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Synthesis and Characterization of Y2O3/SiO2 Composites

Profile image of Anna MusinuAnna Musinu

2004, Zeitschrift für Naturforschung A

Abstract

Impregnation and deposition-precipitation syntheses have been used to obtain Y 2 O 3 /SiO 2 samples. In the deposition method, the urea causes the precipitation of yttrium hydroxide, which leads to the formation of yttria nanocrystalline particles in the final composites. Delaying the silica addition up to the visible muddying of the solution, a relevant formation of yttria nanocrystalline particles with average size of about 12 nm is produced. The composites obtained through the impregnation method are amorphous and contain different amounts of yttrium depending on the kind of solvent, the highest concentration being reached using ethanol. In all the samples important interactions at the molecular level among yttrium and silica are revealed, but less important in composites obtained with the deposition precipitation method.

Figures (10)
Table 1. Preparation conditions and compositions of all the samples. dying, caused by the progressive hydrolysis of urea and then stirred for 90 minutes at the same tempera- ture. The use of nitric acid prevents a sharp increase of the pH, avoiding massive precipitation of yttrium hy- droxide. The solids were removed from the liquid by centrifugation and then dried at room temperature for 20 hours. Two reference samples were also prepared, that is silica free (D Psp) and yttria free (DP rer), follow- ing the same preparation conditions but without the ad- dition of silica in the first case and impregnating silica for three hours with 50 ml of aqueous urea solution in the second one. All the samples were submitted to thermal treatments at 900 °C for 1 hr.
Table 1. Preparation conditions and compositions of all the samples. dying, caused by the progressive hydrolysis of urea and then stirred for 90 minutes at the same tempera- ture. The use of nitric acid prevents a sharp increase of the pH, avoiding massive precipitation of yttrium hy- droxide. The solids were removed from the liquid by centrifugation and then dried at room temperature for 20 hours. Two reference samples were also prepared, that is silica free (D Psp) and yttria free (DP rer), follow- ing the same preparation conditions but without the ad- dition of silica in the first case and impregnating silica for three hours with 50 ml of aqueous urea solution in the second one. All the samples were submitted to thermal treatments at 900 °C for 1 hr.
Fig. 2. TEM bright field micrograph of I1 composite treated at 900 °C. Fig. 1. XRD spectra of all the composites and silica reference samples treated at 900 °C. * Y 203 cubic phase.
Fig. 2. TEM bright field micrograph of I1 composite treated at 900 °C. Fig. 1. XRD spectra of all the composites and silica reference samples treated at 900 °C. * Y 203 cubic phase.
Fig. 3. TEM dark field micrograph of DP2 composite treated at 900 °C.
Fig. 3. TEM dark field micrograph of DP2 composite treated at 900 °C.
Fig. 5. 29Si MAS-NMR spectra of all composites. Fig. 4b. Particle size distribution of the DP1 composite.
Fig. 5. 29Si MAS-NMR spectra of all composites. Fig. 4b. Particle size distribution of the DP1 composite.
C. Cannas et al. - Synthesis and Characterization of Y203/Si02 Composites
C. Cannas et al. - Synthesis and Characterization of Y203/Si02 Composites
Table 2. Qy sites percentage (accuracy 3- 6%) resulted from the simulation of 29Si MAS NMR spectra of all the samples. Q, bands found in the I3 sample as constraints for the deconvolution; the results from the best fit are reported in Table 1. The results found for samples I1 and 12, in terms of the relationship between the amount of yt- trium and the percentage of Q,; groups, show a be- haviour very similar to sample 13, suggesting that yt- trium is totally interacting at a molecular level with the surface of the silica matrix. This can be interpreted with the formation of an amorphous yttrium-based sil- icate phase covering the silica surface, in agreement with the TEM bright field observation (Fig. 2) and the XRD results (Fig. 1).
Table 2. Qy sites percentage (accuracy 3- 6%) resulted from the simulation of 29Si MAS NMR spectra of all the samples. Q, bands found in the I3 sample as constraints for the deconvolution; the results from the best fit are reported in Table 1. The results found for samples I1 and 12, in terms of the relationship between the amount of yt- trium and the percentage of Q,; groups, show a be- haviour very similar to sample 13, suggesting that yt- trium is totally interacting at a molecular level with the surface of the silica matrix. This can be interpreted with the formation of an amorphous yttrium-based sil- icate phase covering the silica surface, in agreement with the TEM bright field observation (Fig. 2) and the XRD results (Fig. 1).
Fig. 7. XRD spectra of the free silica sample (ppt) as a func- tion of thermal treatments. * Y(OH)3 and ° Y203 cubic phase.
Fig. 7. XRD spectra of the free silica sample (ppt) as a func- tion of thermal treatments. * Y(OH)3 and ° Y203 cubic phase.

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