Creep characterization of single crystal silicon in support of the MIT Micro-Engine Project

Abstract

In 1995, a program was initiated at the Massachusetts Institute of Technology (MIT) to develop the technology to design and manufacture a micro-gas turbine generator. The objective of the research is to demonstrate a full-scale, functional micro-gas turbine generator by the year 2000. To meet this milestone, the first engine to test will be constructed of single-crystal silicon. Due to its low brittle-to-ductile transition temperature, silicon is not ideal for a production engine. However, the microfabrication technology for silicon is far more advanced than other candidate materials making it the best short-term option. In support of the micro-engine project, this thesis investigates the high temperature creep capability of silicon. The goal of the investigation is to develop a creep relationship upon which the demonstration engine can be designed. The proposed micro-turbine will initially be designed for a power generation application. With a 20 mm diameter and 4 mm axial length, the micro-engine should be able to generate electrical power at over 10 times the power density of a comparably sized battery. Other applications for this technology have been identified, including; propulsion for small unmanned air vehicles, boundary layer control, a micro-turbine expander for miniature cooling system: a turbo pump for micro-rocket engines, and a micro-motor driven pump or compressor for fluid handling applications. To characterize the creep behavior, representative samples of single-crystal silicon were tested in bending using a silicon carbide 4-point bend fixture. For a given applied stress, tests were run at several temperatures between 600 and 850°C. The creep of specimens in this temperature range was dominated by localized deformation at the inner loading point. Etching of the specimens revealed a high density of slip bands on the < 111 > planes at these locations. The overall specimen deformation proceeded via the formation of "plastic hinges" at the inner loading points. The localized nature of the creep response is not amenable to the application of standard continuum power law models, however an empirical LarsonMiller fit was applied in order to provide some guidance for micro-engine design. Finally, it is important to note the implications that the observed specimen plastic collapse has for the design of the micro-engine. The strain-softening characteristic of yielding silicon must be accounted for in the detailed design. Improper treatment of localized stress concentrations could result in premature engine failure.