Effect of substrate on the atomic structure and physical properties of thermoelectric Ca3Co4O9thin films (original) (raw)
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Applied Physics Letters, 2005
Ca 3 Co 4 O 9 thin films have been grown directly on glass ͑fused silica͒ substrate by pulsed laser deposition. Detailed microstructure analysis showed stacking faults abundant throughout the films. However, the Seebeck coefficient ͑ϳ130 V/K͒ and resistivity ͑ϳ4.3 m⍀ cm͒ of these films on glass substrate at room temperature were found comparable to those of the single-crystal samples. The presence of these structural defects could reduce thermal conductivity, and thus enhance the overall performance of cobaltate films to be potentially used in the thermoelectric devices.
Understanding the effect of thickness on the thermoelectric properties of Ca3Co4O9 thin films
Scientific Reports
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 gr...
High-temperature stability of thermoelectric Ca3Co4O9 thin films
Applied Physics Letters, 2015
An enhanced thermal stability in thermoelectric Ca 3 Co 4 O 9 thin films up to 550 C in an oxygen rich environment was demonstrated by high-temperature electrical and X-ray diffraction measurements. In contrast to generally performed heating in helium gas, it is shown that an oxygen/helium mixture provides sufficient thermal contact, while preventing the previously disregarded formation of oxygen vacancies. Combining thermal cycling with electrical measurements proves to be a powerful tool to study the real intrinsic thermoelectric behaviour of oxide thin films at elevated temperatures. V
High-throughput synthesis of thermoelectric Ca3Co4O9 films
Applied Physics Letters, 2013
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, Ca 3 Co 4 O 9 films are deposited, using the Pulsed Laser Deposition technique, onto Al 2 O 3 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 lV/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. V C 2013 AIP Publishing LLC. [http://dx.
The layered cobaltate Ca 3 Co 4 O 9 is of interest for energy-harvesting and heatconversion applications because of its good thermoelectric properties and the fact that the raw materials Ca and Co are nontoxic, abundantly available, and inexpensive. While single-crystalline Ca 3 Co 4 O 9 exhibits high Seebeck coeffi cient and low resistivity, its widespread use is hampered by the fact that single crystals are too small and expensive. A promising alternative approach is the growth of highly textured and/or epitaxial Ca 3 Co 4 O 9 thin fi lms with correspondingly anisotropic properties. Here, we present a two-step sputtering/ annealing method for the formation of highly textured virtually phase-pure Ca 3 Co 4 O 9 thin fi lms by reactive cosputtering from Ca and Co targets followed by an annealing process at 730 °C under O 2 -gas fl ow. The thermally induced phase transformation mechanism is investigated by in situ time-resolved annealing experiments using synchrotron-based 2D X-ray diffraction (XRD) as well as ex situ annealing experiments and standard lab-based XRD. By tuning the proportion of initial CaO and CoO phases during fi lm deposition, the method enables synthesis of Ca 3 Co 4 O 9 thin fi lms as well as Ca x CoO 2 . With this method, we demonstrate production of epitaxial Ca 3 Co 4 O 9 thin fi lms with in-plane electrical resistivity of 6.44 mΩ cm and a Seebeck coeffi cient of 118 μV K −1 at 300 K.
MRS Proceedings, 2010
The influence of incorporating nanoparticulate additions into Ca3Co4O9 (CCO) thin films prepared by pulsed laser deposition using composite targets of CCO and CCO + 3wt% BaZrO3 (BZO) on Si and LaAlO3 substrates is investigated. X-ray data and high-resolution scanning electron microscopy reveal preferred c-axis orientation of the films deposited on Si substrates with the formation of nanoparticles between ∼ 10 – 50 nm. Preliminary thermoelectric behavior shows an enhancement of the power factor α2/ρ at room temperature. The microstructure and thermoelectric behavior of the CCO films are compared to the BZO-doped films.
Electronic structure and large thermoelectric power in Ca3Co4O9
Journal of Electron Spectroscopy and Related Phenomena, 2004
Resonant photoemission spectroscopy, soft X-ray emission spectroscopy, soft X-ray absorption spectroscopy, and high-resolution (E ≈ 10 meV) ultraviolet photoemission spectroscopy were performed on a layered cobalt oxide, Ca 3 Co 4 O 9 , which has attracted interests as one of the potential thermoelectric materials because of its possession of high thermoelectric power S, relatively low electrical resistivity ρ, and small thermal conductivity κ. A narrow band of 1.5-2 eV in width was observed in the photoemission spectra with its center at 1.0 eV below the Fermi level (E F). This peak turns out to be less significant when Co 2p-3d resonance takes place, indicating that it consists mainly of O 2p and of small amount of Co 3d component. Since E F is located near the high-energy edge of this narrow band, the density of states at E F is finite but negligibly small at room temperature. An energy-gap across E F opens below 50 K with decreasing temperature. This development of the energy-gap causes the insulating behavior; divergence both in electrical resistivity and Hall coefficient. We calculated thermoelectric power S(T) using the photoemission spectra near E F. The calculated S(T) shows almost half of the measured value. Large thermoelectric power up to 200 V/K observed for the Ca 3 Co 4 O 9 is closely related to the metallic electron transport in a less dispersive band with E F near its band-edge.
Improved High Temperature Thermoelectric Properties in Misfit Ca3Co4O9 by Thermal Annealing
Energies
Ca3Co4O9, a p-type thermoelectric material based on transition-metal oxides, has garnered significant interest due to its potential in thermoelectric applications. Its unique misfit-layered crystal structure contributes to low thermal conductivity and a high Seebeck coefficient, leading to a thermoelectric figure of merit (zT) of ≥1 at 1000 K. Conventionally, it has been believed that thermopower reaches its upper limit above 200 K. However, our thermopower measurements on polycrystalline Ca3Co4O9 samples have revealed an unexpected increase in thermopower above 380 K. In this study, we investigate the effects of high oxygen pressure annealing on Ca3Co4O9 and provide an explanation based on the mixed oxide states of cobalt and carrier hopping. Our results demonstrate that annealing induces modifications in the defect chemistry of Ca3Co4O9, leading to a decrease in electron hopping probability and the emergence of a thermal activation-like behavior in thermopower. These findings carr...
Journal of Alloys and Compounds, 2014
Misfit-layered Ca 3 Co 4Àx Cr x O 9+d (0 6 x 6 0.2) powders were synthesized by a simple thermal hydrodecomposition method and then fabricated to form highly dense ceramics using spark plasma sintering. X-ray diffraction, and a field emission scanning electron microscope equipped with an energy dispersive X-ray spectrometer were used to verify the single phase of Ca 3 Co 4 O 9+d for the samples with x = 0, 0.05, 0.10 and 0.15. The characteristic plate-like grain structure was observed. The chemical composition of the fabricated samples was found to be close to nominal composition. The thermoelectric measurement showed that Cr doping has an influence on the thermoelectric properties. The resistivity and the Seebeck coefficient increased while the thermal conductivity was suppressed when Cr was added into the system. Explanation of the change in thermoelectric properties was discussed regarding the difference in the charge states and the ionic radii between Co and Cr ions. The highest ZT of 0.19 at 1073 K was obtained for the Ca 3 Co 3.85 Cr 0.15 O 9+d sample. In addition, the paramagnetic property was observed in all samples at room temperature.