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University of MaineOrono, July 16, 2008
Faculty of Engineering Institute of Materials Science Chair of Materials Science and Biomaterials
Superhydrophobic Aluminum Surfaces:
Preparation Routes, Properties and Artificial Weathering Impact
M. Thieme 1, C. Blank 1, A. Pereira de Oliveira 2, H. Worch 1,R. Frenzel 3, S. Höhne 3, F. Simon 3, H. Pryce Lewis 4, A. J. White 4
1 Technische Universität Dresden (TUD), Institute of Materials Science, Dresden, Germany
2 TUD, now at: Unicamp, Campinas, Brazil3 Leibniz Institute of Polymer Research Dresden, Dresden, Germany4 GVD Corporation, Cambridge, MA, USA
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Superhydrophobicity
Introduction
Theory:
•Type of interaction
•Nature of forces
•Triple line motion
•Design, bionics
Model Systems:
•Model arrays (pillars, hoodoos, …)
•Fractal surfaces
•Prediction - experiment
Analytical Issues:
•Contact angle measurement
•Surface characterization
Preparation Issues:
•Novel pathways
•Switchable systems
•Stability (tensides, irradiation, abrasion, …)
•Application
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Introduction
Outline
Introduction
Our Experimental Attempts
Selected Results
- Two-step Preparation Routes
- Stability Under Artificial Weathering
Summary & Outlook
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Introduction
Background
Superhydrophobicity for self-cleaning purposes
Aluminum as desired base material
Idea: Superhydrophobically furnished Al facades (transparent coating), looking like...
Source: Luxalon, 2000
former general store Centrum Dresden
Source: Kantschew, 2005
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Experimental Measures
Preparation of Coated Inorg.-org. Hybrid
Systems (large area capability)
Micro-roughening+
Chem. Modification
Model Exposure
Artificial Weathering
(cyclic moisturing/ drying & irradiation, total duration 15 d)
‘WTH’
In-situ Characterization
EIS
CA(DCA)
XPS
Cross-sect.
FT-IRRAS
SEM
Cryo-fractur-
ing
Fluor-esc.
micro-scopy
EIS
AFM
EQCM
Ex-situ Characterization
NMR
Micro-hard-ness
LSM
Qual. criteria
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Preparation Routes Micro-roughening
Al substrates:
Alloys
99.5 (1050),
AlMg1 (5005),
AlMgSi0.5 (6060)
Sheet, rod
Sulfuric Acid Anodization
Intensified Conditions
‘SAAi’
HT Micro-Embossing
‘ME’
+ Micro-blasting
‘MB’
Phosphoric-Chromic Acid Anodization[C. Blank et al., Prakt. Metallogr. Sonderband36, 491-496 (2004)]
Laser Ablation
[M. Thieme et al., Adv. Eng. Mat. 3, 691-695
(2001)]
1 2
SAAi:2.3 M H2SO440°C 30 mA cm-2
1200 s
ME:SiC tool (350 °C)285 °C 120 MPa
MB:corundum600 - 1200 grit
SAAu:20°C 15 mA cm-2
1200 s
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Alkylsilanes
‘HTMS’
Fluoroalkylsilane
‘PFATES’
-Si-O-Si-O-O O
Preparation Routes Chemical Modification
Dissolved Compounds
Wet-born Coatings
Aminosilane+ Teflon® AF
‘AS/TAF’
Chitosan ac. (cathodically) + Alkyl maleicanhydride copolymer
‘Chs/POMA’
Tetradecanephosphonic
acid
‘TDPA’
SAMs
1
oxide
-Si-O-Si-O-O O
HO-P=OO
oxide oxide
Hot-filament CVD
(GVD Corp.)
Hexafluoro-propylene
oxide
PTFE
Var. in substrate
temperature, pressure,
post temper
(SC, AC, LP, A1, A2)
FF
NN
NN
oxide
OH
HOCH 2
OO
OHO
CH 2OHNH
NHO
2
2
AO
O
FO
2
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SAAi
1 µm
1 µm
T = 40 °C, j varied, t = 1200 s
5 µm
28 ...42 mA cm-2
35 ...21 ...15 ...SAAu
T varied, j = 28 mA cm-2, t = 1200 s
5 µm
40 °C35 °C30 °C45 °C 50 °C
145°//103° 149°//54°155°//148°
148°//149°150°//150°
114°//36° 116°//32° 154°//150° 158°//156° 155°//91°
Parameter variations
ResultsMicro-roughening by SAAi
• optimal ranges of T, j exist for achieving SH
10 µm
Contact angles for modification using HTMS
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ResultsMicro-roughening by SAAi
SAAi
10 µm
1 µm 1 µm
5 µm
Metal deposited+ cross-sect.
• SAAi: specific morphology & modified oxide compo-sition in comparison to usual anodic oxide SAAu
O-H strAl-Ox
S-Ox?
2 µm
Cryo-fractured
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20 µm
20 µm
20 µm
20 µm
2 µm
20 µm
10 µm
ResultsMicro-roughening by ME/ MB
Tool
ME ME + MB-600
ME + MB-1200 + SAAi
‚Cold‘ ME
HT-ME HT-ME + MB
2 µm
• Purely mechanical & mixed approaches:
specific morphologies very similar to Lotus leaf
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Results Chemical Modification by Solutes
SAAi + wet-born coatings (Chs/POMA, PFATES, AS/TAF)
• CA: superhydrophobicity achievable
• SEM: coatings generally not detectable
θa ≈ 152°
θr ≈ 150°
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Li and Neumann:
θa = 133°, θr = 110°γl = 72.8 mJ m-2 (water),β = 0.0001247 (m2 mJ-1)2;
γs = 5.5 mJ m-2
PTFE, for comparison: γs = 18 mJ m-2
θ advancing angleγs surface energyγl surface tensionβ constant
[ ]2sl
l
s )((exp21cos γγγγθ −−+−= ß
Further approaches:
Zisman: critical surface tension γcrit = 11.6 mJ m-2,
Girifalco-Good-Fowkes-Young: disperse portion γs
d = 10.8 mJ m-2
Results Chemical Modification by Solutes
Surface energies of pickled state (nearly flat) + AS/TAF
• free surface energies of modified surface extremely low
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Results Chemical Modification by Solutes
SAAi + wet-born coatings Chs Chs/POMA
AS/TAF
PFATES
• FT-IRRAS: POMA, PFATES, TAFcan be ±clearly identified viaC-H and C-F vibration bands
• XPS: different carbon bindings incl. >CF2, -CF3
Chs+POMA
PFATESAS/TAF
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Results Chemical Modification by Solutes
2 µm
20 µm
SAAi + Chs
Higher pH, j, t: craters, spots
• SEM, EIS: cathodicallydeposited Chs may be harmful for oxide integrity
NH3+ + HO- ↔ NH2 ↓ + H2O
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ME + MB-600+ SC-1000
SAAi+ SC-1000
Results Chemical Modification by PTFE
• ‘thick PTFE coatings’ (50 nm to 1000 nm) can be deposited on different micro-rough substrates without leveling effects
• SEM: different, controllable morphology(budded protrusions / filmy flakes)
• CA: superhydrophobicity achievable, depending on substrate roughness
flat surface+ SC-1000
SAAi+ A1-500
10 µm
157±1, 155±1
1 µm
152±1, 151±1
2 µm
151±1, 149±1
2 µm
144±3, 94±6
SAAu+ SC-50
2 µm
151±2, 140±2
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SAAi + PTFE
PTFE-LP
PTFE-SCPTFE-SC
PTFE-A1
PTFE-SC on MB
PTFE-LP
Results Chemical Modification by PTFE
• IRRAS: different PTFE deposition conditions with minor deviations in C-F bands
• XPS: generally mostly >CF2 bindings; with PTFE-LP deviations detectable (more C-H and O)
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SAAu + PTFE-SC
Results Chemical Modification by PTFE
• EIS: growth of impedance at medium frequencies with increasing PTFE thickness; low frequency limit ±uniform;
EIS curve shapes/ positions similar to earlier results with SAAi + PFATES and SAAi + AS/TAF[M. Thieme & H. Worch, J. Solid State Electrochemistry 9, 737-745 (2006)]
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50
75
100
125
150
175
SAAi + C
hs +
POMA
SAAi + P
FATES (2
x)
SAAi + A
S/TAF (2
x)
SAAi + S
C (250
-1000
nm, 6
x)
SAAi + C
hs +
SC (250
-1000
nm, 4
x)
SAAi + A
1-500
(2x)
SAAi + A
2-500
SAAi + A
C-500
SAAi + LP
-500
SAAu + S
C (50-1
000 n
m, 3x)
Pickled
+ SC-50
0 SC-10
00
MB-1200
+ SC-10
00
MB-600 +
SC-10
00ME +
SC-1000
ME + MB-12
00 + S
C-1000
ME + MB-60
0 + S
C-1000
Con
tact
ang
le /
°before WTH, adv. CAbefore WTH, rec. CAafter WTH, adv. CAafter WTH, rec. CA
Results Impacts from Artificial Weathering
• Practically no CA changes ( ): SAAi + PFATES, + AS/TAF,+ PTFE-A1, -AC; ME + SC, + MB-1200 + SC, + MB-600 + SC
• Moderate changes ( ): SAAi + PTFE-SC, -A2, -LP; MB-600 + SC
• Considerable changes ( ): SAAi + Chs + PTFE-SC
• Dramatic worsening ( ): SAAi + Chs/POMA, SAAu + SC,Pickled + SC, Flat + SC, MB-1200 + SC
CA Survey
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Results Impacts from Artificial Weathering
SAAi + wet-born coatings before/ after WTH
AS/TAFPFATES
AS/TAF+ WTH
PFATES+ WTH
• PFATES: practically unchanged; portion of F-bound carbon remained on a high level
• AS/TAF: decrease of low-energy bound carbon; agrees with increased F/C concentration ratio aminosilane component undergoes degradation in the course of the exposure!
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Results Impacts from Artificial Weathering
SAAi + PTFE variants before/ after WTH
SAAi + SC
SAAi + SC+ WTH
• PTFE-SC: low, but marked increase of H-bound carbon, along with increase of O content
• PTFE-AC: relatively lowest WTH-related changes acc. to XPS – finding corresponds to little changes in CA!
SAAi + AC+ WTH
SAAi + AC
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Conclusions
Superhydrophobic hybrid systems in multifold variants
Micro-roughening feasible i) by specific anodizationor ii) by micro-embossing/ -blasting
SAAi large-area capable; ME to be developed as a continuous rolling technique
Chemical modification feasible i) by different nanoscalic wet-born coatings or ii) by HFCVD-produced sub-microscalic PTFE layers
Weathering stability of F-containing systems superior to POMA; PTFE-AC better than SC procedure
Resistivity to abrasive effects must be enhanced, in spite of higher ‘buffer’ in case of thicker coatings –subject of further work
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Acknowledgments
Saxon State Ministry of Science and the Fine Art (SMWK)
German Academic Exchange Service (DAAD)
Mr. T. Burkhardt and J. Engelmann (FhG-IWUChemnitz)
Mrs. K. Galle (TUD) and Mrs. B. Schneider (IPF)