Synthesis of nanocrystalline low temperature silica polymorphs

Colloidal silica (Dupont, Ludox 40) has been hydrothermally treated at temperatures between 125 and 220 °C at 100 MPa water pressure for 5 to 90 days. Final synthesis product was low quartz (α-quartz) in all cases, which was obtained directly after five days at the highest experimental temperatures. Silica phases obtained at 145 °C and below were all of nanocrystalline size.At 145 °C and below, the hydrothermal high pressure conversion of colloidal silica takes place in time dependent different steps, starting from a precipitated mixture of an amorphous phase and “phase X2” found after 5 days of synthesis at 145 °C, while at longer synthesis times (21 and 42 days respectively) moganite-low quartz mixtures are found. After 90 days of synthesis phase low quartz is established. At 125 °C the occurrence of these phases is moved to longer synthesis times, however moganite-low quartz mixtures are still present after 90 days.The experiments established that “phase X2” and moganite are low temperature polymorphs of low-quartz stable for kinetic rather than for thermodynamic reasons. Therefore, “phase X2” shall be renamed to “lower low quartz”.

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