Thermochromic VO2 sputtered by control of a vanadium-oxygen emission ratio

The objective of this work is to deposit and characterize coatings containing primarily the VO2 phase. This phase has a thermochromic semiconductor to metal transition at 68 degreesC. The VO2 thus becomes more reflective and conductive above the transition temperature. One application uses the change in infrared optical properties for thermal switches near room temperature. Precise stoichiometry is required to deposit VO2 coatings because of competition from other oxide phases. To achieve this stoichiometric control we deposited VO2 by controlling a plasma emission ratio of vanadium and oxygen. The emission ratio was processed in real time for feedback control. The feedback involved increasing or decreasing the oxygen flow to maintain a desired ratio. Direct current reactive magnetron sputtering was used with a constant current power supply. We varied the deposition temperature between 350 and 650 degreesC and the amount of oxygen injected into the system from 3.0-3.8 seem. X-Ray, resistance and reflectance measurements verified that the majority phase of the coatings was VO2. The resistance results showed a change of about three orders of magnitude due to the semiconductor to metal transition. The reflectance results showed emittance changes in the infrared between 61.2 and 90%, so this is an excellent material for a thermal switch. Space simulation exposures showed that these films could withstand between 3.7 and 37.2 years in space without forming other phases but that their emittance would degrade during this time. (C) 2001 Elsevier Science B.V All rights reserved.