Mechanism of Formation of the Thermoelectric Layered Cobaltate Ca 3 Co 4 O 9 by Annealing of CaO–CoO Thin Films (original) (raw)
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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.
Journal of Physics: Condensed Matter, 2011
The incommensurately layered cobalt oxide Ca 3 Co 4 O 9 exhibits an unusually high Seebeck coefficient as a polycrystalline bulk material, making it ideally suited for many high temperature thermoelectric applications. In this paper, we investigate properties of Ca 3 Co 4 O 9 thin films grown on cubic perovskite SrTiO 3 , LaAlO 3 , and (La 0.3 Sr 0.7 )(Al 0.65 Ta 0.35 )O 3 substrates and on hexagonal Al 2 O 3 (sapphire) substrates using the pulsed laser deposition technique. X-ray diffraction and transmission electron microscopy analysis indicate strain-free growth of films, irrespective of the substrate. However, depending on the lattice and symmetry mismatch, defect-free growth of the hexagonal CoO 2 layer is stabilized only after a critical thickness and, in general, we observe the formation of a stable Ca 2 CoO 3 buffer layer near the substrate-film interface. Beyond this critical thickness, a large concentration of CoO 2 stacking faults is observed, possibly due to weak interlayer interaction in this layered material. We propose that these stacking faults have a significant impact on the Seebeck coefficient and we report higher values in thinner Ca 3 Co 4 O 9 films due to additional phonon scattering sites, necessary for improved thermoelectric properties.
Journal of the European Ceramic Society, 2012
Multilayer Ca 3 Co 4 O 9 (349) thick thermoelectric (TE) materials were fabricated by hot-pressing stacked dense and strongly textured single-layer samples. Microstructure and volume quantitative texture investigations were undertaken by using scanning electron microscopy and neutron diffraction techniques, respectively. The results show a bulk density similar to single-layer samples, but remarkable texture strength reinforcement. The electrical resistivity, ρ, and Seebeck coefficient, S, were reproducibly measured in directions parallel (ρ c and S c ) and perpendicular (ρ ab and S ab ) to the mean c-axis. ρ showed a high anisotropy ratio ρ c /ρ ab of 13.5 and 8.8 at 300 and 900 K, respectively, and ρ ab kept the same values whereas ρ c decreased in the multilayer samples. S ab and S c unexpectedly revealed different values. The thermal conductivity also displayed a significant anisotropy, with ratio κ ab /κ c = 2.7 at 900 K. The resulting figure-of-merit ZT is then noticeably anisotropic, with ratio ZT ab /ZT c = 4.6. ZT ab was found 2 times larger than ZT value of the conventional sintered 349 materials often used for TE modules fabrication. (D. Kenfaui).
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.
Nanomaterials
The layered cobaltates AxCoO2 (A: alkali metals and alkaline earth metals) are of interest in the area of energy harvesting and electronic applications, due to their good electronic and thermoelectric properties. However, their future widespread applicability depends on the simplicity and cost of the growth technique. Here, we have investigated the sputtering/annealing technique for the growth of CaxCoO2 (x = 0.33) thin films. In this approach, CaO–CoO film is first deposited by rf-magnetron reactive cosputtering from metallic targets of Ca and Co. Second, the as-deposited film is reactively annealed under O2 gas flow to form the final phase of CaxCoO2. The advantage of the present technique is that, unlike conventional sputtering from oxide targets, the sputtering is done from the metallic targets of Ca and Co; thus, the deposition rate is high. Furthermore, the composition of the film is controllable by controlling the power at the targets.
Synthesis and Characterization of Bulk Nanostructured Thermoelectric Ca3Co4O9
Journal of Nanoscience and Nanotechnology, 2017
Nanostructuring has been proposed as an effective strategy for the reduction of the phonon contribution to the thermal conductivity, resulting in an increase in the figure of merit of thermoelectric materials. However, obtaining bulk samples presenting high relative density and nanometric grain size can be quite challenging, particularly in the case of ceramic phases. Only few examples have been reported and none in the case of Ca 3 Co 4 O 9. In this work, we used a sol-gel synthesis coupled with ball milling to prepare powders of Ca 3 Co 4 O 9 presenting a grain size as small as 4 nm. These nanopowders were then sintered at different temperature and pressures using the High-Pressure Field-Assisted Sintering Technique (HP-FAST). Relative densities up to 95 vol% where obtained while maintaining a nanometric grain size. The microstructural and electrical properties of the sintered samples have been characterized. The results show that in this oxide a reduction to the nanometric grain size produces a drastic reduction in the electrical conductivity, which cannot be compensated by the reduction in the thermal conductivity. The Seebeck effect, on the other hand, appears to be affected only marginally by the grain size and porosity.
Improved High-Temperature Thermoelectric Properties of Dual-Doped Ca3Co4O9
ACS Omega, 2022
Layered structured Ca 3 Co 4 O 9 has displayed great potential for thermoelectric (TE) renewable energy applications, as it is nontoxic and contains abundantly available constituent elements. In this work, we study the crystal structure and high-temperature TE properties of Ca 3−2y Na 2y Co 4−y Mo y O 9 (0 ≤ y ≤ 0.10) polycrystalline materials. Powder X-ray diffraction (XRD) analysis shows that all samples are single-phase samples and without any noticeable amount of the secondary phase. X-ray photoelectron spectroscopic (XPS) measurements depict the presence of a mixture of Co 3+ and Co 4+ valence states in these materials. The Seebeck coefficient (S) of dual-doped materials is significantly enhanced, and electrical resistivities (ρ) and thermal conductivities (κ) are decreased compared to the pristine compound. The maximum thermoelectric power factor (PF = S 2 /ρ) and dimensionless figure of merit (zT) obtained for the y = 0.025 sample at 1000 K temperature are ∼3.2 × 10 −4 W m −1 K −2 and 0.27, respectively. The zT value for Ca 2.95 Na 0.05 Co 3.975 Mo 0.025 O 9 is about 2.5 times higher than that of the parent Ca 3 Co 4 O 9 compound. These results demonstrate that dual doping of Na and Mo cations is a promising strategy for improving the high-temperature thermoelectric properties of Ca 3 Co 4 O 9 .
Science China Materials
This work presents a short and very efficient method to produce high performance textured Ca 3 Co 4 O 9 thermoelectric materials through initial powders modification. Microstructure has shown good grain orientation, and low porosity while slightly lower grain sizes were obtained in samples prepared from attrition milled powders. All samples show the high density of around 96% of the theoretical value. These similar characteristics are reflected in, approximately, the same electrical resistivity and Seebeck coefficient values for both types of samples. However, in spite of similar power factor (PF) at low temperatures, it is slightly higher at high temperature for the attrition milled samples. On the other hand, the processing time reduction (from 38 to 2 h) when using attrition milled precursors, leads to lower mechanical properties in these samples. All these data clearly point out to the similar characteristics of both kinds of samples, with a drastic processing time decrease when using attrition milled precursors, which is of the main economic importance when considering their industrial production.
Materials Letters, 2019
This work reports the microstructural and thermoelectric characterization of the misfit [Ca 2 CoO 3-δ ] 0.62 [CoO 2 ] compound obtained by a solid-state synthesis using mollusk shells and a proteic sol-gel method, which uses gelatin as a polymerizing agent. The results clearly demonstrate the capability of these routes to produce pure Ca 3 Co 4 O 9 with plate-like morphology. Sintered ceramic samples show randomly oriented grains and relative densities in the range of 63-67%. The obtained microstructures provide reasonable electrical properties and result in competitive thermoelectric performance for the material prepared by the proteic sol-gel synthesis (P.F. of 205 μW/K 2 m at 700 ºC).