Showing papers in "Journal of Optoelectronics and Advanced Materials in 2004"

Chalcogenide glasses for optical and photonics applications

Abstract: A short review of relevant parameters, such as phonon energy, transmission range and refraction index, that distinguish previously and presently studied chalcogenide glasses from silica based ones is given. Chalcogenide glass systems GeSe, GeSeTe and As2Se3 have been prepared and characterized by absorption spectroscopy and low-temperature photoluminescence. Both absorption and low-temperature photoluminescence spectra reveal shifts of absorption edge and/or luminescence band to longer wavelength due to Te → Se substitution. Luminescence band shows little change with increasing temperature up to 200 K and considerable shift to shorter wavelength appears when room temperature is approached. No influence of Te → Se substitution on the position or spectral shape of luminescence band is observed at room temperature.

Current sensors using magnetic materials

Abstract: Precise contactless DC and AC magnetic sensors are required by car industry, chemical industry, for measurement of power and many other applications. The emphasis is given on current sensors based on magnetic materials, but other methods are also mentioned for comparison. Discussed are the factors influencing precision and geometrical selectivity.

Characterisation of carbon nanotube materials by Raman spectroscopy and microscopy - A case study of multiwalled and singlewalled samples

Abstract: Laser excited Raman scattering was measured from various carbon nanotube samples, as well as high purity graphite using an Art laser at different wavelengths and the variation of the band parameters was studied as a function of the excitation laser wavelength. Features in the Raman scattering have been identified and assigned to known structural and dynamical sources. Scanning electron microscopic and tunnelling microscopic pictures of two different nanotube samples contributed to the characterization of the samples as multiwalled and singlewalled carbon nanotube bundles and helped identify Raman spectral features.

Physical properties of PbO-ZnO-P2O5 glasses. I. Infrared and raman spectra

Abstract: Phosphate glasses of composition x PbO - 10 ZnO - (90-x) P 2 O 5 , 30 ≤ x(mol. %) ≤55 (series A); 50 PbO -x ZnO - (50-x) P 2 O 5 , 0 ≤ x(mol %) ≤ 20 (series B); and x PbO - (60 - x) ZnO - 40 P 2 O 5 , 0 ≤ x(mol %) ≤ 60 (series C), were investigated. Depending mainly on the PbO content in glasses, the density varies in the range 3.2 to 5.5 [g/cm 3 ], the glass-transition temperature lies in the region 270 - 394 °C. Both the Raman and infrared spectroscopy were used to study the considerable structural changes in the phosphate network induced by incorporation of PbO and ZnO into a glass network. Depending on the actual system, PbO behaves as a network modifier and/or network former, especially for higher content of PbO. ZnO in most of cases seems to prefer to interact with PO 3 -end groups. In the C-series of glasses the local minimum in the glass-transition temperature T g (PbO) dependence, we speculate, could reflect some tendency of the system to a local nano-phase separation.

Comparison of the dielectric properties for doped and undoped TiO2 thin films

Abstract: The dielectric properties of TiO 2 thin films have been investigated on ITO/TiO 2 /Au structures for a large domain of signal frequencies (10 2 Hz-10 6 Hz). At 100Hz, the values of the electric capacitance increases from 5.22nF to 22nF, by doping with Nb (0.35 at. %) and to 10.2nF by doping with Ce (0.4 at. %). This increase is correlated with the increase of the dielectric constant of the film. At 100 Hz, the dielectric constant increase from 83.6 for the undoped sample, to 179 by doping with Ce and to 108, by doping with Fe. Nb impurities determine a large increase of the dielectric constant, till 371, which is very important in fabricating capacitors. The dependence of dielectric loss versus frequency was also examined.

Chalcogenide thin films deposited by radio-frequency sputtering

Abstract: Several properties of chalcogenide glasses make these materials attractive candidates for integrated optics. These properties include infrared transparency, high refractive index, photosensitivity, low phonon energy and ease in preparation of thin film. We report on the radiofrequency sputtering deposition of thin columnar films of chalcogenide glasses (Ge 3 3 As 1 2 Se 5 5 and Ge 2 8 Sb 1 2 Se 6 0 ) and on the influence of their thickness and morphology on their optical characteristics (optical band gap and refractive index). It is shown that the higher the thickness, the larger the gathering of the columns in clusters, leading to an increase in the porosity (from 18 % for 0.2 μm thick films to 33 % for 5 μm thick ones), and to a decrease in the optical band gap and refractive index (about 10 %).

Modelling of relaxation phenomena in organic dielectric materials. Application of differential and integral operators of fractional order

Abstract: In this work we have used differential and integral operators of fractional order (between 0 and 1) for modelling the real and imaginary parts of E* and e r * considering the three more important relaxation phenomena in semi-crystalline polymers. To justify the validity of the proposed models we have used measurements of E* and e r * under isochronal conditions of a semi-crystalline specimen of PEN in a broad temperature range.

Characterization of ZnAl2O4 nanocrystals prepared by coprecipitation and microemulsion techniques

Abstract: Nanometer-sized zinc aluminate (ZnAl 2 O 4 ) particles were synthesized by coprecipitation method using NH 3 like precipitation agent and the reverse micelle method using like cationic surfactant cetyltrimethylammonium bromide (CTAB), from Zn(NO 3 ) 2 and Al(NO 3 ) 3 an ideal cation stoichiometry for ZnAl 2 O 4 spinel. XRD analysis for their crystal structure and TEM for their texture characterized the materials obtained after heating at 500 - 800 °C. The TEM photographs reveal that both of them, coprecipitation and the reverse micelle method, are made up of primary nanoparticles in the size range 3-15 nm.

Influence of thermal annealing in air on the structural and optical properties of amorphous antimony trisulfide thin films

Abstract: Amorphous antimony trisulfide films were deposited on glass using thermal vacuum evaporation technique at substrate temperatures Ts = 300 K The films were annealed in air at a temperature of 500 K for 30 minutes The structure and chemical composition of films were determined by X-ray and electron microscopy investigations The films are composed of an amorphous structure that includes nanocrystallites of antimony trioxide The amorphous as-deposited films transform into polycrystalline films during treatment above 500 K Absorption coefficients of the films were determined using spectrophotometric measurements of the transmittance, T, at normal incidence in the spectral range 460-1400 nm The optical properties of amorphous film differ substantially from those of polycrystalline films, after heat treatment the direct energy band gap decreases from 246 eV to 240 eV and the indirect band gap from 164 eV to 120 eV, respectively

Crystallization kinetics of some Se-Te-Ag chalcogenide glasses

Abstract: The present paper reports the Differential Scanning Calorimetric (DSC) study of some Ag doped Se - Te chalcogenide glasses. DSC runs were taken at different heating rates. Well -defined endothermic and exothermic peaks were obtained at glass transition and crystallization temperatures. Three different methods have been used for the evaluation of activation energy of crystallization (E c ). The results show that the value of E c is decreased due to addition of Ag in binary alloy Se 7 0 Te 3 0 . This decrease is explained in terms of mean atomic masses of ternary alloys. The compositional dependence of (E c ) in ternary alloys shows a reversal in the trend at 4 at. % of Ag, which is explained in terms of mechanically stabilized structure at this composition.

Microstructural characterization of cylindrical Fe1-xNix thin films

Abstract: Structural and microstructural characterization was performed on soft magnetic Fe 1 - x Ni x thin films on copper and stainless steel cylindrical substrates. Thin films have been prepared using an electrodeposition device with uniform cylindrical deposition ability. Plating baths, containing nickel sulfate and iron sulfate, of variable Fe/Ni ion ratio in the electrolyte have been used in order to control the stoicheiometry. The microstructural characterization and composition analysis were performed using SEM. The structure of the alloys was investigated by X-ray diffraction. All samples have been submitted to thermal treatment at 450 °C at Ar atmosphere.

State of the art and development trends of novel nanostructured elastomagnetic composites

Abstract: The coupling between elastic and magnetic properties of composite materials made of magnetic particles uniformly dispersed into an elastic matrix was revised. The influence of particles content and size was considered. A unified simple model of the direct and inverse elastomagnetic effect was furnished explaining the experimental results in different experimental conditions. Some applications of elastomagnetic materials in both sensors and actuators are reported and their potentiality is discussed. The correlation between strain and electric conduction in this kind of materials is also presented and the first experimental results on this topic are reported.

Simulation of corona discharge in configurations with a sharp electrode

Abstract: Three algorithms are presented to determine the distributions of electric potential and charge density in the case of an injected space charge in a gas. We consider electrode configurations characterised by a sharp electrode injecting charge from the restricted zone where the electric field takes very high values and induces a corona discharge. For the point-plane configuration with a needle having the shape of an axi-symmetric hyperboloid, a change of coordinates transforms the domain of integration into a rectangle, which facilitates the use of finite differences or finite volume methods. For a general axi-symmetric shape of the needle, a finite element method is used to solve the Poisson equation. In these two cases, an adapted method of characteristics makes it possible to solve the charge conservation equation in a way which does not smooth out very high lateral gradients of charge density. In a third approach developed first for the blade-plane electrode configuration, the mesh is redefined with each of the successive approximations. This technique also leads to predictions of current density profile on the plane very similar to the measured ones.

Growth and dissolution of calcium oxalate monohydrate (com) crystals

Abstract: Calcium oxalate monohydrate (COM) crystals were obtained by different methods. The tendency of COM to form splices, twins, spherulites and dendrites was demonstrated. The mechanism and kinetics of growth of two out of three basic faces were investigated by in situ atomic force microscopy. The rate of COM dissolution was revealed to increase in the presence of aluminum and iron ions. Substitution of Aqua RX water for distilled water also increased the dissolution rate.

Physical properties of PbO-ZnO-P_{2}O_{5} glasses: 2: refractive index and optical properties

Abstract: Phosphate glasses of composition x PbO - 10 ZnO - (90-x) P 2 O 5 , 30 ≤ x(mol. %) ≤ 55 (series A); 50 PbO -x ZnO - (50-x) P 2 O s , 0 ≤ x(mol %) ≤ 20 (series B); and x PbO - (60 - x) ZnO - 40 P 2 O 5 , 0 ≤ x(mol %) ≤ 60 (seriesC), were investigated. Experimental values of the refractive index (n) [1.59 ≤ n ≤ 1.87] were compared with the calculated values of n and good results were obtained using the Effective Medium Theory. Optical gap (Eg) varies in the region: 4.6 ≤ E g [eV], 300 K ≤ 4.98, the temperature coefficient β of the optical gap was found around 6.7x10 - 4 [eV/K].

Some physical properties of spray deposited SnO2 thin films

Abstract: Thin films of tin oxide (SnO 2 ) were deposited on various substrates by spray pyrolysis technique using SnCl 2 as precursor. The as-prepared films were characterized for their electrical, optical and structural properties. Films deposited on optical glass showed the better electrical properties as compared to those deposited on other substrates. Further, fluorine- and antimony- doped SnO 2 films were deposited on these substrates. The fluorine- doping resulted in enhancing both the electrical and optical properties of these films. On the other hand antimony- doping enhanced the electrical properties of the films but decreased the optical properties. The preferred orientation (211) of undoped films was found shifted to (200) for both the dopants. The change in orientation was reflected in SEM studies as they have different grain shapes. The minimum sheet resistance of 1.8 and 2.2 Ω/□ obtained for the films SnO 2 :F and SnO 2 :Sb deposited on glass are the lowest among the reported values for these materials prepared from SnCl 2 precursor. The 42 % transmittance of undoped films found increased to 85 % on fluorine- doping (15 wt. %) but decreased to 20 % on antimony-doping (2 wt. %).

Investigation of phase transition mechanism in vanadium oxide thin films

Abstract: Thin film materials with "smart" properties which react with temperature variations, electrical or magnetic fields, and pressure variations, attracted great attention in recent years. Vanadium dioxide thin films belong to this family of "smart materials" by having a first order phase transition metal-semiconductor. These films display a variety of conductivities under heating in the vicinity of Curie temperature. The electrical properties of vanadium dioxide films were studied in past and found to be outstanding. This paper deals with the electrical properties of specific vanadium oxide thin films, deposited on various substrates. We have been able to show that the electrical transport mechanism of the obtained vanadium oxide films differs in low and high electrical fields. In low electrical fields, conductivity is obtained by Schottky transport mechanism, whereas in high electrical fields conductivity ranges from Ohmic mechanism for low fields to Poole-Frenkel for higher fields.

Bulk nanocomposite Re-Tm-M permanent magnets

Abstract: (Pr,Dy) x Fe 7 0 - x Co 1 0 B 2 0 (x=4,5) bulk nanocomposite permanent magnets in the form of rods, of diameter 0.5 mm, and tubes, of outer diameter 3 mm and wall thickness up to 0.3 mm, have been prepared by devitrificationannealing of amorphous precursors produced by suction die casting. The best magnetic properties were achieved for Pr 3 Dy 1 Fe 6 0 Co 1 0 B 2 0 rod and tube heat treated at 670 °C for 10min, as follows: i H c = 287 kA/m, μ 0 M r = 0.92 T, (BH) m a x = 58 kJ/m 3 and i H c = 293 kA/m, μ 0 M r = 1.0 T, (BH) m a x = 66 kJ/m 3 respectively. The bulk samples are compared in terms of their thermal stability and magnetic properties with thick ribbons of the same compositions.

Impedance spectroscopy studies on doped polyanilines

Abstract: Impedance spectroscopy studies on polyaniline doped by various amounts of HCl are reported. The measurements were performed at room temperature, over a wide range of frequencies [1000 Hz to 45 MHz]. The experimental data shows that the resistance dominates the AC behavior of polyaniline confirming that the charge transport occurs through an one-dimensional hopping process.

Spectroscopic characterization of chemical bath deposited cadmium sulphide layers

Abstract: Thin films of cadmium sulphide have been prepared by chemical bath deposition in alkaline ammonia solutions from cadmium acetate and thiourea, using sodium citrate and/or ammonium chloride as chelating agent. The films grown from the bath without citrate are uniform and adherent, have a high transmittance and good reflectance in the visible region, and are about 2 times thicker than the films deposited from the ,,standard" bath. The evolution of optical characteristics of CdS films after annealing process was influenced by the deposition conditions.

DIELECTRIC BEHAVIOUR OF NIOBIUM DOPED Ni-Zn FERRITES

Abstract: Polycrystalline Ni-Zn ferrites with the formula Ni 0 . 6 5 Zn 0 . 2 5 Fe 2 O 4 + x Nb 2 O 5 with x values ranging from 0.0 to 1.5 wt% in steps of 0.3 wt% have been prepared by conventional ceramic technique to investigateand understand the influence of pentavalent niobium additions on the dielectric behavior of Ni-Zn ferrites as a function of composition and frequency. Dielectric constants of these ferrites have been found to be of the order of 10 6 . The dielectric constant of any sample containing niobium addition is higher than the undoped one. However, the dielectric constant is found to decrease by increasing the concentration of the additive. The dielectric loss has been found to be smaller for the samples containing niobium. The dependence of dielectric constant and the variation of tan 6 with frequency exhibit normal behavior up to 4 MHz beyond which they experienced dimensional resonances. The results are explained on the basis of Koop's two layer model and Maxwell-Wagner polarization theory.

A possibility for local targeting of magnetic carriers

Abstract: This work suggests a new procedure for local targeting of magnetic microentities (magnetic oxide particles, ferro-carbon particles, magnetic polymer particles, magnetic amorphous alloys particles, etc.), by placing small sized ferromagnetic wires in the target area or within the area joining the target and applying an external uniform background magnetic field which has to be perpendicular to the magnetizable wires. The possibility to capture the magnetic microparticles and to build deposits is theoretically studied under circumstances similar to the in vivo ones.

Polymorphous-crystalloid nature of vitreous and liquid H2O

Abstract: Based on the concept of polymeric polymorphous-crystalloid (polymeric-polymorphoid) structure of glass and glassforming liquid of individual chemical substances the structure of liquid and vitreous H 2 O has been considered. Experimental data are presented and a number of arguments are suggested which point out that vitreous and liquid water are constructed from structural fragments of crystalline polymorphous modifications (crystalloids or polymorphoids) of H 2 O: ice I, ice II and ice III. Polymorphoids have no long-range order and have an individual intermediate-range order inherent to these polymorphous modifications.

Electrodeposition of CdTe nanorods in ion track membranes

Abstract: CdTe nanowires were electrodeposited in ion track membranes with pore diameters in the range 100 - 2000 nm. Acidic and basic baths were tested for obtaining materials with good stoichiometry. The cyclic voltammograms werecompared for the case of deposition on a carbon rotating disc and on the porous membrane substrates, the differences being attributed to the additional resistance induced by the pores. For larger pores a tendency of growing hollow structures was observed.

The 2D-DCT coefficient statistical behaviour: A comparative analysis on different types of image sequences

Abstract: This paper presents a method designed to analyse the distribution of the 2D-DCT (bi-Dimensional Discrete Cosine Transform) coefficient hierarchy computed over three types of image sequences: colour video, grey-level X-rayCT (computerised tomography) images and computer simulated noise images This method is a statistical approach which combines in a new way the following four tests: (1) the Χ 2 (Chi-square) test on concordance between experimental data and a theoretical probability density function, (2) the p (Ro) test on correlation, (3) the Fisher F test on equality between two variances, and (4) the Student T test on equality between two means Such an approach was compulsory so as to mathematically overcome the dependency existing among successive images in the considered sequences The results obtained on natural sequences (either video or medical images) are compared to those corresponding to computer simulated sequences, interesting differences being pointed out and discussed The overall results may play a central role in a large variety of image/video processing applications: compression, segmentation, retrieval, protection (eg cryptography and watermarking) For instance, we successfully applied them to the design of a new robust watermarking method for colour video

Observation of Meyer-Neldel rule in a- Se60Te20Ge20 thin films in presence of light

Abstract: In general, in case of semiconductors, conductivity (a) varies exponentially with temperature (T), i.e., a = σ 0 exp [-AE/k T]. In most of the materials, σ 0 does not depend on AE. However, in many amorphous and liquid semiconductors and many other class of materials, σ 0 is found to increase exponentially with ΔE. This is called Meyer- Neldel rule. This rule is generally verified by selecting different compositions of different AE in a given class of materials. This opens a possibility of change in various other physical properties apart from ΔE. In the present paper, we report on the observation of Meyer- Neldel rule where ΔE is varied by varying the intensity of light while measuring the photoconductivity in amorphous thin films of Se 6 0 Τe 2 0 Ge 2 0 instead of changing composition of the glassy system. The observation of Meyer-Neldel rule in the present case indicates that this rule is more general and does not cause due to change in density or distribution of defect states or any other physical property due to change in composition.

Mgcu nanocrystalline ceramic with la3+ and y3+ ionic substitutions used as humidity sensor

Abstract: The partial substitution of La and Y for Fe into nanocrystalline Mg 0 . 5 Cu 0 . 5 Fe 2 O 4 compound was studied. The Mg-based ceramics present an attractive set of properties: high electrical resistivity, high Curie temperature and chemical stability. Y 3 - and La 3 + ions affect the structure and magnetic and electric properties of Mg 0 . 5 Cu 0 . 5 Fe 2 O 4 compound. These ions increase the grain size and favour the densification of ceramic. The electrical resistivity decreases with incorporation of La 3 + and Y 3 + ions in Mg 0 . 5 Cu 0 . 5 Fe 2 O 4 .

SIMULATION OF RAMAN SPECTRA OF AsxS100-x GLASSES BY THE RESULTS OF AB INITIO CALCULATIONS OF AsnSm CLUSTERS VIBRATIONS

Abstract: We here present ab initio calculations on small As n S m clusters that are building blocks for the formation of continuous random networks (CRN) generally found in As x S 1 0 0 - x glasses. The calculations were performed by density functional theory (DFT). Geometries, vibrational frequencies, Raman intensities for the clusters have been calculated using the modified Stuttgart RLC ECP basis set at DFT/B3LYP level. Using the calculated and experimental data the relative contributions of different small clusters in the glasses structure matrix and the implications are discussed.

Influence of the experimental parameters on silicon nanoparticles produced by laser ablation

Abstract: Influence of the laser wavelength, fluence, gas and gas pressure on silicon nanoparticles produced by laser ablation is reported. Nanoparticles ranging from 2 nm to 13 nm were generated by ablation of silicon target into He or Ar, (250-550 mbar) with a Nd:YAG laser, (335 and 532 nm, 5 ns), at a fluence of 4-8 J/cm 2 . The shape, structure, and size distribution were determined by transmission electron microscopy. 90% of nanoparticles were in the range 2-10 nm, regardless pressure, wavelength or fluence. The wavelength 355 nm and higher fluence, 8 J/cm 2 , are more suitable for smaller nanoparticle synthesis (2-5 nm). The optimal conditions for an efficient synthesis of minimum sized silicon nanoparticles were 355 nm / 8 J/cm 2 / 550 mbar / He.

Thin magnetic amorphous wires for GMI sensor

Abstract: This paper presents a study of the GMI in amorphous conventional wires with nearly zero magnetostriction (Co 6 8 . 1 8 Fe 4 . 3 2 Si 1 2 . 5 B 1 5 ) having different diameters obtained by cold drawing processes in multiple steps. The purpose of our work is to find the way in which the sensitivity of GMI effect is affected by the decrease of the wires diameter and also the way in which the stress induced during the cold drawing process influence the configuration of the magnetic domains structure through the modification of the magnetoelastic energy. We have performed a study of the giant magneto-impedance effect (GMI) in nearly zero magnetostrictive conventional amorphous wires with diameters between 150-20 μm. The values of the impedance (Z) and GMI ratio (ΔZ/Z) were measured in the frequency range 100 kHz - 10 MHz for wires in the as cast state and cold drawn. We obtained a GMI ratio four times higher for cold drawn wires with diameters of 20 - 30 μm subjected to thermomechanical treatments in comparison with the as cast wire of 130 μm diameter. The results are discussed considering the correlation between induced stresses and modifications in circumferential magnetic domains configuration.