Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
Field of the Invention - This invention relate~ to surface
acoustic wave (SAW) pressure sensors, and more particularly
to vacuum encapsulating structures therefor.
Description of the Prior Art - SAW pressure sensors are
well known in the art, as reported in U. S. Patents 3,978,731
and 4,100,811. Briefly stated, SAW delay lines which include
a planar substrate having two major surfaces with electro-acous-
tic transducers disposed in an active signal region on one
of the surfaces are adapted to provide SAW pressure sensors
by forming a flexible, deformable diaphragm in the active
signal region. The diaphragm is formed between the surface
of the substrate which includes the active signal region and
a parallel surface provided by the end wall of an interior
cylindrical cavity, or bore, formed in the second major sur-
face. The cavity acts as a fluid conduit to the interior
surface of the diaphragm for applied pressure signals which
apply stress to the diaphragm causing it to deform and change
the acoustic wave propagation characteristics in the active
signal region of the substrate. By connecting the SAW delay
line to an external oscillator the change in acoustic wave
propagation velocity may be measured as a change in the fre-
quency of oscillation, all of which is disclosed in the
hereinbefore referenced patents.
When used as absolute pressure sensing devices, the SAW
pressure sensors must be vacuum encapsulated to provide zero
psi on a reference surface of the diaphragm (the active signal
region surface) while permitting access to the opposite sur-
face of the diaphragm (the interior surface formed by the
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cavity end wall) for the sensed pressure signals. The encap-
sulating structure must also permit external electrical con-
nection to the transducers of the delay line and, ideally,
must not induce thermal strain in the SAW active signal region
resulting from temperature cycling of the structure over the
operating temperature range of the sensor. The requirement
to prevent, or minimize induced thermal strain presents diff-
iculties when there are different temperature coefficients of
expansion between the SAW substrate material and the vacuum
encapsulating material. The problem is particularly acute
when the SAW substrate itself comprises piezoelectric material,
such as quartz which has anisotropic temperature coefficients
of expansion. One structure which satisfies all of the require-
ments, especially that of minimizing induced strain, is des-
cribed in a commonly owned, copending application of the same
assignee entitled VAC W M ENCAPSULATION FOR SURFACE ACOUSTIC
WAVE (SAW) DEVICES, Application serial no. 333,053, filed on
August 2, 1979, by D. E. Cullen and R. A. Wagner, wherein
the vacuum structure is ~orrned frorn the sarrle crystal material
comprising the substrate, which results in identical expansion
characteristics over temperature and which is electrically
insulative permitting a bond of the structure directly across
the transducer conductors. As a result, the active signal
region is maintained in a vacuum while the opposite surface
of the diaphragm is readily accessible to the sensed pressure
signals. There are many instances, however, where a metal
vacuum structure would be preferred due to the operating
environment. While suitable metal packaging techniques are
available for providing the electrical interconnection to
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the transducers, the cornbination of the dissimilar materials,
i.e. metal and crystal, results not only in induced strain
in the SAW sensor diaphragm but, for piezoelectric substrates
with anisotropic temperature characteristics, the strain rnay
become so severe as to cause the rupture of the vacuum
seal of the structure to the substrate. At the present time
this provides a definite limitation on both the accuracy and
the maximum operating temperature range of metal encapsulated
SAW pressure sensors.
SUMMARY OF THE INVENTION
Objects of the present invention include providing
an encapsulating structure for maintaining a SAW pressure
sensor in a vacuum environment over an extended temperature
range of operation and for isolating the SAW sensor diaphragm
from induced thermal strain resulting from temperature cycling
of the structure over the same range of temperature.
According to one aspect of the present invention,
the SAW pressure sensor is supported in a vacuum environment
within a vacuum sealing structure by a cyl;ndrical rnetal
sleeve which displaces the sen90r from a rnounting wall of the
structure at a distance ten to twenty times greater than
the value of the cylinder wall thickness, the sleeve being
disposed at one end in a vacuum sealing relationship to the
cavity opening in the S~W substrate and being disposed at
the other end in a vacuum sealing relationship to an orifice
formed through the mounting wall of the structure, the
sleeve providing a fluid conduit for external pressure
signals through the vacuum environment from the orifice to
the interior surface of the SAW diaphragm. According to
another aspect of the present invention the sleeve has an
outer diameter with a minimum value less than the diarneter
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of the SAW diaphragm According to still another aspect of
the present invention the sleeve is comprised of a metal
having a temperature coefficient of expansion which is
intermediate to that of the anisotropic temperature co-
efficients of a piezoelectric SAW substrate In further
accord with the last aspect, the sleeve comprises a metal
having good vacuum characteristics, including low vapor
pressure, high melting point, corrosion resistance, is
easily outgassed, may be machine formed, and which may be
soldered, welded, or brazed, such as nickel.
The vacuum encapsulating structure of the present
invention provides a minimum vacuum of 10 6 torr over an
extended temperature range on the order of 200C, The
structure provides for isolation of the SAW substrate to
minimize the induced strain into the substrate resulting
from temperature cycling of the vacuum structure over the
operating temperature range of the sensor.
In accordance with a specific embodirnent of the
invention, a surace acoustic wave (SAW) prc~.ssllre sensor
structure comprises: a SAW pressure sensor including a
SAW delay line di.sposed in an active signal region on a
first one of two parallel major surfaces of a substrate,
said substrate having a deformable diaphragm formed therein
coextensive with said active signal region, said diaphragm
having a membrane thickness determined by the relative dis-
placement of said first surface from a parallel interior
surface defined by the end wall of a cylindrical cavity
formed in the second major surface of said substrate, a
vacuum sealing enclosure including a base portion and a
cover portion joined in a vacuum sealing relationship, and
adapted to receive said SAW sensor in a vacuum charnber
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formed therebetween, said base portion including an orifice
therethrough adapted for alignrnent with said cavity in said
substrate, and a cylindrical metal sleeve having a central
aperture formed along the length thereof and joined in a
vacuum sealing relationship at opposite ends thereof to
said cavity and to said orifice, for providing a fluid
conduit for external pressure signals through said vacuum
environment from said orifice to said cavity, said sleeve
supporting said SAW pressure sensor in displacement from
said base portion at a distance which is from ten to twenty
tirnes greater than the wall thickness of said sleeve.
In accordance with a further embodiment of the
4 invention, a surface acoustic wave (SAW) pressure sensor
structure comprises: a SAW pressure sensor including a SAW
delay line disposed in an active signal region on a first
one of two parallel major surfaces of a substrate, said sub-
strate having a deformable diaphragm formed therein co-
extensive with said active signal region, said diaphragm
having a membrane thickness determined by the relative dis-
placement of said first surface from a parallel interiorsurface defined by the end wall of a cylindrical cavity
formed in the second major surface of said substrate,~said
cavity having a diarneter deflnitive of the diameter of
said deformable diaphragm, a vacuum sealing enclosure
including a base portion and a cover portion joined in a
vacuum sealing relationship and adapted to receive said SAW
sensor in a vacuum chamber formed therebetween, said base
portion including an orifice therethrough adapted for align-
ment with said cavity in said substrate, and a cylindrical
metal sleeve having a central aperture formed alony the
length thereof and joined in a vacuum sealing relationship
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at opposite ends thereof to said cavity and to said orifice
for providing a fluid conduit for external pressure signals
through said vacuum environment from said orifice to said
cavity, said sleeve having an outer diameter which is equal
to the diameter of said cavity at the second major surface
of said substrate.
In accordance with a still further ernbodiment of
the invention, a surface acoustic wave (SAW) pressure sensor
structure comprises: a SAW pressure sensor including a SAW
delay line disposed in an active signal region on a first
one of two parallel rnajor surfaces of a substrate, said sub-
strate having a deformable diaphragm formed therein coexten-
sive with said active signal region, said diaphragm having a
mernbrane thickness determined by the relative displacement
of said first surface from a parallel interior surface
defined by the end wall of a cylindrical cavity formed in
the second r~jor surface of said substrate, said cavity
having a diameter definitive of the diameter of said
deforrnable diaphragm, a vacuum sealing erlcLosure including
a base portion and a cover portion joined to said base
portion in a vacuum sealing relationship, said base portion
and cover portion being adapted to receive said SAW sensor
in a vacuum chamber formed therebetween, said base portion
including an orifice therethrough adapted for alignment
with said cavity in said substrate, and a cylindrical metal
sleeve having a central aperture formed along the length
thereof and joined in a vacuum sealing relationship at
opposite ends thereof to said cavity and to said orifice,
said sleeve providing mutually matching surfaces at each
end in dependence on the outer diameter and wall thickness
of said sleeve, said central aperture providing a fluid
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conduit for external pressure signals through said vacuum
environment from said orifice to said cavity, said sleeve
being formed from a metal having a low vapor pressure and
high melting point temperature.
These and other objects, features and advantages
of the present invention will become more apparent in the
light of the following detailed description of preferred
embodiments thereof, as illustrated in the accornpanying
drawing,
BRIEF DESCRIPTION OF THE DRAWING
Fig. 1 is a perspective view of a prior art SAW
pressure sensor, as may ~e used in the present invention,
Fig, 2 is a simplified, sectioned side elevation
view of the SAW sensor of Fig. 1,
Fig, 3 is a simplified illustration of one thermal
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expansion characteristic of the SAW pressure sensor structure
of the present invention;
Fig. 4 is a simplified, secticned side elevation view of
one embodiment of a vacuum encapsulated SAW pressure sensor
structure according to the present invention; and
Fig. 5 is a simplified, partial sectioned side elevation
view of an alternative embodiment of the SAW pressure sensor
structure shown in Fig. 4.
DETAILED DESCRIPTION
Referring now to Figs. 1 and 2, a SAW pressure sensor 10
of the type disclosed in U. S. Patent 4,100,811 includes a
planar substrate 11 having first and second major surfaces 12,
14. Pairs of electro-acoustic transducers 16, 18 are disposed
on the first surface in the active signal region 19 which also
includes the deformable diaphragm 20 formed in the substrate
by a cylindrical cavity, or bore, 22 of diameter do. The
thickness of the diaphragm is measured between the first sur-
face 12 and the interior surface 24 formcd in -the substrate
by the end wall of the cavity 22. The diaphragm 20 flexes
in response to pressure on the surface 24 from a fluid pre-
sented into the cavity.
Typically, the substrate is comprised of piezoelectric
material, although piezoelectric material such as zinc oxide
may be deposited in a form of a thin-film coating between the
transducers 16, 18 and the first surface 12. If piezoelectric,
the substrate may comprise any of the known piezoelectric
materials including quartz, lithium niobate or lithium tantalate.
Of these quartz is the most widely used because of its avail-
ability and lower cost. The quartz has anisotropic
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temperature coefficient characteristics, the optic or Z axis
having a temperature coefficient of expansion which is on the
order of twice that in either of the X or Y axes. The quartz
substrate is cut from a bulk quartz crystal in any one of a
number of known crystallographic orientations, such as a Y-cut
or a ST-cut wafer, depending on the particular SAW device
application. For Y cut quartz the 25C temperature coeffi-
cient of expansion in the Z axis is on the order of
13.7xlO 6 ~ and in the X axis, the axis of the SAW propa-
gation, it is on the order of 7.5xlO 6 in C As a result of
the anisotropic temperature characteristics the cylindrical
cavity 22 deforms over the operating range of temperatures in
a generally elliptical fashion, as shown in Fig. 3. The
illustration of Fig. 3 is exaggerated for teaching purposes
to demonstrate the nature of the deformation, where a circ`le
26 represents the shape of the cavity at room tempera-
ture which deforms with increasing temperature to a geometry
which is substantially elliptical as illustrated in phantom
by the ellipsoid 28, the major axis o~ the ellipsoi.d being
along the Z axis of the crys-tal waer.
Referring now to Fig. 4, in a vacuum encapsulated SAW
pressure sensor structure 30 according to the present inven-
tion, the SAW sensor lO is encapsulated by a vacuum enclosure
comprising a cover portion 32 adapted to enclose the sensor
lO within a chamber 34 ormed by the cover 32 and a base
portion 36. The cover and base are formed from vacuum type
material, whether metal or glass, suitable for providing a
minimum vacuum of 10 6 torr within the chamber 34. The
cover is bonded to the base along the mating surface 37 with
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a vacuum seal, such as a solder seal, or weld. The cover 32
includes a small orifice 38 which allows evacuation of the
chamber 34 following the bonding of the cover to the base,
after which the orifice is closed off with a solder seal 39.
The electrical connections between the SAW trans-
ducers 16, 18 and the external oscillator circuits (not
shown) are provided through electrical conductors 40 which
are mounted through the base 36 with feed-thru insulators
41, of a type known in the art, which provide both electrical
insulation of the conductor and a vacuum seal between the
chamber 34 and the outside arnbient. For a metal vacuum
enclosure the base itself may be used as the return current
path for the transducer and internal ground wires 42 may be
provided between the SAW transducers and the base.
The SAW sensor 10 is supported in the chamber by
a cylindrical metal sleeve 44 which displaces the sensor
substrate 11 at a distance, or height (hl) above the inside
surface 46 of the base. The sleeve has a diametQr (D),
and the sleeve aperture 48 provides a fluid passage, or
conduit, between the cavity and an orifice 50 formed through
the wall of the base and accessible to an external source of
pressure signals (not shown). In Fig. 4 the metal sleeve 44
i~ illustrated as a straight walled cylinder having a bear-
ing surface 52 adapted to fit into a counterbore formed in
the substrate 11 along the circumference of the cavity open-
ing in the substrate 11, and having a seating surface 54
adapted to fit into a similar type counterbore provided in
the surface 46 along the circumference of the orifice 50.
Each of the sleeve surfaces are bonded to their respective
mating surfaces by a solder seal. In assembly of the
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structure the sleeve is first sealed to the substrate.
Following a step of RF sputtering thin-films of chrome and
gold to the side wall of the counterbore, a plating of
nickel is applied to the gold film and the bearing surface
52 is soldered in the counterbore with a lead tin solder
having a melting point of approximately 200C. At a later
step the seating surface is soldered to the base with a
lower temperature indium solder having a melting point
temperature of 156C, Each of the solder bonds provide a
vacuum seal of the substrate and base to ~the mating sur-
faces of the sleeve.
The metal sleeve comprises a vacuum type metal
having good vacuum characteristics, such as: low vapor
pressure, high melting point, corrosion resistance, may be
easily outgassed, may be formed by machining, and may be
soldered, welded or brazed. To prevent the rupture of the
vacuum seal between the sleeve and the substrate, the metal
must also be of a type having a temperature coefficient of
expansion which i~; compatible to that of the substrate.
Since the temperature coeficient of the metal is i~o-
tropic, for piezoelectric material substrates having aniso-
tropic temperature characteristics the metal must have a
temperature coe~ficient between those of the optic axis and
the X and Y axes. This allows the sleeve to expand, as
illustrated in Fig. 3, from the solid circle 26 to the
dashed circle 60 while the cavity expands from the circle
26 to the ellipsoid 28. The sleeve expands less along the
Z axis than the substrate but more along the X axis, pro-
- viding an approximate mean expansion to that of the two
axes of the substrate.
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This expansion of the sleeve beyond the substrate
in the X axis, if permitted, would result in strain being
induced into the substrate, and possibly fractures along
the interior surface of the cavity, If, however, the metal
sleeve deforms such that the cylinder walls yield to the
restricted expansion of the sleeve along the substrate X
axis, the sleeve will follow the elliptical distortion
characteristic of the substrate cavity, reducing or even
eliminating the induced strain and maintaining the integrity
of the vacuum seal. Of course, the sleeve must exhibit an
elastic deformation allowing restoration of the sleeve
contour at room ambient along with the restoration of the
cavity, Therefore, in addition to the requirements that
the sleeve comprise a metal which is suitable for providing
a vacuum seal, i.e, high vapor pressure, it must have a
temperature coefficient which is between that of the an-
isotropic characteristics of the quartz and must also
exhibit an elastic deformation characteristic. One metal
which satisfies all of these re~uiremerlts is nickel which
has a 25C ternperature coefficient of 12,6x10 6 iinn C
Nickel exhibits an inherent elastic deformation character-
istic and through suitable sleeve geometry, including wall
thickness, ~leeve length, and sleeve diameter, the sleeve
may be rnade to exhibit the deformation required to conform
in concert with the quartz substrate over the operating
temperature range of the sensor.
The deformation may be provided by selecting a
length for the sleeve which ensures that the substrate 11
is displaced from the base interior surface 46 at a height
(hl) which is ten times greater than the thickness of the
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cylinder walls 62, which are formed to a minimum dimension.
The minimum wall thickness is selected with consideration
given to: providing the sleeve structure with sufficient
rigidity to prevent deformation of the sleeve cylindrical
shape under a maximum pressure differential between sensed
pressure and the zero psi of the chamber 34, and providiny
a vacuum tight seal over the same operating range of sensed
pressures, i.e. that the cylinder walls do not become so
thin as to exhibit a porisity which may provide for a
vacuum leak. A minimum dimension for the cylinder wall
thickne~s for a 50 psi sensor is in the range of .002 to
,003 inches. A more conservative value of wall thickness
for the same sensor is on the order of .005 inches, which
then establishes the height (hl) as .050 inches. The
additional length of the sleeve beyond that of the height
dimension is selected to provide a suitable insertion length
of the sleeve into each of the counterbores for the cavity
22 and orifice S0.
Establi3hing the 10:1 ratio between the helyht
and the wall thickness provides the sleeve with sufficierlt
elasticity to allow the bearing surface 52 and the adjacent
top portion to deform in cooperation with the cavity, how-
ever, the dissimilar temperature coefficients still produce
dimensional differences in expansion which, in turn, induce
some proportional degree of strain in the substrate. The
solder seal along the surface 52 exhibits sufficient
elasticity to maintain the vacuum seal despite the slight
dissimilarities in deformation, To minimize these dimen-
sional differences in expansion the sleeve diameter (~) is
selected at the minimum value possible. This is limited by
two constraints. The first constraint is the diameter of
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the cavity 22, Since the sleeve must be joined to the sub-
strate in a vacuum sealing relationship, the minimum outer
diameter of the sleeve, as illustrated by the sleeve embodi-
ment of Fig. 5 which is described in detail hereinafter, is
limited to a value which is not less than the cavity opening
at the substrate second surface minus the coating thickness of
the solder seal. For the SAW pressure sensor lO of Figs. 2,
4, the diameter of the cavity opening at the substrate second
surface is equal to that of the diaphragm since the substrat~e
ll is a monolith. If instead the SAW sensor comprises a
diaphragm disposed on a separate apertured substrate, such as
that disclosed in the here,inbefore referenced U. S. Patent
3,978,731 where each side of the substrate is accessible for
drilling, then the diameter of the cavity opening at the second
surface may be less than the required diaphragm diameter, such
that the sleeve diameter may be smaller than that of the
diaphragm itself. The second constraint is that the sleeve
must provide the required rigid support of the substrate mass.
The substrate/sleeve mounting i9 in the nature of a pedestal
which may vibrate under sensor operating conditions~ If the
vibration, or oscillation, is severe enough it may result in
a tearing away of the substrate from the sleeve surface 52
resul1:ing in a vacuum leak, or a break in the electrical con-
nections provided to the SAW transducers. A minimum diameter
which satisfies the mechanlcal support requirements is on
the order of one quarter of the maximum dimension of the
rectangular substrate of Fig. l. If a circular substrate
is used the sleeve diameter is on the order of one quarter
that of the substrate. Since the diaphragm diameter is typi-
cally one half that of a circular substrate, or one half themaximum dimension of a rectangular substrate, the minimum
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diameter for the sleeve 44 is on the order of one half the
diameter of the diaphragm.
In summary, the metal sleeve 44 has the characteristics
of: (1) comprising a vacuum type metal having a temperature
coefficient of expansion compatible with that of the aniso-
tropic characteristics of the SAW substrate material, (2)
has as an overall length to wall thickness which provides for
displacement of the substrate from the enclosure base at a
distance which is ten times greater than the wall thickness
of the sleeve, and (3) and has a sleeve diameter which is
equal to or less than the diameter of the diaphragm formed
in the SAW substrate, and which has an optimum minimum diameter
equal to one half that of the diaphragm. As long as these
requirements are satisfied, the sleeve may have a slightly
altered geometry to satisfy alternative mounting requirements
of the sleeve to both the substrate and the orifice formed
in the wall, such as the base 46, of the vacuum structure.
Referring now to Fig. 5, in an alternativ~ embodiment
the sleeve 44' includes a rim, or flange, 70 formed around
the outer surface of the cylindrical wall. The rim provides
a bearing surface 72 for supporting the substrate 11 at the
height (hl) above the surface 46 of the base 36. In this
manner, the substrate ]1 need not have the counterbore formed
along the circumference of the cavity, which may be preferred.
The sleeve 44' comprises the same material as that of the
sleeve 44, having the same requirements of providing a vacuum
seal and an elastic deformation characteristic such that the
sleeve conforms to the deformation of the aperture over tem-
perature. The sleeve provides for a similar fluid conduit 48
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between the orifice 50 and the cavity 22 allowing for fluid
communication between an external source of pressure signals
and the surface 24 of the diaphragm formed in the substrate.
The sleeve 44' also has a seating surface 74 which mates with
a countersink in the base 36 of the enclosure. In Fig. 5,
the seating surface is provided by a shoulder portion 76 of
the sleeve which permits both for enhanced mechanical strength
of the sleeve at the seating surface and also for a reduction
in the diameter of the orifice 50 formed in the base. This
allows for practical considerations in both providing the
thin-walled sleeve with sufficient rigidity for handling, i.e.
to prevent distortion of the sleeve during fabrication which
may result for sleeves having the minimum wall thickness,
and also for providing an opening at the orifice which is
compatible to standard size pressure fittings, such that
the orifice and/or the interior wall of the shoulder 76 may
be threaded to an external fluid conduit. Since the sleeves
44 and 44' each have temperature coefficients which are
compatible with the metal enclosure there is no requirement
that the sleeve exhibit unusual deformation along the seating
surface. Any incidental differences in temperature coeffi-
cients which may induce strain in the base 36 do not provide
any induced strain in the substrate. Therefore, the sleeve
44 shown in Fig. 4 may similarly be provided with the
shoulder 76 illustrated for the sleeve 44' while the bearing
surface 52 of the sleeve 44 remains the same.
The vacuum encapsulated SAW pressure sensor structure
of the present invention provides for both the tight vacuum
encapsulation of the SAW substrate to prevent the
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deterioration or change in prop~gatioll velocity due to ambient
contamination while also providing the required zero psi
reference for an absolute pressure sensor configuration. The
use of the met~l sleeve to support the suhstrate at a clis-
placed dimension from the wall of the structural enclosure
isolates the SAW substrate from any induced strain resulting
from temper;~ture cycling of the structure over the temperature
range o~ operation. The sleeve geometry including length,
wall thickness, and diameter may be altered within the guide-
~o lines recited hereinbefore to provide for higher operating
temperature ranges, such that a minimum wa~l thickness in the
range of 0.002 to 0.003 inches for a sensor having a maximum
pressure differential of 50 psi must be increased to satisfy
higher pressure differentials. For a six hundred psi sensor,
the minimum wall thickness is on the order of 0.003 to 0.004
inches with a typical thickness on the order o~ .008 inches.
The preferred material for the metal sleeve is nickel, although
any matcrial having the requisite characteristicæ ~lescribed
hereinbefore may be used. Similarly, althcugh the invention
has been shown and described with respect to illustrative
embodiments thereof, it should be understood by tho~.e sXilled
in the art that the foregoing and various other changes,
omissions and additions in the form and detail thereof may
be made therein without departing from the spi~it and the
scope of the invention.
