EP1306643A1 - Low energy optical detonator - Google Patents

Abstract

An optical detonator comprises a secondary explosive (8) distributed in a cavity (7), a fibre optic (5) connected by one extremity to a laser source and an optical interface (11) at the other fibre extremity to transmit the laser ray towards the secondary explosive. A layer of an ignition powder (16) is disposed between the explosive and the interface.

Description

The present invention relates to optical detonators low energy in which priming is achieved by a laser source that can be, for example a diode laser.

A detonator is a device designed to prime detonation an external load of secondary explosive downstream; for this, any detonator contains a small amount of secondary explosive (100 mg to 1 g) which must be brought into detonation (at least) in its terminal part from the energy supplied to the entry of the detonator by an external source.

In known manner, the optical detonator is of the type comprising a secondary explosive arranged in a cavity, an optical fiber connected by a first end to a source of laser radiation, and a optical focusing interface located between the other end of the optical fiber and the secondary explosive and adapted to transmit laser radiation to the secondary explosive.

In a very classic way in the field of explosives, secondary explosives are called relatively insensitive explosives, as opposed to primary explosives, eg lead azide, who are very sensitive and therefore dangerous.

In low-energy optical detonators (less than 10 mJ) and low-power detonators (a few watts), the light energy of the laser radiation from a solid laser source in relaxed mode or a quasi-continuous laser diode is used. limited space of 1 cm ³ ) via an optical fiber to ignite the charged secondary explosive at the optical interface.

This heating by absorption of laser radiation at through the optical interface presents a security recognized use of optical detonators compared electric detonators in which the substance explosive near the input interface is in contact intimate and permanent with an electrical wire resistive heating up when passing through it a electrical current and transmitting its heat through thermal conduction to the explosive substance that the coating but can be activated accidentally by unexpected electrostatic discharges or induced currents due to radiation electromagnetic parasites.

Despite this undeniable advantage of the detonators optics, their use poses some problems of secondary explosives used do not absorb not the light emitted in the near infrared, that this either solid state lasers or laser diodes.

Also, in order to overcome this problem, the state of the technique teaches to optically boost the explosive secondary, ie to mix with this explosive secondary (particle size close to 3 μm) between 1 and 3% by weight of ultrafine carbon black (from a particle size between 50 and 200 nm) which absorbs laser light.

Thus, thanks to this optical doping, and by focusing the laser light into a spot of a diameter between 50 and 100 μm, the laser energy threshold is lowered ignition, which ensures ignition thermal of the explosive composition even with laser diodes that deliver a nominal power of one watt for 10 milliseconds.

However, during operational tests that used to validate the use of detonators in severe operational environments (use on planes, missiles, space vehicles ...) and which are made either after thermal shocks intense (room temperature test after submission for 5 hours at temperatures above 100 ° C), or after thermal cycling (from -160 ° C to 100 ° C), it turned out that the laser ignition of the explosive composition optically doped with black carbon was not reliable enough.

This lack of reliability concerns especially the nitramines (octogen and hexogen) which are the secondary explosives the most common for these applications.

In fact, the coefficient of thermal expansion of secondary explosive crystals organic is a lot higher (between 3 and 7 times) than materials used for the construction of the detonator (silica optical interface, stainless steel or inconel loading body). Also, during the release constraints resulting from thermal shocks, cracks appear in the explosive composition compressed in the vicinity of the optical interface and the distribution of carbon black in the composition explosive is no longer homogeneous. Consequently, the secondary explosive is no longer sufficiently coated carbon black, which suddenly increases the energy threshold and reduces the efficiency of optical doping.

The problem is to realize an optical detonator low energy whose effectiveness of the device ignition is reliable and high, especially when a such detonator is intended for use in severe environments.

According to the invention, a layer of ignition powder is disposed in the cavity of the optical detonator of the aforementioned type, between the secondary explosive and the optical focusing interface.

In the prior art relating to ignition powders which are essentially a mixture of a chemical body oxidizing agent and a reducing chemical substance, that these are used to ignite the combustion propellant powders which are especially used to accelerate a projectile.

Indeed, propellant powders are usually used in large quantities - a 120 mm barrel uses about 8 kg of propellant powder in a 10 liter chamber - and ignition of combustion such a large volume is difficult and makes it necessary the use of an igniter containing a powder ignition.

Inflammators used to ignite powders propulsive are electrical igniters in which the ignition powder is ignited by thermal conduction of the heat released by the wires electric, the start of the chemical reaction between the oxidizing body and the reducing body being obtained when a very small amount of the ignition powder has reached the critical temperature of starting this reaction (typically 400 ° C).

It is quite surprising to use a powder ignition to ignite a detonator, the domain detonator technique being totally different from that of the igniters used for the ignition of the propellant powder of guns or propellant thrusters.

In the guns and thrusters it is sought to obtain, with the igniters or igniters, a controlled combustion of a propellant powder generating a fairly low pressure (5000 bar maximum in a gun), the speed of these combustion fronts being at best a few ms ^-1 . In the detonators, it is sought to obtain a detonation, ie an extremely fast combustion generating a very high pressure (between 300 000 and 400 000 bar), the speed of the detonation wave propagating at speeds between 7,000 and 9,000 ms ^-1 .

In addition, the combustion of the ignition powders used in the electrical igniters is generated by the high temperature released by resistive wires. At contrary, in the present invention, the powders are lit by photon absorption of a luminous energy.

Using an optical detonator comprising a ignition powder according to the present invention, their reliability is greatly increased by compared to those using optical dopants, especially with regard to those intended for use in severe environmental conditions.

In addition, the trigger time of detonators according to the present invention is reduced by a factor of 5, 10, compared to optically detonators doped.

Other peculiarities and advantages of this invention will result from the following description.

• la figure 1 est une vue en coupe longitudinale du premier étage d'un détonateur optique selon la présente invention;
• la figure 2 est une vue en coupe longitudinale d'un détonateur optique selon la présente invention, la transition dans le deuxième étage étant du type choc-détonation; et
• la figure 3 est une vue en coupe longitudinale d'un détonateur optique selon la présente invention, la transition dans le deuxième étage étant du type déflagration-détonation.

In the accompanying drawings, given as non-limiting examples:

• Figure 1 is a longitudinal sectional view of the first stage of an optical detonator according to the present invention;
• FIG. 2 is a longitudinal sectional view of an optical detonator according to the present invention, the transition in the second stage being of the shock-detonation type; and
• Figure 3 is a longitudinal sectional view of an optical detonator according to the present invention, the transition in the second stage being of the detonation-detonation type.

As can be seen in the attached figures, the optical detonator 1 has a tip 2, a first floor 3 and a second floor 4.

The tip 2 serves to support an optical fiber 5 which a first end is connected to a laser source, and whose second end 6 is free.

The first floor 3 has a housing 7 inside from which an explosive secondary explosive is confined 8. This confinement is achieved by the walls of the structure 9 of the first stage 3, a device 10 to trigger the transition to detonation in the second floor 4 to a first end, and an optical focusing interface 11 at the other end.

Once the tip 2 secured to the first floor 3 of the detonator 1, the second end 6 of the fiber Optical 5 is in the immediate vicinity of the optical focusing interface 11, this interface 11 serving as a separation between the housing 7 and the optical fiber 5.

The second floor 4 has a housing 12 to inside which is confined a secondary explosive detonating 13. This confinement is achieved by the walls of the structure 14 of the second floor 4, the device 10 for triggering the transition to detonation in the second floor 4 and a plate 15 propelled during the detonation of second floor 4.

According to the invention, an ignition powder 16 is disposed in the housing 7 of the first floor 3, between the explosive secondary explosive 8 and the interface focusing optics 11.

The operation of a detonator 1 according to FIG. is the next :

At first, the laser source is activated.

The laser infrared light is transported by the optical fiber 5 and is focused on the powder ignition 16 by the optical interface of focusing 11 including a glass ball 11b associated with a glass plate 11c.

In a second step, the ignition powder 16 located in the first floor 3 is lit by absorption of laser infrared light and undergoes, as a result, a combustion.

One of the constituents of the ignition powder 16, the oxidant is the reducing agent (the most frequent case), is absorbing the light energy provided by a near infrared radiation. Metals micronized reducing agents exhibit this optical absorption property.

The laser ignition threshold of the ignition powder 16 depends on its loading density, stoichiometry and the particle size of its constituents.

The compaction pressure of the ignition powder 16 will be chosen advantageously equal to that of the explosive secondary explosive 8, the density of loading of this explosive secondary explosive 8 being greater than 80% of its theoretical maximum density.

The use of an ignition powder 16 in conditions close to the stoichiometry allows to lower the ignition energy threshold of the powder However, for safety reasons when handling this ignition powder 16, it's better to have a 15% mix of stoichiometric conditions.

Likewise, the use of an ignition powder 16 of which the particle size is small makes it possible to lower its threshold laser ignition. Effective focus of the task laser by the optical interface 11 necessary for lower the laser ignition energy threshold, reduce the laser spot with a diameter of 50 to 100 μm, so that the reducing metals used are in the form micronized (with a particle size less than 10 μm) for increase the absorption in the near infrared. The oxidizing minerals will preferably have a particle size neighbor.

In general, the setting of these parameters depends on compromise between the job security of explosive substances and the performance of operation.

Thirdly, the secondary explosive Explosion-proof 8 located in the first floor 3 is lit by burning the ignition powder 16 with which he is in contact.

The chemical reaction of combustion of the powder ignition 16 (oxidation-reduction reaction) is exothermic and releases a great heat of reaction to start reliably and immediately the explosion of secondary explosive 8 in contact with this layer of ignition powder 16.

Note that if this ignition powder 16 releases a lot of warmth favorable to the ignition of the explosive in explosion 8, against all alone releases too little gas to replace secondary explosives, which limits its use to ignition of these.

In a fourth step, the secondary explosive detonating 13 located in the second floor 4 is primed in detonation by the transmission of the energy released by the explosive secondary explosive 8.

The transition to the detonation regime is triggered by the blast of the secondary explosive Explosion 8: the explosion causes the compaction dynamics of secondary explosive loading detonating 13. The high porosity of the explosive 13 (the compactness is close to 50%, the explosive having a big grain size and being loaded with a weak density) and the use of the disk 10a (which is in flake and acts as a piston crushing the column detonating secondary explosive 13 porous) promoting the transition deflagration - detonation over a distance scaled down.

In a fifth step, the plate 15 is propelled by detonation of explosive secondary explosive 13, this which detonates the external loading secondary explosive.

The operation of the detonator 1 according to FIG. differs from that illustrated in Figure 3 only by the ignition of the detonating secondary explosive 13 (fourth time).

In the detonator 1 shown in FIG. transition to the detonation regime is triggered by the shock wave that is created during the impact of the 10b projectile disc propelled into the cavity 10c by the explosion of the explosive secondary explosive 8, this wave being focused on the bare surface of the detonating secondary explosive 13 by the configuration of this cavity 10c.

Preferably, in the case of this shock-detonation transition (described in application FR 2 796 172), detonating secondary explosive 13 has a fine granulometry and is loaded with a higher density that of explosive secondary explosives 13 used in blast-detonation transition detonators.

Of course, it is possible to use, as optical focusing interface 11, a bar of gradient glass of index 11a (as shown in FIG. 1) instead of the glass ball 11b associated with the glass plate 11c (as illustrated in FIGS. and 3).

In the prior art, the carbon black used to capture light energy and transmit energy by thermal conduction needed to be mixed with homogeneous way with the explosive secondary explosive 8.

In addition, like carbon black, or any other dopant optically, is chemically inert and does not participate in no way to an exothermic chemical reaction it is necessary to use it in very small quantities for do not decrease the total chemical energy contained in mixing the secondary explosive.

A first advantage of the ignition powders 16 is that they easily absorb the laser light. The 16 ignition powder does not have to be mixed with a any optically doping material, it is lit by its own absorption of light energy.

A second advantage of the ignition powders 16 is that they are chemically reactive. The powder ignition 16 undergoes combustion (chemical reaction exothermic) whose flame initiates the combustion of the explosive secondary explosive 8. The powder ignition 16 does not have to be mixed with the explosive secondary 8, a contact between the ignition powder 16 and the explosive secondary explosive 8 being sufficient.

Since it is not necessary, during the preparation detonator 1, to make any mixture homogeneous (which is quite delicate) with the powder ignition 16 (neither with carbon black nor with a secondary explosive), the preparation of the detonator 1 is greatly facilitated.

Another particularly interesting consequence of the chemical composition of the ignition powders 16 is it is possible to have per unit volume a much higher percentage of absorbent material (the percentage of carbon black being of the order of 1%), significantly increasing the ignition of the explosive secondary explosive 8.

The ignition powder 16 only serves to light the explosion of the explosive secondary explosive 8 which remains the major energy material of the first floor 3. It only takes a thin layer of powder ignition 16 whose thickness is between 4 and 10 times less important than that of the secondary explosive For example, a thickness between 0.5 and 1 mm of ignition powder adjacent to a layer 4 mm explosive secondary explosive 8 (for example octogen) is enough to make a deflagration enabling the priming of the secondary explosive detonating 13.

A third advantage of the ignition powders 16 is that they reduce the time to triggering the detonator by a factor of 5 or 10.

The time taken to ignite the ignition powder 16 by absorption of the laser radiation, for the reaction chemical redox of this powder 16, and for the transmission of heat from this exothermic reaction to the secondary explosive 8 allowing his blast is shorter than the one set for the absorption of the laser radiation by the black of carbon and for conduction transmission thermal energy to secondary explosive allowing his blast.

Indeed, the exothermic chemical reaction of burning of the ignition powder 16 releases a more great heat of reaction (+ 100%) that the reaction of decomposition of the optically doped secondary explosive by carbon black, so this heat of larger reaction allows you to start so quick and immediate explosive blast secondary 8 in contact with this ignition powder 16.

A fourth advantage of the ignition powders 16 is that they are physically stable.

The ignition powder 16 is much more stable physically when she is subjected to holding trials shocks and thermal cycles and therefore remains integrates in contact with the optical interface 11. In fact, the ignition powder 16 has a coefficient of thermal expansion weaker than explosive secondary organic. For example, the zirconium that is one of the reducing metals that can be used in these powders, is ten times less expandable than the octogen.

Advantageously, the ignition powder 16 is a powder redox composed of a mixture of reducing metal and of mineral oxidants. Indeed, these powders 16 absorb easily infrared laser light and have a particularly high flame temperature.

The reducing metals are, for example, zirconium, zirconium-nickel alloys, titanium, hydrides of titanium, aluminum, or magnesium.

The mineral oxidants used are, for example, the potassium perchlorate, ammonium perchlorate, ammonium nitrate, ammonium dichromate, barium chromate, or iron oxides.

• les thermites comprenant de l'aluminium et de l'oxyde de fer,
• les poudre de type ZPP, c'est à dire contenant pour l'essentiel du zirconium et du perchlorate de potassium, par exemple, un mélange comprenant 52% de zirconium, 42% de perchlorate de potassium, 5% de viton et 1% de graphite (pourcentage massique),

Thus, it is possible to use as ignition powder 16:

• thermites comprising aluminum and iron oxide,
• the ZPP-type powder, ie essentially containing zirconium and potassium perchlorate, for example, a mixture comprising 52% of zirconium, 42% of potassium perchlorate, 5% of viton and 1% of graphite (mass percentage),

• une poudre contenant pour l'essentiel du zirconium et du chromate de baryum, par exemple, un mélange comprenant 45% de zirconium, 34% de chromate de baryum, 7% bichromate d'ammonium et 14% de perchlorate d'ammonium (pourcentage massique),
• une poudre contenant pour l'essentiel du titane et du perchlorate de potassium, par exemple, un mélange comprenant 40% de titane et 60% de perchlorate de potassium (pourcentage massique) ou un mélange comprenant 40% d'hydrure de titane TiH_x et 60% de perchlorate de potassium (pourcentage massique), x étant égal à 0,2 ; 0,65 ou 1,65.

It is possible to use other redox powders, such as, for example:

• a powder containing for the most part zirconium and barium chromate, for example, a mixture comprising 45% zirconium, 34% barium chromate, 7% ammonium dichromate and 14% ammonium perchlorate (mass percentage) )
• a powder containing for the most part titanium and potassium perchlorate, for example, a mixture comprising 40% of titanium and 60% of potassium perchlorate (mass percentage) or a mixture comprising 40% of titanium hydride TiH _x and 60% potassium perchlorate (mass percentage), where x is 0.2; 0.65 or 1.65.

Of course, the invention is not limited to powders ignition described above. Other powders absorbing laser light and generating reactions Exothermic substances may be suitable.

Claims (10)

An optical detonator (1) comprising a secondary explosive (8) disposed in a cavity (7), an optical fiber (5) connected at a first end to a laser radiation source, and an optical focusing interface (11) located between the other end (6) of the optical fiber (5) and the secondary explosive (8) and adapted to transmit the laser radiation to the secondary explosive (8), characterized in that a layer of a powder of ignition (16) is arranged in the cavity (7) between the secondary explosive (8) and the optical focusing interface (11). Optical detonator (1) according to claim 1, characterized in that the ignition powder (16) contains a micronized powdered metal. Optical detonator (1) according to claim 1 or 2, characterized in that the thickness of the layer of the ignition powder (16) is between 4 and 10 times less than the thickness of the secondary explosive (8). ). Optical detonator (1) according to one of claims 1 to 3, characterized in that the pressure of compaction of the ignition powder (16) is substantially equal to that of the secondary explosive (8). Optical detonator (1) according to one of claims 1 to 4, characterized in that the composition of the ignition powder (16) is within 15% stoichiometric. Optical detonator (1) according to one of claims 1 to 5, characterized in that the ignition powder (16) is a redox powder composed of a mixture of reducing metal and inorganic oxidants. An optical detonator (1) according to claim 6, characterized in that the reducing metal is zirconium, a zirconium-nickel alloy, titanium, titanium hydride, aluminum or magnesium. Optical detonator (1) according to claim 6 or 7, characterized in that the inorganic oxidant is potassium perchlorate, ammonium perchlorate, barium chromate, ammonium dichromate, ammonium nitrate or an ammonium nitrate. iron oxide. Optical detonator (1) according to one of claims 1 to 8, characterized in that the transition process to detonation triggered in the second stage (4) is of the detonation-detonation type. Optical detonator (1) according to one of claims 1 to 8, characterized in that the transition process to the detonation triggered in the second stage (4) is of the shock-detonation type.

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