Hydrothermal Reaction Sintering (HRS) [11,82] employs as a starting material a metallic pre compacted powder that is oxidised by a supercritical water solvent heated within a millimetre sized metal sealed capsule (made of platinum which is permeable to generated hydrogen) where the self-generated pressure can reach 100 MPa with a temperature approaching 1000 °C (which is well above the 300℃ discussed above, but still well below the traditional consolidation temperatures).