TRANSMISSION ATTENUATING AEROSOL

"Transmission Attenuating Aerosol" The present invention relates to a transmission reducing aerosol particularly for absorption within the IR-range (infrared spectral range).

The background of the invention is that optical techniques are used increasingly, e.g. for reconnaissance» sight and fire directing apparatus, target searching and laser guîded (semì-active) missiles, warning devices and distance measuring devices. The spectral working range for most systems is also being moved towards the long-wave IR-range (7-14 pm). The main reasons are as follows.

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• -<ki • ,ioi The thermal radiation from objects having room temperature (approximately 300 K) reaches its maximum at the wavelength I0 m.

In the long wave IR-range, it is thus possible to locate and observe objects, having a temperature slightly different from the surrounding environment. During the last few years, apart from expected developments relating to laser, image amplífiers and semiconductor technology, a break-through has also occured wíth regard to detector technology. It is now possible to manufacture arrays of electro-optìcal detectors for long wave radiation, which can considerably improve the sensitivity, range, informatíon capacity and rejection of interference in electro-optical systems.

As a result of these improvements in electro-optical apparatus, the electro-optical threat, particularly with regard to milítary objects, has increased significantly. If is now necessary to make comprehensive plans with regard to counter-measures against such apparatus, compared with electro-optical apparatus of previous generation, particularly in view of the fact that the "natura! protection" such as mist, fog or darkness, only to a minor degree, or not at all, influences the range of the new apparatus. Existing conventional artificial transmissíon reducing means, such as conventional smoke or mist, do not reduce, or only reduce slightly, the effectiveness.

Major actions must thus be taken when designing e.g. ships, vehicles or fortifications, e.g. for cooling and screening of exposed surfaces, but even such actions - in combination wìth camouflage methods - hardly reduce the electro-optical threat to previous existing level, particularly with regard to the hìgh speed action when e.g. missiles are used. The present invention relates to an areosol, which can be spread in a very short period of rime and thus reduces the required pre-warning time. As examples of fields of use can be mentioned: attack of IRor laser guided missiles against ships, fortifications, bridges and tanks.

The present invention makes it also possible to control the range and extent of time with regard to transmission reduction. The present invention has thus a great degree of flexibility. A further advantage of the present invention is that it can not only be used as an addition to conventional smoke but also in many cases» as a replacement, sìnce the aerosol covers all of the optical wave length range (0.2 - 14 pm), whereby the aerosol is effective against all types of previously mentioned types of apparatus. The attenuating surface is approximately 50% larger in the visual range than wïthin the !R-r&nge.

According to the present invention there is provided a transmissïon attenuating aerosol havïng a low reflectance index for electro-magnetic radiation useful for the absorption of radiation, particularly withïn the IR-region, which contaìns actïvated carbon particles of irregular configuration and having substantial mïcro-porosity.

The present invention is more fully described below with reference to the accompanying drawing, which shows a diagram of the distrìbution in size for the aerosol particles in a preferred embodiment of the invention. The diagram shows the cumulative number of dïstribution for particles.

The optical transmission reducing aerosol according to the present invention încludes activated carbon, acting partly due to the fact that the complex refractive index has suitable values for amorphous carbon, giving low reflection and relatively high absorption in the relevant wave length range, partly due to the fact that particles of activated carbon have suitable surface properties and morphological structure.

The carbon particles have, due to the activating process, a considerable "micro-porosity", whereby we mean a small scale porosity with holes having a size less than the optical wave length (i.e 0.i m). The porosity causes a lower density of the particles, whereby the particles, without regard to size, can be suspended for a considerable length of time.

Furthermore, the carbon particles also have a very irregular configuratïon, having "large-scale" variations in the shape and surface ïrregularïtìes ranging between one or a few m.

- 3 - I13 741 These properties contribute, as well as the number of partìcles per unit of volume, average particle size and the relatively large imaginary part of complex refractive index, to give the carbon powder the desired properties when it is necessary to efficiently and in a short t oEme achieve a wide band optical transmission reducing aerosol. A short motivatfon is given as follows.

i0 The extinction coefficient, which is a measure of the radiation attenuating ability for an aerosol (per length unit along a radiation beam), is basically the product of two factors - the specific extinction area (characteristic area of attenuation per mass unit of aerosol substance) and aerosol concentration (mass of aerosol substance per volume unìt of aérosol).

The specific extinction area is a function of wave length and is determined by the properties of the aerosol substance.

For wide band optical attenuation it is more desirable to use light, ab- - 3a í .. ,, i • , ,.[ " • . •r "°L.

.4 I13774Z i0 sorbing particles, (i.e. low density and high imaginary parts of complex refractive index) in connection with a suitable distribution of particle size. The latter means, basically, that the main part of the particles should have sizes ranging within the optical wave length interval (0.3 - 14 m) with an average value approximately in the middle of the range.

The irregular surfaces of the particles contribute in an important way to their increase of effective absorption (apart from the factor caused by the complex refractive index), depending partly on the fact that radiation, basically due to repeated reflection, is spread in a random way in the surface regions of the particles, partly due to the fact "that the irregular absorbing surface of the particle in average is considerably larger (up to 1200 m2/g) than e.g.

the absorbing surface of a spherical particle having the same volume. This fact is favourable in two ways, since not only the specific extinction area is increased, but also since the size and spectral variation is less sensitive to particle size distribution. Furthermore, the micro-porosity of the particles also causes a reduction in the effective density, which means that the specific area of extinction is increased.

The irregular shape of the particles also cause a further interesting effect. During the dispersion process, the particles may to a certain extent join together. The "new" particles thus formed, usually obtain highly irregular and "thin" configurations, which to a large extent counteract the otherwise unfavourable influence due to an increase in the mean size of the particles° The concentration of aerosol depends basically on the method of dispersion and should be controlled in such a way that desired properties of the aerosol cloud are achieved, with regard to area of attenuation, time from initiation - 4 - until the area of attenuation is developed and the time during which the aerosol cloud is maintained.

i0 In a preferred embodiment according to the present invention, the carbon particles have a size distribution according to the diagram, which to a large extent contribute to the specific transmission reducing action of the smoke.

The diameter measure for approximately 80% of the particles is mainly distributed between ]. and 9 m, and with approximately 10% above and below these limits.

The distribution in particle size must not be too inhomogenous, partly due to the fact that the method of dispersion can cause a tendency to separate particle sizes in various regions of the aerosol cloud, an effect usually not desired.

The carbon particles can be generated and dispersed Using a number of simple and reliable methods, which makes the substance even more attractive.

A preferred method of dispersion is mechanical dispersion of an existing aerosolic substance. Dispersion can be achieved by means of an explosive charge. Charges of aerosolic substance and explosive substance can be stationary, or they may be launched as projectiles. Since the aerosol is generated frcm primary charges, which separately can be given a limited duration, the action with regard to time and place can be well predecided.

For this purpose, it is extremely suitable to use activated carbon having large mechanical strength. Hereby, it is possible to avoid restrictions relating handling of the charges, during which the distribution in size for the carbon particles otherwise might be changed. The mechanical properties are also of importance, when launching the carbon -- 5 -- . r ,'<.2.

.r I13774 powder charges.

i0 When testing different designs for the cover of the charge, the spherical shape has been found most suitable, but also shapes such as cylindrical shapes, give good results.

Furthermore, when using such covers PET detonating fuse has been more advantageous as explosive means than explosive paste. During tests with spheres of paper having various size, optimal results were achieved using a diameter of 12 cm, when the sphere included 300 g carbon powder. This coincides basically with approximations when using a theoretical model for optimising the amount and the concentration of smoke.

With regard to the amount of explosive material necessary in the charge, a compromise has been made between the necessary fast dispersion of the aerosol cloud and the duration as a homogeneous transmission reducing medium.

The tests have also shown, that desired area of attenuation can be achieved within a second and that the existance can be in the region of a minute, depending on wind strength.

In an embodiment, a smoke grenade having a thin tubular cartridge with a front and a rear wall and the explosive charge in the centre of the grenade can be used.

Mechanical dispersion can also be effected using ejector methods. The carbon powder can be spread stationary way using a - 5a - Ç«"[" v " compressed air device of the same type as s being used for dry painting. The dispersion of the separate grains can also be improved by introduction of an electro-statical field.

Other methods to form and disperse the aerosol include the use of'chemical reactions. Accordingly, certain substances can be arranged to form considerable amounts of carbon smoke by restricted burníng. As an example, a rocket motor, can be used for this purpose. However, it should be emphasized, that this smoke basically consists of mainly spherical particles, having a considerably smaller surface than activated carbon particles and thus less attenuating effect on IR-radiation.

In certain cases, the camouflaging aerosol can be arranged at a high temperature, t us causing a false target effect within the IR-range.

Finally, it should be emphasized, that the present invention discloses an aerosol, which reduces transmission of optical radiation from an object to a detector, and which also, due to its own low reflection ability, makes it difficult to lock a semi-actively guided missile system to a target.

I187741 THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

i. A transmïssion attenuatïng aerosol having a low reflectance index for electro-magnetic radiatîon useful for the aSsorptïon of radïatïon, particularly wïthin the IR-range, which contains activated carbon particles of irregular configuration and havïng substantial micro-porosity.

2. An aerosol as claimed in claim i, in which the average of surface area of the active carbon particles is up to 1,200 m2/g larger than the surface of a spherical particle of the same volume.

3. An aerosol according to claim I, in which 80% of the carbon particles have diameters mainly distributed equally between 1 and 9 um, having approximately 10% above and below these limits.

4. An aerosol according to claim l, 2 or 3, in which the activated carbon particles have high mechanical Strength.

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