Selection of the powdery metal hydride best for producing H- by thermal desorption

To find the metal hydride best for producing H- by thermal desorption, the desorption rates of H-, H-2 and electron (e(-)) directly emitted from a pow dery sample (similar to 1 mg) of NaH, LiH, MgH2, CaH2, SrH2, TiH2, ZrH2, KB H4, LiAlH4, or NaAlH4 heated up to similar to 1000 K were determined simult aneously with a special system. Theoretical analysis of the experimental da ta thus achieved yields the following results. (1) Both H- and e(-) are emi tted from those active spots (mainly consisting of alkali or alkali earth m etal) created by thermal decomposition (e.g., LiH --> Li+H-2/2). (2) The ac tive spots are readily destroyed and reconstructed by admission of H-2 and by stopping the admission, respectively. (3) The work function (phi) of act ivated NaH is very low (similar to 2 eV), but NaH is rapidly depleted owing to its thermal instability. (4) Among the ten hydrides, CaH2 is concluded to be the best as a source material for thermal desorption of H- because ac tivated CaH2 (phi similar or equal to 5 eV) is most stable and strongest in H- (similar to 10(-12) A or 10(-10) A/cm(2) after mass analysis) around 90 0 K. (C) 2000 American Institute of Physics. [S0034-6748(00)59402-7].