Ionization Expansion of Pressure-Compacted Vitreous Silica

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Abstract
In vitreous silica compacted by pressure at room temperature, a large proportion of the compaction is gradually released by ionization. This ionization expansion is shown to be a cooperative effect involving both radiation and thermal processes and is therefore termed a radiation‐annealing effect. Data are given for soft x‐ray radiation annealing at 15° and 100°C, and for radiation annealing after prior thermal annealing for 2 h at 200°C. The amount of compaction released (dilatation) is proportional to the radiation dose rate, nearly proportional to absolute temperature, rises rapidly from 0%–3% compaction, and then falls a little with increasing compaction. For vitreous silica compacted at higher temperature, the radiation‐annealing rate is much reduced. The initial efficiency of the radiation annealing can be very high, ∼10 ionic oxygen volumes for each ionization. The effect seems to be related to transient ionization, which seems capable of releasing compaction over a radius of ∼8 ionic oxygen diameters. Accordingly, it is concluded that the infolding of the loops of Si–O tetrahedra in pressure‐compacted vitreous silica is locked‐in by overlapping strain fields which are unbalanced by transient ionization, and thermal annealing follows. This long‐range locking‐in may be absent in vitreous silica compacted by other means (e.g., ionization), and then the spectacular release of compaction by ionization does not occur. Other peculiarities of the effect are described and explained.