Papers by Author: Jasna Grbović Novaković

Abstract: Magnesium based composites MgH2 + X (X=Ti, Co) were synthesized by ball milling in an argon atmosphere using stainless steel vial and balls. The crystallographic behavior of the resulting powders was examined by XRD. Thermal stability and hydrogen desorption properties were investigated by thermal analysis methods. In order to obtain a deeper insight into bonding mechanisms of the transition metal in MgH2 relaxed structure, ab initio electronic structure calculation of MgH2 + X (X=Ti, Co) was performed using Full Potential Linearized Augmented Plane Wave method, implemented in WIEN2K code. DOS analysis, confirmed by DTA measurements, resulted in the conclusion that, in the composite, in comparison to MgH2, the bonding Mg-H was weakened, on account of the shortening of interatomic distances hydrogentransition metal.

Abstract: Hydrogen, being a regenerative and environmentally harmless fuel, can play a crucial role in the energetic scenario of the near future. In recent years several systems for solid-state hydrogen storage have been investigated, among which a few metals and metal alloys show the most promising properties. Mg and Mg-based micro and nanocomposites are widely studied for this application owing to the high gravimetric storage capacity, even if a proper microstructure, mainly at the material surface, has to be setup in order to overtake kinetic constraints often related to the high surface reactivity. In this paper, we present our approach to this problem and report the main findings. We have examined the influence on hydrogenation/dehydrogenation characteristics of different modifications of the bulk and surface microstructure of MgH2 powders processed by ball milling, which appears to be one the most useful methods for preparing powders suitable for hydrogen storage.

Abstract: The alloying and phase formation in Ni-Hf samples with 0.2-, 2-, and 5-at.% Hf were studied by X-ray diffraction (XRD) technique and scanning electron microscopy (SEM). Both characterization methods, XRD and SEM, reveal the presence of the HfNi5 phase (fcc structure) where the excess Ni atoms are present in the form of Ni or Ni-rich segregations in the sample containing 5-at.% Hf. The sample with 2-at.% Hf is characterized by the presence of the two phases present in the 5-at.% sample and by Hf atoms, which occupy substitutional lattice positions in the Ni lattice. Finally, in the third sample with 0.2-at.% Hf, the Hf atoms mainly substitute the Ni atoms in the lattice. This analysis is being complemented with additional information on the local structure around Hf by extended X-ray absorption fine structure spectroscopy (EXAFS).

Abstract: The influence of the Co addition and synthesis route on desorption properties of MgH2 were investigated. Ball milling of MgH2-Co blends was performed under Ar using different milling intensities and different weight ratios. Microstructural and morphological characterization, performed by XRD and SEM, show a huge correlation with thermal stability and hydrogen desorption properties investigated by DSC. A complex desorption behaviour is correlated with the dispersion of the catalytic particles that appears to play a main role in desorption performances. The optimum catalyst concentration was found to be around 10 wt.%, while the optimum value of the ball to powder ratio was 10:1.

Abstract: The hydrogen storage performances of Mg-C nanocomposites have been studied on materials synthesized by ball milling with and without addition of an organic additive. The main purpose of this work is to study in more detail the cooperative effect observed when both graphite and benzene are added to the milled blend. In fact, when both components are added to Mg in the ball milling process, good catalyzing properties of the composite surface allow improved sorptiondesorption behavior of the synthesized material. The processed materials have been characterized by XRD to assess the details of the phase structure by Rietveld analysis, while surface features have been studied by XPS, which evidences structural modifications of both the surface Mg oxide and the graphite particles. The hydrogen desorption behavior has been correlated with the surface structure which appears to depend on the carbon to benzene ratio. Thermal stability and hydrogen desorption properties were investigated by DSC. Experimental results on nanocomposites with the same Mg to C weight ratio (70:30) show improved performances for a proper choice of carbon to benzene ratio weight (1/3), even after manipulation of the milled material in air.