Polycrystalline particle

Polycrystalline particle of Na-ZSM-5(9) showing individual crystals. Reproduced with permission from Ref. 24, Copyright 1983, American Chemical Society. 

It is no wonder that the particles are spherical but crystalline, if one considers the formation mechanism. The rather smooth surface of the spherical magnetite may be due to the rapid contact recrystallization of the constituent primary particles (5), forming the rigid polycrystalline structure. Flowever, it must be noted that polycrystalline spheres are also prepared by normal deposition of monomeric solute, as shown in the formation of the uniform spherical polycrystalline particles of metal sulfides in Chapters 3.1-3.3. Thus, while we may be able to predict the final particle shape and structure from the formation mechanism, it is risky to conclude the formation mechanism only from characterization of the product. As a rule, scrupulous analyses are needed for concluding the growth mechanism in a particle system. 

Matijevic and Wilhelmy (I) prepared uniform spherical polycrystalline particles of cadmium sulfide (CdS) by reaction of Cd2+ ions with thioacetamide (TAA) in a dilute acidic media (pH < 2), as shown in the TEM and SEM images of Figure... 

The polycrystalline particles consist of much smaller crystallites whose size and morphology were found to be strongly affected by the concentration of TAA, probably due to the adsorption of TAA to the surface zinc ions during their growth i.e., spheres of 20 nm in average diameter at 0.40 mol dm-3 TAA Fibrils of —50 nm in average length at 0.12 mol dm-3 TAA. 

Dea lomeration can take place by two methods comminution and ultrasonification. Comminution is the subject of Chapter 4, so it will not be discussed in detail here. In the comminution of aggregates, the popvdation balance can be used to predict the size distribution as a function of time in a batch mill or as a function of mean residence time in a continuous mill. Aggregates have the same type of birth and death fvmctions for particle breakage as polycrystalline particles but the rate constants are much hi er and the size selectivities for aggregates are different than those for the comminution of crystalline materials. The... 

This has important consequences concerning the spatial resolution of the detector system. Due to scattering and total reflection at the edges of the polycrystalline particles, more phosphorescent light is lost in a thick layer than in a thinner one. In addition, the visible photons are more spread-out laterally in thicker screens. 

The self-diffusivity of propane in the coked polycrystalline grains unveils details of coke formation in polycrystalline particles. Figure 44 shows that coking reduces the translational mobility inside the grains. The effect of intracrystalline coke deposition on the translational mobility of propane is indicated in Fig. 43. After a coking time of 1 h, only a slight increase of Di ,ra with increasing observation times occurs. After 12 h, a decrease is ob-... 

Sand and sandstone are assemblages of quartz, chert, feldspar, mica, fragments of feldspar-rich igneous, sedimentary, and metamorphic rocks, fossils, clay, and other constituents in various percentages. Sandstones contain cements, normally calcite or quartz, or a clay matrix, that bind the grains together. These rocks, therefore, typically contain a mixture of monocrystalline and polycrystalline particles. 

A primary particle is a discrete unit with relatively low porosity, which can be either a single crystal, a polycrystalline particle, or an amorphous/glass. They can be isolated if pores are present. A primary particle could be defined as the smallest unit of the powder with clearly defined surfaces. Therefore, it cannot be broken down into even smaller units by some physical agitations, such as ultrasonic agitation in a liquid. A polycrystalline primary particle consists of liny crystals, which are also known as crystallites or grains. 

Among all parameters influencing RS content, crystal texture is of utmost importance. It is evident that a crystal exhibiting a porous texture will enable the easy escape of a solvent while a compact and dense crystal will retain the solvent inside its structure whatever the type of particles monocrystalline (i.e., monoparticular) or polycrystalline . The term polycrystalline particles will be employed to designate elementary particles that can be composed of agglomerates, spherolites or spherical crystals according to Kawashima. ... 

The drying conditions should also be adapted to the area offered to the evaporation of solvent, particularly in the case of polycrystalline particles presenting a high porosity and a large surface to be dried. This important surface is due to the disordered rearrangement of very small crystals inside the particles. To illustrate this, we shall see in a second example the cases of spherical crystals of meprobamate and ibuprofen agglomerates. In both cases the RS can escape easily from the large surface of polycrystalline particles. 

Monocrystalline particles Mean diameter, Um Polycrystalline particles Mean diameter, um... 

Second example polycrystalline particles of meprobamate and ibuprofen... 

The polycrystalline particles were produced using various crystallization processes and designed to obtain directly compressible particles of pure drug, as tablets cannot be formed by direct compression of the raw materials. 

Depending on the melting point of the drugs, different drying conditions were applied in a ventilated oven. Drastic and progressive temperature conditions were studied for each type of polycrystalline particles (Table 15.2.3.2). 

The residual solvent concentrations of the polycrystalline particles are reported in Table 15.2.3.8. 

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