5. Conclusions

significant increase of the sensitivity as the grain size decreases from 120 to 4 nm. Also, the porous structure promotes the increase of the sensitivity. If the sensor material is porous, the gas will easily penetrate into the internal part of the sintered material, resulting in a large change in the resistance (i.e., a large sensitivity). This may be referred to as a structure effect. These results suggest that the role of tin in MgFe2O4 sample is to facilitate the oxidation of

Due to the oxidizing reaction, in the oxide semiconductors, the oxygen vacancies (point defects) appear, which change the electrical conductivity (the free electron concentration increases for the samples with n-type semiconductor behavior, analog the gaps concentration

For LPFO samples, the sensitivity of ethanol (Figure 5) decreases with increasing Zn2+ ion concentration from 133 for LPFO-0 sample to 10 for LPFO-3 sample. The sensitivity of LPFO-1 and LPFO-2 samples is somewhere between and close to 50 for both samples. The optimum operating temperature remains around 280�C at the samples with x = 0, 0.05 and 0.1, and increases to 330�C for x = 0.2. The effect of Fe3+ ions substitution by Zn2+ ions consists in the

The sensitivity to acetone (Figure 6) increases very much with the increase of the Zn2+ ions concentration from 140 at the sample with x = 0–560 at the sample with x = 0.1. For the concentration x = 0.2, the sensitivity suddenly decreases to 45, as in the case of ethanol. The optimum operating temperature is of 280�C for the samples with x = 0 and 0.05 and increases at 330�C for x = 0.1 and 0.2. The effect of the concentration x of Zn2+ ions is a spectacular increase of the sensitivity up to x = 0.1, after which the sensitivity strongly decreases for higher

When the C2H5OH (ethanol) or C3H6O (acetone) gas is introduced, a chemical reaction occurs

Electrons released from the reaction would annihilate the holes. Hence, the material resistivity increased. As each acetone molecule produces 8n electrons, that is, the highest number among the two gases, and the molar concentration was the same for the studied gases, the increase of the sensing element resistivity in the presence of acetone is the highest [51]. This suggests that La0.8Pb0.2Fe1�xZnxO3 sensors are applicable to detect these gases, especially acetone vapors. The substitution of Fe3+ ions by Zn2+ ions in La0.8Pb0.2FeO3 intervenes directly, but in a different manner in the sensing mechanism of these gases. For ethanol, the sensitivity decreases with an increasing concentration of Zn2+ (x) ions, while for acetone, the sensitivity increases with the increase of x value, but decreases for x > 0.1. Therefore, in order to clarify the mechanism of sensitivity, especially to acetone, further investigations will be

C2H5OHgas <sup>þ</sup> <sup>6</sup> � On�ð Þ! ads <sup>2</sup> � CO2ads <sup>þ</sup> <sup>3</sup> � H2Oads <sup>þ</sup> <sup>6</sup> � ne (12)

C3H6O <sup>þ</sup> <sup>8</sup> � <sup>O</sup><sup>n</sup>�ð Þ! ads <sup>3</sup> � CO2ads <sup>þ</sup> <sup>3</sup> � H2Oads <sup>þ</sup> <sup>8</sup> � ne (13)

between C2H5OH and C3H6O, respectively, and the adsorbed oxygen [51, 53]:

increases for the samples with p-type semiconductor behavior).

diminution of the sensitivity to ethanol [51].

reducing gases [50].

144 New Uses of Micro and Nanomaterials

concentrations [51].

necessary [51].

Nanostructured oxide semiconductor compounds have gained a big importance, in basic and mostly in applicative researches, due to their unique properties, their increased potential of utilization as sensors in various electronic and optoelectronic devices. The development of devices based on semiconductor materials as gas sensors has been visible during the recent years, due to their low manufacturing cost.

The mass spectrometer and the gas chromatograph are the most important systems of spectroscopic gas sensors; yet, at the same time, they are very expensive, hard to implement in reduced spaces and can rarely be used in real time.

Instead, a compact, robust, highly performing, and low-cost gas sensor can be a very attractive alternative to the classical devices used for environment monitoring. A series of recent researches have focused on the development of solid gas sensors having as sensitive element oxide semiconductor materials, among which are the spinels and perovskites, and their performances began to be improved.

In this chapter, the structural, morphological, and sensory characteristics of some porous oxide semiconductor compounds with a spinel-type structure (Mg1xSnxFe2O4; x = 0, 0.1) or with a perovskite-type structure (La0.8Pb0.2Fe1xZnxO3; x = 0, 0.05, 0.1, 0.2) were presented.

These compounds were prepared by the sol-gel self-combustion method. After the thermal treatments in air, the samples attain corresponding crystalline structure (spinel-type or perovskite-type, respectively).

The spinel-type samples are characterized by a very fine structure (100–500 nm) with an accentuated porosity (46–65%) and channels that favor the adsorption or desorption of the gas around particle agglomerates. Samples show a semiconductor behavior with a thermal activation energy between 0.4 and 0.6 eV. The gas sensitivity is strongly related to the working temperature, material composition, mean particle size, and porosity.

In the case of these samples, the gas sensitivity increased with the increasing operating temperature and reached a maximum value at an optimum operating temperature (Top) of about 380C. The sensitivity to acetone vapors is higher than that to ethanol vapors for both samples (x = 0 and x = 0.1). The best sensitivity, 0.82, was obtained for the sample that has tin substitutions (x = 0.1) to acetone vapors at an optimum operating temperature of 380C. The obtained results correlate well with the grain size changes from 500 to 100 nm.

The perovskite-type compounds exhibit orthorhombic symmetry (space group Pnma) and crystallizes in the perovskite-like cell of LaFeO3, having a porous granular and a uniform structure. The average grain size decreases from 250 to 150 nm with the increase of Zn concentration. The porosity of the samples increases with increasing Zn concentration from 31.11 to 46.78%. The sensor elements show p-type-semiconducting properties for all studied gases within the temperature range of 100–380C. Through the substitution of the Fe3+ ions by Zn2+ ions (x = 0.1), the sensor element has the best response to acetone. At a concentration of 400 ppm gas at the operating temperature of 330C, the response to acetone is spectacular (560).
