An incident occurred when Ti-doped sodium alanate was exposed to air, apparently resulting in an unstable compound that experienced a rapid exothermic reaction.
The sample consisted of mechanically milled NaAlH4 with 4% TiCl3 dopant which was prepared in an argon atmosphere. The sample was sealed and placed in the probe head of an NMR magic angle-spinning (MAS) rotor and spun at approximately 9,000-13,000 rpm. During the process, the sealing cap dislodged and exposed the sample to ambient air for a little less than 24 hours. When discovered, the sample was visually inspected and showed no evidence of oxidation. The sample was re-capped and returned to an argon environment for removal. Most of the sample material was removed using a small stainless steel needle, but a residual amount, roughly 25 mg sodium alanate, proceeded to undergo a rapid exothermic reaction. No damage resulted to the tube, the glove box or the scientist.
The lab does not know the composition of the material after exposure to the ambient air nor the ignition energy needed to initiate the reaction. However, it appeared that this material underwent a rapid exothermic reaction requiring very little ignition energy.
Aside from exposing this safety hazard, and the relatively minor incident, laboratory personnel pointed out the advantage of working with small samples.
The lab is planning to study this phenomenon, including the running of a time-of-flight mass spectrographic study, to determine what occurred. It will be important for the community at large to be aware of any potentially unknown hazards of working with these materials. The lab believes that the slow exposure to room air is the greatest concern. While loose powder samples will quickly react with the air and ignite, this tightly packed powder indicated no signs of reactivity.
One needs to take extreme care with both new and supposedly spent hydride samples; the spent materials may contain pockets of unoxidized alanates that could react violently when being transferred.
Work with small samples so if something does go wrong, the possibility of serious injury is low.
The lab believes that the slow exposure to room air is the greatest concern.
Additional discussion about working with reactive metal-hydride materials in the laboratory can be found in the Lessons Learned Corner on this website and in the Hydrogen Safety Best Practices Manual.