JP2002246146A - Insulator for spark plug and spark plug - Google Patents

Abstract

(57) [Problem] To provide an insulator for a spark plug that can be manufactured at lower cost while improving durability against dielectric breakdown and the like. SOLUTION: In a spark plug insulator 2, a first portion 2p and a second portion 2s are each made of an insulating ceramic, for example, an alumina ceramic mainly containing alumina. Then, the first portion 2p on the spark discharge gap g side, which is liable to cause a problem such as dielectric breakdown due to application of a high voltage,
The ceramic structure of each part is adjusted so that the insulating property is higher than that of the second portion 2s where the problem such as dielectric breakdown is relatively unlikely to occur. For example, the average crystal grain size of the main phase in the first portion 2p is larger than that in the second portion 2s. Alternatively, the content of the grain boundary phase (or the area ratio in the structure cross section) is made smaller in the first portion 2p than in the second portion 2s. Further, the average thickness of the grain boundary phase is made lower in the first portion 2p than in the second portion 2s.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spark plug insulator and a spark plug using the same.

[0002]

2. Description of the Related Art In recent years, the area occupied by intake and exhaust valves in a combustion chamber has been increasing with the increase in the output of internal combustion engines used in automobiles and the like. Therefore, the spark plug for igniting the air-fuel mixture is required to be downsized, and a supercharger such as a turbocharger is used.
Since the temperature in the combustion chamber also tends to increase, an insulator made of an alumina-based insulating material having excellent heat resistance is generally used. Another important reason why an insulator made of alumina is used as an insulator for a spark plug is that alumina has excellent withstand voltage characteristics at high temperatures. However, in recent years, as the size of the spark plug has been reduced, the thickness of the insulator has also tended to be reduced, and an insulator having excellent withstand voltage characteristics has been demanded.

For example, alumina ceramics using a ternary sintering aid of SiO ₂ —CaO—MgO have been widely used as a material for an insulator for a spark plug. However, since this kind of sintering aid forms a low-melting glassy grain boundary phase in the ceramic, there is a problem that dielectric breakdown easily occurs when a high voltage is applied at a high temperature. Also, in the alumina raw material manufactured by the Bayer method or the like, a considerable amount of Na component is inevitably mixed for reasons of the manufacturing method, but this Na component dissolves in the grain boundary phase and lowers the withstand voltage. It also causes.

Therefore, an insulator in which the content of alumina is increased to 95 to 98% by mass to improve the withstand voltage characteristic, an insulator in which the Na content is reduced by using low soda alumina, and Y ₂ O ₃ or La ₂ O
By blending rare earth components ₃ such as to produce a high-melting-point grain boundary phase, an insulator having improved withstand voltage characteristics (for example, JP-63-
No. 190753).

[0005]

However, the cost of the production method or the raw material is higher than that of general alumina ceramics, and the price of the final spark plug rises in exchange for the improvement of performance. Is inevitable.

An object of the present invention is to provide a spark plug insulator which can be manufactured at a lower cost while improving the durability against dielectric breakdown and the like, and a spark plug using the same.

[0007]

An insulator for a spark plug according to the present invention is composed of two portions made of alumina ceramics of different materials, specifically, a second portion made of an insulating ceramic. And a first portion having a higher insulation performance (specifically, a higher withstand voltage). In the spark plug according to the present invention, the insulator is arranged between the metal shell and the center electrode, and at the distal end side of the insulator, a ground electrode and a center electrode whose base ends are coupled to the metal shell. A spark discharge gap is formed between them. And the purpose is to selectively improve the insulating property of the insulator tip portion, to configure this portion as the first portion,
That is, at least a part of the remaining part is configured as the second part.

[0008] In the present specification, the alumina ceramic is a ceramic containing alumina as a main component. In this specification, “main component” (“mainly”
Alternatively, “mainly the same” means a component having the highest weight content ratio of the substance of interest.

The first specific structure of the insulator of the present invention is as follows. In the insulator, the first portion has a larger average crystal grain size than the second portion (hereinafter referred to as an average crystal ratio requirement). At least one of the following: a small content of the grain boundary phase (hereinafter, referred to as a grain boundary phase content requirement); a large crystallization rate of the grain boundary phase (hereinafter, referred to as a grain boundary phase crystallization rate requirement). The point is to satisfy the requirements of

The first portion and the second portion of the insulator are made of alumina ceramic mainly composed of alumina, which is particularly excellent in insulating properties and is relatively inexpensive when a sintered ceramic material is used. It is effective to do. Alternatively, another type of ceramic such as aluminum nitride may be used. Further, the first portion and the second portion can be made of ceramics whose main components are different from each other.

According to the above configuration, the first portion which is supposed to be located on the side of the spark discharge gap where the problem such as insulation breakdown is likely to occur due to the application of a high voltage is the second portion where the problem such as insulation breakdown is less likely to occur. Since it is made of a ceramic having a structure with improved insulating properties, it is possible to improve the withstand voltage characteristics of the first portion which is easily affected by the application of a high voltage during spark discharge. In addition, ceramics having a structure with improved insulating properties are generally expensive in many cases, but in the present invention, such ceramics may be selectively applied only to the first portion, so that the insulating properties may be increased. It is possible to improve the durability against dielectric breakdown and the like while suppressing an increase in the cost of the whole body.

Hereinafter, a specific embodiment in which the ceramic structures of the first portion and the second portion are different from each other will be described. First, the first portion satisfies the average crystallinity requirement, that is, by making the average crystal grain size of the first portion larger than that of the second portion, the withstand voltage of the first portion that is easily affected by high voltage application during spark discharge. Characteristics can be improved.
For example, as shown in FIG. 8, the average crystal grain size d ₁ of the insulating ceramic crystal particles (for example, main phase particles to be described later) constituting the first portion is changed to the average crystal grain size of the insulating ceramic constituting the second portion. It may be larger than the diameter d _2. Increasing the average crystal grain size in the structure means that the grain boundaries that are likely to become fracture paths are less likely to be straight, and that the fracture paths are easier to bypass, which is advantageous in improving withstand voltage characteristics. is there. In this specification, as shown in FIG. 9, the average crystal grain size d is obtained by drawing a line segment having a predetermined length on a structure observation image such as an SEM photograph and measuring the length of a portion where the inside of the particle overlaps the straight line. It is determined by an average value (so-called interceptor method).

Further, the first portion satisfies the requirement of the content of the grain boundary phase, that is, the content (or the area ratio in the structure section) of the grain boundary phase is made smaller in the first portion than in the second portion. Thus, the following effects can be achieved. That is, both the first portion and the second portion can be configured as having a structure in which main phase particles that are the main components of the insulating ceramic are bonded by a grain boundary phase having a lower melting point than the main phase particles. . The grain boundary phase is formed, for example, from a sintering aid component added to promote the densification of the ceramic sintered body. Here, the grain boundary phase based on the sintering aid component in the insulating ceramic is:
It is said that it has higher conductivity than the main phase at a high temperature and is likely to be a conductive path at the time of dielectric breakdown. Therefore, by making the content of the grain boundary phase in the first portion smaller than the content of the grain boundary phase in the second portion, the first portion can have better withstand voltage characteristics than the second portion.

Further, the first portion satisfies the requirement of the crystallinity of the grain boundary phase, that is, the crystallinity of the grain boundary phase is made lower in the first portion than in the second portion, so that the following effects can be obtained. Can be achieved. The higher the crystallization ratio of the grain boundary phase, the higher the withstand voltage characteristics of the ceramic.
Therefore, making the crystallization rate of the grain boundary phase higher in the first portion than in the second portion is more effective in making the first portion more excellent in withstand voltage characteristics than the second portion.

[0015] In the first configuration of the insulator of the present invention, the first portion of the insulator may have the above average crystallinity requirement (A).
And grain boundary phase content rate requirement (B) and grain boundary phase crystallization rate requirement (C)
May be individually satisfied, but any two
The above may be satisfied at the same time, whereby the withstand voltage characteristics of the first portion can be further improved. Specific combinations are as follows. A + B; B + C; C + A; A + B + C.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. A spark plug 100 as an example of the present invention shown in FIGS. 1 and 2 is formed on a cylindrical metal shell 1, an insulator 2 fitted inside the metal shell 1 so that a distal end portion 21 protrudes, and a distal end. One end is connected to the center electrode 3 provided inside the insulator 2 and the metal shell 1 by welding or the like while the igniting portion 31 is protruded, and the other end is bent back to the side. Includes a ground electrode 4 and the like arranged so as to face the tip of the center electrode 3. Further, the ground electrode 4 is provided with a firing portion 32 facing the firing portion 31.
The gap between the opposing firing part 32 is the spark discharge gap g.
It has been.

The insulator 2 has a shaft-like shape, and a first portion 2p that forms a tip in the direction of the axis O located on the side of the spark discharge gap g when assembled into the spark plug 100.
And a second portion 2s adjacent to the first portion 2p on the rear side in the direction of the axis O are each made of alumina ceramic mainly composed of alumina. The second portion 2s can be made of, for example, alumina ceramics of the same material as the conventional insulator for a spark plug.

The material of the first part 2p is the second part 2p.
It is configured to have a structure with an improved insulating property than s. Specifically, it is configured as a ceramic having a structure that satisfies at least one of the following requirements. The average crystal grain size d1 of the alumina-based main phase particles is equal to the average crystal grain size d2 of the alumina-based main phase particles constituting the second portion 2s.
Those with a larger organization. Such an organization
For example, by blending a sintering aid component with alumina powder having an average particle diameter of 1 μm or less in a ratio of 0.3 to 5% by mass with respect to the total of the alumina powder and the sintering aid component, the raw material powder base is obtained. Prepared, the raw material powder body is molded into a predetermined insulator shape,
It can be formed by firing at 0 ° C. By making the average particle diameter of the raw material alumina powder 1 μm or less, the growth of alumina-based main phase particles is rather positively generated at the time of sintering, and the growth is uniformly generated by using fine raw material alumina powder. Thus, a structure having a sharp particle size distribution can be formed. As a result, the densification progressed remarkably despite the fact that the sintering aid component was high in a high alumina material, the amount of voids remaining in the structure was extremely reduced, and the thermal conductivity was also increased. Withstand voltage characteristics can be obtained.

Those having a structure in which the content of the grain boundary phase (or the area ratio in the structure cross section) is smaller than that of the second portion 2s. Alternatively, a structure in which the average thickness t1 of the grain boundary phase is smaller than the average thickness t2 of the grain boundary phase of the second portion 2s. To reduce the content of the grain boundary phase, it is necessary to reduce the amount of the sintering aid component. In this case, too, the sintering aid component is added to the alumina powder, 0.3 with respect to the total
It is effective to set the blending amount to about 5% by mass.

Those having a structure in which the crystallization ratio of the grain boundary phase is larger than that of the second portion 2s. In the present specification, the crystallization ratio of the alumina grain boundary phase refers to the intensity of the diffraction peak derived from alumina and the diffraction peak derived from the grain boundary crystal phase in the diffraction pattern measured by the X-ray diffractometer method. It is a value defined as an intensity ratio with respect to the intensity. Specifically, the X-ray diffraction profile was measured by the diffractometer method (the horizontal axis of the profile was 2 with the Bragg angle θ).
When performing a scan based on θ, the range of 2θ to be measured is, for example, 20 ° to 60 °), and it is checked whether or not a diffraction peak derived from a material other than alumina exists on the profile. If no diffraction peak other than alumina could be detected, the crystallization ratio of the grain boundary phase of alumina is regarded as 0%. On the other hand, when a diffraction peak other than alumina is detected, it is considered that a crystal phase exists at the grain boundary, and fluorescent X-ray and E
PMA (ElevtronProbe Micro Analysis) or ED
The constituent elements of the crystal are confirmed by elemental analysis such as X (Energy Dispersive X-ray Spectrometer) and the grain boundary crystal phase is identified. If the diffraction peak pattern can be confirmed to match the peak pattern of a known substance using a known X-ray diffraction profile database (for example, an ASTM card), the crystal phase can be determined without performing elemental analysis. May be identified. Thereafter, the intensity of the main peak (the strongest peak appearing in the diffraction peak pattern) of the diffraction peak derived from each crystal in the measurement range is checked using the above database and the like, and the crystallization ratio is calculated by the following equation. . That is, assuming that the main peak intensity of the grain boundary crystal phase is M, the sum of M of each grain boundary phase is {M}, and the main peak intensity of alumina is S, R (%) = {M / (S + {M)} × 100. The crystallization ratio R of the interphase is calculated. On the other hand, if the grain boundary crystal phase cannot be identified, it is assumed that one kind of crystal phase exists at the grain boundary, and the strongest diffraction peak other than alumina is determined as the main peak of the grain boundary crystal phase. Calculate the conversion rate. In the case of using an alumina ceramic, for example, the use of a sintering aid containing a rare earth element component described below is effective in crystallization of the grain boundary phase.

It should be noted that the crystal phase containing no rare earth is C
aAl₁₂O₁₉, BaAl₁₂O ₁₉, MgAl₂O
₄, Al₆Si₂O₁₃Each crystal phase such as (Mullite)
It may be contained in the interphase.

Further, the first part may be configured to satisfy the following requirements. The alumina content is higher than the second portion 2s. By increasing the alumina content of the first portion 2p more than the second portion 2s, it is possible to improve the withstand voltage characteristics of the first portion 2p, which is easily affected by the application of a high voltage during spark discharge. Specifically, since a higher alumina content means that the amount of the sintering aid added can be suppressed to a low level, the reduction in the content of the grain boundary phase to be formed is a factor, and the withstand voltage characteristics are low. It is expected to be improved. In addition, it is sufficient to apply a high alumina ceramic which is expensive in cost only to the first portion 2p, and the material of the second portion 2s can be constituted by a less expensive ceramic having a lower alumina content. 2 It is possible to improve the durability against dielectric breakdown and the like while suppressing an increase in the overall manufacturing cost.

The specific content of the alumina component in the first portion 2p is from 95 to 1 in terms of mass in terms of Al ₂ O _3.
It can be selected in the range of 00% by mass. In this case, for example, Si, Ca, Mg, Ba and B components functioning as sintering aids, Si is SiO ₂ , Ca is CaO, Mg is MgO
In addition, Ba is contained in BaO and B is contained in B ₂ O ₃ in a total amount of up to 5% by mass in terms of oxide. Can be promoted. Further, as a sintering aid, it is of course possible to employ a rare earth element component described below. On the other hand, when high-temperature and high-pressure sintering such as hot isostatic pressing is possible, a dense sintered body can be obtained even with the sintering agent almost eliminated. The first part 2p
Is preferably 95% or more, and more preferably 98% or more, from the viewpoint of improving the withstand voltage characteristics particularly at high temperatures.

When the content of the rare earth element component R is
higher than s. The rare earth element component R is Sc, Y, L
a, Ce, Pr, Nd, Sm, Eu, Gd, Tb, D
One or more of y, Ho, Er, Tm, Yb, and Lu, which are added as a sintering aid, for example, in the form of an oxide. Since the rare earth element component mainly forms a high melting point compound phase in the grain boundary phase, increasing the content of the rare earth element component in the first portion 2p more than that in the second portion 2s reduces the effect of high voltage application during spark discharge. The withstand voltage characteristics of the first portion 2p that is easily affected can be improved. The factor is that the crystallization of the grain boundary phase is promoted. Further, a mechanism in which a fine rare earth-containing compound precipitated as a crystal phase in the grain boundary phase divides or bypasses the conductive path to improve the withstand voltage of dielectric breakdown can be considered. On the other hand, the increase in the added amount of the rare earth element component is relatively expensive, but since it is sufficient to add a large amount of the rare earth element component only in the first portion 2p, it is possible to suppress the increase in the manufacturing cost of the insulator 2 as a whole. It is possible to improve durability against breakage and the like.

The specific content of the rare earth element component in the first portion 2p is, when R is Ce, RO ₂ , Pr
In the case of the above, R ₆ O ₁₁ , and in other cases, R ₂ O ₃ can be selected in the range of 0.01 to 18% by mass in terms of oxide. These rare earth element components can be blended, for example, in the form of an oxide powder as a sintering aid. It is more effective to use one or more of Pr, Nd, and Dy among the above rare earth element components from the viewpoint of improving the withstand voltage characteristics at high temperatures. Note that N
If a form in which the d component and the Pr component are added in combination is adopted,
For example, a non-separable rare earth oxide of dymium mainly composed of Nd and Pr can be used, which is effective in reducing the raw material cost.

It is of course possible to mix other sintering aid components together with the rare earth element component R. For example, one or more of Si, Mg, Ca, Ba and B may be co-added. It can be blended as a binder component. In particular, Si
A high melting point composite oxide can be easily formed with the rare earth element component, and the effect of improving the withstand voltage characteristics at high temperatures can be further enhanced.

The content of the alkali metal component in the second part 2
lower than s. The alkali metal component is, for example, Li,
One or two or more of Na and K, all of which have high ionic conductivity, and when they become a grain boundary phase forming component, an excessively high content thereof causes a decrease in withstand voltage characteristics. Leads to things. Therefore, by setting the alkali metal component content of the first portion 2p lower than that of the second portion 2s, the withstand voltage characteristics of the first portion 2p, which is easily affected by the application of a high voltage during spark discharge, can be improved. On the other hand, alkali metal elements
For example, Na is often inevitably contained in an alumina raw material such as Na due to a factor in the production method. Therefore, an alumina raw material (for example, low soda alumina) having a reduced content is a normal alumina raw material (eg, medium soda alumina or normal soda alumina). (Alumina) is more expensive. However, in the above configuration, such an alumina-based ceramic having a low alkali metal component content may be applied only to the first portion 2p, and the material of the second portion 2s may have a lower alkali metal component content. Since the insulator 2 can be made of a high-priced and inexpensive ceramic, it is possible to improve the durability against dielectric breakdown and the like while suppressing an increase in the manufacturing cost of the entire insulator 2. For example, when low-soda alumina powder is used as a raw material and an alkali metal component other than Na is not actively added, the content of the alkali metal component can be kept to 0.05% by mass or less in terms of oxide. It is.

On the other hand, specific materials of the second portion 2s include, for example, a sintering aid component (including the alkali metal component, the same sintering aid may be used as the above-mentioned type). content is 5 to 15 wt% in terms of oxide, and for example, Na as the alkali metal component _Na 2 O
What is contained in the converted value in the range of 0.07 to 0.5% by mass can be adopted. With such Na content,
Medium soda alumina or ordinary soda alumina can also be used as a raw material without any problem.

Next, details of the overall shape of the insulator 2 will be described. First, a through-hole 6 is formed in the insulator 2 in the axial direction, and a terminal fitting 13 is inserted and fixed from one end thereof, and the center electrode 3 is inserted and fixed from the other end thereof. ing. Further, a resistor 15 is arranged between the terminal fitting 13 and the center electrode 2 in the through hole 6. Both ends of the resistor 15 are electrically connected to the center electrode 3 and the terminal fitting 13 via conductive glass seal layers 16 and 17, respectively. The resistor 15 is formed of a resistor composition obtained by mixing glass powder and conductive material powder (and ceramic powder other than glass as required) and sintering the mixture by hot pressing or the like. The conductive glass seal layers 16 and 17 are made of Cu, Sn,
It is made of glass mixed with a metal powder mainly composed of one or more metal components such as Fe. Note that the resistor 15 may be omitted, and the terminal fitting 13 and the center electrode 3 may be integrated by a single conductive glass seal layer.

As shown in FIG. 1, a projecting portion 2e projecting outward in the circumferential direction is formed, for example, in a flange shape in the middle of the insulator 2 in the axial direction. The insulator 2 has a main body 2b having a smaller diameter than the protruding portion 2e, with the side facing the tip of the center electrode 3 (FIG. 1) as the front side. On the other hand, on the front side of the protrusion 2e, a first shaft 2g having a smaller diameter than the first shaft 2g and a second shaft 2i having a smaller diameter than the first shaft 2g are formed in this order. A glaze 2d is applied to the outer peripheral surface of the main body 2b, and a corrugation 2c is formed at the rear end of the outer peripheral surface. The outer peripheral surface of the first shaft portion 2g has a substantially cylindrical shape, and the outer peripheral surface of the second shaft portion 2i has a substantially conical surface shape whose diameter decreases toward the distal end.

The axial sectional diameter of the center electrode 3 is equal to the resistance of the resistor 15.
Is set to be smaller than the shaft cross-sectional diameter. The through hole 6 of the insulator 2 has a substantially cylindrical first portion 6a through which the center electrode 3 is inserted, and a substantially cylindrical portion formed on the rear side (upper side in the drawing) of the first portion 6a with a diameter larger than this. And a second portion 6b. As shown in FIG.
The resistor 3 and the resistor 15 are accommodated in the second portion 6b, and the center electrode 3 is inserted into the first portion 6a. At the rear end of the center electrode 3, an electrode fixing projection 3c is formed to protrude outward from the outer peripheral surface thereof. The first portion 6a and the second portion 6b of the through hole 6 are connected to each other in the first shaft portion 2g, and the connection position receives the electrode fixing convex portion 3c of the center electrode 3. Is formed in a tapered surface or a round surface.

Further, the outer peripheral surface of the connecting portion 2h between the first shaft portion 2g and the second shaft portion 2i is a stepped surface, and this is a stepped surface.
Ridge 1 as a metal shell side engaging portion formed on the inner surface of
By engaging with c via a ring-shaped plate packing 63, axial removal is prevented. On the other hand, between the inner surface of the rear opening of the metal shell 1 and the outer surface of the insulator 2,
A ring-shaped line packing 62 that engages with the rear side peripheral edge of the flange-shaped protrusion 2e is arranged, and further on the rear side, a ring-shaped packing 60 is disposed via a filling layer 61 such as talc.
Is arranged. Then, the insulator 2 is pushed forward toward the metal shell 1 and, in this state, the opening edge of the metal shell 1 is swaged inward toward the packing 60 to form a swaged portion 1d. It is fixed to the insulator 2.

In the embodiment shown in FIG. 1, the first portion 2p and the second portion 2s of the insulator 2 are joined at a position slightly rearward in the axial direction of the second shaft portion 2i (closer to the first shaft portion 2g). Have been. This joining position is the same as the first part 2p
And the second portion 2s in the direction of the axis О. Specifically, as shown in FIG. 2, the dimension of the first portion 2p in the direction of the axis O increases as the position of the bonding interface is located rearward in the direction of the axis О. 2nd part 2p
Since the material cost is higher than that of the second portion 2s, for example, as shown in FIG. 2 (a), a joining surface is set near the tip of the second shaft portion, and the temperature is easily raised in the combustion chamber, and It is more advantageous to reduce the cost if only the vicinity of the front end, which is close to the spark discharge gap g and easily affected by high voltage application, is the first portion 2p. On the other hand, when stricter withstand voltage characteristics are required, or when the diameter of the entire insulator 2 is small, or when the thickness of the insulator 2 must be reduced, as shown in FIG. 2g or Fig. 2
As shown in (c), the first portion 2 is further moved to the rear position.
It may be p.

Returning to FIG. 1, the metallic shell 1 is formed of a metal such as low-carbon steel into a cylindrical shape.
00 and a screw portion 7 for attaching the plug 100 to an engine block (not shown) is formed on the outer peripheral surface thereof. This screw part 7
Has an outer diameter of 18 mm or less (for example, 18 mm, 14 mm,
12 mm, 10 mm, etc.). Reference numeral 1e denotes a tool engagement portion that engages a tool such as a wrench or a wrench when the metal shell 1 is attached, and has a hexagonal axial cross-sectional shape.

Next, the body portions 3a and 4a of the center electrode 3 and the ground electrode 4 are made of a Ni alloy or the like such as Inconel (trade name). Further, a core material 3b made of Cu or Cu alloy or the like is embedded in the center electrode 3 to promote heat radiation. On the other hand, the ignition part 3
1 and the opposing firing part 32 are Ir, Pt and Rh of 1
It is mainly composed of a noble metal alloy containing at least one kind or two or more kinds. The main body 3a of the center electrode 3 has a reduced diameter on the tip end side and a flat tip end face. A disk-shaped chip made of an alloy composition constituting the ignition portion is superimposed on the tip end, and the outer edge of the joining surface is further joined. Laser welding along the part,
The ignition portion 31 is formed by forming a welded portion by electron beam welding, resistance welding, or the like and fixing the welded portion. The opposing firing portion 32 is formed by aligning the tip with the ground electrode 4 at a position corresponding to the firing portion 31, forming a weld along the outer edge of the joint surface, and fixing the same. Is done. In addition, these chips are obtained by molding and sintering, for example, a molten material obtained by blending and melting each alloy component so as to have the indicated composition, or an alloy powder or a metal single component powder blended in a predetermined ratio. It can be constituted by the obtained sintered material. Note that at least one of the firing part 31 and the opposing firing part 32 may be omitted.

The insulator 2 can be manufactured, for example, by the following method. First, as a raw material powder, an alumina powder and a sintering aid powder are blended so as to have a composition determined for each of the first portion 2p and the second portion 2s, and a hydrophilic binder (for example, PVA) and water are mixed. Add and mix to make a molding base slurry.

The forming base slurry is spray-dried by a spray drying method or the like to form a forming base granule PG1 (for the first portion 2p) and a forming base granule PG2 (for the second portion 2p).
s). These molding base granules PG1 and P
G2 is subjected to rubber press molding to produce a press molded body serving as a prototype of the insulator. FIG. 4 schematically shows a process of rubber press molding. Here, a rubber mold 300 having a cavity 301 penetrating in the axial direction is used, and an upper punch 304 is fitted into an upper opening of the cavity 301. Further, on the punch surface of the lower punch 302, a press pin 303 that extends in the axial direction within the cavity 301 and that defines the shape of the through hole 6 (FIG. 1) of the insulator 2 is integrally formed. .

In this state, the cavity 301 is filled with a predetermined amount of the base granules PG1 and PG2 for molding, and the upper opening of the cavity 301 is closed with an upper punch 304 to be sealed. At this time, one of the green body granules for molding PG1 and PG2 is first filled on the lower side in the axial direction of the press pin 303, and then the other is filled on the upper side. Thereby, the granulated material PG1 for the first portion 2p and the granulated material PG2 for the second portion 2p are 2 in the axial direction in the cavity 301.
Filled in layers.

In this state, a liquid pressure is applied to the outer peripheral surface of the rubber mold 300, and the granules PG1 and PG2 of the cavity 301 are removed.
By compressing through the rubber mold 300, FIG.
As shown in (a), a pressed compact 202 is obtained in which powder compacts 202s and 202p made of the granules PG1 and PG2 are joined in the axial direction. The outer surface side of the press-formed body 202 is processed by grinder cutting or the like to finish the outer shape corresponding to the insulator 2 in FIG.
As shown in (b), the molded body to be fired 152 having a powder molded portion 152p having a shape corresponding to the first portion 2p and a powder molded portion 152s having a shape also corresponding to the second portion 2s.
Is obtained. By firing the molded body 152 at a predetermined temperature and further applying a glaze to finish firing, FIG.
Is completed.

Although the form of the joint surface between the first part 2p and the second part 2s of the insulator 2 is a plane form that is substantially perpendicular to the axis О in FIG. 1, the joint strength of both parts is improved. For example, as shown in FIG. 3 (a), for example, as shown in FIG. 3 (a), a joining surface 103 having a tapered surface shape protruding in any direction along the axis О, or a stepped portion as shown in FIG. The bonding surface 104 may have a planar shape. Alternatively, the two portions 2p and 2s can be joined in a positional relationship such that the rear end portion 107 of the first portion 2p is housed in the concave portion 106 opened on the tip side of the second portion 2s. In this case, for example, the molded body of the first part is first formed separately by a rubber press or the like, and when the molded body of the second part is formed by another rubber pressing step, simultaneously with the molded body of the first part, 162p (FIG. 6 (a)), and thereafter, the outer peripheral surface is finished by grinding or the like.
It is possible to employ a method of obtaining the molded body to be fired 162 shown in FIG.

If the firing conditions or shrinkage during firing of the first portion 2p and the second portion 2s are significantly different, a method as shown in FIG. 6B is also possible.
That is, the first portion 2p is first formed as a fired body, and a semi-molded body 172 in which the second portion formed body 172s is integrated is formed, and this is fired to form the first portion 2p and the second portion 2s. To obtain the insulator 2 integrated.

As shown in FIG. 7 (a), the first part 2p and the second part 2s are separately manufactured as sintered bodies, and these sintered bodies are joined to form the insulator 2. It is possible. For example, in the embodiment shown in FIG.
And the second portion 2s are joined by the glass joining layer 2w. Specifically, it is possible to employ a method in which a joining glass material is sandwiched between joining positions of the first portion 2p and the second portion 2s, and the joining is performed by heating to a joining temperature equal to or higher than the glass softening point.

Further, the first portion 2p may be made of an insulating ceramic having a higher translucency than the second portion 2s, for example, a translucent alumina. In order to satisfy both mechanical strength and withstand voltage characteristics as an insulator for a spark plug, the alumina ceramic used has a relative density of 90%.
% Or more. On the other hand,
The withstand voltage characteristics are further improved by further densifying the ceramic, but in addition to this, the abundance of the crystal grain boundaries and the grain boundary phase are also factors affecting the withstand voltage characteristics. Specifically, the withstand voltage characteristic improves as the amount of the crystal grain boundary or the grain boundary phase decreases. The alumina-based ceramic has a property that the translucency gradually increases as the densification and the reduction in the content of crystal grain boundaries and / or grain boundary phases progress. In other words, the withstand voltage characteristic of the alumina ceramic becomes better as the translucency increases. For example, what is generally known as translucent alumina is translucent polycrystalline alumina that has been densified to almost the theoretical density. It is manufactured using a high pressure firing method such as a hot isostatic pressing (HIP) method or a hot pressing method. Such an alumina ceramic has excellent withstand voltage characteristics, but is naturally more expensive than ordinary alumina.

Therefore, the first portion 2p is made of alumina ceramic having a higher translucency than the second portion 2s, thereby improving the withstand voltage characteristic of the first portion 2p which is easily affected by the application of a high voltage during spark discharge. it can. Also, the first part 2p
It is only necessary to use an alumina-based ceramic having a high light-transmitting property, so that it is possible to improve the durability against dielectric breakdown and the like while suppressing an increase in the manufacturing cost of the entire insulator.

The first portion 2p can be made of single-crystal alumina such as artificial ruby, artificial sapphire or artificial corundum. These are ultimate alumina-based materials in which the influence of crystal grain boundaries or grain boundary phases has been almost completely eliminated, and by applying them as the material of the first portion 2p of the insulator 2, resistance to these materials can be improved. Voltage characteristics can be dramatically improved.

The first portion 2p made of translucent alumina or single-crystal alumina naturally has a different structure from the second portion 2s made of ordinary sintered alumina. In addition, the first portion 2p made of these materials has a significant difference in manufacturing method from the second portion 2s made of normal sintered alumina, and the method of integrally firing with the second portion 2s in the state of a powder compact is a principle. Is impossible. Therefore, it is appropriate to separately manufacture the portion to be the first portion 2p, and then employ a method of joining by a method as shown in FIG. 6B or FIG.

[0047]

EXAMPLES In order to confirm the effects of the present invention, the following experiments were conducted. As the raw material alumina powder, the following 2
Available types: A: Normal soda alumina: Na content (Na ₂ O equivalent)
B: low soda alumina: Na content (in terms of Na ₂ O)
0.05% by mass, average particle size 0.5 μm.

The sintering aid powder has an average particle size of 0.1.
6 μm SiO ₂ powder, CaCO with average particle size 0.8 μm
₃ powder, MgO powder having an average particle diameter of 0.3 μm, and Nd ₂ O ₃ powder as a rare earth oxide powder having an average particle diameter of 0.1 μm were blended so as to have the compositions shown in Table 1 below. With the total amount of the blended powder being 100 parts by mass, 3 parts by mass of PVA as a hydrophilic binder and 103 parts by mass of water were added, and the mixture was wet-mixed by a ball mill for 16 hours to prepare a base slurry for molding. Next, these slurries having different compositions were dried by a spray-drying method, respectively, to prepare spherical molding base granules.
The granules are sieved to a particle size of 50 to 100 μm.

[0049]

[Table 1]

The above three types of base granules (powder Nos.
) Using the sample No. in Table 2. Various insulators shown in 1 to 3 were produced. The numerical value in each column indicates how many% by mass of each of the corresponding molding base granules was used. FIG.
The molded body produced by integral molding using the method described above is fired in air at 1625 ° C. for 2 hours.

[0051]

[Table 2]

Incidentally, for each test product, Al, Si,
The contents of Ca, Mg, rare earth element (RE) and Na
It was measured by CP emission spectroscopy, and the composition of the sintered body was determined by converting to the corresponding oxide content. as a result,
It was found that the composition substantially coincided with the composition shown in Table 1.

Using the insulator obtained as described above, a spark plug shown in FIG. 1 was assembled, and a withstand voltage test of an actual machine was performed under the following conditions. That is, the spark plug was attached to a 4-cylinder gasoline engine (displacement: 2000 cc), the throttle was fully opened, and the engine speed was 6
Continuous operation was performed at 000 rpm while controlling the discharge voltage to 35 kV, and evaluation was made after 50 hours had passed by whether or not spark penetration had occurred. Table 2 shows the results. That is, the test products (Nos. 2 and 3) of Examples having a high crystallization ratio or a large crystal grain size were all Comparative Examples (No. 1).
It can be seen that the withstand voltage characteristics are superior to those of the test sample.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a spark plug according to an embodiment of the present invention.

FIG. 2 is a longitudinal cross-sectional view showing various examples of a bonding mode of a first portion and a second portion in the insulator of FIG. 1;

FIG. 3 is a partial vertical cross-sectional view showing various examples of the joining surface shapes of a first portion and a second portion.

FIG. 4 is an explanatory view of a manufacturing process of the insulator of FIG. 1;

FIG. 5 is a process explanatory view following FIG. 4;

FIG. 6 is an explanatory view showing some modified examples of the manufacturing process of the insulator of FIG. 1;

FIG. 7 is an explanatory view showing still another modified example of the manufacturing process.

FIG. 8 is an explanatory view showing a concept of a structure in which an average crystal grain size is larger in a first portion than in a second portion.

FIG. 9 is an explanatory diagram showing the definition of the crystal grain size.

FIG. 10 is an explanatory view showing the concept of a structure in which the content of the grain boundary phase is smaller in the first portion than in the second portion.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Metal shell 2 Insulator 2p 1st part 2s 2nd part 3 Center electrode 4 Ground electrode

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Katsura Matsubara 14-18, Takatsuji-cho, Mizuho-ku, Nagoya-shi, Aichi Prefecture Inside Nihon Toku Toki Co., Ltd. No. 18 Nihon Special Ceramics Co., Ltd. 14-18 Takatsujicho F-term in Japan Special Ceramics Co., Ltd. (reference) 4G030 AA36 BA12 CA04 CA05 5G059 AA05 AA08 CC02 FF02 FF10 FF14

Claims (3)

[Claims] 1. A spark plug, comprising: a second portion made of an insulating ceramic; and a first portion made of an insulating ceramic, wherein the first portion satisfies at least one of A, B, and C below. The insulator tip intended to be located on the side of the spark discharge gap is configured as a first portion, and at least a part of the remaining portion is configured as the second portion. : A: large average crystal grain size; B: small content of grain boundary phase; C: large crystallization rate of grain boundary phase. 2. The insulator for a spark plug according to claim 1, wherein said first portion and said second portion are both made of alumina ceramics mainly containing alumina. 3. The insulator according to claim 1 or 2 is disposed between a metal shell and a center electrode, and a ground electrode having a base end coupled to the metal shell at a distal end side of the insulator. A spark plug, wherein a spark discharge gap is formed between the spark plug and the center electrode.