Nickel, particles

Iron, cobalt, and nickel particles also grow in soot deposited on the chamber walls, but graphitic layers wrapping the metals are not so well-developed as those grown in the cathode soot. Figure 7 shows a TEM picture of iron particles grown in the chamber soot. They... 

Bamboo-shaped tubes. A carbon tube with a peculiar shape looking like bamboo, produced by the arc evaporation of nickel-loaded graphite, is shown in Fig. 8. The tube consists of a linear chain of hollow compartments that are spaced at nearly equal separation from 50 to 100 nm. The outer diameter of the bamboo tubes is about 40 nm, and the length typically several /im. One end of the tube is capped with a needle-shaped nickel particle which is in the normal fee phase, and the other end is empty. Walls of each compartment are made up by about 20 graphitic layers[34]. The shape of each compartment is quite similar to the needle-shape of the Ni particle at the tip, suggesting that the Ni particle was once at the cavities. 

McCammon et al. have studied fine nickel particles using Ni Mossbauer spectroscopy [22]. The measured average hyperfine field of 10 nm particles at 4.2 K was 7.7 T for nickel foil, it was found to be 7.5 T. Application of an external magnetic field of 6 T caused a reduction of the hyperfine splitting to 1.5 T as a consequence of the negative hyperfine field at Ni nuclei. 

A similar study using Ni was carried out by Stadnik et al. [23]. Measurements were performed at 4.2 K on spherical nickel particles covered with a protective layer of SiO, with average diameter of 500 and 50 A, respectively. The hyperfine... 

For catalysts reduced at 400°-500°C, average nickel particle diameters in the range 30-45 A (40,43), and 30-200 A (41) have been quoted. Coenen and Linsen (41) have assumed a roughly hemispherical shape for the nickel particles which expose (111), (100), and (110) planes, and this is at least consistent with the very limited electron microscopic evidence. On the whole, it appears to be more difficult to produce a very high degree of metal dispersion with nickel than with platinum, and it is very difficult to obtain an average nickel particle diameter <30 A, although not impossible. 

Morikawa et al. (42) suggest that nickel aluminate itself undergoes hydrogen reduction only to a superficial extent, and then produces extremely small nickel particles as the reduction product. In this circumstance, the nickel particle size distribution in a reduced nickel/alumina catalyst will obviously be much dependent on the preparative details that control the proportions nickel oxide and nickel aluminate and the size of the particles in which these substances exist before reduction. 

During occupational exposure, respiratory absorption of soluble and insoluble nickel compounds is the major route of entry, with gastrointestinal absorption secondary (WHO 1991). Inhalation exposure studies of nickel in humans and test animals show that nickel localizes in the lungs, with much lower levels in liver and kidneys (USPHS 1993). About half the inhaled nickel is deposited on bronchial mucosa and swept upward in mucous to be swallowed about 25% of the inhaled nickel is deposited in the pulmonary parenchyma (NAS 1975). The relative amount of inhaled nickel absorbed from the pulmonary tract is dependent on the chemical and physical properties of the nickel compound (USEPA 1986). Pulmonary absorption into the blood is greatest for nickel carbonyl vapor about half the inhaled amount is absorbed (USEPA 1980). Nickel in particulate matter is absorbed from the pulmonary tract to a lesser degree than nickel carbonyl however, smaller particles are absorbed more readily than larger ones (USEPA 1980). Large nickel particles (>2 pm in diameter) are deposited in the upper respiratory tract smaller particles tend to enter the lower respiratory tract. In humans, 35% of the inhaled nickel is absorbed into the blood from the respiratory tract the remainder is either swallowed or expectorated. Soluble nickel compounds... 

MgO is a basic metal oxide and has the same crystal structure as NiO. As a result, the combination of MgO and NiO results in a solid-solution catalyst with a basic surface (171,172), and both characteristics are helpful in inhibiting carbon deposition (171,172,239). The basic surface increases C02 adsorption, which reduces or inhibits carbon-deposition (Section ALB). The NiO-MgO solid solution can control the nickel particle sizes in the catalyst. This control occurs because in the solid solution NiO has strong interactions with MgO and, as indicated by TPR data (26), the former oxide can no longer be easily reduced. Consequently, only a small amount of NiO is expected to be reduced, and thus small nickel particles are formed on the surface of the solid solution, smaller than the size necessary for coke formation. Indeed, the nickel particles on a reduced 16.7 wt% NiO/MgO solid-solution catalyst were too small to be observed by TEM (171). Furthermore, two additional important qualities stimulated the selection of MgO as a support its high thermal stability and low cost (250,251). 

Low redox cycling tolerance. Redox cycling occurs when the fuel flow to the stack is interrupted. Air leaking in from the environment will oxidize the nickel. Upon recovery of the fuel flow the nickel oxide will reduce again. Such a cycle will cause considerable performance loss due to coarsening of the nickel particles in the anode. 

M. Che, M. Richard, and D. OUvier ferromagnetic resonance study of dispersed nickel particles prepared by reduction of nickel ion-exchanged X-zeolites by hydrogen molecules or hydrogen atom beams, J. Chem. Soc. Faraday Trans. 176,1526-1534 (1980). 

The reaction of AsPhs with Ni/AhOs in n-heptane solution, under hydrogen (12 bar) at between 25 and 200 °C, only takes place on the nickel surface and is characterized by benzene (and cyclohexane, secondary product) evolution [135]. At 80 °C, saturation of the nickel surface has been reached with a ratio As/Nis of 1. At 100 °C, arsenic migration from the nickel surface to the core of the particle is observed. This migration is characterized by a rapid decrease in ferromagnetism of the nickel particles, reaching zero for an As/Ni ofO.45. At 170 °C, NiAs alloy formation has been highlighted by its X rays diffraction pattern (Figure 2.18). 

The mass fraction of nickel powder incorporated into the NP pyrolant was 0.01 and the diameter of the nickel particles was 0.1 pm. The NP pyrolants with and without nickel particles were pressed into pellet-shaped grains 1 mm in diameter and 1 mm in length. The BK pyrolant was pressed into pellet-shaped grains 3 mm in diameter and 3 mm in length. 

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