# Processing and Structure of Carbon Nanofiber Paper

Authors: Zhongfu Zhao; Jihua Gou; Aurangzeb Khan
Journal: Journal of Nanomaterials
 (2009)

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## Abstract

A unique concept of making nanocomposites from carbon nanofiber paper was explored in this study. The essential element of this method was to design and manufacture carbon nanofiber paper with well-controlled and optimized network structure of carbon nanofibers. In this study, carbon nanofiber paper was prepared under various processing conditions, including different types of carbon nanofibers, solvents, dispersants, and acid treatment. The morphologies of carbon nanofibers within the nanofiber paper were characterized with scanning electron microscopy (SEM). In addition, the bulk densities of carbon nanofiber papers were measured. It was found that the densities and network structures of carbon nanofiber paper correlated to the dispersion quality of carbon nanofibers within the paper, which was significantly affected by papermaking process conditions.


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## Body

## 1. Introduction

A variety of nanoparticles have been
incorporated into polymer resins to make nanocomposites for a wide range of
applications [1, 2]. Among them, carbon nanotubes and carbon nanofibers have
attracted significant research interests due to their unique characteristics and properties such as high strength and modulus,
low density, high surface area,
good chemical stability, high electrical and thermal conductivity, and fire
resistance. Although
there are different processing techniques to fabricate polymer nanocomposites,
they are mainly divided into three categories: (a) solution processing [3–13], (b) melt mixing
[14–17] using a batch
mixer or continuous device such as an extruder, and (c) in situ polymerization
[18, 19]. The mechanical and physical properties of polymer nanocomposites have
large variations due to processing-related issues.Generally,
carbon nanotubes and carbon nanofibers consist of graphitic
cylinders with diameters of 1–100 nm and high
aspect ratios of a few
micro lengths, leading to high van der Waals force between adjacent nanotubes or
nanofibers. The high van der Waals forces and high aspect ratios with a combination of high
flexibilities make the nanotubes and nanofibers easily aggregate. As a result, it
is difficult to individually disperse the nanotubes or the nanofibers into
polymer resins. The addition of a small amount of carbon nanofibers will also significantly
increase the viscosity of the resin. Particularly during the processing of fiber-reinforced
polymer composites, the resin flow through the fiber mats becomes very
difficult. The fiber mats will also filter out the nanotubes and nanofibers
during liquid molding process such as resin transfer molding and
vacuum-assisted resin transfer molding [20, 21]. In addition, the presence of
the nanotubes or nanofibers can significantly modify the local crystallization
behavior of polymer resins either through the direct polymer/particle
interaction at the interface or the modification of temperature and stress
field around and between the nanotubes or nanofibers.A
unique concept of making nanocomposites from carbon nanofiber paper has been
explored [22–24]. This
approach involves making carbon nanofiber paper through the filtration of the
suspension of well-dispersed carbon nanofibers under controlled process conditions.
Such a specialty paper has a uniform network structure formed through the
entanglement of the nanofibers. The carbon nanofiber paper can be further
integrated into traditional fiber reinforced composite laminates through liquid
molding processes. The structural characteristics of carbon nanofiber paper
such as pore size and orientation can significantly affect the penetration of
polymer resins. In this study, the correlation between the morphologies and
processing conditions of the paper was systematically studied. Specifically,
the effects of different types of carbon nanofiber, solvent, dispersant, and
acid treatment on the microstructures of carbon nanofiber paper were examined.
The processing conditions were chosen to make carbon nanofiber paper with an
optimum structure.


## 2. Experimental

Vapor-grown carbon nanofiber was
supplied from Applied Sciences, Inc., Cedarville, Ohio, USA. Two types
of carbon nanofibers were used: oxidized carbon nanofiber (OXCNF) and nonoxidized
carbon nanofiber (NOXCNF). Compared to NOXCNF, OXCNF had more functional
groups, such as hydroxyl group (–OH) and carboxylic acid (–COOH). The OXCNF and
NOXCNF were rinsed with diluted nitric acid. The acid-treated carbon nanofibers
were designated as RIN-OXCNF and RIN-NOXCNF. During the rinse of carbon
nanofibers, OXCNF or NOXCNF was first stirred in 2M nitric acid at room
temperature for 2 hours. The solution was then filtered through a 4μm polycarbonate
membrane and rinsed with water until the nitric acid was completely removed. In
this study, deionized water (WA), ethyl alcohol (AL), and acetone (AC)
were used as solvents. The dispersant (DISPERBYK-191; BYK-Chemie, Wesel. Germany) was used to aid the
dispersion of carbon nanofibers. The BYK had amine values of 20 mg KOH/g and acid value of 30 mg
KOH/g and could work through steric stabilization of carbon nanofibers.Carbon
nanofiber paper was fabricated by using the following procedures. The
as-treated or as-received carbon nanofibers of 200 mg were grinded by placing
them into a mortar and adding a small amount of solvent.  After grinding, they were
transferred into 500 mL beaker. and 400 mL solvent was added. The suspension
was sonicated using a high intensity sonicator for 20 minutes. After the
initial sonication, both the solution and the probe were cooled down to room
temperature. 1 mL BYK was then added to the suspension. The suspension was
sonicated for another 20 minutes under the same condition. The as-prepared
suspension was allowed to settle overnight. 300 mL suspension at the upper
level of the beaker was collected. The remaining 100 mL suspension containing
some deposits was mixed with 200 mL solvent and sonicated for another 10 minutes.
The carbon nanofiber paper was made by filtering the suspension through 0.4μm
hydrophilic polycarbonate or hydrophobic Teflon membrane under a high-pressure
filtration system. The carbon nanofiber paper was further dried in oven at
120C° for 2 hours. In this study, eight groups of samples were made under various
combinations of different types of carbon nanofibers, solvents, BYK, suspension, or deposit, as
shown in Table 1. The samples of carbon nanofiber paper were designated based
on their processing conditions. For example, RIN-OXCNF-AC-SUS-BYK represents
that the sample was made from RIN-OXCNF suspension in acetone, which was
dispersed with an aid of dispersant BYK. OXCNF, RIN-OXCNF, NOXCNF, and
RIN-NOXCNF are four types of carbon nanofibers. Three types of solvents
including water, alcohol, and acetone were used. SUS and DEP indicate the
samples from the upper suspension or the deposit, respectively.Table 1
Sample identification and
processing conditions.
Sample IDType of carbon nanofiberType of solventUse of dispersantSuspension or depositOXCNF-WA-SUSOXCNFWaterSUSOXCNF-WA-DEPOXCNFWaterDEPOXCNF-AL-SUSOXCNFAlcoholSUSOXCNF-AL-DEPOXCNFAlcoholDEPOXCNF-AC-SUSOXCNFAcetoneSUSOXCNF-AC-DEPOXCNFAcetoneDEPRIN-OXCNF-AC-SUSRIN-OXCNFAcetoneSUSRIN-OXCNF-AC-DEPRIN-OXCNFAcetoneDEPRIN-OXCNF-AC-SUS-BYKRIN-OXCNFAcetoneDISPERBYK-191SUSRIN-OXCNF-AC-DEP-BYKRIN-OXCNFAcetoneDISPERBYK-191DEPRIN-OXCNF-WA-SUSRIN-OXCNFWaterSUSRIN-OXCNF-WA-DEPRIN-OXCNFWaterDEPNOXCNF-AC-SUS-BYKNOXCNFAcetoneDISPERBYK-191SUSNOXCNF-AC-DEP-BYKNOXCNFAcetoneDISPERBYK-191DEPRIN-NOXCNF-AC-SUS-BYKRIN-NOXCNFAcetoneDISPERBYK-191SUSRIN-NOXCNF-AC-DEP-BYKRIN-NOXCNFAcetoneDISPERBYK-191DEP


## 3. Results and Discussion

### 3.1. Effects of Solvents

OXCNFs were dispersed in water, ethyl alcohol, and acetone. It was found
that OXCNF could be easily dispersed in water and alcohol but not well
dispersed in acetone. The variations in the dispersion quality are due to the
surface functional groups of OXCNFs. The suspension of OXCNFs in water and
ethyl alcohol was stable. But carbon nanofibers easily agglomerated in acetone
once the sonication was over. Both water and alcohol are more polar
solvent than acetone. The oxidized carbon nanofiber has more functional groups
such as OH, COOH, and so forth in the
solution. Therefore, the interaction between the polar groups (i.e., OH groups
on the nanofiber and OH groups of alcohol or water) leads to better quality of
dispersion. Six samples of carbon nanofiber paper were
made: OXCNF-WA-SUS, OXCNF-WA-DEP, OXCNF-AL-SUS, OXCNF-AL-DEP, OXCNF-AC-SUS, and
OXCNF-AC-DEP. Figures1(a)–1(e) show the scanning
electron microscopy (SEM) images of
OXCNF-WA-SUS, OXCNF-AL-SUS, OXCNF-AC-SUS, OXCNF-WA-DEP, and OXCNF-AL-DEP,
respectively.SEM images of carbon nanofiber paper:
OXCNF-WA-SUS, OXCNF-AL-SUS, OXCNF-AC-SUS, OXCNF-WA-DEP, and OXCNF-AL-DEP.
(a)
OXCNF-WA-SUS(b)
OXCNF-AL-SUS(c)
OXCNF-AC-SUS(d)
OXCNF-WA-DEP(e)
OXCNF-AL-DEPFigures1(a) and 1(d) show the SEM images of OXCNF-WA-SUS and
OXCNF-WA-DEP, respectively. It can be seen that no large particles can be found
in Figure 1(a). However, large particles can be clearly observed in Figure 1(d). The large particles came from the aggregates of carbon nanofibers. Carbon
nanofibers with smaller diameters were closely packed in OXCNF-WA-SUS, which
results in a higher bulk density of OXCNF-WA-SUS, as shown in Table 2. The
metal catalyst particles can be found in both OXCNF-WA-SUS and OXCNF-WA-DEP.Table 2
Bulk densities of carbon nanofiber papers.
Sample IDSample numberThickness (mm)Weight (mg)Weight percentage(a) (wt%)Bulk density(b) (g/cm3)OXCNF-WA-SUS10.34615874.50.383OXCNF-WA-DEP0.1495425.50.304OXCNF-AL-SUS20.56314169.50.210OXCNF-AL-DEP0.2746230.50.189OXCNF-AC-SUS30.2567738.50.252OXCNF-AC-DEP0.76812361.50.134OXCNF-AC-SUS-BYK40.38412560.70.272OXCNF-AC-DEP-BYK0.3718139.30.183RIN-OXCNF-AC-SUS-BYK50.29411459.10.324RIN-OXCNF-AC-DEP-BYK0.2887940.90.231RIN-OXCNF-WA-SUS60.23712161.10.426RIN-OXCNF-WA-DEP0.1607738.90.403NOXCNF-AC-SUS-BYK70.845134670.133NOXCNF-AC-DEP-BYK0.46166330.120RIN-NOXCNF-AC-SUS-BYK80.47410852.70.191RIN-NOXCNF-AC-DEP-BYK0.6149747.30.132Note: (a)
Total weight of carbon nanofibers from the suspension and deposit; (b) sample
diameter 39 mm.Similar observation was made for OXCNF-AL-SUS
and OXCNF-AL-DEP, as shown in Figures1(b) and 1(e). It is difficult to
individually disperse and separate carbon nanofibers, and the deposited carbon
nanofibers became the aggregates. The difference between Figures 1(d) and 1(e)
indicates that it was more difficult to separate carbon nanofibers in alcohol
than in water. Larger carbon nanofiber aggregates can be found in Figuer 1(e).
There are more carbon nanofibers with smaller diameters shown in Figure 1(a)
due to better dispersion of carbon nanofibers in water. This observation also
can be made by comparing their weights shown in Table 2. About 30.5 wt% carbon
nanofibers were deposited in OXCNF-AL-DEP. Only about 25.5 wt% carbon
nanofibers were deposited in water. From Figures 1(a)–1(c),
OXCNF-WA-SUS had good network structure of carbon nanofibers, in which no large
carbon nanofiber aggregates were formed, and individual nanofibers were closely
packed within the paper. It seems that the structure of carbon nanofiber paper
was better in OXCNF-AC-SUS than in OXCNF-AL-SUS.


### 3.2. Effects of Dispersant

In order to obtain better dispersion of OXCNFs in acetone, the
dispersant, BYK, was used. For OXCNF-AC-SUS-BYK and OXCNF-AC-SUS, it was found
that less carbon nanofibers were deposited with the aid of the dispersant, as
shown in Table2. The dispersant or surfactant decreases the interfacial surface
tension or surface energy of the solid phase like carbon nanofibers. Therefore,
they can be relatively easily dissolved in the solvent. Under the same
processing conditions, only 38.5 wt% of OXCNFs remained in the suspension for
OXCNF-AC-SUS but 60.7 wt% of OXCNFs for OXCNF-AC-SUS-BYK. Large amount of
OXCNFs were individually dispersed into acetone with the aid of BYK. Figure 2 shows the SEM images of OXCNF-AC-SUS-BYK. Compared to Figure 1(c), Figure 2 clearly indicates that more
uniform network structure was formed in OXCNF-AC-SUS-BYK. In Figure
1(c), carbon nanofiber paper only consisted of straight OXCNFs with larger
diameters. But there were some curved OXCNFs with smaller diameters in Figure
2. Therefore, the use of BYK could improve the dispersion of OXCNFs.Figure 2
SEM images of carbon nanofiber paper:
OXCNF-AC-SUS-BYK.


### 3.3. Effects of Acid Rinse

The diluted nitric acid was used to rinse
carbon nanofibers. Figures3 and 4 show the SEM images of RIN-OXCNF-AC-SUS-BYK, RIN-OXCNF-AC-DEP-BYK,
RIN-OXCNF-WA-SUS, and RIN-OXCNF-WA-DEP. Compared to OXCNF-AC-SUS-BYK in
Figure 2, Figure 3(a) shows uniform network structure without large OXCNF
aggregates. The acid rinse improved the dispersion of OXCNFs within the paper
consisting of short and small nanofibers. From Figure 1(a), it can be seen that
short OXCNFs existed in bundles and filled the pores of carbon nanofiber paper.
The OXCNF-WA-DEP only had
25.5 wt%, but RIN-OXCNF-WA-DEP could reach up to 38.9 wt%, as show in Table 2.
Compared to Figure 1(d), Figure 4(b) shows more carbon nanofiber aggregates,
although the size of carbon nanofiber aggregates in Figure 1(d) was much
larger. It can be concluded that it is not necessary to treat OXCNFs with the
acid rinse if the water is used as solvent. The acid rinse could improve the
dispersion of carbon nanofibers in alcohol and lead to uniform network
structure.SEM images of
carbon nanofiber paper: RIN-OXCNF-AC-SUS-BYK and RIN-OXCNF-AC-DEP-BYK.
(a)
RIN-OXCNF-AC-SUS-BYK(b)
RIN-OXCNF-AC-DEP-BYKSEM images of carbon nanofiber paper:
RIN-OXCNF-WA-SUS and RIN-OXCNF-WA-DEP.
(a)
RIN-OXCNF-WA-SUS(b)
RIN-OXCNF-WA-DEP


### 3.4. Effects of Carbon Nanofibers

There are very few active chemical sites on the surface of
NOXCNFs. Therefore, it is extremely difficult to directly disperse NOXCNFs in
water and acetone. The NOXCNFs were easily aggregated after the sonication. The
filtered carbon nanofiber paper had very rough surface. The NOXCNFs were deposited
quickly on the bottom of the beaker in water. The NOXCNF floccules were formed
in acetone. In order to obtain a stable suspension of NOXCNFs, it is necessary
to use the dispersant BYK in acetone. Figure5 shows the morphologies of
NOXCNF-AC-SUS-BYK and RIN-NOXCNF-AC-SUS-BYK. For NOXCNF-AC-SUS-BYK, there were
large NOXCNF aggregates, and the network structure was not uniform. But for
RIN-NOXCNF-AC-SUS-BYK, the large NOXCNF aggregates disappeared, and the quality
of network structures was improved. Therefore, the acid rinse enhanced the
dispersion of NOXCNF in acetone.SEM images of carbon nanofiber paper:
NOXCNF-AC-SUS-BYK and RIN-NOXCNF-AC-SUS-BYK.
(a)
NOXCNF-AC-SUS-BYK(b)
RIN-NOXCNF-AC-SUS-BYK


### 3.5. Bulk Densities of Carbon Nanofiber Paper

The bulk density is one of the important parameters of carbon nanofiber
paper. The thickness, weight, weight percentage, and density of carbon
nanofiber paper are shown in Table2. The bulk densities are plotted against
the sample number in Figure 6.  It can be
seen that all the samples made from the upper suspension had
higher bulk densities than those from the deposits. This is due to the close
packing of individual nanofibers uniformly dispersed in the suspension. Bars 1,
2, and 3 represent the samples made from the suspension of OXCNF in water,
alcohol, and acetone, respectively. The bulk densities of these samples from
the deposits decreased by following the order: water, alcohol, and acetone. The
sample from the alcohol suspension has the lowest bulk density. This agrees
very well with the SEM images shown in Figure 1. Compared to Sample 3, BYK was
used for Sample 4. In Sample 5, OXCNF was rinsed with the diluted nitric acid and
dispersed with the aid of BYK. Both BYK and acid rinse improved the dispersion
of OXCNFs in acetone. The same phenomenon was observed for the samples made
from the OXCNF aqueous solutions by comparing Samples 1 and 6. After the acid
rinse, their bulk densities increased. For NOXCNF, the bulk densities of the
samples from the deposits have no significant changes. However, the acid rinse
increased their bulk densities for the samples from the suspension.Figure 6
Bulk densities of carbon
nanofiber papers.


## 3.1. Effects of Solvents

OXCNFs were dispersed in water, ethyl alcohol, and acetone. It was found
that OXCNF could be easily dispersed in water and alcohol but not well
dispersed in acetone. The variations in the dispersion quality are due to the
surface functional groups of OXCNFs. The suspension of OXCNFs in water and
ethyl alcohol was stable. But carbon nanofibers easily agglomerated in acetone
once the sonication was over. Both water and alcohol are more polar
solvent than acetone. The oxidized carbon nanofiber has more functional groups
such as OH, COOH, and so forth in the
solution. Therefore, the interaction between the polar groups (i.e., OH groups
on the nanofiber and OH groups of alcohol or water) leads to better quality of
dispersion. Six samples of carbon nanofiber paper were
made: OXCNF-WA-SUS, OXCNF-WA-DEP, OXCNF-AL-SUS, OXCNF-AL-DEP, OXCNF-AC-SUS, and
OXCNF-AC-DEP. Figures1(a)–1(e) show the scanning
electron microscopy (SEM) images of
OXCNF-WA-SUS, OXCNF-AL-SUS, OXCNF-AC-SUS, OXCNF-WA-DEP, and OXCNF-AL-DEP,
respectively.SEM images of carbon nanofiber paper:
OXCNF-WA-SUS, OXCNF-AL-SUS, OXCNF-AC-SUS, OXCNF-WA-DEP, and OXCNF-AL-DEP.
(a)
OXCNF-WA-SUS(b)
OXCNF-AL-SUS(c)
OXCNF-AC-SUS(d)
OXCNF-WA-DEP(e)
OXCNF-AL-DEPFigures1(a) and 1(d) show the SEM images of OXCNF-WA-SUS and
OXCNF-WA-DEP, respectively. It can be seen that no large particles can be found
in Figure 1(a). However, large particles can be clearly observed in Figure 1(d). The large particles came from the aggregates of carbon nanofibers. Carbon
nanofibers with smaller diameters were closely packed in OXCNF-WA-SUS, which
results in a higher bulk density of OXCNF-WA-SUS, as shown in Table 2. The
metal catalyst particles can be found in both OXCNF-WA-SUS and OXCNF-WA-DEP.Table 2
Bulk densities of carbon nanofiber papers.
Sample IDSample numberThickness (mm)Weight (mg)Weight percentage(a) (wt%)Bulk density(b) (g/cm3)OXCNF-WA-SUS10.34615874.50.383OXCNF-WA-DEP0.1495425.50.304OXCNF-AL-SUS20.56314169.50.210OXCNF-AL-DEP0.2746230.50.189OXCNF-AC-SUS30.2567738.50.252OXCNF-AC-DEP0.76812361.50.134OXCNF-AC-SUS-BYK40.38412560.70.272OXCNF-AC-DEP-BYK0.3718139.30.183RIN-OXCNF-AC-SUS-BYK50.29411459.10.324RIN-OXCNF-AC-DEP-BYK0.2887940.90.231RIN-OXCNF-WA-SUS60.23712161.10.426RIN-OXCNF-WA-DEP0.1607738.90.403NOXCNF-AC-SUS-BYK70.845134670.133NOXCNF-AC-DEP-BYK0.46166330.120RIN-NOXCNF-AC-SUS-BYK80.47410852.70.191RIN-NOXCNF-AC-DEP-BYK0.6149747.30.132Note: (a)
Total weight of carbon nanofibers from the suspension and deposit; (b) sample
diameter 39 mm.Similar observation was made for OXCNF-AL-SUS
and OXCNF-AL-DEP, as shown in Figures1(b) and 1(e). It is difficult to
individually disperse and separate carbon nanofibers, and the deposited carbon
nanofibers became the aggregates. The difference between Figures 1(d) and 1(e)
indicates that it was more difficult to separate carbon nanofibers in alcohol
than in water. Larger carbon nanofiber aggregates can be found in Figuer 1(e).
There are more carbon nanofibers with smaller diameters shown in Figure 1(a)
due to better dispersion of carbon nanofibers in water. This observation also
can be made by comparing their weights shown in Table 2. About 30.5 wt% carbon
nanofibers were deposited in OXCNF-AL-DEP. Only about 25.5 wt% carbon
nanofibers were deposited in water. From Figures 1(a)–1(c),
OXCNF-WA-SUS had good network structure of carbon nanofibers, in which no large
carbon nanofiber aggregates were formed, and individual nanofibers were closely
packed within the paper. It seems that the structure of carbon nanofiber paper
was better in OXCNF-AC-SUS than in OXCNF-AL-SUS.


## 3.2. Effects of Dispersant

In order to obtain better dispersion of OXCNFs in acetone, the
dispersant, BYK, was used. For OXCNF-AC-SUS-BYK and OXCNF-AC-SUS, it was found
that less carbon nanofibers were deposited with the aid of the dispersant, as
shown in Table2. The dispersant or surfactant decreases the interfacial surface
tension or surface energy of the solid phase like carbon nanofibers. Therefore,
they can be relatively easily dissolved in the solvent. Under the same
processing conditions, only 38.5 wt% of OXCNFs remained in the suspension for
OXCNF-AC-SUS but 60.7 wt% of OXCNFs for OXCNF-AC-SUS-BYK. Large amount of
OXCNFs were individually dispersed into acetone with the aid of BYK. Figure 2 shows the SEM images of OXCNF-AC-SUS-BYK. Compared to Figure 1(c), Figure 2 clearly indicates that more
uniform network structure was formed in OXCNF-AC-SUS-BYK. In Figure
1(c), carbon nanofiber paper only consisted of straight OXCNFs with larger
diameters. But there were some curved OXCNFs with smaller diameters in Figure
2. Therefore, the use of BYK could improve the dispersion of OXCNFs.Figure 2
SEM images of carbon nanofiber paper:
OXCNF-AC-SUS-BYK.


## 3.3. Effects of Acid Rinse

The diluted nitric acid was used to rinse
carbon nanofibers. Figures3 and 4 show the SEM images of RIN-OXCNF-AC-SUS-BYK, RIN-OXCNF-AC-DEP-BYK,
RIN-OXCNF-WA-SUS, and RIN-OXCNF-WA-DEP. Compared to OXCNF-AC-SUS-BYK in
Figure 2, Figure 3(a) shows uniform network structure without large OXCNF
aggregates. The acid rinse improved the dispersion of OXCNFs within the paper
consisting of short and small nanofibers. From Figure 1(a), it can be seen that
short OXCNFs existed in bundles and filled the pores of carbon nanofiber paper.
The OXCNF-WA-DEP only had
25.5 wt%, but RIN-OXCNF-WA-DEP could reach up to 38.9 wt%, as show in Table 2.
Compared to Figure 1(d), Figure 4(b) shows more carbon nanofiber aggregates,
although the size of carbon nanofiber aggregates in Figure 1(d) was much
larger. It can be concluded that it is not necessary to treat OXCNFs with the
acid rinse if the water is used as solvent. The acid rinse could improve the
dispersion of carbon nanofibers in alcohol and lead to uniform network
structure.SEM images of
carbon nanofiber paper: RIN-OXCNF-AC-SUS-BYK and RIN-OXCNF-AC-DEP-BYK.
(a)
RIN-OXCNF-AC-SUS-BYK(b)
RIN-OXCNF-AC-DEP-BYKSEM images of carbon nanofiber paper:
RIN-OXCNF-WA-SUS and RIN-OXCNF-WA-DEP.
(a)
RIN-OXCNF-WA-SUS(b)
RIN-OXCNF-WA-DEP


## 3.4. Effects of Carbon Nanofibers

There are very few active chemical sites on the surface of
NOXCNFs. Therefore, it is extremely difficult to directly disperse NOXCNFs in
water and acetone. The NOXCNFs were easily aggregated after the sonication. The
filtered carbon nanofiber paper had very rough surface. The NOXCNFs were deposited
quickly on the bottom of the beaker in water. The NOXCNF floccules were formed
in acetone. In order to obtain a stable suspension of NOXCNFs, it is necessary
to use the dispersant BYK in acetone. Figure5 shows the morphologies of
NOXCNF-AC-SUS-BYK and RIN-NOXCNF-AC-SUS-BYK. For NOXCNF-AC-SUS-BYK, there were
large NOXCNF aggregates, and the network structure was not uniform. But for
RIN-NOXCNF-AC-SUS-BYK, the large NOXCNF aggregates disappeared, and the quality
of network structures was improved. Therefore, the acid rinse enhanced the
dispersion of NOXCNF in acetone.SEM images of carbon nanofiber paper:
NOXCNF-AC-SUS-BYK and RIN-NOXCNF-AC-SUS-BYK.
(a)
NOXCNF-AC-SUS-BYK(b)
RIN-NOXCNF-AC-SUS-BYK


## 3.5. Bulk Densities of Carbon Nanofiber Paper

The bulk density is one of the important parameters of carbon nanofiber
paper. The thickness, weight, weight percentage, and density of carbon
nanofiber paper are shown in Table2. The bulk densities are plotted against
the sample number in Figure 6.  It can be
seen that all the samples made from the upper suspension had
higher bulk densities than those from the deposits. This is due to the close
packing of individual nanofibers uniformly dispersed in the suspension. Bars 1,
2, and 3 represent the samples made from the suspension of OXCNF in water,
alcohol, and acetone, respectively. The bulk densities of these samples from
the deposits decreased by following the order: water, alcohol, and acetone. The
sample from the alcohol suspension has the lowest bulk density. This agrees
very well with the SEM images shown in Figure 1. Compared to Sample 3, BYK was
used for Sample 4. In Sample 5, OXCNF was rinsed with the diluted nitric acid and
dispersed with the aid of BYK. Both BYK and acid rinse improved the dispersion
of OXCNFs in acetone. The same phenomenon was observed for the samples made
from the OXCNF aqueous solutions by comparing Samples 1 and 6. After the acid
rinse, their bulk densities increased. For NOXCNF, the bulk densities of the
samples from the deposits have no significant changes. However, the acid rinse
increased their bulk densities for the samples from the suspension.Figure 6
Bulk densities of carbon
nanofiber papers.


## 4. Conclusions

This paper
systematically studied the processing-structure relationship of carbon
nanofiber paper. It is clearly seen that the dispersion of carbon nanofibers in
solvents played an important role in determining the network structures of
carbon nanofiber paper. The good dispersion led to close packing of carbon
nanofibers, which formed the uniform network structure of the paper with a
higher bulk density. The variations in the dispersion quality caused the
changes in the network structures and densities of carbon nanofiber paper. The
types of solvents, dispersant, acid rinse, and types of carbon nanofibers
significantly affected the dispersion quality. It has been found that
OXCNFs were easily dispersed in water and alcohol but not well in acetone due
to the functional groups on the fiber surface. Although there were few functional
groups on NOXCNFs, they were easily dispersed in acetone with the aid of
dispersant. In addition, the treatment with diluted nitric acid improved the
dispersion in the solvents. Therefore, the network structure of carbon
nanofiber paper can be controlled and optimized by choosing appropriate
processing parameters.



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*Source: 325769-2009-04-12.xml*