Hydride Nanoparticles Research Articles

X-ray diffraction and H-storage in ultra-small palladium particles

In situ X-ray diffraction (XRD) and gravimetric hydrogen uptake measurements of d ∼ 2–3 nm spherical PdH x particles have been studied in the temperature and pressure range of 323 < T < 428 K and 0 < P < 10 bar. The Pd particles were protected from sintering with a hydrogen-permeable carbon coating. While only containing ∼300–1000 atoms, the Pd particles were found to exhibit the same fcc structure and lattice constant as the bulk. Our isothermal studies show that, with increasing x, these highly crystalline PdH x nanoparticles also exhibit a complete transformation from the dilute α solid solution phase to the more concentrated β hydride phase. However, we observed that the character of the α–β phase transition in these nanoparticles is very different from that in the bulk. Indeed, the hydrogen uptake isotherm exhibits a noticeable positive slope in the α + β co-existence region. Furthermore, we also observed a noticeable narrowing of the α + β co-existence region ( δx) in the nanoparticles. Also, a significant suppression of the critical temperature T c for the phase boundary was observed: T c(nano) ≈ 430 K while T c(bulk) ≈ 570 K. These results signal a significant change in the thermodynamic behavior of very small hydride nanoparticles that may be common to many other nano-scale metal hydride systems as well.

Analysis of hydrogen adsorption in the bulk and on the surface of magnesium nanoparticles

The stability of magnesium hydride (MgH x ) nanoparticles (x = 0.5, …, 2) is investigated using ab initio calculations. It is shown that for a nanoparticle diameter of D ∼ 5 nm, the internal pressure becomes lower than 3 kbar; for this reason, the structure of hydride nanoparticles coincides with the structure of this hydride in crystalline form. It is found that the phase of partly saturated MgH x hydrides (x < 2) must decompose into the phase of pure hcp magnesium and the α phase of MgH2. The frequencies of jumps of hydrogen atoms within the hcp phase of magnesium and in the α phase of MgH2 are calculated; it is shown that slow diffusion of hydrogen in magnesium is due to the large height of potential barriers for motion of hydrogen within MgH2. To attain high diffusion rates, the structures of Mg53Sc and Mg53Ti crystals and their hydrides are calculated. It is found that the frequency of jumps of H atoms in Mg53ScH108 near the Sc atoms does not noticeably change as compared to the frequency of jumps in the α phase of MgH2, while the frequency of jumps in Mg53TiH108 near Ti atoms is higher by approximately a factor of 2.5 × 106. This means that diffusion in manganese hydride with small admixtures of titanium atoms must be considerably eased. Chemical dissociation of hydrogen molecules on the (0001) surface of hcp magnesium, on the given surface with adjoined individual Ti atoms, and on the surface of a one-layer titanium cluster on the given surface of magnesium is investigated. It is found that dissociation of hydrogen at solitary titanium atoms, as well as on the surface of a Ti cluster, is facilitated to a considerable extent as compared to pure magnesium. This should also sharply increase the hydrogen adsorption rate in magnesium nanoparticles.

Synthesis and characteristics of Ti–Fe nanoparticles by hydrogen plasma–metal reaction

Titanium hydride nanoparticles and five kinds of Ti–Fe nanoparticles were prepared by hydrogen plasma–metal reaction. The morphology, crystal structure and chemical composition of nanoparticles obtained were investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD) and induction-coupled plasma (ICP) spectroscopy. The particle size was determined by TEM and specific surface area. The formation mechanism of nanoparticles was discussed. It was found that all the nanoparticles have spherical shapes, with average particle size in the range of 22–44 nm. The titanium contents in Ti–Fe ultrafine particles (UFPs) are about 1–2 times lower than those in the master alloys. With Ti contents less than 50 at.% in bulk, the evaporation speed of Fe in Ti–Fe alloys is accelerated greatly by titanium addition. Only the UFPs originating from pure Ti react with hydrogen and form TiH1.924 possessing both FCC and BCT structures. Fe and Fe–7 at.%Ti UFP samples have the structure of α-Fe. As to sample Fe–9.9 at.% Ti and Fe–16.6 at.% Ti, some titanium atoms dissolve in α-Fe and others combine with iron atoms to form intermetallic TiFe2, therefore no hydrogen is adsorbed. No TiFe2 is detectable in sample Fe–42.2 at.% Ti which consists of the main phase of α-Fe, plus a small amount of an unidentified phase.

Synthesis and Characteristics of Sm2O3 and Nd2O3 Nanoparticles

A novel method has been used to synthesize rare earth oxide nanoparticles. Sm and Nd hydride nanoparticles were first produced by a hydrogen plasma−metal reaction. The evaporation rate of Sm is about three times that of Nd. Nd2O3 nanoparticles were then directly synthesized from the passivation of Nd hydride nanoparticles and Sm2O3 nanoparticles with a pure cubic structure from the heat treatment of Sm3H7 nanoparticles at 673 K in air. The morphology and crystal structure of the prepared powders were investigated by transmission electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy. Both the Sm2O3 and the Nd2O3 nanoparticles were spherical in shape, with a mean particle diameter of about 40 and 17 nm, respectively. The particle size of the Sm2O3 nanoparticles changed very little after the sintering of the Sm hydride nanoparticles at 473 and 673 K.