Electric heating – Heating devices – With heating unit structure
Reexamination Certificate
2001-02-15
2002-01-22
Hoang, Tu Ba (Department: 3742)
Electric heating
Heating devices
With heating unit structure
C219S548000, C219S552000
Reexamination Certificate
active
06340809
ABSTRACT:
BACKGROUND OF THE INVENTION
1 Technical Field of the Invention
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
2
, NOx, or CO, and more particularly to an improved structure of a ceramic heater used in gas sensors and a manufacturing method thereof.
2 Background Art
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.
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.
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
.
The above ceramic heater
9
and the manufacturing method thereof, however, have the following drawbacks.
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.
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.
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 &bgr; 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 &agr; 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 &agr; and &bgr; 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
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
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.
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
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.
In the preferred mode of the invention, a second outer lead is further joined to the other metallic terminal through a bonding layer.
The metallic terminals are electrically connected to the leads through holes formed in at least one of the covering substrate and the heater substrate.
Each of the metallic terminals is mounted on an area inside edges of the surface of the ceramic square rod.
The bonding layer occupies an area of a surface of the metallic terminal inside edges of the metallic terminal.
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.
The bonding layer may contain 60 Wt % of Au or less.
An Ni-plated layer may be formed on the one of the metallic terminals, having a thickness of 3 &mgr;m or less. The outer lead is joined to the Ni-plated layer through the bonding layer.
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 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.
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.
Each of the bonding layers occupies an area of a surface of one of the metallic terminals inside edges of the metallic terminal.
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.
Each of the bonding layers contains 60 Wt % of Au or less.
An Ni-plated layer formed on each of the metallic terminals, having a thickness of 3 &mgr;m or less. The outer leads are joined to the Ni-plated layers through the bonding layers.
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 surf
Denso Corporation
Hoang Tu Ba
Pillsbury & Winthrop LLP
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