Simona Petroni

SURFACE ACOUSTIC WAVE PRESSURE SENSOR

A sensitive sensor for measurement of pressure and strain is created by fabricating a surface acoustic wave (SAW) delay line on a quartz substrate. The SAW device which acts as a band pass filter has a central frequency of 98MHz. When an external force, strain, pressure is applied to the acoustic gap, then the gap length and IDT finger spacing will change and thus cause a shift of the central frequency and of the phase. Our approach was first to generate a 98MHz signal at the input transducer and to evaluate the relative phase shift due to the pressure or the strain. The second approach was to introduce the SAW device in an oscillator loop to measure the relative variations of the central frequency. A simulation based on finite elements was carried out to compare the length variations and an experimental study of the IDTs position, substrate thickness and orientation on the sensor response.

Electrical response from nanocomposite PDMS–Ag NPs generated by in situ laser ablation in solution

Laser ablation technique is employed in order to generate polydimethylsiloxane (PDMS)/Ag NPs in situ, starting from a silver target in a solution of PDMS prepolymer and toluene. The produced surfactant-free nanoparticles are characterized by high resolution transmission electron microscopy (HRTEM) and scanning TEM-high angle annular dark field (STEM-HAADF) imaging modes, showing the majority of them to be of the order of 4 nm in diameter with a small percentage of larger Ag-AgCl multidomain NPs, embedded into a PDMS matrix. Low concentrations of carbon onion-like nanoparticles or larger fibers are also formed in the toluene-PDMS prepolymer solution. In accordance with this, UV-vis spectra shows no peak from silver NPs; their small size and their coverage by the PDMS matrix suppresses the signal of surface plasmon absorption. Inductively coupled plasma measurements reveal that the concentration of silver in the polymer is characteristically low, ~0.001% by weight. The electrical properties of the PDMS nanocomposite films are modified, with current versus voltage (I-V) measurements showing a low current of up to a few tenths of a pA at 5 V. The surface resistivity of the films is found to be up to ~10(10) Ω/sq. Under pressure (e.g. stress) applied by a dynamic mechanical analyzer (DMA), the I-V measurements demonstrate the current decreasing during the elastic deformation, and increasing during the plastic deformation.

AlN-based MEMS devices for vibrational energy harvesting applications

This paper presents a new AlN-based MEMS devices suitable for vibrational energy harvesting applications. Due to their particular shape and unlike traditional cantilever which efficiently harvest energy only if subjected to stimulus in the proper direction, the proposed devices have 3D generation capabilities solving the problem of device orientation and placement in real applications. Thanks to their particular shape, the realized devices present more than one fundamental resonance frequencies in a range comprised between 500 Hz and 1.5 kHz, with a voltage generation higher than 300μV and an output power up to 0.4 pW for single MEMS device.

Low stiffness tactile transducers based on AlN thin film and polyimide

In this paper, we propose a flexible piezoelectric MEMS transducer based on aluminum nitride thin film grown on polyimide soft substrate and developed for tactile sensing purposes. The proposed device consists of circular micro-cells, with a radius of 350 μm, made of polycrystalline c-axis textured AlN. The release of compressive stress by crystalline layers over polymer substrate allows an enhanced transduction response when the cell is patterned in circular dome-shaped geometries. The fabricated cells show an electromechanical response within the full scale range of 80 mN (≃200 kPa) both for dynamic and static load. The device is able to detect dynamic forces by exploiting both piezoelectric and flexoelectric capabilities of the aluminum nitride cells in a combined and synergistic sensing that occurs as voltage generation. No additional power supply is required to provide the electrical readout signals, making this technology suitable candidate when low power consumption is demanding. Moreover a capacit...

Flexible AlN flags for efficient wind energy harvesting at ultralow cut-in wind speed

Wind and fluid flow represent some of the most attractive renewable energy sources for addressing climate change, pollution and energy insecurity issues. Wind harvesting technologies, in particular, are the fastest-growing electric technologies in the world because of their efficiency and lower environmental impact with respect to traditional energy sources, despite exhibiting major drawbacks such as big infrastructure investment and environment invasiveness, producing high levels of noise and requiring the need of large areas for their installation. A single wind turbine can produce megawatts of power and they have the potential to cover the entire world’s energy demand in the next few years, but they have a technological limit in a cut-in wind speed of about 4 m s−1, below which the turbines do not operate, excluding them as an energy source for slow air flows. At the same time most of the wind available in the environment is below the turbines’ threshold speed. In this paper we show that small flags, made by piezoelectric thin film on flexible polymers and whose shape resembles the dry leaves of trees, can efficiently act as harvesters of energy from wind at extremely low speed, such as from a gentle blow or breath. We demonstrate that piezoelectricity on flexible polymers is achievable by depositing a thin film of piezoelectric aluminium nitride (AlN), sandwiched between metal electrodes with columnar grains coherent through the polycrystalline layers, on Kapton substrates. The prototype flags have a natural curling due to the release of the residual stress of the layers. While the curling is essential for the activation of the maximum flag oscillation, this system is so elastic and light that oscillations start at a cut-in flow speed of 0.4 m s−1, the lowest reported so far, with an open circuit peak to peak voltage of 40 mV. The voltage increases to 1.2 V when the flag is flattened and parallel to the fluid flow lines, with a generated power of 0.257 mW cm−3.

Noise reduction in GaN-based radio frequencysurface acoustic wave filters

In this work we have fabricated and characterized GaN based surface acoustic wave (SAW) delay lines grown by metal organic chemical vapor deposition (MOCVD) on sapphire substrate. The acoustic wave velocity of 0th Rayleigh and Sezawa modes, and the piezoelectric electromechanical coupling constant have been measured for different wave numbers in a 2μm-thick layer. The acoustic velocity resulted to be independent from the layer resistivity, which strongly affects the noise level. Through the introduction of a highly resistive GaN buffer layer, a noise level as low as −70dB has been measured. This result has been attributed to a reduced coupling between the input and output terminals.

Magnetically active polymeric nanocomposites for two-photon stereolithography

We report about the realization of a negative-tone photoresist for two-photon lithography containing superparamagnetic iron-oxide nanoparticles at 0.1 wt.% concentration. The material was characterized in terms of optical transparence in the visible spectral range and the microfabrication process engineered in order to minimize particles agglomeration. The versatility of two-photon direct laser writing allowed us to fabricate three-dimensional structures with sub-micrometer resolution, letting us envision possible applications of this nanocomposite blend for the realization of complex magnetically actuated MEMS.

Piezoelectric soft MEMS for tactile sensing and energy harvesting

Flexible piezoelectric heterostructures based on aluminium nitride (AIN) grown on a polymeric soft substrate are shown to be a powerful material system to produce soft and conformable MEMS. Here we report on two different devices based on soft AlN MEMS technology on top of Kapton substrates, such as tactile sensing for robotic applications and energy harvesters from vibrations and air flow. While extremely flexible, the Mo/AIN/Mostructure results to be crack-free even after the microfabrication procedure, peeling off from its rigid support and several cycles of bending. The fabricated tactile sensors exhibit a linear response as a function of pressure, while energy harvesting devices, structured as piezoelectric flexible cantilever, generate a voltage of about 20 mV under just a gentle breath.