U.S. patent application number 10/256841 was filed with the patent office on 2004-04-01 for optimum switching of a reversible electrochemical mirror device.
This patent application is currently assigned to Innovative Technology Licensing, LLC. Invention is credited to Rowell, Petra V., Tench, D. Morgan.
Application Number | 20040061919 10/256841 |
Document ID | / |
Family ID | 32029373 |
Filed Date | 2004-04-01 |
United States Patent
Application |
20040061919 |
Kind Code |
A1 |
Tench, D. Morgan ; et
al. |
April 1, 2004 |
OPTIMUM SWITCHING OF A REVERSIBLE ELECTROCHEMICAL MIRROR DEVICE
Abstract
Reversible electrochemical mirror (REM) devices typically
comprise a conductive oxide mirror electrode that is substantially
transparent to radiation of some wavelengths, a counter electrode
that may also be substantially transparent, and an electrolyte that
contains ions of an electrodepositable metal. A voltage applied
between the two electrodes causes electrodeposition of a mirror
deposit on the mirror electrode and dissolution of the mirror
deposit on the counter electrode, and these processes are reversed
when the polarity of the applied voltage is changed. Such REM
devices provide precise control over the reflection and
transmission of radiation and can be used for a variety of
applications, including smart windows and automatically adjusting
automotive mirrors. According to the present invention, REM mirror
uniformity is improved with minimal sacrifice in switching speed by
utilizing a lower drive voltage when the mirror electrode sheet
resistance is high, and a higher drive voltage when sufficient
mirror metal is present to appreciably reduce the sheet resistance.
Faster switching without damage to the electrode or decomposition
of the electrolyte is provided by adjusting the applied voltage by
the resistive loss in the electrolyte. Optimum results are provided
by adjusting the drive voltages for the mirror metal plating and
erasure processes based on real-time measurements of cell current,
mirror electrode sheet resistance and temperature. Such sheet
resistance measurements can also be used to monitor and control the
reflectance of the device.
Inventors: |
Tench, D. Morgan;
(Camarillo, CA) ; Rowell, Petra V.; (Newbury Park,
CA) |
Correspondence
Address: |
John J. Deinken
ROCKWELL SCIENTIFIC COMPANY LLC
P.O. Box 1085, Mail Code A15
Thousand Oaks
CA
91358-0085
US
|
Assignee: |
Innovative Technology Licensing,
LLC
|
Family ID: |
32029373 |
Appl. No.: |
10/256841 |
Filed: |
September 27, 2002 |
Current U.S.
Class: |
359/265 |
Current CPC
Class: |
G02F 1/1506
20130101 |
Class at
Publication: |
359/265 |
International
Class: |
G02F 001/15 |
Claims
We claim:
1. A method for optimizing the mirror uniformity and switching
speed of a reversible electrochemical mirror of the type including
a mirror electrode, a counter electrode, and an electrolyte
disposed between and in electrical contact with the mirror and
counter electrodes, wherein the electrolyte contains cations of an
electrodepositable mirror metal, comprising the steps of: applying
a first negative voltage to the mirror electrode relative to the
counter electrode so that mirror metal is deposited onto the mirror
electrode at a first rate; and applying a second negative voltage
more negative than the first negative voltage to the mirror
electrode relative to the counter electrode so that additional
mirror metal is deposited onto the mirror electrode at a second
rate which is faster than the first rate, wherein the second
negative voltage does not exceed a predetermined maximum drive
voltage.
2. The method of claim 1, wherein the second negative voltage is
increased with time so that the rate at which additional mirror
metal is deposited on the mirror electrode increases with time.
3. The method of claim 2, wherein the second negative voltage is
increased with time continuously.
4. The method of claim 2, wherein the second negative voltage is
increased with time in steps.
5. The method of claim 1, wherein the second negative voltage is
increased based on the amount of charge passed between the mirror
electrode and the counter electrode.
6. The method of claim 1, wherein the predetermined maximum drive
voltage corresponds to a current that is less than the
diffusion-limited current for electrodeposition of the mirror
metal.
7. The method of claim 1, further comprising the steps of:
measuring the sheet resistance between two locations on the mirror
electrode; and measuring the current flowing between the mirror
electrode and the counter electrode, wherein the second negative
voltage is such that the multiplication product of the measured
sheet resistance and the measured current is less than a
predetermined maximum sheet IR drop.
8. The method of claim 7, wherein said step of measuring the sheet
resistance comprises the steps of applying an alternating voltage
between two electrical contacts on the mirror electrode and
measuring an alternating current response to the alternating
voltage.
9. The method of claim 7, wherein said step of measuring the sheet
resistance comprises the steps of applying a direct voltage between
two electrical contacts on the mirror electrode and measuring a
direct current response to the direct voltage.
10. The method of claim 7, further comprising the step of:
determining the electrical resistance of the electrolyte between
the mirror electrode and the counter electrode, wherein the first
negative voltage is a substantially safe voltage with respect to
damage to the mirror electrode and breakdown of the electrolyte
when no mirror metal is present on the mirror electrode, and
wherein the magnitude of the second negative voltage is the smaller
of: the predetermined maximum drive voltage; the sum of the safe
voltage and the electrolyte voltage drop, the latter being equal to
the multiplication product of the measured current and the measured
electrolyte resistance; and that which will cause the
multiplication product of the measured sheet resistance and the
measured current to be less than the predetermined maximum sheet IR
drop.
11. The method of claim 10, further comprising the steps of:
measuring the temperature of the mirror; and adjusting the second
negative voltage to account for the temperature dependence of the
electrolyte resistance.
12. The method of claim 10, further comprising the steps of:
measuring the temperature of the mirror; and adjusting the second
negative voltage to account for the temperature dependence of the
mirror electrode sheet resistance.
13. A method for optimizing the mirror uniformity and switching
speed of a reversible electrochemical mirror of the type including
a mirror electrode, a counter electrode, and an electrolyte
disposed between and in electrical contact with the mirror and
counter electrodes, wherein the electrolyte contains cations of an
electrodepositable mirror metal, comprising the steps of: applying
a first positive voltage to the mirror electrode relative to the
counter electrode so that mirror metal is dissolved from the mirror
electrode at a first rate; and applying a second positive voltage
less positive than the first positive voltage to the mirror
electrode relative to the counter electrode so that additional
mirror metal is dissolved from the mirror electrode at a second
rate which is slower than the first rate, wherein the first
positive voltage does not exceed a predetermined maximum drive
voltage.
14. The method of claim 13, wherein the second positive voltage is
decreased with time so that the rate at which mirror metal is
dissolved from the mirror electrode decreases with time.
15. The method of claim 14, wherein the second positive voltage is
decreased with time continuously.
16. The method of claim 14, wherein the second positive voltage is
decreased with time in steps.
17. The method of claim 13, wherein the second positive voltage is
decreased based on the amount of charge passed between the mirror
electrode and the counter electrode.
18. The method of claim 13, wherein the predetermined maximum drive
voltage corresponds to a current that is less than the
diffusion-limited current for dissolution of the mirror metal.
19. The method of claim 13, further comprising the steps of:
measuring the sheet resistance between two locations on the mirror
electrode; and measuring the current flowing between the mirror
electrode and the counter electrode, wherein the second positive
voltage is such that the multiplication product of the measured
sheet resistance and the measured current is less than a
predetermined maximum sheet IR drop.
20. The method of claim 19, wherein said step of measuring the
sheet resistance comprises the steps of applying an alternating
voltage between two electrical contacts on the mirror electrode and
measuring an alternating current response to the alternating
voltage.
21. The method of claim 19, wherein said step of measuring the
sheet resistance comprises the steps of applying a direct voltage
between two electrical contacts on the mirror electrode and
measuring a direct current response to the direct voltage.
22. The method of claim 19, further comprising the step of:
determining the electrical resistance of the electrolyte between
the mirror electrode and the counter electrode, wherein the first
positive voltage is a substantially safe voltage with respect to
damage to the mirror electrode and breakdown of the electrolyte
when no mirror metal is present on the mirror electrode, and
wherein the magnitude of the second positive voltage is the smaller
of: the predetermined maximum drive voltage; the sum of the safe
voltage and the electrolyte voltage drop, the latter being equal to
the multiplication product of the measured current and the measured
electrolyte resistance; and that which will cause the
multiplication product of the measured sheet resistance and the
measured current to be less than the predetermined maximum sheet IR
drop.
23. The method of claim 22, further comprising the steps of:
measuring the temperature of the mirror; and adjusting the second
positive voltage to account for the temperature dependence of the
electrolyte resistance.
24. The method of claim 22, further comprising the steps of:
measuring the temperature of the mirror; and adjusting the second
positive voltage to account for the temperature dependence of the
mirror electrode sheet resistance.
25. A method for optimizing the mirror uniformity and switching
speed of a reversible electrochemical mirror of the type including
a mirror electrode, a counter electrode, and an electrolyte
disposed between and in electrical contact with the mirror and
counter electrodes, wherein the electrolyte contains cations of an
electrodepositable mirror metal, comprising the steps of: applying
a first negative voltage to the mirror electrode relative to the
counter electrode so that mirror metal is deposited onto the mirror
electrode at a first rate, wherein the first negative voltage is a
substantially safe voltage with respect to damage to the mirror
electrode and breakdown of the electrolyte when no mirror metal is
present on the mirror electrode; measuring the sheet resistance
between two locations on the mirror electrode; measuring the
current flowing between the mirror electrode and the counter
electrode; determining the electrical resistance of the electrolyte
between the mirror electrode and the counter electrode; and
applying a second negative voltage more negative than the first
negative voltage to the mirror electrode relative to the counter
electrode so that additional mirror metal is deposited onto the
mirror electrode at a second rate which is faster than the first
rate, wherein the magnitude of the second negative voltage is the
smaller of: a predetermined maximum drive voltage; the sum of the
safe voltage and the electrolyte voltage drop, the latter being
equal to the multiplication product of the measured current and the
measured electrolyte resistance; and that which will cause the
multiplication product of the measured sheet resistance and the
measured current to be less than a predetermined maximum sheet IR
drop.
26. The method of claim 25, further comprising the steps of:
measuring the temperature of the mirror; and adjusting the second
negative voltage to account for the temperature dependence of the
electrolyte resistance and the temperature dependence of the mirror
electrode sheet resistance.
27. A method for optimizing the mirror uniformity and switching
speed of a reversible electrochemical mirror of the type including
a mirror electrode, a counter electrode, and an electrolyte
disposed between and in electrical contact with the mirror and
counter electrodes, wherein the electrolyte contains cations of an
electrodepositable mirror metal, comprising the steps of: applying
a first positive voltage to the mirror electrode relative to the
counter electrode so that mirror metal is dissolved from the mirror
electrode at a first rate, wherein the first positive voltage does
not exceed a predetermined maximum drive voltage; measuring the
sheet resistance between two locations on the mirror electrode;
measuring the current flowing between the mirror electrode and the
counter electrode; determining the electrical resistance of the
electrolyte between the mirror electrode and the counter electrode;
and applying a second positive voltage less positive than the first
positive voltage to the mirror electrode relative to the counter
electrode so that additional mirror metal is dissolved from the
mirror electrode at a second rate which is slower than the first
rate, wherein the magnitude of the second positive voltage is the
smaller of: the predetermined maximum drive voltage; the sum of a
safe voltage with respect to damage to the mirror electrode and
breakdown of the electrolyte when no mirror metal is present on the
mirror electrode plus the electrolyte voltage drop, the latter
being equal to the multiplication product of the measured current
and the measured electrolyte resistance; and that which will cause
the multiplication product of the measured sheet resistance and the
measured current to be less than the predetermined maximum sheet IR
drop.
28. The method of claim 27, further comprising the steps of:
measuring the temperature of the mirror; and adjusting the second
positive voltage to account for the temperature dependence of the
electrolyte resistance and the temperature dependence of the mirror
electrode sheet resistance.
29. A reversible electrochemical mirror device of the type
including a mirror electrode, a counter electrode, and an
electrolyte disposed between and in electrical contact with the
mirror and counter electrodes, wherein the electrolyte contains
cations of an electrodepositable mirror metal, further comprising:
first and second electrical contacts located on the mirror
electrode, wherein the electrical resistance between said
electrical contacts provides a measure of the amount of mirror
metal on the mirror electrode.
30. The device of claim 29, wherein said electrical contacts are
provided on opposite sides of a rectangular mirror electrode.
31. The device of claim 29, wherein at least one of said electrical
contacts is located at the midpoint of at least one of the opposite
sides that does not contain a contact used to apply the voltage
tending to cause the mirror metal to electrodeposit upon or
dissolve from the mirror electrode.
32. The device of claim 29, wherein at least one of said electrical
contacts is also used to apply the voltage tending to cause the
mirror metal to deposit upon or dissolve from the mirror
electrode.
33. The device of claim 29, wherein said electrical contacts are
not in electrical contact with the electrolyte.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is related to U.S. Pat. Nos. 5,903,382;
5,923,456; 6,111,685; 6,166,847; 6,256,135; 6,301,039; and
6,400,491; and to U.S. patent application Ser. No. 10/066,210,
filed Jan. 31, 2002, all of which are assigned to the assignee of
the present application. The teaching of each of these patents and
patent applications is incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] This invention is concerned with devices, such as adjustable
mirrors, smart windows and optical attenuators, for controlling the
reflectance and/or transmission of electromagnetic radiation.
[0003] Sunlight transmitted through windows in buildings and
transportation vehicles can generate heat (via the greenhouse
effect) that creates an uncomfortable environment and increases air
conditioning requirements and costs. Current approaches to
providing "smart windows" with adjustable transmission for use in
various sunlight conditions involve the use of light absorbing
materials. Such approaches are only partially effective since the
window itself is heated so that heat is transferred into the
interior by convection. In addition, these devices, such as
electrochromic devices, are relatively expensive and exhibit
limited durability and cycle life. Certain liquid crystal-based
window systems switch between transmissive and opaque/scattering
states, but these systems require substantial voltages to maintain
the transparent state. There is an important need for an
inexpensive, durable, low-voltage smart window with variable
reflectivity. Reflecting the light, rather than absorbing it, is
the most efficient means for avoiding inside heating. Devices for
effectively controlling transmission of light are also needed for a
variety of other applications. For example, an effective means for
controlling light transmission over a wide dynamic range is needed
to permit use of inexpensive arc lamps as light sources for
projection displays.
[0004] Bright light from headlamps on following vehicles reflected
in automobile rear and side view mirrors is annoying to drivers and
creates a safety hazard by impairing driver vision. Currently
available automatically dimming mirrors rely on electrochromic
reactions to produce electrolyte species that absorb light that
would otherwise be reflected from a static mirror. Such devices do
not provide close control over the amount of reflected light, and
are expensive to fabricate since a very constant inter-electrode
spacing (i.e., cell gap) is required to provide uniform dimming.
Image sharpness is also reduced for electrochromic mirror devices
since the reflected light must pass through the electrolyte
(twice). There is an important need for an inexpensive adjustable
mirror device that provides close control of reflected light with
minimal image distortion.
[0005] There have been attempts in the prior art to exploit
reversible electrodeposition of a metal for light modulation,
primarily for display applications [see for example, J. Mantell and
S. Zaromb, J. Electrochem. Soc. 109, 992 (1962) and J. P. Ziegler
and B. M. Howard., Solar Eng. Mater. Solar Cells 39, 317, (1995)].
In this work, metal, typically silver or bismuth, was reversibly
electrodeposited onto a transparent working electrode, usually
indium tin oxide (ITO), from a thin layer of electrolyte sandwiched
between the working electrode and a counter electrode. Both water
and organic liquids (e.g., dimethylsulfoxide or dimethylformamide)
were employed as solvents. The deposits obtained on the transparent
electrode presented a rough and black, gray, or sometimes colored
appearance (typical of finely-divided metals) and were used to
enhance light absorption by display elements. Pigments were often
added to the electrolyte to provide a white background for improved
contrast. An auxiliary counter electrode reaction (e.g., halide ion
oxidation) was typically employed to provide a voltage threshold
(which is needed for matrix addressing) and/or to avoid metal
deposition on a transmissive counter electrode (which would offset
the light modulation provided by metal deposition on the working
electrode). Such auxiliary reactions introduced chemistry-related
instabilities during long term operation and led to deposit self
erasure on open circuit via chemical dissolution of the metal
deposit. Nonetheless, the key drawback of reversible metal
electrodeposition for display applications was the relatively slow
response for attaining adequate light blocking.
[0006] A reversible electrochemical mirror (REM) device permitting
efficient and precise control over the reflection/transmission of
visible light and other electromagnetic radiation is described in
U.S. Pat. Nos. 5,903,382, 5,923,456, 6,111,685 and 6,166,847 to
Tench et al. In this device, an electrolyte containing ions of an
electrodepositable metal is sandwiched between a mirror electrode
and a counter electrode, at least one of which is substantially
transparent to the radiation. A typical transparent mirror
electrode is indium tin oxide (ITO) or fluorine doped tin oxide
(FTO) deposited on a transparent glass (or plastic) pane which
serves as the substrate. Application of a voltage causes the
electrodepositable metal, e.g., silver, to be deposited as a mirror
on the mirror electrode while an equal amount of the same metal is
dissolved from the counter electrode. When the voltage polarity is
switched, the overall process is reversed so that the
electrodeposited mirror metal is at least partially dissolved from
the mirror electrode. A thin surface modification layer of noble
metal, e.g., 15-30 .ANG. of platinum, on the transparent conductor
is usually required to improve nucleation so that a mirror deposit
is obtained. The thickness of the mirror metal layer present on the
mirror electrode determines the reflectance of the device for
radiation, which can be varied over a wide range.
[0007] The REM technology can be used to provide control of either
light reflectance or transmission, or both. A transmissive REM
device suitable for smart window applications utilizes a noble
metal counter electrode that is locally distributed, as a grid for
example, on a transparent substrate, e.g., glass or plastic, so
that mirror metal deposited thereon does not appreciably increase
the blockage of light. In this case, high light transmission is
provided by a locally distributed counter electrode of relatively
small cross-sectional area and the device reflectance/transmission
is adjusted via the thickness of mirror metal on the mirror
electrode. As described in U.S. Pat. No. 6,166,847 to Tench et al.,
such a transmissive counter electrode is not required for
reflective REM devices used for adjustable mirror applications. An
electrolytic solution, which provides the inherent stability, high
deposit quality, complete deposit erasure, long cycle life, and
reasonably fast switching needed for most practical applications,
is described in U.S. Pat. No. 6,400,491, to Tench et al. This
solution is typically comprised of 1.5 M AgI and 2.0 M LiBr in a
gamma-butyrolactone (GBL) solvent, and may also contain highly
dispersed silica (HDS) added to produce a gelled electrolyte and/or
dispersed carbon added to blacken the electrolyte so as to reduce
background light reflection.
[0008] A significant problem with both electrochromic and REM
devices is that light modulation at constant applied voltage occurs
more slowly toward the center of the device. This reduced
modulation rate results because the voltage is decreased by the
relatively high sheet resistance of the transparent conductor film,
e.g., indium tin oxide, which is used for at least one of the
electrodes. Such "irising" is most noticeable for low light
modulation states and is unacceptable for many applications. The
iris effect can be mitigated by utilizing lower switching currents,
for which the Ohm's law (IR) voltage drop is less, but at the
sacrifice of switching speed. Switching speed of electrochemical
light modulation devices is also limited by the need to avoid
excessive voltages at the electrode interfaces with the
electrolyte, which can cause decomposition of the electrolyte or
damage to the electrode surfaces. A means for uniformly switching
REM devices at relatively fast rates would greatly increase their
utility and provide an additional advantage compared to
electrochromic devices.
SUMMARY OF THE INVENTION
[0009] The method of the present invention provides uniform
switching at relatively fast rates for reversible electrochemical
mirror (REM) devices, which are comprised of an electrolyte
containing electrodepositable metal ions, e.g., silver ions, in
contact with a mirror electrode and a counter electrode. The
electrolyte may be a liquid or solid electrolytic solution, an
ionic liquid electrolyte, or a solid electrolyte. A stiffening
agent may be added to render a liquid electrolyte more viscous,
semi-solid or solid. The mirror electrode is typically comprised of
a very thin layer of noble metal (e.g., platinum) on a layer of a
transparent conducting oxide (e.g., indium tin oxide) on a glass or
plastic substrate. Generally, the counter electrode is a sheet or
layer of the electrodepositable mirror metal for devices that are
designed to control radiation reflection, and is a locally
distributed electrode for devices that also transmit radiation. The
device reflectance is determined by the thickness of the mirror
metal layer on the mirror electrode, which can be adjusted by
applying a voltage of the appropriate polarity to cause mirror
metal electrodeposition or dissolution, while the reverse process
occurs at the counter electrode. The present invention exploits the
fact that the sheet resistance of the mirror electrode decreases as
the thickness of the deposited mirror metal layer increases. This
sheet resistance decrease is unique to REM devices and provides
another significant advantage compared to normal electrochromic
systems.
[0010] According to the method of the present invention, improved
mirror uniformity with minimal sacrifice in switching speed is
attained for REM devices by utilizing lower drive voltages when the
sheet resistance of the mirror electrode is high, and increasing
the drive voltage when the sheet resistance is reduced by an
appreciable thickness of the mirror metal. Good mirror uniformity
is provided since the resistive voltage drop along the mirror
electrode surface is minimized by the lower currents when little or
no mirror metal is present, and by the low sheet resistance when
the mirror metal thickness is appreciable. The overall switching
time can be short since the current, which is directly related to
the switching rate, can be greatly increased for thick mirror metal
deposits without inducing mirror nonuniformity. The improvement
provided is greatest for reflective-type devices with continuous
metal counter electrodes also having low sheet resistance. However,
the invention is also useful for transmissive-type devices
utilizing counter electrodes that are locally distributed or
located outside the light path.
[0011] Large voltages that would otherwise decompose the
electrolyte or damage the electrode surface can be applied to
increase the switching rate when current is flowing. This is
because the voltage drop associated with the resistance of the
electrolyte does not appear as electrode potential across the
electrode-electrolyte interface. Consequently, the drive voltage
can be increased beyond a safe value for the electrode potential by
the magnitude of the resistive voltage loss (IR drop) in the
electrolyte without detrimental effect. Likewise, the drive voltage
is decreased so as to limit the electrode potential to a safe value
as the current decreases in the later stages of mirror erasure.
Such IR-compensated device switching is another aspect of the
present invention.
[0012] In a preferred approach, the REM device is automatically
switched (via a computer) according to a drive voltage algorithm
based on real-time measurements of the electrode sheet resistance,
device switching current and temperature. A method for measuring
the electrode sheet resistance, which also yields the device
reflectance/transmission, is described in U.S. Pat. No. 6,301,039
to Tench. Typically, the computer memory contains data defining the
device current as a function of voltage and temperature, as well as
the mirror electrode sheet resistance as a function of temperature.
This data can be in the form of equations (and appropriate constant
parameters) since the voltage drop in the electrolyte is typically
much larger than the potential drops at the electrodes so that the
device current varies linearly with the applied voltage (to a good
approximation). Since the reciprocal of the electrolyte resistance
is typically linear with temperature, a simple equation can also be
used to determine appropriate adjustments in the applied voltage to
compensate for changes in the device temperature. As a key feature
of the present invention, the device current, preferably for both
plating and erasure, is limited so that the voltage drop along the
electrode (current.times.sheet resistance) does not exceed a value
chosen to provide the best compromise between mirror uniformity and
switching speed.
[0013] A variety of alternative approaches within the scope of the
present invention will be apparent to those skilled in the art. For
example, the charge passed in electrodepositing mirror metal on the
bare electrode provides a measure of the deposit thickness that
could be used to provide feedback on the mirror electrode sheet
resistance in real time. In principle, the electrode sheet
resistance could be known at any given time via the thickness of
the mirror metal deposit by utilizing a charge integration device
and keeping track of all of the charge passed for metal
electrodeposition and dissolution as the mirror state was cycled.
However, as the mirror was subjected to multiple cycles in which
complete erasure of the mirror metal did not occur, measurement
imprecision and minor efficiency imbalances between the metal
electrodeposition and dissolution reactions would introduce
cumulative errors in the calculated thickness and associated
reflectance. However, this approach could be used with devices for
which the mirror deposit is fully erased on a frequent basis.
[0014] In another embodiment of the present invention, a drive
voltage that varies with time is used and no sheet resistance
feedback is needed. In this case, a relatively small negative
voltage is applied to initiate mirror formation and the voltage is
stepped or ramped to more negative values as the mirror metal is
deposited and the electrode sheet resistance decreases. Likewise, a
relatively large positive voltage is applied to initiate mirror
erasure and the voltage is stepped or ramped to less positive
values as the mirror metal deposit is dissolved and the mirror
electrode sheet resistance increases. Excess applied voltage to
compensate for the electrolyte IR drop could also be used in this
case. This approach is most appropriate with devices for which the
mirror is fully erased on each cycle, as is typically the case for
smart windows.
[0015] Further features and advantages of the invention will be
apparent to those skilled in the art from the following detailed
description, taken together with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a cross sectional view depicting a representative
design of a reversible electrochemical mirror (REM) device.
[0017] FIG. 2 is a schematic representation of a REM mirror
electrode (as viewed from the electrolyte side) illustrating a
suitable arrangement of separate contacts for applying the
switching voltage and measuring the mirror electrode sheet
resistance during mirror state switching.
[0018] FIG. 3 gives plots of the difference in reflectance between
the center and a corner of a rectangular REM reflective device
(8.3.times.8.7 cm mirror) as a function of plating time for mirrors
formed at constant voltage (-0.40 V) and by ramping the voltage
from -0.10 V to a maximum of -0.40 V at 50 mV/s.
[0019] FIG. 4 gives plots of the difference in reflectance between
the center and a corner of the REM reflective device of FIG. 3 as a
function of time for erasure of 66% reflective mirrors (formed by
the voltage ramping procedure of FIG. 3) at constant voltage (+0.40
V) and by ramping the voltage from +0.40 V to a minimum of +0.2 V
at 10 mV/s.
[0020] FIG. 5 gives plots of the mirror electrode sheet resistance
for a REM device as a function of the mirror metal (silver) deposit
thickness measured during mirror plating and erasure (using the
mirror electrode configuration of FIG. 2).
DETAILED DESCRIPTION OF THE INVENTION
[0021] Throughout this document, a higher drive voltage means a
more negative voltage, providing faster mirror metal plating, or a
more positive voltage, providing faster mirror metal erasure.
[0022] FIG. 1 is a cross sectional view depicting a representative
design of a reversible electrochemical mirror (REM) to which the
present invention pertains. Some dimensions, particularly layer
thicknesses, are disproportionate in the drawings in order to more
effectively illustrate the structure and function of the device. A
REM device for modulation of reflected radiation is described in
U.S. Pat. No. 6,166,847 to Tench et al. The REM device in the
present example, which provides precise control over the reflection
of electromagnetic radiation, includes a first substrate 102, which
is substantially transparent to the portion of the spectrum of
electromagnetic radiation to be controlled, and a second substrate
104. An electrically conducting film 106, which is also
substantially transparent, is disposed on the first substrate. The
film 106, with the optional addition of an electrochemically stable
surface modification layer 108, functions as the mirror electrode.
The surface modification layer 108 is typically a noble metal
selected from the group consisting of platinum, iridium, gold,
osmium, palladium, rhenium, rhodium and ruthenium. An underlayer of
another metal (e.g., aluminum, chromium, hafnium, molybdenum,
nickel, titanium, tungsten or zirconium) may be used to improve the
adhesion of surface modification layer 108.
[0023] As also shown in FIG. 1, a second electrode 110 is disposed
on the second substrate 104 and functions as the counter electrode.
The counter electrode 110 can alternatively be a bulk electrode, a
metal plate or sheet for example, with sufficient rigidity that the
second substrate 104 would not be needed. For a device that also
transmits radiation, electrode 110 may be a locally distributed
electrode (not shown in FIG. 1), as described in U.S. Pat. Nos.
5,903,382 and 5,923,456 to Tench et al. The counter electrode 110
is electrochemically stable or is covered with a sufficient
thickness of an active metal layer 114 to avoid exposure of the
counter electrode surface to the electrolyte. It may also be
protected from exposure to the electrolyte by a coating of
electrochemically stable metal. Relatively stable metals that might
be used as the counter electrode material or as a protective layer
or coating on the counter electrode include Pt, Ir, Au, Os, Pd, Re,
Rh, Ru, Cr, Ni, Ti and stainless steel. The surface of electrode
110 may be roughened to reduce reflection of radiation from the
electrode or to improve switching speed by lowering the current
density (via increased surface area). The REM cell may be initially
charged with mirror metal prior to assembly by depositing the
metallic layer 114 on the electrode 110, by depositing the layer
120 on the nucleation layer 108 or directly on electrode 106, or,
as depicted in FIG. 1, by depositing partial mirror metal layers on
each of the two electrodes.
[0024] An electrolyte 112, containing electrodepositable mirror
metal ions 116, is located between and in electrical contact with
the electrodes 106 and 110 (or layer 108 or, depending on the
switched state of the device, layers 120 or 114). Metal ions 116,
which contain the same metal atoms as the layers 114 and 120, can
be reversibly electrodeposited on and electrodissolved from the
mirror and counter electrodes. Surface modification layer 108
enhances nucleation for the metal electrodeposition process so that
metal electrodeposited in layer 120 is continuous and fine-grained
so as to exhibit high reflectivity for radiation.
[0025] The electrolyte 112, which contains electrodepositable metal
ions and complexing agents, may contain a solvent or be a
solventless ionic liquid electrolyte. A stiffening agent, highly
dispersed silica (HDS) for example, may be added to render
electrolyte 112 more viscous, semi-solid or solid. Electrolyte 112
may also contain one or more coloring agents to impart a desirable
color to the electrolyte, or absorb light strongly over the
wavelength region of interest to avoid reflection from the counter
electrode in reflectance-type devices. For example, a black color
can be imparted to electrolytic solutions via addition of a small
amount of dispersed carbon black, which is typically used in
conjunction with an electrolyte stiffener to prevent settling under
the influence of gravity.
[0026] Preferred REM electrolytic solutions utilizing nonaqueous
solvents are described in U.S. Pat. Nos. 5,903,382, 5,923,456,
6,111,685, and 6,400,491 to Tench et al. The solvent is preferably
selected from the group consisting of gamma-butyrolactone (GBL),
ethylene glycol (EG), dimethylsulfoxide (DMSO), dimethylformamide
(DMF), and mixtures of these solvents. The electrodepositable metal
is preferably selected from the group consisting of silver,
bismuth, copper, tin, cadmium, mercury, indium, lead, antimony,
thallium and zinc, and may be an alloy. The complexing anions are
preferably selected from the groups consisting of halides (e.g.,
chloride, bromide and iodide) and pseudohalides (cyanide and
thiocyanate), and are typically present in molar excess compared to
the electrodepositable metal cations.
[0027] Ionic liquid electrolytes suitable for use in REM devices
are described in U.S. patent application Ser. No. 10/066,210 (filed
Jan. 31, 2002). Ionic liquid electrolytes containing pyrrolidinium
and N-methylpyrrolidinium cations have been found to provide
particularly high current carrying capability (>10 mA/cm2) for
reversible electrodeposition of silver, copper, zinc and tin in
halide systems, and to offer reasonably high electrical resistivity
(300-2000 ohm-cm). Good mirror uniformity has been obtained with
these cations in REM cells having small electrode spacing (0.2 mm).
Systems incorporating mixtures of the two cations and/or different
halides (chloride and bromide, for example) are apparently stable
over the range from at least -20.degree. C. to 150.degree. C.
Because of the protic nature of the cations, ceramic gelling agents
involving hydrogen bonding, highly dispersed silica (HDS) for
example, can be used to provide more rigid pyrrolidinium-based
ionic liquid electrolytes.
[0028] The REM device is intended for use in conjunction with a
source of voltage 118, which has a reversible polarity and
adjustable or pre-set positive and negative voltage values,
connected between the mirror and counter electrodes 106 and 110
(see FIG. 1). When a negative electrical voltage is applied to the
mirror electrode 106 relative to the counter electrode 110, metal
114 deposited on the counter electrode 110 is dissolved from the
counter electrode into the electrolyte 112, while metal ions 116 in
the electrolyte are electrodeposited from the electrolyte onto the
surface modification layer 108 of the mirror electrode 106. The
surface modification layer 108 causes the metal to deposit in a
substantially uniform layer, forming a mirror surface. When the
polarity of the applied voltage is reversed, such that a positive
voltage is applied to the mirror electrode 106 relative to the
counter electrode 110, deposited metal is dissolved from the mirror
electrode into the electrolyte 112 and dissolved metal is
electrodeposited from the electrolyte onto the counter
electrode.
[0029] The thickness of electrodeposited mirror metal layer 120
present on the mirror electrode determines the reflectivity of the
mirror for radiation, which can be varied over a wide range from
nearly 0% to almost 100% reflective The lower limit of reflectivity
for the REM device is affected by the reflectivities of the
nucleation layer 108, the electrode 106, and the substrate 102.
These reflectivities may be reduced by use of anti-reflection
coatings of the type commonly employed, or by adjusting the layer
thicknesses. Likewise, the maximum reflectivity of the REM device
is affected by light absorption in the substrate 102, the electrode
106, and the nucleation layer 108.
[0030] All of the various layers that affect the overall
reflectivity of the REM device for radiation, particularly the
layer 120 of deposited mirror metal, must typically be very uniform
in thickness to provide the highly uniform reflectance over the
mirror surface required for most applications. When this is the
case, a given mirror metal thickness corresponds to a definite
amount of mirror metal with respect to the charge required for its
electrodeposition or dissolution. Note that these processes
generally occur with nearly 100% charge efficiency for REM
electrolytes.
[0031] In principle, the thickness of the mirror metal deposit
could be known at any given time by incorporating a charge
integration device 119 (FIG. 1) and keeping track of all the charge
passed for metal electrodeposition and dissolution as the mirror
state was cycled. Device 119 could be a coulometer for direct
measurement and integration of charge or an ammeter coupled with a
current integration device. As the mirror is subjected to multiple
cycles in which complete erasure of the mirror metal does not
occur, however, measurement imprecision and minor efficiency
imbalances between the metal electrodeposition and dissolution
reactions can introduce cumulative errors in the calculated
thickness and associated reflectance. These errors could be
mitigated by periodic full erasure of mirror metal layer 120 from
mirror electrode 106 to establish a new starting point for the
charge integration, but this would be impractical for large numbers
of cycles and the necessity of such periodic erasure would be
unacceptable for many applications. In addition, the equipment
required for accurate coulometric tracking is relatively
expensive.
[0032] A more precise approach for determining the thickness of
deposited mirror metal is to measure the sheet resistance of the
mirror electrode. From FIG. 1, it is evident that the measured
sheet resistance will include parallel contributions from mirror
electrode 106, nucleation layer 108, and mirror metal layer 120.
Note that the electrolyte resistance is generally high enough that
the currents flowing along electrolyte layer 112 and counter
electrode layers 110 and 1 14 are small. In addition, nucleation
layer 108 is typically very thin (15-30 .ANG.) and has a minimal
effect on the sheet resistance of mirror electrode 106, which is
about 10 ohm/square for the indium tin oxide typically used.
Consequently, the thickness of mirror metal layer 120 has the
largest effect on the measured sheet resistance.
[0033] Sheet resistance is normally defined as the electrical
resistance per unit area of a layer or sheet of a given material
and is measured in such a way that contact resistances and
contributions from contiguous layers of other materials are
negligible or taken into account. Throughout this document, unless
stated otherwise, the term "sheet resistance" is used to denote the
resistance measured between two separate contacts attached to the
mirror electrode of a REM device and specifically includes
contributions from contiguous and adjacent layers of other
materials. Contact resistances associated with the interfaces
between the electrical contacts and the mirror electrode, which are
in series with the mirror electrode sheet resistance, are typically
small enough to be negligible or are relatively constant with time
so that their effect can be taken into account by periodic
calibration.
[0034] It is a relatively simple matter to measure the mirror
electrode sheet resistance as a function of mirror metal thickness
to provide a calibration curve for future measurements or for other
devices of the same type. By utilizing the change in resistance
produced by the deposited mirror metal and periodically
re-measuring the sheet resistance of the electrode without a mirror
metal deposit, the effects of variations with time and from device
to device can be minimized. The thickness of the mirror metal
deposit can readily be ascertained from the charge passed, using
the bare electrode as the baseline. By also measuring the
reflectance of the REM device as a function of mirror metal
thickness, the device reflectance can then be determined by
measuring the mirror electrode sheet resistance. Note that this
calibration approach will also yield accurate measurements of the
mirror metal thickness and device reflectance for other electrode
geometries and placements of the electrical contacts.
[0035] FIG. 2 illustrates placement of separate electrical contacts
on the mirror electrode to provide internal electrical isolation
for the circuit used to measure the mirror electrode sheet
resistance, thereby permitting the thickness of the mirror metal
deposit to be determined while the mirror state is being switched.
In the illustrated embodiment of this approach, electrical contacts
151 and 152 for measuring the sheet resistance are placed on the
sides of electrode 106 not having contacts 131 and 132, which are
used to apply the mirror switching voltage (circuit not shown). The
resistance between contacts 151 and 152 is measured by applying a
voltage via voltage source 133 and measuring the current response
via current measuring device 134. By making contacts 151 and 152
relatively small and locating them midway between contacts 131 and
132, flow of the measurement current along contacts 131 and 132 is
minimized by the relatively high sheet resistance of layer 106.
Small area contacts also minimize shunting across the contact that
might locally decrease the uniformity of the mirror deposit. The
measured sheet resistance in this case is proportional to the
thickness of the mirror metal layer 120 and can be calibrated to
provide a measure of the mirror reflectance. Further electrical
isolation of contacts 151 and 152 from contacts 131 and 132 can be
attained by placing contacts 151 and 152 on tabs 153 and 154 of
mirror electrode material 106, as indicated by the dashed line in
FIG. 4. Bare substrate areas 155, 156, 157 and 158 minimize current
flow between the measurement contacts (151 and 152) and the buss
bars (131 and 132) used to switch the mirror state. There are
numerous contact arrangements that would provide relative sheet
resistance values for determining the mirror metal thickness. For
example, contact 152 could be eliminated and the resistance between
contact 151 and electrically shorted contacts 131 and 132 could be
measured.
[0036] An alternating current (AC) measurement for determining the
sheet resistance has the advantage of minimizing voltage losses due
to contact resistances, which may vary appreciably with time and
would introduce errors in the measurement. The voltage perturbation
frequency is preferably chosen to minimize the effects of
capacitive and inductive losses, as indicated by a near-zero phase
shift between the applied AC voltage and the AC current response.
In some cases, it may be necessary to take this phase shift into
account to calculate an accurate sheet resistance for the mirror
electrode. The magnitude of the applied voltage perturbation is not
critical but is preferably chosen to yield a current response that
is large enough to enable accurate measurement of the current
response but not so large that functioning or control of the REM
device is impaired, e.g., by Joule heating effects.
[0037] According to the present invention, mirror uniformity and
switching speed are optimized for REM devices by utilizing lower
drive voltages when the sheet resistance of the mirror electrode is
high, and utilizing higher drive voltage when the sheet resistance
is reduced by the presence of an appreciable thickness of the
mirror metal. In this case, the IR voltage drop along the mirror
electrode surface (sheet voltage drop), which is primarily
responsible for mirror nonuniformity, is maintained at a relatively
low value via a combination of low current and low sheet
resistance. Fast switching overall can be attained by using much
higher voltages/currents when the sheet resistance is low to
compensate for the lower voltages/currents needed when the mirror
metal deposit is relatively thin.
[0038] For even faster switching, a safe applied voltage that
avoids electrode damage or electrolyte decomposition can be
augmented by the IR drop in the electrolyte (when sufficient mirror
metal is present to adequately reduce the electrode sheet
resistance). This additional voltage produces no detrimental
effects on the electrode surfaces or the electrolyte since it does
not appear across the electrode-electrolyte interfaces. Such
IR-compensated switching is particularly important for REM devices
employing ionic liquids with pyrrolidinium (P+) and
N-methylpyrrolidinium (MP+) cations. For such devices, a large
drive voltage (typically more than one volt) is required to
overcome the high resistance of the electrolyte but the amine
protons on these cations are reduced at very low electrode
potentials (around 0.1 V). Fast switching in this case is attained
without electrolyte breakdown by increasing the voltage from 0.1 V
by the IR drop in the electrolyte, and removing this IR
compensation as the current decreases during mirror erasure.
[0039] In practice, a negative voltage is applied to initiate
mirror formation on the bare mirror electrode and the voltage is
stepped or ramped to more negative values as the mirror metal is
deposited. A positive voltage is applied to initiate mirror erasure
and the voltage is stepped or ramped to less positive values as the
mirror metal deposit is dissolved. Feedback mechanisms and the
plating and erasure voltage waveforms are chosen to provide the
required level of control and the desired compromise between mirror
uniformity and switching speed. In some cases, especially for
switching between a mirror state and full erasure, simple voltage
ramps as a function of time without feedback of the thickness of
the mirror deposit can provide adequate results. For most
applications, however, it is advantageous to have the applied
switching voltage depend directly on the thickness of the mirror
metal deposit. The latter can be monitored by integrating the
charge passed during mirror formation and erasure. Imbalances in
the anodic and cathodic processes, however, as well as cumulative
measurement errors, can introduce large uncertainties unless the
mirror is frequently erased fully to establish a known starting
point.
[0040] Optimum results with respect to mirror uniformity and
switching speed are provided by closely matching the REM drive
voltage to the mirror deposit thickness and the electrolyte
temperature. As described above and in U.S. Patent 6,301,039 to
Tench, the deposit thickness can be determined accurately from its
effect on the sheet resistance of the mirror electrode, preferably
measured using an alternating current (AC) perturbation applied
between two separate contacts on the mirror electrode outside the
cell seal area. Drive voltages should be limited so that currents
remain below the diffusion-limited currents for mirror metal
electrodeposition and dissolution, and should be lowered at
elevated temperatures. Excessively fast plating rates can lead to
powdery deposits, and excessively fast erasure rates can lead to
salt precipitation in some electrolytes. Higher drive voltages are
needed to compensate for increased electrolyte resistance and
slower reaction rates at low temperatures so as to provide suitably
fast switching rates. Temperature also needs to be taken into
account to provide an accurate measure of the deposit thickness
from the mirror electrode sheet resistance. Thermocouples,
thermistors, and a variety of other devices can be used to measure
the electrolyte temperature.
[0041] The specific voltage algorithm for optimum REM switching
according to the present invention depends on the electrolyte
properties (e.g., conductivity and concentrations of reactants),
electrode spacing and geometry (e.g., shape, size and curvature),
electrical contact configuration and geometry, temperature effects,
and the desired compromise between mirror uniformity and switching
speed. The optimum algorithm for a given device can be determined
empirically, using spatial variations in mirror reflectance for
feedback, or by electrical modeling based on the device geometry,
material properties, and electrochemical characteristics. On the
other hand, significant improvement in mirror uniformity without
undue sacrifice in switching speed can be attained via relatively
simple drive voltage algorithms, e.g., linear voltage variations as
a function of thickness for relatively thin mirror deposits and
constant voltage when the deposit thickness is greater than a
critical value providing an acceptably low electrode sheet
resistance. In any case, it is important to ensure that the erasure
voltage is reduced when the current decreases in the final stages
of erasure so that excessive voltage is not applied to the bare
mirror electrode surface, which might cause damage to the
nucleation layer or electrolyte breakdown.
[0042] In a preferred approach, the REM device is automatically
switched via a computing device (computer or microprocessor, for
example) according to a drive voltage algorithm based on real-time
measurements of the electrode sheet resistance, device switching
current and temperature. Even sophisticated REM drive voltage
algorithms based on feedback from sheet resistance and temperature
measurements can be inexpensively implemented for high-volume
production via modem integrated circuit (IC) and logic chip
technologies. Ideally, a semiconductor control device would apply
the appropriate drive voltage based on almost continuous
measurements of the mirror electrode sheet resistance and the
electrolyte temperature. The control device would include the
capability of measuring the mirror electrode sheet resistance by
applying an AC voltage perturbation across electrical contacts on
the mirror electrode and analyzing the current response.
[0043] Although the approach above focuses on direct control of the
drive voltage for REM devices, indirect control of the voltage via
the device switching current could also be used to practice the
invention. However, this would involve somewhat more complicated
drive circuitry and would provide little or no advantage compared
to direct voltage control.
[0044] Fabrication of a Preferred Embodiment
[0045] The preferred mirror electrode utilizes a glass or plastic
substrate which is uniformly coated on one side with an optically
transparent conductive film, e.g., indium tin oxide (ITO) or
fluorine-doped tin oxide (FTO), which has relatively low
resistivity (about 10 ohm/square) and serves as the mirror
electrode and current collector. A very thin layer of inert metal
(15-30 .ANG. platinum, for example) is preferably sputtered onto
the ITO or FTO surface to enhance the uniformity of nucleation to
provide a mirror deposit.
[0046] The preferred counter electrode depends on whether the
device is designed to control light transmission or reflectance.
For REM devices involving adjustable transmittance, the preferred
counter electrode is locally distributed, as described in U.S. Pat.
No. 5,903,382 to Tench et al. In this case, the counter electrode
comprises an electrochemically inert metal grid or nucleation layer
matrix pattern of relative small overall area, so that metal plated
on the counter electrode blocks only a small fraction of the
radiation. For adjustable reflectivity REM devices, the preferred
counter electrode comprises a reasonably thick (e.g., 1 .mu.m)
layer of mirror metal on an electrochemically stable conducting
substrate, e.g., 50 .ANG. Pt on an ITO/glass or plastic substrate.
Suitable counter electrodes for adjustable reflectivity devices are
described in U.S. Pat. No. 6,166,847 to Tench et al.
[0047] One preferred electrolyte, comprised of silver ions and an
excess of halide anions in a nonaqueous solvent, preferably
gamma-butyrolactone (GBL), is described in U.S. Pat. No. 6,400,491
to Tench et al. A preferred ionic liquid electrolyte, comprised of
silver ions, halide anions and pyrrolidinium-based cations, is
described in U.S. patent application Ser. No. 10/066,210 (filed
Jan. 31, 2002).
[0048] Although the REM device can be fabricated using a liquid
electrolyte, use of an electrolyte stiffener is preferred for many
applications. Preferred electrolyte stiffeners are dispersed
inorganic materials, e.g., highly dispersed silica (HDS) or
alumina, which form thixotropic gels that can be liquefied by
mechanical shearing for facile injection in REM cells, and
typically have minimal effect on the electrolyte conductivity and
REM performance.
[0049] For adjustable mirror applications, a coloring agent is
preferably added to the REM electrolyte so that light reflection is
minimized for the non-mirror state. A preferred coloring agent in
this case is dispersed carbon black, which, in small amounts,
provides high light absorption over a wide spectral range, and
tends to protect the electrolyte from degradation by ultraviolet
light. The carbon black is preferably suspended by ultrasonic
agitation and maintained in suspension by subsequent addition of an
electrolyte stiffener.
[0050] The reversible electrochemical cells pertaining to this
invention can be fabricated using spacers and a polymer sealant, or
using a gasket or o-ring to provide both the proper spacing and a
seal. The preferred electrode separation is about 0.05-3.0 mm. The
electrodes may be planar or curved.
[0051] REM cells may have any geometric shape but those exhibiting
a high degree of symmetry (e.g., rectangles or circles) are more
amenable to uniform mirror switching. The preferred REM cell
geometry is rectangular or square with the electrical contacts for
switching the mirror state being provided by copper strips attached
with conductive adhesive that run the length of two opposite sides.
Contacts are preferably placed outside the seal area so that they
are not in contact with the electrolyte. The same contacts can be
used to measure the sheet resistance of the mirror electrode,
preferably using an applied alternating voltage having a frequency
(e.g., 1-30 kHz) for which the phase shift of the corresponding
current approaches zero. For measuring the sheet resistance while
the REM mirror state is switched, a preferred approach is to
provide separate small-area contacts located midway on the sides of
the device not having the contacts for applying the switching
voltage.
[0052] The sheet resistance is calibrated in terms of the thickness
of mirror metal on the mirror electrode, preferably by measuring
the charge required to deposit a given amount of mirror metal.
After calibration via standard reflectance measurement methods, the
sheet resistance provides an accurate measure of the device
reflectance.
EXAMPLE 1
[0053] Mirror Uniformity Improvement Via Drive Voltage Ramping
[0054] An adjustable reflectivity REM device having a rectangular
viewing area of approximately 8.3.times.8.7 cm was constructed
using a mirror working electrode comprised of a 15 .ANG. sputtered
platinum nucleation layer on a 10-ohm/square ITO film on a glass
substrate (10 cm square). The counter electrode was 60 .ANG.
sputtered Pt on 10 ohm/square ITO on a glass substrate (10 cm
square), which had been electroplated with about 1 .mu.m of silver
from a commercial cyanide bath (Technisilver 2E, Technic Co.) and
annealed at 200.degree. C. for 30 minutes in a reducing atmosphere
(to improve adhesion) prior to cell assembly. A bare Pt/ITO border
was left around the plated silver (via masking with platers' tape)
to permit formation of a good seal with acrylic adhesive tape (VHB
#4910, 3M Company), which also overlapped the plated silver to
protect its edges. This acrylic tape (about 6 mm wide) served as
both the electrode spacer (1 mm) and primary sealant and was
recessed from the edges of the glass panes so as to leave room for
3-mm wide copper buss bars, which were attached to the Pt/ITO layer
around the perimeter of the device with conductive adhesive (C665,
Furon Co.).
[0055] Electrolyte preparation and final device assembly were
performed inside a nitrogen atmosphere glove box to avoid
contamination with oxygen, which reacts electrochemically and can
cause mirror self-erasure via chemical dissolution of the mirror
metal. The electrolyte was injected through the acrylic tape using
a pair of hypodermic needles (inlet and outlet) and a syringe.
Epoxy was used to provide a second seal and to help hold the buss
bars in place. The electrolyte contained 1.5 M AgI+2.0 M LiBr+63
mg/mL highly dispersed silica (M-5 Cab-O-Sil, Cabot Co.)+1.5 mg/mL
carbon black (Vulcan, Cabot Co.) in high-purity GBL solvent (<20
ppm water). Addition of the highly dispersed silica produced a
thixotropic gel that could be liquefied by stirring but became
stiff upon standing. This REM device exhibited excellent mirror
quality (reflectance at 700 nm wavelength of 6.0% minimum, and 80%
with a 400 .ANG. silver deposit) and could be switched repetitively
without change in reflectance for a given amount of silver
deposited on the mirror electrode.
[0056] Constant and ramped drive voltages for REM switching were
provided by a PAR Model 273 potentiostat in the two-electrode mode.
Reflectance measurements were made at the center and as near to one
corner of the REM device as permitted by the required measurement
area (1.5.times.2.5 cm) using a double reflection technique with a
Cary 5 photospectrometer. Mirrors for erasure tests were plated at
-0.30 V until a reflectance of 66% was attained.
[0057] FIG. 3 gives plots of the difference in reflectance between
the center and a corner of the rectangular REM reflective device as
a function of plating time for mirrors formed at constant voltage
(-0.40 V) and by ramping the voltage from -0.10 V to a maximum of
-0.40 V at 50 mV/s. Whereas the maximum reflectance difference for
mirrors formed at the constant voltage was about 14%, this value
was reduced to about 4% when the 50 mV/s voltage ramp was used.
[0058] FIG. 4 gives plots of the difference in reflectance between
the center and a corner of the rectangular REM reflective device as
a function of time for erasure of 66% reflective mirrors (formed by
the voltage ramping) at constant voltage (+0.40 V) and by ramping
the voltage from +0.40 V to a minimum of +0.2 V at 10 mV/s. Whereas
the maximum reflectance difference for mirrors erased at the
constant voltage was about 17%, this value was reduced to less than
4% when the 10 mV/s voltage ramp was used.
[0059] These results with simple voltage ramps demonstrate the
efficacy of the present invention in providing improved REM mirror
uniformity by utilizing lower plating and erasure voltages (less
negative and less positive, respectively) when the mirror electrode
sheet resistance is higher because the mirror metal deposit is
thinner. It is obvious that closely matching the drive voltages to
the current-carrying capability of the mirror metal deposit would
provide the optimum compromise between mirror uniformity and
switching speed.
EXAMPLE 2
[0060] Mirror Electrode Sheet Resistance Measurement during REM
Switching
[0061] The ability to measure the REM mirror electrode sheet
resistance during mirror state switching was demonstrated for a
cell similar to that in Example 1 but having a mirror electrode
with buss bars along only two opposite sides, and two separate
contacts on the other two sides for sheet resistance measurements
(configuration shown in FIG. 2). For this cell, the mirror viewing
area was a 7.4.times.7.3 cm rectangle, the buss bars for applying
the switching voltage ran along the longest side with their inside
edges 9.2 cm apart, and the contacts for measuring the sheet
resistance were 6 mm square and spaced 9.3 cm apart. The sheet
resistance was measured with a 5 mV voltage perturbation at a
frequency of 28 kHz using a Hewlett-Packard Model 4194A Network
Analyzer while a constant voltage (+0.30 V) was applied between the
electrodes via an automobile battery and a voltage divider to
switch the mirror state. The battery arrangement was used to
circumvent equipment grounding difficulties but undoubtedly
introduced errors in the silver thickness determination, which was
based on the charge passed during a given time assuming that the
current remained constant (which was only approximately true).
Switching between plating and erasure was accomplished by switching
the cell leads, which may also have introduced contact resistance
errors.
[0062] FIG. 5 gives plots of the mirror electrode sheet resistance
for the REM device as a function of the mirror metal (silver)
deposit thickness measured during mirror plating at -0.30 V and
erasure at +0.30 V. Note that the absolute value of the measured
sheet resistance depends strongly on geometric factors but is
readily calibrated to provide a reliable measure of the deposit
thickness. Good sensitivity of the sheet resistance to silver
thickness over a wide range is evident. Sensitivity is particularly
good at silver thicknesses below 400 .ANG., which provides nearly
the maximum reflectance. Differences in the plating and erasure
curves are not large but probably result primarily from measurement
errors associated with the relatively crude demonstration apparatus
used. Measurement errors should be negligible for properly
engineered equipment.
[0063] These results demonstrate that optimum REM switching
according to the present invention can be attained by utilizing
measurements of the mirror electrode sheet resistance as feedback
for determining the appropriate drive voltage. It is also necessary
to take temperature into account for both calculation of the silver
deposit thickness from the measured sheet resistance and for
specifying the drive voltage.
EXAMPLE 3
[0064] Computer Programmed Switching of REM Devices
[0065] A computer program was written for a personal computer in
LabView.RTM. version 5.0 (National Instruments, Austin, Tex.) to
automatically switch REM devices according to a drive voltage
algorithm based on real-time measurements of the electrode sheet
resistance, device switching current and temperature. User inputs
to the computer program include the electrolyte resistance as a
function of temperature (slope and intercept for linear plot of
inverse electrolyte resistance vs temperature), maximum allowable
voltage drop for the mirror electrode sheet resistance, safe
voltage to be applied when the device current is negligibly small
(minimum applied voltage), and maximum voltage for avoiding
diffision-limited deposition/dissolution of mirror metal. A version
of this program written for use with a power supply instead of a
potentiostat utilizes a maximum current limitation (instead of a
maximum voltage). The magnitude of the minimum voltage is usually
the same for plating and erasure and this voltage is applied (with
the correct polarity) to initiate mirror plating or erasure. From
the current flowing at the minimum voltage, the electrolyte IR drop
and mirror electrode sheet resistance are calculated. The applied
voltage is increased by the smaller of some pre-determined
percentage of the electrolyte IR drop or the voltage that
corresponds to the maximum allowable sheet voltage drop for the
mirror electrode. A percentage of the electrolyte IR drop
(typically 75%) is used to avoid overshooting the target voltage.
The process of measuring (current, sheet resistance and
temperature), calculating IR drops, and appropriately increasing or
decreasing the applied voltage is repeated continuously and rapidly
throughout the switching operation. During mirror erasure, the IR
compensation is removed as the current decreases, which avoids
electrode damage and electrolyte breakdown in the later stages of
erasure when the current is small.
[0066] This computer program was used (in conjunction with a
personal computer, an electronic potentiostat and a custom-made
device for measuring the mirror electrode sheet resistance) to
switch REM devices between the fully erased and mirror states, and
between intermediate mirror states. The custom-made impedance
measuring device was battery-powered and utilized an 1 kHz AC
voltage perturbation (50-100 mV peak to peak). A commercial
impedance measuring device could be used, with precautions to avoid
grounding problems resulting from the use with other AC
equipment.
[0067] A variety of REM devices were switched according to the
voltage algorithm and computer program described above. For
large-area devices (about 10.times.10 cm square), the programmed
switching provided a significant visual improvement in the
uniformity of the mirror deposits compared to that for switching at
constant voltage.
[0068] The preferred embodiments of this invention have been
illustrated and described above. Modifications and additional
embodiments, however, will undoubtedly be apparent to those skilled
in the art. Furthermore, equivalent elements may be substituted for
those illustrated and described herein, parts or connections might
be reversed or otherwise interchanged, and certain features of the
invention may be utilized independently of other features.
Consequently, the exemplary embodiments should be considered
illustrative, rather than inclusive, while the appended claims are
more indicative of the full scope of the invention.
* * * * *