Workfunction based A/F sensor

An electrical sensing structure having a surface material, for sensing a stoichiometric ratio of an oxidizing and a reducing species in an ambient gas phase including means for measuring a reversible change that occurs at this stoichiometric ratio in a workfunction of said surface material which are alternately exposed to, and interact with, the oxidizing and reducing gaseous species;

• and wherein said surface material includes:
• a thermionically emitting surface and supporting structure including a catalyst for promoting a reaction between the oxidizing and reducing species;
• said emitting surface being such that when the amount of the oxidizing species exceeds the stoichiometric ratio with respect to the reducing species in the gas phase, the oxidizing species is absorbed from the gas phase onto said surface in a manner such that a workfunction of said surface is increased as a result of the adsorption;
• said emitting surface also being such that when the amount of the reducing species exceeds the stoichiometric ratio with respect to the oxidizing species in the gas phase, the reducing species will react with and remove oxidizing species adsorbed on said emitting surface thereby reducing the workfunction of that surface.
• said emitting surface and supporting structure containing a dopant for the thermionic emission of said dopant as a positive ion from that surface into the surrounding ambient;
• an emitter electrode including said emitting surface supporting structure and said dopant, and being one in which the rate of thermionic emission of the dopant reversibly increases/decreases with an increase/decrease in the workfunction of said emitting surface;
• a collector electrode electrically insulated from said emitter electrode and placed so that it can electrically collect the ions emitted from said emitter electrode;
• said collector electrode being spaced from said emitter electrode and so designed so that the ambient gas phase to the sensed can establish itself within a space between said collector and emitter electrodes and be exposed to said emitter electrode;
• a first circuit means coupled to said emitter and collector electrodes to provide a potential difference between said emitter and collector electrodes such that said emitter electrode is at a higher positive potential than said collector electrode and including a current sensing means which has an electrical output proportional to the magnitude of the measured thermionic current;
• and
• a heating element adjacent to said emitter electrode for raising said emitter electrode temperature to a high enough value to allow thermionic emission;
• and
• wherein said dopant is an alkali metal.

An electrical sensing structure for sensing the stoichiometric ratio of an oxidizing and a reducing species in an ambient gas phase including:

• a thermionically emitting surface and supporting structure including a catalyst for promoting a reaction between the oxidizing and reducing species;
• said emitting surface being such that when the oxidizing species exceeds a stoichiometric amount with respect to the reducing species in the gas phase, the oxidizing species is adsorbed from the gas phase onto said surface in a manner such that a workfunction of said surface is increased as a result of the adsorption;
• said emitting surface also being such that when the reducing species exceeds a stoichiometric amount with respect to the oxidizing species in the gas phase, the reducing species will react with and remove oxidizing species adsorbed on said surface thereby reducing the workfunction of said surface;
• said emitting surface and supporting structure containing an alkali metal dopant for the thermionic emission of said dopant as a positive ion from said surface into the surrounding ambient;
• an emitter electrode of a platinum material including said emitting surface supporting structure and said dopant and being one in which the rate of thermionic emission of said dopant reversibly increases/decreases with an increase/decrease in the workfunction of said emitting surface;
• a collector electrode electrically insulated from said emitter electrode and placed so that it can electrically collect the ions emitted from said emitter electrode;
• said collector electrode being spaced from said emitter electrode and so designed so that the ambient gas phase to be sensed can establish itself within a space between said collector and emitter electrodes and be exposed to said emitter electrode;
• a first circuit means coupled to said emitter and collector electrodes to provide a potential difference between said emitter and collector electrodes such that said emitter electrode is at a higher positive potential than said collector electrode and including a current sensing means which has an electrical output proportional to the magnitude of the measured thermionic current;
• and
• a ceramic encapsulated heating element adjacent said emitter electrode for raising the temperature of said emitter electrode to a high enough value to allow thermionic emission.