Isotropic structure

The other type of pyroiytic structure, isotropic carbon, has littie graphitic characteristic and essentiaiiy no opticai activity, it is composed of very fine grains without observable orientation and for this reason, it is known as isotropic carbon rather than isotropic graphite. It is often obtained in fluidized-bed deposition, possibly due to continuous surface regeneration by the mechanical rubbing action of the bed. An isotropic structure, observed by transmission electron microscopy, is shown in Fig. 7.8.P I 

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Whereas in many metals with relatively simple and isotropic crystal structures the parameter / has values between 0.5 and 1, it can have much more extreme values in materials in which the mobile species move through much less isotropic structures with 1-D or two-dimensional (2-D) channels, as is often the case with insertion reaction electrode materials. As a result, radiotracer experiments can provide misleading information about self-diffusion kinetics in such cases. 

Fig. 10. Comparison of the measured refractive indices for PET film with values calculated from the orientation functions determined from n.m.r. assuming transversely isotropic structural units. I, Experimental points predicted values. Reproduced from Polymer by permission of the publishers, Butterworth Co (Publishers) Ltd. (C)...

Total Sn0 for transversely isotropic structural units in trans configuration + 0.00424 —0.00227 + 0.00276... 

The influence of the underlayer was demonstrated by Mirzamaani et al. [74] and by DiMilia et al. [75]. Electroless CoP was deposited from a solution yielding an intrinsically isotropic structure onto NiP that had undergone various pretreatments. When the NiP was exposed to a NaOH treatment before CoP deposition, good inplane magnetic properties resulted, but deposition on a chemically or electrochemically pre-reduced NiP surface gave unacceptable magnetics. These authors found... 

Also, other factors may influence the fracture morphology as is demonstrated by the following examples. Glass fibres having an isotropic structure... 

PSDVB copolymers and their ion exchange derivatives consist of a three dimensional four-connected network structure. Such networks may have a statistically isotropic structure that includes tetrahedral cells, such as the X" unit structure described by Flory (6). The four-connectedness results from the expected pairwise chain connecting function of... 

Carbonization of the oxidized Phalen Seam coal at 550°C resulted in a non-agglomerated char. Dilatation and FSI results for this coal are given in Table II and confirm its non-coking character. The semi-coke obtained from dilatation experiments had an isotropic structure, as shown in Figure 1. 

Lindblom, G. and Rilfors, L. (1989). Cubic phases and isotropic structures formed by membrane lipids - possible biological relevance. Biochem. Biophys. Acta, 988, 221-56. 

Similarly, charged solid particles (such as latex spheres) —kinetically stable lyophobic colloids —may exist in colloidal crystalline phases (with body-centered or face-centered cubic structures) as a consequence of thermodynamically favored reduction in free energies (see Chapter 13). Even neutrally charged spherical particles ( hard spheres ) undergo a phase transition from a liquidlike isotropic structure to face-centered cubic crystalline structures due to entropic reasons. In this sense, the stability or instability is of thermodynamic origin. 

The statistical independence of the eigenvalues at a 0.05 confidence level (a) is also tested. In this way, it is possible to distinguish among orthotropic structure (in which all the three eigenvalues differ each other), transverse isotropic structure (with two eigenvalues statistically equivalent), and isotropic structure (where all the three eigenvalues are equivalent). 

PE/PP blends have also been studied in terms of stability to oxidation. Livanova shown that oxidative degradation reaches a maximum when PP is in an isotropic structure and minimum for an oriented one [90]. 

The body under drying is a moistened capillary-porous solid. This body is assumed here to be of isotropic structure and obeying the viscoelastic Maxwell model of the form... 

Historically most of the microscopic diffusion models were formulated for amorphous polymer structures and are based on concepts derived from diffusion in simple liquids. The amorphous polymers can often be regarded with good approximation as homogeneous and isotropic structures. The crystalline regions of the polymers are considered as impenetrable obstacles in the path of the diffusion process and sources of heterogeneous properties for the penetrant polymer system. The effect of crystallites on the mechanism of substance transport and diffusion in a semicrystalline polymer has often been analysed from the point of view of barrier property enhancement in polymer films (35,36). 

Nearly isotropic structured carbon (e.g., binder coke)... 

The aluminum industry consumes about 0.45 lb. of anode carbon for each pound of aluminum produced. The ideal carbon should have a moderately-isotropic structure with minimum oxidant-accessible surface of low, uniform reactivity, and a maximum ash content, excluding bath salts, of a few tenths of one percent. Industrial anode carbon is a baked composite usually made of calcined petroleum coke filler with a binder of coal-tar pitch coke. While there is no shortage of calcined petroleum coke, the quality is not very good and likely to become worse. Also, coke binders are subject to variability in both quality and supply. 

For the isotropic structure (Figure la) the rose plot is almost a circle (Figure 3c) whereas for the anisotropic image, a gradual variation of the slope with direction gives rise to noncircular patterns (Figure 3d). 

As seen in Table 7.1 and 7.2, there are, in addition to these two examples, several other photoactive cells in which we have identified cubic membranes. In the case of the mitochondrial cubic membrane in Tupaia gUs we can speculate that the use of an isotropic structure allows efficient capture of the incoming light, which has to pass the giant mitochondria before reaching the outer segment. Similar reasons may guide the choice of a cubic membrane system in the lens of certain scaleworms. 

Carbon felt, carbon wool and woven carbon, which due to their isotropic structure only exhibit low mechanical strengths and low elasticity moduli, are manufactured by the pyrolysis of organic textiles. Depending on the form of the starting material utilized e.g. as woven textiles or felt, woven carbon or carbon felt is produced after carbonization. 

The pyrolysis reactions take place at temperatures between 800 and 3000°C, both in the gas phase and on the substrate surface. Their property mix can be varied within wide limits by appropriate choice of process parameters. The production of an isotropic structure is just as possible as the obtaining of spiral, columnar or extremely anisotropic laminar structures. Graphite whiskers can also be produced. 

These are transparent isotropic structured fluids composed of two immiscible phases that are stabilized by surfactants. Often a co-surfactant and a co-solvent are present in the formulation. Microemulsions form spontaneously and are thermodynamically stable. Their transparency is due to the small droplet size (<100 nm) in microemulsions (Flanagan and Singh, 2006 Garti and Aserin, 2007). 

Figure 5.33 shows the microstructure characteristics of C/C composites fabricated by the TG-CVI process. The rough laminar pyrocarbon (RL) is located near the heating source at position 1 in Figure 5.32b and the corresponding microstructure is shown in Figure 5.33a. A mixture of smooth laminar and isotropic structure pyrocarbon (SL+ISO) at position 2 is formed and its microstructure is shown in Figure 5.33b. At and near the depositing zone, the pyrocarbon has a smooth laminar structure (SL), and the related microstructures are shown in Figure 5.33c and d. The structure characteristic of the pyrocarbon is dependent on the deposition temperature, the concentration and nature of the precursor as well as those of intermediates. During the TG-CVI process the width of the depositing zone (8) depends on the deposition temperature (Td), the...

Sample PTEOX-83P, on the other hand, shows a ring-like WAXS pattern (Figure 6C). The reason for the absence of discrete scattering may possibly be attributed to the Isotropic structure of lamella stacking. The amorphous halos in all of the PTEOX samples may be due to the amorphous regions between the lamellae of folded chain crystals. 

A simultaneous IPN of epoxy with unsaturated polyester, cross-linked using microwave radiation, has been reported (1). The radiation was intended to yield a more isotropic structure because of uniform heating also, it could reduce grafting reactions between the two networks and produce a more classical IPN structure. Unfortunately, the mechanical properties and morphology of the network were not reported. 

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