Improvement of textured Bi1.6Pb0.4Sr2Co1.8Ox thermoelectric performances by metallic Ag additions
Introduction
Thermoelectric (TE) materials with high energy conversion efficiencies are strongly required for electric power generation. Thermoelectric energy conversion is now showing very important advantages to harvest waste heat in a wide number of applications. Moreover, it can transform solar energy into electricity at lower cost than photovoltaic energy [1]. The conversion efficiency of such materials is quantified by the dimensionless figure of merit ZT, which is defined as TS2/ρκ (in which S2/ρ is also named power factor, PF), where S is the Seebeck coefficient (or thermopower), ρ the electrical resistivity, κ the thermal conductivity, and T is the absolute temperature [2]. As higher ZT means higher efficiency, an adequate TE material for practical applications must involve high thermopower and low electrical resistivity, with low thermal conductivity.
The discovery of large thermoelectric power in NaxCoO2 [3], which was found to possess a high ZT value of about 0.26 at 300 K, has opened a broad research field and from that moment on, great efforts have been devoted to explore new cobaltite families with high thermoelectric performances. Some other layered cobaltites, such as misfit [Ca2CoO3][CoO2]1.62, [Bi0.87SrO2]2[CoO2]1.82 and [Bi2Ca2O4][CoO2]1.65 were also found to exhibit attractive thermoelectric properties [4], [5], [6], [7], [8]. The crystal structure is composed of two different layers, with the alternate stacking of a common conductive CdI2-type CoO2 layer with a two-dimensional triangular lattice and a block layer, composed of insulating rock-salt-type (RS) layers. Both sublattices (RS block and CdI2-type CoO2 layer) possess common a- and c-axis lattice parameters and β angles but different b-axis length, causing a misfit along the b-direction [9], [10], [11].
As layered cobaltites are materials with a strong crystallographical anisotropy, the alignment of plate-like grains by mechanical and/or chemical processes is necessary to attain macroscopic properties comparable to those obtained on single crystals. Some techniques have been shown to be adequate to obtain a good grain orientation in several oxide ceramic systems, such as template grain growth (TTG) [10], sinter-forging [12], spark plasma [13], and directional growth from the melt [14]. On the other hand, it is interesting to explore cationic substitutions in the RS layer, which can change the misfit relationship between these two layers and, as a consequence, modify the values of the thermopower [7]. From this point of view, it is clear that this kind of substitution can be useful in order to improve thermoelectric performances of ceramic materials [12], as is reported for the substitution of Gd and Y for Ca [15], or Pb for Bi [16]. Moreover, metallic Ag additions have also been shown to improve, in an important manner, the mechanical and thermoelectrical properties of this system [17] and other similar materials [18] which nearly do not react with Ag.
Taking into account these previously discussed effects, the aim of this work is producing high performance TE materials by the addition of metallic Ag to the optimally Pb doped Bi–Sr–Co–O compound [16], [19], followed by a texturing process performed by the laser floating zone (LFZ) technique.
Section snippets
Experimental
The initial Bi1.6Pb0.4Sr2Co1.8Ox with small amounts of silver (0, 1, and 3 wt% Ag) polycrystalline ceramics were prepared from commercial Bi(NO3)3·5H2O (≥98%, Aldrich), SrCO3 (98.5%, Panreac), Co(NO3)2·6H2O (98%, Panreac), and metallic Ag (99%, Aldrich) powders by a sol–gel via nitrates method. They were weighed in the appropriate proportions and suspended in distilled water. Concentrated HNO3 (analysis grade, Panreac) was added dropwise into the suspension until it turned into a clear pink
Results and discussion
Powder XRD patterns for all Bi1.6Pb0.4Sr2Co1.8Ox samples with different amounts of Ag are plotted (from 10° to 40° for clarity) in Fig. 1. They show very similar patterns where the most intense peaks correspond to the misfit cobaltite Bi1.6Pb0.4Sr2Co1.8Ox phase, in agreement with previous reported data [16], [28]. From this figure, it is clear that the cobaltite phase appears as the major one, independently of Ag content. Peaks marked with a ● in the plot correspond to the Co-free Bi0.75Sr0.25O
Conclusions
This paper demonstrates that Bi1.6Pb0.4Sr2Co1.8Ox thermoelectric materials with small Ag additions (0, 1, and 3 wt%) can be directionally grown by the laser floating zone method. It has been determined that the optimal Ag content in the textured materials is 3 wt%. Microstructural evolution shows that this amount of Ag reduces significantly the amount of secondary phases and induces their alignment with the growth direction. Moreover, the electrical resistivity data clearly indicated that this Ag
Acknowledgements
This research has been supported by the Spanish Ministry of Science and Innovation (Project no. MAT2008–00429) and the Universidad de Zaragoza (Project no. UZ2011-TEC-03). The authors wish to thank the Gobierno de Aragón (Consolidated Research Groups T87 and T12) for financial support and C. Gallego, C. Estepa and J.A. Gomez for their technical assistance. Sh. Rasekh acknowledges a JAE-PreDoc2010 grant from the CSIC.
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