= Solution
Disperse a very small amount of the dry powder in a clean volatile solvent compatible with <aluminium oxide>, use brief gentle sonication to separate loose agglomerates, and place a dilute drop on a holey carbon-coated <transmission electron microscopy> grid. Wick away excess liquid and dry thoroughly. Look first at low magnification and select isolated particles or thin edges over holes, rather than an electron-opaque pile. Record a solvent-only grid to identify contamination and compare several independently prepared areas.
This is quick and preserves individual particles without mechanical thinning. However, <spray pyrolysis> can make hollow or porous particles that may fracture during sonication; sedimentation and drying can select sizes and aggregate particles. <aluminium oxide> is insulating, so charging or beam-induced changes can complicate images, and the carbon support can overlap weak contrast. Vary dose and dwell time to test stability. A projected image of one selected thin particle is not an unbiased three-dimensional size or porosity distribution of the whole powder.
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