CO Gas Sensors

CATALYTIC FLAMMABLE GAS SENSORS

The concept of the pellistor is based on the fact that the most foolproof way to determine whether a flammable gas is present in air is to test a sample by trying to burn it. A pellistor consists of a very fine coil of wire suspended between two posts. The coil is embedded in a pellet of a ceramic material, and on the surface of the pellet (or 'bead') there is a special catalyst layer.
 

ELECTROCHEMICAL SENSOR TECHNOLOGY

The carbon monoxide sensor consists of three electrodes immersed in a liquid electrolyte (a non-metallic liquid that conducts electricity, usually through acids or dissolved salts). The three electrodes are the working electrode, the reference electrode, and the counter electrode. The most important of these is the working electrode (WE). The working electrode is made of platinum, which is a catalytic metal to CO (it catalyzes the oxidation of CO to CO2), backed by a gas-permeable but hydrophobic (water-proof) membrane. The CO gas diffuses through the porous membrane and is electrochemically oxidized (Equation 1).

CO + H2O à CO2 + 2H+ + 2e   -------------------------------------------------------(Eq.1) 

The electrons involved in the electrochemical reaction flow from the working electrode through the external circuit, producing the output signal of the sensor. 
 

SEMICONDUCTOR SENSOR’S THEORY OF OPERATION

One of the important properties of a semiconductor is the concentration of charge carriers. As semiconductors in general contain relatively few free charge carriers this facilitates control of their behaviour and concentration by external means. In a clean semiconductor, negative free electrons and positive free holes are present in equal numbers. These electrons and holes are created by the thermal excitation of valence electrons from the valence states, in the crystal, to the conduction band leaving behind positive holes in the valence band. (See Fig1.)

 

This is true for a so-called ‘intrinsic’ semiconductor, for a pure oxide system, and is equally true for Silicon or Germanium. For most commercial applications of semiconductor materials it is necessary to add a dopant, which supplies an excess of holes or electrons that form the majority current carrier, i.e. p or n type semiconductors. For the Chrome Titanium oxide system, Chrome oxide is the intrinsic semiconductor, which, with a valency state of 3+, when doped with Titanium, of valency 4+, now becomes a p-type semiconductor. The Titanium requires an extra electron, which it takes from the Chrome d-band leaving a hole.
The electron it takes is trapped by an oxygen atom forming TiO2.Therefore Oxygen adsorbed at the gas–solid interface removes electrons from the surface region of the solid to form a surface oxygen ion; the oxygen can be thought of as a surface trap for electrons from the Chrome oxide d-band (see Fig 2).

 

Therefore the adsorbed oxygen gives rise to an increase in the hole concentration,
and it follows that any decrease in the surface coverage of Oxygen ions by reacting with, for example, CO to form CO2 would release electrons back into the lower band and decrease the hole concentration and hence lead to an increase in the resistance of the semiconductor material.