Tantalum powders, ranging from 14 to 56 nm in primary particle size, were produced through hydrogen reduction of TaCl5 vapor in a furnace aerosol reactor. The mass of the precursor, solid TaCl5, was instantaneously measured during vaporization with a load cell to determine and control the precursor concentration at the reactor inlet. Tantalum pentachloride to tantalum conversion was 35% and 98% at 1000 and 1400 degrees C, respectively. The primary particle size increased with increasing the evaporator temperature and the hydrogen flow rate. The tantalum powder produced was amorphous at 1000 degrees C and crystalline at 1400 degrees C. The XRD peaks of the as-produced powders shifted towards lower angles due to the partial dissolution of hydrogen in the tantalum. Upon removal of the hydrogen through heat treatment in a vacuum, the shift disappeared. The oxygen content of the powder was 6.0 to 6.9 wt.%, with an oxide layer about 3 nm thick formed on individual particles. Through thermogravimetric analysis in air, the powders were found to be resistant to further oxidation at up to 200 degrees C, but to have been completely oxidized at 400 degrees C. (c) 2006 Elsevier B.V All rights reserved.