Chromogenic Materials

Glazing materials that selectively control the spectral aspect of radiation are now commonplace. Low-emittance coatings supress infrared radiation transfer thereby imparting additional thermal insulation. Modified low-emittance coatings can also reject unwanted heat gain due to solar infrared. Additional energy savings result with dynamic control over the spectral characteristics of the glazing. There are a variety of technologies that can produce the desired effect:

Transition Metal Switchable Mirror

The switchable-mirrors technology was developed by Tom Richardson and Jonathan Slack of Berkeley Lab's Environmental Energy Technologies Division. By using transition metals rather than the rare earth metals used in the first metal-hydride switchable mirrors, Richardson and Slack were able to lower the cost and simplify the manufacturing process. Energy performance is improved as well, because the new windows can reflect or transmit both visible and infrared light. Besides windows for offices and homes, possible applications include automobile sunroofs, signs and displays, aircraft windows, and spacecraft. To see the mirror windows in action, click on the image below:



Reflective Hydrides
Liquid Crystals
Suspended Particle Displays

Electrochromics: Considered to be the most suitable chromogenic technology for energy control in buildings, electrochromics are the subject of intensive research. Electrochromic materials undergo a reversible change in optical properties upon injection of light ions. Typically they consist of two electrodes separated by an ion conductor. Transparent conductors form the contacts. We have an active research program focussing on complementary counterelectrodes in lithium-ion systems. Association with the Berkeley Electrochemical Research Center provides a constant flow of new ideas for materials.

Measurement of optical properties is a specialty, and we have an extensive library of optical indices of electrochromic materials. Realistic images generated by Radiance have been used to visualize an electrochromic office and determine light levels under various conditions. DOE also supports an Electrochromics Initiative whose purpose is to accelerate the development of a window product.

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Hydrides: These materials can be classified as electrochromics, but they are different in several ways from conventional oxide electrochromics. Originally deposited as a metal, they can be converted to a partially transparent hydride by injection of hydrogen from the gas or solid phase. Thus, they switch to a reflective state which has several potential advantages in terms of energy performance and durability, Read more about the newly discovered transition-metal hydrides. back to menu

Liquid Crystals: This familiar technology was commercialized for window use and later discontinued. Liquid crystal windows switch quickly from  a transparent state to a diffuse white state.  The primary function is to provide privacy and control glare as a substitute for conventional shading devices. In the diffuse state liquid crystals are primarily forward scattering so there is little control over solar heat gain.   back to menu

Suspended Particle Displays: Like electrochromics, SPDs are a promising energy control technology. They are reported to have a number of advantages over electrochromics. More information is available from Research Frontiers Inc.  back to menu

Photochromics: As the name implies, these materials darken under the direct action of sunlight. They are not considered as versatile as electrochromics becuase they cannot be manually controlled and because optimum energy performance requires consideration of temperature conditions as well as solar radiation. For example, a photochromic window may darken on a cold sunny day when more solar heat gain is desireable. They are used widely for automatically darkening sunglassesback to menu

Thermotropics: As photochromics respond primarily to light, thermotropics respond to heat. Again this is not as versatile a response as electrochromics. Daylight or view may have a higher priority for the occupant, at least temporarily, than reduction in solar gain.
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For more information about this subject contact Tom Richardson