L. Smardz

Surface analysis of polycrystalline and nanocrystalline LaNi5-type alloys

Abstract The chemical composition and the cleanness of the surface of polycrystalline and nanocrystalline LaNi 5 , LaNi 4.2 Al 0.8 , and LaNi 3 AlCo alloys were studied by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Results showed that the surface segregation under UHV conditions of lanthanum atoms in the mechanically alloyed nanocrystalline samples is significantly stronger compared to that of polycrystalline powders obtained from arc-melted ingots. On the other hand, the level of oxygen impurities trapped in the mechanically alloyed powder during the processing is practically the same as in the arc-melted ingots. Furthermore, we have estimated an average native oxide layer thickness of nanocrystalline LaNi 4.2 Al 0.8 as ∼5 nm. A strong segregation of the Fe impurities to the surface could be responsible for the observed slightly lower hydrogen storage capacity of the mechanically alloyed nanocrystalline LaNi 4.2 Al 0.8 compared to that of polycrystalline sample.

Nanocrystalline LaNi4.2Al0.8 prepared by mechanical alloying and annealing and its hydride formation

Abstract The formation of nanocrystalline LaNi 4.2 Al 0.8 material by mechanical alloying (MA) followed by annealing has been studied by X-ray diffraction, scanning electron microscopy and differential scanning calorimetry. The amorphous phase forms directly from the starting mixture of elements, without formation of other phases. Heating the MA powders at 750°C for 0.5 h in high purity argon resulted in the creation of hexagonal CaCu 5 -type structure. The surface chemical composition of the nanocrystalline LaNi 4.2 Al 0.8 alloy was studied by Auger electron spectroscopy and compared with that of a polycrystalline sample. Results showed that the surface segregation of lanthanum atoms in the MA nanocrystalline LaNi 4.2 Al 0.8 alloy is stronger than that of polycrystalline powders from arc-melted ingots. On the other hand, the level of oxygen impurities trapped in the mechanically alloyed powder during the processing is practically the same as in the arc-melted ingots. Small amounts of Fe impurities, which strongly segregate to the surface, could be responsible for the somewhat lower hydrogen storage capacity of the MA nanocrystalline LaNi 4.2 Al 0.8 alloy if compared with that of polycrystalline samples.

Structure and Electronic Properties of La(Ni,Al)5 Alloys

Nanocrystalline and polycrystalline La(Ni,Al) 5 alloys were prepared by mechanical alloying (MA) followed by annealing and arc melting method, respectively. The amorphous phase of MA samples forms directly from the starting mixture of the elements, without other phase formation. Heating the MA powders at 800 °C for 1 h resulted in the creation of hexagonal CaCu 5 -type nanocrystalline compound with mean crystallite size less than 80 nm. XPS studies showed that the shape of the valence band measured for the arc melted (polycrystalline) LaNi 5 is practically the same compared to that reported earlier for the single crystalline sample. The substitution of Ni in LaNi 5 by Al leads to significant modifications of the electronic structure of the polycrystalline sample. On the other hand, the XPS valence band of the MA nanocrystalline LaNi 4.2 Al 0.8 alloy is considerably broader compared to that measured for the polycrystalline sample. The strong modifications of the electronic structure of the nanocrystalline LaNi 4.2 Al 0.8 alloy could significantly influence on its hydrogenation properties.

Growth Properties of Ti/Co Multilayers

Ti/Co multilayers with either wedge-shaped or constant-thickness Co sublayers were prepared using UHV DC/RF magnetron sputtering. The planar growth of the Co and Ti layers was confirmed by X-ray photoelectron spectroscopy and scanning tunnelling microscopy. Results on structural and magnetic studies showed that the cobalt sublayers grow on 2 and 5 nm titanium sublayers in the soft magnetic nanocrystalline phase up to a critical thickness d crit ∼ 3.0 and 3.3 nm, respectively. For a thickness greater than d crit , the Co sublayers undergo a structural transition to the polycrystalline phase with much higher coercivity.

Magnetic and electrical resistivity study of Tm3Co11B4 compound

Abstract The magnetic (magnetization and coercivity) and transport (electrical resistivity) properties of the Tm 3 Co 11 B 4 is a ferromagnetic with studied. This compound crystallizes in the hexagonal Ce 3 Co 11 B 4 type structure. Results show that the Tm 3 Co 11 B 4 is a ferromagnet with T c =347 K and a compensation point equal to 125 K. The coercivity of the compound was determined from hysteresis measurements in fields up to 4 T. For the Tm 3 Co 11 B 4 compound we have observed a large increase of coercivity at low temperatures, typically from 0.01 T at 295 K to 0.8 T at 30 K. The resistivity at low temperatures shows a T 2 dependence, implying that electron-spin wave scattering is dominant in this temperature range.

Segregation Effect on Nanoscale Mg - Based Hydrogen Storage Materials

The nanocrystalline Mg-based metal hydrides offer a breakthrough in prospects for practical applications. In this work, we study experimentally the structure, electrochemical properties and surface segregation effect of nanocrystalline and microcrystalline Mg2M alloys and Mg2M/M’ (M=Cu, Ni; M’=C, Ni, Pd) nanocomposites. These materials were prepared by mechanical alloying (MA). In the nanocrystalline Mg2Cu powder, discharge capacity up to 30 mA h g-1 was measured. It was found that nickel substituting copper in Mg2Cu1-xNix alloy greatly improved the discharge capacity of studied material. In nanocrystalline Mg2Ni powder, discharge capacities up to 100 mA h g-1 were measured. Additionally, it was found that mechanically coated Mg-based alloys with graphite, nickel or palladium have effectively reduced the degradation rate of the studied electrode materials. Finally, the properties of nanocrystalline alloys and their nanocomposites are compared to that of microcrystalline samples. X-ray photoelectron spectroscopy studies showed that the surface segregation of Mg atoms and valence band width in the nanocrystalline Mg2M alloy are greater compared to those observed in microcrystalline Mg2M. Especially, a strong surface segregation of Mg atoms was observed for the Mg2Ni/M’ composites. In that case, Mg atoms strongly segregate to the surface and form a Mg based oxide layer under atmospheric conditions. The lower lying Ni and M’ atoms form a metallic subsurface layer and could be responsible for the observed relatively high hydrogenation rate. Furthermore, the valence band broadening observed in the nanocrystalline Mg2Ni alloys and Mg2Ni/M’ composites could also significantly influence their hydrogenation properties.

Nanocomposite Hydride LaNi5/A-and Mg2Ni/A-type Materials (A=C, Cu, Pd)

In this work, we have synthesized LaNi5/A and Mg2Ni/A (A = graphite, copper or palladium) nanocomposites. The A elements were distributed on the surface of ball milled alloy particles homogenously and role of these particles is to catalyze the dissociation of molecular hydrogen on the surface of studied alloy. Mechanical coating with graphite or palladium effectively reduced the degradation rate of the studied electrode materials. Results showed a significant broadening of the valence bands of studied nanocomposites compared to those obtained by theoretical band calculations. The reasons responsible for the band broadening of the nanocrystalline LaNi5- and Mg2Ni-type alloys are probably associated with a strong deformation of the nanocrystals in the mechanically alloyed (MA) samples. Normally the interior of the nanocrystal is constrained and the distances between atoms located at the grain boundaries expanded. The valence band spectra of the MA samples could be also broadened due to an additional disorder introduced during formation of the nanocrystalline structure.

Electronic Structure and Transport Properties of UFe_{2} System

The electronic structure of the UFe2 compound was studied by X-ray photoemission spectroscopy and ab initio self-consistent tight binding muffin tin orbital method. This compound crystallizes in a cubic Laves phase. The calculated valence band spectrum is characterized by two peaks due to U(5 f ) and Fe(3d) states. We have found a good agreement between the experimental valence band spectrum and theoretical ab initio calculations. The carrier concentration estimated from the Hall effect amounts to 10 22 cm -3 .