A Novel Piezoelectric RF-MEMS Resonator with Enhanced Quality Factor

This work presents a novel ultra-high frequency (UHF) Lamb mode Aluminum Nitride (AlN) piezoelectric resonator with enhanced quality factors (Q). With slots introduced in the vicinity of the tether support end, the elastic waves leaking from the tether sidewalls can be reflected, which effectively reduces the anchor loss while retaining size compactness and mechanical robustness. Comprehensive analysis was carried out to provide helpful guidance for obtaining optimal slot designs. For various resonators with frequencies ranging from 630 MHz to 1.97 GHz, promising Q enhancements up to 2 times have all been achieved. The 1.97 GHz resonator implemented excellent f × Q product up to 6.72 × 1012 and low motional resistance down to 340 Ω, which is one of the highest performances among the reported devices. The devices with enhanced Q values as well as compact size could have potential application in advanced RF front end transceivers.

Fundamentals, Progress and Perspectives on High-frequency Phononic Crystals

Phononic crystals (PnCs) are capable to manipulate the flow of elastic energy through their periodic structures and emerge as a promising field in the last two decades. Thanks to the advances in microfabrication technologies and developments of multifunctional materials, the engineering of periodic structures moves forward to the nanometer scale. Hence, the relevant frequencies of elastic waves are pushed toward the gigahertz regime where strong photon-phonon interactions trigger the applications of PnCs towards information and communication technologies. In this review, we present the experimental achievements on hypersonic PnCs involving microfabrication technologies to realize the desired structures and characterization of their band structures for unraveling phonon propagation modulation. Some application-orientated research directions are proposed in terms of advances in fabrication and characterization technologies and the development of electro-optomechanical systems.

Modulation Instability of Hydro-Elastic Waves Blown by a Wind with a Uniform Vertical Profile

An interesting physical phenomenon was recently observed when a fresh-water basin is covered by a thin ice film that has properties similar to the property of a rubber membrane. Surface waves can be generated under the action of wind on the air–water interface that contains an ice film. The modulation property of hydro-elastic waves (HEWs) in deep water covered by thin ice film blown by the wind with a uniform vertical profile is studied here in terms of the airflow velocity versus wavenumber. The modulation instability of HEWs is studied through the analysis of coefficients of the nonlinear Schrödinger (NLS) equation with the help of the Lighthill criterion. The NLS equation is derived using the multiple scale method in the presence of airflow. It is demonstrated that the potentially unstable hydro-elastic waves with negative energy appear for relatively small wind speeds, whereas the Kelvin–Helmholtz instability arises when the wind speed becomes fairly strong. Estimates of parameters of modulated waves for the typical conditions are given.

Crack propagation speeds in weak snowpack layers

For the release of a slab avalanche, crack propagation within a weak snowpack layer below a cohesive snow slab is required. As crack speed measurements can give insight into underlying processes, we analysed three crack propagation events that occurred in similar snowpacks and covered all scales relevant for avalanche release. For the largest scale, up to 400 m, we estimated crack speed from an avalanche movie; for scales between 5 and 25 m, we used accelerometers placed on the snow surface and for scales below 5 m, we performed a propagation saw test. The mean crack speeds ranged from 36 ± 6 to 49 ± 5 m s−1, and did not exhibit scale dependence. Using the discrete element method and the material point method, we reproduced the measured crack speeds reasonably well, in particular the terminal crack speed observed at smaller scales. Finally, we used a finite element model to assess the speed of different elastic waves in a layered snowpack. Results suggest that the observed cracks propagated as mixed mode closing cracks and that the flexural wave of the slab is responsible for the energy transfer to the crack tip.