Monodisperse powders

With micromesh sieves, near-monodisperse powders can be generated in the 1 to 10 pm size range [3]. An overview of recent developments in fine sieving in Japan has been presented in two reviews [5,6]. 

Thus, the lower size limit depends upon the nature of the sedimenting system, e.g. density, temperature etc. It also increases if the analysis time is reduced or the measurement height is decreased. The measured distributions for monodisperse powders will be broadened, the effect increasing as the measurement time is reduced [9]. A corollary to this is that analyses of fine powders, in which a scanning system is used, will give different results at different scan speeds. 

The right-hand side of Equation 15.56 coincides with the dissolution contribute employed in Equation 15.41. Since 5p, for a plane and uniform surface, does not change as dissolution develops, we may be sure that Ki is time independent. Unfortunately, this is not the case for a dissolving powder. Indeed, in such hypothesis, the dissolution surface decreases as the time goes on. The initial value of Sp and Spo is equal to the powder surface area and, for a monodisperse powder made up of Np, all equal, spherical particles, is given by... 

Particles of microcrystalline size can also be obtained by spray-drying procedures, resulting in a porous, free-flowing, easily wetted, essentially monodispersed powder. With proper control of process variables, spherical particles are obtained that may be coated with agents to aid suspension and promote stability. However, the process is not normally considered for the preparation of ultrafine powders. 

One of the most important characteristics of fine powders is the area of the surface of the solids this is usually expressed as specific surface area. This is a measure of the fineness of the powder (the specific surface increases with decreasing particle size) as well as of its porosity but it is unable to discriminate between a monodisperse powder and one containing a wide range of particle sizes. 

Another example of dryer selection is related to the choice of a suitable atomizer for a spray dryer. A spray dryer is indicated when a pumpable slurry, solution, or suspension is to be reduced to a free-flowing powder. With proper choice of atomizer, spray chamber design, gas temperature, and flow rate it is possible to engineer powders of desired particle size and size distribution. Table 47.2 shows how the choice of the atomizer affects chamber design, size, as well as energy consumption of atomization and particle size distribution. The newly developed two-fluid sonic nozzles appear to be especially attractive choices when nearly monodisperse powders need to be produced from relatively moderate viscosity feeds (e.g., under 250 cP) at capacities up to 80 t/h by using multiple nozzles. More examples may be found in Kudra and Mujumdar [20]. 

According to sizes and size distributions, powders can be classified into two types (i) polydispersed and (ii) monodispersed powder [7-15]. Polydisperse means... 

NMR supports purity of the gel with correct integration ratio. The gel changed to powder on self-hydrolysis at room temperature. On pyrolysis at 350°C 50°C for 1 h in the presence of argon atmosphere it gives monodispersible powder with good crystallinity. 

Powders that are made from the gas phase or with the sol-gel process are often monodisperse (all the primary particles have the same size). Why Explain how the aggregation model for particle growth can suggest formation of monodisperse powders. 

A vibratory separation method has been used to obtain monodisperse powder fractions [104]. This technique is based on the principle of particle separation into fractions under the influence of vibration of the working surface, which is inclined at a certain angle. As a result of vibrations of the inclined surface, the fine particles shift toward the upper part of the surface, and the coarse particles to the lower part, thus achieving separation of the particles into fractions. 

Vezin and Florence (1980) used monodisperse powdered polymer particles for studying the in vitro degradation of poly(7j-alkyl a-cyanoacrylates), with a range of alkyl side chains and MW. These authors showed that degradation in aqueous buffer depends not only upon the pH and the length of the polymer alkyl side chain, but also critically on the polymer particle surface, particle size, polyoner MW and MW distribution. They concluded that at low MW (below the characteristics of effective tissue adhesiveness), increased water solubility, plasticity and diffusivity may result in a bulk rather than surface polymier degradation. 

The other simplifying assumptions of the models must also be remembered. The extension of the two-sphere geometry to real powder compacts is valid only if the particles are spheres of the same size arranged in a uniform pattern. In practice, this system is, at best, approached only by the uniform consolidation of monodisperse powders by colloidal methods (see, for example, the work of Barringer and Bowen discussed in Chapter 1). Coble (17) considered the effect of a particle size distribution on the initial stage of sintering by considering a linear array of spheres. 

Studies by Ciftcioglu et al. and Milne et al. also demonstrated another important fact about sintering. Monodispersed powders will form ordered areas of close packed particles, which are characterized by uniform and small pore sizes. This condition will favor densification of the material at a lower temperature and in shorter times. It will also lead to smaller and uniform grain sizes within those areas. How-... 

If all the grains have the same shape and the same sizes (monodispersed powder) (Figure 10.1b), at any time, all the grains have the same extent and thus all are covered with the same layer thickness of the formed solid. 

Theorem.- The rate of a monodispersed powder during a reaction with instantaneous nucleation and slow growth is equal to the rate of one grain of this powder. 

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