Crystallization behavior of phosphate glasses and its impact on the glasses’ bioactivity

The crystallization behavior of phosphate glasses of the composition 50P2O5–(40−x)CaO−xSrO–10Na2O with x = 0, 20, and 40 was investigated using non-isothermal differential thermal analysis. The study was performed on three different size fractions (fine powder ( 500 μm)). The DTA thermograms recorded for all glasses exhibited a single and symmetrical crystallization peak. The position and shape of the crystallization varies with the particles size. The activation energy for crystallization, E c, was calculated using equations proposed by Kissinger and Friedman. The calculated E c values were similar for both techniques. E c decreased with increasing the particle size for all glass compositions. The Johnson–Mehl–Avrami exponent n expressing the crystal growth dimensionality was quantified using the methods proposed by Augis–Bennett (AB) and Ozawa. In general, the AB method gave higher value than the Ozawa technique. For all glass composition the n value for the coarse powder suggested bulk crystallization whereas with decreasing particle size, the value of n indicated a complex surface crystallization. Glass monoliths were heat treated in electric oven at several temperatures for various times. XRD suggested crystallization of two phases: Ca(PO3)2 and NaCa(PO3)3 for the glass with x = 0, Sr(PO3)2 and NaSr(PO3)3 for the glass with x = 40, and a solid solution of (Ca,Sr)(PO3)2 and Na(Ca,Sr)(PO3)3 for the glass with x = 20. All crystals were found to preferentially nucleate and grow from the surface. Partially to fully crystallized glass particles were immersed in simulated body fluid for 24, 48, and 72 h. SEM/EDS of the particles and changes in the pH and ion concentrations of the solution (ICP-OES) suggested that crystallization prevents the CaP layer formation for all partially crystallized glasses.