Besides, gas sensors designed for the market must guarantee the stability of their operation, in other
words, they should exhibit a stable and reproducible signal for a period of time. There are several
factors leading to gas sensor’s instability
(1) design errors (which should be avoided);
(2) structural changes, such as variations of grain size or grain network;
(3) phase shifts, which usually refers to the segregation of additives doped with sensing materials;
(4) poisoning triggered by chemical reactions;
(5) variation of the surrounding environment. In order to solve these problems, the following methods could be considered: (1) using materials with chemical and thermal stability;
(2) optimizing elemental composition and grain size of sensing materials;
(3) utilizing specific technology during surface pretreatment of sensors. Among those indicators mentioned above, sensitivity and selectivity are the two main indicators, which will be discussed in Section 5.