Growth-active impurities

The work of Yamamoto (43) with growth-active impurities such as Pb+2, Sn+2, and Mn+2 ions reveals that their presence in very small quantities decreases the probability of nucleation, thus extending the metastable region of aqueous solutions of the alkali halides. These ions of the transition elements because of their screening demands withdraw Cl- ions from solution and form complexes such as (PbCl6) 4 and (MnCl6)-4. Without changing the over-all composition of the solution, these ions lower the effective concentration of the Cl ions and thus decrease the nucleation rate of NaCl. 

In most cases, the activator impurity must be incorporated during crystal growth. An appropriate amount of impurity element is dissolved in the molten Ge and, as crystal growth proceeds, enters the crystal at a concentration that depends on the magnitude of the distribution coefficient. For volatile impurities, eg, Zn, Cd, and Hg, special precautions must be taken to maintain a constant impurity concentration in the melt. Growth occurs either in a sealed tube to prevent escape of the impurity vapor or in a flow system in which loss caused by vaporization from the melt is replenished from an upstream reservoir. 

The level of impurity uptake can be considered to depend on the thermodynamics of the system as well as on the kinetics of crystal growth and incorporation of units in the growing crystal. The kinetics are mainly affected by the residence time which determines the supersaturation, by the stoichiometry (calcium over sulfate concentration ratio) and by growth retarding impurities. The thermodynamics are related to activity coefficients in the solution and the solid phase, complexation constants, solubility products and dimensions of the foreign ions compared to those of the ions of the host lattice [2,3,4]. 

Because crystal growth is a surface phenomena, it is not surprising that impurities that concentrate at crystal faces will affect the growth rate of those faces and hence the crystal shape. With some surface active impurities, small traces, about 0.01%, are all lhat is required to change crystal habit during crystallization. These impurities can ... 

The morphological development of a precipitate is a complex combination of a variety of processes, including nucleation, growth, habit modification, phase transformation, ripening, agglomeration, and so on. The dominant system parameters are supersaturation and the level of active impurities, although in some aqueous systems pH can also exert a profound effect. 

Another important aspect of migration is the concentration of active impurities. For one metal ion concentration and one type of impurity at a given temperature, the critical current density for dendrite growth is (3)... 

The primary cation CH20H is created in the cage reaction under photolysis of an impurity or y-radiolysis. The rate constant of a one link growth, found from the kinetic post-polymerization curves, is constant in the interval 4.2-12 K where = 1.6 x 10 s . Above 20K the apparent activation energy goes up to 2.3 kcal/mol at 140K, where k 10 s L... 

The growth of the catalytic activity of Si02 with respect to the hydrogen-deuterium exchange reaction upon addition of a donor impurity to specimens has also been observed by Taylor and his colloborators (31). 

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