Abstract:
A pyrotechnic device comprises a body of ignitable material ( 66 ). A pair of electrodes ( 44 ) and ( 52 ) provide electrical energy to heat and ignite the body of pyrotechnic material ( 66 ). The electrodes ( 44 ) and ( 52 ) extend through an electrical insulation housing ( 78 ). The electrical insulation housing ( 78 ) has surfaces defining a chamber ( 92 ) through which the electrodes ( 44 ) and ( 52 ) pass. A body ( 94 ) of a solid electromagnetically lossy, substantially gas impermeable material is positioned within the chamber ( 92 ). The lossy material comprises a vitreous ceramic matrix consisting essentially of about 5% to about 50% by weight of a multi-component glass binder and about 50% to about 95% by weight of an electromagnetically lossy ferromagnetic and/or ferroelectric filler. The body ( 94 ) of lossy material is fused to the surfaces defining the chamber ( 92 ) in the electrical insulation housing ( 78 ) and to the electrodes ( 44 ) and ( 52 ).

Description:
TECHNICAL FIELD 
     The present invention relates to an apparatus for inflating a vehicle occupant protection device and particularly relates to an electrically actuatable pyrotechnic igniter for an air bag inflator. 
     BACKGROUND OF THE INVENTION 
     An inflatable vehicle occupant protection device, such as an air bag, is inflated in the event of sudden vehicle deceleration such as occurs in a vehicle collision. The air bag restrains movement of a vehicle occupant during a vehicle collision. The air bag is inflated by inflation fluid from an inflator. The inflation fluid may be stored gas which is released from the inflator and/or gas generated by ignition of combustible gas generating material in the inflator. The inflator uses an electrically actuatable pyrotechnic igniter to open the container and release the stored gas and/or to ignite the gas generating material. 
     The electrically actuatable pyrotechnic igniter contains a charge of ignition material. The pyrotechnic igniter also contains a bridgewire that is supported in a heat transferring relationship with the ignition material. When the pyrotechnic igniter is actuated, an actuating level of electric current is directed through the bridgewire in the igniter. This causes the bridgewire to become resistively heated sufficiently to ignite the ignition material. The ignition material then produces combustion products that open the container and release the stored gas and/or ignite the gas generating material. 
     Radio frequency interference (RFI) suppression filters are commonly incorporated in an electrically actuatable pyrotechnic igniter. RFI suppression filters ensure that unwanted radio frequency (RF) signals are suppressed and allow the passage of direct current and low frequency alternating current. Failure to suppress RF signals might lead to the undesired actuation of the igniter. 
     In many cases, electrically actuatable pyrotechnic devices incorporating these RFI filters are also required to provide a gas-tight seal to protect sensitive components or materials contained within an enclosure. Many electrically actuatable pyrotechnic igniters incorporate a hermetically sealed chamber for their ignitable material that is vulnerable to degradation by the intrusion of water vapor. 
     SUMMARY OF THE INVENTION 
     The present invention is a pyrotechnic device. The pyrotechnic device comprises a body of ignitable material. A pair of electrodes provide electrical energy to heat and ignite the body of pyrotechnic material. The electrodes extend through an electrical insulation housing. The electrical insulation housing has surfaces defining a chamber through which the electrodes pass. A body of a solid electromagnetically lossy, substantially gas impermeable material is positioned within the chamber. The lossy material comprises a vitreous ceramic matrix consisting essentially of about 5% to about 50% by weight of a multi-component glass binder and about 50% to about 95% by weight of an electromagnetically lossy ferromagnetic and/or ferroelectric filler. The body of lossy material is fused to the surfaces defining the chamber in the electrical insulation housing and to the electrodes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features of the invention will become more apparent to one skilled in the art upon consideration of the following description of the invention and the accompanying drawings in which: 
     FIG. 1 is a schematic view of a vehicle occupant protection apparatus embodying the present invention; and 
     FIG. 2 is an enlarged sectional view of a part of the apparatus of FIG.  1 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Referring to FIG. 1, an apparatus  10  embodying the present invention includes an inflator  14  and an inflatable vehicle occupant protection device  26 . The inflator  14  contains a gas generating material  16 . The gas generating material  16  is ignited by an igniter  24  operatively associated with the gas generating material  16 . Electric leads  20  and  22  convey electric current to and from the igniter  24 . An electric current is conveyed to the igniter  24  through a crash sensor  18  from a power source (not shown). The crash sensor  18  acts as a switch in response to vehicle deceleration indicative of a vehicle collision. The current to the igniter  24  causes ignition of the gas generating material  16 . A gas flow means  28 , such as an opening in the inflator  14 , conveys gas, which is generated by combustion of the gas generating material  16 , to the vehicle occupant protection device  26 . 
     A preferred vehicle occupant protection device  26  is an air bag, which is inflatable to help protect a vehicle occupant in the event of a vehicle collision. Other vehicle occupant protection devices which can be used with the present invention are inflatable seat belts, inflatable knee bolsters, inflatable air bags to operate knee bolsters, inflatable head liners, inflatable side curtains, and seat belt pretensioners. 
     Referring to FIG. 2, the igniter  24  includes a header  30 . The header  30  is a generally cylindrical metal member preferably machined from 304L steel. The header  30  has a cylindrical outer surface  32  and flat, parallel, radially extending, circular opposite sides  34  and  36 . A cylindrical opening  40  extends completely through the header  30  parallel to a central axis  42  of the igniter  24  and intersects the opposite sides  34  and  36  of the header  30 . 
     A first electrode  44  is connected with the header  30 . The first electrode  44  is made from a conductive wire material, such as drawn nickel iron alloy wire, and extends parallel to the central axis  42  of the igniter  24 . The first electrode  44  has an inner end  48 , which is brazed to the side  34  of the header  30 , and an outer end  50 , which extends away from the header  30  and protrudes, in the form of a prong  46 , at one end of the igniter  24 . 
     A second electrode  52  extends parallel to the first electrode  44 . The second electrode  52  is made from the same material as the first electrode  44 . The second electrode  52  has an inner end  56  which extends axially through the cylindrical opening  40  in the header  30 . An outer end  58  of the second electrode  52  extends away from the opening  40  and forms a prong  54 , similar to the prong  46  of the first electrode  44 , at the one end of the igniter  24 . 
     A bridgewire  62  extends between the inner end  56  of the second electrode  52  and the side  36  of the header  30 . The bridgewire  62  is formed from a high resistance metal alloy. A preferred metal alloy is a nickel-chromium-iron alloy. Other suitable alloys for forming a high resistance bridgewire include platinum-tungsten and 304L steel. The bridgewire  62  heats up and generates thermal energy when an electrical current of predetermined magnitude passes through the bridgewire  62 . 
     The bridgewire  62  extends through a portion of a pyrotechnic charge  66 . The pyrotechnic charge  66  is a pyrotechnic material, which auto-ignites upon application of sufficient thermal energy. The pyrotechnic material can be any pyrotechnic material typically used in an igniter such as boron potassium nitrate (BKNO 3 ), potassium dinitrobenzofuroxan (KDNBF), barium styphnate monohydrate (BARSTY), cis-bis-(5-nitrotetrazolato)pentaaminecobalt(III)perchlorate (CP), diazidodinitrophenol (DDNP), 1,1-diamino-3,3,5,5-tetrazidocyclotriphosphazine (DATA), cyclotetramethylenetetranitramine (HMX), lead azide, and lead styphnate. 
     The pyrotechnic charge  66  is enclosed in an ignition cup  68 . The ignition cup  68  is a cup-shaped metal member preferably made from drawn 304L stainless steel. The ignition cup  68  has a cylindrical wall  74 , which defines a cavity  76  in which the pyrotechnic charge  66  is disposed. A portion  70  of the wall  74  of the ignition cup  68  overlies most of the cylindrical outer surface  32  of the header  30 . The ignition cup  68  has a frangible end wall  72 , which ruptures on ignition of the pyrotechnic charge  66 . 
     The igniter  24  further includes a housing  78 . The housing  78  is formed from an electrical insulation material. An electrical insulation material is a material that has a high resistance to the passage of current. Preferred electrical insulation materials are molded thermoplastics, such as nylon, and sintered ceramics, such as alumina or zirconia. 
     The housing  78  has a side wall  80 , which extends parallel to the central axis  42  of the igniter between an open end  82  and a closed end  84  of the housing  78 . The side wall  80  of the housing  78  has a cylindrical inner surface  86 , which extends from the open end  82  of the housing  78  to the closed end  84 . The cylindrical inner surface  86  and closed end  84  define a chamber  92 . The closed end  84  of the housing  78  has parallel cylindrical passages  88  and  90  that extend parallel to the igniter axis  42  through the closed end  84  of the housing  78  and open into the chamber  92 . The passages  88  and  90  receive the parallel electrodes  44  and  52 , respectively. 
     The header  30  is seated within the open end  82  of the housing  78  so that the header  30  closes the open end  82 , except for where the cylindrical opening  40  in the header  30  overlaps the open end  82 . 
     A body  60  of gas impermeable glass is positioned in the cylindrical opening  40  of the header  30 . The body  60  encircles the inner end  56  of the second electrode  52  and is encircled by a cylindrical inner surface  38  of the header  30  that defines the opening  40 . The body  60  of gas impermeable glass is positioned in the opening  40 , in a manner to be described, so that it fuses to and forms a gas-tight seal with the surface  38  and the inner end  56  of the second electrode  52 . The body  60  of gas impermeable glass electrically insulates the header  30  from the inner end  56  of the second electrode  52 . 
     A body  94  of electromagnetically lossy, substantially gas impermeable material is positioned within the chamber  92  of the housing  78 . The body  94  of electromagnetically lossy, substantially gas impermeable material is fused to and forms a gas-tight electromagnetically lossy seal with the inner surfaces  84  and  86  of the housing and the side  34  of the header  30 . The body  94  is also fused to and forms a gas-tight electromagnetically lossy seal with the portions of the first electrode  44  and the second electrode  52  that are encircled by the body  94 . The body  94  of electromagnetically lossy, substantially gas impermeable material electrically insulates the first electrode  44  from the second electrode  52 . Also, because it comprises an electromagnetically lossy filler, the body  94  provides RF attenuation for the igniter  24 . 
     In accordance with the present invention, the body  94  of electromagnetically lossy substantially gas impermeable material comprise a dense vitreous ceramic matrix. The matrix consists essentially of a glass binder and a electromagnetically lossy ferromagnetic and/or ferroelectric filler interspersed through the binder. The amount of binder is about 5% to about 50% by weight of the matrix. The amount of filler is about 50% to about 95% by weight of the matrix. 
     Preferred glass binders are lead borosilicate and lead aluminoborosilicate glasses, which include oxides of Al, B, Ba, Mg, Sb, Si, and Zn. These binders are commercially available in the form of finely ground frits. Examples of binders are CORNING (Corning, N.Y.) high temperature sealing glasses nos. 1415, 8165, and 8445, CORNING low temperature ferrite sealing glasses nos.1416, 1417, 7567, 7570, and 8463, and FERRO CORPORATION (Cleveland, Ohio) low temperature display sealing glasses nos. EG4000 and EG4010. 
     Preferred ferromagnetic fillers include spinal structured ferrites having the general formula (AaO) 1−x (BbO) x Fe 2 O 3  where Aa and Bb are divalent metal cations of Ba, Cd, Co, Cu, Fe, Mg, Mn, Ni, Sr, or Zn, and x is a fractional number in the semi-open interval [ 0 , 1 ). Examples of commercially available ferromagnetic fillers are FAIR-RITE PRODUCTS (Wallkill, N.Y.) nos. 73 and 43, which are sintered manganese-zinc and nickel-zinc spinal ferrite powders, respectively. 
     Preferred ferroelectric fillers include perovskite titanates having the general formula (XxO)TiO 2  and perovskite zirconates having the general formula (XxO)ZrO 2  where Xx denotes divalent metal cations of Ba, La, Sr, or Pb. Barium titanate, (BaO)TiO 2 , is a typical species. Other acceptable fillers include electrically lossy La-modified lead zirconium titanate perovskite ceramics known as PLZTs. 
     The body  94  is formed by first preparing an electromagnetically lossy ceramic mixture of 5-50% by weight of the glass binder and 50-95% by weight of the lossy ferromagnetic and/or ferroelectric filler. The mixing is performed wet in a polyethylene ball mill using a ceramic media such alumina or zirconia and a volatile organic carrier such as acetone having a forming agent such as polyvinyl acetate and a fatty acid dispersant such as menhaden fish oil. The resulting mixture is then dried. The dried mixture can be used in either a free-flowing form or as a vitreous preform. A vitreous preform is prepared by pouring the dried mixture into a mold having the desired configuration and heating the mixture to an elevated temperature effective to coalesce the mixture into a solid body. 
     The following Examples illustrate use of the dried mixture and assembly of the igniter. 
     EXAMPLE 1 
     In this Example, the non-conductive housing  78  is made of a thermoplastic, such as nylon. 
     A graphite mold/fixture is provided that has in the mold portion of the mold/fixture the desired configuration of the body  94  of glass. The first and second electrodes,  44  and  52 , and the header  30  are also positioned in the mold/fixture and held in fixed, preset desired positions in the mold/fixture. The dried mixture of electromagnetically lossy filler and glass binder is introduced into the mold/fixture as a vitreous preform. The vitreous preform fills the mold and encircles the electrodes  44  and  52 . A glass preform is introduced into the opening  40 . The glass preform fills the opening  40  and encircles the inner end  56  of the electrode  52 . The graphite mold/fixture, the mixture of electromagnetically lossy filler and glass binder, the glass preform, the electrodes  44  and  52 , and the header  30  are heated to a temperature above the glass working temperature of the glass binder and the glass preform (i.e. about 580° C. to about 800° C.). At this temperature, the electromagnetically lossy filler and glass binder as well as the glass preform soften or melt, wetting the surfaces of the electrodes  44  and  52  and the header  30  in contact with the electromagnetically lossy filler and binder and the glass preform. Upon cooling, the glass preform solidifies into the gas impermeable glass body  60 , and the electromagnetically lossy filler and binder coalesce into the electromagnetically lossy, substantially gas impermeable body  94 . The surfaces of the body  60  are chemically bonded to and form a gas-tight seal with the surface  38  of the header  30  and inner end  56  of the second electrode, which are contacted by the body  60 . The surfaces of the body  94  are chemically bonded to and form a gas-tight electromagnetically lossy seal with the surfaces of the electrodes  44  and  52  and the header  30  that are contacted by the body  94 . 
     The bridgewire  62  is welded to the inner end  56  of the second electrode  52  and the side  36  of the header. The pyrotechnic charge  66  is placed in the ignition cup  68 . The ignition cup  68  is attached to the header  30  so that the pyrotechnic charge  66  is in contact with the bridgewire  62  and the wall of the ignition cup overlies most of the outer surface  32  of the header  30 . 
     The graphite fixture is then removed and the electrodes  44  and  52 , the header  30 , the body  64  and the body  90  are placed in a second mold having a cavity shaped to the shape of the housing  78 . The material of the thermoplastic housing  78  is heated and flowed into the mold cavity around the now solid body  94  so that electrodes extend through the cylindrical passages  88  and  90  in the closed end  84  of the housing  78 . Upon cooling, the thermoplastic housing  78  becomes bonded to and forms a gas-tight electromagnetically lossy seal with the body  94  of electromagnetically lossy, substantially gas impermeable material, the ends  50  and  58  of the electrodes  44  and  52 , and the header  30 . The second mold is then removed from the housing. 
     EXAMPLE 2 
     This Example illustrates use of the dried mixture of glass binder and electromagnetically lossy filler when the housing  78  is made of a sintered ceramic such as alumina. 
     The vitreous preform of binder and electromagnetically lossy filler is seated in the housing  78 . The electrodes  44  and  52  and the header  30  are placed in the housing  78  so that the electrodes  44  and  52  extend through the cylindrical passages  88  and  90  in the closed end  84  of the housing  78 . A glass preform is seated in the opening  40  of the header  30  so that the inner end  56  of the second electrode  52  extends through the opening  40 . The housing  78 , the mixture of electromagnetically lossy filler and binder, the glass preform, the electrodes  44  and  52 , and the header  30  are heated to a temperature above the glass working temperature of the glass binder and the glass preform (i.e. about 580° C. to about 800° C.). At this temperature, the housing  78  retains its shape. Also, at this temperature, the electromagnetically lossy filler and glass binder as well as the glass preform soften or melt, wetting the surfaces of the housing  78 , electrodes  44  and  52 , and the header  30  in contact with the electromagnetically lossy filler and binder and the glass preform. Upon cooling, the glass preform solidifies into the gas impermeable glass body  60 , and the electromagnetically lossy filler and binder coalesce into the electromagnetically lossy substantially, gas impermeable body  94 . The surfaces of the body  60  of gas impermeable glass are chemically bonded to and form a gas-tight seal with the surface  38  of the header  30  and inner end  56  of the second electrode, which are contacted by the body  60 . The surfaces of the body  94  are chemically bonded to and form a gas-tight electromagnetically lossy seal with the surfaces of the housing  78 , electrodes  44  and  52 , and header  30  that are contacted by the second body  94 . 
     The bridgewire  62  is welded to the inner end  56  of the second electrode  52  and the side  36  of the header. The pyrotechnic charge  66  is placed in the ignition cup  68 . The ignition cup  68  is attached to the header  30  so that the pyrotechnic charge  66  is in contact with the bridgewire  62  and the wall of the ignition cup overlies most of the outer surface  32  of the header  30 . 
     From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims.