2. Sensor for CO2 detection with mixed binary oxide CeO2-Nb2O5 sensitive material

Niobium oxide has some properties that make it in principal as promising for catalytic applications. Niobia-based materials are effective catalysts in selective oxidation reactions due to its redox properties. Also, niobia-doped ceria materials have shown a good carbon deposition and excellent properties as solid oxide fuel cell (SOFC) anodes [35]. Nb5+ ions (ionic radius of Nb5+: 78 pm) may initiate the reduction of Ce4+ to Ce3+ by the doping Nb into the CeO2 structure, which results in formation of oxygen vacancies. Using the Kröger-Vink notation, it can mention two mechanisms for the dissolution: one of which occurs by electronic compensation (Eq. 2) and the other by consumption of vacancies (Eq. 3), as shown below [3, 36, 37].

$$\text{Nb}\_2\text{O}\_5 + \text{Ce}\_M\* \to 2\text{Nb}\_M + 4\text{O}\_0^\* + \frac{1}{2}\text{O}\_2 + 2\text{Ce}\_M^\prime \tag{2}$$

$$\text{N}b\_2O\_5 + V\_0^\cdot \to 2\text{N}b\_M^\cdot + 5O\_o^\ast \tag{3}$$

where O<sup>∗</sup> <sup>o</sup> , <sup>V</sup>∙∙ <sup>o</sup> represent oxygen and oxygen vacancies on the oxygen sites, Ce<sup>∗</sup> <sup>M</sup>, Ce<sup>=</sup> <sup>M</sup> represent cerium (Ce4+) and negatively charged cerium ions (Ce3+) on metal sites M, Nb<sup>∙</sup> <sup>M</sup> metal vacancy. Nb2O5 it is known as an n-type oxide semiconductor with a band gap about 3.4 eV. Because of its good physicochemical properties and structural isotropy, it is used in other range of applications such as: in construction of gas sensing, field-emission displays and microelectronics electrochromics display and photoelectrodes [38].

## 2.1. Synthesis of mixed oxides CeO2-Nb2O5 sensitive material

In our case, we used the mixed binary oxides CeO2-Nb2O5 for CO2 detection. Sensitive element is composed from mechanical mixing of CeO2 (97%) and Nb2O5 (3%); both reagents purchased from Merck. The powder oxides were treated with a few drops of ethylic alcohol for ink obtaining and then introduced in a ball mill for homogenization followed by calcination at 500, 600 and 800�C for 1 hour. The powder calcined at 600�C was pressed in disc form at 2 tons force/cm<sup>2</sup> with the dimensions <sup>∅</sup><sup>4</sup> � 1 mm and mounted on the ambasis transistor. The sensor image is showed in Figure 1 [39].

Figure 1. Image of the CO2 sensor made with mixed oxides CeO2-Nb2O5 sensitive material.

#### 2.2. Structural characterization

Calcined mixed powder oxides were characterized by X-ray diffraction using a diffractometertype X Bruker-AXS type D8 ADVANCE in conditions: CuK<sup>α</sup> radiation (λ =1.54059 Å), 40 kV/ 40 mA, filter k<sup>β</sup> of Ni. pas: 0.04 , measuring time on point: 1 s, measure range 2θ = 10–100 . The mixed oxides powder with composition CeO2–3%Nb2O5 was calcined at 500, 600 and 800C for 1 hour. It shows a cubic phase for CeO2 and orthorhombic phase for Nb2O5, Figure 2. Also, for this powder that was calcined at 800C, was identified in addition a hexagonal Ce2O3 phase (Figure 3). It obtain for CeO2 cell parameter a = b = c = 5.407 Å. This is in accord with theoretical value of a = 5.404 Å as well as in according with card number 03–065-5923. The cell parameters for Nb2O5 orthorhombic phase were a = 6.175 Å, b = 29.175 Å and c = 3.930 Å in accord with card number 30-0873 [4]. Corresponding to hkl (Miller indices) 111, 200, 220 and 311, the crystallites size determined with Scherrer formula give values of 160.9, 145.6, 117.4

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The gas sensors testing were performed with the apparatus as presented in Figure 4. It is realized by SYSCOM-18 Romania for National Institute for Research and Development in

2.3. CO2 gas sensor made with mixed oxides CeO2-Nb2O5 sensitive material tested

and 63.5 nm.

in automated process mode

Figure 4. The testing gas sensor installation.

Figure 3. X-ray diffraction of CeO2-Nb2O5 calcinated at 800C.

Figure 2. X-ray diffraction of CeO2-Nb2O5 calcinated at: (a) 500C; (b) 600C and (c) 800C.

Prototyping a Gas Sensors Using CeO2 as a Matrix or Dopant in Oxide Semiconductor Systems http://dx.doi.org/10.5772/intechopen.80801 65

Figure 3. X-ray diffraction of CeO2-Nb2O5 calcinated at 800C.

2.2. Structural characterization

64 Cerium Oxide - Applications and Attributes

40 mA, filter k<sup>β</sup> of Ni. pas: 0.04

Calcined mixed powder oxides were characterized by X-ray diffraction using a diffractometertype X Bruker-AXS type D8 ADVANCE in conditions: CuK<sup>α</sup> radiation (λ =1.54059 Å), 40 kV/

Figure 1. Image of the CO2 sensor made with mixed oxides CeO2-Nb2O5 sensitive material.

Figure 2. X-ray diffraction of CeO2-Nb2O5 calcinated at: (a) 500C; (b) 600C and (c) 800C.

mixed oxides powder with composition CeO2–3%Nb2O5 was calcined at 500, 600 and 800C for 1 hour. It shows a cubic phase for CeO2 and orthorhombic phase for Nb2O5, Figure 2. Also, for this powder that was calcined at 800C, was identified in addition a hexagonal Ce2O3 phase (Figure 3). It obtain for CeO2 cell parameter a = b = c = 5.407 Å. This is in accord with

, measuring time on point: 1 s, measure range 2θ = 10–100

. The

theoretical value of a = 5.404 Å as well as in according with card number 03–065-5923. The cell parameters for Nb2O5 orthorhombic phase were a = 6.175 Å, b = 29.175 Å and c = 3.930 Å in accord with card number 30-0873 [4]. Corresponding to hkl (Miller indices) 111, 200, 220 and 311, the crystallites size determined with Scherrer formula give values of 160.9, 145.6, 117.4 and 63.5 nm.

## 2.3. CO2 gas sensor made with mixed oxides CeO2-Nb2O5 sensitive material tested in automated process mode

The gas sensors testing were performed with the apparatus as presented in Figure 4. It is realized by SYSCOM-18 Romania for National Institute for Research and Development in

Figure 4. The testing gas sensor installation.

Figure 5. Variation of the voltage depending on time for CO2 sensor, made with mixed oxides CeO2-Nb2O5 sensitive material.

Figure 6. A simplified schematic of the AD620 [39].

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Figure 7. AD620 closed-loop gain versus frequency [39].

Electrical Engineering ICPE-CA. The voltage measurements were effected by testing module, in automated process mode. A control panel provides a lot of measuring values, at rate 1/10 s. The bench of testing for the gas sensor consists in an enclosure where there are set of testing conditions for the sensor as well as in connected equipment. The whole process of testing is automated, being controlled by a programmable automaton. The gas for testing is introduced in a controlled way in the testing enclosure, through a mass debit meter. In the testing, enclosure is set a constant temperature, controlled by a temperature regulator.

The gas testing was done in concentration of 10,000 ppm CO2 at the 25, 50 and 70C chamber test temperature. The sensor was developed the voltages values of 48, 50 and 770 mV (Figure 5) [39].

The experimental data shows a good sensor response for CO2 detection with increasing temperature.
