# Several Aspects of Application of Nanodiamonds as Reinforcements for Metal Matrix Composites
## 3 Results and Discussion
On the other hand, if the transformation of nanodiamond nanopowders takes place according to the second scheme, it is necessary to work with the nanopowders obtained via synthesis. Therefore, it is impossible to reduce their size by annealing and cleaning. It should be noted that a detached ND particle does, in fact, transform layer by layer when exposed to local heating by an electron beam in the electron microscope. Figure 5a-c shows annealing stages for a particle heated with an electron beam in a transmission electron microscope. Figure 4: Results of DSC study of nanocomposite “Al + 25% ND”: exo-peak is the result of Al\({}_{4}\)C\({}_{3}\) formation; endo-peak is the result of aluminium melting. However, as investigations show, when massive samples of ND agglomerates are exposed to heat treatment (heating of significant quantities, not local), the transformation process is realized according to the second hypothesis: ND particles from the surface layer of agglomerates transform earlier than nanoparticles located in the central part of the agglomerate. Agglomerated ND powders annealed in vacuum furnaces at 1180 to 1300 \({}^{\circ}\)C were studied using the transmission electron microscopy and electron energy loss spectroscopy (EELS). Figure 6a shows the fracture point of the partially transformed ND agglomerate. Parts with particles that have 100% diamond structure and parts with completely transformed particles are clearly visible. This fact attests to the second hypothesis. A separate investigation of the nanoparticles on the surface of partially annealed agglomerates showed that they had undergone complete transformation (Figure 6b). Figure 7 shows the EEL spectra. The size of the electron beam is within a range of 2-5 nm, and the size of the ND agglomerates is about 50 \(\upmu\)m. The experimental method is  
Figure 5: Transformation of a detached nanodiamond particle exposed to local heating by electron beam; images were produced with an interval of 60 s (TEM JEOL JEM 2100 F/Cs, accelerating voltage 200 kV): (**a**) big diamond core; (**b**) small diamond core; (**c**) complete transformation. Figure 6: TEM images of nanodiamonds after annealing at 1200 \({}^{\circ}\)C: (**a**) fracture of ND agglomerate after partial transformation; and (**b**) complete transformation of ND in surface layers of agglomerate. shown in Figure 7a. Zone 1 mainly includes carbon onions. The EEL spectrum for zone 1 is shown in Figure 7b: the sp\({}^{2}\) peak (\(\pi^{\star}\) peak) is remarkably high. Zone 2 mainly consists of particles from the central layers of the ND agglomerate and a small number of particles from the surface layers. The EEL spectrum for this zone is shown in Figure 7c: the sp\({}^{2}\) peak (\(\pi^{\star}\) peak) has a much lower intensity than zone 1 (enhanced thickness leads to the noisy image). This result suggests that the ND agglomerate has a core of nanodiamond particles. This core is coated with a layer of graphitic particles (carbon onions), i.e., the second hypothesis is valid for transforming agglomerated ND powders. It is probable that several reasons led to this result. In our opinion, the main reason is the energetic state of the nanoparticles involved. It is necessary to note that the energetic state of ND particles located in the surface layer and located in the deeper layers of the agglomerate differs due to several reasons, mainly the difference in the surface energy of the nanoparticles. Nanoparticles on the agglomerate surface have a significant portion of their surface open (free). The free surface of the ND nanoparticles located in deeper layers is much less; i.e., the surface energy is different. This fact leads to the difference  
Figure 7: EEL spectra for the surface and central layers of the ND agglomerate: (**a**) method of data collection; (**b**) EEL spectrum from zone 1 (particles from surface layers); and (**c**) EEL spectrum from zone 2 (mainly particles from central layers). in the energetic state of nanoparticles from the surface and deep layers of agglomerate.