Passive device shield with proximity change alert

Abstract

A method of making a fuel for a portable heater comprises heating a salt of iron with a carboxylic acid at a temperature from about 400° C. to about 700° C. under an inert or reducing atmosphere to produce a composite material comprising carbon and iron nanoparticles. The fuel may be mounted to a substrate, such as a mesh or foil, and incorporated into a self-heating packaging. Typically, the packaging comprises a wall defining a chamber that, in use, contains a substance to be heated, e.g. a food-stuff or liquid. A heating manifold in thermal communication with the chamber contains the fuel in the inert atmosphere to initially inhibit production of heat otherwise arising from substantial oxidation of substantially all the fuel in the manifold in an exothermic reaction. A vent in the manifold is initially sealed and air-tight, but selectively opening of the vent allows oxidation of the iron nanoparticles. The chamber is then heated by the transfer of heat from the fuel to the chamber, the transfer of heat being by at least one of direct thermal conduction through the wall, thermal convection or indirect thermal heating arising from heating of a fluid medium that is introduced into the chamber.

Dependencies are inferred from the claim text.