Monday, May 30, 2011

Carbon Nanotube Nanopaper Window Reduces Aircraft Visibility To Radar

BAE Systems PLC (London, GB) reveals, in U.S. Patent 7,948,674, an electro-optic window that is made of a material substantially transparent to infra-red radiation and is treated to have reduced RF transmission characteristics by the provision of carbon nanotubes within the window.

There are many applications where an infra-red transparent window is positioned in the optical path of electro-optic equipment to protect the equipment during use. Where such windows are used on military vehicles, they can give unwanted radar returns if they pass a substantial proportion of any incident RF transmission, and particularly in the microwave band, which is understood to refer to radiation in the waveband of from 2-18 GHz. This places stringent design criteria on the window so that it transmits in the infra-red and preferably also the visible spectral bands but does not transmit microwaves.

The term `window` is used broadly to mean an element capable of transmitting radiation in the optical and/or infra-red wavebands with or without optical power and so includes lenses as well as refractive and diffractive elements generally. The term `transparent` is used to mean that the window transmits at least 90% at the mid value of the infra-red and/or optical wavebands.

The invention resides in the concept of using the electromagnetic properties of carbon nanotubes in electro-optic windows, and in the methods of incorporating carbon nanotubes within window structures.

Inventors Michael Dunleavy, Sajad Haq, Joseph Maurice Davies and Harry John Finn found that the carbon nanotubes, being relatively inert, do not react adversely with the window material. Also the use of carbon nanotubes allows an electro-optic window to be produced having excellent IR and visible spectra transmission characteristics whilst providing a suitably low level of RF transmission.

The nanotubes can be distributed in or on the window in the form of a coating or a layer, or as a dispersion within the bulk material itself. Where applied as a layer to one side of the window, the thickness of the layer required may be much less than that of the metal used previously and therefore have a reduced deflecting effect on the water droplets or sand particles and therefore reducing water or sand abrasion.

The windows made using nanopaper can be combined with a transparent paint. Such a paint could be applied either before or after the nanopaper has been deposited and shaped on the window. Alternatively the nanopaper could be soaked in the paint prior to being attached to the window.

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