The Difference Between an Alloy and an Amalgam - It's Thermoelectric

The following statement was made by Professor Xu Wang of the 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:

http://book.boot.users.btopenworld.com/setting.htm

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:

http://www.ecgf.uakron.edu/~pan/wang/

Remember, dental amalgam is not a single metal. And it is not even a single alloy. It is an inhomogeneous mixture of dissimilar mixtures of metals. You would expect its thermoelectric behavior to correspond accordingly. (Well I would anyway.)

Keith Walsh