Sol-gel fabrication of compact, crack-free alumina film

Article abstract of DOI:10.1016/j.materresbull.2006.08.005

Compact, crack-free alumina film was fabricated using an alumina sol with a high Al₂O₃ content. With the addition of ethylacetoacetate (CH₃COCH₂COOC₂H₅, EAcAc), the stable sol could be prepared with a molar ratio of aluminum sec-butoxide (Al(O-sec-Bu)₃, ASB) to water up to 1:25. It was found that EAcAc could notably decrease the surface tension of the liquid in the gel pores. The EAcAc modification layer on the colloidal particle retarded greatly the densification of the Al₂O₃ gel film and provided a long-lasting structural relaxation during heating. Therefore, the formation of cracks was effectively prevented in this alumina material. The alumina gel film contained a high Al₂O₃ content and there was a rather small mass loss during sintering. The critical thickness of Al₂O₃ sol-gel film was eight times higher than that could be achieved via the general sol-gel route and a film thicker than 0.8 μm was prepared by a single-step dipping operation.

Full text of DOI:10.1016/j.materresbull.2006.08.005

Materials Research Bulletin 42 (2007) 600–608
www.elsevier.com/locate/matresbu
Sol–gel fabrication of compact, crack-free alumina film
a
a,
Chengbin Jing , Xiujian Zhao , Yongheng Zhang
b
*
a
College of Materials and Environmental Science, Qingdao University of Technology and Science,
3 Zheng-zhou Road, Qingdao 266042, China
5
b
Key Laboratory for Silicate Materials Science and Engineering of Ministry of Education,
Wuhan University of Technology, Wuhan, Hubei, China
Received 6 October 2005; received in revised form 30 July 2006; accepted 10 August 2006
Available online 11 September 2006
Abstract
Compact, crack-free alumina film was fabricated using an alumina sol with a high Al
2 3
O content. With the addition of
3
2
2 5
ethylacetoacetate (CH COCH COOC H , EAcAc), the stable sol could be prepared with a molar ratio of aluminum sec-butoxide
(
Al(O-sec-Bu) , ASB) to water up to 1:25. It was found that EAcAc could notably decrease the surface tension of the liquid in the
3
2 3
gel pores. The EAcAc modification layer on the colloidal particle retarded greatly the densification of the Al O gel film and
provided a long-lasting structural relaxation during heating. Therefore, the formation of cracks was effectively prevented in this
alumina material. The alumina gel film contained a high Al O content and there was a rather small mass loss during sintering. The
2
3
critical thickness of Al
thicker than 0.8 mm was prepared by a single-step dipping operation.
2006 Elsevier Ltd. All rights reserved.
2 3
O sol–gel film was eight times higher than that could be achieved via the general sol–gel route and a film
#
Keywords: A. Thin films; B. Chemical synthesis; B. Sol–gel chemistry; D. Microstructure; D. Surface properties
1
. Introduction
Alumina thin film has numerous electrical, optical and wear-resistant applications. Many techniques have been
applied to synthesize this material, including pulsed laser deposition, chemical vapor deposition (CVD) and sol–gel
method, etc. [1–5]. Among these, the sol–gel process provides a convenient means for coating a wide range of
substrates, including glass, plastics, metals and ceramics. Advantages of method include excellent homogeneity, low
annealing temperature and high purity of the resulting film [6–8].
In protective, optical and wear-resistant applications, the alumina film is required to be compact, crack-free and thick
enough. It was reported that when an excess of water is chosen as the hydrolysis medium of aluminum sec-butoxide (Al
(
thick film, it is necessary to increase the Al O content in the sol. However, a higher Al O content can accelerate the
O-sec-Bu) , ASB), theyieldedmaterialisexpectedtohavethesmallest averageporesizeandporosity[9–11]. Todeposit
3
2
3
2 3
nucleation process and the growth of the colloidal particles resulting in a sol with a rather short lifetime [12]. For this
reason, the molar ratio of aluminum alkoxide to water is usually kept at about 1:100 to obtain an acceptable sol lifetime
[11,13–16]. Due to the low Al O content in the sol, the produced film by a single dipping operation has a thickness
2 3
normally less than 0.1 mm. The limitation for producing the film with high thickness is that the cracks could be easily
*
Corresponding author. Tel.: +86 532 84022814; fax: +86 532 84022814.
E-mail address: jingchengbin@263.sina.com (C. Jing).
0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2006.08.005

C. Jing et al. / Materials Research Bulletin 42 (2007) 600–608
601
formed in the sol–gel film. The maximum film thickness achieved without cracking via a single-step sol–gel coating is
often called ‘‘critical thickness’’. The critical thickness of sol–gel ceramic film is often below 0.1 mm in many cases [17].
From the view point of industrial production, the small critical thickness is a serious drawback.
Recently, many efforts have been made to fabricate PZT thick films from a sol containing the polyvinylpyrrolidone
PVP).ItwasconfirmedthatPVPisaneffectiveagentforimprovingcriticalthickness[17].Forthepreparationofcompact,
(
crack-freeandthickaluminafilm,itisnecessarytoobtainastable,highAl O contentsolandsuppressthecrackformation
2
3
in the sol–gel film during heating. In our previous work, ethylacetoacetate (CH COCH COOC H , EAcAc) has
2 5
3
2
been examined to be able to stabilize greatly an alumina sol with an ASB/EAcAc molar ratio of 1:100. This sol was so
stable that it could be a commercial product on the market [18]. In this work, efforts were made to fabricate Al O film
2
3
from the alumina sol with an ASB/EAcAc molar ratio of 1:25 and investigate the effects of EAcAc on the surface tension
of the sol, the densification of gel materials during sintering and the critical thickness of the produced Al O film.
2
3
2
. Experimental
2
.1. Fabrication of alumina film samples
All chemicals were used as received without further purification. Aluminum sec-butoxide, ASB (98%) was
purchased from Chemat Technology, Inc. Ethylacetoacetate, EAcAc (99%) was obtained from Acros Organics
Company. Nitric acid (65%) was purchased from Shanghai Jiachuan Chemicals Co. Ltd.
Initially, deionized water was heated in a glass beaker until it reached 80 8C. ASB was added by syringe to the water
with continuous mixing. The molar ratio of ASB to water was 1:25. As hydrolysis took place, the temperature of the
solution rose to 91 8C, at which it was maintained for 3 min. Then, EAcAc was added to the mixture solution. The
molar ratio of EAcAc to water was fixed at 1:25. After stirring for 2 min, nitric acid was added into the solution step-
wise with stirring until it was transformed into a transparent solution. The pH value of the as-synthesized sol was about
3. Alumina sol with a molar ratio of ASB to water of 1:100 was also synthesized through the same process as described
above, but without EAcAc addition.
Borosilicate glass slides (softening point 1075 8C) were chosen as the substrates. Al O sol–gel film was deposited
2
3
onto the substrate via dip-coating method. The substrate was immersed into the sol reservoir and kept soaking for 30 s.
Then, the substrate was taken out from the solution. The coated substrate was dried in a clean cabinet at room
temperature for 2 days to produce the alumina gel film. Then, the supported gel films were subjected to thermal
treatment at 700 8C for 4 h in an oxygen atmosphere. The heating and cooling rate was 0.5 8C/min. The sintered films
were studied by using scanning electron microscopy. The green gel films after drying at 30 8C for 72 h were scrapped
off from the substrate for thermogravimetry and differential thermal analyses.
2
.2. Measurements
The viscosity of the sol samples was measured at room temperature using a Brookfield DV-II+ viscometer. Fourier
transform infrared spectroscopy (FTIR, Sadtler) was employed to determine the IR spectra of EAcAc and dried gel
samples. The thermal behaviors of alumina gel samples were studied by using the TG–DTA instrument (NETZSCH
STA 449C). The thickness and morphology of the film were determined by scanning electronic microscope (SEM,
JSM-5610LV).
3
. Results
3
.1. Dependence of sol viscosity on sol aging time
Fig. 1 shows the change in viscosity of the alumina sol with sol aging time. When the molar ratio of ASB to H O
2
was 1:25, it is impossible to obtain a stable alumina sol without EAcAc addition. This sol was found gelled
immediately and its viscosity increased rapidly to 2540 mPa s within 10 h. With the addition of EAcAc, the stable time
of this sol was enormously prolonged. For the sol prepared at an ASB/water molar ratio of 1:100 and without EAcAc
addition, its viscosity still increased more rapidly than that of the sol prepared at an ASB/water molar ratio of 1:25 and
with EAcAc addition. Therefore, EAcAc was effective on stabilizing the sol with a high Al O content.
2
3

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C. Jing et al. / Materials Research Bulletin 42 (2007) 600–608
Fig. 1. Changes in viscosity of alumina sols with sol aging time.
3
.2. IR spectra of alumina sol samples
Fig. 2 shows the IR transmission spectra of EAcAc and the alumina gel samples. The absorption peaks at 3594,
À
1
À
1
3445, 3437 and 3123 cm correspond to the –OH groups. The 2990, 1411, 1379, 1371, 1309 and 538 cm peaks due
to C–H bonds, 1160, 1168, and 798 cm peaks due to C–O bonds, 1042 and 932 cm peaks due to C–C bonds were
À1
À1
À1
observed [19–21]. The peaks at 617, 625 and 476 cm are associated with Al–O–Al bonds. The alumina gels with or
without EAcAc exhibited 1072 peaks due to Al–OH bonds [22]. As we know, EAcAc has the ketonic and enolic
À1
À1
structure forms. In the IR spectrum of EAcAc, there are two characteristic peaks at 1720 cm and 1650 cm which
can be assigned to the C O stretching vibrations of the ketonic and enolic forms, respectively [19,23]. It was found
À1
À1
that the intensity of the peak at 1720 cm was much stronger than that of the peak at 1650 cm . Therefore, EAcAc
was considered to be mainly in the ketonic form. However, when EAcAc was added to the alumina sol, the peak at
À1 À1
1
720 cm disappeared almost completely while the peak at 1650 cm was also weakened. In contrast, two new
À1
À1
sharp peaks located at 1624 cm and 1530 cm appeared. These two sharp peaks could not be observed in the IR
spectrum of the alumina sol without EAcAc. It has been reported that EAcAc can chelate directly with some metal
alkoxide molecules in an organic medium and the formed chelate complex may cause the red-shifts of IR absorption
peaks of EAcAc [24–26]. Here we considered that EAcAc reacted most likely with the surface HO–Al-groups on the
*
Al O colloidal particle instead of with ASB molecules to form the chelate complex. With the p–p transition of this
2
3
Fig. 2. IR spectra of ethylacetoacetate and alumina gels produced with or without ethylacetoacetate.

C. Jing et al. / Materials Research Bulletin 42 (2007) 600–608
603
Fig. 3. Thermogravimetry (TG) and differential thermal analyses (DTA) of alumina gel with ethylacetoacetate.
complex the energy of the conjugate double bonds in the EAcAc molecule could be decreased to some degree.
Accordingly, the IR absorption peaks of EAcAc underwent red-shifts resulting in the two new peaks because of the
interaction between EAcAc and alumina sol.
3
.3. TG–DTA analysis of alumina gel samples
Fig. 3 indicates the thermogravimetry (TG) and differential thermal analysis (DTA) of alumina gel film sample
derived from the sol with EAcAc. The weight loss of the gel sample was observed to be in the temperature range from
2
5 to 700 8C. A big endothermic peak at 135 8C and a small endothermic peak at 180 8C were observed in the DTA
curve which can be assigned to the evaporation of the solvent and the free EAcAc in the gel, respectively. There were
two big exothermic peaks at 294 and 436 8C which were caused by the decomposition of the organic groups in the gel.
Therefore, the weight loss occurred below 200 8C was mainly caused by the evaporation of the solvent and the free
EAcAc while the weight loss occurred within the temperature range from 200 to 700 8C was mainly caused by the
decomposition of the organic groups in the gel. There was a small exothermic peak at around 970 8C due to the
crystallization process of alumina. The notable exothermic effect caused by the crystallization process and a 35.2% of
total mass change implied that this gel sample indeed had a higher Al O content.
2
3
Fig. 4 indicates the TG–DTA curves of the alumina gel film sample derived from the sol synthesized via the general
process (without EAcAc). It can be seen in Fig. 4 that the total weight loss reached 92.2% and most of the mass change
Fig. 4. Thermogravimetry (TG) and differential thermal analysis (DTA) of alumina gel without ethylacetoacetate.

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C. Jing et al. / Materials Research Bulletin 42 (2007) 600–608
was caused by the desorption of the adsorbed moisture and the evaporation of the solvent at 137 8C. There was no more
mass change in TG curve when the temperature was over 500 8C. So, the decomposition of the organic groups almost
finished before 500 8C indicating that the organic compounds were burnout within a relatively narrow heating
temperature range. Within the temperature range from 500 to 1200 8C, there was no thermal effect originated from the
crystallization process of alumina observed because of small amount of Al O derived.
2
3
3
.4. Morphology and thickness of heat-treated films
Fig. 5 shows the SEM photographs of the surface (a) and cross-section (b) morphologies of a single-layer film
obtained from the alumina sol with EAcAc. The film was heated at 700 8C in an oxygen ambient for 4 h in order to
remove completely the organic groups in the film. It can be seen from Fig. 5 that the film was smooth, compact and
crack-free, and the thickness was about 0.8 mm. Fig. 6 gives the SEM photographs of the surface (a) and cross-section
(
b) morphologies of a seven-layer film prepared from the alumina sol with an ASB/water molar ratio of 1:100. It can be
2 3
Fig. 5. SEM micrographs of a single-layer Al O film derived from the sol containing ethylacetoacetate (a) surface morphology and (b) cross-
section morphology.

C. Jing et al. / Materials Research Bulletin 42 (2007) 600–608
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2 3
Fig. 6. SEM micrographs of a seven-layer Al O film derived from the sol without ethylacetoacetate (a) surface morphology and (b) cross-section
morphology.
seen that the film thickness was only about 0.7 mm. That means the thickness achieved by a single-step dipping
deposition from this sol was less than 0.1 mm. In addition, a lot of bumps and small pores were observed in this film.
Therefore, the addition of EAcAc contributed not only to enhancing the critical thickness but also to improving the
quality of the alumina sol–gel film.
4
. Discussion
4
.1. Effect of interaction between EAcAc and Al O particles on the sol stability
2
3
The reaction between EAcAc and the alumina sol played a key role in the synthesis of stable high Al O content sol.
2
3
With the addition of EAcAc, the IR absorption peaks of EAcAc underwent red-shifts. These peak shifts were most
likely caused by the formation of the chelate bonds in the sol. Tohge et al. have disclosed that EAcAc can chelate

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2 3
Fig. 7. Schematic diagram of the interaction between EAcAc and Al O particles in the gel film.
directly with some metal alkoxide molecules in an organic medium and the formed chelate complex may cause the
red-shifts of IR absorption peaks of EAcAc [24,26]. However, it cannot be considered in this study that the chelating
reaction was the same as occurred in the organic medium. In this work, water was used as the solvent, but ASB was
extremely moisture sensitive. So ASB was observed undergoing sufficient hydrolysis and producing a large number of
Al O precipitates in the water medium before nitric acid addition. The Al O precipitates were then peptized into
2
3
2 3
Al O colloidal particles by nitric acid, so we considered that EAcAc reacted most likely with the surface HO–Al-
2
3
groups on the Al O colloidal particle instead of with ASB molecules. Because EAcAc was added after the ASB had
2
3
sufficiently gone through hydrolysis, EAcAc molecules had few opportunities to chelate directly with the ASB
molecules. As a result, a chelate complex ring structure was formed as shown in Fig. 7. So, the active HO–Al-groups
on the colloidal particles were sufficiently capped and the growth of the colloidal particle was effectively prevented
leading to a very stable sol.
4
.2. EAcAc as effective agent for improving the critical thickness
Generally, only the solid particles in a wet sol–gel film contribute to the thickness accumulation of a solid film
material. So, higher oxide content in a sol is expected to yield a thicker solid film. With the addition of EAcAc, the
molar ratio of ASB to water in the sol was increased from 1:100 to 1:25. Therefore, the volume shrinkage of the film
was reduced.
EAcAc contributed largely to suppressing the crack formation in the gel film. It is widely believed that the
cracks in sol–gel materials are mainly caused by the effect of capillary pressure. According to the Laplace’s law,
the liquid inside the pores exerts a stress on the ‘‘walls’’ of the capillaries during drying which is inversely
proportional to the pore diameter, but proportional to the surface tension of the liquid [27]. Therefore, the liquid
with a lower surface tension inside a bigger pore will produce a smaller stress on the ‘‘walls’’ of the capillaries
during drying.
In this work, we examined that the surface tension of the alumina sol obtained without EAcAc addition was
8.3 mN/m. When EAcAc was added to the alumina sol with an ASB/water molar ratio of 1:25, the surface
7
tension of this sol was only 41.7 mN/m. So, EAcAc could notably decrease the stress on the ‘‘walls’’ of the
capillaries.
With the removal of solvent and organic complexes in the gel, the colloidal particles will get closer to form gel
pores. In the alumina gel not containing EAcAc, there were numerous HO–Al-groups on the Al O particle, which
was favorable for Al O particles to combine each other through further dehydration. In addition, the
2
3
2
3
decomposition of the organic groups in this gel went on rapidly, so the densification of this gel progressed quickly
and the very small gel pores could be formed at the early period of heating. At this time, a lot of liquid still
remained in the gel pores and high capillary stress was exerted on the ‘‘walls’’ of gel pores. However, the gel

C. Jing et al. / Materials Research Bulletin 42 (2007) 600–608
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material was not sufficiently sintered yet and was still quite fragile. So the cracking would occur most likely in the
gel. For this reason, the critical thickness of the Al O film prepared from the sol not containing EAcAc was
2
3
found to be smaller than 0.1 mm.
With addition of EAcAc, the active HO–Al-groups on the surface of the Al O colloidal particles were sufficiently
capped by EAcAc that was progressively decomposed in a wide temperature range. Hence, the Al O particles could
2
3
2
3
not be combined together rapidly to form very small gel pores during sintering. So, the densification of the gel film
containing EAcAc was greatly retarded and the high capillary stress on the ‘‘walls’’ of the small gel pores was avoided
during the early period of heating.
The boiling point of EAcAc is about 180 8C. But, there was only a small endothermic peak at 180 8C in the DTA
curve of the gel film due to the EAcAc evaporation, so most of the added EAcAc were involved in the chelating
reaction with the Al O colloidal particles. Because the chelated EAcAc was decomposed within a wide temperature
2
3
range from 200 to 700 8C, this would provide a long-lasting structural relaxation. According to the researches made by
Kozuka and Milne, the structural relaxation contributed largely to preventing the gel film from cracking [17,28]. Due
to the addition of EAcAc, the critical thickness of the gel film was increased to 0.8 mm, which was eight times higher
than that could be achieved from the sol without EAcAc addition.
5
. Conclusion
With the addition of EAcAc, a stable high Al O content sol could be produced. EAcAc chelated with the surface
3
2
HO–Al-groups of the Al O particle and formed a surface modification layer on the colloidal particle. This surface
2
3
layer could prevent the colloidal particle from growing in the sol and decrease the surface tension of the sol. Because of
the chelation complex formation between EAcAc and colloidal particles, the decomposition temperature range of
EAcAc was widened. This would provide a long-lasting structural relaxation and cause a slow densification process of
the gel film. As a result, a compact, crack-free film with a thickness up to 0.8 mm was obtained by a single dip-coating
operation.
Acknowledgements
This work was partly financially supported by the Foundation of Key Teachers of the Ministry of Education, and by
the Doctoral Fund of Qingdao University of Technology & Science, China.
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