Fast preparation route to high-performances textured Sr-doped Ca 3 Co 4 O 9 thermoelectric materials through precursor powder modification

This work presents a short and very efficientmethod to produce high performance textured Ca3Co4O9thermoelectric materials through initial powders modifica-tion. Microstructure has shown good grain orientation, andlow porosity while slightly lower grain sizes were obtained insamples prepared from attrition milled powders. All samplesshow the high density of around 96% of the theoretical value.These similar characteristics are reflected in, approximately,the same electrical resistivity and Seebeck coefficient valuesfor both types of samples. However, in spite of similar powerfactor (PF) at low temperatures, it is slightly higher at hightemperature for the attrition milled samples. On the otherhand, the processing time reduction (from 38 to 2 h) whenusing attrition milled precursors, leads to lower mechanicalproperties in these samples. All these data clearly point out tothe similar characteristics of both kinds of samples, with adrastic processing time decrease when using attrition milledprecursors, which is of the main economic importance whenconsidering their industrial production

Microstructures and Thermoelectric Properties of Sintered Misfit-Layered Cobalt Oxide

Misfit-layered cobalt oxide Ca3Co4O9 is considered to be a prospective material for thermoelectric conversion. The thermoelectric properties are anisotropic owing to its anisotropic crystal structure. The crystal has preferred thermoelectric properties along the a-b plane. Therefore, the thermoelectric properties are improved and controlled by the degree of orientation of the sintered sample. In the present work, Sr-doped misfit cobalt oxide Ca2.7Sr0.3Co4O9 was prepared by solid-phase reaction, followed by uniaxial compression molding and sintering at 1173 K. The Seebeck coefficient α, electrical resistivity ρ, and dimensionless figure of merit ZT were measured as a function of the compression pressure applied in the uniaxial molding. α, ρ, and ZT as functions of the degree of orientation and the relative density are experimentally clarified and explained by calculations using the compound model

Mechanically Robust BiSbTe Alloys with Superior Thermoelectric Performance: A Case Study of Stable Hierarchical Nanostructured Thermoelectric Materials

Bismuth telluride based thermoelectric materials have been commercialized for a wide range of applications in power generation and refrigeration. However, the poor machinability and susceptibility to brittle fracturing of commercial ingots often impose significant limitations on the manufacturing process and durability of thermoelectric devices. In this study, melt spinning combined with a plasma-activated sintering (MS-PAS) method is employed for commercial p-type zone-melted (ZM) ingots of Bi_0.5Sb_1.5Te_3. This fast synthesis approach achieves hierarchical structures and in-situ nanoscale precipitates, resulting in the simultaneous improvement of the thermoelectric performance and the mechanical properties. Benefitting from a strong suppression of the lattice thermal conductivity, a peak ZT of 1.22 is achieved at 340 K in MS-PAS synthesized structures, representing about a 40% enhancement over that of ZM ingots. Moreover, MS-PAS specimens with hierarchical structures exhibit superior machinability and mechanical properties with an almost 30% enhancement in their fracture toughness, combined with an eightfold and a factor of six increase in the compressive and flexural strength, respectively. Accompanied by an excellent thermal stability up to 200 °C for the MS-PAS synthesized samples, the MS-PAS technique demonstrates great potential for mass production and large-scale applications of Bi_2Te_3 related thermoelectrics

Étude des propriétés mécaniques et thermoélectriques des matériaux Ca3Co4O9 texturés pour la conversion d'énergie

Ca3Co4O9 thermoelectric (TE) oxide is very promising for energy conversion applications because of its good TE properties, chemical and thermal stabilities in air at high temperature and the absence of bio-toxicity of its elements. A comparative study of the respective merits of consolidation processes (conventional sintering-CS, hot pressing-HP and spark plasma sintering - SPS) has been undertaken to determine the experimental conditions for obtaining Ca3Co4O9 ceramics with optimal TE characteristics and good reliability in order to be integrated in TE devices. Using the HP process, the optimal conditions (920°C, 30 MPa, 24 h) allowed to elaborate dense (96% of theoretical density) and strongly textured materials, with a maximum of the {001} poles of 22 mrd and significant grain growth in-planes. It results a low electrical resistivity ab (5.25 m.cm at 900 K) and consequently a remarkable power factor PFab (595 µW.m-1.K-2). The mechanical characteristics were drastically enhanced in comparison with the reference ceramics (CS). Denser ceramics (99.6 %) were elaborated in very short time by SPS, but with a weak texture. PFab is lower than that obtained by HP. However, themechanical properties are higher. On the other hand, Ca3Co4O9 thick multilayer materials were elaborated and the obtained TE show anisotropic properties. The resistivity anisotropy c/ab is 13.5 in the 0-350 K range, but decreases beyond 350 K to 8.8 at 900 K. The anisotropy ratio PFab/PFc is 12 at 900 K. The thermal conductivity abis higher than c. The figure of merit is higher in-planes, with. ZTab = 0.16 at 900 K and (ZTab/ ZTc) = 4.6.L'oxyde thermoélectrique (TE) Ca3Co4O9 se place avantageusement pour les applications de conversion d'énergie en raison de ses bonnes propriétés TE, ses stabilités chimique et thermique à l'air à haute température et l'absence de biotoxicité. Une étude comparative des apports respectifs de procédés de consolidation (frittage conventionnel-CS, pressage à chaud-HP et frittage flash-SPS) a été menée pour déterminer les conditions expérimentales d'obtention de céramiques Ca3Co4O9 aux caractéristiques TE et fiabilité optimisées pour être intégrées dans des dispositifs TE. Par le procédé HP, les conditions optimales (920°C, 30 MPa, 24 h) ont permis d'élaborer des matériaux denses (96 %) et fortement texturés, avec un maxima de pôles {001} de 22 mrd et une croissance notable des grains dans les plans (a,b). Il en découle une faible résistivité ab (5.25 m.cm à 900 K) et donc un facteur de puissance PFab remarquable (595 µW.m-1.K-2). Les caractéristiques mécaniques sont considérablement améliorées en comparaison avec la céramique CS. Des céramiques plus denses (99.6 %) ont été élaborées dans des temps très courts par SPS, mais avec une faible texture. PFab est plus faible que celui obtenu par HP. Les propriétés mécaniques sont toutefois meilleures. D'autre part, des matériaux Ca3Co4O9 multicouches ont été élaborés et les propriétés TE obtenues sont anisotropes. L'anisotropie de la résistivité c/ab vaut 13.5 dans la plage 0-350 K, mais diminue au-delà de 350 K et avoisine 8.8 à 900 K. L'anisotropie PFab/PFc vaut 12 à 900 K. La conductivité thermique abest plus élevée que c. Le facteur de mérite est plus élevé dans les plans (a,b), avec ZTab = 0.16 à 900 K et (ZTab/ ZTc) = 4.6

Étude des propriétés mécaniques et thermoélectriques des matériaux Ca3Co4O9 texturés pour la conversion d'énergie

Ca3Co4O9 thermoelectric (TE) oxide is very promising for energy conversion applications because of its good TE properties, chemical and thermal stabilities in air at high temperature and the absence of bio-toxicity of its elements. A comparative study of the respective merits of consolidation processes (conventional sintering-CS, hot pressing-HP and spark plasma sintering - SPS) has been undertaken to determine the experimental conditions for obtaining Ca3Co4O9 ceramics with optimal TE characteristics and good reliability in order to be integrated in TE devices. Using the HP process, the optimal conditions (920°C, 30 MPa, 24 h) allowed to elaborate dense (96% of theoretical density) and strongly textured materials, with a maximum of the {001} poles of 22 mrd and significant grain growth in-planes. It results a low electrical resistivity ab (5.25 m.cm at 900 K) and consequently a remarkable power factor PFab (595 µW.m-1.K-2). The mechanical characteristics were drastically enhanced in comparison with the reference ceramics (CS). Denser ceramics (99.6 %) were elaborated in very short time by SPS, but with a weak texture. PFab is lower than that obtained by HP. However, themechanical properties are higher. On the other hand, Ca3Co4O9 thick multilayer materials were elaborated and the obtained TE show anisotropic properties. The resistivity anisotropy c/ab is 13.5 in the 0-350 K range, but decreases beyond 350 K to 8.8 at 900 K. The anisotropy ratio PFab/PFc is 12 at 900 K. The thermal conductivity abis higher than c. The figure of merit is higher in-planes, with. ZTab = 0.16 at 900 K and (ZTab/ ZTc) = 4.6.L'oxyde thermoélectrique (TE) Ca3Co4O9 se place avantageusement pour les applications de conversion d'énergie en raison de ses bonnes propriétés TE, ses stabilités chimique et thermique à l'air à haute température et l'absence de biotoxicité. Une étude comparative des apports respectifs de procédés de consolidation (frittage conventionnel-CS, pressage à chaud-HP et frittage flash-SPS) a été menée pour déterminer les conditions expérimentales d'obtention de céramiques Ca3Co4O9 aux caractéristiques TE et fiabilité optimisées pour être intégrées dans des dispositifs TE. Par le procédé HP, les conditions optimales (920°C, 30 MPa, 24 h) ont permis d'élaborer des matériaux denses (96 %) et fortement texturés, avec un maxima de pôles {001} de 22 mrd et une croissance notable des grains dans les plans (a,b). Il en découle une faible résistivité ab (5.25 m.cm à 900 K) et donc un facteur de puissance PFab remarquable (595 µW.m-1.K-2). Les caractéristiques mécaniques sont considérablement améliorées en comparaison avec la céramique CS. Des céramiques plus denses (99.6 %) ont été élaborées dans des temps très courts par SPS, mais avec une faible texture. PFab est plus faible que celui obtenu par HP. Les propriétés mécaniques sont toutefois meilleures. D'autre part, des matériaux Ca3Co4O9 multicouches ont été élaborés et les propriétés TE obtenues sont anisotropes. L'anisotropie de la résistivité c/ab vaut 13.5 dans la plage 0-350 K, mais diminue au-delà de 350 K et avoisine 8.8 à 900 K. L'anisotropie PFab/PFc vaut 12 à 900 K. La conductivité thermique abest plus élevée que c. Le facteur de mérite est plus élevé dans les plans (a,b), avec ZTab = 0.16 à 900 K et (ZTab/ ZTc) = 4.6

Toward the enhancement of thermoelectric properties of lamellar Ca3Co4O9 by edge-free spark plasma texturing

International audienceA spark plasma sintering (SPS) device was modified to allow free deformation of grains perpendicular to the loading axis. The aim was to obtain lamellar thermoelectric Ca3Co4O9 ceramics. The new process is referred to as edge-free spark plasma sintering or spark plasma texturing (SPT). SPT materials are compared to those processed by conventional sintering and traditional SPS in terms of microstructure, texture and high-temperature thermoelectric properties. The results highlight the decisive advantages of the SPT technique

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

International audienceThe 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 (GBD(c)) and perpendicular (GBD(ab)) to the mean c*-axis. We found that the edge-free processing fostered material texturing and (a,b) plane grain growth, thereby dropping GBD(ab) and increasing GBD(c). This resulted in a resistivity rho(ab) reduction, leading to a marked enhancement in power factor PFab, which reached 520 mu W.m(-1).K-2 at 800 K for the HP sample. The anisotropy rho(c)/rho(ab) was substantially promoted as the texture was reinforced and the GBD(c)/GBD(ab) ratio increased, rho(c)/rho(ab) (HP) > rho(c)/rho(ab) (SPT) > rho(c)/rho(ab) with (SPS). The Seebeck coefficient S also revealed an anisotropic behavior, with a ratio S-c/S-ab >1 for the SPS-processed materials. This behavior was reversed (S-c/S-ab < 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