Field Ion Microscopy of the Platinum Metals
Field Ion Microscopy of the Platinum Metals
Platinum, iridium, rhodium and palladium are among the few metals that may be examined by field ion microscopy, in which it is possible to magnify the image of a prepared surface so that individual atoms are revealed. This technique, by which such factors as plastic deformation, gas adsorption, fatigue, dislocation structure and the effects of impurities can be examined, may well lead to a better understanding of the valuable properties shown by the platinum metals in their catalytic, electrochemical and high temperature applications.
The field ion microscope is the only device capable of directly showing us the individual atoms at a metal surface (1). Its operation is based on the simple principle of radially projecting the nearly hemispherical specimen, the cap of a needle tip, on to a fluorescent screen by using gas ions. The necessary magnification, of the order of 106, is obtained by using a tip radius between a few hundred and a few thousand ångströms, while the projection distance is about 10 cm. The extremely high degree of geometric perfection of the specimen surface which is necessary to make the projection principle work is achieved by field evaporation. This effect balances the binding energy of the metal ions with the force of the applied electric field. At protrusions the field is locally enhanced, and the metal field evaporates until a smooth curvature is obtained over the entire tip cap.
The resolution of a projection microscope improves with increasing field strength at the emitter. Helium or neon, having the highest ionisation potentials, are, therefore, preferred for imaging. Optimum ionisation occurs at 450 or 350 MV/cm (million volts per centimetre), respectively. Obviously, only those metals that do not field evaporate too fast at these extremely high field strengths will give sufficiently stable images. The evaporation field increases approximately with the sum of the vaporisation energy of the metal and its ionisation energy. Although the theory of field evaporation is not yet too well established (in part due to the inadequacy of manageable linear surface potential models for the description of the conditions at an atomic site where the potential changes by 10 to 20 volts across one atomic diameter), one can estimate, and confirm experimentally, that about twenty pure metals, from tungsten with the highest melting point over the other refractory metals and the platinum metals down to copper and gold, are suitable for field ion microscopy.
Not all of these metals are easily observed. The field exerts a mechanical stress F2/8π which amounts to 1 ton/mm2 at 480 MV/cm. For the possibility of seeing the original atomic lattice structure of the specimen it is decisive whether field evaporation or yield to the field stress occurs first. This must be found out experimentally, the balance of the two effects being uncertain because of the inadequacy of the theories for both.
Surprisingly, it turns out that besides tungsten only iridium, platinum, rhodium, palladium and gold have sufficient strength at their respective evaporation fields to resist the field stress or its shear component. The refractory metals, rhenium, tantalum, molybdenum and niobium, as well as other transition metals such as nickel and iron, all glide in certain crystallographic areas when exposed to the field evaporation stress. With the latter metals the discrimination between intrinsic lattice defects and artifacts due to the shear stress of the field is very difficult.
So far, the outstanding suitability for field ion microscopy of the platinum metals has not been really utilised. During field evaporation, platinum is exposed to a stress exceeding its technical tensile strength by a factor of 30, and yet no inelastic deformation occurs. The surface of the specimen, and by controlled field evaporation also the bulk of the metal, can be observed to determine the vacancy concentration in quenched material by direct counting of empty atoms sites (2) or to see surface vacancies or rearrangement caused by reaction with adsorbed gases (3) (Fig. 1). Cycling field stresses have been used to observe fatigue cracks (4). Radiation damage can be seen in the form of vacancies and interstitials when α particles are used (5) or large damage spikes when heavier ions are used (Fig. 2). Slip bands (6) are easily produced in situ by applying field stress to the heated specimen (Fig. 3). In tips with not too large radii single dislocations (7, 8) often do not move under the field stress, and their core structure and possible decoration with impurity atoms can be seen (Fig. 4). In platinum, iridium and rhodium a certain impurity, most probably oxygen, shows up as a bright blob with an apparent size larger than the metal atoms (Fig. 5).
There is also a promising application to the study of alloys. In random solute alloys field evaporation cannot produce the absolutely perfect surface geometry which is obtained with the pure metals. Randomness of distribution of the kind of nearest neighbours locally varies the binding energy at equivalent lattice sites (9). However, dilute alloys might be suitable for the study of short range order. Of the alloy systems with long range order, 50 atomic per cent cobalt-platinum is a most promising subject (9, 10) for field ion microscopy (Fig. 6).
The reason for neglecting so long the many opportunities which the platinum metals offer for field ion microscopy lies probably in the fact that this technique is fairly new and requires a skilful operator. With the recently increasing interest in this method of inquiry, the availability of an inexpensive commercial instrument (11) and the progress in image interpretation (12, 13) useful new information on the physical metallurgy of the platinum metals should be obtained in the not too far future.
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