Alumina â€“ : a solid with a negative surface energy?
Water absorbed on the crystallite surface is responsible for maintaining extremely small nanoparticles with a thickness of 1â€“50 nm and a surface area of 100 m2/g up to very high temperatures.
The reason is that water adsorption makes the surface energy negative.
The new picture of nano-crystalline alumina is based on detailed density functional theory calculations and has been confirmed in IR experiments at Haldor TopsÃ¸e research laboratories.
The stable high surface area of θ â€“ alumina makes it a perfect support in catalysts.[Above right is an artist's rendition of a] nanocrystallite of θ â€“ alumina.
(note: excerpts below from: Nanomedicine, Volume IIA: Biocompatibility)
First, is alumina powder lethal?
Massive administration of several bioactive ceramic powders were lethal to mice in 5 gm/kg doses when injected intraperitoneally, producing a swollen kidney having an ischemic color, with edema of interstitial tissue in the kidney cortex, severe degenerative changes in the tubular epithelial cells, and hyaline deposits in the renal collecting tubules, along with edema and inflammation in pulmonary tissues.
Ceramics in fine powder form are generally believed to have higher bioactivity and to be associated with higher mortality.
In this experiment, as ceramic particle size was increased the fatal effects in mice decreased.
As for inhalation toxicity, human experience with alumina powders strongly suggests that they are not associated with major specific pulmonary hazards under typical 20th century conditions of occupational inhalation exposure though rodent experiments suggest that clearance of alumina particles from the lung is slow.
OSHA occupational exposure limits for alumina dust are 10 mg/m3 (total fraction) and 5 mg/m3 (respirable fraction), respectively, according to the official Material Safety Data Sheets.
Alumina refinery workers exposed to >100 mg/m3-year of gamma aluminas for >20 years had a 3- to 4-fold excess of individuals with an abnormal forced expiratory volume at 1 second, with abnormal being defined as <80%> of the predicted figure, though smoking had a far more deleterious effect on ventilatory capacity.
Alpha-alumina 100-700 nm particles have only minimal or no fibrogenic reactivity, and only at doses instilled intratracheally that are massive compared to the amount which could reasonably be inhaled in any one breath.
Such massive doses of gamma-alumina in the 20-40 nm size range did produce a fatal fibrosis of the lungs in rats