(Invited) Potentiometric Measurements of Defect Equilibrium of Nonstoichiometric Oxides Under Mechanical Load
Authors: Kawada Tatsuya , Sekizawa Chikara , Sato Ryuta , Watanabe Satoshi , Yashiro Keiji 
Published: May 2020

Nonstoichiometric compounds are widely used for solid state electrochemical devices such as lithium ion batteries and solid oxide fuel cells. Their functions arise from point defects of which the equilibrium concentration varies with temperature, pressure, and chemical potential of the constituent elements in the surrounding environment. In an actual device, materials are used in combination with other compounds among which internal stress appears due to the mismatch of the thermal or chemical strain. The mechanical load, or stress tensor, applied to the solid may also affect the local defect equilibrium. In thermodynamic considerations by Johnson and Schmalzried [1], the modification of the defect equilibrium by the mechanical load was correlated by the chemical strain or the volume change upon variation of defect concentration.
In our previous reports [2,3], we proposed a potentiometric method to monitor the shift of oxygen defect equilibrium of a mixed conducting oxides placed under mechanical load. The measurements were done using a small ball of stabilized zirconia (YSZ) which was used for a potentiometric oxygen sensor as well as for a pushing jig to apply compressive stress on the sample at the measurement point. As a mixed conducting oxide, (La,Sr)(Co,Fe)O3 was chosen and the measurements were done using a universal mechanical testing machine. The emf was monitored over the YSZ ball between the sample and a reference electrode which was placed on the free surface of the ball. The results were as expected, i.e. a negative voltage appeared on the sample side on the instance of applying load, and it gradually approached back to zero. On removing the load, a positive voltage appeared, and it decayed to zero with time. The observed phenomena were explained as follows: On application of the load, the local composition tends to shift to a new equilibrium under the load. However, it does not occur instantly since it accompanies mass transport. Instead, the sample behaves first as a closed system and the local chemical potential varies, which was monitored by the YSZ ball. Equilibration with the gas phase proceeds gradually via chemical diffusion of oxygen in the sample. The maximum voltage in the negative and positive directions showed good agreement with those estimated from the local stress and the chemical expansion coefficient for vacancy formation reaction. The similar measurements were made with (La,Sr)CoO3, (La,Sr)FeO3, and (La,Sr)MnO3, and the similar results were obtained when oxygen nonstoichiometry variation is large enough compared to the thermal fluctuation.
Recently, the similar measurements were made by monitoring a voltage across a mixed conducting oxide itself on pressing its surface with an alumina ball. Gadlinia doped ceria, GDC, was used as the sample, and the surface was pushed with a small alumina ball coated with platinum paste. Reference Pt electrode was placed on the GDC sample at the place far from the applied load. The voltage across GDC was monitored using the Pt coat on alumina ball and Pt electrode on GDC. Under reducing atmosphere, the negative shift of potential followed by a gradual decay was observed. The maximum voltage was as expected if the transference number that is less than unity was taken into consideration. Under oxidizing atmosphere, a quick appearance of positive voltage was observed upon loading. It was similar to that observed with YSZ ball when it was pushed against Pt foil. The oxygen dissolved or adsorbed on Pt or at the interface could be the source of the response. With GDC, a persistent voltage was also observed in some cases, of which the reason is not clear yet.
The similar approach as GDC was explored to a Ba(Ce,Y)O3 based proton conducting oxides. In this case, not only proton but also electron hole and oxide ion vacancy may contribute to the transport and thus the appearance of EMF. The results observed so far was the negative voltage on loading under oxidizing atmospheres, and negative voltage under reducing atmospheres. Rather large persistent volrage was also detected. Details of the results and the explanation will be discussed.
References
[1] W. C. Johnson, H. Schmalzried, J. Am. Ceram. Soc., 76, 1713 (1993)
[2] T. Kawada, T. Masumitsu, Y. Kimura, S. Watanabe, S. Hashimoto, K. Yashiro, K. Amezawa, J. Electroceram. 32(1), 78 (2014)
[3] Y. Gonoi, Y. Kimura, K. Yashiro, S. Watanabe, S. Hashimoto, T. Kawada, J. Electrochem. Soc. 161(11), F3111 (2014)