JP2001002465A - Insulator for spark plug, its production and spark plug provided with the insulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an insulator for spark plug having a high voltage withstanding property even at a high temp. close to 700 deg.C, its production method and the spark plug provided with the insulator. SOLUTION: This insulator 2 for a spark plug consists essentially of alumina (Al2O3), contains at least two optional components of the tree-component system contg. silicon element, calcium element and magnesium element and has a mullite (Al6Si2O13) crystal phase as the crystal phase, and the insulator is formed by an aluminous sintered compact having >=95% relative density. By such a constitution, the insulator 2 for a spark plug hardly causes the puncture of insulation due to the residual pore present in the alumina grain boundary or a low-m.p. glass phase and has a high withstand voltage of >=60 kV/mm from room temp. to a high temp. close to 700 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spark plug used for igniting an internal combustion engine, an insulator used for the same, and a method of manufacturing the same.

[0002]

2. Description of the Related Art Alumina (Al ₂ O ₃ ) based insulating material has been conventionally used as an insulator for a spark plug (hereinafter simply referred to as “insulator”) of a spark plug used in an internal combustion engine such as an automobile engine. Alumina-based sintered bodies made of The reason is that alumina is excellent in heat resistance, mechanical properties, and withstand voltage properties. In particular, the insulator for a spark plug is exposed to heat of about 500 to 700 ° C. from a high-temperature combustion gas due to spark discharge generated in a combustion chamber of an internal combustion engine. It is important to have excellent characteristics. Here, as such an insulator, conventionally, for example, SiO ₂ −
An alumina-based sintered body using a ternary system of CaO-MgO as a sintering aid is used.

[0003]

However, when an insulator is formed by using a raw material powder obtained by mixing the above-mentioned three-component sintering aid with alumina powder, the three-component sintering aid is used. However, they exist as a low-melting glass phase at the alumina crystal grain boundaries after sintering. As a result, at a high temperature such as around 700 ° C., the low melting point glass phase is easily melted, so that not only the mechanical strength of the insulator is impaired, but also when the high voltage is applied to spark-discharge the spark plug. Dielectric breakdown (spark penetration) easily occurs, and it is hard to say that the dielectric strength is excellent.

In order to reduce the low-melting glass phase at the alumina crystal grain boundaries, it is conceivable to simply reduce the amount of the sintering additive to form an insulator. As a result, the densification of the insulator does not proceed, or many pores remain at the alumina crystal grain boundary even if the densification seems to be proceeding. As a result, when a high voltage is applied to cause spark discharge of the spark plug, an electric field concentrates on the pores (residual pores), and dielectric breakdown (spark penetration) of the insulator is likely to occur. It is hard to say that it is excellent.

Therefore, in order to improve the withstand voltage characteristics of the insulator, for example, Japanese Patent Publication No. 63-1262 discloses a method of generally using an alumina-based sintered body as a sintering aid. The withstand voltage characteristics are improved by limiting the blending ratio of the ternary system of SiO ₂ —CaO—MgO present as a low-melting glass phase in the field. Further, for the purpose of densifying an insulator, Japanese Patent Application Laid-Open
In the publication, an ungranulated raw material composition of the raw material powder is blended with a raw material composition obtained by granulating a raw material powder composed of alumina powder and the above-mentioned ternary sintering aid to a predetermined particle size. And sintering, or as disclosed in JP-A-62-14386.
In Japanese Patent Publication No. 6 (1999), by sintering two kinds of alumina powders having different particle diameters and a raw material powder comprising the above-mentioned ternary sintering aid, the residual pores in the alumina-based sintered body are reduced. In addition, the withstand voltage characteristics are improved.

For the purpose of improving the heat resistance (melting point) of the glass phase existing at the alumina crystal grain boundaries, for example, Japanese Patent Publication No. 7-17436 discloses Y ₂ O ₃ , La ₂ O ₃ , and ZrO _2. By forming an alumina-based sintered body using a novel sintering aid, the melting point of the glass phase existing at the alumina crystal grain boundaries is improved while reducing residual pores. Further, in Japanese Patent No. 2564842, an organic metal compound and an aluminum compound are added to alumina powder as a main component to prepare a raw material powder, so that Y ₄ Al ₂ is added to a triple point portion of uniform alumina crystal grains.
The O ₉ crystal phase is uniformly dispersed to improve the withstand voltage characteristics of the alumina-based sintered body.

In recent years, the area occupied by the intake and exhaust valves in the combustion chamber has increased with the increase in the output of the internal combustion engine and the miniaturization of the engine, and the spark plugs have tended to be miniaturized. . for that reason,
It is also required that the thickness of the insulator constituting the spark plug be reduced, and an insulator having even higher withstand voltage characteristics is required. However, in such a demand, even if the insulator is formed by the alumina-based sintered body of each of the above-mentioned publications, the withstand voltage characteristic can be increased to a sufficient level at a high temperature of around 700 ° C. It is hard to say that it is satisfactory,
Therefore, dielectric breakdown of the insulator may occur.

Accordingly, the present invention is intended to suppress the occurrence of dielectric breakdown due to the influence of residual pores and low-melting glass phase existing at the alumina crystal grain boundaries in the alumina-based sintered body constituting the insulator, and to reduce the temperature from room temperature to 700 ° C. It is an object of the present invention to provide a denser insulator that is more excellent in withstand voltage characteristics up to a high temperature such as around ° C. and a spark plug including the insulator.

[0009]

Means for Solving the Problems and Action / Effect To solve the above problems, the insulator for a spark plug according to the first aspect of the present invention comprises a crystal in an alumina-based sintered body constituting the insulator. At least mullite (Al ₆ Si ₂ O)
₁₃ ) By generating a crystal phase, excellent withstand voltage characteristics can be obtained at high temperatures such as around 700 ° C. That is, a point to be noted in the present invention is that an alumina-based sintered body constituting an insulator for a spark plug contains 19
The point is that a mullite crystal phase having a very high melting point such as around 00 ° C. is generated. The location of the mullite crystal phase in the insulator for a spark plug is not particularly limited, and it is preferable that the mullite crystal phase exists even inside the insulator for a spark plug (alumina-based sintered body). More preferably, it exists at the grain boundary and / or triple point.

Further, according to the present invention, the relative density of the insulator is set to 95% or more. This makes it possible to provide an insulator having excellent withstand voltage characteristics because the insulator having a small number of residual pores in which the electric field existing in the alumina crystal grain boundaries tends to concentrate, that is, a dense insulator is formed. The “relative density” is a percentage of the apparent density to the theoretical density of the sintered body.

The invention described in claim 2 is the first invention.
When the Si component, the Ca component, and the Mg component contained in the alumina-based sintered body described in (1) are converted to oxides, and the total of the three components is 100% by weight, the three components (SiO 2
The preferred range of the composition ratio of ( ₂ , CaO, MgO) is defined by a ternary composition diagram.

When the amount of SiO ₂ is less than the above range, the mullite crystal phase is hardly formed, and the withstand voltage characteristics at high temperatures such as around 700 ° C. are low. On the other hand, when the amount is large, the progress of densification of the alumina-based sintered body itself tends to decrease.
Further, when the CaO and MgO equivalent amount is larger than the range of the composition ratio of the present invention, respectively, hardly produce mullite crystalline phase in order to SiO ₂ equivalent amount is decreased, 70
The withstand voltage characteristics at high temperatures such as around 0 ° C. are low. On the other hand, if the CaO conversion amount is smaller than this, the withstand voltage characteristics at high temperatures such as around 700 ° C. will be low. Although the cause is not clear, it is considered that the low melting point glass phase inevitably remains at the alumina crystal grain boundary. That is, by controlling the composition ratio of the above three components, a mullite crystal phase is easily generated in the alumina-based sintered body, and a dense alumina-based sintered body can be formed.

[0013] Further, the invention according to claim 3 provides an alumina-based sintered body according to claim 1 or 2, wherein the Si component, the Ca component, and the Mg component are converted into oxides, respectively, and the three components are converted into oxides. When the total is 100% by weight, a preferable composition range of the three components (SiO ₂ , CaO, MgO) is defined in the three-component composition diagram. By setting the range of the composition ratio, the mullite crystal phase is easily generated more effectively in the alumina-based sintered body, and a denser alumina-based sintered body can be formed. The withstand voltage characteristics of the body can be further improved.

The invention according to claim 4 shows a preferable structure of the alumina-based sintered body according to any one of claims 1 to 3. Here, in the spark plug insulator, a high voltage is applied to the center electrode when spark discharge is performed to burn the air-fuel mixture in the spark plug. If coarse pores are present in the sintered body, an electric field is likely to be locally applied to the pores, and dielectric breakdown is likely to occur at high temperatures.

Therefore, as in the present invention, by setting the major axis of the pores present in the insulator to 10 μm or less in the dimension observed in the cross-sectional structure, good withstand voltage characteristics at high temperatures can be effectively improved. Can be secured. In addition,
Regarding the long diameter of the pores, a part of the insulator was sectioned, mirror-polished, and then subjected to a scanning electron microscope at a magnification of 1000.
A SEM photograph of the doubled structure is taken, pores are observed using the photograph, dimensions are calculated, and a part of the structure is assumed to represent the insulator for the spark plug. In addition, as shown in FIG. 8, the major axis of the pore refers to two parallel lines A and B that are in contact with the contour of the pore observed on the cross section and do not cross the pore. Is defined as the maximum dimension value d of the distance between the parallel lines A and B when various positions are drawn while changing the positional relationship with the pores.

A sixth aspect of the present invention is directed to a method of manufacturing a spark plug insulator according to any one of the first to fourth aspects. According to such a production method, a compact is formed from a raw material powder obtained by mixing each powder of the Si, Ca, and Mg components having a small average particle size of 1 μm or less with respect to an alumina powder having a small average particle size of 1 μm or less. And baking the formed body under the baking conditions according to claim 6, the ratio of each of the powder particles reacting with each other (that is, contacting) per unit surface area during baking becomes large, whereby the powder The interaction between the particles becomes active. As a result, a mullite crystal phase is effectively generated in the alumina-based sintered body constituting the insulator for the spark plug. In addition, since the reaction between the powder particles becomes active, it is possible to suppress the occurrence of voids between the powder particles and to increase the firing shrinkage, and to provide a dense insulator for a spark plug having a relative density of 95% or more. (Alumina-based sintered body).

The alumina powder used to form the insulator constituting the spark plug is generally manufactured by the Bayer method. In the Bayer method, alumina powder is wet-extracted from bauxite, which is a raw alumina ore, and a relatively high concentration of caustic soda (Na) is used as an extraction medium.
OH) aqueous solution is used. Therefore, the obtained alumina-based sintered body often inevitably contains a sodium (Na) component, which is an alkali metal component. The sodium component contained in the alumina-based sintered body has a high ionic conductivity, and when its content is excessive, the withstand voltage characteristics particularly at a high temperature of 500 ° C. or more are deteriorated. Or the mechanical strength may be lost at high temperatures.

Therefore, as in the invention according to claim 5,
The content of the sodium component contained in the alumina-based sintered body according to any one of claims 1 to 4 is 0.05% by weight in terms of oxide in 100% by weight of the alumina-based sintered body.
It is desirable to set the following. As described above, by setting the content of the sodium component (that is, the content of the sodium component in terms of oxide) within a predetermined range, the insulator formed of the alumina-based sintered body can be favorably formed at a high temperature of around 700 ° C. Withstand voltage characteristics and mechanical characteristics can be effectively secured.

Further, the content of the sodium component contained in 100% by weight of the alumina powder used as the raw material powder is 0.07% by weight in terms of oxide.
It is recommended to set the following range. In order to make the content of the sodium component in the alumina powder 0.07% by weight or less in terms of oxide, the alumina powder produced by the Bayer method must have a predetermined sodium content (the sodium component is calculated as oxide). The content can be set by performing a soda removal process until the content reaches the specified value. Although the sodium component contained in the alumina powder may theoretically be 0% by weight, the presence of a very small amount of the sodium component within the above-mentioned range increases the generation rate of the mullite crystal phase, and effectively increases the mullite crystal phase. A mullite crystal phase can be generated in the alumina-based sintered body.

Further, as described in claim 8, an axial center electrode, a metal shell arranged radially around the center electrode, and fixed to one end of the metal shell to face the center electrode The spark plug insulation according to any one of claims 1 to 6, further comprising a ground electrode disposed between the center electrode and the metal shell so as to cover a radial periphery of the center electrode. By providing the body, a spark plug having an insulator having excellent withstand voltage characteristics can be formed.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Some embodiments of the present invention will be described below with reference to the drawings. The spark plug 100 as an example of the present invention shown in FIG.
And an insulator 2 arranged so as to cover the circumference of the center electrode 3 in the radial direction, and a metal shell 4 holding the insulator 2. The metal shell 4 is made of carbon steel (JIS-G350).
7), and one end 5a of the ground electrode 5 is fixed to one end of the tip side 4a by welding or the like. The other end of the ground electrode 5 extends toward the tip-side center electrode 3a and is bent back into a substantially L-shape to connect the center electrode 3 (the tip-side center electrode 3a) and a predetermined spark discharge gap g. Has formed.

The insulator 2, which is a main part of the present invention, is formed vertically long while having a through-hole 6 for fitting the center electrode 3 in the center along the direction of its own center axis O. In addition, as described later, it is made of an insulating material containing alumina (Al ₂ O ₃ ) as a main component. The terminal electrode 7 is inserted and fixed on one end side, and the center electrode 3 is inserted and fixed on the other end side. Further, a resistor 8 is arranged between the terminal electrode 7 and the center electrode 3 in the through hole 6. Both ends of the resistor 8 are connected to the center electrode 3 and the terminal electrode 7 via the conductive glass layers 9 and 10, respectively.
And are electrically connected to each other. Note that the resistor 8
Is formed of a resistor composition obtained by mixing a glass powder and a conductive material powder (and a ceramic powder other than a glass powder, if necessary) and sintering the mixture by hot pressing or the like. Alternatively, the resistor 8 may be omitted, and the center electrode 3 and the terminal electrode 7 may be integrated by a single conductive glass seal layer.

Further, when looking at the insulator 2 in detail, a protruding portion 2 which protrudes outward in the circumferential direction is provided in the middle of the insulator 2 in the axial direction.
e is formed in a flange shape, for example. The insulator 2 has a main body 2b formed so that the side facing the tip of the center electrode 3 is the front side and the rear side of the protrusion 2e is thinner than 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. The glaze 2 is applied to the outer peripheral surface of the main body 2b.
d is given, and a corrugation 2 is provided at the rear end of the outer peripheral surface.
c is formed. 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 shape whose diameter decreases toward the tip.

Next, 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 larger diameter at the rear side (upper side in the figure) of the first portion 6a. And a substantially cylindrical second portion 6b. FIG.
As shown in (2), the terminal electrode 7 and the resistor 8 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 3a 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
The first shaft portions are connected to each other, and a convex portion receiving surface 6c for receiving the electrode fixing convex portion 3b of the center electrode 3 is formed in a tapered surface or an R surface at the connection position.

The outer peripheral surface of the connecting portion 2h between the first shaft portion 2g and the second shaft portion 2i is a stepped portion, which is formed as a convex as a metal shell side engaging portion formed on the inner surface of the metal shell 4. Shape part 4c
And through the annular plate packing 11,
The insulator 2 is prevented from coming off in the axial direction. On the other hand, between the inner surface of the rear opening portion of the metal shell 4 and the outer surface of the insulator 2, an annular wire packing 12 that engages with the rear peripheral edge of the flange-shaped protrusion 2e is disposed. On the side, an annular wire packing 14 is arranged via a powder talc 13. Then, the insulator 2 is pushed forward toward the metal shell 4, and in this state, the opening edge of the metal shell 4 is crimped inward toward the wire packing 14 to form an R-shaped portion 4b. The metal shell 4 is fixed to the insulator 2.

FIGS. 2A and 2B show some examples of the insulator 2. The dimensions of each part are exemplified below. Overall length L1: 30 to 75 mm. The length L2 of the first shaft portion 2g: 0 to 30 mm (however, not including the connection portion 2f with the protrusion 2e but including the connection portion 2h with the second shaft portion 2i). -The length L3 of the second shaft portion 2i: 2 to 27 mm. -The outer diameter D1 of the main body 2b is 9 to 13 mm. The outer diameter D2 of the protrusion 2e: 11 to 16 mm; -The outer diameter D3 of the first shaft portion 2g: 5 to 11 mm. -The proximal end outer diameter D4 of the second shaft portion 2i: 3 to 8 mm. -The outer diameter D5 at the distal end of the second shaft portion 2i (however, if the outer peripheral edge of the distal end surface is rounded or chamfered, the outer diameter at the base position of the R portion or chamfered portion in the cross section including the central axis O) Diameter): 2.5-7 mm. The inner diameter D6 of the second portion 6b of the through hole 6: 2 to 5 mm;・ Inner diameter D7 of first portion 6a of through hole 6: 1 to 3.5 m
m. -The thickness t1 of the first shaft portion 2g is 0.5 to 4.5 mm. -Base end wall thickness t2 of second shaft portion 2i (value in a direction orthogonal to central axis O): 0.3 to 3.5 mm. The thickness of the tip portion 3t of the second shaft portion 2i (a value in a direction orthogonal to the center axis O; however, when R or chamfering is performed on the outer peripheral edge of the tip surface, in a section including the center axis O,
The thickness at the base end position of the R portion or the chamfered portion): 0.2 to 3 mm. The average thickness tA of the second shaft portion 2i ((t1 + t2) /
2): 0.25 to 3.25 mm.

The dimensions of the respective parts of the insulator 2 shown in FIG. 2A are, for example, as follows: L1 = about 60 mm, L2 = about 10 mm, L3 = about 14 mm, D1
= About 11 mm, D2 = about 13 mm, D3 = about 7.3 m
m, D4 = 5.3 mm, D5 = about 4.3 mm, D6 =
3.9 mm, D7 = 2.6 mm, t1 = 1.7 mm, t
2 = 1.3 mm, t3 = 0.9 mm, tA = 1.5 m
m.

The insulator 2 shown in FIG. 2 (b) has a first shaft portion 2g and a second shaft portion 2i each having a slightly larger outer diameter than that shown in FIG. 2 (a). I have. The dimensions of each part are, for example, as follows: L1 = about 60
mm, L2 = about 10 mm, L3 = about 14 mm, D1 = about 11 mm, D2 = about 13 mm, D3 = about 9.2 mm, D
4 = 6.9 mm, D5 = about 5.1 mm, D6 = 3.9 m
m, D7 = 2.7 mm, t1 = 3.3 mm, t2 = 2.
1 mm, t3 = 1.2 mm, tA = 2.7 mm.

Next, a method of manufacturing the insulator 2 will be described.
Then, an example of the process will be described. First, according to the buyer method
Alumina having an average particle size of 1 μm or less (Al₂O
₃) Powder (contained in 100% by weight of alumina powder)
The content of sodium component is 0.07 weight in oxide equivalent
% Or less), silicon (Si) component and calcium
At least the (Ca) component and the magnesium (Mg) component
Each compound powder having an average particle size of 1 μm or less
To prepare a raw material powder. The added S
After sintering, the three components i, Ca, and Mg
When the converted total is 100% by weight, three components
The three components (SiO₂, CaO, MgO)
As shown in FIGS. 3 and 4, the composition ratio of
A (95, 5, 0), B (75, 25, 0), C (7
5,5,20) within the range surrounded by the line connecting the three points,
More preferably, D (92,8,0), E (78,22,
0), F (78, 8, 14)
So that it is added at a predetermined ratio so that
Adjust. The alumina powder constituting the raw material powder was prepared
100% by weight of the raw material powder to be produced
l₂O₃91 to 98% by weight, especially 9
Contained in the range of 4 to 98% by weight requires withstand voltage characteristics
It is preferable from the viewpoint of obtaining an improvement.

In this embodiment, as described later,
Si component is SiO ₂ powder, Ca component is CaCO ₃ powder,
The Mg component is added in the form of MgO powder, but Si, C
For each component of a and Mg, in addition to oxides (or composite oxides) of those components, hydroxides, carbonates, chlorides,
Various inorganic raw material powders such as sulfates, nitrates and phosphates may be used. However, it is necessary to use any of these inorganic raw material powders that can be oxidized by firing at a high temperature in the atmosphere and converted into oxides. Further, the average particle diameter of each of these inorganic raw material powders is 1 μm or less.

Then, a hydrophilic binder (eg, polyvinyl alcohol) and water as a solvent are added to the prepared raw material powder, and the mixture is wet-mixed with a ball mill using alumina balls to form a molding slurry. I do. Then
This molding slurry is spray-dried by a spray drying method or the like to prepare a spherical granulated powder for molding. Then, the obtained granulated powder for molding is poured into a rubber press rubber mold,
Press molding is performed while inserting a press pin for forming the through hole 6, and the outside of the obtained press molded body is cut with a resinoid grindstone to form a molded body having a predetermined insulator shape.
Then, at a temperature of 1450 ° C. to 165
The firing is carried out for 1 hour to 8 hours while maintaining the temperature within the range of 0 ° C., followed by finish firing with glaze to complete the insulator 2 as shown in FIG. 2, for example.

In the insulator 2 thus formed, at least a mullite (Al ₆ Si ₂ O ₁₃ ) crystal phase is generated as a crystal phase in the alumina-based sintered body constituting the insulator 2. The insulator 2 has high insulation (withstand voltage characteristics) at a high temperature such as around 700 ° C. Here, the reason why the mullite crystal phase is formed is that, for alumina powder having a small average particle size of 1 μm or less, each powder of Si, Ca, and Mg components having a small particle size of 1 μm or less is used. The raw material powder is prepared after being added in the composition range described above, and is fired, whereby each powder particle reacts with each other during firing (ie,
This is presumed to be due to the fact that the ratio per unit surface area) increases, and the reaction between the powder particles becomes active. In addition, since the interaction between the powder particles becomes active in this manner, voids are less likely to be generated between the powder particles, and firing shrinkage can be increased, and the insulating density for the spark plug is reduced to a relative density of 95% or more. A body (alumina-based sintered body) can be formed.

The mullite crystal phase generated in the alumina-based sintered body constituting the insulator 2 can be generated by adding mullite powder to raw material powder in advance. In this case, the average particle size of the mullite powder added as the raw material powder is preferably set to 1 μm or less, more preferably 0.5 μm.
It is preferably set to be not more than μm. However, when baking is performed after adding mullite powder as a raw material powder in advance, pores may be generated around mullite crystal grains. Voltage characteristics may be degraded. For this reason, as described above, it is better to add an arbitrary two-component ternary component including an SI component, a Ca component, and a Mg component to alumina powder to generate a mullite crystal phase during sintering. This can be said to be certain in achieving a relative density of 95% or more of the base sintered body).

Regarding the sintering temperature of the compact, if the sintering temperature is lower than 1450 ° C., a sufficiently densified insulator may not be obtained, while 1650 ° C.
If it is higher, the alumina crystal particles may grow abnormally during firing, which not only deteriorates the mechanical properties of the insulator, but also tends to generate coarse pores at the alumina crystal grain boundaries and withstand voltage. This leads to a decrease in characteristics.

Further, as for the firing time of the molded body kept within the above temperature range, it is desirable to keep it for 1 hour to 8 hours. If the firing time is shorter than 1 hour, a sufficiently dense insulator (alumina-based sintered body) cannot be obtained, and at the same time, no mullite crystal phase is generated. On the other hand, if the calcination time is longer than 8 hours, the alumina crystal particles may grow abnormally during calcination, and the breakdown voltage characteristics may be reduced as in the case where the calcination temperature is too high (1650 ° C. or higher). It leads to a decline.

That is, as a preferable firing condition of the molded body, it is maintained at a firing temperature of 1450 to 1650 ° C. for 1 to 8 hours. In holding the molded body within the above-mentioned temperature range, it may be performed for a predetermined time while maintaining the temperature of any one point in the above-mentioned temperature range constant, or may be performed for a predetermined time in the above-mentioned temperature range. It may be performed for a predetermined time while changing the temperature according to the heating pattern.

[0037]

EXAMPLES The following experiments were performed to confirm the effects of the present invention. (Example 1) First, an average particle size of 0.5 μm was used as a raw material powder.
m of alumina (Al ₂ O ₃ ) powder (the content of sodium in 100% by weight of alumina powder is 0.029% by weight in terms of oxide, and the purity is 99.8% or more). SiO _{2 with an} average particle size of 0.6 μm as a binder
Powder (purity 99.9%) and Ca having an average particle size of 0.8 μm
CO ₃ powder (purity 99.9%), average particle size 0.3 μm
MgO powder (purity: 99.9%) is weighed so as to have a quantitative ratio shown in Table 1, and then blended. Then, 2 parts by weight of polyvinyl alcohol as a hydrophilic binder and 70 parts by weight of water as a solvent were added to the total amount of the blended raw material powders as 100 parts by weight, and wet-mixed in a ball mill using alumina balls. Then, a molding slurry is prepared.
Next, this molding slurry is spray-dried by a spray-drying method to prepare a spherical granulated powder for molding, and sieved to a particle size of 10 to 355 μm. Then, the obtained granulated powder for molding is poured into a rubber press rubber mold, and press-molded at a pressure of about 100 MPa while inserting a press pin for forming a through hole 6, and the outside of the obtained press-molded body is formed. Cutting is performed with a resinoid grindstone to form a molded body having a predetermined insulator shape. Thereafter, each compact was fired in an air atmosphere at a firing temperature (maximum firing holding temperature) and a holding time shown in Table 1, and then finished and fired with a glaze to form an insulator as shown in FIG. 2 were each manufactured.

Then, the relative density of the obtained insulator, Al, Si, and C contained in the insulator are obtained by the following method.
Each test and analysis of the oxide conversion value of the component amount in each component of a and Mg, the major axis of pores, the presence or absence of a mullite crystal phase, the withstand voltage value at 700 ° C., and the withstand voltage test of the actual machine were performed. Table 2 shows the results.

Measurement of relative density: The relative density of each insulator was measured by the Archimedes method, and expressed as a ratio to the theoretical density according to the mixing rule.

Al, Si, Ca contained in the insulator
Oxide value of the component amount in each component of Mg and Mg: Analyzing each insulator (alumina-based sintered body) with X-ray fluorescence, and detecting the component amount in each component of Al, Si, Ca and Mg Al ₂ O ₃ , SiO ₂ , CaO and MgO respectively
The value in terms of oxide is shown.

Measurement of long diameter of pores: After a part of the cross section of each insulator is taken and mirror-polished, it is measured with a scanning electron microscope.
A SEM photograph of the tissue at a magnification of 00 was taken and observed and calculated using the photograph. Note that, as shown in FIG. 8, the long diameter of the pore is such that the outline of the pore observed on the cross section is in contact with the outline and does not cross the pore.
The parallel lines A and B are defined as the maximum dimension value d of the distance between the parallel lines A and B when various lines are drawn while changing the positional relationship with the pores. Here, the sample No. of the present embodiment was used. 6 shows an SEM image of Sample No. 3 as a comparative example.
FIG. 7 shows 12 SEM images.

Presence or absence of mullite crystal phase: A cross section perpendicular to the axial direction of each insulator is taken, and the cross-sectional structure is subjected to X-ray diffraction analysis to determine whether or not a spectrum corresponding to JCPDS card No. 15-776 exists. did. In addition,
The X-ray diffraction chart is shown in FIG.

Withstand voltage value at 700 ° C .: In this test, test pieces for withstand voltage value measurement were prepared using the same granulated powder as described above. For details,
The granulated powder is press-molded by die press molding (pressing force of 100 MPa), and the granulated powder is fired under the same conditions as the above-mentioned insulator to obtain a disc-shaped test piece of Φ25 mm × t (thickness) = 0.65 mm. Obtained. As shown in FIG. 6, each test piece 20 is sandwiched between electrodes 21a and 21b, and further fixed with alumina insulators 22a and 22b and sealing glass 23. 700 in 25
C., and a withstand voltage value was measured by applying a voltage to the electrodes using a high voltage generator (CDI power supply) 26.

Actual machine withstand voltage test: Using each insulator,
Each of the spark plugs shown in FIG. 1 is formed. Here, the screw diameter of the metal shell of the spark plug in this embodiment was 12 mm. Then, insert the spark plug into 4
Attached to a cylinder engine (displacement: 2000 cc), with the throttle fully open, engine speed of 6000 rpm, discharge voltage controlled at 35 kV and 38 kV, continuous operation with the tip temperature of the insulator set at around 700 ° C After 50 hours, it was evaluated whether spark penetration occurred in the insulator. In addition, the case where no abnormality was observed in the insulator after 50 hours had elapsed was indicated by ○, and the case where the insulator had undergone dielectric breakdown (spark penetration) in less than 50 hours was indicated by x.

[0045] (Example 2) Sodium converted oxide (Na) component using the five alumina _(Al 2 _{O 3)} powder containing within the scope set forth in Table 3, further each of the _Al 2 O ₃ powder 95 parts by weight, SiO ₂ powder 4.0
Parts by weight, 0.893 parts by weight of CaCO ₃ powder, Mg
A raw material powder was prepared by blending 0.5 part by weight of O powder (corresponding to sample No. 1 in Table 1 in Example 1). The average particle size of each powder was the same as in Example 1 described above. Then, an insulator was manufactured in the same manner as in Example 1 described above. The firing temperature (maximum firing holding temperature) and the holding time were determined under the conditions shown in Table 3. With respect to each of the obtained insulators, a withstand voltage value at 700 ° C. and each test of an actual device withstand voltage test were performed in the same manner as in Example 1 described above. Table 3 shows the results. .

[0046]

[Table 1]

[0047]

[Table 2]

[0048]

[Table 3]

According to Table 2, a mullite crystal phase is formed in the insulator (alumina-based sintered body), and the composition ratio of each component of Si, Ca, and Mg contained in the insulator in terms of oxide. In the ternary composition diagram of (SiO ₂ , CaO, MgO), A (95,5,0), B (7
5, 25, 0) and C (75, 5, 20) are within the range (including the line segment) surrounded by the line segment connecting the three points (see FIGS. 3 and 4), and further, the relative density Is 95% or more and the major axis of the pores remaining in the insulator is 10 μm or less, the withstand voltage value at a high temperature of 700 ° C. is 60%.
It turns out that it shows a high value of kV / mm or more.

As described above, the formation of the mullite crystal phase in the alumina-based sintered body constituting the insulator allows the formation of a mullite crystal phase.
It is considered that the reason why the withstand voltage characteristics are improved at a high temperature such as around 0 ° C. is that the heat resistance of the alumina-based sintered body is improved because the melting point of the mullite crystal phase is very high at around 1900 ° C. Further, the relative density of the insulator (alumina-based sintered body) is 95% or more in order to reduce the residual pores at the alumina grain boundaries, and the withstand voltage is improved due to the denser insulator. It is thought that it is.

In the present embodiment, it is more preferable that the composition ratio of each component of Si, Ca and Mg contained in the insulator in terms of oxide is (SiO ₂ , CaO, Mg)
In the ternary composition diagram of O), D (9
2,8,0), E (78,22,0), F (78,8,1)
This is within the range (including a line segment) surrounded by the three points 4) (see FIGS. 3 and 4). With the composition ratio in this range, the mullite crystal phase is effectively generated in the alumina-based sintered body, and as a result, it is considered that the withstand voltage characteristics are further improved.

On the other hand, in Table 2 No. of the comparative example. In No. 12, although the mullite crystal phase was generated, the withstand voltage was as low as 34 kV / mm because the relative density was low and the long diameter of the pores was large. In addition, the sample No. 13-No.
In No. 18, the mullite crystal phase was not generated, and the low melting point glass phase was present at the alumina crystal grain boundaries, and the withstand voltage value was low.

The sample number No. In No. 19, since the firing temperature was 1450 ° C. or less (1425 ° C.), the relative density was as low as 95% or less (89.3%), and the withstand voltage value was reduced to 30 kV / mm. Further, the sample number No. 20 is a temperature (1) where the firing temperature is 1650 ° C. or higher.
675 ° C.), resulting in oversintering, resulting in a relative density as low as 94.8% and a withstand voltage of 45 kV /
mm.

That is, in Table 2, the sample N according to the present invention was used.
o. 1 to No. The insulator of Sample No. 11 is a sample of Comparative Example. 1
2-No. 20 with higher withstand voltage characteristics compared to the insulator of 20,
It can be seen that it is suitable for a spark plug insulator exposed to a high temperature of around 700 ° C.

Next, according to the results in Table 3, the content of the sodium component in terms of oxide contained in the insulator was determined as follows.
It can be seen that the withstand voltage at 700 ° C. has a slight variation. In particular, in the sample No. In No. 22, the highest withstand voltage value was obtained, and it is presumed from this that an optimum value exists for the content of the sodium component in terms of oxide contained in the insulator. In this example, the content of sodium component in terms of oxide contained in 100% by weight of alumina powder was 0.07% by weight.
The following, namely, the obtained insulator (alumina-based sintered body)
It is assumed that when the content of the sodium component in terms of oxide contained therein is 0.05% by weight or less, the fluctuation in the withstand voltage value becomes small.

[Brief description of the drawings]

FIG. 1 is an overall front sectional view showing an example of a spark plug of the present invention.

FIG. 2 is a longitudinal sectional view showing some embodiments of an insulator for a spark plug.

FIG. 3 shows (SiO ₂ , CaO, MgO) in the present invention.
FIG. 4 is a ternary composition diagram of

FIG. 4 is an enlarged view of a main part of FIG. 3;

FIG. 5 is an X-ray diffraction chart of a spark plug insulator having a mullite crystal phase in a grain boundary phase composed of alumina particle crystals.

FIG. 6 is a SEM photograph of a 1000-fold structure taken with a scanning electron microscope in order to measure the maximum dimension of pores after mirror-polishing a sample (insulator) of an example of the present invention.

FIG. 7 is a SEM photograph of a micrograph at a magnification of 1000 times taken by a scanning electron microscope in order to measure the maximum size of pores after a section of a sample (insulator) of a comparative example is mirror-polished. .

FIG. 8 is a diagram illustrating the definition of the maximum size of pores in FIG. 6;

FIG. 9 is a schematic diagram showing an apparatus used for measuring a withstand voltage value at 700 ° C.

[Explanation of symbols]

 REFERENCE SIGNS LIST 100 spark plug 2 spark plug insulator (insulator) 3 center electrode 4 metal shell 5 ground electrode

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (Reference) H01T 21/02 H01T 21/02 // H01B 17/00 H01B 17/00 Z (72) Inventor Yoshihiro Yamamoto Mizuho Nagoya 14-18, Takatsuji-cho, Ward, Japan Special Ceramics Co., Ltd. (72) Inventor Katsura Matsubara 14-18, Takatsuji-cho, Mizuho-ku, Nagoya City Inside Japan Specialty Ceramics Co., Ltd. No. 18 Japan Special Ceramics Co., Ltd. (72) Inventor Atsushi Shimamori 14-18 Takatsuji-cho, Mizuho-ku, Nagoya City Inside Japan Special Ceramics Co., Ltd. (72) Masaya Ito 14-18 Takatsuji-cho, Mizuho-ku, Nagoya-shi F-term (reference) in Japan Special Ceramics Co., Ltd. AA09 AA11 AB16 AB22 BA01 CB01 CB19 CC14 CC15 DA01 EA02

Claims (8)

[Claims] 1. A method comprising alumina (Al ₂ O ₃ ) as a main component,
An insulator for a spark plug containing at least any two components of a ternary system containing a silicon component, a calcium component, and a magnesium component, having at least a mullite (Al ₆ Si ₂ O ₁₃ ) crystal phase as a crystal phase, An insulator for a spark plug, comprising an alumina-based sintered body having a relative density of 95% or more. 2. The silicon component, the calcium component and the magnesium component contained in the alumina sintered body are each converted into an oxide, and the total of the three components in terms of oxide is calculated as 100.
% By weight, the three components (SiO ₂ , Ca
O, MgO), the composition ratio of the following A, B,
The insulator for a spark plug according to claim 1, wherein the insulator is within a range surrounded by a line connecting three points C. Where A (95,5,0), B (75,25,0), C
(75, 5, 20). 3. The silicon component, the calcium component and the magnesium component contained in the alumina sintered body are each converted into an oxide, and the total of the three components converted into oxides is 100.
%, The composition ratio of the three components in the three-component composition diagram (Si
O ₂ , CaO, MgO) are represented by the following D
The insulator for a spark plug according to claim 1 or 2, wherein the insulator is within a range surrounded by a line connecting three points (E) and (F). However, D (92,8,0), E (78,22,
0) and F (78, 8, 14). 4. The major axis of the pores present in the alumina-based sintered body has a length of 1 in a dimension observed in a sectional structure.
The insulator for a spark plug according to any one of claims 1 to 3, wherein the thickness is 0 µm or less. 5. The insulator for a spark plug according to claim 1, wherein a content of a sodium component in 100% by weight of the alumina-based sintered body is 0. An insulator for a spark plug, wherein the content is not more than 05% by weight. 6. The method for producing an insulator for a spark plug according to claim 1, wherein the alumina compound powder has an average particle diameter of 1 μm or less, a silicon compound powder, a calcium compound powder and magnesium. Mixing a powder of at least one of the compound powders having an average particle size of 1 μm or less to prepare a raw material powder; forming the raw material powder into a predetermined insulator shape; by then retained and fired for 1 hour for 8 hours at a temperature range of to 1650 ° C., the step of generating a mullite _{(Al 6 Si 2 O 1 3} ) crystalline phase in the resulting alumina-based sintered body, consisting of A method for producing an insulator for a spark plug, comprising: 7. The method for producing an insulator for a spark plug according to claim 6, wherein the content of sodium component in 100% by weight of alumina powder used as the raw material powder is 0.1% in terms of oxide. A method for producing an insulator for a spark plug, wherein the content is not more than 07% by weight. 8. A shaft-shaped center electrode, a metal shell arranged radially around the center electrode, and a ground electrode fixed to one end of the metal shell and arranged to face the center electrode. And a cover disposed between the center electrode and the metal shell so as to cover a radial periphery of the center electrode.
A spark plug, comprising: the spark plug insulator according to any one of claims 1 to 5.