Recent Research Projects

Fuel Cells and Piezoelectric Transducers

Over the past few decades, there has been rapid technological development in the area of portable electronic devices and systems. Unfortunately, there is a lack of power generation sources with compact size and sufficient power density to drive these systems. The size of power source for an electronic device sometimes occupies one-third or even half of the overall volume of the whole system. Frequent recharge is necessary to maintain the proper function of the system. Obviously, there is a great need to design and fabricate a long-lifetime, high-efficiency power system with compact size to provide the driving power to the portable electronic systems. Some approaches have been attempted in the past few years to address this demand. For example, various nickel metal hydride batteries, lithium-batteries with compact size has been fabricated and commercially available, but their energy densities are relatively low and frequent recharge is needed to maintain the functionality of the electronic devices. Small-scale combustion-driven system has also been investigated, which can operate more efficiently, and with an energy storage density up to ten times that of a battery, but interconnection problems such as noise and layout are introduced when it is used in conjunction with microelectronic devices.

Among all different type of power generation technologies and processes, a power generation system that utilizes electrochemical fuel cell technology to convert the chemical energy of a reaction directly into electrical energy is particularly attractive due to high energy efficiency. The fuel cell based power generation system combines a fuel storage chamber, a compact fuel processor, and a compact fuel cell. This resultant system will simply use hydrocarbon liquid fuel such as methanol to produce enough energy to power portable electronic systems. The process flow chart of the power generation system is schematically shown in Figure 1.

The centerpiece of the system is a fuel cell. Being able to deliver high power density and operate at low temperature, proton exchange membrane (PEM) fuel cell allows for fast startups and immediate response to changes in the demand for power. Together with lightweight, low cost, and compact volume, miniaturized PEM fuel cell is believed to be the ideal power generation source for powering electronic devices.

A single PEM fuel cell (Fig.2) is primarily composed of a proton-conducting membrane surrounded by two porous electrodes (which are impregnated with platinum catalyst), two backing (or diffusion) layers for uniform diffusion of reactants to the electrode/catalyst layer, and two current collector layers (which also provide reactant flow channels). A PEM fuel cell uses hydrogen and air (or oxygen) to operate. Hydrogen comes into contact with the fuel electrode (the anode) and provides a proton. This frees an electron, which passes through a circuit to the air electrode (the cathode). The proton then passes through the membrane with water molecules, and reacts with oxygen and the electron at the cathode. The product of these reactions is water:

Microelectromechanical system (MEMS), or microsystems technology (MST) is a relatively new technology, which exploits the existing microelectronics infrastructure and other microfabrication technologies to create complex micromechanical systems with micron or tens of microns feature sizes. These micromechanical systems can have many functions, including sensing, communication and actuation. The realization of complex micromechanical systems on a chip, and the integration of these micromechanical systems with on-chip control and communication microelectronics enable the creation of intelligent microsystems which know where they are, what is going on around them, and how to interact with the environments to perform impacts. Extensive research activities are currently being conducted in University laboratories, industrial R & D laboratories, and National Laboratories in the United States and other countries to develop new MEMS fabrication technologies, MEMS devices, and look for more applications

MEMS devices (such as microactuators and microsensors) are mostly designed and fabricated based on silicon or semiconductor microfabrication technologies and using various transduction mechanisms. Piezoelectric materials, such as zinc oxide (ZnO) and lead zirconate titanate (PZT), can provide a direct transformation between electrical and mechanical energy, are highly desirable to be utilized in the design and fabrication of MEMS devices for a variety of applications. The attractive features of piezoelectric materials include large achievable output forces, short response times, a high sensitivity and a low noise level. The opportunities for piezoelectric materials in MEMS are tremendous, especially in applications where ultrasound frequencies, acoustic waves, combined sensing and actuation (transducer), fast and precise actuation, and resonant sensing are involved or required. It is expected that with the unique electromechanical sensing and actuation properties, piezoelectric films can be integrated with silicon microfabrication for many practical applications such as piezoelectric inkjet printheads, piezoelectric underwater acoustic imaging sensor arrays, piezoelectric microphones, micro-pumps, micro valves, etc.

Thin film microactuator and microsensors based on piezoelectric PZT materials have actually been demonstrated over the past few years. In most cases, PZT thin films were fabricated by sol-gel processing. The properties of PZT thin films show smaller effective piezoelectric coefficients than do bulk ceramics of the same composition (i.e., d33 ~ 70-140 pm/V for PZT films where undoped ceramic PZT show a d33 ~ 220 pm/V). Nonetheless, this represents a significant step up from ZnO-based devices. Several types of MEMS devices using PZT films have also been demonstrated, including micromotors (Udayakumar et al., 1992; Muralt et al., 1995; Flynn et al., 1992), micro-pumps (Polla, 1995), underwater acoustic imaging devices (Bernstern et al., 1996), AFM components (Fujii and Watanabe, 1996), and meanderline actuators (Polla and Francis, 1996).

Research efforts in my Lab on piezoelectric MEMS include piezoelectric micro pumps for precise drug delivery, piezoelectric micro power generation devices, piezoelectric micro valves for fuel control in micro fuel cell systems, etc. The heart of these researches is the fabrication and integration of the piezoelectric materials-on-Si micro-devices in the MEMS systems, especially, the design, microfabrication and characterization of thin film piezoelectric materials and devices.