Reaction sintering of rhabdophane into monazite-cheralite Nd1-2xThxCaxPO4 (x=0-0.1) ceramics

The direct sintering of Nd1-xCaxThxPa4 center dot nH(2)O rhabdophanes (x = 0 - 0.1) was achieved for the first time resulting in homogeneous high-density monazite-cheralite pellets. Thanks to their large specific surface area, rhabdophane precursors led to a lower shrinkage temperature compared to samples synthesized through solidstate reactions. The associated activation energy was found between 360 +/- 90 and 530 +/- 90 kJ.mol(-1), depending on the thorium incorporation. Meanwhile, sintering map was built up, showing that densification predominated for T <= 1200 degrees C. Conversely, the complete densification took place at 1400 degrees C concomitantly to grain growth and elimination of open porosity. Moreover, Th-Ca coupled substitution seemed to inhibit both densification and grain growth as the average grain size dropped down one order by one order of magnitude between x = 0.0 and x = 0.1. However, these differences did not affect microhardness, which reached 4.9 +/- 0.8 GPa whatever the chemical composition tested.