Tungsten facetting

Measuring the electron emission intensity from a particular atom as a function of V provides the work function for that atom its change in the presence of an adsorbate can also be measured. For example, the work function for the (100) plane of tungsten decreases from 4.71 to 4.21 V on adsorption of nitrogen. For more details, see Refs. 66 and 67 and Chapter XVII. Information about the surface tensions of various crystal planes can also be obtained by observing the development of facets in field ion microscopy [68]. 

Fig. 14,4. Tip treatment for tunneling spectroscopy. (A) By applying a relatively large positive bias on the sample, a sharp tip generates a field-emission current. (B) When the field-emission current is very high, the tip end melts. (C) The tip end recrystallizes to form facets with low surface energy. In the case of tungsten, the W(llO) facets are preferred. Its surface DOS resembles a free electron metal. (After Feenstra et al., 1987a.)...

The chapter consists of nine sections. Sections II through VII deal with the pterin-containing molybdenum enzymes. Biochemical and model studies of molybdopterin, Mo-co, and related species are described in Section II. In Section III, we briefly survey physical and spectroscopic techniques employed in the study of the enzymes, and consider their impact upon the current understanding of the coordination about the molybdenum atom in sulfite oxidase and xanthine oxidase. Model studies are described in Sections IV and V. Section IV concentrates on structural and spectroscopic models, whereas Section V considers aspects of the reactivity of model and enzyme systems. The xanthine oxidase cycle (Section VI) and facets of intramolecular electron transfer in molybdenum enzymes (Section VII) are then treated. Section VIII describes the pterin-containing tungsten enzymes and the evolving model chemistry thereof Future directions are addressed in Section IX. 

FIGURE 5.24. Well-faceted tungsten crystals obtained at a reduction temperature of 1000°C (powder layer height, 20 mm). 

FIGURE 6.7. SEM image of a non-sag tungsten filament (a) before use, (b) after several hundred hours of operation in a 60-W incandescent lamp. Facetting of the originally round filament occurred due to evaporation of tungsten at the high operation temperature (2400-2500 °C). 

Becker and Hartman (14), using a sample exposing predominantly the (411) plane of tungsten, observed that adsorption of CO at 300°K existed in at least two states, probably an a and /3 phase desorbing between 400-600°K and 1000-1600°K, respectively. This work, coupled with the results of the FEM lends support to the idea that the a and j8 phases coexist on an atomic scale, and are not merely artifacts caused by adsorption on different crystal facets. 

Fig. 16. The defective lattice of tungsten bronze and the arrangement on it of reacting atoms in the dehydration of alcohol [19S) (c) the (100) facet of the lattice of thorium carbide.

Fig. 47. Work function of the (110) facet of a tungsten field emitter at 300 K as a function of silver exposure. The authors interpret this data as follows Below an exposure treshold of about one ML, Ag adatoms migrate to the high-index facets that surround the W(llO) plane, and the emission properties are indistinguishable from those of the bare substrate. At 1 ML the adatoms invade the (110) plane, modifying the surface density of electron states and bringing about an abrupt increase in electron emission. Above 2 ML the additionally evaporated Ag atoms migrate again to the surrounding facets. From [95D].

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