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Chapter VII Reactivity at Dislocation….
KDP crystals compared to pure KDP crystals indicates higher reaction rate for
doped crystal. The doping of amino acid is expected to increase the reactivity at
line defects by facilitating preferential chemical reaction. But for a particular
dopant, the activation energy remains practically constant even if the
concentration of that dopant changes, while the pre-exponential factor does not
show any systematic variation. But this factor is higher for pure KDP crystals,
compared to the doped crystals. In doped crystals the pre-exponential factor is
found higher for L-histidine doped KDP crystals and then it drops for L-threonine
and it has the minimum values for DL-methionine doped crystals. It has been
observed by Joshi and Shah [87] that the shape of molecule and its reactivity
plays important roles in reactivity at line defects. They have studied the effect of
n-propyl alcohol and iso-propyl alcohol containing etchants and found that the
branched iso-propyl alcohol containing etchant exhibits lower reaction rate the
higher values of activation energy compared to n-propyl alcohol.
In the present study, the three selected amino-acids viz. L-histidine, L-
threonine and DL-methionine doped KDP crystals have shown different values of
activation energy, as this can be seen from table (7.1). The values of activation
energy are higher for L-histidine containing KDP crystals than the other two
amino acids containing KDP crystals. The molecular structure of amino acids are
shown in Chapter-6, indicating that L-histidine has closed structure with
imidazole group attached to it and possess basic nature, while L- threonine has
linear structure having hydrophobic nature and unchanged or non-ionic polar side
chains and DL-methionine has also linear structure with non-polar side chains
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