Many-defect systems

B. Chemical Factors Contributing to Aging Many propint systems have built-in defects which contribute to degradation of the polymer system and reduce service life. The poly are ... 

Albert Tauber. I d like to give a reference that everyone knows. Sickle-cell is commonly evoked as a great reductionist victory. I treat many patients with sickle-cell anemia, and I can tell you that the genetic defect is not the disease, because the disease manifestation is highly variable. Some patients have an enormous number of pain-crises. Some patients have a lot of haemolysis that s red-cell destruction. Other patients have pulmonary hypertension. And it s obvious that the disease that we call sickle-cell anemia is an extraordinarily complex phenomenon interacting with many many different systems and many other genes. And the bottom line, from a clinical perspective, which is the phenomenon that we call sickle-cell anemia, is that the sickle-cell gene is necessary but not sufficient for the disease. 

The description theoretical study of defects frequently refers to some computation of defect electronic structure i.e., a solution of the Schrodin-ger equation (Pantelides, 1978 Bachelet, 1986). The goal of such calculations is normally to complement or guide the corresponding experimental study so that the defect is either properly identified or otherwise better understood. Frequently, the experimental study suffices to identify the basic structure of the defect this is particularly true when the system is EPR (electron paramagnetic resonance) active. However, if the computational method properly simulates the defect, we are provided with a wealth of additional information that can be used to reveal some of the more basic and general features of many-electron defect systems and defect reactions. 

MINDO/3 and MNDO, utilized widely by chemists for molecular problems, were introduced to silicon dioxide defect physics by Edwards and Fowler (1985) and to silicon defect physics by DeLeo et al. (1984b). They have since been applied to many defect problems in these systems, with considerable success. Both can be used in restricted or unrestricted Hartree-Fock form. MNDO has a practical advantage in that it is formulated in a way that all parameters are associated with particular atoms in effect,... 

Materials may break in shear or in tension the former occurs in short (e.g., brittle), the latter in long (e.g., rubbery) materials. Another material property is notch sensitivity. In a test piece that is put under tension, notches can be applied, and in several materials the ensuing stress concentration greatly lowers the overall stress needed for fracture propagation. The notch sensitivity is smaller if the material contains defects of the size of a small notch, or if the bonds between structural elements are much stronger in the direction of the applied stress than in a perpendicular direction, as in many fibrous systems. 

Quantum defect theory (QDT) was developed by Seaton [111] and his collaborators, from ideas which can be traced to the origins of quantum mechanics, through the work of Hartree and others. They relate to early attempts to extend the Bohr theory to many-electron systems (see e.g. [114]). 

It is important to note here that the solution of one compound in another is unavoidable — a perfectly pure crystal is a thermodynamic impossibility for the same reason that a defect-free crystal is impossible. " The only legitimate question therefore is. How much solubility is there In many binary systems, the regions of solid solution that are necessarily present do not appear on the phase diagrams. For example, according to Fig. 8.7 or... 

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