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ABSTRACT:
Switchable
mirrors are a new generation of electrochromic windows that
can alternate between a reflecting state and a transparent
or absorbing state when a small voltage is applied. These
energy saving devices have advantages over traditional absorbing
electrochromics for radiant energy control because of their
large dynamic range in both transmission and reflection for
visible light and infrared radiation.
Applications
for the new Berkeley Lab electrochromic materials described
below include architectural glass, sunroofs, rear view mirrors,
aerospace insolation controls, optical switching elements,
and thin film displays.
Thomas Richardson of Berkeley Lab has developed a switchable
mirror system based on lithium ion insertion that is inherently
more stable than systems based on metal hydrides. Hydride
systems suffer from poor life cycle due to the highly reactive
nature of the active metal film which is easily and irreversibly
oxidized by air or water. Because the Berkeley Lab device
is sealed and the component electrode materials are stable
with respect to the non-aqueous electrolyte, irreversible
degradation of the mirror film is avoided. The devices can
be made using less expensive fabrication methods and do not
require the protective palladium layer needed in hydride mirrors.
A typical device consists of a thin reflective film of active
material such as antimony applied to a glass substrate. The
electrolyte may be a solid, liquid, or polymer. A counter
electrode supplies positive lithium ions and may be transparent,
opaque, reflecting, or electrochromic, depending on the application.
When a potential is applied, electrons flow into the active
metallic layer. At the same time, lithium ions are taken up
by the metal to form a new transparent compound. The transition
may be stopped or reversed at any point to achieve a desired
level of transparency or reflectivity. Infrared radiation
is modulated even more effectively than visible light, raising
the prospect of improved energy efficiency.
A video demonstration on the technology is available here (QuickTime required). |
