Fine MoO3 powder was mixed with 4 w

Fine MoO3 powder was mixed with 4 wt. % water using an agate mortar. The mixture was pressed into cylinders (20 mm in diameter and 4 ˜ 5 mm in height) at 100 ˜ 150 MPa at 120 ˜ 150 °C from 10 min to 30 min. Besides, 10 wt% (NH4)6Mo7O24·4H2O was also mixed with MoO3 powders and water (4 wt%) and the same cold sintering process was carried out. To prevent the reaction between MoO3 and the steel, thin PTFE pellets were added inside the die during the cold sintering process. After cold sintering, the samples were dried at 120 °C for 24 h before measurements. Some samples were also annealed at 600 to 700 °C in air after cold sintering.