Elsevier

Journal of Alloys and Compounds

Volume 653, 25 December 2015, Pages 480-485
Journal of Alloys and Compounds

Investigation of the effects of compressive and tensile strain on n-type bismuth telluride and p-type antimony telluride nanocrystalline thin films for use in flexible thermoelectric generators

https://doi.org/10.1016/j.jallcom.2015.09.039Get rights and content

Highlights

  • We assess the potential performances of flexible thin film thermoelectric generators.

  • We prepared Bi–Te and Sb–Te thin films on flexible polyimide substrates.

  • We applied various compressive and tensile strains to the thin films.

  • The performance of both types of thin films decreased with increasing tensile stain.

Abstract

To assess the potential performances of flexible thin film thermoelectric generators, we prepared n-type bismuth telluride and p-type antimony telluride nanocrystalline thin films on flexible polyimide substrates using a RF magnetron sputtering method. We applied various compressive and tensile strains to the films by changing the bending radii of the flexible substrates prior to deposition. The structural and thermoelectric properties of the completed samples were analyzed. The bismuth telluride and antimony telluride thin films had dense granular and a chaff-like surface morphologies, respectively. The shapes of both types of films did not greatly change when strains were applied. Both types of thin films demonstrated higher electrical conductivities when under compressive strain than when under tensile strain. The absolute Seebeck coefficients of the bismuth telluride thin films were slightly higher when under tensile strain than when under compressive strain. In contrast, the antimony telluride thin films demonstrated the opposite trend, having higher values when under compressive strain than when under tensile strain. The power factors of both types of thin films were found to decrease with increasing tensile stain. Therefore, we expect that the thermoelectric performances of flexible thin film generators are not largely changed under compressive strain but they decrease under tensile strain.

Introduction

Power management in energy harvesting technologies, which transform environmental energy sources into electricity, is essential for their ubiquitous and autonomous implementation in energy systems [1]. Compared to other energy harvesting technologies, the thermoelectric effect (Seebeck effect) is one of the simplest processes for interconverting thermal and electrical energy. Thermoelectric devices (thermoelectric generators) use the thermoelectric effect and consist of multiple pairs of n- and p-type materials. Their main advantages are reliability and silent, vibration-free operation because they have no moving fluids or mechanical parts. Research is currently focused on developing portable thermoelectric power generators [2], [3], [4]. Generators for use as wearable biometric monitoring sensors [5] and autonomous wearable sensor nodes [6] are of particular interest. Efforts are being made to increase their energy conversion efficiencies, reduce the sizes of such devices, and minimize their manufacturing costs.

The thermoelectric properties of bismuth-telluride-based alloys, including n-type bismuth telluride and p-type antimony telluride, are some of the best materials for use in portable power generators because they exhibit the excellent performance near room temperature (RT) [7]. In addition, the thermoelectric efficiencies of these materials are known to improve when they are nanostructured, such as in nanocomposites [8], nanoporous materials [9], [10], and nanocrystals [11], [12].

On the other hand, thin film materials have shown promise for use as small thermoelectric generators with low manufacturing costs because they have already been used in microelectromechanical systems [13], [14], [15] and can be produced by batch processes. Another advantage of thin film thermoelectric generators is the possibility of using flexible substrates. Flexible substrates would allow generators to be attached on various places, such as human bodies and tubes. Hence, developing flexible thin film thermoelectric generators has become a major objective in the field of thermoelectrics [16], [17], [18].

An important factor to consider when fabricating flexible thin film thermoelectric generators is the strain that occurs as a result of bending of substrates. The effects of strain on thermoelectric properties have been widely studied; for example, strain has been induced by using a substrate with a mismatched lattice [19], and by applying pressure [20], [21], [22]. Although these methods have shown how strain affects thermoelectric properties, controlling both the magnitudes and directions of applied strains is challenging. We previously reported how to precisely control both the magnitude and direction of a strain applied to a flexible substrate bent around a radius, using a tuning method [23]. However, we only investigated strain effects of n-type bismuth telluride thin films, neglecting the p-type thin films. It is known that there are differences between the strain effects of n- and p-type thermoelectric materials [22]. Therefore, in order to evaluate the potential performances of flexible thin film generators under bending conditions, it is necessary to investigate the properties of both n-type and p-type thin films.

In this study, in order to assess the potential performances of the flexible thin film thermoelectric generators, we applied various compressive and tensile strains to n-type bismuth telluride and p-type antimony telluride nanocrystalline thin films by changing the bending radius of the flexible substrate. Both types of films were deposited using a RF magnetron sputtering method since it is known to result in good adhesion between substrates and sputtered layers. The structural and thermoelectric properties of the resulting films were analyzed. In this paper, we compare the two types of films, discuss how strain affects thermoelectric properties, and predict the potential performances of flexible thin film thermoelectric generators.

Section snippets

Experimental details

The basic experimental procedures were described in our previous study [23]. In brief, n-type bismuth telluride and p-type antimony telluride thin films were deposited by RF magnetron sputtering (Tokuda, CFS-8EP). Polyimide (Kapton) films (size: 100 mm × 20 mm, 125 μm thick) were used as substrates because they are flexible and have heat resisting properties. For the deposition of bismuth telluride thin films, we used a high-purity (99.9%) Bi(40 at.%)–Te(60 at.%) target with a 5 inch diameter

Results and discussion

Fig. 1 shows SEM images of the surface morphologies of n-type bismuth telluride [23] and p-type antimony telluride thin films. Comparing the unstrained bismuth telluride thin film (Fig. 1(c)) with the unstrained antimony telluride thin film (Fig. 1(h)), it is apparent that the two samples had significantly different grain shapes. The unstrained bismuth telluride thin film had a dense granular structure with a grain size of approximately 100 nm. On the other hand, the unstrained antimony

Conclusion

In order to assess the potential performances of flexible thin film thermoelectric generators, we applied various compressive and tensile strains to n-type bismuth telluride and p-type antimony telluride nanocrystalline thin films by changing the bending radii of flexible substrates. Both types of films were deposited by RF magnetron sputtering using a substrate temperature of 200 °C. The shapes of the crystal grains of the bismuth telluride thin films were quite different from those of the

Acknowledgments

The authors wish to thank H. Kiyuna, N. Hatsuta, K. Takayama, T. Inamoto, Y. Sasaki and K. Kurita at Tokai University for providing experimental support.

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