Use aluminum oxide spheres and microspheres when the particle needs to be:
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hard — ceramic contact, abrasion, wear, polishing, or surface-interaction testing
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white/opaque — visible ceramic particles without dye, fluorescence, or coating
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electrically insulating — non-metallic ceramic filler or spacer behavior
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thermally stable — inorganic oxide material where polymers or glass are unsuitable
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moderately dense — 3.8 g/cc particle density without the very high mass of WC or metal spheres
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solid alumina — not hollow, not glass, not polymer, not metallic