Patent Number: 051031035
Section: claims

1. A switchable shield for an electromagnetic device which transmits or receives radio frequency or microwave radiation through said shield, which shield is selectively switchable form a first high transmissivity, high resistivity mode to a second low transmissivity, low resistivity mode, which shield comprises: a substrate formed of a material which is highly transmissive of radio frequency and microwave radiation; and  an active film layer disposed on said substrate, which active film layer is switchable upon heating above a selected temperature from said first high transmissivity, high resistivity mode permitting transmission or receipt of radio frequency or microwave radiation by said electromagnetic device, to said second low transmissivity, low resistivity mode precluding transmission or receipt of radio frequency or microwave energy by said electromagnetic device. 2. A switchable shield as described in claim 1, wherein the active film layer is comprised of vanadium oxide. 3. A switchable shield as described in claim 2, wherein the vanadium oxide consists essentially of VO.sub.2. 4. A switchable shield as described in claim 3, wherein the active film layer in the high transmissivity mode is semiconducting. 5. A switchable shield as described in claim 4, further characterized in that the active film layer in the high transmissivity, high resistivity mode has a monoclinic structure. 6. A switchable shield as described in claim 3, further characterized in that the active film layer in the low transmissivity, low resistivity mode is metallic. 7. A switchable shield as described in claim 6, further characterized in that the active film layer in the low transmissivity, low resistivity mode is tetragonal in structure. 8. A switchable shield as described in claim 2, further comprising a doping agent which alters the selected temperature. 9. A switchable shield as described in claim 8, wherein the doping agent is selected from the group consisting of chromium, molybdenum, titanium, manganese, nickel, iron, cobalt, tungsten and phosphorus. 10. A switchable shield as described in claim 1, wherein the active film layer is discontinuous. 11. A switchable shield as described in claim 1, wherein the active film layer has a thickness in the range of 100 .ANG. to 10000 .ANG.. 12. A switchable shield as described in claim 11, wherein the active film layer is further comprised of a plurality of film sublayers, each having a thickness of at least 100 .ANG.. 13. A switchable shield as described in claim 11, wherein the active film layer is comprised of a single layer, having a thickness in the range of 2000 .ANG. to 10000 .ANG.. 14. A switchable shield as described in claim 13, wherein the single active film layer has a thickness of approximately 4000 .ANG. to 8000 .ANG.. 15. A switchable shield as described in claim 1, further characterized in that the active film layer changes from the first mode to the second mode at approximately 68.degree. C. 16. A switchable shield as described in claim 15, wherein the active film layer is changed from the first mode to the second mode by heating with a forced air flow over the active film layer. 17. A switchable shield as described in claim 15, further characterized in that the active film layer is changed from the first mode to the second mode by passing an electric current therethrough to heat the active film layer. 18. A switchable shield as described in claim 1, wherein the active film layer changes from the first mode to the second mode over a narrow temperature range. 19. A switchable shield as described in claim 18, wherein the narrow temperature range is 3.degree. C. 20. A switchable shield as described in claim 1, wherein the substrate layer is selected from the group consisting of glass, quartz, zirconia, corderite, polycrystalline aluminum oxide, aluminosilicates, aluminum nitride, boron nitride, silicon nitride, silicon carbide and sapphire. 21. A switchable shield as described in claim 20, wherein the alumino-silicates are selected from the group consisting of mullite and celsian. 22. A selectably shield as described in claim 1, wherein the substrate layer is selected from the group consisting of glass, quartz, zirconia, corderite, polycrystalline aluminum oxide, aluminosilicates, aluminum nitride, boron nitride, silicon nitride, silicon carbide and sapphire. 23. A switchable shield as described in claim 22, wherein the alumino-silicates are selected from the group consisting of mullite and celsian 24. A selectably intermittent shield as described in claim 1, wherein the substrate layer is formed from a material which is transparent to infrared electromagnetic radiation and is selected from the group consisting of silicon, germanium, calcium fluoride, barium fluoride, zinc sulfide, zinc selenide, diamond and gallium arsenide. 25. A switchable shield as described in claim 1, further characterized in that the resistivity of the active film layer decreases from approximately 10.sup.6 .OMEGA. per square to approximately 1000 .OMEGA. per square when changed from the first mode to the second mode. 26. A switchable shield as described in claim 1, further characterized in that the transmissivity of the active film layer drops from approximately 90% to approximately 0% when changed from the first mode to the second mode.