E. Wistrela

Characterization of a roof tile-shaped out-of-plane vibrational mode in aluminum-nitride-actuated self-sensing micro-resonators for liquid monitoring purposes

This Letter reports on an advanced out-of-plane bending mode for aluminum-nitride (AlN)-actuated cantilevers. Devices of different thickness were fabricated and characterized by optical and electrical measurements in air and liquid media having viscosities up to 615 cP and compared to the classical out-of-plane bending and torsional modes. Finite element method eigenmode analyses were performed showing excellent agreement with the measured mode shapes and resonance frequencies. Quality factors (Q-factor) and the electrical behavior were evaluated as a function of the cantilever thickness. A very high Q-factor of about 197 was achieved in deionized water at a low resonance frequency of 336 kHz, being up to now, the highest quality factor reported for cantilever sensors in liquid media. Compared to the quality factor of the common fundamental out-of-plane bending mode, a 5 times higher Q-factor was achieved. Furthermore, the strain related conductance peak of the roof tile-shaped mode is superior. Compared ...

Temperature dependent performance of piezoelectric MEMS resonators for viscosity and density determination of liquids

It is the objective of this paper to report on the performance of piezoelectric MEMS resonators for viscosity and density measurements at elevated temperatures. A custom-built temperature controlled measurement setup is designed for fluid temperatures up to 100 °C. Piezoelectric single-side clamped resonators are fabricated, excited in 2nd order of the roof tile-shaped mode (13-mode) and exposed to several liquids (i.e. D5, N10, N35, PAO8, olive oil, ester oil and N100). At the next step, these results are analysed applying a straightforward evaluation model, thus demonstrating that with piezoelectric MEMS resonators the density (i.e. from kg m−3 to kg m−3) and viscosity (i.e. from mPa s to mPa s) values of liquids can be precisely determined in a wide range. Compared to standard measurement techniques, the results show for the first parameter a mean deviation of about 1.04% at 100 °C for all the liquids investigated. For the second parameter, the standard evaluation model implies a systematic deviation in viscosity with respect to the calibration being N35 in this study. This inherent lack of strength has a significant influence on the accuracy, especially at 100 °C due to fluids having a viscosity reduced by a factor of 30 for N100 compared to room temperature. This leads to relative deviations of about 23% at 100 °C and indicates the limits of the evaluation model.

Magnetic and microstructural properties of sputter deposited Cr-doped aluminum nitride thin films on silicon substrates

In this paper, we report on the microstructural and magnetic properties of CrxAl1−xN thin films with Cr concentrations ranging up to x = (8.5 ± 0.5) at. %. The thin films are sputter deposited on silicon substrates and exhibit a wurtzite type microstructure verified by X-ray diffraction measurements. A vibrating sample magnetometer based measurement equipment is used to investigate magnetic properties of the Cr doped thin films in a temperature range of T = 10 K–300 K, revealing a paramagnetic behavior. With increasing temperature, the temperature independent diamagnetic contribution of the substrate material dominates the overall response characteristics. No room temperature ferromagnetism is observed for all samples investigated.

Piezoelectric MEMS resonators for density and viscosity sensing in engine oil with diesel fuel

This work demonstrates the potential of AlN-based resonators as on-line sensors for monitoring lubricant oil dilution with diesel. Two devices are compared, one actuated in the first extensional in-plane mode and the other in an out-of-plane mode (14-mode). Both devices are designed to feature high quality factor in liquid, and allow discriminating variations in density or viscosity in the medium. Sensor resolutions for these two variables are compared in lubricant oil SAE 2.5W, and the device with the best result (14-mode) was also tested in continuous-flow measurements, showing a resolution of 0.5 ppm of diesel contamination in this oil.

Out-of-plane piezoelectric microresonator and oscillator circuit for monitoring engine oil contamination with diesel

Real-time monitoring of the physical properties of liquids is an important subject in the automotive industry. Contamination of lubricating oil by diesel soot has a significant impact on engine wear. Resonant microstructures are regarded to be a precise and compact solution for tracking the viscosity and density of lubricant oils. Since the measurement of pure shear forces do not allow an independent determination of the density and viscosity, two out-of-plane modes for the monitoring of oil dilution with diesel have been selected. The first one (12-mode) is working at 51 kHz and the second mode (14-mode) at 340 kHz. Two parameters were measured: the quality factor and the resonance frequency from which the viscosity and density of the fluids under test can be determined, requiring only a small amount of test liquid. A PLL-based oscillator circuit was implemented based on each resonator. Our results demonstrate the performance of the resonator in oils with viscosity up to 90 mPa·s. The quality factor measured at 25°C was 7 for the 12-mode and 19 for the 14-mode. A better resolution in density and viscosity was obtained for the 14-mode, showing a resolution of 3.92·10-5 g/ml for the density and 1.27·10-1 mPa·s for the viscosity, in pure lubricant oil SAE 0W30. An alternative tracking system, based on a discrete oscillator circuit, was tested with the same resonator, showing a comparable stability and supporting our approach.

Piezoelectric response optimization of multi roof tile-shaped modes in MEMS resonators by variation of the support boundary conditions

This paper investigates strain-related conductance peaks ΔG of advanced roof tile-shaped vibration-modes in piezoelectrically actuated resonators by variation of the support boundary conditions, leading to a complete new class of liquid monitoring sensors. These new vibration-modes feature very high Q-factors in liquid media and enhanced volume-strain values in the device. Combined with an optimized electrode design, the enhanced volume-strain results in very high strain-related conductance peaks ΔG. Furthermore, the impact on the piezoelectric response ΔG/Q is studied, leading to an increased ΔG/Q ratio by ~25% compared to single-side clamped resonators. These features predestinate this new class of vibration-modes for a large variety of challenging resonator-based sensing applications in liquid media exceeding the overall performance of commonly used out-of-plane vibration-modes.

Characterisation of multi roof tile-shaped out-of-plane vibrational modes in aluminium-nitride-actuated self-sensing micro-resonators in liquid media

This letter reports on higher orders of an advanced out-of-plane bending mode in aluminium-nitride (AlN)-actuated cantilever plates achieving the highest quality factors (Q-factor) of cantilever-based MEMS (micro electromechanical system) resonators in liquids up to now. Devices based on a 20 μm thick silicon cantilever were fabricated and characterised by optical and electrical measurements in air and in different liquids. Furthermore, finite element method eigenmode analyses were performed, showing an excellent agreement with the measured mode shape and the electrical characteristics. The highest Q-factor was achieved in deionised water with Q = 366, operated at the 10th order mode at a resonance frequency less than 4 MHz. This is the highest value ever measured in liquid media with a cantilever-based MEMS resonator up to now and exceeds the Q-factors of state of the art resonators in liquids in the given resonance frequency range by a factor of about 4. Furthermore, the strain related conductance peak ...

Piezoelectric resonators and oscillator circuit based on higher-order out-of-plane modes for density-viscosity measurements of liquids

We report the use of two AlN-based piezoelectric microresonators for the monitoring of density and viscosity of liquids and its application to detect lubricant oil dilution with diesel fuel. Two devices designed to resonate in the 4th-order roof tile-shaped vibration mode, but with two different anchor schemes, were fabricated and characterized. Interface circuits were designed to convert the one-port impedance into a resonant two-port transfer function. This allowed us to implement a phase locked loop (PLL)-based oscillator circuit based on the resonators, the interface circuit and a commercial lock-in amplifier. Our results demonstrate the performance of the resonators in fluids having viscosities up to 500 mPa s. The performance of the sensors in terms of sensitivity and resolution are compared for both anchor configurations.

Fabrication and characterisation of ScAlN -based piezoelectric MEMS cantilevers

Scandium (Sc) doping of aluminium nitride (AlN) increases the piezoelectric actuation potential due to substantially enhanced piezoelectric constants. This work demonstrates the fabrication of MEMS cantilevers actuated by sputter deposited ScxAl1-xN thin films (x = 27 %) sandwiched between gold electrodes. Patterning of ScxAl1-xN films is performed by a reactive ion etching process using SiCl4. The dynamic actuation potential of the fabricated devices is evaluated with Laser Doppler Vibrometry and with electrical impedance spectroscopy measurements. When applying the Butterworth Van-Dyke equivalent circuit a significant increase of the effective transverse piezoelectric constant d31 is demonstrated.

Density-viscosity sensor based on piezoelectric MEMS resonator and oscillator circuit

There are many examples of physical sensing in liquid employing resonators, especially involving TSM quartz crystals. Nevertheless, to determine density and viscosity independently, out-of-plane vibration modes are required, what typically results in low quality factors. Non-fundamental out-of-plane vibration modes have not been deeply studied for this application. Here we demonstrate the benefit of the 2nd-order mode of a microplate. In addition, the device needs to be electronically interfaced in order to measure its resonant characteristics that are ultimately translated into density and viscosity. For this, we have implemented an oscillator circuit with discrete components, which supposes a novelty regarding piezoelectric microstructures immersed in liquid. As a result, we present for the first time a complete density-viscosity sensor system based on a MEMS resonator embedded in low-cost circuitry. The sensitivities involved are evaluated, along with the resolutions for density and viscosity, below 4.09·10-6 g/ml and 2.07·10-6 mPa·s respectively.