28-08-2013, 03:46 PM
X-band EMI shielding mechanisms and shielding effectiveness of high structure carbon black/polypropylene composites
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Abstract
The electromagnetic interference (EMI) shielding effectiveness (SE) and EMI shielding
mechanisms of high structure carbon black (HS-CB)/polypropylene (PP) composites in the
X-band frequency range were studied. Composite plates with three different thicknesses and
five different electrical conductivities were studied. The reflection loss and absorption loss of
the composites were quantified based on the electromagnetic radiation power balance. The
results showed that for HS-CB/PP composites, absorption loss contribution to the overall
attenuation is more than the contribution of the reflection loss. The ability of the theoretical
model to predict the EMI shielding by reflection and absorption was found to be a function of
the shielding plate thickness and conductivity.
Introduction
The rapid growth in portable electronics has accelerated the
demand for lightweight enclosures that can reduce electromag-
netic interference (EMI) pollution [1, 2]. Electronics radiate
and are affected by EMI. Protecting electronic devices from in-
coming EMI is paramount to maintain device functionality and
integrity, while controlling electronic devices’ EMI emissions
is essential in complying with electromagnetic compatibility
standards imposed by governmental agencies [3]. Polymers
filled with nano- and micro-conductive fillers including car-
bon black (CB) [4, 5], carbon fibre (CF) [6, 7], metal-coated CF
[8, 9], nickel filaments [10], metal fibres [11–13], carbon nan-
otubes (CNTs) [14–18], carbon nanofibres (CNFs) [19, 20] and
most recently metal nanowires [21, 22] and graphene platelets
[23–25] have been investigated for EMI shielding applications.
Experimental
Materials and composite preparation
The materials used in this study were kindly provided by
the manufacturers. The conductive filler was Black Pearls
2000 HS-CB from Cabot (surface area 1487 m2 g−1 , particle
size 12 nm [35]). The polymer was Huntsman HO-500
PP having a MFI and density of 5 g/10 min and 0.9 g ml−1 ,
respectively. HS-CB/PP composites with 1, 3, 5, 7.5 and
10 vol% HS-CB were compounded in a Haake Rheomix batch
mixer. The volume per cent of HS-CB in PP composites
was calculated using 1.8 g ml−1 as the density of HS-CB.
Prior to compounding, the PP pellets were dried under
vacuum for 16 h at 80 ◦ C. The PP pellets were fed to the
mixer, previously heated to 200 ◦ C, and mixed for 3 min at
50 rpm. Then the required amount of HS-CB was added
and mixed for an additional 5 min at 50 rpm. For EMI SE
and electrical conductivity characterization, rectangular plates
42 × 25 mm2 of three different thicknesses 0.34, 1.0 and
2.8 mm were moulded in a Carver compression moulder at
200 ◦ C for 4 min under 14 MPa pressure. The reported results
represent an average of at least two different specimens for
each formulation.
Results and discussion
Electrical conductivity and EMI SE of the HS-CB/PP
composite
Figure 1 shows the dc electrical conductivity of the HS-
CB/PP composite as a function of HS-CB content. The
electrical conductivity (σdc ) of the composite increased with
the increase in HS-CB concentration. It is apparent that
the electrical percolation threshold is between 1 and 3 vol%
HS-CB.
The increase in electrical conductivity when
increasing filler concentration from 1 to 3 vol% is almost 14
orders of magnitude. This considerable increase is due to the
formation of conductive filler networks within the polymer
matrix [38].
Conclusions
The EMI shielding characteristics and mechanisms of
HS-CB/PP composites were studied. Composite plates
2.8 mm in thickness made of 10 vol% HS-CB/PP composite
showed an EMI SE of 43 dB. HS-CB/PP composites exhibit a
considerable increase in EMI SE with increase in composite’s
conductivity and shielding plate thickness. Regardless of the
sample thickness and conductivity, absorption loss was found
to be the major contributor to the overall EMI SE, contributing
up to 87% of the overall EMI SE. Experimental results showed
that shielding by absorption and that by reflection are mainly
a function of the composite’s conductivity. For the effect
of shielding plate thickness, the increase in shielding by
absorption with increase in shielding plate thickness was less
than the theoretical predictions. The estimated theoretical
overall EMI SE was remarkably lower than the experimental
values indicating the necessity to develop new models to
estimate the EMI SE of composite materials. Generally
speaking, the experimental absorption loss was much higher
than theoretical predictions. Meanwhile, for the reflection loss,
the theoretical model gave reasonable estimation for some of
the composites.