Hydrothermal Synthesis: Energy Dispersive

Hydrothermal Synthesis

II. Energy Dispersive

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Energy Dispersive

Hydrothermal syntheses can also be conveniently followed using energy-dispersive (powder) diffraction (EDD) methods, and as pointed out previously this was, historically, the first method used with X-rays. The relative advantages and disadvantages of the EDD and angle-dispersive modes have been mentioned several times in this course, including the high pressure pages given previously. In the context of hydrothermal synthesis the hit list is:

(1) advantages of EDD for hydrothermal synthesis

the fixed geometry means no moving parts which means more rapid data (better time resolution) up to the limit of EDD detectors;
the fixed geometry and small incident/diffracted beams means that only a small portion of the hydrothermal bomb needs to be thin enough for X-ray transmission;
the white synchrotron X-ray beam is intense and usually extends to high energies on many white beam stations (e.g. 80 - 125 keV), resulting in a very penetrating X-ray wavelength, again simplifying the design of the hydrothermal bomb.

(2) disadvantages of EDD for hydrothermal synthesis

EDD patterns are less sharp than AS diffraction patterns and therefore if the diffraction pattern is crowded (i.e. many peaks) the peaks will overlap making data analysis much more difficult;
Rietveld structure refinement programs for AS diffraction are far more developed and available than the equivalent for EDD, to the point that structural refinement is performed almost exclusively on AS diffraction data.

However in the many cases when these two latter conditions do not apply, there is much benefit in performing hydrothermal synthesis experiments in the EDD mode. Two interesting examples, of considerable industrial significance, are given. The first concerns tricalcium aluminate (known as C₃A) which hydrates very rapidly to form a calcium aluminate hydrate (known as C₃AH₆; where C is CaO, A is Al₂O₃ and H is H₂O). This is of interest because C₃A is present in Portland cement but is prevented there from hydrating too rapidly by the addition of modifiers such as calcium sulphate. When one views this hydration in-situ, using energy-dispersive diffraction on the ESRF synchrotron (Jupe et al. Phys. Rev. B 53, 14697 (1996)) with sufficient time resolution (0.35 seconds or 6 seconds) and viewing a small representative region within the sample, a quite different picture, from that in the previous literature, emerges in which an intermediate hydrate phase always appears for a short time before the final C₃AH₆ hydrate forms (below).

It is now believed that this intermediate phase acts as a nucleating agent for the C₃AH₆ to start forming, and its crucial role was not realised by the traditional cement community due to the insufficient time-resolution and spatial precision of conventional powder diffraction systems. It serves as a good example of the power of in-situ methods. A related example concerns the setting of oil well cements under autoclave conditions (130 - 250°C). In oilwell operations cement is pumped into well bores for many miles passing through gaps as small as one inch (25 mm). However under the prevailing geothermal conditions of high temperature/pressure the cement can set prematurely, thus blocking the cement flow at a cost that can easily rise into the multi-million dollars range. For the first time the setting of cement within bore-hole autoclave conditions has now been studied (J. Syncrotron Radiation 5, 112 (1998) and 7, 167 (2000)) using energy-dispersive diffraction on the Daresbury SRS synchrotron (below).

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From this kind of data one is able to obtain phase-concentration plots during the process (below)

which charts the changes with time, displaying for example a complex interplay between three calcium alumino-sulphate hydrate phases (so-called ettringite, C₃A.3Cs.32H; 14-water monosulphate, C₃A.Cs.14H; 12-water monosulphate, C₃A.Cs.12H; where s = SO₃) and the rapid consumption of one phase (known as brownmillerite) which under normal ambient temperature would remain unaffected. So just one successful in-situ experiment has removed all the guesswork and elucidated the true course of hydration under these conditions.

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Author(s): Paul Barnes
Sally Colston
Simon Jacques
Andrew Jupe
Martin Vickers