Achieving chemical stability in thermoelectric NaxCoO2 thin films

Abstract

Stability issues in thermoelectric Na x CoO 2 thin films have been solved by the addition of an in situ amorphous AlO x capping layer, which prevents previously reported degradation when exposed to air. These chemically stable thin films enable detailed analysis of the intrinsic thermoelectric properties and form a significant progress towards applications. Single phase Na x CoO 2 thin films with a low surface roughness are grown by pulsed laser deposition with either an epitaxial or textured crystal structure on Al 2 O 3 (001) or LaAlO 3 (001) single crystal substrates, respectively. For textured thin films a resistivity and thermopower of 0.99 mV cm and 69 mV K 21 are observed at room temperature, respectively. Based on an estimated thermal conductivity for thin films, the dimensionless figure of merit is very comparable to Na x CoO 2 single crystals, demonstrating the effectiveness of the developed capping layer.

Thermoelectric and thermal transport properties of complex oxide thin films, heterostructures and superlattices

“…These stability issues were addressed using barrier or capping layers and the thermoelectric properties at room temperature were found to be largely stable and comparable to bulk single crystals. 135 The thin film studies also focused on other notable layered cobaltates such as Ca 3 Co 4 O 9 , Bi 2 Sr 2 Co 2 O y etc. We will not focus our discussions on Ca 3 Co 4 O 9 , as this material has received considerable attention in a recent review article, 35 but we will discuss the growth efforts on Bi 2 Sr 2 Co 2 O y and related compounds.…”

“…Synthesis of Na x CoO 2 thin films was carried out primarily using pulsed laser deposition, but other methods such as topotactic exchange, chemical solution deposition. [132][133][134][135][136][137][138][139] Several attempts resulted in epitaxial growth of Na x CoO 2 , on c-plane sapphire, SrTiO 3 , SrLaGaO 4 etc. 132,133,137,140 As sodium in these compounds is a reactive species, these films often suffered from poor stability.…”

“…Although much of the interest in this materials system stemmed from the large thermoelectric power factor in Na x CoO 2 , 21,73 most thin film synthesis efforts were affected by the poor stability of Na x CoO 2 . 135 Hence, only few studies addressed the thermoelectric properties in detail, and often below room temperature. 134,136 Venimadhav et al 134 prepared Na x CoO 2 thin films using two different techniques: one using a two-step topotaxial conversion of Co 3 O 4 films into Na x CoO 2 , and another of direct synthesis.…”

Thermoelectric oxides

“…Detailed thermoelectric characterization of Na x CoO 2 thin films has been hindered by the chemical instability in ambient conditions, which reduced the interest and the number of reports on Na x CoO 2 thin films (Brinks, Heijmerikx, Hendriks, Rijnders, & Huijben, 2012;Zhou et al, 2006). This lack of chemical stability was observed through an increase of the (room temperature) electrical resistivity with time, when samples were exposed to ambient conditions.…”

“…This lack of chemical stability was observed through an increase of the (room temperature) electrical resistivity with time, when samples were exposed to ambient conditions. A method was developed to obtain chemically stable, singlephase Na x CoO 2 thin films by PLD, due to the in-situ deposition of an amorphous AlO x capping layer, enabling the study of the intrinsic properties of these thermoelectric thin films (Brinks et al, 2012). 14.4.3.1 Chemically stable, capped Na x CoO 2 thin films Two possible solutions were proposed to achieve chemical stability in Na x CoO 2 thin films: protecting the thin film from direct contact with the surrounding atmosphere (Zhou et al, 2006) or modifying the reactivity of the material through (partial) substitution of sodium by another cation (Sugiura et al, 2006a). This chemical instability has also been observed in single crystals (Li, Zhang, & Xu, 2013) of Na x CoO 2 and is caused by the reactivity of the sodium with moisture and carbon dioxide from air, which results in the formation of sodium carbonate.…”