Thermoelectric Properties of the Layered Cobaltite Ca<SUB>3</SUB>Co<SUB>4</SUB>O<SUB>9</SUB> Epitaxial Films Fabricated by Topotactic Ion-Exchange Method

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

“…The resistivity ρ ab of the HP-textured sample approached the values recorded in the single crystal (~3.8 mΩ·cm at 800 K) [15,36]. It was also close to the values reported for thin films by Sugiura et al (~3.4 and ~3.9 mΩ·cm at 300 and 800 K, respectively) [37,38] and was even lower than the values published by Sakai et al (~8 mΩ·cm at 300 K) [39]. This result reflects the role of the achieved texture and the low GBD ab in achieving the electrical conduction in the (a,b) planes of the oriented Ca 3 Co 4 O 9 ceramics.…”

“…S ab went from 172 μV·K −1 at 800 K for the SPS-processed sample to 157 μV·K −1 for the HP-textured sample. This is slightly lower than the figures reported for a single crystal (210 μV·K −1 ) [36] or thin films (185 μV·K −1 ) [38]. …”

“…Such an improvement of PF, accompanied by the outstanding time and energy savings brought by the SPT process compared to HP, emphasizes the decisive benefits of the former in texturing lamellar materials like Ca 3 Co 4 O 9 . The PF ab value (520 μW·m −1 ·K −2 at 800 K) noted for the HP sample was, however, among the largest ones reached so far on undoped Ca 3 Co 4 O 9 ceramics [25,26,29,38], but remains 2 and 1.5 times lower compared to the reported single crystal [36] and thin films [38]. …”

Mechanism of Formation of the Thermoelectric Layered Cobaltate Ca3Co4O9 by Annealing of CaO–CoO Thin Films

“…The electrical resistivity (Figure 7a) of the Ca:Co=0.72 film is lower than that of the Ca-rich and Co-rich samples throughout the measured temperature range, owing to its phase purity. The room temperature electrical resistivity of the phase pure sample (6.44 mΩcm) is comparable to most Ca 3 Co 4 O 9 thin films grown by different techniques, [8][9][10][11][12][13][14][15][16][17][18][19][20][21] but somewhat higher than the lowest reported electrical resistivity value for Ca 3 Co 4 O 9 thin films deposited on glass substrates (~4 mΩcm). [ 28 ] The electrical resistivity of the Ca-rich and Co-rich samples increases with increasing deviation of the Ca:Co ratio from the stoichiometric limit, which is attributed to enhanced scattering of charge carriers by the grains of impurity oxide phases.…”

“…2 mΩcm at room temperature) [10] is practically realized in the basal plane of single crystalline [ 12 ] and metal-organic CVD, [ 13 , 14 ] as well as chemical solution deposition methods followed by annealing, [ 15 , 16 , 17 ] and reactive solid-phase epitaxy by topotactic-ion exchange. [18,19] However, PVD has broad advantages compared to chemical methods since PVD operates far from thermodynamic equilibrium and offers atomistic control of the deposition flux. PVD-based synthesis of Ca 3 Co 4 O 9 thin films has been demonstrated with both pulsed laser deposition [ 20 , 21 , 22 , 23 ] and rf-magnetron sputtering [ 24 , 25 , 26 ] from compound targets of Ca-Co-O.…”