Superconducting transition temperature in anodized aluminum

We have measured the superconducting transition temperature of anodized aluminum films of grain sizes ranging from less than 100 to 3000 {}. The transition temperature is 1.8 K for films of grain size 100 {} and decreases monotonically with increasing grain size to 1.2 K for 3000-{} grains. The effect depends only on the volume of the grains.