Surface composition, vaporization

Metals A and B form an alloy or solid solution. To take a hypothetical case, suppose that the structure is simple cubic, so that each interior atom has six nearest neighbors and each surface atom has five. A particular alloy has a bulk mole fraction XA = 0.50, the side of the unit cell is 4.0 A, and the energies of vaporization Ea and Eb are 30 and 35 kcal/mol for the respective pure metals. The A—A bond energy is aa and the B—B bond energy is bb assume that ab = j( aa + bb)- Calculate the surface energy as a function of surface composition. What should the surface composition be at 0 K In what direction should it change on heaf)pg, and why ... 

In Surface Analysis by Laser Ionization (SALI), a probe beam such as an ion beam, electron beam, or laser is directed onto a surfiice to remove a sample of material. An untuned, high-intensity laser beam passes parallel and close to but above the sur-fiice. The laser has sufficient intensity to induce a high degree of nonresonant, and hence nonselective, photoionization of the vaporized sample of material within the laser beam. The nonselectively ionized sample is then subjected to mass spectral analysis to determine the nature of the unknown species. SALI spectra accurately reflect the surface composition, and the use of time-of-flight mass spectrometers provides fast, efficient and extremely sensitive analysis. 

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. 

As an example of composite core/shell submicron particles, we made colloidal spheres with a polystyrene core and a silica shell. The polar vapors preferentially affect the silica shell of the composite nanospheres by sorbing into the mesoscale pores of the shell surface. This vapor sorption follows two mechanisms physical adsorption and capillary condensation of condensable vapors17. Similar vapor adsorption mechanisms have been observed in porous silicon20 and colloidal crystal films fabricated from silica submicron particles32, however, with lack of selectivity in vapor response. The nonpolar vapors preferentially affect the properties of the polystyrene core. Sorption of vapors of good solvents for a glassy polymer leads to the increase in polymer free volume and polymer plasticization32. 

The atmosphere is made up of a number of gases and, near the earth s surface, water vapor as well. The pressure exerted by atmospheric air is referenced to sea level. Average atmospheric pressure is 1013 mbar (equivalent to the atmosphere , a unit of measure used earlier). Table VIII in Chapter 9 shows the composition of the standard atmosphere at relative humidity of 50 % and temperature of 20 °C. In terms of vacuum technology the following points should be noted in regard to the composition of the air ... 

The purpose of this paper is to (1) document published studies of vaporization in vacuum of the Group 4 and 5 transition metal carbides and uranium carbide, and determine the temperature dependence of their equilibrium CVCs (C/MeCVC) and vaporization rates (Vecvc, g/cmzs) (2) document published studies on chemical diffusion of these carbides, and develop and compare data to a model describing the concentration dependence of the chemical diffusion coefficients and (3) develop diffusion-coupled vaporization equations which predict changes of surface composition (Cs, units of C/M and g/cm3 of C), average composition (Cavg units of C/M and g/cm3 of C), and C mass loss (M, units of g/cm2 of C). 

During vaporization of non-stoichiometric refractory carbides each element vaporizes at a different rate which is dependent on surface composition or relevant activities at the surface. When the initial bulk composition is near C/M = 1, the vaporization of C is much greater than that of M. As a result, the surface C content decreases and eventually approaches a constant value, which we will call the steady-state CVC (ssCVC). At the ssCVC, the vapor composition is nearly equal to the initial bulk composition. As C diffusion to the vaporizing surface reduces the C content of the bulk material, the surface composition asymptotically approaches the equilibrium CVC (eCVC). The rate at which eCVC is approached depends on the relative magnitudes of C vaporization and diffusion. When the eCVC has been reached, the surface and bulk C/M ratios are equal to the vapor composition. The intersection of the solid eCVC map with the solidus boundary of the monocarbide phase determines where melting occurs under equilibrium conditions for a particular atmosphere. 

The principal difficulty with the previous approaches to molecular speciation has been less with what was attempted and more with the inability to control the sample surface. A recent study of RF plasma treated surfaces has shown that the surface composition can change on exposure to laboratory air and water vapor (91,92). The development of a sample isolation/ treatment/... 

Various surface analysis techniques show that silicate glasses rapidly develop surface compositional profiles when exposed to water. When water is present as a vapor an alkali-rich layer (presumably a hydrated alkali carbonate) forms over the SiOj-rich layer. Water as a liquid dissolves the alkali and leaves the silica-rich film. As long as this SiC -rich film is stable the rate of corrosion due to diffusion is reduced with exposure time. Addition of multi-valent species to the glass or reactant results in formation of a complex protective surface layer in the glass which may be stable over a wide range of environmental conditions. 

The surface free energy (s.f.e.) of a material is a measure for the work required to enlarge its surface (mJm-2). At constant pressure, constant temperature and if the surface composition remains constant, die s.f.e. equals the surface tension (ysv) of the material against its vapor [26], To determine the value of ysv and hence of the s.f.e. of a solid, contact angles with several liquids with known s.f.e. (yLv) are required. Thomas Young s equation relates the surface free energies and the contact angle (9) based on the force balance at the three-phase boundary [27] ... 

In the liquid phase the loss of light (low C/H atomic ratio) species from the surface causes a concentration profile to be established for each compound. The lighter compounds, being deficient at the surface, diffuse to the surface and the heavier compounds, being concentrated at the surface, diffuse towards the center of the droplet. The combined effects of vaporization and diffusion determine the surface composition and thereby the surface temperature. This combination of temperature and composition determines the relative volatilities of the species present at the surface and hence the vapor phase composition. 

If diffusion is not able to transport enough material to the surface to satisfy the vaporization requirement, the surface composition will shift towards the heavier molecular weight component, which increases its relative volatility. This is seen to happen for the case of acridine, where its high molecular weight results in a smaller diffusivity and its low volatility prevents it from being evolved until its surface concentration has increased sufficiently. However, because of the lower diffusivity the surface concentration will increase more rapidly than equilibrium conditions, permitting the higher rate of vaporization observed. 

Fig. 30. Schematic description of PFSA membrane surface composition in contact with water vapor and with liquid water, as deduced from contact angle measurements. The scheme on the right-hand side describes the change in membrane surface composition during contact angle measurement which results in the large difference between advancing and receding contact angle readings (see Table 7),...

Iwasita et al. [107] also studied the electro-oxidation of methanol on Ru-evaporated Pt(lll) modified electrodes with different Ru coverage. The surface compositions were characterized by cyclic voltammetry and Auger spectroscopy. The topography of the UHV-prepared deposits was observed by STM. Figure 11 shows STM data for a Pt(l 11) electrode without (a) and with (b-f) Ru layers formed by vapor deposition. 

Sieger, H. et al., Gonlrolling surface composition and zeta potential of chemical vapor synthesized alumina-sihea nanoparticles, Chem. Vapor Depos., 10, 71, 2004. 

Phase transition of crystalline a-Te2Mo07to the vitreous p-form, surface composition, and activity in the vapor-phase selective oxidation of ethyl lactate to pyruvate over TeOa-MoOa catalysts... 

A major study of condensing on the outside of vertical enhanced tubes has been carried out at Oak Ridge National Laboratory in connection with geothermal Rankine cycle condensers. About 12 tubes were tested with ammonia, isobutane, and various fluorocarbons. The report by Domingo [163] on R-ll concluded that the best surface was the axially fluted tube, followed, in order, by the deep spirally fluted tube, spiral tubes, and roped tubes. The composite (vapor and tube wall) heat transfer coefficient was as much as 5.5 times the smooth-tube value. This high performance was further improved to a factor of 7.2 by using skirts to periodically drain off the condensate. 

---