Influence of Cu<Superscript>2+</Superscript> ions on structural and magnetic properties of NiZn ferrites

The paper is devoted to the preparation of NiZn ferrite with small substitutions of copper by means of ceramic technology. The influence of small Cu substitution on the microstructural and magnetic properties of NiZn ferrites have been analysed by means of various experimental methods and interpreted from the point of view of preparation technology optimisation and possible applications of such materials. A strong correlation between the substituent content and resulting properties has been observed, thus allowing preparation of material with the properties tailored for any particular application.     

Some electrical transport studies on compounds of the Tl-Te system

Galvanomagnetic, thermoelectric and electrical conductivity measurements were made on single crystal specimens of Tl₂Te₃, TlTe and -phase in the temperature range from 77° K up to 500 to 700° K. All crystals were found to be of p-type conductivity. Measurements of the electrical conductivity versus composition have revealed that -phase represents solid solutions of Tl and Te in the compound with formula Tl₅Te₃. Also, it has been found that both TlTe and -phase samples exhibit a metallic behaviour in contrast to Tl₂Te₃, which has semiconducting properties. In the intrinsic conduction region a thermal energy gap of a 0.68 ± 0.03 eV was found, which was compared with that obtained from optical transmission measurements. The electrical properties of Tl-Te compounds are discussed on the basis of the general valence rule.     

Intratumoral differences in methotrexate levels within human osteosarcoma xenografts studied by microdialysis

A central tenet in oncology is the assumed relationship between drug concentration and cytotoxicity. Determinations of drug levels in tumor tissues are, however, generally not undertaken. Microdialysis is a method where continuous drug monitoring may be achieved by sampling of low molecular weight substances from the extracellular space. By employing this technique it is possible to observe variable drug levels within tissues, including tumors, over time. Herein, we present results from a nude rat model where subcutaneous human osteosarcoma xenografts were established prior to the administration of the antifolate methotrexate as an intravenous infusion. Significant differences in drug exposure within single tumors were evident. Generally, peak drug concentrations were lower and drug efflux slower from the center of the tumors as compared to the periphery. The use of microdialysis could be an important tool for optimizing current strategies in anticancer chemotherapy.     

The effect of couple stresses on the stress field around a penny-shaped crack

The present paper is concerned with the influence of couple stresses on the stress concentration around a penny-shaped crack in an infinite body within the framework of both the micropolar and couple stress (indeterminate) theory. Taking into account the symmetry of the boundary value problem considered this one is converted into solving dual integral equations, and finally into a Fredholm integral equation of the second kind. Approximating the kernel the latter is solved analytically.

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Résumé Dans le cadre de la théorie des contraintes micropolaires et des couples, on étudie l'influence d'un couple et d'une concentration de contraintes sur une fissure en ongle dans un corps infini. En prenant en compte la symétrie qui caractérise les contours du problème considéré, celui-ci est ramené `a la solution d'une double équation intégrale, et ensuite à une équation intégrale de Fredholm du second genre. Celle-ci est résolue par voie analytique en faisant une approximation sur le noyau.

     

Chitosan–silver nanocomposites: New functional biomaterial for health-care applications

Chitosan–silver nanocomposites (CS-HDA-AgNCs) was prepared using chitosan, biogenic silver nanocomposites, and crosslinker, hexamethylene 1,6-di(amino carboxysulfonate) (HDA). The film is flexible and transparent. Its physical, mechanical, thermal, hydrophilicity, and swelling properties were improved by HDA (2.5%). The antimicrobial activity of CS-HDA-AgNCs were not displayed any remarkable zone of inhibition but showed toxic effect in the presence of normal 3T3 fibroblasts and cancer HeLa cells. It decreases to ca. 5–7% for both cell lines. In conclusion, it can be mentioned that the CS-HDA-AgNCs, a kind of new functional biomaterial, could be useful for health-care applications.     

Some considerations regarding finishing by abrasive flap wheels

The diminishing of the surface roughness of a metallic part can be performed by using various finishing methods, polishing by means of abrasive flap wheels being one of them. There are several factors that influence the roughness of surfaces finished by means of abrasive flap wheels. When polishing external cylindrical surfaces by means of abrasive flap wheels, the surface roughness can be changed more easily if the following elements are modified: the peripheral speed of the workpiece and of the abrasive flap wheel, the speed of the longitudinal feed, and the grit size of the abrasive material. Experimental research has allowed for the establishment of power-type functions which highlight the influence exerted by the above-mentioned factors on the size of the surface roughness parameters R _a , RS, RSm, Rku. Analyses of the experimental results have shown that, in the case of the R _a roughness parameter, from among the range of interactions of orders 2 and 3, only the interaction between the rotation speed of the abrasive flap wheel and the speed of the longitudinal feed seems to be significant.     

Mechanical properties and fracture behavior of high-strength steels

Typical thicknesses of high-strength steels (HSS) sheets used in the car industry are inapplicable for standardized testing procedures. The aim of this study is to propose an appropriate methodology for testing and comparing of thin HSS sheets. Microstructures were observed by means of light and scanning electron microscopy. The modified Charpy impact tests and fracture toughness tests were used in order to compare the fracture properties of three different HSS sheets (Docol 1200 M, Multiphase 1200 and BTR 165). Ductile-to-brittle transition curves and tearing resistance (J − Δa) curves were measured. From the fracture toughness linked to the specimen thicknesses the value of fracture toughness K_Ic was estimated. Fractographic analysis of broken specimens has revealed that due to the fine microstructure of mixed ferrite-martensite fracture mechanism remains ductile even at low temperatures (down to −100°C). __________ Translated from Problemy Prochnosti, No. 1, pp. 155–158, January–February, 2008.     

Computation of stresses in axisymmetric elastostatical problems using BEM

The application of the conjugate residual method to the indefinite system resulting from the finite element approximation of the incompressible Navier-Stokes equations is studied. The possibility to use an element-by-element preconditioner for the acceleration of the convergence is also discussed. With this scheme, one can achieve significant savings in storage without ever forming the large total matrix. Furthermore, the hybrid finite element mesh, a mixture of the structured and unstructured mesh subdivisions, is employed to minimize the storage requirement. The numerical simulations of the Karman vortex street, a driven cavity flow and the flow in a safety valve are presented to demonstrate the applicability of the proposed strategies.     

Performance of a Retrofitted Medieval Stone Bridge under a Severe Flood

The very long absence of floods with a return period of more than about 100 years in the Vltava river basin dulled the vigilance of the state administration and self-government bodies. Flood safety measures were mostly related to once-in-a-hundred-years floods. When planning the engineering works before major repairs to the medieval Stone Bridge in Písek in the 1990s, an unpleasant but justified question was raised—what would happen to the Stone Bridge if an extremely powerful 500-year return period flood were to occur? The following case study deals with this question and provides a definitive answer to it. Works were carried out on the Stone Bridge, which was just a few years later exposed to the catastrophic flood that occurred in Bohemia in 2002.