characterizing surface layers in nitinol using xps rebecca christopfel western washington university...
TRANSCRIPT
Characterizing surface layers in Nitinol using XPS
Rebecca Christopfel
Western Washington University
Mentor: Apurva Mehta
August 15, 2007
outline
• Brief explanation of Nitinol
• XPS
• Data collection/analysis
• Results
• Conclusion
• Nitinol is a shape memory alloy
• Manufactured as stents
• Tested– Mechanically polished– Mechanically polished
in solution
• Explanted stents– Thick oxide layer– Thin oxide layer
What causes a thin layer to be more biocompatible?
• Biocompatibility is crucial
• Titanium has a higher affinity for oxygen than nickel.
• Nickel can leak into the body--BAD NEWS!
titanium
nickel
nitinol
titanium oxide
nickel
(metallic or oxidized?)
x-ray photoelectron spectroscopy
X-ray photons
X-ray photoelectrons
x-ray photoelectron spectroscopy
• Why?– Tunability of incident
energy allows us to probe certain depths.
– Great for surface analysis ~ few nanometers
Ti
probing different depths• By changing detector/sample orientation we get
signals from different depths
detector
0 45 70 90
surface
getting started…
• Choose incident photon energy – 1100 eV
getting started…
• Choose incident photon energy – 1100 eV
• Perform an overview scan
C
O
Ti
Ni
P
getting started…
• Choose incident photon energy – 1100 eV
• Perform an overview scan
• Choose scan ranges in a given KE range
Ni
O
Ti
C
P
P
KE (eV)
Inte
nsity
KE (eV)
Inte
nsity
KE (eV)
Inte
nsity
KE (eV)
Inte
nsity
KE (eV)
Inte
nsity
math….
cos)(
2
1 21 dd
CeS
S
212
1
cos
1lnln
ddC
S
S
S1
S2
SteCfh
eIS
dd
coscos ])
cos([),(
what does it mean?
• Sign of slope tells us which species lies above• Magnitude tells us an approximate separation
between species
212
1
cos
1lnln
ddC
S
S
-4
-2
0
2
4
6
8
10
12
0 1 2 3 4 5 6
d1>d2
d2>d1
d1=d2
(d = depth)
data…
P2p - MPPBS
0.00E+00
2.00E-01
4.00E-01
6.00E-01
8.00E-01
1.00E+00
1.20E+00
1.40E+00
1.60E+00
0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3 3.25
1/COS(alpha)
ln(S
1/S
2)
S1S2 S3
S3:S1
S3:S2
S2:S1
conclusion: species associated with signal three is above both two and one; species one and two appear to be coming from the same depth
alumina
hydroxide
phosphate
titanium oxide
partially oxidized nickel
oxidized nickel
metallic nickel
nitinol substrate
a model…
mechanically polished mechanically
polished in solution
summary…
• By varying angle we were able to differentiate the depths of various species
• Mechanically polished sample in PBS solution has a slightly thicker oxide layer
• Phosphorus layer near the surface suggests no nickel leaking
• Implies biocompatibility
thanks to…
• Apurva Mehta, SSRL/SLAC
• Zhi Liu, SSRL/SLAC
• Valeska Schroeder, NDC
• Alan Pelton, NDC
• Hirohito Ogasawara, SSRL/SLAC
• DOE and SLAC
questions?