Nanocrystalline, porous periclase aggregates as product of brucite dehydration

Abstract

Transmission electron microscopy (TEM) techniques were employed to in situ study the electron-beam induced dehydration of brucite Mg(OH)2. Under the electron beam, the hexagonal platelets of brucite immediately decompose and show a morphological shrinkage of 5% and 10–20% in the a and c directions, respectively. The volume contraction occurs first in the rim and then affects the center of grains. Electron energy low-loss spectra reveal a simultaneous change in the local mass thickness of 50–55%. Combining these data, it follows that the porosity in the dehydrated material is 37.5–50%. The decomposition product is composed of numerous, tiny MgO crystallites and voids. Electron diffraction reveals a topotactic relationship between brucite and MgO with [0001]Bru // [111]MgO and [1120]Bru // [110]MgO. Since the porosity of the dehydrated material is slightly smaller than the maximum theoretical porosity (54%), only a small fraction of the voids is transported out of aggregates. Information on the local environment of the oxygen atoms was derived from extended energy-loss fine (EXELFS) and energy-loss near-edge structures (ELNES). In the time course of dehydration the coordination number of oxygen shows the expected increase from 3 for brucite to 6 for MgO. In a transient state the Debye-Waller factor reaches a maximum, indicating a highly disordered intermediate state. These data allow us to model the water loss and to examine reaction kinetics applying the Avrami equation. The decomposition of brucite is interpreted as a complex three-stage process: ( i ) It proceeds first via an interface-controlled process, starting at the rim of brucite; water escapes through the basal plane. ( ii ) The dehydrated lattice collapses then at the rim, whereas the core region is still hydrated. To further dehydrate the grain, the voids have to interconnect and rearrange in the form of a network slowing down the decomposition. At this stage, the process is diffusion-controlled. ( iii ) Finally, the pores are interconnected and reach the surface. The dehydration accelerates and is again an interface-controlled process. Dedicated to Prof. Dr.Wolfgang F.Muller on the occasion of his 60th birthday