JP2000272958A - Insulating material for spark plug, its production and spark plug using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain both an insulating material for spark plug high voltage endurance characteristics at a high temperature in the vicinity of 700 deg.C and a spark plug provided with the insulating material. SOLUTION: This insulating material 2 for a spark plug comprises alumina (Al2O3) to be used for a spark plug 100 as a main component is constituted of an alumina base sintered compact which contains at least a silicon(Si) component and one or more rare earth elements (RE for short) and has >=95% theoretical density ratio. The insulating material 2 for a spark plug has an insulating material controlling the occurrence of dielectric breakdown affected by residual pores existing at an alumina crystal grain boundary and a low-melting glass phase and having excellent voltage endurance characteristics at a high temperature of about 700 deg.C by the constitution.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a spark plug used as an ignition source for an air-fuel mixture in an internal combustion engine,
The present invention relates to a spark plug insulator used therefor and a method of manufacturing the same.

[0002]

2. Description of the Related Art In a spark plug used for an internal combustion engine such as an automobile engine, a spark plug insulator (hereinafter, also simply referred to as "insulator") has been conventionally used.
It is formed of an alumina-based sintered body obtained by firing an alumina (Al ₂ O ₃ ) -based material. The reason is that alumina is excellent in heat resistance, mechanical strength, and withstand voltage characteristics. Conventionally, in forming this insulator (alumina-based sintered body), for example, silicon oxide (SiO ₂ ) -calcium oxide (CaO) -oxide A ternary system composed of magnesium (MgO) is used as a sintering aid.

[0003]

By the way, the insulator for the spark plug is a high-temperature combustion gas (about 2000 to 3000 ° C.) caused by spark discharge generated in the combustion chamber of the internal combustion engine.
Exposed to a temperature of about 500 to 700 ° C. For this reason, it is important for the spark plug insulator to have excellent withstand voltage characteristics in a range from room temperature to the high temperature. In particular, in recent years, with the increase in the output of the internal combustion engine and the miniaturization of the engine, the enlargement of the area occupied by the intake and exhaust valves in the combustion chamber and the use of four valves have been studied, and the spark plug itself has been reduced in size ( (Diameter reduction). Therefore, it is required that the thickness of the insulator is also reduced, and the insulator is required to have more excellent withstand voltage characteristics even when exposed to heat of about 500 ° C. to 700 ° C. It is becoming.

However, when an insulator (alumina-based sintered body) is formed using a three-component sintering aid as described above, the three-component sintering aid (mainly a Si component) is used. )
However, after sintering, a low-melting glass phase is present at the grain boundaries of the alumina crystal particles. Therefore, when the insulator is exposed to heat at about 700 ° C., the low-melting glass phase is affected by the temperature. Softening leads to a decrease in withstand voltage characteristics.
Therefore, for the purpose of reducing the low melting point glass phase, it is conceivable to simply reduce the amount of these sintering aids to form an insulator, but the densification of the insulator does not progress, or at first glance Even if the densification is progressing, many pores remain at the grain boundaries formed by the alumina crystal particles, and these cause a decrease in withstand voltage characteristics.

In other words, if pores (residual pores) exist at the grain boundaries formed by alumina crystal grains, or if the grain boundaries are composed of a low-melting grain boundary phase (low-melting glass phase), the insulator will be heated to about 700 ° C. When a high voltage of several tens of kV is applied to cause spark discharge of a spark plug, an electric field concentrates on residual pores existing at grain boundaries, or a grain boundary phase is formed. It may soften or cause dielectric breakdown (spark penetration) of the insulator.

Accordingly, the present invention suppresses the occurrence of dielectric breakdown due to the influence of the low melting point glass phase at the residual pores existing at the grain boundaries in the insulator (alumina-based sintered body) and the grain boundaries, and to reduce the occurrence of dielectric breakdown.
An insulator that is more excellent in withstand voltage characteristics and is made denser even when exposed to heat at about 700 ° C., and a method of manufacturing an insulator capable of manufacturing such an excellent insulator; Another object of the present invention is to provide a spark plug using the same.

[0007]

According to the first aspect of the present invention, there is provided an insulator for a spark plug according to the first aspect of the present invention, which comprises a spark plug mainly composed of alumina (Al ₂ O ₃ ). Insulator for at least silicon (Si) component and one or more rare earth elements (hereinafter, R
E. FIG. And the theoretical density ratio is 95.
% Or more of an alumina-based sintered body.

According to the present invention, at least an Si component and one or more types of R
E. FIG. By containing these components, an insulator having excellent withstand voltage characteristics at a high temperature of about 700 ° C. can be obtained. The reason for this is that the Si component and R
E. FIG. By containing the components, both components are likely to be melted at the time of firing to form a liquid phase, and function as a sintering aid for accelerating the densification of the insulator (alumina-based sintered body). That is, by the inclusion of the Si component, the ratio of the residual pores in which the electric field tends to concentrate in the insulator is small, that is, a dense insulator can be obtained.

In the insulator of the present invention, as described above, the Si component and the RE. The inclusion of these components makes it possible to make the insulator denser, but it is important that the theoretical density ratio of the obtained insulator be 95% or more. If the theoretical density ratio of the insulator is less than 95%,
This is because the withstand voltage characteristics at a high temperature of about 700 ° C. may be reduced. The "theoretical density" here means
It means the density calculated by converting the content of each element component constituting the sintered body into an oxide and calculating the mixing content from the content of each oxide. The “theoretical density ratio” indicates the ratio of the sintered body density measured by the Archimedes method to the theoretical density.

Incidentally, the above-mentioned Si component functions as a sintering aid for accelerating densification, but also exists as a low-melting glass phase at the grain boundaries of alumina crystal particles. Therefore, in the present invention, one or more types of RE. It is important to include the components. This R
E. FIG. Component is contained in the insulator together with the Si component, so that RE. The component is S
RE. It is considered that a high melting point phase such as a glass phase made of -Si is generated to improve the melting point of the grain boundary phase. And, by the generation of this high melting point phase, 7
Softening of the grain boundary phase at a high temperature of about 00 ° C. can be prevented or suppressed, and the withstand voltage characteristics can be improved. The RE. The components include Sc (scandium), Y (yttrium), and La (lanthanum) to Lu (lutetium) of Group 3a of the periodic table.
The lanthanoid element described above is included, and by including any of the components in the insulator (alumina-based sintered body), the effect of improving the melting point of the grain boundary phase and, consequently, improving the withstand voltage characteristics can be obtained. .

That is, according to the present invention, at least S is contained in the alumina-based sintered body constituting the insulator for the spark plug.
i component and one or more RE. Including ingredients, and
By setting the theoretical density ratio of the sintered body to 95% or more, the withstand voltage characteristic of the insulator at a high temperature of about 700 ° C. is superior to that of the conventional spark plug, and the insulator is small and the thickness of the insulator is thin. When applied to a spark plug or when applied to a spark plug for a high-output internal combustion engine having a high temperature in a combustion chamber, troubles such as dielectric breakdown (spark penetration) can be effectively prevented.

Further, in the insulator for a spark plug according to the present invention, RE. As described in claim 2, when the component is contained in the range of 0.01 to 18% by weight in terms of oxide, the withstand voltage characteristic at a high temperature of about 700 ° C. can be more effectively improved. It is preferable for obtaining.

The insulator for a spark plug according to the present invention may contain at least one of a calcium (Ca) component and a magnesium (Mg) component. In this case, the silicon component (S: unit is% by weight), the calcium component (C: unit is% by weight), the magnesium component (M: unit is% by weight), and the Si component with respect to the total of the oxides. It is preferable to satisfy the relational expression of S / (S + C + M) ≧ 0.7 on a weight basis.

Like the Si component, the Ca component and the Mg component melt during firing to form a liquid phase and function as sintering aids for accelerating the densification of the insulator (alumina-based sintered body). Accordingly, the densification of the insulator can be improved (the theoretical density ratio can be improved). And Si, C
When each component of the ternary system of a and Mg is contained in the insulator, the ratio of the Si component is adjusted as in the above relational expression, so that the Si component is reduced to RE. Together with the components, a high melting point phase can be effectively generated in the grain boundary phase, and the withstand voltage characteristics can be improved.

Further, in the above relational expression,
0.95 ≧ S / (S + C + M) ≧ 0.
By satisfying the relational expression 75, it is preferable to configure an insulator for a spark plug having at least a mullite (Al ₆ Si ₂ O ₁₃ ) crystal phase as a crystal phase. In the present invention, when each component of the ternary system is contained, by adjusting the ratio of the Si component within the range of the relational expression, the Si component is reduced to RE. A high melting point phase can be generated in combination with the components, and a mullite crystal phase having a melting point of about 1900 ° C. can be generated, thereby improving the withstand voltage characteristics of the insulator at a high temperature of about 700 ° C. it can.

In the above relational expression, when the proportion of the Si component is less than 0.75 or more than 0.95, almost no mullite crystal phase is formed. In particular, in the above relational expression, 0.92 ≧ S / (S + C
+ M) ≧ 0.78, the mullite crystal phase can be more effectively generated in the insulator (alumina-based sintered body). The location of the mullite crystal phase in the insulator is not particularly limited, but is preferably present inside the insulator (alumina-based sintered body), and furthermore, two grain boundaries of alumina and / or More preferably, it is present at the triple point.

In addition, RE. It is preferable that at least a neodymium (Nd) component is contained as a component. By containing the Nd component, the variation in the withstand voltage value of each of the insulators at a high temperature of about 700 ° C. when a plurality of insulators are manufactured can be reduced by other RE. The size can be reduced as compared with the case where a component is included. This means that when mass-producing insulators,
An insulator having excellent withstand voltage characteristics at a high temperature of about 700 ° C. can be reliably obtained.

Further, RE. By containing at least an Nd component as a component, as described in claim 5,
RE. -Β-alumina (composition formula: RE.Al
₁₁ O ₁₈ ) structure (hereinafter simply referred to as “RE.-β-
(Referred to as “alumina crystal phase”).

This is because the radius of the ion (Nd ³⁺ ) of Nd is R
E. FIG. Since it is the smallest among the components, it produces a high melting point crystalline phase in combination with the Si component, and also has a RE. It is considered that -β-alumina crystal phase (Al ₁₁ NdO ₁₈ crystal phase) is formed. And this high melting point RE. The generation of the -β-alumina crystal phase can improve the withstand voltage characteristics of the insulator. RE. In insulator
The location of the -β-alumina crystal phase is not particularly limited, and it is preferable that the -β-alumina crystal phase is present even inside the insulator (alumina-based sintered body). More preferably, it is at the point of importance.

The Nd component is described in RE. -Β-
Although identification by X-ray diffraction is not directly possible due to the absence of a JCPDS card for alumina, Nd ³⁺ has almost the same ionic radius as La ³⁺ , so Nd ³⁺
JCPDS card whose component is La-β-alumina (No.
33-699). RE. -Β-alumina crystal phase is Nd
In addition to the components, La and Pr (praseodymium) components can be generated in the case where they are contained in the insulator, but it is necessary to generate a crystal phase including at least the Nd component in consideration of the variation in the withstand voltage value. Is preferable. Also, R
E. FIG. -Β-alumina crystal phase is RE. Although β-alumina can be added in advance as a raw material powder, the crystal phase is formed during firing because the anisotropy of grain growth during firing may hinder densification of the sintered body. Preferably.

Further, the insulator of the present invention is characterized in that at least a silicon compound powder and at least one kind of RE. Mixing the component-based compound powder to prepare a raw material powder, forming the molded body having a predetermined insulator shape using the raw material powder, and bringing the molded body into a temperature range of 1450 to 1650 ° C. And firing for 1 to 8 hours. By controlling the average particle size of each of the powders in this manner and firing under the firing conditions, the reaction (contact) between the powder particles during firing becomes favorable,
As a result, a high melting point crystal phase can be effectively generated in the obtained insulator. Further, since the reaction between the respective powder particles becomes favorable, the firing shrinkage can be increased, and a dense insulator having a theoretical density ratio of 95% or more can be manufactured.

The spark plug according to the present invention, as set forth in claim 8, has a shaft-shaped center electrode, a metal shell disposed radially around the center electrode, and fixed to one end of the metal shell. The ground electrode is disposed so as to face the center electrode, and is disposed between the center electrode and the metal shell so as to cover the radial periphery of the center electrode,
The spark plug having the insulator for a spark plug according to any one of claims 1 to 6 has an excellent withstand voltage characteristic at a high temperature of about 700 ° C., and does not easily cause dielectric breakdown (spark penetration). Can be configured.

[0023]

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, for example, carbon steel (JIS-G3
507), and one end 5a of the ground electrode 5 is fixed to one end of the distal end 4a by welding or the like. And
The other end of the ground electrode 5 extends toward the front end center electrode 4a and is bent back into a substantially L-shape to form a predetermined spark discharge gap g with the center electrode 3 (front end center electrode 3a). I have.

A through hole 6 is formed in the insulator 2 which is a main part of the present invention along the direction of its own central axis O.
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.
The terminal electrode 7 and the center electrode 3
The resistor 8 is disposed between the first and second resistors. Both ends of the resistor 8 are electrically connected to the center electrode 3 and the terminal electrode 7 via conductive glass layers 9 and 10, respectively. The resistor 8 is formed of a resistor composition obtained by mixing glass powder and conductive material powder (and ceramic powder other than glass powder as necessary) and sintering by hot pressing or the like. . Also, omitting the resistor 8,
The central electrode 3 and the terminal electrode 7 may be integrated by one conductive glass seal layer.

The insulator 2 has a through hole 6 into which the center electrode 3 is fitted along the center axis O of the insulator 2.
The whole is made of the insulating material of the present invention. That is,
The insulating material is mainly composed of alumina (Al ₂ O ₃ ), and has at least a silicon (Si) component and at least one kind of RE. Component, especially RE. The component is composed of an alumina-based sintered body contained in the range of 0.01 to 18% by weight in terms of oxide.

Further, when looking at the insulator 2 in detail, as shown in FIG. 1, a protrusion 2e projecting outward in the circumferential direction is formed in the middle of the insulator 2 in the axial direction, for example, in a flange shape. . 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. 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 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. FIG. The dimensions of each part are exemplified below.・ 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.

In the insulator 2 shown in FIG. 2B, the first shaft portion 2g and the second shaft portion 2i each have a slightly larger outer diameter than that shown in FIG. 2A. 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, the insulator 2 is manufactured by the following method, for example. First, alumina (Al ₂ O ₃ ) powder, Si component, and calcium (C
a) component, each inorganic powder of magnesium (Mg) component, and RE. The component-based powder is added and then blended, and a hydrophilic binder (for example, polyvinyl alcohol) and water as a solvent are added and mixed to prepare a molding base slurry.

Alumina powder, which is the main component of the raw material powder,
It is preferable to use those having an average particle size of 2 μm or less.
If the average particle size exceeds 2 μm, it tends to be difficult to sufficiently advance the densification of the sintered body itself, which may lead to a decrease in withstand voltage characteristics of the insulator. The alumina powder constituting the raw material powder is contained in 100% by weight of the alumina-based sintered body after firing, in terms of oxide of Al component,
It is preferable to appropriately adjust the content to fall within the range of 75.0 to 99.7% in order to obtain good withstand voltage characteristics.

RE. As a component-based powder,
E. FIG. There is no particular limitation on the type of the substance as long as the substance can be converted into the oxide of the component. Examples of the powder include oxides of the components and composite oxides thereof. In addition, RE. The component-based powder contains RE.RTM. In 100% by weight of the alumina-based sintered body after firing. 0 in terms of oxide of the component.
It is appropriately adjusted so as to be in the range of 01 to 18% by weight,
It needs to be added. RE. Examples of the component include Sc (scandium), Y (yttrium), and lanthanoid elements from La (lanthanum) to Lu (lutetium) of Group 3a of the periodic table.

Further, the Si component can be added to the alumina powder in the form of SiO ₂ powder, the Ca component can be added to the CaCO ₃ powder, and the Mg component can be added to the alumina powder in the form of MgO powder. In addition, Si, Ca,
Regarding each component of Mg, hydroxide, carbonate, chloride, sulfate,
Various inorganic powders such as nitrates and phosphates may be used. However, it is necessary to use any of these inorganic powders that can be oxidized by baking at a high temperature in the atmosphere and converted into an oxide.

Each of the inorganic powders to be added includes Si component (S: unit is% by weight), Ca component (C: unit is% by weight), and Mg component (M: unit is% by weight) in terms of oxide. The ratio of the Si component to the total component is expressed as S
It is necessary to add and adjust appropriately so as to satisfy the relational expression of /(S+C+M)≧0.7, more preferably 0.95 ≧ S / (S + C + M) ≧ 0.75.
The average particle diameter of each inorganic powder is preferably 1 μm or less for each inorganic powder. When the average particle size of each inorganic powder is within the above range, the alumina powder and RE. It is considered that the reaction (contact) in the sintering process with the component powder is improved, and furthermore, the firing shrinkage is increased, and a dense insulator can be formed.

There is no particular limitation on the water used as a solvent in preparing the molding slurry, and the same water as used in the case of producing a conventional insulator can be used. As the binder, for example, a hydrophilic organic compound can be used, and examples thereof include polyvinyl alcohol (PVA), a water-soluble acrylic resin, gum arabic, and dextrin. Among them, PVA is most preferable. The method of preparing the molding slurry is not particularly limited, and any mixing method may be used as long as the raw material powder, the binder and the water can be mixed to form the molding slurry. The mixing amount of the binder and water is 0.1 when the raw material powder is 100 parts by weight.
1 to 5 parts by weight, especially 0.5 to 3 parts by weight,
0 to 120 parts by weight, especially 50 to 100 parts by weight.

The molding base slurry is spray-dried by a spray drying method or the like to prepare a spherical molding base granule. The average particle size of the granulated product is 30 to 200
μm is preferred, and particularly preferably 50 to 150 μm.
Then, the obtained granulated body for molding is subjected to rubber press molding to produce a press molded body that is a prototype of the insulator. The outside of the obtained press-formed body is cut by a resinoid grindstone or the like to finish the outer shape corresponding to FIG. 2, and then fired at 145 ° C. in an air atmosphere.
The molded body is fired within the range of 0 ° C. to 1650 ° C. for a firing time of 1 to 8 hours, and thereafter, is fired with a glaze and finish fired to complete the insulator 2.

Regarding the firing temperature of the molded body, if the firing temperature is lower than 1450 ° C., a sufficiently dense insulator may not be obtained. On the other hand, when the sintering temperature exceeds 1650 ° C., the alumina crystal particles grow abnormally during the sintering, so that the mechanical strength of the insulator tends to decrease and coarse pores are formed at the grain boundaries. This easily leads to a decrease in withstand voltage characteristics.

The firing time of the green body under the firing temperature conditions is preferably maintained for 1 hour to 8 hours. If the firing time is shorter than 1 hour, a sufficiently dense insulator (alumina-based sintered body) may not be obtained. On the other hand, when the calcination time is longer than 8 hours, the alumina crystal particles grow abnormally during the calcination, so that the withstand voltage characteristics deteriorate as in the case where the calcination temperature is too high (1650 ° C. or more). Leads to In holding the formed body within the firing temperature range, the formed body may be held for a predetermined time while maintaining an arbitrary temperature within the temperature range, or may be heated for a predetermined time within the temperature range. The temperature may be maintained for a predetermined time while changing the temperature according to the pattern.

The operation of the spark plug 100 will be described below. That is, the spark plug 100 is attached to the engine block by the screw portion 4d formed in the metal shell 4, and is used as an ignition source for the air-fuel mixture introduced into the combustion chamber. Here, since the insulator 2 used in the spark plug 100 is made of the insulator of the present invention, the withstand voltage characteristics at a high temperature of about 700 ° C. are improved, and the high temperature in the combustion chamber becomes high. Even when used in an output engine, insulation breakdown (spark penetration) is unlikely to occur, and high reliability can be ensured.

For example, as shown in FIGS. 2A and 2B, a shaft having a smaller diameter and a smaller thickness in the radial direction is provided on the insulator 2 in front of the engaging protrusion 2e. When a portion (in this case, a portion where the first shaft portion 2g and the second shaft portion 2i are combined) is formed, the shaft portion, for example, the second shaft portion 2 is formed.
At i, dielectric breakdown (spark penetration) easily occurs. Therefore, in such an insulator 2, the insulator of the present invention is particularly useful. For example, in the insulator of FIG. 4A, the average thickness tA of the second shaft portion 2i is 1.5 mm, but such a thin portion is formed around the center electrode 3. However, by applying the insulator of the present invention, it is possible to effectively prevent or suppress occurrence of troubles such as dielectric breakdown (spark penetration).

The spark plug to which the insulator of the present invention can be applied is not limited to the type shown in FIG. 1. For example, a spark discharge gap may be provided between a plurality of ground electrodes with their ends facing the side surface of the center electrode. May be formed.
In this case, a semi-creeping type spark plug in which the tip of the insulator enters between the side face of the center electrode and the tip face of the ground electrode may be configured. In this configuration, since spark discharge is performed along the surface of the distal end portion of the insulator, the stain resistance against smoking and the like is improved as compared with the air-discharge type spark plug.

[0044]

EXAMPLES (Example 1) To confirm the effects of the present invention.
Then, the following experiment was performed. First, the alumina shown in Table 1
(Al₂O₃) Powder (all of which have a purity of 99.8% or more)
Among them, the number a having an average particle size of 0.1 to 2.2 μm
Select one of each from Lumina powder
Luminous powder with an average particle size of 0.6 μm SiO₂Powder (purity
99.9%) and CaCO having an average particle size of 0.8 μm.₃Powder
(Purity 99.9%) and MgO powder with an average particle size of 0.3 μm
Powder (purity 99.9%).
RE-1 having an average particle size of 1.0 to 19.0 μm. Oxide
(RE. Component powder) was weighed so as to have a quantitative ratio shown in Table 3.
After being weighed and added, a raw material powder was prepared. In addition, various
RE. The oxide is alumina powder, SiO₂Powder, CaC
O ₃Externally added to the total amount of powder and MgO powder
Was added.

Then, 2 parts by weight of PVA as a hydrophilic binder and 70 parts by weight of water as a solvent are mixed with the total amount of these raw material powders as 100 parts by weight, and wet-mixed in a ball mill using alumina balls. Thus, a molding base slurry is prepared. Next, this molding base slurry is spray-dried by a spray drying method or the like to prepare a spherical molding base granule, and the particle size is 10 to 355 μm using a sieve.
Sized to m. Then, the obtained granulated body for molding is put into a rubber press mold, and subjected to rubber press molding at a pressure of about 100 MPa while inserting a rubber press pin for forming a through hole 6, and the obtained press molded body is formed. The outside is cut with a resinoid grindstone to form a molded body having a predetermined insulator shape. Thereafter, each molded body was fired in the atmosphere at the firing temperature (maximum firing holding temperature) and holding time shown in Table 3, and then finished and fired with glaze.
Insulators 2 as shown in FIG.

Then, by the following method, the theoretical density ratio of the insulator obtained by firing and the RE.
Oxide value of the component, the value of the relational expression of S / (S + C + M) described above, and the RE. Each test and each analysis of the presence / absence of a crystal phase having a -β-alumina structure and a mullite crystal phase, a withstand voltage value at 700 ° C., and an actual withstand voltage test were performed. Table 4 shows the results.

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

[0048] RE. Component oxide
Converted value: Each insulator is analyzed by X-ray fluorescence and detected from it
RE. The amounts of the components were shown as oxide-converted values. This
Here, the Re. Of the components, La, Nd, Dy
(Dysprosium), Er (erbium), Sc and Y
With respect to La₂O₃, Nd₂O₃, Dy ₂
O₃, Er₂O₃, Sc₂O₃, Y₂O₃Converted as
And for Pr ₆O₁₁To be converted as
I do.

The value of the relational expression of S / (S + C + M): Each insulator is chemically analyzed, and the Si component and Ca
The values of the component and the Mg component were converted to oxides, respectively, and were calculated by the above relational expressions.

RE. Presence / absence of -β-alumina crystal phase and mullite crystal phase: In each insulator, take a cross section orthogonal to its own axis, perform X-ray diffraction of the cross-sectional structure, and perform JCP
DS card No. 33-699, no. The determination was made based on whether or not a spectrum corresponding to 15-776 was present.
FIG. 3 shows the Nd-β-alumina crystal phase (Al ₁₁ NdO
₁₈ ) shows an X-ray diffraction chart of Sample No. 10 which is an example having ()). FIG. 4 shows a mullite crystal phase (Si ₂ A
showed X-ray diffraction chart of Sample No. 8 is an example having l ₆ O _13). In addition, even when each crystal phase exists, there is a case where it does not clearly appear as a spectrum by X-ray diffraction due to, for example, a very small amount of the crystal phase. .

Withstand voltage value at 700 ° C .: In this test, the same molding base granules as described above were used.
Test pieces for withstanding voltage value measurement were prepared. Specifically, a green body for molding is molded by die press molding (pressing force of 100 MPa), which is fired under the same conditions as the insulator, and Φ25 mm × t (thickness) = 0.
A 65 mm disk-shaped test piece was obtained. Then, as shown in FIG. 5, each test piece 20 is sandwiched between electrodes 21 a and 21 b, and further fixed by alumina insulating cylinders 22 a and 22 b and sealing glass 23, and a heating box is heated by an electric heater 24. 25 is heated to 700 ° C. and a high voltage generator (CD) for applying a high voltage of about several tens kV to each test piece 20.
A withstand voltage value of each test piece 20 was measured by applying a constant high voltage using an I 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), the temperature of the tip of the insulator (the lower part in the figure) was set to about 700 ° C. while controlling the discharge voltage at 35 kV and 38 kV at a throttle fully open state and an engine speed of 6000 rpm. The continuous operation was performed on the above, and after 50 hours, it was evaluated whether or not spark penetration occurred in the insulator. In the case where no abnormality was observed in the insulator after 50 hours, a mark "○" was applied.
Those in which insulation breakdown (spark penetration) was observed in the insulator in less than 0 hours are indicated by crosses.

[0053]

[Table 1]

[0054]

[Table 2]

[0055]

[Table 3]

[0056]

[Table 4]

According to the results shown in Table 4, the relational expression of S / (S + C + M) satisfies 0.7 or more (in other words, contains the Si component). Component and the theoretical density ratio is 95
% For samples Nos. 1 to 17
The withstand voltage value at 0 ° C. was as good as 60 kV / mm or more.
It can be seen that good results are obtained at both V and 38 kV. Among them, RE. Component content is 0.01
In the range of 18% by weight, the withstand voltage at 700 ° C. showed a better value.

On the other hand, RE. In the case of the sample No. 18 which is a comparative example containing no component, the withstand voltage at 700 ° C. was inferior to 47 kV / mm. Also, R
E. FIG. Sample No. 1 whose content in terms of oxide was within a predetermined range, but whose theoretical density ratio was 95.0% or less.
In 9 and 20, the withstand voltage at 700 ° C. was 43 k each.
V / mm and 32 kV / mm were inferior. This indicates that the effect of improving the withstand voltage value cannot be obtained when the theoretical density ratio is lower than 95%.

(Example 2) Sample No. 4 used in Example 1
, And 10 test pieces for withstand voltage measurement were manufactured by the same method as in Example 1, and with respect to each of the obtained test pieces, the measurement of the withstand voltage value at 700 ° C. was performed as in Example 1. It carried out by the same method. FIG. 6 shows plots of the withstand voltage values of Sample Nos. 4 and 6, respectively.

From the results shown in FIG. 6, it is found that the sample No. 4 containing the Nd component and the sample No. 6 containing the La component both have a withstand voltage at 700 ° C. of 60 kV / mm or better, but N
In the case of the one containing the d component, the variation in the withstand voltage value was smaller than that of the one containing the La component.
It can be seen from the present embodiment that all the zero samples show a withstand voltage value of 60 kV / mm, and the highest and lowest withstand voltage values show higher values than those of the La component. Thereby, even when the insulator of the present invention containing the Nd component is mass-produced,
It can be said that an insulator having excellent withstand voltage characteristics at a high temperature of about 700 ° C. can be reliably obtained.

[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. Nd-β-alumina crystal phase (Al ₁₁ NdO)
₁₈ ) is an X-ray diffraction chart of Sample No. 10 which is an example having ( ₁₈ ).

FIG. 4 is an X-ray diffraction chart of Sample No. 8, which is an example having a mullite crystal phase (Si ₂ Al ₆ O ₁₃ ).

FIG. 5 is a schematic view showing an apparatus used for measuring a withstand voltage value of each test piece of the example at 700 ° C.

FIG. 6 is a diagram showing a variation in a withstand voltage value of each test piece of Sample Nos. 4 and 6 which are examples.

[Explanation of symbols]

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

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yoshihiro Yamamoto 14-18 Takatsuji-cho, Mizuho-ku, Nagoya-shi Inside Japan Special Ceramics Co., Ltd. (72) Inventor Katsura Matsubara 14-18 Takatsuji-cho, Mizuho-ku, Nagoya Japan Special Inside Ceramics Co., Ltd. (72) Inventor Kuniharu Tanaka 14-18, Takatsuji-cho, Mizuho-ku, Nagoya-shi Inside Japan Special Ceramics Co., Ltd. (72) Inventor Masaya Ito 14-18 Takatsuji-cho, Mizuho-ku, Nagoya-shi F-term in Japan Special Ceramics Co., Ltd. AB02 BA11 CA01 CB01 CB06 CB15 CB17 CB22 CB26 CB30 CB40 CB41 CB43

Claims (8)

[Claims] 1. An insulator for a spark plug containing alumina (Al ₂ O ₃ ) as a main component, comprising at least a silicon (Si) component and at least one kind of rare earth element (hereinafter, referred to as RE.) Component. And an alumina-based sintered body having a theoretical density ratio of 95% or more. 2. The RE. The components were added in an amount of 0.
The insulator for a spark plug according to claim 1, which is contained in the range of 01 to 18% by weight. 3. A composition containing at least one of a calcium (Ca) component and a magnesium (Mg) component, and the contents of the silicon component, the calcium component and the magnesium component in terms of oxides, respectively. S (unit: wt%), C (unit: wt%) and M
3. The insulator for a spark plug according to claim 1, wherein a relational expression of S / (S + C + M) ≧ 0.7 is satisfied when (unit:% by weight). 4. 4. The content of each of the silicon component, the calcium component, and the magnesium component in terms of oxides is S (unit:% by weight) and C (unit:% by weight), respectively.
And M (unit: wt%), satisfying the following relationship: 0.95 ≧ S / (S + C + M) ≧ 0.75, and mullite (Al
₆ Si ₂ O ₁₃₎ for a spark plug insulator according to claim 3 having a crystalline phase. 5. The RE. The insulator for a spark plug according to any one of claims 1 to 4, wherein the insulator contains at least a neodymium (Nd) component as a component. 6. RE. -Β- alumina (composition formula: RE.Al _{1 1} O ₁₈₎ alumina insulator for a spark plug according to claim 5 having a crystal phase of the structure. 7. An alumina powder as a main component, at least a silicon compound powder and one or more RE. Mixing the component-based powder to prepare a raw material powder; forming the molded body having a predetermined insulator shape using the raw material powder; and forming the molded body in a temperature range of 1450 to 1650 ° C. 1 to
And baking for 8 hours. A method for manufacturing an insulator for a spark plug, comprising: 8. An axial center electrode, a metal shell disposed 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.
7. A spark plug comprising: the insulator for a spark plug according to any one of claims 6 to 6.