Investigation of the properties of polymer composite

Transcription

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© 2014 Carl Hanser Verlag, München
Zeitschrift Kunststofftechnik
4Autor
Titel (gegebenenfalls gekürzt)
Journal of Plastics Technology
www.kunststofftech.com · www.plasticseng.com
archivierte, peer-rezensierte Internetzeitschrift
archival, peer-reviewed online Journal of the Scientific Alliance of Plastics Technology
10 (2014) 3
eingereicht/handed in:
angenommen/accepted:
19.11.2013
19.05.2014
Dr.rer.nat. Nacera Stübler, Dr.-Ing. Dieter Meiners, Prof. Dr.-Ing. Gerhard Ziegmann,
Institute of Polymer Materials and Plastics Engineering, at the TU-Clausthal
Dr. Thorsen Hickmann
Eisenhuth GmbH & Co. KG, Osterode Lerbach, Germany
Investigation of the properties of polymer
composite bipolar plates in fuel cells
This work presents a comparative study of the effect of the polymer matrix on the thermal, electrical
and mechanical properties of highly-filled graphite polymer composite. This material was destined to
be used as bipolar plates in proton exchange membrane fuel cell (PEMFC). In order to investigate the
synergetic effect between Gr (Graphite) and CB (carbon black) particles the polymer PVDF (poly
(vinylidene fluoride)) was mixed with 15% wt of CB and 70% wt. of Gr. The results were offering a high
value of the electrical conductivity (176 S/cm), while the value of the electrical conductivity of
Gr/PVDF composite is about 87 S/cm. A correlation between the thermal and electrical conductivity
was explored for the samples filled only with graphite (without CB).
Untersuchung der Eigenschaften hochgefüllter
Polymer Composite für Bipolarplatten in
Brennstoffzellen
In diesem Beitrag wurde eine vergleichende Untersuchung des Effekts der Polymer-Matrix auf die
thermischen, elektrischen und mechanischen Eigenschaften von mit Graphit hoch gefüllten Polymer
Composite dargestellt. Dies wird als Bipolarplatten in PEM-Brennstoffzellen prädestiniert. Um den
Synergieeffekt zwischen Gr (Graphit) und CB (Ruß) zu prüfen, wurde exemplarisch die Polymermatrix
PVDF (poly (vinylidene fluoride)) mit 15 Gew % CB und 70 Gew % Gr gemischt. Für diesen
Verbundwerkstoff wurde eine sehr hohe elektrische Leitfähigkeit (176 S/cm) ermittelt, im Vergleich zu
einem Kennwert von 87 S/cm für Gr/PVDF-Composite. Die Korrelation zwischen der thermischer und
der elektrischer Leitfähigkeit wurde für die hochgefüllte Compound-Materialien (ohne Ruß) aufgezeigt.
© Carl Hanser Verlag
Zeitschrift Kunststofftechnik / Journal of Plastics Technology 10 (2014) 3
Stübler, Ziegmann
Characterisation of bipolar plates
Investigation of the properties of
polymer composite bipolar plates in fuel
cells
N. Stübler, D. Meiners, G. Ziegmann, T. Hickmann
1
INTRODUCTION
Conductive polymer composites play a significant role in the application areas
as organic light emitting diodes, polymer photovoltaic cells and fuel cells [1, 3].
A Proton Exchange Membrane Fuel Cell (PEMFC) converts the chemical
energy into electrical energy by electrochemical reactions. Nowadays fuel cells
represent an energy source of choice for stationary and mobile applications as
electric vehicle, speed cameras, and telecommunications.
PEMFC consists of the Membrane Electrode Assembly (MEA) which includes
the membrane, electrode and gas diffusion layer, as well as the bipolar plates.
The most important components in fuel cells are the bipolar plates. They are
multifunctional components in the PEMFC stack. They provide a conductive
medium between the anode and cathode, transfer heat out of the cell and
provide a flow field channel for the reaction gases. Three types of material are
used for bipolar plates: metallic, pure graphite and polymer composites plates.
Metallic bipolar plates of, for example, stainless steel show good mechanical
stability as well as high electrical and thermal conductivity. The main
disadvantage of metal plates is their susceptibility to corrosion. Pure graphite
bipolar plates are already known for their excellent resistance to corrosion and
their low electrical contact resistance. The graphite bipolar plates have several
disadvantages. The graphite is brittle, quite porous, has low mechanical
properties and also high costs. Polymer composites offer a more economical
alternative route for manufacturing of bipolar plates because of their lower cost,
higher flexibility, reduced brittleness, lower gas permeability and light weight
[4,5]. Therefore the development of polymer composite bipolar plates facilitates
the commercialisation of the PEM fuel cells in a wide range of applications.
Vigorous efforts were deployed to optimize the functionality of the polymer
composite plates [6-9].
However the disadvantage of these plates is their low electrical conductivity. An
excessive conductive filler content should be incorporated in the polymer matrix
during the processing to increase the electrical conductivity which leads in many
cases to a decrease of the mechanical properties. For many authors or
suppliers such as DuPont, it is a challenge to meet together good electrical,
mechanical and thermal properties of bipolar plates [9].
Journal of Plastics Technology 10 (2014) 3
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Stübler, Ziegmann
There are two different types of matrix materials to manufacture the graphitebased composite plates: thermoplastic and thermoset polymers. The composite
is created by incorporating a filler or mixture of fillers like graphite (Gr) and/or
carbon black (CB) [7]. Polymer composites are usually manufactured using
conventional polymer processing methods, such as compression molding and
injection molding. The compression molding is favoured for both thermoplastic
and thermoset matrix composite. However there is a difference in the
processing; the thermoset has to be cured by a chemical reaction whereas the
thermoplastic material has to be processed by heating to the melt phase and
cooled in the mould. The thermoset material has a shorter cycle time than
thermoplastic material. Therefore the cost-effective mass production is in this
case more efficient. If the material is injected the processing time is very short
(about 30 s) and the production is automated with high precision of the size.
The big disadvantage using the injection molding lies in the high viscosity of the
material caused by the high load of the conductive filler. The melt flows poorly
and the material after injection could have a poor quality.
The bipolar plates must have a high electrical conductivity and sufficient
mechanical properties to be used in fuel cell stack due to the constant
compressive load. The US Department of Energy (DOE) defined requirements
for an ideal material for bipolar plates [10]. These requirements are represented
as targets for the properties as contact resistance, thermal conductivity, gas
permeability, compression strength etc…
In this paper we focused on the electrical conductivity, thermal conductivity and
mechanical properties. According to the US DOE the minimum value of the
electrical conductivity and the flexural strength are 100 S/cm and 25 MPa,
respectively. The minimum value for the thermal conductivity is 10 W/mK [10].
The polymer matrix, which acts as binder in the material composite is an
insulating material with an electrical conductivity in the order of 10 -15 S/cm. A
gradual incorporation of graphite content creates the conductive network by
increasing the electrical conductivity of the sample. M. Xiao et al. [11] measured
both the electrical conductivity and the flexural strength on the
poly(arylenedisulfide)/graphite nanosheet (NanoG) composite. They observed a
a very slight negative effect of the strength when the NanoG content varies from
20wt% to 60wtl%. However the electrical conductivity increases from 40 S/cm to
190 S/cm. [11]. Ling Du [9] manufactured epoxy composite filled with expanded
graphite. He showed that the flexural strength increased from 40 MPa to 55
MPa when Gr load varied from 40 wt% to 60 wt%.
Zhu Bin et al. [12] manufactured bipolar plate by using polyvinylidene fluoride
(PVDF) as binder and titanium silicon carbide (Ti3SiC2) as conductive filler.
They showed that the electrical conductivity and flexural strength of the
composite bipolar plate with 80 wt.% Ti3SiC2 content, were 28,83 S cm−1 and
24,92 MPa, respectively. These values especially for the electrical conductivity
do not meet the DOE requirements. For bipolar plates the electrical conductivity
should be very high (> 100 S/cm) accompanied with optimal values of the
70
Stübler, Ziegmann
flexural strength. It is important to remember that it is difficult to get high
conductivity and sufficient mechanical properties simultaneously.
In this work a comparative study was performed on high graphite-filled
thermoplastic polymer and thermosetting resin -composites. The samples were
manufactured by using a kneader and then a hydraulic compression press. The
prepared samples have different polymer matrix types and filled with high Gr
contents in volume. Different properties were investigated as the thermal
stability, the in-plane electrical conductivity, the flexure strength and the thermal
conductivity. For the thermal properties both parameters as glass transition and
degradation temperature were determined to get more information about the
thermal stability of the material. The thermal conductivity measurement was
performed within a temperature range 20°C to 150°C where the operating
temperatures of PEM fuel cell (40°C-120°C) are included. The in-plane
electrical conductivity was determined at room temperature by using the 4 pointprobe method. The flexure strength of the composite was measured at room
temperature. Dynamic mechanical analysis (DMA) measurements were added
in order to get a better understanding of the material behaviour at high
temperatures.
2.
EXPERIMENT
2.1.
Material
In this work a series of graphite polymer composite bipolar plates were
developed. Five polymer matrixes were used for the preparation of the plates:
Poly (phenylene sulphide) (PPS), Poly (propylene) (PP), Poly (vinylidene
fluoride) (PVDF), Polysulfon (PSU) and Phenolic resin. A high content of
graphite (Gr) (from 85 wt. % to 92wt. %) was mixed intensively with the polymer
matrix. The volumetric mass density is related to the polymer matrix system,
which leads to different volume contents: 70 vol. % Gr for PP composite, 85 vol.
% Gr for PPS composite, 82 vol. % Gr for PSU composite and 89 vol. % Gr for
phenolic resin. Two Polymer composites were prepared using the PVDF
polymer system. The first one was filled with 85 wt. % Gr, and the second one
with an additional content of carbon black (CB) of about 15 wt. % so that the
sum of the filler content (CB+Gr) reaches the value 85 wt %. The total volume
content of the conductive filler for both materials is nearly similar (82 vol. %). To
summarise the results Gr/PPS, Gr/PSU, CB+Gr/PVDF and CB+Gr/PVDF has
approximately the same filler volume content (82-85 vol. %).
The thermoplastic polymer and the fillers were first dried and mixed in a
kneader. After that the sample was placed in hydraulic compression press and
heated to the high temperature and then cooled carefully for 15-20 min below
the melting temperature. The processing temperatures are different for each
sample; for example for PPS composite the temperature is 300 °C, for PVDF
composite 260 °C and for PP composite 180 °C. For the Phenolic resin
71
composite the sample is cured at 160 °C with a cycle below 10 min. The
thickness of all samples after compression is approximately 3 mm.
2.2. Thermal properties
The thermogravimetric analyser (TGA Q5000, TA Instrument) was used to
determine the degradation temperature T1. The temperature T1 was obtained at
5% weight loss [5]. The measurements were carried out under nitrogen
atmosphere with a heating rate of 10 K/min. In addition the glass transition T g
and the melting temperature Tm were determined by using differential scanning
calorimetry (DSC Q2000, TA Instrument) technique. The measurements were
carried out under nitrogen atmosphere with a heating rate of 20 K/min (DIN EN
ISO 11357). The weight of the sample was approximately 20 mg for TGA and
10 mg for DSC.
2.3. In-plane electrical conductivity
The in-plane (bulk) electrical conductivity was measured with a conventional
four probe method. The measurements were performed at room temperature
using a cylindrical four point probe head and RM3000 Test Unit from Jandel
Engineering Limited. The RM3000 can supply constant currents between 10 nA
and 99,99 mA. The voltage can be measured in a range between 0.01 mV and
1250 mV. The electrical conductivity  of the sample was calculated according
to [8]:
[
Stübler, Ziegmann
Figure 1
( )
( )
( )]
(1)
Diagram of four-point probe resistance method [13]
where s is the probe spacing (s = 1 mm), t is the thickness of the sample, U is
the voltage, I is the current and G is the correction factor depending on the
geometry of the sample. The specimen has a rectangular shape (40×50 mm2)
with a thickness of about 3 mm.
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Stübler, Ziegmann
2.4. Thermal conductivity
In this work the thermal conductivity of the materials was not determined
directly. In a first step, the specific heat parameter Cp was determined by using
modulated DSC where a periodic temperature modulation (± 0,48 °C every 60
s) is superimposed on the constant heating rate of a conventional DSC
measurement. In a second step the thermal diffusivity parameter was
determined by using the equipment of Laser Flash technique from Netzsch 427.
The thermal conductivity  is related to the thermal diffusivity D according to:
(T) = D(T)  Cp(T)  (T)
(2)
In this work; the volume density  is supposed to be independent of the
temperature (
⁄
) over the temperature range 20 °C - 150 °C.
The heating rate for both measurements equals 3 K/min and the temperature
range spread from 22 to 120 °C. The tests of the thermal diffusivity were carried
out on samples with cylindrical shape having a diameter of 12,7 mm, and
thickness of 2 mm. The sample for the Cp – measurements has a diameter of 5
mm and a thickness of 1-2 mm.
2.5. Mechanical properties
The three-point bending technique according to DIN EN ISO 178 was
performed at room temperature in order to obtain the flexural strength of the
materials. Rectangular specimens were cut out for flexural testing. For these
tests, the support span L was calculated using a ratio L/d of 16:1 where d is the
thickness of the specimen.
To get more information about the stiffness at high temperature, the dynamic
mechanical analysis (DMA Q800, TA Instrument) technique was carried out on
the samples. The technique was performed by using the three-point bending
mode. A scanning temperature range from 22 °C to 120 °C was employed with
a heating rate of 3 K/min. The length and the width of the specimens for DMA
test were 50 × 10 mm with a thickness of about 2 mm.
Both the storage modulus E as well as loss modulus E were determined. In
this paper only the results of E´ against the temperature at the frequency 1 Hz
are represented and discussed.
73
(b)
(a)
0,9
98
Gr/PVDF
Gr/PSU
Gr/PPS
Gr/PP
96
Heat flow (W/g)
Weight loss (%)
100
Gr/PPS
Gr/PP
Gr/PVDF
0,8
0,7
0,6
5% weigth loss
0,5
94
0,4
0
200
400
600
Temperature (°C)
100
200
300
Temperature (°C)
Figure 2:
left: Temperature-dependent percent weight for Gr/PPS
(triangle) Gr/PP (square) Gr /PVDF (open circle) and Gr/PSU
(star)
Right: Temperature- dependent heat flow for Gr/PPS, Gr/PP and
Gr/PVDF (b).
Figure 3:
Temperature- dependent heat flow showing the glass transition
temperatures of both samples
left: for Gr/PPS
right: for Gr/PSU
Stübler, Ziegmann
74
Stübler, Ziegmann
3.
RESULTS AND DISCUSSION
3.1
Thermal properties
The quantitative analysis of the weight loss against the temperature was
characterised by TGA measurements under nitrogen atmosphere. The
temperature range used for the analysis is 22 °C - 800 °C. TGA curves of
Gr/PVDF, Gr/PP, Gr/PPS and Gr/PSU composites are displayed in Fig. 2 (a).
The temperature degradation T1 is defined as the onset of a mass change at
which 5% of weight is lost. The TGA-curves show sharp drops at different
temperatures related to the different polymers, for example the degradation of
Gr/PPS and Gr/PSU start later compared to that of Gr/PVDF and Gr/PP
composite. The values of the degradation temperature T 1 for all samples are
reported in Table. 1. The value of T1 for Gr/PPS is about 552 °C, while for
Gr/PP the temperature T1 reaches the value 466 °C. This means that PPS has
a higher thermal stability than PP composite. In addition the filled phenolic resin
composite reveals an excellent heat resistance since T1 is approximately about
577 °C. DSC curves of Gr/PPS, Gr/PP and Gr /PVDF were plotted against the
temperature in Fig. 2 (b). From the DSC analysis two temperatures are
determined Tm and Tg (Fig. 3).
The results of T1, Tm and Tg are summarised in table 1. The DSC analysis
indicates that the melting temperature Tm (170 °C) for Gr/PVDF composite is
shifted slightly towards higher temperatures compared with Gr/PP composite
(Tm = 160 °C). However Gr/PPS composite exhibits a higher Tm (266 °C) and a
glass transition temperature Tg of about 96 °C (Fig. 3(a)). It should be
mentioned that Tg value is included to the operating temperature range (40 °C
to 120 °C) of the fuel cells for Gr/PPS composite. The filled PSU composite is
an amorphous thermoplastic which means it has not a crystalline melting point
while the glass transition temperature reaches the value 220 °C (Fig. 3(b)). The
glass transition is a change in the heat flow showing the shape of a step. Only
the amorphous part contributes to the glass transition leading to a soft material
upon heating or brittle material upon cooling. For PP/Gr, PVDF/Gr and
Gr/phenolic resin composite it was not possible to detect this change. According
to the reference [14,15] Tg should be lower than 0°C for both materials PP/Gr,
and PVDF/Gr. The glass transition for a phenolic resin occurs at high
temperature. The author Theodore N. Morrison [16] reported a value of about
237 °C. He attested that Tg is strongly related to the degree of cure of phenolic
resin.
75
3.2
SEM micrograph of the plates
Scanning electron microscope (SEM) was used to analyse the internal structure
of the compression moulded bipolar plates. The samples were broken and their
cross sectional areas were considered for the SEM analyses.
Composite Filler (vol. %)
PPS
85
PP
70
PSU
82
PVDF
82
Phenolic
resin
89
Table 1:
T1 (°C)
552
466
573
463
577
Tg (°C)
Tm (°C)
96
266
/
160
220
amorphous
/
170
/
thermoset
Glass transition temperature Tg, degradation Temperature T1
and melting temperature Tm determined for all samples. Tg value
for Gr/PP, Gr/Gr and Gr/phenolic resin could not be detected
Figure 4 illustrates the morphology of the samples Gr+CB/PVDF, Gr/PVDF,
Gr/PP, Gr/PSU. The SEM images of Gr/PP, Gr/PSU and Gr/PVDF show dense
materials and no obvious voids are observed. The structure of the Gr+CB/PVDF
composite shows different voids (see the arrow in the picture). For this material
it is not possible to differentiate between CB and Gr particles because both
fillers have approximately the same average particle sizes.
Stübler, Ziegmann
76
Gr+CB/PVDF
Gr/PP
Gr/PVDF
Gr/PSU
Figure 4:
SEM micrograph of fractured cross section of Gr+CB/PVDF,
Gr/PVDF, Gr/PP and Gr/PSU composite
Figure 5:
In-plane electrical conductivity for Gr/PSU, Gr/PP, Gr/PPS,
Gr/PVDF, Gr+CB/PVDF and Gr/phenolic resin composite. For
each composite the volume content in % is given. Dashed line
shows the limit of  =100 S/cm defined by DOE.
Stübler, Ziegmann
77
Stübler, Ziegmann
3.3
Electrical conductivity
The in-plane electrical conductivity of highly filled polymer composites was
determined at room temperature through the conventional four- point- probe
method. The polymer in the composite is acting as an insulator; with increasing
filler concentration an insulator- conductor transition appears. A continuous
increase of the filler content increases strongly the electrical conductivity. The
transition is described by the percolation theory [17,18]. At percolation threshold
concentration the electrical conductivity increases abruptly by several orders of
magnitude. The filler aggregates touch each other enabling electrons to
“percolate”, thus leading to the creation of many percolation pathways. A good
quality of the dispersion of fillers in the polymer matrix contributes to the
increase of the number of the percolation pathways. The percolation threshold
is strongly related to the nature as well as to the aspect ratio as well as the type
of the filler [6].
Figure 6:
In-plane electrical conductivity of Gr/ phenolic resin composite
measured on a rectangular plate (70×80 mm2). The electrical
conductivity was measured on more than 200 positions
In an ideal case, a filled polymer composite for the bipolar plates should have a
high content of graphite with a high dispersion quality in order to meet the US
Department of Energy (DOE) requirements. In plane electrical conductivity  of
Gr/ phenolic resin composite was obtained on a rectangular plate (70×80 mm2).
More than 200 positions were considered to measure the in plane current. The
figure 6 is a typical example showing the distribution of  values on the plate.
The values stretched from 100 S/cm to 200 S/cm. The corresponding standard
78
deviation is approximately 25%. The standard deviation of  for the rest of
samples varies between 15 and 25%. The dispersion of  values is probably
due to the agglomeration effect of filler particles during the processing.
The results of the in-plane electrical conductivity of all samples are represented
in Fig. 5. The volume content of fillers is added for each sample in the figure.
The results show that the value of the electrical conductivity for Gr/PPS,
Gr+CB/PVDF and Gr/phenolic resin composite exceed the limit value of the
electrical conductivity ( = 100 S/cm) defined by DOE. However a value of 87
S/cm was determined for Gr /PVDF. The electrical conductivity for Gr+CB/PVDF
composites reaches a very high value of 176 S/cm. Indeed there is a synergistic
effect on the electrical conductivity between graphite and carbon black. The
measured conductivity is caused by the macroscopic resistance of the filled
polymer composite which depends not only on the graphite aggregates
resistance but also on the resistance of inter- aggregates space. The
incorporation of CB and Gr particles to PVDF matrix created additional
connection between Gr and CB -aggregates.
The Gr/PSU composite shows a value of  of about 21 S/cm despite the high
volume content (82 vol%) of graphite used during the processing of the
material. A similar content of graphite in volume was incorporated to prepare Gr
/PVDF composite. However the electrical conductivity of this sample (87 S/cm)
is nearly 4 times greater than for the Gr/PSU composite. In addition the
electrical conductivity  of Gr/PP reaches the value 46 S/cm with 70 vol. % Gr
load. These results show that the filler load, alone, is not an indicator for a high
electrical conductivity. The electrical conductivity depends also on the
processing parameters (like temperature, pressure …), viscosity of the melt and
especially on the interfacial interaction between graphite/carbon black and
polymer matrix [19].
Stübler, Ziegmann
79
16
(a)
14
1,0
Cp [J(g.°C)-1]
Thermal diffusivity (mm2s-1)
PPS
12
0,8
10
8
20
40
60
80 100 120
Temperature (°C)
Thermal conductivity  (W m-1 K-1)
30
PPS
(b)
25
20
15
10
25
50
75
100
125
Temperature (°C)
Figure 7:
left:
T-dependent thermal diffusivity (square symbol) and
specific heat (circle symbol)
right. T-dependent thermal conductivity against the temperature
(triangle symbol) for Gr/PPS composite (b).
Figure 8:
Temperature-dependent thermal conductivity for Gr/PSU (full
circle), Gr/PP (open circle), Gr+CB/PVDF (full triangle), Gr/PPS
(star), Gr/PVDF (open triangle) and Gr/phenolic resin (square).
Dashed line shows the limit of  =10 W/m K defined by DOE.
Stübler, Ziegmann
80
Stübler, Ziegmann
3.4
Thermal conductivity
One of the main functions of the bipolar plate is the transfer of the waste heat to
a cooling channel in order to maintain a constant operating temperature in the
fuel cell systems and to prevent an eventual degradation of the proton
exchange membrane. Therefore a high thermal conductivity is necessary to
move the heat through the plates rapidly. As mentioned previously the thermal
conductivity is calculated from the thermal diffusivity using the Laser Flash
technique.
Thermal diffusivity D is a measure of the rate at which heat transports through
the material is conducted. The rear surface of the sample is illuminated
uniformly with a pulsed laser (1064 nm) for few ms. The sample absorbs the
laser energy and warm up. The heat diffuses through the thickness of the
sample creating a thermal gradient in one direction. The solution of the Fourier
equation for heat diffusion is [20]:
(
)
[
∑
(
)
(
(
)
)]
(3)
Where L is the sample thickness, D the thermal diffusivity and Tend is the total
temperature variation of the sample.
The Fig. 7(a) shows an example of the evaluation of the thermal conductivity of
Gr/PPS composite. The figure displays both curves; thermal diffusivity D and
specific heat cp against the temperature. Both cp- and D- curves show distinctive
trends. The D values decrease with increasing temperature, while cp values are
increasing. By applying Eq. 2, the thermal conductivity  was calculated and
plotted in Fig. 7(b). The value of  at room temperature is about 21 W m-1 K-1.
By increasing the temperature, a very slight drop of  is observed, which means
that the thermal conductivity is slightly affected within the operating temperature
range of the fuel cell (40 °C-120°C).
The parameter  was plotted as function of the temperature for Gr/phenolic,
Gr/PPS, Gr/PVDF, Gr/PSU, Gr+CB/PVDF and Gr/PP composites (Fig. 8). It can
be observed that all the samples exceed the value of the  parameter 10 W/ m
K defined by US DOE. The pure graphite material is characterised with a high
value of thermal conductivity ( 160 W/m K). However the obtained values of
the filled polymer composites in Fig. 8 are at least 5 times lower although the
samples contain less than 15 vol% of polymer (excepted for Gr/PP).
In addition the results in Fig 8 show that the thermal conductivity is strongly
influenced by the choice of the polymer matrix-type; although the thermal
conductivity  values of the employed polymer matrix are nearly similar (from
0.16 to 0.25 W/mK). The reason is probably due to the interfacial thermal
resistance between Gr/CB and polymer which strongly depends on the
interfacial interaction strength [19]. Tengfei Luo et al. [19] studied the thermal
81
Stübler, Ziegmann
energy transport in graphene/graphite polymer (paraffin wax-C30H62) composite
systems using molecular dynamics simulation. They showed that the graphenepolymer interfacial thermal conductance is greatly enhanced by increasing the
graphene size and polymer density or forming covalent bonds between the
graphite edges and polymer molecules. For Gr/phenolic composite this
interaction strength seems to be stronger leading to a higher value of  (27 W/
m K) at room temperature.
If we consider the figure 5 and 8, by omiting the Gr+CB/PVDF composite, we
can clearly observe the correlation between thermal and electrical conductivity
(Fig. 9).
Figure 9:
The electrical conductivity depending on the thermal conductivity
of highly filled polymer composites having different polymer matrix
systems.
In addition it was expected that the incorporation of 15 wt.% of CB to Gr/PVDF
will increase the thermal conductivity. However the experiment results show the
contrary;  values are approximately 20% higher for Gr/PVDF composite. This
result agrees well with the morphological analysis of Gr+CB/PVDF composite
(Fig. 4) showing voids inside the structure of the material.
Furthermore a pronounced drop of  parameter after 100 °C can be observed
for Gr+CB/PVDF composite. This is may be due to the increasing of the heat
flux into the sample at high temperature which leads to the decreasing of the
intermolecular forces. The conduction of the heat is therefore decreasing.
82
Stübler, Ziegmann
3.5
Mechanical properties
Bipolar plates support the membrane electrode assembly and are exposed to
the constant effect of the compressive load of the PEMFC or exposed to
exterior shocks and vibrations if the fuel cells are used as portable electronics
and transportation systems. Therefore robust bipolar plate having good
mechanical properties is a required condition.
The three-point-bending technique was performed at room temperature in order
to obtain the flexural strength parameter R. The results of the bending strength
are represented in Fig. 10. It can be seen that the R value exceed the value
defined by US DOE (25 MPa) for almost all samples. A value of 26 MPa of the
flexural strength was obtained for the Gr+CB /PVDF and 24 MPa for Gr/PVDF.
By considering the standard deviation of R ( 4 MPa), the values of the flexural
strength are approximately the same which indicates a similar brittleness for
both samples. The flexural strength values of the rest of samples is positioned
in the range 31 MPa - 37 MPa.
Dynamic mechanical analyzer (DMA) tests were carried out on the Gr/phenolic
resin, Gr+CB /PVDF, Gr/PVDF, PP/PVDF and PPS/PVDF (Fig. 11). The DMA
apparatus measures the energy to deform a sample (storage modulus E). A
perfectly elastic sample returns all of the energy. If the energy is partially
returned, the rest of the energy is measured as loss modulus related to the
dissipated energy by molecular motion. The stiffness of Gr/PPS composite in
Fig. 11 keeps a constant value as the temperature goes up, until approximately
80 °C. A sharp drop appears from 80 °C to higher temperature showing the
direct result of an increase in molecular mobility, a visible creep effect occurs in
the sample and the glass transition takes place. The result agrees well with the
value of Tg obtained by DSC measurements (96 °C).
The E-curves for Gr+CB /PVDF, Gr/PVDF and PP/PVDF show a steady pace
drop of the stiffness as the temperature goes up. These samples show a soft
state over the temperature range 20°C to 120 °C because of their low glass
transition temperature (Tg < 0°C).
An ideal polymer composite bipolar plate should have a constant stiffness within
the operating temperature range of the fuel cell (40°C - 120 °C). A physical
change as the glass transition in this range is not desirable (case of Gr/PPS), it
can affect the performance of the fuel cell.
The storage modulus E of Gr/phenolic resin shows nearly a constant value of
E from room temperature to 120 °C indicating non change in the stiffness over
the operating temperature range 40 -120 °C.
83
Figure 10:
Flexural strength for Gr/PSU, Gr/PP, Gr/Epoxy resin, Gr/PPS,
Gr/PVDF, Gr+CB/PVDF, and Gr/phenolic resin composites.
Figure 11:
Temperature-dependent storage modulus for Gr/PP, Gr/PPS,
Gr+CB/PVDF, and Gr/phenolic resin composites
Stübler, Ziegmann
84
Stübler, Ziegmann
4.
CONCLUSION
Highly-filled graphite polymer composites were investigated to be used as
bipolar plates. The effect of the polymer matrix-type on the thermal, electrical
and mechanical properties was examined. The volume content of the
conductive fillers for Gr/PSU, Gr/PPS, Gr+CB/PVDF and Gr/PVDF varies
between 82vol% and 85vol%. However the electrical conductivity (22 S/cm) for
Gr/PSU composite is significantly lower in comparison to that (87 S/cm) of
Gr/PVDF composite. The same tendency was observed for the thermal
conductivity despite that the volume contents were approximately similar for
Gr/PSU and Gr/PVDF. The thermal conductivity depends especially on the
interfacial bonding strength between graphite and polymer.
An additional incorporation of about 15 wt.% CB to Gr +PVDF increased
strongly the electrical conductivity (176 S/cm) although some voids were
observed in the morphological structural of Gr+CB/PVDF. This is a direct result
of the positive synergetic affect between Gr and CB particles leading to the
creation of additional connections between Gr and CB -aggregates.
A correlation was observed between the electrical and thermal conductivity by
considering all samples prepared only with graphite (without Gr+CB/PVDF).
Composite
Gr/Phenolic
resin
Gr/PPS
Gr+CB/PVDF
Gr/PVDF
Gr/PP
Gr/PSU
Gr/Epoxy
Table 2:
Thermal conductivity







electrical
conductivty
Flexural
strength






-




The results of the thermal conductivity, electrical conductivity and
the flexural strength in relation with the requirements defined by
The US Department of Energy (DOE).
Furthermore the results of Gr/phenolic resin, Gr/PPS and Gr+CB/PVDF
composites regarding in-plane electrical conductivity, thermal conductivity and
mechanical properties meet the requirements defined by the US Department of
Energy. However Gr/phenolic resin composite showed the best properties (see
Tab. 2).
85
Stübler, Ziegmann
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[10] Yeetsorn, R.
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Stübler, Ziegmann
88
Stübler, Ziegmann
Stichworte:
hochgefüllte Compound-Materialien auf Graphitbasis, Bipolarplatten
Brennstoffzellen, elektrische Leitfähigkeit, wärmeleitfähigkeit
in
Keywords:
Polymer composite, bipolar plates in Fuel cells, electrical and thermal
conductivity
Autor/author:
Dr.rer.nat. Nacera Stübler
Dr.-Ing. Dieter Meiners
Prof. Dr.-Ing.Gerhard Ziegmann)
Institute of Polymer Materials and
Plastics Engineering, at the TU-Clausthal
Agricolastr. 6
D-38678 Clausthal-Zellerfeld
Dr. Thorsen Hickmann
Eisenhuth GmbH & Co. KG,
Osterode Lerbach, Germany
Herausgeber/Editor:
Europa/Europe
Prof. Dr.-Ing. Dr. h.c. Gottfried W. Ehrenstein, verantwortlich
Lehrstuhl für Kunststofftechnik
Universität Erlangen-Nürnberg
Am Weichselgarten 9
91058 Erlangen
Deutschland
Phone: +49 (0)9131/85 - 29703
Fax.:
+49 (0)9131/85 - 29709
E-Mail-Adresse: [email protected]
Verlag/Publisher:
Carl-Hanser-Verlag
Kolbergerstraße 22
D-81679 München
Tel.: +49 (0)89 99830-613
Fax: +49 (0)89 99830-225
E-Mail-Adresse: Webseite:
[email protected]
Tel.: +49 (0)5323/722426
Fax: +49 (0)5323/722324
Amerika/The Americas
Prof. Prof. hon. Dr. Tim A. Osswald,
responsible
Polymer Engineering Center,
Director
University of Wisconsin-Madison
1513 University Avenue
Madison, WI 53706
USA
Phone: +1/608 263 9538
Fax.:
+1/608 265 2316
E-Mail-Adresse:
[email protected]
Redaktion / Editorial Office:
Dr.-Ing. Eva Bittmann
Christopher Fischer, M.Sc.
Beirat / Advisory Board:
38 Experten aus Forschung und
Industrie, gelistet unter
89

Investigation of the properties of polymer composite

Transcription

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