# Sintering Behavior of a Six-Oxide Silicate Bioactive Glass for Scaffold Manufacturing
## 3 Results and Discussion
In order to explain this, two different hypotheses can be proposed:  
1. [label=_()_]
2. the nucleation of a metastable crystalline phase at 750 \({}^{\circ}\)C, which gradually evolves to a second more stable crystalline system, resulting in a second exothermic signal at temperatures T \(\geq\) 800 \({}^{\circ}\)C;
3. the nucleation of two different metastable phases at 750 \({}^{\circ}\)C and 780 \({}^{\circ}\)C, which evolve to a third crystalline system stable at temperatures \(\geq\) 850 \({}^{\circ}\)C.  
In this regard, XRD analyses performed on sintered 47.5B tablets confirmed the progressive nucleation of crystalline species with increasing sintering temperatures. XRD patterns related to the samples sintered at different temperature are depicted in Figure 2. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sintering Temperature** & **Glass Transition** & **Crystallization Onset** & **Crystallization Peak** & **Maximum Shrinkage** \\ \(T/^{\circ}\)C & \(T_{g}/^{\circ}\)C & \(T_{d}/^{\circ}\)C & \(T_{g}/^{\circ}\)C & \(\Delta H/\%\) \\ \hline
600 & 527 & - & - & 24 \\
650 & 528 & - & - & 28 \\
700 & 528 & - & - & 27 \\
750 & 527 & 690 & 750 & 18 \\
800 & 530 & 707 & 750,783 & 21 \\
850 & 531 & 697 & 772 & 29 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Characteristic temperatures of 47.5B bioactive glass sintered at different temperatures assessed by DTA measurements under isothermal conditions. As expected, specimens sintered below the crystallization onset, at 600 and 650 \({}^{\circ}\)C (Figure 2a,b), revealed no diffraction peaks and were characterized by an amorphous halo typical of silicate glasses  
Figure 2: XRD patterns of 47.5B tablets sintered 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 20 values ranging between 25\({}^{\circ}\) and 35\({}^{\circ}\), thus indicating that no microstructural changes occurred within the material as a result of these sintering treatments. On the contrary, the patterns of samples treated at 700, 750, 800 and 850 \({}^{\circ}\)C presented multiple diffraction peaks, which are typical of glass-ceramic materials, as might be predicted from DTA thermograms. Crystalline phases detected above the crystallization onset are summarized in Table 3.  
The analysis carried out on all the samples sintered at temperatures \(\geq\) 700 \({}^{\circ}\)C revealed the presence of two main crystalline species, i.e., combineite (Na\({}_{2}\)Ca\({}_{2}\)(Si\({}_{3}\)O\({}_{9}\))) and tremolite (CaMg\({}_{3}\)(SiO\({}_{4}\))\({}_{3}\)), together with other sodium-calcium silicates with different stoichiometry, to a lesser extent. The formation of combineite (Na\({}_{4}\)Ca\({}_{4}\)(Si\({}_{6}\)O\({}_{18}\)) and akemanite (Ca\({}_{2}\)Mg(Si\({}_{2}\)O\({}_{7}\)), a calcium-magnesium silicate similar to tremolite, was already observed upon sintering treatment at high temperature (\(\geq\) 950 \({}^{\circ}\)C) in a silica-based experimental composition above, named CEL2, exhibiting the same oxides system of 47.5B [39]. Moreover, the formation of sodium-calcium silicate crystalline phases was also reported in our previous studies on the production of bread-templated and dolomite-foamed 47.5B-based scaffolds sintered at 750 \({}^{\circ}\)C and 800-850 \({}^{\circ}\)C, respectively; moreover, sodium-calcium silicate (combeite-type) phases were also reported to be the main crystalline species nucleating above 550 \({}^{\circ}\)C in 4555 Bioglass(r), which is commonly adopted as the positive control material among BGs in terms of bioactivity and citocompatibility standards [30; 40]. Hot-stage microscopy (HSM) measurements under isothermal conditions were carried out in order to quantify the volumetric shrinkage of glass samples upon sintering at different temperatures.