3. Results and discussion
Fig. 1 shows TEM images of powder containing 2 mol% Eu immediately after synthesis (a) and after annealing at 1000 °C (b). As it is seen from fig. 1a the produced nanopowder is low agglomerated and consists of spherical particles. The specific sur- face of the powder is 70 m2/g. The size distribution is normal log- arithmic with average size 13.1 nm and standard deviation 1.56, when the number of particles taken into account is 5328. The high temperature annealing of the powder leads to its agglomeration (Fig. 1b). The nanoparticles are partially sintered. Therefore deter- mination of particles sizes on the base of the TEM images is com- plicated. The average size of agglomerates reaches hundreds of nanometers. Nanopowders with other contents of Eu have similar morphology and their surface area is in the range 68–74 m2/g. The typical diffraction patterns and the results of their treatment be- fore and after annealing are shown in Fig. 2, Tables 1 and 2. For the other test materials the similar results were obtained. As it is seen from Table 1 the average crystallite size is in good agreement with the average particle size estimated by TEM for the powder di- rectly after the synthesis. This indicates that the particles are single crystals. The average crystallite size increases slightly with the increasing of the annealing temperature up to 1200 °C. However at the annealing temperature 1300 °C the average crystallite size is increased to more than 200 nm.
The phase composition of the powders after the laser evapora- tion and the annealing at different temperatures for one hour in the air under a layer of coal are shown in Table 1. As can be seen from these data the formation of alumina–magnesium spinel takes place in the process of laser evaporation. The fabricated powder contains 5 wt% of MgO and Eu in the oxide form Eu2O3. The ratios of the intensities of the diffraction lines of the spinel phase have intermediate values between the typical ratios observed for the true spinel MgAl2O4 and the defect spinel c-Al2O3. Thus, the result- ing structure of the spinel is highly disordered. With the increasing of the annealing temperature the process of the spinel ordering takes place and ends at 1200 °C. The change of the phase composi- tion (Table 1) shows that the dissolution of Eu2O3 occurs only at temperatures above 1000 °C. Nanopowders with other contents of Eu have similar diffraction patterns. The difference lies only in the relative phase content of Eu2O3.
The values of the crystal lattice periods of the phases present in the powder for the different annealing temperatures are given in Table 2. The crystal lattice period of MgO significantly differs from the equilibrium value and tends to its equilibrium with the
increasing of the annealing temperature. This is consistent with the results obtained in [10] and indicates that during the vapor condensation after the laser evaporation a solid solution on the ba- sis of MgO is formed. According to the state diagram [11] at high temperatures there is the range of solubility of Al2O3 in MgO. Since in our case the vapor condensation occurs at high temperature and cooling rate is about 106 K/min, the produced powder contains such solid solution.
Directly after the laser evaporation the produced nanopowders do not reveal a significant luminescence independently of the euro- pium content. The luminescence appears only after the annealing of the nanopowders at temperatures above 1000 °C. The typical PCL emission spectra are shown in Figs. 3 and 4. Their general form and location of the emission bands correspond to the observed emission spectra obtained during UV excitation of similar powder materials synthesized by other methods [5–7]. However, there are some differences. In our case in the red region the emission bands typical for transitions in Eu3+ are clearly identified: 5D0 ? 7F1 (with a maximum of 590 nm), 5D0 ? 7F2 (with a maximum of 614 nm) and 5D0 ? 7F4 (with a maximum of 698 nm). However, the ratio of bands intensity is different. The transition 5D0 ? 7F4 is more effec- tive in the contrast to the literature data. The nature of the observed effect is not clear. Perhaps this is due to a different method of exci- tation. In the blue region of the spectrum there is the band associ- ated with transitions in the Eu2+. The similar effect was observed in [7] where the heat treatment was carried out in the presence of carbon material. The intensity of this band shows the strong depen- dence on the dopant concentration (Fig. 3), and weak dependence on the thermal treatment temperature (Fig. 4). It indicates that the europium ions in the crystal lattice occupy mainly 3+valence positions, but with the increasing of the concentration of Eu part of them fall into 2+valence positions. In the case of the annealing of the powder in the air under a layer of coal substantial number of oxygen vacancies is formed. The increasing of the annealing tem- perature leads to the increasing of a number of the vacancies. It also leads to the change in the valence state of Eu and the luminescence intensity of the corresponding band is amplified.
An additional difference is the appearance of a broad emission band with a maximum at 520 nm. The intensity of this band strongly depends on the annealing temperature (Fig. 4). It reaches a maximum at 1200 °C. The appearance of this band was observed also in [7]. We associate this with the luminescence of the MgAl2O4 matrix containing oxygen vacancies.