the following description of the presently contemplated best mode of practicing the invention is not to be taken in a limiting sense , but is made merely for the purpose of describing the general principles of the invention . the scope of the invention should be determined with reference to the claims . referring first to fig1 a cross - sectional view is shown of a case 10 and a band 12 ( or case / band assembly 8 ) having been bonded together at a bonding site 14 . similarly , in reference to fig2 a cross - sectional view is shown of the case 10 ( or first structure ) and the band 12 ( or second structure ) having been bonded together at the bonding site 14 . the view shown in fig1 is taken along line b -- b shown in fig2 and the view shown in fig2 is taken along line a -- a shown in fig1 . in both fig1 and 2 , the case 10 is shown as having a &# 34 ; d &# 34 ; shaped cross section . such cross section facilitates implantation and accommodates any electronic components that are to be housed within the case / band assembly 8 , as well as one or more coils that can be housed within the case / band assembly 8 . the case 10 is preferably made from a body - safe ceramic , e . g ., alumina ( alo 2 ) or zirconium oxide ( zo 2 ), and is open at its straight end , i . e ., the straight end of the &# 34 ; d &# 34 ; shape while its curved end and side walls are closed . walls 16 of the case 10 terminate around the open end forming a first annular surface 18 . referring next to fig3 an end view is shown of the band 12 showing a second flat annular surface 20 ( also shown in fig1 and 2 ) to which the first flat annular surface 18 of the case 10 is ultimately bonded . the band 12 is preferably made from a body - safe metal , e . g ., an alloy of titanium - 45 niobium ( i . e ., 55 % ti and 45 % nb ), available from teledyne wha chang of albany , n . y ., or numerous other sources , or any other metal or alloy that readily forms an instant oxide when heated , i . e ., that readily oxidizes when heated in an oxygen - containing atmosphere . note that both the alumina and the titanium - 45 niobium have thermal coefficients of expansion ( tces ) of between 8 and 9 mm 3 /° c . this minimizes the risk of cracking when the case 10 and band 12 are bonded together at high temperature and then cooled . the band 12 has two open ends . side walls 22 of the band case 12 terminate at each of the open ends , forming the second flat annular surface 20 at one of the ends , and having , e . g ., a flanged edge 24 at another of the ends , which can be for receiving a header plate ( not shown ). see , e . g ., u . s . pat . no . 4 , 991 , 582 , previously incorporated herein by reference . referring next to fig4 a perspective view is shown of an outer jig 30 that is used in bonding the case 10 and band 12 together . the outer jig 30 has generally a rectangular three dimensional shape with a rectangular cavity 32 passing therethrough . the upper surface 34 is identical to the lower surface 36 except for a channel 38 in the lower surface 36 that passes from the center of one of the outer side edges of the outer jig 30 to the center of one of the inner side edges of the cavity 32 . the channel 38 is also illustrated in fig9 and is explained more fully below . the dimensions of the outer jig 30 are dictated by the size and shape of the case 10 and band 12 that are bonded together . for the case 10 and band 12 , shown in the figures , the outer jig 30 is preferably made from alumina , available from ici advanced ceramics , and has the following outer dimensions : 8 . 97 × 7 . 06 × 3 . 81 cm . the dimensions of the cavity 32 are preferably : 3 . 89 × 1 . 98 × 3 . 81 cm , and the channel preferably has a cross sectional area of 7 . 70 cm 2 . the outer jig 30 preferably has beveled or rounded edges to improve its appearance and to facilitate its handling . referring next to fig5 a perspective view is shown of an inner jig 40 that is used in conjunction with the outer jig 30 in bonding together the case 10 and band 12 . like the outer jig 30 , the inner jig 40 has generally a rectangular three dimensional shape . the inner jig 40 has a cavity 42 opening on one of its sides that is formed so as to receive the case 10 . when the case 10 is inserted into the cavity 42 it to is held with all of the interior walls of the cavity 42 touching all of the exterior walls of the case 10 . for the preferred embodiment shown in the figures , the inner jig 40 is preferably made from alumina , available from ici advanced ceramics , and has the following outer dimensions : 3 . 81 × 1 . 90 × 3 . 81 cm so that the inner jig 40 can be slid into the cavity 32 of the outer jig 30 . the inner jig 40 preferably has beveled or rounded edges to improve its appearance and handling . in practice , the case 10 is slid into the inner jig &# 39 ; s cavity 42 until it becomes seated against a closed end 44 ( fig6 ) and side walls 46 ( fig6 ) of the cavity 2 . after the case 10 is slid into the cavity 42 , the band 12 is slid into the cavity 42 until the second flat annular surface 20 ( fig1 and 2 ) seats against the first flat annular surface 18 ( fig1 and 2 ) of the case 10 . the band 12 protrudes from the cavity 42 when it is seated against the case 10 , as shown in fig9 below . referring to fig6 a cross sectional view of the inner jig is shown taken along plane c of fig5 . as viewed in fig6 the cavity 42 in the inner jig 40 is substantially &# 34 ; d &# 34 ; shaped so as to accommodate the &# 34 ; d &# 34 ;- shaped case / band assembly 8 of fig1 ( or case 10 and band 12 , before they are bonded together ). referring next to fig7 a side view is shown perpendicular to plane c of fig5 of the inner jig . the inner jig 40 is shown , and the cavity 42 is shown with dashed lines . the cavity 42 also has a &# 34 ; d &# 34 ; shaped cross section as viewed in fig7 which accommodates the &# 34 ; d &# 34 ;- shaped cross section of the case / band assembly 8 as viewed in fig2 ( or case 10 and band 12 , before they are bonded together ). referring next to fig8 a perspective view is shown of a support surface 50 that is utilized in conjunction with the outer jig 30 and the inner jig 40 in bonding the case 10 and band 12 . the support surface 50 has a lip 52 at the periphery of an upper side 51 of the support surface 50 . the lip 52 is used to keep powdered titanium oxide on the support surface 50 . ( use of the powdered titanium oxide powder is described below .) a lower side 53 of the support surface is supported against , e . g ., an alumina plate , which in turn rests against a rack or grill within a vacuum oven , described below . the support surface 50 has a vent hole 54 near its center that allows gasses to readily enter and exit the case / band assembly 8 when the other open end of the band 12 , i . e ., not the end that is against the open end of the case 10 , is aligned over the vent hole 54 . referring to fig9 a partial cross sectional view is shown of the outer jig 30 , the inner jig 40 , the support surface 50 , and the case / bend assembly 8 . the lower surface 36 of the outer jig 30 is held by gravity against the upper side 51 of the support surface 50 with the upper jig &# 39 ; s channel 38 having a central longitudinal axis within the plane of the paper in fig9 and shown to the left of the cavity 32 of the outer jig 30 . the case 10 is inserted into the inner jig &# 39 ; s cavity 42 until it seats against the closed end 44 and sides 46 of the inner jig 40 . next , the band 12 is inserted into the cavity 42 until it seats against the case 10 and the sides 46 of the cavity 40 . the band 12 protrudes from the cavity when seated against the case 10 and sides 46 . before inserting the case 10 and band 12 into the cavity 42 , however , the interior surface of the cavity 42 , as well as the upper side 51 of the support surface 50 , is coated with powdered titanium oxide ( to 2 ) to prevent the case 10 and band 12 from bonding to the inner jig 40 and support surface 50 . the inner jig 40 , with the band 12 protruding therefrom , is inserted cavity - first into the outer jig 30 through the open end of the outer jig &# 39 ; s cavity 32 at the upper surface 34 of the outer jig 30 . the inner jig 40 is inserted into the outer jig &# 39 ; s cavity 32 until the band 12 protruding from the inner jig 30 seats against the support surface . the inner jig 30 does not come into contact with the support surface 50 . while the inner jig 30 is sliding into the outer jig &# 39 ; s cavity 32 , the inner jig &# 39 ; s movement is restricted to movement along a single coordinate axis , which is preferably normal to the support surface 34 , i . e ., the plane defining the upper side 51 of the support surface 50 . the other open end of the band , i . e ., the open end of the band 12 that is not seated against the case 10 , is centered over the vent hole 54 , and a chamber formed by the space within the outer jig &# 39 ; s cavity 32 , below the inner jig 40 , above the support surface 50 and outside the band 12 , is vented by the channel 38 in the outer jig 30 . the support surface 50 , outer jig 30 , inner jig 40 , case 10 and band 12 are placed onto , e . g ., a grate ( not shown ) in a vacuum oven 70 , and a compressive force f is applied along the single coordinate axis to the inner jig 40 in a downward direction , as depicted in fig9 by the downward pointing arrow . this force may be applied by placing weights subject to gravity on top of the inner jig 40 . the weights can be secured by wrapping stainless steel bands over the top of the weights and securing them under the support surface 50 . preferably , four or more bands having a width of 1 . 27 cm and a thickness of 0 . 025 cm are used . the compressive force applied should be from between 950 n / m 2 to 1500 n / m 2 . the compressive force f is translated to the case 10 by the inner jig 40 . note also that an equal force is applied by the support surface 50 , to the band 12 along the single coordinate axis opposite the direction of the compressive force f . the compressive force f and the opposing equal force isodynamically press the case 10 and band 12 together at the sight where the second flat annular surface 20 of the band 12 is seated against the first flat annular surface 18 of the case 10 . next , a sealed chamber of the vacuum oven 70 is evacuated to at least 10 - 5 , preferably 10 - 6 , atmospheres using a vacuum pump 72 . the vacuum oven 70 is then heated by energizing a heating coil 74 using a power supply 76 . the temperature in the vacuum oven is heated at the rate of approximately 5 ° c ./ minute until it reaches a temperature of at least 1000 ° c ., preferably to between 1000 ° c . and 1100 ° c . this temperature is maintained for about 2 hours , i . e ., 120 minutes , by a thermostat 78 that is coupled to the power supply 76 . the thermostat 78 uses a temperature probe 80 to monitor the temperature within the vacuum oven 70 . after the 2 hours , the vacuum oven 70 is cooled at a rate of approximately 1 ° c ./ minute , which generally takes about 17 hours , e . g ., 1000 minutes , at ambient temperature . preferably , no forced cooling is performed , i . e ., no cold gas spray , or other exposure to a cold environment . note that during the cooling of the vacuum oven 70 , the heating coil 74 will generally remain energized , at least partially , in order to assure that the desired slow rate of cooling is achieved , i . e ., 1 ° c ./ minute . during the time the case 10 and band 12 are heated and pressed together , titanium atoms from the band diffuse into the alumina of the case 10 . this is caused by an attraction of the titanium atoms to oxygen atoms that are loosely held by the alumina at the above - mentioned temperatures . when the case 10 and band 12 are cooled , the titanium atoms share the oxygen atoms with the alumina . in this way , a hermetically sealed bond is formed between the case 10 and band 12 , so that the case 10 and band 12 can be safely utilized to house an implantable electronic device . advantageously , the bonding does not degrade or crack the metal or ceramic , and they each maintain their hermeticity . a header plate ( not shown ) is used to seal the other end of the band after electronic circuits , and , e . g ., inductive pickup coils , are inserted into the case / band assembly 8 . the header plate is bonded to the band by , e . g ., welding , as is described in u . s . pat . no . 4 , 991 , 582 , previously incorporated herein by reference . note that because the electronic circuits are not inserted into the case / band assembly 8 until after the cooling , and because the header plate can be sealed to the other open end of the band 12 without the need for heating the entire case / band assembly 8 to high temperatures , the electronics are much less prone to heat damage than with many heretofore utilized techniques for bonding the ceramic case 10 to the metal band 12 . while the invention herein disclosed has been described by means of specific embodiments and applications thereof , numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims .
in a method and apparatus for forming a hermetically sealed bond for use in implantable medical devices , a first structure , made from a first material , is positioned against a second structure , made from a second material . a compressive force directed at the second structure is applied to the first structure , and an equal force directed at the first structure in a direction opposite the compressive force is applied to the second structure so that the first and second structures are isodynamically pressed together . the first and second structures are heated to a diffusion temperature whereat the first material and the second material undergo diffusion , thereby forming a hermetically sealed bond between the first and second materials .