## 3 Results and Discussion
### W-TCP Eutectic
The critical cooling rate (\(\mathrm{R_{c}}\)) for glass formation is correlated to the \(\gamma\) parameter by means of the following expression:  
\[\mathrm{R_{c}}=\mathrm{R_{o}}\;\mathrm{exp}\;[(-\mathrm{ln}\;\mathrm{R_{o}}) \gamma/\gamma_{\mathrm{o}}, \tag{1}\]  
where the constants \(\mathrm{R_{o}}\) and \(\gamma_{\mathrm{o}}\), for oxide glasses, are 8 \(\times\) 10\({}^{27}\) K/s and 0.421, respectively. The devitrification and crystallization process of W-TCP eutectic glass begins at 870 \({}^{\circ}\)C with the crystallization of a Ca-deficient apatite phase [13]. Accounting that the glass transition temperature for this glass was estimated at 790 \({}^{\circ}\)C, the calculated \(\gamma\) parameter and critical cooling rate \(\mathrm{R_{c}}\) were found to be 0.414 and 3 K/s, respectively. This rate corresponds to a solidification rate of about 100 mm/h in the Laser Floating Zone (LFZ) technique for an experimental axial thermal gradient of 10\({}^{5}\) K/m, in good accordance with the growth rate of 150 mm/h reported by [PERSON] et al. for the fabrication of a W-TCP eutectic glass rod of 3 mm in diameter [15]. It is also similar to other critical cooling rates of oxides given in the literature [16]. This eutectic glass, produced by the laser floating zone technique, is transparent as seen in Figure 4a. In this figure, the precursor rod located at the top of the figure and the glass resulting from the manufacturing process at the bottom can be observed. As mentioned before, the fabrication process is carried out downwards. Figure 4b corresponds to a W-TCP rod, grown at two different rates. The translucent part, on the left, corresponds to a glass-ceramic composite and the opaque part, on the right, is totally crystalline; they are separated by a glass portion. We can conclude that it is possible to obtain a range of microstructures from the eutectic composition, with different crystalline phases, through appropriate design of solidification conditions. Figure 4: Growth by LFZ of a glass rod of W–TCP with the eutectic composition (**a**). Eutectic W–TCP sample (**b**) grown at two different conditions (100 mm/h left part and 20 mm/h right part). According to the glass composition, its bioactivity can be predicted by the N\({}_{\rm c}\) factor that corresponds to the number of bridging oxygen bonds per silicon atom. The formula expressing this factor is [17]:  
\[\rm{N_{c}=\frac{4[SiO_{2}]\ -\ 2[M_{2}O\ +\ MO]\ +\ 6[P_{2}O_{5}]}{[SiO_{2}]}} \tag{2}\]  
For this glass N\({}_{\rm c}\) = 2, glasses of N\({}_{\rm c}\) > 2.6 are likely not bioactive due to their resistance to dissolution. The composition of this eutectic (33.33 SiO\({}_{2}\)-58.33 CaO-8.33 P\({}_{2}\)O\({}_{5}\)) corresponds to an \"invert\" glass, as firstly proposed by [PERSON] and [PERSON] [18], where the modifier content (Ca) is larger than the former content (Si + P), which contains less bridging oxygen resulting in isolated parts and shorter chains. The term \"invert\" was proposed since the traditional network forming oxides SiO\({}_{2}\) and P\({}_{2}\)O\({}_{5}\) form continuous molecular/ionic networks in normal conditions. Nevertheless, if the network modifying oxides, like in this glass, are in majority on the molar basis, the glasses are structurally inverted compared to conventional glasses. This structural inversion is reflected in the properties of the glasses as reported by [PERSON] et al. [19]. Their work permitted to show that the phosphorus-containing invert glasses are active, developing the apatite layer in relatively short immersion times (10-16 h) in SBF. The appearance of different crystalline or glassy phases coexisting in the material controlled by the solidification conditions would allow to make a specific material, with the same composition but different mechanical response or biodegradation rate depending on the application.