The Significance of Gas-Phase Mass Transport in Assessment ofk_chemandD_chem

Abstract

In this work, the validity of electrical conductivity relaxation (ECR) as a method for the assessment of chemical surface exchange, k chem , and bulk diffusion, D chem , coefficients is investigated with respect to mass transport limitations in the gas phase. A model encompassing both the oxygen surface exchange, mass transport in the bulk sample and the gas phase was set up and solved under different conditions using finite element software. It is found that the transport of oxygen in the gas phase is insufficient at low oxygen partial pressures, causing a concentration boundary layer at the sample surface to develop. This significantly decreases the driving force for oxygen exchange. The effect of mass transport limitations on the computed apparent transport coefficients is shown to be pronounced and surface exchange coefficients are shown to deviate as much as one order of magnitude from the set values. When mass transport limitations are pronounced, a discrepancy between oxidation and reduction values of the apparent k chem and D chem is evident and modelled apparent activation energies for k chem are shown to decrease significantly. The validity of the apparent transport coefficients can be improved by changing the experimental parameters, however the surface exchange coefficient is extremely sensitive to insufficient transport of oxygen in the gas phase and improvements are in general marginal. A criteria for the validity of D chem is introduced while no such measure could be introduced for k chem . The effect of experimental parameters and material properties on mass transport limitations are presented and general recommendations concerning the assessment of k chem and D chem are given.

Synthesis and oxygen transport properties of La2−ySryNi1−xMoxO4+δ

“…La 2-y Sr y Ni a four-point DC method at temperatures between 600 and 900 ºC using bar samples [30]. The bar-shaped green bodies (based on spray pyrolysed powder) were prepared by uniaxial pressing (0.8kN) followed by cold isostatic pressing (CIP, 200MPa).…”

“…). A small step in oxygen partial pressure will increase the accuracy of the oxygen transport coefficients according to recommendations given in [30]. Both oxidation and reduction steps were performed.…”

“…Eq. 7 can be solved numerically and further details are given elsewhere [30,32]. D chem and k chem were determined by fitting of the electrical conductivity relaxation data.…”

Oxygen Exchange Kinetics of SrTiO3 Single Crystals: A Non‐Destructive, Quantitative Method

“…last but not least is electrochemical conductivity relaxation (ECR), as often used for mixed conducting ceramics. [21][22][23][24][25][26][27][28][29][30] ECR requires reliable electrical contacts at the sample, which may be difficult to obtain depending on the samples surface and temperature range required. Moreover, metallic contacts may influence the rate at which oxygen is exchanged.…”

“…On the other hand, the type and charge of defects after ion implantation is varying throughout the sample series. last but not least is electrochemical conductivity relaxation (ECR), as often used for mixed conducting ceramics . ECR requires reliable electrical contacts at the sample, which may be difficult to obtain depending on the samples surface and temperature range required.…”