Ca3Co4O9 Films Research Articles

Thin-film thermocouples of Ni-joined thermoelectric Ca3Co4O9

Thin-film Ni–Ca3Co4O9 and Ni–Mo thermocouples were prepared by stepwise magnetron-sputtering/annealing synthesis using masks. Compared with Ni–Mo thin film thermocouples, Ni–Ca3Co4O9 thin film thermocouples have higher output voltage due to large positive Seebeck voltage (153 μV/K for single thermocouple and 912 μV/K for 6-series thermocouple). The maximum output voltage from the thermocouple is 70 mV was obtained for a hot-end temperature of 105 °C for Ni–Ca3Co4O9 for a 6-series thermocouple. The stability of Ca3Co4O9 films and the ability to make free-standing films, together with the high Seebeck coefficient, show that these thin-film oxides can be used as p-type leg in thermocouples, with implications for use in free-standing and flexible thermoelectric devices.

Effect of disordered nanoporosity on electrical and thermal properties of layered Ca3Co4O9 films

Independently controlling electronic and thermal transport in solids is a challenge, because these properties are coupled. Here, we show that disordered nanoporosity in Ca3Co4O9 thin films can decrease the thermal conductivity without significantly hampering electronic transport. Scanning thermal microscopy was used to determine the out-of-plane thermal conductivity and estimate the in-plane values. Nanoporous Ca3Co4O9 films exhibit a thermal conductivity of 0.82 W m−1 K−1, which is nearly twofold lower than that obtained from nonporous Ca3Co4O9 films. Nanoporous Ca3Co4O9 exhibit a room-temperature electrical resistivity of 4 mΩ cm, which is comparable to polycrystalline Ca3Co4O9 and twice that reported for single-crystal Ca3Co4O9. Our results suggest that controlling nanoporosity and their degree of disorder can offer a means of decoupling electrical and thermal properties in materials.

Understanding the effect of thickness on the thermoelectric properties of Ca3Co4O9 thin films

We are reporting the effect of thickness on the Seebeck coefficient, electrical conductivity and power factor of Ca3Co4O9 thin films grown on single-crystal Sapphire (0001) substrate. Pulsed laser deposition (PLD) technique was employed to deposit Ca3Co4O9 films with precisely controlled thickness values ranging from 15 to 75 nm. Structural characterization performed by scanning electron microscopy (SEM) and atomic force microscopy (AFM) showed that the growth of Ca3Co4O9 on Sapphire (0001) follows the island growth-mode. It was observed that in-plane grain sizes decrease from 126 to 31 nm as the thickness of the films decreases from 75 to 15 nm. The thermoelectric power measurements showed an overall increase in the value of the Seebeck coefficient as the films’ thickness decreased. The above increase in the Seebeck coefficient was accompanied with a simultaneous decrease in the electrical conductivity of the thinner films due to enhanced scattering of the charge carriers at the grain boundaries. Because of the competing mechanisms of the thickness dependence of Seebeck coefficient and electrical conductivity, the power factor of the films showed a non-monotonous functional dependence on thickness. The films with the intermediate thickness (60 nm) showed the highest power factor (~ 0.27 mW/m-K2 at 720 K).

Atomic layer deposition of thermoelectric layered cobalt oxides

Layered cobalt oxides based on the hexagonal CoO2 layer, e.g., NaxCoO2 and [CoCa3O3]0.62CoO2 (or “Ca3Co4O9”), are promising thermoelectric materials. Here, the authors investigate the atomic layer deposition (ALD) of these materials in a thin-film form; this is not trivial, in particular, for the former compound, as both Na and Co are little challenged as components of ALD thin films. The authors employ diketonate precursors for all the metal constituents and ozone as the source of oxygen. In both cases, a postdeposition heat-treatment in O2 is applied to get crystalline coatings; the processes are found amazingly robust in terms of metal precursor pulsing ratios. A striking difference between the two processes is the resultant morphology: while the Ca3Co4O9 films grow highly homogeneous and smooth, the NaxCoO2 coatings exhibit a rather unique reproducible 10–20 μm scale channel-like island structure for all x values investigated. Finally, the authors characterized their ALD Ca3Co4O9 films for their chemical, structural, and physical property details not previously reported.

The effects of composition deviations on the microstructure and thermoelectric performance of Ca3Co4O9 thin film

This work investigated composition deviations in Ca3Co4O9 (CCO) films during rf-sputtering process and their effects on the properties of post-annealed CCO films. The negative oxygen ions produced during sputtering resulted in a great reduction in deposition rate and preferential re-sputtering of calcium atoms from the substrate due to their lighter weight compared to cobalt. Experiment results showed that the single phase CCO film exhibited the lowest electrical resistivity of ~ 4 mΩ cm at room temperature and a high power factor of ~ 1.1 mW/mK2 at 973 K. Compared with the single phase CCO sample, the resistivity of the Co-rich sample increased slightly and the Seebeck coefficient increased by ~ 20% at high temperature, thereby leading to a high power factor. The upturn of the Seebeck coefficient behaviours in Co-rich samples from 700 K is most likely associated with the strain induced by the mismatch of thermal expansion coefficients (CTE) between CCO and Co3O4 grains.

Nanoporous Ca3Co4O9 Thin Films for Transferable Thermoelectrics.

The development of high-performance and transferable thin-film thermoelectric materials is important for low-power applications, e.g., to power wearable electronics, and for on-chip cooling. Nanoporous films offer an opportunity to improve thermoelectric performance by selectively scattering phonons without affecting electronic transport. Here, we report the growth of nanoporous Ca3Co4O9 thin films by a sequential sputtering-annealing method. Ca3Co4O9 is promising for its high Seebeck coefficient and good electrical conductivity and important for its nontoxicity, low cost, and abundance of its constituent raw materials. To grow nanoporous films, multilayered CaO/CoO films were deposited on sapphire and mica substrates by rf-magnetron reactive sputtering from elemental Ca and Co targets, followed by annealing at 700 °C to form the final phase of Ca3Co4O9. This phase transformation is accompanied by a volume contraction causing formation of nanopores in the film. The thermoelectric propoperties of the nanoporous Ca3Co4O9 films can be altered by controlling the porosity. The lowest electrical resistivity is ∼7 mΩ cm, yielding a power factor of 2.32 × 10–4 Wm–1K–2 near room temperature. Furthermore, the films are transferable from the primary mica substrates to other arbitrary polymer platforms by simple dry transfer, which opens an opportunity of low-temperature use these materials.

Perovskite substrates boost the thermopower of cobaltate thin films at high temperatures

Transition metal oxides are promising candidates for thermoelectric applications, because they are stable at high temperature and because strong electronic correlations can generate large Seebeck coefficients, but their thermoelectric power factors are limited by the low electrical conductivity. We report transport measurements on Ca3Co4O9 films on various perovskite substrates and show that reversible incorporation of oxygen into SrTiO3 and LaAlO3 substrates activates a parallel conduction channel for p-type carriers, greatly enhancing the thermoelectric performance of the film-substrate system at temperatures above 450 °C. Thin-film structures that take advantage of both electronic correlations and the high oxygen mobility of transition metal oxides thus open up new perspectives for thermopower generation at high temperature.

Thermoelectric functionality of Ca3Co4O9 epitaxial thin films on yttria-stabilized zirconia crystalline substrate

Abstract Layered cobaltite Ca3Co4O9 is one of the most interesting materials for energy conversion due to its excellent thermoelectric properties and stability at high temperatures. However, the best performance of this material is achieved for single crystals and epitaxial films. Within this framework, an important role is played by the substrate, which should promote epitaxial growth and that should have suitable thermal properties for its integration in devices. In this work, we report on the preparation of epitaxial thin films of Ca3Co4O9 by sputtering using low cost ceramic target on cubic yttria-stabilized zirconia (YSZ(100)) that is typically the standard for low thermal conductivity material and on hexagonal sapphire (Al2O3(0001)). The quality of the epitaxial films and its microstructure has been studied to explain the thermoelectric properties obtained for both substrates. YSZ(100) substrate provides Ca3Co4O9 films with enhanced Seebeck coefficient and Al2O3(0001) allows improved conductive Ca3Co4O9 films that finally, at 200 °C, results in a substrate-modified Z′T figure of merit one order of magnitude higher for Ca3Co4O9 on YSZ(100) than on Al2O3(0001) substrate.

Enhanced thermoelectric performance in c-axis oriented Ca3Co4O9 films by Ag addition through multiple annealing

The [Formula: see text]-axis oriented Ca3Co4O9 (CCO) films without and with 5 wt.% Ag addition were prepared by chemical solution deposition (CSD) through multiple annealing processing on single crystal LaAlO3 (001) substrates. With Ag addition, the resistivity at 300 K is decreased to 2.25 m[Formula: see text]cm, the Seebeck coefficient at 300 K is enhanced to 106 [Formula: see text]V/K and the power factor at 300 K can reach as high as 0.5 mW[Formula: see text]K[Formula: see text]m[Formula: see text], which is the highest value among CCO films prepared by CSD. The results suggest that Ag addition is a very effective route to improve the thermoelectric properties of CCO films through multiple annealing processing.

High-throughput synthesis of thermoelectric Ca3Co4O9 films

Properties of complex oxide thin films can be tuned over a range of values as a function of mismatch, composition, orientation, and structure. Here, we report a strategy for growing structured epitaxial thermoelectric thin films leading to improved Seebeck coefficient. Instead of using single-crystal sapphire substrates to support epitaxial growth, Ca3Co4O9 films are deposited, using the Pulsed Laser Deposition technique, onto Al2O3 polycrystalline substrates textured by spark plasma sintering. The structural quality of the 2000 Å thin film was investigated by transmission electron microscopy, while the crystallographic orientation of the grains and the epitaxial relationships were determined by electron backscatter diffraction. The use of a polycrystalline ceramic template leads to structured films that are in good local epitaxial registry. The Seebeck coefficient is about 170 μV/K at 300 K, a typical value of misfit material with low carrier density. This high-throughput process, called combinatorial substrate epitaxy, appears to facilitate the rational tuning of functional oxide films, opening a route to the epitaxial synthesis of high quality complex oxides.

Enhancing the thermoelectric properties of Ca3Co4O9 thin films by Nb ion injection

High quality Ca3Co4O9 thin films have been grown epitaxially on single crystal Al2O3 substrates with pulsed laser deposition. Nb was implanted into the Ca3Co4O9 films using an ion beam injection technique. The microstructure of the thin films has been investigated by XRD, SEM and AFM. The epitaxial thin films were grown with the c-axis perpendicular to the substrate surface. The effect of Nb doping by ion beam injection was verified using resistivity measurements at room temperature. Resistivity and the Seebeck coefficient were also measured in the temperature range 150–380K. The results indicate that the power factors of Ca3Co4O9 thin films increase when doped with Nb. When the concentration of doped Nb was 3.65×1019atoms/cm3, the power factor of the thin films reached 0.10mW/mK2 at room temperature, and it approached a maximum of 0.17mW/mK2 at 380K.

(00l)-oriented Bi2Sr2Co2Oy and Ca3Co4O9 films: Self-assembly orientation and growth mechanism by chemical solution deposition

Abstract In this study, two typical cobaltate-based thermoelectric films, Bi 2 Sr 2 Co 2 O y (BSC) and Ca 3 Co 4 O 9 (CCO), with structures of [Bi 2 Sr 2 O 4 ][CoO 2 ] 2 and [Ca 2 CoO 3 ] RS [CoO 2 ], respectively, are prepared by a simple chemical solution deposition on SrTiO 3 (1 0 0), (1 1 0), and (1 1 1) single crystal substrates. X-ray results reveal that all films are c -axis oriented regardless of the orientation of the substrate, suggesting self-assembly orientation. Transmission electron microscopy reveals amorphous/delamination regions for BSC film on SrTiO 3 (1 1 0), and the c -axis stripes of CCO on SrTiO 3 (1 1 1) are inclined at 30° to the interface, whereas the c -axis stripes are parallel to the interfaces for other films. The growth mechanism is established, and the driving force for self-assembly c -axis orientation is attributed to the syneresis stress due to solvent evaporation. The microstructures and properties are also studied and discussed, with the conclusion that self-assembly c -axis oriented layered cobaltates films are good candidates for thermoelectric applications.

Growth of Ca3Co4O9 films: Simple chemical solution deposition and stress induced spontaneous dewetting

Chemical solution deposition as a simple method for preparation of oxide films is used to fabricate misfit structured Ca3Co4O9 thermoelectric films using cheap precursors, Ca and Co acetates, which opens up a simple route to fabricate large-area Co-based oxide thermoelectric films. The results show that when the annealing temperature is higher than 800°C crystallized Ca3Co4O9 films can be obtained on single-crystal LaAlO3 substrates, whereas the Ca3Co4O9 films are amorphous for lower annealing temperatures. Experimental evidence shows that the amorphous Ca3Co4O9 films behave like semiconductors with enhanced resistivity and Seebeck coefficient within the measured temperature range. The magnetoresistance is positive indicating the absence of long-range order of the Co-ion spins. For the crystallized Ca3Co4O9 films, it can be seen that the thinner films show the characteristics of spontaneous dewetting induced by the stress, which suggests that it is possible to prepare nanostructured Ca3Co4O9 films through tailoring the film thickness and the lattice mismatch; whereas, for the thicker films the temperature dependence of the resistivity and the Seebeck coefficient shows that it is possible to prepare excellent Ca3Co4O9 thermoelectric films using chemical solution deposition route, which will broaden the range of preparation methods for misfit structured Co-based thermoelectric films.

In situ growth of c-axis-oriented Ca3Co4O9 thin films on Si (100)

High-quality c-axis-oriented Ca3Co4O9 thin films have been grown directly on Si (100) wafers by pulsed-laser deposition without prechemical treatment of the substrate surface. Cross-sectional transmission electron microscopy shows good crystallinity of the Ca3Co4O9 films. The Seebeck coefficient and resistivity of the Ca3Co4O9 thin films on Si (100) substrate are 126μV∕K and 4.3mΩcm, respectively, at room temperature, comparable to the single-crystal samples. This advance demonstrates the possibility of integrating the cobaltate-based high thermoelectric materials with the current state-of-the-art silicon technology for thermoelectricity-on-a-chip applications.