Ta2O5/SiO2 insulating acoustic mirrors for AlN-based X-band BAW resonators

This work describes the performance of AlN-based bulk acoustic wave resonators built on top of insulating acoustic reflectors and operating at around 8 GHz. The acoustic reflectors are composed of alternate layers of amorphous Ta2O5and SiO2 deposited at room temperature by pulsed-DC reactive sputtering in Ar/O2 atmospheres. SiO2 layers have a porous structure that provides a low acoustic impedance of only 9.5 MRayl. Ta2O5 films exhibit an acoustic impedance of around 39.5 MRayl that was assessed by the picoseconds acoustic technique These values allow to design acoustic mirrors with transmission coefficients in the centre of the band lower than -40 dB (99.998 % of reflectance) with only seven layers. The resonators were fabricated by depositing a very thin AlN film onto an iridium bottom electrode 180 nm-thick and by using Ir or Mo layers as top electrode. Resonators with effective electromechanical coupling factors of 5.7% and quality factors at the antiresonant frequency around 600 are achieved.

Hard transition to chaotic dynamics in Alfven wave fronts

The derivative nonlinear Schrodinger DNLS equation, describing propagation of circularly polarized Alfven waves of finite amplitude in a cold plasma, is truncated to explore the coherent, weakly nonlinear, cubic coupling of three waves near resonance, one wave being linearly unstable and the other waves damped. In a reduced three-wave model equal dampings of daughter waves, three-dimensional flow for two wave amplitudes and one relative phase, no matter how small the growth rate of the unstable wave there exists a parametric domain with the flow exhibiting chaotic relaxation oscillations that are absent for zero growth rate. This hard transition in phase-space behavior occurs for left-hand LH polarized waves, paralleling the known fact that only LH time-harmonic solutions of the DNLS equation are modulationally unstable, with damping less than about unstable wave frequency 2/4 x ion cyclotron frequency. The structural stability of the transition was explored by going into a fully 3-wave model different dampings of daughter waves,four-dimensional flow; both models differ in significant phase-space features but keep common features essential for the transition.

The influence of sensor placement on in-situ ultrasound wave velocity measurement.

Ultrasound wave velocity was measured in 30 pieces of Spanish Scots pine (Pinus sylvestris L.), 90 x 140 mm in cross-section and 4 m long. Five different sensor placement arrangements were used: end to end (V0), face to opposite face, edge to opposite edge, face to same face and edge to same edge. The pieces were successively shortened to 3, 2 and 1 m, in order to obtain these velocities and their ratios to reference value V0 for different lengths and angles with respect to the piece axis for the crossed measurements. The velocity obtained in crossed measurements is lower than V0. A correction coefficient for crossed velocities is proposed, depending on the angle, to adjust them to the V0 benchmark. The velocities measured on a surface, are also lower than V0, and their ratio with respect to V0 is close to 0.97 for distances equal to or greater than 18 times the depth of the beam.