M. Goedbloed

VIBRATION ENERGY HARVESTING WITH ALUMINUM NITRIDE-BASED PIEZOELECTRIC DEVICES

This paper describes the measurement results of piezoelectric energy harvesters with aluminum nitride (AlN) as a piezoelectric material. AlN was chosen for its material properties and for its well-known sputter deposition process. For AlN devices a high optimum load resistance is required, which is favorable due to the high resulting voltage level. The output power harvested from mechanical vibrations has been measured on micromachined harvesters with different geometries. The resonance frequencies ranged from 200 up to 1200 Hz. The packaged devices had limited output powers and quality factors due to air damping caused by the package. A maximum output power of 60 µW has been measured on an unpackaged device at an acceleration of 2.0 g and at a resonance frequency of 572 Hz. The package of the harvester requires special attention, since air damping can significantly decrease the maximum power output.

Vacuum-packaged piezoelectric vibration energy harvesters: Damping contributions and autonomy for a wireless sensor system

This paper describes the characterization of thin-film MEMS vibration energy harvesters based on aluminum nitride as piezoelectric material. A record output power of 85 μW is measured. The parasitic-damping and the energy-harvesting performances of unpackaged and packaged devices are investigated. Vacuum and atmospheric pressure levels are considered for the packaged devices. When dealing with packaged devices, it is found that vacuum packaging is essential for maximizing the output power. Therefore, a wafer-scale vacuum package process is developed. The energy harvesters are used to power a small prototype (1 cm\3 volume) of a wireless autonomous sensor system. The average power consumption of the whole system is less than 10 μW, and it is continuously provided by the vibration energy harvester.

Modeling and characterization of MEMS-based piezoelectric harvesting devices

Vibrational piezoelectric harvesting devices (PHD) provide an autonomous power source for various types of sensors, actuators and MEMS devices. There have been several examples of vibrational energy harvesters published in the literature over the years. However, for many applications the generated power is not yet sufficient. In this paper, a physical model for predicting the generated electric power from piezoelectric harvesting devices is introduced. The model is based on estimating the total charge generated on a piezoelectric material when it is subjected to mechanical strain as a result of bending at the fundamental resonance frequency. Based on Euler–Bernoulli beam theory, the strain can be determined in terms of the beam deflection at purely mechanical excitation. The proposed model extends the current state of the art by consideration of the strain distribution due to the presence of an extended mass volume at the end of the beam. The constitutive equations of piezoelectricity in the sensing mode correlate the strain and the induced charge in the piezoelectric element. Using the device design parameters and the beam deflection as inputs, the power output can be calculated. The results of the model were experimentally verified for MEMS-based PHDs. The model was found to give an accurate prediction of the electrical parameters under various damping conditions. After model validation, a subsequent device optimization has been made to improve the power generation.

A batch process micromachined thermoelectric energy harvester: fabrication and characterization

Micromachined thermopiles are considered as a cost-effective solution for energy harvesters working at a small temperature difference and weak heat flows typical for, e.g., the human body. They can be used for powering autonomous wireless sensor nodes in a body area network. In this paper, a micromachined thermoelectric energy harvester with 6 μm high polycrystalline silicon germanium (poly-SiGe) thermocouples fabricated on a 6 inch wafer is presented. An open circuit voltage of 1.49 V and an output power of 0.4 μW can be generated with 3.5 K temperature difference in a model of a wearable micromachined energy harvester of the discussed design, which has a die size of 1.0 mm × 2.5 mm inside a watch-size generator.

Shock induced energy harvesting with a MEMS harvester for automotive applications

In this work we report shock induced measurement and simulations on AlN based piezoelectric vibration energy harvesters. We compare the result with sinusoidal input vibrations, where we obtain a record power of 489 µW. The vacuum packaged harvesters have high quality factors and high sensitivity. We validate the potential of piezoelectric vibration harvesters for car tire applications by measurements and simulations.

First autonomous wireless sensor node powered by a vacuum-packaged piezoelectric MEMS energy harvester

This paper describes the experimental characterization of piezoelectric harvesters with different dimensions. We present a record level of generated power of 85 µW obtained from an unpacked device. We have developed a wafer-scale vacuum package which shows a 100–200 fold increase in power output compared with packaged devices under atmospheric pressure. A wireless sensor node was fully powered by a piezoelectric harvester. The average power consumption was less than 10 µW while it was operating at 15 seconds duty cycle.

Piezoelectric AlN energy harvesters for wireless autonomous transducer solutions

Piezoelectric energy harvesters with aluminum nitride (AlN) are fabricated by micromachining. Devices with different size and mass were fabricated. The devices were packaged by a top and bottom glass wafer with cavities of 200mum to prevent excessive displacements. The devices are characterized on a shaker by applying an oscillation as mechanical input and measuring the power dissipated in a resistive load. A power output was generated of ~10muW for an acceleration of 8g at the resonance frequency of 1155Hz. The power generation for AlN is comparable with lead zirconate-titanate (PZT), despite the much lower piezoelectric constants. This makes AlN a very suitable material for energy harvesters due to its easier processing.

Effect of length/width ratio of tapered beams on the performance of piezoelectric energy harvesters

Tapering of the beams as a way to increase the generated output power of cantilever piezoelectric energy harvesters has gained popularity in recent years. The tapering increases the average strain in the beam and consequently the charge generated by the piezoelectric material. Different authors claim an improvement of up to 30% in the generated output power. We have investigated the possibility of using tapered beams in MEMS piezoelectric energy harvesters. Numerical simulations did not suggest any increase in the generated output power and the lack of improvement was confirmed in practice. With the help of the numerical simulations it was further found that the tapering does work but only for certain design configurations, namely for cantilevers with long slender beams. For cantilevers with short wide beams, the tapering has no significant effect on the output power of the harvester.

Design improvements for an electret-based MEMS vibrational electrostatic energy harvester

This paper presents several improvements to the design of an electret-based MEMS vibrational electrostatic energy harvester that have led to a two orders of magnitude increase in power compared to a previously presented device. The device in this paper has a footprint of approximately 1 cm2 and generated 175 μW. The following two improvements to the design are discussed: the electrical connection principle of the harvester and the electrode geometrical configuration. The measured performance of the device is compared with simulations. When exited by sinusoidal vibration, a device employing the two design improvements but with a higher resonance frequency and higher electret potential generated 495 μW AC power, which is the highest reported value for electret-based MEMS vibrational electrostatic energy harvesters with a similar footprint. This makes this device a promising candidate for the targeted application of wireless tire pressure monitoring systems (TPMS).

The effect of the built-in stress level of AlN layers on the properties of piezoelectric vibration energy harvesters

In this paper we investigated the effects of built-in stress on the dielectric and piezoelectric properties of sputtered AlN layers, meant to be implemented in micromachined piezoelectric vibration energy harvesters. Test structures including cantilevers, 4-point bending beams and metal–insulator–metal capacitors were manufactured with reactive sputtered AlN layers in a thickness range of 400–1200 nm. Various bias conditions during the deposition process allowed controlling the built-in stress level in the layers, from tensile to compressive. The clamped dielectric permittivity e33S, the voltage response and the piezoelectric coefficient e31 of the deposited AlN layers were measured by performing capacitance, voltage-deflection and 4-point bending measurements, respectively. In addition, we obtained from electrical impedance analyses the generalized electromechanical coupling (GEMC) and the quality factors of the fabricated test cantilevers, which are the critical parameters directly connected to the performance of the device in terms of energy harvesting. It is found that the permittivity e33S and the piezoelectric constant e31 were not significantly affected by the different stress levels for a given layer thickness. However, the GEMC and the quality factor were found to be decreasing for structures that have a larger residual stress. We concluded that large residual stress has to be avoided in order to optimize the output power of AlN-based vibration harvesters.