Effect of Ag addition on the mechanical and thermoelectric performances of annealed Bi2Sr2Co1.8Ox textured ceramics

Abstract

Bi 2 Sr 2 Co 1.8 O x /Ag composites with small amounts of Ag have been synthesized by a sol-gel via nitrates reaction and directionally grown from the melt. Some of the obtained samples were annealed in order to obtain the thermoelectric phase as the major one. As-grown and annealed samples were microstructurally characterized to determine the phases distribution and alignment. Moreover, thermoelectric and mechanical characteristics of annealed samples were determined by the four-probe technique and by three point flexural strength tests, respectively. Scanning electron microscopy revealed that Ag particles appear dispersed among well oriented ceramic grains with large size, providing a plastic flow region which increases the flexural strength for the optimally Ag added samples (1wt.%). The composites electrical resistivity is lower than that of pure Bi 2 Sr 2 Co 1.8 O x while Ag addition does not significantly affect thermopower values. The resistivity reduction leads to power factor improvements of ~50%, compared with pure samples, for Ag additions of 1wt.%Ag.

Role of Ag in textured-annealed Bi2Ca2Co1.7Ox thermoelectric ceramic

“…Consequently, it can be expected that metallic Ag could be found finely dispersed all along the samples in agreement with previous works [19,20,37,38]. Further Ag addition starts to raise the amount of this secondary phase.…”

“…In those micrographs, some Ag particles are indicated by arrows in the sake of clarity, as they show nearly the same contrast observed in the thermoelectric phase. Furthermore, these Ag particles show an elongated shape and appear between the thermoelectric grains, probably improving their electrical connectivity, as reported in previous works in similar textured systems [19,20], or in solid state sintered materials [39]. Moreover, Ag addition seems to slightly decrease porosity of samples, probably due to the formation of a eutectic phase, as it was already found in similar systems [38].…”

Effect of Ag additions on the Bi_1.6Pb_0.4Sr₂Co_1.8O_x thermoelectric properties

“…The values increase almost linearly with temperature in all the measured range, with very similar values for all the samples. This is a clear indication that these small amounts of Ag do not affect thermopower values, as observed in annealed textured samples without Pb [27]. In all cases, the room temperature values are higher than the measured in single crystals with similar composition without Ag (about 100 μV/K) [39].…”

“…From this point of view, it is clear that this kind of substitutions can be useful in order to improve thermoelectric performances of ceramic materials [19], as it is reported for the substitution of Gd and Y for Ca [23], or Pb for Bi [24][25][26]. Moreover, metallic Ag additions have also shown to improve, in an important manner, the mechanical and electric properties of this system [27] and other similar materials [28] which nearly do not react with Ag, as it has been found in some phase diagram studies [29].…”

“…Taking into account these previously discussed effects, the aim of this work is producing high performance TE materials by the addition of small amounts of metallic Ag, determined in previous works [27], to the optimally Pb doped Bi-Sr-Co-O compound studied in previous works [24,30].…”