Compound having exponential temperature dependent electrical resistivity, use of such compound in a self-regulating heating element, self-regulating heating element comprising such compound, and method of forming such compound

A compound having exponential temperature dependent electrical resistivity comprising an electrically insulating bulk material (11), electrically conductive particles (12) of a first kind, and electrically conductive particles (13) of a second kind, wherein

• the electrically insulating bulk material holds the electrically conducting particles of the first and second kinds in place in the electrically insulating bulk material;
• the electrically conducting particles of the second kind are smaller than the electrically conducting particles of the first kind;
• the electrically conducting particles of the second kind are more in number than the electrically conducting particles of the first kind;
• the electrically conducting particles of the second kind have higher surface roughness than the electrically conducting particles of the first kind, wherein the electrically conducting particles of the second kind comprise tips (13a) and the electrically conducting particles of the first kind comprise even surface portions (12a);
• the electrically conducting particles of the first and second kinds are arranged to form a plurality of current paths (14) through the compound, wherein each of said current paths comprises galvanically connected electrically conducting particles of the first and second kinds and a gap (14a) between a tip (13a) of one of the electrically conducting particles of the second kind and an even surface portion (12a) of one of the electrically conducting particles of the first kind, which gap is narrow enough to allow electrons to tunnel through the gap via the quantum tunneling effect;
• the electrically insulating bulk material has a thermal expansion capability such that it expands with temperature, thereby increasing the gap widths (w) of the current paths, which in turn increases the electrical resistivity of the compound exponentially;
• and
• said compound comprises a lubricant (21), such as for example a homo-oligomer, e.g. vinylmethoxysiloxane homo-oligomer, wherein the surface of the electrically conducting particles of the second kind are covered by said lubricant.