nuclei in a very short amount of time. In addition, the high pressure also limits the movement of the atoms or ions and the size of critical nucleation radius. Therefore, the growth rates of the grains decreased greatly, and the nanoparticles formed [19]. It is believed that nanostructure may be one of the most effective ways to improve the performance of TE materials, which can scatter phonons more effectively than electrons [22], hence decreasing the thermal conductivity (κ), and consequently enhancing the value of ZT. Fig. 3(a) and (b) shows the temperature dependence of electrical conductivities of the HPS samples and annealed samples. It can be seen that the HPS samples' properties have three obvious features. The first one is that all the samples show a decrease in electrical conductivity with increasing measuring temperature from 298 K to 473 K, indicating a metallic conducting behavior. The second feature is that the electrical conductivity increases with increasing sintering
temperature, the highest electrical conductivity of 101.68 (Ω mm) 1 is obtained at 298 K for the HPS sample sintered at 673 K as shown in Fig. 3(a). The increase in electrical conductivity can be attributed to the increase in carrier mobility resulting from the grain growth and densification. However, when the sintering temperature was further increased to 693 K, the electrical conductivity decreased to 48.97 (Ω mm) 1. While raising the HPS temperature resulted in the decrease of carrier concentration (n) as presented in Table 2. It is well known that two competing factors of carrier concentration (n) and carrier mobility (μ) determine the electrical conductivity, and the relationship of both can be described by the equation of s ¼ neμ. The values of (n) and (μ) of HPS sample sintered at 693 K are both low as presented in Table 2, and the electrical conductivity of HPS sample sintered at 693 K is the lowest as shown in Fig. 3(a). The third one is that the electrical conductivity of annealed sample is higher than that of the HPS sample as shown in Fig. 3(b). The highest electrical conductivity value of 161.35 (Ω mm) 1 was obtained at 298 K for the annealed sample (sintered at 673 K, and annealed at 633 K). The increase in electrical conductivity after annealing can be attributed to the increase in carrier mobility. The variation of carrier concentration and carrier mobility before and after annealing is listed in Table 2. As presented in Table 2 the carrier concentration of HPS sample decreases with increasing sintering temperature. In addition, the carrier concentration of HPS sample is higher than that of the annealed sample, but the carrier mobility of the HPS sample is lower than that of the annealed sample. The relationship between the energy band gap of Bi2Te3 and temperature can be given as follows [23]: Eg ¼ 0:13 0:95 10 5 ðT 293Þ
ð1Þ
The energy band gap (Eg) of Bi2Te3 decreases with increasing temperature. Assuming that ideal gas state equation is still applicable to solid. PV ¼ nRT
ð2Þ
Please cite this article as: P. Zou, et al., Effect of high pressure sintering and annealing on microstructure and thermoelectric properties of nanocrystalline Bi2Te2.7Se0.3 doped with Gd, Progress in Natural Science: Materials International (2014), http://dx.doi.org/10.1016/j.pnsc.2014.05.009
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Fig. 4. Seebeck coefficient values of samples as a function of temperature (a) HPS sample, and (b) annealed sample.
Fig. 5. Power factor values of samples as a function of temperature (a) HPS sample, and (b) annealed sample.
By substituting Eq. (2) into Eq. (1), we can get PV 293 E g ¼ 0:13 0:95 10 5 nR
ð3Þ
By assuming n is a constant during high pressure sintering and annealing process, and taking a derivative with respect to Eq. (3), we can obtain the following equation: 0:95 10 5 ðdP þ dVÞ ð4Þ nR During sintering and annealing process, P is a constant, so dP ¼ 0. By substituting dP ¼ 0 into Eq. (4), we can get dE g ¼
0:95 10 5 dV ð5Þ nR This means that the energy band gap is only related to volume. Energy band gap is inversely proportional to volume. According to the relationship between carrier concentration and energy band gap [24]
dE g ¼
np ¼ 4ð2πKT=ℏ2 Þ3=2 ðme mh Þ3=2 e Eg =KT
ð6Þ
where n is the electron concentration, p is the hole concentration, K is the Boltzmann constant, T is the absolute
temperature, ħ ¼ h/2π, h is Planck's constant, me is the electron mass, mh is the hole mass, and Eg is the energy band gap (eV). From Eq. (6), it can be seen that carrier concentration is inversely proportional to energy gap. After annealing, since the cell volume decreased (dV o 0), the energy band gap increased in width (according to Eq. (5)), and carrier concentration decreased (according to Eq. (6)). Secondly, the electronegativity of Bi and Te are 2.02 and 2.01, respectively, and hence pure Bi2Te3 should be a covalent compound. The electronegativity of Gd is 1.11, which is much lower than the electronegativity of Te, hence the bonding of Bi2Te3 changes from covalent bonding to ionic bonding after Gd doping. As a result, the electrical conductivity of HPS sample decreased. Thirdly, the HPS sample consists of nanometer grains, which means that there are numerous grain boundaries in the HPS sample. Carrier was scattered by grain boundary and the large amount of residual stress. Therefore, the carrier mobility of HPS sample was low. While after annealing the carrier mobility is significantly increased due to the lattice defects reduction and lattice integrality improvement. Fig. 4(a) and (b) shows the temperature dependence of HPS sample's Seebeck coefficient and annealed sample's Seebeck
Please cite this article as: P. Zou, et al., Effect of high pressure sintering and annealing on microstructure and thermoelectric properties of nanocrystalline Bi2Te2.7Se0.3 doped with Gd, Progress in Natural Science: Materials International (2014), http://dx.doi.org/10.1016/j.pnsc.2014.05.009
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Fig. 6. Temperature dependence of the thermal conductivities of the HPS sample and annealed sample (a) κ, (b) κp , and (c) κbip þ κph.
coefficient respectively. Four phenomena appeared for the sample's properties. The first one is that the negative values of Seebeck coefficients indicate that all the samples are n-type semiconductor. The second one is that the absolute value of Seebeck coefficient of all samples reaches the minimum at room temperature, and increases with increasing temperature. The third one is that the Seebeck coefficient increases with increasing sintering temperature from 593 K to 693 K. Consequently, the highest Seebeck coefficient value of 126.56 μV/K is obtained at 473 K for the HPS sample sintered at 693 K as shown in Fig. 4(a). The relationship between Seebeck coefficient α and carrier concentration n can be expressed as follows [25]: " # KB 2ð2πmn K B TÞ3=2 α¼ 7 ðγ þ 2Þ þ ln ð7Þ e h3 n where7represents the polarity of carrier charge, KB is the Boltzmann constant, e is the electron charge, γ is the scattering factor, mn is the effective mass, h is Planck’s constant, n is carrier concentration, and T is the absolute temperature. From Eq. (7) it can be seen that Seebeck coefficient (α) is proportional to scattering factor (γ), and is inversely proportional to the carrier concentration (n). The increased Seebeck coefficient with increasing sintering temperature probably resulted from the decreased carrier concentration. The fourth one is that the Seebeck
coefficient of annealed sample is much higher than that of HPS sample as shown in Fig. 4(b). The reason can be attributed to the decrease in carrier concentration resulting from energy gap widening after annealing. The maximum Seebeck coefficient value of 182.94 μV/K was obtained at 473 K for the annealed sample (sintered at 693 K, annealed at 633 K). Fig. 5(a) and (b) shows the temperature dependence of the power factor (P) of the HPS sample and annealed sample respectively. It can be seen that as sintering temperature increases from 593 K to 673 K, the power factor gradually increased at first and then decreased when the sintering temperature reached 693 K. From Fig. 5(b) it can be seen that the power factor of annealed sample is much higher than that of the HPS sample. This can be attributed to the electrical conductivity and Seebeck coefficient's significant increase after annealing. The highest power factor of 3.31 mW K 2 m 1 was obtained at 298 K for the sample sintered at 673 K, and then annealed at 633 K for 36 h. For thermoelectric materials, the thermal conductivity can be expressed by the sum of the electronic component (κe) and lattice component (κph), where κe consists of the polar thermal conductivity (κp) and the bipolar components (κbip) [26]. κ ¼ κe þ κph ¼ κp þ κ bip þ κph
ð8Þ
The values of κp can be estimated from Wiedemann–Franz's law as κp ¼ LsT, where L is the Lorenz number
Please cite this article as: P. Zou, et al., Effect of high pressure sintering and annealing on microstructure and thermoelectric properties of nanocrystalline Bi2Te2.7Se0.3 doped with Gd, Progress in Natural Science: Materials International (2014), http://dx.doi.org/10.1016/j.pnsc.2014.05.009
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Fig. 7. Temperature dependence of the figure of merit ZT of the HPS sample and annealed sample.
(L ¼ 2.45 10 8 W Ω K 2 for semiconductor), s is electrical conductivity, and T is absolute temperature. Consequently, κbip þ κph can be obtained from κ and κp. The temperature dependences of the thermal conductivities, including κ, κp and κbip þ κph of the HPS samples sintered at 673 K and annealed sample (sintered at 673 K,annealed at 633 K), are shown in Fig. 6(a)–(c), respectively. From Fig. 6(a) it can be seen that thermal conductivity κ of HPS sample and annealed sample decreases with increasing temperature from 298 K to 423 K, and reaches its minimum value 2.18 Wm 1 K 1, and 1.55 Wm 1 K 1 at 423 K, respectively. The thermal conductivity κ of HPS sample and annealed sample increased when temperature was higher than 423 K. The reason for this is that at low temperature, phonon scattering enhancement due to lattice vibration is intensified with increase in the temperature. Therefore the lattice thermal conductivity (κph) decreased, as a result the value of κ decreased. When temperature was higher than 423 K, the intrinsic excitation increased the carrier concentration, and the bipolar thermal conductivity increased quickly, which lead to a significant increase of κ. From Fig. 6 (b) it can be seen that the polar thermal conductivity (κp) of HPS sample almost linearly increased with the increasing temperature, but κp of annealed sample decreases with increase in the temperature, and is equal to 1.03 Wm 1 K 1 at 473 K. Because of the narrow energy band gap and the high carrier concentration before annealing, the effect of all kinds of scattering on polar thermal conductivity (κp) is relatively weak, thus κp increased with the temperature increment. While after annealing, the energy band gap became wider, carrier concentration decreased and carrier mobility increased, and therefore κp decreased with temperature increasing. From Fig. 6(c) it can be seen that κph þ κbip of HPS sample and annealed sample decreases with increasing temperature between 298 K and 423 K, reaching its minimum value of 1.23 Wm 1 K 1, and 0.60 Wm 1 K 1 at 423 K, respectively. When temperature was higher than 423 K, κph þ κbip increased with increase in the temperature. Furthermore, the bipolar thermal conductivity (κbip) of annealed sample was lower than HPS sample.
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The reason is that when temperature is higher than 423 K, the intrinsic excitation increases the carrier concentration, and the bipolar thermal conductivity increases with increase in the temperature. Before annealing, due to the abundance of lattice defect which originates from high pressure sintering, energy band gap decreased in width, intrinsic excitation increased in intensity, and the anti-site defect increased, thus the κbip was increased. This result indicates that for the HPS sample the effect of κbip on κ is very significant. After annealing, the bipolar effect is suppressed due to energy band gap's increase in width, so κbip decreased. This suggests that a wider energy band gap is useful in terms of decreasing κbip. In addition, κ of annealed sample was lower than that of κ of HPS sample. Fig. 7 shows the temperature dependence of the figure of merit, ZT of HPS sample sintered at 673 K, and annealed sample (sintered at 673 K, and annealed at 633 K) respectively. ZT of HPS sample increased with increase in the temperature. The highest ZT value of 0.23 for HPS sample was obtained at 473 K. ZT value of annealed sample increased with temperature and reached the maximum value at 423 K. For the annealed sample, the maximum ZT of 0.74 was achieved at 423 K, which was enhanced by 85% compared with that of the ZM sample (ZT was 0.4) [27] at 423 K. These results can be applied to the field of thermoelectric power generation materials. The ZT value is improved in the whole temperature range for the annealed sample because of low thermal conductivity and good electrical property. 4. Conclusions N-type Bi2Te2.7Se0.3 compounds doped with Gd fabricated by the high pressure (6.0 GPa) sintering method at 593 K, 633K, 673 K and 693 K respectively. The experimental results indicate that the electrical transport properties of HPS samples are enhanced by increasing the sintering temperature from 593 K to 673 K while the sintering temperature must be at a moderate value. The bipolar diffusion has a strong effect on the thermal conductivity for HPS sample. HPS together with annealing can improve the TE properties of the sample by decreasing the thermal conductivity of the sample with nanostructure. It should be noted that this nanostructure has a good stability at high temperature. After annealing, the electrical conductivity and Seebeck coefficient both increased, while thermal conductivity decreased and ZT increased. ZTmax is 0.74 at 423 K, thus the temperature of the maximum of figure of merit has been increased. These results can be applied to the aspect of thermoelectric power generation materials. References [1] [2] [3] [4]
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Please cite this article as: P. Zou, et al., Effect of high pressure sintering and annealing on microstructure and thermoelectric properties of nanocrystalline Bi2Te2.7Se0.3 doped with Gd, Progress in Natural Science: Materials International (2014), http://dx.doi.org/10.1016/j.pnsc.2014.05.009