# Glass Ceramic with Improved Structure and Properties Made with Wastes from FeNi Production
## 4 Results and Discussion
### Phase Formation and Structure  
The efficiency of the optimal thermal regime was studied using XRD, SEM, TEM and FESEM. Various samples, obtained with and without the nucleation step at 650 \({}^{\circ}\)C for 1 h, and with 1, 5 and 60 min crystallization holds at 730 \({}^{\circ}\)C, were made and analyzed. These specimens were labeled GC0-1, GC0-5, GC0-60, GC60-1, GC60-5 and GC60-60; the first number indicates the nucleation time, and the second, the crystallization time. The XRD pattern of the parent glass (a) and specimens GC60-1 (b), GC60-5 (c) and GC60-60 (d) are shown in Figure 3.  
Some spinel was observed in the parent glass. As a result of the low Cr\({}_{2}\)O\({}_{3}\) solubility in silicate melts, when its concentration is higher than 0.7-1.0 wt %, a part of this oxide spontaneously precipitates into \"preliminary\" spinel crystals during the melt cooling; the residual part eventually might participate in the formation of a \"secondary\" spinel phase at low temperatures. The \"preliminary\" spinel (labelled S1) that was formed in our glass was defined as a solid solution between MgO-Cr\({}_{2}\)O\({}_{3}\) (ICSD171106) and FeO-Fe\({}_{2}\)O\({}_{3}\) (ICSD158742). In sample GC60-1, the formation of some pyroxene phase (labelled P1) was also identified. After 5 min crystallization at 730 \({}^{\circ}\)C, some increasing in its amount were observed. In the final glass-ceramics (see Figure 3d), together with P1, a new pyroxene phase, one which practically becomes the main crystal phase in the material (labelled P2), is formed. According to the structural analysis P1 is characterized with a unit cell with V-428 \(\times\) 10\({}^{6}\) pm\({}^{3}\), whereas in P2, V-422 \(\times\) 10\({}^{6}\) pm\({}^{3}\). This difference is about 1.5%, and it clearly highlights a variation in the chemical compositions of both pyroxene phases. It can be assumed that some larger ions participate in the formation of the P1 phase. Additionally,  
Figure 3: XRD patterns of parent glass (a) and nucleated samples, crystallized for 1 min (b), 5 min (c) and 60 min (d) at 730 \({}^{\circ}\)C.  
some increase of the spinel phase is observed, which must be related to the formation of \"secondary\" magnetite spinel (labelled S2). The results for the non-nucleated samples (GC0-1, GC0-5 and GC0-60) show no significant differences from the nucleated samples. After 1 min at 730 \({}^{\circ}\)C, the phase formation is still in its initial stage and the SEM image, which is presented in Figure 4a, practically shows only \"preliminary\" spinel crystals. Their size is between 3 and 10 \(\upmu\)m, and the EDS analysis reports a quite constant chemical composition (mol %): 36 \(\pm\) 2 iron oxides, 23 \(\pm\) 2 Cr\({}_{2}\)O\({}_{3}\) and 41 \(\pm\) 3 MgO. These results confirm the XRD data for the formation of a solid solution between MgO\(\cdot\)Cr\({}_{2}\)O\({}_{3}\) and FeO\(\cdot\)Fe\({}_{2}\)O\({}_{3}\). Since the molar percentage of MgO in the parent glass is higher than that of Cr\({}_{2}\)O\({}_{3}\), it follows that the iron taking part in the formation of the \"preliminary\" spinel is predominantly in Fe\({}^{3+}\) valence. This indicates that some increase of the Fe\({}^{2+}\)/Fe\({}^{3+}\) ratio in the main glassy matrix can be expected, which must favor the subsequent formation of a \"secondary\" magnetite spinel phase. In addition, considering that the overall volume of the Cr-Mg-Fe spinel phase is at about 4-6%, it might also be concluded that the chromium oxide is mainly concentrated in the \"preliminary\" spinel crystals. Figure 4b shows a photo at higher magnification, with a group of \"preliminary\" spinel crystals in GC60-1. It is evident that around these crystals is formed a \"homogeneous\" crystallization field with a thickness between 5 and 10 \(\upmu\)m.  
However, the main volume of the sample (i.e., at about 80-85%), where the Fe\({}_{2}\)O\({}_{3}\) concentration is high, is characterized with a \"non-homogeneous\" structure due to the formation of tiny spherical particles enriched with iron.