Abstract:
A method for reducing oxidation of indium seals within a substantially sealed housing is disclosed. The method includes installing a getter within the housing, the getter including a getter material to reduce oxygen and water vapor levels within the housing, reducing oxygen and water vapor levels within the housing relative to ambient air, substantially sealing the housing, and activating the getter within the housing.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to extending life cycles of products, and more specifically to, methods and apparatus for removal of certain gases from enclosures. 
   At least one type of known ring laser gyroscope includes electrode seals having a thin layer of indium pressed between an electrode and a glass-ceramic block. Life testing of the gyroscopes has revealed that an effective life of the indium seals are shorter for samples tested in air, as opposed to samples tested in a dry nitrogen environment. In another test, the life of the indium seal was shorter for a sample tested in a higher humidity (water vapor) level, as compared to an indium seal sample located in a vacuum baked housing that was substantially sealed and having a dry nitrogen backfill. 
   It is believed that the shorter life of the indium seal is caused by corrosion of the seal originating at exposed outer diameters of the seal. The hypothesis has been supported by testing in an oxygen rich environment which shows that the corrosion moves radially inward from an outside diameter of the seal annulus. The corrosion can eventually breach the annular width of the seal and cause a leak between a low-pressure laser cavity, within the glass-ceramic block, and the ambient atmosphere. The leak eventually renders the gyroscope inoperative. 
   Current methods of reducing oxygen at the exposed outside diameter of an indium seal include vacuum baking a gyroscope housing to reduce humidity, backfilling the housing with dry nitrogen after the vacuum baking, and substantially sealing the housing. However, gases and humidity tend to penetrate the housing seals over the course of time, exposing the indium seals within the housing to air (oxygen) and humidity. 
   BRIEF SUMMARY OF THE INVENTION 
   In one aspect, a method for reducing oxidation of indium seals within a substantially sealed housing is provided. The method comprises installing a getter, the getter including a getter material to reduce oxygen and water vapor within the housing, reducing oxygen and water vapor levels within the housing relative to ambient air, substantially sealing the housing, and activating the getter within the housing. 
   In another aspect, a ring laser gyroscope is provided which comprises a gyroscope assembly, a housing, and a getter. The gyroscope assembly incorporates indium seals and the housing is configured to accept the gyroscope assembly within a cavity of the housing, which is configured to be substantially sealed. The getter is configured to be mounted within the housing and comprises a getter material to remove oxygen and water vapor from the cavity of the housing. 
   In still another aspect, a housing for a ring laser gyroscope is provided. The housing comprises a first portion, a second portion configured to mate with the first portion in order to define a cavity, and a getter configured to be mounted within the housing. The housing cavity is configured to be substantially sealed, and the getter comprises a getter material to remove oxygen and water vapor from the cavity of the housing. 
   In yet another aspect, a method for reducing oxygen and water vapor levels within a substantially sealed housing is provided. The method comprises installing a getter, the getter including a getter material to reduce oxygen and water vapor levels within the housing, sealing the housing, and activating the getter within the housing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded view of a ring laser gyroscope including a housing and support assembly. 
       FIG. 2  is a top plan view of a ring laser block. 
       FIG. 3  is a top plan view of an indium seal. 
       FIG. 4  is a top plan view of an indium seal which has been somewhat exposed to oxygen or water vapor. 
       FIG. 5  is a top plan view of an indium seal which has been extensively exposed to oxygen or water vapor. 
       FIG. 6  is a cross sectional schematic view of a housing which includes a ring laser gyroscope assembly and a getter including a getter material to remove oxygen and water vapor from the housing. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates an exploded view of an exemplary ring laser gyroscope assembly  10 . Assembly  10  is generally comprised of a laser assembly  12  including a laser block  14 . Assembly  10  also includes a wire harness assembly  16 , a support plate  18 , an electronics circuit card  20 , which includes a flexible conductor  22  for interconnection to an outside system (not shown). Examples of such outside systems include an aircraft avionics suite or a missile guidance system. Assembly  10  further includes a cup shaped housing  24 , a dither suspension mechanism  26 , and a cover  28 . Laser assembly  12  is suspended by dither suspension mechanism  26  between support plate  18  and housing  24 . Wire harness  16  is attached to laser assembly  12  to provide electrical signals from electronics card  20  to various components of laser assembly  12 . Cover  28  includes an opening  30  into which a connector may be mounted, for the interconnection of gyroscope assembly  10  to other systems. Alternatively, individual conductors may be passed through opening  30  to make the interconnections. Whether used with individual conductors or with a connector, opening  30  is sealed, to try to prevent air and moisture from entering assembly  10 . 
     FIG. 2  illustrates laser block  14  which is constructed of a glass-ceramic material and provides a ring laser cavity containing a lasing gas. Between block surfaces  40 ,  42 , and  44  is a plurality of tunnels or cavities (not shown) having a polygon shape in the form of a triangle, in the embodiment shown, with vertices  46 ,  48 , and  50 . Mirror assemblies  52 ,  54 , and  56  are mounted to block surfaces  40 ,  42 , and  44 , respectively. The tunnels or cavities are filled with a lasing gas and ignited or excited by a sufficient voltage between a cathode  58  and each of a pair of anodes  60  and  62 . In turn, a pair of laser beams will counter-propagate along an optical ring path  64  within the laser cavity. Ring laser optical path  64  establishes a ring lasing plane defined by the three vertices  46 ,  48 , and  50 , and also circumscribes an aperture  66  in laser block  14 , which receives dither suspension mechanism  26 . Cathode  58  and anodes  60  and  62  are mounted to laser block  14  through utilization of a seal  68 . In the exemplary embodiment, seal  68  is an indium seal. 
   Pure indium is very ductile (i.e. yields at a low stress value) and readily wets metals and ceramic oxide materials, and therefore is well suited for vacuum sealing applications. A typical vacuum seal requires clean bonding surfaces, clean indium, and a smooth surface finish. A gasket (seal) of indium is placed between the surfaces to be sealed, and sufficient force is applied to spread the indium across a surface to be sealed. The spreading process breaks up an oxide layer on the indium and brings unoxidized indium into contact with the substrate materials. The indium bonds to the substrates, in this case laser block  14  and cathode  58  or anodes  60  and  62 , to form an airtight seal. 
     FIG. 3  illustrates an exemplary seal  68  which surrounds a tunnel or cavity  80  that is located along a side surface  82  of laser block  14 , with cathode  58  (shown in  FIG. 2 ) or anode  60  or  62  (both shown in  FIG. 2 ) removed for clarity. Seal  68  includes a non-corroded area  84 . Seal  68  is configured to prevent entry of air, humidity, and other contaminants into tunnel  80 . Seal  68  is in good condition and shows no corrosion activity. 
     FIG. 4  is an illustration of seal  68  for tunnel  80  as corrosion (oxidation) activities take place due to exposure to oxygen or humidity. Seal  68  includes a non-corroded area  84  and a corroded area  86 . As an outer perimeter of seal  68  is not in contact with side surface  82  of laser block  14  or cathode  58  or one of anodes  60  and  62 , the outer perimeter is susceptible to exposure to surrounding elements. Therefore, corrosion of seal  68  begins at the outer perimeter. 
     FIG. 5  illustrates a seal  68  which has been heavily damaged by corrosion. Non-corroded areas  84  no longer provide a seal for opening  80  as corroded area  86  forms most of seal  68 . 
   Therefore, the life of an indium seal can be extended by preventing or reducing oxidation of the indium after seal  68  is formed. Typical ring laser gyroscopes include an indium seal  68  which bonds dissimilar substrate materials together (i.e., an electrode of aluminum or beryllium, and a zero-expansion glass ceramic). Since coefficients of thermal expansion for the two substrate materials are different, temperature cycling creates thermal stress that tend to cause the indium to yield across an annular width of seal  68 . Indium does not work harden, rather, it is self-annealing, and seal  68  will remain leak tight in spite of thermally induced yielding. Indium is readily oxidized. However, indium oxide is not self-annealing. If oxygen is present, an outer perimeter of seal  68  will become oxidized, and the oxidation of seal  68  allows propagation of a fracture from the outer perimeter to an inner perimeter of seal  68 . 
     FIG. 6  is an illustration of a housing  100  which is configured to house laser assembly  12 , wire harness assembly  16  (shown in FIG.  1 ), support plate  18 , electronics circuit card  20  (shown in FIG.  1 ), and dither suspension mechanism  26 . For simplicity, only laser assembly  12  and support plate  18  are shown within housing  100 . Housing  100  is functionally similar to housing  24  and cover  28  (both shown in  FIG. 1 ) as housing  100  is also hermetically sealed to protect the electrical and optical equipment within. 
   Housing  100  includes an opening  102  through which a signal conductor  104 , for example, flexible conductor  22  (shown in  FIG. 1 ) passes. Signal conductor  104  extends from a connector assembly  106 , which may be a portion of wire harness assembly  16  or electronics circuit card  20 , through opening  102  to an external connector assembly  108 . Opening  102  is sealed with a plug  110 , which also engages signal conductor  104  in forming the seal. External connector assembly  108  is coupled to a connector  112  which provides interconnection to a conductor assembly  114  to provide signals to an external device  116 , for example, other electronics within an aircraft avionics system. 
   Housing  100  includes a first portion  120  and a second portion  122  which are joined together at an interconnection  124  by welding or through utilization of an adhesive  126 , which forms at least a portion of an hermetic seal for housing  100 . First portion  120  and second portion  122 , when joined together, form an interior cavity  128  within housing  100 . In one embodiment, interior cavity  128  is filled with a dry nitrogen or other gas through a backfill opening  130  before opening  130  is filled with a plug  132 , which is held in place with adhesive  126 . However, adhesive  126  and plugs  108  and  130  only provide a substantial sealing, not an absolute sealing, of housing  100 . As time passes, housing  100  will begin to accumulate ambient air (oxygen) and humidity. 
   Housing  100  further includes a getter  140 , which in the embodiment shown, is attached to first portion  120  of housing  100 . Getter  140  includes well known getter materials, for example, an active metal material which eliminates or reduces levels of water vapor (humidity) and oxygen within cavity  128  of housing  100 . In one embodiment, the getter material reduces the levels of water vapor and oxygen through a chemical reaction with the water vapor and oxygen. One group of known getter materials includes zirconium alloys. Over time, adhesive  126  will allow air and water vapor to enter the nitrogen filled environment of cavity  128 , thereby causing damage, in the form of oxidation, to indium seals  68  (shown in  FIGS. 2-5 ) which form a portion of gyroscope assembly  10  as above described. Getter  140  counteracts this seepage of air (oxygen) and water vapor, through the chemical reaction of the getter material with the water vapor and oxygen as described above, thereby reducing oxidation of the seals and adding to a useful life of gyroscope assembly  10 . 
   In one embodiment, getter  140  includes a chemical purifier  142 , for example, the active metal material in a pelletized form, which is installed into a fixture  144 . Fixture  144  is then mounted into housing  100 . In an alternative embodiment, (not shown) getter  140  includes a getter material, for example, an active metal material, that is heated utilizing an electrical current. Once heated, the material will react with oxygen and water vapor, removing the oxygen and water vapor from the atmosphere of housing  100 . In any of the above described embodiments, fixture can be either of a screen which wraps around purifier  142  and a tube within housing  100 . 
   In alternative embodiments, purifier  142  comprises a flowthrough material or a fusion material. Getter  140  therefore removes oxygen from housing  100  by reacting with the oxygen or water vapor thereby purifying the fill gas (dry nitrogen or other non-oxidizing fill gas) within cavity  128 . An alternative embodiment of getter  140  includes a getter material (not shown) which is utilized in gas chromatograph purifiers as an oxygen and moisture (water vapor) trap. 
   Deployment of a getter  140  in a housing  100  therefore provides an active mechanism to extend the life of certain components, both electrical, and electro-optical, which can be damaged by exposure to air (oxygen) and humidity, by extending the life of an indium seal utilized to protect such components. In one exemplary embodiment, an active getter provides a favorable environment for extending the life of an indium seal which is enclosed within a hermetically sealed housing as that seal is inevitably permeated by oxygen and water vapor. 
   While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.