Specific surface area of powder

The most well-known method for determining the specific surface area of powders is based on a theory of multimolecular adsorption of gases developed by Brunauer, Emmett, and Teller (BET) (1). The BET method involves the determination of the quantity of a gas which, when adsorbed on the surface of the solid, would completely cover the solid with a monolayer of the gas. 

BS 4359 1994. Determination of the specific surface area of powders. Part 4 Recommendations for methods of determination of metal surface area using gas adsorption techniques. ... 

The theory of physical adsorption of gas molecules on solid surfaces was derived by Brunauer, Emmett, and Teller (BET). The theory serves as the basis for the most widely used technique to assess specific surface area of powders and solids. It extends the Langmuir isotherm concept. 

Specific surface area of powder (m g ) Oxygen content (in wt%) of ... 

Vitreous silica has the same solubility as other amorphous silica. Because of the small specific surface area of powdered silica glass in comparison with that of microamorphous or colloidal silicas, workers found it difficult to establish solubility equilibrium. Stober (144) found that at pH 8.4 in Ringer s solution (0.9% NaCI, 0.1% NaHCOj) at 25 C, at least 15 days was required to reach equilibrium when 20 m of silica surface was exposed per liter, regardless of particle size Without the use of this solution, which has an optimum catalytic effect, it would probably have been impossible to establish equilibrium. The solubility was found to be about 100 ppm. 

The best-known method for determining the specific surface area of powders was developed by Brunnauer, Emmett, and Teller (BET) (7). The BET method involves the determination of the quantity of gas taken up through adsorption by a solid adsorbent at equihbrium with a gas phase at a pressure P. After a known quantity of gas has been admitted to a chamber containing the sample, adsorption occurs, resulting in a pressure decrease until equilibrium between the adsorbed and gas phases is reached. The quantity of gas adsorbed is determined by the difference between the amount of gas originally admitted and the amount remaining in the gas phase at equilibrium. The quantity of gas originally admitted is calculable from the initial pressure because the volume above the adsorbent was calculated previously. 

BS 4359 Pt 2 (1982) Determination of Specific Surface Area of Powder. Permeametry the Kozeny-Carman Equation. BSI, Gunnersbury, London, UK. 

BS 4359 Methods for the Determination of Specific Surface Area of Powders. 

BS 4359 (1984), Determination of the Specific Surface Area of Powders, Part 1. Recommendations for gas adsorption techniques, 70... 

A fast cooling step can help to limit the size of the particles. For high-power applications, small particle size is generally preferred (diffusion paths of Li shortcuts]. The specific surfaces area of powders are between 10 and 45 m /g. 

Estimate the specific surface area of the quartz powder used in Fig. X-1. Assume that a monolayer of C4H9OH is present at P/P = 0.2 and that the molecule is effectively spherical in shape. 

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 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 difference between the specific surface areas of materials treated by the wet or dry method can also be explained by the rewelding mechanism. The surface area of wet milled powders continuously increases during milling, while extended dry milling causes a decrease in surface area. 

Brown et al. [494] developed a method for the production of hydrated niobium or tantalum pentoxide from fluoride-containing solutions. The essence of the method is that the fluorotantalic or oxyfluoroniobic acid solution is mixed in stages with aqueous ammonia at controlled pH, temperature, and precipitation time. The above conditions enable to produce tantalum or niobium hydroxides with a narrow particle size distribution. The precipitated hydroxides are calcinated at temperatures above 790°C, yielding tantalum oxide powder that is characterized by a pack density of approximately 3 g/cm3. Niobium oxide is obtained by thermal treatment of niobium hydroxide at temperatures above 650°C. The product obtained has a pack density of approximately 1.8 g/cm3. The specific surface area of tantalum oxide and niobium oxide is nominally about 3 or 2 m2/g, respectively. 

Investigations of the plasma chemical decomposition of tantalum-containing fluoride solutions indicated no significant differences in the process and product parameters compared to the corresponding decomposition of niobium-containing fluoride solution [529, 532]. The particle diameter, shape and specific surface area of both niobium oxide and tantalum oxide powders attest to a gas-phase mechanism of the interaction, with sequential condensation and agglomeration of the oxides. 

Based on available results, it can be summarized that the particle size of tantalum powder increases (specific charge decreases) with the increase in temperature, K2TaF7 concentration and excess sodium. In addition, an increase in the specific surface area of the melt and Na/K ratio also leads to the formation of coarser tantalum powder. The most important conclusion is that for the production of finer tantalum powders with higher specific charges, the concentration of K2TaF7 in the melt must be relatively low. This effect is the opposite of that observed in the electrochemical reduction of melts. 

Table 19.1 demonstrates how the type of precursor affects the specific surface area (Sg) of the products. Typically, the specific surface area of the powders exceeds 100 m g-i and is higher compared with commercial products [17]. The ef-... 

Specific surface areas of the catalysts used were determined by nitrogen adsorption (77.4 K) employing BET method via Sorptomatic 1900 (Carlo-Erba). X-ray difiraction (XRD) patterns of powdered catalysts were carried out on a Siemens D500 (0 / 20) dififactometer with Cu K monochromatic radiation. For the temperature-programmed desorption (TPD) experiments the catalyst (0.3 g) was pre-treated at diflferent temperatures (100-700 °C) under helium flow (5-20 Nml min ) in a micro-catalytic tubular reactor for 3 hours. The treated sample was exposed to methanol vapor (0.01-0.10 kPa) for 2 hours at 260 °C. The system was cooled at room temperature under helium for 30 minutes and then heated at the rate of 4 °C min . Effluents were continuously analyzed using a quadruple mass spectrometer (type QMG420, Balzers AG). 

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. 

Various techniques and equipment are available for the measurement of particle size, shape, and volume. These include for microscopy, sieve analysis, sedimentation methods, photon correlation spectroscopy, and the Coulter counter or other electrical sensing devices. The specific surface area of original drug powders can also be assessed using gas adsorption or gas permeability techniques. It should be noted that most particle size measurements are not truly direct. Because the type of equipment used yields different equivalent spherical diameter, which are based on totally different principles, the particle size obtained from one method may or may not be compared with those obtained from other methods. 

Homogeneous LaMn03 nanopowder with the size of 19-55 nm and with the specific surface area of 17-22 m2/g has been synthesized using a surfactant, sodium dodecyl sulphate (SDS) to prevent agglomeration [47], The sonochemically prepared LaMn03 showed a lower phase transformation temperature of 700°C, as compared to the LaMn03 prepared by other conventional methods which has been attributed to the homogenization caused by sonication. Also, a sintered density of 97% of the powders was achieved for the sonochemically prepared powders at low temperature than that of conventionally prepared powders. 

A B.E.T. surface area measurement(37) was carried out on tfie activated Ni powder showing it to have a specific surface area of 32.7 m /g. Thus it is clear that the highly reactive metals have very high surface areas which, when initially prepared, are probably relatively free of oxide coatings. 

FIGURE 2.1 Change of electrical conductivity with respect to Ni content in the Ni-YSZ cermets at 1,000°C. Two types of YSZ were used to prepare the cermets one was Toyo Soda powder with a specific surface area of 23 m2/g and an agglomerate size of -0.3 pm, and the other was Zircar powder with a specific surface area of 47 m2/g and an agglomerate size of -0.1 pm the NiO used has a specific surface area of 3.5 m2/g. (From Dees, D.W. et al., J. Electrochem. Soc., 134 2141-2146, 1987. Reproduced by permission of ECS-The Electrochemical Society.)... 

An advantage of the constant composition technique is that relatively large extents of growth and enhanced crystallinity can be achieved at low supersaturations. Improved crystallinity of the particles during crystallization is reflected in lower specific surface areas of the solid phases x-ray powder diffractograms of the solid phases removed from the crystallization cell also show increases in sharpness. Experiments in which crystal growth was allowed to proceed until five or six times the amount of... 

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