Electrochromic Materials

Electrochromic materials undergo pigment changes and color shifts by redox reactions in an electrochemical cell. By applying a small voltage and current, these cells can switch from transparent to pigmented, for example white or black. There is a wide range of materials and behaviors.

Mass produced examples include car mirrors and window light shutters for thermal control and privacy. The 787 Dreamliner passenger windows are electrochromically adjustable from clear to dark blue.

White to Clear

Adjusting reflectivity changes light pressure. Changing from transparent to a perfect Lambertian white reflector changes the on-axis light pressure from 0 to ( 1 + 1/π ) ( P/c ). Off axis sideways thrust is proportional to sin( 2 θ ) , with a contribution from incident light but none from the reflective light.

Black to Clear over Reflector

If the area just behind the electrochromic material is a shiny reflector (similar to electronic car mirrors), we can switch from blackish near-zero-albedo to a mirror. Tilting the mirror by provides sideways thrust. Sun-direction light pressure at black will be somewhat less than P/c; the electrochromic material may remain transparent in some bands. With a clear window exposing the mirror underneath, sun-direction thrust is < 2(P/c)cos(Θ)cos(Θ) = 2(P/c)cos²(Θ) and the sideways thrust is 2(P/c)cos(Θ)sin(0) = (P/c)sin(2Θ).

If the substrate is reflective/conductive - say aluminum and molybdenum over an insulator - this permits two important things:

Switchable Mirror Transition Metal Hydrides

Transition metal electrochromics can shift all the way from transparent to a mirror. The light pressure is 50% higher, and the off-axis sideways thrust from the mirror doubles.

Here is an Lawrence Berkeley Lab video of a transition metal electrochromic window in action. I do not know how close this material is to deployment, but more traditional electrochromics are still usable for server sky.

These are more speculative - I haven't seen much work over the past few years, and some early switchable mirrors seem to be switched by hydrogen gas. That is way too complicated for thinsats, and hydrogen will leak through the very thin coverglass.

Further, switching all the way to transparent precludes the use thruster area for radio antennas, and clear thrusters are less useful for high speed spin reduction.

Transparent Electrodes

An expensive component of electrochromic devices (and solar cells) is the Indium Tin Oxide (ITO) transparent electrode. Billions of square meters of electrode can use millions of tons of indium; it may be possible to use lensing and strips to slightly focus the light away from narrow, opaque metal electrodes, allowing ultrathin, high sheet resistance ITO electrodes, which conduct laterally only a few micrometers. While this would be unacceptable for human-visible displays and windows, it would benefit solar cells and light shutters, especially combined with reflection-reducing surface treatments. The solar light source is collimated to half a degree, and we can take advantage of this.

We may also find other transparent electrode materials, such as Aluminum Zinc Oxide (AZO) , which are earth (and probably lunar ) abundant. On earth, aluminum is 40,000 times more abundant than tin, and zinc is 1500 times more abundant than indium. On the moon, zinc will not be hydrologically beneficiated into ore bodies like it is on earth - it may turn out that asteroidal indium may be easier to extract. Most likely, for the next century, we will continue to ship zinc from earth, even as we extract many other thinsat materials from lunar regolith.

Notes from Electrochromism and Electrochromic Devices

Monk, Mortimer, and Rosseinsky, Cambridge University Press, 2007

The 1987 Donnelly Corp Lynam-Seah Solid State Mirror Patent

This is a solid state rear view mirror patent. There are probably better systems out there in 2013, but the patent has expired and this shows one way to do it.

The mirror stack: