Examination of wavelength dependent soot optical properties of diesel and diesel/rapeseed methyl ester mixture by extinction spectra analysis and LII measurements

The refractive index of soot is an essential parameter for its optical diagnostics. It is necessary for quantitative interpretation of LII (Laser Induced Incandescence) signals, light scattering or extinction measurements as well as for emissivity calculations. The most cited values have been determined by intrusive methods or without taking into account the soot size distribution and its specific morphology. In the present study, soot generated by the combustion of diesel and diesel/rapeseed methyl ester (RME) mixture (70% diesel and 30% RME) are extensively characterized by taking into account the morphology, the aggregate size distribution, the mass fraction and the spectral dispersion of light. The refractive index m for wavelengths λ between 300 and 1000 nm is determined for diesel and diester fuels by both in-situ and ex-situ methods. The ex-situ method is based on the interpretation of extinction spectra by taking into account soot sizes and fractal morphology with the RDG-FA (Rayleigh–Debye–Gans for Fractal Aggregate) theory. The in-situ approach is based on the comparison of the LII signals obtained with two different excitation wavelengths. The absorption function E(m) and the scattering function F(m) are examined. This study reveals similar optical properties of soot particles generated by both studied fuels even at ambient and flame temperatures. The function E(m) is shown to reach a maximum for λ=250 nm and to tend toward a plateau-like behavior close to E(m)=0.3 for higher wavelength (600<λ (nm)<1000). The function F(m) is found to be quite constant for 400<λ (nm)<1000 and equal to 0.31.     

Spectrally resolved light absorption properties of cooled soot from?a?methane flame

The optical properties of combustion-generated soot, crucial information for quantitative soot emission diagnostics and for climate modeling, have been determined for the particular case of cooled soot from a methane flame. Optical extinction measurements were performed over a wavelength range of 450–750 nm using a novel diffuse-light, spectrally resolved line-of-sight attenuation experiment, and quantified using extractive methods coupled with scanning and transmission electron microscopy in conjunction with a detailed uncertainty analysis. The absorption component of the total measured extinction was isolated by calculating the expected scattering contribution, according to the Rayleigh–Debye–Gans approximation for polydisperse fractal aggregates. In contrast to the large degree of scatter seen in data previously reported in the literature, a consistent trend of negligible variation of the soot absorption refractive index function E(m) with wavelength over the visible was observed (E(m)=0.35±0.03 at wavelengths of 450–750 nm). These new data are also cast in the form of dimensionless extinction, which is independent of the scatter correction, as well as mass absorption cross section, which is independent of the mass density of soot and is commonly used by atmospheric modelers.     

Optical properties of pulsed laser heated soot

To investigate the transient change of soot optical properties resulting from pulsed laser heating of soot in a cooled exhaust plume we have simultaneously performed cw light extinction at 405 and 830 nm and elastic light scattering at 1064 nm. A reversible increase to the 830-nm light extinction of up to 7%, observed during the time period where the soot was hot, suggests a temperature-dependent light absorption refractive index function, E(m _λ ). At low fluence, small permanent increases of E(m _λ ) of <2% were also observed. 405-nm extinction measurements revealed that the soot likely contained material which continued to absorb 405-nm radiation when desorbed, thus complicating measurement interpretation. 1064-nm light scattering measurements showed a gradual decrease of scattering propensity with increasing laser fluence up to the point of material loss, which is consistent with the expected decrease of the structure factor of the soot aggregates as they expand. It is concluded that variations of the optical properties are occurring at the time of laser-induced incandescence (LII) emission, which should be accounted for in time-resolved LII measurement interpretation.     

Laser induced incandescence determination of the ratio of the soot absorption functions at 532 nm and 1064 nm in the nucleation zone of a low pressure premixed sooting flame

In this work, the two-excitation wavelength laser induced incandescence (LII) method has been applied in a low-pressure premixed methane/oxygen/nitrogen flame (equivalence ratio 2.32) to determine the variation of the ratio of the soot absorption functions at 532 nm and 1064 nm E(m,532 nm)/E(m,1064 nm) along the flame. This method relies on the comparison of LII signals measured upon two different excitation wavelengths (here 532 nm and 1064 nm) and with laser fluences selected in such a way that the soot particles are equally laser-heated. The comparison of the laser fluences at 532 nm and 1064 nm leads to an easy determination of E(m,532 nm)/E(m,1064 nm). The reliability of the method is demonstrated for the first time in a low pressure flame in which the soot nucleation zone can be spatially resolved and which contains soot particles acting differently with the laser fluence according to their residence time in the flame. The method is then applied to determine the profile of E(m,532 nm)/E(m,1064 nm) along the flame. A very important decrease of this ratio is observed in the region of nascent soot, while the ratio remains constant at high distance above the burner. Implication on temperature determination from spectrally resolved measurement of flame emission is studied.     

Structural properties of silver nanoparticle agglomerates based on transmission electron microscopy: relationship to particle mobility analysis

In this work, the structural properties of silver nanoparticle agglomerates generated using condensation and evaporation method in an electric tube furnace followed by a coagulation process are analyzed using Transmission Electron Microscopy (TEM). Agglomerates with mobility diameters of 80, 120, and 150 nm are sampled using the electrostatic method and then imaged by TEM. The primary particle diameter of silver agglomerates was 13.8 nm with a standard deviation of 2.5 nm. We obtained the relationship between the projected area equivalent diameter (d _pa) and the mobility diameter (d _m), i.e., d _pa = 0.92 ± 0.03 d _m for particles from 80 to 150 nm. We obtained fractal dimensions of silver agglomerates using three different methods: (1) D _f = 1.84 ± 0.03, 1.75 ± 0.06, and 1.74 ± 0.03 for d _m = 80, 120, and 150 nm, respectively from projected TEM images using a box counting algorithm; (2) fractal dimension (D _fL) = 1.47 based on maximum projected length from projected TEM images using an empirical equation proposed by Koylu et al. (1995) Combust Flame 100:621–633; and (3) mass fractal-like dimension (D _fm) = 1.71 theoretically derived from the mobility analysis proposed by Lall and Friedlander (2006) J Aerosol Sci 37:260–271. We also compared the number of primary particles in agglomerate and found that the number of primary particles obtained from the projected surface area using an empirical equation proposed by Koylu et al. (1995) Combust Flame 100:621–633 is larger than that from using the relationship, d _pa = 0.92 ± 0.03 d _m or from using the mobility analysis.     

Production and characterization of carbon nano colloid via one-step electrochemical method

We present a one-step electrochemical method to produce water-based stable carbon nano colloid (CNC) without adding any surfactants at the room temperature. The physical, chemical, and thermal properties of CNC prepared were characterized by using various techniques, such as particle size analyzer, zeta potential meter, TEM, XRD, FT-IR, turbidity meter, viscometer, and transient hot-wire method. The average primary size of the suspended spherical-shaped nanoparticles in the CNC was found to be ∼15 nm in diameter. The thermal conductivity of CNC compared with that of water was observed to increase up to ∼14% with the CNC concentration of ∼4.2 wt%. The CNC prepared in this study was considerably stable over the period of 600 h. With the assistance of FT-IR spectroscopy analysis, we confirmed the presence of carboxyl group (i.e., O–H stretching (3,458 cm⁻¹) and C=O stretching (1,712 cm⁻¹)) formed in the outer atomic layer of carbon nanoparticles, which (i) made the carbon particles hydrophilic and (ii) prevented the aggregation among primary nanoparticles by increasing the magnitude of zeta potential over the long period.     

Comparison of LII derived soot temperature measurements with LII model predictions for soot in a laminar diffusion flame

Laser-induced incandescence (LII) was used to derive temperatures of pulsed laser heated soot particles from their thermal emission intensities detected at two wavelengths in a laminar ethylene/air co-annular diffusion flame. The results are compared to those of a numerical nanoscale heat and mass transfer model. Both aggregate and primary particle soot size distributions were measured using transmission electron microscopy (TEM). The model predictions were numerically averaged over these experimentally derived size distributions. The excitation laser wavelength was 532 nm, and the LII signal was detected at 445 nm and 780 nm. A wide range of laser fluence from very low to moderate (0.13 to 1.56 mJ/mm²) was used in the experiments. A large part of the temporal decay curve, beginning 12–15 nsec after the peak of the laser excitation pulse, is successfully described by the model, resulting in the determination of accommodation coefficients, which varies somewhat with soot temperature and is in the range of 0.36 to 0.46. However, in the soot evaporative regime, the model greatly overpredicts the cooling rate shortly after the laser pulse. At lower fluences, where evaporation is negligible, the initial experimental cooling rates, immediately following the laser pulse, are anomalously high. Potential physical processes that could account for these effects are discussed. From the present data the soot absorption function, E(m), of 0.4 at 532 nm is obtained. A procedure for correcting the measured signals for the flame radiation is presented. It is further shown that accounting for the local gas temperature increase due to heat transfer from soot particles to the gas significantly improves the agreement in the temperature dependence of soot cooling rates between model and experiments over a large range of laser fluences.     

Sticking efficiency of nanoparticles in high-velocity collisions with various target materials

In order to find reliable collector surfaces for the Mesospheric Aerosol – Genesis, Interaction and Composition (MAGIC) sounding rocket experiment, intended to collect atmospheric nanoparticles, the sticking efficiency of nanoparticles was measured on several targets of different materials. The nanoparticles were generated by a molecular beam apparatus in Jena, Germany, by laser ablation (Al₂O₃ particles, diameter 5–50 nm) and by laser pyrolysis (carbon particles, diameter 10–20 nm). In a vacuum environment (>10⁻⁵ mbar) the particles condensed from the gas phase, formed a particle beam, and were accelerated to ∼ ∼1 km/s. The sticking efficiency on the target materials carbon, gold and grease was measured by a microbalance. Results demonstrate moderate to high sticking probabilities. Thus, the capture and retrieval of atmospheric nanoparticles was found to be quantitatively feasible.     

Encapsulation of iron nanoparticles in alginate biopolymer for trichloroethylene remediation

Nanoscale zero-valent iron (NZVI) particles (10–90 nm) were encapsulated in biodegradable calcium-alginate capsules for the first time for application in environmental remediation. Encapsulation is expected to offers distinct advances over entrapment. Trichloroethylene (TCE) degradation was 89–91% in 2 h, and the reaction followed pseudo first order kinetics for encapsulated NZVI systems with an observed reaction rate constant (k _obs) of 1.92–3.23 × 10⁻² min⁻¹ and a surface normalized reaction rate constant (k _sa) of 1.02–1.72 × 10⁻³ L m⁻² min⁻¹. TCE degradation reaction rates for encapsulated and bare NZVI were similar indicating no adverse affects of encapsulation on degradation kinetics. The shelf-life of encapsulated NZVI was found to be four months with little decrease in TCE removal efficiency.     

A diffusion model of chain-branching reaction of the explosive decomposition of heavy metal azides

A diffusion model of a solid-phase chain reaction of explosive decomposition of heavy metal azides was developed. The dimensional effects of initiation of the reaction were examined: the dependence of the critical fluence of initiation on the microcrystal size H(R) and on the irradiated zone diameter H(d). It was demonstrated that the diffusion model of the chain reaction closely describes the measured H(R) dependence at diffusion coefficients of D ∼ 0.2–0.3 cm²/s, values that correspond to experimentally measured mobility of electronic charge carriers of μ ∼ 10 cm²/(V s). To account for the measured H(d) dependence and the reaction front propagation velocity (V = 1.2 km/s), it is necessary that the diffusion coefficient be three orders of magnitude higher than the experimentally determined value. That the H(R) and H(d) dependences cannot be quantitatively described simultaneously is indicative of the underlying mechanisms of energy transfer being different.     

Acoustic probing of two-temperature relaxation initiated by action of ultrashort laser pulse

Ultrashort laser pulse transfers metal into a two-temperature warm dense matter state and triggers a chain of hydrodynamic and kinetic processes—melting, expansion, stretching, creation of tensile stress and transition into metastable state. We study the response of aluminum film deposited on a glass substrate to irradiation by a pump laser pulse transmitted through glass. Several films with thicknesses from 350 to 1200 nm have been investigated. The smallest thickness is of the order of the heating depth d _T∼100 nm in Al. The d _T-layer and the free rear side of the film are coupled through pressure waves propagating between them. Therefore, the processes within d _T-layer affects the time dependent displacement Δ x _rear(t) of the rear surface. We compare simulated and experimental dependencies Δ x _rear(t) obtained by the pump–probe technique. It allows us to define a thickness of molten Al layer and explore the two-temperature processes occurring inside the heated layer.     

Control of material removal of fused silica with single pulses of few optical cycles to sub-picosecond duration

Surface ablation of a dielectric material (fused silica) by single femtosecond pulses is studied as a function of pulse duration (7–450 fs) and applied fluence (F _th<F<10F _th). We show that varying the pulse duration gives access to high selectivity (with resolution ∼10 nm) for axial removal of matter but does not influence the transverse ablation selectivity, which only depends on the normalized applied fluence F/F _th. The ablation efficiency is shown to be inversely dependent on the pulse duration and saturates with respect to the applied fluence earlier at ultra-short pulse durations (≤30 fs). The deduced optimal fluence F _opt corresponding to the highest ablation efficiency for each pulse width defines two regimes of laser application. Below F _opt, the removed material depth can be accurately adjusted in a large range (∼40–200 nm) as a function of the applied fluence and the morphology of the ablated pattern almost reproduces the Gaussian beam distribution. Above F _opt, the material removal depth tends to saturate and the morphology of the ablated pattern evolves to a top-hat distribution. The coupled evolution of depth and morphology is related to the dynamics of formation of dense plasma at the surface of the material, acting as an ultra-fast optical shutter.     

Mechanical and thermal properties of a nanopowder talc compound produced by controlled ball milling

A powdered compound constituted by over the 95% of talc Mg₃Si₄O₁₀(OH)₂ with MgCO₃ and CaMg(CO₃)₂ as minor phases was mechanically deformed by compaction and shear to a nanosized particulate (crystallite size ~5 nm) in a specifically built planetary ball mill. The mechanical milling was conducted in a controlled thermodynamic environment (25 °C and 0.13 Pa) by using low mechanical load to minimise amorphisation of the material. Mechanical τ(ε) shear analysis and thermo-structural modifications of the nanostructured talc particulate were investigated after selected milling times (0, 1, 5 and 20 h). At the very early stages of milling (1 h) layer flattening, lamination and texturing of the talc particles occurred. For prolonged milling (up to 20 h), a progressive reduction of the TOT talc stacking layer coherence, from about 20–5 nm, and an increase of (001) microstrain from about 0.6–2.2 × 10⁻² nm, as a non-linear function of the treatment time, were observed. A progressive increase of the specific surface area up to 28 m²/g as a consequence of the particle size reduction took place at intermediate milling times (5 h) and reduced to about 10 m²/g at prolonged milling (20 h). Even the thermo-structural behaviour of the particulate was significantly modified. For 20-h milled talc, a severe decrease of the dehydroxylation temperature from about 900–600 °C was observed with a concomitant anticipation of the recrystallisation of talc into MgSiO₃ (enstatite). The τ(ε) behaviour of the compound was strongly affected by the milling treatment changing from a shear-softening regime (untreated and 1 h) to a shear-hardening one (20 h). The observed changes of talc are of great importance to understand the rheology and the thermal transformation kinetics of talc compounds and can be exploited in those industrial applications that required milling of talc, such as in the production of talc-polymers nanocomposites or in medium–high-temperature ceramic processes.     

| m | Partial wave treatment for two-dimensional Coulomb-scattering and Regge pole

The symmetry and |m| partial-wave analysis for two-dimensional (2D) Coulomb-scattering is investigated. As a function of energyE, the |m| partial-wave scattering amplitudef _|m|(θ) is analytically continuated to the, negativeE (complexk) plane, and it is found that the bound state energy eigenvalues (E<0) are just located at the poles off _|m|(θ) on the positive imaginaryk axis as is expected. In addition, as a function of |m|,f _|m|(θ) is analytically continuated to the complex |m| plane, the bound state energy eigenvalues are just located at the poles off _|m|(θ) on the positive real |m| axis.     

Size effects in the exciton energy and first-order phase transitions in CuCl nanocrystals in glass

The absorption spectra and the melting and crystallization kinetics of CuCl nanocrystals in glass are investigated in the range of particle radii 1–30 nm. Three discontinuities are found on the curves representing the size dependence of the melting point T _m(R) and the crystallization point T _c(R). As the particle radius gradually decreases from 30 nm in the range R⩽12.4 nm there is a sudden 60° drop in the temperature T _c in connection with the radius of the critical CuCl nucleus in the melt. A 30° drop in T _m is observed at R=2.1 nm, and a second drop of 16° in the temperature T _c is observed for CuCl particles of radius 1.8 nm. The last two drops are associated with changes in the equilibrium shape of the nanoparticles. In the range of smaller particles, R⩽1.34 nm the T _c(R) curve is observed to merge with the T _m(R) curve, owing to the disappearance of the work of formation of the crystal surface during crystallization of the melt as a result of the zero surface tension of CuCl particles of radii commensurate with the thickness of the effective surface layer. An increase in the size shift of the exciton energy is observed in this same range of CuCl particle radii (1–1.8 nm). The size dependence of the melting and crystallization temperatures of the nanoparticles is attributed to variation of the free energy in the surface layer of a particle. Fiz. Tverd. Tela (St. Petersburg) 41, 310–318 (February 1999)     

Nonlinear optical absorption and refraction in Ru, Pd, and Au nanoparticle suspensions

We present the results of studies of the nonlinear optical properties of Pd, Ru, and Au nanoparticles. We studied the nonlinear refraction and nonlinear absorption of suspensions of these nanoparticles at 1064-nm wavelength. A relatively strong nonlinear absorption of the Pd nanoparticles was observed in the case of 1064-nm, 50-ps pulses (β=2×10⁻⁹ m W⁻¹). The Ru and Pd nanoparticles showed weak negative nonlinear refraction (γ∼−(6–8)×10⁻¹⁶ m² W⁻¹) in this spectral range. In the case of the Au nanoparticles, a saturated absorption at 532 nm dominated over other nonlinear optical processes.     

Synthesis of nanometric iron oxide films by RPLD and LCVD for thermo–photo sensors

Iron oxide films were deposited on <100> Si substrates by reactive pulsed laser deposition (RPLD) using a KrF laser (248 nm). These films were deposited too by laser (light) chemical vapor deposition (LCVD) using continuous ultraviolet photodiode radiation (360 nm). The deposited films demonstrated semiconducting properties. These films had large thermo-electromotive force (e.m.f.) coefficient (S) and high photosensitivity (F). For films deposited by RPLD the S coefficient varied in the range 0.8–1.65 mV/K at 205–322 K. This coefficient depended on the band gap (E _g ) of the semiconductor films, which varied in the range 0.43–0.93 eV. The largest F value found was 44 V_c/W for white light at power density I≅0.006 W/cm². Using LCVD, iron oxide films were deposited from iron carbonyl vapor. For these films, the S coefficient varied in the range −0.5 to 1.5 mV/K at 110–330 K. The S coefficient depended on E _g of the semiconductor films, which varied in the range 0.44–0.51 eV. The largest F value of these films was about 40 V_c/W at the same I≅0.006 W/cm². Our results showed that RPLD and LCVD can be used to synthesize iron oxide thin films with variable stoichiometry and, consequently, with different values of E _g . These films have large S coefficient and high photosensitivity F and therefore can be used as multi-parameter sensors: thermo–photo sensors.     

Optimization of F-doped SnO2 electrodes for organic photovoltaic devices

Fluorine-doped tin oxide (FTO) thin films have been investigated as an alternative to indium tin oxide anodes in organic photovoltaic devices. The structural, electrical, and optical properties of the FTO films grown by pulsed laser deposition were studied as a function of oxygen deposition pressure. For 400 nm thick FTO films deposited at 300°C and 6.7 Pa of oxygen, an electrical resistivity of 5×10⁻⁴ Ω-cm, sheet resistance of 12.5 Ω/□, average transmittance of 87% in the visible range, and optical band gap of 4.25 eV were obtained. Organic photovoltaic (OPV) cells based on poly(3-hexylthiophene)/[6,6]-phenyl-C₆₁-butyric acid methyl ester bulk heterojunctions were prepared on FTO/glass electrodes and the device performance was investigated as a function of FTO film thickness. OPV cells fabricated on the optimum FTO anodes (∼300–600 nm thick) exhibited power conversion efficiencies of ∼3%, which is comparable to the same device made on commercial ITO/glass electrodes (3.4%).     

Photogalvanic effect in an asymmetric system of three quantum wells in a strong magnetic field

The photogalvanic effect (PGE) in an asymmetric undoped system of three GaAs/AlGaAs quantum wells illuminated with white light of various intensities is investigated in magnetic fields up to 75 kOe at temperatures ranging from 4.2 K up to 300 K. A maximum of the spontaneous photogalvanic current J ^PGE as a function of the magnetic field predicted by A. A. Gorbatsevich et al., JETP Lett. 57, 580 (1993), is observed. Analysis of the experimental data shows that the main initial characteristic of the PGE is not the spontaneous current but rather the electromotive force E ^PGE arising in the direction perpendicular to the applied magnetic field. It is determined that this emf is independent of the intensity of the incident light, increases linearly with the size d of the illuminated region, and decreases slowly with temperature: E _max ^PGE ∼0.8 V at 300 K and ∼0.1 V at 4.2 K for d∼3 mm. The curve E ^PGE(H) at room temperature is determined with allowance for the strong transverse magnetoresistance of the nanostructure. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 3, 197–202 (10 February 1996)     

Bethe-Salpeter equation with the sine-Gordon interaction

An attempt is made to study the interaction Hamiltonian,H _int =Gψ ²(x)U(φ(x)) in the Bethe-Salpeter framework for the confined states of theψ particles interactingvia the exchange of theU field, whereU(φ) = cos (gφ). An approximate solution of the eigenvalue problem is obtained in the instantaneous approximation by projecting the Wick-rotated Bethe-Salpeter equation onto the surface of a four-dimensional sphere and employing Hecke’s theorem in the weak-binding limit. We find that the spectrum of energies for the confined states,E =2m+B (B is the binding energy), is characterized byE ∼n ⁶, wheren is the principal quantum number.