NDIR against Metal Oxide Films in CO2 Measurement
NDIR sensors provide definitive CO2 accuracy through direct mid-IR light absorption, while low-cost metal oxide films infer CO2 via prone cross-reactive surface chemistry.
Surface Chemisorption
Bonding Process
Chemical bond formation between gas molecules and a solid surface involves the exchange or sharing of electrons, creating a strong and often irreversible interaction. This process, known as surface chemisorption, is the initial step in the sensing mechanism of many solid state gas detectors. Unlike simple physical adsorption, where molecules are held by weak forces, chemisorption involves a change in the chemical state of the surface.
This leads to a significant shift in the electrical properties of the material, such as its resistance or its work function. The strength of the bond ensures that the sensor can detect even trace amounts of a target gas in the atmosphere. It is the primary reason for the high sensitivity of metal oxide sensors to toxic gases.
Activation Energy
Chemical reactions on the surface require a certain amount of energy to occur, which determines the temperature dependence of the sensor. Surface chemisorption is usually a slow process at room temperature and becomes much faster as the sensor is heated to its operating range. This energy barrier must be overcome for the gas molecules to break their internal bonds and form new ones with the sensing material.
If the temperature is too low, the molecules may simply stick to the surface without reacting, leading to a slow and weak signal. Higher temperatures provide the energy needed for a rapid and strong response. However, too much heat can cause the molecules to detach before they can be measured, reducing the overall sensitivity of the device.
Charge Transfer
During the formation of the chemical bond, electrons are moved between the gas molecule and the semiconductor lattice. This charge transfer is the physical event that changes the electrical resistance of the sensor. In the case of reducing gases, the molecules donate electrons to the material, which increases the conductivity of an n-type semiconductor.
This process of surface chemisorption effectively modulates the number of available charge carriers in the thin film. The magnitude of the resistance change is proportional to the number of molecules that have bonded to the surface. Because the bond is strong, the sensor can maintain its reading even if the gas concentration drops slightly.
This leads to a very stable signal but can also make the sensor slow to recover.
Desorption Phase
Returning the sensor to its original state requires the breaking of the chemical bonds and the removal of the gas molecules. This desorption process often requires more energy than the initial bonding, which is why many sensors use a high temperature pulse to clear the surface. In surface chemisorption, the recovery time can be much longer than the response time if the bonds are particularly strong.
If the molecules cannot be removed, the sensor sites become permanently occupied, leading to a loss of sensitivity over time. This poisoning of the sensor is a common failure mode in industrial environments with high concentrations of sticky chemicals. Regular heating cycles and protective filters are used to extend the life of the sensing element by maintaining the availability of the reaction sites.