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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 4 One drawback of many of these ap- proaches is the necessity to simplify complex or irregular shapes into equiv- alent ideal geometries, analogous to how laser scattering particle size data is fit to equivalent spherical diameters. Another problem with some of these techniques is the limitation of analyzing particles with 2D imaging, where a par- ticle outline or shadow is used to repre- sent an entire particle. A third challenge in microscopy techniques is the limit- ed number of particles that can be ana- lyzed in each image, often on the order of 100 particles or fewer. Nonetheless, dynamic image analysis instrumenta- tion is evolving rapidly and could soon become a benchmark technique. In contrast to these approaches, the use of well-established gas adsorp- tion techniques provides a way to probe bulk quantities of a powder—with parti- cle quantities ranging from thousands to hundreds of thousands. The BET (Brunauer-Emmett-Teller) method uses condensation of krypton or nitrogen gas on a powder surface at cryogen- ic conditions, measuring the resulting gas pressure change to quantify the ex- posed surface area of powder. Well-es- tablished in powder metallurgy [8] , the method is also used extensively in ce- ramics, pharmaceuticals, and geology to characterize particle morphology [9-12] . The specific surface area is reflective of the particle size, shape, texture, and porosity. It can therefore also serve as a way to quantify defects in a large en- semble of particles. A rigorous analysis of the rela- tive contributions of particle size and morphology to BET surface area was conducted by Corning using XRD crys- tallography and electron microscopy to demonstrate the strong correlation between these properties in talc pow- ders [13] . For routine analyses, gas ad- sorption is therefore an excellent way to conduct true 3D, bulk morphological analysis much more efficiently and pre- cisely than other methods. When inves- tigating specific morphology issues, it is useful to couple this with microscop- ic analyses. BULK POWDER PROPERTIES One of the traits of the energet- ics of fine powder surfaces described above is that these forms of materials tend to absorb ambient moisture much more readily than they do in other forms [3] . Both metals and ceramics ex- hibit this behavior, impacting powder flow and final part chemistry and den- sity [14-16] . However, depending on the material, the absolute amount of mois- ture can still be very low and is often less than 0.5 wt% for metal powders. Therefore, conventional oven-based TABLE 3 — PARTICLE SCALE MORPHOLOGY PARAMETER CHARACTERISTICS AND TECHNIQUES Particle scale Macroshape Mesoshape Surface/textural scale Descriptors Sphericity, elongation, etc. Circularity, angularity, etc. Roughness Surface area Nature 3D 2D 2D 3D Basis Equivalent dimensions or volume Equivalent area or perimeter Comparative geometries Geometric analysis (ratios, etc.) Pressure measurement Techniques Dynamic or static digital imaging, optical or electron microscopy, tomography, laser scattering Gas adsorption (BET) Sample representation Microscopy (static imaging): ≈ Hundreds of particles Laser or dynamic Imaging: Tens of thousands of particles Tens of thousands of particles TABLE 4 — MOISTURE ANALYSIS TECHNIQUES AND LIMITATIONS Non-specific Technique Limitations Water-specific Technique Limitations Chemical Karl Fischer Calcium carbide Laborious Complex Spectroscopic TPD/TDS Precision TPD-MS/TDMS Near IR Precision Sample prep Thermogravimetric Oven/vacuum Microwave Halogen IR TGA/DSC Slow >2% H 2 O Decomposition Method RH sensor TGA-MS Slow Sample size, method