From: Jacob Zabicky (zabicky$##$bgumail.bgu.ac.il)
Date: Tue Sep 16 2003 - 11:05:58 EDT
Dear Thomas,
IMHO you cannot state a general rule of thumb for "sensible" activation
energies. Furthermore, the Arhenius equation is sometimes blindly
applied to situations it was not meant for, and the resulting slopes
are called activation energies, whatever that means in that case.
As for local temperatures being largely different from the bulk
temperature, that is a case frequently encountered in ongoing
processes. This may be ascribed to a fast process taking place locally,
which turns the local system practically to an adiabatic one. Some
examples that come to mind:
1. In sonochemical processes there is nucleation of small bubbles,
which on cavitation develop local temperatures claimed to be up to a
few thousand K. This enables relatively low enegy inputs to promote
interesting chemical processes.
2. Milling may have similar effects, promoting mechanochemical
processes. An explanation of why it is usually impossible to comminute
dry ceramic oxide particles in ordinary mills below the micrometer size
range, is that certain particles are locally heated up beyond sintering
temperatures and cling to other particles in touch with them, reverting
the break up. The sparks one gets on knocking two flint stones with one
another is an expression of a similar phenomenon, where tiny stone
particles get heated up to glowing temperatures.
3. Etc...
All the best,
Jacob
On Monday, Sep 15, 2003, at 23:24 Asia/Jerusalem, Hoyer wrote:
> "'Organic List'" <everybody$##$orglist.net>
>
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Institutes for Applied Research
Ben-Gurion University of the Negev
P. O. Box 653, Beer-Sheva, 84105, Israel
Phone: +972-8-6461271; Fax: +972-8-6472969
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