Powder surfaces

The method to be described determines the pore size distribution in a porous material or compacted powder surface areas may be inferred from the results. 

Surface Area Determination The surface-to-volume ratio is an important powder property since it governs the rate at which a powder interacts with its surroundings. Surface area may be determined from size-distribution data or measured directly by flow through a powder bed or the adsorption of gas molecules on the powder surface. Other methods such as gas diffusion, dye adsorption from solution, and heats of adsorption have also been used. It is emphasized that a powder does not have a unique surface, unless the surface is considered to be absolutely smooth, and the magnitude of the measured surface depends upon the level of scrutiny (e.g., the smaller the gas molecules used for gas adsorption measurement the larger the measured surface). 

FIG. 20 66 Contact angle on a powder surface, where y ", are the sohd-... 

Lowell, S., and Shields, J. E. (1991). Powder Surface Area and Porosity, 3rd Ed. Chapman Hall, London. 

Titanium Dioxide Single-Crystal and Powder Surfaces in the Presence and Absence of Platinum... 

Metallic powders are made several different ways. They can be prepared by reducing salts in a stream of a reducing gas, such as hydrogen chlorides of metals are commonly used but oxides are used too. Thermal decomposition in a vacuum of metal carbonyls or metal salts of organic acids, such as formates, produces metal powders. Surface areas of such powders are around 1.5 m2/g. Powders can also be made from electrolytic reduction of salts in organic solvents and by atomization of the metal. 

Figure 3. Vertical cross-section showing equipotential contours inside a conductive cylindrical silo containing a symmetric conical heap of uniformly charged solids. The electrostatic potential maximum exists on the center line somewhat below the powder surface, while the maximum electric field intensity occurs near the wall just above the powder.

S. Lowell and J. E. Shields, Powder Surface Area and Porosity, 2nd ed., Chapman and Hall, London and New York, 1984. 

S. Lowell, in Introduction to Powder Surface Area, J. Wiley Sons, New York, 1979. 

Powdered iron phosphates, 26 215 Powdered sugar, 23 482 Powdered surfaces, diffuse reflectance of radiation from, 24 110 Powder glass-ceramic processing,... 

Particle-Size Analysis Methods for particle-size analysis are shown in Fig. 17-34, and examples of size-analysis methods are given in Table 17-1. More detailed information may be found in Lapple, Chem. Eng., 75( 11), 140 (1968) Lapple, Particle-Size Analysis, in Encyclopedia of Science and Technology, 5th ed., McGraw-Hill, New York, 1982 Cadle, The Measurement of Airborne Particles, Wiley, New York, 1975 Lowell, Introduction to Powder Surface Area, 2d ed., Wiley, New York, 1993 and Allen, Particle Size Measurement, 4th ed, Chapman and Hall, London, 1990. Particle-size distribution may be presented on either a frequency or a cumulative basis the various methods are discussed in... 

Photochemical decomposition can also be carried out in the presence of a suspension of photoactive material such as Ti02 where substrate absorption onto the uv activated surface can initiate chemical reactions e. g. the oxidation of sulphides to sul-phones and sulphoxides [37]. This technology has been adapted to the destruction of polychlorobiphenyls (PCB s) in wastewater and is of considerable interest in environmental protection. Using pentachlorophenol as a model substrate in the presence of 0.2 % TiOj uv irradiation is relatively efficient in dechlorination (Tab. 4.5) [38]. When ultrasound is used in conjunction with photolysis, dechlorination is dramatically improved. This improvement is the result of three mechanical effects of sonochemistry namely surface cleaning, particle size reduction and increased mass transport to the powder surface. 

Transmittance and reflectance data are used in color measurements. Transmittance spectra are used for liquid color measurements, while reflectance spectra are used on solid samples (powders, surfaces) and on opaque liquids (paint). A large number of color scales are in use today. Some are specific to a particular industry, while others enjoy broader application. The most common scales are the CIE L a b (Commission... 

Table 3.4 summarizes the major properties of powders which need to be characterized in pre-formnlation. The effect of micronization (or other high energy processes), which is often applied to the powder (surface), should also be investigated as this may alter the properties of the powder during processing [64]. This may also alter the performance of the formulation. 

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