Ce1−xSrxZnSbO: New thermoelectric materials formed between intermetallics and oxides

Accepted Manuscript Ce1-xSrxZnSbO: New thermoelectric materials formed between intermetallics and oxides Jian Liu, Jian Wang, Chun-lei Wang, Sheng-qing Xia PII:

S0925-8388(16)32264-2

DOI:

10.1016/j.jallcom.2016.07.235

Reference:

JALCOM 38404

To appear in:

Journal of Alloys and Compounds

Received Date: 23 June 2016 Revised Date:

20 July 2016

Accepted Date: 21 July 2016

Please cite this article as: J. Liu, J. Wang, C.-l. Wang, S.-q. Xia, Ce1-xSrxZnSbO: New thermoelectric materials formed between intermetallics and oxides, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.07.235. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Ce1-xSrxZnSbO: New thermoelectric materials formed between intermetallics and oxides

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Jian Liu1, 2*, Jian Wang1, Chun-lei Wang1, 2, Sheng-qing Xia1†

1. State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R. China

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2. School of Physics, Shandong University, Jinan 250100, P. R. China

Abstract

Thermoelectric properties of ZrCuSiAs-typed Ce1-xSrxZnSbO (x=0, 0.04, 0.08, 0.10) ceramic samples were investigated in the temperature range from 330 K to 727 K. Thermal conductivity is extremely low in the pristine CeZnSbO, and increases after Sr doping. In addition, with the rare-earth cations doped by alkaline-earth metals, the

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power factor and figure of merit ZT are significantly enhanced. In our research, a ZT

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value of 0.12 was achieved at 727 K for the sample with a composition of Ce0.92Sr0.08ZnSbO. Although the figure of merits of CeZnSbO materials are not very

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impressive compared to other regular TE materials, it still provide a new type of

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interesting TE compounds formed between intermetallics and oxides.

Keywords: Thermoelectric materials; Ceramics; Electronic properties; Thermoelectric

* †

Corresponding author. E-mail address: [email protected] (Jian Liu) Corresponding author. E-mail address: [email protected] (Sheng-qing Xia) 1

ACCEPTED MANUSCRIPT 1. Introduction Thermoelectric (TE) materials have attracted much attention in recent years

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because of its potential applications on waste heat recovery and environmentally friendly refrigeration [1-3]. The key parameter that evaluates the efficiency of TE materials is the ‘dimensionless figure of merit’ ZT = (S2σ/κ)Τ, where S is Seebeck

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coefficient, σ is electrical conductivity, κ is thermal conductivity, and Τ is absolute temperature, respectively. Intrinsic low thermal conductivity is an important character for a good TE material. In current researches, most of good TE materials, for example Bi2Te3, CoSb3 and PbTe, exhibit thermal conductivities smaller than 3 WK-1m-1 at high temperatures where ZT > 1. In order to further improve TE performance of these

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materials, many efforts have been performed to reduce their thermal conductivity by

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introducing and managing nanostructures [4-10]. Recently, some new promising TE

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materials were discovered to exhibit extremely low thermal conductivity and thus high ZT values, such as Yb14MnSb11 [11], AgPbmSbTem+2 [12], CuGaTe2 [13], and

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SnSe [14]. However, these materials also suffer from the common disadvantages of intermetallic TE materials, such as, poor durability at high temperatures and high production cost.

Oxide materials are regarded as alternatives to overcome these disadvantages, while most of them exhibit larger thermal conductivity and smaller ZT values as compared with state-of-art TE intermetallic compounds [15-18]. In 2010, L. D. Zhao et. al. discovered that BiCuSeO oxyselenide exhibits an extremely low thermal conductivity (as low as 0.5 WK-1m-1 at 900 K), and shows a high figure of merit [19]. 2

ACCEPTED MANUSCRIPT The extremely low thermal conductivity is attributed to its ZrCuSiAs-type crystal structure, which exhibits a low phonon group velocity due to ‘soft’ bonding involving

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heavy atoms [20-22]. Therefore, ZrCuSiAs-type crystal structure could be considered as a promising structure of TE materials.

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REZnSbO (RE = Trivalent Rare-Earth Metals) compounds were reported to crystallize in the ZrCuSiAs structure type as well, for which related optical and magnetic properties have been investigated [23-26]. However, there are few references on the thermoelectric properties about these materials [27]. In this work, the thermoelectric properties of Ce1-xSrxZnSbO series are presented and these materials exhibit very low thermal conductivities originating from their intrinsic

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crystal structures. More interestingly, the crystal structure of CeZnSbO is configured

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by two different types of components: ZnSb intermetallic layer and CeO oxide layer.

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At this aspect, CeZnSbO can be viewed as a special TE candidate formed between intermetallics and oxides, which may bring very different electrical transport

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properties as well. Taking this aim, experiments of doping Sr at Ce sites were carried out, which resulted in significantly enhanced electrical conductivity but decreasing Seebeck coefficient. However, both of the power factor and figure of merit (ZT) were

improved after doping with Sr. Although ZT values of Ce1-xSrxZnSbO are not very impressive (0.12 at 727 K obtained in Ce0.92Sr0.08ZnSbO) as compared with conventional TE materials, this study may still throw some light on the strategy of exploring new TE candidates based on intermetallics and oxides.

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ACCEPTED MANUSCRIPT 2. Experimental section Ce1-xSrxZnSbO (x = 0, 0.04, 0.08, 0.10) were prepared by the two-step solid-state

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reaction techniques described as following. Appropriate amounts of the starting materials (Sr 99%, Ce 99.8%, Sb 99.999%, and ZnO 99%) were weighted and sealed

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in a Niobium tube in a glovebox (in Ar atmosphere, O2<0.1 ppm). The Nb tubes were sealed into a vacuum quartz tube, and dwelled at 900 ºC for 24 hours. After reaction, the products were finely ground and the obtained powder was sealed into a Nb tube for the second time. The samples were annealed at 900 ºC for 96 hours, followed by a cooling-down process to room temperature at a rate of 50 ºC/hr. For the measurements of thermoelectric properties, the resulted samples were pressed into pellets and

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sintered at 625 ºC for 10 minutes under a pressure of 45 MPa by a spark plasma

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sintering (SPS) equipment. The phase purity of the materials was investigated by

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powder X-ray diffraction (PXRD) with a Rigaku D/MAX-2550P diffractometer equipped with Cu Kα radiation (λ = 0.15406 nm).

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The thermal conductivity was calculated based on the thermal diffusivity, specific

heat capacity, and sample density, which were measured by a Laser Flash Apparatus (Netzsch LFA 427), a thermal analyzer (Netzsch STA 449C) and Archimedes’ method, respectively. The sintered discs were cut into rectangular columns (12 mm × 2 mm × 2 mm) for measurements of Seebeck coefficient and electrical conductivity by a Linseis LSR-3/1100 instrument in a helium atmosphere through a modified dynamic method. In the dynamic method, a series of voltages ∆U due to different temperature

gradients ∆T were measured, and by linearly fitting the curve of ∆U~∆T, Seebeck 4

ACCEPTED MANUSCRIPT coefficient was obtained from the slope. 3. Results and discussion

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The XRD patterns of materials Ce1-xSrxZnSbO prepared by SPS methods all represent a single phase as shown in Fig. 1(a). All major Bragg peaks can be indexed

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with the ZrCuSiAs structure type. The crystal structure of CeZnSbO is shown in Fig. 1(b), which is constituted by (Zn2Sb2)2- layers alternately stacked with (Ce2O2)2+ layers along the c axis of the tetragonal cell [23]. The lattice parameters are calculated from the XRD data and shown in Fig. 1(c). The parameter a (=b) decreases slightly and c increases obviously with increasing Sr doping content, which is related to the larger radius of Sr2+ ions (1.18 Å) than that of Ce3+ ions (1.01 Å) [28]. CeZnSbO

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crystallizes in a layered structure, and could therefore exhibit preferential orientation

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after the uniaxial densification process. However, the XRD patterns for

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Ce1-xSrxZnSbO samples along the section parallel to the SPS pressing direction and those of the ground powders do not show any preferential orientation of the

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crystallites. Therefore, the TE transport properties of Ce1-xSrxZnSbO polycrystals

prepared by SPS are assumed to be isotropic. The pristine CeZnSbO samples exhibit low electrical conductivity with a

semiconducting behavior, as shown in Fig. 2. With the increasing Sr content, the electrical conductivity increases significantly and eventually exhibits a metallic transport behavior, for example, 1.3*103 Sm-1 in CeZnSbO and 3.3*104 Sm-1 for Ce0.9Sr0.1ZnSbO at 330 K. This result indicates that Sr doping can effectively enhance the electrical conductivity of Ce1-xSrxZnSbO, which is also very common for 5

ACCEPTED MANUSCRIPT materials with such a structure type [22, 29]. For material CeZnSbO, the insulating (Ce2O2)2+ layers act as the charge reservoir, whereas the conductive (Zn2Sb2)2- layers

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constitute the conduction pathway for carrier transport. With the substitution of Ce3+ by Sr2+, carriers are thus introduced into the insulating (Ce2O2)2+ layers and then

conductivity in Ce1-xSrxZnSbO.

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transferred to the conductive (Zn2Sb2)2- layers, resulting in the enhanced electrical

Fig. 3 shows the temperature dependence of Seebeck coefficient for materials Ce1-xSrxZnSbO. All the samples indicate positive Seebeck coefficient, implying that the electrical transport properties are dominated by holes. With the increasing Sr doping content, the Seebeck coefficient decreases over the whole temperature region

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due to the introduction of holes. The Seebeck coefficient of CeZnSbO (129 µV/K at

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330 K) is generally smaller than that of BiCuSeO (350 µV/K at 293 K), and the

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electrical conductivity of CeZnSbO (1300 Sm-1 at 330 K) is much larger compared to that of BiCuSeO (470 Sm-1 at 293 K) [19]. This is an indication that the band gap of

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pristine CeZnSbO should be much smaller than that of pristine BiCuSeO. The thermoelectric power factor (PF) of Ce1-xSrxZnSbO samples were calculated and shown in Fig. 4. PF values were enhanced significantly by Sr doping, from 44 µWK-2m-1 (CeZnSbO) to 170 µWK-2m-1 (Ce0.9Sr0.1SbO) at 727 K. Fig. 5(a) shows the total thermal conductivity (κtot) of Ce1-xSrxZnSbO samples. κtot of all the Ce1-xSrxZnSbO samples decreases with increasing temperature, indicating that the phonon contribution is predominant to thermal conductivity. The κtot values of

pristine CeZnSbO are very low, varying from 0.9 Wm-1K-1 to 0.6 Wm-1K-1 over the 6

ACCEPTED MANUSCRIPT measured temperature range, which is comparable with that of BiCuSeO [22] or some other state-of-art intermetallic TE compounds [4-10]. The low thermal conductivity of

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Ce1-xSrxZnSbO may originate from the layered ZrCuSiAs-typed structure since phonons can be scattered at the layers interfaces, through the weak bonding between

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layers and the presence of low-phonon conductive heavy elements, similar to BiCuSeO [22]. In combination of the measured electrical conductivity with the Wiedemann–Franz law (κe=LσT, where L=2.45*10-8 WΩK-2 is the Lorenz number), the electronic and lattice contribution of the thermal conductivity could be extracted, which is shown in Fig. 5(b). With the Sr doping content increased, both the electronic thermal conductivity and the lattice thermal conductivity increase. The former can be

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attributed to the increasing of the carrier concentration, which also leads to the

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improvement of the electrical conductivity. The latter is probably due to the reduction

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of the effective mass when Ce is substituted by Sr, which results in the lower efficiency in phonon scattering. However, the total thermal conductivity of

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Ce0.9Sr0.1ZnSbO sample (1.4 Wm-1K-1 at 727 K) is still not very high compared with most oxide-based TE materials. Based on the measured electrical and thermal transport properties, the figure of

merit ZT is calculated and the data are presented in Fig. 6. The ZT values of all samples increase with increasing temperature and are enhanced significantly by Sr doping. A maximum ZT value of 0.12 is achieved at 727 K for material Ce0.92Sr0.08ZnSbO. Although the ZT values of Ce1-xSrxZnSbO samples are lower than that of BiCuSeO system, such antimonide oxide materials with the ZrCuSiAs 7

ACCEPTED MANUSCRIPT structure type represent a completely new family for TE candidates [23-25, 30-33]. And there might be more isomorphic materials which are not discovered till now.

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These materials might lead to high thermoelectric performance due to their intrinsic low thermal conductivity and certainly is worthy of further investigations.

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4. Conclusion

In this work, the thermoelectric properties of materials Ce1-xSrxZnSbO with the ZrCuSiAs structure type were reported. The Seebeck coefficient measurements indicate that these materials are p-type semiconductors with the electrical transport properties dominated by holes. With the Sr doped into the Ce sites, the electrical conductivity is significantly enhanced accompanied with certain reduction of the

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Seebeck coefficient. However, the resulted power factor is enhanced, which leads to

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the improvement of the thermoelectric performance. The pristine CeZnSbO sample

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also exhibits an extremely low thermal conductivity (0.6 Wm-1K-1 at 727 K), due to its complex crystal structure. A highest ZT value of 0.12 is achieved for

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Ce0.92Sr0.08ZnSbO at 727 K.

Acknowledgements This work was supported by the National Natural Science Foundation of China (grant numbers 51271098, 51202132) and China Postdoctoral Science Foundation (grant number 2015M572025).

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ACCEPTED MANUSCRIPT Figure captions Fig. 1 (a) XRD patterns of Ce1-xSrxZnSbO (x=0, 0.04, 0.08, and 0.10) samples, (b)

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crystal structure of CeZnSbO, (c) doping dependence of lattice parameters a and c. Fig. 2 Temperature dependence of electrical conductivity of Ce1-xSrxZnSbO (x=0,

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0.04, 0.08, and 0.10) samples.

Fig. 3 Temperature dependence of Seebeck coefficient of Ce1-xSrxZnSbO samples. Fig. 4 Temperature dependence of (a) total thermal conductivity, and (b) electronic and lattice thermal conductivity for Ce1-xSrxZnSbO.

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Fig. 5 Temperature dependence of the figure of merit ZT for Ce1-xSrxZnSbO.

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Intensity (a. u.)

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ACCEPTED MANUSCRIPT 1. Thermal conductivity is extremely low in CeZnSbO and increases after Sr doping. 2. Power factor and figure of merit ZT are significantly enhanced by Sr doping.

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3. This work provides a new type of interesting TE compounds.