Experimental Methods and Materials

Sintering of Dy2O3 and quenching experiments

Dy2O3 powder (Chempur, 99.99%) was molded into a disk (10 mm diameter, 5 mm thickness) and pressed with a tablet press (Paul-Otto Weber D7064) at 30 MPa. A cold isostatic press (CIP, EPSI B-9140) was used at a pressure of 2000 bar with a dwelling time of 5 min. The samples were then heated with a heating rate of 3 K min-1 and sintered at 1173 K (900 °C) for 12 h. It was cooled down to room temperature at a cooling rate of 3 K min 1. The temperature was measured using a calibrated thermocouple placed directly above the sample. The microstructure of the sintered Dy2O3 was investigated by scanning electron microscopy (SEM, Philips XL30 FEG).

In the quenching experiments, CaCl2 (Aldrich, 99.9%) electrolyte was pre-dried under vacuum for 24 h at 573 K (300 °C). Subsequently, it was melted at 1173 K (900 °C) in a highly pure argon atmosphere (99.999%). The pre-electrolysis process was performed between a Kanthal wire cathode and a graphite anode at a potential of 2.5 V during 2 h to remove moisture and impurities. The CaCl2 salt was remelted in a quartz cup (25 mm diameter, 25 mm height) under Ar gas and a sintered particle of Dy2O3 (6 mm diameter, 2 mm thickness) was dropped into the crucible after the temperature reached 1173 K (900 °C), after which the quartz cup was held at that temperature for 2 h and then quenched in air.

The cold mounted samples were polished with 1 pm diamond paste after grinding with emery paper 80, 320, 600, 800, 1200 and 1400, respectively. The as-polished samples were then platinum coated and analyzed by SEM. Qualitative element maps of calcium, dysprosium, chlorine and oxygen were recorded to determine the composition in the bulk and at the oxide/salt interface.

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