Sintering, surface area effect

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. 

It is important to distinguish clearly between the surface area of a decomposing solid [i.e. aggregate external boundaries of both reactant and product(s)] measured by adsorption methods and the effective area of the active reaction interface which, in most systems, is an internal structure. The area of the contact zone is of fundamental significance in kinetic studies since its determination would allow the Arrhenius pre-exponential term to be expressed in dimensions of area"1 (as in catalysis). This parameter is, however, inaccessible to direct measurement. Estimates from microscopy cannot identify all those regions which participate in reaction or ascertain the effective roughness factor of observed interfaces. Preferential dissolution of either reactant or product in a suitable solvent prior to area measurement may result in sintering [286]. The problems of identify-... 

On the other hand, when the reaction temperature was increased fijrther to 400°C, the reactivity of the absorbent significantly dropped. It was previously reported that for absorbent prepared from coal fly ash, when the absorbent was dried at temperature above 400°C, the reactivity of the absorbent dropped due to the decomposition of the active materials in the absorbent [8]. Since the effect of drying the absorbent above 400°C is similar to exposing the absorbent to reaction temperature above 400°C, therefore it can be concluded that the active materials in absorbent prepared from oil palm ash also decompose at reaction temperature above 400°C resulting in lower reactivity. Apart from that, another possible explanation for the drop in the reactivity of the absorbent at 400°C could be due to the sintering of the absorbent that decreases the surface area of the absorbent. 

In addition to the use of composite anodes and cathodes, another commonly used approach to increase the total reaction surface area in SOFC electrodes is to manipulate the particle size distribution of the feedstock materials used to produce the electrodes to create a finer structure in the resulting electrode after consolidation. Various powder production and processing methods have been examined to manipulate the feedstock particle size distribution for the fabrication of SOFCs and their effects on fuel cell performance have also been studied. The effects of other process parameters, such as sintering temperature, on the final microstructural size features in the electrodes have also been examined extensively. 

The ferric oxide, hematite, used in the present work was a high purity powder reagent with a BET surface area of 27 m2/g 30 mg was employed in each run. Some measurements were made on hematite calcined in air to see the effects of sintering the surface on the chemical structure of the adsorbed metal ions. The hematite samples were checked by Mossbauer absorption and powder X-ray diffraction measurements. The Mossbauer absorption spectra consisted of a magnetic sextet with no superparamagnetic component due to fine particles ( ). 

In the preparation of MgO from chloride salt precursors, it was observed that the presence of any residual chloride had a detrimental effect on the surface area of the resultant MgO 29). This effect has been related to the ability of chloride to aid sintering and grain growth in the oxide 30). 

Generally, high space velocities of ammonia and low temperature ramp rates are required for maximisation of surface area. The former has the effect of reduction in the partial pressure of water vapour generated on reaction, whilst the latter minimises sintering by solid-solid reaction. In a factorial study, Choi... 

Arai and co-workers investigated the effects of cation composition in the mirror plane of Mn-substituted hexaaluminates.5,15 Investigation of a series of rare-earth-based hexaaluminates (AMnAluOi9, A=La, Pr, Nd, Sm) prepared via hydrolysis of alkoxides and calcined at 1200 °C for 5 h showed that surface area increases with the ionic radius of A. The La-substituted sample, the largest cation of the series, showed a surface area of 15 m2/g, which compared with 5 m2/g for the Sm-substituted one, i.e., the smallest cation of the series. These data are consistent with the mechanism of sintering resistance reported above. Indeed, it is expected that the larger the cation, the more difficult its diffusion along the c axis. 

Figure 3.5 shows the effect of calcination on the physical properties of HyCOM TiOz s. The crystallite size of anatase and the BET surface area of as-prepared sample were 11 nm and 140 m2g-1, respectively. Upon elevating the calcination temperature, the crystallite size was increased and the surface area was decreased, reflecting crystal growth and sintering of the anatase crystallites upon calcination. It should be noted that even after calcination at 973 K the sample remained in the anatase phase and had a large surface area of 34 m2g-1. The factor of adsorptivity, [Ag+]ads, was also reduced by the calcination (Fig. 3.6) and almost proportional to the BET surface area (Fig. 3.7). This shows that the density (ca-...

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