Ruichao Xu

Screen printed PZT/PZT thick film bimorph MEMS cantilever device for vibration energy harvesting

We present a MEMS-based PZT/PZT thick film bimorph vibration energy harvester with an integrated silicon proof mass. The most common piezoelectric energy harvesting devices utilize a cantilever beam of a non piezoelectric material as support beneath or in-between the piezoelectric material. It provides mechanical support but it also reduces the power output. Our device replaces the support with another layer of the piezoelectric material, and with the absence of an inactive mechanical support all of the stresses induced by the vibrations will be harvested by the active piezoelectric elements.

MEMS-based thick film PZT vibrational energy harvester

We present a MEMS-based unimorph silicon/PZT thick film vibrational energy harvester with an integrated proof mass. We have developed a process that allows fabrication of high performance silicon based energy harvesters with a yield higher than 90%. The process comprises a KOH etch using a mechanical front side protection of an SOI wafer with screen printed PZT thick film. The fabricated harvester device produces 14.0 µW with an optimal resistive load of 100 kΩ from 1g (g=9.81 m s−2) input acceleration at its resonant frequency of 235 Hz.

Fabrication and characterization of MEMS-based PZT/PZT bimorph thick film vibration energy harvesters

We describe the fabrication and characterization of a significantly improved version of a microelectromechanical system-based PZT/PZT thick film bimorph vibration energy harvester with an integrated silicon proof mass; the harvester is fabricated in a fully monolithic process. The main advantage of bimorph vibration energy harvesters is that strain energy is not lost in mechanical support materials since only Pb(ZrxTi1-x)O3 (PZT) is strained; as a result, the effective system coupling coefficient is increased, and thus a potential for significantly higher output power is released. In addition, when the two layers are connected in series, the output voltage is increased, and as a result the relative power loss in the necessary rectifying circuit is reduced. We describe an improved process scheme for the energy harvester, which resulted in a robust fabrication process with a record high fabrication yield of 98%. The robust fabrication process allowed a high pressure treatment of the screen printed PZT thick films prior to sintering. The high pressure treatment improved the PZT thick film performance and increased the harvester power output to 37.1 ?W at 1 g root mean square acceleration. We also characterize the harvester performance when only one of the PZT layers is used while the other is left open or short circuit.

Cost-effective screen printed linear arrays for medical imaging fabricated using PZT thick films

Screen- and pad-printed single-element ultrasonic transducers have been successfully commercialized over the recent years. Typically, PZT (Lead Zirconate Titanate) thick films are pad- or screen-printed on a curved ceramic substrate acting as integrated backing layer and providing mechanical pre-focus. The center frequency ranges between 8 MHz and 80 MHz. The devices are characterized by good sensitivity as well as high relative bandwidth. The main objective of the presented work has been to apply a similar technology to manufacture multi-element transducers enabling novel cost-effective fabrication of imaging arrays for medical applications. The thick film arrays have been integration-tested using a commercial ultrasound scanner (BK Ultrasound bk3000). The integration test revealed that the 32-element thick film transducers are compatible with a commercial scanner, have a frequency range of 7.5 MHz to 12 MHz, and a TX bandwidth of 70%. Moreover, the transducers support linear array beamforming as well as phased array beamforming. Here 32-element transducers are presented, however the technology can easily be extended to fabrication of transducers with 128 or more elements.

Impedance Based Characterization of a High-Coupled Screen Printed PZT Thick Film Unimorph Energy Harvester

The single degree of freedom mass-spring-damper system is the most common approach for deriving a full electromechanical model for the piezoelectric vibration energy harvester. In this paper, we revisit this standard electromechanical model by focusing on the impedance of the piezoelectric device. This approach leads to simple closed form expressions for peak power frequency, optimal load, and output power without a tedious mathematical derivative approach. The closed form expressions are validated against the exact numerical solution. The electromechanical model contains a set of only five lumped parameters which, by means of the piezoelectric impedance expression, all can be determined accurately by electrical measurements. It is shown how four of five lumped parameters can be determined from a single impedance measurement scan, considerably reducing the characterization effort. The remaining parameter is determined from shaker measurements, and a highly accurate agreement is found between model and measurements on a unimorph MEMS-based screen printed PZT harvester. With a high coupling term K2 Q ≃ 7, the harvester exhibits two optimum load points. The peak power performance of the harvester was measured to 11.7 nW at an acceleration of 10 mg with a load of 9 kQ at 496.3 Hz corresponding to 117 μW/g2.

Characterization of Kerfless Linear Arrays Based on PZT Thick Film

Multielement transducers enabling novel cost-effective fabrication of imaging arrays for medical applications have been presented earlier. Due to the favorable low lateral coupling of the screen-printed PZT, the elements can be defined by the top electrode pattern only, leading to a kerfless design with low crosstalk between the elements. The thick-film-based linear arrays have proved to be compatible with a commercial ultrasonic scanner and to support linear array beamforming as well as phased array beamforming. The main objective of the presented work is to investigate the performance of the devices at the transducer level by extensive measurements of the test structures. The arrays have been characterized by several different measurement techniques. First, electrical impedance measurements on several elements in air and liquid have been conducted in order to support material parameter identification using the Krimholtz–Leedom–Matthaei model. It has been found that electromechanical coupling is at the level of 35%. The arrays have also been characterized by a pulse-echo system. The measured sensitivity is around −60 dB, and the fractional bandwidth is close to 60%, while the center frequency is about 12 MHz over the whole array. Finally, laser interferometry measurements have been conducted indicating very good displacement level as well as pressure. The in-depth characterization of the array structure has given insight into the performance parameters for the array based on PZT thick film, and the obtained information will be used to optimize the key parameters for the next generation of cost-effective arrays based on piezoelectric thick film.

Characterization of linear array based on PZT thick film

Multi-element transducers enabling novel cost-effective fabrication of imaging arrays for medical applications have been presented earlier. Due to favorable low lateral coupling of the printed PZT, the elements can be defined by the top electrode pattern, leading to a kerf-less design with low cross-talk between the elements. The linear arrays have proved to be compatible with a commercial ultrasonic scanner and to support linear array beamforming as well as phased array beamforming. The main objective of the presented work is to investigate the performance of the devices at the transducer level by extensive measurements of the prototype structures. The arrays have been characterized at the transducer level by several different measurement techniques. Firstly, electrical impedance measurements on several elements in air and liquid have been conducted. The arrays have also been characterized by a pulse-echo system. The measured sensitivity is around -60 dB, the fractional bandwidth is close to 60%, while the center frequency is about 12 MHz over the whole array. Finally, laser interferometry measurements have been conducted. The in-depth characterization of the array structure have given insight into the performance parameters for the array based on PZT thick film and the obtained information will be used to optimize the key parameters for the next generation of arrays based on piezoelectric thick film.