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SSL Seminar Series 2005 No.2
Combined talks (two speakers) Date: March 18 (Friday), 2005
Time: 11:00am-12:00pm
Venue: Physics Conference Room (Blk S13 #M01-15)
Speaker I: Dr. Xiong Zhitao
Title:Interaction between NaNH 2 and LiAlH 4 for hydrogen generation at room temperature & Effect of Li/Mg ratio on the reversible hydrogen storage over Li-Mg-N-H complex
Abstract:
Certain amount of NaNH 2 and LiAlH 4 were mechanically milled using a planetary ball miller at several chosen conditions. With only 15 minutes of milling at 200 revolutions per minute (rpm) 4.6% weight percentage of hydrogen was generated at room temperature. NaH and a kind of Li-Al-N-H species were found to be the final solid products which released another 0.8wt% hydrogen upon heating to 300 o C and this part of hydrogen can be rehydrogenated at temperatures below 200 degree C. Mg(NH 2 ) 2 and LiH as the starting material were mechanically milled with Mg/Li ratio at 1/1, 1/2 and 1/3, respectively. Temperature Programmed Desorption (TPD) revealed that sample Mg(NH 2 ) 2 -2LiH presents a single peak signaled quick hydrogen release in large amount before 250 degree C with no ammonia emission. With less LiH mixed with Mg(NH2)2 –N or –NH units in reacting mixture are rather unstable, while with more LiH hydrogen desorption can only be completed at higher temperatures with multi desorption steps. X-ray powder diffraction and pressure-composition (P-C) isotherms were employed to investigate the structure and performance of the dehydrogenated species or so-called Li-Mg-N-H complex for reversible hydrogen storage.
Speaker II: Dr. Gao Xingyu
Title: Thickness-dependence of X-ray absorption and photoemission in Fe thin films on
Si(001)
Abstract:
To investigate the initial growth of Fe films on Si(001) and their interface, Fe films at various
thicknesses have been systematically studied by soft X-ray absorption spectroscopy (XAS) and Xray
photoemission spectroscopy (XPS). The Fe L edge XAS spectrum shows a strong thickness
dependence with broader line-width for thinner films. Detailed analysis of the Fe absorption signal
as a function of the thickness shows that the broad linewidth of Fe L edge XAS spectra is mostly
contributed by the first Fe layer at the Fe/Si interface. In contrast to XAS, Fe 2p photoemission
spectra for these films are identical. However, valence band photoemission also shows a strong
thickness dependence. Comparing valence band photoemission spectra of the thin Fe/Si(001) films
with that of of pure Si and that of the thickest Fe film, the difference spectra at all the thicknesses
show almost identical shape which indicates its same origin: Fe/Si interface. Thus, it is mainly
the first Fe layer at Fe/Si layer that is reactive with the Si substrate which changes its electronic
structure compared with that of metallic Fe.
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