Hanqing Li

A piezoelectric microvalve for compact high-frequency, high-differential pressure hydraulic micropumping systems

A piezoelectrically driven hydraulic amplification microvalve for use in compact high-performance hydraulic pumping systems was designed, fabricated, and experimentally characterized. High-frequency, high-force actuation capabilities were enabled through the incorporation of bulk piezoelectric material elements beneath a micromachined annular tethered-piston structure. Large valve stroke at the microscale was achieved with an hydraulic amplification mechanism that amplified (40/spl times/-50/spl times/) the limited stroke of the piezoelectric material into a significantly larger motion of a micromachined valve membrane with attached valve cap. These design features enabled the valve to meet simultaneously a set of high frequency (/spl ges/1 kHz), high pressure(/spl ges/300 kPa), and large stroke (20-30 /spl mu/m) requirements not previously satisfied by other hydraulic flow regulation microvalves. This paper details the design, modeling, fabrication, assembly, and experimental characterization of this valve device. Fabrication challenges are detailed.

Micro hydraulic transducer technology for actuation and power generation

The paper introduces a novel transducer technology, called the solid-state micro-hydraulic transducer, currently under development at MIT. The new technology is enabled through integration of micromachining technology, piezoelectrics, and microhydraulic concepts. These micro-hydraulic transducers are capable of bi-directional electromechanical energy conversion, i.e., they can operate as both an actuator that supplies high mechanical force in response to electrical input and an energy generator that transduces electrical energy from mechanical energy in the environment. These transducers are capable of transducing energy at very high specific power output in the order of 1 kW/kg, and thus, they have the potential to enable many novel applications. The concept, the design, and the potential applications of the transducers are presented. Present efforts towards the development of these transducers, and the challenges involved therein, are also discussed.

Fabrication and Testing of a High-Speed Microscale Turbocharger

A microelectromechanical system (MEMS) turbocharger has been designed, fabricated, and tested as part of a Massachusetts Institute of Technology program aimed at producing a microfabricated gas turbine engine for portable power applications. A gas turbine engine requires high-speed high-efficiency turbomachinery operating at tip speeds of several hundred meters per second. This MEMS turbocharger serves to demonstrate these requirements. The turbochargers silicon rotor, which is supported on hydrostatic gas thrust and journal bearings in a silicon stator housing, was spun to 480 000 rpm, corresponding to a tip speed of 200 m/s. This paper discusses critical fabrication processes that enabled the capabilities of this device. Operational issues and test results are also presented. The turbochargers compressor demonstrated a pressure ratio of 1.21 at a mass flow rate of 0.13 g/s, with a combined compressor-turbine spool efficiency of 0.24. Under these conditions, the turbine produced about 5 W of power. Results from the simultaneous operation of a spinning rotor and burning combustor within the microscale turbocharger are also presented. Experimental results compare well with analytical models and computations.

Design, fabrication, and testing of a piezoelectrically driven high flow rate micro-pump

Towards the development of novel class of miniature transducers with very high specific power, a high frequency and high flow rate hydraulic micro-pump with passive check valves was fabricated and tested. The micro-pump features a small piezoelectric (PZT-5H/PZN-PT) cylinder integrated into micromachined silicon and pyrex chips. The piezoelectric cylinder bonded to a thick circular disk serves as the drive element in the pump chamber, and two axisymmetric silicon membranes with hollow annular bosses serve as system check valves. Using silicone oil as the working fluid, the performance of the micro-pump was tested by varying voltages from 0 to 1600 V and frequencies from 1 kHz to 12 kHz. The micro-pump with PZT-5H element achieved a high flow rate of 2.5 ml/min at 1200 V and 4.5 kHz.

A single-stage micromachined vacuum pump achieving 164 torr absolute pressure

We are developing a micromachined vacuum pump for portable analytical instruments that is comprised of a mechanical roughing pump integrated with a micro-machined ion pump. The focus of this paper is on the development of the roughing pump. We have fabricated two generations of micromachined displacement vacuum pumps to explore various design options. Through optimization of design and operation, we can now report a pump that achieves 164 torr absolute pressure, which is to our knowledge the lowest measured pressure in a micro-machined vacuum pump operated from atmospheric pressure.

High-Speed Operation of a Gas-Bearing Supported MEMS-Air Turbine

Silicon based power MEMS applications require the high-speed micro-rotating machinery to operate stably over a large range of operating conditions. The technical barriers to achieve stable high-speed operation using micro-gas-bearings are governed by: (1) stringent fabrication tolerance requirements and manufacturing repeatability, (2) structural integrity of the silicon rotors, (3) rotordynamic coupling effects due to leakage flows, (4) bearing losses and power requirements, and (5) transcritical operation and whirl instability issues. Over the past few years, a large body of research was conducted at MIT to address these technical challenges; many lessons were learned and new theories were developed related to the dynamic behavior of micro-gas journal and thrust bearings. Based on the above mentioned experience, a gas-bearing supported micro-air turbine was developed with the objectives of demonstrating repeatable, stable high-speed gas-bearing operation and verifying the new micro-gas-bearing analytical models. The key challenge in this endeavor involved the synthesis and integration of the newly-developed gas-bearing theories and insight gained from extensive experimental work. The focus of this paper is on the process and the outcomes of this synthesis, rather than the details and results of the underlying theoretical models which have been previously published. The characteristics of the new micro-air turbine include a four-chamber journal bearing feed system to introduce stiffness anisotropy, labyrinth seals to avoid rotordynamic coupling effects of leakage flows, a reinforced thrust bearing structural design, a redesigned turbine rotor to increase power, a symmetric feed system to avoid flow and force non-uniformity, and a new rotor micro-fabrication methodology. A large number of test devices were successfully manufactured demonstrating repeatable bearing geometry. More specifically, three sets of devices with different journal bearing clearances were produced to investigate the dynamic behavior as a function of bearing geometry. Experiments were conducted to characterize the “as fabricated” bearing geometry, the damping ratio, and the natural frequencies. Repeatable high-speed bearing operation was demonstrated using isotropic and anisotropic bearing settings reaching whirl ratios between 20 and 40. A rotor speed of 1.7 million rpm (equivalent to 370 m/s blade tip speed or a bearing DN of 7 million mm-rpm) was achieved demonstrating the feasibility of MEMS based micro-scale rotating machinery and validating key aspects of the micro-gas-bearing theory.Copyright

Design of a piezoelectrically-driven hydraulic amplification microvalve for high pressure, high frequency applications

This paper reports the design of a piezoelectrically-driven microfabricated valve for high frequency control of large pressure fluid flows. The enabling concept of the valve is the ability to convert the small displacement of a piezoelectric element into a large valve cap stroke through the use of a hydraulic fluid, while maintaining high force capability. The current valve design, with operating frequency of 24 kHz and valve stroke of 40 micrometer, has been tailored for use in microhydraulic actuation and energy-harvesting devices, which require high-frequency regulation of approximately 1 ml/sec fluid flows across pressure differentials of 1-2 MPa.

Silver nanoparticle structures realized by digital surface micromachining

We report a new surface micromachining process using commercial silver nanoparticle inks and digital fabrication methods. This process is entirely digital (non-lithographic patterning), the feature sizes can be ≪20µm, and the maximum temperature is less than 250°C. Furthermore, we have completed materials property characterization in order to enable design in this process. As a demonstration, silver cantilevers were fabricated using this process and subsequently characterized to determine the silver films mechanical properties.

Fabrication of a microvalve with piezoelectric actuation

The fabrication of an active MEMS micro valve driven by integrated bulk single crystal piezoelectric actuators is reported. The valve is a nine-layer structure of glass, Si, and silicon on insulator (SOI) assembled by wafer level fusion bonding and anodic bonding, die level anodic bonding and eutectic bonding. Valve head strokes as large as 20/spl mu/m were realized through hydraulic amplification of small strokes of piezoelectric actuators. A flow rate of 0.21ml/s was obtained at 1 kHz. The fabrication, bonding and assembly processes, as well as some test results are described.

A High-Frequency, High-Stiffness Piezoelectric Micro-Actuator for Hydraulic Applications

A piezoelectric micro-actuator capable of high stiffness actuation in micro-hydraulic systems was fabricated and experimentally tested to frequencies in excess of 100 kHz. The actuator was fabricated from a bonded stack of micromachined silicon-on-insulator (SOI) and borosilicate glass layers. Actuation was provided by 1mm sized piezoelectric cylinders, which were integrated within a tethered piston structure and electrically and mechanically attached using a thin-film AuSn eutectic bond. Die-level anodic bonding techniques were developed to assemble the supporting structural silicon and glass layers. The microfabrication, device assembly, experimental testing procedures, and actuator performance are discussed in this paper. Issues such as piezoelectric material preparation, requisite dimensional tolerancing, micromachining of the silicon tethered structures, and integration of multiple piezoelectric elements within the micro-actuator structure are detailed.