A single-crystal silicon micromachined air turbine supported on gas-lubricated bearings has been operated in a controlled and sustained manner at rotational speeds greater than 1 million revolutions per minute, with mechanical power levels approaching 5 W. The device is formed from a fusion bonded stack of five silicon wafers individually patterned on both sides using deep reactive ion etching (DRIE). It consists of a single stage radial inflow turbine on a 4.2-mm diameter rotor that is supported on externally pressurized hydrostatic journal and thrust bearings. This work presents the design, fabrication, and testing of the first microfabricated rotors to operate at circumferential tip speeds up to 300 m/s, on the order of conventional high performance turbomachinery. Successful operation of this device motivates the use of silicon micromachined high-speed rotating machinery for power microelectromechanical systems (MEMS) applications such as portable energy conversion, micropropulsion, and microfluidic pumping and cooling.

High-speed microfabricated silicon turbomachinery and fluid film bearings

MIT is developing micro-electro-mechanical systems (MEMS)-based gas turbine engines, turbogenerators, and rocket engines. Fabricated in large numbers in parallel using semiconductor manufacturing techniques, these engines-on-a-chip are based on micro-high speed rotating machinery with power densities approaching those of their more familiar, full-sized brethren. The micro-gas turbine is a 2 cm diameter by 3 mm thick Si or SiC heat engine designed to produce about 10 W of electric power or 0.1 N of thrust while consuming about 15 grams/hr of H2. Later versions may produce up to 100 W using hydrocarbon fuels. This paper gives an overview of the project and discusses the challenges faced in the design and manufacture of high speed microrotating machinery. Fluid, structural, bearing, and rotor dynamics design issues are reviewed.

/pdf/shirtbutton-sized-gas-turbines-the-engineering-challenges-of-1m787a5i2n.pdf

Shirtbutton-sized gas turbines: the engineering challenges of micro high speed rotating machinery

A single-crystal silicon air turbine supported on gas lubricated bearings has been operated in a controlled and sustained manner at rotational speeds greater then 1 million rpm and power levels approaching 5 W. The device is a second-generation version of the microbearing rig first reported by Lin et al. (1), and is the first micromachine to operate at circumferential tip speeds of hundreds of meters per second, comparable to conventional scale turbomachinery. To achieve this level of peripheral speed, microfabricated rotors must withstand large induced stresses, need a sufficient power source to drive them, and require stable, low friction bearings for support. This paper focuses on process improvements in the microbearing device fabrication and on the advances in operating micro-gas bearings and micro- turbomachinery. The successful operation of this device motivates the use of this technology for high-power density MEMS.

/pdf/demonstration-of-a-microfabricated-high-speed-turbine-1i2iwg5f9r.pdf

Demonstration of a Microfabricated High-Speed Turbine Supported on Gas Bearings

Numerical prediction of slip flow effect on gas-lubricated journal bearings for MEMS/MST-based micro-rotating machinery

This paper reviews the current state-of-the-art surrounding macro scale machining; it discusses how these factors will influence the future development of micro and nano scale machining. The paper begins by reviewing machining theory, and then discusses high speed machining, tool wear, tool coatings, micromachining, meso machine tool design and future applications and research directions. Tool wear is a factor that determines the economy of the machining process. Therefore, an extensive part of the paper is devoted to the development of materials used to coat tools; in turn, it is anticipated these coatings will used in future micro machining applications. Consideration is also given to machine structures that are required to use these cutting tools at speeds in excess of one million revolutions per minute. This review provides a timely explanation of the literature surrounding the factors required for successful micro and nano machining.

A review of machining theory and tool wear with a view to developing micro and nano machining processes

A macro-scale experimental test facility for investigating high-speed gas bearings for MEMS devices such as the MIT Micro-Engine is presented along with results from subsequent experiments. It is shown that the bearings required by such MEMS devices fall outside the usual range of design parameters for conventional gas lubrication systems. Due to the unorthodox design of the bearings, a new "hybrid" mode of operation is introduced along with the traditional hydrodynamic regime. The new hybrid mode is exploited to implement a novel in-situ rotor balancing scheme which enables hydrodynamic operation. Analysis for both modes of operation is presented along with experimental results. A high-order, efficient scheme for computing both the steady and unsteady hydrodynamic properties of the fully coupled, rotor/gas film dynamical system is presented along with comprehensive calculations for this class of plain, cylindrical, gas journal bearing. The scheme is then used to perform a generalized eigenvalue analysis on the compressible, unsteady system which reveals a new type a hydrodynamic instability. From a fundamental understanding of the bearing physics, strategies for operating MEMS devices with this class of bearing are deduced and minimum requirements for the accompanying measurement systems are established. Ancillary issues such as axial equilibrium of the rotor are discussed in detail. Thesis Supervisor: Prof. Kenneth S. Breuer Title: Visiting Associate Professor

Doyle Jay Orr

Papers

Macro-scale investigation of high speed gas bearings for MEMS devices