Surface structure and adsorption properties of ultrafine porous carbon fibers

Ultrafine porous carbon fibers (UPCFs) were successfully synthesized by chemical activation of electrospun polyacrylonitrile fibers. In the current approach, potassium hydroxide was adopted as activation reagent. UPCFs were systematically evaluated by scanning electron microscope and nitrogen adsorption. The mass ratio of potassium hydroxide to preoxidized fibers, activation temperature and activation time are crucial for producing high quality UPCFs. The relationships between porous structure and process parameters are explored. UPCFs were applied as adsorbent for nitrogen monoxide to be compared with commercial porous carbon fibers.     

Investigation of thermal influence on the bandgap properties of liquid-crystal photonic crystal fibers

We have analyzed the thermal influence on the bandgap properties of liquid-crystal photonic crystal fibers. The bandgap parameters which affect the transmission conditions have been investigated. It is observed that the photonic bandgap can be thermally tuned, i.e. the red or blue shift of the bandgap results from the temperature dependence of the refractive index of the liquid crystal. For the planar alignment of liquid-crystal filled cladding, the ordinary refractive index plays a major role in determining the bandgap properties; the extraordinary refractive index comes into influence while the ordinary refractive index is relatively constant of temperature. The analyses agree well with the experiments results.     

Composites from Brazilian natural fibers with polypropylene: mechanical and thermal properties

Brazil has a well established ethanol production program based on sugarcane. Sugarcane bagasse and straw are the main by-products that may be used as reinforcement in natural fiber composites. Current work evaluated the influence of fiber insertion within a polypropylene (PP) matrix by tensile, TGA and DSC measurements. Thus, the mechanical properties, weight loss, degradation, melting and crystallization temperatures, heat of melting and crystallization and percentage of crystallinity were attained. Fiber insertion in the matrix improved the tensile modulus and changed the thermal stability of composites (intermediary between neat fibers and PP). The incorporation of natural fibers in PP promoted also apparent T _c and ΔH _c increases. As a conclusion, the fibers added to polypropylene increased the nucleating ability, accelerating the crystallization process, improving the mechanical properties and consequently the fiber/matrix interaction.     

Optical and mechanical properties of hollow-core fibers with cobweb cladding structure

This paper explores the possibility of realizing a hollow-core optical fiber, whose cladding is composed of cylindrical alternating layers of air and high-index material with supporting structure. The optical properties and the design criteria of the proposed fiber are evaluated by the compact two-dimensional (2D) finite-difference time-domain (FDTD) method. In particular, the influence of the number and width of supporting strips on the leakage loss of the fiber is investigated. Furthermore, the mechanical performances of the fiber are estimated by finite-element method, confirming that hollow-core fibers with a reasonable size and number of supporting strips are reliable.     

Thermal degradation mechanism and thermal mechanical properties of two high-performance aromatic polyimide fibers

Two organo-soluble aromatic polyimides have been synthesized by introducing pendant groups of trifluromethyl or methyl onto the 2- and 2′-positions of a biphenyl diamine and reacting each of them with biphenyltetracarboxylic dianhydride (BPDA): 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB) or 2,2′-dimethyl-4,4′-diaminobiphenyl (DMB). These two polyimides, BPDA-PFMB and BPDA-DMB, are prepared via a one-step poly condensation method, and they can be fabricated into fibers, films, and other application forms. Our research shows that in fiber applications, these two fibers exhibit excellent mechanical properties and outstanding thermal and thermooxidative stability. Their long-term mechanical tensile performance at high temperatures is found to be critically associated with the type of side pendant groups at the 2- and 2′-positions of the diamines. High-resolution pyrolysis-gas chromatography/mass spectrometry (PyGC-MS) results show that the pyrorgams of these two polyimides possess more than 16 pyrolyzates. Based on the compositions and distributions of the pyrolyzates at different temperatures, the thermal degradation mechanisms for both of the polyimides are found to be the same.     

Effects of variable thermal diffusivity on the structure of convection

The structure of multiscale convection in a thermally stratified plane horizontal fluid layer is investigated by means of numerical simulations. The thermal diffusivity is assumed to produce a thin boundary sublayer convectively much more unstable than the bulk of the layer. The simulated flow is a superposition of cellular structures with three different characteristic scales. In contrast to the largest convection cells, the smaller ones are localised in the upper portion of the layer. The smallest cells are advected by the larger-scale convective flows. The simulated flow pattern qualitatively resembles that observed on the Sun.     

The mode structure and spectral properties of supercontinuum emission from microstructure fibers

The mode structure and spectral properties of supercontinuum emission generated by femtosecond pulses of Ti: sapphire laser radiation in microstructure fibers are studied. The long-wavelength (720–900 nm) and visible (400–600 nm) parts of supercontinuum emission are shown to be spatially separated in microstructure-fiber modes, which can be isolated with an appropriate spectral filtering. The spatial modes thus isolated in spectrally sliced supercontinuum emission possess a spatial quality sufficient for further efficient frequency conversion. The possibility of achieving a high spectral quality of supercontinuum emission is also demonstrated. We explore the ways to control the spectrum of supercontinuum emission by matching parameters of the pump pulse with the parameters of a microstructure fiber and by tuning the initial chirp of the pump pulse. The results of our studies show that supercontinua produced in microstructure fibers offer new approaches to designing a new generation of optical parametric amplifiers and broadband radiation sources for spectroscopic, metrological, and biomedical applications.     

The effect of molecular weight on the structure and thermal properties of polyethylene

The morphology of polyethylene single crystals prepared isothermally in solution was found to be independent of molecular weight. The enthalpy of fusion, lamellar fold period, and optical appearance were invariant for samples grown from fractions ranging from 20,000 to 2,000,000 in molecular weight. The mass fraction of lamellae which thicken during heating decreased linearly with increasing log molecular weight. The melting temperature of the crystals was also nearly independent of molecular weight.

The superheating of polyethylene crystals was observed to be a function of molecular weight and morphology. At a comparatively high molecular weight the heating rate of the calorimeter exceeded the crystal melting rate, which shifted the observed melting temperature to an anomalously high value. The incorporation of defects within the crystals by irradiation-induced cross-links or chain entanglements increased the melting rate of the high molecular weight samples and thereby minimized the effects of superheating.

The apparent heat of fusion of melt crystallized polyethylene decreased linearly with increasing log molecular weight. In contrast to this behavior the crystallinity of single crystals from dilute solution was independent of molecular weight.

In previous papers we have shown that reorganization of polymer single crystals is suppressed by cross-linking [1—3]. With the appropriate selection of heating rate and irradiation dose, the melting temperatures of solution grown crystals of various morphologies were determined in the absence of lamellar thickening. The observed melting temperatures of polyethylene single crystals with different X-ray fold periods were found to fit the following expression:

T_m = T_m0[1—2σ_e/H_f?] with an equilibrium melting temperature (T_m0) of 145.8 ± 1.0°C and a surface free energy (σ_e) of 89 ± 5 ergs cm^?2 for a polyethylene crystal of infinite dimensions. In addition, at a constant heating rate it was observed that the fraction of crystals which thickened prior to melting decreased with increasing fold period.

Since cross-linking polyethylene increases the molecular weight of the material, it is instructive to investigate the reorganization characteristics of single crystals prepared from polyethylene fractions. Single crystals were prepared in xylene from molecular weight fractions of polyethylene and the effect of molecular weight upon the structure and thermal properties of the crystals was determined.     

Effects of ultrasound-assisted enzyme hydrolysis on the microstructure and physicochemical properties of okara fibers

This work focuses on the effects of different ultrasound power densities on the microstructural changes and physicochemical properties of okara fibers, which are composed of carbohydrate-based polymers. Okara suspensions were treated with ultrasound at different power densities (0, 1, 2, 3, 4, and 5 W/mL) for 30 min, after which the ultrasound-treated okara were hydrolyzed by trypsin to obtain okara fibers. The ultrasound treatment of the okara fibers induced structural disorganization and changes, evidenced mainly in their morphological characteristics and their relative crystallinity degrees. Increasing the ultrasound power broke the okara fibers into flaky and stacked structures. When the ultrasound power density reached 4 W/mL, the parenchyma became compact and the hourglass structure fractured. The mean particle size of the okara fiber was reduced from 82.24 µm to 53.96 µm, and the homogeneity was enhanced significantly. The relative crystallinity of the okara fibers was reduced from 55.14% to 36.47%. The okara fiber surface charge decreased when the ultrasound power was increased. However, after ultrasound treatment at 4 W/mL (800 W), the okara fiber suspension exhibited the highest viscosity value and a higher swelling capacity, water-holding capacity, and oil-holding capacity. Therefore, the results indicated that the selection of processing conditions for okara fibers is critical and that okara fiber modification using a high ultrasound treatment might improve their use in potential applications.     

Effects of pressure on the structure and the thermal decomposition kinetics of RDX

Abstract

High pressure and temperature structural changes for RDX were investigated to 7.0 GPa and 570 K in a diamond anvil cell apparatus using FTIR absorption, optical microscopy, and energy-dispersive powder x-ray diffraction techniques. Three distinct solid phases were observed. The effects of pressure on the thermal decomposition kinetics as a function of RDX pressure were investigated using an infrared absorption technique. Solid phase I was found to have a pressure enhanced reaction rate with an energy and volume of activation of 51 Kcal/mole and -5.6 cc/mole respectively. Solid II was not observed to react and the observed reaction rate of Solid III decreased with increasing pressure.     

Effects of thermal annealing on the properties of N-implanted ZnS films

N-ion-implantation to a fluence of 1×1015ions/cm2was performed on ZnS thin films deposited on glass substrates by using the vacuum evaporation method. The films were annealed in flowing nitrogen at 400?C–500?C after N-ionimplantation to repair the ion-beam-induced structural destruction and electrically activate the dopants. Effects of ionimplantation and post-thermal annealing on ZnS films were investigated by X-ray diffraction(XRD), photoluminescence(PL), optical transmittance, and electrical measurements. Results showed that the diffraction peaks and PL intensities were decreased by N-ion-implantation, but fully recovered by further annealing at 500?C. In this experiment, all films exhibited high resistivity due to the partial dopant activation under 500?C.     

Influence of thickness and crystalline structure on thermal and optical properties of ZnO thin films

Measurements of the temperature dependence of refractive index of ZnO thin films and thermal diffusivity using photothermal deflection technique are presented. Thin film thickness and surface homogeneity were found to be the effective parameters on optical and thermal properties of the thin films. High refractive index gradient with temperature was found for films of a nonuniform distribution and gathered in clusters, and a high predicted value for thermal diffusivity. Optical properties of the thin films revealed that films with disorder in the deposition and gathered clusters showed poor transmittance in visible region with a pronounced peak in the near IR, and also a reduction in the band gap. A detailed parametric analysis using analytical solution of one-dimensional heat equation had been performed. A discontinuity in the temperature elevation at the ZnO-glass interface was found.     

Effects of thermal oxidation on the photoluminescence properties of porous silicon

The effects of thermal oxidation on the photoluminescence (PL) properties of powdered porous silicon (PSi) are studied using X-ray photoelectron spectroscopy (XPS). It is found that the PL intensity is steeply quenched after annealing at and recovered at above . The XPS intensity of oxides formed on the PSi surface is also found to strongly depend on the annealing temperature. The comparison between the annealing temperature dependence of PL intensity and that of the oxide XPS intensity suggests that the formation of thin disordered SiO₂ layer accompanies the quenching of the PL intensity, and that the formation of thick high-quality SiO₂ layer results in the PL intensity recovery. These results indicate that the thickness and quality of SiO₂ layer play a crucial role in the PL properties of thermally oxidized PSi.     

Effects of surface charges on phonon properties and thermal conductivity in GaN nanofilms

Surface charges can modify the elastic modulus of nanostructure, leading to the change of the phonon and thermal properties in semiconductor nanostructure. In this work, the influence of surface charges on the phonon properties and phonon thermal conductivity of GaN nanofilm are quantitatively investigated. In the framework of continuum mechanics,the modified elastic modulus can be derived for the nanofilm with surface charges. The elastic model is presented to analyze the phonon properties such as the phonon dispersion relation, phonon group velocity, density of states of phonons in nanofilm with the surface charges. The phonon thermal conductivity of nanofilm can be obtained by considering surface charges. The simulation results demonstrate that surface charges can significantly change the phonon properties and thermal conductivity in a GaN nanofilm. Positive surface charges reduce the phonon energy and phonon group velocity but increase the density of states of phonons. The surface charges can change the size and temperature dependence of phonon thermal conductivity of GaN nanofilm. Based on these theoretical results, one can adjust the phonon properties and temperature/size dependent thermal conductivity in GaN nanofilm by changing the surface charges.     

Influence of thermal annealing on bonding structure and dielectric properties of fluorinated amorphous carbon film

The bond structure and dielectric properties of fluorinated carbon films after thermal annealing in N₂ ambience were studied. The results show that dielectric constant and dielectric loss increased, and optical gap decreased with increasing annealing temperature. The composition and bonding structure of the films were obtained by FTIR and XPS analysis. The data indicate that fluorine-to carbon ratio decreased and CC group increased in the films after the films were annealed. It suggests that the structural and dielectric property changes correlate with the release of fluorine and increase of cross-linking during the annealing.     

Effect of surface on the optical structure and thermal properties of organically capped CdS nanoparticles

Cadmium sulfide (CdS) nanoparticles were synthesized by a novel wet chemical route with various organic thiol stabilizers. Systematic experimental studies, including X-ray diffraction (XRD), ultraviolet–visible (UV–vis) spectroscopy, Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), photoluminescence (PL) spectroscopy, and time-resolved photoluminescence (TRPL), have evidenced that the stability, crystallinity, and optical properties of the CdS nanoparticles are affected by the organic groups which generate significant effects in surface reconstruction. Particle size was evaluated from UV–vis spectroscopy using the effective mass approximation (EMA) method and from XRD patterns based on Scherrer?s formula. The S–H vibrations are not detectable in the infrared (IR) spectra of any of the bound ligands, which are expected for thiols covalently bound to the surface of nanoparticles. PL studies reveal that the emission from the nanostructures is not much influenced by the surface states, indicating a good passivation of the particle?s surface. The time-resolved measurements reveal a biexponential decay behavior. The fast decay component is attributed to the recombination of core states, while the slow decay component of PL is associated with the charge-carrier recombination process with the involvement of surface states.     

Crystal structure and magnetic properties of lepidocrocite upon thermal transformation to hematite

Synthesized lepidocrocite (-FeOOH) has a unit cell of the rhombic system (a = 3.86 Å, b = 12.49, and c = 3.36). In the 230–250°C region the structure of lepidocrocite is reconstructed to cubic (a = 8.331 ± 0.003 A) maghemite (-Fe₂O₃), which at 375°C is transformed completely to hematite (-Fe₂O₃) with a hexagonal structure which becomes more perfect with further heating (a = 5.0330 ± 0.002 Å, c = 13.748 ± 0.002 Å). Lepidocrocite occurs in a paramagnetic state. The intermediate substance of its thermal transformation, maghemite, as a consequence of high dispersion, is characterized by superparamagnetic properties and the end product, hematite, owing to the presence of Dzialoshinski's moment, has weak ferromagnetism.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 84–90, May, 1972.     

Effects of thermal annealing on the properties of GaN MSM UV photodetectors

Metal-semiconductor-metal-structured GaN ultraviolet photodetectors have been fabricated on sapphire substrates by metalorganic chemical vapor deposition. The properties of GaN photodetectors have been improved through thermal annealing. With a 3 V bias, the very low dark current is about 200 pA, the maximum responsivity of 0.19 A/W is achieved at 362 nm, and the corresponding detectivity is 1.2×10¹¹ cm Hz^1/2/W. The physical mechanism of the effects of thermal annealing also has been studied.