Soot volume fraction measurements in aero-engine exhausts using extinction-calibrated backward laser-induced incandescence

Control and reduction of soot particle emissions from aeronautic turbines requires a monitoring system suitable for quantification of these emissions. Currently, such emissions are estimated using the technique of smoke number. This is an extractive method, which is not sensitive enough for the low emission levels of modern gas turbines. Within a recent European project, AEROTEST, part of the project aimed at investigating an alternative soot monitoring technique, laser-induced incandescence (LII) as an in-situ optical diagnostic for quantification of soot emissions. For aero-engine applications, especially those involving large-scale turbines, it is necessary to perform the measurements at long distance from the turbine. The LII technique is favourable in this respect as it provides for non-intrusive measurements and, by detecting the isotropic LII signal along the same axis as the incoming laser beam (so called backward LII), both the laser and the detector can be built inside one system located several meters from the turbine. The concept was initiated in the previous European projects, AEROJET I and II. This paper describes the modified version of the system and the procedure developed to achieve reliable and quantitative soot volume fraction measurements in the exhausts of aero-engines. Application of the backward LII technique is demonstrated in the exhaust of a military turbojet engine for different engine speeds.     

Laser-induced incandescence of titania nanoparticles synthesized in a flame

Laser induced incandescence experiments were carried out in a flame reactor during titania nanoparticle synthesis. The structure of the reactor employed allowed for a rather smooth particle growth along the flame axis, with limited mixing of different size particles. Particle incandescence was excited by the 4th harmonic of a Nd:YAG laser. The radiation emitted from the particles was recorded in time and checked by spectral analysis. Results were compared with measurements from transmission electron microscopy of samples taken at the same locations probed by incandescence. This was done covering a portion of the flame length within which a particle size growth of a factor of about four was detected. The incandescence decay time was found to increase monotonically with particle size. The attainment of a process control tool in nanoparticle flame synthesis appears to be realistic.     

Spatial characterization of the laser-induced plasma plumes generated by IR CO2 pulsed laser on carbon targets

We have used ferrocene and paraffin wax as novel precursor and solvent for the growth of iron oxide nanoparticles. The proposed method of growth has several advantages over existing methods of growth using iron pentacarbonyl a precursor. Highly crystalline and monodispersed particles are obtained which assemble in two- and three-dimensional hexagonal closed packed superlattices. Growth kinetics has been studied by varying concentration of the precursor and time of growth. A phenomenological model has been proposed to explain the growth kinetics.     

II–VI semiconductor nanoparticles synthesized by laser ablation

Nanoparticles of the II–VI semiconductors CdTe, CdSe and ZnTe were synthesized by laser ablation (387 nm, 180 fs, 1 kHz, pulse energy of 7 μJ (fluence of 2 J/cm²)) of the target materials in methanol, de-ionized water and acetone. The nanoparticles size distributions follow log-normal functions with median diameters between about 6 and 11 nm for the several materials. The nanoparticles have the same crystalline structure as that of the corresponding bulk material and under the present conditions of ablation are rich in the higher volatility element of the two in the binary alloy and oxidized. Photoluminescence emission in the green-yellow (∼570 nm) was detected from CdSe nanoparticles.     

Dye-sensitized solar cells based on TiO2 nanotube/porous layer mixed morphology

Titania porous layer has been fabricated on titania nanotubes for dye sensitized solar cells and the photovoltaic performance of solar cells with mixed morphology has been investigated. The porous layer results in a similar improvement in the short circuit current density to conventional TiCl₄ treatment, although the mechanisms responsible for the observed increase in the efficiency are different. This enables further improvements of the photovoltaic performance by combining the TiCl₄ treatment and porous layer deposition, so that the efficiency in the case of ∼5 μm long tubes increases on average from ∼1.6 to ∼2.2%.     

Structural properties of size-selected FeCo nanoparticles deposited on W(110)

Size-selected iron and iron–cobalt alloy clusters have been studied with high resolution transmission electron microscopy (HRTEM) and scanning tunneling microscopy (STM). The clusters were produced by a continuously working arc cluster ion source and subsequently size-selected by an electrostatic quadrupole deflector. The crystalline structure of pure clusters has been investigated with HRTEM to ensure a reliable determination of the lattice parameter for the alloy clusters. The composition of the alloy clusters was checked with energy dispersive X-ray spectroscopy (EDX). The height of the deposited FeCo clusters on the (110) surface of tungsten was determined via STM. These results were compared with the lateral size distribution being investigated by TEM and allow a conclusion on the shape of the deposited alloy clusters. Furthermore, the behavior of the alloy clusters on the W(110) surface at elevated temperatures has been examined, at which the clusters show anisotropic spreading.     

Transient 3D/2D simulation of laser-induced ablation of silicon

A numerical software has been developed to simulate heating, enthalpy-based phase changes and ablation of silicon during pulsed or continuous-wave laser irradiation. The unsteady heat transfer equation is solved by finite differences in two or three dimensions with full resolution of the thin liquid layer. An intelligent adaptive grid refinement and a semi-analytic treatment of the surface elements have been implemented to simulate laser cuts with lots of laser pulses in moderate computing time. The code has been successfully verified by comparisons with an analytic solution and with experimental data. Details of the mathematical model, the implementation in Matlab^®and comparisons with experimental laser cuts are presented in this paper.     

Derivation of a temperature-dependent accommodation coefficient for use in modeling laser-induced incandescence of soot

This paper presents a derivation of an expression to estimate the accommodation coefficient for gas collisions with a graphite surface, which is meant for use in models of laser-induced incandescence (LII) of soot. Energy transfer between gas molecules and solid surfaces has been studied extensively, and a considerable amount is known about the physical mechanisms important in thermal accommodation. Values of accommodation coefficients currently used in LII models are temperature independent and are based on a small subset of information available in the literature. The expression derived in this study is based on published data from state-to-state gas-surface scattering experiments. The present study compiles data on the temperature dependence of translational, rotational, and vibrational energy transfer for diatomic molecules (predominantly NO) colliding with graphite surfaces. The data were used to infer partial accommodation coefficients for translational, rotational, and vibrational degrees of freedom, which were consolidated to derive an overall accommodation coefficient that accounts for accommodation of all degrees of freedom of the scattered gas distributions. This accommodation coefficient can be used to calculate conductive cooling rates following laser heating of soot particles.     

Hydrothermal growth of large-scale macroporous TiO<Subscript>2</Subscript> nanowires and its application in 3D dye-sensitized solar cells

Large-scale macroporous TiO₂ nanowires (MTN) were directly grown on spiral-shaped titanium wires as photoanodes of dye-sensitized solar cells (DSSCs) via a facile hydrothermal reaction without any seeds, templates, and TiO₂ powder. The MTN thin film was characterized by SEM, XRD and TEM. The studies revealed that the MTN thin film had better mechanical properties and provided an efficient pathway for the diffusion of liquid electrolyte. The efficiency of 0.86% for the 3D DSSC was obtained with a J _sc of 2.30 mA/cm², V _oc of 616 mV, and FF of 0.61. This MNT-based mini 3D DSSC is a promising photovoltaic device for applications in the fields of high-integrated micro-electronic equipment.     

Plasmonic slow light waveguide and cavity

In this work, we present a detailed analysis of optical properties of metal–insulator and metal–insulator–metal (MIM) structure-based surface-plasmon waveguides and cavities. It is shown from the dispersion relation, the field distribution, the quality factor Q, and the transmission spectrum that the MIM structure can be designed as a high-Q cavity, supporting slow light with tolerable pulse distortion and lower propagation loss in the axial direction.     

The structure and photoluminescence properties of TiO<Subscript>2</Subscript>-coated ZnS nanowires

The structure and photoluminescence properties of TiO₂-coated ZnS nanowires were investigated. ZnS nanowires were synthesized by thermal evaporation of ZnS powder and then coated with TiO₂ by using the metal organic chemical vapor deposition (MOCVD) technique. We performed scanning electron microscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy, and photoluminescence (PL) spectroscopy to characterize the as-synthesized and TiO₂-coated ZnS nanowires. TEM and XRD analyses revealed that the ZnS core and the TiO₂ coatings had crystalline zinc blende and crystalline anatase structures, respectively. PL measurement at room temperature showed that the as-synthesized ZnS nanowires had two emissions: a blue emission centered in the range from 430 to 440 nm and a green emission at around 515 nm. The green emission was found to be dominant in the ZnS nanowires coated with TiO₂ by MOCVD at 350°C for one or more hours, while the blue emission was dominant in the as-synthesized ZnS nanowires. Also the mechanisms of the emissions were discussed.     

Highly oriented electrospun polycaprolactone micro/nanofibers prepared by a field-controllable electrode and rotating collector

Novel ZnO tetrapod-shaped nanostructures with pearl-necklace-shaped arms were successfully synthesized using mixture of Zn, ZnO, and carbon powder as source. The definite supersaturation ratio provided by Zn, ZnO, and carbon powder was considered as the crucial factor of determining the formation of this kind of structure, and a negative feedback growth model combined with octahedral nucleation mechanism was proposed. Two other comparative experiments were also conducted to study the growth behavior of reagent species under different supersaturation ratios. Our experiments provided a beneficial experimental exploration in controlled growth of nanostructures through modulating supersaturation ratio by source, and these obtained novel nanostructures were also expected to have potential application as functional blocks in nanodevices. Furthermore, the study of photoluminescence indicated that the physical properties were strongly dependent on the crystal structure.     

Synthesis and characterization of silica nanosprings by a low temperature chemical vapor deposition

Silica nanosprings were synthesized using a simple, low temperature, chemical vapor deposition method via a vapor–liquid–solid mechanism. Nanosprings with excellent uniformity and helicity in high and repeatable yields have been observed. The morphology and crystal structure of the nanosprings were characterized by scanning electron microscopy and transmission electron microscopy. The chemical composition of the nanosprings was determined using the energy-filtered transmission electron microscopic method. The as-grown nanomaterials were confirmed to be amorphous silica with irregularly shaped Au catalytic particles located at the tips. In addition, we propose a spontaneous spinning growth model to explain the formation of such helical nanostructures.     

Inverse pulsed laser deposition

Since the advent of pulsed laser deposition (PLD), several different target-substrate arrangements have been proposed. Besides the most common on-axis PLD, several off-axis geometries were studied, mainly to protect the substrate from the agglomerated species (clusters, droplets, particulates) of the plasma plume, which are detrimental to the homogeneity of films. Recently we introduced a novel geometry, termed inverse pulsed laser deposition (IPLD), in which the substrate is placed parallel to and slightly above the target plane. In this paper we summarize our results on this new geometry, and show how it can extend the perspectives of pulsed laser deposition, e.g., by improving the surface morphology of the films. Effects of ambient pressure are presented and exemplified on metallic and compound IPLD films, including Ti, CN _x , and Ti-oxides. AFM topographic images are used to prove that under optimized conditions IPLD is capable of growing compact and smooth films that are superior to PLD ones. A special—but easy-to-implement—IPLD arrangement is also introduced that considerably improves the homogeneity of IPLD films. In this geometry, the properties (e.g., deposition rate and roughness) of the films grown in the 1–25 Pa pressure domain are examined.     

Morphology and size dependence of silver microstructures in fatty salts-assisted multiphoton photoreduction microfabrication

The morphology and size dependence of silver microstructures in a novel microfabrication process, fatty salts-assisted multiphoton photoreduction (MPR), were investigated by using the fatty salts with different carbon chain lengths (C _n : n=4,5,7,9) under varied powers and irradiation times of a femtosecond near-infrared laser with the wavelength of 800 nm. Not only the feature size of the silver structures was reduced but also the surface smoothness was improved by increasing the chain length of the fatty salts. The highest resolution of a silver line was obtained to be 285 nm, which exceeded the diffraction limit. The fatty salts-assisted MPR microfabrication approach would provide an efficient protocol for fabricating metallic micro/nanostructures with fine morphology and size and could play an important role in the fabrication of the metallic micro/nanostructures for applications in photonics and electronics as well as in sensors.     

Simultaneous excitation of cavity resonance and surface plasmon resonance in Ag/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Ag layer structure

Simultaneous excitation of cavity resonance (CR) and surface plasmon resonance (SPR) was observed in the angular spectrum by substituting Ag/Al₂O₃/Ag layers for the metal film in a Kretschmann structure. Two reflective valleys, elicited respectively by CR and SPR, appeared at different positions in the angular spectrum. The former is the sum of enhanced transmission of CR and absorption of the metal, expressed in the reflection spectrum and extremely insensitive to the changes of the surface environment (refractive index). The latter behavior is like that when two metal films are stuck together: it has almost the same resonance depth and width, and is extremely sensitive to the changes of the surface environment. Moreover, two SPR peaks could be excited simultaneously at one angle but with different wavelengths in the frequency spectrum, which is not seen in traditional Kretschmann structures.     

Methods for controlling of the laser-induced absorption in a BTO crystal by using of cw-laser radiation

The iron-atom concentration distribution as well as the gas-phase temperature was measured via laser-induced fluorescence (LIF) during iron-oxide nanoparticle synthesis in a low-pressure hydrogen/oxygen/argon flame reactor using ironpentacarbonyl (Fe(CO)₅) as precursor. Temperature measurements based on multi-line NO-LIF imaging are used to correct for temperature-dependent ground-state populations. The concentration measurement is calibrated based on line-of-sight absorption measurements. The influence of the precursor on the flame is observed at precursor concentrations larger than 70 ppm as the flame front moves closer to the burner surface with increasing Fe(CO)₅ concentration.     

Filamentation of ultrashort light pulses in a liquid scattering medium

We have studied filamentation of 1-ps laser pulses in a scattering medium (aqueous suspension of 2-μm polystyrene microspheres) and compared filamentation dynamics to that in pure water. Our results indicate that light scattering does not alter filamentation dynamics in general, but rather results in farther position of the nonlinear focus, shorter filament length, and the development of speckle structure in the peripheral part of the beam. The experimental observations are qualitatively reproduced by the numerical model which accounts for diffraction, self-focusing, multiphoton absorption, and light scattering introduced through a stochastic diffusion and diffraction term.     

Femtosecond-laser nanofabrication onto silicon surface with near-field localization generated by plasmon polaritons in gold nanoparticles with oblique irradiation

Nanohole fabrication process with gold nanoparticles irradiated by femtosecond laser at different incident angles is investigated. Nanoparticles with diameter of 200 nm and laser irradiation with center wavelength of 800 nm are used in the present study. The analysis of the electromagnetic field distribution in the near-field zone of the particle is made by simulations based on finite-differential time domain (FDTD) method. It is shown that when gold nanoparticle is irradiated by laser pulse surface plasmon excitation can be induced, and associated with it, high-intensity near field is produced in a limited area around the particle. It is found that the change of the irradiation conditions by means of irradiation from various incident directions gives a possibility of laser nanoprocessing with tunable characteristics. Our results show that enhanced optical intensity is able to be induced on the substrate surface regardless of incident direction of the laser due to the image charge interaction with the substrate. Furthermore, the use of p-polarized laser irradiation at a certain angle gives a minimum of the spatial dimensions of the enhanced zone on the substrate which is about two times smaller than that obtained at normal incidence.