Dislocation charge

Soliton sa-lo- tan [solitary + -on] (1965) n. A solitary wave (as in a gaseous plasma) that propagates with little loss of energy and retains its shape and speed after colliding with another such wave. Solitons have had many physical applications included order-disorder phase transitions, cr) tal dislocations, charge density waves, and Josephson junction transmission lines. Ku CC,... 

Intrinsic defects (or native or simply defects ) are imperfections in tire crystal itself, such as a vacancy (a missing host atom), a self-interstitial (an extra host atom in an otherwise perfect crystalline environment), an anti-site defect (in an AB compound, tliis means an atom of type A at a B site or vice versa) or any combination of such defects. Extrinsic defects (or impurities) are atoms different from host atoms, trapped in tire crystal. Some impurities are intentionally introduced because tliey provide charge carriers, reduce tlieir lifetime, prevent tire propagation of dislocations or are otlierwise needed or useful, but most impurities and defects are not desired and must be eliminated or at least controlled. 

The ultimate trapping site for a photoelectron is influenced by the high dielectric constant of silver haUde (ca 12.5, 11.15, and 7.15 for AgBr, AgCl, and P-AgI, respectively), the negative surface charge, and relative trap depths. Interior traps located at point defects on dislocation lines are probably not as... 

In the last chapter we examined data for the yield strengths exhibited by materials. But what would we expect From our understanding of the structure of solids and the stiffness of the bonds between the atoms, can we estimate what the yield strength should be A simple calculation (given in the next section) overestimates it grossly. This is because real crystals contain defects, dislocations, which move easily. When they move, the crystal deforms the stress needed to move them is the yield strength. Dislocations are the carriers of deformation, much as electrons are the carriers of charge. 

When other elements dissolve in a metal to form a solid solution they make the metal harder. The solute atoms differ in size, stiffness and charge from the solvent atoms. Because of this the randomly distributed solute atoms interact with dislocations and make it harder for them to move. The theory of solution hardening is rather complicated, but it predicts the following result for the yield strength... 

The kinds of substitution mechanisms that may be relevant to super-low concentration elements such as Pa involve intrinsic defects, such as lattice vacancies or interstitials. Vacancy defects can potentially provide a low energy mechanism for heterovalent cation substitution, in that they remove or minimise the need for additional charge balancing substitutions. Formation of a vacancy per se is energetically unfavourable (e.g., Purton et al. 1997), and the trace element must rely instead on the thermal defect concentration in the mineral of interest, at the conditions of interest. Extended defects, such as dislocations or grain boundaries, may also play a key role, but as these are essentially non-equilibrium features, they will not be considered further here. 

In general terms, as has already been mentioned, plastic deformation is a transport process analogous with electrical and thermal conductivity. These involve an entity to be transported, a carrier that does the transporting, and a rate of transport. In the case of electrical conductivity, charge is the transport entity, electrons (or holes) are the carriers, and the electron net velocities determine the rate. In the case of plastic deformation, displacement, b (cm) is the transport entity, dislocations are the carriers, N ( /cm2), and their velocities, v (cm/sec) determine the shear deformation rate, d8/dt. In two dimensions, the latter is given by the Orowan Equation ... 

Another special factor in ionic crystals is that dislocation cores in them acquire net charge. As a result, plastic bending of an ionic crystal causes the top and bottom regions to become charged relative to the middle. This is easily demonstrated because such specimens preferentially attract fine insulating powders. It has been studied in some detail by Li (2000). 

Before a dislocation on one of the glide planes passes through the complex, the distance between the two charge centers is d = b = a0/>/2. After it has passed by the distance is d = V2(b) = a0. Therefore, if K is the static dielectric constant, and q = electron s charge, the energy difference between the before and after states is AU = (q2/Ka0)(V2-l). 

As mentioned earlier in this chapter dislocations in ionic crystal may carry a net electric charge. Therefore, their motion may be influenced by applied electric fields, and may generate observable fields external to a specimen during plastic flow. These effects have been studied by Li (2000) and others. 

J. C. M. Li, Charged Dislocations and Plasto-electric Effect in Ionic Crystals, Mater. Sci. Eng., A287,265 (2000). 

In an ionic material such as sodium chloride the dislocation can bear a charge, which will depend upon the half-planes inserted. If the tip has an excess of cations, it will be positively charged whereas if the tip consists of anions, it will be negatively charged (Fig. 3.19b). Similarly, when the dislocation reaches the surface, a charge may be present, which may enhance chemical reactivity at this... 

Burgers vector b, [101], is the shortest vector connecting two identical atoms in this structure. (.b) An edge dislocation in the NaCl structure, consisting of two extra half-planes of atoms. If these are ionic, the tip will bear a charge, depending upon the ions in the termination, and slip will occur on 110. ... 

Many phenomena such as dislocations, electronic structures of polyacetylenes and other solids, Josephson junctions, spin dynamics and charge density waves in low-dimensional solids, fast ion conduction and phase transitions are being explained by invoking the concept of solitons. Solitons are exact analytical solutions of non-linear wave equations corresponding to bell-shaped or step-like changes in the variable (Ogurtani, 1983). They can move through a material with constant amplitude and velocity or remain stationary when two of them collide they are unmodified. The soliton concept has been employed in solid state chemistry to explain diverse phenomena. 

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