Charge Interaction Effects in Epoxy with Cation Exchanged Montmorillonite Clay and Carbon Nanotubes. Page: 4
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between the hexagonal close-packed oxygen lattices. The most common cations
are silicon with a radius of 0.040-nm and aluminum with 0.055-nm. Because
these cations fit tightly into the spaces between the oxygen anions, they are
protected against migration by packing, size, and the high ionic bond strength
due to the high number of charges on each atom. Internal cations are therefore
not replaced by weathering or ion transport.
The most commonly found element at the center of the tetrahedral
structure is silicon, which falls near the middle of the oxygen : cation radius ratio
range at 1 : 0.30. This intermediate value allows tolerance for a few internal
tetrahedral sites to be substituted by cationic impurities of different radius ratios
and fewer unit charges during the crystal formation. These substitutions are
sufficiently infrequent to permit continuity of the overall crystal structure. One
common impurity in the tetrahedral layer is iron in the form Fe+2.
The octahedral unit cell maintains 6 nearest neighbor oxygen atoms to the
central cation. Each octahedral oxygen atom is shared with a neighboring cation
outside of that cell. The octahedral unit cell has a characteristic radius ratio
between 1 : 0.414 and 1 : 0.732. Aluminum is the most common impurity element
in octahedral clay layers. This cation has a radius ratio of 1 : 0.41. Aluminum is
somewhat larger than silicon, and has one fewer positive charge, yet the overall
octahedral crystal structure maintains a 6-fold coordinated nearest-neighbor
environment of oxygen atoms around this impurity cation.4
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Butzloff, Peter Robert. Charge Interaction Effects in Epoxy with Cation Exchanged Montmorillonite Clay and Carbon Nanotubes., dissertation, May 2005; Denton, Texas. (https://digital.library.unt.edu/ark:/67531/metadc4786/m1/17/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; .