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eutectics - simple???

updated thu 9 jul 98


Laura Conley on wed 8 jul 98

Eutectic - when one mixes several ceramic oxides together that individually
melt at high temperatures (3000F, 3500F, 2800F for example) they now melt at
much lower temperatures (such as 1800F, 2200F). The ratio of these oxides
that causes the lowest temperature melt is called the Eutectic. Every group
of oxides has a different eutectic.


To complicate this, a particular group of oxides can actually have several
eutectics, one being (what I would call) the primary eutectic and the others
being secondary. The primary one would have the lowest melting temp. and
others would also be eutectics, but would be higher temps. For anyone that
took calculus, the eutectics are minima of the function. For the rest of
us, imagine the electrocardiogram from tv shows, or the printout of an
earthquake-sensing machine, any wiggly line that has sharp dips. The bottom
point of the largest dip is the primary eutectic (the exact ratio at that
point). The wiggly line goes upward to the left and the right of that point
because by changing the ratio slightly the melting temperature increases.
Any other dips that do come to a point, but not as low a point, would be
secondary eutectics.

A chemical example, extremely simplified, would be if you took sodium and
silica and mixed them:

100 parts silica to 1 part sodium might melt at (to simplify) say 100F
100 parts silica to 5 parts sodium might melt at 90F
100 parts silica to 25 parts sodium might melt at 60F
100 Si to 26 Na might melt at 55F***********
100 Si to 27 Na might melt at 60F
100 Si to 30 Na might melt at 62F
100 Si to 50 Na might melt at 70F
100 Si to 100 Na might melt at 85F
100 Si to 101 Na might melt at 80F*****
100 Si to 102 Na might melt at 85F
100 Si to 150 Na might melt at 90F
100 Si to 200 Na might melt at 95F

55F at a ratio of 100:26 would be the primary eutectic
80F at a ratio of 100:101 would be a secondary eutectic - there are other
ratios that melt at lower temps than this one (such as 100:50), but because
if you adjust the ratio either direction of 100:101, the temp goes up, this
is a eutectic.

Laura Conley
Boulder, CO

carrie jacobson wrote:

> ----------------------------Original message----------------------------
> Hi all. For those of us who are novices, could someone provide a simple
> explanation of eutetics?
> Thanks, Carrie
> Carrie Jacobson
> Pawcatuck, CT