Jung-Hyun Park

The design, fabrication and evaluation of a MEMS PZT cantilever with an integrated Si proof mass for vibration energy harvesting

A microelectromechanical system (MEMS) piezoelectric energy harvesting device, a unimorph PZT cantilever with an integrated Si proof mass, was designed for low vibration frequency and high vibration amplitude environment. Pt/PZT/Pt/Ti/SiO2 multilayered films were deposited on a Si substrate and then the cantilever was patterned and released by inductively coupled plasma reactive ion etching. The fabricated device, with a beam dimension of about 4.800 mm × 0.400 mm × 0.036 mm and an integrated Si mass dimension of about 1.360 mm × 0.940 mm × 0.456 mm produced 160 mVpk, 2.15 μW or 3272 μW cm−3 with an optimal resistive load of 6 kΩ from 2g (g = 9.81 m s−2) acceleration at its resonant frequency of 461.15 Hz. This device was compared with other demonstrated MEMS power generators.

Electrical transport properties and defect structure of SrFeCo{sub 0.5}O{sub x}

Mixed-oxide conductors find wide application in high-temperature, solid-state electrochemical devices such as solid-oxide fuel cells, batteries, and sensors. By using a gastight electrochemical cell with flowing air as the reference environment, the authors were able to achieve an oxygen partial pressure (P{sub O{sub 2}}) and temperature. It was found that in the high-P{sub O{sub 2}} range, interstitial oxygen ions and electron holes are the dominant charge carriers, while in the low-P{sub O{sub 2}} range, oxygen vacancies and electrons are dominant. At 800 C in air, total conductivity and ionic conductivity of SrFeCo{sub 0.5}O{sub x} are 17 and 7 S cm{sup {minus}1}, respectively, and the ionic transference number is 0.4. A semiconductor-metal-semiconductor transition is found in this system in a reduced-oxygen environment. Defect dynamics in this system can be understood by means of the trivalence-to-trivalence transition of Fe ions when P{sub O{sub 2}} is reduced. A defect model has been proposed. By using the conductivity results, the authors were able to estimate oxygen permeation through a ceramic membrane made of Sr FeCo{sub 0.5}O{sub x}. The oxygen permeability they calculated is consistent with that measured at the conversion reactor. To confirm the ionic transference number measured by electron-blocking method, electromotive force measurementmorexa0» was carried out and obtained consistent results.«xa0less

Determination of chemical diffusion coefficient of SrFeCo0.5Ox by the conductivity relaxation method

A conductivity relaxation experiment has been conducted on an SrFeCo0.5Ox sample by abruptly changing the oxygen partial pressure in the atmosphere and monitoring the change of conductivity as a function of time. The re-equilibrium process obeys Ficks second law. By fitting the relaxation data to the solution of the diffusion equation with appropriate boundary conditions, we could determine the oxygen chemical diffusion coefficient and the activation energy. The oxygen diffusion coefficient is 8.9 × 10−7 cm2/s at 900 °C and it increases with increase in temperature. Measured activation energy is 0.92 eV, which is slightly lower than that of other oxides in the system SrFe1 − xCoxOy.

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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Abstract The formation of etch pits during heteroepitaxial growth of 3Cue5f8SiC on silicon is a common occurrence. A comparison of the morphology of 3Cue5f8SiC epilayers grown on silicon at both atmospheric and low pressures shows that the problem is more severe when the epilayers are grown at low reactor gas pressure. It is found that the density of etch pits is affected by the condition of the susceptor for both atmospheric and low pressure processes. At low pressure, the composition of the process gas during buffer layer growth plays an important role in suppressing the formation of etch pits. The nature of etch pits and various attempts to suppress the formation of etch pits are discussed.

d_{33}

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.

Modes for Vibration Energy Harvesting

Oxides in the system Sr-Fe-Co-O exhibit both electronic and ionic conductivities. Recently, Sr-Fe-Co-O system attracted great attention because of the potential to be used for oxygen permeable membranes that can operate without the electrodes or external electrical circuitry. Electronic and ionic conductivities at various temperatures have been measured on two compositions in Sr-Fe-Co-O system named SFC-1 and SFC-2. The electronic transference number is much greater than the ionic transference number in SFC-1 sample, while the electronic and ionic transference numbers are very close in SFC-2 sample. At 800{degrees}C, the electronic conductivity and ionic conductivity are {approx}76 S{center_dot}cm-1 and =4 S-cm-1, respectively, for SFC-1. While, for SFC-2, the electronic and ionic conductivities are =10 S-cm-1 and {approx}7 S-cm-1, respectively. By a local fitting to {sigma}{center_dot}T = A exp(-E{sub {alpha}}/{kappa}{Tau}), we found that the oxide ion activation energies are 0.92 eV and 0.37 eV respectively for SFC-1 and SFC-2 samples. Oxygen diffusion coefficient of SFC-2 is {approx}{times}10{sup {minus}7} cm{sup 2}/sec at 900C.

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

The conductivity and weight changes of SrFeCo{sub 0.5}O{sub x} in various different oxygen partial pressures (p{sub O{sub 2}}) were measured simultaneously by the four-point dc method and a thermogravimetric analysis apparatus, respectively. The apparent oxygen diffusion coefficients were derived from the weight and conductivity relaxation experiments by abruptly changing the p{sub O{sub 2}} in the surrounding atmosphere and monitoring the change of weight and conductivity as functions of relaxation time. At 950 C in air, the obtained apparent oxygen diffusion coefficient of SrFeCo{sub 0.5}O{sub x} is {approx} 1 {times} 10{sup {minus}6} cm{sup 2}/s. Conductivity increases while the weight decreases with increasing temperature within the experimental p{sub o{sub 2}} range (1 {ge} p{sub O{sub 2}} {ge} 10{sup {minus}18} atm), and the activation energy decreases while conductivity increases with increasing p{sub O{sub 2}}. The p{sub O{sub 2}}-dependent conducting behavior can be understood by considering the trivalent-to-divalent transition of transition metal ions in the sample. The oxygen stoichiometry of SrFeCo{sub 0.5}O{sub x} in air is x = 3 at 950 C.

Reduction of etch pits in heteroepitaxial growth of 3CSiC on silicon

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.