The high power RF device performance decreases as operation temperature increases (e.g. fall of electron mobility impacting the cut-off frequencies and degradation of device reliability). Therefore the determination of device temperature is a key issue for device topology optimisation. This work presents the comparison between pulsed I-V at different temperature and DC measurements of AlGaN/GaN HEMTs grown on two different substrates: sapphire and silicon. This technique allows the determination of mean channel temperature and the device thermal resistance. The thermal resistance is a classical way to define the average channel temperature as a function of the dissipated power. In this work the thermal resistance ratio of the HEMT grown on sapphire compared to the one grown on silicon is found to be 1.7 instead of 3 as expected from straightforward thermal conductivity ratio. This lower difference is clearly attributed to the contribution of the GaN buffer layer.

Magnetooptical and optical experiments are performed on size sorted maghemite ($\gamma-{\rm Fe}_2{\rm O}_3$) ferrofluids. Water based ferrofluid samples are synthesised by the coprecipitation method followed by a size sorting process. All the samples have the same particles volume fraction: 0.2%. Transmittance and Faraday rotation are found to be size dependent. The saturation of the Faraday rotation is decreasing when particles mean diameter increases. Faraday rotation curves are described by orientation Langevin theory and the size dependence of their magnitude is discussed using nanoparticles internal field.

In this work, a low-frequency dynamic hysteresis model is proposed and applied to nonoriented Fe-Si steel. The model based on Preisach theory can be used when skin effect is negligible over the lamination thickness. In our model, the Preisach engine that usually takes the magnetic field as input variable must be inverted. Some B(H) symmetric loops are calculated and compared with measurements in the rolling direction both in quasistatic and dynamic operations under sinusoidal induction.

Here is reported a new scheme to accurately determine the vapor pressure of undercooled, liquid, and high temperature solid materials. The method relies on an imaging technique that measures the time variation of the radius of an electrostatically levitated sample. This scheme, compared to other techniques, offers unique opportunity to explore not only the liquid above the melting point but also the undercooled states of highly reactive materials in a contamination free environment. This was exemplified in this paper with titanium. For the first time, we report the vapor pressure $(V_{\rm p})$ of its liquid phase over a large temperature range, covering the undercooled region. Over the 1700 to 2050 K temperature range, it was measured as Log $V_{\rm p}(T) = 9.154 - 17978\ T^{-1}$ (3%). Similarly, for high temperature solid titanium, the vapor pressure could be expressed as Log $V_{\rm p}(T) = 16.634 - 32960\ T^{-1}$ (6%) over the 1770 to 1940 K temperature interval. From these data, the average latent heats of vaporization and sublimation were calculated respectively as 344.8 kJ/kg (8%) and 632.1 kJ/kg (6%) respectively.

In this paper we demonstrate that the presence of thermoelectric anisotropy, no matter how small, can be exploited to unmask the presence of cracks and holes in metals. This is in direct contrast with situations involving isotropic metals where such detection is not possible. Although such a phenomenon will occur for all types and shapes of cracks and holes, for simplicity we shall demonstrate our theoretical and numerical modeling on the more mathematically tractable case of a cylindrical hole aligned along an axis of symmetry of an anisotropic metal medium.

The energies and charge-state fractions of carbon plasma ions produced with the pulsed arc-discharge system have been measured using the Doppler-shifts in the line spectra emitted by carbon ions. The arc-discharge voltages ranged from 3 to 4 kV and the corresponding currents from 7.5 to 10 kA. Pulse duration times were about 30 μs. Strong populations of doubly and triply ionized carbon ions were observed. The maximum ion energy for the triply ionised carbon ions was 250 eV. The Doppler-shift measurements were performed with a crossed-dispersion spectrograph (theoretical spectral resolution 0.01 nm) and the lines were photographed.

The aim of this paper is to model the printed connections in static power converters. An analytical technique, based on the transmission line theory has been proposed previously (Labarre et al., Eur. Phys. J. AP 3, 169 (1998)). This paper shows that this modeling technique, named the thin wire method (TWM) is well adapted to predict the behaviour of various configuration of printed circuit board with a ground plane: isolated microstrip lines exhibiting a constant or not width, parallel or non-parallel lines of any length. New analytical formulas are proposed in any layout configuration. However, the total circuit modeling needs to combine the features of different electromagnetic models: MTL, PEEC, CTL with a numerical technique: the two dimensional transmission line matrix method (TLM2D).

A mathematical model is described and applied to simulate sudden total one-dimensional dam-break flow over wetted bed. The dam collapse takes place in a rough sloping non-prismatic channel of various cross-sections. The water parameters to be instantaneously calculated are the height h, the discharge Q, the mean velocity u and the pressure force P. The mentioned flow is governed by the Saint-Venant shallow water equations and the computation process, on the basis of rectangular grid of points, consists of two complementary solutions: (a) at the first instant after the collapse, an analytical procedure is considered. The calculated parameters are taken as initial values in the water stream embraced by the flood wave. Outside this zone, initial conditions are those which preexist before the rupture. (b) Beyond this time, a numerical computation is carried out by using an iterative explicit method of characteristics. (c) Every time stage of calculation starts by determining the discontinuity (wave front) parameters namely its abscissa xδ, height hδ, celerity cδ and alert delay tδ. The former is the discontinuity arrival time at considered station. Typical results are obtained and compared with similar ones already published by others in the literature.

We describe an optical fiber based interferometer to measure velocity profiles in sheared complex fluids using dynamic light scattering (DLS). After a review of the theoretical problem of DLS under shear, a detailed description of the setup is given. We outline the various experimental difficulties induced by refraction when using a Couette cell. We also show that homodyne DLS is not well suited to measure quantitative velocity profiles in narrow-gap Couette geometries. On the other hand, the heterodyne technique allows us to determine the velocity field inside the gap of a Couette cell. All the technical features of the setup, namely its spatial resolution (≈ 50 − 100 µm) and its temporal resolution (≈ 1 s per point, ≈ 1 min per profile) are discussed, as well as the calibration procedure with a Newtonian fluid. As briefly shown on oil-in-water emulsions, such a setup permits one to record both velocity profiles and rheological data simultaneously.

Shear viscosity measurements with dilute suspensions of red blood cells are interpreted using a microrheological model that relates the bulk measurements to the physical properties of the suspended cells. It is thus possible to quantify the average deformability of a RBC population in terms of a mean value of the membrane shear elastic modulus Es. The values obtained for normal cells are in good agreement with those given in the literature. The method allows to discriminate between normal and altered (diamide or glutaraldehyde treated) cells or pathological cells (scleroderma). The predictions of the microrheological model, based on analytic calculations, are also compared with the numerical results of Ramanujan and Pozrikidis (JFM 361, 1998) for dilute suspensions of capsules in simple shear flow.