Deposit resistance

There is great interest in developing soft lens materids that have high oxygen permeabdity and good wettabdity. Such lenses will provide patients with good comfort, high deposit resistance, and superior corneal health, particularly for extended wear appHcations. 

The deposit resists atmospheric tarnish even in the presence of high pollution by sulphur dioxide (in contrast to nickel) and hydrogen sulphide, and coatings exposed to the outdoor atmosphere remain bright indefinitely, sometimes taking on a slightly more pink colour as the oxide film thickens. 

Aside from the reactant gases that can be used, another factor influencing the nature of the films is ion bombardment. If ion bombardment is increased at a given deposition temperature, it is possible to convert an amorphous film to one which is microcrystalline. As ion bombardment continues to increase, the volume of crystallites display lower resistivity, as would be expected. The as-deposited resistivity of PECVD polysilicon has been shown to have a value of 0.78 x 106 S2-cm, as compared to similar film done by LPCVD which is 13.8 x 106 2-cm. 

In the previous chapter, it was shown that it is possible to deposit quality tungsten films by CVD for either selective or blanket applications at moderate temperatures. As-deposited resistivities are low, so post-deposition high-temperature anneals are not essential. 

Figure 16 As-deposited resistivity and sheet resistance of versus deposition temperature.16...

As we saw in the TiSi2 case just covered, it may be possible to deposit an amorphous film and anneal, if necessary, to achieve a desirable grain size. In fact, PECVD of aluminum films composed of small crystals with random orientation have been obtained23 with as-deposited resistivities of 5 to 10 jUf2-cm. 

An early study by Bixler and Rappe [88] showed that glass beads (up to 100 p,m size) added to a stirred cell UF of a macrosolute were able to significantly enhance flux. The mechanism was probably eddy formation and thinning of the concentration boundary layer by particle interaction. Similar effects were reported by Fane [89] who noted that enhancement required significantly supramicron particles and that smaller particles could in fact add to the deposit resistance. 

On the other hand, it is known that catalyst support exerts a great influence on the catalytic properties of the metallic particles deposited on it during the carbon dioxide reforming of methane. So for a given metal, catalytic activities can be changed [5], product selectivities modified [6] and carbon deposition resistivity altered [7]. Also the addition of certain promoters can improve the catalytic behavior of a given supported metal catalyst. In particular we have shown the benefit of the MgO addition to cobalt catalyst [ 8, 9]. 

Figure 9.16. As deposited resistivity as a function of the deposition temperature. SiH2Cl2AVF6 ratio 32. [Tom Wu236, reprinted with permission].

Assuming that the major accumulation of erythromycin in S. aureus cells is determined by uptake dependent on the amounts of ribosome present in the cells, these observations of drug accumulation can be easily accounted for. As described in a previous section, erythromycin molecules can bind to ribosome at a certain equilibrium state expressed as a dissociation constant [39]. For example, on the supposition that an intracellular concentration of erythromycin increases by 30 times the level of that in an extracellular medium (1 pg/ml), the nonprotonated molecules of erythromycin (p T = 8.8) are able to occupy no more than 1% of a drug concentration present in S. aureus cell. That is why it is expected that intracellular pH would be 6.8 or less [200] and that erythromycin (even as a free base) with p T = 8.8 is one of the most water-soluble (i.e., deposition-resisting) drugs in the macrolide antibiotics (Table I). 

Each step contributes to Vd or the dry deposition resistance r = 1/Vd- According to the three layers, the total resistance r is given from the partial resistances (Fig. 4.22), the aerodynamic r, the quasi-laminar r and the surface resistance r ... 

The binder used to link the material to the support is a glass material (its fusion temperature is low, about 800°C). In order not to exorbitantly increase the deposit resistivity, we have chosen a sodic glass material (mass proportions 26.1% Na20, 12.3% AI2O3, 61.6% SiOz). 

Cake Filtration model. This model treats both the membrane resistance and deposit resistances in a very similar way to that described in Chapter 2. Equation (10.10) is the starting point for fiirther development which is concerned initially with the in ease in cake resistance to some equilibnum value. This is analogous to accounting for the increasing cake d th in convoitional dead-end filtration, see Section 2.5. In membrane filtration a mass balance over the deposit layer is performed ... 

Suzuki et have developed Ru-YSZ anodes which show significantly lower polarization compared with the conventiorral anodes using Ni-YSZ cermets. Moreover, tests using Ru/AljOj have revealed that imder SOFC anode operational conditions, Ru metal has a high-steam reforming reaction activity, carbon deposition resistance, and sintering resistance. Dees et al. ° have prepared Ni-YSZ cermets by mixing NiO and zirconia. Then NiO is reduced to... 

In most vacuum deposition applications a given amount of material (charge) is heated. In some cases the material is vaporized to completion while in others the vaporization is stopped when a specific amount of material has been deposited. Resistive heating is the most common technique for vaporizing material at temperatures below about 1500°C, while focused e-beams are most commonly used for temperatures above 1500°C. Suggested vaporization sources for a variety of materials have been tabulated by a number of suppliers of source materials and in publications. ... 

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