US3815507A

US3815507A - Electrical initiator

Info

Publication number: US3815507A
Application number: US00286384A
Authority: US
Inventors: C Irish; J Silva; R Cowles
Original assignee: Olin Corp
Current assignee: Olin Corp
Priority date: 1970-01-21
Filing date: 1972-09-05
Publication date: 1974-06-11
Anticipated expiration: 1991-06-11

Abstract

An initiator having a body of nitrocellulose material having a surface characterized by thin fibres or thin webs with a film of a conductive material adhered to said surface.

Description

United States Patent [191 Irish, Jr. et a1.

ELECTRICAL INITIATOR Inventors: Charles G. Irish, Jr., Cheshire;

Joseph Silva, Northford; Raymond l. Cowles, Woodbridge,

all of Conn.

Assignee: Olin Corporation, New Haven,

Conn.

Filed: Sept. 5, 1972 Appl. No.: 286,384

Related US. Application Data Continuation of S'er. No. 4,480, Jan. 21, 1970, abandoned.

US. Cl 102/103, 102/46', 102/70.2 A lnt. Cl. F42b 1/00 Field of Search 102/38, 39, 43, 43F, 46,

in] 3,815,507 [451 Junell, 1974 References Cited Primary Examiner-Robert F. Stahl I Attorney, Agent, or Firm-Donald R. Motsko; H. Samuel Kieser; William W. Jones [5 7] ABSTRACT An initiator having a body of nitrocellulose material having a surface characterized by thin fibres or thin webs with a film of a conductive material adhered to said surface.

8 Claims, 7 Drawing Figures mammal l w 3815507 SHEET 10F 2 FIG -5 1 ELECTRICAL INi'rlATon ing or explosive reaction. Initiators are classifiedaccording to the nature of the input stimulus which can be either mechanical or electrical. lnitiators may be further classified according to their intended output function as primers, squibs, or detonators. As will be disclosed, this invention concerns an initiator which responds to an electrical input impetus and whose output characteristics are such as to enable it to act as a primer, a squib, or the first element of a detonator. f

Electric initiators may be further categorized by the type of transducer used to convert the electrical input energy into heatenergy. Transducers that may be used with electrical initiators include a hot bridgewire or an exploding bridgewire, a conductive mix, a conductive film bridge, anda spark gap. The input sensitivity varies sharply with the type of transducer; each type havinga characteristic threshold firing energy and current requirements. This invention concerns that type of electric initiator wherein the transducer is a conductive It is thought that an understanding of the nature, construction, and electrical input characteristics of a conductive film bridge initiator which existed prior to this invention would be helpful in understanding the preserit invention. Conductive films maybe applied'to the surface of insulators by a variety of techniques to produce bridges which can be heated or exploded'to initiate explosive reactions. The only type of film bridge applied in standard fuzes hasbeen the low-energy graphite or carbon bridge. These bridges are all made by essentially the same process. A droplet of a colloidal suspension of graphite in water is deposited on a surface which consists of two or more metal electrodes separated by, and usually imbedded' in, an insulator. Be-

cause graphite has a negative resistance temperature coefficient and because of the inherent variable film thickness and path length, the electric current is channeled intoa path which is only a small part of the total volume of the-film. Only this small portion of the film is heated. Characteristically, the resistance spread is very wide ranging from 700 to 14,000 ohms. Because of the channeled path, the energy required for ignition is low, in the order of only a few hundred ergs. As a further consequence of the channeled path, the mass of graphite heated is small. To be effective as an ignitor, this small mass of graphite must be in intimate contact with a primary explosive which is thermally sensitive.

and has good propagation characteristics. The primary explosives, lead azide and lead styphnate, are universally used as spotting charges not only for graphite or carbon bridge devices but also for every type of present initiatorexcept the exploding bridgewire whose greater energy release permits the use of secondary explosives under special conditions of particle size andconfinement.

" We may summarize the salient characteristics of currently employed conductive film initiators as follows:

1. The conductive film may be metallic or nonmetallic but non-metallic carbonaceous materials are most usual.

2. The carbonaceous film'bridge is electrically characterized by a high resistance of'700 to 14,000 ohms and is very sensitive to electric energy input requiring, at the most, a few hundred ergs for initiation.

3-. All present conductive film bridge initiators require the presence of aprimary explosive, commonly lead styphnate or leadazide, in direct and intimate contact with the bridge.

4. Generally, the conductive film bridge is applied to the surface of an insulating material into which are imbedded two or more electrodes.

The critical dimensions, the non-uniformity, and, above all, the susceptibility of the low-energy carbon bridge to accidental discharge, as, for example, by the static electricity accumulated on a person, or that induced by radio frequency or radar energy sharply limit the uses of the carbon bridge conductive film device and it is found that bridgewire devices are almost universally used'to fulfill the present day needs for electric initiators.

The bridgewire initiator requires an assembly which isrelatively complex and costly. Further (except in. the special case of the-exploding bridgewire) it universally requires the presence of a primary explosive to be in intimate contact with the wire'to act as the initiator for 7 what is usually a train of several explosive materials.

In view of the above, it is an object of the presentinvention to provide an electric initiator that does not require-the presence of a primary explosive material. A further object of the present invention is to provide an electric initiator of the conductive film type which possesses the electrical and safety characteristics of the bridgewire type without the attendant complexity and high cost.

Yet another object of the present invention is the provision of an electric initiator which does not require a primary explosive and which may be used by itself as a source of power todrive projectiles, bolts, studs or the like, or which can be used as a means for igniting additional powder, pyrotechnic material or as the first element in a train of explosives.

Generally, the initiator of the present invention comprises a body of nitrocellulose material having a surface characterized by thin fibres or, alternately, thin webs with a film of a semiconducting material adhered to said surface.

The objects and advantages of the present invention will become more apparent by reference to the following description of several preferred embodiments and to the accompanying drawing in which:

FIG. I is a cross-sectional view of an initiator constructed in accordance with the present invention;

FIG. 2 is a cross-sectional view of one embodiment of a shotshell incorporating the initiator of the present invention;

FIG. 3 is a cross-sectional view of a second embodiment of a shotshell incorporating the initiator of the present invention;

FIG. 4 is a cross-sectional view showing a projectile attached to the initiator of thepresent invention;

FIG. 5 is a sectional view showing the initiator used in conjunction with caseless ammunition;

FIG. 6 is a cross-sectional view showing the initiator used in an industrial type fastening tool; and

FIG. 7 is a schematic diagram of an electrical circuit which may be used to supply the power for the initiator.

Referring to the drawings, and in particular FIG. 1, the initiator 2 of the present invention includes a body 4 of either fibrous or porous nitrocellulose having a surface characterized by either thin fibres or thin webs of nitrocellulose. A film 6 of a semi-conductive, nonmetallic material is adhered to the surface with the thin fibres or thin webs being intimately coated with the conductive material.

In practicing the present invention, anynitrocellulose may be used which when formed into its desired shape will have a fibrous, stringy, or porous structure, such that the overall density of the nitrocellulose body will be between about 0.85 and 1.40 grams per cubic centimeter. The receptive surface of such a body to which the conductive film is applied is characterized by having either exposed thin fibres'or exposed thin webs of nitrocellulose which are intimately coated with the conductive material. It is these thin fibres and thin webs of nitrocellulose which are readily heated to their ignition point by the transfer of heat from the conductive film as the film is heated by the applied electrical en ergy. 1

One type of material ideally suited for the purposes of this invention is bulk powder as is fully described in U.S. Pat. No. 3,463,086. This powder is characterized as being fibrous, stringy, or fuzzy and may be compacted in the dry, state to form pellets, grains, charges or shapes which faithfully retain the form into which they are compacted and whose ignition and combustion characteristics may be varied. This type of powder, when compacted, will have a surface which includes exposed thin fibres which can be intimately coated with the conductive material.

However, other means are available of attaining the proper receptive surface. Fibrous, non-gelatinized, virgin nitrocellulose may be used. The sponge-like irregular particles of precipitated nitrocellulose provide exposed thin fibres on the surface which is receptive to the conductive coating. Preferably, precipitated nitrocellulose may be made by first dissolving nitrocellulose in a solvent such as acetone to form a lacquer and then pouring the lacquer into an excess of water under vigorous agitation. Under this treatment the nitrocellulose precipitates out as solid particles which, under microscopic examination, are of irregular shape and of sponge-like character.

In addition, nitrocellulose prepared by the wash out" process may be utilized in the forming of the body 4 of the initiator 2. In this process, nitrocellulose together with a stabilizer and a suitable water-soluble salt is gelatinized with a solvent, extruded or molded into a pellet shape, extracted with hot water to remove the water-soluble salt and dried. A homogeneous structure results which, under microscopic examination, resembles a slice of bread, i.e. irregular pores formed by relatively thin webs. An additional method of providing a receptive surface on a nitrocellulose body which is otherwise fully gelatinized, is by mechanically abrading or scoring the surface or by treating the surface with a sol vent.

The conductive film 6 which is applied to the nitrocellulose 4 is formed from a material of moderate electrical resistivity which also has a negative temperature coefficient of electrical resistivity. Of particular value because of their ready availability and low cost are graphite, partially graphitized carbon black, and finely divided lead peroxide. Other materials which might be employed would include the more exotic semiconductors such as germanium and silicon which also have a negative temperature coefficient of electrical resistivity.

The preferred materials for the present invention are graphite, graphitized carbon black and lead peroxide. These are preferably applied as colloidal or semicolloidal suspensions in an aqueous or non-aqueous media together with suitable organic or non-organic binders. These are applied in a manner such that the dried, adherent film has a thickness which may be varied from 0.5 mil to 2.0 mils and may, with the electrode employed, show resistance from about 2 ohms 'to about ohms with the range of 5 ohms to 20 ohms being preferred.

If desired, an oxidizing material may be incorporated into the coductive film 6 or into the receptive surface of the nitrocellulose body to enhance the ignition. Suitable oxidizers include the nitrates, chromates and dichromates of barium, potassium and sodium, the peroxides of barium and strontium, the chlorates, perchlorates and sulfates of potassium, and the oxides and peroxides of lead. I

The initiator 2, either alone or supplemented by addi tional propellant, may be used as a source of power to drive projectiles, bolts, studs, nails, etc. It may also be used as a power capsule to start a small gasoline engine or the like or it may be used as a means of igniting a larger propellant mass, a mass of pyrotechnic composition or as the first element in a train of explosives. FIGS. 2-6 depict various ways of utilizing the initiator of the present invention.

In FIG. 2, a standard shotshell 8 comprises a plastic body 10 having a head portion 12 and open cavity 14. Within the cavity 14 is placed a propellant charge 16 of suitable composition. A cup wad l8 and fibrous wad 20 overlie the powder charge 16 in a manner well known in the art. The remainder of the cavity 14 is filled with shot 22 and theend of the tube (notshown) closed in any conventional manner. A metallic head 24 overlies the plastic head 12 of the shotshell and has a portion 26 inturned into the primer orifice 28. A battery cup 30 extends through the primer orifice 28 and communicates with the powder charge 16.- The initiator 2 is mounted in the battery cup'30 in a position adjacent to propellant charge 16. The conductive film 6 of the initiator 2 is in contact with an electrode structure 32 mounted in the rearward end of the battery cup 30. The electrode structure 32 includes a hollow, cylindrical band 34 of conductive material which is in electri cal contact with the battery cup 30 and a central electrode 36 of suitable conductive material separated from the outer electrode 34 by suitable insulating material 40. This type of shotshell may be mounted in a cartridge chamber of a firearm having a breech 42 provided with a suitable electrode which comprises an outer electrode 44 generally circular in transverse cross-section which is adapted to be placed in electrical contact with the metallic head 26 and a central electrode 46 which is adapted to engage the central electrode 46 of the electrode structure 32. The

electrodes

44 and 46 are separated from each other by means of battery cup with the initiator 2 and electrode structure 32 could also be used'in place of the percussion primer normally used in conventional rifle and pistol ammunition to fire a single projectile.

In FIG. 3, a second embodiment of a shotshell is shown which includes as in FIG. 2, a plastic case 10 having a head portion 12 and

suitable wadding

18 and 20. A suitable amount of shot 22 is provided in the forward end of the shell.'lmmediately below the wadding l8'is placed a compacted or bonded pellet 52 formed of an appropriate smokeless powder. A hollow 54 is formed in the rearward end of the pellet 52 and an initiator 2 inserted into the pellet with the conductive film 6 facing the opening in which the primer would normally be inserted. With this particular shotshell, the use of a metallic head 24 is optional. The shotshell 50 is adapted to be placed in the cartridge chamber of a firearm which is equipped with an electrode probe 56 adapted to extend through the primer opening into intimate contact with the conductive film 6 of the initiator 2. The electrode 56 may take the form of a hollow, cy-

lindrical, outer electrode member 58 and a central rodlike electrode 60 spaced from the outer electrode 58 by suitable insulation 62. It is to be noted that as in the modification shown in FIG. 2, the modification of FIG.

3 could be used in connection with centerfire rifle ammunition to propel a bulletQ As another alternative, instead of the additional powder being in the formof a pellet 52, it is possible to form such powder as part of the initiator body 2 whereupon the conductive film 6 would simply be ofsuch a size'as to'insure that it would come into intimate contact with the electrode.

FIG. 4 shows the use of the initiator 2 as a propellant for ammunition. In this, particular case, the initiator body 4 is in the form of a pellet. One surface of the pellet is attached to a bullet 64 by mechanical means, by adhesives or in a number of other ways well known to those skilled in the art. The opposite surface of the initiator body 4 contains the conductive film 6 as heretofore described.

FIG. 5 shows yet another form of caseless ammunition in which the initiator 2 of the present invention may be utilized. A generally cylindrical, hollow, molded propellant body 66 may be provided as mentioned in U.S.-Pat. No. 3,3 1 1,057. The usual projectile 68 may be adhesively or otherwise secured in the forward end of the propellant body 66 and an initiator 2 of the present invention secured within the rearward end of the body 66 with the conductive film 6 being exposed for contact with the electrode structure of an appropriate firearm.

Another application of the initiator 2 of the present invention is shown in FIG. 6 wherein it is desired' to utilize the initiator 2 as a source of power for driving the piston 70 of an industrial type, powder-actuated driving tool. The piston 70 is generally mounted in a suitable barrel 72 and the fastener 74 which is desired to be driven inserted into the muzzle end thereof. The nitrocellulose body 4 of the initiator 2 is generally pelletshaped .with the conductive film 6 applied to the rearward end thereof. The initiator 2 is inserted into a suitable pellet chamber 76 with the conductive film 6 being positioned so that it will be intimately engaged by a suitable-electrode 78 of the type shown in FIG. 3.

A preferred electrical circuit for providing a source of electrical energy for ignition of the initiator 2 is 6 shown in FIG. 7. Such circuit may include a suitable battery 80 having a capacitor 82in parallel therewith. The conductive film 6 of the initiator 2 is represented by the resistance 84. A suitable on-off switch 86 is provided between the capacitor 82 and the two electrodes 88 and 90 for firing control. It is to be noted, however, that any direct current source of equivalent power and energy may be used. The capacitances and voltages may be varied widely but should be so selected that the total energy available for initiation is about 1 joule. This effect is shown in the following table which represents Bruceton type testing, at three values of capacitance, to determine the all-fire voltage required.

The following examples are intended -to illustrate some of the many uses to which the initiator 2 of the present invention may be put as well as illustrate the various parameters used for its successful functioning.

EXAMPLE I Pellets of propellant were made by compacting dense bulk powder to a diameter of about 0.650 inch and a thickness of about 0.250 inch. The pellets weighed between about 20 to 24 grains and had a density of about 1.3 grams per cubic centimeter. On one of the flat faces of each pellet a drop of a diluted suspension of colloidal graphite in. water was placed. The graphite suspension was oven-dried for 2 hours at 150F. A portion ofthe graphite suspension permeated between and adhered to the exposed fibres of the pellets. The remainder of the graphite suspension formed a film on the surface of the pellets. The films had a thickness of between about 0.5 to 1:0 mil. With concentric electrodes having a center conductor of 0.040 inch outer diameter and an insulation thickness between the inner and'outer electrodes of about 0.040 inch, the films displayed an individual resistance ranging from about 3 ohms to ohms with a predominating majority being between 5 ohms and 15 ohms- Each pellet was placed in the end of a-shotshell in place of the regular powder charge and primer. The shotshells were provided with conventional type waddingand shot pellets. The shotshells were placed into a test vehicle for firing which included a batterycapacitor circuit with suitable switching to permit the charging of the capacitor from the battery and, subsequently, the discharge of the capacitor through the electrode which was placed in firm contact with the conductive film on the pellet. The capacitor, having a capacitance of 480 uF, was charged to a voltage of 65 volts. Thus the energy in the power supply was equal to 1 joule. The pellets ignited within a period of l to 2 milliseconds after application of the electrical energy and burned as a propellant yielding the pressure and velocity characteristics normally expected in a shotshell. In the case of graphite films having a thickness of 0.5 mil, a burn out of the conductive film occurred before the energy was drained from the capacitor. In these instances, the actual energy consumed for ignition was of the order of 0.2 to 0.5 joule.

EXAMPLE II In a manner similar to Example I, an ignition pellet was dry compacted from bulk powder to a density of about 1.2 grams per cubic centimeter and a diameter and thickness of about 0.2 inch. One face of the pellet was coated with a graphite film as explained in connection with Example I. The electrode employed had a gap dimension of about 0.020 inch. The pellet was placed in a suitable text fixture for firing a .22 caliber rifle bullet. Utilizing the power supply of Example I, the pellet ignited and burned yielding the pressure and velocity characteristics normally expected from the firing of conventional .22 caliber ammunition. The ignition delay after application of the energy was in the order of l to 2 milliseconds and the energy consumed less than I joule.

EXAMPLE III film in a manner mentioned in connection with Example I. The pellet was placed in a test fixture resembling an industrial type powder-actuated fastening tool of the piston type similar to that shown in FIG. 6. On application of electrical energy by use of the circuit mentioned in Example I, the pellet ignited and burned and successfully .drove the piston which in turn drove the fastener into the work surface.

EXAMPLE IV An ignition pellet was compacted into the form of a wafer from bulk powder to a density of 1.25 grams per cubic centimeter. One surface of the wafer was coated with an aqueous dispersion of graphite as heretofore mentioned. A conventional shotshell was modified by removing the primer and the ignition pellet was inserted into the opening leaving room for the insertion into the primer opening of an electrode. Over the ignition pellet was placed a propellant charge of conventional granular propellant and the shell charged normally with shot and wads. On application of about 1 joule of electrical energy, the pellet initiated the combustion of the granular propellant yielding pressure and velocity characteristics normally associated with a conventional shotshell.

EXAMPLE V A conventional shotshell was modified by substituting for the standard percussion primer a battery cup closed at one end with the electrode of Example I into which was pressed a cylindrical ignition pellet of compacted bulk powder having a face coated with graphite film in intimate contact with the electrode. This arrangement is depicted in FIG. 2 of the drawings. Upon the application of about I joule of electrical energy, the pellet initiated the combustion of the granular propellant yielding expected pressure and velocity characteristics normal to shotshells.

. EXAMPLE VI A densely colloided smokeless powder was formed into a pellet using the wet solvent process taught by US. Pat. No. 3,092,525. An initiator pellet of the type described in Example III was placed into a suitable hollow molded into the pellet. In this instance, the weight of the bulk powder initiator pellet was about 3 grains and the weight of the colloided smokeless powder pellet was about 18 grains. The pellet and initiator were loaded into a shotshell in a'manner shown in FIG. 3. Upon application of electrical energy from the aforementioned power supply and electrode, the initiator functioned to ignite the main charge of propellant in a manner which yielded the pressure-time relationship and shot velocity expected of a shotshell.

EXAMPLE VII A shotshell was constructed in accordance with Example VI except that lead peroxide was used in place of the graphite film. The lead peroxide film was about 2 mils in thickness and showed, with the electrode employed, had bridge resistance of about 10 to 15 ohms. Upon application of electrical energy as in Example VI, the initiator functioned to satisfactorily ignite the main charge of propellant.

EXAMPLE VIII A pellet was fabricated from virgin, fibrous, nongelatinized nitrocellulose (13.1 percent nitrogen) by forming a relatively loose mat or wad from a slurry of fibres and water on a filter and then compacting the relatively loose wad in a press to a density of about 1.2 to 1.3 grams per cubic centimeter. The resulting pellet was of the same dimensions as the pellet of Example III. A graphite film was applied to one face of the virgin nitrocellulose pellet according to the previously cited Examples. The pellet was placed in the test fixture of Example III and upon the application of electrical energy ignited and burned with the necessary characteristics required for industrial driving tools.

EXAMPLE IX A pellet was made by dry compacting in a press a material made by first dissolving nitrocellulose in a solvent such as acetone to form a lacquer and'then pouring the lacquer into an excess of water under vigorous agitation. Under this treatment, the nitrocellulose precipitates out as solid particles. Pellets compacted from this precipitated nitrocellulose were treated exactly the same as the pellets cited in Example III and yielded the same desirable characteristics.

EXAMPLE X A porous pellet was fabricated by the wash-out process as hereinbefore described. Pellets made by this process were treated exactly as the pellets cited in'Example III and yielded the same desired performance.

The functioning time of the conductive film initiator described in this disclosure is desirably short. The func tioning time may be defined as that time which elapses from the first application of electrical energy to the first evidence of a pressure rise caused by the burning of the propellant body. This functioning time is between 1 and 2 milliseconds.

It is to be understood that the initiator of the present invention with a suitable amount of nitrocellulose body 4 may be used by itself as a replacement for the powder charge and primer in conventional ammunition. It may also be used alone to provide the energy to drive a bul- -9 let, to drive the piston of an industrial type fastening tool or to start a small internal combustion engine. It may be used as a replacement for the conventional primer to ignite additional propellant or to serve as a means of igniting a mass of pyrotechnic composition or as the first element in a train of explosives.

What is claimed is:

1. An initiator comprising a body of fibrous nitrocellulose having a surface characterized by a multiplicity of exposed fibres and a conductive film adhered to said surface in intimate contact with said fibres, said film having a negative temperature coefficient of electrical resistivity, a thickness of between about 0.5 mil and 2.0 mils, and a resistance of between about 2 ohms and 100 ohms.

2. The initiator of claim 1 wherein said conductive film is selected from the group consisting of graphite,

partially graphitized carbon black and lead peroxide.

3. The initiator of claim 2 wherein the density of said nitrocellulose body is between about 0.85 and about 1.40 grams per cubic'centimeter.

4. An initiator comprising a body of porous nitrocellulose having a surface characterized by exposed webs between pores and a conductive film adhered to said surface in intimate contact with said exposed webs, said conductive film having a negative temperature coefficient of electrical resistivity, a thickness of between about 0.5 mil and 2.0 mils, and a resistance of between about 2 ohms and ohms.

5. The initiator of claim 4 wherein said conductive film is selected from the group comprising graphite, graphitized carbon black and lead peroxide.

6. The initiator of claim 5 wherein the density of said nitrocellulose body is between about 0.85 and about L40 grams per cubic centimeter.

7. An initiator comprising a body of dry compacted bulk powder formed to a predetermined configuration including a face and a conductive film adhered to said face, said film having a negative temperature coefficient of electrical resistivity, a thickness of between about 0.5 mil and 2.0 mils, and a resistance of between 2 ohms and 100 ohms.-

8. A caseless pellet comprising dry compacted bulk powder formed to a predetermined configuration, an electrically conductive coating on one face of said pellet, said coating having a negative temperature coefficient of electrical resistivity, a thickness of between about 0.5 mil and 20 mils, and a resistance of between about 2 ohms and 100 ohms, and being selected from a group consisting of graphite, graphitized carbon black and lead peroxide.

Claims

2. The initiator of claim 1 wherein said conductive film is selected from the group consisting of graphite, partially graphitized carbon black and lead peroxide.
3. The initiator of claim 2 wherein the density of said nitrocellulose body is between about 0.85 and about 1.40 grams per cubic centimeter.
4. An initiator comprising a body of porous nitrocellulose having a surface characterized by exposed webs between pores and a conductive film adhered to said surface in intimate contact with said exposed webs, said conductive film having a negative temperature coefficient of electrical resistivity, a thickness of between about 0.5 mil and 2.0 mils, and a resistance of between about 2 ohms and 100 ohms.
5. The initiator of claim 4 wherein said conductive film is selected from the group comprising graphite, graphitized carbon black and lead peroxide.
6. The initiator of claim 5 wherein the density of said nitrocellulose body is between about 0.85 and about 1.40 grams per cubic centimeter.
7. An initiator comprising a body of dry compacted bulk powder formed to a predetermined configuration including a face and a conductive film adhered to said face, said film having a negative temperature coefficient of electrical resistivity, a thickness of between about 0.5 mil and 2.0 mils, and a resistance of between 2 ohms and 100 ohms.
8. A caseless pellet comprising dry compacted bulk powder formed to a predetermined configuration, an electrically conductive coating on one face of said pellet, said coating having a negative temperature coefficient of electrical resistivity, a thickness of between about 0.5 mil and 2.0 mils, and a resistance of between about 2 ohms and 100 ohms, and being selected from a group consisting of graphite, graphitized carbon black and lead peroxide.