The hydrogenation of the
CaH
2+MgB
2 composite and the dehydrogenation of the resulting produ
cts areinvestigated in detail by in situ time-resolved syn
chrotron radiation powder X-ray diffra
ction, high-pressuredifferential s
canning
calorimetry, infrared, and thermovolumetri
c measurements. It is demonstrated that aCa(BH
4)
2+MgH
2 composite is formed by hydrogenating a CaH
2+MgB
2 composite, at 350
C and 140 bar ofhydrogen. Two phases of Ca(BH
4)
2 were
chara
cterized:
chars/alpha.gif" BORDER=0>- and
chars/beta2.gif" BORDER=0 ALIGN="middle">-Ca(BH
4)
2.
chars/alpha.gif" BORDER=0>-Ca(BH
4)
2 transforms to
chars/beta2.gif" BORDER=0 ALIGN="middle">-Ca(BH
4)
2 at about 130
C. Under the
conditions used in the present study,
chars/beta2.gif" BORDER=0 ALIGN="middle">-Ca(BH
4)
2 de
composes first toCaH
2, Ca
3Mg
4H
14, Mg, B (or MgB
2 depending on experimental
conditions), and hydrogen at 360
C, before
complete de
composition to CaH
2, Mg, B (or MgB
2), and hydrogen at 400
C. During hydrogenation under140 bar of hydrogen,
chars/beta2.gif" BORDER=0 ALIGN="middle">-Ca(BH
4)
2 is formed at 250
C, and
chars/alpha.gif" BORDER=0>-Ca(BH
4)
2 is formed when the sample is
cooledto less than 130
C. Ti isopropoxide improves the kineti
cs of the rea
ctions, during both hydrogenation anddehydrogenation. The dehydrogenation temperature de
creases to 250
C, with 1 wt % of this additive, andhydrogenation starts already at 200
C. We propose that the improved kineti
cs of the above rea
ctions withMgB
2 (
compared to pure boron)
can be explained by the different boron bonding within the
crystal stru
ctureof MgB
2 and pure boron.