P. Pernod

Band gap tunability of magneto-elastic phononic crystal

The possibility of control and tuning of the band structures of phononic crystals offered by the introduction of an active magnetoelastic material and the application of an external magnetic field is studied. Two means to obtain large elastic properties variations in magnetoelastic material are considered: Giant magnetostriction and spin reorientation transition effects. A plane wave expansion method is used to calculate the band structures. The magnetoelastic coupling is taken into account through the consideration of an equivalent piezomagnetic material model with elastic, piezomagnetic, and magnetic permeability tensors varying as a function of the amplitude and orientation of the applied magnetic field. Results of contactless tunability of the absolute bandgap are presented for a two-dimensional phononic crystal constituted of Terfenol-D square rod embedded in an epoxy matrix.

Magnetoelectric write and read operations in a stress-mediated multiferroic memory cell

Magnetic memory cells associated with the stress-mediated magnetoelectric effect promise extremely low bit-writing energies. Most investigations have focused on the process of writing information in memory cells, and very few on readout schemes. The usual assumption is that the readout will be achieved using magnetoresistive structures such as Giant Magneto-Resistive stacks or Magnetic Tunnel Junctions. Since the writing energy is very low in the magnetoelectric systems, the readout energy using magnetoresistive approaches becomes non negligible. Incidentally, the magneto-electric interaction itself contains the potentiality of the readout of the information encoded in the magnetic subsystem. In this letter, the principle of magnetoelectric readout of the information by an electric field in a composite multiferroic heterostructure is considered theoretically and demonstrated experimentally using [ N × ( TbCo 2 / FeCo ) ] / [ Pb ( Mg 1 / 3 Nb 2 / 3 ) O 3 ] ( 1 − x ) − [ PbTiO 3 ] x stress-mediated ME heter...

A robust thermal microstructure for mass flow rate measurement in steady and unsteady flows

A silicon micro-machined thermal gas flow sensor operating in anemometric mode has been designed, fabricated and investigated for continuous and pulsatile flows. The sensor is specifically designed to achieve high sensitivity, fast response time and high robustness. It is composed of four metallic resistors interconnected to form a Wheatstone bridge. Two of them act simultaneously as the heating and sensing elements and the two others are used as a temperature reference. The heating element consists of a metallic wire of platinum Pt (2 µm width, 2 mm length) maintained on each lateral side by periodic silicon oxide SiO2 micro-bridges. Finite element simulations show that this structure achieves a fast thermal response time of 200 µs in constant current operating mode and a coefficient of temperature rise close to 25 °C/120 µW based on bulk electrical resistivity and when the Pt wire and SiO2 thicknesses are close to 100 nm and 500 nm, respectively. This design allows the fabrication of a robust thermal flow sensor with heating elements as long as possible, which enables accurate measurements with high signal to noise ratio. The sensor is then characterised experimentally; its electrical and thermal properties are obtained in the absence of fluid flow. These results confirm the effectiveness of the thermal insulation as predicted by the simulations. In a second step, the fluidic characterizations are reported and discussed for both continuous and pulsatile flows. In continuous mode, the sensor response was studied for gas flow rate ranging from 0 L min−1 to 10 L min−1. In pulsatile mode, the sensor is integrated inside a channel of a micro-valve actuated at 200 Hz. The measurements are compared with those obtained by a classical commercial hot wire.

A millimeter-wave frequency tunable microstrip antenna on ultraflexible PDMS substrate

At millimeter waves, reconfigurable antennas are increasingly needed for high-speed wireless communications and high-resolution sensing systems [1]. Recently there has been a growing interest for frequency agile antennas due to the multiplication of wireless standards in close proximity to each other. Microstrip patch antennas are widely used because they are low-cost, light-weight, low-profile and compatible with MMIC technology. Various approaches have been implemented to tune the resonance frequency of printed antennas; they are based on electrical or mechanical reconfiguration means. In the early eighties, Lee et al. [2] proposed a manually-reconfigurable circular microstrip antenna with an air gap. In [3], an electrostatically-actuated micromachined copper ground plane was used to tune a microstrip antenna from 16.8 to 17.82 GHz and in [4] electrostatic actuation was implemented to move a microstrip patch antenna fabricated on a flexible Kapton substrate to achieve a tunability between 16.91 and 16.64 GHz. One of the key issues for elaborating mechanically reconfigurable antennas at mm-waves consists in finding materials with suitable electromagnetic and mechanical properties. In this work, we use Polydimethylsiloxane (PDMS) as an antenna substrate. PDMS is an extremely flexible polymer with very low Youngs modulus (EYoung=2 MPa) and is compatible with a number of silicon micromachining techniques. PDMS exhibits many attractive features: it is low-cost, light-weight, biocompatible and chemically resistant. The feasibility and performance of PDMS-based millimeter-wave transmission lines and microstrip antenna arrays have been demonstrated in [5]-[7]. In this paper we describe a PDMS-membrane-based frequency-tunable microstrip antenna in the 60-GHz band where a pneumatic actuation is used to reconfigure the PDMS membrane over an air-filled cavity of variable height.

A micro-scale hot wire anemometer based on low stress (Ni/W) multi-layers deposited on nano-crystalline diamond for air flow sensing

A linear array of microscale thermal anemometers has been designed, fabricated and characterized. The sensitive element consists of a self-compensated-stress multilayer (Ni/W) patterned to form a wire with length, width, and thickness close to 200 μm, 5 μm and 2 μm respectively. The wire is deposited and supported by prongs made of nano-crystalline diamond (NCD) of about 2 μm in thickness. Due to its high Youngs modulus, NCD allows a very high mechanical toughness without the need for thicker support for the hot wire. Also, depending on grain size, the NCD is able to present thermal conductivity smaller than 10 W mK−1, providing good thermal insulation from the substrate and less conductive end losses to the prongs. The sensor was characterized experimentally. Its electrical and thermal properties were obtained first in the absence of fluid flow. The results confirm the effectiveness of thermal insulation and the mechanical robustness of the structure. The fluidic characterizations were performed and analysed in the case of an airflow with velocities of up to 30 m s−1.

Ferromagnetic resonance and magnetoelastic demodulation in thin active films with an uniaxial anisotropy

The results of experimental and theoretical studies of ferromagnetic resonance (FMR) and magnetoelastic excitations near the spin reorientation transition (SRT) in an uniaxial TbCo2/FeCo layered nanostructure and a La0.7Sr0.3MnO3 film are reported. Experimental dependences of the amplitude of the reflected microwave signal versus the external magnetic field strength are presented in comparison with the theoretical ones. An increase in FMR reflectivity in the vicinity of SRT is clearly demonstrated. Low frequency magnetoelastic excitation of flexural vibrations of the samples by means of modulated microwave electromagnetic field is observed experimentally using a laser beam deflection technique. The increase in amplitude of vibrations at modulation frequency under combined FMR and SRT conditions is observed in agreement with the theory.

Measurement of the Thermal Conductivity of Polydimethylsiloxane Polymer Using the Three Omega Method

Polydimethylsiloxane (PDMS) has widely appeared in different electronic and medical applications. The knowledge of the thermal properties of PDMS and especially its thermal conductivity is required while processing PDMS to design a particular device. In this paper measurement of the thermal conductivity of PDMS using the three omega method is presented at different temperatures. The three omega method has been chosen because of its ease of use and accuracy. It requires the fabrication of metallic lines which act as heaters and thermometers on the surface of the material under test. A different procedure is introduced in this paper through which the metallic lines are embedded in the surface of PDMS. Experimental results are then compared to Cahills approximate solution and to the results obtained by numerical simulations using a finite element method.

High temperature gradient micro-sensor for wall shear stress and flow direction measurements

We present an efficient and high-sensitive thermal micro-sensor for near wall flow parameters measurements. By combining substrate-free wire structure and mechanical support using silicon oxide micro-bridges, the sensor achieves a high temperature gradient, with wires reaching 1 mm long for only 3 μm wide over a 20 μm deep cavity. Elaborated to reach a compromise solution between conventional hot-films and hot-wire sensors, the sensor presents a high sensitivity to the wall shear stress and to the flow direction. The sensor can be mounted flush to the wall for research studies such as turbulence and near wall shear flow analysis, and for technical applications, such as flow control and separation detection. The fabrication process is CMOS-compatible and allows on-chip integration. The present letter describes the sensor elaboration, design, and micro-fabrication, then the electrical and thermal characterizations, and finally the calibration experiments in a turbulent boundary layer wind tunnel.

Magnetostatic micro-actuator based on ultrasoft elastomeric membrane and copper — Permalloy electrodeposited structures

This paper presents different designs of magnetostatic micro-actuators, based on both conventional and integrated micro-coils. A 3-dimensional magnetic circuit made of Permalloy is proposed in order to improve their efficiency. The mobile parts of the micro-actuators are made of ultrasoft elastomeric membranes, bearing deformations up to 850%. This allows the use of pulse actuation, in order to get higher displacement amplitudes, that are up to -100/+200 μm for instantaneous forces of 55 mN. A fully integrated high aspect ratio micro-coil design, based on copper and Permalloy electrodeposited structures, is proposed as an alternative to conventional coils at millimetric size.

Supercritical dynamics of magnetoelastic wave triad in a solid

Parametric coupling of three traveling waves is studied numerically on an example of magnetoelastic waves in a highly anharmonic antiferromagnetic crystal. The physical mechanism of coupling is explained as a result of modulation of the nonlinear elastic moduli of the crystal by RF electromagnetic pumping. Parametric interaction of a coupled wave triad with homogeneous pumping field results in an instability of explosive type. Above the threshold of instability, the amplitudes of waves increase to occurrence of singularity in finite time. Explosion is accompanied by spatial localization of wave envelopes. The supercritical dynamics of a wave triad is simulated numerically taking into account the third- and fourth-order magnetoelastic anharmonicity of the medium. Violation of the explosive scenario by nonlinear phase mismatch between the coupled waves and pumping field is demonstrated. Modulation of the pumping phase in time is considered as a tool to compensate for the nonlinear mismatch and recondition the explosive amplification and spatial localization of wave triads. A proper phase modulation law is found in a numerical experiment.