A. Khoury

Fabrication and characterization of an evanescent wave fiber optic sensor for air pollution control

The fiber optic evanescent wave sensors provide low-cost integrated optics sensors for the monitoring of air pollution and species concentrations in various environments. This sensor offers the possibility to be linked to fiber optics for remote measurements and dangerous locations. It offers multiplexing possibilities and presents perfect immunity to electromagnetic perturbations. A fraction of the power transmitted along the core of the fiber propagates as an evanescent field in the cladding, so the transmission of this fiber depends on the absorption of the cladding. The sensor is obtained by removing a small length of the cladding of a standard commercial single-mode fiber silica/silica and replacing it by an absorbing medium. The evanescent field in the cladding interacts with the absorption medium surrounding the core and the power transmitted by the fiber is attenuated. In this paper, we describe a simple method for removing the cladding by chemical etching with HF. The transmission characteristics of this sensor is analyzed to determine the diameter of the remaining core. Due to the core capillarity to HF, a tapered profile is produced at the ends and the geometry is optically determined. A metallic or polymer layer can be deposited to increase the response of the sensor and adapt it to the detection of a specific pollutant.

Electrical and optical properties of pyrolytically electrostatic sprayed fluorine-doped tin–oxide: Dependence on substrate–temperature and substrate–nozzle distance

Tin–oxide (SnO2) thin films were prepared by a modified spray technique called electrostatic spray pyrolysis. Their electrical and optical properties dependence on fluorine concentration, substrate temperature Ts, and the substrate–nozzle distance Dsn was studied. Dsn and Ts seem to be interrelated for the optimization of good quality fluorine-doped SnO2 films. Films of about 9×10−4 Ω cm resistivity and high visible 85% transparency were obtained for a substrate temperature of 550 °C, a 6 cm substrate nozzle distance, and 80% of hydrofluorhydric acid HF in the starting solution. These data are discussed in light of the kinetics reaction.

Iron-based 1D nanostructures by electrospinning process

Iron-based 1D nanostructures have been successfully prepared using an electrospinning technique and varying the pyrolysis atmospheres. Hematite (Fe(2)O(3)) nanotubes and polycrystalline Fe(3)C nanofibers were obtained by simple air or mixed gas (H(2), Ar) annealing treatments. Using the air annealing treatment, a high control of the morphology as well as of the wall thickness of the nanotubes was demonstrated with a direct influence of the starting polymer concentration. When mixed gases (H(2) and Ar) were used for the annealing treatments, for the first time polycrystalline Fe(3)C nanofibers composed of carbon graphitic planes were obtained, ensuring Fe(3)C nanoparticle stability and nanofiber cohesion. The morphology and structural properties of all these iron-based 1D nanostructures were fully characterized by SEM, TEM, XRD and Raman spectroscopy.

Diode laser sensor for process control and environmental monitoring

Absorption spectroscopy with tunable diode lasers (TDLAS) in the infrared region is a well-known technique for the chemical analysis of gas mixtures. The laser provides a high selectivity, which is important in industrial environments such as in-line stack monitoring, where complex gas mixtures are present. A wavelength tunable diode laser in the near infrared region has been utilized as a light source in absorption measurements of air pollution resulting from energy usage for industry. The emission frequency can be varied over a relatively wide spectral range by changing the current and temperature of the diode.

Fabrication of tin oxide (SnO 2 ) thin film by electrostatic spray pyrolysis

Electrostatic spray pyrolysis (ESP) technique has been used for thin film deposition of fluorine-doped tin oxide, which is used for optoelectronic applications. The results show that films are polycrystalline, highly conductive (ρ=9.3×10−4 Ω cm), and have a high transmission (85%) in the visible range.

Fabrication and study of photovoltaic material CuInxGa1-xSe2 bulk and thin films obtained by the technique of close-spaced vapor transport

Abstract We present the technique of preparation of the material photovoltaic CuInxGa1−xSe2 thin films for the fabrication of solar cells. Bulk materials were synthesized by the Bridgman technique using the elements, Cu, Ga, In and Se. We have realized thin films by close-spaced vapor transport (CSVT) in a closed tube where the iodine is the transport agent. The CSVT technique is simple [J. Appl. Phys. 84 (1998) 1; Thin Solid Films 226 (1993) 254], cheap and allows the production of samples with good crystalline qualities, in order to produce photovoltaic cells for solar energy conversion. The bulk samples and thin films were characterized by energy dispersive spectrometry, scanning electron microscope, hot point probe method and X-ray diffraction.

Degradation of the diode ideality factor of silicon n–p junctions

Abstract An investigation of the effects of bulk and interface degradation processes on the junction ideality factor is presented. The ideality factor is extracted from a description of the simulated current–voltage characteristics of the junction, using a double exponential model. The junction ideality factor appears as a sensitive parameter appropriate to characterize the effect of recombination losses and to quantify the magnitude of junction devices degradation.

X-ray diffraction studies of electrostatic sprayed SnO2: F films

Fluorine-doped tin oxide films were deposited by electrostatic spray pyrolysis technique (ESP), on 1cmx1cm Corning 7059 substrates. The structural and electrical properties of the deposited films with different doping levels are studied. Relative variations in the structural properties were explained on the basis of structural factor calculations. The results show that the incorporation of fluorine atoms took place only at substitutional sites leading to an increase in free carrier concentration.

The thermodynamic compensation law

A demonstration of the thermodynamic compensation law is given; this law is shown to be a consequence of experimental approximations. It has been shown that the Meyer-Nelder rule is a consequence of experimental methods that consider a straight-line approximation of the In(X) versus 1/T plot of measurements of a physical quantity X in a limited temperature range. The ability of the Meyer-Neldel rule to give meaningful results is linked to the quality of the measurements and approximations made, but it appears that no genuine physical effect or fundamental relationships or universal quantities underlie its applicability.

Switching times variation of power MOSFET devices after electrical stress.

The switching performance of three power MOSFET devices with different oxide thicknesses is studied after several periods of electrical stress. The thickest oxide reveals a large accumulation of positive charges in the oxide bulk after small periods of stress. These charges affect the switching parameters by increasing the rise time and by decreasing the fall time. Larger periods of stress reduce the effect of positive charges by increasing the number of interface states. The threshold voltage is decreased by the effect of a positive oxide charge and increases with the appearance of interface states. All these phenomena are less observable as we reduce the oxide thickness.