2. Reports from the specialized literature on a series of oxide semiconductor compounds used for resistive gas sensors

The specialized literature of the recent years reports a series of spinels and perovskites used to obtain gas sensors [25–39].

The spinel-type oxide semiconductors with a general formula of AB2O4 have demonstrated to be good materials for the detection of both oxidizing and reducing gases [25–33]. Kapse [35] conducted a study on the sensitivity of spinelic oxide compounds (NiFe2O4, ZnFe2O4, MgFe2O4, ZnAl2O4, CoAl2O4 and MgAl2O4) synthesized by citrated sol–gel technique for various gases (H2S, NH3, C2H5OH, LPG). The author obtains the best values of the magnesium spinel sensitivity (MgFe2O4) for H2S (4.8), C2H5OH (12.4), LPG (6.3) at an operating temperature of 325C and within the CoAl2O4 spinel for NH3 (1.3) at an operating temperature of 150C and a concentration of 50 ppm. Sutkaa et al. [36] investigated the nickel ferrite with zinc substitutes (Ni1xZnxFe2O4), p-type semiconductors with increased porosity, predominantly open pores. The samples were synthesized by the sol–gel self-combustion method. For the NiFe2O4 sample, for a concentration of 500 ppm acetone vapors in the air, they obtain a sensitivity S of 3.7 at an optimal operating temperature of 275C.

order of microseconds) under the form of an autonomous combustion. The substances necessary for combustion result from the reaction of hydroxides formation itself, if adequate

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Because the reaction lasts a few seconds, the crystals do not have enough time to grow. The final dimension of the particle of oxide semiconductor material, as well as its structure and properties, is obtained after a heat treatment, during which the process of material crystallization and crystallites growth up to the necessary size occurs. The treatment temperature and duration are, in all the cases, smaller than those necessary for sintering according to classical

This method provides a good product homogenization. At the same time, due to the absence of settling, filtering, or washing operations, one has the certitude of material composition.

The sol-gel self-combustion method permits to obtain an ultra-fine, homogeneous powder, with particles of nanometric size, within a narrow dimension range, and a pronounced porosity (as

reagents are used in the precipitation reaction.

methods, due to the high homogeneity of the mix.

Figure 1. Stages of obtaining materials by sol-gel self-combustion method.

Regarding a series of perovskites of type ABO3, Wang et al. [37] reported the utilization of the nanocrystalline BaMnO3 perovskite having an n-type semiconductor behavior as a sensor selective to O2, with low operating temperatures. Hara et al. [38] have reported a study on perovskites from the SrTiO3 family as O2 selective sensors, working at the room temperature. The undoped material exhibits high sensitivity, but its resistivity is extremely high, which makes it unusable in practice. It has been found that by Nb5+ doping, the sensor resistivity decreases, but its sensitivity also decreases. The same happens for Fe3+ doping. By doping with Cr3+, a high decrease of resistivity was obtained, while the sensitivity remains the same. Gaudhari et al. [39] carried out a study on the Ba-doped nanostructured perovskite SmCoO3 as selective sensor for CO2, working at a temperature of 425C. For an Sm substitution with Ba (Sm0.9Ba0.1CoO3), a decrease in the optimum operating temperature from 425 to 370C and a diminution of the response time are obtained.
