Small Icosahedral Particles Research Articles

Особенности поведения электролитических медных порошков различных морфологических форм в температурных полях

An important goal of materials science is the development of copper powders, which are used as catalysts in reactors based on fluidization bed technology. These reactors are more economical and efficient than the reactors with fixed catalytic layer. However, such reactors put special requirements to the catalysts. The catalyst should be stable in the temperature fields, have high thermal conductivity and wear resistance, certain shapes and sizes of the active particles. Сopper powders with approximately the same average particle size, but with different internal structures and surface morphologies were obtained by electrolytic method in this work. A comparative analysis of the results of exposure to temperature fields was also performed. The heating of powders was carried out in a differential scanning calorimeter (X-DSC 7000). Investigations of surface morphology and phase composition changes of the copper powders particles were performed using scanning electron microscopy and x-ray diffraction. It has been shown that particles with different initial structures and morphologies have similar morphological and phase transformations in the process of annealing in air (sintering and loss of particles faceting, the growth of whiskers and oxidation, the formation of cavities inside and pores on the surface). Nevertheless, for the icosahedral small particles of copper, there is an increased release of energy during the heating in DSC, and this energy activates and accelerates structural-phase transformations in the particles. By authors’ opinion, this can be related to particular features of the internal structure and morphology of their surface.

Phase transformations in icosahedral small copper particles during their annealing in different gas media

Results from experiments on the annealing of icosahedral small particles (ISPs) of copper in different gas media are presented. The growth of nanowhisker structures and nanopores on the surfaces of ISPs is observed. The multilayer internal structure of annealed ISPs that incorporate a metallic core, cavities, and copper oxide layers is revealed.

Overview on Sobemoviruses and a Proposal for the Creation of the Family Sobemoviridae.

The genus Sobemovirus, unassigned to any family, consists of viruses with single-stranded plus-oriented single-component RNA genomes and small icosahedral particles. Currently, 14 species within the genus have been recognized by the International Committee on Taxonomy of Viruses (ICTV) but several new species are to be recognized in the near future. Sobemovirus genomes are compact with a conserved structure of open reading frames and with short untranslated regions. Several sobemoviruses are important pathogens. Moreover, over the last decade sobemoviruses have become important model systems to study plant virus evolution. In the current review we give an overview of the structure and expression of sobemovirus genomes, processing and functions of individual proteins, particle structure, pathology and phylogenesis of sobemoviruses as well as of satellite RNAs present together with these viruses. Based on a phylogenetic analysis we propose that a new family Sobemoviridae should be recognized including the genera Sobemovirus and Polemovirus. Finally, we outline the future perspectives and needs for the research focusing on sobemoviruses.

Stress relaxation in icosahedral small particles via generation of circular prismatic dislocation loops

A theoretical model is developed that describes stress relaxation in icosahedral small particles through generation of circular prismatic dislocation loops. It is shown that loop generation is energetically favorable in the equatorial section of the particle, the radius of which is larger than the critical one. The dislocation loop then extends until it reaches its optimal radius which increases with particle size. Both the critical particle radius and optimal loop radius strongly depend on dislocation core energy.

On the habitus modification of silver microcrystals by electrolityc origin

We present experimental facts on the role of electrolyte pH in shape control of electrodeposited small silver microcrystals. Increasing electrolyte pH value, i.e. the transition from acidic to alkali solutions, causes the change of microcrystal habitus from strongly distorted (pH ∼ 4) through faceted (pH ∼ 7) to smooth hemispheric (pH ∼ 10) ones. At neutral pH ∼ 7 the observed silver microcrystals demonstrated shapes with pentagonal symmetry including icosahedral microcrystals with incoming pentagonal facets at the icosahedron vertices that is unique for single‐component small metallic particles. Such faceting has previously been reported only for bicompounent B6O icosahedral small particles [H. Hubert et al., Nature 391, 376 (1998)].

Structural Phase Transformations and Morphological Changes of Icosahedral Small Copper Particles in Temperature Fields and Reactive Media

The influence of temperature fields and different reactive media on the structure and morphology of small icosahedral particles (ISPs) are investigated. It is shown experimentally that ISPs of micron size can be transformed into advanced functional materials with a developed surface. The leading role of disclination defects for the transformations occurring in ISPs is established. Potential fields of application of the new functional materials based on transformed ISPs are discussed.

The formation of voids in icosahedral small particles during electrocrystallization

A model describing the formation of voids in icosahedral small particles of electrolytic origin is proposed. The model is based on the assumption that a void grows as a result of the diffusion of vacancies, which are formed during electrocrystallization at the periphery of a particle and then drift toward its center under the action of the elastic stress field created around a disclination-type defect.

Nanoviruses, small plant viruses: similarities and differences with geminiviruses

Nanoviridae family comprises genus Nanovirus (viruses infecting legume plants) and Babuvirus (viruses infecting banana plants). Nanovirus genomes consist of multiple circular single-stranded DNAs of about 1 kb each. They are encapsidated individually in small icosahedral particles. Each DNA molecule encodes only one protein. Nanoviruses replicate in the nucleus of infected plants by a rolling-circle replication mechanism initiated by viral Rep protein. Multiple DNAs encoding similar but clearly distinct Rep proteins have been described for each nanovirus. All Rep proteins of a given nanovirus are functional and each initiates replication of the DNA by which it is encoded (auto-replication). Only the Master Rep protein is able to initiate replication of other genome components (trans-replication). Induction of nanovirus disease by cloned genome components was established only for Faba bean necrotic yellows virus.

Voids in icosahedral small particles of an electrolytic metal

Results of an experiment aimed at the detection of voids in small icosahedral particles of electrolytic copper are presented. The chosen experimental technique is theoretically justified. The presence of voids revealed in the experiment gives strong evidence that the formation of small particles and microcrystals with pentagonal symmetry is governed by the disclination mechanism.

Survival of indicators of bacterial and viral contamination in wastewater subjected to low temperatures and freezing: application to cold climate waste stabilisation ponds

The survival of bacterial and viral pollution indicators and Salmonella in urban wastewaters under freezing conditions (-14 degrees C for up to 60 days) is reported. Presumptive, total and faecal coliforms (PC, TC, FC), salmonellae and coliphages were tested. The dynamics of somatic coliphage (E. coli C) and F-pili specific coliphage inactivation were compared at 4 degrees C and 25 degrees C over various run times. On freezing of the wastewater, it was found that PC, TC and FC showed a first rapid phase (days) of inactivation followed by a slower second phase (up to 4 weeks) and then stabilisation at between 1-10% of the initial population size, depending on the wastewater sample used. Salmonella spp. were detectable in 0.1 ml of raw wastewater and were still detected up to 2 days after freezing but none were detected in 100 ml samples after 4, 42 and 60 days, although microbiologically similar but antigenically different forms were found. Viral indicators of pollution showed a slow but constant decrease in viability during the first month but then stabilised at between 10-20% survivors (10% in somatic E. coli C phages, 15.8% in somatic Salmonella phages and 17.9% in F-pili specific coliphages). Using electron microscopy, no difference in susceptibility to freezing could be detected with respect to morphological phage types, which were either small icosahedral particles or complex tailed phages. The study of viral indicators at 4 degrees C versus 25 degrees C showed a higher survival of the various coliphages over time at 4 degrees C. F-pili specific leviviridae were particularly susceptible to the antiviral factors at 25 degrees C and no viable units per ml were detected after one month at that temperature, whereas somatic coliphages were detected in higher numbers after this period, especially at 4 degrees C.

Crystallization and preliminary X-ray analysis of human rhinovirus serotype 2 (HRV2).

Human rhinoviruses, the major cause of mild recurrent infections of the upper respiratory tract, are small icosahedral particles. Over 100 different serotypes have been identified. The majority (91 serotypes) use intercellular adhesion molecule 1 as the cell-attachment site; ten serotypes (the minor group) bind to members of the low-density lipoprotein receptor. Three different crystal forms of the minor-group human rhinovirus serotype 2 (HRV2) were obtained by the hanging-drop vapour-diffusion technique using ammonium sulfate and sodium/potassium phosphate as precipitants. Monoclinic crystals, space group P2(1), diffracted at least to 2.8 A resolution, and two complete virus particles were located in the crystal asymmetric unit. A second type of crystals had a compact cubic like morphology and diffracted beyond 2.5 A resolution. These crystals belong to a primitive orthorhombic space group, with unit-cell parameters a = 309.3, b = 353.5, c = 759.6 A, and contain one virus particle in the asymmetric unit. A third type of crystals, with a prismatic shape and belonging to space group I222, was also obtained under similar crystallization conditions. These latter crystals, with unit-cell parameters a = 308.7, b = 352.2, c = 380.5 A, diffracted to high resolution (beyond 1.8 A) and contained 15 protomers per asymmetric unit; this requires that three perpendicular crystal twofold axes coincide with three of the viral particle's dyad axes.

Voids and channels in pentagonal crystals

Formation and growth conditions of voids are considered for pentagonal whiskers and for icosahedral small particles. Formation of such voids is shown to be due to relaxation of elastic fields of disclinations which are inherent in pentagonal crystals. Experimental observations of voids in pentagonal whiskers of CdTe are reported.

Use of two-dimensional single electron detector for electron diffraction studies in tem

The application of a position-sensitive detector, capable of registering single electrons, to a TEM is described. The type of detector used was the Mepsicron, developed by Firmani and co-workers [C. Firmani, E. Ruiz, C.W. Carlson, M. Lampton and F. Paresce, Rev. Sci. Instr. 53 (1982) 570]. It is shown that very high resolution diffraction patterns can be obtained with this detector. Patterns from small icosahedral particles and layered materials are shown as examples of applications. The fine structure of these patterns including the central spot is clearly resolved. Some possible applications of this detection system to TEM studies of materials are pointed out.

Relaxation of Mackay icosahedra

Multilayer icosahedra, first introduced about twenty years ago by Mackay [Acta Cryst. (1962), 15, 916-918], are no longer considered a geometrical curiosity, as small icosahedral particles have been observed in a great number of experiments. The hard-sphere models, previously considered, are not really suited to the study of physical properties because they fail to express the important stresses due to the icosahedral structure. Therefore, Mackay icosahedra, made of atoms interacting through a Lennard-Jones potential, were constructed and allowed to relax freely. Results of the calculation are given, consisting of a detailed description of relaxed icosahedra with up to nine layers, i.e. with up to almost 3000 atoms.

A study of the self-assembly process in a small spherical virus. Formation of organized structures from protein subunits in vitro.

The arrangement of the morphological units of cowpea chlorotic mottle virus (CCMV), which has a dry diameter of 250 A, is consistent with that expected for a 32-unit structure, the units being situated on the vertices of a rhombic triacontahedron. The hydrodynamic behaviour of CCMV is pH dependent. The particles “swell” near neutrality and in doing so become susceptible to attack by nucleases. After treatment with pancreatic ribonuclease, ellipsoidal particles of 210 A diameter and 280 A length, small icosahedral particles of 160 A diameter, double-shelled particles of 340 A, and long tubes with a diameter of 160 A and rounded ends are produced. All these particles are also found after treatment with takadiesterase T1 and in addition, small spherical particles with a double shell with an outer diameter of 250 A, and tubes with a diameter of 250 A which may enclose 160 A tubes, are produced. The production of these new forms are described and discussed and the products are related to model structures. In addition, a possible reconstitution system for CCMV and brome mosaic virus is outlined.