U.S. patent number 3,779,167 [Application Number 05/286,387] was granted by the patent office on 1973-12-18 for electrical initiator.
This patent grant is currently assigned to Olin Corporation. Invention is credited to Raymond I. Cowles, Charles G. Irish, Jr., Joseph W. Silva.
United States Patent |
3,779,167 |
Irish, Jr. , et al. |
December 18, 1973 |
**Please see images for:
( Certificate of Correction ) ** |
ELECTRICAL INITIATOR
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.
Inventors: |
Irish, Jr.; Charles G.
(Cheshire, CT), Silva; Joseph W. (Northford, CT), Cowles;
Raymond I. (Woodbridge, CT) |
Assignee: |
Olin Corporation (New Haven,
CT)
|
Family
ID: |
26673058 |
Appl.
No.: |
05/286,387 |
Filed: |
September 5, 1972 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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4480 |
Jan 21, 1970 |
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Current U.S.
Class: |
102/202.9;
102/472 |
Current CPC
Class: |
F42B
5/08 (20130101) |
Current International
Class: |
F42B
5/08 (20060101); F42B 5/00 (20060101); F42b
005/08 () |
Field of
Search: |
;102/38,46,28,7.2R,86.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Stahl; Robert F.
Parent Case Text
This is a division, of application Ser. No. 4,480, filed Jan. 21,
1970.
Claims
What is claimed is:
1. In combination, a conductive cup closed at one end by an
electrode structure, an initiator in the other end of said cup,
said initiator comprising a body of nitrocellulose having a surface
characterized by a plurality of exposed fibres, a conductive film
adhered to said surface in intimate contact with the fibres, said
electrode structure including a first electrode in intimate contact
with said conductive cup and said film and a second electrode
electrically insulated from said first electrode and being in
intimate contact with said film.
Description
This invention relates generally to the field of initiators. More
particularly, this invention relates to an initiator of a smokeless
propellant which is capable of being electrically ignited.
An initiator is the first element in a device using explosive,
propellant, or pyrotechnic materials. The initiator responds to an
input stimulus and starts the burning or explosive reactions.
Initiators are classified according to the nature of the input
stimulus which can be either mechanical or electrical. Initiators
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.
Electrical initiators may be further categorized by the type of
transducer used to convert the electrical input energy into heat
energy. Transducers that may be used with electrical initiators
include a hot bridgewire or an exploding bridgewire, a conductive
mix, a conductive film bridge, and a spark gap. The input
sensitivity varies sharply with the type of transducer; each type
having a characteristic threshold firing energy and current
requirements. This invention concerns that type of electric
initiator wherein the transducer is a conductive film bridge.
helpful in understanding the present invention. Conductive films
may be 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 has been 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.
Because graphite has a negative resistance temperature coefficient
and because of the inherent variable film thickness and path
length, the electric current is channeled into a 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 charged not only for graphite or carbon bridge devices but
also for every type of present initiator except the exploding
bridgewire whose greater energy release permits the use of
secondary explosives under special conditions of particle size and
confinement.
We may summarize the salient characteristics of currently employed
conductive film initiators as follows:
1. The conductive film may be metallic or non-metallic 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 a priimary explosive, commonly lead styphnate or lead
azide, 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 is relatively
complex an 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 what is usual train of several explosive
materials.
In view of the above, it is an object of the present invention to
provide an electric insulator 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 to drive
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 of 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. 1 is a cross-sectional view of an initiator constructed in
accordance with the present invention;
FIG. 2 is a cross-sectional view of on 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 the present invention;
FIG. 5 is a sectional view showing the initiator usee 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, non-metallic 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, any nitrocellulose 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
energy.
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 gelatanized, is by mechanically abrading
or scoring the surface or by treating the surface with a
solvent.
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
semi-conductors such as germanium an 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 semi-colloidal 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 100 ohms with the range of 5 ohms to 20 ohms
being preferred.
If desired, an oxidizing material may be incorporated into the
conductive 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.
The initiator 2, either alone or supplemented by additional
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 porotechnic
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
18 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 the end of the tube (not shown) 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 electrical
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 suitable
insulation 48. It is to be understood that the battery cup 30 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 50 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. Immediately below the
wadding 18 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,
cylindrical, outer electrode member 58 and a central rod-like
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 bullet. As another
alternative, instead of the additional powder being in the form of
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 of
such 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. 3,311,057. 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 pellet-shaped 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 shown in FIG. 7. Such
circuit may include a suitable battery 80 having a capacitor 82 in
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
one 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.
TABLE I
Voltage for 99.95 Energy for 99.95 Capacitance Probability to
Probability to Fire FIre 30 microfarads 251 volts 0.94 joules 120
microfarads 110 volts 0.73 joules 480 microfarads 57 volts 0.77
joules
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 two
hours at 150.degree.F. A portion of the 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 on individual resistance ranging from about 3 ohms
to 100 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 wadding and shot pellets. The
shotshells were placed into a test vehicle for firing which
included a battery-capacitor 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 one joule. The pellets ignited within a period of 1 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 iginition 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 0.22 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 0.22 caliber ammunition. The ignition
delay after application of the energy was in the order of 1 to 2
milliseconds and the energy consumed less than one joule.
EXAMPLE III
In a manner similar to Examples I and II, an ignition pellet was
dry compacted from bulk powder to a density of about 1.2 grams per
cubic centimeter and a diameter of about 0.4 inch and a thickness
of about 0.150 inch. One face of the pellet was coated with a
graphite 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 one 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 shotwhell 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 one 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 U.S. 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 three
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 two 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, non-gelatinized
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 functioning 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 by 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 bullet, 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.
* * * * *