Extrinsic barrier

The structural materials used by engineers are not soft, but only deform plastically at large applied stresses. These result from a variety of extrinsic barriers to dislocation motion. Thus dislocations move freely between the barriers, but then stop until enough stress is applied to overcome the barriers. 

The gastrointestinal epithelium forms an extrinsic and an intrinsic barrier against diffusion of toxins and pathogens. The extrinsic barrier is characterized by secretion of mucus, which hinders colonization and accelerates clearance of pathogenic organisms. The importance of mucus as a barrier to drug absorption is discussed in Chapter 2 of this book. In addition, the intestinal immune... 

The gastrointestinal barrier is considered to comprise two components the intrinsic barrier, composed of epithehal cells lining the digestive tube and the tight junctions that tie them together and the extrinsic barrier, consisting of secretions and other influences that are not physically part of the epithehum but which affect the epithelial cells and maintain their barrier function (Table 4.1). 

Associated with item 2, above. Is the rotational relaxation barrier Imposed by the environment upon the probe molecule, as has been discussed for stllbene (90). This extrinsic barrier can be much higher than the Intrinsic barrier In Item 2, but results in the same observation of temperature dependence of luminescence of the probe except that the temperatures at which discontinuities In Arrhenius plots occur will be solvent dependent (39,90,91). 

Low contact resistance values are required for most superconductor applications. Actual values of contact resistance are determined both by intrinsic electronic contributions and by extrinsic barriers. In the former area, problems resulting in higher interface resistance values may result from interface reflections due to carrier density mismatch. For example, the carrier density of YBa2Cu307-3 is 6x 10 cm", while the carrier density of Ag is about an order of magnitude higher. This carrier mismatch can lead to the possibility of interface reflection, which results in an increase in the value of the contact resistance. Lee and Beasley [91] estimated the intrinsic contact resistivity of a YBa2Cu307- /Ag interface based on the carrier mismatch using the expression... 

The crystal structure of NiAl is the CsCl, or (B2) structure. This is bcc cubic with Ni, or A1 in the center of the unit cell and Al, or Ni at the eight comers. The lattice parameter is 2.88 A, and this is also the Burgers displacement. The unit cell volume is 23.9 A3 and the heat of formation is AHf = -71.6kJ/mole. When a kink on a dislocation line moves forward one-half burgers displacement, = b/2 = 1.44 A, the compound must dissociate locally, so AHf might be the barrier to motion. To overcome this barrier, the applied stress must do an amount of work equal to the barrier energy. If x is the applied stress, the work it does is approximately xb3 so x = 8.2 GPa. Then, if the conventional ratio of hardness to yield stress is used (i.e., 2x3 = 6) the hardness should be about 50 GPa. But according to Weaver, Stevenson and Bradt (2003) it is 2.2 GPa. Therefore, it is concluded that the hardness of NiAl is not intrinsic. Rather it is determined by an extrinsic factor namely, deformation hardening. 

Thus, the fat globules are surrounded, at least initially, by a membrane typical of eukaryotic cells. Membranes are a conspicuous feature of all cells and may represent 80% of the dry weight of some cells. They serve as barriers separating aqueous compartments with different solute composition and as the structural base on which many enzymes and transport systems are located. Although there is considerable variation, the typical composition of membranes is about 40% lipid and 60% protein. The lipids are mostly polar (nearly all the polar lipids in cells are located in the membranes), principally phospholipids and cholesterol in varying proportions. Membranes contain several proteins, perhaps up to 100 in complex membranes. Some of the proteins, referred to as extrinsic or peripheral, are loosely attached to the membrane surface and are easily removed by mild extraction procedures. The intrinsic or integral proteins, about 70% of the total protein, are tightly bound to the lipid portion and are removed only by severe treatment, e.g. by SDS or urea. 

Fig, 3. Extrinsic (a) and intrinsic (b) electron-transfer reactions at a bare and a modified electrode. Ep = Fermi potential, AE = height and d = thickness of energy barrier. - are phase numbers, o and are oxidized and reduced redox centers in phase . A reduction is shown in both cases. 

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