Mg(BH4)2 sentezi ve hidrojen depolama özelliklerinin incelenmesi ve MgB2 yapısal modifikasyonunun incelenmesi

Complex metal borohydrides, such as MBH4 (M+ = Li, Na, K), M(BH4)2 (M2+ = Mg, Ca, Zn, Sn, Cu) and M (BH4)3 (M3+ = Al) have attracted a great deal of interest for their potential use as solid-state hydrogen storage materials due their high gravimetric and volumetric storage capacities. One of the most promising and extensively investigated candidate in this series is Mg(BH4)2 which is available only in small quantities on preparative scale. The complex synthesis procedure and the comparatively high price of the material has been one of the main drawbacks in its more widely use, both in research and for technical applications. In the present study, five different multi steps mechanochemical and wet mixing synthesis – all based on the metathesis reaction of MgX2 (X = Cl, Br) with NaBH4 – were performed in a planetary ball-mill or in diethyl ether to develop an easy and convenient preparation route for Mg(BH4)2. In addition, a novel and expedient approach was tested at relatively high pressure and high temperature in an autoclave using diethyl ether as solvent. Mg(BH4)2 was extracted from the reaction product by Soxhlet method overnight. Highly pure Mg(BH4)2 with 63% yield was obtained after solvent removal. In contrast to MgCl2, MgBr2 is at least partially soluble in diethyl ether. Thus, using bromide should enhance the reaction rate and the yield of Mg(BH4)2. The synthesis results surprisingly not only in formation of pure magnesium borohydride and NaBr but yielding also the mixed phases NaBr1-x(BH4)x and/or Na(BH4)1-yBry, as consequence of solid solution formation. The effect of milling speeds, time and reactant ratios for sample prepared by ball milling and various temperatures, time and reactant ratios for mixtures prepared in autoclave are investigated in detail with respect to solid solution formation by XRPD and ATR-FTIR. Finally, dehydrogenation behavior of Mg(BH4)2 has been investigated in combination with metal amides and MgBr2. For the systems of Mg(BH4)2-NaNH2, Mg(BH4)2-LiNH2, Mg(BH4)2-Mg(NH2)2, the decomposition pathway were predicated using XRPD, DTA-TG-MS, DSC and ATR-FTIR spectroscopy. The mentioned systems show an improvement in H2 onset temperature release while NH3 and B2H6 liberation is decreased in a great extend or suppressed. Interestingly, hydrogen desorption in Mg(BH4)2-Mg(NH2)2 mixtures occurs in two steps, unlike the other blends and pure Mg(BH4)2 which release their hydrogen in three and four steps, respectively. MgB2 is the ultimate product of thermal decomposition of Mg(BH4)2 at 600 °C. This simple compound is a binary superconductor with a great potential for several engineering applications. In the second part of this thesis, investigation of the synthesis of nano structured MgB2, as well as its structural modification will be coming to the fore. Micron sized MgB2 is obtained from the boron and magnesium powders by solid-state reaction route under Ar. However, the synthesis of nano-sized material succeeds only when nano sized boron powder was used as precursor, as proved by SEM and TEM images. Rietveld analysis revealed also a reduction in MgO content when nano boron was employed. The defect structures of MgB2 bulk and nano particles, have been investigated mainly by the aid of X-band electron paramagnetic resonance (EPR) spectrometer which proved the presence of Mg vacancies in nanoscale MgB2. In addition, the microscopic defect structures of carbon-doped MgB2 were systematically investigated by means of XRPD, Raman spectroscopy and EPR. Mg vacancies and C-related dangling-bond active centers were distinguished and sp3-hybridized carbon radicals were detected. By the help of EPR in the temperature range of 40 to 10 K, the reduction in the critical temperature Tc was observed due to C doping.