2.3.3. Characterization of Composite Nanostructured Samples
The characterization of the nanostructured samples composed of ferrite-resin was carried out both before and after the additive manufacturing process, in order to know the chemical and magnetic characteristics of the materials, as well as to verify if the nanoparticle magnetic properties are preserved or modified once the process is finished.
Figure 11a shows the Raman spectra corresponding to BaFe
12O
19–IRIX resin and BaFe
12O
19– ANY Cubic Green resin nanocomposites. In the spectrum, in addition to the polymer’s signals (
Figure S1), the three characteristic signals reported in the literature corresponding to BaFe
12O
19 can be observed in both materials. In the case of BaFe
12O
19–IRIX resin, the signals are slightly displaced, suggesting the existence of the interaction between the barium ferrite nanoparticles and the resin, which limits the vibrational modes in the ferrite molecule, displacing them towards the zone of lower energy; although they remain the same signals for barium ferrite, which indicates that a change in the NPs after the additive manufacturing process is not significant by this technique.
In the case of the CoFe
2O
4–IRIX resin and CoFe
2O
4–ANY Cubic Green resin nanocomposites (
Figure 11b), the characteristic signals of the resins are observed, as well as the signals corresponding to CoFe
2O
4. The shifting of the ferrite signals is marginal to CoFe
2O
4–IRIX resin. This indicates that there is not strong enough interaction between the ferrite and the resin, once the photopolymerization process has been carried out. On the contrary, for CoFe
2O
4–ANY Cubic Green nanocomposite, the displacement is significant, indicating a greater interaction between the ferrite and the resin.
By comparing the Raman spectra of the both materials, it can be determined that in the case of BaFe12O19 nanocomposites and resin signals are present. Therefore, it can be assumed that the distribution of these nanoparticles is in the bulk.
On the contrary, in the CoFe2O4 nanocomposites it is observed that the signals corresponding to the polymer do not appear in the spectra. Hence, it can be proposed that CoFe2O4 has a greater interference with Raman laser light than the polymer, thus minimizing the latter's signals in the spectrum, which may indicate that the distribution of the CoFe2O4 nanoparticles is found on the surface of the pieces.
The EPR analysis of the nanocomposites was carried out, to know the magnetic characteristics of the compounds involved in the experiment (resin, ferrites, nanostructured-composite materials), as well as the changes that the materials may have in their magnetic properties once the additive manufacturing process has been carried out.
Figure 12a shows the comparison of the EPR spectra of the nanostructured materials composed of BaFe
12O
19-IRIX White resin prior to the AM process. The concentration of BaFe
12O
19 increases upward. In all the spectra that contain resin, a signal located at 160 mT, assigned to the paramagnetic centers of Fe
3+, is observed. In each graph, the characteristic signal of barium ferrite between 315.5 and 358.5 mT is observed, which suggests the presence of BaFe
12O
19 nanoparticles. However, there is a variation in the linewidth of the spectra. It is known that the greater the linewidth amplitude (H
dc), the greater the concentration of magnetic moments and the greater the interaction between them. That is, it can be inferred that the particles are closer to each other. By "diluting" the ferrite in the resin, the magnetic moments are separated, and although there is still an interaction between them, by increasing the distance between the magnetic moments, the line width becomes smaller. The variation in the line widths obtained in the spectra (independently of the concentration of nanoparticles) suggests that the dispersion of ferrite in the resin is different for each sample.
In
Figure 12b, the EPR spectra of the BaFe
12O
19–Irix White resin nanocomposites after photopolymerization are shown. Like the samples without treatment, the signal corresponding to the magnetic dipolar interactions of BaFe
12O
19 is observed in all the spectra; however, the line width (H
dc) of each graph is different. This variation in the size of the curve suggests, again, that the dispersion of BaFe
12O
19 nanoparticles is different for each sample even after the additive manufacturing process.
In the case of the piece containing BaFe12O19 at 0.5% w/w, a signal corresponding to a splitting or hyperfine interaction is observed. In other words, due to the effect of light, a rearrangement of the electronic levels was generated in the resin, which allows the spin of the nucleus to interact directly with the spin of the unpaired electrons in the resin molecules, whereby it is possible that the sample was exposed to light before the additive manufacturing process.
Comparing the EPR results shown in
Figure 12, it was possible to determine that the signal corresponding to the Fe
3+ ions and the characteristic signal of BaFe
12O
19 are present in all the spectra, both before and after the photopolymerization process. Therefore, both substances, the resin and the nanoparticles, retained their magnetic properties after the additive manufacturing (AM) process.
Figure 13a shows the EPR spectra of the CoFe
2O
4-IRIX White resin nanocomposites before the AM process. In all the spectra containing resin, the signal located at 160 mT, corresponding to the paramagnetic centers of Fe
3+ is observed. Furthermore, in each graph, the characteristic signal of cobalt ferrite is located between 100 and 460 mT, which reveals the presence of CoFe
2O
4 nanoparticles in the samples. In this case, the line width corresponding to CoFe
2O
4 could not be obtained, since the signal is incomplete due to the measurement limitations of the equipment used. However, an increase in the intensity of the signal is observed (in all the spectra) as the concentration of NPs increases. This indicates that the higher the concentration of the nanoparticles, the higher concentration of magnetic moments and therefore greater interaction between them, that is, the nanoparticles are closer to each other. By "diluting" the ferrite in the resin, the magnetic moments are separated and the interaction between them diminishes.
Spectra of CoFe
2O
4-IRIX White resin nanocomposites after photopolymerization are displayed in
Figure 13b. Again, in all the spectra the signal corresponding to the Fe
3+ paramagnetic centers of the resin is evident. However, the characteristic signal belonging to the nanoparticles cannot be observed in the spectra. Although in the Raman spectrum for this composite nanostructured material (
Figure 11), the peaks associated with the ferrite nanoparticles are observed, these NPs are sufficiently dispersed within the material that the signal corresponding to the magnetic interactions between them are not visible in the spectrum.