Calculation of the Optical Properties of Quartz Ceramics Based on Data on Its Structure

The optical parameters of quartz ceramics from a previously proposed identification method and simulation using different optical models of the material are compared. The identification method is based on deliberately measuring hemispherical spectral reflectances for layers of different thicknesses and solving the inverse problem using asymptotic formulas. Mathematical models are constructed based on the Mie theory on the assumption of independent scattering of electromagnetic radiation by fragments of the material. The material is considered as a polydisperse packing of spheres, the sizes of which are determined by data on the material structure. Both a grain surrounded by gas and a pore in monolithic material are considered as a scatterer. Data on the material structure were gathered using optical microscopy, static laser scattering, and mercury porosimetry. The best agreement with the results of the identification method is demonstrated by the model of ceramics in the form of a glass monolith with spherical voids. Comparative analysis eliminates uncertainty in the form of the scattering phase function and shows that the scattering is close to isotropic.     

Measurement of optical properties of highly scattering ceramic materials

Radiation transfer in highly scattering ceramics is described by a diffusion approximation based on the asymptotic relation of the radiant flux and the radiant energy density in material depth. In this approximation, a calculation of the effective absorption coefficient k and the radiation diffusion coefficient D is based on the measurement of normal-hemispherical transmission of specimens shaped as various-thickness disks. Taken into account are radiation field, twodimensionality, and the radiation boundary reflection effect. The optical property measurements have been performed on the experimental apparatus based on an integrating sphere, a collimated radiation source, and a twochannel data acquisition and processing system. Results of the measurements of k and D for the silica ceramic are given at room temperature.     

Structure, mechanical and functional properties of aluminum nitride-silicon carbide ceramic material

Two-phase ceramic composites of the dielectric-semiconductor type having different semiconducting phase content (aluminum nitride ceramics with uniformly distributed inclusions of silicon carbide of a certain size) have been produced by pressureless sintering. These composites are characterized by Vickers hardness HV (150 N) 9.5–15.8 GPa, Palmqvist fracture toughness 3.0–4.2 MPa m^0.5, bending strength 132–209 MPa, thermal conductivity 37–82 W/(m K), and by a coefficient of the microwave electromagnetic energy attenuation to 36.3 dB/cm. It has been found that as the size of silicon carbide grains in aluminum nitride-based ceramics increases, the thermal conductivity increases and microwave energy attenuation decreases, which is indicative of the decisive role of grain boundaries in scattering both phonons and microwave radiation.     

Efficiency factors and radiation characteristics of spherical scatterers in an absorbing medium

The radiative properties of bubbles or particles embedded in an absorbing medium are investigated. We aim first to determine the conditions under which absorption by the surrounding medium must be accounted for in the calculation of the efficiency factors by comparing results from Mie theory and the far-field and near-field approximations. Then, we relate these approximations for a single particle to the effective radiation characteristics required for solving the radiative transfer in an ensemble of scatterers embedded in an absorbing medium. The results indicate that the efficiency factors for a spherical particle can differ significantly from one model to another, in particular for large particle size parameter and matrix absorption index. Moreover, the effective scattering coefficient should be expressed based on the far-field approximation. Also, the choice of the absorption efficiency factor depends on the model used for estimating the effective absorption coefficient. However, for small void fractions, absorption by the matrix dominates, and models for the absorption coefficient and efficiency factor are unimportant. Finally, for bubbles in water, the conventional Mie theory can be used between 0.2 and 200 mum except at some wavelengths at which absorption by water must be accounted for.     

Diffuse reflection of ceramics coated with dielectric thin films

We have studied the diffuse reflection properties of ceramics in the presence of dielectric thin films on the surface. A simple optical model was proposed in which interference effects in a thin film were considered for light scattered out of a ceramic in various directions. Measurements were performed on angle-resolved reflection spectra of a thin-film-coated alumina ceramic in the case of normal incidence. They showed that the presence of the thin film on the ceramic's surface modified the angular distributions of scattered radiation from that of a bare ceramic, which suggested a way to tailor the scattering properties of a diffuse reflector as needed.     

The effect of clustering of particles on Rayleigh scattering of radiation in a turbulent flow

The effect of clustering of particles under conditions of homogeneous isotropic turbulence on Rayleigh scattering of radiation is analyzed. It is demonstrated that, similarly to the diagnostics of molecular structure of liquid using X-ray scattering, the radial distribution function of macroscopic particles, which characterizes the clustering phenomenon, may be determined by way of measuring the intensity of scattering of infrared or microwave radiation. An approximate relation for clustering coefficient is obtained, which may be employed for estimating the increase in reflected radiation of microwave radar from cumulus clouds owing to turbulent clustering of water drops in the cloud.     

The effect of vacuum on the reflectivity of alumina ceramics under conditions of melting by concentrated laser radiation and subsequent free cooling

The results are given of measurements of the normal-hemispherical reflectivity of alumina ceramics in the semitransparency region for the wavelengths of 0.488, 0.6328, 1.15, and 3.39 μm. The measurements are performed in vacuum both in the process of fast heating by concentrated radiation of a CO₂ laser from room temperature to ～2900 K with the formation of a thin layer of melt and in the process of subsequent cooling after the heating radiation is switched off. The heating time is approximately 2.25 s, with the density of heating radiation flux of approximately 1600 W/cm². The effect of abrupt variation of the absorption coefficient during melting and solidification on the thermal-radiation characteristics is analyzed. The results demonstrate that the absorption coefficient of the melt in the semitransparency region is higher in vacuum than in the air.     

Spectral radiative properties of a living human body

Spectral radiative properties of the human body were studied experimentally in the region from the ultraviolet to the far-infrared to know the thermal response of the human body exposed to solar radiation and infrared radiation. The measuring equipment for reflectance and transmittance of a semitransparent scattering medium was developed and measurement on a living human skin was performed in vivo. The measured parts are forearm, cheek, dorsum hand, hip, and hair. The values obtained by the present study are much different from those of previous in vitro measurements. Fairly large values for hemispherical reflectances are observed in the visible and near-infrared regions but very small values for hemispherical reflectances are observed in the infrared region, below 0.05. By applying the four-flux treatment of radiative transfer, the absorption coefficient and scattering coefficient in the human skin are determined. The scattering coefficient is large in the visible region but negligible in the infrared region. The absorption coefficient is very close to that of water and large in the infrared region.     

Effective absorption coefficient of a selective medium within a finite spectral range with regard for scattering

The effect of scattering on the lineshape of outgoing radiation for the regular absorption band model is investigated for the model of a homogeneous plane-parallel radiating, absorbing, and scattering layer with transparent boundaries. The investigation is based on numerical solution of the integrodifferential equation of radiation transport. Based on an analysis of results obtained, methods more accurate than the known ones are proposed for calculation of the absorption coefficient by a selective component of the medium averaged over a finite spectral range.     

Transient cavity growth in ceramics under compression

The transient cavity growth behaviour of liquid phase-sintered ceramics subject to compressive loads is examined. Three possible sources of transient behaviour are suggested, and their ranges of applicability evaluated. By considering the values of the characteristic time for individual transient modes, it has been determined that transient cavity growth in ceramics probably originates from transient grain-boundary sliding. Assuming that the creep-induced cavities nucleate and grow on grain boundaries that are parallel to the loading axis, a transient cavity growth model is developed on the basis that the local stress which drives cavity growth is induced by transient sliding of adjacent grain boundaries. Results of the proposed model are compared with small-angle neutron scattering measurements of a hot-pressed silicon carbide and a liquid phase-sintered alumina, both of which contain a continuous, amorphous grain-boundary phase. The different cavity growth behaviours observed in these ceramics are discussed in conjunction with transient grain-boundary sliding.     

Effect of laser-radiation polarization on the nonlinear scattering of light in nanodiamond suspensions

The effect of laser radiation polarization on the nonlinear scattering of light in aqueous suspensions of detonation nanodiamonds (DNDs) in a regime of optical power limiting (OPL) has been studied. It is established that the nonlinear transmission coefficient of DND suspension in the OPL regime in a field of nanosecond laser pulses with a wavelength of 532 nm is independent of the polarization of incident radiation. The nonlinear scattering of light observed at an angle of 90° in the plane perpendicular to the plane of polarization of the incident radiation depends on the polarization angle in accordance with a trigonometric law. It is shown that the ratio of the signals of scattered radiation for the vertical and horizontal polarizations exhibits nonmonotonic dependence on the laser-beam power density. The results are explained by the Rayleigh-Mie scattering and a change in the size of scattering centers as a result of the effect of a laser upon the DND suspension.     

Si含量对反应烧结SiC陶瓷热电性能的影响

采用可控溶渗反应烧结法制备了致密SiC陶瓷，研究了不同Si含量对反应烧结SiC陶瓷热电性能的影响。经研究发现，反应烧结SiC陶瓷中Si的存在使SiC陶瓷的电阻率急剧下降，大大改善了SiC陶瓷的电学性能；同时Si也改变了SiC陶瓷塞贝克系数随温度的变化趋势，即没有添加Si元素的SiC陶瓷的塞贝克系数随温度的升高逐渐增大，而添加Si元素的SiC陶瓷的塞贝克系数随温度的升高逐渐减小；总的来看，随着Si含量的增加，SiC陶瓷的塞贝克系数扣电导率不断增大，因此Sic陶瓷的功率因子不断提高，而且随着温度的升高，Si含量对SiC陶瓷热电优值的影响越来越明显，当含量为15％时，材料的热电优值是SiC烧结体的30倍。     

Scattering of the transverse magnetic modes from an abruptly ended strongly asymmetrical slab waveguide by an accelerated integral equation technique.

We study the problem of the scattering of the first TM guided mode from an abruptly ended strongly asymmetrical slab waveguide by an improved iteration technique, which is based on the integral equation method with "accelerating" parameters. We demonstrate that the values of these parameters are related to the variational principle, and we save approximately 1-2 iterations compared with the case in which these parameters are not employed. The tangential electric-field distribution on the terminal plane, the reflection coefficient of the first TM guided mode, and the far-field radiation pattern are computed. Furthermore, a simple technique based on the Aitken extrapolation procedure is employed for faster computation of the higher-order solutions of the reflection coefficient. Numerical results are presented for several cases of abruptly ended waveguides, including systems with variational profile, while special attention is given to the far-field radiation pattern rotation and its explanation.     

Reflectivity of buried slab waveguides

The scattering properties of an abruptly ended buried slab waveguide for both TE and TM modes are examined by an improved iteration technique that is based on the integral equation method with "accelerating" parameters. The waveguide is considered a symmetrical slab, for which the weakly guiding conditions are invalid, and it is embedded in a different dielectric material. The tangential electric field distribution on the terminal plane, the reflection coefficient of the first TE and TM guided modes, and the far-field radiation pattern are computed. Numerical results are presented for several ended waveguides, while special attention is given to the far-field radiation pattern rotation and the terminal field distributions.     

On the validity of the anomalous diffraction approximation in light-scattering studies of soft biomedical tissues

In simulations of light propagation in soft biomedical tissues, the tissue is generally modeled as a turbid medium. In one such model, suspended scatterers are assumed to be homogeneous spheres of size following a fractal size distribution law. In this paper, we examine, within the framework of this model, the accuracy of a well-known approximation known as the anomalous diffraction approximation. The deployment of this approximation allows the tissue scattering coefficient to be expressed in a closed analytic form. It is demonstrated that use of the anomalous diffraction approximation yields reasonably good results for the tissue scattering coefficient, the phase function and the asymmetry parameter. It is also shown that the experimentally observed wavelength dependence of the scattering coefficient follows analytically from the formulas obtained using this approximation. The results are important from the point of view of constructing more realistic tissue models which may include non-spherical scatterers.     

Thermoradiation characteristics of refractory oxides upon heating by concentrated laser radiation

The results of experimental investigations and numerical calculations of thermoradiation characteristics of refractory oxides in a wide range of weak absorption (from 0.5 to 5.4 μm), obtained over more than 20 years of research, are analyzed and generalized. Experimental data have been obtained for ceramics made of magnesium oxide, cubic zirconia stabilized by calcium and yttrium oxides, and aluminum oxide. The experiments performed have shown that the temperature distributions in the reflecting and emitting layers must be taken into account when studying thermoradiation and optical properties upon heating by laser radiation up to the intense-evaporation temperatures and that it is important to take into account the influence of the heating radiation flux rate and the possible influence of scattering from condensate droplets formed in the surrounding air.     

Simple Mathematical Models for Temporal,Spatial, Angular,and Attenuation Characteristics of Light Propagating Through the Atmosphere for Space Optical Communication:

Abstract

Mathematical models are developed to characterize propagation through a turbid medium at three different wavelengths in the visible and near infrared spectral range. These models are based upon relations between the temporal, angular, and spatial spread of electromagnetic unpolarized radiation, geometrical path length, particle size distribution, and the medium's propagation parameters such as Mie scattering, and absorption coefficients, Mie phase-function, and optical thickness. Calculations of the radiation characteristics were carried out using Monte Carlo simulations. Here, atmospheric particulates are used to model turbid media for optical thickness between 1 and 6, emphasizing optical communication applications, The advantage of this work is the ability to predict simply and in real time important radiation parameters relevant to any optical communication system. Results indicate very high correlation between optical thickness and propagation characteristics. For transmission, comparison is made to Bucher's model. Results are similar except for absorption effects which are not included in Bucher's model. Some important conclusions are derived such as the prediction that it is advantageous to use longer wavelength radiation through the atmosphere. In addition, there is a very dominant back scattering effect, involving up to 50% of transmitted power for optical densities as low as 6. On the other hand, power density of received scattered light is very low for conventional distances relevant to satellite optical communication, and can be neglected. On the basis of simulation results, the received radiation is of unscattered light only for any optical communication application. The dominant mechanism relating to radiation attenuation is scattering rather than absorption.     

Sources of scattering in cervical tissue: determination of the scattering coefficient by confocal microscopy

Most models of light propagation through tissue assume that the scattering properties of various tissue layers are the same. We present evidence that the scattering coefficient of cervical epithelium varies by a factor of 3 within the epithelium owing to variations in nuclear density and to the presence of keratin. We estimated the scattering coefficient from regions of normal and precancerous cervical epithelium by fitting reflectance measurements from confocal images to an exponential function of depth based on Beer's law of attenuation. The results suggest that the normal cervix is characterized by highly variable scattering in the superficial epithelium, low scattering in the intermediate epithelium, and high scattering in the basal and stromal regions. In high-grade dysplasia, high scattering from high-density nuclei is observed throughout the entire epithelium.