Enhanced Thermoelectric Power Factor of Na_xCoO₂ Thin Films by Structural Engineering

Abstract

engineering can be used to tune the specifi c thermoelectric material properties.Detailed thermoelectric characterization of Na x CoO 2 thin fi lms has been hindered by the chemical instability in ambient conditions. [ 7,8 ] However, a recently developed method to obtain chemically stable, single-phase Na x CoO 2 thin fi lms by pulsed laser deposition due to the in situ deposition of an amorphous AlO x capping layer enables us to exploit the intrinsic properties of these thermoelectric thin fi lms. [ 8 ] Here, we show that by structural engineering in chemically stable Na x CoO 2 thin fi lms the thermoelectric properties can be controlled and enhanced as compared to bulk samples. By changing the single crystalline substrate material we can control the structural properties and as a consequence the electronic and thermal properties of the thermoelectric thin fi lms. Tuning of the grain size within the Na x CoO 2 thin fi lms signifi cantly infl uences the achievable Seebeck coeffi cient. We demonstrate that preservation of the crystallinity in these thin fi lms with enhanced Seebeck coeffi cient results in minimal reduction of the electrical conductivity and, therefore, leads to a doubling of the thermoelectric power factor at room temperature.Here, structural engineering is applied as a tool to obtain improved control over the thermoelectric properties of Na x CoO 2 thin fi lms, which is unique for epitaxial thin fi lms and cannot be obtained in single crystal or polycrystalline samples. To study this effect, Na x CoO 2 thin fi lms were grown by pulsed laser deposition (PLD) on various single crystal substrates. All Na x CoO 2 thin fi lms were deposited under the same conditions and have a thickness of 60 nm. Independent of the substrate material and structure, all thin fi lms showed a preferred growth orientation with the (00l) direction parallel to the surface normal.Previously it was shown that the crystallinity of Na x CoO 2 thin fi lms does not strongly depend on the deposition temperature, [ 8 ] and an optimum deposition temperature of 430 °C was determined. However, the effect of oxygen deposition pressure on the crystallinity was not systematically studied yet. Here, we observe a signifi cant decrease in crystallinity when the deposition pressure was reduced by one and two orders of magnitude from the previously reported value of 0.4 mbar, [ 8 ] resulting in an increased resistivity by a factor of fi ve, together with a strong reduction of the Seebeck coeffi cient. Based on these results we can conclude that, although the deposition pressure can clearly be used to tune the crystallinity of these Na x CoO 2 thin fi lms, it will not provide the required enhanced control over the thermoelectric properties. Therefore the optimized deposition parameters [ 8 ] are used, which result in a combination of the optimum crystallinity and thermoelectric properties. Furthermore, all thin fi lms have been cooled down after growth in 1 atm. of oxygen at a rate of 10 °C min -1 to optimize the oxidation level.

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

“…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.…”

“…The reported thermopower and electrical conductivity were lower than the values reported for single crystals, presumably due to difficulties associated with controlling the sodium content in the films, which has profound influence on the thermoelectric properties. 73 Brinks et al 136 studied the effect of substrate symmetry and lattice constants on the thermoelectric properties of Na x CoO 2 films, and found that even cubic substrates were suitable for epitaxial growth of Na x CoO 2 films. The highest ZT obtained was ;0.17 at room temperature for films grown on LSAT substrates.…”

“…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. Both methods used pulsed laser deposition for the synthesis.…”

High-temperature stability of thermoelectric Ca3Co4O9 thin films

“…As substrate material, (001) oriented single crystals of Al 2 O 3 and (LaAlO 3 ) 0.3 (Sr 2 AlTaO 6 ) 0.7 (LSAT) were used, which were annealed beforehand at 1050 C for, respectively, 1 h and 10 h to obtain a smooth surface with predominantly unit-cell-height steps. 12 Hexagonal Al 2 O 3 was chosen because of the good structural match with the hexagonal Ca 3 Co 4 O 9 inplane crystal structure, whereas LSAT was used because of the recent observation of significant enhancement of the thermoelectric properties in Na x CoO 2 thin films. 12 To study the high-temperature structural properties, temperature dependent X-Ray diffraction (XRD) measurements were performed in air, using a Bruker D8 and a Panalytical XPert Pro MRD diffractometer, equipped with an Anton Paar domed hot stage.…”

“…12 Hexagonal Al 2 O 3 was chosen because of the good structural match with the hexagonal Ca 3 Co 4 O 9 inplane crystal structure, whereas LSAT was used because of the recent observation of significant enhancement of the thermoelectric properties in Na x CoO 2 thin films. 12 To study the high-temperature structural properties, temperature dependent X-Ray diffraction (XRD) measurements were performed in air, using a Bruker D8 and a Panalytical XPert Pro MRD diffractometer, equipped with an Anton Paar domed hot stage. High-temperature measurements of the in-plane electrical resistivity and Seebeck coefficient were performed using a Linseis LSR3-1100 system.…”