Surface cleanliness

In Fig. 37, a large number of representative desorption spectra are presented for simple adsorption systems in which the adsorbate desorbs from the surface without decomposition or reaction. Even a simple molecule such as NO can undergo disproportionation reactions on surfaces to yield N2 into the gas phase. The influences of surface structure and surface cleanliness on desorption spectra are discussed below. Section 4.2.3 deals with a specialised branch of thermal desorption known as temperature programmed reaction spectroscopy. 

Once again, Pt will be used as a sample system to illustrate the effect of one adspecies on the desorption of another. Single crystals of Pt often contain large surface impurities of S, P, Ca, C, Cl and O and these adatoms are often very difficult to remove. The influence of S at the surface on the desorption of CO from Pt is shown in Fig. 40 from the work of Bonzel and Ku [325]. Clearly, S substantially affects the desorption, decreasing both the desorption energy of the adsorbate and the amount adsorbed. Oxygen adsorbs on platinum to form an oxide and McCabe and 

Some desorption spectra from the simpler adsorbate/substrate combinations. References are given with the spectra. (See Table 2.) 

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Xps ndAes Instrumentation. The instmmentation required to perform xps and aes analyses is generally sophisticated and expensive (19). The need for UHV conditions in order to retain surface cleanliness for a tractable period of time was mentioned above. Beyond this requirement (and the hardware that accompanies it), the most important components of an electron spectrometer system are the source, the electron energy analyzer, and the electron detector. These will be discussed in turn below. 

Surface cleanliness of the substrate is important for all thermal spray processes. Degreasing, which formerly often reHed on freons or chlorinated... 

Heat Transfer from Various Metal Surfaces In an early work, Pridgeon and Badger [Jnd. Eng. Chem., 16, 474 (1924)] pubhshed test results on copper and iron tubes in a horizontal-tube evaporator that indicated an extreme effect of surface cleanliness on heat-transfer coefficients. However, the high degree of cleanhness needed for high coefficients was difficult to achieve, and the tube layout and... 

Clean surfaces are almost always easier to protect chemically from corrosion. Chemical inhibition and inhibitors are often tested on relatively clean surfaces, for a variety of reasons. The effectiveness of almost all commonly used corrosion inhibitors increases as surface cleanliness improves. 

The propensity toward dealloying decreases as surface cleanliness increases. Flow should be maintained at rates high enough to prevent... 

Analysis of surface crystallography and microstructure surface cleanliness... 

The degree of surface cleanliness or even ordering can be determined by REELS, especially from the intense VEELS signals. The relative intensity of the surface and bulk plasmon peaks is often more sensitive to surface contamination than AES, especially for elements like Al, which have intense plasmon peaks. Semiconductor surfaces often have surface states due to dangling bonds that are unique to each crystal orientation, which have been used in the case of Si and GaAs to follow in situ the formation of metal contacts and to resolve such issues as Fermi-level pinning and its role in Schottky barrier heights. 

Cause, R.L., A nonconiacting scanning photoelectron emission technique for bonding surfaces cleanliness inspection. NASA Technical Memorandum NASA TM-100361, 1989. Schirato, R.C., Polichar, R.M. and Shreve, D.C., In Proc JANNAF Nondestructive Evaluation Subcommittee Meeting. Chemical Propulsion Information Agency, Columbia, MD, 1992. 

Surface finish is increasingly referred to as surface cleanliness . This can be misleading because the standards refer to the appearance of the blasted steel and do not deal with chemical contamination. Site tests for assessing the level of soluble salts on freshly blast-cleaned surfaces, and which allow the semi-quantitative determination of the chlorides, soluble sulphates and soluble iron salts, are urgently needed. 

Referring to Table 4.2 (boiler heat transfer surface cleanliness) and the supporting italicized notes in Chapter 4 concerning tolerance for deposit thickness, it can be seen that ... 

We have undertaken a series of experiments Involving thin film models of such powdered transition metal catalysts (13,14). In this paper we present a brief review of the results we have obtained to date Involving platinum and rhodium deposited on thin films of tltanla, the latter prepared by oxidation of a tltanliua single crystal. These systems are prepared and characterized under well-controlled conditions. We have used thermal desorption spectroscopy (TDS), Auger electron spectroscopy (AES) and static secondary Ion mass spectrometry (SSIMS). Our results Illustrate the power of SSIMS In understanding the processes that take place during thermal treatment of these thin films. Thermal desorption spectroscopy Is used to characterize the adsorption and desorption of small molecules, In particular, carbon monoxide. AES confirms the SSIMS results and was used to verify the surface cleanliness of the films as they were prepared. 

The other vibrational spectroscopies, laser Raman and magic angle spinning NMR, have also been useful. Despite its low resolution, high resolution EELS has been usefiil in UHV work for assessment of surface cleanliness and for the identification of adsorbed species. 

For the present purpose, we take the term ultrathin to refer to an evaporated metal film where the concentration of metal on the substrate is low enough for the film to consist of small isolated metal crystals. If the average concentration of metal atoms on the substrate is of the order of a monolayer or less, the metal crystals are small enough for ultrathin films to serve as models for highly dispersed metal catalysts, but where surface cleanliness and catalyst structure can be better controlled. 

Recently, ultrathin evaporated films have been used as models for dispersed supported metal catalysts, the main object being the preparation of a catalyst where surface cleanliness and crystallite size and structure could be better controlled than in conventional supported catalysts. In ultrathin films of this type, an average metal density on the substrate equivalent to >0.02 monolayers has been used. The apparatus for this technique is shown schematically in Fig. 8 (27). It was designed to permit use under UHV conditions, and to avoid depositing the working film on top of an outgassing film. ... 

The use of hydrogen annealing to ensure homogenization and surface cleanliness is an attractive procedure in alloy film preparation but for the question of surface enrichment. The possibility of surface enrichment in... 

Smooth polycrystalline Au, Pt and Ir thin-layer electrodes were utilized (10-11). Electrodes were cleaned between trials by sequential electrochemical oxidation above 1.2 V [Ag/AgCl (1 M Cl-) reference] and reduction below -0.2 V in 1 M H2SO4 surface cleanliness was verified with the aid of cyclic voltammetry in the same molar sulfuric acid solution. Experiments were carried out in 1 M H2SO4, 1 M NaC104 buffered at pH 7 and 10, and in 1 M NaOH solutions were prepared with pyrolytically triply distilled water (12). Surface reagents employed were iodide, hydroquinone (HQ), 2,5-dihydroxythiophenol [DHT (13)1. and 3,6-dihydroxypyridazine (DHPz). 

In the first set of measurements the rate of carbon build-up on a Ni(lOO) surface was measured at various temperatures as follows (1) surface cleanliness was established by AES (2) the sample was retracted into the reaction chamber and exposed to several torr of CO for various times at a given temperature (3) after evacuation the sample was transferred to the analysis chamber and (4) the AES spectra of C and Ni were measured. Two features of this study are noteworthy. First, two kinds of carbon forms are evident - a carbidic type which occurs at temperatures < 650 K and a graphite type at temperatures > 650 K. The carbide form saturates at 0.5 monolayers. Second, the carbon formation data from CO disproportionation indicates a rate equivalent to that observed for methane formation in a H2/CO mixture. Therefore, the surface carbon route to product is sufficiently rapid to account for methane production with the assumption that kinetic limitations are not imposed by the hydrogenation of this surface carbon. 

ISO 8501 (all parts). Preparation of steel substrates before application of paints and related products - Visual assessment of surface cleanliness. 

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