Patent Description:
The invention could also be used in applications in fields like aeronautics or automotive industry, for providing customized heating parts regarding localized areas, such as for example in defrost applications.

In the context of current military conflicts, and mainly asymmetric conflicts, the protection of troops and equipment is essential. Military strategies in mission theatres are also permanently in evolution, mainly due to the emergence of increasingly efficient and sophisticated technologies. This essential aspect of protection for the troops is now reinforced by calls for proposals of the European Defence Industrial Development Programme (EDIDP).

On the ground, the main objective in terms of protection and therefore stealth is, according to danger order :.

Day or night, a widely used means of detection remains infrared thermography, which enables to perceive the thermal signature of a soldier, (armoured) vehicle, etc..

The idea at the basis of the present invention is therefore to be able to couple both camouflage in the visible while being stealthy in the infrared, either by blending into the environment (i.e. becoming "invisible") or by adjusting the thermal signature to dupe the enemy.

Document <CIT> (or <CIT>) discloses a number of systems and assemblies for simultaneous adaptive camouflage, concealment and deception. The assemblies that can be used in the systems include a vinyl substrate layer and a miniaturized thermoelectric device array secured to the vinyl substrate layer. The miniaturized thermoelectric device array is configured to provide an adaptive thermal signature to a side of the miniaturized thermoelectric device array that faces outward from the vinyl substrate layer. A flexible image display matrix can be secured on the vinyl substrate layer, said flexible image display matrix being configured to display visual images. A laminate layer can be secured over the vinyl substrate layer covering the flexible image display matrix and the miniaturized thermoelectric device array to provide protection and strengthen the assemblies. One or more nanomaterials can be disposed on the vinyl substrate layer or the laminate layer to provide thermal or radar suppression. This disclosure seems to be only a concept, a suggestion for combining a succession of new technologies not yet turned into practice, which would be expensive and lacking flexibility.

Document <CIT> relates to a thermal vision countermeasure system to enable concealment of objects from identification by thermal imaging night vision systems, including a screen made of thermoelectric (Peltier effect) modules, disposed between the target object and an IR detector. The screen, formed of a number of thermoelectric units, is coupled to the target object, and the thermoelectric unit includes a thermoelectric cooler (TEC) module coupled to a plate formed of a material selected from aluminium, copper, or aluminium with copper, the plate being substantially larger than the TEC module. This technology allows full coverage with interchangeable and bee structure modules, providing IR cartography and radar stealth. However this solution is complex, expensive to develop, provides poor cartography resolution and does not allow visible dissimulation.

Document <CIT> discloses a device for signature adaptation, comprising at least one surface element arranged to assume a determined thermal distribution, wherein said surface element comprises at least one temperature generating element arranged to generate a predetermined temperature gradient to a portion of said surface element. The surface element also comprises at least one radar suppressing element, wherein said radar suppressing element is arranged to suppress reflections of incident radio waves. The invention also concerns an object provided with a device for signature adaptation. A full coverage is obtained thanks to interchangeable and honeycomb-like hexagonal unitary panels structure, with good spatial resolution. However, this solution is complex, expensive to develop, not easy to attach on site and does not provide visible dissimulation (not embedded).

Document <CIT> discloses a device provided for camouflaging an object from an infrared detection apparatus. The device includes a cloak positionable between the object and the infrared detection apparatus. The cloak includes a layer of infrared absorptive material including a plurality of silicon nanowires. A flexible substrate has a first surface operatively connected to an inner surface of the layer. The substrate includes a heat dissipation arrangement for dissipating heat generated by the cloak during operation. An array of infrared emitters is operatively connected to a second surface of the substrate. The array of infrared emitters selectively radiates an infrared pattern to disguise the object to the infrared detection apparatus. The heat dissipation arrangement includes a channel formed in the flexible substrate and adapted for receiving a cooling fluid therein. The heat dissipation arrangement further includes a pump for recirculating the cooling fluid through the channel. Each IR emitter is electrically connected to a corresponding contact by a corresponding line, each contact is operatively connected to a processing unit, e.g. a controller. The controller is configured to selectively actuate each IR emitter such that each actuated IR emitter transmits an infrared signal that is visible to IR reader. This disclosure is very conceptual and poor in details.

Document <CIT> discloses a fibre optics network having a grid of fibre optic cables with a section having the sleeve removed and fed from an input point. Different infrared bands are generated and radiated from one of the optical lines. The system measures the background radiance in the different infrared bands and sets the radiated levels to provide camouflage. The optical cables are embedded in a flexible outer section.

Document <CIT> relates to a carbon material based double-sided active infrared emissivity adjustment thin film, in particular to a thin film material based on a carbon material (including graphene, carbon nanotubes, amorphous carbon, carbon black and the like) and ionic liquid. A flexible device with double-sided infrared radiation control can be realized through a voltage regulation mode, with low working voltage and power consumption, large emissivity adjusting amplitude and simple structure. The adjustment thin film is suitable for large-scale production, has good mechanical bending performance, can be widely applied to infrared camouflage or stealth on the surfaces of automobiles, ships, airplanes, satellites and the like, and can also be applied to the surfaces of batteries, micro-nano satellites and the like to realize temperature control.

<NPL>), demonstrate an active cloaking device capable of efficient thermal radiance control, which consists of a vanadium dioxide (VO<NUM>) layer, with a negative differential thermal emissivity, coated on a graphene/carbon nanotube (CNT) thin film. A slight joule heating drastically changes the emissivity of the device, achieving rapid switchable thermal camouflage with a low power consumption and excellent reliability. This device is intended to find wide applications not only in artificial systems for infrared camouflage or cloaking but also in energy-saving smart windows and thermo-optical modulators.

Prior art IR stealth solutions generally present the following drawbacks:.

The present invention aims to provide an efficient active IR stealth system, which is inexpensive, light and not bulky as well.

Another goal of the invention is to provide a flexible, easy-to-attach, embedded, customizable in terms of shape and/or pattern and object-matching solution.

Furthermore, the invention is intended to provide a spatial IR mapping with suitable resolution.

The above aims of the invention are solved by an ultrathin, multilayer and encapsulated surface element according to claim <NUM> as well as the use of one or more surface elements according to claim <NUM>.

A first aspect of the present invention relates to an ultrathin multilayer and encapsulated surface element for providing thermal signature adaptation with the purpose of infrared stealthing, and being also suited for camouflage in the visible, the element being flexible and comprising :.

the heat dissipation elements having a size and being organized according to a spatial arrangement so as to provide a predetermined infrared spatial resolution, when a current is fed into said dissipation elements.

According to preferred embodiments, the surface element additionally comprises one or a suitable combination of the following features :.

A second aspect of the present invention relates to the use of one or more surface elements as described above, for providing an object or a person with a cover, sheet, blanket, casing or roofing capable of adapting the thermal signature of said object or person with the purpose of infrared stealthing, deception, camouflage, decoying or concealment.

Preferably, the plurality of heat dissipation elements selectively radiate a surface infrared pattern allowing to avoid the person or the object covered by said one or more surface elements to be detected by an infrared detection device.

The present invention is based on an active (i.e. controllable) system <NUM> of heating resistors <NUM> printed on a thermally and electrically resistive substrate <NUM>, as shown in <FIG>. For example the resistive heating layer <NUM> printed on the resistive substrate <NUM> can be made of metal itself (e.g. silver conductive paste) or of a carbon-based material with power feeds under the form of a conductive layer <NUM> made of silver (or copper, aluminium, etc.). The underlying principle is that the heating element <NUM> is based on different resistivity between carbon and silver. Useful printing techniques are for example serigraphy (also called screen printing or silkscreen), inkjet, flexography, sintering and other printing electronic deposition methods, possibly combined with heating or radiation (oven, IPL, IR, UV, laser, etc.). In addition, other new 3D printing electronic deposition methods can also be used, such as spray (with stencil), micro spray, 3D inkjet , ink dispensing, etc. In this case, direct printing can be performed on 3D objects.

The whole multilayer system is finally encapsulated by an insulating protective layer <NUM> obtained for example by serigraphy or spraying and made of off-the-shelf dielectric components such as oxides (e.g. Al<NUM>O<NUM>, ZnO, TiO<NUM>, etc.), polymers (polycarbonate, polyimide, PE, PP, PET, PVC, etc.) or ceramic-based materials.

The IR stealth technology according to the invention is intended to provide two functions :.

The following examples provide matrix/2D architectures that illustrate the general idea of the present invention. A first example (<FIG>) shows 3x3 independent modules <NUM> (from the point of view of power feeding) and a second example (<FIG>) shows 10x10 multiplexed modules <NUM>. The independent modules <NUM> have each an independent power feed while in the multiplexed module <NUM>, a specific module is chosen by power feeding a specific line and a specific column (matrix power feed).

Further, considering a multiplexed configuration of functionally independent modules/cells, one module <NUM> equals one pixel (<FIG>). The resistive elements <NUM> (e.g. made of carbon containing material) are under the form of a regularly spaced studs layout embedded in conductive tracks <NUM> (e.g. made of silver) under the form of combs (see detailed description below). In this array configuration, each cell or pixel <NUM> can be activated independently.

In another example of multiplexed modules configuration (<FIG>), each module may comprise <NUM> pixels. A power supply distributes the current to the pixels either sequentially or by selecting/programming a specific pixel to heat in a matrix way (selection of a row and a column). Each cell or pixel has a multiplexed connection with the power supply, it means that each cell or pixel of the two-dimensional array is connected to a "column" input electrode and to a "row" output electrode (or vice versa) so that a determined pixel in position (X, Y) is powered by selecting respective row and column powering tracks in the two-dimensional array. Row and column powering tracks suitably overlap in the array thanks to positioning dielectric elements <NUM> preventing electric contacts at the crossover points of the row and column powering tracks (see below).

In a first embodiment (<FIG>), the inventors used a metallic support <NUM> as a substrate covered with an insulating layer (dielectric) <NUM>. The dielectric layer <NUM> was a PVC material layer having a thickness up to <NUM>. Conductive tracks <NUM> were obtained by screen printing of silver-based microparticles. The layout (architecture) was made of linear simple tracks. Simulations showed an important heat dissipation through the underlying metal substrate and a non-uniform increase of temperature in a hexagonal full silver motif/heater module was observed (not shown).

In a second embodiment (<FIG>), the metallic substrate covered with a dielectric insulating layer was replaced with a glass substrate <NUM>, which is a good thermal and electric insulator. The conductive tracks <NUM> and architecture layout were unchanged.

The use of a substrate material having a drastically reduced thermal conductivity led to a much better spatial IR resolution (but with some local increase of temperature). Silver based heater tracks gave high ΔT but also gave uniform T mapping and further good IR spatial resolution. Different motifs/heater modules were investigated (hexagonal Ag/C, spiral Ag,. ) with the occurrence of non-uniform temperature and current distribution (not shown). This showed the need of further increasing IR spatial resolution.

In a third embodiment, while still utilizing a glass substrate <NUM> as a thermal insulator, an active material <NUM>, under the form of a ferroelectric positive temperature coefficient (PTC) material, was used as carbon resistive material, in combination with conductive silver tracks <NUM>, as described above (<FIG>). Each cell belongs to an array making a heat dissipation element and is obtained by the superposition of a silver layout <NUM> on a carbon layout <NUM> (<FIG>), which will be described with more details here below.

Each cell <NUM>, <NUM> comprises a module of conductive tracks <NUM> having an input electrode <NUM>, <NUM> and an output electrode <NUM>, <NUM>, having the form of two interdigitated combs, and comprising an array of heat dissipation elements <NUM> connected between the respective teethes <NUM>, <NUM> of the interdigitated combs. Note that respective <NUM>, <NUM>, <NUM>, etc. and <NUM>, <NUM>, <NUM>, etc. reference signs refer to the embodiments with 3X3 independent heater modules and 10x10 multiplexed heater modules. The "cell" element can also be referred to as a "pixel" with reference to the IR spatial resolution of the device.

Preferably, each cell or pixel <NUM>, <NUM> is obtained by firstly printing, for example by screen printing, the module of conductive tracks <NUM> on the insulating substrate <NUM>, <NUM> and secondly printing the array of heat dissipation elements <NUM> onto both insulation substrate <NUM>, <NUM> and module of conductive tracks <NUM>, so that the heat dissipation elements are brought into close electrical contact with the module of conductive tracks <NUM>. Each heat dissipation element has preferably a squared shape with an upper surface and a lower surface, said lower surface being provided with a side recess <NUM> on two edges, so that the heat dissipation element can be inserted between adjacent teethes <NUM>, <NUM> of the first electrode <NUM>, <NUM> and the second electrode <NUM>, <NUM> respectively (see <FIG>).

The non-multiplexed heater motif size is: 4x4 cm<NUM> (see <FIG>). The material is composed of silver tracks having outside input/output electrode width of <NUM> and interdigitated width of <NUM>. The PTC carbon resistor is made of <NUM> small square units (<NUM>×<NUM><NUM>). A temperature of about <NUM> was obtained after <NUM> with a current of 210mA. A high spatial resolution is obtained with self-regulation, low current, homogeneity. The drawbacks are wide current feeds and a non-flexible substrate (glass).

In a fourth embodiment, Kapton® (polyimide film, DuPont™) has been used as a thermal insulator and flexible substrate <NUM> (<FIG>). The active material is again PTC (ferroelectrics) material <NUM> combined with conductive silver tracks <NUM>.

The coupling of track width reduction with a PTC effect allows to perfectly localize the increase of temperature and thus provides high spatial resolution, with good time response (ΔT/Δt high, low current) and self-regulation (I~40mA, V~24V). The tracks were initially chosen very large (<NUM>) outside the patch but their width could be reduced/optimized later on up to <NUM> times (not shown). As an additional advantage, the device is flexible.

In an example demonstrator with 3x3 independent cells (not shown), a heater motif of <NUM>×<NUM><NUM> is provided with <NUM> stealth cells with Ag tracks of <NUM> width, Ag interdigitated tracks of <NUM> width and <NUM> units PTC-C resistors pixels of <NUM>×<NUM><NUM>. The substrate is Kapton®.

The (T, I, V) characteristics are the following :.

An example of obtainable IR pattern is shown on <FIG>.

In an alternative embodiment (<FIG>), the architecture is a 10x10 multiplexed configuration. The multiplexed solution has a number of advantages : save space, reduce the number of connectors needed to power the device (for 10x10 multiplexed, <NUM> connectors instead of <NUM> connectors for 10x10 independent cells and no space available in the center), easier control of IR cartography. Further, in this embodiment, a dielectric layer <NUM> (e.g. Al<NUM>O<NUM>) is provided for electrode separation (<FIG>).

In an example demonstrator with 10x10 multiplexed cells (not shown), the stealth cells are provided with Ag tracks of <NUM> width, Ag interdigitated of <NUM> width and <NUM> units PTC-C resistors pixels of <NUM>×<NUM><NUM>. The substrate is Kapton®. The heater motif unit is <NUM>×<NUM><NUM>.

Claim 1:
An ultrathin, multilayer and encapsulated surface element (<NUM>) for providing thermal signature adaptation with the purpose of infrared stealthing, and being also suited for camouflage in the visible, the element being flexible and comprising :
- a lower layer made of an insulation substrate (<NUM>, <NUM>) ;
- an intermediate layer comprising a plurality of conductive tracks (<NUM>) connectable to a power supply ;
- a plurality of active heat dissipation elements (<NUM>, <NUM>) connected to the conductive tracks (<NUM>) in the intermediate layer and capable to irreversibly provide a temperature increase by Joule effect in a given time interval when a current is fed into said heat dissipation elements, wherein each dissipation element (<NUM>, <NUM>) comprises an electric conductive path extending between a first connexion and a second connection connecting respectively said dissipation element (<NUM>, <NUM>) to least two of the conductive tracks (<NUM>), said dissipation element (<NUM>, <NUM>) being capable of providing the temperature increase throughout said path from the first connection to the second connection when the current is fed, the heat dissipation elements (<NUM>, <NUM>) and the conductive tracks (<NUM>) being printed on the insulation substrate (<NUM>, <NUM>) ;
- an upper layer made of a protective layer (<NUM>) ;
the heat dissipation elements (<NUM>) having a size and being organized according to a spatial arrangement so as to provide a predetermined infrared spatial resolution, when a current is fed into said dissipation elements.