Polycrystalline foils, preparation

The 3 mm disc was covered by a 3 pm thick polycrystalline nickel foil, then a polycrystalline molybdenum disc was placed on top of the nickel foil. This composite sandwich was placed between two alumina supports and heated for 30 min at 1350°C. The cross-sections used for microanalysis were prepared by cutting with a wire saw in a direction perpendicular to the grain boundary, then mechanically and chemically polished. 

The above ideas that anion-cation pair sites are the surface sites for CO and CO2 adsorption on magnetite was verified directly by Udovic and Dumesic (43 ). These authors prepared films of magnetite on polycrystalline iron foils and varied the oxidation state of the surface by vacuum-annealing at different temperatures. In short, it was shown by Auger electron spectroscopy and X-ray photo-... 

Figure 3. Surface preparation of the polycrystalline iron foil as received, with damage from cold work (a), ion etched, to remove 2 fan of metal (b) after annealing for 2 hat 500°C to remove etch structure (c), and development of step structure after further annealing for 3 hat 600°C (d).

It can be seen that the activities of the amorphous alloys are lower than those of the polycrystalline catalysts. Formation of the corresponding diol was not observed on the amorphous catalysts, while the crystalline catalysts either produced the diol selectively, or a mixture of the diol and the hydroxy ketone was formed. The fundamental reason for the lower activity and higher selectivity of the amorphous alloys is their rather small surface area. Of the amorphous alloys studied, Ni-B and Ni-P alloy powders prepared by chemical reduction exhibited higher activities than those of Ni-P alloys prepared by electrolytic reduction or rapid quenching. This difference in activity can be attributed to an oxide layer covering the surface of m-P foils [Ij. It is necessary to point out, however, that the comparison of activities is based on unit catalyst weight. Obviously, this comparison does not take into account the real surface area of the nickel samples, nor active site densities. 

A different approach to the preparation of preferentially oriented surfaces on polycrystalline systems has been demonstrated by Sumino and Shibata. In References 57-59 it has been shown that a fast-cycling treatment is not required for obtaining deposits with preferential orientation. A platinum electrode with a single-crystal (100) surface was obtained by annealing a thin platinum film electrodeposited on a polycrystalline platinum foil. 

The polycrystalline materials were prepared at FSU, where Ba, Fe, Ni (or Co), and As were mixed together and wrapped with Nb foil, and then sealed in stainless steel vial. The sealed samples were heat treated at 1120 °C for 12 h and cooled to 900 Cat the rate of4°Ch and held at 900 °C for 12 h and cooled to room temperature at the rate of l50°Ch . The phase purity of the samples was checked by powder X-ray diffraction (XRD) and energy-dispersive X-ray analysis (EDX). 

Another source for different experimental results may be the enormous sensitivity of the Lj]i absorption to the local chenjistry of the absorbing lanthanide atom. Crystalline defects or small amounts of other phases below the detection limit of the usual structural analysis or other measurements might explain different valence numbers from nominally identical materials. Most of the Lm measurements were performed on finely ground polycrystalline material. Even the few materials available as single crystals were powdered in order to prepare the required thin absorption foils (about 10 pm thick). Clearly this procedure may induce defects and imperfections which may alter the elastic and electronic properties. In particular powdered... 

Figure 14. XPS Ag 3d spectra recorded (a) from the as-prepared AgO powder sample, (b) after annealing at 100°C, (c) after annealing at 200 C. (d) after annealing at 300°C, (e) after annealing at 40()°C and (f) from a polycrystalline Ag foil after annealing at 250°C and sputtering with 2 keV Ar. (From Ref. 45.)...

Most experiments on the surface free energy which have been discussed in the preceding sections have been performed on polycrystalline material. One exception is the zero creep technique when applied to foils which can be prepared as single crystal surfaces [67Hon], Absolute values for the surface free energy of a specific orientation have also been obtained by the cleavage technique, see data in section 4.4.7.1. 

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