Solid-film resists

Resists as different as naturally occurring colloids (such as gelatin, albumin, and shellac) or synthetic polymers (such as polyvinyl alcohol, polyacrylics, polyvinyl pyrrolidone, and polyvinyl hutyral) rendered photosensitive by dichromate or other means, as well as synthetic photosensitive polymers such as polyvinyl cinnamates (Kodak positive resists), diazonaphthoquinone/novolac, and solid-film resists such as Riston (introduced by Dupont in 1968) have all been used in PCB fabrication. Some of these resists are still being used today. 

The tenn tribology translates literally into the study of nibbing . In modem parlance this field is held to include four phenomena adhesion, friction, lubrication and wear. For the most part these are phenomena that occur between pairs of solid surfaces in contact with one another or separated by a thin fluid film. Adhesion describes the resistance to separation of two surfaces in contact to while friction describes their tendency to resist shearing. Lubrication is the phenomenon of friction reduction by the presence of a fluid (or solid) film between two surfaces. Finally, w>ear describes the irreversible damage or defonnation that occurs as a result of shearing or separation. 

The wide range of soHd lubricants can generally be classified as either inorganic compounds or organic polymers, both commonly used in a bonded coating on a matching substrate, plus chemical conversion coatings and metal films. Since solid-film lubricants often suffer from poor wear resistance and inabihty to self-heal any breaks in the film, search continues for improved compositions. 

Later publications have been concerned with mass transfer in systems containing no suspended solids. Calderbank measured and correlated gas-liquid interfacial areas (Cl), and evaluated the gas and liquid mass-transfer coefficients for gas-liquid contacting equipment with and without mechanical agitation (C2). It was found that gas film resistance was negligible compared to liquid film resistance, and that the latter was largely independent of bubble size and bubble velocity. He concluded that the effect of mechanical agitation on absorber performance is due to an increase of interfacial gas-liquid area corresponding to a decrease of bubble size. 

The relatively high volatility of Tg[CH = CH2]8 has enabled it to be used as a CVD precursor for the preparation of thin films that can be converted by either argon or nitrogen plasma into amorphous siloxane polymer films having useful dielectric propertiesThe high volatility also allows deposition of Tg[CH = CH2]g onto surfaces for use as an electron resist and the thin solid films formed by evaporation may also be converted into amorphous siloxane dielectric films via plasma treatment. ... 

Toshiyuki, Hayashi, Akihito, Matsumuro, Mutsuo, Mura-matsu, Masao, Kohzaki, and Katsumi, Yamaguchi, "Wear Resistance of Carbon Nitride Thin Films Formed by Ion Beam Assisted Deposition," Thin Solid Films, Vol. 376, No. 1-2, 2000,pp. 152-158. 

In either equation, k° is given by Eq. (16-84) for parallel pore and surface diffusion or by Eq. (16-85) for a bidispersed particle. For nearly linear isotherms (0.7 < ft < 1.5), the same linear addition of resistance can be used as a good approximation to predict the adsorption behavior of packed beds, since solutions for all mechanisms are nearly identical. With a highly favorable isotherm (R —> 0), however, the rate at each point is controlled by the resistance that is locally greater, and the principle of additivity of resistances breaks down. For approximate calculations with intermediate values of R, an overall transport parameter for use with the LDF approximation can be calculated from the following relationship for solid diffusion and film resistance in series ... 

The complexity of eqn. (58) arises because it was not assumed that any one of the three rate processes identified is rate determining. The relation between time and conversion is considerably simplified one of the rate processes dominates the overall process. When reactant gas flows through a fixed bed of solid particles, the gas film resistance to conversion is... 

Nonisothermal Effects. We may expect temperature gradients to occur either across the gas film or within the particle. However, the previous discussion indicates that for gas-solid systems the most likely effect to intrude on the rate will be the temperature gradient across the gas film. Consequently, if experiment shows that gas film resistance is absent then we may expect the particle to be at the temperature of its surrounding fluid hence, isothermal conditions may be assumed to prevail. Again see Example 18.1. 

Cooney defined qmgx as the solid concentration in equilibrium with C0. This number is dimensionless, and thus the concentration units are the same for the liquid and solid phase (e.g. mg/g). Cooney pointed out that for Bi < 0.5, complete dominance of the liquid-film resistance exists, while for Bi > 30, reasonably complete dominance of intraparticle resistance exists. The only restriction is that the isotherm be favorable. 

In the case of solid diffusion control, even in the absence of agitation where the mass transfer coefficient is at its minimum value, sufficient agitation should be provided in order to avoid the negative effect of the liquid-film resistance. The effect of agitation should be taken into account in both the design and application stage. 

This model covers the case where we have combined resistances to diffusion (fluid-film and solid diffusion). In this case, the concentration in the main phase of the fluid (bulk concentration) is different from the one at the interface due to the effect of the fluid film resistance. The following equations can be used for Langmuir and Freundlich equilibrium equations (Miura and Hashimoto, 1977). The solutions of the fixed-bed model are the following ... 

Essentially, the above model is empirical as in real systems, both solid and fluid-film resistances play an important role in the adsorption process. An improved BDST model is found elsewhere (Ko et al., 2000, 2002). Finally, qm a and could be flow rate-dependent parameters (Walker and Weatherley, 1997). 

As described previously, the reactant has to first reach the external surface of the catalyst. For simplicity, we take into consideration the case of a gas reacting on a solid catalytic surface. Owing to gas film resistance, the concentration of the reactant at the catalytic surface (Cs) is lower than that in the bulk of the fluid (Cb). This difference depends on... 

Reaction Scheme of CMS Resists. The transient absorption spectrum shown in Figure 6 and observed for irradiated CMS films is mainly composed of two components as based on pulse radiolysis data of solid films of CMS and polystyrene, and CMS and polystyrene solutions in cyclohexane, chloroform, and carbon tetrachloride. An absorption with a maxima at 320 nm and 500 nm as due to the charge transfer radical-complex of the phenyl ring of CMS and chlorine atom (see Figure 14) and an absorption with maxima at 312 and 324 nm is due to benzyl type radicals (see Figure 11). 

The problem is also more complex when heterogeneous catalysed reactions are considered. With porous catalyst pellets, reaction occurs at gas- or liquid-solid interfaces at the outer or inner sphere. When the reactants diffuse only slowly from the bulk phase to the exterior surface of the catalyst, gas or liquid film resistance must be taken into account. Pore diffusion resistance may be involved when the reactants move through the pores into the pellet. 

The individual mass transfer and reaction steps occurring in a gas-liquid-solid reactor may be distinguished as shown in Fig. 4.15. As in the case of gas-liquid reactors, the description will be based on the film theory of mass transfer. For simplicity, the gas phase will be considered to consist of just the pure reactant A, with a second reactant B present in the liquid phase only. The case of hydro-desulphurisation by hydrogen (reactant A) reacting with an involatile sulphur compound (reactant B) can be taken as an illustration, applicable up to the stage where the product H2S starts to build up in the gas phase. (If the gas phase were not pure reactant, an additional gas-film resistance would need to be introduced, but for most three-phase reactors gas-film resistance, if not negligible, is likely to be small compared with the other resistances involved.) The reaction proceeds as follows ... 

The importance of adsorbent non-isothermality during the measurement of sorption kinetics has been recognized in recent years. Several mathematical models to describe the non-isothermal sorption kinetics have been formulated [1-9]. Of particular interest are the models describing the uptake during a differential sorption test because they provide relatively simple analytical solutions for data analysis [6-9]. These models assume that mass transfer can be described by the Fickian diffusion model and heat transfer from the solid is controlled by a film resistance outside the adsorbent particle. Diffusion of adsorbed molecules inside the adsorbent and gas diffusion in the interparticle voids have been considered as the controlling mechanism for mass transfer. 

The model based on the concept of pure limiting film resistance involves the steady-state concept of the heat transfer process and omits the essential unsteady nature of the heat transfer phenomena observed in many gas-solid suspension systems. To take into account the unsteady heat transfer behavior and particle convection in fluidized beds, a surface renewal model can be used. The model accounts for the film resistance adjacent to the heat transfer... 

Chloromethylated polystyrene and chloromethylated poly(a-methyIstyrene) are negative type resists having high sensitivity and high resolution. In the pulse radiolysis of solid films of this polymer, the absorption spectra of substituted benzyl-type polymer-radical and the charge transfer complex between phenyl rings and chlorine atoms were observed (Fig. 17) [59], The benzyl-type radical may be produced by the dissociative electron attachment to the benzyl part of chloromethylated polystyrene (CMS). 

The average dimensionless solid phase concentration Y can be given409 the numerical value of 0.5, and if the effect of the film mass transfer is negligible, i.e., if K f - 00, then adsorption with the nonporous HPLC sorbent in a well-packed bed is controlled by second-order kinetics.169,399 When the external film resistance Kf controls the adsorption, equilibrium is assumed to exist between the polypeptide or protein and the polypeptide- or protein-ligate complex at each point on the particle surface. 

Normally, KSa and Ksc are assumed to be equal. Once again, conditions kG - oo, k,.oo, and KSA, Ksc - co imply that the gas-film resistance, the liquid-film resistance for the mass transfer at the gas-liquid interface, and the liquid-film resistance for the mass transfer at the liquid-solid interface are negligible. When all the mass-transfer resistances are important, the typical concentration distributions of A and C are as shown in Fig. 2-6. 

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