U.S. patent application number 09/783589 was filed with the patent office on 2001-12-13 for gas sensor with ceramic heater.
This patent application is currently assigned to DENSO CORPORATION. Invention is credited to Kobayashi, Masayuki, Kuroki, Hisao, Shirai, Makoto, Yamada, Hirokazu.
Application Number | 20010050280 09/783589 |
Document ID | / |
Family ID | 27277926 |
Filed Date | 2001-12-13 |
United States Patent
Application |
20010050280 |
Kind Code |
A1 |
Yamada, Hirokazu ; et
al. |
December 13, 2001 |
Gas sensor with ceramic heater
Abstract
An easy-to-manufacture structure of a ceramic heater having a
high durability and a manufacturing method thereof are provided.
The ceramic heater may be used in an oxygen sensor for automotive
air-fuel ratio control systems and includes a ceramic square rod
formed with a laminate of a heater substrate on which a
heater-patterned layer consisting of a heater element and leads is
formed and a covering substrate covering the heater-patterned
layer. Metallic terminals are connected electrically to the leads
of the heater-patterned layer, respectively, and mounted on
surfaces of the ceramic square rod opposed to each other in a
direction of lamination of the heater substrate and the covering
substrate, respectively. At least one outer lead is joined to one
of the metallic terminals through a bonding layer.
Inventors: |
Yamada, Hirokazu; (Nagoya,
JP) ; Shirai, Makoto; (Kuwana-shi, JP) ;
Kuroki, Hisao; (Kuwana-shi, JP) ; Kobayashi,
Masayuki; (Kuwana-shi, JP) ; Yamada, Hirokazu;
(Nagoya, JP) |
Correspondence
Address: |
Pillsbury Winthrop LLP
Intellectual Property Group
1600 Tysons Boulevard
McLean
VA
22102
US
|
Assignee: |
DENSO CORPORATION
|
Family ID: |
27277926 |
Appl. No.: |
09/783589 |
Filed: |
February 15, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09783589 |
Feb 15, 2001 |
|
|
|
09365173 |
Aug 2, 1999 |
|
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|
6194693 |
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Current U.S.
Class: |
219/543 ;
219/548; 219/552 |
Current CPC
Class: |
G01N 27/4067
20130101 |
Class at
Publication: |
219/543 ;
219/552; 219/548 |
International
Class: |
H05B 003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 3, 1998 |
JP |
10-219031 |
Jan 14, 1999 |
JP |
11-8185 |
Apr 27, 1999 |
JP |
11-120248 |
Claims
What is claimed is:
1. A ceramic heater comprising: a ceramic square rod formed with a
laminate of a heater substrate on which a heater-patterned layer
consisting of a heater element and leads connected to the heater
element is formed and a covering substrate covering the
heater-patterned layer of the heater substrate; metallic terminals
connected electrically to the leads of the heater-patterned layer
of the heater substrate, respectively, said metallic terminals
being mounted on surfaces of said ceramic square rod opposed to
each other in a direction of lamination of the heater substrate and
the covering substrate, respectively; and at least one outer lead
joined to one of said metallic terminals through a bonding
layer.
2. A ceramic heater as set forth in claim 1, further comprising a
second outer lead joined to the other metallic terminal through a
bonding layer.
3. A ceramic heater as set forth in claim 1, wherein said metallic
terminals are electrically connected to the leads through holes
formed in at least one of the covering substrate and the heater
substrate.
4. A ceramic heater as set forth in claim 1, wherein each of said
metallic terminals is mounted on an area inside edges of the
surface of said ceramic square rod.
5. A ceramic heater as set forth in claim 1, wherein the bonding
layer occupies an area of a surface of the metallic terminal inside
edges of the metallic terminal.
6. A ceramic heater as set forth in claim 1, wherein the bonding
layer contains 40 Wt % of Cu or more.
7. A ceramic heater as set forth in claim 1, wherein the one of the
metallic terminals contains 70 Wt % of W or more, and the bonding
layer contains 40 to 98 Wt % of Cu and 2 to 20 Wt % of Ni.
8. A ceramic heater as set forth in claim 7, wherein the bonding
layer contains 60 Wt % of Au or less.
9. A ceramic heater as set forth in claim 8, further comprising an
Ni-plated layer formed on the one of said metallic terminals,
having a thickness of 3 .mu.m or less, and wherein the outer lead
is joined to the Ni-plated layer through the bonding layer.
10. A ceramic heater comprising: a ceramic square rod formed with a
laminate of heater substrates each having formed thereon a
heater-patterned layer consisting of a heater element and first and
second leads connected to the heater element and a covering
substrate interposed between the heater substrates; first and
second metallic terminals connected electrically to the first and
second leads of the heater-patterned layers of the heater
substrates, respectively, said metallic terminals being mounted on
surfaces of said ceramic square rod opposed to each other in a
direction of lamination of the heater substrates and the covering
substrate; and outer leads joined to said first and second metallic
terminals through bonding layers, respectively.
11. A ceramic heater as set forth in claim 10, wherein said first
metallic terminal is connected to the first leads of the heater
substrates through conductive material-coated holes formed in the
covering substrate and one of the heater substrates, and the second
metallic terminal is connected to the second leads of the heater
substrates through conductive material-coated holes formed in the
covering substrate and the other heater substrate.
12. A ceramic heater as set forth in claim 10, wherein each of the
bonding layers occupies an area of a surface of one of the metallic
terminals inside edges of the metallic terminal.
13. A ceramic heater as set forth in claim 10, wherein each of the
bonding layers contains 40 Wt % of Cu or more.
14. A ceramic heater as set forth in claim 10, wherein each of the
metallic terminals contains 70 Wt % of W or more, and each of the
bonding layers contains 40 to 98 Wt % of Cu and 2 to 20 Wt % of
Ni.
15. A ceramic heater as set forth in claim 14, wherein each of the
bonding layers contains 60 Wt % of Au or less.
16. A ceramic heater as set forth in claim 15, further comprising
an Ni-plated layer formed on each of said metallic terminals,
having a thickness of 3 .mu.m or less, and wherein the outer leads
are joined to the Ni-plated layers through the bonding layers.
17. A method of manufacturing ceramic heaters comprising the steps
of: preparing a first green sheet; preparing a second green sheet;
printing a first surface of said second green sheet an array of
heater-patterned layers each consisting of a heater element and
leads connected to the heater element; printing a second surface of
said second green sheet opposite the first surface with an array of
metallic terminals; attaching said first green sheet to said second
green sheet so as to cover the first surface of said second green
sheet to form a laminate; baking said laminate to form a ceramic
board; joining outer leads to the metallic terminals through
bonding layers, respectively; and cutting said ceramic board into a
plurality of square rods constituting units of the ceramic
heaters.
18. A method as set forth in claim 17, further comprising a step of
forming through holes in said first green sheet for electrical
connections of the leads of the heater-patterned layers and the
metallic terminals.
19. A method as set forth in claim 17, further comprising a step of
forming grooves in a surface of the ceramic board between adjacent
two of the units of the ceramic heaters to be cut by said cutting
step.
20. A method of manufacturing ceramic heaters comprising the steps
of: preparing a first green sheet; preparing second green sheets;
printing a first surface of each of said second green sheets an
array of heater-patterned layers each consisting of a heater
element and leads connected to the heater element; printing a
second surface of each of said second green sheets opposite the
first surface with an array of metallic terminals; interposing said
first green sheet between said second green sheets so as to cover
the first surfaces of said second green sheets to form a laminate;
baking said laminate to form a ceramic board; joining outer leads
to the metallic terminals formed on at least one of said second
green sheets through bonding layers, respectively; and cutting said
ceramic board into a plurality of square rods constituting units of
the ceramic heaters.
21. A method as set forth in claim 20, further comprising a step of
forming through holes in said first green sheet for electrical
connections of the leads of the heater-patterned layers and the
metallic terminals.
22. A method as set forth in claim 20, further comprising a step of
forming grooves in a surface of the ceramic board between adjacent
two of the units of the ceramic heaters to be cut by said cutting
step.
23. A gas sensor comprising: a gas sensing element having a
gas-exposed portion, said gas sensing element having formed therein
a chamber; a ceramic heater disposed within the chamber of said gas
sensing element to heat said gas sensing element, said ceramic
heater including, (a) a ceramic square rod formed with a laminate
of a heater element substrate on which a heater-patterned layer
consisting of a heater element and leads connected to the heater
element is formed and a covering substrate covering the
heater-patterned layer of the heater substrate, (b) metallic
terminals connected electrically to the leads of the
heater-patterned layer of the heater substrate, respectively, said
metallic terminals being mounted on surfaces of said ceramic square
rod opposed to each other in a direction of lamination of the
heater substrate and the covering substrate, respectively, and (c)
at least one outer lead joined to one of said metallic terminals
through a bonding layer; a first cylindrical holder fitted in the
chamber of said gas sensing element, said first holder including a
heater holding portion for holding said ceramic heater and a sensor
contact in contact with an inner wall of said gas sensing element,
the sensor contact having a sensor signal output terminal; a second
cylindrical holder mounted on an outer wall of said gas sensing
element, having a sensor signal output terminal; and slit formed in
said first holder to define a C-shaped cross section, said slit
being located substantially 90.degree..+-.20.degree. apart from the
sensor signal output terminal of said first cylindrical holder.
24. A gas sensor as set forth in claim 23, wherein the sensor
signal output terminal of said first cylindrical holder is located
180.degree. apart from the sensor signal output terminal of said
second cylindrical holder.
25. A gas sensor as set forth in claim 23, wherein said slit is
located substantially 90.degree. apart from the sensor signal
output terminal of said first cylindrical holder.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field of the Invention
[0002] The present invention relates generally to a gas sensor
which may be employed in an air-fuel ratio control system for
automotive vehicles for measuring the concentration of gas such
O.sub.2, NOx, or CO, and more particularly to an improved structure
of a ceramic heater used in gas sensors and a manufacturing method
thereof.
[0003] 2. Background Art
[0004] FIGS. 1(a) and 1(b) show one example of conventional ceramic
heaters which is built in an oxygen sensor for use in air-fuel
ratio control of automotive internal combustion engines. The
ceramic heater 9 serves to heat a sensor element up to an elevated
temperature to minimize a variation in measured value.
[0005] The ceramic heater 9 consists of a ceramic square rod 10
made of a laminate of heater substrates and a covering substrate
and metallic terminals 3 mounted on side surfaces 15 of the rod 10.
The metallic terminals 3 connect electrically with leads of a
heater-patterned layer in the rod 10 and joined to outer leads 4
through solders 5, respectively.
[0006] In manufacturing the ceramic heater 9, green sheets 101 and
102, as shown in FIG. 2(a), whose main component is alumina are
first prepared. Next, a conductive paste is applied to the surface
of each of the green sheets 101 to form a heater-patterned layer 2
consisting of pairs of a heater element 21 and a lead 22. The two
green sheets 101 and the covering green sheet 102 are laid to
overlap each other to form a three-layer laminate. The three-layer
laminate is cut into several pieces as shown in FIG. 2(b). The
metallic terminals 3 are formed on the side surfaces 15 of each
piece which communicate electrically with the leads 22 to make an
intermediate. Subsequently, the intermediate is baked, after which
the outer leads 4 is, as shown in FIG. 2(c), welded to the metallic
terminals 3 through the solder 5. Finally, welded portions of the
outer leads 4 are, as indicated at numeral 6 in FIG. 1(b), plated
with Ni to make the ceramic heater 9.
[0007] The above ceramic heater 9 and the manufacturing method
thereof, however, have the following drawbacks.
[0008] The metallic terminals 3 are, as described above, mounted on
the side surfaces 15 of the ceramic heater 9. It is, thus, only
possible to attach the metallic terminals 3 to the square rod 10
after the three-layer laminate is cut as shown in FIG. 2(b). In
other words, a large number of terminal attachment processes are
required in mass-production of ceramic heaters.
[0009] In addition, the performance of the ceramic heater 9 is
usually inspected after the outer leads 4 are mounted thereon. A
large number of individual inspections are also required in the
mass-production of ceramic heaters, thus resulting in an increase
in manufacturing cost.
[0010] Another problem is also encountered in that the ceramic
heater 9 is lower in durability than a round rod heater 91 as shown
in FIG. 3(a). The results of heat cycle tests show that portions of
the ceramic heater 9 welded to the outer leads 4 and the metallic
terminals 3 tend to be cracked as compared with the round rod
heater 91. This is because the angle .beta. which each of the
metallic terminals 3 of the ceramic heater 9, as shown in FIG. 4,
makes with the outer surface of the solder 5 is greater than the
angle .alpha. which each of the metallic terminals 3 of the round
rod heater 91, as shown in FIG. 3(b), makes with the outer surface
of the solder 5. The difference between the angles .alpha. and
.beta. depends upon the geometry of the heaters 9 and 91 and thus
is difficult to eliminate. The use of solder which is soft enough
to absorb internal stress ensures substantially the same durability
of the portions of the rod 10 welded to the leads 4 as that of the
round rod heater 91, however, square rod heaters exhibiting higher
durability even in use of harder solder is sought.
SUMMARY OF THE INVENTION
[0011] It is therefore a principal object of the present invention
to avoid the disadvantages of the prior art.
[0012] It is another object of the present invention to provide an
easy-to-manufacture ceramic heater used in gas sensors which has a
high durability and a manufacturing method thereof.
[0013] According to one aspect of the invention, there is provided
a ceramic heater which may be employed in an air-fuel ratio control
system for automotive vehicles for measuring the concentration of
gas such O.sub.2, NOx, or CO. The ceramic heater comprises: (a) a
ceramic square rod formed with a laminate of a heater substrate on
which a heater-patterned layer consisting of a heater element and
leads connected to the heater element is formed and a covering
substrate covering the heater-patterned layer of the heater
substrate; (b) metallic terminals connected electrically to the
leads of the heater-patterned layer of the heater substrate,
respectively, the metallic terminals being mounted on surfaces of
the ceramic square rod opposed to each other in a direction of
lamination of the heater substrate and the covering substrate,
respectively; and (c) at least one outer lead joined to one of the
metallic terminals through a bonding layer.
[0014] In the preferred mode of the invention, a second outer lead
is further joined to the other metallic terminal through a bonding
layer.
[0015] The metallic terminals are electrically connected to the
leads through holes formed in at least one of the covering
substrate and the heater substrate.
[0016] Each of the metallic terminals is mounted on an area inside
edges of the surface of the ceramic square rod.
[0017] The bonding layer occupies an area of a surface of the
metallic terminal inside edges of the metallic terminal.
[0018] The one of the metallic terminals contains 70 Wt % of W or
more. The bonding layer contains 40 to 98 Wt % of Cu and 2 to 20 Wt
% of Ni.
[0019] The bonding layer may contain 60 Wt % of Au or less.
[0020] An Ni-plated layer may be formed on the one of the metallic
terminals, having a thickness of 3 .mu.m or less. The outer lead is
joined to the Ni-plated layer through the bonding layer.
[0021] According to the second aspect of the invention, there is
provided a ceramic heater. The ceramic heater comprises: (a) a
ceramic square rod formed with a laminate of heater substrates each
having formed thereon a heater-patterned layer consisting of a
heater element and first and second leads connected to the heater
element and a covering substrate interposed between the heater
substrates; (b) first and second metallic terminals connected
electrically to the first and second leads of the heater-patterned
layers of the heater substrates, respectively, the metallic
terminals being mounted on surfaces of the ceramic square rod
opposed to each other in a direction of lamination of the heater
substrates and the covering substrate; and (c) outer leads joined
to the first and second metallic terminals through bonding layers,
respectively.
[0022] In the preferred mode of the invention, the first metallic
terminal is connected to the first leads of the heater substrates
through conductive material-coated holes formed in the covering
substrate and one of the heater substrates. The second metallic
terminal is connected to the second leads of the heater substrates
through conductive material-coated holes formed in the covering
substrate and the other heater substrate.
[0023] Each of the bonding layers occupies an area of a surface of
one of the metallic terminals inside edges of the metallic
terminal.
[0024] Each of the metallic terminals contains 70 Wt % of W or
more. Each of the bonding layers contains 40 to 98 Wt % of Cu and 2
to 20 Wt % of Ni.
[0025] Each of the bonding layers contains 60 Wt % of Au or
less.
[0026] An Ni-plated layer formed on each of the metallic terminals,
having a thickness of 3 .mu.m or less. The outer leads are joined
to the Ni-plated layers through the bonding layers.
[0027] According to the third aspect of the invention, there is
provided a method of manufacturing ceramic heaters which comprises
the steps of: (a) preparing a first green sheet; (b) preparing a
second green sheet; (c) printing a first surface of the second
green sheet an array of heater-patterned layers each consisting of
a heater element and leads connected to the heater element; (d)
printing a second surface of the second green sheet opposite the
first surface with an array of metallic terminals; (e) attaching
the first green sheet to the second green sheet so as to cover the
first surface of the second green sheet to form a laminate; (f)
baking the laminate to form a ceramic board; (g) joining outer
leads to the metallic terminals through bonding layers,
respectively; and (h) cutting the ceramic board into a plurality of
square rods constituting units of the ceramic heaters.
[0028] In the preferred mode of the invention, a step is further
provided which forms through holes in the first green sheet for
electrical connections of the leads of the heater-patterned layers
and the metallic terminals.
[0029] A step is further provided which forms grooves in a surface
of the ceramic board between adjacent two of the units of the
ceramic heaters to be cut by the cutting step.
[0030] According to the fourth aspect of the invention, there is
provided a method of manufacturing ceramic heaters which comprises
the steps of: (a) preparing a first green sheet; (b) preparing
second green sheets; (c) printing a first surface of each of the
second green sheets an array of heater-patterned layers each
consisting of a heater element and leads connected to the heater
element; (d) printing a second surface of each of the second green
sheets opposite the first surface with an array of metallic
terminals; (e) interposing the first green sheet between the second
green sheets so as to cover the first surfaces of the second green
sheets to form a laminate; (f) baking the laminate to form a
ceramic board; (g) joining outer leads to the metallic terminals
formed on at least one of the second green sheets through bonding
layers, respectively; and (h) cutting the ceramic board into a
plurality of square rods constituting units of the ceramic
heaters.
[0031] According to the fifth aspect of the invention, there is
provided a gas sensor which comprises: (a) a gas sensing element
having a gas-exposed portion, the gas sensing element having formed
therein a chamber; (b) a ceramic heater disposed within the chamber
of the gas sensing element to heat the gas sensing element; (c) a
first cylindrical holder fitted in the chamber of the gas sensing
element, the first holder including a heater holding portion for
holding the ceramic heater and a sensor contact in contact with an
inner wall of the gas sensing element, the sensor contact having a
sensor signal output terminal; (d) a second cylindrical holder
mounted on an outer wall of the gas sensing element, having a
sensor signal output terminal; and (e) a slit formed in the first
holder to define a C-shaped cross section, the slit being located
90.degree..+-.20.degree. apart from the sensor signal output
terminal of the first cylindrical holder. The ceramic heater
includes, (a) a ceramic square rod formed with a laminate of a
heater substrate on which a heater-patterned layer consisting of a
heater element and leads connected to the heater element is formed
and a covering substrate covering the heater-patterned layer of the
heater substrate, (b) metallic terminals connected electrically to
the leads of the heater-patterned layer of the heater substrate,
respectively, the metallic terminals being mounted on surfaces of
the ceramic square rod opposed to each other in a direction of
lamination of the heater substrate and the covering substrate,
respectively, and (c) at least one outer lead joined to one of the
metallic terminals through a bonding layer.
[0032] In the preferred mode of the invention, the sensor signal
output terminal of the first cylindrical holder is located
180.degree. apart from the sensor signal output terminal of the
second cylindrical holder.
[0033] The slit is located 90.degree. apart from the sensor signal
output terminal of the first cylindrical holder.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] The present invention will be understood more fully from the
detailed description given hereinbelow and from the accompanying
drawings of the preferred embodiments of the invention, which,
however, should not be taken to limit the invention to the specific
embodiments but are for the purpose of explanation and
understanding only.
[0035] In the drawings:
[0036] FIG. 1(a) is a perspective view which shows a conventional
ceramic heater;
[0037] FIG. 1(b) is a cross sectional view taken along the line A-A
in FIG. 1(a);
[0038] FIGS. 2(a), 2(b), and 2(c) are perspective views which show
a sequence of manufacturing processes of a conventional ceramic
heater;
[0039] FIG. 3(a) is a perspective view which shows a conventional
ceramic heater made of a round bar;
[0040] FIG. 3(b) is a cross sectional view taken along the line B-B
in FIG. 3(a);
[0041] FIG. 4 is a sectional view which shows a welded angle of an
outer surface of an end of a bonding layer with a metallic
terminal;
[0042] FIG. 5(a) is a perspective view which shows a ceramic heater
according to the invention;
[0043] FIG. 5(b) is a sectional view taken along the line C-C in
FIG. 5(a);
[0044] FIG. 6 is an exploded view which shows the ceramic heater in
FIG. 5(a);
[0045] FIGS. 7(a), 7(b), and 7(c) are perspective views which show
a sequence of manufacturing processes of a ceramic heater;
[0046] FIGS. 8(a) and 8(b) show modifications of an outer lead
connected to a ceramic heater;
[0047] FIG. 9 is a graph which shows the relation between the
hardness of solder and a component ratio of Au to Cu of the
solder;
[0048] FIG. 10 shows the second embodiment of the manufacturing
processes of the ceramic heater 1.
[0049] FIGS. 11(a) and 11(b) show manners to measure the surface
roughness of a metallic terminal of the invention and a
conventional metallic terminal;
[0050] FIG. 12 is a vertical sectional view which shows an oxygen
sensor in which the ceramic heater shown in FIGS. 5(a) and 5(b) is
built;
[0051] FIG. 13(a) is a perspective view which shows a minus holder
for holding a gas sensing element;
[0052] FIG. 13(b) is a perspective view which shows a plus holder
for holding a ceramic heater;
[0053] FIG. 14(a) is a plan view of a plus holder;
[0054] FIGS. 14(b) and 14(c) are side views of the plus holder in
FIG. 14(a);
[0055] FIGS. 15(a) and 15(b) are side views of a plus holder in
which a ceramic heater is fitted;
[0056] FIG. 16 is a plan view which shows a plus holder in which a
ceramic heater is fitted;
[0057] FIG. 17 is a plan view which shows a plus holder holding
therein a ceramic holder fitted in a gas sensing element and a
minus holder;
[0058] FIGS. 18(a) and 18(b) are plan views which a comparative
example; and
[0059] FIG. 19 is a sectional view which shows a welded angle of an
outer surface of an end of a bonding layer with a metallic
terminal.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0060] Referring now to the drawings, wherein like numbers refer to
like parts in several views, particularly to FIG. 5(a) and 5(b),
there is shown a ceramic heater 1 of an oxygen sensor according to
the invention which is employed in automotive air-fuel ratio
control systems to measure an oxygen content in exhaust gasses of
an internal combustion engine. Note that the present invention is
not limited to an oxygen sensor and may alternatively be used with
a variety of gas sensors such as HC, CO, and NOx sensors.
[0061] The ceramic heater 1 includes a ceramic square rod 10 which
is, as clearly shown in FIG. 6, made of a laminate of two heater
substrates 1 1 and a covering substrate 12. Each of the heater
substrate 11 has formed thereon a heater-patterned layer 2
consisting of a heater element 21 and leads 22 connected to the
heater element 21. The covering substrate 12 is interposed between
the heater substrates 11 to cover the heater-patterned layers
2.
[0062] The ceramic heater 1 also includes a pair of metallic
terminals 3 which are attached to upper and lower surfaces 17 and
18, as viewed in FIGS. 5(a) and 5(b), of the heater substrates 11
and which are electrically connected to the leads 22. Outer leads 4
are welded to the terminals 3 through bonding layers 5,
respectively.
[0063] The covering substrate 12 and the heater substrates 11, as
clearly shown in FIGS. 5(b) and 6, have conductive material-coated
through holes 71, 72, 73, and 74, respectively, to establish
electrical communication between the heater-patterned layers 2 of
the heater substrates 1 1 and the metallic terminals 3.
[0064] The metallic terminals 3 are, as clearly shown in FIG. 5(b),
disposed on flat portions of the surfaces 17 and 18 of the heater
substrates 11 so that side ends 31 thereof may be located inside
side edges 171 and 181 of the heater substrates 11,
respectively.
[0065] The bonding layers 5 are formed with solder made of, for
example, Cu/Si, Cu/Au, or Cu/Ni material and, as can be seen in
FIG. 5(b), formed on flat surfaces of the terminals 3 so that side
edges 51 thereof may be located inside the side ends 31 of the
terminals 3.
[0066] The ceramic square rod 10 has, as shown in FIG. 6, an
overall length L of 54 mm, an overall width W of 2.9 mm, and a
thickness T of 1.6 mm (see FIG. 5(a)). The length C of the heater
element 21 of each of the heater-patterned layers 2 is 9 mm. The
length D of each of the leads 22 is 42 mm.
[0067] The leads 22 formed on each of the heater substrates 11
extend in parallel at an interval F2 of 0.228 mm away from each
other. Each of the leads 22 is disposed at an interval F1 of 0.25
mm away from the side of the heater substrate 11 and at an interval
F3 of 1 mm away from a rear end of the heater substrate 11.
[0068] The through holes 71 to 74 are arrayed with a pitch P1 of
3.6 mm in a lengthwise direction of the heater substrate 11 and a
pitch P2 of 1.4 mm in a widthwise direction of the heater substrate
11 and have a diameter of 3 mm. The metallic terminals 3 each have
a length E1 of 5.5 mm and a width E2 of 2.3 mm.
[0069] A sequence of manufacturing processes of the ceramic heater
1 will be discussed below with reference to FIGS. 7(a), 7(b), and
7(c).
[0070] A powdered raw material containing about 92 Wt % of
Al.sub.2O.sub.3 and a total of about 8 Wt % of SiO.sub.2, CaO, and
MgO is first prepared to make slurry.
[0071] Next, a green sheet is formed with the slurry using the
doctor blade and then punched by a punch press to form green sheets
101 measuring 120 mm.times.120 mm for making the heater substrates
11 and a green sheet 102 measuring 120 mm.times.120 mm for making
the covering substrate 12. The through holes 71 to 74 are formed in
the green sheets 101 and 102.
[0072] The making of the green sheets 101 and 102 may alternatively
be achieved with the extrusion molding.
[0073] A conductive paste whose main constituent is metal such as W
or Mo is prepared and coated on surfaces of the green sheets 101 to
form heater-patterned layers 2, as shown in FIG. 7(a), and inner
walls of the through holes 71 to 74 using printing techniques. The
heater-patterned layers extend parallel to each other.
[0074] On a surface of each of the green sheets 101 opposite to the
heater-patterned layers 2, a conductive paste is coated to form the
metallic terminals 3 in line using printing techniques. The
conductive paste is made of a main constituent of metal containing
70 Wt % or more of W and a remaining content of Mo, but may be
identical with that used in forming the heater-patterned layers
2.
[0075] The two green sheets 101 are arranged so that the
heater-patterned layers 2 may face each other. Subsequently, the
green sheet 102 is interposed between the green sheets 101 to form
a three-layer laminate. The three-layer laminate is baked at 1400
to 1600.degree. C. in a reducing atmosphere of N.sub.2 and H.sub.2
gasses to make an intermediate.
[0076] The outer leads 4 are, as shown in FIG. 7(b), soldered to
the metallic terminals 3, respectively. The soldering is achieved
by placing solder and the outer leads 4 on the metallic terminals 3
and heating them at 1000 to 1200.degree. C. to form the bonding
layers 5.
[0077] Each of the outer leads 4 may be made either of a round bar,
as shown in FIG. 7(b), or of a square bar, as shown in FIGS. 8(b)
and 8(b).
[0078] The overall surface of each of the bonding layers 5 is, as
clearly shown in FIG. 5(b), covered with an Ni-plated layer 6.
[0079] The intermediate is, as shown in FIG. 7(c), cut into several
pieces, i.e., units of the ceramic heaters 1.
[0080] Finally, an end of each of the ceramic heaters 1 opposite to
the outer leads 4 is rounded using a grinding machine.
[0081] Note that after the three-layer laminate is braked, the
intermediate is tested for heater performance.
[0082] Each of the bonding layers 5 may contain 40 to 98 Wt % of Cu
and 2 to 20 Wt % of Ni. The metallic terminals 3, as described
above, contains W, thus resulting in improved wettability between
the bonding layers 5 and the metallic terminals 3, which eliminates
the need for the metallic terminals 3 to be plated with Ni in
conventional manufacturing processes.
[0083] When the content of Cu in the bonding layers 5 is small,
less than 40 Wt % and when the leads 4 do not contain Ni, it will
cause no Ni to be diffused from the leads 4 to the bonding layers
5, so that the content of Ni in the bonding layers 5 will be
smaller than that when the content of Cu is more than 40 Wt %,
which results in lowered wettability of the bonding layers 5 to the
metallic terminals 3 and a decrease in strength of joints of the
bonding layers 5 and the metallic terminals 3.
[0084] When the content of Cu in the bonding layers 5 is greater
than 98 Wt %, the content of Ni in the bonding layers 5 will be
smaller than that in the metallic terminals 3, thereby causing the
wettability of the bonding layers 5 to the metallic terminals 3 to
be lowered, which results in a decrease in strength of the joints
of the bonding layers 5 and the metallic terminals 3.
[0085] When the content of Ni in the bonding layers 5 is less than
2 Wt %, it will cause the wettability of the bonding layers 5 to
the metallic terminals 3 to be lowered, resulting in a decrease in
strength of the joints of the bonding layers 5 and the metallic
terminals 3. Alternatively, when the content of Ni in the bonding
layers 5 greater than 20 Wt %, it will cause a W--Ni intermetallic
compound to be precipitated during manufacture, resulting in a
decrease in strength of joints of the bonding layers 5 and the
metallic terminals 3.
[0086] The metallic terminals 3 contain, as described above, 70 Wt
% of W or more (including 100 Wt % of W) and thus have good
conformability to a ceramic particularly containing alumina (i.e.,
the square rod 10 of the ceramic heater 1) and good heat
resistance. When the content of W is less than 70 Wt %, it may
result in decreases in strength of a joint of the metallic
terminals 3 and the square rod 10 and heat resistance.
[0087] The bonding layers 5 may contain 60 Wt % of Au or less for
avoiding precipitation of a W--Ni intermetallic compound to
increase the strength to join the leads 4 to the metallic terminals
3. When the content of Au in the bonding layers 5 is more than 60
Wt % the content of Cu will be decreased. Thus, when the leads 4 do
not contain Ni, it will cause no Ni to be diffused from the leads 4
to the bonding layers 5, so that the content of Ni in the bonding
layers 5 will be smaller than that when the content of Au is less
than 60 Wt %, which results in lowered wettability of the bonding
layers 5 to the metallic terminals 3 and a decrease in strength of
joints of the bonding layers 5 to the metallic terminals 3.
Specifically, when the content of Au is, as shown in FIG. 9, 60 to
90 Wt %, the hardness of the solder forming the bonding layers 5
becomes too high, thus resulting in a decrease in durability
against cyclic changes in ambient temperature. When the content of
Au is greater than 90 Wt %, the hardness of the solder is lower,
but manufacturing costs will increase.
[0088] A major surface of each of the metallic terminals 3 to which
the leads 4 are to be joined through the bonding layer 5 may be
plated with Ni. The thickness of the Ni-plated layer is 3 or less
.mu.m. The formation of the Ni-plated layer improves the
wettability of the bonding layer 5, thereby decreasing the welded
angle which the outer surface of each side end of the bonding layer
5 makes with the metallic terminal 3, resulting in a decrease in
thermal stress contributing to cracks. When the thickness of the
Ni-plated layer is more than 3 .mu.m, a metallic alloy will be
produced between the Ni-plated layer and the metallic terminal 3
which decreases the strength to join the bonding layer 5 and the
metallic terminal 3.
[0089] The laminate produced in the process shown in FIG. 7(a) may
consist only of the single green sheet 101 and the green sheet 102.
In this case, the metallic terminals 3 are also formed on a surface
of the green sheet 102 opposite to a surface covering the
heater-patterned layers 2 of the green sheet 101.
[0090] As can be seen from the above discussion, the metallic
terminals 3 and the outer leads 4 are disposed on the surfaces 17
and 18 of the square rod 10 opposed in a direction of lamination of
the substrates 11 and 12, thereby allowing the joining process
wherein the outer leads 4 are joined to the metallic terminals 3,
respectively, to be performed before the intermediate is cut into
units of the ceramic heaters 1 in the course of manufacture. This
will result in great rationalization of the manufacturing
processes.
[0091] In addition, the performance test may be, as described
above, performed before the intermediate is cut into unit of the
ceramic heaters 1, thus resulting in rationalization of procedure
of the test.
[0092] The metallic terminals 3 and the bonding layers 5 are, as
described above, arranged on the surfaces 17 and 18 of the square
rod 10 out of alignment of side ends with each other, thus avoiding
concentration of stress on the side edges 171, 181, 31, and 51,
which will result in improved durability of the ceramic heater
1.
[0093] One of the metallic terminals 3 of the ceramic heater 1 may
be connected directly to a connector leading to, for example, a
ground terminal without use of the outer lead 4. In this case, the
single outer lead 4 may be joined to either of the metallic
terminals 3.
[0094] FIG. 10 shows the second embodiment of the manufacturing
processes of the ceramic heater 1.
[0095] Before the three-layer laminate of the green sheets 101 and
102 is braked, cutting notches or grooves 7 are machined in upper
and lower surfaces of the three-layer laminate which extend
parallel between adjacent two of the metallic terminals 3 for
facilitating ease of cutting the three-layer laminate into units of
the ceramic heaters 1 after being baked.
[0096] The formation of the cutting grooves 7 is achieved by
grooving the upper and lower surfaces of the three-layer laminate
to a depth less than half a thickness of the laminate using a
cutting machine.
[0097] Other manufacturing processes are identical with those of
the first embodiment, and explanation thereof in detail will be
omitted here.
[0098] Ten samples of the ceramic heater 1 made in the
manufacturing processes of the first embodiment were tested for the
surface roughness of the metallic terminals 3 which may be thought
of as one of factors of improvement of durability. The measurement
of the surface roughness was accomplished, as shown in FIG. 11(a),
by scanning the surface of the metallic terminal 3 of each sample
over 0.8 mm in a direction, as indicated by S in FIG. 11(a). For
comparison, the same tests were performed, as shown in FIG. 11(b),
for ten conventional ceramic heaters identical with the one shown
in FIGS. 1(a) and 1(b). The results of the tests are shown in table
1 below.
1TABLE 1 Sample No. Prior art (.mu.m) Invention (.mu.m) 1 3.642
1.481 2 3.932 1.098 3 2.47 1.018 4 3.782 0.978 5 3.146 1.294 6
2.858 1.893 7 3.431 1.149 8 3.278 1.19 9 2.685 1.435 10 2.891 1.215
Average 3.212 1.275
[0099] The table 1 shows that the surface roughness (Rz) of the
metallic terminals 3 of the ceramic heater 1 is greatly improved as
compared with the conventional ceramic heaters. The improvement of
the surface roughness of the metallic terminals will facilitate
flow of solder on the surfaces of the metallic terminals 3 when the
outer leads 4 are joined to the metallic terminals 3, thereby
increasing an area of the bonding layers 5, which results in
improvement of initial strength to join the outer leads 4 to the
metallic terminals 3 and a decrease in thermal stress acting on the
joints produced by cyclic temperature changes, thus improving the
durability of the ceramic heater 1.
[0100] FIG. 12 shows an oxygen sensor 8 in which the ceramic heater
1 is built.
[0101] The oxygen sensor 8 is used in an automotive internal
combustion engine control system and includes a gas sensing element
81 with a gas-exposed portion 811 exposed to the gas to be
measured.
[0102] The gas sensing element 81 is of a cup-shape having formed
therein an inner chamber 810. Within the inner chamber 180, the
ceramic heater 1 is disposed for heating the gas sensing element
81.
[0103] On outer and inner surfaces of the gas sensing element 81,
minus and plus holders 86 and 87 are installed which have sensor
signal output terminals 869 and 879, respectively. The pulse holder
87 includes, as shown in FIGS. 13(b) and 14(a) to 14(c), a heater
holding portion 871 for holding the ceramic heater 1 and a sensor
contact 873 for making contact with the inner surface of the gas
sensing element 81. The sensor signal output terminal 879 extends
from an end of the sensor contact 873. The heater holding portion
871 and the sensor contact 873 have formed therein slits 877 and
878 to define C-shape in section so that they may be elastically
deformable to have spring properties. The slits 877 and 878 extend
in a lengthwise direction of the pulse holder 87 and are shifted
approximately 90.degree. away from each other.
[0104] The heater holding portion 871 and the sensor contact 873
are joined through a frusto-conical connector 872. The connector
872 has formed therein an L-shaped slit which connects the slits
877 and 878. The heater holding portion 871 and the sensor contact
873 are eccentric so that the ceramic heater 1 may be coaxial with
the gas sensing element 81 when the plus holder 87 is fitted in the
gas sensing element 81.
[0105] The slit 878 formed in the sensor contact 873 is, as can be
seen in FIG. 13(b), diametrically opposed to the sensor signal
output terminal 879 and thus is located at an angular interval of
90.degree. away from the slit 877 formed in the heater holding
portion 871.
[0106] The sensor contact 873 has formed on the end thereof a
plurality of claws 874 which engage an upper end of the gas sensing
element 81 for orientation to the gas sensing element 81.
[0107] The sensor contact 873 has an outer diameter slightly
greater than an inner diameter of the gas sensing element 81 so
that the sensor contact 873 may be installed elastically within the
gas sensing element 81 by a press fit. The heater holding portion
871 has an inner diameter slightly smaller than a maximum outer
diameter of the ceramic heater 1 for establishing tight engagement
with the ceramic heater 1 when fitted in the heater holding portion
871.
[0108] The minus holder 86, as clearly shown in FIG. 13(a), has
formed therein a slit to have spring properties like the plus
holder 87. In order to enhance the spring properties, the plus
holder 87 and the minus holder 86 are both made of a heat resisting
spring steel.
[0109] FIGS. 15(a), 15(b), and 16 show the plus holder 87 in which
the ceramic heater 1 is fitted.
[0110] As clearly shown in FIG. 16, the ceramic heater 1 is
disposed in the plus holder 87 with one of the surfaces on which
the outer leads 4 are installed facing the slit 877 so that the
outer leads 4 may be both located 90.degree. apart from the sensor
signal output terminal 879.
[0111] FIG. 17 shows the plus holder 87 holding therein the ceramic
holder I fitted in the gas sensing element 81 and the minus holder
86 installed on the outer surface of the gas sensing element 81.
The sensor signal output terminal 869 of the minus holder 86 is
located approximately 180.degree. away from the sensor signal
output terminal 879 of the plus holder 87. The sensor signal output
terminals 869 and 879 are, therefore, arranged at angular intervals
90.degree. away from the outer leads 4, respectively.
[0112] The gas sensing element 81 has, as shown in FIG. 12, a
reference gas chamber 812 formed in the inner chamber 810 and
defines a gas chamber 813 between itself and a protective cover
assembly 82. An outer electrode 815 and an inner electrode 814 both
made of platinum are installed on the gas-exposed portion 811 and
the inner surface of the gas sensing element 81 in connection with
the minus holder 815 and the plus holder 87, respectively.
[0113] The sensor signal output terminals 869 and 879 of the
holders 86 and 87 and the leads 4 of the ceramic heater 1 are
electrically connected to four leads 891 to 893, respectively,
through connectors 995 and 896. The connectors 995 and 895 are
disposed in an insulator 85 at regular intervals of 90.degree. for
avoiding interference with each other.
[0114] The gas sensing element 81 is installed in a sensor mount 84
which is used in mounting the oxygen sensor 8 in an exhaust pipe of
an automotive engine. The protective cover assembly 82 is mounted
on an end of the sensor mount 84 to cover the gas sensing element
81. A dust cover 83 is mounted on the sensor mount 84.
[0115] The sensor mount 84 has a cylindrical wall which extends
upward from the flange thereof and in which an insulator 881, a
talc 882, and a ring spacer 883 are disposed to retain the gas
sensing element in the sensor mount 84. An end 841 of the
cylindrical wall of the sensor mount 84 is crimped inward to
elastically press the ring spacer 883 downward, as viewed in FIG.
12. A float packing 884 is interposed between an inner wall of the
sensor mount 84 and an outer wall of the gas sensing element 81 to
seal the gas chamber 813 hermetically.
[0116] The sensor mount 84 has formed in the end 842 thereof an
annular groove 843 to form an outer skirt 844 and an inner skirt
845. The protective cover assembly 82 consists of an outer cover
821 and an inner cover 822 both made of a cup-shaped member. The
outer and inner covers 821 and 822 have flanges 828 and 829 which
are retained in the groove 843 of the sensor mount 84 by crimping
the outer skirt 844 inward. The outer and inner covers 821 and 822
have formed in side walls thereof a plurality of holes through
which a gas to be measured passes to enter the gas chamber 813.
[0117] The dust cover 83, as shown in FIG. 12, consists of a
small-diameter cylinder 831, a large-diameter cylinder 832, and a
shoulder portion 833 connecting the cylinders 831 and 832. The dust
cover 83 is, as described above, welded at a circumferential
portion 834 thereof to a boss of the sensor mount 84 and retains
therein the insulator 85.
[0118] A cylindrical cover 839 is mounted on the periphery of the
small-diameter cylinder 831 of the dust cover 83 by crimping. A
water-repellent filter 857 is installed between the cylindrical
cover 839 and the small-diameter cylinder 831. The cover 839 and
the dust cover 83 have formed therein first air vents 858 and
second air vents 859, respectively, which communicate with the
reference gas chamber 812 formed in the gas sensing element 81 to
fill the reference gas chamber 812 with air.
[0119] A heat-resisting rubber bush 895 is mounted in the end of
the small-diameter cylinder 831 of the dust cover 83 to retain the
leads 891 to 893 at angular intervals of 90.degree..
[0120] The insulator 85 consists of a sleeve 851 in which the leads
891 to 893 are disposed and a flange 852 greater in diameter than
the sleeve 851. The small-diameter cylinder 831 of the dust cover
83 has the inner diameter greater than the outer diameter of the
sleeve 851 of the insulator 85 and smaller than the outer diameter
of the flange 852. The large-diameter cylinder 832 of the dust
cover 83 has the inner diameter greater than the outer diameter of
the flange 852 of the insulator 85.
[0121] The insulator 85 is retained in the large-diameter cylinder
832 of the dust cover 83 in engagement of an upper end of the
flange 852 with the shoulder portion 833 of the dust cover 83 by a
stop ring 899 press-fitted in the large-diameter cylinder 832.
[0122] The gas sensing element 81 produces the electromotive force
as a function of a difference in oxygen concentration between the
air in the reference gas chamber 812 and the gas in the gas chamber
813 and outputs a signal indicative thereof through the leads 891
and 892. The operation of the oxygen sensor 8 is well known in the
art, and explanation thereof in detail will be omitted here.
[0123] The operation and effects of this embodiment will be
described below.
[0124] The four connectors 896 and 995 are disposed in an insulator
85 at regular intervals of 90.degree. for avoiding interference
with each other. The sensor signal output terminals 879 and 869 of
the holders 86 and 87 and the leads 4 of the ceramic heater 1 are,
therefore, located at regular intervals of 90.degree. away from
each other.
[0125] The sensor signal output terminal 879 installed on the
sensor contact 873 of the plus holder 87 is, as described above,
located approximately 90.degree. away from the slit 877 formed in
the heater holding portion 871, thereby allowing the ceramic heater
1 to be, as shown in FIGS. 16 and 17, fitted firmly in the heater
holding portion 871 of the plus holder 87 so that the leads 4 of
the ceramic heater 1 may be located at angular intervals of 90 away
from the sensor signal output terminal 879.
[0126] For comparison with this embodiment, a plus holder 97 used
in conventional oxygen sensors is shown in FIGS. 18(a) and 18(b).
The plus holder 97 has a slit 977 formed in a heater holding
portion 971 at an angular interval of 180.degree. away from a
sensor signal output terminal 979. The slit 977 is located at the
same angular position as that of a slit 978 formed in a sensor
contact 973 of the plus holder 97. Arranging the leads 4 of the
ceramic heater 1 90.degree. apart from the sensor signal output
terminal 979 requires, as shown in FIG. 18(a), retaining side walls
of the ceramic heater 1 between vertical edges 999 and an opposite
inner wall of the heater holding portion 971 defining the slit 977,
thus resulting in instability of installation of the ceramic heater
1.
[0127] The stable installation of the ceramic heater 1 in the plus
holder 97 requires, as shown in FIG. 18(b), retaining the side
walls of the ceramic heater 1 between opposite portions of the
inner wall of the plus holder 97 located 90.degree. apart from the
slit 977. In this case, the leads 4 are oriented in alignment with
the sensor signal output terminal 979, so that they are twisted
undesirably when connected to the connectors 896 and 995.
[0128] The structure of this embodiment allows, as described above,
the leads 4 of the ceramic heater 1 to be located 90.degree. apart
from the sensor signal output terminal 879 without compromising the
installation of the ceramic heater 1 in the plus holder 87.
[0129] The positional relation between the sensor signal output
terminal 879 of the sensor contact 973 and the slit 877 of the
heater holding portion of the plus holder 87 is not limited to
90.degree., but may be within an angular range of
90.degree..+-.20.degree.. This also achieves firm installation of
the ceramic heater 1 in the plus holder 87 without interfering the
connectors 896 and 995 with each other.
[0130] The inventors of this application analyzed the relation
between the durability of the ceramic heater 1 and a welded angle
which the outer surface of each side end of the bonding layer 5
makes with the metallic plate 3. The analysis was made by preparing
samples whose welded angles .gamma., as shown in FIG. 19, are
25.degree. to 60.degree. and performing a temperature cycle test a
hundred times in which each sample was subjected to intense heat at
450.degree. C. for four minutes and then left at room temperature
for four minutes. After the hundred temperature cycle tests, each
metallic terminals 3 was checked for cracks, and the strength of a
joint of the bonding layer 5 and the metallic terminal 3 was
measured. The measurement of the strength was performed in tensile
tests. The results of the tests are shown in table 2 below.
2TABLE 2 Welded Angle Joint Strength .gamma. Cracks (kgf)
Evaluation 60.degree. many 1 or less X 50.degree. many 1 or less X
40.degree. few 3 .DELTA. 30.degree. few 4 .largecircle. 20.degree.
few 4.5 .largecircle.
[0131] where .largecircle. indicates excellent durability, .DELTA.
indicates allowable durability, and X indicates lack of
durability.
[0132] The table 2 shows that the ceramic heater 1 has high
durability when the welded angle .gamma. is 40.degree. or less.
[0133] While the present invention has been disclosed in terms of
the preferred embodiments in order to facilitate better
understanding thereof, it should be appreciated that the invention
can be embodied in various ways without departing from the
principle of the invention. Therefore, the invention should be
understood to include all possible embodiments and modifications to
the shown embodiments which can be embodied without departing from
the principle of the invention as set forth in the appended
claims.
* * * * *