Add time:08/11/2019         Source:sciencedirect.com

Here, we report on a size-controlled synthesis of highly crystalline germanium particles, via the self-sustained reaction of GeO2 with Zn, Mg, and NaBH4. The thermodynamic analysis suggests that the GeO2 + 2Zn and GeO2 + NaBH4 systems are characterized by moderate heats of reaction (−121 and −204 kJ per mole of Ge, respectively), while GeO2 + 2Mg exhibits a much higher reaction enthalpy (−623 kJ). Magnesium reduction has been found not to be suitable for the preparation of germanium crystals due to the high reaction temperature, which exceeds 2000 °C. The partial substitution of Zn by Mg, however, enables increasing the overall reaction temperature in the low caloric GeO2 +Zn system. By using different reducers and their mixtures, the reaction temperature was optimized to be in the 600–1100 °C range. Such temperature modifications allow for control of the germanium particle sizes, ranging from 200 nm to 2 mm. Thermal analysis of the reacting mixtures and electron microscopy examination of the products indicates that dissolution-precipitation is the dominant formation mechanism of the germanium crystals in the GeO2 + Zn system. The higher reaction temperatures in the GeO2 + Zn + Mg and GeO2 + NaBH4 systems cause melting, and subsequent coalescence of the primarily precipitated germanium resulting in much larger particles.

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