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
![](/images/entities/deg.gif)
C and 140 bar ofhydrogen. Two phases of Ca(BH
4)
2 were
chara
cterized:
![](/images/gif<font color=)
chars/alpha.gif" BORDER=0>- and
![](/images/gif<font color=)
chars/beta2.gif" BORDER=0 ALIGN="middle">-Ca(BH
4)
2.
![](/images/gif<font color=)
chars/alpha.gif" BORDER=0>-Ca(BH
4)
2 transforms to
![](/images/gif<font color=)
chars/beta2.gif" BORDER=0 ALIGN="middle">-Ca(BH
4)
2 at about 130
![](/images/entities/deg.gif)
C. Under the
conditions used in the present study,
![](/images/gif<font color=)
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
![](/images/entities/deg.gif)
C, before
complete de
composition to CaH
2, Mg, B (or MgB
2), and hydrogen at 400
![](/images/entities/deg.gif)
C. During hydrogenation under140 bar of hydrogen,
![](/images/gif<font color=)
chars/beta2.gif" BORDER=0 ALIGN="middle">-Ca(BH
4)
2 is formed at 250
![](/images/entities/deg.gif)
C, and
![](/images/gif<font color=)
chars/alpha.gif" BORDER=0>-Ca(BH
4)
2 is formed when the sample is
cooledto less than 130
![](/images/entities/deg.gif)
C. Ti isopropoxide improves the kineti
cs of the rea
ctions, during both hydrogenation anddehydrogenation. The dehydrogenation temperature de
creases to 250
![](/images/entities/deg.gif)
C, with 1 wt % of this additive, andhydrogenation starts already at 200
![](/images/entities/deg.gif)
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.