Thermoelectric Power of Silver and a Proton Conductor

Abstract

Thermoelectric power measurements have been reported on many oxides over the past several decades. In many of these studies, difficulties were encountered in achieving sample and thus electromotive force ͑emf͒ equilibration. Thermoelectric power in the present work was measured on dense and porous samples of two materials: silver, a noble metal, and Ba 3 Ca 1.18 Nb 1.82 O ͑9−␦͒ or BCN18, a high-temperature proton conductor. The objective of the work on porous BCN18 was to facilitate rapid sample equilibration by substantially increasing diffusion kinetics because BCN18 can exchange oxygen and/or water vapor with the atmosphere. Silver is not expected to exhibit any exchange of matter with atmosphere. Upon change of temperature, the time required for equilibration of emf was about the same for dense and porous silver samples, consistent with expectations. Correspondingly, the measured thermoelectric power was also about the same for both samples. In BCN18, however, kinetics of emf equilibration was much faster in porous samples compared to that in dense samples. Thus, it was not possible to accurately measure thermoelectric power of BCN18 using dense samples, especially at low temperatures ͑below 700°C͒. The vast difference in equilibration kinetics between dense and porous BCN18 samples is attributed to differences in solid-state diffusion distances between porous samples ͑on the order of a few micrometers͒ and dense samples ͑on the order of a millimeter͒. Above ϳ700°C, thermoelectric power measured using dense and porous BCN18 was nearly the same, since experiments could be conducted for a long enough time to ensure equilibration even when using dense samples. Thermoelectric power on porous BCN18 was measured between 500 and 800°C over a range of partial pressures of water vapor between ϳ0.023 and ϳ0.31 atm. Issues relating to kinetics of emf equilibration in dense and porous samples of silver and BCN18 are discussed.

(Invited) Transport Properties of Mixed Ionic Electronic Conductors under Thermodynamically Equilibrated Conditions: Measurements on Porous Bodies

“…Experiments were also conducted on dense and porous samples of silver. Figure 3 compares the EMF measurements on dense and porous samples of silver [2]. The average sample temperature was 400 o C in both experiments.…”

“…oxygen, sample equilibration may not occur in a reasonable time. For example, if O D ~ is the chemical diffusion coefficient of oxygen, the a bar or disc-shaped sample of thickness 2 . Suppose the O D ~~10 -7 cm 2 s -1 and the sample thickness is 2 mm, then the equilibration time is t ~27 h. If the temperature is lower, the equilibration time is even longer.…”