Ostwald ripening process

Kizling and coworker [21] suggested that salts in the aqueous phase stabilised w/o HIPEs by two means. First, the Ostwald ripening process is inhibited due to the decreased solubility of the aqueous solution in the continuous oil phase. Secondly, the attractive forces between adjacent aqueous droplets are lowered, as a result of the increase in refractive index of the aqueous phase towards that of the oil phase. When the refractive indices of the two phases are matched, the attractive forces are at a minimum and highly stable, transparent emulsions are formed. The attractive force, A, is given by ... 

Nevertheless, the microstructure cannot be directly correlated to the initial /1-content in the starting powder. The experimentally determined particle density in sintered samples indicates that only a part of the initial /I particles are able to grow [283]. The number of growing /1-particles depends on a critical particle diameter dcrit (Fig. 18). Particles below dcrit will dissolve in the oxide nitride liquid during phase transformation and reprecipitate on the overcritical /1-particles according to an anisotropic Ostwald ripening process [284, 292, 293]. 

In 1962, Higuchi and Misra examined the quantitative aspects of the rate of growth of the large droplets and the rate of dissolution of the small droplets in emulsion for the case in which the process is diffusion controlled in the continuous phase [4]. It was proposed that unstable emulsions may be stabilized with respect to the Ostwald ripening process by the addition of small amounts of a third component, which must distribute preferentially in the dispersed phase [4]. The obtained stability in miniemulsions is said in the literature to be metastable or fully stable. The stabilization effect by adding a third component was recently theoretically described by Webster and Cates [5]. The authors considered an emulsion whose droplets contain a trapped species, which is insoluble in the continuous phase, and studied the emulsion s stability via the Lifshitz-Slyozov dynamics (Ostwald ripening). 

Fig. 18.5 TEM images and Gaussian fits of the size distribution of the palladium nanoparticles initially (a, b), after being exposed to the solvent (1 3 water acetonitrile) and the base (sodium acetate SA) (c, d), after being exposed to the base and phenylboronre acid (PA) (e, f), and after being exposed to the base and iodobenzene (I) (g, h). It can be seen that the Ostwald ripening process is inhibited upon exposure to the phenylboronic acid while exposure to iodobenzene results in the growth of the nanoparticles. In addition, refluxing the nanoparticles in the solvent alone also results in the Ostwald ripening of the particles. Reprinted with permission from [40]. Copyright 2003, American Chemical Society...

Ostwald ripening process dominates. Furthermore, enlargement of residual pores containing trapped gases leads to swelling of the compact if the final stage is too prolonged. 

A second-phase that forms a liquid at the firing temperature can provide a fast diffusion path for densification but grain growth by the Ostwald ripening process may also be enhanced. In this case, high density is normally accompanied by appreciable grain growth. This commonly used fabrication approach is the subject... 

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