Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DESCRIPTION
MAKING ELECTRICAL CONTACT BETWEEN METALS
5AND RESISTIVE ELEMENTS
This invention relates to electrical devices
comprising conductive members having different
resistivities.
Many electrical devices, particularly heaters,
comprising conductive members having different
resistivities are known. Such devices may comprise,
for example, a metallic member used in conjunction with
a resistive element such as a conductive polymer, i.e.
a mixture comprising a conductive filler and an organic
polymer (this term being used to include
polysiloxanes), the filler being dispersed in the
organic polymer or otherwise held together by the
organic polymer, or a ceramic. The conductive polymer
may exhibit PTC behavior. Documents describing
conductive polymer compositions and devices comprising
them include U.S. Patents Nos. 2,952,761, 2,978,665,
253,243,753, 3,351,882, 3,571,777, 3,757,086, 3,793,716,
3,823,217, 3,858,144, 3,861,029, 3,950,604, 4,017,715,
4,072,848, 4,085,286, 4,117,312, 4,177,376, 4,177,446,
4,18~,276, 4,237,441, 4,242,573, 4,246,468, 4,250,400,
4,252,692, 4,255,698, 4,271,350, 4,272,471, 4,304,987,
304,309,596, 4,309,597, 4,314,230, 4,314,231, 4,315,237,
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4,317,027, 4,318,881, 4,327,351, 4,330,704, 4,334,351,
4,352,083, 4,388,607, 4,398,084, 4,413,301, 4,425,397,
4,426,339, 4,426,633, 4,427,877, 4,435,639, 4,429,216,
4,442,139, 4,459,473, 4,481,498, 4,476,450, and
4,502,929; J. Applled Polymer Science 19, 813-815
(1975), Klason and Kubat; Polymer Engineering and
Science 18, 649-653 (1978), Narkis et al; and commonly
assigned U.S. Serial Nos. 601,424 now abandoned,
published as German OLS No. 1,634,999; 732,792 (Van
Konynenburg et al), now abandoned, published as German
OLS No. 2,746,602; 798,154 (~orsma et al), now
abandoned, published as German OLS No. 2,821,799;
134,354 and 184,642 (Lutz), published as European
Appication No. 28,142; (Lutz); 141,984 (Gotcher et al),
published as European Application No. 38,718; 141,988
(Fouts et al), published as European Application No.
38,718, 141,989 (Evans), published as European
Application No. 38,713, 141,991 (Fouts et al),
published as European Application No. 38,714, 150,909
(Sopory), published as UK Application No. 2,075,106A,
250,491 (Jacobs et al) published as European
Application No. 63,440, 272,854 and 403,203 (Stewart
et al), published as European Patent Application No.
67,679, 274,010 (Walty et al), published as European
Application No. 68,688; 300,709 and 423,589
(Van Konynenburg et al), published as European
Application No. 74,281, 369,309 (Midgley et al),
published as European Application No. 92,406; 483,633
(Wasley), published as European Application No.
123,540; 493,445 (Chazan et al), published as European
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Patent Application Publication No. 128,664, 606,033,
(Leary et al), published as European Application No.
119,807, 509,897 and 598,048 (Masia et al) published as
European Application Publication No. 133,748, 524,482
(Tomlinson et al) published as European Application
Publication No. 134,145, 534,913 (McKinley), and
535,449 (Cheng et al) published as European Application
No. 138,424, 552,649 IJensen et al) published as
European Application No. 144,187 and 904,736, published
as UK Patent Nos. 1,470,502 and 1,470,503. The
application contemporaneously filed with this
application (our reference MP0959-COM) corresponding to
U.S. Serial No. 650,918 and Canadian Patent Application
No. 472607.3 corresponding to U.S. Serial No. 573,099
(MP0897, Batliwalla et al).
Care is required to ensure satisfactory electrical
contact, with a minimum of contact resistance, between
two members of different resistivities. This is
especially true when a large and/or long contact area
is needed, as for example in strip heaters and large
sheet heaters, where contact is to be made, for
example, between a metallic member and a resistive
element composed of a conductive polymer. Methods have
been proposed for achieving such contact between a
metallic member and a resistive element. Some of those
methods involve heating the metallic member and the
conductive polymer in contact therewith at a
temperature above the melting point of the conductive
polymer; the molten conductive polymer can be contacted
with a suitably preheated metallic member, and/or the
metallic member and conductive polymer can be heated
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after they have been brought into contact. It is also
known to coat the metallic member with a highly
conductive polymer, e.g., containing a relatively high
concentration of silver or graphite, before contacting
it with the conductive polymer of the resistive
element. Other proposed methods involve the use of
conductive adhesives, staples or rivets (or other low
resistance connection member).
We have now discovered that if a thin contact
layer, composed of a material whose resistivity is
between that of two conductive members having different
resistivities is sandwiched between the two conductive
members and is bonded to the surface of the highest
resistivity member, improved electrical contact between
the said two members is achieved.
A first aspect of the present invention provides an
electrical device which comprises:
(1) a resistive element composed of a first
material which has a resistivity at 23C of 1
to 500,000 ohm.cm;
(2) a contact layer which is directly bonded to a
surface of the resistive element, and is
composed of a second conductive material
having a resistivity at 23C which is less
than the resistivity at 23C of the first
material; and
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(3) a further member which is composed of a third
conductive material having a resistivity at
23C which is less than the resistivity at
23C of the second material, preferably a
metal, said further member being in direct
physical contact with the contact layer and
being maintained in such contact substantially
only by means of pressure over a connection
area which is at least 1.61 cm3 (0.5 inch2) in
area or which has at least one dimension
greater than 2.54 cm (1 inch),
the components of the device being positioned such that
the device can be connected to a source of electrical
power so that an electrical path exists from the
further member to the resistive element through the
contact layer.
With such an arrangement good electrical contact
between the resistive element and the further member,
that is the lowest resistivity member, can be achieved
merely by pressing the further member against the
contact layer, even when the connection area is large
and/or long and even when the pressure is sufficiently
low to allow the further member to be moved relative to
the contact layer. In one preferred embodiment the
further member provides a connection means for
connection, for example to a power supply.
A second aspect of the present invention provides
an electrical device which comprises:
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(1) a resistive element composed of a first
conductive material, which has a resistivity
at 23C of 1 to 500,000 ohm.cm;
(2) a contact layer which is supported by and
bonded to a surface of the resistive element,
and is composed of a second conductive
material having a resistivity at 23C which is
less than the resistivity at 23C of the first
material; and
(3) a further member which is composed of a third
conductive material having a resistivity at
23C greater than lx10-5 ohm.cm but less than
the resistivity at 23C of the second
material, the further member being in direct
physical contact with, and preferably being
bonded to, the contact layer,
the components of the device being positioned such that
the device can be connected to a source of electrical
power so that an electrical path exists from the
further member to the resistive element through the
contact layer.
A third aspect of the present invention provides an
electrical device which comprises
(1) a laminar resistive element composed of a
first conductive material having a resistivity
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at 23C of 1 to 500,000 ohm.cm and comprising
spaced apart substantially flat surfaces,
which flat surfaces are in the same plane;
(2) interdigitated contact layers, composed of a
second conductive material having a
resistivity at 23C which is less than the
resistivity of the first material, the contact
layers being directly bonded to respective
ones of the substantially flat surfaces; and
(3) interdigitated further members composed of a
third conductive material having a resistivity
at 23C greater than lx10-5 ohm.cm but less
than the resistivity at 23C of the second
material, the further members being bonded to
respective ones of the contact layers to
provide a plurality of interdigitated
electrodes, which are positioned and shaped
such that when they are connected to a source
of electrical power, they cause current to
flow between them, through and in the plane of
the resistive element.
In the devices of the invention, there is
preferably no direct physical contact between the
resistive element and the further member.
The resistive element in the devices of the
invention is preferably composed of a conductive
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polymer. When the device is a heater, the conductive
polymer preferably exhibits PTC behavior, thus
rendering the heater self-regulating. The preferred
range of resistivity at 23C depends upon the
dimensions of the heater and the power supply to be
used, e.g. 5 to 50 ohm.cm for voltages up to 6 volts
DC, 50 to 500 ohm.cm for 4 to 60 volts DC, 500 to
10,000 ohm.cm for 110 to 240 volts AC and 10,000 to
100,000 ohm.cm for voltages greater than 240 volts AC.
The conductive filler in the conductive polymer usually
comprises, and preferably consists essentially of,
carbon black.
The contact layer preferably also is composed of a
conductive poiymer. The contact layer can exhibit PTC,
substantially ZTC or NTC behavior in the operating
temperature range of the device. The ratio of the
resistivity of the resistive layer material to the
resistivity of the contact layer material is preferably
at least 20:1, preferably at least 100:1, especially at
least 1000:1, or even higher, e.g. at least 100,000:1.
The contact layer can be applied to the resistive layer
by printing a conductive ink thereon, or through use of
polymer thick film technology, or by a process
comprising an etching step, or in any other way. The
contact layer can be present only between the most
conductive member and the resistive element, or can
extend beyond the connection member, in which case it
may act as a preferential current carrier.
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In the device according to the first aspect of the
present invention, wherein the lowest resistivity
member is preferably metal and preferably functions as
a connection means, it is preferred that the contact
layer extends beyond the lowest resistivity member in
which case the contact layer can provide one or more
electrodes which extend beyond the connection member.
The electrodes provided by the contact layer are
preferably arranged in a manner similar to that
disclosed in our copending Application No. 85300415.8
filed 85/01/22 corresponding to U.S. Serial No.
573,099 (MP0897), i.e. there are a plurality of ribbon-
shaped electrodes which are dimensioned and positioned
on a surface of the resistive heating element (in our
case the highest resistivity layer) so that
~a) when current passes between the electrodes, a
substantial proportion of the current is
parallel to the faces of the resistive
element, and
(b) the ratio of the average width of the
electrodes, measured parallel to the faces of
the resistive element, to the average distance
between ad]acent electrodes between which
current passes, measured parallel to the faces
of the resistive element, is at least 0.01:1,
particularly at least 0.1:1.
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Preferably the electrodes are so positioned and
dimensioned that, at all points, the distance between
adjacent electrodes between which current passes,
measured parallel to the faces of the resistive
element, is not more than three times the average
distance between adjacent electrodes between which
current passes, measured parallel to the faces of the
resistive element. It is particularly preferred that
the ratio of the average width of the electrodes to the
average distance between the electrodes between which
current passes is from 0.4:1 to 5:1, especially an
arrangement in which the electrodes comprise a
plurality of parallel bars which are preferably spaced
apart from each other by substantially the same
distance. Preferably adjacent electrodes are less than
1 inch apart. When the resistive element is conductive
polymer which has been melt-extruded, the electrodes
are preferably arranged so that the current flows along
the direction of extrusion.
In the device according to the second aspect of the
present invention, wherein the further member has a
resistivity greater than lx10-5 ohm.cm, and is
therefore non-metallic, it is preferred that the
contact layer has the same configuration as, and
extends slightly beyond, the further member, so that
there is no direct contact between the further member
and the resistive element. In this case the further
member, may itself provide one or more electrodes. The
devices of the present invention each provide three
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components arranged relative to each other so that an
electrical path can exist from the component having the
lowest resistivity of the three components to the
component having the hiyhest resistivity of the three
components through the other, intermediate resistivity
component. The devices may comprise more than three
components of different resistivity. Where there are
more than three components, the components are
preferably arranged sequentially in order of their
resistivity~ so that the electrical contact between any
two components is improved by the presence of an
intermediate resistivity layer between them. For
example, a preferred electrical device comprises four
components of different resistivities in which the
component having the lowest resistivity of the four
comprises a metal connection member for connection to
an electrical power source. It contacts a second
higher resistivity member, which preferably extends
beyond the connection member to provide electrodes, and
in turn contacts a third higher resistivity layer,
which preferably has the same configurations, but
extends slightly beyond the second layer. The third
layer in turn contacts a higher resistivity layer which
preferably provides a substrate resistive element. The
device according to the third aspect of the invention
comprises four members of sequentially increasing
resistivity.
By arranging one or more intermediate resistivity
layers between the members of different resistivities
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in this way, good electrical contact may be achieved
between members having resistivities differing by lO10
ohm.cm, and even up to 1012 ohm.cm.
In preferred devices according to the invention,
particularly in preferred devices according to the
first aspect of the present invention, the contact
layer preferably comprises a conductive polymer in
which the conductive filler consists of or contains a
metal, preferably silver, or a mixture of silver with
graphite or silver with graphite and carbon black. In
this case the contact layer preferably has a
resistivity in the range 2.5x10-5 to lx10-3 ohm.cm. In
other preferred devices according to the invention,
particularly in devices according to the second aspect
of the present invention, the contact layer preferably
comprises a conductive polymer in which the conductive
filler consists of graphite and/or carbon black, or a
mixture of graphite and/or carbon black with a metal,
for example silver, wherein there is more graphite
and/or carbon black than silver. In this case the
contact layer preferably has a resistivity in the range
0.5xlO-2 to 0.1 ohm.cm.
Preferred features of the further member in devices
according to the invention are now discussed.
Particularly in devices according to the first aspect
of the present invention, wherein the further member
preferably provides a connection member, that member is
preferably composed of at least one metal, e.g. copper,
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which is usually preferred for reasons of economy,
aluminum, nickel, silver or gold, or a coating of one
metal on another, e.g. nickel-coated or tin-coated
copper, and is usually a wire or sheet or tape, and may
be straight or bent or folded. Generally there are two
or more connection members in each device, the members
being connectable to a power supply to cause current to
pass through the resistive element. Often the
connection area between each connection member and a
contact layer is at least 0.5 inch2 preferably at least
5 inch2, e~g. at least 10 inch2, in area and can be
very much more. The connection area often has at least
one dimension greater than 0.5 inch2, preferably
greater than 1 inch and can be much more, e.g. at least
5 inch. Preferably the connection member makes
substantially continuous contact with the contact
layer, but this is not essential.
In the devices according to the invention and
particularly in devices according to the second aspect
of the present invention wherein the further member has
a resistivity greater than lx10-5, and is therefore
non-metallic, that member is preferably composed of a
conductive polymer. The member can exhibit PTC,
substantially ZTC or NTC behavior in the operating
temperature range of the device. In certain
embodiments of devices according to the second aspect
of the invention the ratio of the resistivity of the
contact layer to the resistivity of the further member
may be from as little as 5:1 to as much as 10,000:1,
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preferably it is in the range 10:1 to 1,000:1, for
example 100:1.
The further member has a resistivity less than that
of the contact layer but greater than lx10-5 ohm.cm.
Preferably the further member has a resistivity in the
range lx10-5 to lx10-2 ohm.cm, more preferably in the
range lx10-4 to lx10-3 ohm.cm. In a preferred
embodiment the resistivity is about 5x10-4 ohm.cm.
Where the further member comprises a conductive
polymer, it may be applied to the contact layer in the
same way that the contact layer is applied to the
resistive layer, that is by printing a conductive ink
on the contact layer, through the use of polymer thick
film technology, or by a process comprising an etching
step or it may be applied in any other way.
Devices according to the first aspect of the
invention include (i) sheet heaters, e.g. a sheet
heater wherein the resistive element is a laminar
element comprising spaced-apart substantially flat
surfaces to which the contact layers are bonded and in
particular include sheet heaters wherein the further
members are connection members, the connection members
having substantially flat surfaces which are pressed
against the respective contact layers, and the contact
layers extend beyond the areas of contact with the
connection members to provide a plurality of
electrodes; and (ii) strip heaters wherein the
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resistive element is in the form of a strip comprising
spaced-apart concave surfaces to which the contact
layers are bonded, and the connection members have
substantially complementary convex surfaces which are
pressed against the respective contact layers.
Devices according to the second aspect of the
present invention include sheet heaters, wherein the
further member itself provides a plurality of
electrodes and wherein the contact layer is at least
coextensive with the electrodes and preferably extends
slightly beyond the electrodes. Preferably the contact
layer has the same configuration as the electrodes.
The contact layer and the electrodes are preferably
each in the form of conductive inks that are applied
sequentially to the resistive layer by a printing
process.
Devices according to the second aspect of the
invention preferably also include a metal connection
member, for connection to an electrical power source.
The connection member is preferably in contact with the
electrodes, and preferably has all the preferred
features attributed to the further member of the
devices according to the first aspect of the invention.
In devices according to the second aspect of the
invention, the resistive element is preferably a
laminar element comprising spaced-apart substantially
flat surfaces to which respective contact layers are
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bonded, to which, in turn, respective further members
are bonded, the further members providing a plurality
of electrodes, which, when connected to a source of
electrical power, cause current to flow through the
resistive element, preferably in the plane of the
resistive element. Preferably the spaced apart,
substantially flat surfaces are in the same plane, and
the electrodes are interdigitated. The contact layers
preferably have the same general configuration as the
electrodes, but extend beyond the electrodes. In the
case of interdigitated electrodes the contact layers
are preferably from l.5 to 3 times as wide as the
electrodes, for example about twice as wide.
Devices according to the present invention
preferably include a dielectric layer, covering and
intimately bonded to at least part of the electrodes.
Devices according to the invention, especially devices
which are heaters, preferably also comprises a laminar
polymeric insulating element which is adjacent to, but
not secured to, the electrodes or dielectric layer (if
present), or to the electrode bearing face of the
resistive element. Preferably the insulating element
is arranged in a manner described in the Patent
Application Serial No. 490,552 filed contemporaneously
with this application corresponding to U.S. Serial No.
650,918 (MP0959, Batliwalla et al).
In the device according to the first aspect of the
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present invention the connection area between the
resistive element and the further member is at least
2.54 (1), preferably at least 12.70 square centimeters
(5 square inches) in area. The connection area
preferably has at least one dimension greater than
7.62 cm (3 inches).
An advantage of devices according to the invention
is that they can be used in applications where it is
necessary for the device to carry a current of at least
5, and in some situations at least 10 Amps.
In devices according to the second aspect of the
present invention, particularly in sheet heaters,
wherein the further members pre~erably provide a
plurality of electrodes, for example interdigitated
electrodes, on a surface of a laminar resistive
element, and the respective contact l.ayers provide an
intermediate resistivity layer between the electrodes
and the resistive element, the presence of the contact
layers not only improves the electrical contact between
the electrodes and resistive elements, but also
significantly improves the voltage stability of the
devices, as compared with devices in which there are no
intermediate contact layers and the electrodes directly
contact the resistive element. The voltage stability
of a device indicates how the resistivity of the device
changes with voltage. It is conventionally recorded in
terms of a linearity ratio (LR), that is the ratio of
the resistance at a low voltage (typically 30mV) to the
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resistance at a high voltage (typically lOOV).
Ideally, for a completely stable material the linearity
ratio is 1. The improvement in the voltage stability
in devices according to the second aspect of the
invention, as compared to identical devices in which
there is no intermediate resistivity layer between the
electrodes and the resistive layer, is particularly
substantial where the device has been subjected to
in-rush currents or to temperature ageing.
A comparative test was carried out to show the
improvement in voltage stability o~ a device according
to the second aspect of the present invention
(incorporating an intermediate resistivity layer
between the electrodes and the resistive element), as
compared to a comparative, control, device (with no
intermediate resistivity layer), after submitting the
devices to a cycling voltage treatment or an ageing
treatment. In the test, comparative control devices
(with no intermediate resistivity layer) were prepared
by printing on a conductive polymer resistive element a
singIe layer of interdigitated electrodes, comprising a
vinyl based conductive ink containing silver graphite
and carbon black, and devices according to the
invention (with an intermediate layer) were prepared by
sequentially printing onto an identical resistive
element interdigitated contact layers, and respective
interdigitated electrodes over each contact layer, the
contact layer comprising a vinyl based conductive ink
containing graphite and carbon black only and having a
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resistivity intermediate to that of the electrodes and
the resistive element. In the control devices, the
interdigitated electrodes were 0.16 cm (1/16 inch) wide
and separated by 0.64 cm (1/4 inch). In the devices
according to the invention the electrodes were again
0.16 cm (1/16 inch) wide, the contact layers were
0.32 cm (1/8 inch) wide, and adjacent contact layers
were separated by 0.64 cm (1/4 inch).
Three sets of test and control devices were
prepared. The first set of devices were maintained as
virgin samples. The second set of devices were
subjected to a cycling voltage input in which a current
at 240 Volts was pulsed on and off at 15 minute second
intervals. The pulsing was carried out at 21C (70F),
for 250 cycles. The cycling represent the in-service
treatment of the devices which are continually switched
on and off and therefore sub~ected to so-called
"in-rush" currents each time they are switched on. A
third set of devices were powered continuously at 240V
and aged for 1 week at 107C (225F). The resistivity
of each set of devices was measured at 21C (70F) at
30mV and lOOV continuous current, and the linearity
ratio of each set calculated. The results are set out
in the Table below.
Table
Linearity Ratio Linearity Ratio
Control Samples Test Sample
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(no intermediate (including inter-
layer) mediate resistivity
layer)
Virgin samples 1.005 1.005
Cycled samples 1O036 1.006
Aged samples 1.034 1.008
As can be seen the linearity ratio of the control
devices is significantly and detrimentally increased by
the cyclin~ and ageing treatments, while the linearity
ratio of the test devices is only slightly increased.
Embodiments of the present invention will now be
described by way of example, with reference to the
accompanying drawings, wherein:
Figure 1 is a cross-section through a first sheet heater
according to the invention,
Figure 2 is a plan view of the resistive element,
contact layers and connection members of Figure 1,
Figure 3 is a cross-section through a strip heater of
the invention;
Figure 4 is a cross-section through a second sheet
heater according to the invention; and
Figure 5 is a plan view of the resistive element contact
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layers, further members, and connection members of
Figure 4.
Referring now to the drawings, Figures 1 and 2
illustrate a heater which comprises a heating element
comprising a laminar conductive polymer resistive
element 11 having printed on the top surface thereof
inter-digitated electrodes 12 and 13 which are composed
of a conductive polymer composition containing a metal,
e.g. silver, as the conductive filler and having
substantially lower resistivity than the conductive
polymer in element 11. Bus bars 15 and 16, composed of
expa~ded metal mesh, are folded around marginal
portions of the element 11 and the electrodes 12 and 13
respectively, the marginal portions of the electrodes
providing the contact layers of the present invention.
An insulating jacket is formed around the heating
element and bus bars by a polymeric bottom sheet 17 and
a polymeric top sheet 18. Sheet 17 is secured to the
bottom of the resistive element, to the bottom of the
bus bars and to edge portions of the top sheet by a
substantially continuous layer of adhesive 21 (as
shown), or by melt bonding (not shown). The top sheet
is adjacent to, but not secured to, the bus bars,
electrodes and resistive element. On top of the top
sheet there is a metallic, e.g. copper, foil 19 which
is maintained in position by an outer polymeric
insulating sheet 20, whose marginal portions are
secured to the marginal portions of the sheet 18 by
adhesive layers 22 and 23 (as shown), or by melt
i25~9
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bonding (not shown). As shown in Figure 2, the
electrodes have a width t and a length 1, and are
separated by a distance d, and the bus bars have a
width x. Typical values for these variables are
t 0.08 - 0.51 cm (0.03 - 0.2 inch~
1 6.35 - 15.24 cm (2.5 - 6.0 inch)
d 0.25 - 0.76 cm (0.1 - 0.3 inch)
x 1.02 - 2.04 cm (0.4 - 0.8 inch)
Figure 3 is a cross-section through a
self-regulating strip heater having a constant
cross-section along its length. An elongate strip 1 of
PTC conductive polymer has concave edges which are
coated with contact layers 2 and 3 of a ZTC conductive
polymer whose resistivity at room temperature is
several times less than that of the PTC conductive
polymer. Elongate wires 5 and 6, which may be solid or
stranded, are pressed against the contact layers 2 and
3 respectively by means of polymeric insulating jacket
7.
Figures 4 and 5 illustrate a heater similar to that
shown in Figures 1 and 2 which comprises a heating
element comprising a laminar conductive polymer
resistive element 11. Printed on the top surface of
the resistive element 11 is an interdigitated pattern
of a resistive conductive polymer composition 30 which
contains carbon black, or a mixture of graphite and
carbon black, as the conductive filler, and has
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substantially lower resistivity than the conductive
polymer in the element 11. Printed over the resistive
pattern 30 are interdigitated electrodes 32 which are
composed of a conductive polymer containing a metal
e.g. silver, as the conductive filler and having lower
resistivity than the conductive polymer in the
resistive pattern 30. The configuration of the
electrodes 32 is identical to that of the underprint
layer 30, but the electrodes are narrower than the
underprint layer. Thus the layer 30 extends between
the electrodes 32 and the resistive element 11 and
extends slightly beyond the electrodes 32. Bus bars 15
and 16, as used in the device of Figures 1 and 2 are
provided. An insulating jacket in the form of a
polymeric bottom-sheet 17 and a polymer top sheet 18
which is secured by adhesive 21 or by a melt bond, is
also provided as in the device illustrated in Figures 1
and 2, as is a metallic foil 19 which is held in place
by polymeric insulating sheet 20 secured to sheet 18 by
adhesive layers 22 and 23 or by a melt bond. The width
t and length 1, of the electrodes 32 are the same as
those for the electrodes 12 and 13 illustrated in
Figure 1. The width t' and the separation distance d'
of the underprint layer 30 are
5
t' 0.15 - 1.02 cm (0.06 - 0.4 inch)
d' 0.25 - 0.76 cm (0.1 - 0.3 inch(
The invention is further illustrated by the
following Examples.
:12S~S~)9
-24- MP0961-CA
Example 1
A heater as illustrated in Figures 1 and 2 was made
in the following way.
The ingredients listed below were compounded
together and melt-extruded at 232C (450F) as a sheet
0.04 cm (0.0175 inch) thick.
Ingredient% by weiqht
Polyvinylidene fluoride 79.7
("Kynar")
Carbon Black 10.2
(Vulcan XC-72)
Fillers and other additives 10.1
The sheet was irradiated to a dose of 14 megarads, thus
cross-linking the polymer. The resistivity of the
cross-linked composition at 23C was 3,500 ohm.cm. The
sheet was then heated and split into strips 18.42 cm
(7.25 inches) wide. An electrode pattern as
illustrated in Figure 1 was deposited on the strips, by
screen-printing a graphite-and-silver-containing
composition onto the strip, followed by drying. The
resistivity of the printed composition, after it had
dried, was about 10-4 ohm.cm. The distance (d) between
adjacent electrodes was 0.64 cm (0.25 inch); the width
l~S~5~9
-25- MP0961-CA
(t) of each electrode was 0.16 cm (0.0625 inch); and
the length (1) of each electrode was 13.72 cm (5.4
inches).
Bus bars of nickel-coated copper expanded metal,
3.81 cm (1.5 inch) wide, were folded around the edges
of the electrode-bearing strip, and the assembly
laminated between (A) a bottom sheet of ethylene-
chlorotrifluoroethylene copolymer ("Halar", a
trademark of Allied-Signal) 21O6 cm (8.5 inch) wide
and 0.05 cm (0.020 inch) thick, coated on the whole
of its top surface with a layer 0.005 cm (0.002 inch)
thick of a silicone adhesive sold by Adhesives
Research Corporation under the trade name "Arclad",
and lB) a top sheet of ethylenechlorotrifluoro-
ethylene ("Halar") 21.6 cm (8.5 inch) wide and 0.025
cm (0.010 inch) thick, placed in contact with the
printed electrodes, which was coated on 1.27 cm (0.5
inch) wide edge portions of its bottom surface with
a layer 0.005 cm (0.002 inch) thick of the same
adhesive. Lamination was carried out at 52C (125F)
and 690 KPa (100 psi). There was no adhesive between
the top sheet and the bus bars, or between the top
sheet and the conductive polymer sheet, or between the
top sheet and the electrodes. A sheet of copper,
0.005 cm (0.002 inch) thick and 18.24 cm (7.25 inch)
wide, was placed on the exposed surface of the top
sheet, and an outer sheet of ethylene-chlorotrifluoro-
ethylene ("~alar"), 21.6 cm (8.5 inch) wide and
0.01 cm (0.005 inch) thick, was placed over the
copper sheet and laminated [at 52C (125F) and
1~51~
-26- MP0961-CA
690 KPa (100 psi)] to the edge portions of the bottom
sheet (but not the copper foil), through 1.27 cm (0.5
inch) wide layers of 0.005 cm (0.002 inch) thick
"Arclad" adhesive on edge portions of the outer sheet.
There was no adhesive between the outer sheet and the
copper foil.
~xample 2
A heater as illustrated in Figure 4 was made in a
same way to the heater illustrated in Figures 1 and 2
as described in Example 1, except that before the
electrode pattern was deposited on the strips, an
underprint layer comprising a graphite containing
composition, having a resistivity of about 0.1 ohm.cm,
i.e., intermediate between the resistivity of the
resistive element and the electrodes, was deposited on
the strips by screen printing, and then dried. The
electrodes were then screen printed directly to overlie
the underprint layer. The interdigitated portions of
the underprint layers were twice as wide as the
electrodes. Thus the width (t) of each electrode was
0.16 cm (0.0625 inch) and the width (t') of each of the
interdigitated portions of the underprint layer was
0.32 cm (0.125 inch). The distance (d') between
adjacent interdigitated portions of the underprint
layer was 0.64 cm (0.25 inch).
