B. Fogarassy

Structure and properties of fine-grained electrodeposited nickel

Abstract The structure and some physical properties (electrical resistivity and its temperature coefficient, thermoelectric power, Curie temperature) were investigated for electrodeposited Ni foils. The values of these parameters deviated from those of equilibrium, coarse-grained physical properties could be ascribed to the nanocrystalline structure of the deposits which was verified by transmission electron microscopy and X-ray diffraction.

Microstructure and growth of electrodeposited nanocrystalline nickel foils

In the present work, the structure of electrodeposited pure Ni foils has been investigated by X-ray diffractometry, transmission electron microscopy and by measuring their electrical transport properties. It was found that the as-deposited Ni foils have a nanocrystalline structure covered by a thin amorphous Ni layer on the substrate side: the growth of the electrodeposited foils starts in amorphous form followed by nanocrystalline layers. To explain the formation of the amorphous Ni layer, it is supposed that foreign atoms are incorporated into the nucleating Ni films.

Hydrogen sorption and hydrogen-induced phase separation in a nearly equiatomic NiZr amorphous alloy

Abstract The influence of hydrogen on some physical properties and on the micro-structure of a nearly equiatomic, melt-quenched NiZr amorphous alloy was investigated. Hydrogen absorption from the gas phase at 190 °C and 35–40 atm pressure was measured by weight as a function of the charging time. Considerable hydrogen absorption occurred only for charging times around 10 h and was completed within a few hours. The hydrogen content remained constant for a hydrogen-to-metal ratio of about one for prolonged charging. The room-temperature thermopower changed considerably upon hydriding and a local thermopower probe revealed that the activation time varies considerably along the ribbon length. It was found that, during further heat treatment in a hydrogen gas atmosphere after a hydrogen-to-metal ratio of approximately one was reached, the magnetic state of the amorphous NiZr samples changed progressively from paramagnetic to superparamagnetic. The density and size of the magnetic clusters, which were nickel-rich segregations induced by the hydriding process, were determined by magnetic and scanning transmission electron microscopic measurements. After desorbing hydrogen at elevated temperatures, magnetic measurements indicated the disappearance of the superparamagnetic clusters.

Electronic Transport in Nanocrystalline Metals: A Study of Electrodeposited Nickel Foils

The electrical resistivity (ρ), its temperature coefficient (a) and the thermoelectric power (S) has been studied at room temperature for nanocrystalline electrodeposited Ni foils prepared under different deposition conditions. With decreasing crystallite size, ρ increased, α decreased and S became less negative in agreement with the few previously reported results on other nanocrystalline metals. The temperature dependence of ρ was measured from 4.2 K to 300 K for some samples with small (≈50 nm) crystallite size. A large residual resistivity was observed and the temperature variation of ρ was also different from that of well-annealed, defect-free Ni with large crystallite size. It was concluded that the large residual resistivity cannot be completely ascribed to crystallite boundaries only but the presence of a high density of other types of lattice imperfections due to the nanocrystalline structure must also be assumed in the electrodeposited Ni foils whereas the different temperature behavior can be explained by considering that the electronic mean free path of the matrix is higher than the average crystallite size at low temperatures and the situation is reversed at room temperature.

Hydrogen-induced phase-separation in the amorphous Ni-Zr system

Abstract It is found that the magnetic state of the amorphous Ni 60 Zr 40 samples changes from paramagnetic to superparamagnetic upon hydriding. The density and the size of the magnetic clusters induced by the hydriding process was determined by magnetic and scanning transmission electron microscopic (STEM) measurements.

On the formation of a phase with tenfold symmetry in amorphous Ni–Zr alloys

Abstract The splat-quenched Ni50Zr50 alloy has a rosette-shaped phase with tenfold symmetry. The structure of this phase has been interpreted (Yang, 1985) as “tenfold twins” of the orthorhombic NiZr phase. The formation and the feature of the phase with tenfold symmetry have been investigated in Ni50Zr50, Ni50−xCuxZr50 and Ni50−xCoxZr50 amorphous alloys. It was found that this phase forms first during the crystallization process. The structure comes nearer to the structure of a quasi-crystal through the influence of Cu and Co atoms.

AMORPHOUS-CRYSTALLINE TRANSITION IN Ni-P-B METALLIC GLASSES

The crystallization behaviour of melt-quenched Ni81.5P18.5-xBx (0 ≤ × ≤ 18.5) metallic glasses was studied by calorimetry, electron microscopy and measurements of electrical transport properties. For x = 0 a two-step, for 1.8 ≤ × ≤ 18.5 single-step crystallization was observed. The crystallization temperature first increased, then decreased with B content which could be explained by the different mobility and solubility of B and P atoms. On the P-rich side Ni and Ni3P whereas on the B-rich side Ni, Ni3P and Ni3B phases were found in the crystallized samples. A significant grain-coarsening upon heat-treating the crystallized samples could be observed which was accompanied by the precipitation of excess Ni out of the Ni-metalloid phases indicated also by the transport measurements.