Effect of grain size and porosity on phonon scattering enhancement of Ca3Co4O9

Influence of a mechano-chemical treatment on the synthesis and characteristics of p-type thermoelectric Ca 3 Co 4 O 9 ceramics

“…Recently, we reported that modestly textured Ca349 ceramics with a density of about 72% had ZT values of 0.31 at 627 C [7]. Similar ZT values up to 0.29 were also reported for the sol-gel prepared un-doped Ca349 ceramics with fine, plate-like grains [10]. For the Ca349 ceramics with addition of various dopants, values of ZT up to 0.3 were also reported [11].…”

“…6), which increases the grainboundary scattering. However, the influence of the defects, likely introduced by the mechano-chemical milling, should not be excluded, especially when explaining the difference between the more porous UT-760-12 h sample with the higher k and the MC-920 sample, which actually had a lower k. This low k of the MC-920 sample is among the lowest reported for conventionally sintered samples with a random grain orientation and was previously achieved only in the case of sol-gel prepared samples [10]. As was anticipated, the lowest k of 0.4 W/mK was observed in the MC-760-12 h sample because of the inhomogeneous microstructure of the fine, plate-like grains that were locally agglomerated.…”

“…%) after heat treatment of the samples from powder mixture of as-purchased, original reagents (OR), powder mixture from attrition-milled reagents (UT), and mechano-chemically activated powder mixture from attrition-milled reagents (MC2-15). synthesis of single-phase Ca349 ceramics already after a calcination for 4 h at 760 C; however, the microstructure is not homogeneous, even after a longer calcination of 12 h. According to reports, the finer grain size of the single-phase Ca349 powder is expected to allow the preparation of better Ca349 ceramics, with improved TE characteristics, i.e., higher ZT values [10,22]. Hence, the calcinated MC2-15 powders were reused for further processing at higher temperatures.…”

“…According to literature reports the Ca349 ceramic sintered with a classic ceramic procedure is usually prepared in two steps from CaCO 3 and Co 3 O 4 ; with calcination at temperatures of 900e920 C for around 24 h, followed by repeated annealing with intermediate grinding at 900e920 C [11,13,17e21]. However, the sol-gel Pechini synthesis method from Ca-and Co-nitrates [10,22e24] and a thermal hydrodecomposition method of acetate salts [25] enable the preparation of fine-grained powders of single-phase Ca349 with shorter firing times and at much lower temperatures in the range from 650 to 800 C. Pellets prepared from such powders were additionally sintered with temperatures at least 850 C and higher [10,24].…”

Synthesis and Characterization of Ca₃Co₄O₉ Thermoelectric Ceramics using the Slurry Sintering Method

“…2 Such generation of electric power does not emit greenhouse gases, being as much ecofriendly as solar panels and other alternatives. [4][5][6][7][8][9][10][11] The calcium-cobalt oxides remain stable during a long-term use at high temperature; in addition, they are superior to other compound semiconductors in low health hazard, abundance, and low price. 3 Ca 2 Co 2 O 5 , Ca 3 Co 2 O 6 , Ca 3 Co 4 O 9 , and other calciumcobalt oxide thermoelectric materials feature high power factor (PF) at medium to high temperatures, and many research groups are recently engaged in exploration of synthesis methods as well as electric and thermoelectric properties of these materials.…”