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Transition Metal Switchable Mirrors



  2004 R & D 100 Winner
  • Commerical and residential building windows
  • Auto and airplane window glass
  • Aerospace applications


  • Can save up to 25% of the energy lost through building windows
  • Reflects infrared and visible light
  • Switches between reflective and transparent states in seconds
  • Significantly less expensive than electrochromic mirrors made with rare earth metals


Berkeley Lab's Transition Metal Switchable Mirrors (TMSMs) utilize a thin film made from an alloy of magnesium and one or more transition metals. This film enables the glass to which it is bonded to be reversibly converted between reflecting and transparent states either by applying an electrical current or exposing it to hydrogen gas. A window using this technology can be programmed to respond to local sunlight and weather conditions, optimizing the amount of daylight entering a building, while controlling the gain or loss of infrared radiation or heat. When the sun is bright, TMSMs switch to a highly reflective state; in lower-light, the window can be switched to a partially-reflective state. This new technology could save a fourth or more of the energy lost through windows, currently about $33 billion a year. (continued below the image)


Images of Berkeley Lab's Transition Metal Switchable Mirror as it transforms from the reflective state (left) to transparent state (right)

The use of transition-metals instead of rare earth metals could significantly lower the cost of these windows. The transition metal mirrors are less expensive and easier to manufacture, as they use fewer and thinner coatings. Berkeley Lab's technology performs better than absorbing electrochromic (AE) windows in a number of ways. Their greater dynamic range, both in transmission — from 50% to 0.5% or lower — and in reflection — from 75% to 10% — gives them considerable advantages over AEs in providing user comfort and energy savings. While current electrochromic window materials can darken a window, they have little effect on infrared radiation, which accounts for almost half of incident energy. TMSMs reflect both infrared and visible light, providing much better energy conservation.

Applications include windows for offices, homes, autos and airplanes; spacecraft components; and optical switching elements. The technology can also be used in helmets for pilots and motorcyclists. TMSMs offer increased safety by controlling glare. They also reduce a vehicle's heat intake, enabling reductions in the size and weight of air conditioning units, which in turn reduces fuel use.

The Berkeley Lab films are expected to be more resistant to oxidation than the rare-earth based films. This characteristic would reduce the need for a full area palladium (Pd) oxidation-resistant coating which has the disadvantage of reducing transparency.

"Low-e" windows, the first generation of energy-efficient windows, were developed by Berkeley Lab and its commercial partners during the 1980s.

A video demonstration on this technology is available here (QuickTime required).


  • U.S. Patent #6,647,166. Available for licensing or collaborative research


"Electrochromism in Copper Oxide Thin Films," T.J. Richardson, J.L. Slack, M.D. Rubin, Electrochimica Acta, 46, 2281-2284 (2001)

“Switchable Mirrors Based on Nickel-Magnesium Films,” T. J. Richardson, J. L. Slack, R. D. Armitage, R. Kostecki, B. Farangis, and M. D. Rubin, Appl. Phys. Lett., 78, 3047 (2001).

"Mixed Metal Films with Switchable Optical Properties," T. J. Richardson, J. L Slack, B. Farangis, and M.D. Rubin, Appl. Phys. Lett., 80 (8), 1349-1351 (Feb. 2002)

"Calculation of Thermodynamic, Electronic, and Optical Properties of Monoclinic Mg2NiH4," W. R. Myers, .L-W. Wang, T. J. Richardson, and M. D. Rubin, J. Appl. Phys. 91, 4879 (2002)

"New electrochromic mirror systems," Thomas J. Richardson, Solid State Ionics, 165 (2003), 305– 308

"Sb–Cu–Li electrochromic mirrors,"Gao Liu, Thomas J. Richardson, Solar Energy Materials & Solar Cells, 86 (2005), 113–121

"Metal hydride switchable mirrors: Factors influencing dynamic range and stability," Jonathan L. Slack, James C.W. Locke, Seung-Wan Song, Jason Ona, Thomas J. Richardson, Solar Energy Materials & Solar Cells, 90 (2006), 485–490




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