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Energies 2022, 15, 6452 8 of 21 3.1.2. X-ray Diffraction XRD measurements were also performed to confirm the crystalline phase/phases obtained from Raman spectroscopy measurements. As seen in Figure 3a, the as- synthesized ZrO2 nanopowder presents a mixture of ZrO2 tetragonal and monoclinic phases, considering the ICDD card numbers 00-037-1484 and 00-050-1089 for the ZrO2 monoclinic and tetragonal phases, respectively. Figure 3. (a) In situ XRD diffractograms of the ZrO2 nanopowder synthesized under microwave Figure 3. (a) In situ XRD diffractograms of the ZrO2 nanopowder synthesized under microwave irradiation with different annealing temperatures ranging from RT to 800 ◦C with a plateau time of irradiation with different annealing temperatures ranging from RT to 800 °C with a plateau time of 15 min. For comparison, the simulated monoclinic, tetragonal and cubic zirconia structures are also 15 min. For comparison, the simulated monoclinic, tetragonal and cubic zirconia structures are also presented. (b) XRD diffractogram of ZrO2 nanopowder synthesized under microwave irradiation and calcinated at 800 ◦C for 15 min. (c) In situ XRD diffractograms of the ZrO2 nanopowder at 800 ◦C for different annealing times. Green arrows represent an increase of monoclinic phase, while the red arrow indicates a decrease of tetragonal phase. Calcination treatment after microwave synthesis is an important step in transforming an amorphous to crystalline phase or a single/mixture of crystalline phases into a specific crystalline phase, inducing optical and structural’ modifications to the material [13,84]. In this regard, XRD diffractograms of the ZrO2 nanopowder were recorded at different temperatures to infer the thermal behavior of the ZrO2 nanomaterials and thus analyze the phase transition from tetragonal to monoclinic, Figure 3a. As can be observed, the intensity of the diffraction maximum at 30.176◦, characteristic of the tetragonal phase, decreases with increasing temperature. At 800 ◦C, the peaks detected in the diffractogram of the ZrO2 nanopowder (Figure 3b) were fully assigned to the monoclinic ZrO2 phase (ICDD 00-037-1484). However, when performing XRD studies from 27◦ and 33◦ (Figure 3c) with a fixed temperature of 800 ◦C but different annealing times (each minute was recorded up to 15 min), it is evident that the conversion to monoclinic is accomplished at 800 ◦C with a minimal presence of the tetragonal phase. No other crystalline phases or impurities were detected with either of these two techniques. The phase transformation of ZrO2 tetragonal to monoclinic at 800 ◦C has been previ- ously reported in the literature. Horti et al. [13] prepared ZrO2 nanoparticles by chemical co-precipitation method and further annealed at different temperatures for 2 h, whose phase transition of tetragonal to monoclinic was seen to occur at the calcination temper- ature of 800 ◦C, with an evident mixture of both phases. In another study [85], ZrO2 nanoparticles were produced by conventional and ultrasound-assisted precipitation in an alkaline medium followed by calcination at different temperatures ranging from 400 ◦C to 900 ◦C for 3 h. At 800 ◦C, a mixture of three ZrO2 phases was reported, i.e., tetragonal- monoclinic-cubic phases. The present study has the advantage of having monoclinic ZrO2 as the dominant phase after a fast calcination treatment of 15 min instead of hours.PDF Image | Comparison between Solution-Based Synthesis Methods of ZrO2
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