Method for rapid and uniform heating of a transparent and/or reflecting multilayer optical system with polymeric solid electrolyte

The optical system, which is comprised of a thin layer of ion conducting, macromolecular material, or solid polymer electrolyte sandwiched between two electrodes presenting each a conducting deposition carried by a support plate such that at least one of the conductor depositions and the associated support plate are transparent, is heated by applying between the conductor depositions of said electrodes an electric voltage signal of which at least one portion includes an alternating component which has an amplitude between 0.05 and 100 volts and a frequency lower than 5 kHz and preferably between 2 and 2000 Hz so as to generate within said optical system an alternating ionic current susceptible of producing a heating of the ion conducting macromolecular material by Joule effect. The optical system to be heated, which may be an electrochromium device, may have particularly a glazing type structure or a mirror type structure.

FIELD OF THE INVENTION 
The invention relates to a method for rapid and uniform heating of a 
transparent and/or reflecting multilayer optical system containing a 
polymeric solid electrolyte, the said system being in particular an 
electrochromic system. 
BACKGROUND OF THE INVENTION 
An electrochromic system with polymeric solid electrolyte generally 
comprises two electrodes between which is sandwiched a thin layer of an 
ion-conducting macromolecular material (polymeric solid electrolyte), 
especially a conductor of protons or of alkali metal ions, at least one of 
the said electrodes containing a material, called an electrochromic 
material, which is placed facing the layer of ion-conducting 
macromolecular material and in which the insertion or deinsertion of ions, 
especially protons or alkali metal ions, results in a change in the light 
absorption and/or reflection spectrum of the said material, these 
electrodes, on the one hand, being in direct contact with the layer of 
polymeric solid electrolyte to which they adhere strongly and, on the 
other hand, each comprising a currentconducting deposit which is in 
contact with one of the facing sides of two support plates situated on 
both sides of the said electrodes and at least one of which and the 
associated conducting deposit are transparent. In particular, each 
electrode comprises a transparent current-conducting deposit, for example 
a deposit based on mixed indium tin oxide, which is in contact with one of 
the facing sides of two plates made of an inorganic or organic transparent 
material such as glass or transparent plastic, which are situated on both 
sides of the said electrodes and are coupled to form a window-type 
structure. 
When a suitable potential difference, generally lower than 5 volts, is 
established between the conducting deposits of the electrodes, the 
appearance of a permanent colouring of the electrochromic system is 
observed, and this system can again become colourless on the application 
of an electrical voltage, generally of opposite sign to that which 
produced the colouring. 
An electrochromic system of the abovementioned type can be employed as a 
window with variable light transmission for domestic use or for motor 
vehicles or as a mirror or rear-view mirror with variable light 
reflection. 
It is advantageous to be able to heat such a glazed surface uniformly, to 
permit demisting, defrosting and/or a faster response of the colour change 
to the electrical voltage which is applied 
The use of an external source of heat for heating an electrochromic system 
of the abovementioned type does not allow the required result to be 
obtained because operating in this way results in the appearance of a 
temperature gradient inside the electrochromic system, due to the poor 
diffusion of heat in the multilayer structure forming the said system, and 
this is reflected in a nonhomogeneous operation of this system. 
It has already been proposed, as described in reference GB-A-2,065,027, to 
perform the heating of a polymeric composition forming a thin layer and 
containing an ion-conducting macromolecular material consisting of a 
polyether coupled with an ionisable salt by relying on a heating technique 
using dielectric losses, which consists in subjecting the said composition 
to the action of electromagnetic waves of very high frequencies, namely 
frequencies of the order of 10.sup.6 to 10.sup.8 hertz. 
Such a heating technique using dielectric losses is not suitable for 
heating an electrochromic system such as referred to above, which 
comprises a thin layer of an ion-conducting macromolecular material 
sandwiched between two structures with high electronic conduction, namely 
the electrodes of said system, because, apart from the difficulties linked 
with its implementation and the disadvantages which it entails for the 
environment owing to the use of electrical signals of very high frequency, 
this technique does not lend itself well to heating multilayer structures 
comprising a number of layers with high electronic conduction which are 
close to each other. 
SUMMARY OF THE INVENTION 
A method has now been found for rapid and uniform heating of an 
electrochromic system comprising at least one thin layer of an 
ion-conducting macromolecular material intercalated between two 
electrodes, forming two structures with high electronic conduction, so as 
to be in intimate contact with the said electrodes, which makes it 
possible to overcome the disadvantages of the heating methods employing an 
external source of heat or dielectric losses Such a method can be employed 
more generally for heating any transparent and/or reflecting multilayer 
optical system which comprises two electrodes between which is sandwiched 
a layer of an ion-conducting macromolecular material (polymeric solid 
electrolyte), especially a conductor of protons or alkali metal ions, and 
transparent in the thicknesses employed, these electrodes, on the one 
hand, being in direct contact with the layer of polymeric solid 
electrolyte to which they adhere strongly and, on the other hand, each 
comprising a current-conducting deposit which is in contact with one of 
the facing sides of two support plates situated on both sides of the said 
electrodes and at least one of which and the associated conducting deposit 
are transparent, the said optical system being in particular such that 
each electrode comprises a current-conducting transparent deposit, for 
example a deposit based on mixed indium tin oxide, which is in contact 
with one of the facing sides of two plates made of an inorganic or organic 
transparent material such as glass or transparent plastic, which are 
situated on both sides of the said electrodes and are coupled to form a 
window-type structure. 
The method according to the invention is characterised in that between the 
conducting deposits of the electrodes of the optical system which are 
situated on both sides of the layer of ion-conducting macromolecular 
material, an electrical voltage signal is applied, at least part of which 
comprises an alternating component which has a frequency of less than 5 
kHz and an amplitude, that is to say a difference between its maximum and 
mean values, of between 0.05 and 100 volts, so as to generate in the 
optical system an alternating ion current capable of producing a heating 
of the ion-conducting macromolecular material by Joule effect. 
The frequency of the alternating component of the electrical voltage signal 
applied between the conducting deposits of the electrodes of the optical 
system is advantageously more particularly between 2 and 2000 Hz and 
preferably is between 10 and 500 Hz. In addition, the preferred values of 
the amplitude of the said alternating component are between 0.05 and 30 
volts. 
The alternating component of the electrical voltage signal employed 
according to the invention may be sinusoidal or nonsinusoidal and may be 
uninterrupted or noncontinuous. 
This alternating component may consist especially of a sinusoidal 
electrical voltage of a frequency equal to 50 or 60 Hz, generated by the 
sinusoidal alternating voltage supplied by the electricity supply systems. 
A person skilled in the art will be easily capable of adjusting the 
electrical power to be supplied to the terminals of the optical system, 
especially an electrochromic system, of the abovementioned type with a 
polymeric solid electrolyte, which it is desired to heat, to reach the 
desired temperature in a specified time by taking into account the size 
and the geometry of the said system to be heated, its heat capacity and 
its heat loss to the external environment. 
In fact, the heat power dissipated in the polymeric solid electrolyte 
because of the alternating motion of the ions which it contains is of the 
formula U.sub.A.sup.2 /Ri,.sub.A being the effective value of the 
alternating component of the applied electrical voltage signal and Ri 
denoting the ion resistance of the layer of polymeric solid electrolyte of 
the optical system to be heated. This same ion resistance is given by the 
relationship Ri=K.times.t/S, in which K is the ionic resistivity of the 
polymeric solid electrolyte and t and S denote the thickness and the 
surface area respectively of the layer of polymeric solid electrolyte of 
the optical system. The heat power dissipated in the layer of polymeric 
solid electrolyte is therefore of the form U.sub.A.sup.2 /Ri or 
K.times.U.sub.A.sup.2 .times.S/t. 
The alternating voltage to be applied to the optical system in order to 
heat it with a given heat power is therefore proportionally lower the 
greater the surface area and the smaller the thickness of this system. 
Similarly, heating a set of n identical optical systems requires the 
application of an alternating voltage which is higher (coupled with a 
lower current) when these n systems are connected in series than when 
these n systems are connected in a parallel configuration. 
When the optical system is being heated, the intensity of the alternating 
current which is generated within the ion-conducting macromolecular 
material as a result of the application of the electrical voltage signal 
with an alternating component between the conducting deposits of the 
electrodes of the said system tends to increase with temperature because 
of the decrease in the resistance of the ion-conducting material. If need 
be, the temperature within the said ion-conducting material can be 
monitored while the optical system is being heated, in order not to exceed 
a predetermined value, it being possible for the said monitoring to be 
carried out either by employing an electrical voltage signal whose 
alternating component has a constant effective value and by limiting the 
intensity of the alternating current generated and/or by keeping constant 
the intensity of the alternating current flowing in the ion-conducting 
material and by limiting the amplitude of the alternating component of the 
electrical voltage signal. These techniques of thermal control of the 
temperature of a conductor are well-known in the art and will not 
therefore be described in detail.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to FIG. 1, the optical system which is heated using the 
method according to the invention comprises a thin layer of an 
ion-conducting macromolecular material 10 (polymeric solid electrolyte) 
sandwiched between two electrodes 12, 14. 
A "thin layer" of the ion-conducting macromolecular material 10 means that 
a layer of the said material whose thickness, which actually corresponds 
to the distance separating the two electrodes 12, 14 situated on both 
sides of the ion-conducting macromolecular material 10, low in relation to 
the areas of this macromolecular material which are in contact with the 
adjacent layers formed by the electrodes 12, 14. The thickness of the thin 
layer of the ion-conducting macromolecular material 10 is advantageously 
between 5 .mu.m and 2000 .mu.m, it being necessary for the said thickness 
to be as uniform as possible. 
The ion-conducting macromolecular material 10 may be any one of the 
polymer-based materials capable of simultaneously exhibiting an ion 
conductivity of at least 10.sup.-7 siemens/cm at room temperature and an 
electron conductivity of less than than 10.sup.-10 siemens/cm. 
The ion-conducting macromolecular material 10 may, in particular, consist 
of a solid solution of at least one ionisable salt, especially an alkali 
metal salt and in particular a lithium salt, in a plastic polymeric 
material made up at least partly of one or more polymers and/or copolymers 
of monomers containing at least one heteroatom, especially oxygen or 
nitrogen, capable of forming bonds of the donor/acceptor type with the 
cation of the ionisable salt, the said polymer(s) being chosen in 
particular from polyethers and especially from ethylene oxide or propylene 
oxide homopolymers (cf. EP-A-0,013,199). In the improvements made to the 
solid solutions of the abovementioned type the plastic polymeric material 
may consist in particular of a copolymer of ethylene oxide and of at least 
one other cyclic oxide, the said copolymer having either the structure of 
a random copolymer (U.S. Pat. No. 4,578,326) which may be optionally 
crosslinked (FR-A-2,570,224) or else the form of a network of the urethane 
type resulting from the reaction of a block copolymer of ethylene oxide 
and of at least one other cyclic oxide with a coupling agent consisting of 
an organic polyisocyanate (FR-A-2,485,274). In addition, the ionisable 
salts mentioned in reference EP-A-0,013,199 may be partly or wholly 
replaced by ionisable salts such as alkali metal closoboranes 
(FR-A2,523,770), alkali metal tetrakistrialkylsiloxyalanates 
(FR-A-2,527,611), bis(perhaloalkylsulphonyl)imides or alkali metal 
bis(perhaloacyl)imides (FR-A-2,527,602), alkali metal tetraalkynylborates 
or aluminates (FR-A2,527,610), alkali metal derivatives of 
perhaloalkylsulphonylmethane or perhaloacylmethane compounds 
(FR-A2,606,218) or else alkali metal salts of polyethoxylated anions 
(EP-A-0,213,985). 
The ion-conducting macromolecular material 10 may further consist of a 
solid solution of an ionisable salt, for example a salt such as described 
in the abovementioned references, in a polymeric material consisting of an 
organometallic polymer in which at least two polyether chains are linked 
by a metal atom chosen from Al, Zn and Mg (FR-A-2,557,735) or from Si, Cd, 
B and Ti (FR-A-2,565,413) or else of a polymeric material consisting of a 
polyphosphazene carrying two polyether groups such as polyoxyethylene 
groups on each phosphorus atom. 
The ion-conducting macromolecular material 10 can also be chosen from 
mixtures of polymers of polar nature and/or solvating with any salt, acid 
or base sufficiently dissociated in the polymer to obtain the appropriate 
ion conductivity or else from polymers carrying ionisable functional 
groups producing anions or cations attached to the macromolecular chains 
or else from protonic conductors such as those described in reference 
FR-A2,593,328 or mixtures of inert polymers with inorganic or organic 
ion-conducting materials dispersed in the polymeric matrix. 
If need be, the ion-conducting polymeric material 10 may also contain one 
or more additives of a plasticising nature, especially one or more 
sulphones or sulphonamides such as tetraethylsulphonamide. 
With reference to FIG. 2, when the optical system to be heated is an 
electrochromic system, at least one of the electrodes of the said system 
contains a material 26, 28, known as an electrochromic material, which is 
arranged in contact with the layer of polymeric solid electrolyte 10 and 
in which the insertion or the deinsertion of ions, especially alkali metal 
ions, in particular lithium, or of protons, results in a change in the 
light absorption and/or reflection spectrum of the said material. Such an 
electrochromic material 26, 28 may be especially based on a transition 
metal oxide or on a mixture or a solid solution of transition metal 
oxides, and in particular based on an oxide of a metal such as tungsten, 
molybdenum, vanadium or on a mixture or a solid solution of oxides of such 
metals. . 
As for the transparent conducting deposit 16, 18 which at least one of the 
electrodes 12, 14 of the optical system comprises and which is in contact 
with the inner face of the corresponding support plate 20, 22 of the 
inorganic or organic transparent material, this is generally based on tin 
oxide and consists, for example, of mixed tin indium oxide or of tin 
cadmium oxide or else of tin oxide doped with antimony oxide or with 
fluorine. 
When the optical system to be heated contains an electrode provided with a 
transparent conducting deposit and an electrode comprising a 
nontransparent conductor, the latter may be especially made of a 
current-conducting material capable of forming a reflecting layer and, for 
example, made of a metal such as Ag, Al, Ni,, Li, Cr or stainless steel. 
When one of the electrodes 12, 14 comprises a nontransparent current 
conductor, this conductor may take the form of a deposit on the associated 
support plate 20, 22 or may also consist of the said support plate 20, 22 
, which is then chosen so that it will conduct. The conducting deposit may 
be produced on the appropriate face of the support plate 20, 22 by any 
method which is suitable for this purpose and especially by chemical or 
physical vacuum deposition. 
The heating method according to the invention lends itself very well to the 
heating of an optical, especially electrochromic, system with polymeric 
solid electrolyte of the abovementioned type, since it permits rapid and 
uniform heating without altering in any way the transparency of the 
surfaces to be demisted or defrosted, and this is particularly useful for 
defrosting windows and/or rear-view mirrors of motor vehicles in 
wintertime. This is obviously not the case with the resistance heater 
wires printed at uniform intervals on the inner face of rear windows of 
motor vehicles which are currently being employed by most manufacturers 
and which cannot be employed for defrosting windscreens because of the 
interference with visibility due to the presence of these resistance 
wires. 
When the optical system to be heated is an electrochromic system the 
alternating component of electrical voltage which is applied between the 
conducting deposits 16, 18 of the electrodes 12, 14 of the said 
electrochromic device, in order to heat this device, it can be coupled or 
otherwise to the electrical voltage 24 determining the colour of the 
electrochromic device. 
When the optical device is being heated, the intensity of the alternating 
current generated within the polymeric solid electrolyte 10 by the 
application of the alternating voltage between the conducting deposits 16, 
18 of the electrodes of the said device tends to increase because of the 
decrease in the resistance of the polymeric solid electrolyte If need be, 
the temperature during the heating can be controlled so as not to exceed a 
predetermined value, it being possible for this control to be carried out 
as indicated above by controlling the effective voltage and/or the current 
of the alternating signal. 
The alternating voltage which can be employed for heating the optical 
system may be generated by any known alternating voltage source 24 capable 
of delivering an alternating electrical voltage in the shape of an 
uninterrupted signal or a noncontinuous signal exhibiting the frequency 
and amplitude characteristics defined above. This alternating voltage 
source 24 is connected to the conducting deposits 16, 18 the electrodes 
12, 14 of the optical system to be heated. When the optical system is of 
the electrochromic system type the alternating voltage source 24 can be 
integrated into the system which controls the voltage for controlling the 
said electrochromic system. 
The invention is illustrated by the following examples, which are given 
without any limitation being implied. 
EXAMPLE 1 
An electrochromic device was produced, comprising two windows, each with a 
thickness of 3 mm and each of whose facing sides served as a support for 
an electrode, the said electrodes consisting, one of a transparent ITO 
(indium tin oxides) deposit and the other of a transparent ITO deposit 
coated with a layer of WO.sub.3 and both being separated by a 30-.mu.m 
layer of a polymeric solid electrolyte to which they adhere strongly, the 
WO: layer capable of inserting lithium reversibly under the effect of an 
electrical field being facing the polymeric solid electrolyte. The said 
electrolyte consisted of a solid solution containing 7% by weight of 
LiClO.sub.4 in a copolymer of ethylene oxide and butylene oxide, 
containing 70% by weight of ethylene oxide, this electrolyte being 
transparent to visible light and having an ionic conductivity, expressed 
in siemens/cm, ranging from 10.sup.-6 at 0.degree. C. to 10.sup.-4 at 
80.degree. C. 
When a direct voltage of 3 volts was applied between the conducting 
deposits of the electrodes of the electrochromic device thus produced, 
this device changed colour at 20.degree. C. after a period of 
approximately 300 seconds. 
This test was repeated by superposing onto the 3-volt direct voltage 
employed to control the colour change of the device, an alternating 
electrical voltage with an amplitude of 5 volts at a frequency of 50 Hz. A 
very rapid rise in the temperature of the electrochromic device was 
observed, its core reaching a temperature of approximately 60.degree. C. 
after approximately 10 seconds and the change in the colour of the said 
device was produced after approximately 60 seconds. The kinetics of the 
heating phenomenon resulting from the application of the alternating 
voltage are substantially the same for a given alternating voltage, 
whatever the surface area of the device; only the distributed current and 
hence the power dissipated by a Joule effect varies proportionally to the 
said surface area. 
EXAMPLE 2 
A "triplex" device was produced, consisting of two glass panes, each 3 mm 
in thickness, bonded together with a transparent adhesive. Each of the 
facing sides of the two glass panes was coated with a conducting 
transparent deposit based on mixed indium tin oxide (ITO), the said 
deposits forming the electrodes of the device. The adhesive bonding the 
two glass panes together consisted of a polymeric solid electrolyte 
consisting of a solid solution containing 7% by weight of LiClO.sub.4 in a 
polymeric matrix of polyetherurethane obtained by the action of an 
aliphatic triisocyanate on a random .alpha.,.omega.-dihydroxylated 
copolymer derived from ethylene oxide and butylene oxide. 
The device thus produced formed a window whose surface area was 25 cm.sup.2 
and the thickness of the electrolyte adhesive was 50 .mu.m. 
When a sinusoidal alternating voltage which had an effective value of 10 
volts was applied between the conducting deposits or electrodes of the 
said device and a frequency of 50 Hz an increase of 20.degree. C. in the 
temperature of this device was observed after a few seconds.