Polycrystalline Tetragonal Zirconia of the Form ZrO₂: 3 mol% Re₂O₃ (Re-TZP) for Use in Oxygen Sensors: Synthesis, Characterization and Ionic Conductivity

Abstract:

Oxygen ion conductors of zirconia based ceramics are a class of materials with technological applications in several application areas: sensors of chemical species, oxygen pumps, solid oxide fuel cells among others [1]. For these applications, the zirconia must possess the fluorite type crystal structure, or close to it. Such oxides with this structure are the classic oxygen ion conductors [2]. The fluorite structure consists of a cubic lattice of oxygen ions surrounded by cations. The cations are arranged in a face centered cubic structure with anions occupying tetrahedral positions. This leads to an open structure with large empty octahedral interstices.

• Periodical: Materials Science Forum (Volumes 798-799)
• Pages: 145-153
• DOI: https://doi.org/10.4028/www.scientific.net/MSF.798-799.145
• Online since: June 2014
• Authors: R.A. Muñoz*, Paola Cristina Cajas, J.E. Rodriguez, A.C.M. Rodrigues, Cosme Roberto Moreira Silva
• Keywords: Activation Energy, Grain Size, Ionic Conductivity, Oxygen Sensors, Tetragonal Polycrystalline Zirconia

References

[12] The microstructure of sintered ZrO2: 3% Mol Re2O3 ceramics was observed by scanning electron microscopy (SEM), in a Jeol JSM-7001F microscope (Scanning Electron Microscope), of the institute of Biology of the University of Brasilia. Measurements of average grain size and interfacial area per unit of volume were carried out (Figure 5).

[13] counting the number of intersections between the grain boundary and straight lines with known length, which were designed on the image with the program ImageJ of free access. The number of intersections per image was more than 400, aiming to achieve better measurement accuracy. In the case of ceramics sintered with the curve S1, figure 5 (a), the average grain size was approximately 574 nm and the interfacial area per unit of volume was 3. 48 x 10-3 nm2/nm3. For ceramics sintered with the curve S2, figure 5 (b), the average grain size was almost 280 nm and the interfacial area per unit of volume was 7. 38 x 10-3 nm2/nm3. Qualitative analysis of the difference between these obtained values is related to the sintering curve, since the two types of samples were processed identically. Other microstructural features are visible at the micrographs such as low porosity and uniform distribution of grain sizes and shapes (all with the same size and similar shape). The sintering curve S1 strongly promotes the grain growth, resulting in the reduction of the density of grain boundaries, fact that could increase its electrical behavior. (a) (b) Figure 5. Analyses by SEM of the surface of the sintered specimens (a), sintered with curve S1, and, (b) using conventional sintering curve S2. Finally, the electrical response of sintered solid electrolyte was evaluated by impedance spectroscopy. That characterization technique allows to establish the dependence of the electrical behavior with temperature, in different regions of the ceramic, grain and grain boundary. The samples have apparent density to 96. 97% of theoretical. Therefore, the electrical results obtained are related mainly to the properties and characteristics of the electrolyte.

[14] The impedance spectroscopy measurement for samples was carried out on a frequency range of 1 MHz to 1 Hz, and a voltage of 1000 mV, using a Solartron 1260 equipment. The temperature range used was between 125 and 400 °C, with measurements at every 25 degrees, giving a total of twelve (12) measurements for analysis. Paste electrodes of platinum Pt-paste Demetron 308-A were applied on the parallel faces of the samples and cured at 1100 °C for 20 minutes. Figure 6 shows a diagram of typical impedance obtained for the samples analyzed at 300 ° C. It is clearly observed two different semicircles that in the study of ceramic materials, are related to the grain contributions (high frequencies) and at contributions of the grain boundaries (low frequencies). From the diameter of these semicircles, the intragranular and intergranular resistivity were calculated, respectively. It is also evident from Figure 6 (b) that the intragranular resistivity are slightly affected by the heat treatment sintering, while the intergranular resistivity experiences a great decline in value. (a) (b) Figure 6. Typical impedance diagrams of the samples at a temperature of 300 °C, (a) showing the decrease in resistivity attributed to grain boundary, (b) expanding the zone of high frequencies. (Numbers denote the logarithm of frequency). Thus it is clear that the decrease in the density of grain boundaries significantly affects the electrical behavior of ceramics under study. With resistivity values ​​obtained was possible to observe the character thermally activated as a function of test temperature for the grain, grain boundary and total conductivities, Figure 7(a), (b) and 8(a) respectively. The dependence of the conductivity with temperature is shown in Figure 7, where one can see a detailed discussion of this increased conductivity due to the used sintering curve. (a) (b) Figure 7. Dependence of conductivity with temperature for the grain (a) grain boundary (b) To complete the electrical analysis, Arrhenius plots (log σ vs. 1000 / T) were constructed, where is possible to obtain the activation energy for the conduction process, figure 8 (b). It was observed for both samples a single slope in the temperature range 125-400 ° C and that they do not deviate the Arrhenius type behavior. From the slope of these lines total activation energies were obtained, resulting values ​​of 1. 00 and 1. 01 eV for the samples sintered with the curve S1 and S2 respectively. These values ​​of activation energy are in complete agreement with the values found in the literature for ion oxygen conductors based on zirconium oxide that are in the range from 1 to 1. 2 eV.

[15] (a) (b) Figure 8. Dependence of conductivity with temperature for (c) total conductivity and ( b) Arrhenius plots for total conductivity. Conclusions It was possible to stabilize the tetragonal zirconia polycrystalline form ZrO2 3% Mol Re2O3 at a temperature lower than 650 ° C with crystallite size of approximately 24 nm. Transmission electron microscopy revealed that the ceramic powder obtained was composed of nanoparticles with sizes smaller than the 50 nm. Some particles were agglomerated, requiring the grinding step for correct specimens conformation. Ceramics developed from this raw material had densities higher than 96% of theoretical with grain size dependent upon the used sintering schedule. The increase in grain size of 280 to 574 nm caused a decrease in interfacial area per unit volume which is reflected in a decrease in the grain boundaries densities. This result provides beneficial effects on the electrical behavior of the material attributed to the resistivity reduction of the grain boundary which causes an increase in the total conductivity of the ceramic from 1. 35 E-5 E-5 to 2. 15 Ω-1 cm-1 at 400 ° C. Bibliography.

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