U.S. patent number 3,793,920 [Application Number 05/307,002] was granted by the patent office on 1974-02-26 for process for making a conductive-mix electrical initiator.
This patent grant is currently assigned to E. I. du Pont de Nemours and Company. Invention is credited to Carl P. Sheran.
United States Patent |
3,793,920 |
Sheran |
February 26, 1974 |
PROCESS FOR MAKING A CONDUCTIVE-MIX ELECTRICAL INITIATOR
Abstract
A conductive-mix electrical initiator is made more
energy-sensitive by applying to at least one electrode surface
therein which is to be adjacent the conductive ignition charge,
e.g., the end surface of the electrode seated within the initiator
body, a coating which increases the resistance of the initiator.
Preferably, the coating is applied by chemical or electrochemical
conversion coating of the surface, e.g., by acid chromate treatment
or a light anodic oxidation.
Inventors: |
Sheran; Carl P. (Bloomingdale,
NJ) |
Assignee: |
E. I. du Pont de Nemours and
Company (Wilmington, DE)
|
Family
ID: |
23187819 |
Appl.
No.: |
05/307,002 |
Filed: |
November 16, 1972 |
Current U.S.
Class: |
86/1.1; 149/16;
102/202.8; 149/35; 149/92 |
Current CPC
Class: |
F42B
3/14 (20130101) |
Current International
Class: |
F42B
3/14 (20060101); F42B 3/00 (20060101); C06b
021/02 () |
Field of
Search: |
;149/14,15,16,35,92
;86/1R ;102/7.2R,7.2A,28RP,46 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lechert, Jr.; Stephen J.
Claims
I claim:
1. In a method of making a conductive-mix electrical initiator
wherein an electrically conductive explosive charge comprising a
mixture of finely divided heat-sensitive explosive and a
finely-divided electrically conductive material is positioned
between two metal electrodes, the improvement which comprises
applying to a surface of at least one of said electrodes, which
surface is to be positioned adjacent said conductive explosive
charge, a coating which is less electrically conductive than said
electrodes and said conductive material in said explosive charge,
and thereafter bringing the intact coating on said metal surface
into contact with said explosive charge.
2. A method of claim 1 wherein the composition and thickness of
said coating are such that the resistance of the conductive path
from one of said electrodes to the other through said electrically
conductive explosive charge is within the range of about from 0.4
to 110 ohms when measured at 10 milliamperes of test current with a
conductive charge comprising 70 percent by weight of lead azide and
30 percent by weight of silver within one hour of contact with the
coating.
3. A method of claim 1 wherein said coating is about from 0.00002
to 0.0001 inch thick.
4. In a method of making a conductive-mix electrical initiator
wherein a solid metal body is seated within the mouth of a metal
shell in a manner such that one end surface thereof is exposed and
the other end surface thereof is within said shell adjacent an
electrically conductive explosive charge comprising a mixture of a
finely divided heat-sensitive explosive and a finely divided
electrically conductive material, said explosive charge being
adjacent the inner wall of said shell, and adjacent surfaces of
said metal body and said metal shell being electrically insulated
from one another, the improvement which comprises applying to at
least one of the metal surfaces which is to be positioned adjacent
said explosive charge a coating which is less electrically
conductive than said solid metal body, said shell, and said
conductive material in said explosive charge, and thereafter
bringing the intact coating on said metal surface into contact with
said explosive charge.
5. A method of claim 4 wherein said coating is applied to the end
surface of said solid metal body.
6. A method of claim 4 wherein said coating is applied to the inner
wall of said shell.
7. A method of claim 5 wherein said end surface to which said
coating is applied is substantially flat.
8. A method of claim 1 wherein said electrically conductive
explosive charge is comprised of about from 45 to 90 percent by
weight of a metallic azide and 55 to 10 percent by weight of a
conductive material selected from the group consisting of the noble
metals and carbon.
9. A method of claim 8 wherein said electrically conductive
explosive charge is in a continuous reaction train with a secondary
high explosive.
10. A method of claim 8 wherein said conductive material is
silver.
11. A method of claim 2 wherein said coating is applied by a
conversion coating process.
12. A method of claim 11 wherein said electrode is made of aluminum
and said coating is applied by treatment of said electrode surface
with a solution containing a source of hexavalent chromium and a
source of fluoride.
13. A method of claim 5 wherein said solid metal body is made of
aluminum and said coating is applied by anodic oxidation of the end
surface of said body.
14. In a method of making a conductive-mix electrical initiator
wherein a solid metal body is seated within the mouth of a metal
shell in a manner such that one end surface thereof is exposed and
the other end surface thereof is witin said shell adjacent an
electrically conductive explosive charge comprising a mixture of
finely divided heat-sensitive explosive and a finely divided
electrically conductive material, said explosive charge being
adjacent the inner wall of said shell, and adjacent surfaces of
said cylindrical body and said shell being electrically insulated
from one another, the improvement which comprises employing a
conductive explosive charge comprising 45 to 90 percent by weight
of lead azide and 55 to 10 percent by weight silver and applying to
at least one of the metal surfaces which is to be positioned
adjacent said explosive charge a coating which is less electrically
conductive than said solid metal body, said shell, and silver, and
thereafter bringing the intact coating on said metal surface into
contact with said explosive charge, said charge having been
prepared by contacting a silver nitrate with an aqueous suspension
of finely divided lead azide and magnesium powder, whereby silver
is precipitated, and separating the solids.
15. A method of claim 14 wherein said coating is applied to the end
surface of said solid metal body by a conversion coating process.
Description
BACKGROUND OF THE INVENTION
This invention relates to a method of producing a conductive-mix
electrical initiator, e.g., a detonator or squib, which method
provides increased energy sensitivity with a given conductive
mix.
Conductive-mix electrical initiators contain, as an ignition mix,
an electrically conductive explosive charge comprising a mixture of
a heat-sensitive explosive and an electrically conductive material.
These initiators function by the application of a voltage to an
electrode having one of its ends exposed and its other end adjacent
the ignition mix within the body, the ignition mix forming a
conductive path through which current flows between this electrode
and the initiator body (the other electrode) and the explosive
becoming ignited by the heat thereby generated in the mix.
Initiators of this type can be miniaturized more easily than
standard bridge-wire type initiators, and can be fired at
relatively low energy levels, (e.g., below about 5,000 ergs, as
contrasted to 50,000-100,000 ergs for standard-size bridge-wire
type initiators) making them especially suitable for use in
miniaturized form in applications where small initiators and small
firing devices are required.
The ignition sensitivity requirements for low-firing-energy
initiators depend on the specific intended use as well as on safety
considerations. Ordinarily, response to firing energy on the order
of about 2,500 ergs or less may be required for initiation by
capacitor discharge (e.g., by the discharge of a 2.2 microfarad
capacitor charged to 15 volts), or on the order of about 1.5 volts
constant voltage. At the same time, minimum firing energy and
current restrictions are imposed for safety reasons, e.g., it may
be required that the initiators not fire when subjected to about 40
ergs by capacitor discharge and that they sustain a current of
about 10 milliamperes for 30 seconds without firing. The
sensitivity to firing energy is directly related to sensitivity to
capacitor discharge firing voltage, according to the relationship W
= 1/2CV.sup.2, where W is energy in joules, C is capacitance in
farads, and V is the voltage across the capacitor, and therefore
the term "energy sensitivity" when used herein implies "voltage
sensitivity" (capacitor discharge) and vice versa.
Whether or not an initiator of a given design is acceptable for
field use depends on the degree of reliability of the initiator
with respect to its response to firing energy, as well as to its
ability to perform the required work, e.g., to bring about the
detonation of a secondary explosive or the ignition of a
deflagrating composition under a given set of conditions. Regarding
response to an energy input, maximum reliability means that as many
of the initators as possible after subjection to the handling and
storage conditions encountered in use, (a) respond to the selected
firing energy level, or pass the all-fire test; and (b) fail to
fire at the energy and current levels where firing is undesired, or
pass the no-fire test. For maximum reliability, not only must the
structure and composition of the initiator be such as to afford
consistency of functioning, i.e., functioning of large numbers of
initiators, but the potentially active components in the initiator
mus be sufficiently stable to assure functioning of the initiator
after prolonged storage periods.
Various conductive explosive charges for initiators are known in
the arts e.g., those described in U.S. Pats. Nos. 2,918,871 and
3,155,553. Such charges are mixtures of fine particles of a primary
explosive, such as lead azide, and of an electrically conductive
material, such as a noble metal, carbon, etc. The amount of energy
needed to ignite the mixture by application of a voltage to an
electrode adjacent thereto depends on the specific explosive
employed, the specific conductive material used, the relative
proportions of explosive to conductive material, the particle
sizes, etc. While greater voltage sensitivity can be achieved to a
certain extent by use of a better conductor, and/or a higher
conductor/explosive ratio within a specific range, it often is not
desirable, in terms of initiator output, or possible, to alter the
composition of the mix to achieve a required increase in
sensitivity. Thus, a means is needed to increase the voltage
sensitivity of a conductive-mix initiator without sacrifice in the
long-term no-fire characteristics (current tolerance) of the
initiator.
SUMMARY OF THE INVENTION
This invention provides an improvement in a method of making a
conductive-mix electrical initiator wherein an electrically
conductive explosive charge comprising a mixture of a finely
divided heat-sensitive explosive and a finely divided electrically
conductive material is positioned between two electrodes, the
improvement comprising applying to a surface of at least one of the
electrodes, which surface is to be positioned adjacent the
conductive explosive charge, a coating which is less electrically
conductive than the electrodes and the conductive material in the
explosive charge, and thereafter bringing the intact coating on the
metal surface into contact with the explosive charge.
One of the electrodes is a metal shell within which the explosive
charge is located, a portion of the charge being adjacent the inner
wall of the shell, and the other electrode is a solid metal body
seated within the mouth of the metal shell in a manner such that
one of the solid body's end surfaces is exposed and the other is
within the shell adjacent the explosive charge, adjacent surfaces
of the solid body and shell being electrically insulated from one
another. The coating is applied either to the end surface of the
solid metal body to be positioned adjacent the explosive charge, or
to the inner wall surface of the shell where the wall is to be
adjacent the explosive charge, or to all metal surfaces which are
to be adjacent the charge, coating of the end surface, of the solid
metal body being preferred. The nature of the coating material and
its thickness are such that the resistance of the conductive path
from one of the electrodes to the other through the conductive
explosive charge is within the range of about from 0.4 to 110 ohms
when measured at 10 milliamperes of test current with a conductive
charge comprising 70 percent by weight of a lead azide and 30
percent by weight of silver within one hour of contact with the
coating.
In a preferred process of this invention, the electrically
conductive explosive charge comprises a mixture, in finely divided
form, of about from 45 to 90 percent by weight of a metallic azide,
most preferably lead azide, and about from 55 to 10 percent by
weight of silver, the mixture having been prepared most preferably
by the displacement of silver from a silver salt by magnesium
powder in a stirred slurry of fine lead azide substantially as
desired in Example 1; and the electrode which is seated within the
shell is made of aluminum and has a thin coating, e.g., less than
about one-tenth of a mil, applied to its end surface by a
conversion coating process, e.g., anodic oxidation or acid chromate
treatment.
BRIEF DESCRIPTION OF THE DRAWING
The attached drawing is a cross-sectional view of a detonator which
can be made by the process of this invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to the drawing, a metal shell is comprised of an inner
shell component 1a, made, for example, of aluminum or steel, and a
surrounding outer shell component 1b, made, for example, of
aluminum, steel, or stainless steel. The two-component, or
dual-walled, shell structure is a convenient one for use in the
mechanical assembling of the detonator. The closed bottom end of
the shell is formed by the closed end 2 of outer shell component
1b. The open top end of the shell is located in the inner shell
component 1a, which has a smaller internal-diameter portion near
the top end, and a larger interal-diameter portion for the
remainder of its length. Solid metal body 3, e.g., a wire, made of
aluminum or steel, for example, is seated within the opening in the
top end of the shell, fitting in the smaller internal-diameter
portion of shell component 1a, and has one exposed end surface 4
and an internal end surface which has a thin coating 5, e.g., less
than about one-tenth of a mil, of a material which is less
electrically conductive than the metal from which solid body 3 and
shell component 1a are made, e.g., an oxide or other
oxygen-containing compound of a metal, coating 5 being in contact
with the ignition mixture, i.e., an electrically conductive
explosive charge 6, located within the larger internal-diameter
portion of shell component 1a, e.g., a mixture of about 70 percent
finely divided lead azide and about 30 percent finely divided
silver (by weight). Ignition charge 6 is in contact with a priming
charge 7, e.g., lead azide, which in turn is in contact with the
base charge of secondary explosive 8, e.g., HMX
(cyclotetramethylenetetranitramine). Thus, there is a continuous
reaction train containing ignition charge 6, primary charge 7, and
base charge 8.
Solid metal body 3 (one of the electrodes) has a layer of
electrically insulating material 9, e.g., "Formvar," a polyvinyl
formal insulating resin, applied to its convex peripheral surface
where this surface is adjacent to the internal surface of shell
component 1a (the other electrode). Thus, when voltage is applied
to electrode 3, current flows from one electrode to the other
through coating 5 and conductive ignition charge 6, the heat
generated in ignition charge 6 causing the ignition of the
explosive in the mixture, which in turn causes the detonation of
priming charge 7 and base charge 8. A moisture-resistant seal 10,
e.g., an epoxy resin, surrounds electrode 3 in the area where it
emerges from the shell, the seal covering the ends of inner and
outer shell components 1a and 1b, respectively, and extending
beyond the end of insulating layer 9. Annular notch 11 in the end
surface of shell component 1a is produced by staking the surface,
thereby laterally deforming shell component 1a so as to produce an
annular depression 12 in the periphery of electrode 3, which thus
becomes locked securely in position.
In the present process, a metal surface in a conductive-mix
initiator which is to be adjacent to the conductive ignition
charge, e.g., an inner wall surface of the initiator body and/or
the end surface of the electrode seated within the body, has
applied thereto a coating which has a lower electrical conductivity
than the electrodes, and the conductive material in the ignition
charge, thereby increasing the resistance of the conductive path
between the electrodes through the ignition charge. The resistance
of this path depends on the composition of the ignition charge, and
in the absence of the coating, is essentially zero with a charge
containing a noble metal such as silver or gold as the conductive
component, and slightly higher with charges containing a material
such as carbon as the conductive component. It has been found that
increasing the resistance of the initiator to a controlled degree
has the effect of increasing the voltage sensitivity of the
initiator without unduly increasing the long-term sensitivity to
firing current. The composition and thickness of the
resistance-increasing coating are such that the resistance of the
conductive path from one electrode to the other through the
ignition charge is greater than about 0.4 ohm, and preferably at
least about 0.8 ohm, and no greater than about 110 ohms, these
resistances being measured under the conditions specified above.
The suitability of a given coating can be determined by making a
resistance measurement under the above-specified conditions
although it will be understood that the resistance obtained with a
coating which meets the above-specified requirement may be above
the 0.4-110 ohms range when the coating is used in an initiator
containing a different ignition charge than the 70/30 lead
azide/silver charge used for the test. The requirement that the
test resistance measurement be made within one hour of the time
that the coating and test charge are brought into contact is
prescribed to offset the possibility that with some charges the
resistance may increase with time.
When the coating provides insufficient resistance to produce more
than 0.4 ohm in the above-specified test, the initiator cannot be
fired reliably when a 2.2 microfarad capacitor charged to 30 volts
(5,000 ergs) is discharged through it, whereas reliable initiation
at less than 30 volts is achieved when the coating provides a
resistance of more than 0.4 ohm in the test. A coating which
provides sufficient resistance to produce at least an 0.8 ohm
resistance in the above-specified test is preferred inasmuch as an
initiator having such a coating can be fired reliably when a 2.2
microfarad capacitor charged to less than 15 volts is discharged
through it. The coating applied to the electrode surface(s) should
not be so resistant as to produce a resistance above about 110 ohms
with the lead azide/silver charge, because when the resistance
becomes this high, the minimum firing current drops excessively,
and the interior begins to show some tendency to fire when
subjected to a current of about 10 milliamperes for 30 seconds. The
coating can be applied to any or all of the metal surfaces of the
electrodes which are to be located adjacent the conductive charge
in the initiator, e.g., the inside wall surface of the shell or
initiator body (shell component 1a in the drawing), the end surface
of the solid metal body seated within the shell, or both.
Preferably, the coating is applied to the smaller area surface,
e.g., the end surface of the solid metal body, inasmuch as the
required resistance increase may be more readily achieved
therewith.
The chemical composition per se of the coating is not a critical
feature of the process of the present invention, provided that its
electrical conductivity is low enough to afford the resistance
required to achieve the desired voltage sensitivity, and also
provided that it can be applied in a thin enough layer, generally
less than about one ten-thousandths of an inch, to assure that an
excessively high resistance will not result. In any case, the
coating should be sufficiently nonreactive with the electrically
conductive explosive charge employed as the conductive ignition
mixture that the resistance does not become excessive over
prolonged storage periods.
Various procedures can be used to apply the coating to the
electrode surface, e.g. dipping or immersion in a film-forming
material of suitable resistance and stability. A convenient
technique is to apply a conversion coating, which is a film formed
by a reaction in which a portion of the base metal is converted to
one of the components of the film. The conversion coating can be
chemical or electrochemical. For example, with an aluminum
electrode, any of the well-known chemical conversion coating
processes can be used, e.g., those which produce amorphous
phosphate or chromate coatings, such coatings being described more
fully, for example, in U.S. Pat. No. 2,796,370; in a paper entitled
"Surface Preparation of Aluminum for Painting," presented by R. F.
Reeves et al. at the Aluminum Finishing Seminar sponsored by the
Aluminum Association, Detroit, Michigan, Jan. 30--Feb. 1, 1968; and
in Aluminum Vol. III, Fabrication and Finishing, K. R. Van Horn;
Ed., American Society for Metals, 1967, Chapter 17. The chemical
conversion procedure which produces an amorphous chromate coating
employs a bath which contains at least two ingredients, i.e., a
source of hexavalent chromium and a source of fluoride; that which
produces an amorphous phosphate coating additionally contains
phosphate. The coatings are postulated to contain aluminum in the
form of an oxide or hydrated oxide, and a chromium salt (e.g., a
chromate or phosphate). When the coating is formed by a chemical
conversion coating procedure, e.g. with a solution of an "Alodine"
chemical (Amchem, Ambler, Pa.), the thickness of the coating
produced, depends on the coating conditions used, e.g.,
concentration of the solution, temperature of the solution, and
contact time. In general, a concentration of about 7.5 grams of
"Alodine" 1200S per liter of water, with a 3-minute contact at room
temperature, is sufficient to provide the required resistance,
usually in a thickness of about 0.00002-0.00007 inch.
Concentrations in the range of about from 3 to 15 grams per liter
have been employed successfully, as well as temperatures up to
180.degree.F. and contact times of up to 10 minutes. It may be
desirable to avoid the use of a combination of the most strenuous
conditions, however, to assure that not too thick, and therefore
too resistant, a coating is produced.
Another suitable method of applying the coating consists in
producing an electrochemical or anodic conversion coating. For
example, an aluminum electrode may be subjected to a light anodic
oxidation, the anodizing current and time being controlled so as to
produce a thin (less than one ten-thousandths of an inch) aluminum
oxide coating on the aluminum. This can be accomplished by making
aluminum the anode in a suitable electrolyte, e.g., sulfuric,
chromic, or oxalic acid, with a metal or carbon cathode, and
passing an electric current through the cell. The procedure is
described more fully in the above-cited book edited by Van Horn,
Chapter 19. For coating an aluminum electrode in the present
process, a suitable coating of oxide, i.e., a layer about from
0.00002 to 0.0001 inch thick, is produced with an 0.5 milliampere
current, an anodizing time of about from 2 to 10 minutes, and a 15
percent sulfuric acid solution.
Whatever the method employed to produce the coating on the metal
surface, it is essential that the coating be thin, that it have the
required conductivity relative to the other conductors in the
initiator, and that it be a preformed coating made under
controllable conditions, keeping in mind that too thick, i.e., too
resistant, a coating results in increased current sensitivity, and
finally leads to complete failure of the initiator.
One of the electrodes is a solid (i.e., not hollow) metal body, the
surface configuration of which can be as desired. For miniature
initiators, e.g., detonators no more than about 0.5 inch long and
0.33 inch in diameter, the electrode usually is a wire, pin, or
screw. The end surfaces of the electrode can be flat, curved, or
pointed, a substantially flat surface adjacent the conductive
explosive charge being preferred.
The coating applied to the metal surface which is to be adjacent
the conductive ignition charge in the initiator generally is a
substantially continuous layer, although in view of the extreme
thinness of the coating, discontinuities and/or nonuniformities may
be detected on a microscopic scale in the as-applied coating. Such
imperfections are entirely acceptable, having no deleterious effect
with respect to the achieving of the required resistance and
sensitivity. On the other hand, the presence of the less-conductive
coating in the electrode-to-electrode circuit is critical in the
present process, and it is neither necessary nor desirable to
remove a portion of the coating after the application or formation
thereof so as to expose a portion of the surface for contact with
the ignition charge. Therefore, in the present process the intact
coating is contacted with the ignition charge, i.e., there is no
intentional removal of a portion of the coating after its formation
on, or application to, the electrode's surface to the extent that
the resistance is decreased thereby to a value below about 0.4 ohm
in the test specified above.
The application of a coating as specified above to the electrode in
a conductive-mix initiator increases the voltage sensitivity of the
initiator regardless of the specific conductive explosive charge
employed. Therefore, the process can be employed with any
conductive charge desired, e.g., a mixture of a detonating
explosive and a conductive material when the initiator is a
detonator, and a mixture of a detonating or deflagrating explosive
and a conductive material when the initiator is a squib. For a
given charge, the voltage sensitivity is increased when the present
process is employed to produce the initiator. A preferred ignition
charge, especially for detonators, is a mixture of about from 45 to
90 percent by weight of a finely divided metal azide, e.g., lead
azide or silver azide, and about from 55 to 10 percent by weight of
a finely-divided noble metal, e.g., silver or gold, or carbon. A
particularly preferred charge is one which comprises a mixture of
about from 60 to 80 percent by weight of finely divided lead azide
and 40 to 20 percent by weight of finely divided silver, prepared
by the displacement of silver from a water-soluble silver salt such
as silver nitrate by magnesium powder in a stirred slurry of fine
lead azide. Where maximum stability of the firing characteristics,
especially the sensitivity to current, of the initiator over
prolonged storage periods is desired, the lead azide/silver mixture
can be prepared as described in Example 1, i.e., by contacting the
silver salt with an aqueous suspension of the lead azide, to which
magnesium powder has been added, thereby precipitating silver. The
components of the ignition charge should be in the finely-divided
form, e.g., smaller than about 20 microns, and well mixed.
The initiator made by the process of the present invention can be
any device which features, in a metal shell, an
electrode/conductive ignition mixture assembly, wherein the
ignition mixture can be the sole explosive charge present, or it
can be in initiating relationship with another explosive charge
which is located at the bottom of the shell, the bottom charge
being a detonating, or high, explosive in the case of an electric
detonator or blasting cap, and a deflagrating explosive composition
in the case of a squib. Regardless of the number of explosive
charges in the shell, the nature of the bottom charge and the
initiator's output capability, in the initiator produced by the
present process the ignition mixture is an electrically conductive
explosive charge which is in contact with a coating on an
electrode, the coating being less electrically conductive than the
electrodes and the conductive material in the ignition mixture, and
causing the resistance of the conductive path from one electrode to
the other through the ignition mixture to be in the range of about
from 0.4 to 110 ohms when measured as described previously, the
resistance remaining within this range after prolonged storage
periods in initiators containing maximum stability ignition
charges. This means that these initiators meet low firing-voltage
specifications along with no-fire current requirements as-made as
well as for extended periods after production. This stability of
firing characteristics is dependent, of course, not only on the
proper mode of application or production of the coating, but also
on the provision of an ignition mixture which is substantially
stable with respect to the coating in contact therewith.
The conductive ignition charge can be the sole explosive charge in
the initiator, or it can be present in a continuous reaction train
with another explosive charge located at the bottom or base of the
initiator. The nature of the difference between the ignition and
base charges can vary and depends on the intended use for the
initiator. For example, in a detonator, the base charge will be a
secondary high (detonating) explosive and thus usually will be
chemically different from the ignition charge, which is a mixture
of a primary high explosive and a conductive material. A detonator
also will usually have a priming charge of high explosive between
the ignition and base charges. In a squib made by the process of
this invention, the ignition charge may be chemically different
from the base charge, or the difference may be a quantitative one,
with the base charge being of the same chemical structure as the
ignition charge but of different density.
The body structure of the initiator is not critical, provided it
affords the required protection of the active ingredients from the
environment, as well as from possible damage in handling. Although
not required, the dual-walled shell structure shown in the
accompanying drawing is a preferred one both from the standpoint of
strength as well as adaptability to mechanized loading techniques.
Firm seating of the electrode within the body, so that the
electrode is unable to move from its required position adjacent the
conductive ignition charge, is essential. For this reason, an
initiator in which the end surface of the body has been staked so
as to cause the electrode to be swaged in place, as is shown in the
drawing, is preferred. As is shown in the following examples, the
initiator made by the process of this invention is particularly
suited for use in miniature form, making it particularly useful in
military applications requiring small fuze train components
responsive at low energy levels. However, the initiator's utility
should not be construed as being limited to initiators of miniature
size.
The following examples describe different ways in which the process
of the invention can be employed to produce a miniature detonator
having the structural features shown in the drawing.
EXAMPLE 1
Solid metal body or electrode 3 is made from a 0.209-inch long,
0.0508-inch diameter wire made of 5056-0 aluminum having a
0.0031-inch-thick "Formvar" insulation coating on its peripheral
surface, and having bare substantially flat end surfaces. Inner
shell component 1a (the other electrode) is fabricated from an
open-ended shell of 2024-T4 aluminum, which is 0.249-inch long, and
has an outer diameter of 0.120 inch and an internal cavity having a
0.0545-inch diameter for 0.079 inch of length and a 0.100-inch
diameter for 0.170 inch of length. The change in diameter of the
cavity is effected through a tapered portion which forms a
30.degree. angle with the horizontal axis.
The insulated wire, the end surfaces of which are free of corrosion
products, heavy oxide, grease, etc., is placed in a solution of
"Alodine" 1200S, a conversion coating chemical for aluminum
produced by Amchem Products Inc., Ambler, Pa., and containing a
source of hexavalent chromium and a source of fluoride. The
concentration of the solution is 7.5 grams of "Alodine" 1200S
powder per liter of cold tap water. The wire is maintained in the
stirred solution for three minutes, after which time it is removed
from the solutions rinsed in running water for five minutes, and
dried at 120.degree.F for about 4 hours. This treatment applies to
the end surfaces of the wire a thin coating (about 0.00002 inch),
yellowish to tan in color, of an amorphous chromate. The coated
wire then is positioned in the smaller-internal-diameter portion of
the cavity in shell component 1a and swaged in place by applying an
annular stake to the end surface of shell component 1a around the
wire and coaxial therewith. Staking of this surface produces an
annular notch 11 in the end surface and lateral deformation of
shell component 1a so as to produce an annular depression 12 in the
periphery of the wire, thereby locking the wire securely in place.
The "Formvar" insulation is stripped from the protruding portion of
the wire (now electrode 3) to within 0.020 inch from the end
surface of shell component 1a, and the thin coating on the
protruding end surface 4 of electrode 3 is removed.
Ignition charge 6 then is loaded into shell component 1a and
pressed tightly against coating 5 at a pressure of about 12,800
psi. Charge 6, weighing 10 milligrams, is a mixture of 70 percent
finely divided lead azide and 30 percent finely divided silver
prepared by the displacement of silver from silver nitrate by
magnesium powder in a stirred slurry of fine lead azide according
to the following procedure:
In a 2.5-liter vessel, a suspension of lead azide is prepared at
room temperature by rapidly adding 401 milliliters of a sodium
azide solution containing 89.69 grams/liter sodium azide to 398
milliliters of a lead nitrate solution containing 229.9 grams/liter
lead nitrate with vigorous stirring to assure fast mixing. After
about two minutes of stirring, the suspended material is allowed to
settle, the liquid is decanted off, and the solids washed four
times, with 1.3 liters deionized water in each wash, by reslurrying
and decantation. Following the washes, about 800 milliliters
deionized water is added to the solids, and the stirrer restarted.
After about two minutes of stirring, 3.89 grams of magnesium powder
in sufficient (S.D.A.-3A) denatured alcohol to permit suspension of
the powder is poured slowly into the suspension. Immediately
thereafter, a solution of 54.33 grams of silver nitrate in 200
milliliters deionized water is added, and stirring is continued for
one hour. The solids are allowed to settle and are washed four
times (about 1.3 liters each wash) with deionized water by
reslurrying and decantation followed by two washings (about one
liter each wash) with denatured alcohol by reslurrying and
decantation. The solids are then dried under ambient conditions.
Photomicrographs of the solid material show that it consists of a
homogeneous mixture of about 70 percent by weight of lead azide
particles about 2.5-5 microns in size and about 30 percent by
weight of dendritic silver particles about 0.1-10 microns in
size.
Twenty-eight milligrams of priming charge 7 is pressed into shell
component 1a against ignition charge 6 also at a pressure of about
12,800 psi. Charge 7 consists of the so-called RD-1333 lead azide,
a finely divided product which meets military specification No.
MIL-L-46225A, dated Mar. 29, 1963. The base charge 8 of the
detonator is 18.5 milligrams of HMX and is pressed into shell
component 1a against charge 7 at a pressure of about 12,800
psi.
The loaded shell component 1a is inserted into an AISI 305
stainless steel cup, which forms outer shell component 1b. The
thickness of the bottom of the cup, which forms closed end 2 of the
shell, is 0.007 inch. The cup is 0.272 inch long and has an outer
diameter of 0.136 inch and an inner diameter of 0.122 inch. The top
of the cup is roll-over crimped to the top surface of shell
component 1a, and the top end of the assembly is sealed by applying
an epoxy resin sealant 10 around electrode 3 and the ends of
components 1a and 1b. The length of the exposed portion of
electrode 3 (from sealant 10 to the exposed end surface 4) is about
0.100 inch.
In a typical sampling (e.g., about 100) of miniature detonators
made as described above, the resistances measured within one hour
of assembly, at 10 milliamperes of test current, fall within the
range of about from 0.4 to 35 ohms. Detonators in such a sampling
which have a resistance of at least about 0.8 ohm all fire (the
explosive in the conductive charge ignites) in 10 microseconds when
a 2.2 microfarad capacitor charged to 15 volts (ca. 2,500 ergs) is
discharged through the electrode/conductive charge/electrode
circuit, the specific firing voltage range being about from 3 to 13
volts. Detonators at the lower end of the resistance range, i.e.,
about from 0.4 to 0.8 ohm, generally fire when the 2.2 microfarad
capacitor is charged to 30 volts, and some fire at less than 15
volts. All of the detonators in a typical sampling fire when
subjected to a constant voltage of 1.5 volts from a battery or
other power supply.
None of the detonators in the typical sampling fire when a 10
milliampere current is applied to the electrode-to-electrode
circuit for 30 seconds, or when a 2.2 microfarad capacitor charged
to 2 volts is discharged therethrough. The minimum firing current
for these detonators, within one hour of assembly, is about from 62
to above 500 milliamperes.
After storage for about six months at ambient conditions, the
detonators in the sampling have resistances in the range of about
from 3.8 to 100 ohms. The firing voltage range is essentially
unchanged. All of the detonators still pass the 10 milliampere
current no-fire test, the minimum firing current range being about
from 90 to above 500 milliamperes.
With respect to the output characteristics, the detonators in the
sampling, as-assembled as well as after six months' storage
consistently detonate a pellet of pressed tetryl over an air gap of
0.045 inch and through a mild steel barrier 0.008 inch thick.
CONTROL EXPERIMENT 1
A detonator is made according to the procedure described in Example
1 except that the application of coating 5 to the end surface of
electrode 3 is omitted. This detonator, in a typical sampling, has
a resistance, as-made, of 0 to 0.4 ohm, and does not fire when a
2.2 microfarad capacitor charged to 30 volts is discharged
therethrough.
EXAMPLE 2
The procedure of Example 1 is repeated except that coating 5 is
applied to the end of electrode 3 by an electrolytic procedure in
which a length of the aluminum wire is connected to the positive
side of a power supply and a lead strip to the negative pole. Both
metals are placed in a 15 percent sulfuric acid solution, and a 0.5
milliampere current passed through for two minutes. This treatment
produces an approximately 0.00003-inch coating of aluminum oxide
(resistivity at 25.degree.C. is about 10.sup.14 ohm-cm.) on the
bare end of the aluminum wire. The resistivity of aluminum at
25.degree.C. is 2.6 microhm-cm.
In a sampling of 10 detonators made as described in Example 1 with
the electrode coated by anodic oxidation, resistances measured
within one hour of assembly, at 10 milliamperes of test current,
are within the 0.4 to 110 ohm range. The firing voltage range, with
a 2.2 microfarad capacitor, is about from 3 to 8 volts.
CONTROL EXPERIMENT 2
When the procedure described in Example 2 is repeated with the
exception that a 1.0 milliampere current is passed through for 15
minutes, the oxide coating has infinite resistance and all 5
detonators in a sampling fail to fire at 25 volts.
EXAMPLE 3
The procedure of Example 1 is repeated except that ignition charge
6 is a mechanical mixture of 70 parts by weight of RD-1333 lead
azide and 30 parts by weight of channel black. The resistances of a
ten-detonator sampling measured with the ignition charge described
in Example 1 are the same as given in Example 1. With the lead
azide/channel black ignition charge, the detonators fire by the
discharge of a 2.2 microfarad capacitor charged to 9-14 volts.
CONTROL EXPERIMENT 3
Detonators made as described in Example 3, except that the coating
on the end surface of the electrode is omitted, fire by the
discharge of a 2.2 microfarad capacitor charged to 17-28 volts. One
out of ten of the detonators tested is not fired by discharge of
the same capacitor charged to 30 volts.
EXAMPLE 4
The procedure described in Example 1 is repeated with the exception
that inner shell component 1a also is treated with the "Alodine"
1200S solution so as to produce an amorphous chromate conversion
coating on the internal and external surfaces thereof. Thus, charge
6 is in contact with coating 5 on the end surface of electrode 3 as
well as with the coating on the internal surface of shell component
1a.
The resistances of ten detonators prepared in this manner, measured
within one hour of assembly at 10 milliamperes of test current,
range from about 1.8 to 60 ohms. All of the detonators fire in 10
microseconds when a 2.2 microfarad capacitor charged to 15 volts is
discharged through them, the specific firing voltage range being
about from 5 to 10 volts.
EXAMPLE 5
The procedure described in Example 1 is repeated with the exception
that the application of coating 5 to the end surface of electrode 3
is omitted, and inner shell component 1a alone is treated with
"Alodine" 1200S solution, so that charge 6 is in contact with the
amorphous chromate coating on the internal surface of the shell
component 1a. Detonators prepared in this manner and having
resistances above about 0.8 ohm fire in the range of about from 6
to 14 volts (2.2 microfarad capacitor discharge).
* * * * *