Nickel cleaning surface

A part of Figure 3 in Ref. 207, reproduced on the right, reports radial EXAFS data around the S Is absorption edge for sulfur adsorbed on the (100) plane of a g nickel single-crystal surface. The top trace corresponds to the deposition of atomic S sulfur by dehydrogenation of H2S, while g, the bottom data were obtained by adsorb- M ing thiophene on the clean surface at 100 K. Based on these data, what can be learned about the adsorption geometry of thiophene Propose a local structure for the sulfur atoms in reference to the neighboring nickel surface. 

Typically, coatings most often in use as intermediate layers are silver, nickel, copper, and gold however, silver is used by far the most often. This is so because of the low dissociation temperature of silver oxide, making it relatively easy to obtain clean surfaces. Also, the typical thickness range of electroplates used, in practice, for diffusion welding is about 15 to 40/rm, but thicknesses as great as 130 )um must sometimes be used. A considerable variety of steel types as well as aluminum and a host of other difficult-to-join metals and even beryllium have been and continue to be diffusion bonded with the use of electroplated intermediate layers. 

G. Nickel. Nickel (nip 1,453°C), finds its primary use in the construction of apparatus to handle fluorine and volatile fluorides. In this situation the metal is rendered passive by a fluorine coating. Nickel plating is easily performed and provides a means of imparting corrosion resistance. The metal may be machined, silver soldered, copper brazed, or welded. However, the weld should be performed on clean surfaces because the presence of impurities containing lead, sulfur, phosphorus, and various low-melting metals leads to embrittlement and failure at the weld. 

Figure 2 also shows a d-band, arising from the four nickel atoms with d electrons explicitly included, extending downward from about -0.5 a.u. for the clean surface, adsorbed CH and coadsorbed CH and H cases. In a Ni atom, for this basis, the average d orbital energy is -0.44 a.u., a value close to the Hartree-Fock result. Photoemission measurements position the d ionization peaks of nickel near the Fermi level, a result also obtained by most density functional treatments of nickel clusters. Application of Koopmans theorem would therefore suggest that the present d-ionization... 

Pretreatment is necessary to achieve a clean surface on the piece to be coated. To remove fat, oxides, and other impurities from the surface, defatting, etching, and descaling operations are carried out, which are then generally followed by a precoating step. In the case of aluminum plating, a prelayer of nickel, cobalt or iron or alloys of these metals (0.5-2 pm) is applied. Each treatment step is followed by one or more rounds of washing and conducted in such a manner as to minimize loss and to recycle valuable material [28, 29, 33, 127]. 

This is a specialised technique which has been applied in field emission and field ion microscopy (see Section 2.1.5c). It is achieved by giving the tip a positive potential. Tungsten can then be removed at liquid helium temperatures with an applied field of 5.7 x 10 V.cm Perfectly regular surface structures are exposed containing many different lattice planes. Clean surfaces have been produced on tungsten, nickel, iron, platinum, copper, silicon and germanium. It is potentially applicable to a wide range of materials, but the area of clean surface exposed is only about 10 ° cm . 

Figure 3 compares the molecular (left) and acetone (right) TPD data for the reaction of 3.0 L of oxygen (approximately 0.30 ML of O atoms, or 60% of monolayer saturation) with varying amounts of 2-propyl iodide on Ni( 100). A 0.5 L exposure of 2-propyl iodide leads to the desorption of hydrogen, propene and propane, but not acetone, and results in TPD traces quite similar to those obtained from the same 2-C3H7I dose on the clean surface. The onset of acetone formation is seen as a small peak around 350 K only after a 2.0 L alkyl halide dose, and the molecular desorption data shows that monolayer saturation of 2-propyl iodide on this surface occurs between 2.0 and 4.0 L. Notice in particular that the 2.0 L marie corresponds to the point at which all the nickel sites become occupied (see Figure 1). This suggests that, in order for acetone to be produced, a particular surface ensemble is required with the 2-propyl groups adsorbed next to oxygen atoms [19-21].

The idea that adhesion is responsible for metallic friction was advanced as far back as 1724 [1], but the acquisition of evidence for the role of clean surfaces in the friction of metals had its systematic beginning in the work of F. P. Bowden and his collaborators. Bowden and Young [2], who studied the cleansing of nickel surfaces by heating under vacuum pumping at 133 uPa (10 torr) and the subsequent frictional behavior vac-uo at room temperature, were able to achieve such large-... 

With Pt-Ni catalysts, under reducing atmosphere, clean surfaces of bulk alloys are enriched with platinum when oxygen treatments show that the surface is then enriched with nickel (2). On Pt-Ni/A O catalyst with only 0.2wt% Pt - 1.5wt% Ni, the crystallites may be small enough to prevent any segregation phenomenon. 

The adsorption of CO on nickel has received extensive study since large area nickel films are comparatively simple to prepare. The nickel system therefore offered the possibility of producing a clean surface on a metal crystallizing with a face centered cubie structure. The results obtained on this surface could then be compared with those measured on tungsten, which crystallizes with a body centered,cubic structure. Much of the earlier work carried out for CO adsorption on the elements nickel, palladium, and platinum was reviewed by Gundry and Tompkins in 1960 135). 

A striking example is diffusion of nickel atoms on a Si(lll) surface. It is known (340a, 383) that Ni causes the complicated Si(lll)-7 clean surface structure to change to a V19 structure. When a Si(lll) crys-... 

However, there also appears to be a fault in the argument by which Professor Selwood has tried to establish the cleanliness of his catalysts. Even if we accept that the magnetic behavior of the materials reported on is essentially that of bulk nickel and that the admission of O2 or H20 results in a measurable effect, this does not establish that at the start of such an experiment the surfaces are clean. It shows only that if there were impurities present, they could not be detected by this technique. To establish cleanliness, however, it would be necessary to start with a demonstrably clean surface, to contaminate this to a known extent and to follow the magnetization. It is of interest here that in Professor Selwood s paper there are data showing that, under some circumstances, adsorption does not affect the magnetization initially. Thus, it may be advisable to qualify Professor Selwood s claim to cleanliness with the proviso as judged by magnetic measurements, and to subject these to further tests. 

P. M. Gundry (BuckneU University) In connection with the relative merits of nickel films and nickel-on-silica catalysts in presenting a clean surface, I ould like to draw attention to a remark made by Professor Muller in a comment to Dr. Cunningham s paper. He said that he had observed in the emission microscope that glass was freely mobile on a tungsten surface at 400°. It is not possible that some of the silica of the... 

---