Ivor and Olive Lewis on wed 14 feb 07
phase diagrams and the melt space--another reply to Dan
Dear Friends,
Defining "Eutectic Point" depends to a great extent on the perspective =
from which you approach the task. It depends on Context. One extreme is =
the precise scientific way of speaking of things. At the other end is a =
rough and ready idea of melting or fusing with a great degree of ease. =
In between is a practical appraisal of telling us about something =
useful.
Singer and Singer distinguish between the former and the latter with =
statements that seem to be ignored by recent authors and use terms that =
discriminate between the two.
The Reference is Singer and Singer, "Industrial Ceramics" p 220
Quote (sic):
Deformation-Eutectic : That composition within a system that develops a =
sufficient amount of liquid to cause deformation at the lowest =
temperature.
Equilibrium-Eutectic : The composition and temperature of a melt in a =
system of n components, in equilibrium with n crystal phases, and on =
cooling the system there is a positive crystallisation in n crystal =
phases without changing the composition and temperature of any of these =
phases.
My comment
If you read the Equilibrium -Eutectic definition with care you will see =
an editorial error. The term "cooling" is used, implying a diminishing =
temperature. This is incorrect. It is a loss of energy at constant =
temperature, a Thermodynamic Reaction. Liquid changes to solid with the =
loss of energy, solids change to liquid as energy increases. Temperature =
in the system never alters until one phase disappears.
There is also an Exclusion Principle involved which has been disregarded =
in modern commentary. Two or more Eutectic Compositions cannot be =
compounded from the same set of Compounds (Even though those compounds =
contain ingredients from the same set of molecular oxides).
Taking Dave's example of two eutectic points from the K2O-Al2O3-SiO2 =
System. The eutectic point at 710 deg C must be compounded from Quartz =
(SiO2), Potassium Tetra-Silicate (K2O.4SiO2) and Potash felspar =
(K2O.Al2O3.SiO2). The other, at 695 deg C is compounded from Potassium =
Tetra-Silicate (K2O.4SiO2), Potassium Di-Silicate (K2O.2SiO2) and Potash =
Felspar (K2O.Al2O3.SiO2). The differences are slight in terms of Oxides, =
about 1% Al2O3, 8% Na2O and 7% SiO2. It is unfortunate that we cannot =
test this system but the example given by Michael Cardew is =
accessible for testing.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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