Reminder: Ph.D. Dissertation Defense - (Jan. 23) - Jeremy Cheng

Jeremy Cheng acopland at stanford.edu
Mon Jan 22 15:08:40 PST 2007


"Effects of Ion Irradiation on Solid Oxide Fuel Cells"

Jeremy Cheng
Department of Materials Science and Engineering

Advisor: Professor Fritz B. Prinz

Date:  Tuesday, January 23rd, 2007
Time:  9:30AM (Refreshments served at 9:15AM)
Place: CISX 101 (Auditorium)

Abstract:

The solid oxide fuel cell (SOFC) is an electrochemical device that
converts chemical to electrical energy.  It is usually based
around an oxide conducting ceramic electrolyte that requires
temperatures above 800C to operate.  There are many advantages to
lowering this operation temperature such as more gas sealing
options and more efficient startup.  One of the key limitations is
in the transport of ions across the electrolyte.  The most common
electrolyte material used is Yttria-Stabilized Zirconia (YSZ).
The ionic conductivity can be greatly affected by grain
boundaries, dislocations, and point defects.

In this study, dislocations were introduced by heavy ion
irradiation.  Irradiation with Xe+ or Ar+ produced a large number
of point defects and dislocations via a mechanism similar to Frank
partial dislocation formation.  The dislocation density was on the
order of 10^12/cm^2 and the Burgers vector was 1/2<110>, as
confirmed by cross-sectional high-resolution TEM and X-ray
diffraction.  Heat treatment at temperatures from 800-1400C
changed the defect structure, eliminated point defects, and
allowed dislocations to react and grow.

Thin films of YSZ were deposited on silicon substrates using
pulsed laser deposition (PLD).  Films deposited on a metallized
substrate were polycrystalline while films deposited directly onto
conductive silicon could be epitaxially grown.  Conductivity was
measured using Electrochemical Impedance Spectroscopy (EIS).  In
both polycrystalline and epitaxial films, ion irradiation caused
the film conductivity to drop by a factor of 2-3 due to additional
point defects in the film.  In polycrystalline samples, heat
treatment removed these point defects allowing the conductivity to
recover.  In epitaxial samples heat treatment may increase the
conductivity beyond the as-deposited values.  However, formation
of silica at the Si-YSZ interface obscured these results.

A novel method was developed to produce freestanding YSZ membranes
without a silicon substrate by using the Focused Ion Beam (FIB).
Thick, single-crystal YSZ pieces were mechanical polished then
thinned using in-situ X-Ray Energy Dispersive Spectroscopy (EDS)
for end point detection.  The final membranes were single crystal,
less than 350nm thick, and pinhole free.  The measured
conductivity was similar to the polycrystalline films; no
conductivity improvement was seen following heat treatment.

In addition to the ohmic losses from the electrolyte, ion
implantation also affected the activation losses of the
electrode.  Single crystal electrolytes were implanted with
various species.  The exchange current density improved by an
order of magnitude following irradiation with Na+ and Xe+ on the
cathode side.



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