Crystal shape impurities

Growth rates depend on the presence of impurities, system temperature, solvent, mixing, and supersaturation, and the importance of each may vary from one crystal face to another. Consequently, an alteration in any or all of these variables can result in a change of the crystal shape. 

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 ... 

Hematites with a particular crystal shape (plates, needles, spindles, pseudocubes, peanuts) and a narrow size distribution (monodisperse) can be obtained by adding various chemicals (shape controllers) to the system. The mechanism behind the control of shape is most likely to be the adsorption of impurities on certain crystal faces thereby reducing their growth rate in favor of that of the other faces. Internally these crystals may be either mono- or polydomainic, depending on the type and concentration of the additive. It must be kept in mind that higher additive concentrations may lead to product contamination. Some examples are summarized in the following section. 

As the next step in understanding the factors influencing crystal shape, and the effect of solvents and impurities, models... 

Thirdly, the other compounds may lead to an impure product due to inclusions that contain the mother liquor or to adhesion onto the crystal surface. Furthermore, other compounds may influence the mass transfer phenomena of the crystallizing substance, which leads to the need for a better understanding of multicomponent diffusion [13], Other components may adsorb onto a certain facet of the crystal surface, thereby affecting the crystal growth rate and the final crystal shape. 

The production rate may vary widely, but a common design factor is 6.5 tonnes of P205/mVday. The filtration rate is affected primarily by the size and shape of gypsum crystals which, in turn, are affected by conditions in the reaction section including the type of phosphate rock, use of crystal shape modifiers, control of reaction conditions, etc. Insoluble impurities in the rock, such as clay, may affect filtration rates adversely [i9 The filtration rate is also affected by the temperature, concentration, viscosity of the acid, and the desired recovery. While many plants strive for maximum recovery, in specific plants there is often an economic optimum operating rate at which increased production is attained at some sacrifice of recovery. 

White et al. [ 109] report that crystalline structure has little effect on the initiation of EMs by a shock wave, van der Heijden et al. [110], using the example of RDX, HMX and HNIW, state that the following crystal parameters play a role in determining the sensitivity towards a shock stimulus (a) internal product quality, (b) mean particle size, (c) surface smoothness/shape of the explosive particle. Like the impact sensitivity, also the shock sensitivity is affected by the density defect content (dislocations, grain boundaries, voids, impurities, inclusions). The shock initiation tests with HMX, for example, have clearly demonstrated a relationship between the average crystal density and shock initiation pressure. The findings from this area are very important for practice, because a modification of quality and crystal shape of, for example, RDX can give a product with increased resistance to impact and shock (I-RDX [110]). 

The crystal shape depends on the relative growth rate of the crystal faces. Major impact on the face growth rate is due to the effect of impurities (desired, for example, additives, and undesired) as well as solvents. 

The attachment energy model in general provides good predictions of vapor-grown crystals or crystals grown in systems in which the solvent does not interact strongly with the solute. However, in many instances, the simulated crystal shapes differ from experimental because the kinetic effects due to supersaturation, solvent, and impurities dominate the crystal growth process. 

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