Low temperature thermoelectric properties of Na-substituted Bi2Ca2Co2Oy ceramics fabricated via LFZ technique
Introduction
The present awareness about the global warming, and the necessity of reducing the use of energy sources based on fossil fuels signed in Glasgow in 2021, and the global increment in energy demand, all scientific studies related to alternative and renewable energy sources become very important to fulfill these agreements. In this scenario, the use of ceramic materials for technological applications can help to reach these objectives due to their intrinsic properties, as they are chemically and thermally very stable, when compared to other materials (metals, polymers, etc.). Moreover, a broad number of applications can be found for these materials to be applied, as in photocatalytic purifiers [[1], [2], [3], [4], [5], [6], [7]], hydrogen or energy storage [[8], [9], [10], [11], [12]], or filtering devices [13]. Among these ceramic materials different families display excellent electronic properties, as the ferrites [14,15], high-temperature superconductors [16,17], or thermoelectric materials [18,19].
In this situation, thermoelectric materials (TE), which are able to interconvert thermal and electric energies directly via thermoelectric generators, are called to play an important role. These systems make possible harvesting waste heat from energy transformation systems and produce electrical energy. The efficiency of these materials is quantified by the dimensionless figure of merit, ZT, defined as TS2/ρκ, where T is the absolute temperature, S Seebeck coefficient, ρ electrical resistivity, and κ thermal conductivity [20]. This ZT formula can be divided into two different parts: the thermal and the electrical ones, given by T/κ and S2/ρ, respectively. The electrical part is also known as power factor (PF) and indicates the electrical performances of thermoelectric materials.
TE materials are used at present for some practical applications in different areas such as refrigeration, waste heat harvesting in automobiles exhausts, or in the aero-spatial sector [21,22]. Most of these applications use alloys and/or intermetallic TE materials as Bi2Te3, PbTe and CoSe3 [21]. As drawbacks, it should be mentioned that these compounds are found in small amounts in the earth's crust [23,24], and can be degraded and/or release heavy metals at elevated temperatures [21]. On the other hand, the discovery of large TE power in NaxCoO2 [25], opened a broad research field. This study has triggered the discovery of various p-type cobalt oxide layered materials, namely Ca–Co–O [26], Bi–Ca–Co–O [27], or Bi–Sr–Co–O [28]. Later on, TiO- and MnO-based materials, showing n-type conduction were discovered and accompanying these p-type materials as their counterpart in thermoelectric modules [29,30]. These TE oxides are abundant and cheap [24,31], and are thermally and chemically very stable.
Among these thermoelectric materials, misfit layered cobalt oxides, having a perfect combination of high thermoelectric power and electrical conductivity, have been heightened research interest from both scientific and technological points of view, since synthesized for the first time in late 1990s [32,33]. Moreover, it is well-known that crystal structure of these Co-based thermoelectric materials consists of two different layers; CdI2-type CoO2 conductive and rock salt (RS) Bi2X2O4 (X = Ca, Sr and Ba) insulating layers, alternatively stacked. These two conductive and insulating layers display the same a- and c-axis lattice parameters with different b-axis length, causing a misfit along the b-direction [34,35], leading to a large anisotropy. Thus, this disorder affects electrical properties and as well as the Seebeck coefficient of the material. From this point of view, in order to improve the thermoelectric parameters, different cation substitution, and/or synthesis techniques [[36], [37], [38], [39], [40], [41], [42], [43]] have been widely used. In previous studies, high temperature thermoelectric properties of Bi2Ca2-xNaxCo2Oy samples prepared by well-known solid state technique followed by laser floating zone (LFZ) texturing [44] have been determined. In this study, we aim to investigate the variation of the microstructure and thermoelectric parameters as resistivity, Seebeck coefficient, thermal conductivity and figure of merit, at low temperatures.
Section snippets
Experimental procedure
Bi2Ca2-xNaxCo2Oy with x = 0, 0.05, 0.10, and 0.125 thermoelectric ceramics were produced via the well-known conventional solid state route using Bi2O3 (purity 98 + %, Panreac), CaCO3 (purity 98.5%, Panreac), CoO (purity 99.99%, Sigma-Aldrich), and Na2CO3 (purity 99.8%, Panreac) starting powders. After homogenously mixing and ball milling of powders, at 300 rpm for 30 min, in distilled water media, IR lamps were used for drying the suspension, and manually milled to crack the agglomerates. In
Results and discussion
Representative XRD patterns of powder specimens are taken at 300 K and presented in Fig. 1. As can be seen, all patterns are quite similar and major peaks correspond to the (00l) diffraction planes of Bi2Ca2Co2Oy thermoelectric phase, independently of Na substitution, in agreement with previously published studies [48,49]. In addition, low intensity diffraction peaks, identified as *, indicate the presence of Co-free Ca4Bi6O13 phase in small amounts, which appears as unreacted product from the
Conclusion
In this work, Bi2Ca2-xNaxCo2Oy with x = 0.0, 0.05, 0.10, and 0.125 ceramic thermoelectric materials have been prepared by the conventional classical solid state route, followed by a texturing process through the laser floating zone method. XRD patterns were quite similar each other and major peaks correspond to the (00l) diffraction planes of the thermoelectric Bi2Ca2Co2Oy phase, independently of Na substitution. In addition, only small amount Co-free Ca4Bi6O13 secondary phases were identified.
CRediT authorship contribution statement
B. Özçelik: Leader, Writing – original draft, Writing – review & editing, Visualization. G. Çetin: Contributer, Data curation, Formal analysis, Visualization. M. Gürsul: Contributer, Data curation, Formal analysis, Visualization. C. Özçelik: Material preparation, Data curation, Formal analysis. T. Depci: Leader, Writing – original draft, Writing – review & editing, Visualization. M.A. Madre: Formal analysis, Design, Contributer, Visualization. A. Sotelo: Leader, Writing – original draft,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was carried out within the scope of Cukurova University Scientific Research Projects Unit FBA-2020-13007. A. Sotelo and M. A. Madre wish to thank the Spanish MINECO-FEDER (MAT2017-82183-C3-1-R), and Gobierno de Aragón (Research Group T54-20R) for funding. The authors wish to acknowledge the use of Servicio General de Apoyo a la Investigación-SAI, Universidad de Zaragoza.
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