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Thermally Conductive Polymer Composites for Electronic Packaging Applications
by
Muhammad Omer Khan
A thesis submitted in conformity with the requirements for the degree of Master of Applied Science
Mechanical and Industrial Engineering University of Toronto
Chapter 3 Effects of Size and Structure of Micro- and Nano- Sized Carbon Fillers on Composite’s Properties ............................................................................................................ 28
3.3 Results and Discussion ..................................................................................................... 32
3.3.1 Effects of Carbon Fillers on the Effective Thermal Conductivity ........................ 32
3.3.2 Effect of Carbon- Based Fillers on the Coefficient of Thermal Expansion .......... 35
3.3.3 Effect of Carbon- Based Fillers on Glass Transition Temperature ....................... 38
3.3.4 Effects of Carbon- Based Fillers on the Electrical Properties by Measuring Electrical Conductivity ......................................................................................... 40
3.3.5 Effect of Carbon- Based Fillers on the Dielectric Constant ................................. 42
3.3.6 Effect of Carbon- Based Fillers on the Mechanical Properties ............................. 44
3.4 Micromechanical Modeling of Thermal Conductivity ..................................................... 46
Chapter 4 Composites Embedded with Hybrid Fillers to Tailor Thermal and Electrical Properties .................................................................................................................................. 51
5.2 Results and Discussion ..................................................................................................... 80
5.2.1 Effect of Temperature and Orientation on the Alignment of Polymer Fibers ...... 80
5.2.2 Effect of GNP Content on the Effective Thermal Conductivity of the LCP/GNP Composites Fabricated by Aligning Polymer Fibers ........................... 84
5.2.3 Effect of Alignment of Polymer Fibers on the Impedance of LCP/GNP Composites ............................................................................................................ 87
vii
5.2.4 Effect of Alignment of Polymer Fibers on the Dielectric Constant of LCP/GNP Composites .......................................................................................... 88
individual CFs, agglomerates of MWNTs and stacks of GNPs would have negligible effect
on the Tg of the PPS based composites as shown in Figure 3-5. Hence, these composited
would be suitable for use in the electronic packaging applications, as the addition of fillers
does not affect the Tg negatively.
Figure 3-5: Tg of PPS filled with micro- and nano- sized carbon fillers
40
3.3.4 Effects of Carbon- Based Fillers on the Electrical Properties by Measuring Electrical Conductivity
The electrical conductivity (σ), taken at the lowest frequency i.e. 10-2 Hz, of the composites
embedded with CF, MWNT, and GNP were measured. The effects of different fillers on
PPS-based composites are summarized in Figure 3-7 and Table 3-5. The dramatic change in
the σ by increasing the filler content indicates a percolation threshold for the fillers. Above
the compositions of the percolation threshold, the σ of the composites does not change
significantly. The electrical conductivity data were fitted to statistical percolation threshold
curves in the form of
.......................... 3-1
where, is Electrical Conductivity in S/cm, is the volume fraction, is the percolation
threshold and and t are constants. The σ curves of all the composites show that the
percolation thresholds for all the three fillers are below 5 vol.% and they are very close to
each other. However, the aspect ratio and the number of individual particles of the fillers
present in the matrix affect their formation of 3-D network in the matrix. MWNTs due to
their large aspect ratio and high number of particles should form the 3-D network at very
low filler content where as percolation in PPS/CF composites should be achieved at high
filler content. GNP, which has the highest electrical conductivity among the three, increases
the σ of the PPS/GNP composites less effectively than MWNT and CF even though it should
have a lower percolation threshold. The better dispersion of the nanofillers and their better
adhesion to the PPS matrix are needed for the high electrical conductivity of the PPS based
composites. The curve fitting is only an approximation; however, it should be noted that the
percolation threshold depends upon the aspect ratio, number of particles, inherent electrical
conductivity, degree of dispersion of filler in the matrix, nature of interface and interaction
between the filler and matrix [96].
Figure 3-6 shows the SEM micrographs of PPS/MWNT composite with 20 wt.% of MWNT
at magnifications of 5000X, 20000X and 50000X, respectively. Figure 3-6(a) and (b)
indicate the presence of MWNT agglomerates in the PPS matrix. In Figure 3-6(c), individual
MWNTs of diameter about 20 to 40 nm entangled with each other and representing a
proportion of the aggregate can be observed. Together with the measurements of various
41
multifunctional properties, it is believed that mere presence of enough amounts of
electrically conductive fillers in the polymer matrix is a sufficient condition to promote the
material’s electrical conductivity. High electrical conductivity requires the existence of a
good network of electrically conductive fillers.
Figure 3-6: SEM micrographs of PPS/MWNT composite with 20 wt.% MWNT at (a) 5000X (b) 20000X and (c) 50000X
Figure 3-7: Change in electrical conductivity as the filler conent increases for PPS embedded
with carbon based fillers
42
Table 3-5: Percolation threshold calculated by fitting a curve to the experimental data
Composite Percolation Threshold
PPS/CF 4.55 wt.%
PPS/MWNT 4.76 wt.%
PPS/GNP 5.00 wt.%
3.3.5 Effect of Carbon- Based Fillers on the Dielectric Constant
Use of carbon- based filler reinforced polymer composites have been studied as they exhibit
relatively high electrical properties in addition to high mechanical and thermal properties.
High aspect ratio of filler helps in attaining the percolation threshold at very low filler
concentrations, i.e., 0.05-0.1 wt.% for polystyrene/CNT composites [4] and 1-2 wt.% for
polyphenylene sulphide PPS/CNT composites [97]. Even though the percolation threshold
values are relatively low for MWNT reinforced polymer composites, the difficulty of
dispersing MWNT along with their high cost make them less than ideal fillers for ULSI, IC,
and capacitor materials. Composites with high dielectric constant (ε), i.e. greater than 100,
are suitable for capacitors; ε less than 3 is desirable for ULSI; whereas 3 < ε <100 is needed
for electronic packaging applications.
Figure 3-8 shows the dielectric constants for the neat PPS and its carbon composites
measured at 300 kHz. It should be noted that these are the effective dielectric constants i.e.
they do not take into consideration the presence of conductive fillers in the samples. It can
be observed that the dielectric constants (ε) of composites increased with increasing filler
content. Highest improvement in the ε is demonstrated by GNP reinforced PPS based
composites followed by MWNT and then CF. The increase in composites’ ε is primarily due
to the interfacial polarizations [98]. Other factors affecting the ε of different carbon-based
fillers reinforced PPS composites can be attributed to fillers’ dispersion. As we can see from
the figure, GNP shows the highest improvement in ε followed by MWNT and then CF
similar to the impedance results due to the level of connectivity of the fillers and their
electrical conductivity, as mentioned in the previous section. Since the high ε would be
43
beneficial for flexible capacitors, the PPS/GNP composites would be best for these
applications.
Considering the different abilities by CF, MWNT, and GNP to enhance the composites’ keff
and ε, it can be concluded that the choice of them as fillers hinges on the targeted
applications of the composites. For applications where both thermal and electrical
conductivity are desired, it seems that GNP is a more appropriate choice of fillers. However,
if the applications demand thermally conductive but electrically insulating properties (e.g.,
electronic packaging), it is apparent that CF is a natural choice because of its relatively high
keff and lower ε and σ. Furthermore, PPS/GNP composites with filler content more than 10
wt.% may be used for capacitors, and PPS/MWNT may be suitable for electronic packaging
applications.
Figure 3-8: Effect of carbon-based fillers on the dielectric constant at 3×105 Hz
44
3.3.6 Effect of Carbon- Based Fillers on the Mechanical Properties
Figure 3-10 shows the effect of filler types and contents on the elastic modulus (E) measured
in compression tests. It can be seen that the addition of CF and MWNT yielded small
improvement in E, while the addition of GNP led to no change in E. However, it must be
noted that the friction between the specimen and the testing block as the specimen axially
expanded under compression might have led to potential errors in the measurements. The
compressive modulus of neat PPS was not readily available; however, the tensile modulus of
neat PPS as reported by the supplier is 3800 MPa. The results in Figure 3-10 clearly show
that the 1-D structure of CF and MWNT efficiently contribute to the composites elastic
modulus whereas the 2-D structure of GNP seems to have no effect on the PPS/GNP
composite’s E. Furthermore, the increase in the E of the composites is relatively low when
the fillers’ E is taken into consideration. This huge difference can be attributed to the fact
that the fillers in the composites are randomly dispersed and not aligned axially along the
samples’ length. It is believed that difference in the rigidity of the fillers may have been one
of the factors for different E. As mentioned in the earlier sections, it is believed that the
micro size of CFs make them more rigid than the other two fillers resulting in relatively
higher E of the composites. It is believed that the tube-like structure of the MWNT makes
them more rigid than the nano- sized “wrinkled” stacks of GNP resulting in the higher
composites’ E of PPS/MWNT than PPS/GNP composites. However, taking all of these
factors into consideration, the 1-D structure of CF and MWNT had more pronounced
improvement on composites’ E over neat polymer.
Figure 3-9 illustrates the SEM micrographs at magnifications of 50X, 2000X, and 13,000X
of PPS/GNPs composite filled with 20 wt.% of GNP. Figure 3-9 shows the presence of
voids in the composite. These voids are circular in shape with a diameter of about 100 µm.
Therefore, the thermal conductivity of PPS/GNP composites can be further improved if the
presence of these air voids that act as barriers to heat conduction through the composite can
be minimized. Figure 3-9(b) shows the SEM micrograph of the sample composite at higher
magnification of 2000X. The graphene nanoplatelets, which are 25 µm in size, are uniformly
dispersed in the polymer matrix. The electrical conductivity of the composite may be
increased by the presence of electrically conductive fillers. However, in order to increase the
thermal conductivity of the composite, the interfacial resistance between the polymer and the
45
filler must be minimized. Figure 3-9(c) illustrates a magnified image of a single GNP
showing presence of wrinkled GNPs. Wrinkled GNPs could also have been one of the
reasons for no change in the E of the PPS/GNP composites.
Figure 3-9: SEM micrograph of PPS/GNP nanocomposite with 20 wt% GNP at 50X, 2000X, and 13,000X
Figure 3-10: Compressive Modulus of PPS filled with micro- and nano- sized carbon fillers
46
3.4 Micromechanical Modeling of Thermal Conductivity
Micromechanical models, as described in Section 2.4, were compared to experimental data
for PPS based composites filled with three different fillers i.e. CF, MWNT, and GNP, as
shown in Figure 3-11 to Figure 3-13. The series and parallel models represent the maximum
and minimum theoretical keff, while the remaining curves are contained within these limits.
For all the three fillers, the geometric mean model predicts the keff more accurately than the
series and parallel models. Maxwell theoretical model that uses potential theory to obtain an
“exact” solution for the conductivity of randomly distributed and non-interacting
homogenous spheres in a homogeneous continuous medium underestimates the keff for
composites filled with the three carbon-based fillers. The reason of this big discrepancy
could be that neither the three fillers are spherical in shape nor are they randomly distributed.
Furthermore, as evident by the SEMs, fillers are present in the polymer matrix in aggregates.
The only model that takes into consideration the effect of the shape of the particles and the
orientation of packing for a two-phase system is the Nielsen model. For this model, the
packing factor of ∅ 0.52, which is the constant for three dimensional random fibers, was
chosen. For the type of fillers, all three fillers i.e. CF, MWNT, and GNP were assumed to
randomly oriented rods with very different aspect ratios. Based on these assumptions, the
Nielsen model predicts the keff most accurately for PPS embedded with carbon-based fillers
at low filler content. At high filler content, the Nielsen model starts to deviate significantly
from the experimental data due to the assumption that the fillers are dispersed uniformly and
that they do not form aggregates. Since none of the models takes into consideration the
formation of aggregates and the possibility of filler-filler and polymer-filler interfacial
thermal resistance, more accurate models are desired to better predict the thermal
conductivity of the composites.
47
Figure 3-11: Theoretical predictions for thermal conductivity compared to experimental
results for PPS/CF composites
48
Figure 3-12: Theoretical Predictions for thermal conductivity compared to experimental
results for PPS/MWNT composites
49
Figure 3-13: Theoretical predictions for thermal conductivity compared to experimental
results for PPS/GNP composites
3.5 Summary
As the electronic industry moves toward miniaturization, one of the most important
challenges is to remove the heat generated as the chip’s power consumption increases. Thus,
thermal management in electronic circuits is becoming an integral part of the design. This
chapter investigates the increase in effective thermal conductivity (keff), electrical
conductivity (σ), and compressive modulus (E) of poly-phenylene sulphide (PPS) filled with
carbon-based fillers such as carbon fibers (CF), multi-walled carbon nanotubes (MWNT),
and graphene nanoplatelets (GNP). PPS/GNP composites seemed to have the highest keff
whereas PPS/MWNT and PPS/CF showed similar keff values for the same filler loading. The
σ for PPS/CF composites was the lowest whereas PPS/GNP showed higher values for the
50
same filler content. After comparing σ, keff, E, Tg, CTE and SEM micrographs of the three
types of composites, it was concluded that different requirements are needed in CF, MWNT,
and GNP filler network to promote different functional performances of the composites.
Even though the filler contents were higher than their percolation threshold, keff was not
increased significantly, i.e., a sudden jump in the thermal conductivity was not observed.
This potentially implies that there are other underlying factors such as filler-filler contact
resistance, polymer-filler interfacial resistance, and filler orientation that govern the keff. It
was also concluded that the nano-size of GNP and its 2-D structure, both of which
contributed to significantly higher surface area, were among the reasons for higher keff
allowing more phonons to transfer through the PPS/GNP composite. Fillers with large
surface area provide more interaction with the polymer matrix resulting in the formation of
good conductive network. The difference in the σ of the composites was solely dependent on
the amount of filler particles present in the PPS matrix as well as the electrical conductivity
of the filler. As long as the filler content was over the percolation threshold, the σ increased
dramatically due to better conductive network. However, the percolation threshold itself was
influenced by the size of the fillers, i.e., micron size CF achieved the percolation threshold at
relatively higher filler content than nano- size MWNT and GNP. Fillers with a low aspect
ratio played a role in changing the compressive modulus (E) and coefficient of thermal
expansion (CTE) of the composites. CFs, having the lowest aspect ratio among the three
fillers, showed significant change in the E and CTE of the composites. Individual CFs,
agglomerated MWNTs, and stacks of GNPs, all of which are micron- sized, did not affect
the Tg of the composites since much smaller size of the fillers is believed to cause enough
hindrance in the mobility of polymer chains that would increase the Tg. SEMs of the three
types of the composites showing the dispersion of individual CFs, agglomerated MWNTs,
and stacks of GNPs in the PPS matrix gave another perspective on the differences in their
results. Therefore, imperfect alignment of the fillers, imperfect wetting of the fillers by the
polymer, imperfect bonding in the interface between the fillers and the polymer and
imperfect bonding between individual fillers affected the multifunctional properties of the
PPS based carbon filler reinforced composites. It can be concluded that the GNP fillers are
the most efficient fillers among all other carbon-based filler to promote thermal conductivity
for electronic packaging applications.
51
Chapter 4 Composites Embedded with Hybrid Fillers to Tailor Thermal
and Electrical Properties
4 Introduction
The future of Integrated Circuits with three-dimensional chip architecture hinges on the
development of practical solutions for heat management to the excessive amount of heat
generation. In this context, new polymer-matrix composites (PMCs), which have good
processibility, high effective thermal conductivity (keff), and low but tailored electrical
conductivity (σ) would be needed. This chapter aims to explore the synergy of hybrid filler,
hBN platelet with carbon-based fillers, on promoting the keff of the polyphenylene sulfide
(PPS) composites. This chapter also explores promotion of interconnectivity among the
fillers in the PPS matrix, leading to higher keff, by the uses of hybrid fillers. The opportunity
to use carbon-based fillers as the secondary fillers to tailor the PMCs’ σ is discussed.
Furthermore, the effects of hybrid fillers on the PMCs’ coefficient of thermal expansion are
presented. Few SEM micrographs of PPS embedded with hybrid fillers are shown in Figure
4-1.
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Table 3-2; however, the physical properties of hBNPTX60 have been summarized here in
Table 4-1.
PPS was chosen as the matrix material because of its high service temperature (i.e., 200°C),
which is needed in various heat management applications. For the primary filler, hBNPTX60
(i.e., spherical agglomerates) was chosen for several reasons: (i) it is electrically insulating;
(ii) it resembles the layered structure of graphite, making it extremely soft, and thereby
easier to be compounded at high loading; and (iii) its spherical geometry leads to isotropic
filler properties. For the secondary fillers, CF, MWNT, and GNP were chosen because of
their high aspect ratios and excellent thermal and electrical properties.
Table 4-1: Physical properties of hBNPTX60
Physical Properties PTX60
Density (ρ) 2280 kg/m3
Thermal Conductivity (k) 300+ W/m-K
Dielectric Strength 53 kV/mm
Shape Spherical agglomerates
Size 60 µm
4.1.2 Surface Modification
Amino silane ((C2H5O)3SiC3H6NH2), which is a versatile coupling agent and provides a
superior bond between inorganic fillers and organic polymers, was used to modify the
surface of hBN. Amino silane was first dispersed in a 50-50 ethanol-water solution. hBN
powders were then added to the solution and mixed by a magnetic stirrer for 10 minutes. The
samples were filtered and dried for over 12 hours at ~60ºC. For all the surface modifications
of hBN, 8 parts of amino silane were added to 100 parts of hBN as suggested by supplier of
the silane.
54
4.1.3 Composite Preparation
PPS powders were dry-blended with desired volume fractions and compositions of fillers,
which are summarized in Table 4-2 to Table 4-4. All samples have 33.3 vol.% of fillers as
this was the maximum compoundable filler content. The mixtures were then melt-
compounded in a micro-compounder (DSM Xplore 15) at 300 °C and 50 rpm for 6 minutes.
For hBNPTX60-MWNT hybrid fillers, it was impossible to compound the mixture with the
hBNPTX60:MWNT ratio of 2:1 because of the mixture’s high viscosity. The extruded
composites were cooled in a water bath at room temperature. The extrudates were pelletized
and ground into fine powders using a pelletizer and a mill freezer (SPEX CertiPrep Group,
model 6850 Freezer/Mill), respectively.
Table 4-2: Compositions of PPS-hBN-CF composites
Composite Name
Vol. Fraction of PPS
Vol. Fraction of hBNPTX60
Vol. Fraction of CF
Vol. Ratio of hBNPTX60:CF
BNCF1 0.667 0.266 0.067 4:1
BNCF2 0.667 0.250 0.083 3:1
BNCF3 0.667 0.222 0.111 2:1
Table 4-3: Compositions of PPS-hBN-MWNT composites
Composite Name
Vol. Fraction of PPS
Vol. Fraction of hBNPTX60
Vol. Fraction of MWNT
Vol. Ratio of hBNPTX60:MW
NT
BNMWNT1 0.667 0.266 0.067 4:1
BNMWNT2 0.667 0.25 0.083 3:1
55
Table 4-4: Compositions of PPS-hBN-GNP composites
Composite Name
Vol. Fraction of PPS
Vol. Fraction of hBNPTX60
Vol. Fraction of GNP
Vol. Ratio of hBNPTX60:GNP
BNGNP1 0.667 0.266 0.067 4:1
BNGNP2 0.667 0.50 0.083 3:1
BNGNP3 0.667 0.222 0.111 2:1
4.1.4 Test Specimen Preparation
The fabrication method for these samples was similar to the previous ones. The melt-
compounded composites were compression-molded, at 310°C, into disc-shaped samples of
20 mm diameter. Samples prepared for thermal conductivity measurements were 10 mm
thick while those prepared for dielectric analysis were 2 mm thick.
4.2 Characterization
Surface modifications of hBN and the fillers morphologies of the composites were studied in
this work. Furthermore, multifunctional properties and different characteristics of various
samples, including effective thermal conductivity (keff), electrical conductivity (σ), and
coefficient of thermal expansion (CTE) of all samples were investigated. All reported
measurements were determined by analyzing three samples.
4.2.1 Surface Modifications of hBN
A thermogravimetric analyzer (TGA, TA Instruments, model Q50) was used to qualitatively
investigate the amount of amino silane bonded to the hBN fillers’ surfaces. hBNPTX60 (i.e.,
spherical agglomerates), with or without surface modifications, were heated to 900°C at
20°C/min under nitrogen gas flow protection. The weight loss upon the heating were
recorded and analyzed.
56
4.2.2 Composite Morphologies
The dispersion of the filler systems in the compression molded samples was examined on a
scanning electron microscope (SEM, JEOL, model JSM6060) operated at 20 kV. Sample
cross-sections were obtained by cooling and fracturing the composites in liquid nitrogen.
The cross-sections were sputter coated with platinum prior to the SEM analyses.
4.2.3 Effective Thermal Conductivity
A thermal conductivity analyzer (in accordance to ASTM E1225-04) [99] was used to
measure keff at 150°C. The analyzer measures the sample’s k by comparing the temperature
gradient across the sample to that across a pair of stainless steel 304 reference bars. Heat
sink silicone compound was applied at the interfaces between the sample and the reference
bars to enhance the thermal contact and reduce the thermal resistance.
4.2.4 Coefficient of Thermal Expansion
The coefficient of thermal expansion (CTE) was measured using a Thermomechanical
Analyzer (TMA, TA Instruments Q400). The composites were heated to 200°C at a rate of
10 °C/min. The CTEs were determined from the slope of the plot at 50°C and 150°C,
depicting change in dimension and temperature.
4.2.5 Impedance
The Impedance (Z) of the composites were obtained by a dielectric/impedance analyzer
(Novocontrol Technologies, model Alpha-N). A root-mean-square (rms) voltage of 1V was
applied over a frequency of 10-2 to 10-5 Hz. The measurements were used to determine if the
electrical insulating properties of the PPS had been suppressed by the inclusion of various
filler systems.
57
4.3 Results and Discussion
4.3.1 Surface Modification of hBN
Amino silane ((C2H5O)3SiC3H6NH2) is a common coupling agent used to improve the bond
between inorganic and organic fillers. It was used in this study to modify the surface of some
of the composites containing hBN. As previously discussed, high thermal interfacial
resistance exists when filler materials are added into a polymer matrix. One suggested
method of improving the interfacial adhesion was through the chemical functionalization of
both the polymer and filler. Upon evaluating data regarding the thermal conductivity of
treated polymer composites, the results were not as expected. As seen in Figure 4-2, the
addition of amino silane to the hBNPTX60 did not yield favorable results. Studies have also
shown decreases in thermal conductivity with excessive amounts of silane [3]. If the coating
of silane on the surface of the particle is too large, then it may act as a thermal barrier and
decrease the thermal conductivity. In a study by Yung et. al., it was reported that 1 wt.%
coupling agent was sufficient to enhance thermal conductivity, but 2 wt.% was
counterproductive causing decreased thermal conductivities [3]. The opposite may also be
true, where hBN was not coated enough to have strong polymer-filler interfaces in the
composites.
The fractional loss in weight of hBNPTX60, with and without surface modification by amino
silane, is shown in Table 4-5. Since the organic coupling agent is volatile compared to hBN,
the fractional loss in weight of the silane-treated hBN particles after heating to a high
temperature relates to the amount of silane present. TGA results revealed that the amounts of
volatile/decomposable materials on the as-received hBN particles were negligible. For the
treated hBN particles, the amounts of weight loss on the silane treated hBNPTX60 particles
were much higher.
58
Figure 4-2: Effect of using a silane-based coupling agent on PPS-hBN composites
Table 4-5: Fractional loss in mass of hBN particles upon heating to 900 ˚C
hBNPTX60 Filler Fraction Mass Loss
as received < 0.05%
surface modified 1.13%
4.3.2 Effect of Hybrid Fillers on the PMC’s Effective Thermal Conductivity
Figure 4-3 shows the effect of filler compositions on keff of the PPS-based composites
embedded with 33.3 vol.% of hybrid fillers. The keff PPS-hBN composites filled with
hBNPTX60 only were increased from 0.22 W/mK of the neat PPS to 1.77 W/mK, respectively.
SEM micrographs of PPS-hBNPTX60 composite are shown in Figure 4-4. Although hBNPTX60
particles are 60 µm spherical agglomerates, Figure 4-4 reveals that the agglomerates had
been broken down into very fine (< 10 µm) during compounding and compression molding.
The smaller hBN platelets in the PPS-hBNPTX60 composite would lead to a higher filler
59
population density than the PPS-hBNPTX60 composite if the fillers remained spherical. This
would enhance the formation of thermally conductive network in the PPS matrix, and
thereby increased the PMC’s keff.
Furthermore, the effects of adding electrically conductive secondary fillers with high aspect
ratios (i.e., CF, MWNT, and GNP) to the PPS-based composites filled with hBNPTX60 on keff
were investigated. Figure 4-3 indicates that the inclusion of CF as secondary filler led to
limited increases in the PMC’s keff when hBNPTX60-to-CF volume ratios were 27:6.3 and
25:8.3 despite the bridging of hBN fine platelets by CF as illustrated in Figure 4-5. It is
believed that hBNPTX60 and CF would not be an appropriate hybrid filler combination to
promote PMC’s keff. In contrast, keff measurements indicated that MWNT and GNP were
effective secondary fillers. Figure 4-3 shows that PPS-based composites filled with
hBNPTX60 and MWNT and hBNPTX60 and GNP exhibited substantial promotion in the
composite’s k. Comparing to keff of 1.77 W/mK for the PPS-based composites with single
hBNPTX60 fillers, the values of keff for composites filled with 27:6.7, 25:8.3, and 22:11
volume ratios of hBNPTX60 and MWNT, and hBNPTX60 and GNP hybrid fillers were
increased to as much as 2.7 W/mK.
60
Figure 4-3: Effects of hybrid filler compositions on PMC's keff
Comparing the three combinations of hybrid fillers, which include (i) hBNPTX60 with CF, (ii)
hBNPTX60 with MWNT, and (iii) hBNPTX60 with GNP, the synergistic effects were found to
be the best for the composites with hBN with GNP, followed by that with hBN with MWNT.
However, hybridizing hBN with CF showed limited improvement over composites filled
with the same loading of single hBN fillers. Because the ratios of filler’s thermal
conductivity to the matrix’s thermal conductivity are over 100 times for hBN, CF, MWNT,
and GNP the additional improvement in the PMC’s keff values for composites filled with
hBNPTX60 with GNP should not be caused by the higher k of the GNP [37]. In contrast, the
promotion of PMC’s keff is believed to be attributed to the shapes and sizes of the secondary
fillers. Among the three types of secondary fillers (i.e., CF, MWNT, and GNP), GNP has the
highest aspect ratio (i.e., ~2500). For CF and MWNT, their aspect ratios (i.e., ~20, and
~1000 respectively) are less than that of GNP; however, MWNT have higher aspect ratio
than CF, leading to its higher surface area in the PMC than CF. Using the dimensions of the
fillers, their total surface area in the composites were estimated, and the results are shown in
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Figure 4-3. The figure confirms that for the same volume fraction of secondary fillers, GNP
had the highest total filler surface area, followed by MWNT, while CF had the lowest total
filler surface area. The larger total surface area may have caused more occurrences of
connection between primary and secondary fillers leading to improved keff. Together with the
experimental measurements of PMC’s keff, it is believed that effectiveness of secondary
fillers to promote PMC’s keff increased with higher aspect ratios and smaller filler sizes as
discussed in the previous chapter.
Figure 4-4: SEM Micrographs of (a) PPS-PTX60 with 33.3 vol.% hBN
Figure 4-5: SEM Micrographs of PPS-hBN-CF composites with 33.3 vol.% hBNPTX60-CF
hybrid fillers with hBNPTX60-to-CF ratios of (a) 27:6.7; (b) 25:8.3; and (c) 22:11
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Figure 4-6: SEM Micrographs of PPS-hBN-MWNT Composites with 33.3 vol.%
hBNPTX60-MWNT Hybrid Fillers with hBNPTX60-to-MWNT ratios of (a) 27:6.7 and (b)
25:8.3
Figure 4-7: Total surface area of secondary fillers (filler content = 33.3 vol.%)
4.3.3 Effect of Hybrid Fillers on the PMC’s Coefficient of Thermal Expansion
Figure 4-8 shows the effect of filler compositions on the PMC’s coefficient of thermal
expansion (CTE) below the glass transition temperature (Tg). It was observed that the CTEs
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of all PMCs were lower than that of the neat PPS. The particle size distribution and size have
been reported as factors that might influence the CTE of the composite system [100].
Moreover, the hybrid fillers also allowed more efficient packing of the fillers in the PPS
matrix, leading to slightly lower CTE. This efficient packing of fillers is more obvious when
CTE of BN:GNP is studied. The similar structure but very different size of BN and GNP
allowed for more efficient packing of hybrid fillers. SEM micrographs in Figure 4-9 confirm
the presence of efficient packing of BN and GNP in the polymer matrix. Nevertheless, since
the total filler contents for all composites fabricated in this work were maintained to be
constant (i.e., 33.3 vol.%), their CTEs were very close to each other for the same set of
hybrid fillers.
Figure 4-8: Effects of hybrid filler composition on PMC's CTE (below the glass transition
temperature of PPS)
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Figure 4-9: SEM Micrographs of PPS-hBN-GNP Composites with 33.3 vol.% hBNPTX60-GNP Hybrid Fillers with hBNPTX60-to-GNP ratios of (a) 27:6.7; (b) 25:8.3; and (c) 22:11
4.3.4 Effect of Hybrid Fillers on the PMC’s Impedance
The impedance (Z) of the PMCs embedded with different hybrid filler systems was
measured and the results are plotted in Figure 4-10 to Figure 4-12. All of the graphs show
that Z of PPS-based composites filled with only hBN remained low and similar to that of
neat PPS. In other words, the electrical insulating properties of PPS would not be
compromised with the addition of hBN. Figure 4-10 shows that, when the hBNPTX60 to CF
volume ratios were 26:6.3 and 25:8.3, the impedance of the PPS-hBNPTX60-CF composites
were also very similar to that of PPS-based composites filled with only hBN. However,
when the hBNPTX60 to CF volume ratio changed to 22:11, the PMC’s Z decreased
significantly. For this filler composition, the volume fraction of CF was 11.1 vol.%, which
was higher than the percolation threshold (i.e., 9.0 vol.% for CF) reported by Chingerman et
al [101], resulting in the dramatic decrease in the PMC’s Z. Furthermore, for the two PMCs
filled with hBNPTX60 and MWNT, their Z were significantly lower than those of all other
composites. The volume fractions of MWNT in the PPS-hBNPTX60-MWNT composites were
6.7 vol.% and 8.3 vol.% when the hBNPTX60 to MWNT volume ratios were 26:6.3 and
25:8.3, respectively. Therefore, the MWNT contents in both composites were significantly
higher than the percolation threshold reported in literature [102]. Furthermore, the frequency
independent behavior of PPS-hBN-MWNT composites’ Z reinforced the idea of the
formation of continuous conductive pathways that interconnected the MWNTs between the
two electrodes in the dielectric analyzer. In short, the Z measurements of various hybrid
filler system suggested that the choice and amount of secondary fillers can not only promote
the PMC’s keff but also serve as parameters to tailor the its Z. Similar phenomenon was
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observed with the PPS-hBNPTX60-GNP composites where 6.7 vol.% of GNP showed similar
Z to that of Neat PPS and PPS-hBNPTX60 composites. Whereas, 11 vol.% of GNP showed
drastic decrease in Z of the PPS-hBNPTX60-GNP composites showing that the GNP content is
well above the percolation threshold. Furthermore, Figure 4-12 reveals that the percolation
threshold of GNP in PPS-hBNPTX60-GNP is somewhere between 6.7 vol.% and 8.3 vol.% of
GNP, which is significantly higher than that of MWNT and significantly lower than that of
CF. This is believed to be due to the structure of MWNT that allows it to form connective
pathways in the composites at very low filler content, as discussed in the previous chapter.
Even though the amount of MWNT in PPS-hBNPTX60-MWNT composites was above the
percolation threshold, it only led to few orders of magnitude decrease in Z without resulting
in the same degree of increase in the PMC’s keff. It is believed that the inter-filler contacts
(i.e., hBNPTX60-hBNPTX60, MWNT-MWNT, and hBNPTX60-MWNT) might also have
considerable thermal contact resistance, suppressing the potential to significantly enhance
PMC’s keff by the thermally conductive fillers. In other words, strategies to reduce such
thermal contact resistance among embedded fillers would need to be further investigated to
unleash the full potential of PMCs in the electronic packaging industry as the addition of
silane did not seem to be efficient.
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Figure 4-10: Effects of hybrid filler compositions on PMC's Impedance for PPS-hBNPTX60-
CF composites
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Figure 4-11: Effects of hybrid filler compositions on PMC's Impedance for PPS-hBNPTX60-
MWNT composites
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Figure 4-12: Effects of hybrid filler compositions on PMC's Impedance for PPS-hBNPTX60-
GNP composites
4.3.5 Dielectric Constant
For electronic packaging applications, low electrical conductivity is required; however,
measure of dielectric constant can be beneficial in static charge dissipation. Figure 4-13
shows the dielectric constants of PMC embedded with different hybrid fillers. It is observed
that the composites filled with only hBNPTX60 did not have an increase in dielectric constant
(ε) at all. This shows that PMC’s embedded with only hBNPTX60 fillers can be used for
applications where high thermal conductivity and low dielectric constant (< 3) is desired i.e.
for Ultra Large Integrated Circuits (ULSI). No significant increase in ε of PPS-hBNPTX60-CF
composites was observed and the keff of these composites was very similar to that of PPS-
hBNPTX60 composites. However, ε did increase to about 10 for the composites with 11 vol.%
of CF fillers. This shows that these composites may be used for electronic packaging
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materials, for which 3 < ε < 50 is sought to dissipate static discharge. PPS-hBNPTX60-
MWNT composites showed the most increase in ε with the value reaching 80 for 11vol.% of
MWNT. These composites can; therefore, be used for capacitors as well as electronic
packaging applications. On the other hand, ε of PPS-based composites filled with GNP
showed a range of 10-40. This shows that PPS-hBNPTX60-GNP composites are good
materials for electronic packaging application. The volume fractions of GNP in the PPS-
hBNPTX60-GNP composites were 6.6%, 8.25%, 11%, which are beyond its percolation
threshold [102]. It is concluded that the ε of PPS filled with hybrid fillers can be tailored by
varying the secondary filler content in the composites. As seen in the figure, not only
increasing the secondary filler content increased the ε but the type of filler also played major
role. Therefore, a particular combination of hBN and electrically conductive secondary filler
may be chosen to obtain thermal and electrical properties for ULSI, capacitor, and/or
electronic packaging applications.
Figure 4-13: Dielectric constant of PPS-based hybrid fillers
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4.4 Micromechanical Modeling of Thermal Conductivity
Figure 4-14 to Figure 4-16 depict the results of Nielsen, parallel, and geometric models
compared to the experimental data. Since Nielsen and geometric-mean models had shown
more accurate results for PPS embedded with carbon-based filler, they were used here along
with parallel model to show the lower bound of the keff. As we know, the series model
overestimates and parallel model underestimates the thermal conductivity of the composites
as they are the theoretical upper and lower limits, respectively. However, geometric-mean
model very closely predicts the thermal conductivity of all the composites. All of these
models show a trend of increase in thermal conductivity with the increase in secondary filler
content i.e. CF, MWNT, and GNP. The reason for this trend is that the secondary fillers have
significantly higher thermal conductivity than hBN. However, none of the models takes into
consideration the interaction between the hybrid fillers and the resulting conductive
pathways. A more accurate model that takes into account the presence of two or more fillers
along with their size, structure, and orientation would more accurately predict the thermal
conductivity of these composites.
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Figure 4-14: Theoretical predictions for thermal conductivity compared to experimental
results for PPS-hBN-CF composites
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Figure 4-15: Theoretical predictions for thermal conductivity compared to experimental
results for PPS-hBN-MWNT composites
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Figure 4-16: Theoretical predictions for thermal conductivity compared to experimental
results for PPS-hBN-GNP composites
4.5 Possibility of Replacing Kapton with the Polymer Composites
In natural convection and radiation, the optimum configuration of heat sink can be
investigated by numerical study. Some measurements were conducted to see the effect of
replacing currently used Kapton tape with the polymer composites. It was observed that
aluminum heat sinks with polymer composite base plate showed higher decrease in the
maximum temperature than the heat sink with Kapton base plate. Some infra-red images of
this new heat sink design are shown in Figure 4-17 and Figure 4-18 . Since the proposed
polymer composites are electrically insulative and thermally conductive, they will be ideal
materials to replace currently used Kapton tape.
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Figure 4-17: The maximum temperature of the heat source measured for Aluminum heat
sink with a. no base plate b. with Kapton base plate and c) with PPS-33vol.% hBN
Figure 4-18: The maximum temperature of the heat source measured for Aluminum heat
sink with a. PPS-33 vol.% (hBN:MWNT=3:1), b. PPS-33 vol.% (hBN:CF = 2:1), and c.
PPS-33 vol.% (hBN:CF = 3:1) base plates
4.6 Summary Polyphenylene sulfide (PPS) based composites filled with hybrid fillers, consisting of
hexagonal boron nitride (hBN) with multi-walled carbon nanotube (GNP) were found to
have enhanced effective thermal conductivity (keff), while composites filled with hBN and
pitch-based carbon fiber (CF) had limited increase in keff. In the hybrid system, the secondary
filler promotes the interconnection among the primary fillers through the formation of a
structured network to facilitate the heat conduction across the materials. On the other hand,
for each combination of hybrid filler systems, there exists an optimal volume ratio between
the hybridizing fillers. Experimental results demonstrated that secondary fillers that have
higher aspect ratio and smaller size (e.g., GNP) were more effective in promoting the
composite’s thermal conductivity.
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Furthermore, when carbon-based secondary fillers (e.g., CF, MWNT, and GNP) were used,
the composites’ impedance (Z) decreased significantly when the volume fractions of these
electrically conductive fillers were over the percolation threshold. This phenomenon was
caused by the formation of electrically conductive pathways throughout the composite
samples. In other words, it would be possible to control the type and loading of electrical
conductive filler as the secondary filler in order to tailor the electrical conductivity of a
polymer matrix composite (PMC).
Despite the significant decrease in Z for PMCs filled with GNP as the secondary filler, these
PMCs did not exhibit the same degree of increases in their keff. Such results suggest that
considerable thermal contact resistance might also exist at the polymer-filler contacts. In
other words, continuous research efforts by adding more efficient compatiblizer would need
to be made in developing strategies to lower this resistance in order to take advantage the
full potential of polymer PMCs in the electronic packaging industry.
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Chapter 5 Composites with Aligned Polymer Fibers
5 Introduction
Polymer composite materials embedded with various conductive fillers with many
application possibilities have extensively been developed recently. The advantages of these
materials over traditional materials (metals and other) are reduced density and increased
processability due to polymer matrix [66]. However, addition of large amount of conductive
fillers to increase the thermal conductivity makes the composite counter-productive as the
processibility is severely affected and electrical conductivity is increased. For electronic
packaging applications, new composite materials that have high thermal but low electrical
conductivities are desired. It is known that the thermal and electrical conductivity of a
material depends on two different mechanisms i.e. by lattice vibration or by the presence of
free electrons, respectively. Therefore, it is proposed that polymer fibers may be introduced
and aligned in a composite in order to increase the thermal conductivity but keeping the
electrical conductivity low. This chapter discusses the fabrication of composites with aligned
polymer fibers and their effect on composite’s thermal conductivity while keeping it
electrically insulative with the addition of filler content below the percolation threshold.
Liquid crystal polymer (LCP), which has higher thermal conductivity than PPS as well as
other unique properties that help in fiber formation, was chosen as the matrix material. GNP
was chosen as the filler material due to its ability to improve thermal conductivity
remarkably better than other fillers (such as BN, CF, and MWNT).
5.1 Experimental
5.1.1 Materials
Due to the high thermal stability, shear-thinning effect, and the ability to align the molecules,
liquid crystal polymer (LCP) was selected as the matrix of the composite. High thermal
stability was an important factor in the selection of matrix because electronic components
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generally operate at relatively higher temperatures. Shear thinning effect of LCP would
enable melt compounding higher content of filler. Potential for LCP molecules to align
during processing was made use of by fabricating composites with aligned polymer fibers.
LCP with a commercial name of Vectra A950 was obtained from Ticona. It is a highly
ordered thermoplastic copolymer consisting of 73 mol-% hydroxybenzoic acid (HBA) and
23 mol-% hydroxynaphtholic acid (HNA). Its high service temperature along with the long,
rigid, rod-like molecular structure, which may help orient the polymer fibers in the
composite, make it a potential matrix for electronic packaging applications. Graphene
nanoplatelets (GNPs) were used as the filler material in LCP/GNP composites. GNPs were
purchased from CheapTubes Inc. LCP and GNPs were used as received without performing
any further modifications or treatment. GNPs were in powder- form where as LCP was
acquired in pellets. Table 5-1 and Table 3-2 summarize the physical properties of LCP and
GNP, respectively.
Table 5-1: Physical Properties of LCP
Property Value Unit
Density (ρ) 1400 Kg/m3
Melting Temperature (Tm) 280 °C
Maximum Service Temperature (Tmax) 200 °C
Thermal Conductivity (k) 0.42 W/m-K
Dielectric Constant – 10kHz 3.2 -
Coefficient of linear thermal expansion 40 m/m/°C
Elastic Modulus (E) 10.6 GPa
Tensile Strength (σt) 182 MPa
5.1.2 Sample Preparation
In order to utilize the extraordinary thermal and mechanical properties of GNP to its full
potential, uniform dispersion of GNP fillers in LCP matrix was needed. GNPs were melt-
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compounded with LCP in DSM Xplore 15 microcompounder to uniformly disperse the
fillers in the matrix. In order to keep the composites’ electrical conductivity low, the GNP
content in LCP was kept below 5 wt.%. As discussed in Section 4.3.4, the percolation
threshold of GNP lie between 5 wt. % and 10 wt.%. LCP and GNP were melt- mixed in the
micro-compounder at 300 ˚C for 10 minutes at 100 rpm. The twin-screws were rotated at a
high rpm in order to get the stacked graphene sheets separated into individual sheets
resulting in its uniform dispersion. The drawing system depicted in Figure 5-1 was used to
extrude composite fibers from the compounder. After melt-mixing LCP and GNP in the
compounder at 100 rpm for 10 mins, the rpm was reduced to 1 rpm and the drawing system
was utilized to pull out the fibers and spin them into spools. Spools of neat LCP, 1 wt.%
GNP/LCP and 5 wt.% GNP/LCP were compounded and spun.
Figure 5-1: Fiber drawing system
To be consistent with our previous experimental studies, it was required that cylindrical
samples, 1 cm in height and 2 cm in diameter, were compression molded or sintered for
thermal conductivity measurements. The extruded fibers were cut into 10 cm long stacks and
were sintered for 1 hr into thin films using a compression molder at 250˚C at 5000 psi. The
sintered films were then cut into 1 cm long thin sheets with variable widths. These thin
sheets were stacked in a cylindrical mold such that the fibers were aligned vertically (i.e.
along the heat flux direction). The mold was stacked with 3 g of sintered films because this
was the maximum mass that could be stacked. The remaining mold volume corresponding to
a mass of about ~1.5 g was filled with individual extruded fibers, with a draw ratio of 50, cut
into small pieces of about 1-3 cm in length. A total mass of ~ 4.5 g was required for each
sample. The draw ratio can be defined as the ratio between the nozzle’s diameter to the
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extruded fiber’s diamter. These samples were then compression molded by heating them up
to 310 ˚C at 3000 psi for 20 mins. The top and bottom surfaces of these samples were sanded
to make them flat. After the samples were tested for thermal conductivity, they were cut in
half. One half was used for SEM images and the other half was sanded to be used for
electrical conductivity measurements.
There was a concern that the composites may lose the vertical alignment of polymer fibers in
the disks during the compression molding process. Therefore, it was proposed that a set of 1
wt.% GNP/LCP be made by compression sintering them to 250˚C (SA) and a set of samples
be made by compression molding them to 310˚C (MA2), to comprehensively study the
effect of alignment of polymer fibers on the thermal and electrical properties. Furthermore,
in order to see the true effect of alignment of the thermal and electrical properties, two more
sets of 1 wt.% GNP/LCP were made i.e. (1) by conventional method that was used in the
previous studies (C) and (2) by compression molding and compression sintering randomly
oriented extruded fibers (MR, and SR respectively). To completely study the effect of the
filler content on the material’s properties neat LCP (MA1) and 5wt.% GNP/LCP (MA3)
composites with aligned polymer fibers were compression molded. Therefore, the
comparison between the samples MA1, MA2, and MA3 would provide the effect due to
change in filler content. The comparison between the samples C, MA2, and MR would
provide the effect of alignment. By comparing samples MA2 and SA, and MR and SR effect
of melting and sintering the samples could be obtained. And lastly, comparing samples MA2
and MR, and SA and SR effect of aligning extruded fibers along the heat flow direction
versus randomly orienting the extruded fibers could be observed.
5.1.3 Characterization
The multifunctional properties studied in this chapter are the effective thermal conductivity
(keff), impedance (Z), and dielectric constant (ε). The morphology of LCP/GNP composites
was also explored to study the possible correlation between the phase morphology and the
multifunctional properties. The k of the composites was measured by a thermal conductivity
analyzer (ASTM E1225-05) [99] at 150 ˚C. The Z and ε were measured using a dielectric
analyzer (Alpha-N-Novocontrol Technologies) over a frequency range of 10-2 to 105 Hz with
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an applied AC voltage of 1 V. The morphology of the composites was investigated by
looking at the cross-section of the composites that were fractured in liquid nitrogen and then
analyzed with a scanning electron microscopy, JEOL SEM model JSM6060.
5.2 Results and Discussion
5.2.1 Effect of Temperature and Orientation on the Alignment of Polymer Fibers
Figure 5-2 shows the thermal conductivity (k) of four sets of 1 wt.% GNP/LCP composites
fabricated by different methods. Extruded fibers with a draw ratio of 50 were used to
compression mold/sinter all four sets of samples i.e. MA2, SA, MR, and SR. MA2 and SA
sets of samples were fabricated by aligning the extruded fibers with a draw ratio of 50 along
the direction of heat flux i.e. axially along the disk. MA2 was heated up to 310 ˚C i.e. it was
compression molded whereas SA was heated up to 250 ˚C i.e. it compression sintered. Sets
of samples MR and SR were fabricated by randomly filling the cavity of the mold with 1 – 3
cm long extruded fibers of draw ratio 50. MR was heated up to 310 ˚C i.e. it was
compression molded whereas SR was heated up to 250 ˚C i.e. it was compression sintered.
Figure 5-2 illustrates that the compression molded samples showed better results than the
compression sintered samples for both sets of composites filled with aligned and randomly
oriented fibers. The reason could be the strong polymer-polymer interface in the
compression molded composites resulting in higher phonon transfer across the sample. The
compression sintered compoisites would have poor polymer-polymer iterface due to lack of
continuous polymer chain networks. Furthermore, sets of samples with aligned polymer
fibers showed better keff than randomly oriented fibers. As predicted, the improvement can be
attributed to the alignment of polymer fibers along the heat flux promoting the transfer of
phonon more efficiently.
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Figure 5-2: Effect of Compression Sintering and Compression Moldingon the Thermal Conductivity
The difference in keff values are supported by the SEM images shown in Figure 5-3 through
Figure 5-6. Figure 5-3 and Figure 5-4 show the SEM micrographs of compression molded 1
wt.% GNP/LCP composites filled with aligned and randomly oriented fibers, respectively.
The common feature in both composites was the presence of voids. As discussed in earlier
chapters, these voids are the result of limitation of the fabrication method; i.e. when the
material is melted in the mold, the material starts to leak leaving less than required amount
of material in the mold. Hence, voids develop in the samples when the mold is cooled down.
These voids are of more importance for this study as they tend to distort and affect the
alignment of polymer fibers. Another common feature in theses samples is that the fracture
surfaces look very similar. The aligned samples show very little evidence of global
alignment of fibers even though local polymer fiber alignment can be observed in both
alinged and random samples. The presence of voids may have distorted the possibility of
having global polymer fiber alignment in molten-compression molded samples.
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Figure 5-3: SEMs of 1 wt.% GNP/LCP composites filled with aligned fibers that were
molten-compression molded; (a) 100X and (b) 1000X
Figure 5-4: SEMs of 1 wt.% GNP/LCP composites filled with randomly oriented fibers that
were molten-compression molded; (a) 100X and (b) 1000X
Figure 5-5 and Figure 5-6 show SEM micrographs of compression sintered 1 wt.%
GNP/LCP composites filled with aligned and randomly oriented fibers. One common feature
in both composites is the absence of any voids. This is due to the fact that the samples were
not melted during compression molding and hence there was no leakage of material that
would cause formation of voids in the sample. It can be easily observed that the aligned
compression sintered samples preserved the polymer fiber alignment whereas randomly