Polycrystalline surfaces

Pt is the most efficient catalyst for ORR and has been extensively studied as single crystal surfaces, polycrystalline, alloys, and NP materials. To reduce the cost and enhance the efficiency, Pt has been obtained in highly dispersed form on different high surface area conductive supports. Among various supports, carbon is the most investigated one. Details of work carried out on the ORR on Pt-based materials prior to 2008 or so have been comprehensively reviewed by Antolini et al. [119]. 

Surfaces having heterogeneities due to impurities or polycrystallinity over... 

Most fiindamental surface science investigations employ single-crystal samples cut along a low-index plane. The single-crystal surface is prepared to be nearly atomically flat. The surface may also be modified in vacuum. For example, it may be exposed to a gas that adsorbs (sticks) to the surface, or a film can be grown onto a sample by evaporation of material. In addition to single-crystal surfaces, many researchers have investigated vicinal, i.e. stepped, surfaces as well as the surfaces of polycrystalline and disordered materials. 

Paal Z, ErtI G and Lee S B 1981 Interactions of potassium, nitrogen, and oxygen with polycrystalline iron surfaces Appl. Surf. Sc/. 8 231... 

Flame spraying is no longer the most widely used melt-spraying process. In the power-feed method, powders of relatively uniform size (<44 fim (325 mesh)) are fed at a controlled rate into the flame. The torch, which can be held by hand, is aimed a few cm from the surface. The particles remain in the flame envelope until impingement. Particle velocity is typically 46 m/s, and the particles become at least partially molten. Upon impingement, the particles cool rapidly and soHdify to form a relatively porous, but coherent, polycrystalline layer. In the rod-feed system, the flame impinges on the tip of a rod made of the material to be sprayed. As the rod becomes molten, droplets of material leave the rod with the flame. The rod is fed into the flame at a rate commensurate with melt removal. The torch is held at a distance of ca 8 cm from the object to be coated particle velocities are ca 185 m/s. 

Purification of Silicon. Chemical purity plays an equally important role in the bulk of materials as on the surface. To approach the goal of absolute stmctural perfection and chemical purity, semiconductor Si is purified by the distillation of trichlorosilane [10025-78-2] SiHCl, followed by chemical vapor deposition (CVD) of hulk polycrystalline siUcon. 

Thin films of metals, alloys and compounds of a few micrometres diickness, which play an important part in microelectronics, can be prepared by die condensation of atomic species on an inert substrate from a gaseous phase. The source of die atoms is, in die simplest circumstances, a sample of die collision-free evaporated beam originating from an elemental substance, or a number of elementary substances, which is formed in vacuum. The condensing surface is selected and held at a pre-determined temperature, so as to affect die crystallographic form of die condensate. If diis surface is at room teiiiperamre, a polycrystalline film is usually formed. As die temperature of die surface is increased die deposit crystal size increases, and can be made practically monocrystalline at elevated temperatures. The degree of crystallinity which has been achieved can be determined by electron diffraction, while odier properties such as surface morphology and dislocation sttiicmre can be established by electron microscopy. 

In the case of most nonporous minerals at sufficiently low-shock stresses, two shock fronts form. The first wave is the elastic shock, a finite-amplitude essentially elastic wave as indicated in Fig. 4.11. The amplitude of this shock is often called the Hugoniot elastic limit Phel- This would correspond to state 1 of Fig. 4.10(a). The Hugoniot elastic limit is defined as the maximum stress sustainable by a solid in one-dimensional shock compression without irreversible deformation taking place at the shock front. The particle velocity associated with a Hugoniot elastic limit shock is often measured by observing the free-surface velocity profile as, for example, in Fig. 4.16. In the case of a polycrystalline and/or isotropic material at shock stresses at or below HEL> the lateral compressive stress in a plane perpendicular to the shock front... 

A typical shock-compression wave-profile measurement consists of particle velocity as a function of time at some material point within or on the surface of the sample. These measurements are commonly made by means of laser interferometry as discussed in Chapter 3 of this book. A typical wave profile as a function of position in the sample is shown in Fig. 7.2. Each portion of the wave profile contains information about the microstructure in the form of the product of and v. The decaying elastic wave has been an important source of indirect information on micromechanics of shock-induced plastic deformation. Taylor [9] used measurements of the decaying elastic precursor to determine parameters for polycrystalline Armco iron. He showed that the rate of decay of the elastic precursor in Fig. 7.2 is given by (Appendix)... 

Nucleation in solids is very similar to nucleation in liquids. Because solids usually contain high-energy defects (like dislocations, grain boundaries and surfaces) new phases usually nucleate heterogeneously homogeneous nucleation, which occurs in defect-free regions, is rare. Figure 7.5 summarises the various ways in which nucleation can take place in a typical polycrystalline solid and Problems 7.2 and 7.3 illustrate how nucleation theory can be applied to a solid-state situation. 

Surface-sensitive diffraction is, for the most part, restricted to analysis of surfaces of single crystals and overlayers and films on such surfaces. If a polycrystalline sample is illuminated using a beam of low-energy electrons, each crystallite surfiice exposed will create its own diffraction pattern, all of which will be superimposed on the fluorescent screen detector. If more than a few orientations are illuminated by the beam, the pattern becomes too complicated to analyze. Flowever, if the size of the... 

Other applications of REELM include monitoring variations like oxidation, segregation, and hydration in the surface chemistry of polycrystalline materials. Differences of 1 /10 of a monolayer in oxygen coverage due to variations in grain... 

Usually, particle size has relatively little effect on Raman line shapes unless the particles are extremely small, less than 100 nm. For this reason, high-quality Raman spectra can be obtained from powders and from polycrystalline bulk specimens like ceramics and rocks by simply reflecting the laser beam from the specimen surface. Solid samples can be measured in the 90° scattering geometry by mounting a slab of the solid sample, or a pressed pellet of a powder sample so that the beam reflects from the surface but not into the entrance slit (Figure 3). 

The obvious application of microfocus Raman spectroscopy is the measurement of individual grains, inclusions, and grain boundary regions in polycrystalline materials. No special surface preparation is needed. Data can be obtained from fresh fracture surfeces, cut and polished surfaces, or natural surfeces. It is also possible to investigate growth zones and phase separated regions if these occur at a scale larger than the 1-2 pm optical focus limitation. 

The sputtering process is frequendy used in both the processing (e.g., ion etching) and characterization of materials. Many materials develop nonuniformities, such as cones and ridges, under ion bombardment. Polycrystalline materials, in particular, have grains and grain boundaries that can sputter at different rates. Impurities can also influence the formation of surface topography. ... 

If a sample of polycrystalline material is rotated during the sputtering process, the individual grains will be sputtered from multiple directions and nonuniform removal of material can be prevented. This technique has been successfully used in AES analysis to characterize several materials, including metal films. Figure 9 indicates the improvement in depth resolution obtained in an AES profile of five cycles of nickel and chromium layers on silicon. Each layer is about 50 nm thick, except for a thinner nickel layer at the surface, and the total structure thickness is about 0.5 pm. There can be a problem if the surface is rough and the analysis area is small (less than 0.1-pm diameter), as is typical for AES. In this case the area of interest can rotate on and off of a specific feature and the profile will be jagged. 

The rate (or kinetics) and form of a corrosion reaction will be affected by a variety of factors associated with the metal and the metal surface (which can range from a planar outer surface to the surface within pits or fine cracks), and the environment. Thus heterogeneities in a metal (see Section 1.3) may have a marked effect on the kinetics of a reaction without affecting the thermodynamics of the system there is no reason to believe that a perfect single crystal of pure zinc completely free from lattic defects (a hypothetical concept) would not corrode when immersed in hydrochloric acid, but it would probably corrode at a significantly slower rate than polycrystalline pure zinc, although there is no thermodynamic difference between these two forms of zinc. Furthermore, although heavy metal impurities in zinc will affect the rate of reaction they cannot alter the final position of equilibrium. 

Such simple considerations led Scholten and Konvalinka to confirm the form of the dependence of the reaction velocity on the pressure, as had been observed in their experiments. Taking into account a more realistic situation, on the polycrystalline hydride surface with which a hydrogen molecule is dealing when colliding and subsequently being dissociatively adsorbed, we should assume rather a different probability of an encounter with a hydride center of a /3-phase lattice, an empty octahedral hole, or a free palladium atom—for every kind of crystallite orientation on the surface, even when it is represented, for the sake of simplicity, by only the three low index planes. 

Indirect methods used can profit by the thermodynamic data of a particular metal-hydrogen system. The determination of the H/Me ratio after complete desorption of hydrogen from a sample, despite an apparent simplicity of the method, gives adequate results only when the bulk metal sample was entirely saturated with hydrogen, and that is a very rare case. The metal catalyst crystallites can be saturated in a nonuniform way, not through their whole thickness. The surface of this polycrystalline sample varies to such extent in its behavior toward interaction with hydrogen that hydride forms only in patches on its surface. A sample surface becomes a mosaique of /3-hydride and a-phase areas (85). 

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