Transparency optimum

The various techniques which may be used to provide optimum conditions for the examination of specimens have been described [202—205]. If the sample is opaque, then microscopic investigation is limited to the surface. The depths of penetration for the study of transparent crystals are controlled by the limited depth of field of the optical microscope at high magnifications. This limitation can sometimes be overcome by cleavage of the crystal at an appropriate value of a and examination of the surfaces exposed [120],... 

If the mixture to be separated contains fairly polar materials, the silica may need to be deactivated by a more polar solvent such as ethyl acetate, propanol or even methanol. As already discussed, polar solutes are avidly adsorbed by silica gel and thus the optimum concentration is likely to be low, e.g. l-4%v/v and consequently, a little difficult to control in a reproducible manner. Ethyl acetate is the most useful moderator as it is significantly less polar than propanol or methanol and thus, more controllable, but unfortunately adsorbs in the UV range and can only be used in the mobile phase at concentrations up to about 5%v/v. Above this concentration the mobile phase may be opaque to the detector and thus, the solutes will not be discernible against the background adsorption of the mobile phase. If a detector such as the refractive index detector is employed then there is no restriction on the concentration of the moderator. Propanol and methanol are transparent in the UV so their presence does not effect the performance of a UV detector. However, their polarity is much greater than that of ethyl acetate and thus, the adjustment of the optimum moderator concentration is more difficult and not easy to reproduce accurately. For more polar mixtures it is better to explore the possibility of a reverse phase (which will be discussed shortly) than attempt to utilize silica gel out of the range of solutes for which it is appropriate. 

With all pigments, the particle size distribution of the pigment as synthesised is the most important single factor in ensuring its optimum potential performance in use, but this potential can only be achieved if the particles are properly dispersed. To understand why particle size is so important it is necessary to look in some detail at the optical behaviour of tiny pigment particles dispersed in a transparent organic medium. The exact nature of this medium depends on whether the end-use is a paint, a printing ink or a plastic. 

Many of the stated restrictions can be lifted (at the cost of using more complicated formulas) when these conditions are not adequately met. For other systems, it may be more appropriate to use approximations based on assumptions different from those in the model described above, which applies to systems with approximately optimum covering power. Such an approximation was presented by Hoffmann [9] and simplified by Schmelzer. It is employed for highly transparent coatings where the substrate plays a significant role specifically, it describes the behavior of printing inks. 

Estimates of oCbiend using a rule-of-mixtures relationship are 3.0 X 102 and 7.2 X 103 cm lor 0.2 and 5.0% polyimide, respectively. This dependence of the optimum absorption coefficient (in terms of ablation rate), OVx on fluence is consistent with the observations of Chuang et al.6% for ablation of several UV-transparent (at 308 nm) polymers sensitized with low-molecular-weight dopants, e.g., PMMA doped with pyrene. For the pyrene-PMMA system, Chuang et al.6S reported maximum etch rates for 1.2 J/cm2 at a = 7 X 102 cm 1. It should not be expected that different dopant-matrix systems would yield the same optimum absorption coefficient for a given fluence level since the thermal properties for different polymers may vary significantly. 

Styrolux is an example of a nanostructured polymer which is used in food packaging. It is a polystyrene-polybutadiene block copolymer where polymer chains are build up of alternating polystyrene and polybutadiene blocks. These blocks appear as dark lamellae in the TEM image due to the staining of the polybutadiene with OSO4. This structured nanoscale architecture of the pol5mier, which can be controlled during manufacture, allows the optimum combination of impact resistance and transparency. 

As a pigment, each iron oxide has an optimum particle size which is that with the maximum scattering cross section. This optimum particle size is lower, the higher the refractive index of the mineral. For hematite, the size corresponding to the maximum in scattering/absorption cross section is ca. 1 pm. As the particle size decreases, the relative scattering cross section drops to zero and the relative absorption cross section levels out. As a result, very small particles of hematite are transparent. 

Film electrodes are generally fabricated from conducting or semiconducting materials, which may be deposited as a result of either a physical or a chemical process (or some combination) onto a suitable substrate, which is typically an insulator. Key factors governing the desired thickness of the film are the electrical resistivity (p) or conductivity (k = 1/p) of the film material, which is a practical consideration in almost all cases, and the optical transparency or reflectance of the material if optical transmission or reflection is also desired. The optimum film thickness for an application involving both electrical and optical considerations will require a trade-off, since a decrease in resistivity (usually desirable) normally is also accompanied by a decrease in light transmission (undesirable for an optically transparent electrode). 

Food-grade CMC is a cellulose carboxylic acid ether with an optimum DS = 0.4-0.7. The higher the DS within this range, the more hydrophilic is the polyanion. Uniformity of substitution makes CMC more compatible with dissolved salts and less inclined to thixotropy than uneven distribution (Feddersen and Thorp, 1993). This gum does not precipitate from a 50% ethanol solution. Below approximately pH 4 in water, the polyanions revert to the un-ionized, water-insoluble acid. CMC viscosity-hysteresis has already been described (Fig. 2 in Chapter 3). CMC dispersions and films have the extra advantage of transparency relative to many other polysaccharide dispersions. The films are resistant to oils, grease, and organic solvents (Hercules, Inc., 1980). 

In conclusion, for both AP-CVD and LP-CVD processes, only a narrow range of temperatures can be identified for optimum performance (a range that is typically 40°C-wide). Within this narrow temperature range highly oriented films are obtained that have electrical and optical properties suitable to act as transparent conductors in solar cells. The typical substrate temperature is around 400°C for the AP-CVD process, whereas it is around 160°C for the LP-CVD process. The two processes yield film orientations that are perpendicular to each other. 

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