THESIS
2015
xi, 67 pages : illustrations ; 30 cm
Abstract
Graphene foams (GFs) with a cellular structure are prepared using the template-based
chemical vapor deposition (CVD) method. They have unique properties, such as very
low densities, excellent electrical conductivities, and high elasticity and flexibility.
GF/poly(dimethyl siloxane) (PDMS) composites are prepared by infiltrating ethyl
acetate solvent-diluted PDMS into the porous GFs. The unique three-dimensional (3D)
interconnected porous structure of the composites with inherent percolation can find
multi-functional applications in absorption of both electromagnetic and sound waves.
High electrical conductivities and interconnected conductive channels are beneficial to
electromagnetic interference (EMI) shielding, and efficient sound absorption requires
the materials and struct...[
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Graphene foams (GFs) with a cellular structure are prepared using the template-based
chemical vapor deposition (CVD) method. They have unique properties, such as very
low densities, excellent electrical conductivities, and high elasticity and flexibility.
GF/poly(dimethyl siloxane) (PDMS) composites are prepared by infiltrating ethyl
acetate solvent-diluted PDMS into the porous GFs. The unique three-dimensional (3D)
interconnected porous structure of the composites with inherent percolation can find
multi-functional applications in absorption of both electromagnetic and sound waves.
High electrical conductivities and interconnected conductive channels are beneficial to
electromagnetic interference (EMI) shielding, and efficient sound absorption requires
the materials and structures to have open-cell channels so as to attenuate and dissipate
wave energies.
Composites with different porosities and densities are prepared to investigate their
effects on EMI shielding effectiveness (SE) and low-frequency sound absorption. The GF/PDMS composites show increasing electrical conductivities and EMI SEs with
increasing porosity, and the GF/PDMS composite with a 90.7% porosity has the highest
electrical conductivity of 6.74 S/cm, leading to the highest EMI SE of ~25 dB in the
X-band frequency of 8.2-12.4 GHz. The composite with a 51.3% porosity exhibits an
exceptional absorption peak of ~0.7 at a low frequency range of 100-150 Hz.
Further studies are made by incorporating well-dispersed multi-walled carbon
nanotubes (MWCNTs) in the PDMS matrix to form a novel GF/CNT/PDMS
electrically conductive, hybrid structure with open channels of two different scales. The
hybrid composites display significant enhancements in both EMI shielding
performance and low-frequency sound absorption, compared to the GF/PDMS
composites. The GF/MWCNT/PDMS composite with 2 wt.% CNTs and 90.8%
porosity delivers an excellent electrical conductivity of 31.5 S/cm and a remarkable
EMI SE of ~75 dB, equivalent to nearly 200% increment against the GF/PDMS
composite with the same porosity. There is significant synergy between GFs and CNTs
in the hybrid composites: the GFs force the incident microwaves to be either reflected
or attenuated through the electrically conductive, microscale channels, while the CNTs
facilitate the attenuation of microwaves through the nanoscale channels by multiple
scattering and interfacial electric polarization.
The addition of MWCNTs also significantly improves the sound energy dissipation by
friction between the CNTs and polymer matrix, leading to a commercially viable sound
absorption coefficient of more than 0.3 over the low frequency range of 100-1000Hz.
The microscale and nanoscale carbon channels in the composites are responsible for the
better performance over 100-1000 Hz, because pores and holes with multiple sizes
within the materials are beneficial for broadening the effective frequency band.
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