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EP: Extraterrestrische Physik

EP 11: Gas, Staub und kleine Körper

EP 11.4: Vortrag

Freitag, 24. März 2000, 12:30–12:45, N3110

Charge Generation and Exchange among Mineral Dust Grains - Basis for Grain Agglomeration — •Friedemann Freund¹ and Minoru M. Freund² — ¹SETI Institute/NASA Ames Reseach Center — ²NASA Goddard Space Flight Center

Microscopic physical properties and charge transfer processes inside dust grains are fundamental to understanding the agglomeration of interstellar dust clouds. Mineral (oxide/silicate) grains that condense in the presence of water vapor incorporate hydroxyl. Pairs of hydroxyl undergo an electronic rearrangement inside the host matrix, results in hydrogen plus peroxy. This is a classical redox reaction, where hydrogen represents the reduced, and peroxy the oxidized moiety, respectively. The peroxy in an oxide/silicate matrix represents a positive hole pair (PHP), which is spin-coupled and electrically inactive. When PHPs are activated, they inject holes into the valence band, causing the mineral to become a p-type semiconductor. This hole is chemically equivalent to an oxygen radical in an oxide matrix. Any grain-grain contact will then lead to a transfer of charges, because there are no two grains with exactly the same charge carrier density, and consequently with the same chemical potential. A consequence of the charge transfer is that the grains become electrostatically bonded and dipolar. The dipole induces filamentary aggregation as demonstrated in microgravity experiments.