Re: [ATM] Why not metal mirrors? (was Beryllium)

[Mark Holm <mdholm@telerama.com>]

Advantages of, some, metals:

stiffer
lighter (some metals) (has a lot to do with stiffness)  Really glass 
isn't all that dense.
higher thermal conductivity
easier to machine

Disadvantages of metals:

Polycrystalline  This is a big problem.  It is hard to get a really good 
smooth polish on a polycrystalline surface.  Gold is an exception.  It 
smooths right out.

Hard to get isotropic, especially in large sizes.  This has to do with 
crystallizing taking preferred orientations as the melt cools.  Some 
times I have seen this directly by visual inspection of a cast part.  
Didn't even require any sample preparation.  The crystal "grain" was 
directly visible on the cast surface.

Difficult to renew reflectivity without repolishing and then 
refiguring.  A pretty obvious drawback.  The reason is chemistry.  If 
you evaporate on a metal coating, it is hard to remove it without 
damaging the substrate.  Much easier when the substrate is fairly inert 
like glass.  If you don't coat, then you are forced to repolish when the 
original surface tarnishes.

High thermal expansion.  Much higher than glass.  Partly offset by 
higher thermal conductivity.

Relatively high chemical reactivity.  Glass is pretty darned inert.  
Most useful mirror metals are not.  The light weight ones, magnesium and 
aluminum are highly reactive.  Aluminum only stays good because of the 
oxide layer that forms on it.  I don't know what protects magnesium.  If 
it is pure, I think it goes quickly.  Something in the alloys protects it.

Poor reflectivity.  Aluminum is about the best in this regard except in 
the infrared gold is very good.  A lot of infrared mirrors are gold 
coated.  Lasts a long time!

(Alloys) Hard to get uniform composition in large castings.  Thermal 
gradients tend to lead to concentration gradients in a large casting.  
Advancing crystallization front pushes impurities and non-equilibrium 
alloying elements ahead of the front.

Mark Holm
mdholm@telerama.com

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