Seon-Bae Kim

Comparison of MEMS PZT Cantilevers Based on

<i>d</i> ₃₁ and <i>d</i>₃₃ mode microelectromechanical systems piezoelectric energy harvesters (PEHs) were fabricated and compared to investigate their output powers converted from vibration. Both types of devices have the same dimensions in a cantilever structure and aim to effectively couple vibration from ambient conditions. The resonant frequencies of the cantilevers are 243 Hz. Two types of devices were compared using mathematical equations based on an equivalent circuit model. The output power of the <i>d</i>₃₁ mode PEH was 2.15 μW and 2.33 μW in experiment and modeling, respectively. The <i>d</i>₃₃ mode PEHs generated output power ranging between 0.62 and 1.71 μW when the width of the interdigital electrode (IDE) is ranging from 8 to 16 μm and finger spacing is varied from 4 to 16 μm. The output power of the <i>d</i>₃₃ mode device strongly depends on the dimensions of IDE. The analysis of material constant and electrode design was conducted in conjunction with developing a mathematical equation. The result predicts that the output power of <i>d</i>₃₃ mode PEH can be higher than that of <i>d</i>₃₁ mode PEH when the finger width is reduced to 2 μm and finger spacing is between 8 and 20 μm.

d_{31}

Vertically aligned ZnO nanorods were grown on flexible polyimide films by a thermolysis assisted chemical solution method to fabricate an ethanol sensing material. Flexible ZnO nanorod sensors were examined to monitor ethanol gas by varying the working temperature from 300 to 125°C and by changing the ethanol concentration in a range from 100 to 10 ppm in synthetic air. A flexible ZnO nanorod sensor can detect 100 ppm of ethanol gas with a sensitivity of 3.11 at 300°C. When compared with ZnO nanorod sensors fabricated on a silicon dioxide hard substrate, its sensing performance exhibited competitive sensitivity.

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The performance of piezoelectric vibration energy harvesters was studied as a function of environment temperature. The devices fabricated by soft or hard PZTs were used to investigate the effect of material parameters on the thermal degradation of the devices. PZT MEMS device was also prepared and compared with the bulk devices to investigate scaling effect on the thermal degradation. All devices were heated up to 150^oC in an insulating chamber. Output power was estimated by Roundys equivalent circuit model and compared with experimental data. The output power of all devices decreased with the increase of the temperature. The output power as a function of temperature can be predicted by the change of piezoelectric coupling coefficient that is proportional to piezoelectric constant and inverse of square root of dielectric constant. Such combined influence on the output power leads to a lower thermal degradation rate of the soft PZT-based device at a lower temperature. For MEMS scale device based on PZT films, temperature dependence of the output power was reduced. This result can be attributed to decreased temperature dependence of dielectric and piezoelectric constants mainly due to constrained domain motions.

d_{33}

Much interest in energy harvesters has been focused on maintaining their conversion efficiency during scaling down via the micromachining process. The piezoelectric PZT-based MEMS energy harvester was designed and fabricated to increase the fraction of the material strained during deflection since the geometric change of the cantilever shape can change the strain distribution on the beam and improve the output power. The generated power during beam deflection was separately collected from individual electrodes located at different positions of the cantilever, and they had good agreement with the strain estimation from finite element analysis. The trapezoidal shape showed 39% higher power than that of rectangular one.

Modes for Vibration Energy Harvesting

A microfabricated d33 mode piezoelectric energy harvester (PEH) with unimorph cantilever structure patterned by interdigital electrodes was designed, fabricated and analyzed. New analytical expression for estimating output power was developed based on the modification of conformal mapping and Roundys analytical modeling. The results using the new analytical equation can explain the effects of electrode design on the output power of d33 PEH. The output power increases with a longer finger width to a certain limit, and then decreases presumably due to depoling and polarization reversal by charge injection and charged defects in a piezoelectric layer.

Vertically Aligned ZnO Nanorod Sensor on Flexible Substrate for Ethanol Gas Monitoring

The efficiency of micro-electro-mechanical systems (MEMS) energy harvesters using transverse and longitudinal piezoelectric modes was studied. Since the emergence of piezoelectric MEMS energy harvesters, studies have mainly focused on improving the conversion efficiency at a given strain on a transduction layer. The transverse piezoelectric mode has been applied to use a higher d33 piezoelectric coefficient of a lead zirconate titanate oxide (PZT) material, but few studies have verified its distinguished efficiency compared to that of transverse mode using d31. PZT thin films were deposited on Pt/Ti/SiO2 and ZrO2/SiO2 substrates as the mode by a chemical solution deposition method. The rectangular cantilever with a Si proof mass was fabricated on an SOI wafer, and it has 20 μm of Si device layer. The cantilevered energy harvesters in the same size were respectively fabricated for longitudinal {3-1} and transverse {3-3} mode using parallel electrode plates and interdigitated electrodes (IDE). The generated voltage and power were analyzed considering piezoelectric constants of the PZT films in transverse and longitudinal modes. Since the efficiency of the transverse energy harvester seems to be strongly affected by IDE configuration, a study of this is accompanied. This study might give the direction of choosing the piezoelectric mode for the higher efficiency of MEMS transducers based on the material study.

Temperature effects on output power of piezoelectric vibration energy harvesters

Energy harvesting using piezoelectric devices has received considerable attention in the past few years. The most commonly used devices have been cantilevered bimorphs with a large proof mass attached to it. The goal of this paper is to discuss the effects of varying geometry to enhance the average strain through a material via shape optimization into triangular type geometries from a quantitative point of view, while studying the internal strain energy and the stress distribution over the surface when a cantilevered device is loaded. When a triangular cantilever is compared to a rectangular counterpart with the same volume, the stress over its surface is linear, as it has a more constant radius of curvature, and its loading capacity effectively doubles. These concepts are explored numerically using ANSYS. The concept of internal strain energy per unit area over the length span of the beam is used to evaluate the amount of average energy stored in the material over the surface, and shows that regardless of geometry, the value is strictly a function of volume of the device. The linear stress distribution over the length of triangular beams, and their relations with the volume when compared to a rectangular cantilever is the standout property that would allow much more reliable operation of cantilevered brittle piezoelectric ceramic devices.

Analysis of Stress Distribution in Piezoelectric MEMS Energy Harvester Using Shaped Cantilever Structure

Piezoelectric power generators can be effective power suppliers for small devices by collecting energy from ambient vibration and converting it into electrical energy. Since such small devices are usually mobile, their operation environments can vary widely. For example, the conditions such as vibration frequency and temperature energy highly affect harvesting efficiency of piezoelectric power generators. If the ambient vibration frequency is away from the resonance frequency, there will be a decrease of output power. The ambient temperature can be a possible factor to change the resonance frequency of the power generating device since temperature influences material constants of the constituent components in a power generator such as dielectric constant, piezoelectric strain coefficient, and the stiffness Therefore, the effect of temperature on output power is studied. Since the common piezoelectric material is PZT and it is known that hard- or soft-type PZT exhibits different dependence of materials properties as a function of ambient temperature, this paper investigates the output power of the soft and hard PZTbased power generator depending on ambient temperature.

Modeling and evaluation of d33?mode piezoelectric energy harvesters

KNN, (K0.5Na0.5)NbO3 powder was synthesized from raw materials, and the PZT powder was prepared from commercial PZT powder (Alpha ceramics, Inc.). After a ball milling process, each KNN and PZT powder was uniformly deposited on Pt coated alumina plates up to 200µm and 230µm thick films, respectively. Several chemical additives such as PEI dispersant, PVB as polymer binder, glacial acetic acid and tetrumethyl ammonium hydroxide were varied to investigate their roles in the sintering process and the stability of chemical solution for electrophoretic deposition. The effect of additives on the sintering was not significant, but the stability of the solution and uniformity were strongly influenced by additives. Deposited films were sintered ranging from 850°C to 1100°C. The densest film was obtained at a sintering temperature of 1050°C. Excess addition of raw material K2CO3 could eliminate secondary phases in KNN thick films after sintering. Electrical properties of thick films were investigated by characterizing the ferroelectric hysteresis loops, dielectric properties, and piezoelectric properties.