Oxygen-18, aluminium-26 and silicon-30 self diffusion in aluminosilicate glasses

¹⁸O, ²⁶Al, and ³⁰Si tracer diffusion in model aluminosilicate glasses with varying Al³⁺/Na⁺ ratios was investigated using secondary ion mass spectrometry (SIMS) depth profiling. For aluminium tracer diffusion measurement the long-lived radioactive isotope 26 Al served as a pseudo-stable isotope (half-life 7·4×10⁵ years). The tracer diffusion measurements above the glass transition (T _g) at temperatures from 490 to 821°C clearly show that the mobility of aluminium is about two orders of magnitude lower than that of oxygen in the considered aluminosilicate glasses. The mobility of silicon is a further order of magnitude lower. The activation enthalpy of diffusion is largest for silicon, followed by aluminium and is lowest for oxygen. For both oxygen and aluminium the mobility decreases (and the activation enthalpy increases) with increasing Al³⁺/Na⁺ ratio. For the glass with the highest Al³⁺/Na⁺ ratio the tracer diffusivities of ¹⁸O and ²⁶Al were also measured below T _g. Possible diffusion mechanisms are discussed for all three elements.