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A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 1 9 2 3 AM alloy, as well as two sieve frac- tions within a recycled stainless steel powder [5] . Standard practices for represen- tative powder sampling at all scales are well established [6,7] . At the laborato- ry level (typically <1 kg sample), chute splitters and rotary rifflers are neces- sary to produce smaller sub-samples for testing. Similarly, due to high sur- face area-to-volume ratios, powder ma- terials often tend to absorb moisture more readily than in the bulk state [3] . Therefore, in addition to testing as-re- ceived materials, it is often important to condition powders (via humidifica- tion or drying) prior to analysis. MATERIAL PROPERTIES The fundamental properties of materials are independent of form, at least hypothetically, so these will only be mentioned briefly. The most com- mon chemical analysis includes multi- elemental inductively-coupled plas- ma (ICP) spectroscopy, inert gas fusion, and combustion methods for C, S, O, N, and H elemental analysis, as well as conventional wet chemistry techniques for certain species. There is also a gen- eral trend toward faster, direct powder analysis via x-ray fluorescence (XRF) spectroscopy, whenever possible. PARTICLE PROPERTIES Particle-level characterization in- cludes chemical analyses, size distribu- tion measurements, andmorphological analysis. In cases where particle chem- istry differs from bulk material chem- istry, SEM-EDS analysis of whole or cross-sectioned particles can provide detailed elemental distributions on the surface and interior of particles. Vari- able pressure FE-SEM instruments are best suited to EDS analysis, as con- ductive coatings are not necessary. To avoid the small population sampling in- herent inmicroscopic analyses, XPS can be used to analyze larger powder sam- ples, although it is limited to the parti- cle surface. Despite a vast difference in mea- surement basis, sieve analysis and la- ser scattering techniques remain the most common methods for character- izing particle size distributions in bulk powders. Whichever technique is used, it is important to understand the basis for how the particle sizes are derived. As shown in Table 2, the various size analysis techniques fall into one of the three categories listed here. Depend- ing on how the size data is to be used, one type of method may provide more meaningful results than another. How- ever, in most cases the most useful re- sults are obtained from a combination of methods, often including a micro- scopic technique coupled with an en- semble or separation method. Obtaining particle morphology in- formation that is both representative and useful has long been a challenge in powder technology. Table 3 shows one way to categorize particle shape and texture parameters along with typical techniques employed to derive them. TABLE 1 — POWDER PREPARATION AND CHARACTERIZATION AT VARIOUS SCALES Scale Composition Density and porosity Size and morphology Flow behavior Sampling and conditioning Drying/humidification, splitting, riffling Material properties Spectroscopy, inert gas fusion, gravimetric SEM, He pycnometry Particle properties SEM-EDS N 2 porosimetry Kr/N 2 adsorption, sieve, laser, microscopy Bulk properties Gravimetry, TGA/DSC, halogen IR He pycnometry, loose/apparent, tapping/packing Dynamic rheometry, flow meters TABLE 2 — PARTICLE SIZE AND DISTRIBUTION ANALYSIS TECHNIQUES Method type Ensemble methods Counting methods Separation methods Particles are analyzed simultaneously. Particles are analyzed individually. These rely on the segregation of particles. Primary methods Laser diffraction, dynamic light scattering Optical imaging, microscopy, Coulter/Electrozone Sieve analysis, X-ray sedimentation Specialty methods Air permeability, ultrasonic attenuation Light obscuration, time-of-flight counting Disc centrifuge, capillary fractionation, flow field fractionation, scanning mobility analysis