Transparent pane with radar-reflecting properties

SWEDEN (12) PATENT (13) C2 (ii) 523 348

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(19) SE (st)

International class ⁷

C03C 17/34, F41H 5/26 (21) Patent and Patent Application (45) (41) (22) (24) (62) (86) (86) (83)

Patent issued 2004-04

Application generally available 2004-02 The patent application was filed in 2002-08

Running day 2002-08

Tribal application number

International filing day Filing date for European patent application Deposit of microorganism

<td> 13</td><td></td><td>number 0202432-1</td>

<td> 16</td><td></td><td></td>

<td> 15</td><td colspan="2">Application received as:</td>

<td> 15</td><td>X</td><td>Swedish patent application</td>

<td></td><td></td><td>completed international patent application with number</td>

<td colspan="2"> □</td><td>converted European patent application with number</td>

(30) Priority information (73) (72) (74) (54) (56) (57)

PATENT HOLDER Total Defense Research Institute, 172 90 Stockholm SE INVENTOR Örjan Staaf, Uppsala SE, Mikael Georgson, Linköping SE OMBUD

NAME Transparent box with radar reflecting properties

CALLED PUBLICATIONS: - SUMMARY:

The warfare boxes (1) of a warship are often coated with a radar reflecting layer (20) to reduce the radar signature of the vehicle. At the same time, this layer increases the ability of an enemy to recognize the vehicle during passive IR detection, since the layer also significantly increases the reflectance for IR radiation. As a result, the intensity of IR radiation that an enemy receives from the boxes is much less than that of the vehicle in general, which can be used with signal processing to increase the possibility of recognition of the vehicle. In addition, according to the invention, it is proposed that the panels (1) on the outer surface (8) have a second layer (21, 22) for increasing the emissivity especially within the IR range 2-20 µm used for scouting and commonly used. In particular, this second layer (21, 22) comprises two coatings, one of which is effective in the IR range 3-5 µm and the other in the IR range 7-14 µm. The first coating (21) is proposed to include tin dioxide and the second coating (22) is silicon nitride oxidized.

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The numbers in brackets indicate international identification code, INID code. Letters in clamps indicate international document code.

523 348

SUMMARY

The warfare boxes (1) of a warship are often coated with a radar reflecting layer (20) to reduce the radar signature of the vehicle. At the same time, this layer increases the ability of an enemy to recognize the vehicle during passive IR detection, since the layer also significantly increases the reflectance for IR radiation. As a result, the intensity of IR radiation that an enemy receives from the boxes is much less than that of the vehicle in general, which can be used with signal processing to increase the possibility of recognition of the vehicle. In addition, according to the invention, it is proposed that the panels (1) on the outer surface (8) have a second layer (21, 22) for increasing the emissivity especially within the IR range 2-20 µm used for scouting and commonly used. In particular, this second layer (21, 22) comprises two coatings, one of which is effective in the IR range 3-5 µm and the other in the IR range 7-14 µm. The first coating (21) is proposed to include tin dioxide and the second coating (22) is silicon nitride oxidized.

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Technical area

The invention relates to a box according to the preamble of the first claim. The box is intended to be used mainly as a sighting box in a military vehicle eg. a combat vehicle or a naval vessel. To reduce eg. For example, in the radiation to a radar transmitter, the glass-made box is preferably provided with an outer surface reflecting for radar radiation. For this purpose, the box is usually coated with a layer of metal or metal oxide.

The prior art

Vessels of various kinds are equipped with windows to enable the crew of the vehicle to look from the inside of the vehicle, cover the sight, headlights and the like. The panes, which are transparent to the beneficial radiation used for the purpose, which may be IR or UV radiation but preferably visible to the eyes, are usually made of glass. When IR radiation is used, they are made of e.g. germanium. If the boxes are used in combat vehicles, they should have a good defense capability. They therefore have a considerable thickness, often more than 50 mm, and are made of laminated glass, e.g. a plurality of sheets of tempered glass with intermediate layers of any polymer.

Modern combat vehicles should have stealth properties obtained through eg. a. A suitable design with flat hard surfaces. The surfaces are arranged to reflect as much as possible of the enemy radiation in a direction away from the enemy transmitter / receiver. This first hostile radiation is usually a radar radiation used for scouting and by target seekers. But can also be another electromagnetic radiation such as UV or laser radiation. If the vessels have untreated glass panes that pass through radar radiation, they enter the interior of the vehicle. Inside, it is reflected one or more times before returning more or less randomly to the transmitter. In addition, if the radiation is reflected by corner reflectors, which are often present in the spaces, a significant portion of the radiation is reflected, with consequent deterioration of the creeping properties.

This disadvantage is prevented by the glass panes of the prior art being arranged to reflect the radar radiation so that it is directed away from the transmitter in the same way as described above for the other parts of the vehicle. The reflection is achieved

523 348 by using the box ex. includes a radar radiation reflecting layer, which, like the box itself, is transparent to the utility radiation used for the purpose.

If this layer is inserted into the box some distance from its outer surface, 5 corner reflectors are obtained in the corner of the box between two of the adjacent edge surfaces of the box and the reflective layer. With this layer far down in a thick box, the re-reflection from these corner reflectors will be significant.

Of course, some corner reflectors are not formed if the reflecting layer according to the prior art is arranged on the outer surface of the pane or if the pane is arranged reflective in some other way. The reflection may act by interference or holography but is preferably provided by an electrically conductive layer e.g. of metal or metal oxide. The layer is of such a thickness that the screen is not obstructed but still has so little resistance that an effective reflection for radar radiation is obtained. Examples of materials used are gold, indium oxide and tin oxide. The latter coatings are usually preferred, as they do not, as gold, give any color to the transmitted light.

A radar high reflective outer surface causes the disadvantage that the surface can have a very large reflectance even for one of the other radiation used by the enemy. This is usually also an electromagnetic radiation, which, like the first hostile radiation, is often used for scouting and targeting purposes. Specifically, it is an IR radiation that is commonly used passively and for which the grid surface, with a metal or metal oxide coating, has a very high reflectance. Since the glass material that is usually present in a box does not pass IR radiation from the interior of the vehicle, the radiation from the box, which can be perceived by an IR instrument, depends partly on the intrinsic radiation of the box and partly reflected radiation. Intrinsic radiation is dependent on body surface temperature and emission. In general, the emission is very small as the reflectance is very large as in this case.

The other parts of the vehicle have a much larger emission than the radar reflecting box, usually 0.8 - 0.9 instead of the box 0.2 or less. The temperature of the other parts is usually about the same as the box, since they are both largely determined by the temperature of the common interior spaces. These other parts will therefore emit a radiation intensity which is thus 4-5 times greater than the box, since the intrinsic temperatures are the same and the intensity determined by the emitters of the surfaces.

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As mentioned, the reflectance of the coated box is very large for IR radiation. The radiance of the box therefore consists largely of a reflection of the IR radiation coming from the surroundings. On stealth vehicles, the sides are often inwardly inclined, which is why the squares will reflect IR radiation from the sky. Especially during cloudless nights, very little radiation is obtained from it. This small radiation thus provides a very small addition to the small emitted radiation.

Thus, the total radiation from the boxes in the vehicle will be much smaller than that from the other parts of the vehicle. Through signal processing, this difference can be used in the production of images, display of symbols for different targets, programming of weapons or ammunition, etc. For example, in the production of images, a color change, usually dark areas, is obtained for the boxes on an otherwise bright picture of the vehicle. The size and location of these different colored areas in the image of the vehicle facilitates its recognition. Since recognition is the basis for an enemy's choice of a successful combat method, with a. Choosing suitable tactics and weapons, the presence of these different colored areas is of great detriment to the vehicle in a combat situation. The approach is different if the target is a heavily armored tank or a lightly armored bandwagon.

Disclosure of the Invention

Technical problem

The object of the invention is to provide a box of the type initially specified for use especially for military vessels such as combat vehicles and military vessels. With the box of the invention, the detrimental effects of recognition due to the smaller emittance of the box should be suppressed to such an extent that they cannot facilitate this recognition. This without affecting the window's reflectivity for example. radar radiation is reduced so much that the risk of the vehicle's detection is adversely increased.

Solution

The purpose is achieved by giving the box the characteristics that appear in any of the following claims.

According to the invention, it is proposed that the window comprises a screen which is arranged to increase the emission of the window for the other hostile radiation used in the present case, usually IR light. The layer should preferably increase the emissivity to roughly correspond to that of the parts of the vehicle adjacent to the box. Since the box, as mentioned above, has about the same temperature as the surrounding ambient

523 348 parts, the heat radiation will thereby be the same as for these parts. The enemy's ability to recognize by drawing attention to the mentioned areas with less emitters is hampered by signal processing.

IR instruments of the kind are usually arranged to detect IR radiation within the wavelength ranges that can penetrate atmospheres loaded with battle smoke and moisture. Their sensitivity is therefore adapted for the range of about 2 - 20 µm, especially the two so-called IR windows 3-5 µm and 7-14 µm. Therefore, for an increase in the emission, the layer must be effective especially for the areas of these windows.

The layer skates as well as the means for the reflection of the first hostile radiation, e.g. the radar radiation, being transparent to the utility radiation used for the purpose, usually a light visible to the eye. If the layer is applied to the outer surface of the pane, this should be scratch resistant to withstand wear from, for example, windscreen wipers. In the preferred embodiment, the layer is of a certain type of material of a certain thickness which gives a desired emittance.

The layer may be a glass or a polymer. However, such a layer can have major drawbacks such as having too low an emission and being brittle and scratch-sensitive. According to a development of the invention, therefore, the use of a layer comprising materials which is more suitable is proposed. These materials often have effect only in a limited wavelength range. By using at least one, preferably two, such materials, which for practical reasons are preferably included in their respective coatings and arranged to be operative in each of their important areas, e.g. in the aforementioned IR25 windows 3 - 5 µm and 7 - 14 µm, a screen of the desired properties can be obtained.

An example is a layer applied to the outer surface of the box comprising one or more of both of the following coatings: A first coating comprising a metal oxide, preferably a material of the kind of metal oxides having a low electrical conductivity and which is particularly present in a near stoichiometric form.

Such materials are often effective especially within the infrared window 3-5 gm. They operate by having free charge carriers, which, when illuminated, absorb the materials, especially within said window with short-wave IR35 radiation. A greater absorption, according to the laws of physics, means that the material surface has a greater emission. Examples of suitable metal oxides which may have a near stoichiometric form are titanium oxide, zirconia, hafnium oxide, magnesium oxide and tin oxide, wherein

523 348 their dioxides are often the most useful. Other materials, which have a close stoichiometric shape, other than metal oxides are conceivable for the purpose.

A second coating comprising a preferably hard material and which may be a ceramic. It has been found that materials suitable for producing infrared light in a certain band by so-called residual rays are also suitable for this purpose. If some of these materials, which are thus of the kind having residual band band properties, are irradiated with IR light, oscillations of the material's lattice atoms occur especially for the light having a slightly longer wavelength such as that within the window 7-14 pm. In this way, energy is absorbed and the material surface also receives a greater emittance in this case. Examples of suitable materials with residual radiation band properties are silica, e.g. quartz, beryl oxide and berylium silicate, silicon carbide, sialon, cubic boron nitride and silicon nitride. Also, other materials having residual beam band properties than those mentioned are conceivable for the purpose.

These coatings may, from an optical point of view, be applied in any order on the box, but the layer is usually the easiest to manufacture and / or have the best mechanical properties when the coatings are applied in a specified sequence, which may be determined by means of tests.

benefits

According to the preferred embodiment, coatings of undoped tin oxide and oxidized silicon nitride are used, which are effective in each of the IR windows. The issuance in these windows has thereby been increased to about 0.8, ie to a value approximately equal to the value of the parts of the vehicle surrounding the box. On the enemy's IR instrument, the agent means that some different colored areas of the vehicle's windows are no longer displayed.

This makes identification of the target more difficult. The added coatings do not reduce the radar radiation's reflection in a harmful way. The preferred agent prevents transparency very little and has proven to be very scratch resistant.

Figure Description

A preferred embodiment will be described in more detail with reference to the accompanying figures whose reference numerals denote corresponding parts in the figures.

Fig. 1 is a cross-sectional view of a part of a wartime sighted frame.

Figure 2 shows, in an enlargement of the area II of Figure 1, the screen of the coatings of the invention.

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Figures 3 and 4 show diagrams of the spectral properties of the surfaces applicable in the context.

Preferred embodiment 5

Figure 1 shows a view box 1 according to the invention for a warship. The sight pane is inserted into a wall 2 of a superstructure belonging to the ship's hull, to be one of the command bridge sight panes. The screen is held in place in an opening 3 in the wall 2, which is of course continuous, by means of a metal fastening frame 4.

The mounting frame is made up of corners welded T-profile-shaped lengths to obtain an outwardly flat surface 5.

The screen pane consists of a laminate comprising prior art a number of sheets 6 of tempered glass and intermediate layers 7 of polymer, preferably polyvinyl butyral.

With a total thickness of about 50 mm, it is reinforced by the reinforcing properties of the polymer layers, a protective capability that can prevent splits from entering the vessel. The sight pane has an outer surface 8 and an inner surface 9 which are flat and parallel and edge surfaces 10 perpendicular to the outer surface. Around the periphery, the outer surface of the sieve pane has an immersion 11 of such a design that an inner flange 12 of the mounted fastening frame is recessed into the sieve pane so much that the outer flat surface 5 of the fastening frame is aligned with the outer surface 8 of the inserted sieve pane. The rim flange 13 is configured to mount the sight pane with a suitable clearance to the edge surface 10 of the sight pane.

The aperture 3 arranged in the hull for the sight pane can accommodate the outer frame 4 in the outer part. The wall is submerged around the aperture to enable the flange of an outer flange 14 to be folded in such a way that its outwardly facing surface 5 is aligned with the outer surface of the hull wall 2 outside this immersion. Thus, the outer surfaces of the hull, attachment frame and sight pane will lie in a common plane, which is favorable for the ship's stealth properties. Ev. unwanted intermediate gaps are filled with electrically conductive or radar absorbing material.

Within the retraction of the outer flange, the opening has dimensions such that it can accommodate the mounting flange 13 of the fastening frame with a suitable clearance. A portion below this area is the aperture constricted to form along the entire edge a bracket formed by the hull wall with a plane parallel to the outer surface of the wall 16. gasket 17 is inserted between the viewing window

523 348 and the abutment surface. The mounting frame is held in the hull wall by means of indicated screws

18.

The screen has at its outer surface a layer 19, the structure of which is shown in detail in Fig. 2.

The layer is shown disproportionately thick in the two figures for the sake of illustration.

The layer consists of three coatings, the innermost being a radar-reflecting coating 20 of the prior art having the characteristics described in the introduction. The coating mainly comprises doped tin dioxide of a thickness of about 0.5-1 microns. For best effect, it is grounded to the vessel wall 2 by extending it even over the submerged portion 11 of the sight pane. There it abuts against the mounting frame in electrical contact with it by means of a sealing and electrically conductive putty.

In addition to the radar reflective coating 20 are two coatings 21, 22 of the invention for increasing the visibility of the sight pane. The coatings extend over the entire screen except the submerged portion 11. A first coating 21 of undoped tin oxide (SnO₂) with a thickness of 0.3 - 0.8 µm, preferably about 0.5 µm, which is applied directly to the radar reflective coating, increases the emission of light especially in the range of the IR window from 3 to 5 µm. Ultimately, on top of this first coating, there is a second coating 22 which increases the emission of light especially in the area of ​​the IR window 7-14 pm. This coating is of oxidized silicon nitride (SiO_xNew) and has a thickness of 0.5-1.5 µm, preferably about 1.0 µm.

The radar reflective coating and the first emitting enhancing coating are applied by a Physical Vapor Depositiori method e.g. sputtering. Hereby, a noble gas such as argon with mixed oxygen is used under a very low pressure in a space between the sieve pane and a cathode comprising tin doped with 10% antimony for the radar reflective coating and undoped tin for said first coating. When an electrical voltage is applied across the sieve pane and the cathode, the argon is ionized and material migrates from the cathode to the sieve pane while the tin reacts with the oxygen gas to form tin oxide. In tests, a flow volume ratio of argon to oxygen of about 5: 3 has produced good results.

These two coatings can also be applied by a Chemical Vapor

Depositary method ex. pyrolysis. In this way, the radar reflector is sprayed

523 348 coating a tin chloride-alcohol solution together with a solution containing fluorine or antimony, e.g. an aqueous solution containing ammonium fluoride on a heated screen at 450 ° C where the tin chloride reacts with oxygen in air to form doped tin oxide. The spray volume uses a flow volume ratio of the tin chloride solution to the ammonium fluoride solution of about 2: 3. For the first coating, only one tin chloride-alcohol solution is sprayed on the sieved pane heated to 380 ° C. Large variations from these values ​​can be allowed without but for the result. The reason why the reflective tin oxide coating is doped is because it thereby forms free charge carriers in the material. Thereby, the coating achieves so much electrical conductivity that a sufficient radar reflectivity is obtained.

With regard to the first coating, as previously mentioned, the tin oxide is used undoped and preferably with a deficit of oxygen in order to obtain the near stoichiometric tin oxide with low electrical conductivity. This material absorbs as already described radiation in the IR range 3-5 pm.

The second coating, which is of oxidized silicon nitride, is applied by sputtering. Nitrogen and oxygen are used as sputter gases in the flow volume ratio approx

17 : 1 to form the oxidized silicon nitride. Silicon is used as the material in the cathode. The said flow volume ratios are only guide values ​​and depend on the equipment used. However, large variations from these values ​​can be allowed without but for the result.

To further complicate detection, the sight pane can be provided with an anti-reflex treatment that reduces the surface reflexes of the pane. Especially at oblique viewing angles, which can occur when the sight pane is integrated in signature-adapted vehicles, the surface reflexes can become large and thus clear the vehicle. In addition, the antireflective treatment results in increased visual transmission through the sight pane, thereby increasing the contrast in the environment viewed by the eye or sensor.

An example of anti-reflex treatment is to apply an anti-reflex layer of magnesium fluoride on both sides of the screen, ie. on the inner surface 9 and on the outer surface layer 19. The thickness of the anti-reflex layers can be optimized for different

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<td></td><td>viewing angles, e.g. an optimum thickness for the viewing angle is 60 degrees 0.12 pm. This anti-reflex treatment reduces the reflectance from 25% to 10%.</td>

<td> 5</td><td>Another example of antireflective treatment is to apply an antireflective layer consisting of a four-layer coating on the outer surface of the box 8. Four-layer coating alternately comprises layers of titanium dioxide (TiO₂) and magnesium fluoride (MgF). One anti-reflex layers optimized for 60 degrees have a thickness of 0.088 µm TiO₂; 0.238 pm MgF 0.083 µm TiO₂ and 0.114 µm MgF seen from layer 19 onwards.</td>

<td> 10</td><td>Figures 3 and 4 show diagrams of the spectral properties of the different the coatings for the IR region without anti-reflex treatment. In the diagrams, λ denotes wavelength and R reflectance. The reflectance has been used instead of this one more relevant issuance because reflectance is easier to measure. With knowledge of</td>

<td> 15</td><td>however, the reflectance is easy to calculate. Curve A in Figure 3 shows the reflectance of a sight pane with only the radar reflective coating. Curve B in the same figure shows the reflectance of a pre-coated sight pane according to it preferred embodiment, that is, with also the two coatings for increasing the issuance, affixed. The curve shows that the mean reflectance is about 0.2 in the IR window 3 - 5 pm compared to about 0.75 for the radar reflective coating alone.</td>

<td> 20</td><td>Corresponding reflectances for the IR window 7-14 pm are 0.2 and 2, respectively. 0.85. A reflection of 0.2 is equivalent to an issuance of 0.8 in this use, which is the same endeavored as above.</td>

<td> 25 »</td><td>Curve C in Figure 4 shows the reflectance of only the first coating of near stoichiometric tin oxide for the lower IR window 3-5 pm, along with it radar reflective coating. The curve shows a reflectance minimum at 4 pm in the middle this area.</td>

<td> :· 30</td><td>In the same figure, curve D shows the reflectance of only the second coating of oxidized silicon nitride intended for the upper IR window 7-14 pm, together with the</td>

<td> *</td><td>radar reflective coating. The curve shows a minimum at 11 pm which is in</td>

<td> • » « • *</td><td>area. It can be read that even though no part of the curves reaches further down than approx 0.4 at the wavelength of 4 pm, so the curve B of the finished layer reaches all the way down to one average of about 0.2 in the range of 3-5 pm. The combination of these two coatings</td>

523 348 is thus much more efficient than each coating individually. The combination effect may be due to interference. The tendency is the same even for the IR window 7 -14 μιτι.

When sampling the radar reflectance of the radar after applying the two coatings, a decrease of less than 1% was measured. As the decrease is negligible in this context, no more accurate measurements were made.

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