The resulting mixture was wet milled in a planetary ball milling machine QM 3SP4J Nangjing China for 1 5 or 10 h at a speed of 170 rpm in a zirconia bottle After milling the slurry was dried completely in a convection oven at 150 °C for 24 h Following that the dried powder was heated to 1000 °C at a rate of 10 °C min nbsp }