Tunability of Atomic Layer Deposition of Yttrium-Stabilized Zirconium Oxide for use in Solid Oxide Fuel Cells

Anna WalterUniversity of Delaware, Newark, Delaware

Runshen Xu, Christos G. Takoudis, and Gregory Jursich University of Illinois at Chicago, Chicago, Illinois

Advanced Materials Research Laboratory (AMReL)

Solid Oxide Fuel Cells (SOFCs)

• Fuel cells can replace many of our current methods of obtaining energy[1]

• SOFCs are more efficient and stable than other types of fuel cells[2]

• High operating temperatures - electrolyte can conduct oxygen ions[2]

• Lower temperatures desired[1] – must reduce electrolyte resistance

• Must use different materials or make a thinner electrolyte[1] [1] Brett, Daniel. Chem. Soc. Rev. Vol. 37, 1568-1578, 2008. [2] Jacobson, Allan. Chem. Mater. Vol. 22, 660-674 , 2009.

Atomic Layer Deposition (ALD)• ALD: alternating gas-phase reactants (precursors) to create thin film

of solid product (can be a molecule)• One ALD cycle:

ALD Advantages• Self-limiting process – surface area is limiting factor in the

reaction

• Capable of depositing very thin, highly conformal films; good thickness control[3]

• Less than one monolayer is deposited per cycle[4] - steric hindrance

• Composition of composite films can be controlled by changing the number of cycles used to deposit each component[3]

[3] George, Steven. Chem. Rev. Vol. 110, 111-131, 2011 [4] Sherman, Arthur. ALD for Nanotech. Ivoryton Press, Ivoryton CT, 2008

Project Goals

• We are investigating:• use of new ALD precursors in making yttrium-stabilized

zirconium oxide (YSZ) • tunability of this process - how composition of film is

a function of cycle ratio

• New precursors hopefully leave no impurities – thin films

• Tunability important for composition-dependent uses such as SOFCs – 8% yttria is ideal[5]

[2] Jacobson, Allan. Chem. Mater. Vol. 22, 660-674 , 2009. [5] Pomfret, Michael, et. al. Anal. Chem. Vol. 77, 1791-1795, 2005.

Materials

• Oxygen source: ozone

• Yttrium source: tris(isopropyl-cyclopentadienyl)yttrium

• Zirconium source: tris(dimethylamino) cyclopentadienylzirconium

Custom-Built Hot-Wall ALD Reactor

• Y pulse: 8 s• Zr pulse: 5 s• purge: 10 s• O3 pulse: 1 s

X-ray photoelectron spectroscopy (XPS)

• Determine composition

• Each element has a characteristic binding energy (eV)

• XPS measures the energy of the electrons released from the sample

• Number of electrons – intensity - related to amount of element present

[6] http://www.rrc.uic.edu/instruments/12

XPS Data

[7] Scherrer, Barbara, et. al. J. of Power Sources. Vol. 196, 7372-7382, 2011

020040060080010001200

Binding Energy v. Intensity

Y:Zr = 1:1

Y:Zr = 1:2

Y:Zr = 1:7

Binding Energy (eV)

Inte

nsity

(arb

. uni

ts)

Y 3d

Zr 3d

O

ALD Tunability

0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.550

0.1

0.2

0.3

0.4

0.5

0.6

f(x) = 1.07120655737705 x − 0.00249098360655736R² = 0.999938277326152

Linear ALD Tunability

Y Cycle Fraction (Y cycles / total cycles)

Ato

mic

Y F

racti

on (Y

/(y+

Zr))

Zirconium Composition

172177182187192197202

XPS Data - Zr Peaks Fitted

1:7 raw data1:7 sum of peaks1:7 background1:7 ZrO2 peak1:2 raw data1:2 sum of peaks1:2 background1:2 ZrO2 peak1:1 raw data1:1 sum of peaks1:1 background1:1 ZrO2 peak

Binding Energy (eV)

Inte

nsity

(arb

. uni

ts)

cycle ratio = 1:1

cycle ratio = 1:2

cycle ratio = 1:7

ZrO2

Cycle ratio = yttria cycles : zirconia cycles

Yttrium Composition

148153158163168173

XPS Data - Y Peaks Fitted

1:7 raw data1:7 sum of peaks1:7 background1:7 Y2O3 peak1:2 raw data1:2 sum of peaks1:2 background1:2 Y2O3 peak1:2 YOH peak1:1 raw data1:1 sum of peaks1:1 background1:1 Y2O3 peak1:1 YOH peak

Binding Energy (eV)

Inte

nsity

(arb

. uni

ts)

cycle ratio = 1:7

cycle ratio = 1:2

cycle ratio = 1:1

Y2O3YOH

Cycle ratio = yttria cycles : zirconia cycles

Summary

• YSZ was successfully deposited by ALD of these precursors

• This ALD process is highly tunable – linear relationship

• Decreasing number yttria cycles compared to zirconia cycles (decreasing cycle ratio): • Decreases relative amount of yttria / increases relative amount of

zirconia• Decreases amount of surface YOH impurity

Questions?

• Financial support from the National Science Foundation, EEC-NSF Grant # 1062943

• Additional financial support from the National Science Foundation, CBET-NSF Grant # 1067424

• Other AMReL members.

Acknowledgements: