Tailoring alumina thin film properties using hydrophilic/hydrophobic copolymer additives

Article abstract of DOI:10.1002/pssa.200779429

Thin films of alumina were deposited onto aluminium-treated substrates by pneumatic spray pyrolysis at 250 °C. As precursors, solutions of Alcl ₃ dissolved in the mixture water: ethanol = 1:1 were used. As morphology controlling agents, maleic acid based copolymers (HFB/HFL) and/or acetylacetone were added in spraying solutions. The absorptance (α) and emittance (ε) values are those expected for alumina films, as possible matrix in a cermet, used as a solar-thermal selective absorber. The films have amorphous-crystalline structure and porous morphology. The alumina film composition and crystallinity can be tailored by adding acetylacetone in the spraying solution, when predominantly amorphous structures are obtained. The addition of hydrophobic/hydrophilic co-polymer in the precursor solution favours the obtaining of porous aggregates with large, open structures (3.5-4 μm). These porous morphologies are suitable for further infiltration of metallic particles, for preparation of alumina-based cermets.

Full text of DOI:10.1002/pssa.200779429

phys. stat. sol. (a) 205, No. 10, 2413–2416 (2008) / DOI 10.1002/pssa.200779429
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applications and materials science
Tailoring alumina thin film properties
using hydrophilic/hydrophobic copolymer additives
L. Isac*, 1, A. Duta**, 1, E. Purghel1, G. C. Chitanu2, S. Mitrea3, and I. Pelin2
1
Centre: Product Design for Sustainable Development, “Transilvania” University of Brasov, Eroilor 29, 500036 Brasov, Romania
2
“Petru Poni” Institute of Macromolecular Chemistry, Al. Gr. Ghica Voda 41A, 6600 Iasi, Romania
3
ICPE-CA Institute, Splaiul Unirii 313, 030138 Bucuresti, Romania
Received 15 October 2007, revised 26 July 2008, accepted 28 July 2008
Published online 3 September 2008
PACS 61.05.cp, 68.37.Ps, 68.55.–a, 81.05.Je, 81.15.Rs
**
Corresponding author: e-mail isac.luminita@unitbv.ro, Phone: +40 268 475 597, Fax: +40 268 475 597
**
e-mail a.duta@unitbv.ro, Phone: +40 268 475 597, Fax: +40 268 475 597
Thin films of alumina were deposited onto aluminium-treated porous morphology. The alumina film composition and
substrates by pneumatic spray pyrolysis at 250 °C. As precur- crystallinity can be tailored by adding acetylacetone in the
sors, solutions of AlCl3 dissolved in the mixture water:ethanol spraying solution, when predominantly amorphous structures
= 1:1 were used. As morphology controlling agents, maleic are obtained. The addition of hydrophobic/hydrophilic co-
acid based copolymers (HFB/HFL) and/or acetylacetone were polymer in the precursor solution favours the obtaining of
added in spraying solutions. The absorptance (α) and emit- porous aggregates with large, open structures (3.5–4 µm).
tance (ε) values are those expected for alumina films, as pos- These porous morphologies are suitable for further infiltration
sible matrix in a cermet, used as a solar-thermal selective ab- of metallic particles, for preparation of alumina-based cer-
sorber. The films have amorphous–crystalline structure and mets.
© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Nowadays, alumina (Al O ) thin films spray pyrolysis deposition (SPD) [11, 13]. SPD can be
2
3
are considered materials of interest because of their excel- considered one of the simplest, lowest-cost and up-scalable
lent properties, such as high dielectric constant, high ther- techniques used for the deposition of nano-, mezzo- or mi-
mal conductivity, low permeability of alkali ions, high crostructured layers with controlled properties. Within this
hardness (Mohs 9) and high chemical and thermal stability. approach, SPD is a suitable method for the preparation of
Due to these properties, alumina thin films have a wide alumina-based cermets with properties strongly dependent
range of applications: microelectronic devices, refractory, on alumina layer properties, especially thickness and mor-
anticorrosive and antireflection coatings [1, 2], catalysis phology. These properties can be tailored by varying the
sensors [3], waveguide lasers [4], buffer layers for super- precursor composition (aluminium ion concentration, sol-
conductors [5] and ceramic–metal composites (cermets) vents and additives) and the deposition parameters (tem-
[6, 7]. More recently, the possible applications of alumina- perature, number of spraying sequences, etc.). In this work,
based composites (bioceramic materials) in bioengineering, we report the preparation and characterization of alumina
for the production of temporary or permanent implant de- thin films deposited by pneumatic spray pyrolysis onto
vices, were reported [8]. Thin films of alumina have been aluminium substrates, at 250 °C, from solutions of AlCl
3
prepared by various physical and chemical vapour deposi- (0.15 M) in water:ethanol (1:1) mixture. The influence of the
tion techniques, such as magnetron sputtering [9], filtered precursor solution composition, obtained by addition of dif-
vacuum arc [10], electron beam evaporation [11], metal ferent additives (acetylacetone and copolymers with maleic
organic chemical vapour deposition (MOCVD) [1] and anhydride), on the composition, surface morphology and op-
plasma-enhanced chemical vapour deposition (PECVD) tical properties (α and ε) of the alumina films required for
[12]. Amorphous or a mixture of amorphous and crystal- further preparation of alumina-based cermets (Al O /NiO /
2
3
x
line alumina thin films can be easily prepared by chemical Ni), used as solar-thermal selective absorbers, was studied.
© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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L. Isac et al.: Tailoring alumina thin film properties using hydrophilic/hydrophobic copolymer additives
2 Experimental
2.1 Reagents and substrates Solutions containing
AlCl ⋅ 6H O (95–99.8%, Scharlau Chemie) dissolved in a
3
2
mixture of water:ethanol = 1:1, in volumes, were used as
precursors. Deionized water (W) and absolute ethanol (Et,
99.9%, Scharlau Chemie) were used as solvents. In the
spraying solutions were added, as morphology controlling
agents (Ad), volumes (2.5 ml) of acetylacetone (AcAc,
99%, Sigma Aldrich), hydrophobic copolymer (sodium
maleate–methyl methacrylate, HFB) and hydrophilic co-
polymer (sodium maleate–vinyl acetate, HFL), prepared at
“Petru Poni” Institute, Iasi, Romania. The synthesis of the
two copolymers was reported elsewhere [14].
The precursor solutions with 0.15 M concentration
were sprayed onto preheated aluminium substrates (30 mm
× 15 mm × 0.5 mm, 99.5%, Beofon Bucuresti). Aluminium
was used as substrate for the alumina thin film deposition,
because of its good thermal conductivity, low cost and
good properties in a cermet-based solar absorber. The me-
tallic substrates (A) were ultrasonically cleaned with etha-
Figure 1 Reflectance UV–VIS spectra of alumina thin films
(samples A1–A6) deposited onto Al substrate (A) treated with
HNO 10% (A0).
3
where n is the refractive index of the alumina thin film
(n = 1.66 at λ = 300 nm [17]) and λ and λ are the wave-
1
2
length positions of two successive interference minima.
The solar absorptance (a) of alumina films was calculated
[18] from reflectance spectra recorded in the wavelength
range 290–1100 nm, Fig. 1, using a UV–VIS spectropho-
tometer. The thermal emittance (e) of as-deposited films
was determined by measuring the spectral reflectance in
the spectral range from 2500 nm to 16500 nm with a FTIR
spectrophotometer, model Spectrum BX. The reflectance
values in the wavelength 1100–2500 nm were estimated
by interpolation.
nol and then immersed in HNO 10% solution for 24 h
3
(A0) in order to increase the substrate surface roughness,
thus favouring the alumina film growth.
2.2 Deposition method and parameters The
alumina thin films were deposited onto aluminium sub-
strates by pneumatic spray pyrolysis, which has been pre-
viously described [15]. The deposition was performed in
open atmosphere at 250 °C (deposition temperature). The
nozzle–substrate distance (H = 20 cm), the pressure of car-
rier gas (air, p = 1.4 bar) and the number of spraying se-
quences (55) were kept constant during all deposition
processes. The breaks between two spraying pulses were
varied from 30 s to 60 s. All the deposition parameters
were previously optimized [16].
As observed from Table 1, the alumina thin film ab-
sorptance value increases from 0.607 (sample A4) to 0.889
(sample A5), almost following the average thickness: from
276.6 nm (sample A4) to 525.4 nm (sample A2). The
films’ emittance values are conveniently low (excepting
film A3), as is expected for a refractory material. The
further embedding of metal particles will improve both
optical properties.
2.3 Characterization methods The alumina thin
films were investigated by X-ray diffraction (XRD, Bru-
In order to evaluate the alumina thin film composi-
tion, X-ray diffraction patterns were recorded for samples
A1–A6 (Fig. 2). At the deposition conditions, no signifi-
cant differences can be observed, all the films containing a
mixture of amorphous and crystalline phases.
ker AXS D8 with Cu K radiation), Fourier transform in-
α
frared (FTIR) spectrophotometry (Spectrum BX Perkin
Elmer spectrophotometer, λ = 2500–16000 nm), UV–VIS
spectroscopy (Lambda 25 Perkin Elmer spectrophoto-
meter, λ = 190–1100 nm) and atomic force microscopy
(AFM/STM NTEGRA Probe Nanolaboratory micro-
scope).
Table 1 Influence of the precursor solution composition on
thickness (d) and optical properties (α, ε) of alumina thin films
prepared by pneumatic spray pyrolysis.
3 Results and discussion The thickness and optical
properties (solar absorptance, α, thermal transmittance, ε)
of alumina films deposited by pneumatic spray pyrolysis,
at 250 °C, from precursor solutions containing different
organic additives, are shown in Table 1. The average value
of the film thickness (d) was calculated from the interfer-
ence effects in UV-VIS transmittance spectra using the
relation [11]
*
d
sample precursor solution
α
ε
AlCl /W:Et:Ad
3
[nm]
(V)
A1
A2
1:1:0
354.3
525.4
0.787
0.881
0.011
0.035
10:10:1(AcAc)
A3
A4
A5
A6
*
10:10:1(HFB)
10:10:1(HFL)
292.2
276.6
0.841
0.607
0.889
0.857
0.153
0.041
0.058
0.057
10:10:1(AcAc):1(HFB) 488.2
10:10:1(AcAc):1(HFL) 510.1
λ λ
2
1
d =
,
(1)
2n(λ - λ )
2
d represents the average thickness of alumina thin films (samples).
1
© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Original
Paper
phys. stat. sol. (a) 205, No. 10 (2008)
2415
Figure 3 Reflection FTIR spectra of alumina thin films (samples
A1–A6) deposited onto Al substrate (A) treated with HNO 10%
3
Figure 2 XRD patterns of alumina thin films (samples A1–A6)
deposited onto Al substrate (A) treated with HNO 10% (A0).
3
(A0).
The predominant amorphous phase is alumina, con- all these spectra show two main absorption bands: 920–
–1
950 cm (attributed to amorphous Al O [10]) and 1500–
2 3
firmed by Refs. [12, 13] and FTIR spectra (Fig. 3).
–1
The polycrystalline phases identified in the diffracto- 1750 cm (due to carboxylate groups [12]). The broad ab-
–1
grams can be attributed to hexagonal (H O) Al O (PDF sorption band around 3000 cm (samples A1 and A6) can
3
2
22 34
01-070-1204), orthorhombic AlO(OH) (PDF 01-074-1895) be caused by Al–OH bonds associated through hydrogen
and β-Al O with hexagonal structure (PDF 00-010-0414). bonding [12]. The stretching vibrations of terminal Al=O
2
3
In addition, all the samples contain an amount of carbon and Al–O that appear in FTIR spectra of films A3
–1
–1
(graphite, PDF 01-074-2328) from the high carbon content (ν = 881 cm ) and A6 (ν = 650–881 cm ) can be attrib-
of the additives and/or solvent used in spraying solutions. uted to the film surface, which is passive at lower substrate
The films A1, A2, A5 and A6 have higher carbon content, temperatures (<400 °C) [11].
due to the presence of an extra sharp peak at 2θ = 7.9° (ab-
Depending on the spraying solution composition, po-
sent in the X-ray spectra of aluminium and treated alumin- rous surface morphology alumina films (Figs. 4 and 5),
ium substrates). The carbon content, varying from 11.4% with different pore size and distribution, are obtained by
(sample A5) to 49.2% (sample A1), results from thermal SPD, at 250 °C.
decomposition of the organic additives (AcAc and HFB/
The surface morphology of sample A2 is different and
HFL) and/or, in a lower extent, of ethanol in spraying solu- has the largest estimated open structure varying from the
tion. Considering the further applications, the graphite con- other three samples, due to its predominantly amorphous
tent does not represent a drawback since C has also a very (glassy) structure. The root mean square (RMS) surface
good heat storage capacity. It also may be responsible for roughness is 112.5 nm. The pore aggregates develop large,
the good emittance values, ε = 0.011–0.058.
open structures of 1.2–3.7 μm able to host metal particles
In addition, FTIR spectroscopy confirms and extends in the final cermet. The films A1, A5 and A6 surface mor-
the information obtained from X-ray diffraction analysis, phology shows a certain distribution of the pore aggregates,
related to the alumina films composition. Figure 3 shows from 3.5 μm (A6) to 4 μm (A5), for a RMS surface rough-
the reflection FTIR spectra of alumina thin films A1–A6 ness of 153.4 nm (A6) and 200.6 nm (A5). It is obvious
deposited onto a treated Al substrate (A0). Generally, that the addition of organic additives, such as AcAc and
Figure 4 (online colour at: www.pss-a.com) AFM images of alumina thin films deposited onto treated Al substrates from solutions
containing AlCl in W:Et = 1:1 (A1), AlCl in W:Et:AcAc = 10:10:1 (A2), AlCl in W:Et:AcAc:HFB = 10:10:1:1 (A5) and lCl
3
3
3
3
in W:Et:AcAc:HFL = 10:10:1:1 (A6).
www.pss-a.com
© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

a
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L. Isac et al.: Tailoring alumina thin film properties using hydrophilic/hydrophobic copolymer additives
HFB/HFL copolymers, in aqua:ethanolic precursor solu- and growth) from precursor solution containing HFL/HFB
tions strongly influences the surface morphology of alu- copolymers and/or AcAc as additives.
mina thin films. The hydrophilic polymer favours the de-
velopment of more regular structures, due to a stronger in-
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2
3
predominant amorphous phase. Depending on the additives
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