Alloy films characterization

Vapor quenching provides a method of bridging the miscibility gap which exists in many alloy systems, and makes a range of novel alloys available for study. Such films, of course, would not be ideal for catalytic studies. They could not be used at high temperatures, and indeed the heat of reaction might be sufficient to induce a transformation to a more stable structure. In addition, characterization by X-ray diffraction would be difficult, even for the crystalline films, because of line broadening by the small crystallites. Nevertheless, alloy films which are metastable above room temperature can be prepared, and their high surface area would... 

In discussing the principles involved in alloy film formation, reference had to be made to alloy systems which are uncommon or unused in studies of adsorption and catalysis. This section is specifically concerned with the characterization of alloy films prepared for such purposes. However, the various aspects of alloy film structure mentioned in Section II have to be kept in mind when discussing results of catalytic experiments using evaporated alloy films. 

The characterization of evaporated alloy films can be carried out at widely different levels of sophistication. At the very least, it is necessary to determine the bulk composition, probably after the film has been used for an adsorption or catalytic experiment. Then various techniques can be applied, e.g., X-ray diffraction, electron diffraction, and electron microscopy, to investigate the homogeneity or morphology of the film. The measurement of surface area by chemisorption presents special problems compared with the pure metals. Finally, there is the question of the surface composition (as distinct from the bulk or overall composition), and a brief account is given of techniques such as Auger electron spectroscopy which might be applied to alloy films. 

Some other points worth noting in connection with alloy film composition are The loss in weight from separate sources is a guide to mean composition but not an exact measure because the sources become themselves alloyed. It is often important to determine the composition of the actual specimen on which other characterizing measurements have been made. If there is confidence that the films are reasonably homogeneous, lattice constants determined by X-ray diffraction can be used to examine the uniformity of composition (69), but the change of lattice constant with composition may be inconveniently small. 

Further progress in the study of the Cu-Ni system awaited the preparation and careful characterization of alloy films of known bulk and surface composition. The essential step was taken by Sachtler and his co-workers 28, 88, 114) who prepared Cu-Ni alloy films by successive evaporation of the component metals in UHV. After evaporation the films were homogenized by heating in vacuum at 200°C. The bulk composition of the alloys was derived from X-ray diffraction, and the photoelectric work function of the films was also measured. A thermodynamic analysis, summarized by Fig. 13, indicated that alloy films sintered at 200°C should consist, at equilibrium, of two phases, viz., phase I containing 80% Cu and phase II containing 2% Cu. Evidence was presented that alloys within the... 

In one of the earliest reports of the use of clean evaporated alloy films in surface studies, Stephens described the preparation and characterization of Pd-Au films and presented some results for the adsorption of oxygen on them 46). Films of pure Pd and 60% Au were evaporated directly from wires, while films of 80% Au and pure Au were evaporated from a pre-outgassed tungsten support wire. The films were evaporated in a UHV system and the pressure was kept below PC8 Torr during evaporation. After evaporation, the films were stabilized by cycling between —195° and 30°C four times. They w ere characterized by X-ray diffraction and chemical analysis surface areas were measured by the BET method using krypton adsorption. 

The decomposition of formic acid over evaporated Pd-Au alloy films has been studied by Clarke and Rafter (69) the same reaction on Pd-Au alloy wires was studied by Eley and Luetic (128). The alloy films were prepared in a conventional high vacuum system by simultaneous evaporation of the component metals from tungsten hairpins. The alloy films were characterized by X-ray diffraction and electron microscopy. The X-ray diffractometer peaks were analyzed by a method first used by Moss and Thomas (SO). It was found that alloys deposited at a substrate temperature of 450°C followed by annealing for one hour at the same temperature were substantially homogeneous. Electron microscopy revealed that all compositions were subject to preferred orientation (Section III). 

There is now available a substantial amount of information on the principles and techniques involved in preparing evaporated alloy films suitable for adsorption or catalytic work, although some preparative methods, e.g., vapor quenching, used in other research fields have not yet been adopted. Alloy films have been characterized with respect to bulk properties, e.g., uniformity of composition, phase separation, crystallite orientation, and surface areas have been measured. Direct quantitative measurements of surface composition have not been made on alloy films prepared for catalytic studies, but techniques, e.g., Auger electron spectroscopy, are available. 

X-ray diffraction work showed the existence of two phases. The work function data suggest that the copper-rich alloy in the two-phase system is located at the surface and the nickel-rich phase below the surface. To check this, CO was admitted at a pressure of 10 8 Torr. The gas is strongly adsorbed on nickel, but not on copper at such low pressures. The work function of copper was not altered. The binary alloys showed a constant increase in work function between 0.04 and 0.11 eV. Therefore, the adsorbing surface belonged to the copper-rich phase. Chemisorption of H2 on Ni-Cu films (40) leads to essentially the same conclusions. At temperatures below the miscibility gap, several classes of alloy systems characterized by their concentration ranges can be distinguished (4c), as illustrated in Fig. 3 ... 

A model P4-18-SPM scanning tunneling microscope (NT-MDT, Russia) was employed to investigate the structure, in atmosphere, of nanometer-scale thin film materials and also to measure the thickness of the film. A setup [28] combining electrochemical studies and X-ray photoelectron spectroscopic (XPS) analysis served to characterize the surface composition of alloy films. 

Hoang, H.T., Tong, H.D., Gielens, F.C., Jansen, H.V., and Elwenspoek, M.C. Fabrication and characterization of dual sputtered Pd-Cu alloy films for hydrogen separation membranes. Materials Letters, 2004, 58, 525-528. 

Platinum-Ruthenium alloy films of various compositions have been sputter-deposited and characterized in half-cells and full cells. 

Mousinho AP, Mansano RD, Salvador MC. Nanostrnctnred diamond-like carbon films characterization. J Alloy Compd 2010 495 620-4. 

RBS has also been used to characterize palladium and tin catalysts on polyetherimide surfaces [229], titanium nitride thin films [230], silicon oxynitride films [231], and silicon nitride films [232]. and to study the laser mixing of Cu-Au -Cu and Cu - W - Cu thin alloy films on Si3N4 substrates [233], and the annealing behavior of GaAs after implantation with selenium [234]. 

In order to understand the mechanism of improved oxidation resistance of nanocrystalline Fe-lOCr alloy, the composition, including Cr content of the thin oxide films developed on the nanocrystalline and microcrystalline alloys was characterized. The thin oxide films formed over nanocrystalline Fe-lOCr and microcrystalline Fe-10wt%Cr alloys at 300,350 and 400°C in air were characterised by SIMS depth profiling [12,39]. 

Dual sputtering deposition technique was used to prepare submicron thin Pd-Cu alloy films, which allowed a high composition control of the layer (Hoang et al, 2004). The composition, surface morphology and phase structure of the sputtered layers were investigated by EDS, X-ray, XPS, SEM, TEM and XRD. For example, the XRD data proved that the Pd-Cu layers were an alloy of Pd and Cu. Subsequently, the characterized Pd-Cu alloy layers were deposited on a silicon support structure to create a 750 nm thin Pd-Cu membrane for H2 separation. The reported membrane obtained a high H2 flux of 1.6 moF(m s) at a temperature of452°C, while the selectivity was at least 500 for H2/He. 

The 10 volumes in the Series on characterization of particular materials classes include volumes on silicon processir, metals and alloys, catalytic materials, integrated circuit packaging, etc. Characterization is approached from the materials user s point of view. Thus, in general, the format is based on properties, processing steps, materials classification, etc., rather than on a technique. The emphasis of all volumes is on surfaces, interfaces, and thin films, but the emphasis varies depending on the relative importance of these areas for the materials class concerned. Appendixes in each volume reproduce the relevant one-page summaries from the Encyclopedia and provide longer summaries for any techniques referred to that are not covered in the Encyclopedia. 

Corrosion products formed as thin layers on metal surfaces in either aqueous or gaseous environments, and the nature and stability of passive and protective films on metals and alloys, have also been major areas of XPS application. XPS has been used in two ways, one in which materials corroded or passivated in the natural environment are analyzed, and another in which well-characterized, usually pure metal surfaces are studied after exposure to controlled conditions. 

It is now well established that in lithium batteries (including lithium-ion batteries) containing either liquid or polymer electrolytes, the anode is always covered by a passivating layer called the SEI. However, the chemical and electrochemical formation reactions and properties of this layer are as yet not well understood. In this section we discuss the electrode surface and SEI characterizations, film formation reactions (chemical and electrochemical), and other phenomena taking place at the lithium or lithium-alloy anode, and at the Li. C6 anode/electrolyte interface in both liquid and polymer-electrolyte batteries. We focus on the lithium anode but the theoretical considerations are common to all alkali-metal anodes. We address also the initial electrochemical formation steps of the SEI, the role of the solvated-electron rate constant in the selection of SEI-building materials (precursors), and the correlation between SEI properties and battery quality and performance. 

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