Deposition Step

A two-step carbon deposition process using isobutylene was developed by Cabrera and Armor (1991). Numerous optimized deposition processes have been described by Cabrera et al. (1993). In the two-step scheme, the carbon support with pore sizes of about 4.5 to 20 A is contacted with two different concentrations of a hydrocarbon. The concentration in the first step is larger than that in the second step. In this fashion, the pore openings of the support micropores are narrowed successively in two distinct steps without excessive pore filling. 

Because the finishing step in producing CMS s is carbon deposition in an inert atmosphere at a moderately high temperature, the surface of CMS s is quite uniformly covered by carbon. Unlike activated carbon that has a considerable amount of surface functionality, CMS s not have detectable surface functionality (Armor, 1994). Moreover, they should have fewer exposed inorganic compounds than activated carbon and not have cations. Consequently, adsorption of gas molecules on CMS s involves only nonspecific dispersion forces (see Chapter 2). For these reasons, CMS s should also be more hydrophobic than activated carbon. 

The main use for CMS s is nitrogen production from air and CH4/CO2 separation, both by PSA. The latter is applied for (1) landfill gas that contains approximately 50% each of CH4/CO2, and (2) tertiary oil recovery where the effluent gas contains 80% CO2 and 20% of CH4 plus other light hydrocarbons. The PSA separation of CH4/CO2 with Bergbau Forschung CMS has been discussed in detail by Kapoor and Yang (1989) and by Baron (1994), who also discussed several other possible applications. 

The difference in the kinetic diameters of N2 and O2 is 0.2 A. That between CH4 and N2 is also 0.2 A. Given the importance of CH4/N2 separation in the field of natural gas upgrading, it is surprising that a suitable CMS has not yet been developed. Attention is certainly warranted for developing such a CMS. 

In kinetic separation, both equilibrium amount and diffusion rates are important. A sorbent selection parameter (Sk) has been given in Chapter 3. It involves both diffusivity and equilibrium constant. The equilibrium isotherms and diffusion time constants for a number of important sorbates on CMS s are given below. 

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The experimental design for cathodic stripping voltammetry is similar to that for anodic stripping voltammetry with two exceptions. First, the deposition step in-... 

In adsorptive stripping voltammetry the deposition step occurs without electrolysis. Instead, the analyte adsorbs to the electrode s surface. During deposition the electrode is maintained at a potential that enhances adsorption. For example, adsorption of a neutral molecule on a Hg drop is enhanced if the electrode is held at -0.4 V versus the SCE, a potential at which the surface charge of mercury is approximately zero. When deposition is complete the potential is scanned in an anodic or cathodic direction depending on whether we wish to oxidize or reduce the analyte. Examples of compounds that have been analyzed by absorptive stripping voltammetry also are listed in Table 11.11. 

A.STM.B764, Std. Methodfor Simultaneous Thickness and Electrochemical Potential determination of Individual Eayers in Multi-layer Mickel deposit (STEP test), American Society for Testing and Materials, Philadelphia, Pa., 1986. 

Essentially, stripping analysis is a two-step technique. The first, or deposition, step involves die electrolytic deposition of a small portion of the metal ions hi solution into die mercury electrode to preconcentrate the metals. This is followed by die shipping step (the measurement step), which involves die dissolution (shipping) of die deposit. Different versions of stripping analysis can be employed, depending upon die nature of the deposition and measurement steps. 

Additional SEM pictures taken after various deposition steps show the evolution of a completely disordered, highly amorphous three-dimensional surface coverage (Fig. 20) [93],... 

The primary goal of the researchers has been to produce Q-dots possessing all of the attributes of the Q-dots prepared using liquid-phase synthetic methods (that is adjustability of the nanocrystal identity and diameter and size monodispersity) and also the technological utility of Q-dots prepared by MBE (specifically, the deposition of nanocrystals with a defined orientation and an electrical output contact). It was shown that the E/C-synthesized 5-CuI and CdS Q-dots were indeed epitaxial with narrow size distribution and strong photoluminescence tunable by the particle size. Qne of the advantages of the E/C method is that it can be made size selective. The key point is that the size as well as the size dispersion of product nanoparticles are directed actually by the corresponding properties of the metal nanoparticles therefore the first deposition step assumes special importance. 

In general, a preparation of mixed monolayer can be realized by either a kinetic control or a thermodynamic control (Figure 1, left). Kinetic control is based on a suggestion that for an initial deposition step the desorption rate is ignorable in comparison with the adsorption rate. In this case, the concentration ratio of the adsorbed species A and B on the surface corresponds to the ratio of products of their adsorption rate constant ( a or b) and concentration (Ca or Cb) A aCa/A bC b. The validity of the initial assumption on low desorption rate means that the total surface coverage obtained under kinetic control is essentially lower than 100%. This non-complete coverage does not disturb most of optical applications of the... 

During the deposition step, some fraction of the total analyte is deposited into the mercury electrode by electrolysis for a given length of time. An exhaustive electrolysis, in which all of the analyte is deposited into the electrode, is time consuming and generally unnecessary, since adequate concentrations can usually be deposited into... 

Anodic SV consists in a concentration (pre-electrolysis or cathodic deposition) step, in which the analyte metal ion is reduced with stirring and at a... 

When the direct-on process is utilized, surface preparation requirements are more critical to ensure effective enamel adhesion. The acid etch is often deeper and the nickel deposition is always thicker. Typically, the nickel coating is 0.01 to 0.02 g/m2 for direct-on coating as compared to 0.002 to 0.007 g/m2 for two-coat applications. A few porcelain enamelers prefer to omit the nickel deposition step. Although the nickel enhances enamel bonding, product quality requirements may not require nickel deposition. The omission of the nickel step necessitates the utilization of a heavy acid etch to ensure a clean, properly conditioned surface for enamel bonding.3-6... 

The processed pulp is converted into a paper product via a paper production machine, the most common of which is the Fourdrinier paper machine (see Figure 21.6). In the Fourdrinier system,3 the pulp slurry is deposited on a moving belt (made from polyester forming fabrics) that carries it through the first stages of the process. Water is removed by gravity, vacuum chambers, and vacuum rolls. This waste water is recycled to the slurry deposition step of the process due to its high fiber content. The continuous sheet is then pressed between a series of rollers to remove more water and compress the fibers. 

Ideally, CdTe could be formed using reductive UPD of Te and Cd. If, however, acidic HTeOj solutions are used, reductive Te UPD requires a potential of near 0.0 V (Figure 9B) in order to avoid bulk deposition. Cd UPD is optimal between —0.4 and —0.6 V. Cd atomic layers, however, will strip during the Te deposition step. On the other hand, Te can be deposited at —0.5 V, where the Cd remains stable, but some bulk Te is formed along with the Te(UPD) ... 

Mehlin, M. Rammasch, J. Fritz, FI. P. 1994. Preparation of CuGaSe2 thin-film solar-cells comprising an electrochemical gallium deposition step. Zeitschrift Fur. Naturforschung 49b 1597-1605. 

Figure 11.16. Overview of 3D NW circuit integration, (a) Contact printing of NWs from growth substrate to prepatterned substrate, (b) Three-dimensional NW circuit is fabricated by the repetition of the contact printing, device fabrication, and separation layer deposition steps N times. [Reprinted with permission from Ref.75. Copyright 2007 American Chemical Society.] (See color insert.)...

In CSV, the anions are deposited as the Hg2A2 (s) salt on a mercury electrode in the deposition step and then reduced to Hg (1) in... 

In simple terms, the crystallisation process may be considered to take place in two stages—a diffusional step in which solute is transported from the bulk fluid through the solution boundary layer adjacent to the crystal surface, and a deposition step in which adsorbed solute ions or molecules at the crystal surface are deposited and integrated into the crystal lattice. These two stages which are shown in Figure 15.13, may be described by ... 

The surface structure and characteristics (density and acidity) of the hydroxyl groups presented in Fig. 13.21 (using CrystalMaker 2.1.1 software) give very useful information to understand the reactivity of the surface of the particles, particularly when adsorption of another complex is desired to synthesize a bimetallic catalyst, or to control the interaction with an oxide carrier (the deposition step). The isoelectric point calculated with the model (5.9) is in fair agreement with the experimental value (4.3). 

Despite the vast quantity of data on electropolymerization, relatively little is known about the processes involved in the deposition of oligomers (polymers) on the electrode, that is, the heterogeneous phase transition. Research - voltammetric, potential, and current step experiments - has concentrated largely on the induction stage of film formation of PPy [6, 51], PTh [21, 52], and PANI [53]. In all these studies, it has been overlooked that electropolymerization is not comparable with the electrocrystallization of inorganic metallic phases and oxide films [54]. Thus, two-or three-dimensional growth mechanisms have been postulated on the basis that the initial deposition steps involve one- or two-electron transfers of a soluted species and the subsequent formation of ad-molecules at the electrode surface, which may form clusters and nuclei through surface diffusion. These phenomena are still unresolved. 

For each cathodic stripping mechanism, the dimensionless net peak current is proportional to the amount of the deposited salt, which is formed in the course of the deposition step. The amount of the salt is affected by the accumulation time, concentration of the reacting ligand, and accumulation potential. The amount of the deposited salt depends sigmoidally on the deposition potential, with a half-wave potential being sensitive to the accumulation time. If the accumulation potential is significantly more positive than the peak potential, the surface concentration of the insoluble salt is independent on the deposition potential. The formation of the salt is controlled by the diffusion of the ligand, thus the net peak current is proportional to the square root of the accumulation time. If reaction (2.204) is electrochemically reversible, the real net peak current depends linearly on the frequency, which is a common feature of all electrode mechanism of an immobilized reactant (Sect. 2.6.1). The net peak potential for a reversible reaction (2.204) is a hnear function of the log(/) with a slope equal to typical theoretical response... 

For example, our preliminary MALDI MS studies employed a somewhat unconventional sample preparation approach in which the protein deposition step preceded the matrix deposition step. (In the conventional approach, the protein is co-deposited with the MALDI matrix solution.). .. Thus, in the first/next/colleclively group of experiments, we will explore different sample preparation methods to determine if other protocols also lead to an inverse relationship between surface-protein binding affinity and the MALDI ion signal. 

Lobanov et al. electroplated curium (ultimately converted to Cm02) in a series of electrodeposition steps from solutions of curium nitrate in isobutanol [158]. The electrochemical setup consisted of a Ti foil cathode, Ti or At foil anode, potential difference of 600 V, and a plating time of 10-15 min for each plating step. Between each deposition step the deposited curium was converted to Cm02. The multistep procedure allowed targets of curium with the desired thickness (300-400 pg cm ) to be obtained with a deposition yield of no less than 90%. Ramaswami and coauthors electroplated curium from isopropanol solution [159]. A 100 pL aliquot... 

Chemical vapor infiltration (CVl) is similar to CVD in that gaseous reactants are used to form solid products on a substrate, but it is more specialized in that the substrate is generally porous, instead of a more uniform, nominally flat surface, as in CVD. The porous substrate introduces an additional complexity with regard to transport of the reactants to the surface, which can play an important role in the reaction as illustrated earlier with CVD reactions. The reactants can be introduced into the porous substrate by either a diffusive or convective process prior to the deposition step. As infiltration proceeds, the deposit (matrix) becomes thicker, eventually (in the ideal situation) filling the pores and producing a dense composite. 

The overall process of metal deposition and crystal growth involves several steps. One is the diffusion of ions in the solution to the metal surface. Another is the cathodic deposition step, i.e., the removal of the ion across the interfacial region to land somewhere on a terrace on the metal surface. 

For the deposition step, the working electrode is maintained at a potential cathodic (by at least 0.4 V) of the standard potential of the least easily reduced ion to be determined. Forced convection (via rotation, stirring, or flow) is usually used to facilitate the deposition step. Quiescent solutions may be employed in connection with ultramicroelectrodes. The deposition time required is dependent on the sample concentration, with 1- to 10-min periods usually being sufficient for measurements in the range of 10-7 M to 1 x 10 9 M. Because only a small fraction of the metal ions is deposited, it is essential that all experimental parameters be as reproducible as possible during a series of measurements. 

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