Ion Irradiation Effects on Yttria-stabilized Zirconia Conductivity
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0885-A04-03.1
Ion Irradiation Effects on Yttria-stabilized Zirconia Conductivity Jeremy Cheng, Kevin Crabb, Rojana Pornprasertsuk, Hong Huang, Yuji Saito, Fritz Prinz
Abstract The performance of solid oxide fuel cells is limited largely by ion transport in the electrolyte. Thin film electrolytes of yttria-stabilized zirconia were deposited by pulsed laser deposition. The electrolyte material was subjected to heavy ion irradiation and heat treatment and the effects on conductivity were measured using electrical impedance spectroscopy. Following irradiation there is a drop in conductivity by a factor of 3-4. After heat treatment at 800°C, the conductivity recovers to the as-deposited value. Introduction The solid oxide fuel cell (SOFC) is an electrochemical device that directly converts chemical to electrical energy. It is based around an oxide conducting ceramic electrolyte which requires operation at temperatures above 800°C. There are many advantages to lowering this operation temperature such as more gas sealing options, lower thermal stresses, and more efficient startup. However, several factors limit lower temperature operation; one of the key limitations is the resistance to transport of ions across the electrolyte. The most common electrolyte material used is Yttria-Stabilized Zirconia (YSZ). Its conductivity may be affected by dislocations.1,2 In an ionic crystal, the dislocation core carries a net charge, which results in local space charge effects.3 Furthermore, around the dislocation core is a dilatation of the crystal lattice which can lower the activation energy for vacancy hopping.4 Dislocations can be introduced into YSZ using high energy ion irradiation.5-9 Ion damage appears in three stages versus fluence, with stage 3 occurring at doses above 10 displacements per atom (DPA).10 This stage 3 damage consists of a dense dislocation network and a high concentration of point defect clusters. However, YSZ has unique radiation resistance properties. A dose of 1 DPA means that, on average, each atom at the peak damage depth has been displaced from its lattice position 1 time, as estimated by Monte Carlo simulations.10 Most of these displaced ions will immediately relax back to lattice positions. YSZ never becomes amorphized, even with high doses at high energies.11 Note that the depth for even high energy implants is rather small, around 150 nm, so thin film electrolytes must be used. Experiment YSZ was deposited using PLD in the structures shown in Figure 1. Structure A was built on a silicon substrate with sputtered electrodes. Structure B was the same, except by using a polished single crystal YSZ substrate, there is less thermal expansion mismatch and the thin films can withstand higher temperature heat treatment.
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Figure 1. Diagram of the impedance structures used in this study.
In structure C, YSZ was deposited directly onto a conductive Si substrate to allow for epitaxial growth. The substrate was n-type (100) low resistivity silicon (ρ = 0.01 Ω-cm, Silicon, Inc.) which was etched i
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