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Transparent Concrete
rakerman writes: "The Economist reports in How to see through walls that development is underway on translucent concrete, with hopes of eventually developing transparent concrete. Can transparent aluminium be far behind?"
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Well, it's nice to see innovation within the construction sector isn't dead. Even for something that seems so off the wall as transparent (or currently, translucent) concreat can give birth to innovative new designs and possibilities from architechs.
I mean, I can just see a wall done with a bubble effect (with slighly differnt opacities in the aggitates and clear binding coumpound).
Only thing is, once transparent concreate is perfected... how are the mobsers going to get rid of bodies if they can't throw them in the foundation of a new building anymore...
Can transparent aluminium be far behind?
It's already here, although in the form of an oxide rather than the pure metal.
There is a very big difference between "transparent" and "translucent". The former means that light passes through the material almost completely unchanged (a certain amount of distortion is okay, but the point is that you can make out what's behind it). Translucent means that light is transmitted, but it's diffuse and you can't make out what's behind the material. This concrete is translucent. It's not transparent (read the article).
The reason you will never see transparent aluminum is not because of a lack of crystalline structure--in fact, I think metals generally are crystalline or at least have a crystalline microstructure. The reason that aluminum, and basically all metals, are opaque is the same reason that metals tend to be shiny. Because there are a lot of free electrons in metals (which is why they conduct electricity well), the electric field of light expends energy driving these free electrons (therefore metals are opaque), which in turn reradiate light back in the direction of the incident light (therefore metals are shiny). The amount of light that gets through goes as e^-ax where a is a constant and x is the thickness of the metal, so in a very thin metal film (e.g. mirrored sunglasses) you can still get some light through, but for any measurable thickness of metal (e.g. aluminum foil and anything thicker), the amount of transmitted light is negligible.
I know this is a very hand-wavy explanation, but it's hard to explain without a pretty advanced background in electromagnetics. If you want an explanation of this from a rigorous electromagnetic point of view you can try wading through Chapter 14 of Principles of Optics by Max Born and Emil Wolf, but its mostly math with very little physical intuition or explanation.
"It take 9 months to bear a child, no matter how many women you assign to the job."
It's a shame that electrochromic windows haven't taken off. I first read about them in Popular Science, probably about 10-15 years ago, and if I recall correctly, they were used in a concept car by Ford (I could be mixing two Popular Science articles together), but they allow you to electrically darken and lighten windows, and they actually reflect light and heat (unlike liquid crystals, which just scatter light and heat but still let them through). I'm not sure, but they might also be wavelength-independent, i.e. reflecting all colors of light equally. The obvious barriers to their widespread adoption are probably cost and the ability to make panes large enough to use as windows.
"It take 9 months to bear a child, no matter how many women you assign to the job."
I'll try to re-state Hal-9001's post in a little different form:
Electromagnetic waves consist of oscillating electric and magnetic fields in alignment so as to be self-perpetuating. The changing magnetic field creates an electric field a little further on, and the changing electric field creates a magnetic field still further on, etc.
First consider a radar beam approaching a metal surface. The E-field will cause the free electrons in the metal to move. This transfers the energy of the beam into electron motion. And with several pages of math that I went through once and never want to again, it can be shown that the electrons move so as to create a mirror-image field, re-transmitting the beam at the angle of incidence -- in other words, a reflection.
Due to resistance to electron movement, the reflected beam will be somewhat weaker, the missing energy being absorbed as heat. If the metal is extremely thin there might not be enough free electrons to fully absorb the incident beam, so part of it passes through. In an insulating material, electrons are tightly bound to molecules, and so cannot range far enough for strong interactions with the beam, and so most of the beam will pass through (the material is "transparent" to radar). However, electrons can shift around within the molecules, which causes refraction, partial reflections, and absorption.
Things are different for x-rays, because the individual photons are pretty energetic and the wavelength (size of one photon) is close to the size of an atom. So it's more likely to be the inner electrons still bound to the atoms that wind up trying to capture the x-ray, and only rarely does this succeed -- most of the x-rays get through several inches of all but the densest materials.
Visible light photons are in-between in size, large enough to interact well with the free electrons (reflection), but small enough to also be affected by bound electrons. (Selective absorption by the bound electrons gives copper and gold their color.)
Most insulators are not transparent to visible light, except as very thin films. Most insulators (like metals) consist of irregular aggregations of tiny crystals. The interactions with the electrons bound in molecules will reflect some light, absorb some, and refract all the rest. In most insulators, the interaction varies with the polarization of the photon and the angle of the crystal; since each crystal is oriented differently, each interface between crystals refracts and reflects light in different directions, so the light that isn't reflected from the external surface is scattered and (mostly) bounces around inside the material until absorbed rather than passing through.
Most transparent materials are glasses, with no crystal structure, and so no grain boundaries to scatter the light. Single crystals may also be transparent, although it's pretty hard to grow a single crystal as big as a windowpane. Multi-crystalline insulators can be translucent if sufficiently free of the atoms or molecules that absorb light, that is if the light is scattered but not absorbed eventually it will find it's way back out of the material. Concrete could be translucent if both the aggregate and the cement were free of light-absorbing materials, but I think the price would be extremely high.
Possibly a multi-crystal insulator could be transparent if the refractive index did not depend on orientation of the crystal or polarization of the light, and if all the crystals fit together neatly and had the same refractive index. Or use glass beads for aggregate and somehow make the cement match the glass?
Metals by definition have free electrons, which strongly reflect and absorb visible light. If it's transparent, it's not a metal.
You can form Al2O3 into fairly large crystals, and maybe it could be a glass too. It's stronger and much harder than silica-based glass, so it would make a great windshield, if you didn't mind the cost of using diamonds for cutting and polishing.
modern windows...have a higher R factor than a lot of walls
Not bloody likely. Even triple pane windows aren't much more than R-3 or R-4, even if you add in Low-E and all that, you don't get much more. 6" walls (USA) easily get R-38 with insulation. Maybe if you had simple plank walls your windows would be higher.
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