Mechanism of isotope exchange of dihydrogen with liquid ammonia catalyzed, by amide ions

The interacting bonds method (IBM) has been used for the analysis of hydrogen bonds in liquid ammonia, including estimates of the resonance components of the H=bonds of ammonia molecules with ammonium and amide ions. According to the E_a estimates for the interaction of a hydrogen molecule with an amide-ammonia solution (AAS) and the data on liquid ammonia structure, a kinetic mechanism for isotope exchange (IE) of hydrogen with liquid ammonia is formulated.

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(MBC) , -, - -. E_a - (AAP). () .

     

A fluctuation theory of normal crystal growth in the case of one-component metallic macrosystems

A Model of spontaneous fluctuations of the limited spectrum solid state particle concentrations in the case of one-component metallic macrosystems is considered. Within the small supercoolings' region of the melt-crystal macrosystem a normal law of crystallization is established due to the action of fluctuation mechanism. For some metallic macrosystems a comparison on the crystallization kinetics between the theory and experiments has been realized. Some quantitative restrictions of the melt-crystal system supercoolings' region within which the mechanism of spontaneous fluctuations is expected to be realized are established for a collection of one component metallic macrosystems. Two-phase transitional zone length's influence on the values of admitted supercoolings in presented.     

Fluctuation mechanism of normal crystal growth during solidification of binary metallic melts

A recently published theory on the solidification of a one-component melt has been extended to the more complex case of binary systems. The theory is based on the model of a two-phase transitional zone existing between the crystalline phase and the melt. The concentration of solid state atoms within each mono-atomic layer of the transitional zone are assumed to fluctuate due to thermal fluctuations. A crystal growth law has been derived expressing the crystallization velocity in terms of probability functions describing these concentrations fluctuations. When certain restricting conditions concerning the atomic interaction energies within the transitional zone and the distribution of the atoms among the solid and liquid phases at supercooling are fulfilled the crystal growth law attains a simple form predicting for small supercoolings a growth rate proportional to supercooling (linear growth law), roughly proportional to physical parameter θ_AA, and with a weak dependence on another parameter Δ.     

On normal kinetics in the case of some refractory and transition metals' crystallization

In given work the crystal growth kinetics connected with some refractory and transition metals is considered. For melts of Fe, Co, Ni, Cr, Zr, Ti, Ag, Cu, and Pb in the vicinity of melting temperatures the solid phase growth is described by means of a fluctuation theory applied for the macro-systems. By virtue of the “unlimited” and limited spectrum solid state particle concentration fluctuations' theory some effective kinetic coefficients have been estimated in the work. The effective kinetic coefficients are specific in the case of normal crystal growth kinetics. In the case of the crystallization of Fe, Co, Ni, Cr, Zr, Ti, Ag, Cu, and Pb-melts a comparison between two effective kinetic coefficients (based on the fluctuation theory and so-called equilibrium growth points density) has been realized. In most cases a satisfactory agreement between the effective kinetic coefficients has been observed.     

A fluctuation mechanism of crystallization processes for the one-component metallic melts

In given work a mechanism of spontaneous fluctuations of the „unlimited”︁ spectrum solid state particle concentrations acting in some monolayers of two-phase transitional zone within the small supercoolings' region is considered. This mechanism is applied for some one-component metallic macrosystems crystallizing from the supercooled melts. It is established that given mechanism in the case of macrosystems transforms into action of only one spontaneous fluctuation occuring in each monolayer of two-phase transitional region as a result of which the solid state particle concentration becomes equal to nearly 1/2. It is shown that in this case within the small supercoolings' region given mechanism leads to so-called normal law of crystallization connected with one-component metallic macrosystems. In the particular scheme of mechanism presented an effective kinetic coefficient is estimated. In the vicinity of melting temperature for some metallic melts a comparison of theoretical and experimental data concerning the mean crystallization velocities has been fulfilled. In the case of spontaneous fluctuations of the „unlimited”︁ spectrum solid state particle concentrations two-phase transitional zone's sizes do not have influence on the one-component metallic melts' crystallization kinetics within the small supercoolings' region.     

Strange-quark mass from tau-lepton decays with O(alpha3s) accuracy

The first complete calculation of the quadratic quark mass correction to the correlator of the two currents relevant for the strangeness-changing semihadronic tau-decay rate is presented including its real part at the four-loop level. This allows us to perform the extraction of the strange-quark mass m(s) from the decay width of the tau-lepton with full O(alpha3s) accuracy. In agreement with previous estimates, the newly computed alpha3s term proves to be rather large. This justifies inclusion of the similarly estimated alpha4s term in phenomenological analysis. Combined with an updated value of V us and an "improved" version of the renormalization group improvement of the perturbative series, this leads to an increase of the central value of m(s) by about 20% and a partial reduction of the theoretical uncertainty by about 50%.     

Physical and technological aspects of ultrasonic imaging of brain structures through thick skull bones: 1. Theoretical and model studies

The basic effects that restrict the potentialities of the ultrasonic imaging of brain structures by echo sounding through thick skull bones are considered. It is demonstrated that the main interfering factors are the echo signals from multiple reflections in the bone, its unknown inhomogeneity in depth, and the strong attenuation. To eliminate these effects, the use of a matched spatial processing and complex wideband signals with their subsequent correlation compression is proposed. A simulation of the proposed signal processing techniques confirmed the possibility of brain structure imaging with a spatial resolution of about 1 mm by ultrasonic echo sounding with acceptable intensities through bones up to 20 mm thick at frequencies of 1–2 MHz.     

Regularities of oxygen and water thermal desorption from barium cerate doped by neodymium,samarium, and gadolinium

Processes of thermal desorption of oxygen molecules and water from BaCe_1–x M _x O_3–δ, where M= Nd, Sm, and Gd, presintered in air at the temperature of 650°C are studied. It is found that oxygen is desorbed only from neodymium–doped barium cerate and is almost not evolved from barium cerate doped by samarium and gadolinium. The amount of desorbed oxygen features a square dependence on cationic doping by neodymium. At similar degrees of cationic doping, the amount of water desorbed from neodymium–doped barium cerate is always lower than that from the cerate doped by samarium and gadolinium. The obtained experimental data on thermal desorption and analysis of literature data served as a basis for the conclusion as to the mixed valency of neodymium Nd(III)–Nd(IV) in BaCe_1–x Nd _x O_3–δ. In this case, at similar doping degrees x, the hydration degree of BaCe_1–x Nd _x O_3–δ is lower and the oxygen index is higher than in BaCe1_–x (Sm,Gd) _x O_3–δ. The differences become more pronounced at high degrees of cationic doping and must decrease at an increase in temperature.     

Hydrogen in perovskites: Mechanism of solubility, chemical state, effect on electron subsystem, phase transformation

The phenomena of hydrogen/water solubility in YBa₂Cu₃O_7-δ and BaCeO₃ undoped and doped by Y and Nd are considered. New experimental data as well as recently published data have been accumulated to discuss the chemical state of protons, taking into account the fairly strong interaction of the protons with the electron subsystem. Phase transformation of the studied oxides induced by hydrogen/water intercalation were revealed by the investigations of thermal effects, X-ray diffraction, Raman spectroscopy, inelastic neutron scattering and nuclear quadrupole resonance. Paper presented at the 5th Euroconference at Solid Statre Ionics, Benalmádena, Spain, Sept. 13–20, 1998.