**10.3. Phosphate glasses and glass-ceramics**

In the past decades, optical waveguides have raised great interest, as they are the most fundamental and integral part of integrated optic circuits. Glass-based integrated optical devices have several obvious advantages over other technologies such as low intrinsic absorption in near-infrared region of the spectrum, minimized coupling losses to optical fibers and no intrinsic material birefringence compared to crystalline semiconductors. Phosphate glasses are regarded as excellent glass host for the waveguide laser fabrication mainly because of high solubility of rare-earth ions compared to other oxide glasses, which allows for high doping concentrations without significant lifetime reduction, resulting in high gain in short waveguides or cavities and a desirable feature in single-frequency lasers. High-performance

<sup>6</sup> The rate depends on the geothermal degree, which in Europe is about 33 m·°C−1 [48].

waveguide amplifiers or lasers are fabricated in various earth-ion-doped phosphate glasses and commercial phosphate glasses such as Kigre Q89 and Schott-IOG 1 [51].

Glass-ceramics are polycrystalline materials with an inorganic–inorganic microstructure, which are prepared from the base glass by controlled crystallization. This can be achieved by subjecting glasses to regulated heat treatment, which results in the nucleation and growth of one or more crystal phases within the glass [52]. Once a stable crystal nucleus has formed and begun to grow, there are a number of possible crystal growth mechanisms and these deter‐ mine the final crystal morphology [53],[54]:


Bioactive glass-ceramics are an alternative to synthetic HAP (**Section 10.9**) for the use in vivo both in restorative dental applications and in bone implantation [54],[55]. Artificial materials implanted into the bone defects are generally encapsulated by fibrous tissue isolating them from the surrounding bone. This is the normal response of the body towards inert artificial materials. However, some ceramics, such as bioglass, A-W glass-ceramics and sintered hydroxyapatite form a bone-like apatite on their surfaces in the living body and bond to living bone through this apatite layer. This bone-bonding ability is called the bioactivity [56].

These bioactive ceramics are already used clinically as important bone-repairing materials. Their bone-bonding ability is achieved by the formation of a biologically active apatite layer after the reaction of the ceramics with surrounding body fluid. Controlled surface reaction of the ceramics is an important factor governing its bioactivity as well as its biodegradability [56]. The preparation of glass-ceramics with high CaO/P2O5 ratio, containing large amounts of calcium phosphate crystals, is believed to be one of the best approaches to obtain the biocer‐ amic implants suitable for bone replacement/regeneration [57].
