Thermoelectric Transport Properties of Cu Nanoprecipitates Embedded Bi 2 Te 2 . 7 Se 0 . 3

1Korea Institute of Ceramic Engineering and Technology, Icheon Branch, Icheon 467-843, Republic of Korea 2School of Energy, Materials and Chemical Engineering, Korea University of Technology and Education, Cheonan 330-708, Republic of Korea 3Energy and Environmental Division, Korea Institute of Ceramic Engineering and Technology, Seoul 153-801, Republic of Korea 4Department of Nano Applied Engineering, Kangwon National University, Chuncheon 200-701, Republic of Korea


Introduction
Thermoelectric (TE) power generation is a key technology for clean renewable energy harvesting and reduction of greenhouse gas.Widespread use of TE power generation systems requires enhancement in performance of TE materials, evaluated in terms of a dimensionless figure merit, defined as  =  ⋅  2 ⋅ /, where  is the electrical conductivity,  is the Seebeck coefficient, and  is the total thermal conductivity at a given absolute temperature ().Among TE materials, the Bi 2 Te 3 -based solid solution, such as p-type Bi 2−x Sb x Te 3 (BST) and n-type Bi 2 Te 3−y Se y (BTS), is known to be the best material among those used around room temperature.Although Bi 2 Te 3 -based TE materials are widely used for small-scale and high-density cooling applications, materials with higher  are required for the extension of application, including domestic cooling and power generation from lowgrade heat.Recently, high-performance p-type Bi 2 Te 3 -based bulk TE materials have been developed with the introduction of nanotechnology, which reduce the lattice thermal conductivity ( lat =  −  ele , where  ele is the electronic contribution) by intensified interface phonon scattering.Poudel et al. reported a significant improvement of  that resulted from a reduced grain size [1].High  of 1.4 was obtained at 373 K owing to the reduced  lat .The key feature of the  lat reduction in this nanograined composite is the high density of the grain boundaries to scatter phonons [1,2].However, for the nanostructured n-type BTS, the  value still remains about 1.04 at 398 K even in nanograined composite [3].Moreover, the carrier concentration () increased by formation of point defects (antisite defects and vacancies) generated by heavy deformation during ball milling (BM) process [4].This uncontrollable defect in BTS is known to bring about severe reproducibility issue; thus there have been a lot of efforts to simultaneously enhance the  and improve the reproducibility through the composite-type materials with metallic dispersions such as Cu, In, and Cu-Te [5][6][7][8].
Mixing with Recently, Liu et al. demonstrated that excess Cu could be intercalated in the interlayer space of n-type BTS, and intercalated Cu can improve the reproducibility, increase the carrier mobility (), and decrease the  lat [9].On the other hand, introduction of Cu nanoprecipitates has the potential to enhance the  due to the enhanced  by carrier filtering effect and reduced  lat by phase boundary phonon scattering.
In the present study, Cu nanoprecipitates embedded BTS bulk nanocomposites (Cu-BTS) were fabricated by combined technique of chemical reaction and spark plasma sintering in order to clarify the effect of Cu nanoprecipitates on the transport properties of BTS.We investigated their TE properties and demonstrated the origin of the enhancement of electronic and thermal transport properties.

Experimental
The nanocomposite powder of Cu nanoparticle (1.0, 3.0, and 5.0 vol%) and BTS nanosheet was prepared by precipitating Cu nanoparticle in colloidal suspension of as-exfoliated BTS as shown in Figure 1.The colloidal suspension of the exfoliated BTS was prepared by reaction of lithiated BTS with distilled water.Details for the formation of BTS nanosheets have been reported in our previous study [10].For the hybridization, the colloidal suspension of the exfoliated BTS (5 wt%, 40.0 mL) was mixed with aqueous CuCl 2 solution and was refluxed with 60.0 mL of hydrazine monohydrate (98%, Junsei) for 180 min at 120 ∘ C to eliminate oxygen on the surface of the exfoliated BTS.After repeated washing, centrifugation, and drying in a vacuum, the resulting hybrid powders were loaded into a graphite die.Disk-shaped polycrystalline bulk samples (10 mm in diameter and 13 mm in thickness) were fabricated by spark plasma sintering (SPS) under 30 MPa and at 420 ∘ C for 2 min in a vacuum.
The microstructure of the sintered samples was investigated using transmission electron microscopy (TEM, JEOL JEM-4010).The measurements for  and  were carried out in a perpendicular direction to the SPS press in order to ensure measurement of the correct TE properties. and  measurements from 300 K to 480 K were performed using an ULVAC ZEM-3 system.The  values ( =   ⋅   ⋅ ) were calculated from separated measurements: sample density (  ), heat capacity (  ), and thermal diffusivity (), measured in a vacuum by the laser-flash method (TC-9000, ULVAC, Japan).The densities of the sintered samples by SPS were found to range from 7.52 to 7.56 g/cm 3 (>96% of the theoretical density).Low temperature (100 K-390 K)   values were collected using a Quantum Design physical properties measurement system, and   was used at a constant value of 0.155 J⋅g −1 ⋅K −1 estimated from the Dulong-Petit fitting.The Hall effect measurements were carried out using a commercial system (7600 Electromagnet Series, Lake Shore Cryotronics, USA) with a magnetic field of 2 T and an electrical current of 30 mA.The  and  values were estimated by the one-band model using  = −1/(  ⋅ ) and  =  ⋅  ⋅ , where   and  are the Hall coefficient and electron charge.

Results and Discussion
Recently, a few researches about the Cu addition effects on the TE properties of Bi 2 Te 3 -based alloys including single crystals [5][6][7] and polycrystalline bulks [8,9] have been reported.Three different effects were demonstrated by Cu addition: First, the substitutional doping of Cu on Bi site forms an antisite defect (Cu Bi 2− ) and generates a hole.Second, the occupation of the interstitial site generates a donor with Cu +  .Cu is placed at the interstitial site between two quintets (intercalation) and achieves an electrical connection, resulting in an increase of the  of carriers and a decrease in  lat [5].Third, Cu can be precipitated as a nanoinclusion in matrix and/or grain boundary region.In this case, the heterointerface between the Cu nanoprecipitates and Bi 2 Te 3 -based matrix is generated.This heterointerface might act as an energy barrier for carriers as well as phonon scattering center.In order to clarify the mechanism for  enhancement in this nanoinclusion-type nanocomposite, we prepared Cu nanoprecipitate embedded BTS (Cu-BTS) bulks and evaluated their TE properties.Figure 2 shows TEM images of 3.0 vol% and 5.0 vol% Cu-BTS bulk samples.The well-dispersed Cu nanoprecipitates were clearly observed in BTS matrix, and their size remained <40 nm, indicating that Cu-BTS bulk was successfully synthesized.The average size of Cu nanoprecipitates, which was estimated from Figure 2(b), was about 15 nm.For 3 vol% Cu-BTS, the calculated number of Cu nanoprecipitates is ∼1.7 × 10 16 particles/cm 3 .Also, we calculated population of Cu nanoprecipitates from Figure 2(b).The population was ∼1.4 × 10 16 particles/cm 3 .The number of Cu nanoprecipitates calculated from nominal composition is well consistent with that from TEM image.
The electronic transport properties of Cu-BTS samples and those of pristine BTS were measured to demonstrate the effect of Cu nanoprecipitates on TE properties.Figure 3   shows the temperature dependence of ,  and power factor values for BTS and Cu-BTS samples.The  values slightly decreased (Figure 3(a)), while absolute values of  increased (Figure 3(b)) in the presence of Cu nanoprecipitates.To examine the behavior of  and , we evaluated the  and  at 300 K and represented them in Table 1.The  values of BTS and Cu-BTS samples were almost the same (6.07 × 10 19 cm −3 -6.40 × 10 19 cm −3 ).This is considered to be related to reaction between Cu and BTS.Because substituted Cu on Bi site would be as an acceptor, small amount of Cu might be diffused into the BTS matrix.However, a greater part of Cu nanoprecipitates should remain as unreacted Cu (Figure 2), which is confirmed by the numerical analysis for the microstructure of Cu-BTS.As represented in Table 1, there is a decrease in  of Cu-BTS mainly due to the reduction in .This result is another evidence for the presence of Cu nanoprecipitates, which cause electron filtering at the interface between Cu and BTS.It should be noted that  values of Cu-BTS were rather larger than that of pristine BTS despite moderate decrease in  and exhibited peak value in 3.0 vol% Cu-BTS.This is considered to be related to the carrier filtering effect.The interface between metallic Cu and semiconducting BTS might induce an energy dependent carrier scattering effect by introducing a well and energy barrier which filter the carrier with small energy [11,12].The work function (Φ) of Cu is known to be 4.53 eV-5.10 eV, while the electron affinity () of BTS is ∼4.50 eV.If the energy barrier by Cu and BTS interface is generated in the BTS matrix, the electron transfer should be changed by band bending.In this heterostructured interface, Φ of Cu is higher than  of BTS; thus a Schottky barrier is created.This barrier makes the low energy charge carrier be effectively scattered, while making the high energy charge carrier go easily through the barrier.Thus, appropriate size and density of barrier can cause an increase in the TE power factor ( ⋅  2 ) [11].Inset of Figure 3(b) shows the temperature dependence of calculated power factor for BTS and Cu-BTS.The maximum power factor of 2.84 mW⋅m −1 ⋅K −2 at 300 K was obtained in 3.0 vol% Cu-BTS, which is 17% up from the value of pristine BTS (2.42 mW⋅m −1 ⋅K −2 at 300 K).This result indicates that Cu is an effective element for carrier filtering effect in BTS.
Because the carrier filtering effect is strongly correlated with the interface density as well as band alignment, more detailed study using well-controlled material system such as monodispersed uniform nanoparticle embedded thin film is required to clarify the mechanism of  enhancement by a carrier filtering effect.Figure 4(a) shows the temperature dependence of  for BTS and Cu-BTS.We calculated  lat from the calculation of

2 Figure 1 :
Figure 1: Schematic representation of exfoliation and reassembly processes for the mixture of Cu NP and Bi 2 Te 3 nanosheet.

Figure 4 :
Figure 4: Temperature dependence of (a) the thermal conductivity () and lattice thermal conductivity ( lat ) and (b) dimensionless figure merit  of BTS and Cu-BTS.