U.S. patent application number 08/927196 was filed with the patent office on 2001-07-12 for spark plug and producing method thereof.
Invention is credited to SUGIMOTO, MAKOTO, TANAKA, YUTAKA.
Application Number | 20010007196 08/927196 |
Document ID | / |
Family ID | 26531168 |
Filed Date | 2001-07-12 |
United States Patent
Application |
20010007196 |
Kind Code |
A1 |
SUGIMOTO, MAKOTO ; et
al. |
July 12, 2001 |
SPARK PLUG AND PRODUCING METHOD THEREOF
Abstract
In a spark plug in which a space between a center electrode at
the top end side and a terminal electrode at a rear end side which
are arranged to be opposite to each other in an axial hole of an
insulator, the glass seal is effected at a temperature within the
range of 500 to 1000.degree. C. and under a condition where the
concentration of oxygen is 12 vol % or less.
Inventors: |
SUGIMOTO, MAKOTO; (AICHI,
JP) ; TANAKA, YUTAKA; (AICHI, JP) |
Correspondence
Address: |
MORGAN LEWIS AND BOCKUS
1800 M STREET N W
WASHINGTON
DC
20036
|
Family ID: |
26531168 |
Appl. No.: |
08/927196 |
Filed: |
September 11, 1997 |
Current U.S.
Class: |
65/32.1 ;
313/134; 313/135; 313/136; 65/32.2; 65/59.31; 65/59.35 |
Current CPC
Class: |
H01T 21/02 20130101;
H01T 13/34 20130101 |
Class at
Publication: |
65/32.1 ;
313/136; 313/134; 313/135; 65/32.2; 65/59.31; 65/59.35 |
International
Class: |
H01T 013/20; H01T
013/04; H01T 013/05; C03C 027/02 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 12, 1996 |
JP |
PHEI8241503 |
Aug 29, 1997 |
JP |
PHEI 9233714 |
Claims
What is claimed is:
1. A spark plug comprising: an insulator having an axial hole; a
center electrode provided at the top end side of said spark plug; a
terminal electrode provided at the rear end side thereof, said
center electrode and said terminal electrode being arranged to be
opposite to each other in the axial hole; and seal glass filled in
said axal hole between said center electrode and said terminal
electrode; wherein glass sealing of said seal glass is effected at
a temperature in the range of 500 to 1000.degree. C. at an oxygen
concentration of not more than 12 vol %.
2. A spark plug according to claim 1, wherein said terminal
electrode has a terminal portion having a plating layer thereon,
and said plating layer is not rusted for not less than 70 hours in
neutral brine spray test.
3. A spark plug according to claim 1 further comprising: a
cylindrical metallic shell having a projecting ground electrode
disposed on its top end face; wherein said insulator is fixed in
said metallic shell, said axial hole of said insulator has a step
seat to which a base portion of said center electrode is fitted,
and said terminal electrode has a terminal portion protruding from
an end surface of said insulator.
4. A spark plug according to claim 3, wherein a space between said
center electrode and said terminal electrode is filled with the
seal glass on the base portion of said center electrode, a resistor
and the seal glass in this order to glass-seal said terminal
electrode; further wherein the glass sealing of said seal glass is
effected at a temperature in the range of 500 to 1000.degree.
C.
5. A spark plug according to claims 1, wherein the glass sealing is
effected by glass having softening point of not less than
450.degree. C. at a temperature 50 to 150.degree. C. higher than
the softening point.
6. A spark plug according to claim 1, wherein said terminal
electrode comprises a low carbon steel plated with nickel or
zinc.
7. A spark plug according to claim 1, wherein the glass sealing is
effected in either an electric furnace having an inert gas
atmosphere or a gas surface having a reducing gas atmosphere,
either of said atmospheres having an oxygen concentration of not
more than 12 vol %.
8. A method for producing a spark plug, wherein a space between a
center electrode provided at a top end side and a terminal
electrode provided at a rear end side, which are arranged in a
axial hole of an insulator, is glass-sealed at a temperature of 500
to 1000.degree. C. at an oxygen concentration of not more than 12
vol %.
9. A method for producing a spark plug according to claim 8,
wherein the glass sealing is effected at a temperature of 800 to
1000.degree. C.
10. A method for producing a spark plug according to claim 8,
wherein the glass sealing is effected by glass having softening
point of not less than 450.degree. C. at a temperature 50 to
150.degree. C. higher than the softening point.
11. A method for producing a spark plug according to claim 8,
wherein the glass sealing is effected in either an electric furnace
having an inert gas atmosphere or a gas surface having a reducing
gas atmosphere, either of said atmospheres having an oxygen
concentration of not more than 12 vol %.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a spark plug of a type that
is to be mounted on an internal combustion engine, and a producing
method thereof.
[0003] 2. Description of the Related Art
[0004] A glass sealable spark plug is conventionally known and it
is manufactured by a process comprising the steps of: filling a
seal glass material or a combination of seal glass material, a
resistive material and seal glass material as a glass sealing
material within a space between a center electrode at a top end
thereof and a terminal electrode at a rear end thereof which are
arranged in a axial hole of an insulator; melting the glass of the
glass sealing material by heating; pressing the terminal electrode;
and cooling them to solidify.
[0005] (A) When glass sealing is effected at a comparatively low
temperature using glass sealing materials of low softening point,
the energy cost is reduced and yet high operating efficiency is
provided.
[0006] On the other hand, if the sealed portions (i.e., the top end
of the terminal electrode and the rear end portion of the center
electrode) are exposed to high temperature, the glass sealing
materials will soften and both the terminal and center electrodes
will loosen to impair the airtightness of the spark plug.
[0007] In addition, the binding force of the electrically
conductive substance mixed in the sealing materials and the
resistive material drops to produce a higher resistance.
[0008] (B) When glass sealing is effected at high temperature
exceeding 800.degree. C. using glass sealing materials of high
softening point exceeding 750.degree. C., the terminal electrode
having a plate of nickel or zinc applied to a low carbon steel is
oxidized to corrode in the process of glass sealing.
[0009] If the terminal electrode is oxidized to corrode, the plate
will come off the steel to cause rust formation on the latter.
[0010] If rust forms, the electrical connection of the terminal
electrode to the plug cap will deteriorate. In addition, the rust
stains the barrel portion of the insulator to cause flashover.
SUMMARY OF THE INVENTION
[0011] It is an object of the invention to provide a spark plug
that can be produced without causing oxidation and corrosion of the
terminal electrode during glass sealing and which is protected
against the increase in the resistance between the terminal and
center electrodes.
[0012] A spark plug according to the present invention is comprised
of an insulator having an axial hole; a center electrode provided
at the top end side of the spark plug; a terminal electrode
provided at the rear end side thereof, the center electrode and the
terminal electrode being arranged to be opposite to each other in
the axial hole; and seal glass filled in the axal hole between the
center electrode and the terminal electrode; wherein glass sealing
of the seal glass is effected at a temperature in the range of 500
to 1000.degree. C. at an oxygen concentration of not more than 12
vol %.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] In the accompanying drawings:
[0014] FIG. 1 is a sectional view of the spark plug having a
resistor therein using the resistive material composition according
to the present invention;
[0015] FIGS. 2A to 2C are an explanation view of the glass sealing
process of the center electrode and the terminal electrode into the
axial hole of the insulator according to the present invention;
[0016] FIG. 3A is a schematic view of the gas furnace used in the
glass sealing in the present invention;
[0017] FIG. 3B is a sectional view of A-A line; and
[0018] FIG. 4 is a sectional view of the spark plug without the
resistive body.
DETAILED DESCRIPTION OF THE INVENTION
[0019] Detailed description of the present invention will be
described as follows.
[0020] In a spark plug of the present invention in which a center
electrode at the top end side thereof and a terminal electrode at
the rear end side thereof which are arranged to be opposite each
other in an axial hole of an insulator, a space between the center
electrode and the terminal electrode is glass-sealed; the glass
sealing is effected in a temperature range of 500 to 1,000.degree.
C. at an oxygen concentration of not more than 12 vol %.
[0021] In the spark plug of the present invention, it is preferable
that the plating layer of the terminal portion of the terminal
electrode is not rusted for not less than 70 hours in neutral brine
spray test.
[0022] The spark plug of the present invention may have a
cylindrical metallic shell having a projecting ground electrode
disposed on its top end face; an insulator with an axial hole that
is fixed within the metallic shell; the center electrode arranged
at the top end side which is divided by a step seat of the axial
hole, a base portion of the center electrode fitting to the step
seat; the terminal electrode arranged at the rear end side, having
a terminal portion at rear end which protrudes from the end surface
of the insulation glass; and seal glass which seals a space between
the center electrode and the terminal electrode.
[0023] In the spark plug of the present invention, it is preferable
that the space between both electrodes is filled with the seal
glass, the resistor and the seal glass in this order and the glass
sealing is effected in a temperature in the range of 800 to
1000.degree. C.
[0024] In the spark plug of the present invention, preferably, the
glass sealing is effected, by using glass having softening point of
not less than 450.degree. C., at a temperature 50 to 150.degree. C.
higher than the softening point.
[0025] In the spark plug of the present invention, the terminal
electrode is preferably composed of a low carbon steel plated with
nickel or zinc.
[0026] In the spark plug of the present invention, the glass
sealing is effected in either an electric furnace having an inert
gas atmosphere or a gas furnace having a reducing gas atmosphere,
either of said atmospheres having an oxygen concentration of not
more than 12 vol %.
[0027] If the glass sealing is effected at less than 500.degree.
C., the seal glass is softened during operating the combustion
engine, because a seal glass having low softening temperature is
used. On the other hand, if the glass sealing is effected at more
than 1000.degree. C., the terminal electrode is oxidized and
corroded at the time of glass sealing and it is made difficult to
prepare the seal glass suitable for such a high temperature glass
sealing.
[0028] In the present invention, glass sealing is effected under
the atmosphere in which the temperature of the glass sealing
portion is within the range of 500 to 1000.degree. C. as well as
the oxygen concentration is not more than 12 vol. %. Accordingly,
it is possible to suppress the oxidation and corrosion of the
terminal portion of the terminal electrode. In addition, the
plating layer of the terminal portion of said terminal electrode is
not rusted for not less than 70 hours in neutral brine spray
test.
[0029] In the spark plug in which a space between the center
electrode and the terminal electrode in the axial hole of the
insulator is glass-sealed by the seal glass, resistor and seal
glass, the glass sealing is effected at a high temperature of 800
to 1000.degree. C. and under the condition that the concentration
of oxygen is not more than 12 vol. %. Accordingly, the plating
layer of the terminal portion of the terminal electrode is not be
oxidized to corrode.
[0030] Since the seal glasses do not soften, neither the terminal
electrode nor the center electrode will loosen and, hence, the
airtightness of the spark plug is not impaired. In addition, the
binding force of the electrically conductive substance mixed in the
seal glasses and the resistor will not drop and, hence, the
resistance between the terminal and center electrodes will not
increase.
[0031] If glass sealing is effected at less than 800.degree. C.,
the resistor glass does not soften sufficiently. Therefore, it is
easy to be quenching-shrunk and a conductive path is cut by spark
energy, thereby increasing resistance value. On the other hand, if
glass sealing is effected at more than 1000.degree. C., the
terminal electrode may be oxidized or corroded during glass sealing
and it is made difficult to prepare seal glass suitable for such a
high temperature glass sealing.
[0032] If the softening point is not less than 450.degree. C. and
the glass sealing is effected at the temperature 50 to 150.degree.
C. higher than the softening point, the seal glass sufficiently
melts, thereby ensuring glass sealing. In addition, even if the
temperature of the glass sealing portion increases (300 to
330.degree. C.), the sealing portion is not softened and the
terminal electrode is not loosen, thereby not impairing the
airtightness.
[0033] A low carbon steel on which nickel or zinc is plated is used
as the terminal electrode of a spark plug.
[0034] When glass sealing is effected in air at high temperature
not less than 500.degree. C., the terminal electrode is oxidized to
corrode.
[0035] However, when the concentration of oxygen is not more than
12 vol. %, the terminal electrode is not oxidized to corrode during
glass sealing. Particularly, the nickel plating is preferable.
[0036] If the oxygen concentration is not more than 12 vol. %, said
glass sealing can be effected in either an electric furnace having
an inert gas atmosphere or a gas surface having a reducing gas
atmosphere, and the plating layer of the terminal portion of the
terminal electrode is not oxidized to corrode during glass
sealing.
[0037] The present invention will be described more detail in the
following embodiments.
[0038] Spark plug R having the design features according to the
present invention will now be described with reference to FIG.
1.
[0039] As FIG. 1 shows, spark plug R includes a cylindrical
metallic shell 1, insulator 2 with an axial through-hole 21 that is
fixed within the metallic shell 1, a terminal electrode 3 inserted
into the axial hole 21, a center electrode 4 fixed within the axial
hole 21 such that its distal end portion protrudes from the top end
face 221 of the insulator 2, seal glasses 5 and 6 which seal the
seal portion 31 of the terminal electrode 3 and the base portion 41
of the center electrode 4, respectively, within the axial hole 21,
and a resistor 7 positioned between the seal glasses 5 and 6. The
spark plug R having this construction is threaded onto the cylinder
head (not shown) of an internal combustion engine via a gasket 11
and the plug cap (not shown) is fitted over the terminal portion 32
for supply of high voltage.
[0040] The metallic shell 1 is formed of a low carbon steel and
consists of a screw portion 12 having a male thread 121 formed on
the outer circumference, a barrel portion 13 having the gasket 11
disposed on the front side, and a hexagonal portion 14 which is to
be gripped with a plug wrench. Shown by 141 is a packing, 142 and
143 are each a ring, and 144 is talc.
[0041] The insulator 2 consists of a insulator nose portion 22 that
is formed of an alumina based ceramic sinter and which is
positioned inside of the screw portion 12, a large-diameter portion
23 positioned inside of the metallic shell 1 in an area extending
from the hexagonal portion 14 to the barrel portion 13, and a head
portion 24 having a corrugation 241 formed on the outer
circumference. The axial hole 21 is formed through the insulator 2
along its axis. That part of the axial hole 21 which is positioned
in an area extending form the head portion 24 to the large-diameter
portion 23 is formed in a large diameter (4.5.phi.) and that part
of the axial hole 21 which is positioned in an area corresponding
to the insulator nose portion 22 has a slightly larger diameter
than the center electrode (of which the diameter is 2.6.phi.).
[0042] The terminal electrode 3 is constituted by a low carbon
steel (C: not more than 0.3%) plated with nickel (in a thickness of
5 .mu.m) and it consists of a seal portion 31 that is glass sealed
within the axial hole 21 of the insulator 2, a terminal portion 32
projecting from the end face 242 of the head portion of the
insulator 2 and a rod-shaped portion 33 which connects the terminal
portion 32 and the seal portion 31.
[0043] The terminal portion 32 has a smaller diameter in the center
than in the other parts in order to ensure that the plug cap (not
shown) will not slip out after it has been fitted over the terminal
portion 32.
[0044] The seal portion 31 is threaded or knurled on the outer
circumference and sealed within the axial hole 21 of the insulator
2 by means of the seal glass 5.
[0045] The center electrode 4 is composed of a sheath member made
of a nickel alloy and a core member made of a good heat conductor
metal such as copper that is embedded in said sheath member. Having
this structure, the center electrode 4 is fitted into the axial
hole 21 of the insulator 2 such that its distal end portion 42
projects from the top end face 221 of the insulator 2, with the
base portion 41 being fitted to a step seat 222 and sealed within
the axial hole 21 via the seal glass 6.
[0046] The seal glasses 5 and 6 have electrical conductivity since
they are prepared by melting and solidifying a conductive glass
powder which is a 1:1 mixture of a copper powder and a calcium
borosilicate glass powder having the softening point of 780.degree.
C. The center electrode 4 is electrically connected to the terminal
electrode 3 via the seal glass 6, resistor 7 and seal glass 5 in
this order.
[0047] The resistor 7 which should have a resistance of 5 k.OMEGA.
is prepared by the following procedure.
[0048] 17.3 wt % of ZrO.sub.2 powder, 0.2 wt % of alumina powder,
2.0 wt % of carbon black, 80 wt % of glass powder (containing 50 wt
% of SiO.sub.2, 29 wt % of B.sub.2O.sub.3, 4 wt % of Li.sub.2O and
17 wt % of BaO: Softening point: 820.degree. C.) and 0.5 wt % of
PVA (polyvinyl alcohol) binder are mixed by a mixer to prepare the
resistor 7.
[0049] Sealing materials to provide the seal glasses 5 and 6 are
prepared in the following manner.
[0050] To 50 parts by weight of a calcium borosilicate glass
powder, 50 parts by weight of a metallic copper powder and one part
by weight of a binder PVA are added, and the respective ingredients
are mixed thoroughly with a mixer. The mixture is dried at
100.degree. C. to form the seal glass material.
[0051] Next, the glass sealing process of the spark plug R having
resistor as shown in FIG. 1 will be described referring to FIGS. 2A
to 3B.
[0052] I) As shown in FIG. 2A, the center electrode 4 is inserted
from the upper into the axial hole 21 of the insulator 2 which is
formed by applying and baking a glaze on the surface of the heat
portion 24 of a sintered body mainly composed of alumina, so that
the base portion 41 having large diameter is fitted to the step
seat 222.
[0053] II) As shown in FIG. 2B, 0.3 g of the seal glass material 6
using the above described calcium borosilicate glass powder
(softening temperature: 780.degree. C., G.sub.2 glass) is filled in
the axial hole 21, and the seal glass material 6 is pressed under
the pressure of 140 MPa.
[0054] On the seal glass material 6, 0.3 g of the resistor 7 is
filled in the axial hole 21 and pressed under the pressure of 140
MPa. After the pressing, on the resistor 7, once 0.3 g of the
resistor 7 is filled in the axial hole 21 and also pressed under
the pressure of 140 MPa, so that 0.6 g of the resistor 7 is filled
in the axial hole 21 on the seal glass material 6.
[0055] Moreover, 0.3 g of the seal glass material 5 is filled in
the axial hole 21 on the resistor 7 and pressed under the pressure
of 140 MPa. Thereafter, the terminal electrode 3 is inserted in the
axial hole 21.
[0056] III) Next, the insulator 2 into which the terminal electrode
3 is inserted is inserted into a cylindrical receiving base 8 as
shown in FIG. 2B made of alumina ceramic so as to fit the seat face
of the large diameter portion 23, and is disposed in a furnace 9
which the fuel is LNG. Part of the seal glass portion of the large
diameter portion 23 and the heat portion 24 is heated at 800 to
1000.degree. C. for about 20 min. to melt the glass of the seal
glass materials 5, 6 and resistor 7 (glass seal portion G). Then,
the terminal electrode 3 is pressed under the load of 100 Kg, and
the load force is maintained in 20 Kg until 700.degree. C.
Thereafter, the insulator 2 is cooled to room temperature.
[0057] Then, as shown in FIG. 2C, the seal portion 31 of the
terminal electrode 3 and the base portion 41 of the center
electrode 4 is fixed in the axial hole 21, thereby finishing the
glass sealing process.
[0058] Incidentally, in FIGS. 3A and 3B, reference numeral 91
designates a furnace having fire resistance; 92, a burner device;
and 93, oxygen sensor for detecting oxygen concentration. Two
oxygen sensor 93 arc used so that a controller 94 controls the
oxygen concentration to be less than 12 vol. % (preferably 0.1 to
12 vol. %). The temperature is controlled by adjusting the flowing
gas amount of LNG of the burner device 92 to be in constant.
Accordingly, it is possible to prevent the oxidation and the
corrosion of the plating layer (nickel plating: thickness of 5
.mu.m) of the terminal portion 32 of the terminal electrode 3.
[0059] The insulator 2 which the glaze is previously applied and
baked on the head portion 24, etc. in this embodiment. However, it
is possible to use non-baked insulator 2 which the glaze is merely
applied to the head portion 24, and the glaze is baked thereon
during the glass sealing process. In this case, it is possible to
save cost because the baking is made only one time.
[0060] The insulator 2 which the glass sealing is finished is
fitted in the metallic shell 1 via the packing 141, and the rings
142, 143, the talc 144 is also inserted therein, and the caulking
portion 145 is caulked so that the insulator 2 is assembled to the
metallic shell 1.
[0061] FIG. 4 shows a spark plug S of another embodiment according
to the present invention.
[0062] This spark plug S does not have the resistor 7 of the spark
plug R of the above embodiment, in which a space between the center
electrode 4 and the terminal electrode 3 provided to be opposite to
each other in the axial hole 21 of the insulator 2 is filled with
and sealed by the seal glass 10 at the glass sealing temperature of
500 to 1000.degree. C.
[0063] The seal glass material 10 is a mixture of copper powder and
glass powder, and the softening temperature of the glass is a wide
range of 450 to 950.degree. C. For example, the seal glass material
5, 6 in the above embodiment and the G.sub.1 glass described later
is used as the seal glass material 10 in the present embodiment,
and the oxygen concentration is controlled to be not more than 12
vol. %. Therefore, it is possible to effectively prevent from
oxidizing and corroding the nickel plating layer of the terminal
portion 32 and the terminal electrode 3.
EXAMPLES
[0064] Examples are shown in Table 1. In Table 1, two types of
spark plugs are used, namely, the spark plug R having the resistor
7 and the spark plug S having no resistor are used. Estimations
were made with respect to used seal glass, glass seal temperature,
atmosphere of the furnace, oxygen concentration, rust generation
time (Hr) on the surface of the terminal portion 32 of the terminal
electrode 3, and the variation ratio (%) of the resistance
value.
1TABLE 1 A B C D E F G H I J 1 R G2 880 100 AIR ELECTRIC *19.8 48
+35 2 R G2 890 110 N.sub.2 ELECTRIC 4.0 86 -21 3 R G2 920 140 --
LPG GAS 8.5 72 -13 4 R G2 890 110 -- LPG GAS 12.0 90 -25 5 R G2 900
120 -- LPG GAS *13.0 58 -16 6 S G1 550 90 AIR ELECTRIC *20.0 68 +15
7 S G1 550 90 N.sub.2 ELECTRIC 5.0 75 -15 8 S G2 *480 50 AIR
ELECTRIC 20.5 98 +55 9 S G2 600 *170 N.sub.2 ELECTRIC 2.0 88 +200
10 R G2 950 *170 N.sub.2 ELECTRIC 2.5 80 +75 11 R G2 810 *30 -- LNG
GAS 7.5 98 # 12 S G1 600 140 -- LNG GAS 6.8 88 -11 13 S G2 830 50
-- LNG GAS 8.5 90 -5 14 R G2 830 50 -- LNG GAS 8.0 90 -21 15 S G1
510 50 -- LNG GAS 7.5 92 -20 16 S G1 880 100 AIR ELECTRIC *20.5 50
-10 *:out of range of the present invention. #: glass sealing is
impossible. A) Sample No. B) Spark plug type (R or S) C) Used seal
glass (G1 or G2) D) Glass sealing temperature E) Difference between
glass sealing temperature and glass softening point F) Atmosphere
G) Type of furnace H) Oxygen concentration (vol %) I) Rust
generation time (hours) J) Variation ratio of resistance value
(%)
[0065] The glass compositions (wt %) of the seal glass materials
G1, G2 and G3 used in the test were: G1 contained 33% of SiO.sub.2,
10% of B.sub.2O.sub.2, 6% of Na.sub.2O and 51% of PbO and had the
softening point of 460.degree. C.; G2 contained 55% of SiO.sub.2,
30% of B.sub.2O.sub.2, 5% of Na.sub.2O, 5% of PbO and 5% of CaO and
had the softening point of 780.degree. C; and G3 contained 28% of
SiO.sub.2, 12% of B.sub.2O.sub.2, 5% of Na.sub.2O and 55% of PbO
and had the softening point of 430.degree.C.
[0066] The variation ratio of resistance value (%) is measured in a
manner that the spark plugs R, S was mounted on 4-cycle, 4-valve
engine, and endurance tests were conducted under a condition of
5000 rpm X full-throttle. The results are exhibited as the
difference ratio (%) between the resistance value before the test
and that after the test. Then, plus (+) designates an increase of
the resistance value, and minus (-) designates an decrease of the
resistance value. As the judgement standard of the resistance value
variation ratio, the range within .+-.30% of the resistance value
load life test defined by JIS B8031 was defined as good.
[0067] Further, the rust generation time (hours) of the surface of
the terminal portion 32 of the terminal electrode 3 was measured
based on the neutral brine spray test method defined by JIS H8502.
As the judgement of the rust generation time, the time of not less
than 70 hours was defined as good.
[0068] Next, advantages of the spark plugs produced based on the
conditions of samples in the tests according to the present
invention.
[0069] (a) The spark plugs R produced based on the conditions of
sample Nos. 2, 3, 4 and 14 (corresponding to claims 1, 2, 4, 5, 6
and 7) can prevent to oxidize and corrode the surface of the
terminal portion 32 of the terminal electrode 3.
[0070] The spark plugs S produced based on the conditions of sample
Nos. 7, 12, 13 and 15 (corresponding to claims 1, 2, 3, 5, 6 and 7)
also can prevent to oxidize and corrode the surface of the terminal
portion 32 of the terminal electrode 3.
[0071] Accordingly, since the plating of the terminal electrode 3
does not peel off, rust caused by peeling the plating does not
generate. Accordingly, deficiencies such as a connection inferior
with a plug cap caused by the rust generation and a flush over do
not occur.
[0072] (b) The spark plugs R produced based on the conditions of
sample Nos. 2, 3, 4 and 14 are free from softening the seal glass
materials 5, 6 even if the seal portion (seal portion 31 and base
portion 41) is exposed to high temperature during normal use.
[0073] Accordingly, the terminal electrode 3 and the center
electrode 4 are not loosen, i.e, the airtightness can be
maintained. In addition, since it is possible to suppress to lower
the coupling force of copper contained in the seal glass materials
5, 6, the resistance value (5 k.OMEGA.) between the terminal
electrode 3 and the center electrode 4 is not extremely
increased.
[0074] Further, the spark plugs produced based on the conditions of
sample Nos. 7, 12, 13 and 15 are free from softening the seal glass
material 10 even if the seal portion is exposed to high
temperature.
[0075] Therefore, the terminal electrode 3 and the center electrode
4 are not loosen.
[0076] Incidentally, the spark plug S of the sample No. 8 which the
glass sealing temperature is not more than 500.degree. C. decreases
the resistance value during the engine test.
[0077] In addition, like as the sample No. 11, if the glass sealing
temperature is not 50.degree. C. or more higher than the softening
point (780.degree. C.), the glass sealing is not possible. Further,
like as the sample Nos. 9, 10, if the glass sealing temperature is
150.degree. C. or more higher than the softening point, the
conductive material (copper powder) and glass become in disorder.
Consequently, the resistance value varies widely, and the
resistance value variation ratio after the engine test is extremely
larger than +30%, thereby being not preferable.
[0078] In the electric furnace capable of flowing an inert gas
(sample Nos. 2, 7) and the gas furnace the fuel of which is LPG or
LNG (sample No. 3, 4, 12, 13, 14 and 15), since the oxygen
concentration is made 12 vol % or less, the oxidation and the
corrosion of the surface of the terminal portion 32 of the terminal
electrode 3 can be effectively suppressed.
[0079] The present invention include the following examples in
addition to the above examples.
[0080] (a) Seal glass raw material may be barium borate glass,
lithium borate-calcium glass.
[0081] (b) The terminal electrode 3 may be a zinc plated low carbon
steel (chromete treatment).
[0082] (c) The seal glass 5, 6 may be a known seal glass which
includes a metal oxide and a metal carbide such as TiO.sub.2, TiC,
B.sub.4C and the like in addition to the glass powder and the metal
powder such as copper in the above examples.
[0083] (d) As the resistor 7, various known glass resistive
material can be used in addition to the above examples.
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