Determination of the Oxide Layer Thickness on Beryllium Metal Particles

The hypothesis was tested that beryllium metal particles have a uniformly thick surface coating of beryllium oxide and that smaller particles should have a higher oxide fraction by mass because they have a higher surface to volume ratio. The mass fraction of oxygen, physical density, and specific surface area were determined for size-fractionated samples of respirable beryllium metal particles. The largest particles analyzed (count median diameter 4.6 µm with geometric standard deviation 1.6) contained 7%±1% beryllium oxide by mass; had a physical density of 1.90±0.02 g/cm3; and had a specific surface area of 4.0±0.3 m2/g. The smallest particles analyzed (count median diameter 0.4 µm with geometric standard deviation 1.8) contained 31%±3% beryllium oxide by mass; had a physical density of 2.00±0.17 g/cm3; and had a specific surface area of 20.8±2.1 m2/g. These shifts in density and oxide content with size and surface area are consistent with a beryllium metal core of density 1.84±0.02 g/cm3 (1.848 g/cm3 is theoretical); a beryllium oxide layer of density of 2.53±0.16 g/cm3 (3.025 g/cm3 is the perfect crystalline density); and an oxide layer thickness of 49±4Å for all particle sizes. These results indicate that the inhalation toxicity of beryllium metal particles may be similar to that of beryllium oxide formed at low temperatures.