Application of the Luikov Approximate Solutions to Mathematical Description of the Process of Self-Freezing of Materials by Decreasing Pressure

A mathematical model for the process of self-freezing of moist materials in a continuously increasing vacuum which is developed on the basis of the approximate solutions obtained by A. V. Luikov for drying of capillary-porous media is presented. Results of experimental investigations into the self-freezing of a moist cloth are given, from which the temperature of supercooling of the surface material layer, the temperature coefficient, and the velocity of deepening of the freezing surface are determined; all of these parameters are used in mathematical simulation of the above process.     

Determination of the composition of copolyester based on glycolide and dℓ-lactide by chromatography

Conclusions A procedure has been developed for determining the composition of copolyesters based of glycolide and dl-lactide.The procedure has reasonable analytical accuracy and can be used for operative analytical control in copolymer synthesis.Translated from Khimicheskie Volokhna, No. 4, pp. 59–61, July–August, 1989.     

Photoluminescence decay rate of silicon nanoparticles modified with gold nanoislands

We investigated plasmon-assisted enhancement of emission from silicon nanoparticles (ncs-Si) embedded into porous SiO _x matrix in the 500- to 820-nm wavelength range. In the presence in the near-surface region of gold nanoisland film, ncs-Si exhibited up to twofold luminescence enhancement at emission frequencies that correspond to the plasmon resonance frequency of Au nanoparticles. Enhancement of the photoluminescence (PL) intensity was attributed to coupling with the localized surface plasmons (LSPs) excited in Au nanoparticles and to increase in the radiative decay rate of ncs-Si. It has been shown that spontaneous emission decay rate of ncs-Si modified by thin Au film over the wide emission spectral range was accelerated. The emission decay rate distribution was determined by fitting the experimental decay curves to the stretched exponential model. The observed increase of the PL decay rate distribution width for the Au-coated nc-Si-SiO _x sample in comparison with the uncoated one was explained by fluctuations in the surface-plasmon excitation rate.

PACS

78. 67. Bf; 78.55.-m     

Thermal conductivity of highly porous Si in the temperature range 4.2 to 20?K

We report on experimental results of the thermal conductivity k of highly porous Si in the temperature range 4.2 to 20 K, obtained using the direct current (dc) method combined with thermal finite element simulations. The reported results are the first in the literature for this temperature range. It was found that porous Si thermal conductivity at these temperatures shows a plateau-like temperature dependence similar to that obtained in glasses, with a constant k value as low as 0.04 W/m.K. This behavior is attributed to the presence of a majority of non-propagating vibrational modes, resulting from the nanoscale fractal structure of the material. By examining the fractal geometry of porous Si and its fractal dimensionality, which was smaller than two for the specific porous Si material used, we propose that a band of fractons (the localized vibrational excitations of a fractal lattice) is responsible for the observed plateau. The above results complement previous results by the authors in the temperature range 20 to 350 K. In this temperature range, a monotonic increase of k with temperature is observed, fitted with simplified classical models. The extremely low thermal conductivity of porous Si, especially at cryogenic temperatures, makes this material an excellent substrate for Si-integrated microcooling devices (micro-coldplate).

PACS

61.43.-j; 63.22.-m; 65.8.-g     

Quantitative estimates of gas maldistribution and methods for their localization in absorption columns

In the present work, we carry out evaluation of gas flow maldistribution in different types of gas distributing devices (GDD) and packings by using experimental data for the output velocity profiles for both the absorption pilot column and industrial column. Two GDD construction types—with circular and with local tube gas feed, together with their modifications—are investigated. Two pilot columns—with random packing RSRM 1.5″ and with structured ceramic Honey-comb packing, and one industrial contact economizer with Honey-comb and inclined rings—have been studied. Six types of quantitative estimations of GDD and packings maldistribution are determined by the help of MATHCAD. Then, their sensitivity toward several factors, such as the type of GDD and packing, the initial gas flow velocity, the number and dimension of measuring cells on the column cross-section, the measurement error, and so on, is investigated. It is found that only two of the quantitative maldistribution estimations take into account the formation of maldistribution clusters, which deteriorates the uniform gas distribution. A novel method for quantitative determination of these clusters and also for identification of the zones at column cross-section, in which they appeared, has been developed. It is established that the measurement error and the dimension of measuring cell deeply influence the number, the area, and spatial distribution of these zones on the cross-section, as well as the values of maldistribution estimates.     

Radon programmes and health marketing

Being aware of negative health effects of radon exposure, many countries aim for the reduction of the radon exposure of their population. The Czech radon programme was commenced >20 y ago. Since then experts have gathered a lot of knowledge, necessary legislation has been enacted, tens of thousands of inhabitants have been offered free measurement and subsidy for the mitigation. Despite the effort, the effectiveness of the radon programme seems to be poor. Newly built houses still exhibit elevated radon concentrations and the number of houses mitigated is very low. Is it possible to enhance the effectivity of radon programme while keeping it on a voluntary basis? One possible way is to employ health marketing that draws together traditional marketing theories and science-based strategies to prevention. The potential of using marketing principles in communication and delivery of radon information will be discussed.     

Effects of cola drinks on the morphology and elastic modulus of dentin

The mechanical and morphological degradation that human dentin undergoes after prolonged exposure in cola drinks was examined using nanoindentation and AFM. The elastic modulus of the dentin prior cola exposure was 18.28 ± 1.7 GPa, while after exposure it had decreased to 4.25 ± 1.4 GPa on the surface that was in direct contact with the drink, and to 13.38 ± 2.9 GPa on the surface that was in indirect contact with the drink. AFM documented the corrosion effects of the cola through topography images and histograms of the RMS roughness.     

Properties of TiO2 films on silicon substrate

Influence of the substrate material and the effect of oxygen plasma on the microprofile and electrical properties of TiO₂ films deposited by radio-frequency magnetron sputtering are studied. It is shown that the most continuous films with the smallest roughness are obtained when deposited onto silicon substrates. The change in the capacitance-voltage and conductance-voltage characteristics of the structures upon their exposure to oxygen plasma is accounted for by the diffusion of oxygen atoms across the titanium-dioxide film and by the appearance of a SiO₂ layer at the Si-TiO₂ interface.