# Sintering Behavior of a Six-Oxide Silicate Bioactive Glass for Scaffold Manufacturing
## 3 Results and Discussion
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Sintering Temperature** & **600** & **650** & **700** & **750** & **800** & **850** \\ _T/C_ & & & & & & \\ \hline Diameter _D_/mm & 8.61 \(\pm\) 0.27 & 8.01 \(\pm\) 0.63 & 10.62 \(\pm\) 0.41 & 8.85 \(\pm\) 0.14 & 9.68 \(\pm\) 0.12 & 8.70 \(\pm\) 0.47 \\ Height H/mm & 7.04 \(\pm\) 0.14 & 6.11 \(\pm\) 0.11 & 6.24 \(\pm\) 0.43 & 6.00 \(\pm\) 0.58 & 7.51 \(\pm\) 0.39 & 6.65 \(\pm\) 0.32 \\ Cross sectional area _A_/mm\({}^{2}\) & 58.31 \(\pm\) 3.70 & 51.07 \(\pm\) 7.73 & 88.84 \(\pm\) 6.86 & 61.77 \(\pm\) 2.02 & 73.67 \(\pm\) 1.80 & 59.68 \(\pm\) 6.30 \\ Mass _m_/g & 0.250 \(\pm\) 0.02 & 0.243 \(\pm\) 0.01 & 0.285 \(\pm\) 0.02 & 0.325 \(\pm\) 0.03 & 0.249 \(\pm\) 0.03 & 0.259 \(\pm\) 0.02 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Geometrical characterization of foam-replicated scaffolds used for mechanical testing. Values are expressed as mean value \(\pm\) standard deviation, calculated on three samples for each sintering group. Figure 1: Differential thermal analysis (DTA) simulating the sintering treatment at different temperatures: 600 \({}^{\circ}\)C (**a**), 650 \({}^{\circ}\)C (**b**), 700 \({}^{\circ}\)C (**c**), 750 \({}^{\circ}\)C (**d**), 800 \({}^{\circ}\)C (**e**) and 850 \({}^{\circ}\)C (**f**). For each curve reported in Figure 1, glass transition temperature \(\mathrm{T_{g}}\) was defined within the range 527-531\({}^{\circ}\)C, in correspondence of the inflection point, while the crystallization onset \(\mathrm{T_{x}}\) was identified at the onset point of the exothermic peaks (Figure 1d-f), between 690 and 707 \({}^{\circ}\)C, in good agreement with the temperature ranges previously reported in non-isothermal conditions [34]. Glass powders sintered below the crystallization onset (Figure 1a,b) did not undergo any crystallization and DTA curves referred to these systems were characterized by no exothermic peaks. A steplike increase was observed in correspondence of the beginning of the dwell stage, most likely attributed to the shift to isothermal conditions in the program settings. A similar trend, in fact, was also observed in Figure 1c and d but was completely hidden for the glasses heated up to 800 and 850 \({}^{\circ}\)C (Figure 1e,f), due to the overlap of the exothermic crystallization peaks. A mild crystallization was observed for a sintering temperature of 700 \({}^{\circ}\)C, after about 30 min from the beginning of the dwelling stage (Figure 1c); compared to the previous DTA thermograms acquired at 600 \({}^{\circ}\)C and 650 \({}^{\circ}\)C, which presented a flat trend in stasis, this curve evolved in a broad hump, suggesting the beginning of devitrification phenomena. This hypothesis was strongly supported by the range of crystallization onset previously provided. As expected, the \"hump\" observed for temperature close to the glass crystallization onset evolved into sharp exothermic peaks approaching higher sintering temperatures (Figure 1d-f). As a first approximation, it seemed that the exocrine signals observed at 750 and 800 \({}^{\circ}\)C presented analogous features. Both of them, in fact, consisted of two different peaks, closely located. However, some additional considerations could be made if the position of these signals with respect to the temperature-time dashed curve is considered. In the system treated at 750 \({}^{\circ}\)C, the two exothermic peaks are both registered at the end of the heating ramp, at a constant temperature of 750 \({}^{\circ}\)C (Figure 1d). On the contrary, the peaks observed in Figure 1e developed upon heating at 750 and 783 \({}^{\circ}\)C, respectively, with a time delay of about 6 min. It is worth observing that the curve related to the glass sintered at 850 \({}^{\circ}\)C (Figure 1f) presented a single peak centered at 772 \({}^{\circ}\)C, ending in correspondence of the beginning of the 850 \({}^{\circ}\)C dwell, indicating that glass devitrification is already completed before the beginning of the dwelling stage.