Scholarly article on topic 'Effect of Cd Isoelectronic Substitution on Thermoelectric Properties of Zn0.995Na0.005Sb'

Effect of Cd Isoelectronic Substitution on Thermoelectric Properties of Zn0.995Na0.005Sb Academic research paper on "Nano-technology"

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Abstract of research paper on Nano-technology, author of scientific article — Jingchao Zhou, Lihong Huang, Zhengyun Wang, Zihang Liu, Wei Cai, et al.

Abstract ZnSb as a kind of material with abundant resource and low cost has a low thermal conductivity and a high Seebeck coefficient, giving the potential of high thermoelectric properties. In this paper, Cd isoelectronic substitution was adopted to further improve the thermoelectric performance by reducing the lattice thermal conductivity of ZnSb. The results show that Cd substitution reduces the lattice thermal conductivity and increases the electrical conductivity. A high ZT value of 1.22 is achieved at 350 °C for Zn0.915Na0.005Cd0.08Sb.

Academic research paper on topic "Effect of Cd Isoelectronic Substitution on Thermoelectric Properties of Zn0.995Na0.005Sb"

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Materiomics

Effect of Cd Isoelectronic Substitution on Thermoelectric Properties of Zn0.995Na0.005Sb

Jingchao Zhou, Lihong Huang, Zhengyun Wang, Zihang Liu, Wei Cai, Jiehe Sui

PII: S2352-8478(16)30032-6

DOI: 10.1016/j.jmat.2016.08.003

Reference: JMAT 70

To appear in: Journal of Materiomics

Received Date: 24 March 2016 Accepted Date: 29 August 2016

Please cite this article as: Zhou J, Huang L, Wang Z, Liu Z, Cai W, Sui J, Effect of Cd Isoelectronic Substitution on Thermoelectric Properties of Zn0.995Na0.005Sb, Journal of Materiomics (2016), doi: 10.1016/j.jmat.2016.08.003.

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Effect of Cd Isoelectronic Substitution on Thermoelectric Properties

of Zn0.995Na0.005Sb

Jingchao Zhoua, Lihong Huangb, Zhengyun Wangb, Zihang Liu,a Wei Caia and Jiehe

0 50 100 150 200 250 300 350 400 450

Temperature (°C) 1.6-

0.0 . ................

0 50 100 >150 200 250 300 350 400 450

Temperature (°C)

The Cd isoelectrically substituted Na0.005Na0.995Sb samples were prepared by ball milling and hot press. The total thermal conductivity of Na0.005Na0.995Sb was decreased by Cd isoelectronic substitution caused by the mass and size fluctuation between Zn2+ and Cd2+. A high ZT value of 1.22 was obtained at 350 °C for Zn0.9i5Na0.005Cd0.08Sb sample, which was greater than that of other ZnSb based thermoelectric materials.

Effect of Cd Isoelectronic Substitution on Thermoelectric Properties

of Zn0.995Na0.005Sb

Jingchao Zhoua, Lihong Huangb, Zhengyun Wangb, Zihang Liua, Wei Caia and Jiehe Suia*

aNational Key Laboratory Precision Hot Processing of Metals, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China bCenter for Advanced Materials and Energy, Xihua University, Chengdu, Sichuan 610039, China Abstract

ZnSb as a kind of material with abundant resource and low cost has a low thermal conductivity and a high Seebeck coefficient, giving the potential of high thermoelectric properties. In this paper, Cd isoelectronic substitution was adopted to further improve the thermoelectric performance by reducing the lattice thermal conductivity of ZnSb. The results show that Cd substitution reduces the lattice thermal conductivity and increases the electrical conductivity. A high ZT value of 1.22 is achieved at 350 °C for Zn0.9i5Na0.005Cd0.08Sb.

Keywords: ZnSb; Isoelectronic substitution; Lattice thermal conductivity; Thermoelectric properties.

"Corresponding author.

E-mail address: suijiehe@hit.edu.cn. (Jiehe Sui).

1. Introduction

Thermoelectric (TE) material is known as a promising kind of new energy material. The reuse of waste heat has attracted much recent attention. For the middle-temperature field, the most widely used TE material is PbTe [1-4]. However, Pb is toxic, and Te is scarce and expensive. ZnSb based TE materials have been developed due to the abundant resource and relatively high conversion efficiency since the discovery of the Seebeck effect [5-12].

The performance of TE material is usually determined by the dimensionless figure of merit, ZT = (oS2/k)T, where S, a, k and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature, respectively. It is well known that the electronic (S, a) and thermal (k) transport properties are interdependent, changing one will negatively affect the others. Therefore, improving the ZT value has become a challenge [13-20].

ZnSb is a p-type semiconductor with a Pbca space group, an orthorhombic crystal structure and a band gap of about 0.2 eV. According to phase diagram, there is no phase transformation from room temperature to the melting temperature of 819 K. ZnSb is much more stable than other kinds of Zn-Sb compounds such as Zn4Sb3 [7]. Much effort had been made to improve the thermoelectric performance of ZnSb. For instance, a mechanical grinding method was applied to reduce the thermal conductivity, and the ZT value was increased from 0.2 to 0.9 at 550 K [8]. The maximum ZT value of 1 at 630 K was obtained by Sn acceptor doping and Cd isoelectronic substitution in the ZnSb system [21]. ZnSb with 0.2% Ag doping had a ZT value as high as 1.15, but Ag doping caused the massive cracks [22].

Recently, Na as an acceptor doping improves the electrical conductivity and

power factor and reduces the lattice thermal conductivity. The ZT value of 1 at 350 °C

is obtained for the optimal composition Zn0995Na0 05Sb [23]. However, the lattice thermal conductivity is relatively high (i.e., ~1.75 W/m-K at room temperature). Isoelectronic substitution is an effective way to reduce the lattice thermal conductivity, thereby leading to improve the ZT value, which has been confirmed in the thermoelectric materials, such as Half-Heusler, GeSi, Bi2Te3 [24-26]. In the case of ZnSb, Zn and Cd belong to the same column, and Cd has a greater ionic radius and a heavier atomic mass, compared to Zn. Therefore, an enhanced ZT value is expected due to the reduced lattice thermal conductivity caused by the size and mass fluctuation

between Cd and Zn. In this paper, the effect of Cd isoelectronic substitution on the thermoelectric properties of Zn0.995Na0.005Sb is investigated.

2. Experimental

Na (99.99%), Zn (99.99%), Cd (99.99%), and Sb (99.99%) were weighted and sealed in evacuated quartz tubes according to the formula of Zn0.995-xNa0.005CdxSb (x = 0, 0.04, 0.08, 0.12). The quartz tubes were heated at 923 K for 10 h, and then quenched in cold water. The powder was obtained after ball milling for 2 h. The obtained powder was hot pressed at a sintering temperature of 673 K for 2 min under a pressure of 60 MPa.

The crystal structures of Zn0.995-xNa0.005CdxSb were characterized by X-Ray diffraction (XRD) and the lattice parameters were calculated by peak fitting using the JADE 5.0 program. The grain size was determined by scanning electron microscopy (SEM).

The electrical resistivity and the Seebeck coefficient were measured by ZEM-3 (UlvacRiko ZEM-3) in Ar atmosphere. The hall coefficient (RH) was measured by a four probe method. The carrier concentration (n) and mobility (p) at room temperature were calculated by the equations of n = 1/eRH and p = oRH. The thermal diffusivity (D) was measured on a laser flash apparatus (Netzsch LFS 457) with flowing argon gas protection. The specific heat capacity (Cp) was calculated by using the Dulong-Petit law of Cv = 3NR/M, where N is the number of atoms per molecule, R = 8.314 J-mol"1-K"1 and M is the atomic mass per molecule. The densities (p) of all samples were measured by an Archimedes method, and the relative densities of all samples are greater than 96%. The thermal conductivity (k) was calculated using the equation of k = DpCp.

3. Results and discussion

Figure 1 shows the powder XRD patterns of the Zn0.995-xNa0.005CdxSb (x = 0,

0.04, 0.08 and 0.12). All the major Bragg peaks show an excellent match to the

simulated pattern of ZnSb (PDF#37-1008) and can be indexed as the Pbca space

group. No obvious impurity phase appears within the detectability limit of XRD. The

peaks of XRD patterns slightly shift to the lower diffraction angle when the Cd

content increases. Correspondingly, the lattice parameters of the samples are

calculated and listed in Table 1. Clearly, Cd substitution increases the lattice

parameters due to the difference of ionic radius between Cd2+ (0.95 A) and Zn2+ (0.74 A). Therefore, it can be concluded that the Cd is incorporated into the lattice in

Zn0.995-xNa0.005CdxSb.

Table 2 shows the room temperature carrier concentration and mobility. As ZnSb is a kind of p-type semiconductor, a small amount of Na acceptor doping can result in a significant increase in the carrier concentration. From Table 2, the carrier concentration increases when the Cd substitution content increases to 8%. However, the carrier concentration decreases obviously when the Cd substitution content increases to 12%. The mechanism for the enhancement of carrier concentration due to the Cd substitution is unclear until now. The carrier mobility decreases with the increase of the Cd substitution content due to the increased defect scattering. In addition, the change of both carrier concentration and carrier mobility as a function of Cd content can further confirm that Cd is incorporated into the lattice in Zn0.995-xNa0.005CdxSb.

Figure 2 shows the fracture morphologies of Zn0 995-xNa0 005CdxSb. The grain size ranges from 0.8 p,m to 1.2 p,m for all the samples. This indicates that Cd substitution has no obvious influence on the grain size. The grain size is obtained after ball milling for 2 h and hot press. It is expected that the grain size can be further reduced by prolonging ball milling time, thereby leading to the decrease of the lattice thermal conductivity.

Figure 3 shows the temperature dependence of electrical properties for Zn0.995-xNa0 005CdxSb samples. In Figure 3(a), the electrical resistivity firstly decreases from 2.63 x 10-5 Q-m as x = 0 to 2.07 x 10-5 Q-m as x = 0.08 at room temperature, and then increases to 3.55 x 10-5 Q-m as x = 0.12 with increasing the Cd content, which is similar to the change tendency of carrier concentration. Considering that the carrier mobility decreases with the increase of Cd substitution, we find that the increased electrical conductivity of Zn0 995-xNa0005CdxSb is mostly due to the increased carrier concentration, according to the equation of a = nep.

In Figure 3(b), the positive Seebeck coefficients indicate p-type semiconductor for ZnSb-based materials. Similarly, the Seebeck coefficients at room temperature first decrease and then increase with increasing the Cd content, consistent with the tendency of electrical resistivity. All the samples exhibit the peak values of the Seebeck coefficient at 350 °° showing the typical characteristic of bipolar diffusion effect.

Figure 3(c) shows the power factor (PF = oS) calculated from the measured electrical resistivity and Seebeck coefficient. The power factor of Zno.995. xNao.oo5CdxSb samples (as x = 0.04 and 0.08) increases in the whole measured temperature range due to the decreased electrical resistivity. The maximum power factor reaches 20.6 [j,W/cm-K2 for Zn0.915Na0.005Cd0.08Sb at 200 °C, which is greater than that of the sample without Cd substitution (i.e., -18.5 [j,W/cm-K2 at 200 °C).

Figure 4(b) shows the total thermal conductivity (/cfof) as a function of temperature for Zno.995-xNao.oo5CdxSb samples, which is calculated by the thermal diffusivity shown in Figure 4(a) and the density shown in Table 3. Normally, the ktot consists of three parts, i.e., lattice thermal conductivity (/c/af), electronic thermal conductivity (Keie), and bipolar thermal conductivity (Kh,P). k,.\,. can be easily estimated from the Wiedemann-Franz relationship (k,.\,. = LoT), where L is the Lorenz number, as shown in Figure 4(c). The Lorenz number is obtained by fitting the respective Seebeck coefficient values with an estimate of the reduced chemical potential using a single parabolic band (SPB) model (Eqs. 1-3) [27], where fe is the Boltzmann constant, h is the Plank constant, e is the electron charge, /•„(?/) is the //h order Femi

integral, 77 is the reduced Fermi energy, % is the variable of integration, rather than

8 2* using a constant value of 2.45 x 10" W-Q-K"" for degenerate semiconductor.

i - {he\2 nzfo) _ f2f1(y)\2]

L-\e) UW UortJJ ()

- e VFoOj) V V '

Fn(Jl) = i:-£^dx (3)

In general, k\at can be estimated by directly subtracting ke\e from km. In this case, because of the intrinsic excitation occurred at a high temperature, ke\e and k\at are only calculated before the onset of bipolar effect, as shown in Figures 4(d) and 4(e). It is easy to find that the total thermal conductivity decreases with the increase of the Cd content. The ke\e has a similar tendency with the change of electrical resistivity caused by Cd substitution, as shown in Figure 3(a). The k\at decreases from 1.73 W/m-K for Zn0.995Na0.005Sb to 1.1 W/m-K for Zn0.875Na0.005Cd0.12Sb at room temperature with increasing the Cd content. The low thermal conductivity can be attributed to the strain fluctuation caused by the mass and size fluctuation between Zn2+ (i.e., 65.38, 0.74 A) and Cd2+ (i.e., 112.41, 0.95 A).

Figure 5 shows the ZT value calculated based on the electrical and thermal transport properties. The maximum ZT value is 1.22 at 350 °C for Zn0.915Na0.005Cd0.08Sb sample, which is greater than that of 0.99 for Zn0.995Na0.005Sb sample and 0.45 for ZnSb sample. The present ZT value of 1.22 is greater than that of other ZnSb based thermoelectric materials [17-19]. The improved ZT value can be attributed to the enhanced power factor caused by the enhanced electrical conductivity, as shown in Figure 3, and the reduced thermal conductivity, as shown in Figure 4. In addition, the average ZT value is improved from 0.7 of the Zn0 995Na0 005Sb sample to 0.912 of the Zn0915Na0005Cd008Sb sample due to the Cd substitution.

4. Conclusions

The effect of Cd isoelectric substitution for Zn in Na0 005Cd008Sb on the thermoelectric properties was investigated. The Zn0915Na0005Cd008Sb showed a significant enhancement of ZT value from 0.45 for ZnSb to 1.22 at 350 °C, which could be ascribed to the enhanced power factor and the reduced thermal conductivity caused by the Cd substitution.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (Nos. 51622101, 51471061 and 51271069).

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List of Figure and Table Captions

Figure 1 XRD patterns of ZnSb and Zn0.995-xNa0.005CdxSb (x = 0, 0.04, 0.08 and 0.12).

Figure 2 Fracture morphologies of Zn0 995-xNa0005CdxSb (a) x = 0, (b) x = 0.04, (c) x = 0.08, (d) x = 0.12.

Figure 3 Electrical transport properties of Zn0995-xNa0.005CdxSb (a) Electrical resistivity, (b) Seebeck coefficient and (c) Power factor.

Figure 4 Thermal transport properties of Zn0 995-xNa0005CdxSb (a) Thermal diffusivity,

(b) Total thermal conductivity, (c) Lorenz number, (d) Electronic thermal

conductivity, (e) Lattice thermal conductivity.

Figure 5 ZT values of ZnSb and Zn0 995-xNa0005CdxSb.

Table 1 The lattice parameters of ZnSb and Zn0 995-xNa0005CdxSb samples.

Table 2 Room temperature carrier concentration and mobility as a function of Cd

content for Zn0.995-xNa0.005CdxSb samples.

Table 3 Density of Zn0 995-xNa0.005CdxSb samples.

Jingchao Zhou

Dr. Lihong Huang

Zhengyun Wang

Zihang Liu

Dr. Jiehe Sui

20 25 30 35 40 45 50 55 60

Figure 1 Powder XRD patterns of the ZnSb and the Zn0.995-xNa0.005CdxSb (x = 0, 0.04,

0.08 and 0.12).

(a) V f^- 1 ^ y 3fim r ' ' 1 - ' w \ll \ ■ .

(c) / £ fi 1*1 _j r f CW- T ; V / i s M * i /r ^ - ' Ilk / -I - ^ A \ N '0' —- - N iLXi

Figure 2 Fracture morphologies of Zn0.995-xNa0.005CdxSb (a) x = 0, (b) x = 0.04, (c) x = 0.08, (d) x = 0.12.

x=0 x=0.04 x=0.08 x=0.12 Zn Ag Sb [22]

0.998 &0.002 L V

50 100 150 200 250 300 350 400 450 Temperature ( °C )

Sb [22]

-0.05Ag0.0015Zn0.9485Sn0.02Sb~ J_I_._I_._I_._I_._1_

0 50 100 150 200 250 300 350 400 450 Temperature (C )

o S* 18

LJH r 12

£ o 10

•(c)

......

- —•- x=0.04

-A- x=0.08

...... -T- x=0.12 ............

0 50 100 150 200 250 300 350 400 450 Temperature (C )

Figure 3 Electrical transport properties of Zn0 995.xNa0005CdxSb.

(a) Electrical resistivity, (b) Seebeck coefficient and (c) Power factor.

Temperature (°C )

) 1.63

a £ 1.61

o 1.59

X 1.58

J 1.57

—•- x=0.04

-A- x=0.08

..... —x=0.12 ..........

50 100 150 200 250 300 350 400 Temperature (C )

—■— x=0

—•- x=0.04

-A- x=0.08

..... -T- x=0.12 .........

50 100 150 200 250 300 350 400 Temperature (C )

1.8 -(e)

1.6 -■- x=0

—•- x=0.04

1.4 - \ \ -A- x=0.08

—T- x=0.12

1.2 - a

1.0 - NXJ

0.8 - ^

0.6 ..... ..........

0 50 100 150 200 250 300 350 400 Temperature (°C )

Figure 4 Thermal transport properties of Zn0.995-xNa0.005CdxSb. (a) Thermal diffusivity, (b) Total thermal conductivity, (c) Lorenz number, (d) Electronic thermal conductivity, (e) Lattice thermal conductivity.

2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

■ x=0

■ x=0.04

■ x=0.08

■ x=0.12

^0.998^0.002^ t22l

0 50 100 150 200 250 300 350 400 450 Temperature (°C)

Figure 5 ZT value of both ZnSb and Zn0.995_xNa0.005CdxSb.

Table 1 Lattice parameters of ZnSb and Zn0.995-xNa0.005CdxSb samples

Sample a (Ä) b (Ä) c (Ä)

ZnSb 6.205 7.737 8.085

Zno.995Nao.oo5Sb 6.208 7.743 8.095

Zn0.955Na0.005Cd0.04Sb 6.216 7.756 8.101

Zno.915Nao.oo5Cdo.o8Sb 6.218 7.759 8.105

Zno.875Nao.oo5Cdo.12Sb 6.223 7.766 8.110

Table 2 Room temperature carrier concentration and mobility as a function of Cd

doping for Zn0.995-xNa0.005CdxSb

Sample Carrier concentration (cm- ) Carrier mobility (cm /Vs)

Zn0.995Na0.005Sb 6.12 x 1018 289.9

Zn0.955Na0.005Cd0.04Sb 8.69 x 1018 288.4

Zn0.915Na0.005Cd0.08Sb 10.07 x 1018 262.5

Zn0.875Na0.005Cd0.12Sb 6.15 x 1018 209.3

Table 3 Sample density for Zn0.995. ,xNa0.005CdxSb

Sample Density (g/cm3)

Zn0.995Na0.005Sb 6.328

Zn0.955Na0.005Cd0.04Sb 6.334

Zn0.915Na0.005Cd0.08Sb 6.339

Zn0.875Na0.005Cd0.12Sb 6.440

Jingchao Zhou is a Ph.D. candidate in the School of Materials Science and Engineering from Harbin Institute of Technology, China. He received his master's degree in the Department of Materials Physics and Chemistry from Harbin Institute of Technology. His research focuses on synthesis and characterization of nanostructured thermoelectric materials.

Dr. Lihong Huang is currently a research faculty in the Center for Advanced Materials and Energy at Xihua University of China, and also was a visiting scholar in the Department of Physics and TcSUH at the University of Houston. She obtained her Ph.D. degree in Materials Science from Sichuan University in 2012. Her current research interesting covers the fabrication, characterization of nanostructured thermoelectric materials, especially half-Heuslers.

Zhengyun Wang is currently a researcher in the Center for Advanced Materials and Energy, Xihua University, China. His current research is mainly on synthesis and characterization of phase change functional materials.

Zihang Liu is currently a Ph.D. candidate in the Department of Material Physics and Chemistry at Harbin Institute of Technology, China. He received his Bachelor degree from Material Science and Engineering at Inner Mongolia University of Technology. His current research is mainly on nanostructured thermoelectric materials and devices.

Dr. Wei Cai is currently a Professor and Head of the Material Physics and Chemistry at Harbin Institute of Technology. He obtained his Ph.D. degree from Harbin Institute of Technology in 1994. His research interests focus mainly on shape memory materials, nanomaterials and thermoelectric materials.

Dr. Jiehe Sui is currently a Professor of the Material Physics and Chemistry at Harbin Institute of Technology. He received his Ph.D. degree in the Department of Materials Physics and Chemistry from Harbin Institute of Technology, China. His current research is mainly on thermoelectric materials and devices.