Frédérick Roussel

Electro-optic and dielectric properties of optical switching devices based on liquid crystal dispersions and driven by conducting polymer [poly(3,4-ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS)]-coated electrodes

We report on the electro-optic and dielectric properties of optical switching devices based on poly(3,4-ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS) electrodes sandwiching a polymer-dispersed liquid crystal (PDLC) layer. We demonstrate that the frequency dependence of the driving electric field on the optical properties of these devices allows the fabrication of flexible bandpass light modulators. The (PEDOT:PSS) electrodes are characterized using UV-Vis, scanning electron microscopy, Raman, conductive atomic force microscopy, and linear four probe technique. The PEDOT:PSS/PDLC-based displays exhibit similar electro-optical performances to those of ITO-based devices. In addition, it can function as a bandpass light modulator. This behavior resulted from depolarization fields (Maxwell–Wagner–Sillars effects) occurring (i) at the polymer/LC interface (low frequency) and (ii) between nanometer-sized conductive PEDOT-rich domains and poorly conductive PSS-rich areas present in the bulk and at th...

Transport and thermoelectric properties of polyaniline/reduced graphene oxide nanocomposites

Polyanilines (PANI)/reduced graphene oxide (RGO) nanocomposites are chemically synthesized. Their structure and morphology are characterized by scanning and transmission electron microscopies, x-ray diffraction and Raman spectroscopy. In addition, the nanocomposites electrical, thermal and thermoelectric (TE) transport characteristics are investigated as a function of RGO content. The power factor and figure of merit (ZT) of PANI/RGO hybrids are deduced from measurements of the electrical conductivity (σ), Seebeck coefficient (α) and thermal conductivity (κ). Experimental results reveal that the properties of PANI/RGO composites are inherently dependent on the volume fraction of RGO. It is observed that electrical percolation follows a 2D conduction process which takes place for samples having 0.099 vol% RGO content. Unlike electrical conductivity, the thermal conductivity of PANI/RGO increases only slightly with the RGO fraction and is successfully fitted using a modified MG-EMA model which provides an interfacial (PANI/RGO nanoplatelets) resistance (Rk) of 4.9 × 10(-10) m(2) K W(-1). This low Rk value is attributed to good interactions between the planar geometry of RGO platelets and PANI aromatic rings through π-π stackings as evidenced by Raman spectroscopy and x-ray studies. Compared to that of pure PANI, the TE performance of PANI/RGO composites exhibits a ZT enhancement of two orders of magnitude.

The photothermoelectric technique (PTE), an alternative photothermal calorimetry

The recently introduced photothermoelectric (PTE) effect is proposed as an alternative for measuring dynamic thermal parameters of solid samples. The front PTE configuration, together with the thermal-wave resonator cavity method as a scanning procedure, was used to measure the value of thermal effusivity. The back PTE configuration, together with the chopping frequency of incident radiation as a scanning parameter, leads to the direct measurement of thermal diffusivity. A theory based on the above two detection configurations was developed and its application to solids, covering a large range of typical values of thermal parameters (aluminum and copper alloys, glass, teflon, polyethylene, LiTaO3), was described in order to demonstrate the suitability of the method. Experimental support for other well-known techniques (photopyroelectric and infrared lock-in thermography) has validated the results obtained with the novel method.

Evidence of interfacial charge trapping mechanism in polyaniline/reduced graphene oxide nanocomposites

Relaxation mechanisms in polyaniline (PANI)/Reduced Graphene Oxide (RGO) nanocomposites are investigated using broad band dielectric spectroscopy. The multilayered nanostructural features of the composites and the intimate interactions between PANI and RGO are evidenced by field emission scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. Increasing the RGO fraction in the composites results in a relaxation process observed at a frequency of ca. 5 kHz. This mechanism is associated with an electrical charge trapping phenomenon occurring at the PANI/RGO interfaces. The dielectric relaxation processes are interpreted according to the Sillars approach and the results are consistent with the presence of conducting prolate spheroids (RGO) embedded into a polymeric matrix (PANI). Dielectric permittivity data are analyzed within the framework of the Kohlrausch-William-Watts model, evidencing a Debye-like relaxation process.

Horizontally-aligned carbon nanotubes arrays and their interactions with liquid crystal molecules: Physical characteristics and display applications

We report on the physical characteristics of horizonthally-grown Single-Walled Carbon Nanotubes (h-al-SWNT) arrays and their potential use as transparent and conducting alignment layer for liquid crystals display devices. Microscopy (SEM and AFM), spectroscopic (Raman) and electrical investigations demonstrate the strong anisotropy of h-al-SWNT arrays. Optical measurements show that h-al-SWNTs are efficient alignment layers for Liquid Crystal (LC) molecules allowing the fabrication of optical wave plates. Interactions between h-al-SWNT arrays and LC molecules are also investigated evidencing the weak azimuthal anchoring energy at the interface, which, in turn, leads to LC devices with a high pretilt angle. The electro-optical reponses of h-al-SWNT/LC cells demonstrate that h-al-SWNT arrays are efficient nanostructured electrodes with potential use for the combined replacement of Indium Tin Oxyde and polymeric alignment layers in conventional displays.

Cu1–xFexO: hopping transport and ferromagnetism

Single phase, sol–gel prepared Cu1–xFexO (0u2009≤u2009xu2009≤u20090.125) powders are characterized in terms of structural, electronic and magnetic properties. Using dielectric and magnetic studies we investigate the coupling of electron and spin. The electrical conductivities and activation energies are studied with increasing Fe content. Modelling of experimental conductivity data emphasizes a single hopping mechanism for all samples except xu2009=u20090.125, which have two activation energies. Hole doping is confirmed by confirming a majority Fe3+ substitution of Cu2+ in CuO from X-ray photoelectron spectroscopy studies (XPS). Such a substitution results in stabilized ferromagnetism. Fe substitution introduces variation in coercivity as an intrinsic magnetic property in Fe-doped CuO, and not as a secondary impurity phase.

On the sign of the relaxation activation energy for interfacial polarization in reduced graphene oxide–based nano-composites

The dielectric relaxation mechanism associated with the interfacial polarization in polyaniline/reduced graphene oxide (PANI/RGO) nano-composites is found to shift towards lower frequencies on increasing temperature. Accordingly, the effective activation energy value is negative. Basic concepts of the Sillars dielectric theory of a heterogeneous medium are revisited for a material consisting of conducting platelets dispersed in a semi-insulating matrix in order to explain the negative sign of the relaxation energy. A plausible explanation to this observation involves a thermally activated de-trapping mechanism through the effective potential barrier at the interfaces between RGO and PANI. This results in an enhancement of the density of charge carriers which contributes to dc conductivity at the expense of the density of charge carriers that relax within RGO inclusions. Subsequently, the intensity of the dielectric peak is suppressed on heating which results in a systematic modification of the shapes of t...

Use of Photothermally Generated Seebeck Voltage for Thermal Characterization of Thermoelectric Materials

A simple and accurate experimental procedure to measure simultaneously the thermal properties (conductivity, diffusivity, and effusivity) of thermoelectric (TE) materials using their Seebeck voltage is proposed. The technique is based on analysis of a periodically oscillating thermoelectric signal generated from a TE material when it is thermally excited using an intensity-modulated laser source. A self-normalization procedure is implemented in the presented method using TE signals generated by changing the laser heating from one side to another of the TE material. Experiments are done on a polyaniline carbon nanohybrid (6.6 wt.% carbon nanotubes), yielding a thermal conductivity of 1.106 ± 0.001 W/m-K. The results are compared with the results from photothermal infrared radiometry experiments.

Thermal parameters of carbon nanotubes and potassium bromide composites

Photothermal radiometry is employed to investigate the thermal parameters (diffusivity, effusivity, conductivity, and heat capacity) of carbon nanotubes [single-walled (SWNT) or multiple-walled (MWNT)] and potassium bromide (KBr) pressed pellets as a function of SWNT or MWNT mass fraction. A significant enhancement of the thermal conductivity for carbon nanotubes (CNTs) contents up to 2 wt. % was observed. Above 3 wt. % CNT, a morphological transition from a compacted to an unconsolidated granular media occurs leading to a sharp decrease of the thermal conductivity (k) caused by the presence of air interfaces. A geometrical model based on interpenetrating continua is applied to describe the unusual evolution of the thermal conductivity. The behavior of k is also discussed in regard to the latest theoretical reports.

Anisotropic charge transport in ion-conductive photoresponsive polyethylene oxide–based mesomorphic materials

The mechanism of charge motion in conductive and photosensitive mesogenic block copolymers containing polyethylene oxide (PEO) segments is investigated over a wide frequency and temperature range with the broadband dielectric spectroscopy technique. It is found that the ultraviolet (UV) irradiation, the UV intensity, and the anchoring conditions of mesogenic unit in the cells produce changes in conductivity properties and in the molecular arrangement. The anisotropic nature of the conductivity is established.