Abstract
The controlled precipitation of strontium fluoroapatite crystals was studied in four base glass compositions derived from the SiO2–Al2O3–Y2O3–SrO–Na2O–K2O/Rb2O/Cs2O–P2O5–F system. The crystal phase formation of these glasses and the main properties of the glass-ceramics, such as thermal and optical properties and radiopacity were compared with a fifth, a reference glass-ceramic. The reference glass-ceramic was characterized as Ca-fluoroapatite glass-ceramic. The four strontium fluoroapatite glass-ceramics showed the following crystal phases: (a) Sr5(PO4)3F – leucite, KAlSi2O6, (b) Sr5(PO4)3F – leucite, KAlSi2O6, and nano-sized NaSrPO4, (c) Sr5(PO4)3F – pollucite, CsAlSi2O6, and nano-sized NaSrPO4, and (d) Sr5(PO4)3F – Rb-leucite, RbAlSi2O6, and nano-sized NaSrPO4. The proof of crystal phase formation was possible by X-ray diffraction. The microstructures, which were studied using scanning electron microscopy, demonstrated a uniform distribution of the crystals in the glass matrix. The Sr-fluoroapatites were precipitated based on an internal crystallization process, and the crystals demonstrated a needle-like morphology. The study of the crystal growth of needle-like Sr-fluoroapatites gave a clear evidence of an Ostwald ripening mechanism. The formation of leucite, pollucite, and Rb-leucite was based on a surface crystallization mechanism. Therefore, a twofold crystallization mechanism was successfully applied to develop these types of glass-ceramics. The main focus of this study was the controlled development of glass-ceramics exhibiting high radiopacity in comparison to the reference glass-ceramic. This goal could be achieved with all four glass-ceramics with the preferred development of the Sr-fluoroapatite – pollucite-type glass-ceramic. In addition to this main development, it was possible to control the thermal properties. Especially the Rb-leucite containing glass-ceramic showed the highest coefficient of thermal expansion (CTE). These glass-ceramics allow optical properties, especially the translucency and color, to be tailored to the needs of biomaterials for dental applications. The authors conclude that it is possible to use twofold crystallization processes to develop glass-ceramic biomaterials featuring different properties, such as specific radiopacity values, CTEs, and optical characteristics.
Highlights
The precipitation of fluoroapatites, in glass-ceramics using the methods of controlled nucleation and crystallization of glasses, is well known
X-ray diffraction (XRD) studies of all four monolithic glass samples, showed the formation of primary crystal phases, precipitated during cooling of the glass melts. The microstructures of these glasses No 1–4 were characterized by nano-sized spherical crystals of different types: nano-sized Sr5(PO4)3F crystals in glasses No 1 and 2 and nano-sized NaSrPO4 crystals in glasses No 3 and 4
The authors of this paper used the nano-glass-in-glass phase separation as a primary stage of the internal nucleation process, which was followed by the internal crystal phase formation and crystal growth of uniform phases in the volume of the glass-ceramics
Summary
The precipitation of fluoroapatites, in glass-ceramics using the methods of controlled nucleation and crystallization of glasses, is well known. Fluoroapatite glass-ceramics were reported by Clifford and Hill (1996) and Moisescu et al (1999). Bioactive glass-ceramics, which bond to living bone, contain fluoroapatite crystals. Such a product was developed by Kokubo (1991) who applied the method of twofold nucleation to create a fluoroapatite–wollastonite glass-ceramic. Dejneka and Pinckney (1998) developed fluoroapatite glassceramics with special optical properties All these different types of apatite glass-ceramics were discussed and described on the basis of their chemical nature and microstructure formation by Höland and Beall (2012). The growth of needle-like Ca5(PO4)3F crystals was discovered as an Ostwald ripening process by Müller et al (1999) and Höland et al (2000) and in a different chemical system by Höche et al (2001)
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