Textured Ca3Co4O9 Research Articles

Synthesis of textured discontinuous-nanoisland Ca3Co4O9 thin films.

Controllable engineering of the nanoporosity in layered Ca3Co4O9 remains a challenge. Here, we show the synthesis of discontinuous films with islands of highly textured Ca3Co4O9, effectively constituting distributed nanoparticles with controlled porosity and morphology. These discontinuously dispersed textured Ca3Co4O9 nanoparticles may be a candidate for hybrid thermoelectrics.

Engineering Faceted Nanoporosity by Reactions in Thin-Film Oxide Multilayers in Crystallographically Layered Calcium Cobaltate for Thermoelectrics

Introducing porosity is attractive for tailoring electronic, thermal, and mechanical properties of inorganic materials. Nanoporosity is typically either inherent in crystallographic channels in the structure or obtained by external templating during synthesis and sintering. However, controllably engineering porosity in materials with laminated crystal structures without channels remains a challenge. Here, we demonstrate the realization of faceted and oriented nanopores in textured Ca3Co4O9—a laminated ceramic with a misfit-layered structure of importance for thermoelectric applications—from chemical reactions in CaO/Co3O4 multilayers. We show that CaO conversion to Ca(OH)2 and the cobalt oxide stoichiometry are key determinants of nanoporosity. Adjusting the unreacted CaO fraction alters the nanopore size and fraction and the thermoelectric properties of Ca3Co4O9. The preferred orientation of Ca3Co4O9 is underpinned by the texture of the reactant multilayers and reactant–product crystallographic relationships and density difference. Oriented pore formation is attributed to basal plane removal driven by local densification of textured Ca3Co4O9 nuclei through growth and impingement. These findings point to possibilities for controllably engineering nanoporosity and properties in a variety of inorganic materials with laminated crystal structures.

Enhanced thermoelectric performance of Ca3Co4O9 ceramics through grain orientation and interface modulation

Interface modulations, including a “core–shell” interface and zigzag edge, enhanced the thermoelectric performance of the textured Ca3Co4O9 ceramics.

Fabrication of highly textured Ca3Co4O9 ceramics with controlled density and high thermoelectric power factors

Ca3Co4O9 ceramics have been studied as an alternative p-type thermoelectric material. Thermoelectric properties of the ceramics would be improved by either orientation of grains or introduction of pores. In this study, we fabricated textured Ca3Co4O9 ceramics with controlled density by a reactive-templated grain growth method combined with a hot-forging technique. A powder precursor obtained by mixing β-Co(OH)2 as a template and CaCO3 as a matrix was uniaxially pressed into pellets and sintered under hot-forging pressures up to 5.0 MPa. The relative density of the resulting ceramics was varied between 41.0 and 83.8 % while all the ceramics showed excellent c-axis orientation. The in-plane electrical conductivity of our ceramics could be kept relatively higher than that ever reported previously due to the orientation. Because Seebeck coefficient did not depend on the relative density, the higher electrical conductivity of our ceramics led directly to improved thermoelectric power factors between 67.0 and 409 μW·m−1 K−2.

Fast preparation route to high-performances textured Sr-doped Ca3Co4O9 thermoelectric materials through precursor powder modification

This work presents a short and very efficient method to produce high performance textured Ca3Co4O9 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.

Anisotropy of Transport Properties Correlated to Grain Boundary Density and Quantified Texture in Thick Oriented Ca3Co4O9 Ceramics

The misfit-layered Ca3Co4O9 oxide is being seen as a potential thermoelectric (TE) candidate for high-temperature power generation in air. Given the very small size and low strength exhibited by single crystals, grain-oriented Ca3Co4O9 ceramics are worth elaborating to capitalize on their anisotropy. However, the usual textured pellets are too thin to probe the TE properties along their principal crystallographic directions. In this paper, we report on the anisotropy of TE properties in the 350–860 K range within thick textured Ca3Co4O9 ceramics fabricated by moderately pressing at 1173 K stacks of pellets primarily textured using spark plasma sintering (SPS), spark plasma texturing (SPT), and hot pressing (HP). The texture was quantitatively assessed, and the influent microstructural parameters were identified, particularly the grain boundary density parallel (GBDc) and perpendicular (GBDab) to the mean c*-axis. We found that the edge-free processing fostered material texturing and (a,b) plane grain growth, thereby dropping GBDab and increasing GBDc. This resulted in a resistivity ρab reduction, leading to a marked enhancement in power factor PFab, which reached 520 μW·m−1·K−2 at 800 K for the HP sample. The anisotropy ρc/ρab was substantially promoted as the texture was reinforced and the GBDc/GBDab ratio increased, with ρc/ρab (HP) > ρc/ρab (SPT) > ρc/ρab (SPS). The Seebeck coefficient S also revealed an anisotropic behavior, with a ratio Sc/Sab >1 for the SPS-processed materials. This behavior was reversed (Sc/Sab <1) for the more textured SPT and HP specimens. It therefore resulted in a PF anisotropy PFc/PFab (HP) < PFc/PFab (SPT) < PFc/PFab (SPS). The PFab/PFc ratio attained 13.6 at 800 K for the thick HP sample, which is the largest ratio recorded thus far on undoped Ca3Co4O9 ceramics.

Anisotropy of the thermoelectric figure of merit (ZT) in textured Ca3Co4O9 ceramics prepared by using a spark plasma sintering process

Textured Ca3Co4O9 ceramics were prepared by spark plasma sintering (SPS) from commercial CaCO3 and Co3O4 powders to investigate the thermoelectric anisotropy of Ca3Co4O9. The degree of orientation of the textured specimens was evaluated by using the Lotgering factor method, and the SPS process was confirmed to be an effective technology to prepare textured Ca3Co4O9 ceramics. Both the power factors and the thermal conductivities were measured in two different directions, parallel (out-of-plane) and perpendicular (in-plane) to the SPS press axis. The difference in anisotropy between the electrical conductivity and the thermal conductivity was investigated for the textured Ca3Co4O9 ceramics. The maximum figure of merit (ZT) value along the direction perpendicular to the SPS press axis was more than 20% higher than it was along the direction parallel to the SPS press axis. It is noted that the strong thermoelectric anisotropy can give rise to an overestimates of ZT when preparing samples in different directions for power factor and thermal conductivity measurements.

Development of multilayer textured Ca3Co4O9 materials for thermoelectric generators: Influence of the anisotropy on the transport properties

Abstract 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 a b 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 a b / κ c = 2.7 at 900 K. The resulting figure-of-merit ZT is then noticeably anisotropic, with ratio Z T a b / Z T 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.

Granular and Lamellar Thermoelectric Oxides Consolidated by Spark Plasma Sintering

Using the spark plasma sintering (SPS) technique, dense nanostructured Ca0.95Sm0.05MnO3 (n-type) and textured Ca3Co4O9 (p-type) ceramics were prepared. Nanoceramic powders of doped n-type were synthesized using two routes: coprecipitation and solid-state reaction. The SPS method has been used to control the samples’ densification and grain growth. Microstructural investigations reveal that the SPS technique results in high bulk density and homogeneous morphology for Ca0.95Sm0.05MnO3, and grain alignment for Ca3Co4O9. The thermoelectric and mechanical properties were investigated, showing a dependence on the starting grain size and being governed by the SPS conditions.