Graphene is a one-carbon atom thick sheet with a hexagonal lattice. The chemical structure of graphene is dependent on its processing and synthesis methods. Indeed, graphene obtained via mechanical exfoliation of graphite may yield defect-free single layer sheets, whereas chemically produced graphene results in defective graphene that is highly functionalized with different chemical groups. In the first part of my talk, I will review the major techniques employed for the production of graphene sheets in laboratory and industrial scales. Then, I will discuss their resulting physical and chemical characteristics. In the second part of my talk, I will review and discuss the broad applications of graphene and the most recent achievements.

We have covalently grafted tetrazine derivatives to graphene oxide through nucleophilic substitution. Since the tetrazine unit is electroactive and nitrogen-rich, with a reduction potential sensitive to the type of substituent and degree of substitution, we used electrochemistry and X-ray photoelectron spectroscopy to demonstrate clear evidence for grafting through covalent bonding. Chemical modification was supported by Fourier transform infrared spectroscopy and thermal analysis. Tetrazines grafted onto graphene oxide displayed different mass losses compared to unmodified graphene and were more stable than the molecular precursors. Finally, a bridging tetrazine derivative was grafted between sheets of graphene oxide to demonstrate that the separation distance between sheets can be maintained while designing new graphene-based materials, including chemically bound, redox structures.

Composites of carbon-based fillers and elastomeric matrices are at the heart of developing technologies such as high-strain sensors/actuators and stretchable electronics due to their unique combination of electrical and mechanical properties. Production of these composites typically includes dispersion of filler particles into an uncross-linked polymer matrix such as liquid polydimethylsiloxane (PDMS) and a subsequent cross-linking of that matrix. We show here that, in the cross-linking of PDMS elastomer, carbon-based fillers such as carbon blacks and functionalized graphene can diminish the extent of cross-linking via a deactivation of small molecule catalysts and cross-linking agents. This deactivation is evidenced by the relationship between the filler loading, the composition at which gelation is observed, and the elastomer cure time. We have studied composite mechanical properties over a broad range of cure mixture compositions, and we demonstrate that materials with a high degree of cross-linking can be obtained when corrections are applied for this deactivation effect. Mechanical and electrical properties of these composites are explored with stretchable conductor applications in mind.

Elastic instabilities, when properly implemented within soft, mechanical structures, can generate advanced functionality. In this work, we use the voltage-induced buckling of thin, flexible plates to pump fluids within a microfluidic channel. The soft electrodes that enable electrical actuation are compatible with fluids, and undergo large, reversible deformations. We quantified the onset of voltage-induced buckling, and measured the flow rate within the microchannel. This embeddable, flexible microfluidic pump will aid in the generation of new stand-alone microfluidic devices that require a tunable flow rate.

Buckling of plates and rods are often avoided in design of structures; however, proper implementation of these
instabilities lead to advanced material functionality. In this work, we use the buckling of circular plates for pumping
fluids with a specific flow rate. Dielectric elastomers have been widely used in many applications such as soft
robotics, opto-electro-mechanical systems, and energy harvesting devices due to their unique mechanical and
electrical properties as well as their ability to convert electrical signals to mechanical actuations and vice versa. If we
confine a thin dielectric elastomeric plate at its edge and expose it to an electric field, it will undergo buckling. When
embedded within a microfluidic device, the out-of-plane deformation can be used as a pumping mechanism to inject
the fluids above or below the plate into channels. In order for the device to be compatible with fluids, the dielectric
film was sandwiched between two thin, flexible, solid electrodes that can not only be in direct contact with fluids, but
also undergo significant deformation without losing their functionality. We conducted experiments to quantify the
voltage-induced buckling instability and measured the flow rate as a function of voltage. In addition, the effect of
other boundary conditions such as volume and pressure difference between two sides of the thin film was investigated.
We also show that these pumps can be used in series and/or parallel to enhance the flow rate besides the pump
efficiency. Finally, we offer an analytical prediction that uses plate buckling theory to estimate the critical voltage for
buckling of confined dielectric plates.

Fluid flow can be directed and controlled by a variety
of mechanisms within industrial and biological environments. Advances in
microfluidic technology have required innovative ways to control fluid flow on a
small scale, and the ability to actively control fluid flow within microfluidic devices
is crucial for advancements in nanofluidics, biomedical fluidic devices, and digital
microfluidics. In this work, we present a means for microfluidic control via the electrical
actuation of thin, flexible valves within microfluidic channels. These structures
consist of a dielectric elastomer confined between two compliant electrodes that can
be actively and reversibly buckle out of plane to pump fluids from an applied voltage.
The out-of-plane deformation can be quantified using two parameters: net change
in surface area and the shape of deformation. Change in surface area depends on
the voltage, while the deformation shape, which significantly affects the flow rate, is
a function of voltage, and the pressure and volume of the chambers on each side of
the thin plate. The use of solid electrodes enables a robust and reversible pumping
mechanism that will have will enable advancements in rapid microfluidic diagnostics,
adaptive materials, and artificial muscles.

Dielectric elastomers are well-known for their superior
stretchability and permittivity. A fully-clamped thin elastomer will buckle when it
is compressed by applying sufficient electric potentials to its sides. When embedded
within soft, silicone rubbers, these advanced materials can provide a means for a biocompatible
pumping mechanism that can be used to inject bio-fluids with desired
flow rates into microfluidic devices, tissues, and organs of interest. We have incorporated
a dielectric film that is sandwiched between two thin, flexible, solid electrodes
into a microfluidic device and utilized a voltage-induced out-of-plane buckling instability
for pumping of fluids. We experimentally quantify the voltage-induced plate
buckling and measure the fluid flow rate when the structure is embedded in a microchannel.
Additionally, we offer an analytical prediction that uses plate buckling
theory to estimate the flow rate as a function of applied voltage.

A carbon-nanotube architecture based on ceramic microparticles allows for strikingly reducing the number of thermal contact resistances between carbon nanotubes (CNT). The result is a 130% enhancement of the thermal conductivity of the nanocomposites at a remarkably low CNT mass fraction of 0.15 wt%.

Carbon nanotubes (CNTs) are ideal candidates to reinforce thermoset polymers due to their exceptional intrinsic properties. The resulting multifunctional nanocomposite has electrical, thermal and mechanical properties sensitively higher than pristine polymer. Therefore, this new material possesses various potential applications, and particularly in the domain of electronics and aerospace. The aim of this PhD thesis is oriented towards two directions. In the first one, we establish efficient techniques to produce composite materials with multifunctional properties. Then, the objective consists in the enhancement of these properties by proposing valuable alternatives to previous results cited in the litterature. In the first chapter, we present the state of the art research concerning the materials studied during this work. Among these, there are in particular: CNTs, hybrids constituted of CNTs and alumina microparticles, electronically conducting and thermoset polymers. Moreover, this chapter deals with the characteristics of each material, i.e. elaboration techniques, structures and properties. The second chapter of the manuscript contains first, the elaboration techniques allowing the synthesis of high quality nanocomposites according to international standards. Then, we analyze the properties of these nanomaterials, and particularly in terms of electrical and thermal transports. Further characterization procedures allow better understanding of the obtained structures in a domain ranging from macroscopic to atomic scales. This is realized using scanning/transmission electron microscopy, Raman spectroscopy, EELS, XPS, and AFM. Electrical and thermal conductivity measurements obtained on these new materials give prominence to the necessity of some improvements. Thereby, we have focused our research on the physico-chemical phenomena at the matrix/filler interface. We have proposed to modify the surface of CNTs, in order to favour the matrix/filler cohesion, but also and mainly to decrease contact resistances between the randomly distributed CNTs within the polymer matrix. Finally, the last chapter deals with the surface functionalization of CNTs using electrochemistry. First, we have implemented an accurate technique to deposit a nanometric layer of electronically conducting polymer on the surface of CNTs. This conducting polymer, namely polypyrrole (Ppy) is in the meantime biocompatible. The accuracy and efficiency of our approach are demonstrated through various characterization techniques, and particularly using transmission electron microscopy. Further studies using AFM coupled with a resiscope indicate the electrical resistance distribution performed on CNT-Ppy hybrids. In the second part of this chapter, we present our method to control precisely the thickness of the Ppy layer around the CNTs.

A cavity microelectrode (CME) was used to perform an electrochemical synthesis of hybrid materials made of carbon nanotubes (CNTs) and conducting polymers. The confinement of the CME is used to produce a uniform nanometric coating of an electronically conducting polymer such as poly(N-methylpyrrole) (Pmpy) on multiwalled carbon nanotubes. The CME also allows easy characterization of the presence of the polymer layer on the surface of the CNTs by cyclic voltammetry. Transmission electron microscopy allowed us to measure the thickness and confirm the homogeneity of the Pmpy coating around the CNTs. Finally
Raman spectroscopy brings additional information on the electrogenerated hybrid materials.

Broad-frequency dielectric behaviors of multiwalled carbon nanotubes MWCNTs embedded in room temperature vulcanization silicone rubber RT-SR matrix were studied by analyzing alternating current ac impedance spectra, which would make a remarkable contribution for understanding some fundamental electrical properties in the MWCNT/RT-SR nanocomposites. Equivalent circuits of the MWCNT/RT-SR nanocomposites were built, and the law of polarization and mechanism of electric conductance under the ac field were acquired. Two parallel RC circuits in series are the equivalent circuits of the MWCNT/RT-SR composites. At different frequency ranges, dielectric parameters including conductivity, dielectric permittivity, dielectric loss, impedance phase, and magnitude present different behaviors.