Phase changes and hydrogen release during decomposition of sodium alanates

We have constructed a hermetically sealed high-temperature cell for an X-ray diffractometer which allows in situ identification of structural phase changes during the several thermal decomposition stages of sodium alanate materials. Comparing X-ray data with thermogravimetry measurements allows identification of phase changes that correlate with hydrogen release. Measurements made while uniformly ramping up the temperature of purified NaAlH4 indicate that all of the hydrogen release corresponds to the formation of Al or the formation and decomposition of the Na3AlH6 phase. The melting of the NaAlH4 phase does not correlate well with sample mass loss in the purified material. For as-received technically pure NaAlH4, hydrogen release lags both Al formation and the formation and decomposition of the Na3AlH6 phase. In ball-milled as-received NaAlH4 (ball-milled with diamond powder to achieve similar to2 mum crystallite size) there is good correspondence between hydrogen release and either Al formation or the formation and decomposition of the Na3AlH6 phase, except that hydrogen release lags the phase changes by a few degrees. For purified NaAlH4 ball-milled in either a steel or tungsten carbide jar, there is a much smaller lag in hydrogen release. For Ti doped material, the phase changes to NaH are completed at the lowest temperature observed in any of the samples, 215 degreesC, but the TGA measurements are not reliable because of mass loss from decomposition of the catalyst butoxide anion. Among these samples, the delay in hydrogen release does not correlate well with crystallite size; indicating that the delay is not due to hydrogen diffusion, but the delay is definitely decreased by ball milling or adding catalyst material or even impurities. (C) 2002 Elsevier Science B.V. All rights reserved.