University of Akron in response to a question regarding the
relationship between the thermoelectric properties and the
electromagnetic behavior of metal amalgam dental fillings.
"Theoretically electromagnetic field will be generated by thermal
gradient. But the thermoelectric coupling parameter in most metals is
very low (0.001-0.01). So I don't think the induced electromagnetic
field is significant enough to influence the neurological tissue
nearby."
Amalgams, including dental amalgams, are not like most other metals in
at least one crucial respect; they have a much greater degree of
material inhomogeneity.
This is true when compared either with pure metals, such as copper,
silver, etc., or with true alloys such as brass.
The explanation for the difference in the material homogeneities of
amalgams and true alloys lies in the difference between the methods by
which the two types of material are formed.
When a true alloy is formed, the component metals are mixed together
at a temperature which is greater than the melting point of all of
them. Then, after having been mixed thoroughly in its fully liquid
state, the mixture is allowed to solidify by cooling at a controlled
rate.
By contrast, in an amalgamation process, bits of solid metal, which
may themselves be of either pure metal or an alloy, are mixed together
with a liquid metal at a temperature which is BELOW THE MELTING POINT
of the solid component(s). (And in the case of dental amalgam, where
mercury is used as the liquid metal amalgamating agent, this process
is normally performed at room temperature.)
In the setting process of such an amalgam, the liquid mercury becomes
part of the solid material not as a result of any subsequent reduction
in temperature, but by joining in solid solution with the outer layer
of the solid particles of metal with which it was mixed. But of
course, not all of the volume of the solid particles is involved in
this process and, as a result, the microstructure of the resulting
solid amalgam is as depicted in the schematic diagram at:
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In this diagram the lumps of "unreacted alloy" (denoted "gamma") are
the cores of the original grains of solid silver-tin alloy which have
not mixed with any of the mercury during the amalgamation process.
At this scale it is not possible to show the spatial relationship
between the atoms of silver and the atoms of tin in these alloy
"cores". The alloy has too great a degree of homogeneity for this to
be done.
However, the relative inhomogeneity of the "amalgam" is clearly
depicted by the sizes of the unreacted alloy cores. (These being held
together by a solid matrix of a dissimilar mixture of metals (denoted
"gamma-1") which does have mercury in it, and which may be presumed
therefore to have dissimilar physical properies.)
Now, I have it on good authority from Professor David B Mahler of The
Oregon Health & Science University School of Dentistry that the median
size of the "unreacted" grains of original solid alloy in dental
amalgams is in the order of 30 microns. Scientists with experience of
electrical phenomena at the nano scale might provide some testimony to
the significance of this figure.
The question which remains unanswered is this; whilst it may be
appropriate to quote a "coupling parameter" for the local
electromagnetic effect arising purely from temperature difference in a
homogeneous metallic material, such as a pure metal or an alloy of
metals (for example the type of alloy which may be mixed with liquid
mercury to form an amalgam), is it not the case that the dominant (and
potentially much larger) electromagnetic effect arising as a result of
temperature differences in a more inhomogeneous mixture of dissimilar
metals, such as an amalgam, is more likely to be that caused by the
establishment of thermoelectric eddy currents which would be necessary
for maintaining physical equilibrium against temperature gradient in
such an inhomogeneous medium?
Can anyone think of any experimental procedure which might be employed
in order to demonstrate the case either way?
And is it possible that Professor Wang was failing to take into
account the degree of inhomogeneity of metal amalgams, which is much
greater than "most metals", when estimating the size of the
electromegnetic disturbance produced by thermoelectric effects in
dental amalgams?
Professor Wang's home page is at:
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Keith Walsh
PS, Any dentists out there who don't know anything about the
thermoelectric behavior of dental amalgams can always quote professor
Wang's "coupling parameter" statement if pressed. But I would strongly
advise you to make sure that he knows what he's talking about first.