Growth impurities

Since many undesirable changes in pharmaceutics arise from nucleation and crystal growth, impurities (either added intentionally or inadvertently) can modulate or suppress these processes. This may give rise to inhibition of crystal growth in suspensions, emulsions, and ointments, and may also inhibit polymorphic transitions. 

Crystal habit (shape and aspect ratio) differs from specs wrong temperature during growth/impurities especially surfactants/supersaturation level too high. 

The visible crystals that develop during a crystallization procedure are built up as a result of growth either on nuclei of the material itself or surfaces of foreign material serving the same purpose. Neglecting for the moment the matter of impurities, nucleation theory provides an explanation for certain qualitative observations in the case of solutions. 

Water as an impurity accelerates the oxidation rate. Figure 4 compares growth curves for Si02 under dry and steam conditions. Halogens can also be introduced to the oxidation process, thereby reducing sodium ion contamination. This improves dielectric breakdown strength, and reduces interface trap density (15). 

The stmcture of the polysihcon depends on the dopants, impurities, deposition temperature, and post-deposition heat annealing. Deposition at less than 575°C produces an amorphous stmcture deposition higher than 625°C results in a polycrystalline, columnar stmcture. Heating after deposition induces crystallization and grain growth. Deposition between 600 and 650°C yields a columnar stmcture having reasonable grain size and (llO)-preferred orientation. 

Lead—silver alloys show significant age hardening when quenched from elevated temperature. Because of the pronounced hardening which occurs using small amounts of silver, the content of silver as an impurity in pure lead is restricted to less than 0.0025 wt % in most specifications. Small additions of silver to lead produces high resistance to recrystaUization and grain growth. 

Stress corrosion cracking, prevalent where boiling occurs, concentrates corrosion products and impurity chemicals, namely in the deep tubesheet crevices on the hot side of the steam generator and under deposits above the tubesheet. The cracking growth rates increase rapidly at both high and low pH. Either of these environments can exist depending on the type of chemical species present. 

Fig. 15. Excess carrier concentration in HgCdTe in a saturated Hg vapor as a function of temperature where the dashed line represents Hg vacancies. The extrinsic impurity concentration can be adjusted in the growth process from low 10 up to mid-10. Low temperature annealing reduces Hg vacancy...

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 advantage of this test is that it is quick and easy to do, and gives an indication of biodegradation potential. However, the test is not definitive, because any impurities in the plastic, such as plasticizers and solvents, may interfere with the test by promoting growth, and thus give false positive results. 

For adding dopiag impurities duriag vapor-phase growth, a gaseous or easily vaporizable Hquid compound is metered, added to the siUcon source gas stream, and reduced along with the siUcon compound. Typical examples are diborane, 2 phosphine, and boron tribromide, BBr. ... 

Microstrainers. Microstrainers are rotating steel screens with extremely fine stainless steel mesh (85—170 perforations per square centimeter (13—26/in. )). The flowing Hquid enters the open end of the dmm and passes through the mesh to the effluent end. The mesh traps soHd impurities and rotates with the dmm. A wash-water spray washes the trapped soHds into a hopper for final disposal. The mesh is washed with filtered effluent discharged from jets fitted into the dmm and then exposed to uv radiation to inhibit microbial growth. The mesh is washed with chlorine water at intervals of 7 to 28 days in order to control slime growth removal efficiencies are 30—55% of the appHed BOD and 40—60% of suspended soHds. 

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