By Michael Thieme,* Ralf Frenzel,
Sylvia Schmidt, Frank Simon, Anja Hennig,
Hartmut Worch, Klaus Lunkwitz, and
Dieter Scharnweber
In the last few years, significant efforts have been made to
provide surfaces, which are undergone by dirt contamination
from the atmosphere or other sources, with self-cleaning
properties. This approach utilises principles which have been
discovered in nature now being transformed into technology
(Bionics'). Referring to the perhaps most prominent natural
prototype Barthlott and co-workers[1, 2] created the term
Lotus-effect', which has received a great public attention.
The leaves of the lotus plant (Nelumbo nucifera), but also of a
big number of other herbs were found that their pronounced
water-repellence and clean appearance is due to both micromorphological and chemical features of the leaf surface. In
the case of the lotus leaf, the morphological component is represented by papillose protrusions of the epidermal cells
(appr. 10 lm) and, one order of magnitude lower, rod-shaped
wax crystalloids. The latter provide also for the necessary
hydrophobic properties.
The transformation of this strategy to metallic surfaces is a
scientifically and technologically challenging target. On the
one hand, the micro-morphological features must be implemented into the surface of the metallic substrate, if the hydrophobic modification will be accomplished solely by a trans-
±
[*] Dr. M. Thieme, Prof. H. Worch, Dr. D. Scharnweber
Technische Universität Dresden,
Institut für Werkstoffwissenschaft,
D-01062 Dresden (Germany)
Dr. R. Frenzel, S. Schmidt, Dr. F. Simon, A. Hennig,
Prof. K. Lunkwitz
Institut für Polymerforschung Dresden e.V.,
Hohe Straûe 6,
D-01069 Dresden (Germany)
[**] The authors are obliged to Fh-IWS Dresden, Fh-IWU Chemnitz
and GeSIM GmbH Groûerkmannsdorf for their cooperation.
This work was financially supported by the Sächsisches
Staatsministerium für Wissenschaft und Kunst under contract
no. 4-7533-70-821-98/3.
ADVANCED ENGINEERING MATERIALS 2001, 3, No. 9
parent thin film uniformly covering the roughened surface.
On the other hand, the chemical binding of the hydrophobic
substance is restricted to the coupling with the superficial
mono-atomic hydroxylation layer. Additionally, both the
morphological and the chemical components of the modification must be sufficiently stable under the conditions of the
intended application.
One of the potential fields of application is considered to
be facade construction elements, for which aluminium is one
of the most important metals. If successful, the clean look will
be preserved for a long time, solely by the activity of rain or
pure-water treatment. Scrubbing may be avoided as well as
the use of detergents. No doubt, this would be attractive from
an economical as well as from an ecological point of view.
Ultrahydrophobicity as the near-field target for aluminium
is defined here by the occurrence of an extremely water-repellent behaviour characterised by advancing contact angles of
150-160 and a negligible contact angle hysteresis. Water
droplets will roll off the flat surface even in case of a minute
inclination. All this reflects a strongly reduced surface
energy.
Our research was directed onto (i) the investigation of different routes for the generation of the necessary micro-morphological properties of Al and (ii) the treatment of the
micro-profiled Al surface using various hydrophobic substances. This publication focuses onto the micro-profiling
investigations. Details concerning the chemical modification
will be described separately.[3]
For micro-profiling, five technological approaches will be
outlined in this survey. They may be grouped into classes,
where the surface is remodelled, where material is locally
removed or locally added. The aptitude for the generation of
ultrahydrophobic properties was characterised by dynamic
contact angle (DCA) measurements following the chemical
modification using hexadecyltrimethoxysilane (HTMS) as a
standard substance.
Likewise, in the field of chemical modification a great
number of hydrophobic compounds and systems were applied to flat and micro-profiled Al surfaces. These coatings
belong to quite different compound classes and represent
low- as well as high-molecular substances. Functionalised
reactive silanes, tenside complexes and fluoropolymers may
be mentioned as examples. DCA measurements were carried
out using also non-aqueous liquids to comprehensively evaluate the wetting behaviour and the surface energies.
Generally, pure aluminium 99.5 % (EN AW-1050) was
used. In special cases, the alloys Al Mg1 (EN AW-5005) and
Al MgSi0.5 (EN AW-6060) were used in addition.
Micro-Profiling: (i) Equimorphological replication: Replicas were produced in order to test the influence of the substitution of the natural chemical features by a chemically modified Al surface. The two-step replication was accomplished
using two-component siloxane rubbers and low-viscosity
epoxy resin, followed by thermally coating the cured surface
with a thin Al film (Fig. 1).
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Generation of Ultrahydrophobic
Properties of Aluminium ±
A first Step to Self-cleaning
Transparently Coated Metal
Surfaces**
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Thieme et al./Generation of Ultrahydrophobic Properties of Aluminium
Fig. 1. Scanning electron microscopy (SEM) images a) of an epoxy replica of the upper
side of a lotus leaf using a low-viscosity condensation-curing siloxane type in the first
replication step and b) the original leaf surface.
Fig. 2. SEM images (35 tilted) of a) a laser-treated hard-metal plate (hexagonal array,
pitch 50 lm, crater depth 20 lm) and b) a micro-embossed aluminium specimen (hexagonal array, pitch 40 lm).
(iii) Photolithographic pre-structuring and wet-etching: A chromium-on-quartz mask was designed and produced in such
an manner that hexagonal arrays of resist islands could be
finally formed on the sheet specimens divided into quadrants
with varied diameter of the islands (15, 22.5, 37.5 and 75 lm,
respectively) along with increasing pitch (20-100 lm).
The material was etched in hot acid media for different
times to examine the influence of the changing shape of the
remaining bumps. Figure 3 shows that bumps with flat plateaus were formed in the case of a relatively short etching
duration, whereas more or less pointed peaks could be
observed for a high degree of under-etching and small diameters of the resist islands.
The CA data showed fairly high advancing angles scattering in a wide range (100-150), whereas the receding angles
were much lower only exceptionally exceeding 100. Surprisingly, this was true also in the case of arrays of pointed peaks
(Table 1).
(iv) Laser ablation: Aluminium sheet was directly lasertreated using a focussed scanning beam or an excimer laser
setup provided with appropriate masks and
optics. The surface displays arrays of differTable 1. Advancing (Hadv) and receding (Hrec) contact angles for aluminium specimens after micro-profiling
ently shaped bumps, similar to papillae or
(different variants) and chemical modification using hexadecyltrimethoxysilane (standard').
truncated pyramids (Fig. 4). The pitch varied
between 18 and 50 lm.
Hrec []
Method of aluminium micro-profiling
Hadv []
All of the tested variants proved to be
Epoxy replica coated with aluminium (Al ca. 30 nm)
109 ± 06
015 ± 06
suitable for producing ultrahydrophobic surMicro-embossing (pitch 30, 50 lm)
150±160
pinning
faces (Table 1) so that a patent was applied
Photolithographical pre-structuring and wet-etching
140 ± 06
070 ± 20
for.[4]
(pitch 30 lm, etching time 6 min)
(v) Electrochemical treatment: The tentative
Photolithographical pre-structuring and wet-etching
121 ± 18
023 ± 11
application of conditions for the growth of
(pitch 20 lm, etching time 12 min)
oxide layers in the thickness ranges of 50Nd:YAG laser treatment (pitch 45 lm)
156 ± 01
159 ± 01
100 nm (borate buffer pH = 7.4)[5, 6] and
10±15 lm (sulphuric acid c = 2.3 mol L±1, T =
Excimer laser treatment (pixel mask, pitch 47 lm)
157 ± 01
157 ± 01
18 C, j = 15 mA cm±2, t = 1500 s)[7, 8] made
151±161
144±160
Sulphuric acid anodised under intensified conditions
2± ±1
±2
clear
that the more or less microscopically
(2.3 mol SO4 L , 40 C, 28 mA cm , 1,200 s)
flat
oxide
surfaces do not represent a suffi020 ± 05
016 ± 06
For comparison: Sulphuric acid anodised
under intensified conditions (2.3 mol SO42± L±1, 40 C,
cient prerequisite for ultrahydrophobicity.
28 mA cm±2, 1,200 s), without silane modification
Even the conditions given by Tsuji and co110 ± 01
022± 05
For comparison: Sulphuric acid anodised according
workers[9] for a micro-profiled surface failed
to Tsuji [9] (0.5 mol SO42± L±1, 20 C, 10 mA cm±2, 10,800 s)
(Table 1). However, in the case of 2.3 mol L±1
080 ± 04
026 ± 08
For comparison: Sulphuric acid anodised as usually [7, 8]
sulphuric acid the systematic increase of
(2.3 mol SO42± L±1, 20 C, 15 mA cm±2, 1,500 s)
both temperature and current density led to
the formation a characteristic micro-rough
Comparing the replicas with the original surface, a considerable deficiency of sub-lm details is revealed independent
of the viscosity (1900-8000 mPa s) and the type of curing (condensation, addition) of the primarily applied siloxane. Here,
the reason is seen for the completely unsatisfactory wetting
behaviour after chemical modification (Table 1).
(ii) Micro-embossing: Tools were made from small hardmetal plates (8 ” 8 mm2) by laser treatment using a scanning
focussed Nd:YAG laser. Pitches of 30, 40 and 50 lm with
orthogonal or hexagonal crater arrangements were adjusted
(Fig. 2a). Using 99.5 % pure aluminium sheet (1 mm thick)
preliminary micro-embossing experiments were accomplished at ambient temperature with maximum forces of
35 kN. The generated profile was successfully transferred
onto the Al surface (Fig. 2). However, problems observed in
the CA measurement after HTMS modification (Table 1) suggest that there is a lack of fine roughness portions. Moreover,
measures must be taken to avoid shearing effects which were
observed in the edge regions due to plastic flow of the
pressed material.
692
ADVANCED ENGINEERING MATERIALS 2001, 3, No. 9
Thieme et al./Generation of Ultrahydrophobic Properties of Aluminium
Fig. 6. Non-contact AFM images of Al anodically treated with varied polarisation
time; a) 1,000 s, b) 1,100 s.
Fig. 4. SEM images (35 tilted) of aluminium after different laser treatments; a) a
scanning focussed beam of a Nd:YAG laser (pitch 45 lm) and b) setup with excimer laser and pixel mask (pitch 47 lm).
Fig. 7. SEM images (35 tilted) of micro-profiled Al after intensified anodic treatment
following a) micro-embossing and b) photolithographic/etch treatment, respectively.
surface, which gave the desired ultrahydrophobic properties
subsequently to the treatment with HTMS (Table 1) or a number of other substances (see below).[4] SEM and AFM indicated non-regularly ordered structure elements of typically
1±2 lm in distance and height with an average layer thickness of about 10 lm (Fig. 5). Stripping of the oxidic layer just
in the formation solution led to a specific mass of 2.3 mg cm±2
corresponding to a density of about 2.3 g cm±3. The cross-section indicates a non-homogeneous micro-structure of the
layer with a fiber-like zone adjacent to the metallic substrate,
similarly to the situation with usually anodised oxide.
This well-balanced occurrence of oxide growth and
dissolution was found for T = 35±50 C, j = 28±42 mA cm±2,
t > 1,100 s. The polarisation time proved to be a crucial quality leading to a sharp change in the microscopic shape
between 1,000 and 1,100 s (Fig. 6). Further details will be published elsewhere.[10]
Additionally, it should be noted that the anodic treatment,
as described above, may be used to improve the hydrophobic
properties obtained by the procedures according to (iii) and
(iv). Here, the original surface shapes are remodelled in the
way displayed in Figure 7. Chemically modified, they show a
complete ultrahydrophobicity.
The efficiency for the formation of the micro-profiled oxidic layers was assessed from the measurement of Al(III)
concentration in the processing solution and from oxide stripping along with coulometric data. These calculations showed
that the efficiency falls from a level of about 0.5 for the region
before the onset of micro-profiling (t = 800 s) to about 0.25 for
long polarisation (t = 1,900 s).
Information on composition and structure of this layer
type was derived from X-ray photoelectron spectrometry
(XPS), infrared spectrometric studies (FT-IRRAS), thermogravimetric measurements (TG), electrochemical impedance
spectrometry (EIS) and X-ray diffraction (XRD).[10]
Different synthetic concepts were applied for chemical
modification of aluminium surfaces. In addition to the frequently used alkyl silanes, x-functionalised alkyl silanes,
co-polymers, and polyelectrolyte/surfactant complexes were
employed. It was shown that the desired ultrahydrophobicity
was generated under certain
conditions only, depending on
the actual type of micro-profiling as well as on the manner
of modification.
Figure 8 shows schematically three variants of chemical
modifications
which
proved to be suitable to produce ultrahydrophobic propFig. 5. SEM images of anodically micro-profiled specimens; a) Al Mg1, no tilt, inlens detector for upper part, b) ditto, 35 tilted,
c) Al99.5, Cu-contrasted, cross-sectioned and Weck-etched.
erties of laser-treated or elec-
ADVANCED ENGINEERING MATERIALS 2001, 3, No. 9
693
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Fig. 3. SEM images (35 tilted) of photolithographically pre-structured and wet-etched
aluminium produced under varied experimental conditions; a) starting diameter of the
islands 22.5 lm, pitch 30 lm, etching time 6 min and b) \'o15 lm, pitch 20 lm,
12 min.
COMMUNICATIONS
Thieme et al./Generation of Ultrahydrophobic Properties of Aluminium
surements. According to the formula given
by Cassie and Baxter[14]
cos rough ˆ tp cos smooth ÿ …1 ÿ tp †
(2)
a level of 0.1 is obtained for ultrahydrophobic surfaces. The corresponding force for the
droplet to start rolling on such a surface was
estimated to be in the sub-lN region. This is
the reason that water droplets roll off the flat
surface even in case of a minute inclination.
In view of a practical application of these
systems the durability to water is of fundamental importance. Therefore, the different
types of modified surfaces have been underFig. 8. Successful routes for chemical modification of laser-treated or electrochemically micro-profiled aluminium surfaces.
gone prolonged water spraying (up to
100 h). The fluorine containing systems were
superior to the pure alkyl silan system. The following rank
trochemically micro-profiled surfaces as shown in Figures 5
was found based on the wetting behaviour within the waterand 6b. All these treatments were finished by tempering
contacted zones:
(max. 160 C). Normally, the original microstructure of the
fluoroalkyl silane > (AAPS + Teflon1 hexadecyltrimethoxsurface was not changed.
In the case of the application of alkyl silanes as displayed
ysilane.
on the left-hand side, it was shown that silanes containing
Further work will be concentrated on the improvement of
three alkoxy or chlorine groups are more suitable to cover the
the mechanical stability of the total system consisting of oxide
surface than silanes with only one reactive group. Probably,
and hydrophobic film. Of course, other important properties
the former are able to generate covalently bonded networks
must be evaluated in the context of potential application
fields.
on aluminium surfaces. The length of alkyl chains were typically between C8 and C18, where fluorinated alkyl chains
Received: March 07, 2001
Final version: May 28, 2001
allowed to decrease the chain length.
Alternatively, x-functionalised alkyl silanes, as [3-(2-aminoethyl)aminopropyl]trimethoxysilane (AAPS), were used to
fix a further hydrophobic agent. Here, the fluoropolymer Tef±
[1] W. Barthlott, C. Neinhuis, Planta, 1997, 202, 1.
lon AF [poly(tetrafluoroethylene-co-2,2¢-bis(trifluoromethyl)[2] C. Neinhuis, W. Barthlott, Annals of Botany 1997, 79, 667.
4,5-difluoro-1,3-dioxole)] revealed to be a reliable agent to
[3] R. Frenzel, S. Schmidt, D. Pleul, M. Thieme, U. Lappan,
open the way to produce hydrophobic and oleophobic surF. Simon, Aluminium surfaces with non-adhesion properties,
faces.
Journal of Adhesion Science and Technology, submitted.
To evaluate the ability for conferring the surface hydro[4] M. Thieme, R. Frenzel, S. Schmidt, H. Worch, D. Scharnphobic properties surface energies were calculated according
weber, U. Lappan, A. Lenk, K. Lunkwitz, M. Panzner,
to numerous procedures.[11-13] However, they are not defined
F. Simon, Ultrahydrophobe Oberflächen, Verfahren zu deren
for a composite surface, as in the case of micro-profiled speciHerstellung sowie Verwendung. Application filed
mens. For instance, slightly pickled Al covered with AAPS/
07.06.2000.
Teflon1 AF gave advancing angles of Hadv = 125-133. This
[5] C. A. Gervasi, J. R. Vilche, in Proc. Symp. 'Oxide Films on
corresponds to surface energies of cs = 6-8 mJ m±2 based on
Metals and Alloys', Proc. Vol. 92-22, Pennington, NJ, The
the equation by Neumann and Li :[11]
s
Electrochem. Soc. 1992, 139.
h
i
cs
2
(1)
cos adv ˆ ÿ1 ‡ 2 exp ÿb …cl ÿ cs †
[6] C. Ikegami, H. Takahashi, M. Seo, ibid., 414.
cl
[7] Oberflächenveredlung von Aluminium - Die anodische Oxywhere cl is the surface tension of water as the test liquid
dation, Firmenschrift 6. 104, VEB Mansfeld Kombinat
(72.8 mJ m±2) and b is a constant (b = 0.0001247 (m2 mJ±1)2).
Eisleben, 1977.
It should be noted that this level of the surface energy is
[8] T. W. Jelinek, Oberflächenbehandlung von Aluminium,
equal to the value given for the trifluoromethyl group (ca.
Saulgau, Leuze, 1997.
6 mJ m±2)[9], whereas it is considerably lower than for PTFE
[9] K. Tsuji, T. Yamamoto, T. Onda, S. Shibuichi, Angew.
(18.5 mJ m±2).[9]
Chem., 1997, 109, 1042.
Returning to micro-profiled and chemically modified sub[10] K. Grundke, A. Hennig, R. Frenzel, M. Thieme, Ultrahystrates, the profile-bearing area of the produced material-air
drophobic surfaces: relation between roughness and contact
composite to a water droplet may be assessed by DCA meaangle hysteresis, J. Adhesion Sci. Technol., in prep.
694
ADVANCED ENGINEERING MATERIALS 2001, 3, No. 9
Nanocrystalline Ti-Doped
Li3AlH6 as a Reversible
Hydrogen Storage Material**
By Jun Chen,* Nobuhiro Kuriyama,
Hiroyuki T. Takeshita, and Tetsuo Sakai
The family of alkali non-transition-metal complex hydrides such as LiAlH4 and NaAlH4 offer high hydrogen
weight percents.[1] For the purpose of creating lightweight
hydrogen storage materials with high H2-uptake capacity
and low desorption temperature, such complex hydrides
have been considered. The main problem is that these
hydrides cannot be easily prepared by direct reaction of metals with gaseous hydrogen, but are usually synthesized by
various solvent chemical reactions.[2±5] Although Li3AlH6,[6]
Na3AlH6,[7] and Na2LiAlH6[8] were prepared recently by
grinding mixtures of LiAlH4 or NaAlH4 and LiH or NaH in
the solid state,[9±11] they still have been discounted from consideration as reversible hydrogen storage system due to the
critical conditions required for their rehydriding. The challenge is how to make this class of complex hydrides more
reversible. Recent progress has been made in this direction
for the catalyzed Na±Al hydrides. The method proposed by
Bogdanovic et al.[12,13] requires the use of special solvents. The
work carried out by Jensen and co-workers[14,15] has been
extensively focussed on the dehydrogenation kinetics rather
than the rehydrogenation properties. It is thus clear that
further work is needed and should be directed in improving
the kinetics, typically for the catalysis function of the rehydro-
genation process. Here we report nanocrystalline titaniumdoped lithium aluminum hydride (Ti-doped Li3AlH6)
obtained by high-energy ball milling technique and its potential utilization as a reversible hydrogen storage medium.
Since the doping procedure of preparing catalyzed complex hydrides has been proven to play an important role in
developing and optimizing the reversible hydrogen storage
system, and in addition, Li3AlH6 has a relatively higher
hydrogen capacity than Na3AlH6, in the present communication we selected titanium(IV) isopropoxide, Ti(OPr)4, as the
catalyst that was added into the stoichiometric mixture of
2 LiH + LiAlH4, and then modified them by high energy ball
milling. The effect of phase change on the dehydriding/rehydriding properties was also investigated for the aim of clarifying the relation between the reversible hydrogen storage and
catalysis function.
Figure 1 shows the X-ray diffraction (XRD) spectra of
3 mol-% Ti(OPr)4 + 2 LiH + LiAlH4 mixture before (denoted
as sample a) and after (denoted as sample b) ball milling.
Interval experimental analyses reveal that after 1 h of ball
milling, the initial LiH and LiAlH4 phases have been used to
synthesize Li3AlH6 that has the monoclinic crystal structure.[16,17] This result along with the reported data[9±11] confirm
that the solid state reaction in Equation 1 can proceed
through the mechanical milling.
3 mol-% Ti(OPr)4 + 2 LiH + LiAlH4 ±? Ti-doped Li3AlH6 (1)
In addition, it is very clear that the diffraction peaks of the
milled phase are obviously broadened. A detailed analysis
indicates that the broadening of the diffraction peaks results
mainly from the contribution of the grain size. Taking the
width of the diffraction peak and using the Scherrer equation,
the average grain size was calculated to be 40 nm, which is
much smaller than that of the initial LiH or LiAlH4 phase.
This is further confirmed by the observations of scanning
±
[*] Dr. J. Chen, Dr. N. Kuriyama, Dr. H. T. Takeshita,
Dr. T. Sakai
National Institute of Advanced Industrial Science and
Technology (AIST)
Special Division of Green Life Technology
Ikeda, Osaka 563-8577 (Japan)
E-mail: [email protected]
[**] The authors acknowledge NEDO funding.
ADVANCED ENGINEERING MATERIALS 2001, 3, No. 9
Fig. 1. XRD spectra of a) 3 mol-% Ti(OPr)4 + 2 LiH + LiAlH4 mixture: the peaks
related to LiH are marked by arrows, the others are related to LiAlH4. b) Such a mixture after 1 h of ball milling.
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COMMUNICATIONS
[11] D. Y. Kwok, A. W. Neumann, Advances in Colloid and
Interface Sci., 1999, 81, 167.
[12] W. A. Zisman, Contact angle, wettability and adhesion, in:
Advances in Chemistry Series, American Chemical Soc.,
Washington, 1964, 43.
[13] D. K. Owens, R. C. Wendt, J. Appl. Polym. Sci., 1969, 13,
1741.
[14] A. B. D. Cassie, S. Baxter, Trans. Farad. Soc., 144, 3, 16,
in: J. P. Youngblood, T. J. McCarthy, Macromolecules,
1999, 32, 6800.