The effect of a partially inserted absorbing control rod on the neutron flux density distribution

This paper presents a method of calculating the effectiveness of a partially inserted absorbing, cylindrical control rod in a reactor without a reflector. The method makes it possible to give a comparatively simple estimate of the distortion of the neutron flux density near the control rod. The calculations are made using the one group equation + B² = 0, with special boundary conditions on the surface of the rod. As an example of the use of the method, the values of B² and the corresponding flux distributions have been calculated for several positions of the rod in the reactor.We wish to thank R. Zezula, I. Svatosh, and M. Prazhskaya for carrying out the numerical calculations. particularly great initiative was shown by R. Zezula, who also calculated the tables of the function Zk'(a. B²)/Zk(a, B²) on the electronic BÉSM setup in the Calculational Center of the Academy of Sciences of the USSR.     

Comparison of selected physical and mechanical properties of densified beech wood plasticized by ammonia and saturated steam

Gaseous ammonia treatment in combination with densification of wood has been known for several decades, but these days there is no industrial production of materials modified in this way; also, little has been published in this area of wood science. In this study, selected physical and mechanical properties, i.e. density profile, bending strength, hardness and moisture absorption were investigated for Lignamon (1), which was obtained from the Czech industrial production. Selected properties were also investigated using steam-densified beech (2) and native beech (3) and compared with each other. Densitometry of Lignamon showed a large variability in the density profile compared to that of only densified beech. It is affected by the degree of densification, temperature and moisture gradients, and their relationship to the glass transition of the wood cell wall. Modulus of elasticity, hardness, moisture exclusion and anti-swelling efficiency of Lignamon are enhanced compared to densified beech. The enhanced dimensional stability and lower hygroscopicity of Lignamon are probably caused by heat treatment during the process. Further investigation will be carried out with self-produced Lignamon samples.     

Triaxial Compression of Sand Reinforced with Fibers

Results from drained triaxial compression tests on specimens of fiber-reinforced sand are reported. It is evident that the addition of a small amount of synthetic fibers increases the failure stress of the composite. This effect, however, is associated with a drop in initial stiffness and an increase in strain to failure. Steel fibers did not reduce initial stiffness of the composite. The increase in failure stress can be as much as 70% at a fiber concentration of 2% (by volume) and an aspect ratio of 85. The reinforcement benefit increases with an increase in fiber concentration and aspect ratio, but it also depends on the relative size of the grains and fiber length. A larger reinforcement effect in terms of the peak shear stress was found in fine sand, compared to coarse sand, when the fiber concentration was small (0.5%). This trend was reversed for a larger fiber concentration (1.5%). A model for prediction of the failure stress in triaxial compression was developed. The failure envelope has two segments: a linear part associated with fiber slip, and a nonlinear one related to yielding of the fiber material. The analysis indicates that yielding of fibers occurs well beyond the stress range encountered in practice. The concept of a macroscopic internal friction angle was introduced to describe the failure criterion of a fiber-reinforced sand. This concept is a straightforward way to include fiber reinforcement in stability analyses of earth structures.     

Guided assembly of nanoparticles on electrostatically charged nanocrystalline diamond thin films

We apply atomic force microscope for local electrostatic charging of oxygen-terminated nanocrystalline diamond (NCD) thin films deposited on silicon, to induce electrostatically driven self-assembly of colloidal alumina nanoparticles into micro-patterns. Considering possible capacitive, sp² phase and spatial uniformity factors to charging, we employ films with sub-100 nm thickness and about 60% relative sp² phase content, probe the spatial material uniformity by Raman and electron microscopy, and repeat experiments at various positions. We demonstrate that electrostatic potential contrast on the NCD films varies between 0.1 and 1.2 V and that the contrast of more than ±1 V (as detected by Kelvin force microscopy) is able to induce self-assembly of the nanoparticles via coulombic and polarization forces. This opens prospects for applications of diamond and its unique set of properties in self-assembly of nano-devices and nano-systems.     

Quantitative phase analysis of metastable structure in a laser melted Fe-C alloy

Pure Fe-3.5wt% C alloy was surface melted using a cw C0₂ laser and the microstructure of the laser tracks was investigated by scanning and transmission electron microscopy. Structure of the melted zones consisted of dendrites of partially transformed primary austenite and of very fine lamellar ledeburite. The secondary dendrite arm spacings were indicative of a cooling rate of 10⁵ Ks^–1, in good accord with calculations based on the model of a moving Gaussian beam. Using methods of quantitative metallography the volume fraction of dendrites within the melted zone and the volume fractions of the carbide and ferrite phases in the decomposed ledeburite were estimated. These data were combined with the results of a quantitative X-ray diffraction phase analysis (see Part II) and compared with the equilibrium phase diagram. It was found that the volume fraction of dendrites was near the equilibrium value while the volume content of cementite in the rapidly solidified structures was considerably higher than predicted from the equilibrium phase diagram.     

Syntheses and magnetic properties of trinuclear trithiocyanurato-bridged manganese(II) complexes involving bidentate aromatic N-donor heterocycles

Trinuclear manganese(II) complexes of the compositions [Mn₃(phen)₆(ttc)](ClO₄)₃ (1), [Mn₃(dmbpy)₆(ttc)](ClO₄)₃? 2H₂O (2) and [Mn₃(bpy)₆(ttc)](ClO₄)₃? 3H₂O (3), where phen = 1,10-phenanthroline, dmbpy = 4,4′-dimethyl-2,2′-bipyridine, bpy = 2,2′-bipyridine and H₃ttc = trithiocyanuric acid (2,4,6-trimercapto-1,3,5-triazine), were prepared and characterized by elemental analysis, FTIR and Raman spectroscopies, MALDI-TOF mass spectrometry, magnetic and conductivity measurements. The magnetic analysis incorporating simultaneous fitting of the temperature dependence and the field dependence of the magnetization using the isosceles triangle spin Hamiltonian model revealed a weak antiferromagnetic exchange within the trinuclear units.     

Illumination-induced charge transfer in polypyrrole–diamond nanosystem

Charge transfer in polypyrrole–diamond system is studied by conductivity measurements of a channel made of H-terminated diamond in the dark and under illumination. Micrometer-sized channels of conductive hydrogen-terminated diamond are lithographically fabricated on the surface of monocrystalline intrinsic diamond crystal by selective oxygen plasma treatment. The channels are interrupted by electrochemically synthesized micrometer-sized polypyrrole clusters. Similar characterizations are performed on channels interrupted by surface oxidation. Based on the increase and spectral response of the photo-current in the system, doping of diamond by charge carriers from polypyrrole is deduced.     

Synthesis,structure, and opto-electronic properties of organic-based nanoscale heterojunctions

Enormous research effort has been put into optimizing organic-based opto-electronic systems for efficient generation of free charge carriers. This optimization is mainly due to typically high dissociation energy (0.1-1 eV) and short diffusion length (10 nm) of excitons in organic materials. Inherently, interplay of microscopic structural, chemical, and opto-electronic properties plays crucial role. We show that employing and combining advanced scanning probe techniques can provide us significant insight into the correlation of these properties. By adjusting parameters of contact- and tapping-mode atomic force microscopy (AFM), we perform morphologic and mechanical characterizations (nanoshaving) of organic layers, measure their electrical conductivity by current-sensing AFM, and deduce work functions and surface photovoltage (SPV) effects by Kelvin force microscopy using high spatial resolution. These data are further correlated with local material composition detected using micro-Raman spectroscopy and with other electronic transport data. We demonstrate benefits of this multi-dimensional characterizations on (i) bulk heterojunction of fully organic composite films, indicating differences in blend quality and component segregation leading to local shunts of photovoltaic cell, and (ii) thin-film heterojunction of polypyrrole (PPy) electropolymerized on hydrogen-terminated diamond, indicating covalent bonding and transfer of charge carriers from PPy to diamond.