Sinter rates

Milnes and Mostaghaci [5.5] compared the consequences of different drying methods on the density, the sinter rate and micro structures of sublimated TiO-, suspensions. Evaporation of water in a micro-oven and by radiation heating leds to strongly bound agglomerates, while freeze drying resulted in softly bound secondary clusters. The freeze dried powder reached in 2 h of sintering 98 % of the theoretical density, while differently dried powders needed twice as much time and had a less fine microstructure. 

A variation of eq. (5.453) has been proposed by Fuentes et al. (1991) for quantitatively determining the effect of temperature, time, and atmosphere on the sintering rate of supported metal catalysts ... 

Because the surface energy per volume is larger for small particles and because the fundamental driving force for sintering is surface-energy reduction, compacts composed of smaller powders will typically sinter more rapidly. Smaller powders are more difficult to produce and handle therefore, predictions of sintering rate dependence on size are used to make choices of initial particle size. Herring s... 

Suppose that two sintering systems, S and B, are identical in all aspects except their size.15 Each length dimension of system B is A times as large as the corresponding dimension of system S. Under identical conditions and provided that the same sintering mechanism is operative, the ratio of sintering rates can be determined from the relative sizes of the specimens. 

Any of the various mechanisms for sintering identified in Table 16.1 may contribute to the sintering rate. Which of the mechanisms contributes most to sintering depends on, among other things, particle size and temperature. Sometimes certain... 

Suppose that a body made up of fine particles can sinter by either the crystal diffusion mechanism BS XL or the grain-boundary diffusion mechanism BS B as illustrated in Fig. 16.7. How will the relative sintering rates due to these two mechanisms vary as ... 

The sintering rate due to boundary diffusion and crystal diffusion will be proportional to Db and DXL, respectively. The scaling laws show that the sintering rate due to boundary diffusion will decrease by the factor A 4 when the particle size is increased by the factor A. The corresponding factor for sintering by crystal... 

Sometimes the addition of a very small amount of a second material greatly increases the rate of sintering. Usually this can be attributed to the formation of a phase with a much lower melting point in which the diffusion is much faster. Figure 14.9 shows that the sintering rate of tungsten is drastically increased by enough of certain elements to form a four-atom-thick layer. 

Nevertheless efforts to understand, treat and model sintering/thermal-deactivation phenomena are easily justified. Indeed deactivation considerations greatly influence research development, design and operation of commercial processes. While catalyst deactivation by sintering is inevitable for many processes, some of its immediate drastic consequences may be avoided or postponed. If sintering rates and mechanisms are known even approximately, it may be possible to find conditions or catalyst formulations that minimize thermal deactivation. Moreover it may be possible under selected circumstances to reverse the sintering process through redispersion (the increase in catalytic surface area due to crystallite division or vapor transport followed by redeposition). 

Studies of sintering and redispersion of supported metal catalysts have been reviewed by several authors [M8] most of these reviews focus on early kinetic studies of sintering of supported metal catalysts using a simplified power law expression (SPLE). Unfortunately this crude approach does not permit sintering kinetics to be presented in a consistent way nor does it enable (1) useful extrapolation of the data to other conditions (2) useful quantitative comparisons between different studies, or (3) physically meaningful kinetic parameters to be obtained. The ultimate result has been confusion regarding the effects of reaction parameters such as atmosphere and temperature and of catalyst properties such as support promoters, etc., on sintering rates. 

From previous experimental studies of sintering [2,9 11 12] it is evident that sintering and redispersion are strong functions of temperature time atmosphere and support. Sintering/redispersion rates are also significantly affected by choice of metal and/or promoter metal loading, and catalyst preparation. The discussion below of previous work will focus on how sintering rates are affected by these variables. 

Correlation of sintering rate data through rate equations... 

Sintering rates have been historically correlated by an empirical rate equation involving either surface area S or dispersion D in a simple power law expression (PLE) of the form ... 

Figure 4. Effects of hydrogen and oxygen atmospheres and of metal loading on sintering rates of 0.6% and 5% Pt/alumina catalysts [28,331.

Figure 5. Effects of hydrogen and nitrogen atmospheres on sintering rate of 20 Pl/carbon at 873 K [31J.

Figure 6. Effects of dilute NO and O2 atmospheres on sintering rate of 1% Pt/AhOs at 773 K over a short period of lime [37, ... 

Second Order Sintering Rate Constants and Activation Energies for Pt Catalysts... 

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