High-temperature oxidation of intermetallics formed by group IV transition metals with chromium

We have studied the oxidation resistance of chromium intermetallics: TiCr₂, HfCr₂, and ZrCr₂, in the temperature interval 873–1473 K with isothermal holding of the specimens for up to 20 h. We have shown that the intermetallics TiCr₂ and HfCr₂ are oxidized 2–3 times more slowly than pure titanium and hafnium, but more than 1–2 orders of magnitude faster than chromium at T≤1273 K and 4–6 times faster at 1473 K. In this temperature interval, the intermetallic ZrCr₂ is rapidly oxidized at an increasing rate as the heating temperature rises. The scaling resistance of intermetallics of Group IV d metals with chromium is determined by formation of scales in which the major role is played by the oxides TiO₂, HfO₂, and ZrO₂, with weak protective properties.     

Numerical solution for shape optimization of an impactor penetrating into a semi-infinite target

The shape of the impactor with the maximum depth of penetration (DOP) for a given impact velocity is found using a numerical procedure for solving a corresponding non-classical variational problem. It is shown that the optimum shape in a general case is close to a blunt cone. The variation of the optimal shape of the impactor and the dependence of the DOP vs. the initial (impact) velocity and friction coefficient is studied. The analysis is performed also for optimal conical impactors.     

An example of a GAS system which can be destabilized by an integrable perturbation

A construction of a globally asymptotically stable time-invariant system which can be destabilized by some integrable perturbation is given. Besides its intrinsic interest, this serves to provide counterexamples to an open question regarding Lyapunov functions.     

Simulation of the Accelerating Systems of Cyclotrons for Applications

The results of a comparative numerical investigation of three types of resonator systems of a cyclotron for acceleration of negative hydrogen and deuterium ions up to maximum energy 30 and 15 MeV, respectively, are presented. It is shown that a vertical system with two half-wave resonators excited in-phase has the smallest active losses and the smallest differential of the amplitude of the accelerating voltage along the edge of the dees and is effective for accelerating particles at dual working frequencies.     

Kolmogorov's Contributions to the Foundations of Probability

Andrei Nikolaevich Kolmogorov was the foremost contributor to the mathematical and philosophical foundations of probability in the twentieth century, and his thinking on the topic is still potent today. In this article we first review the three stages of Kolmogorov's work on the foundations of probability: (1) his formulation of measure-theoretic probability, 1933; (2) his frequentist theory of probability, 1963; and (3) his algorithmic theory of randomness, 1965–1987. We also discuss another approach to the foundations of probability, based on martingales, which Kolmogorov did not consider.     

An exploratory study of adolescent's perceptions of the Web

Abstract The purpose of this exploratory study was to investigate the influence of two individual characteristics (Web experience and academic focus) of adolescents on the Web perception, using off-line questionnaires (a Lickert response scale) constituted on the basis of a series of interviews. Questions concerned: perceptions about the nature of information found in the Web; 'strategies' of access to the interesting Internet sites and the reliability of different information resources (libraries, television, Web, etc.). Results lead to the assumption that adolescents with high Web experience became more critical, less confident and less enthusiastic than adolescents with low Web experience and that, in some dimensions, perceptions of literature students are different to those of science students. Even if some interesting results were obtained, further research is needed to explore users' perceptions related to individuals' characteristics and to determine the generalisability of the influences identified in this exploratory study.     

Characteristic curves of hydrogenated amorphous silicon based solar cells modeled with the defect pool model

Using the improved expression of the defect pool model proposed by Powell and Deane we match the experimental current-voltage and the spectral response characteristic curves of hydrogenated amorphous silicon solar cells. We compare the electrical parameters resulting from using the different defect pool models published in the literature and from assuming a uniform density of dangling bond in every device layer. We discuss the applicability of the algorithm derived by Schumm for the stabilized state exploring its sensitivity to the sample history. Finally we propose an expression for stabilized cells adapting Schumm's ideas to the expression derived by Powell and Deane.     

Micromechanism of corrosion fracture of the plates of heat exchangers

We have studied the micromechanism of corrosion fracture of heat-transfer plates of a heat exchanger. The operation of heat exchangers under conditions of contamination of the plates with a sediment from water leads to their local corrosion under the sediment. The conditions of operation of the heat exchangers being equal, heat-transfer plates made of AiSi316Ti steel worked 2.36 times as long as those of AiSi304L steel. The corrosion fracture of plates under the sediment began from pitting initiation near nonmetallic inclusions of a certain nature. Afterwards, these pittings built up into corrosion spots by the mechanism of stress corrosion and hydrogen embrittlement. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 1, pp. 110–116, January–February, 2007.