The Direct Measurement of Ionic Piezoresistance
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The Direct Measurement of Ionic Piezoresistance Stuart N. Cook1 and Harry L. Tuller1 1 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 USA ABSTRACT Ionic piezoresistance, the effect of lattice strain on ionic conductivity, is an important concept that needs to be harnessed to engineer the next generation of fast ionic conductors. To date there have been many reports of strain affecting changes in the level of ionic conductivity in solid electrolytes. The fundamental understanding is, however, still lacking, with limited experimental quantification of the magnitude of the effect. Here, we propose using the ionic piezoresistive coefficient, the constant of proportionality between the strain state and the change in conductivity, as a quantitative measure of this effect and detail a novel technique we have developed to quantify this in high temperature ionically conducting materials. INTRODUCTION Rapid ion conduction at low temperatures, approaching room temperature, is a highly desirable property that is currently unattainable in the majority of solid state materials. Solid state ionic conductors find widespread use in many energy applications, such as batteries and solid oxide fuel cells (SOFC) and electrolytic cells (SOEC), all of which are essential technologies in the move away from fossil fuel dependence. Identifying routes towards the maximization of ionic conduction levels will provide significant advances in device operation and facilitate new device architectures such as micro-SOFCs. The primary mechanism of ionic conduction in crystalline solids is the migration of crystallographic point defects, either vacancies or interstitials. These defects have an effective charge when compared with the perfect lattice and therefore their migration presents a current. Typical methods of reaching the highest levels of ionic conduction include optimizing crystal structure, to allow low energy ion migration and high defect concentration, through the introduction of dopants. This becomes clear when we consider the basic relationship between conductivity (σ), number of charge carriers (n), their charge (q) and mobility (μ) (Eq. 1). (1) All the materials systems reported to feature the highest levels of ionic conduction therefore have both the defect mobility and concentration optimized. For example, the fluorite structure features in several of the fastest oxygen ion conductors, including the stabilized zirconias and rare-earth doped cerias. In both cases the addition of dopant, and the associated vacancy defects, does not provide a linear increase in conductivity with dopant concentration due to increased defect-defect interactions, and in fact leads to a maximum conductivity at a vacancy concentration of just 5-10% [1,2]. In the past few decades, an effective limit has been reached in how far this optimization can go, with the fastest ever reported oxygen ion conductor, the delta polymorph of Bi2O3, featuring a fluorite structure with 25% of the oxygen s
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