Studies of layered uranium (VI) compounds. V. Mechanisms of densification of hydrogen uranyl phosphate tetrahydrate (HUP): Pressure-induced planar glide and solution phase sintering
The good proton conductor HUO 2PO 4·4H 2O (HUP) can be easily fabricated into disks or films suitable for use in electrochromic displays, fuel cells, or batteries. Kinetic studies of the room-temperature densification reveal the presence of two mechanisms. At applied pressures of 20 MN m -2 the observed rapid densification was attributed to the gliding of the structural layers over each other to achieve essentially 100% of the theoretical density. When the powder had been wettened by solution, slow sintering occurred even in the absence of applied pressure, and we attribute this to a solution-phase sintering mechanism. The study is one of the few to be reported on platelet-shaped crystals rather than the usual spherical-shaped particles. This fact, together with the ease of cleavage of the layers, largely accounts for the excellent fabrication properties. The optimum conditions for pressing large clear robust disks are described.