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This invention generally relates to electrical primers and more
particularly to an electrical primer adapted for use in automotive airbag
inflators.
Many types of electric primers are known. These electric primers generally
fall into two categories. The first involves the use of a bridge wire in
conjunction with an explosive primer mixture. In this type, an applied
voltage causes the bridge wire to resistively heat to a point where
ignition occurs. Another variation of the bridge wire device is an
exploding bridge wire primer. In this device the voltage applied is high
enough to cause almost instantaneous vaporization of the bridge wire
creating a plasma. The shock wave produced by this plasma detonates a high
explosive. These devices are relatively insensitive to low shock levels,
but have the disadvantage of requiring a relatively large and cumbersome
voltage source.
The second type of electric primer involves the use of an explosive primer
mixture having a conductive substance such as noble metal filings, noble
metal powder, or some form of carbon mixed therein to provide many small
conduction paths. Like the bridge wire primer, a current passing through
the conducting powder causes localized heating and/or a spark which in
turn detonates the explosive primer mixture.
In the case of bridge wire devices, the bridge wire can tolerate a
generally high amount of radio frequency (RF) radiation or electrostatic
discharge (ESD) stimulus. However, bridge wire devices have a tendency to
burn without detonating the explosive mix thus causing misfires. To remedy
this, redundancy is usually employed requiring a multitude of bridge wires
in contact with the mixture. This substantially increases the
manufacturing problems. Secondly, bridge wire devices require more parts
and more steps to assemble and thus are more costly to produce than
conductive mix type devices.
In contrast, the prior art conductive mix type primers provide an enormous
number of redundant electrically conductive paths and thus theoretically
may be much more reliable. Electric primers utilizing a conductive mixture
composition generally includes an explosive component and a finally
divided noble metal powder or carbon black. One such patent is U.S. Pat.
No. 3,090,310 issued to Peet et al. The conductive mixture in Peet et al
comprises zirconium, zirconium hydride, barium nitrate, lead peroxide, and
pentaethyeride tetranitrate (PETN). This is an explosive primer mixture
having a substantial percentage of PETN, approximately 20%. This mixture
also requires a substantial input of total energy for detonation.
Other examples of electric primers utilizing conductive mixtures are those
described in U.S. Pat. Nos. 3,793,920, 3,320,164, and 3,155,553. These
patents each disclose a composition having primarily a finely divided
detonating material such as lead azide or other explosive and either
carbon in some form or a finely divided noble metal powder. These metals
and carbon act as conductors and do not act primarily as a fuel and
therefore their content is minimized in order to achieve the desired
results.
Since these mixtures contain primarily initiating explosives such as lead
styphnate or lead azide, there is a substantial explosive hazard during
manufacture and handling. In addition, when designed to operate at
relatively low voltages, immunity to radio frequency radiation and
electrostatic discharge is minimal.
A conductive mixture without an explosive utilized in an electric primer is
disclosed in U.S. Pat. No. 4,070,970 issued to Scamaton. This patent
discloses an electric ignitor having good resistance to low voltage
accidental triggering. However, the mixture requires a relatively high
initiation voltage and relies upon breakdown of the dielectric presented
by copper oxide. The mixture disclosed in this patent is a mixture of
copper oxide and aluminum powder, which is a thermite mixture, having a
pre-breakdown DC resistance of at least 1 megohm. Thus, the voltage
required for operation of this ignitor is substantial. In fact, this
mixture requires an applied voltage of approximately 1700 volts to achieve
dielectric breakdown and ignition. Accordingly, this mixture is not useful
at low voltages.
U.S. Pat. No. 4,522,665, issued to Yates et al, discloses a percussive
priming mixture whose composition is similar to that of the present
invention. However, this patent teaches the use of a mixture of titanium
and potassium perchlorate not in an electric primer application, but in a
percussive primer where a substantial level of impact energy is required
for ignition. The Yates Patent does not teach the use of titanium and
potassium chlorate in electric primer applications at low voltages. In
fact, U.S. Pat. No. 4,522,665 specifically teaches that this mixture
requires the use of a separate electrical heating element for electrical
ignition, as in bridge wire primers.
The electrically conductive primer mixtures of the prior art are not found
to be entirely satisfactory for use in automotive airbag inflators for
several reasons. First, the firing voltage threshold required in an
automotive application is preferably on the order of 9 to 12 volts. Most
prior art electric primers require voltages on the order of 80 volts or
higher. At lower voltages, the prior art primer mixtures are generally
overly sensitive to discharge of stray electrostatic energy. In addition,
the presence of an explosive material such as lead styphnate and lead
azide is undesirable. The hazard of manufacturing and handling these
explosives is substantial. Accordingly, a mixture which minimizes or
eliminates the use of explosive materials is advantageous for use in
airbag inflators in automotive passive restraint systems.
It is therefore an object of the present invention to provide an
intrinsically conductive pyrotechnic mix for an electric primer without
utilizing a separate heating element such as a bridge wire or non-reactive
metal powder.
It is another object of the present invention to provide an intrinsically
conductive mix primer without utilizing a primarily explosive component.
It is another object of the present invention to provide an intrinsic
conductive mix primer which operates reliably at low voltages.
It is another object of the present invention to provide an intrinsically
conductive pyrotechnic mix primer that has a reduced sensitivity to RF
radiation and ESD.
It is still another object of the present invention to provide an electric
primer utilizing an intrinsically conductive mixture to reliably ignite
the propellant in a gas generator utilized in an airbag inflator for an
automotive passive restraint system.
These and other objects of the present invention are advantageously
achieved in an electric primer using an intrinsic conductive pyrotechnic
mixture according to the present invention consisting essentially of about
15% to 50% dry weight of a finely divided metal powder fuel, about 75% to
40% dry weight of an alkaline oxidizer, and about 2% to 15% dry weight of
at least one secondary sensitizing fuel mixed together. The mixture may
also include a binder to hold the mixture together in certain
applications.
The mixture may be compacted to establish particular desired resistivity
values, total energy required for ignition and predictable repeatability
values that may be required for a specific design.
The metal fuel in the mixture according to the present invention may be any
oxidizable metal powder which can serve as the primary fuel and provide a
conductive path for electrical current through the mixture. More
specifically, the metal powder fuel is preferably selected from the group
consisting essentially of titanium, zirconium, uranium, and aluminum. A
specifically preferable metal powder fuel for use in the mixture of the
invention for application in automotive airbag inflator electric primers
is titanium.
The alkaline oxidizer advantageously utilized in the mixture according to
the present invention may be an alkaline oxidizer selected from the group
consisting essentially of an alkali metal or an alkaline earth metal
chlorate or perchlorate. Both chlorates and perchlorates must be used with
care as they are very reactive and verge on being an explosive themselves.
Illustrative examples in this group include potassium, sodium, and calcium
chlorate. More specifically, a preferred oxidizer for use in the mixture
of the invention for automotive airbag inflator primers is potassium
chlorate.
The secondary fuel and sensitizer utilized in the mixture of the present
invention is preferably selected from the group consisting essentially of
boron, sulphur, and lead thiocyanate. One specifically preferable
sensitizer for the mixture of the invention for automotive airbag inflator
primers is boron. Alternatively, another preferred embodiment of the
mixture of the invention includes both boron and lead thiocyanate as
secondary fuel sensitizers.
One preferred embodiment of the intrinsic conductive mixture according to
the present invention consists of essentially titanium metal powder in
about 27% dry weight, potassium chlorate in about 68% dry weight, and 5%
dry weight of boron as the secondary fuel and sensitizer.
In order to obtain repeatability and specific desired resistance and total
energy input values in a particular design application, the mixture
according to the present invention is compacted to at least 1000 psi.
Compaction ensures that the mixture is in firm electrical contact with the
electrodes used to pass electrical current through the primer mixture to
cause ignition. Preferably, for applications at low voltages such as are
utilized in automobiles, the mixture is compacted at a pressure of between
about 3000 psi and 150,000 psi using conventional primer mix compaction
techniques.
The electric primer with an intrinsic conductive pyrotechnic mix in
accordance with the present invention is advantageously designed for
igniting a propellant in a gas generator which is in turn utilized in an
automatic airbag inflator for an automobile passive restraint system. The
electric primer includes a cup shaped outer electrode having a central
bore through the bottom of the cup and an inner button shaped electrode
disposed within the cup and spaced from the outer electrode. The inner
electrode has a terminal portion extending through the bore, out of the
cup for external connection to an electrical power source, typically the
vehicle battery. An insulator sleeve is placed between the button portion
of the inner electrode and the outer electrode thus spacing and separating
the electrodes from one another.
The pyrotechnic mixture in accordance with the present invention, as
described above, of a metal powder fuel, an alkaline oxidizer, and a
sensitizing fuel are compacted together of a pressure of preferably about
3000 psi within the cup for consistent repeatability. The mixture is
positioned and compressed against the button and the inner wall of the
outer cup electrode so as to span the insulator. In this configuration the
primer exhibits a resistance of about 5 ohms.
The electric primer is ignited by impressing nominal vehicle battery
voltage of 9 to 11 volts across the two electrodes. This impressed voltage
causes a current to flow through the conductive mix, igniting the mix, and
thus causes ignition of a propellant such as sodium azide within the gas
generator.
The intrinsic conductive pyrotechnic mixture according the present
invention may contain metal fuels from the group of titanium, zirconium,
uranium, and aluminum. The oxidizer may be an alkaline metal chlorate or
alkaline earth metal chlorate or perchlorate. The perchlorates must be
used with care, however, as they are extremely reactive and verge on being
an explosive.
The third ingredient, usually a fuel used to supplement the metal fuels
when a higher resistance is desired, can be boron, sulphur, or lead
thiocyanate. The mixture is consolidated by application of a force between
3000 psi and 150,000 psi. The magnitude of consolidation pressures applied
affects the thermal stability and the ignition sensitivity of the mix as
well as the repeatability and hence reliability of the assembly.
These and other objects, features and advantages of the present invention
will become readily apparent upon consideration of the following detailed
description when taken in conjunction with the drawing and appended claims
.
FIG. 1 is a sectional view of one preferred embodiment of an electric
primer in accordance with the present invention.
Turning now the drawing, an electric primer 10 according to a preferred
embodiment of the present invention is shown FIG. 1. The electric primer
10 is utilized in a gas generator for an automatic airbag inflator (not
shown) in an automobile passive resistant system. The electric primer 10
is operative at a nominal vehicle battery voltage to ignite a propellant
such as sodium azide within the gas generator.
Electric primer 10 includes a cup shaped tubular outer electrode body 12
made of a conductive metal such as copper, brass, steel, or aluminum. The
cup shaped body 12 has a central bore 14 through the bottom of the cup
body 12. An inner electrode 16 having a terminal stud portion 18 and a
button portion 20 is positioned in cup 12 with the terminal stud portion
18 extending out of cup 12 through bore 14. A power supply 22 such as a
vehicle battery or a charged capacitor bank is shown schematically
externally connected to stud portion 18 and outer cup 12. The power supply
22 applies a voltage across electrodes 12 and 16 to cause electrical
current to flow between the electrodes, as described below. The power
supply has a nominal voltage between 9 and 12 V DC.
Spaced between and separating button portion 20 of inner electrode 16 from
outer electrode 12 is an insulator 24. Insulator 24 may be a ceramic,
plastic, or glass material and is sleeve shaped so as to space the disk
shaped button portion 20 internally from the cup shaped outer electrode
12.
Placed within the cup shaped outer electrode 12 and against the button
portion 16 is an intrinsically conductive pyrotechnic mixture 26 in
accordance with the present invention. This mixture consists essentially
of a powdered metal fuel, an alkaline chlorate oxidizer, and at least one
secondary fuel which acts as a sensitizer. The presence of a fuel
sensitizer fine tunes the firing threshold of the mixture as will be
subsequentially described.
Against the mixture 26 is a closure wad 28. This closure wad 28 may be made
of any suitable material such as paper, nitrocellulose, or cellulose
acetate. The primary purpose of wad 28 is to retain the mixture in the cup
12 and separate the primer mixture from the propellant to be ignited (not
shown). Frictionally disposed in cup shaped outer electrode 12 and against
closure wad 28 is a support cup 30 which has a centrally located flash
hole 32 therethrough for use as a flame exit. The support cup 30 prevents
the outer electrode body 12 from collapse when the electrical primer is
pressed into a metal cavity (not shown) in the cartridge or gas generator
casing. The support cup 30 is frictionally secured within the outer cup
12. Thus the conductive mixture 26 is packaged between outer cup 12,
button portion 20, and support cup 30.
The intrinsic conductive mixture in accordance with the present invention
preferably includes a metal powder fuel, an alkaline chlorate oxidizer,
and at least one secondary fuel which acts as a sensitizer to adjust the
firing potential of the mixture. A preferred embodiment of this mixture
consists essentially of titanium metal powder, potassium chlorate, and
either boron or lead thiocyanate as a sensitizer fuel.
This mixture is a pyrotechnic mixture and does not include primarily any
explosive or other material that is susceptible to independent detonation.
The metal fuel may be from the group consisting of titanium, zirconium,
uranium and aluminum. The alkaline oxidizer may be from the group
comprising potassium chlorate, sodium chlorate, and calcium chlorate. The
secondary fuel may be boron, sulphur, or lead thiocyanate.
The intrinsic conductive mixture, including titanium, potassium chlorate,
and boron is preferably formulated with 20-45% titanium powder 75-40%
potassium chlorate, and 2-15% secondary fuel sensitizer. In addition, a
binder material of 1-3% dry weight may be added. Finally, an additional
fuel sensitizer may be utilized, such as lead thiocyanate, to further
adjust the sensitivity and firing threshold of the mix for a particular
design and for a given temperature range.
One exemplary preferred embodiment of the mixture comprises 27% dry weight
titanium powder, 68% dry weight potassium chlorate, and 5% dry weight
boron. This composition compacted to at least 3000 psi ignites readily
when a voltage of 9-12 volts is applied to the electrodes 12 and 16 by the
power supply 22. This power supply may be vehicle battery or may be a
separate source of DC voltage and current according to a particular
inflator design. An optimum DC resistance of this mixture has been found
to be about 5 ohms in vehicle airbag inflator applications.
In addition to the three major constituents of the mixture, a binder may be
added to bind the mixture together. Finally, more than one sensitizer such
as lead thiocyanate may be utilized in small amounts to fine tune or
adjust the sensitivity of the mixture.
The intrinsically conductive mixture in accordance of the present invention
is compacted within the outer cup 12 after assembly of the inner electrode
16 and insulator 24 therein. The mixture 26 is compacted under a pressure
ranging from about 3000 psi to 150,000 psi. The compaction pressure
significantly affects the energy required to initiate mixture ignition.
For example, the energy required for is approximately 0.272 Joules at 3000
psi and approximately 0.9 Joules at 150,000 psi.
The presence of a secondary fuel sensitizer such as boron and/or lead
thiocyanate is not absolutely necessary for ignition of the intrinsically
conductive mixture according to the present invention. However, the
presence of one or more of these sensitizers lowers the total energy
required for ignition. However, the total energy required is substantial
and provides a low sensitivity to RF radiation and ESD even when the
resistance is about 5 ohms. For example, the mixture including 5% boron
readily ignites when subjected to an applied voltage of 12 volts. In
contrast, a mixture without any sensitizer requires a substantially higher
voltage level for ignition, on the order of 24 volts.
Unlike prior art conductive mixes, the metal component in the mixture of
the present invention not only serves as a conductive material but also
serves as the major fuel. In addition, no primary explosive is used in the
mixture according to the present invention.
The voltage sensitivity of the mixture is not significantly effected by the
absence of a secondary fuel or sensitizer. Only the total energy required
to ignite the fuel and oxidizer of the mixture is greater where the
sensitizer is absent. Moreover, the total energy required to ignite the
fuel and oxidizer in the mixture is high while the resistance is low,
which permits the use of the mixture at relatively low voltages.
The electric primer utilizing an intrinsically conductive pyrotechnic
mixture consisting essentially of a metal powder fuel, an alkaline metal
oxidizer, and at least one sensitizer fuel in accordance with the present
invention may also be advantageously used in other electric primer
applications than that specifically described herein. For example, the
primer mixture of the present invention may be used in many electric
primer applications where stable mixtures and low operating voltage
thresholds are required. What has been described is a preferred embodiment
of the invention. Variations and equivalents are within the scope of the
present invention. The foregoing description is to be clearly understood
as being given by way of illustration and example only. The spirit and
scope of this invention is intended to be limited only by the scope of the
following appended claims.
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