Pyroelectric Properties of Potassium and Rubidium Titanyl—Arsenate Single Crystals in the Temperature Range of 4.2–300 K

The temperature dependences of the pyroelectric coefficients of KTiOAsO₄ and RbTiOAsO₄ single crystals grown by flux crystallization have been investigated in the temperature range of 4.2–300 K. With an increase in temperature, superionic conductivity first arises in KTiOAsO4 (at T > 200 K) and then (at T > 270 K) in RbTiOAsO₄. This conductivity is much higher in the samples polarized at T = 4.2 K. An exponential change in the crystal resistivity along the polar direction is simultaneously observed. The results of measurements in the range of 4.2–200 K indicate larger values of pyroelectric coefficients when compared with potassium and rubidium titanyl-phosphate crystals. A correlation between the pyroelectric coefficients and a change in the lattice constants at isomorphic substitutions of K atoms for Rb and P atoms for As has been revealed within the symmetry approach.     

A study of internal heat transfer in nonuniform porous structures

The results of theoretical and experimental studies of heat transfer and pressure drop in nonuniform porous materials and systems are presented. In experiments, measurements were made of the air flow rate, inlet and outlet air pressures, and air and porous sample temperatures. Experimental determination of the heat transfer coefficient in porous structures is associated with certain difficulties. The problem of determining a temperature difference between coolant and porous skeleton is the most complex. As a rule, under laboratory conditions this difference is small and cannot be found with sufficient accuracy. In the present work, the method of determination of the internal heat transfer coefficient is based on solving the inverse unsteady heat transfer problem for porous structures. Using this approach, the heat transfer coefficient is calculated indirectly or on the basis of the porous material temperature variation over time.     

Features of the twin structure of YBa2Cu3O7−x epitaxial films

X-ray diffraction is used to investigate YBa₂Cu₃O_7?x (YBCO) films on NdGaO₃(110) and a ${{(100)CeO_2 } \mathord{\left/ {\vphantom {{(100)CeO_2 } {(1\bar 102)}}} \right. \kern-0em} {(1\bar 102)}}Al_2 O_3 $ heterostructure. Symmetric, asymmetric, and axial geometries for θ and θ/2θ scans are used to obtain diffraction spectra from different crystallographic planes. The orientational and quantitative twinning characteristics of the films are determined. While the crystallographic parameters of these two types of film (the films are c-axis oriented with c=11.67 Å) are similar, there are differences in the twin structure. In particular, the features of the NdGaO₃ structure lead to the appearance of an angle differing from 90° (90.20°) between the possible (110) and θ/2θ twin planes in a YBCO film and a different number of twin components in each system of twins. It is concluded from an analysis of the broadening of reflections, which are sensitive to twinning, that there is not twinning in a 60% film of YBCO on Al₂O₃ with a CeO₂ buffer layer.     

Influence of the composition and morphology of nanosized transition metal sulfides prepared using the Anderson-type heteropoly compounds [X(OH)6Mo6O18]n? (X = Co, Ni, Mn, Zn) and [Co2Mo10O38H4]6? on their catalytic properties

Using the Anderson-type heteropoly compounds (HPCs) [X(OH)₆Mo₆O₁₈] _n− (X = Co, Ni, Mn, Zn) and [Co₂Mo₁₀O₃₈H₄]₆₋ and cobalt (or nickel) nitrate, XMo/Al₂O₃ and Co(Ni)-XMo/Al₂O₃ catalysts were prepared. The catalysts were studied by low-temperature nitrogen adsorption, X-ray diffraction, and high-resolution transmission electron microscopy. The average length of the active-phase particles of the catalysts was 3.5 to 3.9 nm, and the average number of MoS₂ layers in a packet was 1.4 to 2.1. The catalytic properties of the samples, which were estimated in dibenzothiophene (DBT) hydrodesulfurization and in the hydrotreating of the diesel fraction, are considerably dependent upon both the type and composition of the HPC, and the nature of the applied promoter (Ni or Co). As compared to the Ni-promoted catalysts, the Co-promoted samples exhibit a higher desulfurization activity, whereas the hydrogenation ability of the Ni-XMo/Al₂O₃ catalysts surpasses that of the Co-XMo/Al₂O₃ ones. The catalytic properties depend on the morphology of the nanostructured active phase. With a growing number of MoS₂ layers in the packet of the catalysts’ active phase, the DBT hydrodesulfurization rate constants for both the direct desulfurization route and the preliminary hydrogenation rote rise linearly and the selectivity falls linearly for the hydrogenation route. The selectivity of Ni-XMo/Al₂O₃ decreases to a greater extent than that of Co-XMo/Al₂O₃. The dependences of the catalytic properties on the morphology of the catalysts’ active phase are consistent with the “dynamic” model of the functioning of the active sites of transition metal sulfides.     

Photosensitivity of paramagnetic centers in radiolyzed solid hydrogen cyanide

The photolysis of solid hydrogen cyanide and the effects of UV light on⁶⁰Co--irradiated HCN at 77 K were studied using an ESR technique. As in the case of radiolysis, the HZ₂C=N radical formed due to sticking of the H atom to the triple bond of the HCN molecule is the main radical product of low-temperature HCN photolysis. The C=N^– radicals are accumulated at 77 K in insignificant amounts (3 %). It was established that radical and ionic products stabilized in y-irradiated HCN possess photochromism and under the action of UV light enter photochemical reactions leading to their decomposition. Upon photobleaching, the concentration of H₂C=N^– radicals first increases two- to threefold because of the decomposition of H₂C=N^– ions and then decreases. The presence of radicals and ions formed upon the low-temperature radiolysis of HCN broadens the optical absorption band of the system, and the boundary of the action spectrum shifts from 280 nm (for nonirradiated HCN) to the visible region at 400–440 nm.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 859–863, April, 1996.     

Oxygen diffusion and isotope exchange behaviour of NdBa2Cu3O7−x

The oxygen diffusion and exchange behaviour of NdBa₂Cu₃O_7–x is investigated by means of isothermal and dynamic-thermal analytical techniques. From the results it is concluded that oxygen anions move along vacancies in the CuO₂-layers of the perovskite-like structure. In temperature regions where the diffusion rate is low (665 K<T<773 K) heterogeneous oxygen exchange reactions take place on the phase boundary. The higher the exchange rate the more the partially heterogeneous exchange is favoured in comparison to the completely heterogeneous isotope exchange. Depending on the oxygen deficiency of NdBa₂Cu₃O_7–x the out-diffusion of oxygen becomes dominating at higher temperatures.     

Three-dimensional simulation of dynamic processes of formation of microclusters in a crystalline matrix

A discrete computer model of multistage processes of formation of porous clusters in a crystalline matrix is developed and constructed. A package of computer programs simulating dynamic processes of formation of a cluster in the bulk of crystals is developed taking into account the processes occurring at the surface, external fields, and chemical reactions accompanying these processes. The morphological pattern of formed pores is correlated with the regularities associated with the shape of the current-voltage characteristic of anodization.     

Nearly backward reflection of bulk acoustic waves at grazing incidence in a TeO2 crystal

Reflection of bulk acoustic waves in a TeO₂ acoustooptic single crystal is studied for the case of a grazing incidence on the free crystal-vacuum boundary. The propagation and reflection of elastic waves is considered in the XOY plane of the material cut out in the form of a rectangular prism. An extraordinary case of reflection at the grazing incidence, when the energy flow of one of the two reflected waves in the crystal is directed opposite to that of the incident wave, is studied. It is shown that the transformation of the incident elastic energy into the energy of the backward-reflected wave can occur with an efficiency close to 100% and can be observed in a wide range of crystal cut angles. An abrupt change of the reflection coefficients in the vicinity of the critical angle is predicted.     

Use of time-of-flight mass spectrometry for monitoring helium microcontamination in the industrial manufacture of extra high purity inert gases

A time-of-flight EMG-20-9 mass spectrometer was designed for the online monitoring of helium impurity at the 0.05 ppm level. The spectrometer was introduced for the continuous control of the gas mixture composition in the technology of neon purification from the helium impurity in spite of the large difference in the concentrations of components. The instrument was certified as a measuring facility and introduced into the State Register of Measurement Instrumentation of the Russian Federation. The characteristics of the mass spectrometer and its modifications were overviewed and its possibilities for monitoring xenon and krypton production from air were shown.