## 3 Experimental Results
### Sensing Properties Analysis  
Figure 6 shows the response of the NiSb\({}_{2}\)O\({}_{6}\) pellets as a function of the CO concentration at the operating temperatures 100, 200, and 300 \({}^{\circ}\)C. It can be clearly verified in the graph that the pellets are sensitive to the concentrations of the gas. The high response of the material is closely related to the chemical adsorption of the gas on the pellets' surface, as well as to the increase of the test gases' concentration and of the temperature. This is corroborated in Figure 6 since as the concentration of the gas and the operating temperature increase, the efficiency of the pellets to detect the CO concentrations improves substantially. The results obtained in the gas are attributed to the fact that the increase in the operating temperature favors the reaction kinetics between the surface of the pellets and the test gas [20], contributing to a higher oxygen desorption during the adsorption of the test gas [20; 22; 24; 30], which leads to changes in the material's electrical resistance and, consequently, on the high response obtained during the detection tests. The adsorption of different types of oxygen species at high temperatures has been reported in the literature [36]. It can be seen in Figure 6 that the maximum sensitivity magnitudes, \(\sim\)0.05 and \(\sim\)0.35, were respectively obtained at 200 and 300 \({}^{\circ}\)C for a carbon monoxide concentration of 300 ppm. The response of the NiSb\({}_{2}\)O\({}_{6}\) pellets is associated to the mechanism of oxygen desorption at high temperatures [36], which implies that at a temperature lower than 150 \({}^{\circ}\)C, the thermal energy is not enough to provoke the oxygen desorption reactions so that no electrical response occurs regardless of the gas concentration. In general, the most abundant oxygen species at temperatures below 150 \({}^{\circ}\)C are the O\({}_{2}^{-}\) ions. Above 150 \({}^{\circ}\)C (in our case 300 \({}^{\circ}\)C) the formation of oxygen species such as the O\({}^{-}\) and O\({}^{2-}\) ions takes place, which are more reactive [22; 24; 28; 30; 36]. Furthermore, the increase of the temperature causes a rise in the solid-gas interactions [19; 22; 30], leading to a good material's response in CO atmospheres. Figure 6: Response of NiSb\({}_{2}\)O\({}_{6}\) pellets as a function of (a) CO and concentrations. In order to know the variations of the electrical response (resistivity) in the presence of different carbon monoxide (CO) concentrations, the changes in the resistivity were estimated as a function of the gas concentration and the operating temperature. The calculation was made with the formula [20; 37]: \(\rho\) = RA/t, where R is the electrical resistance in the test gases, A is the area of the cross-section, and t is the thickness of the pellets (=0.5 mm, diameter = 12 mm). The results are shown in Figure 7.  
According to the results, the electrical resistivity of the material in CO atmospheres decays as the concentration and the operating temperature increase. This means that the electrical conductivity of the material increases proportionally as the resistivity decreases (see Figure 7a,b). This behavior is mainly attributed to the fact that, due to the increase of temperature, the charge carriers show more mobility on the surface of the pellets, leading to an increase in the conductivity [20; 24]. In addition, it is evident that increasing the concentration of the test gases and the operating temperature, it favors the decrease in resistivity. This phenomenon occurs because the kinetic activity of the gas molecules on the pellets' surface is increased by the rise in operating temperature [24] (in our case at 200 and 300 \({}^{\circ}\)C).