Experimental Modeling of Sterilization Effects for Atmospheric Entry Heating on Microorganisms

Silicon was chosen for sample coupons.

The objective of this research was to design, build, and test an experimental apparatus for studying the parameters of atmospheric entry heating, and the inactivation of temperature-resistant bacterial spores. The apparatus is capable of controlled, rapid heating of sample coupons to temperatures of 200 to 350 ºC and above. The vacuum chamber permits operation under vacuum or special atmospheric gas mixtures.

Experimental Apparatus consists of a vacuum chamber (left) and the stand for the silicon chips (right)." class="caption" align="right">A radiant heating system using tungsten- halogen lamps was chosen to heat the spores to the desired temperatures. This method of heating was preferred because there was no physical contact between the heater and the sample coupons, the radiant heat can be controlled more precisely than heating methods by conduction and convection, and halogen light bulbs are readily available. The design allowed for the bulbs to radiantly heat the backside of the sample coupons, avoiding possible sterilization of the spores by a method other than just heating, such as ultraviolet radiation.

The material chosen for the sample coupons was silicon, due to its favorable properties for this application. Silicon is chemically and biologically inert, and has very high thermal conductivity. Furthermore, silicon has high emissivity in the visible and near-infrared portion of the electromagnetic spectrum, and has a lower emissivity in the mid-infrared range. This means that the silicon coupons are able to absorb a significant portion of the radiation output by the halogen light bulbs, but not re-radiate much midinfrared radiation at the sample temperatures. This unique property of silicon allows for the sample coupons to be heated very quickly and accurately using the radiant heat from the halogen light bulbs. Furthermore, due to the widespread use of silicon in the microelectronics industry, silicon was available in very thin wafers. The low thermal mass of the thin wafers helped them heat up very quickly.

This work was done by Wayne W. Schubert and James A. Spry of Caltech; Paul D. Ronney and Nathan R. Pandian of the University of Southern California; and Eric Welder of Stanford University for NASA’s Jet Propulsion Laboratory. NPO-48091

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