Nonlinear optical activity of anhydrous and hydrated sodium p-nitrophenolate

Differently hydrated sodium p-nitrophenolate (NPNa) crystals were obtained while growing them from different solvents such as methanol and water. Thermal analysis and powder X-ray diffraction studies were carried out on these crystals. Kurtz powder SHG technique was used for qualitative assessment of their nonlinear optical (NLO) activity. From the detailed single-crystal X-ray diffraction studies it is established that NPNa has three different forms, of which only one is found to possess NLO activity. Additionally, a new NLO active crystal was also found to grow from aqueous solution. (C) 1999 Elsevier Science B.V. All rights reserved.

Relaxation behavior of twin nonlinear optical chromophores: Effect of the spacer length

A new series of twin nonlinear optical (NLO) molecules, having two 4-nitrophenol chromophores that are linked via a flexible polymethylene spacer of varying length [(CH2)(n), n = 1-12], were synthesized. Powder second harmonic generation measurements of these twin samples indicated a pronounced odd-even oscillation, with the odd twins exhibiting a high SHG value while the even ones gave no measurable SH signal. This behavior reflects the crystal packing preferences in such twin NLO systems that have odd and even numbers of atoms linking them - the even ones appear to prefer a centrosymmetric packing arrangement. The orientational/disordering dynamics of these twin NLO molecules, doped in a polymer (poly(methyl methacrylate)) matrix, has also been studied using SHG in electric field poled samples. Interestingly, the maximum attainable SH signal, chi((2)), in, the poled samples also showed an odd-even oscillation; the odd ones again having a higher value of chi((2)) This unprecedented odd-even oscillation in such molecularly doped systems is rationalized as being due to the intrinsically greater ease of a parallel alignment of the two chromophores in the twins with an odd spacer than in those with an even one. Further, the temporal stability of the SHG intensity at 70 degrees C, after the removal of the applied corona, was also studied. The relaxation of all the twin chromophores followed a biexponential decay; the characteristic relaxation time (tau(2)) for the slow decay component suggests that while the twin with a single methylene unit relaxes relatively slowly, the relaxation is significantly faster in cases where n = 2 and 3. In the twins with even longer spacer segments, the relaxation again becomes slower and reaches a saturation value. The observed minimum appears to reflect the interplay of two competing factors that affect the chromophore alignment in such twin systems, namely, the electrostatic repulsion between neighboring oriented dipoles and the intrinsic flexibility of the spacer.

Studies on the variations in compression strengths of graphite powders bearing glass-epoxy composites exposed to humid conditions

The moisture absorption and changes in compression strengths in glass-epoxy (G-E composites without and with discrete quantities of graphite powders introduced into the resin mix prior to its spreading on specific glass fabric (layers) during the lay-up (stacking) sequence forms the subject matter of this report. The results point to higher moisture absorption for graphite bearing specimens. The strengths of graphite-free coupons show a continuous decrease, while the filler bearing ones show an initial rise followed by a drop for larger exposure times. Scanning Fractographic features were examined for an understanding of the process. The observations were explained invoking the effect of matrix plasticizing and the role of interfacial regions.

Wavelet based compression and denoising of optical tomography data

Two methods based on wavelet/wavelet packet expansion to denoise and compress optical tomography data containing scattered noise are presented, In the first, the wavelet expansion coefficients of noisy data are shrunk using a soft threshold. In the second, the data are expanded into a wavelet packet tree upon which a best basis search is done. The resulting coefficients are truncated on the basis of energy content. It can be seen that the first method results in efficient denoising of experimental data when scattering particle density in the medium surrounding the object was up to 12.0 x 10(6) per cm(3). This method achieves a compression ratio of approximate to 8:1. The wavelet packet based method resulted in a compression of up to 11:1 and also exhibited reasonable noise reduction capability. Tomographic reconstructions obtained from denoised data are presented. (C) 1999 Published by Elsevier Science B.V. All rights reserved,

Aryl monosulphides and symmetrical disulphides based second order nonlinear optical chromophores with transparency in the visible

A series of aryl monosulphides and disulphides have been synthesized and characterized. Their molecular hyperpolarizability (beta) has been measured in solution with the hyper-Rayleigh Scattering technique and also calculated by semiempirical AMI method. The trend in the observed and calculated values of first hyperpolarizability of these compounds has been found to be in good agreement. These compounds show moderate P values and excellent transparency in the visible region.

Pulsed laser deposited ZnO/ZnO:Li multilayer for blue light emitting diodes

Thin films of ZnO, Li doped ZnO (ZLO) and multilayer of ZnO and ZLO (ZnO/ZLO) were grown on silicon and corning glass substrates by pulsed laser deposition technique. Single phase formation and the crystalline qualities of the films were analyzed by X-ray diffraction and Li composition in the film was investigated to be 15 wt% by X-ray photoelectron spectroscopy. Raman spectrum reveals the hexagonal wurtzite structure of ZnO, ZLO and ZnO/ZLO multilayer and confirms the single phase formation. Films grown on corning glass shows more than 80% transmittance in the visible region and the optical band gaps were calculated to be 3.245, 3.26 and 3.22 eV for ZnO, ZLO and ZnO/ZLO, respectively. An efficient blue emission was observed in all films which were grown on silicon (1 0 0) substrate by photoluminescence (PL). PL measurements at different temperatures reveal that the PL emission intensity of ZnO/ZLO multilayer was weakly dependent on temperature as compared to the single layers of ZnO and ZLO and the wavelength of emission was independent of temperature. Our results indicate that ZnO/ZLO multilayer can be used for the fabrication of blue light emitting diodes. (C) 2011 Elsevier B.V. All rights reserved.

Embrittlement of a bulk metallic glass due to low-temperature annealing

Embrittlement of a bulk La-based metallic glass due to isothermal and isochronal annealing below the T-g was investigated. Results show that the impact toughness decreases with increasing annealing time or temperature, accompanied by a change in fracture morphology. Reasons for this are discussed in terms of structural relaxation. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Evolution of ferroelectric SrBi2Nb2O9 phase in the Li2B4O7-SrO-Bi2O3-Nb2O5 glass system

Glasses of various compositions in the system (100 - x)Li-2 B-4 O-7 - x (SrO-Bi2O3-Nb2O5) (10 less than or equal to x less than or equal to 60) (in molar ratio) were prepared via a conventional melt-quenching technique. The glassy nature of the as-quenched samples was established by Differential Thermal Analyses (DTA). X-ray powder diffraction (XRD) and Transmission Electron Microscopic (TEM) studies confirmed the amorphous nature of the as quenched and crystallinity in the heat-treated samples. The formation of nanocrystalline layered perovskite SrBi2Nb2O9 (SBN) phase, in the samples heat-treated at temperatures higher than 550degreesC, through an intermediate fluorite phase in the LBO glass matrix was confirmed by both the XRD and High Resolution Transmission Electron Microscopy (HRTEM). The samples that were heat-treated at two different temperatures, 550 and 625degreesC, (containing 0.35 and 0.47 mum sized SBN crystallites) exhibited broad dielectric anomalies in the vicinity of ferroelectric to paraelectric transition temperature of the parent SBN ceramics. A downward shift in the phase transition temperature was observed with decreasing crystallite size of SBN. The observation of pyroelectric and ferroelectric properties for the present samples confirmed their polar nature.

A chemical approach to understand fragilities of glass-forming liquids

Fragility is viewed as a measure of the loss of rigidity of a glass structure above its glass transition temperature. It is attributed to the weakness of directional bonding and to the presence of a high density of low-energy configurational states. An a priori fragility function of electronegativities and bond distances is proposed which quite remarkably reproduces the entire range of reported fragilities and demonstrates that the fragility of a melt is indeed encrypted in the chemistry of the parent material. It has also been shown that the use of fragility-modified activation barriers in the Arrhenius function account for the whole gamut of viscosity behavior of liquids. It is shown that fragility can be a universal scaling parameter to collapse all viscosity curves on to a master plot.

Cobalt oxide thin films prepared by metalorganic chemical vapor deposition from cobalt acetylacetonate

Thin films of cobalt oxide have been deposited on various substrates, such as glass, Si(100), SrTiO3(100), and LaAlO3(100), by low pressure metalorganic chemical vapor deposition (MOCVD) using cobalt(IL), acetylacetonate as the precursor. Films obtained in the temperature range 400-600 degreesC were uniform and highly crystalline having Co3O4 phase as revealed by x-ray diffraction. Under similar conditions of growth, highly oriented thin films of cobalt oxide grow on SrTiO3(100) and LaAlO3(100). The microstructure and the surface morphology of cobalt oxide films on glass, Si(100) and single crystalline substrates, SrTiO3(100) and LaAlO3(100) were studied by scanning electron microscopy. Optical properties of the films were studied by uv-visible-near IR spectrophotometry.

Compressive behavior and fracture features of rubber bearing glass-epoxy composites exposed to aqueous media

Epoxy systems containing HTBN rubber material and reinforced with E-glass fibres, exposed to a fixed time duration in three separate media were subjected to compressive mode of deformation. The yield stress and fractographic features noted on the compression failed samples are reported in this work. The experiment reveals that the seawater exposed sample exhibits a drop in strength compared to dry (unexposed) sample. This kind of drop is maintained if the media is changed from seawater to distilled water. When HCl is included in seawater. the experiment shows a small rise in strength value. These changes have been attributed to various factors like medium ingress into samples assisting interface failure, the larger-sized Cl- influencing the extent of diffusion of medium into system and finally their participation in the deformation phenomena. The fractographic features reveal interface separations that show either scattered debris or a cleaner surface or display a whitish-coated matrix region depending on whether the tests are done on unexposed samples or on ones following the immersion in the media.

Dielectric anomaly in strontium borate-bismuth vanadate glass nanocomposite

Transparent glass nanocomposites in the pseudo binary system (100 - x) SrB4O7 (SBO)-x Bi2VO5.5 (BiV) (0 less than or equal to n less than or equal to 70) were prepared by the splat quenching technique. The nano-crystallization of bismuth vanadate (BiV) in 50 SBO-50 BiV (in mol%) glass composite has been demonstrated. These were characterized for their structural, thermal and dielectric properties. As-quenched composites under study have been confirmed to be amorphous by X-ray powder diffraction (XRD) studies. The glass transition temperature (T-g) and crystallization temperatures (T-er) were determined using differential thermal analyses (DTA), High resolution transmission electron microscopic (HRTEM) studies carried out on heat-treated samples reveal the presence of spherical nanosize crystallites of Bi2VO5.5 (BiV) dispersed in the glassy matrix of SrB4O7 (SSO). The dielectric constant (epsilon (r)) and the dielectric loss (D) measurements were carried out on the as-quenched and heat-treated glass nanocomposite samples in the frequency range 100 Hz-10 MHz. The as-quenched and the heat-treated at two different temperatures (720 and 820 K) samples exhibited broad dielectric anomalies in the vicinity of the ferroelectric-to-paraelectric transition temperature of the parent BiV ceramics. The Curie-Weiss law was found to be valid at a temperature above the transition temperature, establishing the diffused nature of the transition. (C) 2001 Elsevier Science Ltd. All rights reserved.

Effect of absorption in aqueous and hygrothermal media on the compressive properties of glass fiber reinforced syntactic foam

Absorption due to immersion in aqueous media consisting of either saline or seawater or due to exposure to water vapor conditions and the attendant effect on the compressive properties of syntactic foam reinforced with E-glass fibers in the form of chopped strands were studied. Whereas the compressive strength decreased in samples exposed to water vapor, the saline or seawater immersed samples showed increase when compared to the dry sample. The decrease in strength in the vapor-exposed case is ascribed to higher absorption of water and to debonding and damaged features for interfaces. The enhancement of strength values for the samples immersed in saltish media is traced to the larger size of the chloride ion and resultant changes in the stress state around the fiber-bearing regions. Recourse to an analysis of scanning electron microscopic pictures of the compression-failed samples is taken to explain the observed trends.

Effect of lithium ion irradiation on the transport and optical properties of Bridgman grown n-type InSb single crystals

Single crystal (100) wafers of n-InSb were implanted with 50 MeV Li3+ ions at various fluences ranging from 10(10) to 10(14) ions/cm(2) at room temperature. Investigations of the optical, electrical, and structural properties of the as-grown, irradiated, annealed wafers were carried out by infrared and Raman spectroscopies, Hall measurements, and high resolution x-ray diffraction (HRXRD). In the case of samples irradiated with an ion fluence of 1.6x10(14) ions/cm(2), electrical measurements at 80 K reveal that there is a decrease in carrier concentration from 8.5x10(15) (for unirradiated) to 1.1x10(15)/cm(3) and an increase in mobility from 5.4x10(4) to 1.67x10(5) cm(2)/V s. The change in carrier concentration is attributed to the creation of electron trap centers induced by ion beam irradiation and the increase in mobility to the formation of electrical inactive complexes. Nevertheless, even with the irradiation at 1.6x10(14) ions/cm(2) fluence the crystalline quality remains largely unaffected, as is seen from HRXRD and Raman studies. (C) 2001 American Institute of Physics.