Powder characterization surface area

S. Lowell, J. E. Shields, M. A. Thomas, M. Thommes, Characterization of Porous Solids and Powders Surface Area, Pore Size and Density, 2nd edition, Springer-Verlag, Berlin, 2006. 

Although the characterization of the powder surface area and its role in processing have received considerable attention for decades, it is only within recent years that the importance of surface chemistry in ceramic processing has begun to be recognized. As we have outlined in Chapter 1, the consolidation of fine ceramic powders from liquid suspensions to produce more uniform green bodies has been shown to produce significant benefits in fabrication. In this case, the quality of the microstructure of the consolidated body is controlled by the dispersion behavior of the powder and the interaction between the particles in the liquid, which, in turn, are controlled by the surface chemistry. 

The various characterization techniques employed for catalysts are Brunauer-Emmet-Teller (BET) for surface area characterization, Bopp-Janeso-Heinz-inger (BJH) pore size measurements, N2 adsorption-desorption and temperature programmed desorption (TPD) for texture characteristics, X-ray powder diffraction (XRD) for crystalline structure, scaiming electron microscopy (SEM) measmement for morphology characteristics. Energy dispersive X-ray spectrophotometer (EDX), X-ray fluorescence (XRF), ICP-MS, atomic... 

A fundamental requirement in powder processing is characterization of the as-received powders (10—12). Many powder suppHers provide information on tap and pour densities, particle size distributions, specific surface areas, and chemical analyses. Characterization data provided by suppHers should be checked and further augmented where possible with in-house characterization. Uniaxial characterization compaction behavior, in particular, is easily measured and provides data on the nature of the agglomerates in a powder (13,14). 

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. 

Analysis. Excellent reviews of phosphate analysis are available (28). SoHds characterization methods such as x-ray powder diffraction (xrd) and thermal gravimetric analysis (tga) are used for the identification of individual crystalline phosphates, either alone or in mixtures. These techniques, along with elemental analysis and phosphate species deterrnination, are used to identify unknown phosphates and their mixtures. Particle size analysis, surface area, microscopy, and other standard soHds characterizations are useful in relating soHds properties to performance. SoHd-state nmr is used with increasing frequency. 

The most commonly measured pigment properties ate elemental analysis, impurity content, crystal stmcture, particle size and shape, particle size distribution, density, and surface area. These parameters are measured so that pigments producers can better control production, and set up meaningful physical and chemical pigments specifications. Measurements of these properties ate not specific only to pigments. The techniques appHed are commonly used to characterize powders and soHd materials and the measutiag methods have been standardized ia various iadustries. 

Characterization. Ceramic bodies are characterized by density, mass, and physical dimensions. Other common techniques employed in characterizing include x-ray diffraction (XRD) and electron or petrographic microscopy to determine crystal species, stmcture, and size (100). Microscopy (qv) can be used to determine chemical constitution, crystal morphology, and pore size and morphology as well. Mercury porosknetry and gas adsorption are used to characterize pore size, pore size distribution, and surface area (100). A variety of techniques can be employed to characterize bulk chemical composition and the physical characteristics of a powder (100,101). 

Section 2 of this chapter describes the characterization of carbonaceous materials by powder X-ray diffraction, small-angle-X-ray scattering (SAXS), measurements of surface area, and by the carbon-hydrogen-nitrogen (CHN) test, a chemical analysis of composition. In this section, we also describe the electrochemical methods used to study carbonaceous materials. 

What is observed is that there are significant changes in specific surface, but that they are relatively modest and cannot account for large changes in reaction rates in shocked powders. The observed behavior can be characterized into typical behaviors as summarized in Fig. 7.1. If comminution is the dominant behavior, the specific surface area will be observed to increase. Such a behavior is called Type a. If consolidation is the dominant behavior, specific surface area will be observed to decrease. Such a behavior is called Type b. In the most typical case, the specific surface increases at low pres-... 

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. 

Johnson, Christian, and Tiedemann (Ref 27) evaluated the Sorptometer vs the Micromero-graph and the microscope for particle size and surface area determinations to characterize powdered materials used in solid propints. Table 12 compares the surface area of A1 powder samples calculated from Micromerograph and microscopic data with that measured using a Sorptometer... 

Specific Activity (SA) and Mass Activity (MA) of Pt Electrocatalysts Supported on Different Carbon Powders Characterized by Specific Surface Area (S) and Particle Size (d)... 

In the pharmaceutical industry, surface area is becoming more important in the characterization of materials during development, formulation, and manufacturing. The surface area of a solid material provides information about the void spaces on the surfaces of individual particles or aggregates of particles [5], This becomes important because factors such as chemical activity, adsorption, dissolution, and bioavailability of the drug may depend on the surface on the solid [3,5]. Handling properties of materials, such as flowability of a powder, can also be related to particle size and surface area [4],... 

There are many ways to characterize the structure and properties of carbonaceous materials. Among these methods, powder X-ray diffraction, small angle X-ray scattering, the BET surface area measurement, and the CHN test are most useful and are described briefly here. To study lithium insertion in carbonaceous materials, the electrochemical lithium/carbon coin cell is the most convenient test vehicle. 

Four samples of faujasite were synthesized at Si/Al ratios of 2.61, 2.80, 2.97 and 3.03 using published methods from seeded slurries (8-9) and using proprietary methods. One additional sample of Si/Al ratio 2.58 was purchased from Union Carbide. The samples were characterized by X-ray powder diffraction, by surface area measurements, and by wet chemical analysis. The results of these measurements are contained in Table I. 

The respirable powders of a DPI cannot be characterized adequately by single-particle studies alone bulk properties must also be assessed since they contribute to ease of manufacture and affect system performance. Primary bulk properties include particle size, particle size distribution, bulk density, and surface area. These properties, along with particle electrostatics, shape, surface morphology, etc., affect secondary bulk-powder characteristics such as powder fiow, handling, consolidation, and dispersibility. 

As described above, XAS measurements can provide a wealth of information regarding the local structure and electronic state of the dispersed metal particles that form the active sites in low temperature fuel cell catalysts. The catalysts most widely studied using XAS have been Pt nanoparticles supported on high surface area carbon powders,2 -27,29,so,32,33,38-52 represented as Pt/C. The XAS literature related to Pt/C has been reviewed previ-ously. In this section of the review presented here, the Pt/C system will be used to illustrate the use of XAS in characterizing fuel cell catalysts. 

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