# Formation and Characterization ofTiO2/CNT Nanomaterials Dried under Supergravity Conditions

Authors: Minerva Vargas; Marina E. Rincón; Eduardo Ramos
Journal: Journal of Nanomaterials
 (2009)

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## Abstract

The elaboration of bilayerTiO2/CNT films dried under terrestrial gravity conditions (g) and on a centrifuge with 1.3g and 7g is reported. The changes in microstructure and thickness of these coatings under supergravity environment cause a red-shift tendency in the optical properties at increasing values of acceleration. Experiments of a drop under enhanced gravity force in the range of 3.7 < Bo (bond number) < 51.5 suggest the incomplete elimination of surfactant-water molecules in the TiO2/CNT bilayer film. Increasing acceleration up to 14g will widen the optical differences found, proving the layer-by-layer solution-chemical method in combination with these drying protocols, an alternative to produce thickness-sensitive solar-selective absorbing coatings.


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## Body

## 1. Introduction

Materials processing in centrifuges have two
interesting motivations:
first, opportunities for production of unique and improved materials
that cannot be prepared under normal earth conditions or in space; second, such research
improves our understanding of the influence of acceleration and thermophysical
mechanisms on materials processing. Research on processing materials in high
gravity has been increased in
the last twenty years due to the fact that even a modest increase in
acceleration gives dramatic effects on the physical properties of the materials
[1].
Few examples are the improved compositional homogeneity of
semiconductors [2, 3]: the increased nucleation rates, growth rates, and
coverage area for diamond film deposition [4, 5]; the improved
quality of protein and high-energy crystals [6]; the large-scale production of
nanoparticles [7, 8].On the other hand,
mesoporous materials based on carbon nanotubes (CNTs) are of great interest
because they possess many of the unique electronic and mechanical properties of
the carbon nanotubes [9]. In these complex materials, dispersion and pore
penetration are the qualities affecting conductivity, optical properties, and
strength. Recently, bilayer and composites systems based on TiO2/CNT
are being investigated as sensors [10], photocatalysts [11], and spectral
selective absorbing films [12]. In this work, we
use stainless steel substrates coated with sol-gel mesoporous titania, followed
by a cast layer of carbon nanotube-surfactant film to create a tandem system consisting of a
reflector/porous matrix/absorbing layer. The optical properties of this tandem
system were reported to be sensitive to the morphology resulting from the
annealing process [12]. Here, instead of thermally drying the samples, we
proposed drying under terrestrial gravity conditions (g) and also under  acceleration in the range of 1g to 7g. Moreover, to better understand
the drying process, images of a drop under enhanced gravity force in the range
of 3.7  < Bo (bond number)  < 51.5 will be presented and discussed in the
context of the TiO2/CNT results.


## 2. Experimental

The experimental methodology
consists of three parts: sample preparation, drying, and characterization.


### 2.1. Sample Preparation

#### 2.1.1. MesoporousTiO2 Film

This is done on 2 cm  × 8 cm mirror-polished stainless
steel substrates by sequential immersion/drying cycles of the substrates in a
2-propanol solution of titanium
tetraisopropoxide and polyethylene
glycol (PEG, Mn:400). The approximate concentration ofTiO2 in the
solution is 20 mg/mL. A complete description of this procedure can be found
elsewhere [12]. After TiO2 deposition,
the coated substrates are air annealed at 450°C for 15 minutes.


#### 2.1.2. Deposition of Carbon Nanotubes

The carbon nanotubes were
provided by Professor Gilles Flamant from Processes, Materials and Solar Energy
Laboratory (PROMES-CNRS), France. They are nanotube-rich soot that has been elaborated in a 50 kW solar reactor
operating at the focus of the solar furnace at CNRS, Odeillo, France. A suspension made of 3 g
Triton X-100, 3 g PEG, and 0.05 g of carbon nanotubes was prepared (Scheme1).
The suspension was sonified for 90 minutes, and then 30 mL of 2-propanol were
added and centrifuged again for 30 minutes. The supernatant liquid was
discarded and the nanotubes redissolved in water and centrifuged again. In the
third centrifugation, the nanotubes could no longer be precipitated, and the ink was stable
for weeks. A small volume of the carbon nanotubes emulsion
(0.05 mL) was cast and let dry for 24 hours on the TiO2 coated substrates.Scheme 1
Interactions between PEG/Triton and
carbon nanotubes-rich soot: (a) covalent bonding through the functionality of
the carbon surface; (b) noncovalent wrapping of the carbon surface.


### 2.2. Drying Process

#### 2.2.1.TiO2/CNT Films

Two drying protocols were
designed: in the first, witness samples
were dried under the influence of terrestrial gravity, lying flat on a
horizontal plane; in the second protocol, wet samples
(2 hours after deposition) were mounted on a
centrifuge and spun during 50 minutes at rates of 22 rpm and 60 rpm. The distance
of the samples to the centrifuge rotation axis was 170 cm which amounted to a
centrifugal acceleration of 1.2g (samples spun at 22 rpm) and 6.8g (samples spun at 60 rpm). The
magnitudes of the total acceleration a and gravity plus centrifugal
force were 1.3g (at 22 rpm) and 6.9g (at 60 rpm). The
samples were mounted at one end of one arm of the centrifuge such that they
were always perpendicular with respect to the total acceleration. This is shown
schematically in Figure 1. A more detailed description of the centrifuge can be found elsewhere [13].(a) Centrifuge used for drying the samples under supergravity
conditions. (b) Articulated boat holding the substrates.g: Gravity acceleration; ω2R: Centrifugal acceleration; ω: Angular velocity; R: Distance from the sample
to the rotation axis; a: Total acceleration.
(a)(b)


#### 2.2.2. Drop Behavior in a Centrifuge

In order to assess the gravity force effects
on the shape of a static drop sitting on a solid surface, experiments were
performed in the range of 3.7  < Bo (bond number)  < 25.7. Here,Bo=aρd2/γ, where a is the total acceleration, ρ is the fluid density, d is the drop footprint diameter, which
for our experiment was 5.2 mm,
and γ is the
fluid surface tension. The bond number is a relation between gravity and
surface tension effects. Another important parameter is the capillary length κ−1=(γ/aρ)1/2, which gives information about
the drop shape. Images were captured by an
experimental arrangement integrated by a video camera (Sony CCD-TRV87), a beam-splitter
cube, and two low-intensity 20 W lamps to avoid evaporation. The changes on drop
height and area when the water drop was subjected to centrifugation were analyzed
in the interval of 1g<a<14g. Image views of the side and the
bottom of the drop were captured (see Figure 2), digitized, and processed by a
Visual Builder V2.0 from (National Instruments, Austin, Tex, USA).Figure 2
Sketch of the side
and bottom views of a static drop captured by a video camera.


### 2.3. Films Characterization

Film thickness was measured
with an alpha-step system.
The microstructure was obtained by scanning electron microscopy (JEOL model JMS-5600LV) and the optical properties with a Shimadzu
UV-3101PC spectrophotometer equipped with an integration sphere. An aluminum mirror was used as a reference
mirror for the measurements of specular reflectance, while barium sulfate was
used as a reference coating for the measurements of diffuse reflectance.


## 2.1. Sample Preparation

### 2.1.1. MesoporousTiO2 Film

This is done on 2 cm  × 8 cm mirror-polished stainless
steel substrates by sequential immersion/drying cycles of the substrates in a
2-propanol solution of titanium
tetraisopropoxide and polyethylene
glycol (PEG, Mn:400). The approximate concentration ofTiO2 in the
solution is 20 mg/mL. A complete description of this procedure can be found
elsewhere [12]. After TiO2 deposition,
the coated substrates are air annealed at 450°C for 15 minutes.


### 2.1.2. Deposition of Carbon Nanotubes

The carbon nanotubes were
provided by Professor Gilles Flamant from Processes, Materials and Solar Energy
Laboratory (PROMES-CNRS), France. They are nanotube-rich soot that has been elaborated in a 50 kW solar reactor
operating at the focus of the solar furnace at CNRS, Odeillo, France. A suspension made of 3 g
Triton X-100, 3 g PEG, and 0.05 g of carbon nanotubes was prepared (Scheme1).
The suspension was sonified for 90 minutes, and then 30 mL of 2-propanol were
added and centrifuged again for 30 minutes. The supernatant liquid was
discarded and the nanotubes redissolved in water and centrifuged again. In the
third centrifugation, the nanotubes could no longer be precipitated, and the ink was stable
for weeks. A small volume of the carbon nanotubes emulsion
(0.05 mL) was cast and let dry for 24 hours on the TiO2 coated substrates.Scheme 1
Interactions between PEG/Triton and
carbon nanotubes-rich soot: (a) covalent bonding through the functionality of
the carbon surface; (b) noncovalent wrapping of the carbon surface.


## 2.1.1. MesoporousTiO2 Film

This is done on 2 cm  × 8 cm mirror-polished stainless
steel substrates by sequential immersion/drying cycles of the substrates in a
2-propanol solution of titanium
tetraisopropoxide and polyethylene
glycol (PEG, Mn:400). The approximate concentration ofTiO2 in the
solution is 20 mg/mL. A complete description of this procedure can be found
elsewhere [12]. After TiO2 deposition,
the coated substrates are air annealed at 450°C for 15 minutes.


## 2.1.2. Deposition of Carbon Nanotubes

The carbon nanotubes were
provided by Professor Gilles Flamant from Processes, Materials and Solar Energy
Laboratory (PROMES-CNRS), France. They are nanotube-rich soot that has been elaborated in a 50 kW solar reactor
operating at the focus of the solar furnace at CNRS, Odeillo, France. A suspension made of 3 g
Triton X-100, 3 g PEG, and 0.05 g of carbon nanotubes was prepared (Scheme1).
The suspension was sonified for 90 minutes, and then 30 mL of 2-propanol were
added and centrifuged again for 30 minutes. The supernatant liquid was
discarded and the nanotubes redissolved in water and centrifuged again. In the
third centrifugation, the nanotubes could no longer be precipitated, and the ink was stable
for weeks. A small volume of the carbon nanotubes emulsion
(0.05 mL) was cast and let dry for 24 hours on the TiO2 coated substrates.Scheme 1
Interactions between PEG/Triton and
carbon nanotubes-rich soot: (a) covalent bonding through the functionality of
the carbon surface; (b) noncovalent wrapping of the carbon surface.


## 2.2. Drying Process

### 2.2.1.TiO2/CNT Films

Two drying protocols were
designed: in the first, witness samples
were dried under the influence of terrestrial gravity, lying flat on a
horizontal plane; in the second protocol, wet samples
(2 hours after deposition) were mounted on a
centrifuge and spun during 50 minutes at rates of 22 rpm and 60 rpm. The distance
of the samples to the centrifuge rotation axis was 170 cm which amounted to a
centrifugal acceleration of 1.2g (samples spun at 22 rpm) and 6.8g (samples spun at 60 rpm). The
magnitudes of the total acceleration a and gravity plus centrifugal
force were 1.3g (at 22 rpm) and 6.9g (at 60 rpm). The
samples were mounted at one end of one arm of the centrifuge such that they
were always perpendicular with respect to the total acceleration. This is shown
schematically in Figure 1. A more detailed description of the centrifuge can be found elsewhere [13].(a) Centrifuge used for drying the samples under supergravity
conditions. (b) Articulated boat holding the substrates.g: Gravity acceleration; ω2R: Centrifugal acceleration; ω: Angular velocity; R: Distance from the sample
to the rotation axis; a: Total acceleration.
(a)(b)


### 2.2.2. Drop Behavior in a Centrifuge

In order to assess the gravity force effects
on the shape of a static drop sitting on a solid surface, experiments were
performed in the range of 3.7  < Bo (bond number)  < 25.7. Here,Bo=aρd2/γ, where a is the total acceleration, ρ is the fluid density, d is the drop footprint diameter, which
for our experiment was 5.2 mm,
and γ is the
fluid surface tension. The bond number is a relation between gravity and
surface tension effects. Another important parameter is the capillary length κ−1=(γ/aρ)1/2, which gives information about
the drop shape. Images were captured by an
experimental arrangement integrated by a video camera (Sony CCD-TRV87), a beam-splitter
cube, and two low-intensity 20 W lamps to avoid evaporation. The changes on drop
height and area when the water drop was subjected to centrifugation were analyzed
in the interval of 1g<a<14g. Image views of the side and the
bottom of the drop were captured (see Figure 2), digitized, and processed by a
Visual Builder V2.0 from (National Instruments, Austin, Tex, USA).Figure 2
Sketch of the side
and bottom views of a static drop captured by a video camera.


## 2.2.1.TiO2/CNT Films

Two drying protocols were
designed: in the first, witness samples
were dried under the influence of terrestrial gravity, lying flat on a
horizontal plane; in the second protocol, wet samples
(2 hours after deposition) were mounted on a
centrifuge and spun during 50 minutes at rates of 22 rpm and 60 rpm. The distance
of the samples to the centrifuge rotation axis was 170 cm which amounted to a
centrifugal acceleration of 1.2g (samples spun at 22 rpm) and 6.8g (samples spun at 60 rpm). The
magnitudes of the total acceleration a and gravity plus centrifugal
force were 1.3g (at 22 rpm) and 6.9g (at 60 rpm). The
samples were mounted at one end of one arm of the centrifuge such that they
were always perpendicular with respect to the total acceleration. This is shown
schematically in Figure 1. A more detailed description of the centrifuge can be found elsewhere [13].(a) Centrifuge used for drying the samples under supergravity
conditions. (b) Articulated boat holding the substrates.g: Gravity acceleration; ω2R: Centrifugal acceleration; ω: Angular velocity; R: Distance from the sample
to the rotation axis; a: Total acceleration.
(a)(b)


## 2.2.2. Drop Behavior in a Centrifuge

In order to assess the gravity force effects
on the shape of a static drop sitting on a solid surface, experiments were
performed in the range of 3.7  < Bo (bond number)  < 25.7. Here,Bo=aρd2/γ, where a is the total acceleration, ρ is the fluid density, d is the drop footprint diameter, which
for our experiment was 5.2 mm,
and γ is the
fluid surface tension. The bond number is a relation between gravity and
surface tension effects. Another important parameter is the capillary length κ−1=(γ/aρ)1/2, which gives information about
the drop shape. Images were captured by an
experimental arrangement integrated by a video camera (Sony CCD-TRV87), a beam-splitter
cube, and two low-intensity 20 W lamps to avoid evaporation. The changes on drop
height and area when the water drop was subjected to centrifugation were analyzed
in the interval of 1g<a<14g. Image views of the side and the
bottom of the drop were captured (see Figure 2), digitized, and processed by a
Visual Builder V2.0 from (National Instruments, Austin, Tex, USA).Figure 2
Sketch of the side
and bottom views of a static drop captured by a video camera.


## 2.3. Films Characterization

Film thickness was measured
with an alpha-step system.
The microstructure was obtained by scanning electron microscopy (JEOL model JMS-5600LV) and the optical properties with a Shimadzu
UV-3101PC spectrophotometer equipped with an integration sphere. An aluminum mirror was used as a reference
mirror for the measurements of specular reflectance, while barium sulfate was
used as a reference coating for the measurements of diffuse reflectance.


## 3. Results and Discussion

### 3.1.TiO2/CNTS-PEG400 Coatings

Figure3 shows the film
thickness of TiO2/CNTS-PEG400 coatings after centrifugation.
Measurements were taken along the longitudinal and transversal axes at the positions
indicated in the left side of the figure. The samples were rested for two hours
at ambient conditions prior to the mounting in the centrifuge where they were
spun for 50 minutes at 60 rpm. From the figure, it is clear that a slight mass enrichment
is found at position 3, although in general for this drying protocol the films
have uniform thickness with the values of the longitudinal axis following
closely the ones found for the transversal axis. Samples dried at 1g for
more than 24 hours had an average thickness value of 1.68 μm, which is almost 50% higher than the average value of Figure 3. It is interesting to notice that
although the spun coatings look dried, they are approximately twice as thick as
those reported in previous work, where the films were thermally dried [12]. Therefore,
it is reasonable to assume that although an important amount of PEG400 was
removed at 7g, some remains inside the porous films. Apparently, the interaction
between CNTs and the surfactants can be broken during annealing but not necessarily under centrifugation
at 7g.Figure 3
Thickness of films dried ata  = 1g (triangles) and a  = 7g (squares). Positions (a–d) selected along
the longitudinal axis correspond to (1–4) (solid
symbol). Positions (e–h) along the
transversal axis correspond to (1–4) (empty
symbol).The microstructure of the
tandem system (stainless steel/mesoporousTiO2/CNTS-PEG400) was
obtained by scanning electron microscopy at various magnifications. Figure 4 shows images of films dried at ambient conditions (Figure 4(a)) and at 60 rpm (Figure 4(b)). Indicating that, under rotation, there is redistribution of the mobile phase made up by carbon nanotubes soot and PEG400/Triton X-100; the films are still wet, and the features of the carbon nanotubes soot are not resolvable. Additionally, the comparison of SEM images (not shown) of
films dried under different angular velocities but similar rotation time shows
the TiO2 layer as a
background collection of nanoparticles, while the appearance of the CNTS-PEG400
layer depends on the angular velocity.SEM images of mesoporousTiO2/CNTS-PEG400 tandem films: (a) dried at ambient conditions
and (b) at 60 rpm.
(a)(b)Figure5 shows the
reflectance of the stainless steel mesoporous TiO2/CNTS-PEG400
system as a function of light wavelength in the interval
of 0.25 μm to 22 μm. The curves
correspond to total reflectance in the UV-visible-near infrared region. The total reflectance of the samples centrifuged either
at 22 rpm or 60 rpm shows good reproducibility
(empty and solid symbols). Interestingly, the differences due to the drying
protocol, terrestrial or supergravity, are clearly observed at 60 rpm but not at 22 rpm angular
velocity. The reflectance for tandem systems dried under terrestrial
gravity for 24 hours is also shown in the figure (red curve). It resembles the
reflectance curve of the coatings spun at 22 rpm. In general, all the systems dried
at 60 rpm give lower values of reflectance than those spun at 22 rpm, in
agreement with a thinner and more uniform distribution of the absorbing layer.
Although these films still require optimization, our results indicate that
larger angular velocities (14g) and/or superior centrifugation time can yield
an even thinner and well-dispersed pigment, increasing the absorbance of the
films in the near infrared region. Nevertheless, the present results are in the
range of solution-chemically derived spectrally selective absorbers with more
complex structures [14].Figure 5
Spectrally selective stainless steel/TiO2/CNTS-PEG400
system as a function of the angular velocity (22 rpm black symbols and 60 rpm
blue symbols). Empty symbols correspond to duplicate experiments. The solid red
line represents the system dried under terrestrial
gravity on a 5° tilted plane. The black line corresponds to the reflectance
curve of stainless steel/mesoporus TiO2.


### 3.2. Drop Behavior in a Centrifuge

To better understand the
drying process, images of a water drop under enhanced gravity force in the
range of 3.7  < Bo  < 51.5 are presented in Figure6. The
sequence describes the drop under supergravity environments, from 1g to 14g. In these experiments, the initial drop volume was 20 μL, the bottom
area 21 mm2, and the height 1.5 mm. From the
figure, it is clear that changes occur even at 1.7g. The normalized height
of the drop (h/ho, where ho is the height of the drop at 1g) decreased with increasing
acceleration as shown by the relation plotted in Figure 7.Water drop at different total accelerations: (a) 1.0g, (b) 1.7g, (c) 3.7g, (d) 6.9g, (e) 10.4g, and (f) 14g.
(a)(b)(c)(d)(e)(f)Figure 7
Normalized height of
the drop as a function of total acceleration. The error bars correspond to the instrument uncertainty.The theory for large drops predictsh~a−0.5 [15, 16], but the trend
indicated by the points in Figure 7 indicates that the height drops faster when
the total acceleration increases from 1g to 14g. Although the
discrepancy can be attributed to the changes in the contact angle and
evaporation, a full analysis is outside the scope of the present work, particularly
because the mechanisms that govern the hydrodynamics controlling wetting or
evolution of drop into films are still not fully understood. Nevertheless, the
shape of the drop follows the condition expected for drops with flattened tops
(d>κ−1) at all a, assuring the formation of thin
films.The results presented in Figures6 and 7 agree with the observation that TiO2/CNTS-PEG400
coatings are still thick at 7g but can be substantially thin out at 14g. In this context, we conclude that thickness-sensitive solar-selective
absorbing coatings can be prepared under supergravity environment in the range
of 1g to 14g.


## 3.1.TiO2/CNTS-PEG400 Coatings

Figure3 shows the film
thickness of TiO2/CNTS-PEG400 coatings after centrifugation.
Measurements were taken along the longitudinal and transversal axes at the positions
indicated in the left side of the figure. The samples were rested for two hours
at ambient conditions prior to the mounting in the centrifuge where they were
spun for 50 minutes at 60 rpm. From the figure, it is clear that a slight mass enrichment
is found at position 3, although in general for this drying protocol the films
have uniform thickness with the values of the longitudinal axis following
closely the ones found for the transversal axis. Samples dried at 1g for
more than 24 hours had an average thickness value of 1.68 μm, which is almost 50% higher than the average value of Figure 3. It is interesting to notice that
although the spun coatings look dried, they are approximately twice as thick as
those reported in previous work, where the films were thermally dried [12]. Therefore,
it is reasonable to assume that although an important amount of PEG400 was
removed at 7g, some remains inside the porous films. Apparently, the interaction
between CNTs and the surfactants can be broken during annealing but not necessarily under centrifugation
at 7g.Figure 3
Thickness of films dried ata  = 1g (triangles) and a  = 7g (squares). Positions (a–d) selected along
the longitudinal axis correspond to (1–4) (solid
symbol). Positions (e–h) along the
transversal axis correspond to (1–4) (empty
symbol).The microstructure of the
tandem system (stainless steel/mesoporousTiO2/CNTS-PEG400) was
obtained by scanning electron microscopy at various magnifications. Figure 4 shows images of films dried at ambient conditions (Figure 4(a)) and at 60 rpm (Figure 4(b)). Indicating that, under rotation, there is redistribution of the mobile phase made up by carbon nanotubes soot and PEG400/Triton X-100; the films are still wet, and the features of the carbon nanotubes soot are not resolvable. Additionally, the comparison of SEM images (not shown) of
films dried under different angular velocities but similar rotation time shows
the TiO2 layer as a
background collection of nanoparticles, while the appearance of the CNTS-PEG400
layer depends on the angular velocity.SEM images of mesoporousTiO2/CNTS-PEG400 tandem films: (a) dried at ambient conditions
and (b) at 60 rpm.
(a)(b)Figure5 shows the
reflectance of the stainless steel mesoporous TiO2/CNTS-PEG400
system as a function of light wavelength in the interval
of 0.25 μm to 22 μm. The curves
correspond to total reflectance in the UV-visible-near infrared region. The total reflectance of the samples centrifuged either
at 22 rpm or 60 rpm shows good reproducibility
(empty and solid symbols). Interestingly, the differences due to the drying
protocol, terrestrial or supergravity, are clearly observed at 60 rpm but not at 22 rpm angular
velocity. The reflectance for tandem systems dried under terrestrial
gravity for 24 hours is also shown in the figure (red curve). It resembles the
reflectance curve of the coatings spun at 22 rpm. In general, all the systems dried
at 60 rpm give lower values of reflectance than those spun at 22 rpm, in
agreement with a thinner and more uniform distribution of the absorbing layer.
Although these films still require optimization, our results indicate that
larger angular velocities (14g) and/or superior centrifugation time can yield
an even thinner and well-dispersed pigment, increasing the absorbance of the
films in the near infrared region. Nevertheless, the present results are in the
range of solution-chemically derived spectrally selective absorbers with more
complex structures [14].Figure 5
Spectrally selective stainless steel/TiO2/CNTS-PEG400
system as a function of the angular velocity (22 rpm black symbols and 60 rpm
blue symbols). Empty symbols correspond to duplicate experiments. The solid red
line represents the system dried under terrestrial
gravity on a 5° tilted plane. The black line corresponds to the reflectance
curve of stainless steel/mesoporus TiO2.


## 3.2. Drop Behavior in a Centrifuge

To better understand the
drying process, images of a water drop under enhanced gravity force in the
range of 3.7  < Bo  < 51.5 are presented in Figure6. The
sequence describes the drop under supergravity environments, from 1g to 14g. In these experiments, the initial drop volume was 20 μL, the bottom
area 21 mm2, and the height 1.5 mm. From the
figure, it is clear that changes occur even at 1.7g. The normalized height
of the drop (h/ho, where ho is the height of the drop at 1g) decreased with increasing
acceleration as shown by the relation plotted in Figure 7.Water drop at different total accelerations: (a) 1.0g, (b) 1.7g, (c) 3.7g, (d) 6.9g, (e) 10.4g, and (f) 14g.
(a)(b)(c)(d)(e)(f)Figure 7
Normalized height of
the drop as a function of total acceleration. The error bars correspond to the instrument uncertainty.The theory for large drops predictsh~a−0.5 [15, 16], but the trend
indicated by the points in Figure 7 indicates that the height drops faster when
the total acceleration increases from 1g to 14g. Although the
discrepancy can be attributed to the changes in the contact angle and
evaporation, a full analysis is outside the scope of the present work, particularly
because the mechanisms that govern the hydrodynamics controlling wetting or
evolution of drop into films are still not fully understood. Nevertheless, the
shape of the drop follows the condition expected for drops with flattened tops
(d>κ−1) at all a, assuring the formation of thin
films.The results presented in Figures6 and 7 agree with the observation that TiO2/CNTS-PEG400
coatings are still thick at 7g but can be substantially thin out at 14g. In this context, we conclude that thickness-sensitive solar-selective
absorbing coatings can be prepared under supergravity environment in the range
of 1g to 14g.


## 4. Conclusions

The layer-by-layer solution-chemical method in combination with some of the drying
protocols investigated in this work has proved successful in producing coatings with good spectrally
selective properties. Among the issues learned, we found that surfactant polymers used as
CNTs dispersant are not completely eliminated under the present centrifugal
force, requiring either thermal treatment or highera values. Nevertheless,
the microstructure and thickness changes of these coatings under supergravity
environment in the range of 1g to 7g cause a red-shift tendency
at increasing values of a, which could constitute an alternative method
to produce thickness-sensitive solar-selective absorbing coatings. Further work
is in progress varying the centrifugation time and the elimination/carbonization
of the remaining surfactants to improve even more the absorbance of these
films.



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*Source: 879016-2009-03-29.xml*