Search

The results on thermal stability of the microstructure and mechanical properties of Zr – 1 wt.% Nb and Ti – 45 wt.% Nb ultrafine-grained alloys subjected to long-term thermal annealing at a temperature of 400°С are presented. It is shown that in a Zr – 1 wt.% Nb ultrafine-grained alloy an increase in the annealing duration from 5 to 360 hr leads to a growth of the structural elements (grains, subgrains, fragments) of the α-Zr matrix phase and β-Nb particles. This is a consequence of the recrystallization processes, which gives rise to softening of the alloy and a decrease in their microhardness and yield stress. It is found out that an annealing treatment for as long as 360 hr does not affect the structural element size of β-phase in the Ti – 45 wt.% Nb alloy but favors a noticeable grain growth in α- and ω-phases. It is demonstrated that disordering of the Ti – 45 wt.% Nb alloy and a decrease in its mechanical characteristics are due to the recovery processes at the grain boundaries, an increase in the nanosized grains of α- and- ω-phases, and their decreased contribution to dispersion hardening.

The stage-like character of the stress-strain curves and microstructure in the fracture zone of coarse-grained and ultrafine-grained specimens of the VT1-0 and Zr–1Nb alloy specimens under quasistatic loading are investigated using the temperature curves obtained by the method of IR-imaging. New experimental data are obtained, which provide evidence on a considerable influence of the ultrafine-grained state on the evolution of plastic deformation and fracture in the VT1-0 and Zr–1Nb alloys.

Analytical expressions for spontaneous and induced by blackbody radiation energy shifts and widths of atomic circular states are derived. The features of asymptotic behavior of the total width and energy shifts for highly excited Rydberg circular states in the blackbody radiation field are described. Approximate equations are proposed for simple evaluations of energy characteristics and an absence of temperature-independent spontaneous contribution to the total energy width in a range of high temperatures and principal quantum numbers is demonstrated.

In this paper, we experimentally studied the limiting conditions for the combustion of hydrogen, methane, and propane in mixtures with air or oxygen at pressures below atmospheric. A significant effect is shown for the size of the reaction vessel and the ignition energy on both the minimal pressure value and the change in the concentration range of flame propagation. The dimensions of the reaction vessel and the magnitude of the ignition energy, the increase of which does not lead to a change in the minimal pressure and concentration region of flame propagation, are established. The minimal pressures Pmin of gas mixtures, below which flame propagation is impossible, were determined. A qualitative interpretation of the data is presented.

The results of investigation of the influence of the structural element size and phase state of the Ti – 45 wt.% Nb and Zr – 1 wt.% Nb binary alloys on their mechanical properties are presented. The structural states of the alloys with the structural elements of different dimensions were formed from the ultrafine-grained state via annealing. The curves of dependence of the yield strength and microhardness on the average size of the structural elements, d –1/2, are obtained using the Hall–Petch relation. It is shown that for the Zr – 1 wt.% Nb alloy, the Hall–Petch relation is fulfilled in the entire range of sizes under study, from ultrafine-grained to finegrained states 0.2–1.9 μm. For the Ti – 45 wt.% Nb alloy, in the analysis of the Hall–Petch relationship within the structural element size range 0.43–45 μm, its phase composition is taken into consideration, specifically, the presence of dispersion-strengthened β-phase grains, α-phase subgrains, nonequilibrium nanosized ω- phase, and the bimodal grain-subgrain structure.

Ti–Nb alloy with 40 wt % of Nb is obtained from a composite Ti–Nb powder by means of selective laser melting. The Ti–Nb alloy has a two-phase microstructure. The main β-phase of the solid titanium–niobium solution forms grains ranging in size from ~2 to 20 μm. A nonequilibrium α″-phase is found in the forms of lamellar, globular, and packet martensite inside the grains of the β-phase and along their boundaries.

Phase composition and microstructure of Ti–Nb alloy produced from the composite titanium and niobium powder by selective laser melting (SLM) method were studied in the present research. Ti-Nb alloy produced by SLM is a monolayer and has zones of fine-grained and medium-grained structure with homogenous elemental composition within the range of 36–38 wt.% Nb. Phase state of the alloy corresponds to the main phase of β- solid solution (grain size of 5–7 μm) and non-equilibrium martensite α″-phase (grain size of 0.1–0.7 μm). Grains of the α″-phase are localized along the boundaries of β-phase grains and have a reduced content of niobium. Microhardness of the alloy varies within the range of 4200–5500 MPa.

Results of analyzing the overall balance of materials, the oxygen balance, and the thermal balance within a 120-t arc smelting furnace are presented. It is found that the heat influx in the oxidation of iron and other batch components is so great that the thermal conditions and the process as a whole become autogenous. The significant losses in complete chemical combustion indicate unsatisfactory organization of the aerodynamic fluxes within the furnace. The aerodynamic fluxes and temperature fields in the region of the cold batch between the electrodes and the inner surface of the furnace wall are discussed on the basis of SolidWorks Flow Simulation software. Recommendations are made regarding the configuration of the gas–oxygen burners, the oxygen lances, and the units for injection of the carbon-based materials into the bath.

The microstructure and mechanical properties of bioinert titanium, zirconium, and niobium alloys in the ultrafine-grain state are investigated. The ultrafine-grain structure is obtained by severe plastic deformation, including multicyclic abc pressing at specified temperatures, multipass rolling in shaped rollers at room temperature, and low-temperature non-recrystallizing annealing. Annealing increases the plasticity of the ultrafine-grain alloys, without changing the grain size. As a result of two-stage treatment—severe plastic deformation and annealing—ultrafine-grain structure with grains and subgrains of mean size 0.16–0.25 μm is formed. That considerably improves the mechanical properties (ultimate strength, yield point, and microhardness) of the alloys. At the same time, the formation of ultrafine-grain structure in the alloys does not change the elastic modulus, even with considerable increase in the ultimate strength and plasticity.

The temperature distribution in the working space of an electric-arc steelmaking furnace built by the company “Danieli” and equipped with radially positioned fuel-burning devices is analyzed. It is concluded that the temperature field is quite nonuniform, and the suggestion is made to reposition the gas-oxygen burners into a tangential configuration. Computer modeling is used to determine the temperature distribution in the furnace’s working space for radially and tangentially arranged burners.

Results are presented from an analysis of the motion of gas flows created by fuel-burning devices in existing electric-arc steelmaking furnaces and recommendations are made on how to efficiently arrange this equipment in the furnace. Controlled annular motion of the aerodynamic flows in the part of the working space between the furnace wall and the electrodes increases convective heat transfer to the cold solid charge, forms a reliable and uniform slag crust on the water-cooled wall, and alleviates the deposition of process dust on the electrodes. A numerical study is performed using the SolidWorks Flow Simulation program and boundary conditions that reflect the actual operating conditions in the furnaces.

Total probabilities Anl of spontaneous radiative transitions, which determine lifetimes τnl = 1/Anl of nS-, nP-, nD- and nF-states in singly charged ions of the group IIa (Be+, Mg+, Ca+, Sr+, Ba+) and IIb (Zn+, Cd+, Hg+) elements, were calculated in the single-electron Fues’ model potential (FMP) method. An asymptotic dependence is determined for highly excited states with small angular momentums and presented in the form τnl = τl(0)n3Ql(1/n) and numerical values of the factors τl(0) and coefficients of the cubic polynomial Ql(x) are calculated, which approximate results of numerical computations with relative error below 2% in the range of states with principal quantum numbers from n = 7 to n ≈ 2000. The comparison of the numerical results with data in literature is presented, and applicability and reliability of single-electron model calculations are discussed in detail for positive singly charged ions of the group II elements.

The synthesis of 1-[(2-R-o-carboran-1-yl)methyl]benzotriazoles (R = Me, Ph) is described. α-Deprotonation of these compounds followed by reactions of the generated carbanions with MeI give the corresponding α-methylated products. Even at room temperature, they exist in solution as an equilibrium mixture of two conformers due to steric crowding around the Cα atom. In contrast, the anions generated by deprotonation of α-ferrocenyl derivatives of 1-methyl- and 1-ethylbenzotriazoles are ambident and are attacked by electrophiles at both the Cα and N(3) atoms.

Blackbody-radiation-induced transitions from Rydberg states in S-, P-, D-series of neutral potassium are investigated at temperatures T = 100 - 2000 K . Key features of such transitions are described. The improved approximation for rates of blackbody-radiation-induced decays and excitations from Rydberg states is proposed with an adaptation for wide range of temperatures.

General properties were determined and asymptotic approximation formulas for probabilities of spontaneous decay and photoionization cross-sections were derived on the basis of numerically calculated amplitudes of bound-bound and bound-free radiation transitions from Rydberg nS-, nP- and nD-states of alkali atoms with high principal quantum numbers, up to n = 1000. The departure of data from the asymptotic formulas for states with principal quantum numbers in the range between n = 10 and n = 2000 from originally calculated data does not exceed 0.1–1%, and may be useful for estimations of natural radiation widths for arbitrary high Rydberg levels.

The objective of the present work is to develop recommendations for controlling the safety of nuclear power plants on the basis of risk assessments and safety certification of nuclear power plants. The Kursk nuclear power plant is considered as an example of a nuclear power plant with an RBMK reactor. The concept of risk assessment of a nuclear power plant consists in constructing a set of scenarios of the appearance and development of possible accidents followed by an evaluation of the realization frequency and determination of the scales of the consequences of each one. The result of an analysis is an evaluation of a system of risk indicators in accordance with the requirements of the safety compliance certificate of the nuclear power plant as well as the development of recommendations for increasing plant safety. In risk assessment, the consequences are divided into categories of the seriousness of the damage, for which their probability is evaluated separately. The graphical interpretation of risk due to any dangerous object consists of frequency-consequences curves. Recommendations are developed on the basis of the results of risk analysis.

The LTRretrotransposon gtwin of Drosophila mel� anogaster was first cloned from the laboratory strain G�32, which is characterized by a strong amplification of copies of this transposable element (1). In addition, strain G�32 has a number of other unique features, such as the presence in the majority of studied species of a complex extended inversion in the third chromo� some, within which gtwin insertions are accumulated (2), as well as an increased frequency of occurrence of singlestranded DNA breaks, which cause recombina� tion processes and gene conversion (3). Currently, gtwin transpositions in strain G�32 are not observed; however, extrachromosomal circular copies of this ele� ment containing two polymorphic sites in the binding sequence of the tRNA primer for the synthesis of the minus strand of DNA were identified in the genome. Among the analyzed clones containing relevant 5'� untranslated region of circular gtwin molecules, the group with a mutant binding site for the tRNA primer represented by noncomplementary nucleotides at positions 497 and 503 was found to be the most numerous. In addition, four fulllength gtwin copies cloned from the genomic library of strain G�32 also belonged to the subfamily with the "incorrect" bind� ing site for the tRNA primer (4). In this work we performed molecular analysis of different copies of LTR retrotransposon gtwin, which were cloned from strain G�32, and showed that, although its genome contains variants with either nor� mal or mutant binding site for the tRNA primer for the synthesis of the minus strand of DNA, it is the mutant subfamily that was amplified. In addition, we searched different gtwin variants in 14 laboratory strains of D. melanogaster and in D. melanogaster Schneider 2 cells. We demonstrated the existence of two subfam� ilies of the

Regioselective bromination of 14β-hydroxydihydroquinopimaric acid methyl ester in acetic acid and methanol gave the corresponding 16S-bromo-, 19R-bromo-, and 16S,19R-dibromo-14β,20-epoxy derivatives whose structure was determined by X-ray analysis and NMR spectroscopy. The reactions of 16S-bromo ketones with thiourea afforded 2,1,3-thiadiazoles fused to diterpene fragment.

Direct aromatic C—H bond activation in the (Sa)-BINOL-derived phosphite (Sa)-HL afforded the dimeric cyclopalladated complex (Sa,Sa)-{Pd(η2-L)(μ-Cl)}2 (2) which is the first optically active PC-palladacycle bearing a phosphorus atom in an axially chiral environment. ortho-Palladated structure of dimer 2 was confirmed by spectral (1H and 31P NMR) examination of its mononuclear derivatives and by X-ray diffraction analysis of the phosphine adduct (η2-L)PdCl(PPh3) (4). The enantiomeric purity of the starting ligand remained unchanged in the PC-palladacycle under the thermal conditions used for the cyclopalladation (∼110 °C); this fact was confirmed by the 31P NMR spectroscopy after chiral derivatization in situ of dimer (Sa,Sa)-2 with the (RC)-valinate chiral auxiliary (Val). trans(N,C)-Configuration of the valinate complex (η2-L)Pd(Val) (5) was established by 1H NMR and supported by DFT calculations. The chirality transfer in the new PC-palladacycle was discussed on the basis of X-ray diffraction data for the phosphine adduct rac-4 and DFT calculations performed for both phosphine and valinate mononuclear derivatives.

The rare olean-18(19)-ene triterpenoids moradiol and moronic acid were synthesized from betulin, and their antiviral properties were investigated.

42 Mobile genetic elements account for a considerable part of eukaryotic genome and significantly affect its stability. They are involved in mutagenesis, regulation of gene expression, and occurrence of genomic rearrangements. In view of this, the study of the mechanisms by which the activity of transposable elements is regulated by cellular factors is of great interest. For this purpose, it is convenient to use model objects in the genome of which mobile genetic elements are transposed and amplified. The behavior of mobile genetic elements in different Drosophila species has been described sufficiently well so far. There are many Drosophila strains characterized by genetic instability caused by amplification and transposition of mobile genetic elements. In such strains, an increased frequency of mutations and reversions as well as occurrence of chromosomal rearrangements is observed. A characteristic feature of D. melanogaster laboratory strain G32 is unusual behavior of some mobile elements. In particular, this strain characterized by substantial amplification of the gtwin retrotransposon, which is closely related to gypsy , one of the most comprehensively studied mobile genetic elements. Evidences indicating that currently gtwin undergoes transpositions have not been obtained [1]. In addition, we detected a large chromosomal aberration in this linea with an increased number of sites where gtwin and some other mobile genetic elements are located [2]. It is known from published data that mobile genetic elements are inhomogeneously distributed over the genome, which leads to occurrence of clusters playing a major role in the regulation of transposition activity [3]. Taking into account this information and the fact that the number of gtwin copies in the G32 strain was approximately 30 (which is one order of magnitude larger than the commonly detected number of copies of this element), we assumed that different gtwin copies may be located in the immediate vicinity of one another. In this study, we tested a series of D. melanogaster strains for the presence of neighboring gtwin copies by PCR using genomic DNA as a template. The annealing sites for PCR primers were located at both ends of gtwin in the vicinity of its long terminal repeats (LTRs). Synthesis proceeded in the direction of the genomic environment of the element.

A determination of enantiomeric purity of 1-substituted (Me, Ph, and Pri) 3-amino-1,2dicarba-closo-dodecaboranes by HPLC on chiral Chiralcel OD-H and Chiralpac AD stationary phases involving preliminary phthaloylation of 3-aminocarboranes has been suggested as a general method.

Distribution of two structural functional variants of the gypsy(MDG4) mobile genetic element was examined in 44 strains of Drosophila melenogaster. The results obtained suggest that less transpositionally active gypsyvariant is more ancient component of the Drosophilagenome. Using Southern blotting, five strains characterized by increased copy number of gypsywith significant prevalence of the active variant over the less active one were selected for further analysis. Genetic analysis of these strains led to the suggestion that some of them carry factors that mobilize gypsyindependently from the cellular flamencogene known to be responsible for transposition of this element. Other strains probably contained a suppressor of the flam –mutant allele causing active transpositions of the gypsy. Thus, the material for studying poorly examined relationships between the retrovirus and the host cell genome was obtained.

To study the mechanism of precise excision ofgypsy from genomic sites, the integrase domain ofgypsy pol was cloned and expressed inEscherichia coli. The endonuclease activity of recombinant integrase was assayed with synthetic substrates corresponding to 3′-U5 ofgypsy LTR and to the known genomic insertion sites ofgypsy. Integrase nicked the 5′-A ⇓ YR-3′ triplet in the (+) strand of the double-stranded substrates; cleavage of a single-stranded substrate was nonspecific. Cleavage proved to be affected by the local conformation of the substrate: the (+) strand was cleaved more efficiently when the (−) strand had an unpaired base in the triplet and was not cleaved when the (−) strand was interrupted or branched. The triplet corresponded to the consensus region ofgypsy insertion (5′-YRYR ⇓ YR-3′), the site of cleavagein vitro coinciding with the site of insertionin vivo. The unique mechanism ofgypsy excision was assumed to depend to a great extent on the enzymic properties of its integrase.

Rates Pnl of photoionization from Rydberg ns-, np-, nd-states of a valence electron in Cs, induced by black-body radiation, were calculated on the basis of the modified Fues model potential method. The numerical data were approximated with a three-term expression which reproduces in a simple analytical form the dependence of Pnl on the ambient temperature T and on the principal quantum number n. The comparison between approximate and exactly calculated values of the thermal ionization rate demonstrates the applicability of the proposed approximation for highly excited states with n from 20 to 100 in a wide temperature range of T from 100 to 10,000 K. We present coefficients of this approximation for the s-, p- and d-series of Rydberg states.

New features of the blackbody-induced radiation processes on Rydberg atoms were discovered on the basis of numerical data for the blackbody-induced decay Pdnl(T), excitation Penl(T) and ionization Pionnl(T) rates of nS, nP and nD Rydberg states calculated together with the spontaneous decay rates Pspnl in neutral hydrogen, and singlet and triplet helium atoms for some values of the principal quantum number n from 10 to 500 at temperatures from T = 100 K to 2000 K. The fractional rates Rd(e, ion)nl(T) = Pnld(e, ion)(T)/Pspnl equal to the ratio of the induced decay (excitation, ionization) rates to the rate of spontaneous decay were determined as functions of T and n in every series of states with a given angular momentum l = 0, 1, 2. The calculated data reveal an essential difference between the asymptotic dependence of the ionization rate Pionnl(T) and the rates of decay and excitation Pd(e)nl(T)∝T/n2. The departures appear in each Rydberg series for n > 100 and introduce appreciable corrections to the formula of Cooke and Gallagher. Two different approximation formulae are proposed on the basis of the numerical data, one for Rd(e)nl(T) and another one for Rionnl(T), which reproduce the calculated values in wide ranges of principal quantum number from n = 10 to 1000 and temperatures between T = 100 K and T = 2000 K with an accuracy of 2% or better. Modified Fues' model potential approach was used for calculating matrix elements of bound–bound and bound–free radiation transitions in helium.

Approximation formulae in the form of quadratic polynomials in combination with Planck's distribution are proposed for the thermal ionization rates of Rydberg states in helium and alkali atoms. The coefficients of the polynomials were determined by fitting the data for the rates calculated by integrating the photoionization cross sections obtained in the model potential approach for the S, P and D states with the principal quantum number ranging from n= 10 to n= 45, in product with Planck's frequency distribution for the flux of blackbody photons at temperatures from 100 K to 2000 K. Tables of the polynomial coefficients are presented for the atoms indicated. The deviations of the data given by the approximation formula from that of the straightforward integration of cross sections is less than 7% for states with n from 20 up to 55 in the temperature range from 70 K to about 1000 K, not exceeding 40?50% for states with n= 20?100 in the temperature range extending up to 10?000 K, thus providing rather simple and accurate estimates for experiments and for practical applications.