Effect of process parameters of microwave activated hot pressing on the microstructure and thermoelectric properties of Bi2Te3-based alloys

Accepted Manuscript Effect of process parameters of microwave activated hot pressing on the microstructure and thermoelectric properties of Bi2Te3 based alloys Xi’an Fan, Zhenzhou Rong, Fan Yang, Xinzhi Cai, Xuewu Han, Guangqiang Li PII: DOI: Reference:

S0925-8388(15)00148-6 http://dx.doi.org/10.1016/j.jallcom.2015.01.075 JALCOM 33125

To appear in:

Journal of Alloys and Compounds

Received Date: Revised Date: Accepted Date:

2 December 2014 13 January 2015 13 January 2015

Please cite this article as: X. Fan, Z. Rong, F. Yang, X. Cai, X. Han, G. Li, Effect of process parameters of microwave activated hot pressing on the microstructure and thermoelectric properties of Bi2Te3 based alloys, Journal of Alloys and Compounds (2015), doi: http://dx.doi.org/10.1016/j.jallcom.2015.01.075

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Effect of process parameters of microwave activated hot pressing on the microstructure and thermoelectric properties of Bi2Te3 based alloys Xi'an Fan*1, 2, Zhenzhou Rong1, 2, Fan Yang1, 2, Xinzhi Cai1, 2, Xuewu Han1, 2, Guangqiang Li1, 2 1 The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China 2 Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology, Wuhan 430081, China

*Corresponding author: Xi'an Fan Address: P. O. Box 185#, School of Materials and Metallurgy, Wuhan University of Science and Technology, 947 Heping Road, Qingshan District, Wuhan, 430081, P. R. China. E-mail address: [email protected] Tel: +86-27-68862529

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Abstract: In this paper, fine-grained and highly dense n-type Bi2Te2.7Se0.3 alloys were prepared via mechanical alloying and a new molding technology named microwave activated hot pressing (MAHP) with relatively low sintering temperature and short holding time. The effects of sintering temperature and holding time on the microstructure and thermoelectric properties of sintered samples were investigated in detail. The results indicated that the sintered alloys could achieve nearly complete densification. Laminated microstructure dominated in the as-MAHPed samples and their size increased gradually with increasing sintering temperature. At the same time, although Seebeck coefficients decreased slightly, their power factors increased obviously due to the drastic decrease of electrical resistivities. And a maximum power factor of 1.81 × 10-3 Wm-1K-2 was achieved at room temperature for the sample sintered at 693 K for 10 min. A minimum thermal conductivity of 0.61 Wm-1K-1 was obtained for the sample sintered at 533 K for 10 min owing to the drastic boundary-phonons scattering effect. A highest ZT value of 0.71 was obtained around 423 K for the Bi2Te2.7Se0.3 sample sintered at 613 K without holding time. It confirms that the MAHP technology introduced here can achieve low temperature and rapid sintering process well in preparations of Bi2Te3-based alloys, and has widely potential applications in other functional materials. Keywords: Bi2Te3; Microwave activated hot pressing; Mechanical alloying; Thermoelectric properties 1. Introduction Bismuth telluride based alloys have been widely used as thermoelectric materials for thermoelectric power generators and coolers around the room temperature [1,2]. Commercial Bi2Te3-based alloys are widely produced by unidirectional crystal growth methods, such as zone melting (ZM) and Bridgman techniques [3,4]. Most of these 2

alloys are monocrystalline or oriented polycrystalline. They usually have high electrical properties along the grain growth direction but also have high thermal conductivities. Although their thermoelectric figure-of-merit (ZT) values can reach 1.0 with a thermoelectric conversion efficiency of ~5%, the commercial Bi2Te3-based alloys have poor machining properties. They are easy to fracture along the basal plane due to the weak Van der Waals bonding between Te(1)-Te(1) layers [5], which result in a low material utilization efficiency (<60%) and much difficulties in their preparations and applications. At present, powder metallurgy techniques have been widely favored by researchers for the possibility to achieve the industrial productions, and applied in the preparations of polycrystalline Bi2Te3-based alloys to overcome the poor mechanical performance, decrease the thermal conductivity and improve the thermoelectric properties [1,3,4,6-15]. The traditional sintering techniques, such as hot pressing (HP), spark plasma sintering (SPS), hot isostatic pressing (HIP), are helpful to prevent the grain growth and preserve the nanostructures during sintering processes. This is an effective way to reduce the lattice thermal conductivity by enhancing phonon scattering to improve the ZT value [14,15]. For example, in the work of Poudel et al. [1], p-type nanocrystalline Bi-Sb-Te bulk alloys with relatively low thermal conductivity were prepared by mechanical alloying (MA) combined with HP process, and a highest ZT value of 1.4 was obtained at 373 K. Microwave sintering technology, one of pressureless sintering methods, has been preliminarily introduced to prepare Bi2Te3-based alloys [16-21]. For some materials, microwave can improve the mechanical and physical properties owing to its unique heating mechanisms, such as bulk heating, selective heating, non-thermal effect, and hybrid heat effect [22]. In the work of Kim-Hak et al. [16] and Delaizir et al. [17], the

3

cold-pressed powders were sintered by microwave heating in a specially designed multimode

cavity

to

prepare

p-type

Bi2Te3-based

alloys.

A

maximum

room-temperature power factor of 2.9 × 10−3 WK−2m−1 and a highest ZT value of 0.7 were obtained, respectively. However, the relative densities of these alloys processed by microwave sintering were only 86% ~ 90%. Comparing with pressureless sintering, pressure-assisted sintering is beneficial to mass transfer process, such as powders contact, powders flow and elements diffusion etc. To combine the merits of pressure-assisted sintering and microwave heating, the microwave activated hot pressing (MAHP) technology was developed in our lab. The MAHP equipment holds a uniaxial pressure in the whole microwave heating process, achieving pressure-assisted sintering and microwave heating at the same time. In the previous works [20,21], p-type Bi0.4Sb1.6Te3 alloys and n-type Bi2Te2.85Se0.15 alloys were preliminarily prepared by MA combined with MAHP. Their relative densities could reach ~ 99%, and the highest ZT values could reach to 1.04 and 0.57, respectively. However, the sintering temperature and holding time we selected were imperfect, and further works are needed to explore the optimum process parameters to improve the thermoelectric properties. In the present work, n-type Bi2Te2.7Se0.3 alloys were prepared via MA-MAHP process with lower sintering temperature and shorter holding time on the basis of our previous works [12,20,21]. The effects of sintering parameters on the microstructures and thermoelectric properties of Bi2Te2.7Se0.3 alloys were investigated in detail. 2. Materials and methods Based on the stoichiometry of Bi2Te2.85Se0.3, high-purity powders (≥99.99 wt.%, 200 mesh) of Bi, Se, Te were weighed and then sealed into a planetary ball mill (QM-4F, Nanjing Nanda Instrument) for the MA process. The weight ratio of ball to 4

powder was 15:1, and the process was conducted at a speed of 400 rpm for 10 h under argon atmosphere. Subsequently, the as-MAed powders were loaded into a graphite die with an inner diameter of 20 mm, which was surrounded by a circle SiC mold. The consolidation process was performed in the MAHP equipment with an inputting microwave frequency of 2.45 GHz. The heating rate was fixed at 40 K/min and the sintering processes were performed at 533, 573, 653, 693 K (an error of ± 3 K) for 10 min and 613 K (an error of ± 3 K) for 0, 10, 30 min in argon atmosphere, respectively. The temperature measurement point of infrared thermometer was horizontal to the center of sintered sample, and the temperature of outer surface of SiC was collected as sintering temperature. The uniaxial pressure was fixed at 50 MPa. Finally, some small cylindrical alloys with a typical size of φ20 × 13 mm were obtained. The X-ray diffraction (XRD) in a Philips X’pert Pro diffractometer with Cu Kα radiation (λ=1.5418 Å) was applied to examine the phases of the as-MAed powders and sintered samples. The fracture morphologies of alloys were observed by a field-emission scanning electron microscopy (FESEM, FEI/Nova400 NanoSEM). The compositions of the as-MAed powders and alloys were analyzed by energy dispersive X-ray spectroscopy (EDS). The volume densities (d) of sintered alloys were measured by Archimedes method. The Seebeck coefficient was measured by a dynamic method and the ratio of Seebeck voltage and differential temperature (5 ~ 10 K) was fitted by a least square method. The electrical resistivity was measured using a Four-Point Probe method. A Ecopia HMS 5500 Hall system was applied to measure the room-temperature Hall coefficient (RH) by using the van der Pauw method with a magnetic field strength of 0.550 T, and the carrier concentration (n) and mobility (μ) were calculated according to the equations: RH = 1 / ne and ρ = 1 / nμe , where e is

the electrical charge of electron [23]. The thermal diffusivity (D) and specific heat (Cp) 5

were measured by a laser flash method (LFA 457, Netzsch) and the thermal conductivity was calculated from the formula of κ = DC p d . The measurement directions of electrical properties and thermal properties were all perpendicular to the direction of sintering pressure for all the samples. 3. Results and Discussions

The XRD patterns of the as-MAed powders and sintered samples are shown in Fig. 1. All the diffraction peaks are consistent with the standard JCPDS card of Bi2Te3 (JCPDS 15-0863) and can be indexed to the rhombohedral lattice (space group R 3 m). However, comparing with the patterns of Bi2Te3, diffraction peaks of the powders and sintered samples shift integrally to higher angles, indicating that the Se element doped into the matrix of Bi2Te3 successfully since the Se atom has a smaller atomic radius than Te atom. Furthermore, the XRD peaks of sintered alloys become sharper and stronger after MAHP progresses, indicating the improvement of crystallinity and the release of crystalline defects and remnant stress resulted from MA process. The relative densities of the sintered samples are shown in Fig. 2, and the theoretical density of Bi2Te2.85Se0.3 is decided as 7.769 g/cm3 (JCPDS 50-0954). When the sintering temperature was fixed, extending the holding time appropriately contributed to the densification, and a maximum relative density of 99.36% was obtained for the sample sintered at 613 K for 30 min. On the other hand, when the holding time was fixed at 10 min, the relative density improved slightly from 98.7% to 99.12% with increasing the sintering temperature until 613 K. Comparing with the samples prepared by traditional microwave sintering method [16,17], the samples sintered by MAHP are highly dense, and their relative densities are 9%~14% higher than the former’s. It suggests that the applied uniaxial pressure plays an important role on improving mass transfer and element migration to promote densification during the 6

MAHP process. However, with further increasing sintering temperature, the relative density reduced slightly to 98.52% of the 653 K sample and 97.90% of the 693 K sample due to the serious volatilization of Se and Te elements. The saturated vapor pressure of Bi, Te and Se elements are 10-5, 1 and 100 Pa at 640 K, respectively [23]. It suggests that Te and Se elements are much easier to volatilize than Bi element. According to the results of EDS in inset of Fig. 2, the content of Bi changed little at relatively low sintering temperature, but enhanced a lot when the sintering temperature was relatively high, implying that the volatilization of Te and Se elements exacerbated with increasing sintering temperature. The fracture FESEM photographs of the sintered samples are indicated in Fig. 3. The most grains show a thin laminated structure. It can be found that the grain size increases with increasing sintering temperature, but its increase is not obvious with increasing holding time. There are still some fine grains (<85 nm) existing independently in the sample sintered at 533 K, suggesting that the grain growth is not complete. The sample sintered at 613 K for 10 min has a uniform thin sheet structure with a nanoscale monolayer. It can be concluded that the fracture morphology of n-type Bi2Te2.85Se0.3 alloys prepared by the MAHP technology are similar to those of samples prepared by traditional HP or SPS process [4,6-9]. The sample sintered at a higher sintering temperature such as 653 K or 693 K experienced a very significant grain growth and the sides of the long dimension of thin sheet reached to micro-size. Especially, there are large amount of pores with submicron size distributed in the matrix of lamellar structure in the sample sintered at 693 K. This may be resulted from the volatilization of elements such as Te or Se. Despite partially oriented lamellar structures can be observed at some regions, there are no obvious large-scale preferred orientations in these samples, which is also consistent with the XRD results

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shown in Fig. 1. Fig. 4 illustrates the testing temperature dependence of Seebeck coefficient (α) for the samples sintered at different conditions. The negative values of αconfirm that the electrons are the main carriers. For all the samples, their absolute values of αincrease with increasing testing temperature at first, but subsequently decrease after 423~443 K due to the intrinsic excitation. The absolute value of room-temperature αdecreases from 164.13 μV/K to 130.92 μV/K with the increase of sintering temperature from 533 K to 693 K, and it also declines slightly from 158.46 μV/K to 150.24 μV/K with extending the holding time from 0 min to 30 min. It’s well known that α can be expressed as Eq. (1) [24]:

α ∝ γ − ln n

(1)

Where γ is scattering factor and n is carrier concentration. As shown in table 1, when the sintering temperature was increased or the holding time was extended, the room-temperature carrier concentration enhanced gradually from 6.31 × 10-19 cm-3 to 9.22 × 10-19 cm-3 or from 7.32 × 10-19 cm-3 to 8.03 × 10-19 cm-3, which could cause the decrease of the room-temperature α. The variation of carrier concentration should be attributable to the apparent fluctuation of compositions under different sintering conditions. Increasing sintering temperature or extending holding time would lead to the aggravated volatilization of selenium and tellurium elements in different degrees. With occupation of Bi atoms into Te sites, the progress can be simplified as Eq. (2) [15,25]: Bi2Te3=2BiTe+2VBi+VTe+(3/2)Te2(g)+2h

(2)

Where BiTe is an anti-site defect, VBi is a Bi vacancy, VTe is a Te vacancy, and h is a hole. Moreover, when Bi atoms occupy the Se sites in Bi2(Te,Se)3, electrons are

8

generated as Eq. (3) [25,26]: 3Bi2Se3=4BiBi+2BiSe+7VSe+2V1.5Bi+(9/2)Se2(g)+2e

(3)

Where Bi(Bi,Se) is an anti-site defect, VSe is a Se vacancy, and e is the produced electron. Due to the higher saturated vapour pressure of Se element, it is much easier to evaporate than Te element and more VSe can be formed than VTe in Bi2TexSe3-x compounds. Therefore, more carriers (electrons) will be produced with the aggravated volatilization of Se elements, resulting in the decrease of room-temperature α. Another view considers that the interaction of vacancies with the anti-site defects will also lead to the increase of electrons, shown as Eq. (4) [12,27]: 2VBi+3VTe+BiTe=VBi+BiBi+4VTe+6e

(4)

As a result, the carrier concentration increases with the increase of sintering temperature or holding time. Fig. 5 presents the testing temperature dependence of electrical resistivity (ρ) for the sintered samples. The ρ increases monotonically with testing temperature except for the sample sintered at 533 K. Unexpectedly, the ρof the sample sintered at 533 K decreases as testing temperature. And this kind of typical semiconductor behavior has been reported in other works [20,28-31], which may be resulted from the potential barrier scattering at grain boundary and annealing effect. According to the thermionic emission, the electrical conductivity (σ) across the grain boundaries can be described as σ(T)~T-1/2exp(-EB/kT), where EB is the height of grain boundary potential barrier [27]. Because the grains in the sample sintered at 533 K is relatively small, the potential barrier scattering at grain boundary may lead to a negative temperature dependence of resistivity. On the other hand, some fine grains (<85 nm) still exist independently in this sample (Fig. 3a), and it may be not consolidated fully at a

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relatively low sintering temperature. Then the annealing effect with increasing testing temperature would enhance the contact between the granules, which may also decrease the electrical resistivity [31]. Moreover, the room-temperature ρdecreases with sintering temperature from 533 K to 693 K or holding time from 0 min to 30 min. Theoretically, ρcan be defined as [24]: ρ = 1 / neμ , where μ is carrier mobility. It means that ρshould be affected by the simultaneous variation of n and μ. With increasing sintering temperature, the obvious grain growth weakens carrier scattering effect. As a result, the room-temperature μ increases from 19.53 cmV-1S-1 to 71.35 cmV-1S-1 (as shown in Table 1), which can partly decrease the ρfrom 5.07 × 10-5 Ωm to 0.94 × 10-5 Ωm. With extending holding time, the room-temperature μ varies slightly from 48.51 cmV-1S-1 to 55.20 cmV-1S-1 owing to the unobvious grain growth. Besides, the increasing n with the enhancement of sintering temperature or holding time can also make contributions to the decrease of ρ. Fig. 6 shows testing temperature dependence of the power factor (α2/ρ) for the sintered samples. It can be noted that the alloy sintered at the lowest temperature of 533 K shows a very poor power factor of 5.3 × 10-4 Wm-1K-2. With a further increase in sintering temperature or holding time, the room-temperature power factor shows a gradual increase, indicating that the increase of sintering temperature or holding time is beneficial to improve the electrical transport properties. And a maximum power factor of 1.81 × 10-3 Wm-1K-2 at room temperature was obtained for the 693 K sample. The testing temperature dependences of thermal transport properties for the sintered samples are presented in Fig. 7. As shown in Fig. 7(a), with the increase of testing temperature, the thermal conductivities decrease at first because of the 10

intensified phonon-phonon coupling. Then they turn to increase at a higher temperature range (over 383 K) due to an ambipolar contribution arising from the diffusion of electron-hole pairs with the onset of intrinsic contribution [18]. The minimum thermal conductivity of 0.61 Wm-1K-1 was achieved around 343 K for the sample sintered at the 533 K due to its smallest grains. The thermal conductivity (κ) is the sum of electronic thermal conductivity (κe) and the lattice thermal conductivity (κl). According to Wiedemann-Franz relation, κe can be calculated as: κe = L0T / ρ , with the Lorenz number L0 taken as 2.45 × 10-8 V2K-2 [32,33]. The results of κl are shown in Fig. 7(b). The room-temperature κ and κl increase with sintering temperature or holding time for the samples sintered below 693 K. This should be resulted from the decrease of carrier-grain boundary scattering and phonon-grain boundary scattering owing to the grain growth (as shown in Fig. 3). However, with the further increase of sintering temperature from 653 K to 693 K, the room-temperature κdecreases abnormally from 1.42 to 1.40 Wm-1K-1. Despite the 693 K sample experienced a significant grain growth, the submicro scale pores dispersed in the matrix may enhance the scattering of phonons, leading to a reduction in the κl from 0.71 to 0.62 Wm-1K-1 [21,34]. Finally, the temperature dependences of ZT for the sintered samples are displayed in Fig. 8. With increasing testing temperature, the ZT values of all the samples increase firstly and then decrease. A maximum ZT values can be achieved around 423 K. When the holding times were fixed at 10 min, the ZT values around 423 K increased firstly with increasing sintering temperature before 613 K, and then they turned to decline. Moreover, when the sintering temperatures were fixed at 613 K, the ZT values around 423 K also declined with prolonging holding time owing to the

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increase of thermal conductivities. The highest ZT value of 0.71 could be achieved around 423 K for the sample sintered at 613 K for 0 min. Besides, the ZT value in the temperature range of 380 K ~ 500 K is more than 0.6 for the sample sintered at 613 K for 0 min, which implies that these alloys can be useful not only for cooling device, but also for low temperature power generation. 4. Conclusions

In the present work, n-type Bi2Te2.7Se0.3 alloys were successfully prepared via MA-MAHP process with relatively low sintering temperature and short holding time. The most grains showed fine laminated structure and experienced an obvious increase process in size with increasing sintering temperature. The variation of grain size is not obvious with prolonging holding time. Nevertheless, a relatively high sintering temperature of 693 K resulted in serious volatilizations of Te and Se elements, leaving fine pores distributed uniformly in the matrix of sample. A maximum power factor of 1.81 × 10-3 Wm-1K-2 was obtained at room temperature for the sample sintered at 693 K for 10min, indicating that extending the holding time or increasing sintering temperature appropriately is beneficial to improve the electrical performance. What gratified is that the fine-microstructures scatter not only carriers but also phonons, leading to the low electronic thermal conductivity and lattice thermal conductivity. The alloy sintered at 533 K for 10 min had the smallest grain size and the minimum thermal conductivity of 0.61 Wm-1K-1. The highest ZT value of 0.71 was achieved around 423 K for the Bi2Te2.7Se0.3 alloy sintered at 613 K for 0 min owing to its relatively low thermal conductivity and appropriate power factor. In addition, the ZT values for the sample sintered at 613 K for 0 min are more than 0.6 in the temperature range of 380 K ~ 500 K, implying these alloys can be useful not only for cooling device, but also for low temperature power generation. Comparing with the 12

conventional HP and microwave sintering technologies, the new molding technology of MAHP introduced here can achieve a low temperature and rapid sintering in a greater degree. It gives us a new selection to obtain alloys with fine microstructure, which is beneficial to decrease the thermal conductivity of thermoelectric materials. Acknowledgement

This work was supported by the National Natural Science Foundation of China (NSFC, 11074195). References

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Figure captions

Fig. 1. XRD patterns of the as-MAed powders and the sintered alloys.

Fig. 2. Variation of the relative densities and content of Bi element for the sintered samples with sintering temperature.

Fig. 3. SEM photographs of the sintered samples: (a) 533 K, 10 min; (b) 613 K, 0 min; (c) 613 K, 10 min; (d) 613K, 30min; (e) 653 K 10 min; (f) 693 K, 10 min.

Fig. 4. The testing temperature dependence of Seebeck coefficient for the sintered samples.

Fig. 5. The testing temperature dependence of electrical resistivity for the sintered samples.

Fig. 6. The testing temperature dependence of power factor for the sintered samples.

Fig. 7. The thermal transport properties of the samples sintered at different conditions: (a) thermal conductivity, (b) lattice thermal conductivity.

Fig. 8. The temperature dependence of ZT value for the sintered alloys.

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Table Table 1 The room-temperature carrier concentration (n) and mobility (µ) of the as-MAHPed samples sintered at different conditions. Sintering temperature / K

Holding time / min

n / × 1019 cm-3

µ / cm2V-1s-1

533

10

6.31

19.53

573

10

7.18

35.38

613

0

7.32

48.51

613

10

7.78

48.78

613

30

8.03

55.20

653

10

8.59

69.96

693

10

9.22

71.35

19

20

21

22

23

24

25

26

27

28

Highlights

1. Fine-grained Bi-Te-Se alloys were prepared by microwave activated hot pressing. 2. Microwave activated hot pressing is a new method with low temperature and quick consolidation. 3. Microwave activated hot pressing is helpful to reduce the thermal conductivity of materials. 4. The sample sintered at 533 K for 10 min had a minimum thermal conductivity of 0.61 Wm-1K-1. 5. The maximum ZT value of 0.71 was obtained for the sample sintered at 613 K for 0 min.

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