CuO nanocrystal formation during sintering of Cu-doped bioactive silicate glass powder surfaces

CuO nanocrystals were found to grow on polished, fractured, and glass powder particle surfaces of the copper-doped bioactive glass BG F3-Cu (44.8 SiO 2 –2.5 P 2 O 3 –35.5 CaO–6.6 Na 2 O–6.6 K 2 O–3 CaF 2 –1 CuO; mol%). Our results indicate that this CuO formation is driven by an inhibited Cu + /Cu 2+ redox equilibrium. Since Cu 2+ (CuO) is unstable during melting, a Cu + (Cu 2 O) excess is frozen during cooling. Due to the limited oxygen availability, Cu + diffuses to the surface to get oxidized to Cu 2+ . Such oxidation, however, also occurs close beneath the melt surface during casting. As this reduces the CuO formation driving force, no CuO was found on the as-cast glass surface. The large low-temperature driving force of Cu oxidation also explains why CuO nanocrystals can easily grow during prolonged annealing well below the onset of the dominating surface crystallization of combeite during heating at 10 K/min. The minor influence of CuO surface crystal formation on sintering and overall crystallization allows for its control largely independent of the achievable densification and phase composition of the sintered BG compacts.