Large-signal device simulation in time- and frequency-domain: a comparison

The aim of this paper is to compare the most common time- and frequency-domain numerical techniques for the determination of the steady-state solution in the physics-based simulation of a semiconductor device driven by a time-periodic generator. The shooting and harmonic balance (HB) techniques are applied to the solution of the discretized drift-diffusion device model coupled to the external circuit embedding the semiconductor device, thus providing a fully nonlinear mixed mode simulation. The comparison highlights the strong and weak points of the two approaches, basically showing that the time-domain solution is more robust with respect to the initial condition, while the HB solution provides a more rapid convergence once the initial datum is close enough to the solution itsel

A Java-Based Agent Platform for Programming Wireless Sensor Networks

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A generalized drift-diffusion model for rectifying Schottky contact simulation

We present a discussion on the modeling of Schottky barrier rectifying contacts (diodes) within the framework of partial-differential-equation-based physical simulations. We propose a physically consistent generalization of the drift-diffusion model to describe the boundary layer close to the Schottky barrier where thermionic emission leads to a non-Maxwellian carrier distribution, including a novel boundary condition at the contact. The modified drift-diffusion model is validated against Monte Carlo simulations of a GaAs device. The proposed model is in agreement with the Monte Carlo simulations not only in the current value but also in the spatial distributions of microscopic quantities like the electron velocity and concentratio

Studio teorico e sperimentale sull’usura adesiva e sulla lubrificazione in ingranaggi per impieghi aeronautici

L'usura adesiva è uno dei principali meccanismi di danneggiamento degli ingranaggi. Questa tesi, svolta in collaborazione con Avio Propulsione Aerospaziale e con la società spin-off dell’Università di Pisa AM Testing, riguarda un’analisi teorico sperimentale per la caratterizzazione della resistenza ad usura adesiva di ingranaggi per impieghi aeronautici. È stata eseguita una estensiva campagna sperimentale su un banco a ricircolo di potenza in cui sono stati indagati gli effetti del materiale, della finitura superficiale, dell’angolo di pressione, della velocità e della temperatura del lubrificante sul danneggiamento. Il modello analitico preso a riferimento è stato quello della temperatura totale di contatto, che rappresenta lo standard progettuale proposto dalle vigenti normative di riferimento. Per migliorare l’affidabilità di tale modello, sono state effettuate apposite misure della temperatura del corpo ruota nelle reali condizioni di funzionamento. Sono presentati i risultati sperimentali e le attività di analisi, validazione e taratura del modello. Particolarmente critica è risultata l’interazione tra l’effetto della rugosità, lo sviluppo del meato di lubrificante e l’insorgenza del danneggiamento. Per la valutazione delle problematiche relative al contatto lubrificato tra superfici rugose, è stato sviluppato un apposito programma di calcolo

Stomatal conductance and leaf water potential responses to hydraulic conductance variation in Pinus pinaster seedlings

In this study, tree hydraulic conductance (K tree) was experimentally manipulated to study effects on short-term regulation of stomatal conductance (g s), net photosynthesis (A) and bulk leaf water potential (Ψleaf) in well watered 5–6 years old and 1.2 m tall maritime pine seedlings (Pinus pinaster Ait.). K tree was decreased by notching the stem and increased by progressively excising the root system and stem. Gas exchange was measured in a chamber at constant irradiance, vapour pressure deficit, leaf temperature and ambient CO2 concentration. As expected, we found a strong and positive relationship between g s and K tree (r = 0.92, P = 0.0001) and between A and K tree (r = 0.9, P = 0.0001). In contrast, however, we found that the response of Ψleaf to K tree depended on the direction of change in K tree: increases in K tree caused Ψleaf to decrease from around −1.0 to −0.6 MPa, but reductions in K tree were accompanied by homeostasis in Ψleaf (at −1 MPa). Both of these observations could be explained by an adaptative feedback loop between g s and Ψleaf, with Ψleaf prevented from declining below the cavitation threshold by stomatal closure. Our results are consistent with the hypothesis that the observed stomatal responses were mediated by leaf water status, but they also suggest that the stomatal sensitivity to water status increased dramatically as Ψleaf approached −1 MPa

Modeling the interplay of glycine protonation and multiple histidine binding of copper in the prion protein octarepeat subdomains

The octarepeat region of the prion protein can bind Cu(2+) ions up to full occupancy (one ion per octarepeat) at neutral pH. While crystallographic data show that the HGGG octarepeat subdomain is the basic binding unit, multiple histidine coordination at lower Cu occupancy has been reported by X-ray absorption spectroscopy, EPR, and potentiometric experiments. In this paper we investigate, with first principles Car-Parrinello simulations, the first step for the formation of the Cu low-level binding mode, where four histidine side chains are coordinated to the same Cu(2+) ion. This step involves the further binding of a second histidine to an already HGGG domain bonded Cu(2+) ion. The influence of the pH on the ability of Cu to bind two histidine side chains was taken into account by simulating different protonation states of the amide N atoms of the two glycines lying nearest to the first histidine. Multiple histidine coordination is also seen to occur when glycine deprotonation occurs and the presence of the extra histidine stabilizes the Cu-peptide complex. Though the stabilization effect slightly decreases with the number of deprotonated glycines (reaching a minimum when both N atoms of the two nearest glycines are available as Cu ligands), the system is still capable of binding the second histidine in a 4N tetrahedral (though slightly distorted) coordination, whose energy is very near to that of the crystallographic square-planar 3N1O coordination. This result suggests that at low metal concentration the reorganization energy associated with Cu(II)/Cu(I) reduction is small also at pH approximately 7, when glycines are deprotonated