Improved thermoelectric performances in textured Bi1.6Pb0.4Ba2Co2Oy/Ag composites
Introduction
Due to the present and future energetic challenges, thermoelectric (TE) energy conversion can be exploited for harvesting wasted heat in the classical energy transformation systems, such as automobile, thermal power plants, etc. The use of TE systems can raise the efficiency of these systems, decreasing the fossil fuels consumption and, consequently, the release of greenhouse gases. On the other hand, for these applications TE materials with high energy conversion efficiency, quantified through the dimensionless Figure-of-Merit, ZT (=TS2/ρκ, where S, ρ, κ, and T, are Seebeck coefficient, electrical resistivity, thermal conductivity, and absolute temperature [1]), are necessary. Nowadays, different metallic alloys possess high performances (ZT>1) [2] which can make them attractive for these practical applications. However, the most common, as PbTe [2] or Bi2Te3 [3] are based on heavy and/or toxic elements which can be oxidized or released when working at high temperatures.
In 1997, a broad research field has been opened by the discovery of relatively high thermoelectric properties in NaxCoO2 ceramic material (ZT∼0.26 at 300 K) [4]. Since then, many works have been performed on CoO-based materials with attractive thermoelectric performances [5], [6], [7], [8], [9]. Structural studies of these compounds have shown that their crystalline structure can be described as two different alternately stacked layers: 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 [10], [11].
The coexistence of these two sublattices produces a strong crystallographical anisotropy and anisotropic properties. As a consequence, the grains alignment is necessary when trying to achieve bulk properties close to the determined on single crystals. Among the successful techniques previously used in oxide ceramics, it is worth to mention the template grain growth (TTG) [11], sinter-forging [12], spark plasma [13], laser floating zone (LFZ) [14], or the electrically assisted LFZ [15]. On the other hand, cationic substitution has been found to be beneficial for the thermoelectric performances in previous works [16], [17], leading to improved electrical behaviour. Moreover, metallic Ag additions have also shown to enhance the thermoelectric properties of these CoO-based compounds without any evident reaction with the thermoelectric phases [18].
Taking into account previous results on textured [19], Pb doped [20], and Ag added [18] Bi2Ba2Co2Ox thermoelectric materials, the aim of this work is producing high performance TE materials by the simultaneous Ag addition to the optimally Pb doped Bi2Ba2Co2Ox compound, followed by an adequate texturing process through the well stablished LFZ technique.
Section snippets
Experimental
The initial Bi1.6Pb0.4Ba2Co2Oy+x wt% Ag (with x=0, 1, 3, and 5) polycrystalline ceramics were prepared from commercial Bi(NO3)3·5H2O (≥98%, Aldrich), PbO (Aldrich, 99%), BaCO3 (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 dissolved in a mixture of distilled water and concentrated HNO3 (analysis grade, Panreac). Once a clear pink solution has been obtained, citric acid
Results and discussion
Powder XRD patterns for all Bi1.6Pb0.4Ba2Co2Oy samples with different amounts of Ag are plotted (from 10 to 40 degrees for clarity) in Fig. 1. They show very similar patterns where the most intense peaks correspond to the misfit cobaltite Bi1.6Pb0.4Ba2Co2Oy phase, in agreement with previous reported data [25]. From the graph, it is clear that the cobaltite phase appears as the major one, independently of Ag content. Moreover, the peaks appearing at around 20.5 and 28.5 degrees, marked by a ✶,
Conclusions
This paper demonstrates that Bi1.6Pb0.4Ba2Co2Oy thermoelectric materials with small Ag additions (0, 1, 3, and 5 wt%) can be directionally grown by the laser floating zone method. It has been determined that 3 wt% is the optimal Ag content in the Bi1.6Pb0.4Ba2Co2Oy LFZ grown materials. Microstructural evolution shows that Ag slightly reduces the amount of secondary phases and improves the grain alignment. Thermoelectric data clearly show that Ag addition significantly decreases electrical
Acknowledgements
This research has been supported by the MINECO-FEDER (Project MAT2013-46505-C3-1-R). The authors wish to thank the Gobierno de Aragón (Consolidated Research Groups T87 and T12) for financial support. Authors would like to acknowledge the use of Servicio General de Apoyo a la Investigación-SAI, Universidad de Zaragoza.
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