Effect of sol-gel and solid-state synthesis techniques on structural, morphological and thermoelectric performance of Ca3Co4O9

Effects of calcination temperature and time on the Ca3Co4O9 purity when synthesized using starch-assisted sol-gel combustion method

“…According to this study, no systematic study had been done on the thermoelectric properties affected by the calcination procedure. Conversely, the advantages of sol-gel combustion method are simple, easy control of homogeneity and stoichiometry, and [12][13][14]. Reproduced with permission from Ref.…”

Thermoelectric Properties of Inhomogeneous Ceramics Based on the Layered Calcium Cobaltate

“…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).…”

“…where σ 0 is temperature independent constant, E A is apparent activation energy of electrical conductivity, k B is Boltzmann's constant, and T is absolute temperature [5,23]. The power factor (P) values were calculated by the formula P = S 2 ⋅σ.…”

“…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.…”