THERMOELECTRIC PROPERTIES OF Cu-DISPERSED Bi 2 Te 2.7 Se 0.3 - PDF document

18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS THERMOELECTRIC PROPERTIES OF Cu-DISPERSED Bi 2 Te 2.7 Se 0.3 NANOCOMPOSITES Il-Ho Kim 1 *, Soon-Mok Choi 2 , Won-Seon Seo 2 , Dong-Ik Cheong 3 , Hyung Kang 3 1 Department of Materials Science and Engineering, Chungju National University, Chungju, Chungbuk 380-702, Korea 2 Energy Materials Lab., Green Ceramic Division, Korea Institute of Ceramic Engineering and Technology, Seoul 153-801, Korea 3 The 4 th R&D Institute-4, Agency for Defense Development, Daejeon 305-600, Korea * Corresponding author (ihkim@cjnu.ac.kr) Keywords : thermoelectric, bismuth telluride, dispersion, nanocomposite increasing charge carrier scattering, the figure of 1 Introduction merit can be enhanced [3,4]. Thermoelectric materials are potential sources of In general, however, the thermal conductivity electrical power from heat energy. Superior reduction by phonon scattering accompanies the thermoelectric materials require a high Seebeck electrical conductivity reduction by charge carrier coefficient ( α ), a high electrical conductivity ( σ ) and scattering due to inhomogeneous distribution and a low thermal conductivity ( κ ) at application agglomeration of nanoparticles [5,6]. Conventional mixing process such as ball milling cannot provide temperature ( T in Kelvin) for a high figure of merit appropriate nanostructure to realize the PGEC effect ( Z ): effectively in nanocomposites. In this study, a novel and simple approach was employed to disperse α σ 2 T = ZT (1) nanoparticles (Cu) uniformly in the matrix κ (Bi 2 Te 2.7 Se 0.3 ), and the thermoelectric properties were evaluated for the Cu-dispersed Bi 2 Te 2.7 Se 0.3 which is related to thermoelectric energy conversion nanocomposites. efficiency. The quantity α 2 σ is called the power factor and is the key to achieve high performance. The thermal conductivity has contributions from 2 Experimental Procedure lattice vibrations and charge carrier transportations, Polycrystalline Bi 2 Te 2.7 Se 0.3 ingot was prepared by and a low thermal conductivity is needed to maintain melting at 1073 K for 4 h with high purity a temperature difference between hot and cold (99.999 %) Bi, Te and Se granules in an evacuated junctions of thermoelectric materials. quartz ampoule. The ingot was crushed into powder There are two approaches to increase ZT [1]. One and sieved to obtain < 75 µ m diameter particles. is to maximize the power factor by developing new Bi 2 Te 2.7 Se 0.3 powder was dry-mixed with Cu(OAc) 2 classes of thermoelectric materials, optimizing powder. The mixture of Bi 2 Te 2.7 Se 0.3 and Cu(OAc) 2 existing materials through doping, and exploring was then transferred to an alumina crucible and nanoscale materials. The other is to minimize the heated at 573 K for 3 h in a vacuum to decompose thermal conductivity by developing materials with Cu(OAc) 2 . It was converted into Cu nanoparticles, intrinsically low thermal conductivity, solid-solution which were chemically bonded to Bi 2 Te 2.7 Se 0.3 alloying, and realizing nanostructure engineering. powder. Cu-dispersed Bi 2 Te 2.7 Se 0.3 nanocomposites Phonon glass and electron crystal (PGEC) concept is were hot-pressed in a cylindrical graphite die with considered to reduce the thermal conductivity with an internal diameter of 10 mm at 673 K under a maintaining the high power factor by introducing pressure of 70 MPa for 1 h in a vacuum. nanostructure or nanocomposite [2]. If the Scanning electron microscopy (SEM: FEI nanoparticles are well-dispersed and sufficiently Quanta400) equipped with an energy dispersive small to intensify phonon scattering without

spectrometer (EDS) was used to observe the Fig. 2 presents a SEM image of Cu-dispersed microstructure. Phase analysis was performed by X- Bi 2 Te 2.7 Se 0.3 nanocomposite prepared by Cu(OAc) 2 ray diffraction (XRD: Bruker D8 Advance) using Cu decomposition. Mean particle size of Cu is Kα radiation (40 kV, 40 mA). Diffraction patterns approximately ฀ 50 nm, and Cu nanoparticles are were measured in the θ -2 θ mode (10 to 90 degrees) well-dispersed and bonded to the Bi 2 Te 2.7 Se 0.3 powder surface. with a step size of 0.02 degree, a scan speed of 0.2 degree/minute and a wavelength of 0.15405 nm. Hall effect measurements were carried out in a constant magnetic field (1 T) and electric current (50 mA) with Keithley 7065 system at room temperature to examine the carrier concentration and mobility. The Seebeck coefficient and electrical conductivity were measured using temperature differential and 4- probe methods, respectively, with Ulvac-Riko ZEM3 equipment in a helium atmosphere. The thermal conductivity was estimated from the thermal diffusivity, specific heat and density measurements using a laser flash Ulvac-Riko TC9000H system in a vacuum. The thermoelectric figure of merit was evaluated. Fig.2. Cu-dispersed Bi 2 Te 2.7 Se 0.3 nanocomposite 3 Results and Discussion prepared by Cu(OAc) 2 decomposition. Fig. 1 shows the XRD patterns of Cu-dispersed Fig. 3 shows the electrical conductivity of Cu- Bi 2 Te 2.7 Se 0.3 nanocomposites consolidated by hot dispersed Bi 2 Te 2.7 Se 0.3 . The electrical conductivity pressing. Diffraction peaks were well-matched with did not change significantly by Cu nanoparticle the ICDD standard data. All samples were dispersion. It showed high-10 4 S/m all temperatures polycrystalline and Bi 2 Te 2.7 Se 0.3 phase was examined, and which means that all specimens are in successfully synthesized in this process. Diffraction a degenerate state. peaks for Cu particles were not identified because the amount of Cu was too small to identify. 6 10 BTS: Bi 2 Te 2.7 Se 0.3 BTS + 0.1wt% Cu(OAc) 2 BTS + 0.3wt% Cu(OAc) 2 Electrical conductivity, σ (S/m) BTS + 0.5wt% Cu(OAc) 2 BTS + 0.5wt% Cu(OAc) 2 Intensity (arb. units) BTS + 0.3wt% Cu(OAc) 2 5 10 BTS + 0.1wt% Cu(OAc) 2 BTS: Bi 2 Te 2.7 Se 0.3 4 10 300 350 400 450 500 550 Temperature (K) 10 20 30 40 50 60 70 80 90 Diffraction angle, 2 θ (deg.) Fig.3. Electrical conductivity of Cu-dispersed Fig.1. X-ray diffraction patterns of Cu-dispersed Bi 2 Te 2.7 Se 0.3 . Bi 2 Te 2.7 Se 0.3 nanocomposites.

PAPER TITLE In order to examine the electronic transport mass [8]. Therefore, as shown in Fig. 5, the power properties, the Hall coefficient ( R H ), carrier factor values for Cu-dispersed Bi 2 Te 2.7 Se 0.3 were concentration ( n ) and mobility ( μ ) were measured. maintained higher in the whole temperature range, Table 1 lists the electronic transport properties of and the maximum power factor at 323 K reached Cu-dispersed Bi 2 Te 2.7 Se 0.3 around two times higher than that of Bi 2 Te 2.7 Se 0.3 at room temperature. The . sign of the Hall coefficient was negative for all specimens and it means that the electrical charge -300 was transported mainly by electrons. The carrier concentration and mobility did not change -250 significantly with Cu dispersion, which indicates V/K) that Cu nanoparticles are too small to introduce the ( µ -200 Seebeck coefficient, α charge carrier scattering. -150 Table 1. Electronic transport properties of Cu- dispersed Bi 2 Te 2.7 Se 0.3 at room temperature. BTS: Bi 2 Te 2.7 Se 0.3 -100 BTS + 0.1wt% Cu(OAc) 2 n μ m * R H BTS + 0.3wt% Cu(OAc) 2 specimen (cm 3 /C) (cm 2 /Vs) (cm -3 ) (m o ) BTS + 0.5wt% Cu(OAc) 2 -50 4.7 × 10 19 BTS -0.134 80.4 0.91 0 BTS + 3.5 × 10 19 300 350 400 450 500 550 -0.177 73.1 1.10 0.1 wt% Cu(OAc) 2 Temperature (K) BTS + 4.3 × 10 19 -0.145 99.9 1.12 0.3 wt% Cu(OAc) 2 Fig.4. Seebeck coefficient of Cu-dispersed BTS + 3.5 × 10 19 Bi 2 Te 2.7 Se 0.3 . -0.177 101.0 1.10 0.5 wt% Cu(OAc) 2 4 Fig. 4 presents the Seebeck coefficient of Cu- BTS: Bi 2 Te 2.7 Se 0.3 dispersed Bi 2 Te 2.7 Se 0.3 . All specimens had a BTS + 0.1wt% Cu(OAc) 2 negative Seebeck coefficient, which confirmed that 2 ) BTS + 0.3wt% Cu(OAc) 2 2 σ (mW/mK 3 the electrical charge was transported mainly by BTS + 0.5wt% Cu(OAc) 2 electrons as shown in Table 1. The absolute Seebeck coefficient of Bi 2 Te 2.7 Se 0.3 was almost constant with 2 Power factor, α temperature ranging 323-523 K. However, it was remarkably increased by Cu dispersion and slightly reduced with increasing temperature. The Seebeck 1 coefficient is affected by the carrier concentration ( n ) and the effective mass ( m * ) [7]: 0 300 350 400 450 500 550 π π 2 / 3   2 2 8 k B α =   * m T Temperature (K) (2)   2 3 eh 3 n Fig.5. Power factor of Cu-dispersed Bi 2 Te 2.7 Se 0.3 . where k B , h and e are the Boltzmann constant, Fig. 6 shows the thermal conductivity of Cu- Planck constant and electrical charge, respectively. dispersed Bi 2 Te 2.7 Se 0.3 . It was slightly increased In this study, because the carrier concentration did with increasing temperature because of the not change significantly, the increase in the Seebeck electronic contribution, and Cu dispersion could not coefficient was due to the increase in the effective reduce it. The thermal conductivity (κ) is the sum of mass of a carrier, which is one of the critical factors the lattice thermal conductivity (κ L ) by phonons and determining the Seebeck coefficient. It is speculated the electronic thermal conductivity (κ E ) by carriers, that the charge-carrier energy filtering effect of the and it is given by Eq. 3: nanoparticles causes the increase in the effective 3