## 3 Results
### Analysis of the Dynamic Response of the \(ZnAl_{2}O_{4}\)  
To prove the spinel ZnAl\({}_{2}\)O\({}_{4}\)'s ability to detect gas concentrations, dynamic direct current (DC) tests were performed with the prepared pellets (\(\sim\)0.5 mm thick), which were subjected to extra-dry air (500 mL/min) and propane (1000 ppm) flows for measuring the variations in electrical resistance. The experiments were performed in four steps: (1) the pellets were put inside the measurement system placing two electrodes (two-point method) on the ohmic contacts; (2) then, the measuring chamber was heated at a constant temperature of 250 \({}^{\circ}\)C, and 500 mL/min of extra-dry air (20% O\({}_{2}\) and 80% N\({}_{2}\)) were injected for five minutes to stabilize the surface of the pellets; (3) after stabilization, 1000 ppm of propane were injected for five minutes, recording variations in the material's electrical resistance immediately; (4) after that, the propane was removed and extra-dry air was injected into the measuring chamber, which caused the electrical resistance to return to its initial values (when the pellets were stabilized in air, step 1). This process was repeated for several cycles until obtaining the results shown in Figure 2, where electrical resistance (Figure 2a) and sensitivity (Figure 2b) were plotted as a function of time. Using the results of Figure 2a, we calculated the sensitivity of the pellets with the formula \(S=\left(R_{O}-R_{G}\right)/R_{G}\), where \(R_{O}\) is the electrical resistance in extra dry air and \(R_{G}\) is the electrical resistance in propane. By considering the reciprocal of the electrical resistance (1/electrical resistance), we calculate [1; 23]: \(S=\left(G_{G}-G_{O}\right)/G_{O}\), where \(G_{G}\) and \(G_{O}\) are the propane and air conductances, respectively. From Figure 2a, when the propane molecules interacted with the surface of the pellets, a decrease in electrical resistance was recorded as the exposure time was extended. That was repeated on several occasions (cycles), which indicated that the pellets maintained a stable process of reversibility and stability in air-propane atmospheres [1]. The excellent response of the ZnAl\({}_{2}\)O\({}_{4}\) was attributed mainly to the reaction that took place on the pellets' surface between the oxygen and the test gas because of the operating temperature (250 \({}^{\circ}\)C) [15; 23; 24]. The temperature provoked the activation of the charge carriers (electrons) [1], making them move faster on the sensor's surface [23; 25], causing an almost immediate change in electrical resistance and an increase in the sensitivity of the pellets [1]. In addition, the temperature favored the diffusion of the test gas [26], which contributed to an increase in the oxide's sensitivity (Figure 2b). The variation in electrical resistance (\(\Delta\)R) was calculated in the range of 1520.04-647.17 k\(\Omega\), with an average of 816.85 k\(\Omega\) (Figure 2a). The sensitivity variations (\(\Delta\)S) were estimated in the range of 0.11-1.33, with maximum peaks recorded at \(\sim\)1353 (Figure 2b). The response time (200 s) was estimated considering  
Figure 1: X-ray diffraction pattern showing the pure phase of the ZnAl\({}_{2}\)O\({}_{4}\) at 250 \({}^{\circ}\)C.  
90% of the response in propane; the recovery time (189 s) was calculated by considering 10% of the value when the material was exposed to air [1]. The results depicted in Figure 2a,b were consistent with similar semiconductors studied as gas sensors [1; 9; 26; 27; 28]. The excellent dynamic response and good sensitivity shown in Figure 2a,b indicate that the oxide is an n-type semiconductor [1]. The oxide's behavior was due to the reaction between the ionsorbed oxygen [29] on the material's surface and the propane gas since there was an imbalance between them, leading to the variations in conductivity and, therefore, to the increase in sensitivity.