Surface area of a powder

Dye adsorption from solution may be used to estimate the surface area of a powdered solid. Suppose that if 3.0 g of a bone charcoal is equilibrated with 100 ml of initially 10 Af methylene blue, the final dye concentration is 0.3 x 10 Af, while if 6.0 g of bone charcoal had been used, the final concentration would have been 0.1 x Qr M. Assuming that the dye adsorption obeys the Langmuir equation, calculate the specific surface area of the bone charcoal in square meters per gram. Assume that the molecular area of methylene blue is 197 A. ... 

The determination of the specific surface area of a powder by air permeability methods essentially involves the measurement of the pressure drop across a bed of the powder under carefully controlled flow conditions. The data obtained are substituted in the Kozeny-Carman equation to estimate the specific surface area. Permeability methods have certain advantages, one of them being that the equipment used for carrying out the measurements is cheap and robust. Another advantage is that sample problems are minimized because a large sample of powder is required to be used for analysis. 

Specific surface area of a powder or porous material (m2 kg-1), parameter... 

The specific surface area of a powder can be used to estimate the average particle size if the particle shape is known. 

Powder diffraction provides an alternative approach to obtaining this information since the diffraction pattern from an adsorbed gas contains information about the surface area of a powder and the contributions that are made to it by different surface planes. It also leads directly to the spatial separation between the atoms or molecules of an adsorbate allowing interesting comparisons to be made with the common assumption of the B. E. T. analysis. In certain special cases powder diffraction can also provide information about the sites at which chemisorption occurs. 

Calculated surface area the surface area of a powder calculated from its particle size distribution. 

The relative surface area of a POWDER measured in m /g or area m /cm. ... 

An experimental check of these conclusions showed that the apparent absolute rate of decomposition of powders (Jp ), related to the external surface area of a powder pellet, is always higher than that of crystals (Jc). It was also found that the difference between the decomposition rates is independent of temperature, residual air pressure in the reactor, sample mass and powder particle size. The mean value of Jp /Jc was 2.8 0.4. This value somewhat exceeds the theoretical estimate, which may be due to the rough external surface of the powder. Evidently, the area of a pimpled surface formed by spherical particles should be at least twice that of the flat surface. 

Thus, the true absolute rate of decomposition of powders (Jp) is evaluated by dividing the change in sample mass per unit time. Am/At, measured under isothermal conditions, by the external surface area of a powder pellet at the instant of measurement, s, multiplied by a factor of 2.8, i.e.. 

As seen from the table, during an 80 min decomposition period in which a increases from 0.216 to 0.665, the variations of Jp do not exceed 1%, with the mean value Jp = (6.14 0.04) x 10 kg m s. Thus, Eq. 15.5 allows the variation of the surface area of a powder pellet in the course of decomposition to be taken into account quite adequately. 

Also, variation of the surface area of a powder pellet in the course of decomposition (see Sect. 15.1) was taken into account with Eq. 15.5 instead of Eq. 15.2. These refinements ultimately led to a slight increase in the final values of E. The mean value of E for HgO was 187.5 0.3kJ moP which is 7% lower than that given by the thermochemical calculation (201 kJ moP ), whereas... 

Surface area of a powder increases geometrically with decreasing particle size, so that the volume fraction of the outermost layer of ions on the surface increase significantly, which has a significant effect on properties of the powder. With the development of nanotechnology, it is readily to synthesize powders with nanosized particles (1-100 nm). Therefore, characterization of surface properties becomes more and more important. Specifically for ceramics or transparent ceramics, the consolidation of fine ceramic powders with liquid suspensions to produce more uniform green bodies has been shown to play an important role in the fabrication ceramics, especially when special or complex structures are required. Because the quality of microstructure of the consolidated body is determined by the dispersion behavior of the powder and the interaction between the particles in the suspension, which is closely related to the surface properties of the particles, controlling the physical and chemical properties of particles is a critical to ceramics fabrication. 

Usually the first powder characteristic that we are concerned about in our laboratory is specific surface area (more commonly referred to simply as surface area.) The surface area of a powder is a measure of its size, shape, and irregularity (such as the presence of voids that are open to the surface). There are several excellent books about surface area and its measurement.The most commonly used technique for determining powder surface area is the BET method using the adsorption of a monolayer of a gas such as nitrogen on the powder surface. The common unit of measure for surface area is mVg (area/unit mass). Most powders fall in the range of 1 to 50 mVg, with the vast majority of highly sinterable powders falling between 5 and 15 mVg. 

The most common procedure for determining the surface area of a powder is to derive the amount of adsorbed inert gas (typically nitrogen) at monolayer... 

A procedure for the determination of the total surface area of a powder or of a porous solid by measurement of the volume of gas (usually N2) adsorbed on the surface of a known weight of the sample. The mathematical basis of the method was developed by S. Brunauer,... 

FIGURE 7.30 The specific surface area of a powder, as a function of the grinding time, t, (1) for pure zinc and for zinc in the presence of gallium (2) 1%, (3) 3%, (4) and 10%. (Redrawn from Pertsov, A.V., The studies of dispersion processes under conditions of the strong decrease in the free interfacial energy, Canadian Science thesis, Izd. MGU, Moscow, Russia, 1967.)... 

The specific surface area of a powder is conveniently determined through a Brunauer-Emmett-Teller (BET) analysis. Here, the adsorption of nitrogen is determined and then analysed. Since the cross-sectional surface area of a nitrogen molecule is known, the total area of a powder sample can therefore be determined. BET analysis is conveniently carried out by using commercial instruments build solely for this purpose. In some cases, the solid sample cannot be treated (dried) to give a dry powder. In such cases, the specific surface area can be determined by the adsorption of a surfactant, with a known cross-sectional surface area, that adsorbs as a monolayer at that specific surface. This method is, of course, rather uncertain since it requires the assumption of monolayer adsorption. 

Figure 10.6. Appropriate interpretation of gas adsorption data can enable one to deduce the fractal dimension of a rough surface, a) In gas adsorption, the surface area of a powder is estimated from the number of molecules required to cover the surface. This estimate will vary depending on the size of the molecule used, b) The fractal dimension of a surface can be derived from surface area estimates obtained for a range of different sized molecules.

The specific surface is the surface area of a powder per unit mass (or volume). It can be calculated from the Sauter Mean size... 

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