US3713385A - Electroexplosive devices - Google Patents

Abstract

An electroexplosive device consisting of an intimate mixture of an explosive substance and an electrically conducting material in a fibrous form in sufficient proportion and so distributed to provide an electrically conducting path through the mixture and means for passing an electrical current through the mixture to initiate the explosive substance.

Description

United States Patent [1 1 Lovecy [1 1 3,713,385 1 Jan. 30, 1973 [54] ELECTROEXPLOSIVE DEVICES [75] Inventor: Albert Leonard Lovecy, London,

England [73] Assignee: Minister of Technology in ller Britannic Majestys Government, London, England [22] Filed: Jan. 30, 1970 [21] Appl. No.1 7,239

[52] US. Cl. ..l02/28 R [51] Int. Cl. ..F42c 19/12 [58] Field of Search ..102/28, 28 M, 28 EB, 48;

[561 References Cited UNITED STATES PATENTS 2,918,871 12/1959 Taylor ..102/46X 3,056,350 10/1962 Lindblad ..102/28 M 3,062,143 11/1962 Savitt et al 3,109,372 11/1963 Stresau ..102/28 3,125,954 3/1964 Vilbajo ..102/28 3,167,014 1/1965 Kopito ..102/28 EB 3,374,127 3/1968 Jenner et al. ..149/43 Primary Examiner-Verlin R. Pendegrass Attorney-Stevens, Davis, Miller & Mosher [57] ABSTRACT 7 Claims, No Drawings ELECTROEXPLOSIV E DEVICES The present invention relates to electroexplosive devices which carry out the initiation of explosives by the application of electrical energy. Explosives have been initiated by electrical means by providing an electrically conducting path in contact with the explosive and passing an electric current through it of sufficient strength to cause ignition of the explosive.

Hitherto the conducting path has been either in the form of a continuous metal bridgewire of suitable electrical resistance or in the form of an electrically conducting powder, such as carbon powder, admixed with the explosive in sufficient proportion to provide an adequate network of conducting paths.

There is a marked contrast between these two forms of device and between them they do not provide a completely satisfactory choice to meet all requirements; the bridgewire having the disadvantage that it is relatively fragile and practical difficulties in fabricating restrict its application as regards type of explosive and loading density while the conducting powder requires strict control of proportions in accordance with particle sizes and other characteristics of the ingredients in the mixture.

An object of the invention is to provide an electroexplosive device which has operational advantages over the forms of electroexplosive devices used hitherto.

It has now been found that the use of conductive material in the form of discrete fibers or short filaments is advantageous, both as compared to the bridgewire system and to the use of conductive particles of a substantially rounded or equant type, i.e., the length of a particle not differing greatly from its breadth or its thickness. On the one hand there is no bridgewire to be fixed and protected during filling; on the other, the formation of a electrical network and the manner in which the intensity of energy dissipation is localized by virtue of the points of contact filaments is substantially independent of the nature of the non-conducting ingredient or ingredients present.

In accordance with the invention, therefore, an electroexplosive device comprises an intimate mixture of an explosive substance and an electrically conducting material in a fibrous form in sufficient proportion and so distributed that it provides an electrically conducting path through the mixture, and means for passing an electrical current through the mixture to initiate the explosive substance.

The proportion of conducting fibers in electroexplosive devices in accordance with the invention may vary over a wide range, but proportions between 5 and 35 percent by weight of the conducting fiber/explosive mixture have been found to be particularly satisfactory. in general, proportions of fiber less than 5 percent by weight give less than the acceptable minimum of conducting paths for electrical initiation whereas proportions in excess of 35 percent offer no advantage since they result merely in duplication of these essential conducting paths.

The size of the conducting fibers which may be used depends inter alia upon the desired electrical resistance and the nature of the explosive substance but in any case the fibers should be predominantly shorter than the least distance between the electrode surfaces with which contact is made. The conducting fibers may be in the form of fibrous carbon or very fine metallic or metal-coated filaments such as, for example, asbestos or synthetic polymer fibers coated with a metal, for example, silver.

A major advantage resulting from the use of conductive fibers, i.e., particles having a high axial i.e., length to breadth ratio, 10:] or greater, is that fewer fibers suffice to fonn a satisfactory electrical network than is the case when equant particles (similar in individual mass) are accommodated in the given space. Consequently, fewer interparticle contacts are present so that for a given current and voltage rating, a more intensive energy dissipation at individual contacts is made possible. The use of discrete filamentary conductive material in accordance with the invention makes it possible to explode by direct electrical action a wide range of explosive substances in a simple and straightforward manner which is readily applicable to the design of electroexplosive devices for a great variety of purposes.

Secondary explosives are those explosives which are normally not directly initiated by electrical or mechanical energy but rather through the agency of a much more sensitive (and consequently more hazardous) primary explosive that is itself capable of initiation by electrical or mechanical energy. The detonation wave transmitted by a primary explosive is sufficiently energetic to achieve detonation in the secondary explosive. It is clearly desirable from the point of view of safety, ease of manufacture and cost to avoid primary explosive if means can be found to initiate a secondary explosive directly and, in accordance with a feature of the present invention, it may be possible to achieve this aim by providing an electroexplosive device comprising an intimate mixture of a secondary explosive and an electrical conducting material in a fibrous form in sufficient proportion and so distributed that it provides an electrically conducting path through the mixture, and means for passing an electrical current through the mixtureto initiate the secondary explosive substance.

It has been found that initiation of a secondary explosive in such an electroexplosive device is favored by compression of the mixture of secondary explosive and electrical conducting material to reduce void space in the mixture to a minimum and also by restricting the freedom of the mixture to disperse. Both conditions tend to restrict the expansion of gaseous products from the decomposition of secondary explosive so that a relatively high proportion of the energy from the gaseous products is usefully available to raise the temperature' at localized hot-spots rather than being lost in providing work of expansion. These hot spots may be then sufficiently energetic to achieve detonation in adjacent secondary explosive. A further advantage of the use of carbon fibers is that these fibers have a negative temperature coefficient. As a result, the resistance to electrical flow is lowered once a conducting path is established.

Where initiation and ignition, but not detonation, is achieved'in the adjacent secondary explosive it is possible in accordance with U.S. application Ser. No. 6,305, filed Jan. 20, 1970 to provide a further charge of explosive separated from the adjacent secondary explosive by, for example, an air gap and to arrange that the separation is such that the stress front and the reaction front generated by the initiated secondary explosive substantially coincide on reaching the separated explosive with a combined effect sufficient to produce detonation therein. Thus, an extremely safe electroexplosive device capable of detonating a secondary explosive may comprise merely an admixture of conducting fiber and secondary explosive, an adjacent charge of secondary explosive, means for passing an initiating electrical current through the admixture and means for directing the stress front and reaction front produced from the initiated secondary explosive to a desired detonation position. In employing such a device as a detonator it would thus be necessary only to place the explosive to be detonated in this detonation position. Before an explosive was placed in the detonation position in this way the electroexplosive device could not detonate but merely ignite, when initiated, in a relatively mild manner. The hazards associated with accidental initiation of such an electroexplosive device before being placed in the detonation position would be expected to be considerably less than the hazards from conventional detonators employing primary explosives.

It has been shown that admixture of conducting fibers such as carbon fibers with the secondary explosive nitrocellulose gives a felted mixture which is excellently suited for use in the formation of sheets or the like which can be disposed between conducting surfaces (e.g., metal foil, wire gauze, the surface of a container, or a workpiece) whereby on the passage of an electric current an explosion over an area can be produced directly.

Graphite or carbon in the required form can be prepared from fibrous material by known means, for example carbonization.

Specific additional ingredients, and composite systems which employ the conductive mixtures described above, could also be used.

Admixture of fibrous conductive materials with cellulose, and subsequent conversion to nitrocellulose by known methods of nitration, is an alternative but less convenient procedure for producing electroexplosive mixtures based on nitrocellulose in accordance with the invention.

By way of example, the construction and testing of experimental electroexplosive devices in accordance with the invention will now be described.

Example 1 Fibrous carbon, for instance the commercially available thermal insulation, was admixed to the proportion of about ten per cent by weight with nitrocellulose fiber, made from cotton, 13.8 percent nitrogen content. Approximately two milligrams of this mixture, graphited guncotton, was loaded into a steel tube of Va inch bore fitted with an electrode consisting of a short length of wire passing axially through an insulating plug which closed one end of the tube. Further guncotton was added and the whole consolidated in a press to form a column half an inch long. A tight-fitting plug carrying an ionization-probe was then inserted to make contact with the top of the guncotton column. The steel tube and the central electrode were connected respectively to the leads of a suitable test circuit with an electronic timer which showed the guncotton exploded completely in less than one millisecond for a firing energy of less than millijoules.

Example 2 A mixture was made of pentaerythritol tetranitrate, commonly named PETN, with approximately ten per cent by weight of fibrous carbon. Approximately three milligrams of this mixture was loaded into a steel tube of l/l6 inch bore fitted at one end with a short electrode of 22 SWG wire protruding axially inside the tube end and with an electrical connection passing to the outside, heremetically sealed by an insulating plug. Additional PETN was put in and the whole contents were firmly consolidated in a press to form a column 10 mm long, leaving a further 5 mm of the tube empty. On passing electric current from a 20-volt battery through the central electrode, the tube and its contents, the latter exploded with violence sufficient to initiate detonation in a separate pellet of PETN placed in contact with the open end of the tube. The total time of action from closing the firing circuit to the detonation of the separate pallet of PETN was 45 microseconds. In similar tests at voltages up to 36 volts, action times recorded were from 33 to 53 microseconds.

Example 3 A polyacrylic nitrile filament was coated with silver by well-known means and 10 percent by weight of this silver-coated filament was mixed with PETN Approximately 5 milligrams of this mixture was loaded into a tube of 2.4 mm bore fitted with an electrode at one end as in Example 2. Additional PETN was put in and the whole contents firmly consolidated in a press to form a column 8 mm. long. On passing an electric current from a lp.F capacitor charged to 400 volts, the PETN exploded within 10p. seconds as recorded by an ionization-detector mounted adjacent to the end of the column.

In other experiments with charges from 6 mm. to 10 mm. long, the explosion times recorded were from 9 to 13;]. seconds.

Example 4 Silicon nitride fibers were silver-coated as in Example 3 and mixed with PETN in the proportion 10 percent by weight of the silver-coated fiber.

2mg. of this mixture was pressed into a steel tube 1.6 mm. bore with a central electrode and further PETN added as in Example 3. The resistance measured was below 30 ohms. On passing a current from a 60 volt source, the PETN exploded.

In a further experiment, 4 mg. of the above mixture was loaded in a similar way. The resistance measured was 8 ohms and on passing a current from a 60 volt source, the PETN exploded.

The above experiments were successfully repeated using silver-coated asbestos fibers.

What I claim is:

1. An electroexplosive device comprising an intimate mixture of an explosive substance and an electrically conducting material having fibers with a length to breadth ratio of at least 10 to l in sufficient proportion and so intermixed that it provides an electrically conducting path through the mixture, and means for passing an electrical current through the mixture to initiate the explosive substance.

2. An electroexplosive device according to claim 1 wherein the explosive substance is a secondary explosive.

wherein the proportion of electrically conducting material in fibrous form constitutes between 5 and 35 per cent by weight of the intimate mixture.

7. An electroexplosive device according to claim] wherein a secondary explosive is positioned adjacent to the intimate mixture of the explosive substance and electrically conducting material in fibrous form so that the secondary explosive is initiated by the said explosive substance after the latter is initiated.

I I I I II

Claims (6)

1. An electroexplosive device comprising an intimate mixture of an explosive substance and an electrically conducting material having fibers with a length to breadth ratio of at least 10 to 1 in sufficient proportion and so intermixed that it provides an electrically conducting path through the mixture, and means for passing an electrical current through the mixture to initiate the explosive substance.

2. An electroexplosive device according to claim 1 wherein the explosive substance is a secondary explosive.

3. An electroexplosive device according to claim 1 wherein the explosive substance is selected from the group consisting of nitrocellulose and pentaerythritol tetranitrate.

4. An electroexplosive device according to claim 1 wherein the electrically conducting material consists of fibrous carbon.

5. An electroexplosive device according to claim 1 wherein the electrically conducting material is a metallic or a metal coated filament.

6. An electroexplosive device according to claim 1 wherein the proportion of electrically conducting material in fibrous form constitutes between 5 and 35 per cent by weight of the intimate mixture.