A thin films

The long-range van der Waals interaction provides a cohesive pressure for a thin film that is equal to the mutual attractive force per square centimeter of two slabs of the same material as the film and separated by a thickness equal to that of the film. Consider a long column of the material of unit cross section. Let it be cut in the middle and the two halves separated by d, the film thickness. Then, from one outside end of one of each half, slice off a layer of thickness d insert one of these into the gap. The system now differs from the starting point by the presence of an isolated thin layer. Show by suitable analysis of this sequence that the opening statement is correct. Note About the only assumptions needed are that interactions are superimposable and that they are finite in range. 

A thin film of hydrocarbon spread on a horizontal surface of quartz will experience a negative dispersion interaction. Treating these as 1 = quartz, 2 = n-decane, 3 = vacuum, determine the Hamaker constant A123 for the interaction. Balance the negative dispersion force (nonretarded) against the gravitational force to find the equilibrium film thickness. 

The coefficient of friction between two unlubricated solids is generally in the range of 0.5-1.0, and it has therefore been a matter of considerable interest that very low values, around 0.03, pertain to objects sliding on ice or snow. The first explanation, proposed by Reynolds in 1901, was that the local pressure caused melting, so that a thin film of water was present. Qualitatively, this explanation is supported by the observation that the coefficient of friction rises rapidly as the remperarure falls, especially below about -10°C, if the sliding speed is small. Moreover, there is little doubt that formation of a water film is actually involved [3,4]. 

In moist enviromnents, water is present either at the metal interface in the fonn of a thin film (perhaps due to condensation) or as a bulk phase. Figure A3.10.1 schematically illustrates another example of anodic dissolution where a droplet of slightly acidic water (for instance, due to H2SO4) is in contact with an Fe surface in air [4]. Because Fe is a conductor, electrons are available to reduce O2 at the edges of the droplets. 

Figure Bl.24.13. A thin film of LaCaMn03 on an LaA103 substrate is characterized for oxygen content with 3.05 MeV helium ions. The sharp peak in the backscattering signal at chaimel 160 is due to the resonance in the scattering cross section for oxygen. The solid line is a simulation that includes the resonance scattering cross section and was obtained with RUMP [3]. Data from E B Nyeanchi, National Accelerator Centre, Fame, South Africa.

We are now in a position to calculate the reflections from multiple mterfaces using the simple example of a thin film of material of thickness d with refractive index n.2 sandwiched between a material of refractive index (where this is generally air witii n = ) deposited onto a substrate of refractive index [35, 36], This is depicted in figure Bl.26.9. The resulting reflectivities for p- and s-polarized light respectively are given by ... 

Figure C2.17.3. Close-packed array of sub-micrometre silica nanoparticles. Wlren nanoparticles are very monodisperse, they will spontaneously arrange into hexagonal close-packed stmcture. This scanning electron micrograph shows an example of this for very monodisperse silica nanoparticles of -250 nm diameter, prepared in a thin-film fonnat following the teclmiques outlined in [236].

Pure boron in the form of a thin film can also be obtained by heating diborane to 1000 K ... 

Drying oils. Place 3 ml. of hnseed oil in a test-tube, add about 01 g. of Utharge and boil the mixture gently for 10 minutes. When cold, pour a httle of the product on a watch glass and spread the oil into a thin film with the aid of a smah piece of paper. Pour a little of the untreated hnseed oil on another watch glass and spread it out as a thin film. Compare the times taken for the films to become dry. 

Not all synthetic polymers are used as fibers Mylar for example is chemically the same as Dacron but IS prepared in the form of a thin film instead of a fiber Lexan is a polyester which because of its impact resistance is used as a shatterproof substitute for glass It IS a polycarbonate having the structure shown... 

Liquid samples are analyzed in one of two ways. For nonvolatile liquids a suitable sample can be prepared by placing a drop of the liquid between two NaCl plates, forming a thin film that typically is less than 0.01 mm thick. Volatile liquids must be placed in a sealed cell to prevent their evaporation. 

A typical Ag/AgCl electrode is shown in figure 11.9 and consists of a silver wire, the end of which is coated with a thin film of AgCl. The wire is immersed in a solution that contains the desired concentration of KCl and that is saturated with AgCl. A porous plug serves as the salt bridge. The shorthand notation for the cell is... 

One of the most important advances in column construction has been the development of open tubular, or capillary columns that contain no packing material (dp = 0). Instead, the interior wall of a capillary column is coated with a thin film of the stationary phase. The absence of packing material means that the mobile phase... 

To minimize the multiple path and mass transfer contributions to plate height (equations 12.23 and 12.26), the packing material should be of as small a diameter as is practical and loaded with a thin film of stationary phase (equation 12.25). Compared with capillary columns, which are discussed in the next section, packed columns can handle larger amounts of sample. Samples of 0.1-10 )J,L are routinely analyzed with a packed column. Column efficiencies are typically several hundred to 2000 plates/m, providing columns with 3000-10,000 theoretical plates. Assuming Wiax/Wiin is approximately 50, a packed column with 10,000 theoretical plates has a peak capacity (equation 12.18) of... 

In this cross-flow arrangement, a thin film of sample solution is obtained as it flows around the edge of a small opening, through which there is a fast linear flow of argon. The liquid film is rapidly nebulized along the rim of the orifice. 

The aim of breaking up a thin film of liquid into an aerosol by a cross flow of gas has been developed with frits, which are essentially a means of supporting a film of liquid on a porous surface. As the liquid flows onto one surface of the frit (frequently made from glass), argon gas is forced through from the undersurface (Figure 19.16). Where the gas meets the liquid film, the latter is dispersed into an aerosol and is carried as usual toward the plasma flame. There have been several designs of frit nebulizers, but all work in a similar fashion. Mean droplet diameters are approximately 100 nm, and over 90% of the liquid sample can be transported to the flame. 

This arrangement provides a thin film of liquid sample solution flowing down to a narrow orifice (0.007-cm diameter) through which argon flows at high linear velocity (volume flow is about 0.5-1 1/min). A fine aerosol is produced. This particular nebulizer is efficient for solutions having a high concentration of analyte constituents. 

Reversed-phase columns are used to separate polar substances. Although in LC the stationary phase is a solid, it is necessary to bear in mind that there may be a thin film of liquid (e.g water) held on its surface, and this film will modify the behavior of sample components equilibrating between the mobile and stationary phases. A textbook on LC should be consulted for deeper discussion on such aspects. 

Figure 3.18 Total reflection of radiation in a medium of refractive index 2 by a thin film of refractive index wj, where 2...

Ultraviolet (uv) light has also been used to stetili2e the water in aquaculture systems. The effectiveness of uv decreases with the thickness of the water column being treated, so the water is usually flowed past uv lights as a thin film (alternatively, the water may flow through a tube a few cm in diameter that is surrounded by uv lights). Uv systems require more routine maintenance than o2one systems. Uv bulbs lose their power with time and need... 

The real utility of d comes in the analysis of thin films. Consider a substrate of refractive index supporting a thin film of thickness d and refractive index in contact with an internal reflection element (the prism) of refractive index as shown in Figure 24. In this case, d depends on the polarization of the incident light beam and is given by... 

Atr—ftir can be readily performed on most commercial ftir spectrometers through the use of an attachment for atr spectroscopy. These devices provide ir-transparent internal reflection elements that are typically made of Ge, KRS-5, ZnSe, or ZnS. These internal reflection elements are made of materials that are of extremely high purity to avoid losses from absorption by impurities in these devices. Coupling of a thin film or surface sample to one of these reflection elements is accompHshed by pressing the sample against the element while acquiring the spectmm. 

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