Abstract
High quality NaCo2O4 thermoelectrics are challenging to process due to the volatile nature of Na, the slow densification kinetics, and degradation of NaCo2O4 above 900–950 °C leading to the formation of Na-poor second phases. Fine grained sol-gel derived powders have been used to enhance the densification kinetics while pre-treatment of the NaCo2O4 powder with NaOH, to provide a Na rich environment, has been shown to mitigate Na loss at elevated temperatures. While insufficient to compensate for Na loss at processing temperatures of 1000 °C and above, at lower temperatures it is able to enhance densification and facilitate the formation of complex crystal structures yielding low thermal conductivity (0.66 Wm−1K−1) coupled with high electrical conductivity (3.8 × 103 Sm−1) and a Seebeck coefficient of 34.9. The resultant room temperature power factor and ZT were 6.19 × 10−6 Wm−1K−2 and 0.0026, respectively.
Highlights
Thermoelectric (TE) materials have the potential to play a significant role in the development of sustainable energy-efficiency technologies due to their ability to directly convert heat into electricity with no moving parts [1]
In-situ X-ray diffraction (XRD) of NaCo2O4 precursor materials has shown that NaCo2O4 can be formed at temperatures from around 650 C, but that the material degrades rapidly at temperatures above 950 C with a clear degradation of materials observed at temperatures of 1000 C and above
While NaCo2O4 forms around 650 C it does not begin to sinter until at least 900 C, with 950 C being required to bring about appreciable densification, resulting in a very small processing window
Summary
Thermoelectric (TE) materials have the potential to play a significant role in the development of sustainable energy-efficiency technologies due to their ability to directly convert heat into electricity with no moving parts [1]. ZT should ideally be > 1 to achieve conversion efficiencies greater than 10% [1]. As such there is a significant drive to increase ZT, at high temperatures. At elevated temperatures traditional TE materials, such as Bi2Te3, are known to melt and oxidize in the presence of air [5,6]. Such materials are composed of toxic, heavy and rare elements leading to active research into replacement materials such as oxide ceramics [5]. There are a number of candidate materials including NaCo2O4, Ca3Co4O9 and modified SrTiO3 that have been shown to exhibit useful properties, they contain volatile elements and necessitate high processing temperatures, typical of ceramics, which can lead to loss of stoichiometry and grain growth which are detrimental for thermoelectric properties
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