Recrystallization effect during the dehydration of magnesium hydroxide

Abstract

The thermal decomposition of finely divided Mg(OH)2 does not occur smoothly. Several maxima of the gas evolution rate are observed, when microgram quantities of Mg(OH)2 are heated under ultrahigh vacuum conditions. This phenomenon is attributed to the formation of an overlayer of partially decomposed hydroxide Mg(OH)2-x Ox,2 [[]x,~ with x--+ 2 ([--] : anion-sized neutral vacancy left by H20 molecules in the hydroxide lattice). This defect layer represents a diffusion barrier for further H20 molecules. When a critical thickness is reached, it spontaneously recrystallizes to cubic MgO exposing fresh Mg(OH)2 surface.The differential thermal analysis of magnesium hydroxide, Mg(OH)2, in the usual quantities, namely a few milligrams, yields a smooth endothermic reaction just as one would expect for the simple dehydration reaction Mg(OH)2 ~ MgO + + H20. However, we have repeatedly observed [1,2] that the dehydration of microgram quantities of magnesium hydroxide under ultrahigh vacuum conditions, as measured mass-spectroscopically by the gas evolution rate, does not result in a smooth curve. ExperimentalThe samples consisted of very high purity MgO-powder (total cation impurity content <5 ppm) either superficially hydroxylated or bulk hydroxylated and deuteroxylated to Mg(OH)2 and Mg(OD)2 respectively. The diameter of the individual hexagonal platelets was well below 1 pro, and their thickness in the 0.1 pm range.The samples were heated in a fused-silica capillary tube attached to an oil-free, all-glass ultrahigh vacuum system operating at a base pressure of 10 -~~ mbar. In case A a step mode with 25 ~ intervals and 7 min. isothermal heating periods was used [1 ] whilst in case B a linear heating rate of 2~[2] was used. The gas evolution from the sample was monitored by a memory-free, all platinum Omegatron mass spectrometer [3]. In case A the gases evolved were pumped away rapidly through a wide tube of large conductance, whilst in case B pumping was carried out more slowly through a capillary of known conductance for quantitative evaluation.

Revealing the effect of water vapor pressure on the kinetics of thermal decomposition of magnesium hydroxide

“…Each elementary particle of Mg(OH)2 is usually a hexagonal thin plate, and the 2D shrinkage of the reaction interface in each particle, formed initially at the edge surfaces, is consistent with previous studies. 51,53,[55][56][57] However, the Mg(OH)2 sample in the present study is characterized as agglomerates of crystalline particles with diameters of approximately 50 μm. 71 The apparent best fit using the SR-PBR(3) model likely indicates the overall kinetics of thermal decomposition of the agglomerates.…”

In situ synchrotron XRD investigation of the dehydration and high temperature carbonation of Ca(OH)2

“…24,26 As discussed in the introduction, the formation of such a structure, which is likely to be permeable, can be expected to play an important role in controlling the migration of molecular water from the bulk to the surface of the crystal and so is of interest for this study. Such a delay can be expected to lead to the temporary existence of an intermediate structure, perhaps in the form of a defective CaIJOH) 2 lattice.…”

“…As discussed in the introduction, based on the observations of Freund et al it is possible to postulate that this intermediate phase could be a defective CaIJOH) 2 lattice with a significant number of vacancies instead of OH − anion groups, 26 which can be written as CaIJOH) 2−x O x/2  x/2 , where  designates a neutral anion-sized lattice vacancy.…”

“…Such a delay can be expected to lead to the temporary existence of an intermediate structure, perhaps in the form of a defective CaIJOH) 2 lattice. 24,26 As discussed in the introduction, the formation of such a structure, which is likely to be permeable, can be expected to play an important role in controlling the migration of molecular water from the bulk to the surface of the crystal and so is of interest for this study. Such a delay was observed in both experiments.…”

“…In this context Freund et al 26,27 postulated this intermediate phase takes the form of a highly deficient crystal structure which, in this case, can be written as CaIJOH) 2−x O x/2  x/2 , where  designates a neutral anion-sized lattice vacancy.…”