Transition Oxide
A versatile semiconductor material exhibits a wide variety of crystal phases and distinct sensing properties. Tungsten trioxide is frequently employed for the detection of nitrogen oxides and ammonia due to its high sensitivity and chemical stability. It undergoes a reversible color change and a distinct change in electrical resistance when exposed to specific gases.
Structural Phase
The arrangement of atoms shifts from monoclinic to triclinic or orthorhombic as the temperature increases. Each phase has a unique electronic band structure that influences the gas sensing performance of the material.
Oxygen Stoichiometry
Nonstoichiometric forms of the oxide contain high densities of oxygen vacancies. These defects act as donor sites for electrons, increasing the baseline conductivity of the sensor.
Field Performance
Reliable operation in industrial environments requires the material to be resistant to poisoning by sulfur compounds. Sensors are qualified by measuring their recovery time after exposure to a high concentration of the target gas. This recovery is often facilitated by a high temperature pulse delivered via an integrated heater.
Accuracy is verified against a reference analyzer using a certified gas mixture. The drift in the sensitivity coefficient is monitored over several months to determine the appropriate interval for recalibration.
