Stages of surface erosion under ion irradiation

Initial stages of surface erosion have been studied for NaCl and LiF single crystals bombarded by Ar⁺ ions with 20 keV. For irradiation with doses D=10¹⁰-10¹¹ ions/cm², exoelectron emission has been used, whereas for higher doses, we have used electron microscopy. Two stages of initial surface erosion have been identified; for small doses, there is slow development of atomic-scale roughening, which reaches its peak when areas damaged by closest incident ions start to overlap, and then, beginning with D=10¹⁶ ions/cm², there is rapid etching, deep into the crystal, followed by the emergence of secondary microscopic structure, i.e. caverns, concentric closed steps and terraces. Ion-induced surface structure of alkali halide crystals has been shown to depend strongly on the presence of foreign particles on the surface, as well as on segregation. Topography of thin carbon films and some metals have been discussed.     

Synthesis of magnesium borate nanorods

Single-crystalline magnesium borate Mg₂B₂O₅ nanorods have been synthesized via a simple route based on the calcinations of mixed powders containing Mg(OH)₂ and H₃BO₃. The nanorods have the typical diameters in the range of 70-120 nm and the lengths up to a few micrometers. An optimal synthesis temperature for Mg₂B₂O₅ nanorods was obtained, and the possible growth mechanism was also presented.     

Characterization of two-phase nickel aluminides produced by pressure-assisted combustion synthesis

Two-phase (B2+L1₂) nickel aluminide intermetallic compounds were synthesized by the pressure-assisted volume combustion synthesis (CS). The production and characterization of the samples containing NiAl+Ni₃Al were investigated. Aluminum (99% pure, 15 μm) and carbonyl nickel (99.8 pure, 4-7 μm) powders were used. The production of intermetallic compound was carried out at 1050 °C under 150 MPa uniaxial pressure in open air atmosphere in an electrical resistance furnace for 60 min. The formation temperature of intermetallic compound was determined by differential scanning calorimeter (DSC) analysis, and exothermic temperature of powder mixture was determined as 653 °C. The characterization of samples was confirmed by optical microscope, SEM and XRD analysis. It was observed that the structure of compound has very low porosity and the formation of NiAl was completed successfully. The relative density of test materials measured according to Archimedes’ principle was 98.04%. The microhardness of test materials was about 351 HVN₁.     

Why are Websites co-linked? The case of Canadian universities

This study examined why Websites were co-linked using Canadian university Websites as the test set. Pages that co-linked to these university Websites were located using Yahool. A random sample of 859 co-linking pages (the page that initiated the co-link) was retrieved and the contents of the page, as well as the context of the link, were manually examined to record the following variables: language, country, type of Website, and the reasons for co-linking. The study found that in over 94% of cases, the two co-linked universities were related academically; many of these cases (38%) showed a relationship specifically in teaching or research. This confirms results, from previous quantitative studies, that Web co-links can be a measure of the similarity or relatedness of sites being co-linked and that Web co-link analysis can thus be used to study relationships among linked Websites.     

Grating Couplers Fabricated by Electron-Beam Lithography for Coupling Free-Space Light Into Nanophotonic Devices

Grating couplers with nanoscale periodicity have been fabricated on silicon-on-insulator (SOI) substrates by electron beam lithography and reactive ion etching. A versatile experimental apparatus has been implemented to measure the efficiency of these gratings in coupling free-space radiation into planar waveguides. This coupling efficiency has been measured as a function of grating depth and the angle and wavelength of incident radiation. Coupling efficiencies of at least 5% and as high as 20% are demonstrated for wavelengths in the vicinity of 1550 nm and incident angles around 45deg.     

Functions for relative maximization

We introduce functions for relative maximization in a general context: the beta and alpha applications. After a systematic study of different kinds of regularities, we investigate how to approximate certain values of these functions using periodic orbits. We also show that the differential of an alpha application determines the asymptotic behavior of the optimal trajectories.     

Interaction of optical radiation with GaAs nanowhisker arrays

We have studied the reflectivity spectra of doped GaAs nanowhiskers (NWs) with various morphologies, which were grown by molecular-beam epitaxy. It is established that the character of the reflectivity spectra of NWs is determined by the shape of nanocrystals. NWs with droplike and pointlike vertices differently interact with electromagnetic radiation. Laser radiation produces a spectraly selective action on the NW array, which leads to a change in the NW height and induces “caking” of their vertices, thus modifying the reflectivity of the sample. This phenomenon can be used for the creation of local microstructures with preset characteristics in large NW arrays.     

High-temperature long-term strength of shells in aggressive media with regard for the mass exchange

We study the long-term strength of long cylindrical shells placed in aggressive media under the conditions of complex stressed states. The influence of these states on the long-term strength of the shells is described with the help of equivalent scalar stresses. The time to fracture of the shell is found according to the Rabotnov kinetic theory by using two parameters, namely, the degree of damage to the material and the concentration of the medium in the material of the shell. As the boundary condition on the fracture front, we use the condition of mass exchange. We study the interaction between the diffusion and fracture fronts. The time to fracture of the shell is found by using the solution of a system of integrodifferential equations. Under the assumption of mean level of concentration of the medium, we deduce explicit relationships between the times to fracture of the shell in aggressive and neutral media. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 2, pp. 17–24, March–April, 2007.