Sol–gel synthesis and characterization of unexpected rod-like crystal fibers based on SiO2–(1-x)CaO–xSrO–P2O5 dried-gel

Sol–gel technique has several benefits for the preparation of glass, and morphology can be better controlled compared to conventional methods. In this research, new sol–gel derived bioactive glasses based on SiO2–CaO–SrO–P2O5 dried-gel were synthesized and characterized. Herein, a series of 58S bioactive glasses with the composition of 60%SiO2–36%(CaO/SrO)–4%P2O5 (mol%) were synthesized, and the effect of adding strontium (Sr) to the glass structure SiO2–(1-x)CaO–xSrO–P2O5 (where x=0, 0.5, 0.1, 0.25, 0.5 and 1) was investigated by gradually substitution of Sr with calcium (Ca). The obtained results indicated that the Sr free sample totally takes amorphous state indicative of the internal disorder, glassy nature and non-crystalline states of this material. Surprisingly, after further addition of Sr to the glass structure, the X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) micrographs showed unexpected significant rod-like crystal fibers, and the major diffraction peaks of Sr(NO3)2, SrCO3 and Sr2Si2O4 became sharper and more apparent up to the final addition of Sr. The complicated and contradictory results underscore the need for better knowledge of how impurities act upon by growing rod-like crystals. In addition, totally understanding the effect of Sr on the morphology of samples from the bottom up is a daunting challenge. Surprisingly, after further addition of strontium (Sr) to the glass chemical structure, it was observed that unexpected significant rod-like crystal fibers created and became more apparent up to the final addition of Sr. This significant changes in the morphology of samples might be due to the ionic nature of the Sr precursor and excess HNO3, which results in the pH value of the sol being below the isoelectric point, and thus HO–Si bonds are dominant instead of Sr+…-O–Si bonds, and after drying at 140°C, strontium nitrate is deposited onto the surface of the silica network. [Display omitted] ► Sr presents significant effect, which control growth orientation and morphology. ► Sr0 totally takes amorphous state indicative of internal disorder and glassy nature. ► After further addition of Sr, crystallite phases as Sr compounds were observed. ► Sr100 shows the most crystalline phases and unexpected rod-like crystal fibers.