Ion thinning

Ion-pair formation lowers the concentrations of free ions in solution, and hence the conductivity of the solution. It must be pointed out that ion-pair formation is not equivalent to the formation of undissociated molecules or complexes from the ions. In contrast to such species, ions in an ion pair are linked only by electrostatic and not by chemical forces. During ion-pair formation a common solvation sheath is set up, but between the ions thin solvation interlayers are preserved. The ion pair will break up during strong collisions with other particles (i.e., not in all collisions). Therefore, ion pairs have a finite lifetime, which is longer than the mean time between individual collisions. 

Figure 5. A stereo view of the gellan double-helix featuring the intrachain hydrogen bonds (thin dashed lines), interchain hydrogen bonds (thick dashed lines), potassium ions (filled circles), and water molecules (open circles) and the six ligands attached to each potassium ion (thin lines).

The characteristic products of the late stage of hydration are Types 111 and IV C-S-H, and more CH. STEM examination of Type III material in ion-thinned section (JIO) shows that it, too, consists of interlocked and interleaved thin foils (Fig. 5.4), Type IV material is almost featureless even at the 100-nm level, though a fine pore structure has been observed (D13,G35,G36). 

Transmission electron microscopy of ion-thinned sections provides data at higher resolution than can be obtained with polished sections. Rodger and Groves (R24) described regions which had probably formed in situ from the ferrite phase, and which consisted of C-S-H, a hydrotalcite-type phase and a poorly crystalline phase containing iron that could have been the precursor of a hydrogarnet. The particles of this last constituent were almost spherical and some 200 nm in diameter. The same investigation also showed that much of the product formed in situ from alite or belite was essentially pure calcium silicate hydrate. 

Other foreign ions in the anhydrous phases, and the extents to which they tend to pass into the pore solution on hydration. If the C-S-H constituent of the gel is assumed to have the same Ca/Si ratio as in calcium silicate pastes, one would expect that the ratio would be about 1.9 (Si/Ca = 0.53) in a gel with Al/Ca = 0.07. This agrees with some, but not all, of the data in Table 7.1. The hypothesis might explain the observations of Rayment and Lachowski (R29) on the bimodal distribution of Ca/Si ratio in the in situ gel and the relative constancy of its Ca/(Si + Al) ratio. It probably offers the most satisfactory explanation of the existing data but needs to be further tested. Continued studies by TEM of ion-thinned sections may be expected to yield valuable data in this respect. 

Studies using ion-thinned sections, wet cells and backscattered electron images of polished sections show that a space develops between the shell and the anhydrous material (S40,S41,S68) (Fig. 7.6c). In this respect, the hydration of cement differs from that of C3S, in which the C-S-H grows directly over the C3S surfaces, without any detectable separation (S41). By 12 h, the spaces are up to 0.5 pm wide. They are likely to be filled with a highly concentrated or colloidal solution, and the shells are evidently sufficiently porous at this stage that ions can readily migrate through them (S68). The existence of spaces shows that reaction proceeds by dissolution and precipitation further evidence for this is provided by the fact that the C-S-H also deposits on the surfaces of pfa particles, if these are mixed with the cement (D28). Some other relatively unreactive or inert admixtures behave in the same way. 

Fia. 8. Defect in the (001) plane of o-CraOs Double oiroles are surface hydroxide ions. Thin oiroles are oxide ions in the bulk except for one at the defect. 

Figure 3.11 Ion thinning process (a) dimple grinding and (b) ion milling. (Reproduced with kind permission of Springer Science and Business Media from D. Shindo and T. Oikawa, Analytical Electron Microscopy for Materials Science, Springer-Verlag, Tokyo. 2002 Springer-Verlag GmbH.)...

Figure 6 The structure of NaC102,3H20. The water oxygens and one of the oxygens of CIOJ form a distorted octahedron around the sodium ion. Thin lines represent hydrogen bonds, heavy solid lines the O—H and O—Cl chemical bonds, and double lines describe the octahedron of oxygens in van der Waals contact with the sodium (Reproduced by permission from Acta Cryst., 1975, B31, 2146)...

Semicokes prepared at various temperatures were examined in the transmission electron microscope, but few features could be seen in the lower- and higher-temperature samples. Because of the porous and brittle nature of semicoke, it was difficult to prepare good ion-thinned sections as we did for coal. Therefore, features such as spheres and rods may have been destroyed during grinding to prepare the sample. 

The ion-thinning process also preferentially removes the lighter atoms and may deposit what it removes somewhere else on the sample. 

Ion thinning can implant Ar into loose, or open, structures (like GBs). 

Ion thinning may thus both preferentially thin the GB (forming a groove) and then deposit material in that groove. (Figure 14.35 shows the problem.)... 

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