Nitridation of Bioresorbable Phosphate Glass
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Glass Preparation The phosphate glass with a composition of 35Na2O l0CaO 55P 20 5 was prepared from reagentgrade NaH 2PO 4H 20, CaCO 3, and NH 4H 2P0 4. The powdered batch was melted in alumina crucible under air at 750 'C for I hr. The glass melt was quenched into a clean high-purity graphite boat which was transferred into the tube furnace shown in Fig. 1.Then the glass was remelted at 900 'C under dry nitrogen for 1 hr. Previous work had shown that this treatment was sufficient to remove most chemically dissolved water [7]. Afterwards, the atmosphere inside the silica glass tube was changed to anhydrous ammonia, and the glass was reacted at different temperatures for different times. After nitridation, the graphite boat was removed from the furnace and the furnace sealing cap
graphite boat
silica glass tube
Fig. 1. The tube furnace for nitridation. 501 Mat. Res. Soc. Symp. Proc. Vol. 455 01997 Materials Research Society
glass was quenched into a metal mold and annealed at about 320 'C for 1 hour in air. Characterization of the Glass and Glass Fiber About 0.5 g glass powder of CNP-NI was remelted at 700 'C in air. Glass fibers were drawn from glasses CNP-N1 and CNP-N5 at about 700 'C to 740 'C in a platinum bushing under air. The glass transition temperatures were measured by DSC with heating rate of 10 °C/min using the intercept method for the onset of the transition. 1 g powder sample of the bulk glass and glass fiber was dissolved in 100 ml distilled or de-ionized water at room temperature. The dynamic pH values of the solutions were measured. The equilibrium pH values were measured after 5 months. By using DSC, the peak crystallization temperatures were measured with different temperature increasing rates. RESULTS AND DISCUSSION The Nitrogen Content and Glass Transition Temperature The nitrogen content was calculated by assuming that every two nitrogen atoms replace three oxygen atoms. The chemical reaction of the nitridation for the parent glass is: Ca0 Na 0o 7P. 1 032 + xNH 3 -> Cao (1) 0 Nao-7 P. 1 0 3.2-1. 5xN + 1.5xH 20 The nitrogen contents were calculated from the original weight of the glass and the weight after Table I. Nitrogen content and glass transition temperature. Glass CNP CNP-Nl CNP-N2 CNP-N3 CNP-N5 CNP-N1 Fiber CNP-N5 Fiber
Nitriding Temp., 'C
Nitriding Time, hr
750 735 720 750
4 4 8 6
N Content (wt%) 0 4.95 3.10 2.30 6.24
Tg, C
Tc, C
260 342 307 308 355 343
356 none none 436 none none I
356
none I
I
nitridation and are shown in Table I. As indicated before [2], the nitrogen content increases with increasing the nitriding time or temperature. Table I also shows that the glass transition temperature increases when the nitrogen content increases, since nitrogen increases the cross linking of the glass network. Nitrogen Stability and Fiberization As stated before, fiberization is very important for bioresorbable materials. But for nitrided phosphate glass, it is unknown if nitrogen is retained during fiberization , especially when the fiber is drawn in air. It is shown in Fig.
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