Particle growth, spherical

Three-dimensional growth (Spherical particles of reactant) ... 

The concept of growth functions is covered in more detail in Sec. 3.6. Liu and Litster (1993a) looked at two cases ofEq. (7). The first assumed that k = 0, which is equivalent to assuming that, on average, each particle spends the same amount of time in the spray over a long period of time. Thus, G should be proportional to the projected surface area of the particle (assumed spherical) or... 

Monodisperse oxide particles were only formed at the narrow concentration range in alcohol solution. Monodisperse particles, which have submi-crometer order with narrow size distribution, were amorphous and hydrated. The spherical morphology was retained after the crystallization. The rate-determining step of particle growth was polynuclear layer growth of first order from Nielsen s chronomal analysis. 

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. 

Wet scrubbing uses liquid droplets to remove fine dust in a gas stream. In all types of wet scrubbing, the basic cleaning mechanism involves the attachment of particles to the droplets. The function of the droplets in scrubbers is similar to that of spherical fibers in filtration. Likewise, the primary collection mechanisms in scrubbing are similar to those in filtration, i.e., inertial impaction, interception, and diffusion [e.g., Fan, 1989]. Secondary collection mechanisms include thermophoresis due to temperature gradients, coagulation of particles due to particle electrification, and particle growth due to liquid condensation. 

The process by which vapor-deposited material forms two-dimensional arrays of spherical particles just beneath the substrate surface has been studied in detail. A model has been proposed in which the numbers and sizes of particles are determined by the coupled processes of particle growth (by capture of diffusing molecules) together with particle coalescence. Expressions have been derived for particle size and number density as functions of deposition parameters. Experimental evidence is presented in support of the model for the case of selenium physically vapor-deposited onto a heated thermoplastic substrate. Finally, the technological application of the deposit morphology as dry microfilm is reviewed. 

During diffusionally controlled particle growth the rate at which the radius of the spherical particle, r, increases, is related to the total flux of substance to its surface, js (moles s 1), as... 

A comparison of this expression with eq. (V.12) indicates that diffusion towards a spherical particle is similar to diffusion towards a flat surface through a solution layer of thickness 8 = r. Substitution of eq. (IV.20) into eq. (IV. 19) yields the expression for the rate of diffusionally-controlled particle growth ... 

The possible organisation of the amines in THF could influence the growth of the ruthenium particles in the solution and their tendency to present different forms such as spherical particles and nanorods. In the case ofO.leq HDA/Ru the particles are spherical, while with 0.2 eq HDA/Ru the beginning of the formation of elongated particles can be observed, continuing with time with 0.5 eq H DA/Ru, nanorods were present. 

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