Numerical Analysis of the Nb(C,N) Precipitation Kinetics in Microalloyed Steels

Precipitation kinetics of Nb(C,N) in microalloyed steels is crucial for the achievement of favoured steel properties. Therefore, numerous experimental studies have been performed in the past and various theoretical models have been developed to describe Nb(C,N) precipitation. However, the experimental data is sometimes contradictory and even the thermodynamic data for NbC solubility in austenite have a large scatter. In this paper, experimental results on the Nb(C,N) and NbV(C,N) precipitation kinetics in deformed and undeformed austenite are reviewed. Based on these data and with the precipitation kinetics module of the software package MatCalc, computer simulations are performed. The predicted interfacial energy of precipitates is adjusted to match the observed kinetics. A comparison between experimental information and simulation, i.e. time ‐ temperature ‐ precipitation (TTP) diagrams, is drawn and discussed. The results of the computer simulations using modified interfacial energies are in good agreement with the experiments.     

Cooperative controllers for highways based on human experience

The AUTOPIA program has been working on the development of intelligent autonomous vehicles for the last 10 years. Its latest advances have focused on the development of cooperative man?uvres based on communications involving several vehicles. However, so far, these man?uvres have been tested only on private tracks that emulate urban environments. The first experiments with autonomous vehicles on real highways, in the framework of the grand cooperative driving challenge (GCDC) where several vehicles had to cooperate in order to perform cooperative adaptive cruise control (CACC), are described. In this context, the main challenge was to translate, through fuzzy controllers, human driver experience to these scenarios. This communication describes the experiences deriving from this competition, specifically that concerning the controller and the system implemented in a Citröen C3.     

Assessment of Flooding Risk to Cultural Heritage in Historic Sites

Fluvial flooding in August 2002 affected a number of structures in the Czech Republic. Considerable damage was observed particularly in the historic city of Prague. Extensive investigations indicated that main observed causes of damage could be classified into geotechnical aspects, inadequate structural properties, and insufficient communication. After the flooding responsible authorities have considered permanent and temporary protective measures to reduce adverse consequences of flooding in the future. Decisions concerning expensive measures should be preferably based on risk optimization, taking into account potential societal and economic consequences and losses of cultural heritage values. General framework of the risk assessment is thus proposed considering specific issues of cultural heritage. Such an assessment needs a theoretical model suitable for predicting flows and extents of future floods. For that reason, the authors statistically analyzed hydrologic data for annual maximum flows of the Vltava River in Prague dating back to 1827. Pearson III and lognormal distributions seem to be suitable models for a considered sample. Estimations of extreme flows, needed for assessment of flooding risk to endangered sites and decisions on protective measures, are provided for different return periods.     

Hydrogen-induced microstructural changes of Pd films

The development of the microstructure in nanocrystalline, polycrystalline and epitaxial Pd films loaded with hydrogen is investigated. Structural changes in Pd films loaded with hydrogen were characterized by positron annihilation spectroscopy combined with electron microscopy and X-ray diffraction. It was found that hydrogen charging causes plastic deformation which leads to an increase of the defect density in all Pd films studied. Moreover, the formation of buckles was observed in nanocrystalline and polycrystalline Pd films loaded above a certain critical hydrogen concentration. Buckling leads to detachment of the film from the substrate and this is accompanied with in-plane stress relaxation and plastic deformation of the film.     

Low frequency noise as a tool to study optocouplers with phototransistors

An approach to analyze the optocouplers with phototransistors by low frequency noise measurements is presented in this paper. The analysis is based on the measurements of LED and output noises of optocouplers. We have shown that the corner frequency in output noise can be caused by frequency dependence of dynamic current gain of phototransistor under conditions of open base circuit. A noise parameter as an indicator of optocoupler's phototransistor current gain degradation is proposed. The contributions of noise sources of LED and phototransistor to the frequency dependence of the output noise spectrum are also considered.     

The Effect of an Fe-doped GaN Buffer on off-State Breakdown Characteristics in AlGaN/GaN HEMTs on Si Substrate

An Fe-doped GaN buffer layer was employed in the growth of AlGaN/GaN high-electron mobility transistors (HEMTs) on Si substrates. In order to investigate the effects of an Fe-doped GaN buffer on OFF-state breakdown characteristics, HEMT devices with an Fe-doped GaN buffer on Si substrates were fabricated along with conventional devices utilizing an unintentionally doped GaN buffer on Si substrates. The device characteristics were compared. While HEMT devices with the conventional structure showed an extremely unstable OFF-state breakdown behavior due to punchthrough to the Si substrate, it was demonstrated that an Fe-doped GaN buffer layer on a Si substrate successfully suppressed the premature failure caused by Si-induced breakdown. As a result, the AlGaN/GaN HEMTs with an Fe-doped GaN buffer on Si substrates exhibited much more consistent and enhanced breakdown voltages, when compared with the conventional devices. Consequently, it is highly desirable that AlGaN/GaN HEMTs on Si substrates have an Fe-doped GaN buffer layer in order to achieve stable and robust OFF-state breakdown characteristics     

Homogenization of liquids in tanks stirred by multiple impellers

The process of homogenization of liquids in a tall vessel equipped with a multiple impeller was studied. Up to four standard Rushton turbines and/or six pitched-blade turbines were used. The mixing time in the system was measured by the conductivity method. A continuous time history of the tracer concentration at several points in the system was recorded. The data were interpreted by means of an adapted cell model of the flow within the stirred vessel with several impellers based on the assumption of well mixed cells and intercellular flow of liquid. The liquid transfer flow rate between the cells, as a parameter of the model, was calculated from experimental data. A good agreement between the time dependence of concentration obtained experimentally and that calculated from theory was obtained. A direct relationship between the flow numbers between cells and those of impellers was established.     

Pulling of glass microcapillaries: Theory and experiment

An approximate analytical solution is obtained which predicts a microcapillary form during pulling which is close to that observed in experiments.Institute of Mechanics and Biomechanics, Bulgarian Academy of Sciences, Sofia. Insitute for Problems of Mechanics, Russian Academy of Sciences, Moscow. Translated from Inzhenerno-fizicheskii Zhurnal, Vol. 63, No. 6, pp. 722–727, December, 1992.Indices 0, p, and 1 denote R, h, and T referring to the initial section (x=0), heating zone boundary (x=l), and fiber section on the receiving device (x=L).     

Evaluation of the cochlear aqueduct patency: comparison of 2 noninvasive techniques

Variations in cephalo-rachidian fluid pressure can be transmitted to the middle ear through the cochlear aqueduct (CA). This gives us a non-invasive manner to evaluate any changes in fluid pressure by measuring middle ear impedance (impedancemetry). The present study compared two indirect methods for measuring intracranial pressure: a) impedancemetry during evoked jugulo-tympanic reflex (JTR) and b) study of the tympanic membrane (TM) fine motility using a MMS-10 analyzer. The latter is a new procedure involving the indirect evaluation of the fluid pressure. In fact, when the CA is open the labyrinthine fluid pressure is transmitted to the oval window, the stapes platina and, finally, to the tympanic membrane where it can be measured with an MMS-10 unit. This equipment can measure nanoliter shifts in the TM. In particular, comparison between the clinostatic and orthostatic tympanic motility measurements enable one to establish whether the CA is patent or not. In the present study 15 subjects were examined using both a) impedancemetry during jugular compression and b) analysis of the TM shift using an MMS-10 unit. In 14 of the 15 cases there was good correlation between the data obtained using both methods: in all but one case it proved possible to record a JTR-induced variation in impedance whenever the MMS-10 indicated that the CA was open. The results suggest that, in clinical practice, the two methods can be used in parallel for non-invasive monitoring of variations in intracranial pressure in patients with neurological involvement. On the other hand, in the E.N.T. field these techniques could be used to study inner ear pathologies causing dynamic alterations of the endolabyrinth fluids (endolymphatic hydrops, labyrinthine fistula).