New records of spiders from pond littorals in the Czech Republic

Tmeticus affinis (BLACKWALL, 1855), Tetragnatha shoshone LEVI, 1981, Clubiona juvenis SIMON, 1878, Marpissa Canestrinii NINNI, 1868, and Theridiosoma gemmosum (L. KOCH, 1877) are new records for the Czech Republic. New data about Enoplognatha caricis (FICKERT, 1876), Theridion hemerobium SIMON, 1914, Rugathodes instabilis (O. P. CAMBRIDGE, 1871), Tetragnatha striata L. KOCH, 1862, and Dolomedes plantarius (CLERCK, 1757) are given. The validity of the name Enoplognatha caricis (FICKERT, 1876) is supported

Correlation Attack on A5/1

Popis šifry A5/1, základních kryptografických útoků a rozbor útoku korelační metodou. Tato práce bude prospěšná každému, kdo se bude zajímat o proudové šifry, základní myšlenky kryptografických útoků a speciálně myšlenku útoku korelační metodou aplikovanou na šifru A5/1.Description of A5/1 cipher, basics of cryptographic attacks and analysis of correlation attack. This thesis is helpful for everyone who is intrested in stream ciphers, basic ideas of cryptographic attacks and correlation attack on A5/1 in particular

Modelling in Engineering

Cílem této bakalářské práce bylo najít optimální návrh oběžného kola čerpadla pro zadaný návrhový bod tak, abychom minimalizovali třecí a diskové ztráty. Návrh jsme provedli pomocí matematického modelu v programu Matlab. Bakalářská práce obsahuje teoretickou a praktickou část. V prvních čtyřech kapitolách byly teoreticky popsány technické poznatky, které se dále využívaly a jejichž znalost byla nezbytná. V páté kapitole bakalářské práce byl popsán matematický model, pomocí nějž byly provedeny už konkrétní návrhy a výpočty oběžných kol čerpadla.The aim of this bachelor thesis is to determine the optimal parameters of pump impeller in order to minimize friction loss and disc loss. The proposal has been implemented by a mathematical model in Matlab. The thesis consists of theoretical and practical part. The first four chapters summarize the theroretical background that is necessary for creating the mathematical model for the losses of an impeller. In the fifth chapter the implementation of the mathematical model in Matlab is presented and results for particular types of pumps are discussed.

Preferential site occupancy of alloying elements in TiAl-based phases

First principles calculations are used to study the preferential occupation of ternary alloying additions into the binary Ti-Al phases, namely $\gamma$-TiAl, $\alpha_2$-Ti$_3$Al, $\beta_{\mathrm{o}}$-TiAl, and B19-TiAl. While the early transition metals (TMs, group IVB, VB , and VIB elements) prefer to substitute for Ti atoms in the $\gamma$-, $\alpha_2$-, and B19-phases, they preferentially occupy Al sites in the $\beta_{\mathrm{o}}$-TiAl. Si is in this context an anomaly, as it prefers to sit on the Al sublattice for all four phases. B and C are shown to prefer octahedral Ti-rich interstitial positions instead of substitutional incorporation. The site preference energy is linked with the alloying-induced changes of energy of formation, hence alloying-related (de)stabilisation of the phases. We further show that the phase-stabilisation effect of early TMs on $\beta_{\mathrm{o}}$-phase has a different origin depending on their valency. Finally, an extensive comparison of our predictions with available theoretical and experimental data (which is, however, limited mostly to the $\gamma$-phase) shows a consistent picture.Comment: 7 figures, 1 tabl

Alloying-related trends from first principles: An application to the Ti--Al--X--N system

Tailoring and improving material properties by alloying is a long-known and used concept. Recent research has demonstrated the potential of ab initio calculations in understanding the material properties at the nanoscale. Here we present a systematic overview of alloying trends when early-transition metals (Y, Zr, Nb, Hf, Ta) are added in the Ti$_{1-x}$Al$_x$N system, routinely used as a protective hard coating. The alloy lattice parameters tend to be larger than the corresponding linearised Vegard's estimation, with the largest deviation more than 2.5% obtained for Y$_{0.5}$Al$_{0.5}$N. The chemical strengthening is most pronounced for Ta and Nb, although also causing smallest elastic distortions of the lattice due to their atomic radii being comparable with Ti and Al. This is further supported by the analysis of the electronic density of states. Finally, mixing enthalpy as a measure of the driving force for decomposition into the stable constituents, is enhanced by adding Y, Zr and Nb, suggesting that the onset of spinodal decomposition will appear in these cases for lower thermal loads than for Hf and Ta alloyed Ti$_{1-x}$Al$_x$N.Comment: 9 pages, 6 figure

Ab initio study of the alloying effect of transition metals on structure, stability and ductility of CrN

The alloying effect on the lattice parameters, isostructural mixing enthalpies and ductility of the ternary nitride systems Cr1-xTMxN (TM=Sc, Y; Ti, Zr, Hf; V, Nb, Ta; Mo, W) in the cubic B1 structure has been investigated using first-principles calculations. Maximum mixing enthalpy due to large lattice mismatch in Cr1-xYxN solid solution shows a strong preference for phase separation, while Cr1-xTaxN exhibits a negative mixing enthalpy in the whole compositional range with respect to cubic B1 structured CrN and TaN, thus being unlikely to decompose spinodally. The near-to-zero mixing enthalpies of Cr1-xScxN and Cr1-xVxN are ascribed to the mutually counteracted electronic and lattice mismatch effects. Additions of small amounts of V, Nb, Ta, Mo or W into CrN coatings increase its ductility.Comment: 19 pages, 3 figure

Ab initio study of point defects in NiTi-based alloys

Changes in temperature or stress state may induce reversible B2$\leftrightarrow$(R)$\leftrightarrow$ B19' martensitic transformations and associated shape memory effects in close-to-stoichiometric nickel-titanium (NiTi) alloys. Recent experimental studies confirmed a considerable impact of the hydrogen-rich aging atmosphere on the subsequent B2 austenite $\leftrightarrow$ B19' martensite transformation path. In this paper, we employ Density Functional Theory to study properties of Ar, He, and H interstitials in B2 austenite and B19' martensite phases. We show that H interstitials exhibit negative formation energies, while Ar and He interstitials yield positive values. Our theoretical analysis of slightly Ni-rich Ni--Ti alloys with the austenite B2 structure shows that a slight over-stoichiometry towards Ni-rich compositions in a range 51--52\,\text{at.%} is energetically favorable. The same conclusion holds for H-doped NiTi with the H content up to \approx6\,\text{at.%}. In agreement with experimental data we predict H atoms to have a strong impact on the martensitic phase transformation in NiTi by altering the mutual thermodynamic stability of the high-temperature cubic B2 and the low-temperature monoclinic B19' phase of NiTi. Hydrogen atoms are predicted to form stable interstitial defects. As this is not the case for He and Ar, mixtures of hydrogen and the two inert gases can be used in annealing experiments to control H partial pressure when studying the martensitic transformations in NiTi in various atmospheres.Comment: 7 pages, 7 figure