Transparent Metals 2

Recently I’ve been reading more about anharmonic properties of crystals, for instance metals. There are phenomena, such as thermal expansion, or electrical conductivity (resistivity), that appear to primarily be related to imperfections in the crystal structure, or to non-harmonic behaviour of the material. A material with a purely harmonic response should have no resistance, and should have no dimensional change when heated or cooled. Instead of thinking of the flow of electrons through a material, one might think of a wave function for the distributed gas of electrons.

This is somewhat prompted by a reading of Blakemore’s book on solid state, mentioned in my post about a year ago (April 20th, 2014) on the topic of transparent metals. I had occasion to return to Blakemore’s book again because of a new investigation. And he is as stimulating as ever!

So here’s the basic idea: Instead of thinking about the flow of electrons (as particles), think about propagation of a wave. That leads one to thinking about optics, and indexes of refraction, and the possibility of focusing etc. For instance, it is standard to talk about the speed with which electrons flow through a metal, and compare different models in terms of the wildly different speeds they predict. Do the three noble metals — copper, silver and gold — have the same speeds of electron flow? Otherwise said, would they have the same index of refraction? Can we make a copper-silver-gold alloy that can act as a lens or a waveguide?

Conversely, one can think of traditional wave models, and consider them in terms of particle flow models, eg the resistance (or conductivity) of glass.

Just some late-night idle musing. Maybe something will intrigue you.

Best wishes,
Ken Roberts
14-Mar-2015

Transparent Metals

Can metals be transparent at certain wavelengths? My first reaction is No Way! It’s a myth, sort of like the transparent aluminum which is mentioned in the Star Trek whale movie. But then … reading the book Solid State Physics by J. S. Blakemore (2nd revised edition of 1985, published by Cambridge), I noticed in the table on page 24, under the list of properties of solids with various bonding types, the entry for metallic bonding type, this: “opaque and highly reflective in infrared and visible light; transparent in U.V.”. The three example materials listed for that row of the table are Sodium, Silver, and Nickel. So I have to suppose there must be some frequencies at least, at which at least one of those metals is transparent.

Now I’m really curious. A preliminary search for “transparent silver ultraviolet” turns up an article, at the URL http://www.telfor.rs/telfor2004/radovi/PEL-9-9.PDF which describes silver-dielectric crystals which allow some wavelengths through. An example application of such frequency-dependent transparency or opacity is a material which might be used as a microwave oven window, which would be transparent to visible light but opaque to microwaves. OK. My horizons have just expanded. A new concept, and some application possibilities.

Consider, for instance, what might be done with solar cells. Capture long wavelength radiation (greenhouse effect in micro) in an enclosure, achieve high temperature, and black body distribution provides shorter wavelength photons to kick photoelectrons off a material. Thermoelectric applications too. This is really neat! Got to do some reading.

I still don’t know about transparent pure metals, whether silver or aluminum or whatever. But at least it’s not an automatic dismissal of the concept anymore.

Best wishes,
Ken Roberts
20-Apr-2014