Abstract
The hydrogenation of the CaH2+MgB2 composite and the dehydrogenation of the resulting products are investigated in detail by in situ time-resolved synchrotron radiation powder X-ray diffraction, high-pressure differential scanning calorimetry, infrared, and thermovolumetric measurements. It is demonstrated that a Ca(BH4)2+MgH2 composite is formed by hydrogenating a CaH2+MgB2 composite, at 350 °C and 140 bar of hydrogen. Two phases of Ca(BH4)2 were characterized: α- and β-Ca(BH4)2. α-Ca(BH4)2 transforms to β-Ca(BH4)2 at about 130 °C. Under the conditions used in the present study, β-Ca(BH4)2 decomposes first to CaH2, Ca3Mg4H14, Mg, B (or MgB2 depending on experimental conditions), and hydrogen at 360 °C, before complete decomposition to CaH2, Mg, B (or MgB2), and hydrogen at 400 °C. During hydrogenation under 140 bar of hydrogen, β-Ca(BH4)2 is formed at 250 °C, and α-Ca(BH4)2 is formed when the sample is cooled to less than 130 °C. Ti isopropoxide improves the kinetics of the reactions, during both hydrogenation and dehydrogenation. The dehydrogenation temperature decreases to 250 °C, with 1 wt % of this additive, and hydrogenation starts already at 200 °C. We propose that the improved kinetics of the above reactions with MgB2 (compared to pure boron) can be explained by the different boron bonding within the crystal structure of MgB2 and pure boron.
DOI Link to Publication