Formation of an icosahedral structure during crystallization of nickel nanoclusters

The crystallization of nickel nanoclusters is investigated using a molecular dynamics simulation with tight-binding potentials. The formation of a cluster structure depends on the cooling conditions. Slow cooling results in the formation of a face-centered cubic structure, whereas fast cooling, according to the data obtained in 40% of the simulation experiments, leads to the formation of an icosahedral structure. The molecular dynamics simulation experiments demonstrate the possibility of controlling the formation of a structure of nickel nanoclusters during crystallization.     

Preparation of nickel nanopowder through evaporation of the initial coarsely dispersed materials on an electron accelerator

The experimental studies have shown that, during evaporation of coarsely dispersed carbonyl nickel, a nanodispersed powder of high purity can be obtained, which is promising for use in different technological applications. The process has a high efficiency and a relatively high output (of the order of 100 g/h). The specific feature of the synthesis is the separation of obtained nanoparticles into two fractions with average sizes of approximately 100 and 200 nm. The computer analysis of the system evolution has demonstrated that this separation can occur as a result of the aggregation of the initial clusters of sufficiently large sizes.     

On structural transitions in nanoparticles

Using two independent computer programs based on isothermal molecular dynamics method, we studied size effects during the melting and crystallization of Lennard-Jones nanoparticles and metallic clusters, the interaction in which was described by the cooperative potentials. We found that at nanoparticle radii exceeding a certain characteristic value, the melting point of nanoparticles is higher than their crystallization temperature.     

Model of a martensitic transformation of a two-dimensional crystal

A model of a configuration transition is proposed which describes the characteristic features of the martensitic transformation of a two-dimensional crystal. Such transformations are observed in certain microorganisms.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 80–84, February, 1992.     

Effect of temperature on structural transformations of nickel nanoclusters

The gas-phase condensation of nickel nanoclusters is simulated by the molecular dynamics method with the use of tight-binding potentials. It is revealed that subsequent heating of the synthesized clusters to temperatures of 400–500 K leads to a substantial improvement of their internal structure with a hexagonal close-packed phase predominating. Upon heating of the nanoparticles above the melting point and subsequent gradual cooling, the formation of a cluster structure depends strongly on the cooling rate. The inference is made that heating of the nanoclusters synthesized from a gas phase can be used for the controlled formation of nickel nanoparticles with a predicted structure.     

A study of melting of various types of Pt–Pd nanoparticles

The melting processes of various Pt–Pd nanoparticles (binary alloy, core–shell, D ≤ 4.0 nm) with different percent platinum atom content are investigated via the molecular dynamics using the embedded atom method potential in order to establish the thermal stability of simulated particle structure. In accordance with the data obtained, the most thermally stable are Pt–Pd nanoalloys with a diameter above 2.0 nm and core–shell Pd@Pt particles. As is shown, heating of binary Pt–Pd cluster alloys with the particle diameters less than 2.0 nm may cause the transition to pentagonal symmetry structures and core–shell-like complex formations.

     

Simulation of the processes of structuring of copper nanoclusters in terms of the tight-binding potential

The processes of melting and crystallization of copper nanoclusters with a radius ranging from 0.69 to 3.05 nm have been investigated using the molecular dynamics simulation. The performed simulation has shown that the melting begins with the surface of the cluster. Another feature of this phase transition is that it occurs in a temperature range where the liquid and solid phases can coexist. However, it is found that, for small copper clusters, the melting and crystallization temperatures coincide with each other. Moreover, it is established that the parent face-centered cubic structure of these small clusters (N < 150 atoms) transforms into a structure with fivefold symmetry even at temperatures of the order of 150–170 K. The behavior of some thermodynamic characteristics of copper nanoclusters is investigated in the vicinity of the solid-liquid phase transition. Analysis of the data obtained has revealed a number of regularities that are in agreement with the results of analytical calculations. In particular, the melting and crystallization temperatures of copper nanoparticles are linear functions of N ^?1/3. However, the melting heat ΔH _m and the melting entropy ΔS _m vary in a more complex manner. It is noted that the formation of a cluster structure depends on the conditions used for cooling from the liquid phase. Slow cooling results predominantly in the formation of a face-centered cubic phase, whereas rapid cooling in the majority of cases leads to the formation of an icosahedral modification. Therefore, the simulation performed has demonstrated the possibility of controlling the formation of a structure of copper nanoclusters during crystallization.     

Identification and semiquantitative estimation of antibiotics added to complete feeds, premixes, and concentrates

Classical microbiological methods for determining antimicrobial compounds in feeds are nonspecific. Thus, there is a need to identify biological activity, and bioautography is used for this purpose. A routine method for detecting the following antimicrobial substances in feeds is described: avilamycin, avoparcin, Zn-bacitracin, erythromycin, flavomycin, furazolidone, lasalocid, monensin, narasin, penicillin, salinomycin, spiramycin, tetracyclines, tylosin, and virginiamycin. Carbadox can be detected by UV light examination of the plates prior to bioautography. Semiquantitative estimations of antibiotic content are compared with quantitative determinations of the above mentioned substances in feeds, except erythromycin, penicillin, and tetracyclines. Detection limits range from 0.1 mg/kg (chlortetracycline) to 20 mg/kg (lasalocid). The method involves agar diffusion of buffered samples, a neutral extraction of polyether antibiotics followed by thin-layer chromatography (TLC), and an acid extraction for other antibiotics followed by TLC. Five test bacteria were used for the main detection by agar diffusion: Micrococcus luteus ATCC 9341, Staphylococcus aureus ATCC 6538P, Corynebacterium xerosis NCTC 9755, Bacillus cereus ATCC 11778, and B. subtilis ATCC 6633. Identification after TLC was achieved by bioautography with the most sensitive microorganism(s). This method allows one laboratory technician to analyze up to 30 feed samples within 2.5 working days, provided that feeds of the same category are analyzed in the same run, and that labels of additives are available. Qualitative and semiquantitative information are valuable when performing a quantitative antibiotic determination and it provides proof that the activity determined is due to the tested substance. This last feature is essential from the perspective of quality assurance of results.     

Leontopodic acid—a novel highly substituted glucaric acid derivative from Edelweiss (Leontopodium alpinum Cass.) and its antioxidative and DNA protecting properties

Leontopodic acid—a novel full substituted hexaric acid derivative, was isolated from the aerial parts of Edelweiss (Leontopodium alpinum Cass.) as one of the major compounds. The complex structure of leontopodic acid-2-[(3S)-3-hydroxybutanoate]-3,4,5-tris-[(2E)-3-(3,4-dihydroxyphenyl)-2-propenoate]-d-glucaric acid—was elucidated by mass spectrometry, 1D- and 2D NMR spectroscopy and HPLC monitored transesterification. Leontopodic acid exhibited pronounced antioxidative effects in the Briggs-Rauscher (BR) model [(r.a.c.)_m 3.4±0.5] and the trolox equivalent antioxidant capacity (TEAC) method (TEAC value of 1.53±0.11). The antioxidative properties of this compound were confirmed by the 3D method, an in vitro assay evaluating DNA protection against oxidative damage (IC₅₀: 1.89 μM).     

Molluscicidal Saponins from Talinum tenuissimum DINTER

Two new saponins, β-D -glucopyranosyl 3-O[O-βD -xylopyranosyl-(1→3)-O-(β-D -glucopyranosyluronic acid)]oleanolate ( 1 ) and 3-O-[O-β-D -xylopyranosyl-(1→3)-O-(β-D-glucopyranosyluronic acid)]oleanolic acid ( 2 ), have been isolated from the tubers of Talinum tenuissimum. The structures have been established mainly by ¹³C-NMR and FAB-MS. The monodesmosidic saponin 2 exhibits very strong molluscicidal activity against the schistosomiasis-transmitting snail Biomphalaria glabrata.