Particle coarsening

Figure C2.11.6. The classic two-particle sintering model illustrating material transport and neck growtli at tire particle contacts resulting in coarsening (left) and densification (right) during sintering. Surface diffusion (a), evaporation-condensation (b), and volume diffusion (c) contribute to coarsening, while volume diffusion (d), grain boundary diffusion (e), solution-precipitation (f), and dislocation motion (g) contribute to densification.

The characteristics of WC, especially grain size, are determined by purity, particle shape and grain size of the starting material, and the conditions employed for reduction and carburization. The course of the reaction WO3 — W — WC is dependent on temperature, gas flow rates, water-vapor concentration in the gas, and the depth of the powder bed. All these factors affect the coarsening of the grain. 

A further problem is possible if the reinforcements are very small. Coarsening of the particles or whiskers may occur driven by Ostwald ripening, in which large particles grow through diffusional transport at the expense of smaller ones. This can be minimized by choosing matrices in which the reinforcement elements have very low solid solubilities and diffusion coefficients. Platelets, however, have been shown to be more resistant to coarsening than particles or whiskers. 

Figure 5.4 Coarsening of the internal structure differs from sintering or fusing together of catalyst particles.

Low redox cycling tolerance. Redox cycling occurs when the fuel flow to the stack is interrupted. Air leaking in from the environment will oxidize the nickel. Upon recovery of the fuel flow the nickel oxide will reduce again. Such a cycle will cause considerable performance loss due to coarsening of the nickel particles in the anode. 

Coarsening of fine precipitates kept in the mother liquor for protracted time was observed in numerous instances the number of very fine particless in the precipitate decreased in time, and the volume of several large particles simultaneously grew. This effect, after 1899, was frequently referred to as Ostwald ripening. 

It must be noted that short contact time reactors are typically operated under adiabatic conditions with outlet temperatures of the order of 700-1000 °C. Under such conditions, with respect to other noble metals, Rh is believed to be especially stable due to a low vapor pressure and an increased resistance to carbon formation even under severe operating conditions. The use of low surface area oxides such as (Z-AbO j and ZrO2 as support materials has been reported to improve the catalyst stability by limiting the coarsening of Rh particles while avoiding incorporation of Rh within the oxide structure [148]. 

The sol is made of a stable suspended solution of metal salts or solvated metal precursors containing solid particles of nanometer diameter. Polycondensation or polyesterification results in the appearance of particles in a new phase called the gel . Aging, drying and dehydration are steps required to achieve solid-form ultra-fine particles. Coarsening and phase transformation occur simultaneously with aging. Gel drying is associated with the... 

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