Rate of sintering

The characteristics of a powder that determine its apparent density are rather complex, but some general statements with respect to powder variables and their effect on the density of the loose powder can be made. (/) The smaller the particles, the greater the specific surface area of the powder. This increases the friction between the particles and lowers the apparent density but enhances the rate of sintering. (2) Powders having very irregular-shaped particles are usually characterized by a lower apparent density than more regular or spherical ones. This is shown in Table 4 for three different types of copper powders having identical particle size distribution but different particle shape. These data illustrate the decisive influence of particle shape on apparent density. (J) In any mixture of coarse and fine powder particles, an optimum mixture results in maximum apparent density. This optimum mixture is reached when the fine particles fill the voids between the coarse particles. 

The predominate role of the 2inc and aluminum oxides in the ICI catalyst is to reduce the rate of sintering and loss of metallic copper surface area, which, in addition to poisoning, is one of the modes of activity loss with time for this catalyst. 

Describe the effects on the rate of sintering of Fe203 produced by the specific origin of the change in conductivity. 

Solution. The rate of sintering is given by Eq. 16.16. Using the formalism of Section 16.3.4, where all dimensions in the B system are A times larger than in the S system,... 

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. 

If pressure is applied at the sintering temperature, plastic deformation by creep increases the rate of sintering. This is referred to by the term HIPping. It is used on castings as well as powder compacts. 

XRD spectra of composites sintered at different temperatures are obtained by using a defractometer. Samples sintered at a particular temperature indicate the structural behaviour as either amorphous or crystalline in nature. Such indication is of immense help to researchers for improving the properties of the resultant materials by optimizing the rate of sintering temperature. 

Data from Bartholomew et al. [23,27] for Ni/alumina catalysts (Table 2 and Figure 8) provide perspective regarding the role of alumina surface area, structure, and pretreatment on the thermal stability of nickel. From Figure 8 it is evident that at any given temperature the rate of sintering is greater for Ni supported on 5-alumina (SA = 98 m2/g) than for Ni on y-... 

Species such as, Cl, or F that increase the mobility of metal atoms may cause either redispersion or increased sintering rates. The role of Cl in redispersion has been discussed elsewhere [16]. There is evidence that S and F poisons increase rates of sintering. For example, Erekson and Bartholomew [57] found that an unsupported Ni powder with particles having diameters of 2-6 pm was relatively stable during reduction in H2 at 725-775 K over a period of 18 h. However, after exposure for just 6 h to 0.2 ppm H2S/H2 at either 725 or 775 K, (but not below 725 K) most of the small particles had agglomerated to large (100-250 pm)... 

The rate of sintering dS/dt is generally related to the time by a rate-power law... 

From eqn. (95) it follows that sintering of a Bingham solid should take place only if the product Jf(g) is smaller than unity. If A is small (low yield point), this condition is met for all densities and sintering to unit relative density occurs within a finite period of time. If A > 0.89, the rate of sintering decreases to zero as the material density increases towards a value given by equation f(e) = i/A. 

This is a purely empirical relationship based on Tammann s discovery in 1932 (ref. 78) that there was a minimum temperature at which a solid would undergo a solid-solid interaction. Since that time other workers reported that the rates of sintering of oxides increased markedly at about half the value of the absolute melting temperature. It was also found that under these circumstances defects in the surface of a solid became mobile enabling the surface migration of ions to take place. It should be realized that the Tammann temperature does not represent a discontinuity of behavior, but rather a temperature in the vicinity of which a rapid change in the rate of motion of ions or atoms occurs. 

Rates of sintering have traditionally been correlated by power functions of the type [10]... 

In general terms, Che role of foreign ions is easy Co explain. Thermal reorganisation involves diffusion via defects and vacancies [10]. If foreign ions occupy these sites, the rate of sintering will be affected. Alternatively, if foreign ions react with the host lattice to form a new chemical compound, then the reorganisation of Che new compound will occur at a different rate. 

Since the rate of sintering depends strongly on temperature, a careful study of the effect of temperature on catalyst deactivatiort was necessary. The slurry reactors in both the laboratory and the pilot plant ware essentially Isothermal they provided an ideal vehicle for a controlled. easy-to interpret study of temperature effects. 

How do various experimental factors (i.e. reaction conditimis and catalyst jMoperties) affect the rate of sintering Of redispersion Under what coiditions do certain factc become rate controlling ... 

However, the interpretation of these results in terms of a mechartism is a matter of speculation. Correia and Van Reijen hypothesized that the low rate of sintering after 10 h was due to vapex phase transport of Ni, possibly as NiCl2. This hypothesis was based on two observations (1) XPS analysis indicating traces of chltxide and (2) a dependence of sintering rate on position of sanples in their furnace which was interpreted in terms of vapor-phase transport of... 

The sintering [15] of calcium oxide produced by decomposition of Ca(OH)2 was rapid only after formation of the true oxide from the initially-generated pseudohydroxide. The unexpectedly low rate of sintering of SrO and BaO at 1270 K (oxides again produced from the hydroxides) was ascribed to the influences of CaO and SrO impurities, respectively. 

On cooling some devitrification occurs and the rate of sintering depends largely on the ratio of glassy phase to crystalline species in the ash. The flame volatile sodium captured by the vitrified silicate particles can initiate the coalescence of deposited ash by viscous flow and the rate of sintering is markedly increased by the alkali-metal dissolved in the glassy phase. 

Equation (9) shows that the rate of ash sintering, i.e. the development of cohesive strength of a deposit matrix is proportional to the surface tension and inversely proportional to the viscosity. The surface tension and particle size are not markedly changed by dissolution of sodium, iron or calcium oxides in the glassy phase of silicate ash. However, the viscosity is markedly changed by the oxides. In particular, an enrichment of sodium in the surface layer of the silicate ash particles can lead to a high rate of sintering. 

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