Mean Free Path Length Research Articles

Recombination luminescence of LaPO4-Eu and LaPO4-Pr nanoparticles

The study of the spectral-luminescence parameters of LaPO4-Eu and LaPO4-Pr nanoparticles upon excitation by the synchrotron radiation with photon energies 4–40 eV was performed. The differences of the luminescence intensity dependence on the size for LaPO4-Eu and LaPO4-Pr nanoparticles excited at the range of matrix transparency, the range of band-to-band transitions, and the range of electronic excitation multiplication were revealed. The observed regularities are explained in terms of the electron-phonon and electron-electron scattering, surface losses, and exciton diffusion. The ratio between the length of thermalization and electron mean free path and the size of nanoparticle is determinative for the luminescence intensity upon excitation in the range of fundamental absorption of matrix and X-ray excitation.

Exciton luminescence in krypton cryocrystals with an admixture of molecular deuterium

Data on the VUV and UV cathodoluminescence spectra of the Kr-based solid solutions, Kr-D2, Kr-D2-O2, and Kr-Xe-O2, as functions of dopant concentration are presented. Introducing deuterium impurity into krypton crystals produces no new spectral features, which indicates that electron bombardment of these crystals does not cause excitation or dissociation of D2. The intensity of the intrinsic emission from the matrix increases substantially, the more so for higher concentrations of D2. The observed intensity increase is found to be caused by localization of matrix excitons within a limited volume of the crystal as they undergo quasielastic scattering by impurity deuterium molecules, which leads to a substantial reduction in the mean free path and diffusion length for the excitons, as well as to their faster self-localization. Possible mechanisms for luminescence quenching in pure krypton cryocrystals are discussed. It is shown that quenching is caused by annihilation of excitons as they interact among themselves or with other electronic excitations of the crystal.

Dispersion–orientation effects of fulleropyrrolidine in zone annealed block-copolymer films toward optimizing OPV interfaces

Introduction of an insulating polystyrene block (BCP) polymethylmethacrylate copolymer (PS-b-PMMA) layer between the poly poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate) (PEDOT:PSS) and the photoactive layer (fullerene or functionalized fullerene/poly-3-hexylthiophene) has been reported to improve solar cell performance. We explore how the morphological structure of this ordered interfacial BCP layer may be modified with novel synthesized electron accepting fulleropyrrolidine nanoparticles (f-NP), processed via a novel dynamic zone-annealing (ZA) method. N-methyl-2-(4-nitro phenyl) fulleropyrrolidine and N-methyl-2-(4-cyano phenyl) fulleropyrrolidine were synthesized by 1,3-dipolar cycloaddition of azomethineylides to fullerene and characterized by 1H NMR, 13C NMR, MALDI-TOFMS, cyclic voltammetry, and thermogravimetry. The newly synthesized f-NP's exhibited higher thermal stability and equivalent electronic properties compared to conventionally used [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM) for photoactive layer. f-NP filled PS-b-PMMA thin films processed using uniform oven annealing promoted phase segregation driven aggregation of f-NP located at the defect junction points of the block copolymer films. In contrast, ZA of f-NP filled BCP films led to the homogeneous dispersion of f-NPs within the films, however the f-NP had a synergistic orientation effect on BCP films, switching PMMA cylinders from vertical to parallel orientation in the ZA films. This effect is presumably due to a lowering of thermal conductivity of the BCP film by nanoparticles that scatter the phonons thereby decreasing the mean free path length of phonon propagation. These results may be important for the self-assembly of thermally stable and interfacially insulating BCP films for improved solar cell devices.

Investigation of comminution in a Wiley Mill: Experiments and DEM Simulations

Particle size reduction of dry granular material by mechanical means, also known as milling or comminution, is undoubtedly a very important unit operation in pharmaceutical, agricultural, food, mineral and paper industries. Particle size reduction rate was studied by conducting parametric studies experimentally and computationally using Discrete Element Method (DEM). Studies were performed with lactose non-pareils (spheres) to understand the effect of blade speed (rotational), feed rate, and blade-wall tolerance in a Wiley mill. The size and shape of the resulting progeny of particles were analyzed by sieve analysis and microscope/image analysis techniques respectively. The feed rate determines the hold up of material in sizing chamber and hence energy required for size reduction. Greater size reduction was observed at higher speeds and low feed rates owing to the greater centrifugal force experienced by the particles and longer mean free path lengths respectively. Particle shape analysis revealed fragmentation to be the dominant mechanism of size reduction at higher speeds. Increase in blade-wall tolerance resulted in accumulation of powder bed which was found to be significant at low impeller speeds. Single particle impact studies were performed using Dynamic Mechanical Analyzer to determine the force required to break the granule. In this method, the granules were subjected to a quasi-static compression process and the corresponding force was continuously recorded. The flow and fragmentation of non-pareils were simulated using DEM, an explicit numerical technique scheme, which calculates interaction forces between grains for each grain-grain contact, and the resulting motion of each grain. The fragmentation of each spherical particle was simulated using the Grady's Algorithm. The diameter of the resultant progeny particle was evaluated based on the fracture toughness of material, propagation velocity of longitudinal elastic waves in the material, and the induced strain rate.

Thermal diffusivity and mechanical properties of polymer matrix composites

Polypropylene–iron-silicon (FeSi) composites with spherical particles and filler content from 0 vol. % to 70 vol. % are prepared by kneading and injection molding. Modulus, crystallinity, and thermal diffusivity of samples are characterized with dynamic mechanical analyzer, differential scanning calorimeter, and laser flash method. Modulus as well as thermal diffusivity of the composites increase with filler fraction while crystallinity is not significantly affected. Measurement values of thermal diffusivity are close to the lower bound of the theoretical Hashin-Shtrikman model. A model interconnectivity shows a poor conductive network of particles. From measurement values of thermal diffusivity, the mean free path length of phonons in the amorphous and crystalline structure of the polymer and in the FeSi particles is estimated to be 0.155 nm, 0.450 nm, and 0.120 nm, respectively. Additionally, the free mean path length of the temperature conduction connected with the electrons in the FeSi particles together with the mean free path in the particle-polymer interface was estimated. The free mean path is approximately 5.5 nm and decreases to 2.5 nm with increasing filler fraction, which is a result of the increasing area of polymer-particle interfaces. A linear dependence of thermal diffusivity with the square root of the modulus independent on the measurement temperature in the range from 300 K to 415 K was found.

Coherent growth of superconducting TiN thin films by plasma enhanced molecular beam epitaxy

We have investigated the formation of titanium nitride (TiN) thin films on (001) MgO substrates by molecular beam epitaxy and radio frequency acitvated nitrogen plasma. Although cubic TiN is stabile over a wide temperature range, superconducting TiN films are exclusively obtained when the substrate temperature exceeds 710 °C. TiN films grown at 720 °C show a high residual resistivity ratio of approximately 11 and the superconducting transition temperature (Tc) is well above 5 K. Superconductivity has been confirmed also by magnetiztion measurements. In addition, we determined the upper critical magnetic field (μ0Hc2) as well as the corresponding coherence length (ξGL) by transport measurements under high magnetic fields. High-resolution transmission electron microscopy data revealed full in plane coherency to the substrate as well as a low defect density in the film, in agreement with a mean-free path length ℓ ≈ 106 nm, which is estimated from the residual resistivity value. The observations of reflection high energy electron diffraction intensity oscillations during the growth, distinct Laue fringes around the main Bragg peaks, and higher order diffraction spots in the reciprocal space map suggest the full controlability of the thickness of high quality superconducting TiN thin films.

Imaging properties of small-pixel spectroscopic x-ray detectors based on cadmium telluride sensors

Spectroscopic x-ray imaging by means of photon counting detectors has received growing interest during the past years. Critical to the image quality of such devices is their pixel pitch and the sensor material employed. This paper describes the imaging properties of Medipix2 MXR multi-chip assemblies bump bonded to 1 mm thick CdTe sensors. Two systems were investigated with pixel pitches of 110 and 165 μm, which are in the order of the mean free path lengths of the characteristic x-rays produced in their sensors. Peak widths were found to be almost constant across the energy range of 10 to 60 keV, with values of 2.3 and 2.2 keV (FWHM) for the two pixel pitches. The average number of pixels responding to a single incoming photon are about 1.85 and 1.45 at 60 keV, amounting to detective quantum efficiencies of 0.77 and 0.84 at a spatial frequency of zero. Energy selective CT acquisitions are presented, and the two pixel pitches' abilities to discriminate between iodine and gadolinium contrast agents are examined. It is shown that the choice of the pixel pitch translates into a minimum contrast agent concentration for which material discrimination is still possible. We finally investigate saturation effects at high x-ray fluxes and conclude with the finding that higher maximum count rates come at the cost of a reduced energy resolution.

Drying Process Optimization for an API Solvate Using Heat Transfer Model of an Agitated Filter Dryer

Drying an early stage active pharmaceutical ingredient candidate required excessively long cycle times in a pilot plant agitated filter dryer. The key to faster drying is to ensure sufficient heat transfer and minimize mass transfer limitations. Designing the right mixing protocol is of utmost importance to achieve efficient heat transfer. To this order, a composite model was developed for the removal of bound solvent that incorporates models for heat transfer and desolvation kinetics. The proposed heat transfer model differs from previously reported models in two respects: it accounts for the effects of a gas gap between the vessel wall and solids on the overall heat transfer coefficient, and headspace pressure on the mean free path length of the inert gas and thereby on the heat transfer between the vessel wall and the first layer of solids. A computational methodology was developed incorporating the effects of mixing and headspace pressure to simulate the drying profile using a modified model framework within the Dynochem software. A dryer operational protocol was designed based on the desolvation kinetics, thermal stability studies of wet and dry cake, and the understanding gained through model simulations, resulting in a multifold reduction in drying time.

Track structure of light ions: experiments and simulations

To study the track structure of light ions, a measuring device has been developed at the Legnaro National Laboratory of INFN, which can be used to investigate separately the penumbra region of particle tracks and the track-core region, which is a few nanometres in diameter. The device is based on single-electron counting techniques by means of a gas detector; it simulates a ‘nanometre-sized’ biological volume of about 20 nm in diameter that can be moved with respect to a narrow particle beam to measure the ionization-cluster-size distributions caused within the target volume by the passage of single primary particles, as a function of the impact parameter. To investigate the ionization-cluster-size formation caused by primary particles of medical interest when they penetrate through or pass by the target volume at a specified impact parameter, measurements and Monte Carlo simulations were performed for 20 MeV protons, 16 MeV deuterons, 48 MeV 6Li-ions, 26.7 MeV 7Li-ions and 96 MeV 12C-ions. The detailed analysis of the resulting distributions showed that in the track-core region their shape is mainly determined by the mean free ionization path length of the primary particles, whereas in the penumbra region the shape of the distributions is almost independent of the impact parameter, and also of the particle type and velocity.

Dependence of the piezoactivity of zinc oxide films on the conditions of synthesis in critical regimes of glow discharge

The influence of the gas pressure in a magnetron sputtering system on the efficiency of volume hypersound generation in the synthesized ZnO films has been experimentally studied at pressures close to the transition from glow discharge to Townsend regime, whereby the ion and electron mean free path length increases and a drift component appears in the flux of deposited particles. It is established that the efficiency of hypersound generation (at a frequency above 1 GHz) in ZnO films deposited under these conditions increases. Based on a comparison to the properties of films deposited in a classical diffuse glow discharge, this result is explained by a lower nucleation texture and higher density of the films grown in a critical regime.

Relaxation of electronic excitations in CaF2 nanoparticles

The luminescence properties of CaF2 nanoparticles with various sizes (20–140 nm) are studied upon the excitation by VUV and x-ray quanta in order to reveal the influence of ratio of mean free path and thermalization length of charge carriers and nanoparticle size on the self-trapped exciton luminescence. The luminescence intensity for exciting quantum energies corresponding to optical creation of exciton and to the range of electronic excitation multiplication is not so sensitive to nanoparticle size as for quanta with energy of Eg < hν < 2Eg. The dependences of luminescence intensity on nanoparticle size at the excitation by quanta of various energies are discussed in terms of electron-phonon and electron-electron scattering lengths and energy losses on surface defects.

Ultrahigh speed graphene diode with reversible polarity

The ability to tune the carrier-type concentration ratio with applied field along with a mean-free path length approaching 1μm in graphene enables a new diode in which the diode polarity can be reversed. The diode consists of a thin graphene film with a geometric asymmetry that determines a preferred direction for charge-carrier transport, independent of whether the carriers are electrons or holes. We fabricated submicron geometric diodes by patterning and etching exfoliated graphene. Applying field-effect voltages to the substrate, we reversed the carrier type and demonstrated reversal of the diode polarity. The graphene geometric diodes exhibited rectification at 28THz, opening the way to ultrahigh speed applications for these versatile devices.

Penetration depth of linear polarization imaging for two-layer anisotropic samples

Polarization techniques can suppress multiply scattering light and have been demonstrated as an effective tool to improve image quality of superficial tissues where many cancers start to develop. Learning the penetration depth behavior of different polarization imaging techniques is important for their clinical applications in diagnosis of skin abnormalities. In the present paper, we construct a two-layer sample consisting of isotropic and anisotropic media and examine quantitatively using both experiments and Monte Carlo simulations the penetration depths of three different polarization imaging methods, i.e., linear differential polarization imaging (LDPI), degree of linear polarization imaging (DOLPI), and rotating linear polarization imaging (RLPI). The results show that the contrast curves of the three techniques are distinctively different, but their characteristic depths are all of the order of the transport mean free path length of the top layer. Penetration depths of LDPI and DOLPI depend on the incident polarization angle. The characteristic depth of DOLPI, and approximately of LDPI at small g, scales with the transport mean free path length. The characteristic depth of RLPI is almost twice as big as that of DOLPI and LDPI, and increases significantly as g increases.

Impurity Gas Analysis of the Decomposition of Complex Hydrides

This study aims at an investigation of the impurity gases emitted during the decomposition of borohydrides. For this we have set up a quantitative gas analysis based on a combination of FTIR spectroscopy and gravimetry. We show that the emission of various intermediates, in particular diborane, depends sensitively on the reaction conditions, including gas mean free path lengths, hydrogen backpressure, and sample pretreatment. Adduct-free Mg(BH 4)2 and LiBH4 emit diborane only at the impurity level, while for LiZn2(BH4)5 diborane is the main decomposition product. The decomposition reaction of LiZn 2(BH4)5 proceeds via a collision-induced dissociation of Zn(BH4)2 in Ar at ambient pressures. Various additives were tested aiming at catalyzing the decomposition of the desorbed diborane. © 2011 American Chemical Society.

C+emission from the Magellanic Clouds

We study the 158 micron [CII] fine-structure line emission from star-forming regions as a function of metallicity. We have measured and mapped the [CII] emission from the very bright HII region complexes N 11 in the LMC and N 66 in the SMC, as well as the SMC HII regions N 25, N 27, N 83/N 84, and N 88, with the FIFI instrument on the Kuiper Airborne Observatory. In both the LMC and SMC, the ratio of the [CII] line to the CO line and to the far-infrared continuum emission is much higher than seen almost anywhere else, including Milky Way star-forming regions and whole galaxies. In the low metallicity, low dust-abundance environment of the LMC and the SMC, UV mean free path lengths are much greater than those in the higher-metallicity Milky Way. The increased photoelectric heating efficiencies cause significantly greater relative [CII] line emission strengths. At the same time, similar decreases in PAH abundances have the opposite effect, by diminishing photoelectric heating rates. Consequently, in low-metallicity environments the relative [CII] strengths are high but exhibit little further dependence on actual metallicity. Relative [CII] strengths are slightly higher in the LMC than in the SMC, which has both lower dust and lower PAH abundances.

A Numerical Study of Microscale Flow Behavior in Tight Gas and Shale Gas Reservoir Systems

Various attempts have been made to model flow in shale gas systems. However, there is currently little consensus regarding the impact of molecular and Knudsen diffusion on flow behavior over time in such systems. Direct measurement or model-based estimation of matrix permeability for these “ultra-tight” reservoirs has proven unreliable. The composition of gas produced from tight gas and shale gas reservoirs varies with time for a variety of reasons. The cause of flowing gas compositional change typically cited is selective desorption of gases from the surface of the kerogen in the case of shale. However, other drivers for gas fractionation are important when pore throat dimensions are small enough. Pore throat diameters on the order of molecular mean free path lengths will create non-Darcy flow conditions, where permeability becomes a strong function of pressure. At the low permeabilities found in shale gas systems, the dusty-gas model for flow should be used, which couples diffusion to advective flow. In this study we implement the dusty-gas model into a fluid flow modeling tool based on the TOUGH+ family of codes. We examine the effects of Knudsen diffusion on gas composition in ultra-tight rock. We show that for very small average pore throat diameters, lighter gases are preferentially produced at concentrations significantly higher than in situ conditions. Furthermore, we illustrate a methodology which uses measurements of gas composition to more uniquely determine the permeability of tight reservoirs. We also describe how gas composition measurement could be used to identify flow boundaries in these reservoir systems. We discuss how new measurement techniques and data collection practices should be implemented in order to take advantage of this method. Our contributions include a new, fit-for-purpose numerical model based on the TOUGH+ code capable of characterizing transport effects including permeability adjustment and diffusion in micro- and nano-scale porous media.

Dependence of the self-sputtering yield upon ion incidence angle

Solution of the problem of angular dependence of sputtering yield is divided into three parts: calculation of the number of cascade particles as a function of their path length, definition of the path length distribution of reflected particles, and convolution of the two results obtained. The theory is valid for the case of equal ion and target atom masses (self-sputtering) and contains two parameters: the ratio of the transport path length to the mean free path length and the parameter of inelastic energy losses.

Role of Finite Vacancy Relaxation Rate at SHS Reactions in Nanosized Multilayers

Rate of SHS (self-propagating high-tеmperature synthesis) reactions in solid nano-sized multilayers is controlled by the time and temperature dependent vacancy concentration. The increase of reaction temperature is typically faster than the rate of vacancy generation. Therefore, the finite relaxation rate of vacancies leads to drastic slowing down of SHS. On the other hand, as-prepared vacancy supersaturation due to fast deposition on the cold substrate may lead to a certain acceleration of SHS. Influence of (1) vacancy mean free path and (2) initial vacancy supersaturation on the SHS rate is investigated numerically. In wide region of parameters the front velocity appears to be inversely proportional to the square root of vacancy mean free path length.

Conductance of functionalized nanotubes, graphene and nanowires: from ab initio to mesoscopic physics

AbstractWe review recent theoretical results aiming at understanding the impact of doping and functionalization on the electronic transport properties of nanotubes, nanowires and graphene ribbons. On the basis of ab initio calculations, the conductance of micrometer long tubes or ribbons randomly doped or grafted can be studied, allowing to extract quantities at mesoscopic length scales such as the elastic mean free path and localization length. While the random modification of a 1D conducting channel leads generaly to a significant loss of conductance, strategies can be found to either exploit or limitate such a detrimental effect. Spin‐filtering in transition metal doped nanotubes, the opening of a mobility gap in graphene ribbons, and the choice of molecules to limitate backscattering in covalently functionalized tubes are examples that will be discussed.magnified image Symbolic representation of a nanotube filled with Cobalt atoms or clusters with subsequent optimal spinvalve effect (see text).

A STUDY ON PENETRATION DEPTH OF POLARIZATION IMAGING

Optical clearing improves the penetration depth of optical measurements in turbid tissues. Polarization imaging has been demonstrated as a potentially promising tool for detecting cancers in superficial tissues, but its limited depth of detection is a major obstacle to the effective application in clinical diagnosis. In the present paper, detection depths of two polarization imaging methods, i.e., rotating linear polarization imaging (RLPI) and degree of polarization imaging (DOPI), are examined quantitatively using both experiments and Monte Carlo simulations. The results show that the contrast curves of RLPI and DOPI are different. The characteristic depth of DOPI scales with transport mean free path length, and that of RLPI increases slightly with g. Both characteristic depths of RLPI and DOPI are on the order of transport mean free path length and the former is almost twice as large as the latter. It is expected that they should have different response to optical clearing process in tissues.