graham h. bratzel - washington state universityreu-mse.mme.wsu.edu/past/2008/reu poster ghb...
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IV. Results Sample preparation was improved. Sonicating the CNF between each step
dispersed agglomerate clumps. Sonicating the composite in solution reduced the size and number of bubbles and cracks in the thin films. Casting the thin film at a
0.010 inch thickness allowed the dichloromethane solvent to evaporate from the
body of the film before the surface of the film trapped bubbles of solvent vapor.
The electrical properties of the thin films have begun to be tested. Preliminary analysis of the volume resistivity of the thin films are plotted in chart 1 and chart 2.
Error bars on each chart show a 95% confidence interval. In chart 1, pure PEI
shows the highest volume resistivity of !8E+17 "-m. A 2% untreated CNF
concentration shows the lowest volume resistivity of !1E+12 "-m. The sharp drop
in resistivity between a 0.5% and 0.75% concentration indicate an approach towards the percolation threshold.
In chart 2, a 3% CNF (treated for 3 hours) concentration shows the highest
volume resisitivity of !3E+14 "-m. Higher resistivity correlates to shorter
nanofibers, a result of sonication treatment. A 3% untreated CNF concentration shows the lowest volume resisitivity of !3E+12 "-m. Lower resistivity correlates to
longer nanofibers and greater fiber agglomeration compared to treated CNF.
The SEM scans of the extruded pieces show that the extrusion process mixes
the CNF evenly throughout the PEI. CNF treated with suspended agitation (see figure 5, right) appear shorter than untreated CNF (see figure 5, left). Verifying the
distribution using multiple fracture sites and an overlaid grid is needed for a larger
confidence in the distribution effects.
V. Conclusion
Thin film sample preparation was refined over the course of the experiment. Solvent ratios, dispersion techniques, and film thicknesses were explored in order
to avoid bubbles, cracks, and other deformities in a sample. A 0.010 inch (2.54
mm) film thickness allows the greatest uniformity.
The electrical conductivity of the nanocomposite increases as the CNF concentration increases. For untreated CNF, the percolation threshold appears at
concentrations greater than 0.50% CNF. This is indicated by a sharp decrease in
resistivity. Empirical data shows a minimum resistivity at 2.0% CNF, but finding a
definite optimal concentration requires further experimentation.
Sonicating suspended CNF correlates to an increase in volume resistivity. SEM
scans of treated and untreated CNF in the same concentrations suggest that
treated CNF is better dispersed throughout the matrix than untreated CNF. Studies
on the effect of sonication on CNF length would require TEM scans of bare fibers
undamaged by tensile fracture of the composite.
Acknowledgements
Faculty Advisor: Dr. Katie Zhong Project Supervisor: Dr. Gang Sui
Laboratory Assistant: Loren Baker
Financial support for this work was provided by the National Science Foundation’s Research Experience for
Undergraduates Site Program in the Division of Materials Research under grant number DMR 0755055, Characterization of Advanced Materials.
Chart 1: Volume Resistivity of
Untreated CNF + PEI Thin
Chart 2: Volume Resistivity of 3%
Treated CNF + PEI Thin Films
Rectangular Panel
Six composite mixes were prepared in different concentrations and methods in
order to explore the effects of such differences on the properties of the panel. The
components were mixed manually and by the extrusion process.
The first method mixed 0.5%, 1.0%, and 3.0% concentrations of untreated CNF
in PEI as dry powders. The second method mixed 1.0% concentrations of CNF treated for 60 minutes or 180 minutes in PEI as dry powders. The third method
mixed high concentration films with additional powdered PEI to a final weight. The
films were 10% concentrations of untreated CNF or CNF treated for 60 minutes in
PEI in solution. Additional powdered PEI was added for a final 1.0% CNF
concentration.
These samples were mixed and extruded by a polymer extruder, then formed in
a mold using a hot press, shown in figure 2, at 650°F into a uniform panel, shown in figure 3.
SEM Study A sample of each thin film and panel extrusion was fixed to a glass slide with
conductive tape and was sputtered with gold, shown in figure 4. The natural
surface of the polymer prevented a clear image of the embedded CNF, so the thin film samples were torn and the extrusion pieces were fractured in tension. These
fracture sites exposed the CNF, seen as bright strands in figure 5.
Preparation and Properties of Carbon-
Nanofiber / Polyethylamine Nanocomposites Graham H. Bratzel
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Table 1: Thin Film Concentrations
Figure 2: Resistivity Test Fixture
I. Abstract The polymer Polyethylamine (PEI) was mixed with carbon nanofibers (CNF) to
create a polymer-carbon nanocomposite with near-isotropic and enchanced properties. The volume resistivities of thin films of the nanocomposite were
determined for various CNF concentrations and compared to the known properties
of pure PEI. The effects of CNF sonication on volume resistivity was determined
for a 3% CNF concentration. SEM scans explored nanofiber size and distribution.
II. Background
Nanofiber Reinforced Polymer Composite
As composite materials are being used more in public aerospace applications,
e.g. the Boeing 787 Dreamliner (50% composite), isotropic and generally enhanced
mechanical, electrical, and thermal properties are all being investigated for stronger, lighter, and safer materials.
As a particle’s dimension is reduced to nanoscopic levels, the macroscopic
properties of the material give way to new properties. Polymers reinforced by
nanoparticles exhibit highly isotropic behavior, and at discrete concentrations as low as a few percent, the properties of the composite reflect the properties of the
nanoparticle filler more than that of the polymer matrix. This has been seen in
electrical properties using a conductive filler like carbon. Carbon nanofibers (CNF)
have a high aspect ratio (surface area/diameter ratio) such that electrons are
naturally on the surface, improving electrical conductivity. At concentrations of a few percent in a polymer matrix, electrons can jump among the CNF through the
dielectric matrix, allowing current flow. Volume conductivity improves as the CNF
concentration approaches the percolation threshold.
III. Method Polyethylamine (PEI) was chosen for the polymer matrix because it dissolves
easily and is flame retardant. Thin film samples were prepared to test the electrical
conductivity of the composite. Rectangular panels were prepared to develop nano-
pore foam in following stages of research. Samples were studied using an SEM to observe nanofiber distribution. In each case, sample preparation methods were
examined and refined.
Van der Waals and other adhesion forces cause the CNF to agglomerate into
clumps. These clumps were dispersed in order to improve distribution throughout the PEI matrix. CNF was suspended in acetone and was agitated by a horn-type
sonicator for 1 hour or 3 hours.
Thin Film
The thin film samples were prepared by dissolving the PEI and carbon nanofibers (CNF) in a common solution of dichloromethane (CH2Cl2) and pouring
the mix onto glass plates. Each pour was cast at a 0.010 inch (0.254 mm)
thickness. Seven concentrations were prepared, detailed in Table 1. Five films
were collected for each concentration. Of the five collected films, the film that was
smoothest, most uniform, and had the least bubbles was tested for surface and volume resistivity with an alternating voltage of 50V and a variable nano-ampere
current using a resistivity test fixture, shown in figure 1. The permittivity and other
electrical properties of the films were also tested and will be analyzed in further
stages of research.
Volume resistivity measures the resistance of a material through a known
thickness and across a known area, or , where ! is the resistivity (in ohm
meters, "-m); R is the electrical resistance of a uniform sample of the material (in
ohms, "); A is the cross-sectional area of the sample or the electrodes, whichever
is smaller (in square meters, m#); and l is the thickness of the sample (in meters, m). Conductivity is the inverse of resistivity and has the units "-1-m-1 or Siemens
per meter, S-m-1.
! =RA
l
Figure 5: SEM Scans of 1% untreated CNF (left) and 1% 3hr treated
CNF (right) at Extruded Piece Fracture Site
Figure 4: Fractured Thin Film (left) and Extruded Pieces (right) for SEM
Figure 3: Steel Mold and
Composite Panel
Figure 2: Hot Press for Panel Molding
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