A Micro-Insulation Concept for MEMS Applications

Small scale, thermally driven power sources will require appropriate insulation to achieve sufficiently high thermal conversion efficiencies. This paper presents a micro-insulation design, which was developed for a thermionic microbattery, which converts the decay heat from radioactive isotopes directly to electricity using a vacuum thermionic diode. The insulation concept, which is suitable for any small scale application, separates two planar surfaces with thin, semicircular posts, thus reducing conduction heat transfer and increasing the relative radiation heat transfer. In this case, the surfaces are silicon wafers and the columns are SU-8, a photoresist material. The experimental results indicate that this design is adequate for a practical power source concept, and they are supported by a numerical model for the effective thermal conductivity of the structure. The results show that a typical design of $20columns/cm2$ with a $200μm$ diameter and a $10μm$ wall thickness has an apparent thermal conductivity on the order of $10−4W/mK$ at a pressure of 1 Pa. System models of a thermionic power source indicate that this is sufficiently low to provide practical efficiency.