Thermoelectric properties of Ca3–x Bi x Co4O9 + δ (0.0 ⩽ x ⩽ 1.5) ceramics

Thermoelectric Properties of Inhomogeneous Ceramics Based on the Layered Calcium Cobaltate

“…The values of electrical conductivity and coefficient of thermo-EMF of ceramics, in the whole, were increased at increasing of the cobalt oxide content in the samples ( Table 5). The values of the apparent activation energy of electrical conductivity of the samples of A serie varied within 0.069-0.081 eV, which is typical for ceramics based on the layered calcium cobaltate [12,14,16] (excluding the samples having Ca 3 Co 3.8 O 9+δ , Ca 3 Co 4 O 9+δ composition, which had been sintered at 970°С, whose E A values were equal to 0.274 and 0.125 eV respectively and were abnormally high, which was probably due to the peculiarities of their microstructure).…”

“…The values of the apparent activation energy of electrical conductivity of the samples of N serie varied within 0.050-0.072 eV, which is typical for ceramics based on the layered calcium cobaltate [12,14,16], slightly depended on the cationic compostiton of the samples and temperature of their sintering, and were less than for the samples which had been sintered in air or in oxygen.…”

“…According to the XRD data ( Figure 1, a, [26] with the lattice constants given in the Table. 1. As can be seen from the Table 1, creation both excess and deficiency of cobalt oxide in the layered calcium cobaltate leads to the increasing of b 2 parameter (repeatability period in the ab-plane of CoO 2 -layers of Ca 3 Co 4 O 9+δ crystal structure [26] (1,2,7,8), Ca 3 Co 3.8 O 9+δ (3,4,9,10), and Ca 3 Co 4.2 O 9+δ (5,6,11,12) samples, annealed in air (a) and oxygen (b) at 920°C (1,3,5), 970°C (2,4,6,7,9,11), and 1020°C (8,10,12 (7) -*According to the X-ray data # According to [24], at 970°С in air this phase is thermodynamically unstable, but can form at slow cooling of the sample to the room temperature due to the peritectoid reaction of Са 3 Со 2 О 6 + (Со,Са)О → Са 3 Со 4 О 9+δ .…”

“…Samples which were sintered in oxygen were also p-type (S > 0) semiconductors (∂σ/∂T>0) ( Figure 5, middle block), although their electrical conductivity values at temperature gaining increased less significant than for the samples, sintered in air. (1, 2, 7, 8, 13, 14), Ca 3 Co 3.8 O 9+δ (3,4,9,10,15,16), and Ca 3 Co 4.2 O 9+δ (5,6,11,12,17,18) ceramics, annealed in air (1 -6), oxygen (7 -12) and nitrogen (13 -18) at 890°C (13), 920°C (1,3,5,15,17), 970°C (2,4,6,7,9,11,14,16,18), and 1020°C (8, 10, 12) The highest electrical conductivity was demonstrated by ceramics having Ca 3 Co 4 O 9+δ composition which had been sintered at 1020°C, and the highest value of the thermo-EMF coefficient was observed for ceramics of Ca 3 Co 3.8 O 9+δ composition which had been sintered at 970°C ( Table 5). It is interesting to note that the values of the thermo-EMF coefficient of ceramics containing an excess of both calcium oxide (Ca 3 Co 3.8 O 9+δ ) and cobalt oxide (Ca 3 Co 4.2 O 9+δ ) were higher than for the basic Ca 3 Co 4 O 9+δ phase with stoichiometric relation of cations (Table 5).…”

“…The indicated drawbacks are largely absent for oxide thermoelectrics, including materials based on the layered calcium cobaltate Ca 3 Co 4 O 9+δ , which in recent years are considered as a promising basis for the development of p-branches of high-temperature thermoelectric generators [2,3]. The functional (thermoelectric) characteristics of ceramics based on the Ca 3 Co 4 O 9+δ can be essentially improved by using instead of the traditional ceramic methods the "soft" low-temperature synthesis methods [4,5], using special techniques for sintering ceramics -hot pressing [6], spark plasma sintering [7][8][9], and also by partial substitution of calcium ions in it by bismuth [3,[10][11][12] or rare earth elements ions [3,13,14] or of cobalt ions by 3d-metal ions [3,15,16]. It was shown in [11,[17][18][19] that thermoelectric characteristics of oxide ceramics can be improved by creating phase inhomogeneity in it.…”