# Glass Ceramic with Improved Structure and Properties Made with Wastes from FeNi Production
## 4 Results and Discussion  
### Evaluation of the Optimal Crystallization Heat Treatment  
Evaluation of the suitable crystallization regime is essential for the successful conversion of the initial glass into a fine crystalline glass-ceramic. Typically, the times and temperatures for nucleation and crystallization stages are estimated by sophisticated microscopic studies and quantitative XRD analysis [1; 24], which, unfortunately, are expensive and time-consuming. However, other techniques can also be applied. The most popular alternative for estimating nucleation temperature and time is use of non-isothermal DTA experiments [12; 24]. In this case, the temperature shifts, \(\Delta\)T, of the exothermic crystallization peak temperature, T\({}_{\rm P}\), to inferior temperatures is used as an indicator for the effectiveness of the nucleation process. This is a consequence of the fact that the formation of a significant number of new nuclei can result in a smaller size of crystals being formed, which accelerates the rate of crystallization. The higher the number of formed crystals, the smaller their size and the larger the temperature shift. The DTA method is also widely used to estimate the crystallization tendency (i.e., bulk or surface). In this case, the ability for bulk nucleation and growth is evaluated by a simple comparison of the DTA runs of bulk and fine powder samples. At a satisfactory tendency for bulk crystallization, the T\({}_{\rm p}\) of the bulk sample can be similar to that of the powder sample. To the contrary, when the tendency for bulk nucleation is scarce, the crystallization exo-peak is larger, with lower intensity, and it appears at a higher temperature. The DTA-TG data of the powder sample (below 75 \(\mu\)m) and that for non-nucleated and nucleated bulk samples of the discussed glass are shown in Figure 2. The DTA results elucidate a glass transition temperature of 615-620 \({}^{\circ}\)C (estimated as the first onset of the heat flow signal in the glass transition range) for all samples, and T\({}_{\rm p}\) at \(\sim\)816, \(\sim\)798 and 786 \({}^{\circ}\)C for powder sample, non-nucleated and nucleated bulk samples, respectively. Additionally, the traces of the powder sample show an exotherm with low intensity in the interval 720-780 \({}^{\circ}\)C, which is connected with a weight increase of about 0.5 wt %. This effect can be associated with the surface oxidation of Fe\({}^{2+}\) into Fe\({}^{3+}\) which takes place before the crystallization in the glass powder. Similar DTA peaks and TG signals are not present in the bulk samples due to their low specific surfaces. Since a complete oxidation of the iron oxides is expected in the glass transition temperature interval, the value of TG gain might be used to evaluate the ratio Fe\({}^{2+}\)/Fe\({}^{3+}\) in the parent glass [9; 14]. For our composition, the weight rise due to FeO oxidation is \(\sim\)0.55 wt %, which corresponds to an initial FeO amount of \(\sim\)5% and a FeO/F\({}_{2}\)O\({}_{3}\) ratio of \(\sim\)0.34. This ratio is typical for similar glass compositions and melting procedures and is not very different from the magnetite (FeO-Fe\({}_{2}\)O\({}_{3}\)) ratio of 0.5, one which can provoke the most exhaustive formation of iron spinel. The higher crystallization temperature of the powder samples is unusual behavior, consequent to the surface oxidation Fe\({}^{2+}\) into Fe\({}^{3+}\), leading to an increase of viscosity and to a significant decrease of the magnetite (FeO-Fe\({}_{2}\)O\({}_{3}\)) spinel [5] formation in the glass powders. The viscosity variation is result of the different structural roles of Fe\({}^{2+}\) (which is a typical \"modifier\") and Fe\({}^{3+}\) (acting as \"intermediates\") in glass and melt structures. The intense DTA peak of the non-nucleated bulk sample at 798 \({}^{\circ}\)C, only about 200 \({}^{\circ}\)C higher than the glass transition temperature, indicates a notable bulk crystallization, even without nucleation heat-treatment.