Transcript Understanding oxidation levels in Nitinol using XPS

Characterizing surface layers
in Nitinol using XPS
Rebecca Christopfel
Western Washington University
Mentor: Apurva Mehta
August 15, 2007
outline
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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!
nitinol
titanium
nickel
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…
O
• Choose incident
photon energy – 1100
eV
• Perform an overview
scan
Ti
C
Ni
P
getting started…
O
• Choose incident
photon energy – 1100
eV
• Perform an overview
scan
• Choose scan ranges
in a given KE range
Ti
Ni
C
P
P
KE (eV)
Intensity
KE (eV)
Intensity
KE (eV)
Intensity
KE (eV)
Intensity
KE (eV)
Intensity
math….
S ( ,  )  I  [e
S2

 d
cos 
(
h
)Cf ]e
cos 

d
 cos
St
S1
1
S
 Ce
2
S

( d1  d 2 )
 cos
 S1 
1  d1  d 2 
ln 2   lnC  


S
cos




 
what does it mean?
12
10
d2>d1
8
S 
1  d1  d 2 
ln 2   lnC  


cos   
S 
1
6
4
d1=d2
2
0
(d = depth)
0
-2
1
2
3
4
5
d1>d2
-4
• Sign of slope tells us which species lies above
• Magnitude tells us an approximate separation
between species
6
data…
S2
S3
P2p - MPPBS
S1
1.60E+00
S3:S1
1.40E+00
ln(S1/S2)
1.20E+00
S3:S2
1.00E+00
8.00E-01
6.00E-01
S2:S1
4.00E-01
2.00E-01
0.00E+00
0.75
conclusion: species associated
with signal three is above both two
and one; species one and two
appear to be coming from the same
depth
1
1.25
1.5
1.75
2
2.25
1/COS(alpha)
2.5
2.75
3
3.25
a model…
alumina
hydroxide
phosphate
titanium oxide
partially
oxidized nickel
oxidized nickel
metallic nickel
mechanically
polished
nitinol
substrate
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…
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Apurva Mehta, SSRL/SLAC
Zhi Liu, SSRL/SLAC
Valeska Schroeder, NDC
Alan Pelton, NDC
Hirohito Ogasawara, SSRL/SLAC
• DOE and SLAC
questions?