WO2000019190A1

WO2000019190A1 - Method and apparatus for improved gas sensor

Info

Publication number: WO2000019190A1
Application number: PCT/US1999/022465
Authority: WO
Inventors: Edmond Chu; Marguerite Thomlinson
Original assignee: Teledyne Industries Incorporated
Priority date: 1998-09-30
Filing date: 1999-09-28
Publication date: 2000-04-06
Also published as: AU6165999A

Abstract

A gas sensor comprising a housing (6) including a first wall (30) and a cavity (28), the housing defining a passage between an opening (36) and the cavity, at least a portion of the passage being defined by the first wall, the first wall extending into the cavity, a first electrode (16) positioned within the cavity; and a second electrode positioned at the opening. The attachement of a clamp (22) ensures intimate contact between the sensing membrane (20) and the working electrode, and is accomplished via snap lock engagement with the housing. The present invention also reduces the number of parts in the assembly steps while increasing reliability of the components by providing a cathode contact (2) comprising a base (42) and a substantially rigid cathode tail (44) connected to the cathode base, and an anode contact (4) comprising an anode base (46) and a plurality of projections (48) engaging the base and extending outwardly therefrom.

Description

METHOD AND APPARATUS FOR IMPROVED GAS SENSOR

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to an improved gas sensor and its method of manufacture,

and more particularly to a micro fuel cell gas sensor such as, for example, a micro fuel

cell oxygen sensor, having a reduced parts count, reduced potential for electrolyte

leakage, and reduced cost.

Brief Description of the Invention Background

Figure 1 illustrates a typical micro fuel cell oxygen sensor 110 such as, for

example, Teledyne Analytical Instruments' R-22 sold by Teledyne Electronic

Technologies, City of Industry, CA. The oxygen sensor 110 consists of a cathode 102

and an anode 104 sealed in a housing 106 filled with appropriate electrolyte solution.

Oxygen diffuses into the interior of the sensor housing 106 through a thin sensing

membrane 108. A flexible expansion membrane 112 at the opposite end of the sensor

110 permits expansion or contraction of the electrolyte volume. The sensing membrane

108 is sealed in place either by means of press fit or O-rings. The expansion membrane

112 is sealed in place using O-rings. Reduction of oxygen at the cathode 102 causes current to flow from the cathode 102 to the anode 104 through an externally connected

sensing circuit (not shown).

For effective use of the lead anode material utilized in the R-22 sensor it is

important to have a large surface area to volume ratio for the anode 104. A technique,

used in the R-22 sensor, to address that need is to make the anode 104 so as to include an

anode body 116 composed of sintered lead pellets or lead tape. In either case, the

porosity of the anode body 116 increases its effective surface area. A drawback with

using lead pellets or lead tape is that fragments of either material can become separated

from the anode body 116 and come into contact with the cathode 102 causing spurious

sensor output signals. Another disadvantage of the prior art sensor 110 is that there is

limited contact between the anode body 116 and the wire 125 that completes the anode

circuit. The ratio of the surface area of contact to the total volume of the anode body 116

is not optimized, resulting in less effective anode utilization.

Generally, both the cathode and anode wires 113, 115, respectively, are fed

through holes in the wall of the sensor housing 106. Epoxy is often used to seal the holes

after the wires 113, 115 are fed through the holes into the interior of the sensor.

Alternately, interference fit metal cylinders are employed as electrical feedthroughs for

the cathode and anode wires 113, 115. The cylinders are shown in Figure 1 as cylinders

117, 119. The wires 113, 115 typically are generally arc welded to the cylinders 117,

119, respectively, prior to press fitting the cylinders into the wall of the sensor housing

106. Whether utilizing epoxy or press fit cylinders, the exit points of wires 113, 115

through the wall of the housing 106 may allow leakage of the electrolyte. Also, the

additional step of arc welding the wires 113, 115 to the metal cylinders 117, 119 increases manufacturing time and cost. Another potential electrolyte leakage location in

the sensor 110 is the interface between the sensing membrane 108 and the housing 106.

Existing oxygen sensors, such as the R-22 sensor 110 of Figure 1, also have a

number of components requiring multiple assembly steps. As with other prior art

sensors, sensor 110 is time-consuming and costly to assemble. For example, as described

above, use of the cathode wire and anode wire 113, 115, respectively, requires running

the wires through openings in the wall of the housing 106 at two points, spot welding the

wires to their respective contacts, and sealing the openings to prevent electrolyte leakage.

Furthermore, sealing the sensor's gaseous inlet requires manual placement of the cathode

102, the sensing membrane 108, a clamp cushion 118, and a clamp 122 over the inlet and

securing the several components in place via a retainer ring 120.

Moreover, in addition to requiring a significant expenditures of time and cost, the

construction of the prior art sensors and their related assembly steps may reduce the

effectiveness of the sensor to monitor the gaseous stream. Such reduced effectiveness

may occur as a result of, for example, electrolyte leakage through the seals associated

with wire feed-throughs or through the sensing and expansion membranes. Sensing

effectiveness also may be compromised by oxidative degradation of the contact integrity

between the anode and its contact wire. In addition, as noted above, fragments of the lead

anode body may become loose and detach from the anode body and contact the cathode,

causing undesirable spikes in the output signal of the sensor.

Accordingly, the need exists for an improved gas sensor that, for example,

includes a reduced number of parts, is designed to prevent loose anode particles from coming into contact with the cathode, assures consistent output, and has a diminished

potential for electrolyte leakage.

BRIEF SUMMARY OF THE INVENTION

The present invention addresses the above-mentioned needs by providing a gas

sensor comprising a housing including a first wall and a cavity, and wherein the housing

defines a passage between an opening and the cavity. At least a portion of the passage is

defined by the first wall, and the first wall extends into the cavity. A first electrode is

positioned within the cavity, and a second electrode is positioned at the opening.

The present invention also is directed to a gas sensor comprising a housing

including a cavity and a first wall, and wherein the housing defines a passage between an

opening in the housing and the cavity. At least a portion of the passage is defined by the

first wall, and the passage extends into the cavity. A first electrode is positioned within

the cavity, but does not extend over the passage. A second electrode is positioned at the

opening.

The present invention is additionally directed to a gas sensor comprising a

housing including a cavity and a first wall. The housing defines a passage between an

opening and the cavity, and the passage extends into the cavity. A first electrode is

positioned within the cavity, a second electrode is positioned at the opening, and the first

wall provides a barrier to the migration of all or portions of the first electrode to the

second electrode.

Furthermore, the present invention is directed to a gas sensor comprising a

housing including a cavity and a first wall. The housing defines a passage between an opening and the cavity, and the passage extends into the cavity. A first electrode is

wall provides a barrier inhibiting migration of a portion of the first electrode to the

second electrode.

Moreover, the present invention also provides a gas sensor comprising a housing,

a first electrode positioned within the housing, a second electrode, an electrolyte solution

within the housing and in contact with the first electrode and the second electrode, and an

electrode contact in contact with the first electrode and comprising a base, a tail

connected to the base, and a plurality of contact projections connected to and extending

from the base.

The present invention is also directed to a gas sensor comprising a first electrode,

a housing having a first end, the electrode positioned within the housing, the first end

defining an opening, and a clamp in snap lock engagement with the opening.

In addition, the present invention is directed to a gas sensor comprising a housing

having a first wall, a first electrode contact, and a second electrode contact. At least one

of the first electrode contact and the second electrode contact includes a rigid tail that is

as least partially encapsulated within the first wall.

Moreover, the present invention is directed to a gas sensor comprising a housing,

a first electrode, a second electrode, and a spacer, all of which are within the housing, the

spacer separating the first electrode from the second electrode.

The present invention also is directed to a fuel cell comprising a gas sensor

constructed according to the present invention. A method for forming component parts of a gas sensor also is disclosed herein.

The method includes placing one or both of a first electrode contact and a second

electrode contact in a mold, and introducing thermoplastic inside the mold to thereby

encapsulate portions of one or both of the electrode contacts within the thermoplastic

material.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The characteristics and advantages of the present invention may be better

understood by reference to the accompanying drawings, wherein like reference numerals

designate like elements and in which:

Figure 1 is a cross-sectional view of a prior art oxygen sensor;

Figure 2 is a cross-sectional view of an embodiment of the gas sensor of the

present invention in the form of a micro fuel cell oxygen sensor and illustrates the

arrangement of the embodiment's individual components;

Figure 3 is a cross-sectional view in perspective of the housing of the gas sensor

depicted in Figure 2;

Figure 4 is a perspective view of the housing of the gas sensor depicted in Figure

2 and showing the first opening;

Figure 5 is a perspective view of the housing of the gas sensor depicted in Figure

2 and showing the second opening;

Figure 6 is a perspective view of an embodiment of the cathode contact of the

present invention and which is included in the gas sensor illustrated in Figure 2; Figure 7 is a perspective view of an embodiment of the anode contact of the

present invention and which is included in the gas sensor illustrated in Figure 2;

Figure 8 is a cross-sectional perspective view of an embodiment of the clamp of

the present invention and which is included in the gas sensor illustrated in Figure 2;

Figure 9 is a cross-sectional view of another embodiment of the gas sensor of the

present invention and which incorporates a clamp having an alternate configuration

relative to that of Figure 1;

Figure 10 illustrates aspects of a method of assembling components of an

embodiment of a gas sensor of the present invention wherein a spacer is used during the

molding process to integrally form the housing;

Figure 11 is a cross-sectional view of another embodiment of the gas sensor of the

present invention; and

Figure 12 is a cross-sectional view of yet another embodiment of the gas sensor of

the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION In the present detailed description of the invention, the invention will be

illustrated in the form of a gas sensor adopted for use as a micro fuel cell oxygen sensor.

It will be understood, however, that the invention is not limited to embodiment in such

form and may have application in any gas sensor. Thus, while the present invention is

capable of embodiment in many different forms, for ease of description this detailed

description and the accompanying drawings disclose only specific forms as examples of

the invention. Those having ordinary skill in the relevant art will be able to adapt the

invention to application in other forms not specifically presented herein based upon the

present description.

Also, for ease of description, the invention and devices to which it may be

attached may be described herein in a normal operating position, and terms such as upper,

lower, front, back, horizontal, proximal, distal, etc., may be used with reference to the

normal operating position of the referenced device or element. It will be understood,

however, that the apparatus of the invention may be manufactured, stored, transported,

used, and sold in orientations other than those described.

In addition, for ease of description, the terms "anode" and cathode" are used

herein to refer to the electrodes of the present invention. It will be understood that the

terms "anode" and "cathode" are used herein to refer to the electrodes of only one

embodiment of the present invention and, in particular, are used to refer to electrodes that

may be incorporated as components of an oxygen sensor. It will be understood that the

invention has applicability to gas sensors including electrodes identified as other than

anodes and cathodes. For example, as is known in the art, other types of gas sensors may incorporate electrodes in the forms of a "sensing" or "working" electrode (the cathode) and a counter electrode (the anode).

Typical micro fuel cell oxygen sensors consist, in part, of a machined plastic body

filled with electrolyte solution, a cathode (working electrode) manufactured from

perforated sheet metal such as brass and plated with an appropriate noble metal such as,

for example, rhodium, gold, silver, or nickel, and an anode (counter electrode) made out

of compressed lead pellets. The electrolyte solution may be potassium hydroxide. A

gaseous stream enters the body by diffusing through a synthetic membrane positioned at

an inlet and is transported through a thin electrolyte layer to the working electrode. The

oxygen is reduced to form hydroxyl ions at the working electrode. The potential applied

on the cathode provides the driving force for the reduction of oxygen. Simultaneously,

anode material, such as lead, is continually oxidized at the anode. Thus, the following

set of electrochemical reactions occur at the cathode and the lead anode:

Cathode: C>2+2H2θ+4e^~ -> 40H^~

Anode: 2Pb -> 2Pb²⁺ + 4e^~

Lead oxide formed, though soluble in the potassium hydroxide electrolyte

initially, will eventually deposit on the lead anode as the electrolyte becomes saturated

with lead ions. When the cathode and the anode are electrically connected external to the

sensor, an ionic current flows through the sensor. The current is proportional to the rate

of oxygen consumption. The electric current can be measured via a resistor connected in

series with the cathode and anode to produce a voltage drop across the resistor.

Connection between external sensing circuitry and the cathode is typically achieved by arc welding a small diameter (typically ~0.01 inch) nickel wire to the cathode.

Connection between the same external sensing circuitry and the anode is accomplished by

compressing (sintering) lead pellets around a small coil of nickel wire in an attempt to

maximize the contact surface area between the wire and the lead particles.

Referring now to the drawings, which are for the purpose of illustrating

embodiments of the invention and not for the purpose of limiting the same, Figure 2

depicts an embodiment of a gas sensor constructed according to the present invention and

in the form of a micro fuel cell oxygen sensor 10. As will be apparent from the following

description, the sensor 10 improves upon the construction of the known micro fuel

oxygen sensors with respect to at least the design of the sensor housing, the cathode, the

anode, and the sensor's sealing characteristics. The sensor 10 may include a housing 6, an

anode contact 4, a cathode contact 2, an anode body 16, an expansion or back membrane

18, a sensing membrane 20, a clamp 22, and a printed circuit board 24.

As illustrated in Figures 2 and 3, the housing 6 may be an open ended cylinder

which comprises an anode cavity 28 defined by an external wall 32, and an internal wall

30, and a third wall in the form of a housing base 40. The housing base 40, the external

wall 32, and the internal wall 30 may be separate components that are secured together by

any known method such as, for example, heat sealing, welding, or press fit. All such

components that form the housing 6 of sensor 10 also may be integrally formed as a

single unit through processes such as, for example, pouring or injection molding. The

base 40 and external and internal walls 32, 30 may be fabricated from, for example, any

resilient, insulating material, which material includes thermoplastic material such as, for example, polyethylene. The housing 6 may be any shape, for example, it may be

cylindrical with the internal and external walls 30, 32 being generally coaxial, as

illustrated in Figures 2 - 5. The housing 6 may have any suitable dimensions, and as

incorporated in micro fuel oxygen sensor 10 may have, for example, a longitudinal length

of about 1.10 inches and a diameter of about 0.8 inches. Other housing dimensions will

follow from the application for which the sensor is adopted.

As best shown in Figures 3, 4, and 5, the housing 6 includes a first end 7 defining

a first opening 36 and a second end 9 defining a second opening 38. The first opening 36

receives the entering stream of gas to be sensed. A first rim 37 forms a periphery at the

first end 7 of the housing 6 and defines a recessed portion 25 within which is positioned

the first opening 36. As adapted for sensor 10, the first rim 37 may be, for example,

about 0.1 inches thick and about 0.23 inches deep into the recessed portion 25. The first

opening 36 may be circular and defined by annular lip portion 31 , which may be centrally

positioned within the recessed portion 25. The first opening 36, however, also may have

any suitable shape defined by a suitably shaped lip portion. Thus, the lip portion 31 need

not be circular, but may be any shape, such as, for example, oval, rectangular, square, or

hexagonal. As provided in sensor 10, the lip portion 31 may have, for example, a

diameter of about 0.5 inches and a height of about 0.15 inches above the base 27 of the

recessed portion 25. The lip portion 31 may be integrally formed with the housing 6, and

for example, may be formed in the above-described housing manufacturing processes.

As best shown in Figures 2 and 3, a protrusion 26 extends around the periphery of the lip

portion 31 for engagement with a clamp 22, as further described below. As best illustrated in Figures 3 and 5, the second opening 38 of the housing 6 is

opposite the first opening 36 and is defined by the external wall 32. The external wall 32

includes a ledge 34 that defines a boundary of the anode cavity 28. The ledge 34 has a

grooved portion 35 around its circumference for receipt of, and for engagement with, a

protrusion 19 of the back membrane 18 (shown in Figure 2), which is described below.

External wall 32 includes a second rim 54 that extends above the ledge 34. As adapted

for use in sensor 10, the second rim 54 may be, for example, about 0.07 inches thick and

have a height of about 0.06 inches. The height of the second rim 54 is substantially equal

to the total thickness of the back membrane 18 and the printed circuit board 24 so that a

surface of the printed circuit board 24 may be substantially flush with an edge 43 of the

second rim 54 after being placed over the back membrane 18 and sealed thereto to

complete the back portion of the sensor 10.

The external wall 32 has an inside surface 39 that extends from the housing base

40 to the ledge 34. As adapted for use in sensor 10, the portion of the external wall 32

defined in part by the inside surface 39 may be, for example, about 0.14 inches thick.

Referring to Figures 2 and 3, the internal wall 30 defines an open ended chamber

12 that extends from the first end 7 to an opposite end into the anode cavity 28, thereby

providing fluid communication through the electrolyte for the gases entering the first

opening 36. As adapted for use in sensor 10, the internal wall 30 may be, for example,

about 0.03 inches thick, positioned about 0.26 inches from the inside surface 39 of the

external wall 32, and extends from the housing base 40 into the anode cavity 28 a

distance of about 0.2 inches to an internal wall edge 49. It will be understood that the

anode cavity 28 is an annular chamber that is defined by the external wall 32 as its outer periphery, the internal wall 30 as its inner periphery, the housing base 40 and the back

membrane 18 when the back membrane 18 is positioned in engagement with the ledge

34.

Figures 2 and 6 illustrate the anode contact 4 of the present invention. The anode

contact 4 is positioned in the anode cavity 28. The anode contact 4 rests on the base 40 of

the housing 6 and is positioned between the external wall 32 and the internal wall 30, but

the anode contact 4 does not extend over the open end of the chamber 12. The anode

contact 4 comprises an anode base 46, a number of anode projections or fingers 48, and

an anode tail 50. The anode base 46 defines a central void 47. As adapted for use in

sensor 10, the anode base 46 may have, for example, an outer diameter of about 0.9

inches, an inner diameter of about 0.4 inches, and a thickness of about 0.01 inches. The

anode fingers 48 are protrusions that may be distributed, evenly or otherwise, around the

anode base 46, as shown, providing redundant electrical connections between the anode

body 16 and the anode contact 4. The anode fingers 48 may be sized so as to extend

significantly into the anode body 16 so as to provide a larger surface contact area with the

anode body, as shown in Figure 2, and the anode fingers 48 extend above the base 46

when the anode contact 4 is in its final formed position. Although any number of anode

fingers 48 may be used, in the embodiment of anode contact 4 in sensor 10, six anode

fingers 48 are distributed around an outer periphery of the anode base 46 and six anode

fingers are distributed around an inner periphery of the anode base 46. The large surface

area of the anode fingers 48 reduces the contact resistance with the anode body 16

providing more stable sensor performance. The anode contact 4 may be fabricated from

any suitable electrically conductive material. Such materials include, for example, the noble metals, which include, for example, silver and nickel, and metal alloys, which

include, for example, stainless steel.

The anode tail 50 may be integrally formed with the anode base 46. As adapted

for use in sensor 10, the anode tail 50 may have, for example, a length of about 0.53

inches and extends above the anode base 46 through the external wall 32 and beyond the

ledge 34 to connect to the printed circuit board 24. By such a construction, at least a

portion of the anode tail 50 may be encapsulated in and extend through the housing 6,

thereby minimizing the number of exit and entrance holes in the housing 6 and

eliminating the need for anode wiring. As a result, the present invention greatly reduces

the possibility of electrolyte leakage through the external wall 32 of the housing 6. Also,

the elimination of anode wiring (such as anode wire 115 of the prior art sensor 110)

reduces the parts count for the anode assembly and eliminates the associated welding

step.

It will be understood that the anode contact 4 may be fabricated in a variety of

manners, including those wherein one or more of the anode base 46, anode fingers 48,

and anode tail 50 are separate elements attached to the remaining elements to provide

anode contact 4. As provided for sensor 10, the anode contact 4 is manufactured by

stamping a flat pattern from a sheet of a noble metal such as, for example, nickel. The

anode fingers 48 and anode tail 50 are then bent in position by one or more of a variety of

methods such as, for example, the use of a progressive die. It is also contemplated that

the anode base 46, anode fingers 48, and the anode tail 50 may be individually

manufactured members that are attached together by any known method, such as, for example, spot welding, to produce the anode contact 4. Other manufacturing methods for

providing anode contact 4 will be apparent upon review of this disclosure.

In addition, for a more effective use of the lead anode material, the anode contact

4 may be provided with a large surface area to volume ratio. As described above, the

anode body 16 may be manufactured from, for example, sintered lead tape or sintered

lead pellets to increase the surface area to volume ratio of the anode body 16. The anode

fingers 48, described above, may be dimensioned to provide a relatively large anode

surface area to volume ratio so as to optimize sensor performance.

The anode body 16 is positioned inside the anode cavity 28 and generally

conforms to the shape of a portion of the anode cavity 28 so that contact is established

with the anode contact 4. The anode body 16 may be any anode material know in the art,

and in sensor 10 is formed from sintered lead pellets. As further described below, in

fabricating sensor 10 lead pellets are poured into the anode cavity 28 and over the anode

contact 4 so that the anode fingers 48 are surrounded by the pellets. The lead pellets are

then adhered together, such as by sintering, to form a cohesive anode body 16. As

illustrated in Figure 2, the anode body 16 may be constructed to have a thickness which

is, for example, less than the height of the internal wall 30 so that no portion of the anode

body 16 extends beyond the edge 49 of internal wall 30 or over the opening defined by

the internal wall 30.

As illustrated in Figures 2 and 7, the cathode contact 2 comprises a cathode base

42 and a cathode tail 44. The cathode contact 2 may be constructed of, for example, a

noble metal such as nickel or silver, or a substrate plated with, for example, silver or

rhodium. As provided in sensor 10, the cathode base 42 may be, for example, 0.46 inches in diameter and 0.01 inches thick. The cathode base 42 may include a curved surface 53

that may be, for example, concave relative to the anode cavity 28 when assembled, and

that includes a number of small perforated holes therethrough. A solid peripheral ring 45

is preferably positioned around the outer edge of the cathode base 42 encircling the

perforations. A number of cathode projections or fingers 51 extend from the outer ring

45 of the cathode base 42. The cathode fingers 51 may be, for example, evenly

distributed around the solid peripheral ring 45 of the cathode base 42, as shown. When

the housing 6 is formed through plastic injection molding, the cathode fingers 51 provide

greater surface area and act to grip and hold the cathode contact 2 in place and are

encapsulated in the housing 6, as further described below.

At least a portion of the cathode tail 44 may be encapsulated in, and extend

through, the external wall 32, beyond the ledge 34 to connect to the printed circuit board

24 when the cathode base 42 is positioned over the first opening 36, as illustrated. Thus,

cathode tail 44 may be positioned through the housing 6, thereby minimizing the number

of exit and entrance holes in the housing 6 and eliminating the need for cathode wiring

(as in known sensor 110). As a result, the present invention greatly reduces the

possibility of electrolyte leakage through the walls of the housing 6. Also, the

elimination of cathode wire reduces the parts count for the cathode assembly and

eliminates the associated welding process used with, for example, known sensor 110, to

secure cathode wire 113 to plug 117.

Although those skilled in the art may, upon considering the present disclosure,

readily appreciate numerous ways to form the cathode contact 2, as provided in sensor 10

the cathode contact 2 is manufactured by photo-chemically etching a flat pattern of the cathode contact from a sheet approximately 0.01 inches thick. The sheet may be any

suitable material, such as, for example, nickel or brass. The cathode base 42 is then

shaped to include concave surface 53 as illustrated, the cathode tail 44 is bent in a stepped

fashion as shown, and the cathode fingers 51 are bent outwardly in position by using one

or more of a variety of methods including, for example, the use of a progressive die. The

cathode may then be plated with an appropriate cathode material, such as, for example,

rhodium, gold or silver.

The sensing membrane 20 is positioned directly over and in intimate contact with

the cathode contact 2. As incorporated in sensor 10, the sensing membrane 20 is a Teflon

film. The sensing membrane 20, however, may be constructed of any of the various types

of hydrophobic, gas permeable materials known in the art. The permeability of the

material is such that gas may pass therethrough, but electrolyte solution will not.

Figure 2 shows the second opening 38 sealed with a back membrane 18 that

allows for expansion or contraction of the electrolyte volume. The back membrane 18

may be any suitable resilient material, such as a thermoplastic, including polyethylene.

The back membrane 18 may be sealed to the housing 6 by any means known in the art,

but in sensor 10 the back membrane 18 is thermally sealed to the housing 6.

Figures 2 and 8 illustrate the clamp 22 of sensor 10, which provides for improved

sealing between the sensing membrane 20 and the housing 6. The clamp 22 comprises a

clamp mesh 58, which protects the cathode contact 2, and a clamp restraint 54. The

clamp 22 also includes a groove 23 around the inner periphery of clamp restraint 54 that

is in snap lock engagement with, and is held in place by, the protrusion 26 on housing 6.

The positive locking action between the protrusion 26 and the membrane clamp 22 also provides the pressure required for effective sealing between the sensing membrane 20 and

the housing 6. The clamp 22 is placed over the sensing membrane 20 and thereby seals

the first opening 36 with the sensing membrane 20. The clamp restraint 54 may be

constructed of, for example, an elastomer or thermoplastic material that produces

sufficient pressure against the sensing membrane 20 and housing 6 such that sensing

membrane 20 creates a seal with housing 6. The clamp mesh 58 may be fabricated of, for

example, a metallic or non-metallic woven mesh, perforated metal, or other material. In

one possible alternate configuration, illustrated in Figure 9, the clamp 222 may have a

protrusion 226 and the recessed lip portion 231 may have a groove 223 for snap lock

engagement therewith.

One skilled in the art will, upon considering this description of the invention,

contemplate numerous other arrangements to provide engagement between the clamp 22

and the housing 6. For example, in another possible alternate configuration of the sensor

of the present invention, illustrated in cross-section in Figure 11, sensor 305 includes

clamp 322 having a clamp restraint 340 including an annular protrusion 326 formed about

its outer periphery. The housing 306 of the sensor 305 includes a clamp recess 350

having an annular groove 323. At least a portion of the clamp restraint 340 may be

positioned in the clamp recess 350 such that the groove 323 receives the protrusion 326

for snap lock engagement therewith. A variation of the design of the sensor 305 is

illustrated in Figure 12, wherein sensor 405 includes a clamp 422 having a clamp restraint

440 including a groove 423 formed about its outer periphery. Sensor 405 includes

housing 406 having a clamp recess 450 including an annular protrusion 426 for snap lock

engagement with the groove 423 of clamp 422. As shown in each of Figures 11 and 12, clamps 322 and 422 include opening 358, 458, respectively, in place of the wire mesh

used in the clamp 22 of Figure 8.

Aspects of one method of fabricating the housing of the sensor 10 is illustrated in

Figure 10. The dry sub-assembly generally consists of assembling the housing 6, anode

contact 4, cathode contact 2, and a short tubular spacer 52. The tubular spacer 52 has an

outer diameter and a central hub approximately the same size as the respective

dimensions of the anode base 46. Figure 10, which is a cross-section through the several

depicted elements, shows half of the spacer 52. As utilized in the fabrication of sensor

10, the spacer 52 is approximately 0.24 inches in height. The spacer 52 is used to

properly position the anode contact 4 relative to the cathode contact 2. The properly

oriented cathode contact 2, spacer 52, and anode contact 4 are then positioned in a plastic

injection mold. All three of the parts are vertically aligned in the mold in the mentioned

order, with the cathode contact 4 being at one end. The mold is then closed to receive the

injected material, thereby forming the thermoplastic housing 6, which encapsulates the

anode contact 4, the spacer 52, and the anode contact 2 therein in the proper orientation

and forms the injected material into the sensor housing 6 in the shape shown in Figures 4

and 5. As indicated in Figure 2, the anode contact 4 is attached to the housing 6 by

encapsulating the anode tail 50 in the external wall 32. The cathode contact 2 is attached

to the housing 6 by the cathode tail 44 extending through the external wall 32 and by the

cathode fingers 51.

After the dry sub-assembly is formed, the wet sub-assembly completes the

formation process. Lead pellets are poured into the anode cavity 28 of the housing 6, and

are evenly distributed between the external wall 32 and the internal wall 30, around the anode fingers 48, and preferably below the internal wall edge 49. The pellets are then

formed into a porous unitary mass by being sintered and/or compressed. Any suitable

process for forming a porous unitary mass from the pellets may be used, and in forming

sensor 10, for example, a temperature in the range of 22° C and pressure in the range of

500 - 1000 psi may be used. The pellets are thereby compressed into an annular mass in

intimate contact with each other and the anode contact 4. After the anode body 16 is

formed, the back membrane 18 may be attached by, for example, thermal bonding, to the

housing 6 so that the protrusions 19 of the back membrane 18 engage the grooved portion

35 of the ledge 34. The housing 6 may then be vertically placed in a vacuum chamber

with the cathode contact 2 oriented so that the anode fingers 48 are disposed vertically

upward. The chamber is filled with electrolyte up to the height of the housing 6 so that

the portion of the anode cavity 28 (not already filled by the anode body 16) and the

chamber 12 are completely filled with electrolyte solution. By using a vacuum chamber

to fill the sensor with electrolyte, air bubbles are greatly minimized and the electrolyte

solution may more readily permeate the porous anode body 16. The vacuum chamber

also enables multiple sensors 10 to be filled simultaneously, thus greatly increasing

manufacturing efficiency. The sensing membrane 20 is then stretched over the cathode

contact 2 to seal the electrolyte solution inside the housing 6. The clamp 22 is then

positioned over the sensing membrane 20 and press fit to the housing 6 so that the clamp

restraint 54 tightly engages the first rim 37 such that the groove 35 on the clamp 22

engages the protrusion 26 on the lip portion 31 of the housing 6 in snap-lock

arrangement. Finally the printed circuit board 24 is attached to the back membrane 18 to

complete the assembly. The printed circuit board 24 may be installed by a variety of methods known by those skilled in the art. In the case of sensor 10, the printed circuit

board 24 is installed by means of an interference fit between the printed circuit board 24

on the sensor housing 6.

With the printed circuit board 24 in place, the anode tail 44 and the cathode tail 50

make contact with the negative and positive terminals, respectively, on the printed circuit

board 24, as shown in Figure 2. Contact with the terminals on the printed circuit board

24 can be made in various ways known to those skilled in the art such as, for example, by

soldering the anode tail 44 and cathode tail 50 to the terminals. In the case of sensor 10,

the terminals on the printed circuit board 24 are located on the vertical edge of the printed

circuit board 24.

During the life of the micro fuel cell oxygen sensor 10, the anode body 16 is

continuously oxidized. Lead oxide formed on the surface of the lead particles of the prior

art oxygen sensors may isolate part of the anode from further participation in the oxygen

sensing process. As a result, some of the lead will not be consumed and the life of the

sensor is reduced. Although certain known sensors have incorporated an anode of

sintered lead pellets, a drawback associated with that approach as adapted for use in the

known sensors is that lead pellets can occasionally become separated from the anode

body and come in contact with the cathode causing spurious sensor output signals.

However, because the anode body 16 of the present invention is not disposed over an

opening of the chamber 12 defined by the internal wall 30, any fragments or particles that

become dislodged from the anode body 16 will remain between walls 30 and 32 and will

not pass through chamber 12 and come in contact with the cathode contact 2 causing

spurious sensor output signals. Thus, the internal wall 30 inhibits loose anode fragments or particles from contacting the cathode contact 2 and improves the long-term

performance of the sensor 10. Although sensor 10 incorporates an anode body of

consolidated lead particles, as noted above, other suitable anode bodies may be used. The

benefit provided by the internal wall 30 may be realized in all such alternative

constructions in that the internal wall 30 will inhibit any detached portion of the anode

body from contacting the cathode contact 2. Also, although only a single cavity design is

shown in Figure 2, it should be obvious to one of ordinary skill in the art on reading this

disclosure that variations of cavity design that discourage migration of anode particles to

the cathode contact 2 without interfering with the transfer of charges between the two

electrodes may be provided. It will be understood that all such variations are within the

scope of the present invention as provided in the appended claims.

Accordingly, it will be understood that the sensor 10 includes a reduced number

of parts relative to existing sensors adapted for like application, and sensor 10 lacks for

example, lead wires (such as wires 113, 115 of sensor 110) and their associated housing

plugs (such as plugs 117, 119 of sensor 110), having instead a uniquely configured anode

contact 4 and cathode contact 2. The internal wall 30 and the anode cavity 28 also are

advantageous inasmuch as they inhibit contact between the cathode contact 2 and loose

portions of the anode body 16 or loose oxide detached from the anode body 16. The

anode contact 4 includes anode fingers 48 which reduce the contact resistance through the

increased amount of surface area contact with anode contact 4. The multiple anode

fingers 48 also reduce the possibility that the anode contact 2 will lose contact with the

anode body 16 during the life of sensor 10. Furthermore, the design of sensor 10 reduces

the possibility of electrolyte leakage by providing cathode and anode contacts 2, 4, respectively, each having respective rigid tails 44, 50 that pass through the housing 6. In addition, clamp 22 is provided to seal the sensing membrane 20 via snap lock

engagement.

Although the foregoing description has necessarily presented a limited number of

embodiments of the invention, those of ordinary skill in the relevant art will appreciate

that various changes in the configurations, details, materials, and arrangement of the

elements that have been herein described and illustrated in order to explain the nature of

the invention may be made by those skilled in the art, and all such modifications will

remain within the principle and scope of the invention as expressed herein in the

appended claims. In addition, although the foregoing detailed description has been

directed to an embodiment of the gas sensor of the invention in the form of a micro fuel

cell oxygen sensor, it will be understood that the present invention has broader

applicability and, for example, may be used in connection with the construction of

multiple electrode gas sensors for use in additional applications. All such additional

applications of the invention remain within the principle and scope of the invention as

embodied in the appended claims.

Claims

1. A gas sensor, comprising:

a housing including a first wall and a cavity, said housing defining a

passage between an opening and said cavity, at least a portion of said passage

being defined by said first wall, said first wall extending into said cavity;

a first electrode positioned within said cavity; and

a second electrode positioned at said opening.

2. The gas sensor of claim 1 wherein said first electrode is an anode and said

second electrode is a cathode.

3. The gas sensor of claim 1 wherein the gas sensor is a micro fuel cell

oxygen sensor.

4. The gas sensor of claim 2 wherein said cathode is positioned over said

opening.

5. The gas sensor of claim 1 wherein said first wall is annular.

6. The gas sensor of claim 1 further comprising a second wall and a base,

said cavity being defined by said first wall, said second wall, and said base.

7. The gas sensor of claim 1 wherein said first electrode is an anode

positioned within said cavity and adjacent said first wall.

8. The gas sensor of claim 1 further comprising an electrolyte within said

housing and in contact with said first electrode and said second electrode; and

an electrode contact in contact with said first electrode and comprising: a base;

a tail connected to said base; and

a plurality of contact projections connected to and extending from

said base.

9. The gas sensor of claim 8 wherein at least a portion of each said contact

projection is in contact with said electrode.

10. The gas sensor of claim 8 wherein each said contact projection is at least

partially embedded within said first electrode.

11. The gas sensor of claim 8 further comprising a second wall wherein at

least a portion of said tail is within said second wall.

12. The gas sensor of claim 1 wherein said housing defines a recess and said

passage includes a first end and a second end, said first end positioned within said

cavity, said second end positioned within said recess.

13. The gas sensor of claim 1 further comprising: a clamp; and

a gas permeable member positioned on said opening, said clamp in

snap lock engagement with said housing and securing said gas permeable member

on said opening.

14. The gas sensor of claim 13 wherein said opening includes a protrusion and

said clamp includes a groove, and wherein said protrusion is disposed and

retained within said groove to provide said snap lock engagement.

15. The gas sensor of claim 13 wherein said opening includes a groove and

said clamp includes a protrusion, and wherein said groove is disposed and

retained within said protrusion to provide said snap lock engagement.

16. A gas sensor, comprising:

a housing including a first wall and a cavity, said housing defining a

passage between an opening and said cavity, at least a portion of said passage

being defined by said first wall, said passage extending into said cavity;

a first electrode positioned within said cavity, but not over said passage;

and

a second electrode positioned at said opening.

17. The gas sensor of claim 16 wherein said first electrode is an anode and said second electrode is a cathode.

18. The gas sensor of claim 16 wherein said passage is defined by said first

wall extending into said cavity.

19. The gas sensor of claim 18 wherein said first wall is annular.

20. The gas sensor of claim 16 wherein said cavity is defined by said first

wall, a second wall, and a base.

21. A gas sensor, comprising:

a housing including a cavity and a first wall, said housing defining a

passage between an opening and said cavity, said passage extending into said

cavity;

a first electrode positioned within said cavity;

a second electrode positioned at said opening; and

said first wall being a barrier to migration of portions of said first electrode

to said second electrode.

22. The gas sensor of claim 21 wherein said first electrode is an anode and

said second electrode is a cathode.

23. The gas sensor of claim 21 wherein said first wall is annular.

24. The gas sensor of claim 21 further comprising a second wall and a base,

said cavity being defined by said first wall, said second wall, and said base.

25. A gas sensor, comprising:

a housing including a cavity and a first wall, said housing defining a

passage between an opening and said cavity, said passage extending into said

cavity;

a first electrode positioned within said cavity;

a second electrode positioned at said opening; and

said first wall being a barrier inhibiting migration of a portion of said first

electrode to said second electrode.

26. The gas sensor of claim 25 wherein said first electrode is an anode and

said second electrode is a cathode.

27. The gas sensor of claim 25 wherein said first wall is annular.

28. The gas sensor of claim 25 further comprising a second wall and a base,

said cavity being defined by said first wall, said second wall, and said base.

29. A fuel cell, comprising a gas sensor, said gas sensor including: a housing including a first wall and a cavity, said housing defining a

passage between an opening and said cavity, at least a portion of

said passage being defined by said first wall, said wall extending into said cavity;

a first electrode positioned within said cavity; and

a second electrode positioned at said opening.

30. The fuel cell of claim 29 wherein said first electrode is an anode and said

second electrode is a cathode.

31. The fuel cell of claim 29 wherein said gas sensor is a micro fuel cell

oxygen sensor.

32. The fuel cell of claim 29 wherein said first electrode is an anode

positioned within said cavity and adjacent said first wall.

33. A gas sensor comprising:

a housing;

a first electrode positioned within said housing;

a second electrode;

an electrolyte within said housing and in contact with said first electrode

and said second electrode; and an electrode contact in contact with said first electrode and comprising:

a base;

a tail connected to said base; and

a plurality of contact projections connected to and

extending from said base.

34. The gas sensor of claim 33, wherein at least a portion of each said contact

projection is in contact with said first electrode.

35. The gas sensor of claim 34, wherein each said contact projection is at least

partially embedded within said first electrode.

36. The gas sensor of claim 34, wherein:

said housing includes a first wall; and

at least a portion of said tail is within said first wall.

37. The gas sensor of claim 36, wherein at least a portion of said tail is

encapsulated within said first wall.

38. The gas sensor of claim 37 wherein said first wall comprises a

thermoplastic material and said portion of said tail is encapsulated within said

thermoplastic material.

39. The gas sensor of claim 34, wherein said housing defines a cavity, and

wherein said first electrode, said base, and said contract projections are disposed

within said cavity.

40. The gas sensor of claim 39, wherein said housing further comprises a

second wall and a third wall, said cavity defined by said first wall, said second

wall, and said third wall.

41. The gas sensor of claim 40 wherein:

said first wall is an outer wall;

said second wall is an inner wall; and

said base is positioned adjacent said third wall and between said

first wall and said second wall, said base defining a void, said inner

wall extending through said void.

42. The gas sensor of claim 41, wherein both said outer wall and said inner

wall are annular and coaxial.

43. The gas sensor of claim 37, wherein said tail includes an end and said end

projects from said first wall.

44. A gas sensor, comprising: a first electrode;

a housing having a first end, said first electrode positioned within said housing, said first end defining an opening; and

a clamp in snap lock engagement with said opening.

45. The gas sensor of claim 44 wherein said opening is defined by a first wall

having a groove, said clamp includes a protrusion, and said protrusion is disposed

within said groove to provide said snap lock engagement.

46. The gas sensor of claim 44 wherein said opening is defined by a first wall

having a protrusion, said clamp includes a groove, and said protrusion is disposed

within said groove to provide said snap lock engagement.

47. The gas sensor of claim 44, further comprising a second electrode, said

second electrode positioned over said opening and intermediate said opening and

said clamp.

48. The gas sensor of claim 47, wherein said first electrode is an anode and

said second electrode is a cathode.

49. The gas sensor of claim 47, wherein said clamp includes a gas permeable

membrane.

50. The gas sensor of claim 44 wherein said housing includes a recess having

a groove, said clamp includes a clamp restraint having a protrusion, and said

protrusion is disposed within said groove to provide said snap lock engagement.

51. The gas sensor of claim 44 wherein said housing includes a recess having

a protrusion, said clamp includes a clamp restraint having a groove, and said

protrusion is disposed within said groove to provide said snap lock engagement.

52. A method of forming a gas sensor, the method comprising:

placing one of a first electrode contact and a second electrode contact in a

mold; and introducing molten thermoplastic into the mold to encapsulate at least a

portion of the one of the first and second electrode contacts within the

thermoplastic.

53. The method of claim 52 wherein the first electrode contact is an anode

contact and the second electrode contact is a cathode contact.

54. The method of claim 52, wherein the act of placing further comprises

placing a spacer in the mold, the spacer separating the first electrode contact from

the second electrode contact.

55. The method of claim 54 wherein in the act of introducing thermoplastic

into the mold, the spacer is encapsulated in the thermoplastic.

56 The method of claim 52, further comprising curing the molten

thermoplastic to form a sensor housing.

57. The method of claim 56, wherein the first electrode contact includes at

least one contact projection, the method further comprising:

placing particulate lead within the sensor housing about the contact

projection; and

applying at least one of heat and pressure to the particulate lead to form a

porous lead mass in contact with the contact projection.

58. The method of claim 57, further comprising:

introducing a liquid electrolyte into the sensor housing; and

sealing the sensor housing to retain electrolyte therein.

59. The method of claim 58, wherein the acts of introducing liquid electrolyte

and sealing the sensor housing occur in a vacuum chamber.

60. A gas sensor, comprising:

a housing having a first wall;

a first electrode contact positioned with said housing; and a second electrode positioned on said housing, at least one of said first

electrode contact and said second electrode contact having a rigid tail at least

partially encapsulated in said first wall.

61. The gas sensor of claim 60, wherein said first electrode contact includes

said rigid tail.

62. The gas sensor of claim 61 wherein said second electrode contact includes

said rigid tail.

63. A gas sensor, comprising:

a housing;

a first electrode positioned within said housing;

a second electrode positioned within said housing; and

a spacer positioned within said housing and separating said first electrode

from said second electrode.

64. The gas sensor of claim 63 wherein said first electrode is an anode and

said second electrode is a cathode.

65. The gas sensor of claim 63 wherein at least a portion of at least one of said

first electrode, said second electrode, and said spacer is encapsulated in said

housing.