JP2001313148A - Spark plug - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a spark plug that has an insulator having a high voltage resistance characteristics under such a high temperature as 700 deg.C or the proximity and that satisfies small size. SOLUTION: The spark plug 10 has a small size (the thickness of the insulator 2 having been thinned) to satisfy the following formula, B-A<=2.0 mm, and 0.4 mm<=(A-C)/2<=2.0 mm, wherein, A is the outside diameter of the base of the long leg portion 2i of the insulator 2, B is the minimum inner diameter of the fitting engagement portion 4c of the main fitting 4 and C is the inner diameter of the penetrating hole 6a inside the long leg portion 2i for arranging a center electrode 3 therein. The insulator 2 is composed mainly of alumina (Al2O3) and contains at least one or more kinds of rare earth elements (as expressed RE hereafter). Thereby the spark plug 100 has an excellent voltage resistance characteristics at a high temperature of 700 deg.C or so suppressing occurrence of insulation destruction by the effect of lower melting point glass phase and has an insulator 2 which is suited and preferable for thinness.

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 as an ignition source for an air-fuel mixture in an internal combustion engine.

[0002]

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

[0003]

Incidentally, this kind of insulator is about 500-700 ° C. due to the effect of high-temperature combustion gas (about 2000 ° C.-3000 ° C.) due to spark discharge generated in the combustion chamber of the internal combustion engine. It is exposed to the heat. Therefore, it is desired that the insulator has excellent withstand voltage characteristics from room temperature to the above high temperature. Above all,
In recent years, as the output of an internal combustion engine has been increased and the engine has been downsized, an increase in 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 (smaller diameter). Tend to be. For this reason, the thickness of the insulator tends to be reduced, and an insulator having higher withstand voltage characteristics even when exposed to heat at about 700 ° C. has been required. I have.

However, when an insulator is formed by using a ternary sintering aid as described above, the ternary sintering aid (mainly Si component) is converted to alumina after sintering. It exists as a low-melting glass phase at the grain boundaries of crystal grains,
When the insulator is exposed to heat of about 700 ° C., the low melting point glass phase softens, leading to a decrease in withstand voltage characteristics. Therefore, in order to reduce the low-melting glass phase, it is conceivable to simply reduce the amount of these sintering aids to form an insulator, but densification of the insulator does not progress,
Or at first glance, even if densification is progressing, many pores will remain at the grain boundaries composed of alumina crystal particles,
These factors lead to a decrease in withstand voltage characteristics.

When the withstand voltage characteristic of the insulator is deteriorated in this way, the insulator is exposed to heat of about 700 ° C. and a high voltage of several tens of kV is applied to cause spark discharge of the spark plug. When the electric field is applied, the electric field may be concentrated on the residual pores existing in the grain boundary, or the grain boundary phase may be softened, and spark penetration (dielectric breakdown) may occur in the insulator. .

Accordingly, the present invention provides an insulator whose wall thickness is reduced, has sufficient withstand voltage characteristics at a high temperature of about 700 ° C., suppresses the occurrence of spark penetration, and is thus compact. An object of the present invention is to provide a spark plug capable of responding to a demand for a fuel cell.

[0007]

A spark plug according to the present invention for solving the above problems surrounds an insulator having a through hole in its own axial direction and a radial periphery of the insulator. A spark plug comprising: a metal shell; a center electrode fixed to a through hole of the insulator; and one or more ground electrodes that form a spark discharge gap between the center electrode. A metal fitting side engaging portion for engaging the insulator is formed on the inner peripheral surface of the inner peripheral surface so as to protrude inward. The insulator is made of alumina (Al ₂ O).
₃ ) as a main component and at least one or more rare earth elements (hereinafter, referred to as RE.) Component, and the side where the spark discharge gap is located in the axial direction of the insulator is positioned forward. A leg is formed at the most forward side with the side opposite to the rear as a rear side, while a middle trunk portion engaged with the fitting side engaging portion is formed adjacent to the rear side of the leg length. Further, when the base outside diameter of the leg portion of the insulator is A, the minimum inside diameter of the metal fitting side engaging portion of the metallic shell is B, and the inside diameter of the through hole in the leg portion is C, −A ≦ 2.0 mm and 0.4 mm ≦ (AC) /
It satisfies 2 ≦ 2.0 mm 2.

Generally, in the insulator provided in the spark plug, as shown in FIG. 3, a leg long portion 2i is formed on the most front side (lower side in the figure), and is adjacent to the rear side. In this manner, a middle body portion 2g is formed which is engaged with the metal fitting side engaging portion 4c formed on the metal shell 4. here,
It has been confirmed that the withstand voltage characteristics of the insulator 2 have a correlation with the wall thickness. However, spark penetration (dielectric breakdown) in the insulator 2 caused by insufficient withstand voltage characteristics is caused by the metal side of the metal shell 4. It is known that this is likely to occur at the leg portion 2i where the distance between the joint portion 4c and the leg portion 2i in a positional relationship facing the metal fitting side engaging portion 4c is the shortest.

In particular, the outer diameter of the base end of the leg portion 2i of the insulator 2 is A, and the minimum inner diameter B of the metal fitting portion 4c of the metal shell 4.
When B−A ≦ 2.0 mm, the electric field strength increases near the portion where the distance between the fitting side engaging portion 4c and the leg length 2i is the shortest, and the insulator 2 (the leg length 2
The probability of occurrence of spark penetration in i) increases. On the other hand, the probability of occurrence of spark penetration also correlates with the thickness of the insulator 2 (leg length 2i) as described above. However, in recent years, with the demand for downsizing of the spark plug, it has been considered to reduce the thickness of the insulator. Specifically, the base outer diameter of the leg portion 2i of the insulator 2 is A, and the leg portion 2i When the inside diameter of the through hole 6a in the inside is C, it is studied to reduce the thickness to (A−C) /2≦2.0 mm. As a result, if an insulator is simply formed by using a ternary sintering aid with respect to alumina, which is a main component, as in the conventional case, the insulator (leg length) may be exposed to a high temperature of 700 ° C. or more. When exposed to the above, there is a high probability that spark penetration will occur at the leg length where the distance between the fitting side engaging portion and the leg length is the shortest.

Therefore, in the present invention, at least one kind of RE. By containing the component, the withstand voltage characteristic at a high temperature of about 700 ° C. is remarkably improved, and furthermore, spark penetration (dielectric breakdown) between the leg long portion and the metal fitting side engagement portion is less likely to occur, and as a result, the size is reduced. Thus, a spark plug having a thin insulator can be provided.

RE. The components include Sc (scandium), Y (yttrium) and L of Group 3a of the periodic table.
Lanthanide elements from a (lanthanum) to Lu (lutetium) are listed. By including any of the components in the insulator, the withstand voltage characteristics at a high temperature of 700 ° C. can be improved. RE. The reason that the withstand voltage characteristics are improved by including the components is as follows.
At the grain boundary constituted by the alumina crystal particles, RE. It is considered that a grain boundary phase having a component is generated and the melting point of the grain boundary phase is improved.

However, the value of (AC) / 2 is 0.4
mm, the RE. Even when a component is contained, the thickness of the insulator (leg portion) itself is too thin, and the effect of withstand voltage characteristics cannot be sufficiently exhibited. The lower limit value of the above (BA) may theoretically be 0 mm, but usually takes a value larger than 0 mm in consideration of the ease of assembling the insulator and the metal shell. .

Incidentally, as shown in FIG.
The middle body portion 2g of the insulator 2 and the leg long portion 2i are connected via a connecting portion 2h which is a stepped surface, and the connecting portion 2h is connected to a metal fitting side engaging portion 4c formed on the metal shell 4. Engage (specifically, engage via the annular plate packing 11). Here, in this specification, the connection portion 2h is treated as belonging to the middle trunk portion 2g. Further, a portion adjacent to the front side of the connection portion 2h is treated as a leg length portion 2i. Then, when a longitudinal section including the axis O of the insulator 2 is taken, the first imaginary line 21 extending the outer peripheral side surface of the leg long portion 2i
And an intersection 23 between the first virtual line 21 and the second virtual line 22 as a second virtual line 22 extending the outer peripheral surface of the connection portion 2h.
Is defined as the “base end” of the leg portion, and the diameter at this base end is defined as “base outside diameter A of the leg portion”. In addition, the "base outer diameter A of the leg long portion" can be measured by using, for example, a projector, and the "minimum inner diameter B of the metal fitting side engaging portion of the metal shell" can be measured by using, for example, a pin gauge. it can. further,
The “inner diameter C of the through hole in the leg portion” refers to the inner diameter of the through hole at the “base end” of the leg portion as defined above, and the inner diameter C of this through hole uses, for example, a hole test (inside micrometer). Can be measured.

Next, in the spark plug of the present invention, the RE. The component may be in the range of 0.01 to 18% by mass in terms of oxide at 700 ° C.
This is preferable in that the withstand voltage characteristics can be effectively improved at a high temperature.

[0015] The RE. The component is preferably composed of one or more of lanthanum (La), praseodymium (Pr), and neodymium (Nd) components.

These La, Pr, and Nd components have an ionic radius of a trivalent ion of another RE. Component (RE. Element), it has a small R
E. FIG. -Β-alumina crystal phase (hereinafter simply referred to as “RE.
-Β-alumina crystal phase) in an insulator (alumina-based sintered body). And
The generation of this crystal phase makes it possible for the insulator to further improve withstand voltage characteristics at a high temperature of about 700 ° C. RE. The location of the -β-alumina crystal phase is not particularly limited, but is preferably present even inside the insulator (alumina-based sintered body),
Further, it is more preferably present at the two-grain boundary and / or triple point of alumina.

The Pr and Nd components are La
Unlike the RE. Due to the absence of -β-alumina JCPDS cards, identification by X-ray diffraction is not directly possible. However, since the ionic radius of Pr ³⁺ and Nd ³⁺ is almost equal to that of La ³⁺ , for the Pr and Nd components, JCPDS of La-β-alumina is used.
9 shows an X-ray diffraction spectrum similar to that of a card (No. 33-699). This RE. -Β-alumina crystal phase is RE. Although -β-alumina can be added in advance as a raw material powder, in this case, the densification of the sintered body may be hindered due to the large anisotropy of grain growth during firing. For this reason, it is preferable that the crystal phase is generated during firing.

Here, RE. With β-alumina crystal phase
No means take a cross section orthogonal to the axis of the insulator itself, and
X-ray diffraction of the tissue was performed and the JCPDS card No. 33-
699, no. There is a spectrum corresponding to 15-776.
It can be determined based on whether or not it exists. In FIG.
As an example, the Nd-β-alumina crystal phase (Al₁₁NdO
₁₈2) shows an X-ray diffraction chart. Note that RE. -Β-
When the alumina crystal phase is present in the insulator, for example,
Due to the very small proportion, the spectrum is clearly
May not appear, but the La, Pr, and Nd components
Withstand voltage characteristics due to one or more of the
The effect of improving the properties can be expected.

Next, the spark plug of the present invention comprises alumina, which is a main component of the insulator, and RE. In addition to the components, the insulator may include a silicon (Si) component and at least one of a calcium (Ca) component and a magnesium (Mg) component. In this case, the silicon component (S: unit is mass%), the calcium component (C: unit is mass%), the magnesium component (M: unit is mass%), and the silicon component with respect to the total amount of the oxide-converted components Ratio of S / (S + C + M) ≧
It is preferable to satisfy the relational expression of 0.7.

Each component of the ternary system is contained in an insulator, so that each component is melted at the time of firing to form a liquid phase, thereby promoting densification of the insulator (alumina-based sintered body). Since it functions as a binder, it is effective in densifying the insulator. However, while the Si component functions as a sintering aid for accelerating densification, it exists as a low-melting glass phase at the grain boundaries of alumina crystal phase particles. However, in the present invention, even when the ternary component is contained, by improving the ratio of the Si component as in the above relational expression, the withstand voltage characteristic of the insulator at a high temperature of about 700 ° C. is improved. The effect can be obtained. The reason is that the Si component constituting the low melting point glass phase at the grain boundaries of the alumina crystal particles in the insulator (alumina-based sintered body) is an essential component of the present invention. R together with the ingredients
E. FIG. 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.

The relational expression is: 0.95 ≧ S
/(S+C+M)≧0.75 is more preferably satisfied. By adjusting the ratio of the Si component within the range of this relational expression, the Si component becomes RE. A high melting point crystal phase is generated in combination with the components, and a mullite (Al ₆ Si ₂ O ₁₃ ) crystal phase having a melting point of about 1900 ° C. can be generated. Thereby, the withstand voltage characteristics of the insulator at a high temperature of about 700 ° C. can be further improved.
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.

[0022]

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, and one end 5a of a ground electrode 5 is fixed to the tip end 4a by welding or the like. The other end of the ground electrode 5
The center electrode 4a extends toward the front end surface and is bent back into a substantially L-shape to form a predetermined spark discharge gap g with the center electrode 3 (front end side center electrode 3a).

A through hole 6 is formed in the insulator 2 which is a main part of the present invention in the direction of its own axis O. Hereinafter, the side where the spark discharge gap g is formed in the direction of the axis O of the insulator 2 is referred to as a front side, and the opposite side is referred to as a rear side. The terminal electrode 7 is inserted and fixed on the rear end side of the insulator 2, and the center electrode 3 is inserted and fixed on the front side. A ceramic resistor 8 is arranged between the terminal electrode 7 and the center electrode 3 in the through hole 6. Both ends of the ceramic resistor 8 are connected to the conductive glass layers 9, 10.
Are electrically connected to the center electrode 3 and the terminal electrode 7 respectively. The ceramic 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. You. Alternatively, the ceramic 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.

The insulator 2 is entirely made of the insulating material of the present invention. Specifically, the insulator ₂ has alumina (Al ₂ O ₃ ) as a main component and at least one kind of RE. It is composed of an insulating material containing the components in an amount of 0.01 to 18% by mass in terms of oxide.

Further, as shown in FIG.
A protruding portion 2e that protrudes outward in the circumferential direction is formed in a flange shape in the middle of the insulator 2 in the direction of the axis O. The insulator 2 has a main body 2b having a smaller diameter than the protruding portion 2e. On the other hand, on the front side of the protruding portion 2e, a middle body portion 2g having a smaller diameter than this,
A long leg portion 2i having a smaller diameter than the middle trunk portion 2g is formed adjacently. 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 leg long portion 2g located on the frontmost side in the direction of the axis O of the insulator 2 has a substantially conical shape whose diameter decreases toward the front end.

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 (1), the terminal electrode 7 and the ceramic 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. Then, the first portion 6a and the second portion 6 of the through hole 6 are formed.
b are connected to each other in the first shaft portion, and at the connection position, 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. Have been. The connection position between the first portion 6a and the second portion 6b of the through hole 6 is located at a position rearward of the metal-side locking portion of the metal shell 4.

The outer peripheral surface of the connecting portion 2h between the middle trunk portion 2g and the leg long portion 2i has a stepped surface, and this surface is in contact with a metal fitting side engaging portion 4c formed on the inner surface of the metal shell 4. The engagement of the insulator 2 through the annular plate packing 11 prevents the insulator 2 from being pulled forward in the axial direction. In this specification, the connection portion 2h is treated as belonging to the middle trunk portion 2g. On the other hand, between the inner surface of the rear opening 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. 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. -Length L2 of the middle trunk portion 2g: 0 to 30 mm (however, not including the connecting portion 2f with the protruding portion 2e but including the connecting portion 2h with the second shaft portion 2i). -Length L3 of leg long part 2i: 2-27 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 middle trunk 2g: 5 to 11 mm. -The outer diameter D4 of the base end (rear end) of the leg long portion 2i (however, the axis O
In the vertical cross section including (1), the outer diameter at the intersection of the first imaginary line extending the outer peripheral side surface of the leg long portion 2i and the second imaginary line extending the connecting portion 2h of the middle trunk portion 2g is indicated): 3 to 8 mm. The front end outer diameter D5 of the leg long portion 2i (however, if the outer peripheral edge of the front end face is rounded or chamfered, the outer diameter at the base end position of the R portion or chamfered portion in the longitudinal section including the axis O) Point): 2.5-7 mm. The inner diameter D6 of the second portion 6b of the through hole 6: 2 to 5 mm; The inside diameter of the first portion 6a of the through hole 6 (the inside diameter at the base end of the leg portion 2i): D7: 1 to 3.5 mm. -The thickness t1: 0.5 to 4.5 mm of the middle body 2g. The thickness t2 of the base end (rear end) of the leg portion 2i (value in a direction orthogonal to the axis O; however, in a longitudinal section including the axis O, the base outer diameter of the leg portion is A, When the inner diameter of the through hole (first portion) 6a is C, (C−
A) / 2 value): 0.4 to 2.0 mm. The thickness t3 of the distal end of the leg portion 2i (value in a direction perpendicular to the axis O; however, when the outer peripheral edge of the distal end face is R or chamfered, the R or chamfer in the vertical section including the axis O) (Refers to the thickness at the base position of the part): 0.2
~ 1.5 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.

In the insulator 2 shown in FIG. 2B, the middle trunk portion 2g and the leg long portion 2i each have a slightly larger outer diameter than that shown in FIG. 2A. The dimensions of each part are, for example, as follows: L1 = about 60 m
m, L2 = about 10 mm, L3 = about 14 mm, D1 = about 1
1 mm, D2 = about 13 mm, D3 = about 9.2 mm, D4
= 6.8 mm, D5 = about 5.1 mm, D6 = 3.9 m
m, D7 = 2.8 mm, t1 = 2.7 mm, t2 = 2.
0 mm, t3 = 1.2 mm.

Next, the insulator 2 is manufactured by the following method, for example. First, as a raw material powder, an alumina powder (average particle size 2.0 μm) obtained by a Bayer method was used.
Below), silicon (Si), calcium (Ca), and magnesium (Mg) inorganic powders, 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.

Here, the alumina powder used as a raw material is N 2
It is preferable to use those having a component a content of 0.07% by mass or less in terms of oxide. As a result, the content of the Na component contained in the obtained insulator 2 is reduced to 0.1 in terms of oxide.
It is less than 05% by mass (a part of the Na component may be burned out during firing). When the content of Na component contained in the insulator 2 (content in terms of oxide) exceeds 0.05% by mass, the content of Na component increases by 50% due to the effect of ionic conductivity by Na ions.
The withstand voltage characteristics at a high temperature of 0 ° C. or more may be reduced. Therefore, by setting the content of the Na component within the above range, RE. The effect of improving the withstand voltage characteristics due to the inclusion of the component can be effectively maintained. On the other hand, the Na component is theoretically desirably 0% by mass. However, since alumina is usually purified by the Bayer method, an extremely small amount (nano-order level) of the Na component is inevitably contained. The content of the Na component in the alumina powder was 0.07 in terms of oxide.
In order to make the mass% or less, it is possible to adjust by repeating the de-soda treatment until the alumina purified by the Bayer method (Bayer alumina) has a predetermined Na component content.

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 the withstand voltage characteristics of the insulator 2. Here, the densified insulator 2 has a theoretical density ratio of 95.
% Is preferable. By satisfying such a theoretical density ratio, residual pores in which an electric field existing in the alumina crystal grain boundaries are liable to be concentrated are reduced. The effect of improving the withstand voltage characteristics due to the inclusion of the component can be effectively maintained.
The “theoretical density” is calculated by converting the content of each element component constituting the insulator (alumina-based sintered body) after firing into an oxide, and calculating the content of each oxide by a mixing rule. It refers to density. The “theoretical density ratio” as used herein indicates a ratio of the sintered body density measured by the Archimedes method to the theoretical density.

As the alumina powder constituting the raw material powder, 100% by mass of the alumina-based sintered body after firing is used.
It is preferable to appropriately adjust the Al component so as to be in the range of 75.0 to 99.7% in terms of oxide, in order to obtain good withstand voltage characteristics.

On the other hand, RE. The component powder is R
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 amount of the component-based powder is determined by adding RE.RTM. To 100% by mass of the insulator (alumina-based sintered body) after firing. It is necessary to appropriately adjust the amount of the component to be in the range of 0.01 to 18% by mass in terms of oxide. RE. The components include Sc (scandium) and Y in Group 3a of the periodic table.
(Yttrium) and La (lanthanum) to Lu
Each component of the lanthanoid element up to (lutetium) can be mentioned.

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.

The amount of each inorganic powder to be added is expressed in terms of oxide of Si component (S: unit is% by mass), Ca component (C: unit is% by mass), and Mg component (M: unit is% by mass). The ratio of the Si component to the total of each
It is necessary to adjust appropriately so as to satisfy the relational expression of 0.95 ≧ S / (S + C + M) ≧ 0.75 so as to satisfy /(S+C+M)≧0.7. 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-based powder is improved, and the shrinkage of firing is increased, so that a dense insulator can be formed.

There is no particular limitation on the water used as a solvent for 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 forming base slurry is spray-dried by a spray drying method or the like to prepare a spherical forming 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 compact under the above-mentioned firing temperature condition 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. And, regarding the spark plug 100 of the present invention, the insulator 2 (for example, the insulator 2 shown in FIGS. 2A and 2B) constituting the spark plug 100 is at 700 ° C.
Even when exposed to such a high temperature, the withstand voltage characteristic is excellent, so that occurrence of troubles such as spark penetration (dielectric breakdown) can be effectively prevented or suppressed, and high reliability can be secured.

The spark plug to which the insulator of the present invention can be applied is not limited to the type shown in FIG.
The spark discharge gap may be formed between the plurality of ground electrodes such that the tips of the ground electrodes face the side surfaces of the center electrode. 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 The following experiments were conducted to confirm the effects of the present invention.
Was done. First, alumina (Al) having an average particle size of 0.6 μm
₂O₃) Powder (purity 99.8% or more)
Various RE. Oxide (RE component powder)
Was weighed so as to have a quantitative ratio shown in Table 1, and then added.
g of each raw material powder, and the raw material powder except a
SiO with an average particle size of 0.6 μm₂Powder (purity 99.9%)
And CaCO having an average particle size of 0.8 μm₃Powder (purity 99.
9%) and an MgO powder having an average particle diameter of 0.3 μm (purity: 9%).
9.9%) and the raw material powder is further prepared.
Was. In addition, various RE. The oxide is alumina powder, SiO
₂Powder, CaCO ₃To the total amount of powder and MgO powder
On the other hand, it was added in an external formulation.

Then, with the total amount of these raw material powders being 100 parts by weight, 2 parts by weight of PVA as a hydrophilic binder and 70 parts by weight of water as a solvent are mixed and wet-mixed in a ball mill using alumina spheroid. 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 granulated product, and the particles are sieved to a particle size of 10 to 355 μm. 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 compact was fired at a firing temperature (maximum firing holding temperature) of 1550 ° C. for 2 hours in an air atmosphere, and then finished and fired with glaze.
Insulators 2 as shown in FIG.

The insulator 2 made of each raw material powder
As shown in Table 3, the dimensions of each part (the base outer diameter A of the leg portion corresponds to D4 in FIG. 2 (a), and the inner diameter C of the through hole in the leg portion is shown in FIG. 2 (a). D7) were adjusted to various values, and 11 types were prepared. Further, the spark plugs shown in FIG. Here, the minimum inner diameter of the insulator locking portion of the metal shell was also adjusted for each of the 11 types of insulators 2 as shown in Table 3. In addition, the screw name of the metal shell was M12.

Then, RE.RTM. Contained in the insulator obtained by firing by the following method. Oxide equivalent of the component,
The value of the above-mentioned relational expression of S / (S + C + M), each analysis of the withstand voltage test of the actual machine, and each test were performed. The results are shown in Tables 2 and 3.

[0048] RE. Component: RE. The oxide value of the component is determined by analyzing each insulator with fluorescent X-rays and detecting RE. The amounts of the components were shown as oxide-converted values. Here, the Re. Of the components, La, Nd, and Dy (dysprosium) are respectively converted as La ₂ O ₃ , Nd ₂ O ₃ , and Dy ₂ O ₃ , and Pr is converted as Pr ₆ O ₁₁ .

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.

Table 3 shows the results of each analysis.
Adjust the dimensions of each part to various values as shown in 11
Among the various types, the insulators a to g corresponding to sample No. 5 were analyzed, and the results of the analysis (shown in Table 2) were used for sample numbers 1 to 4 and 6 to 11
Of the insulators a to g corresponding to the above.

Actual machine withstand voltage test: Each spark plug was attached to a four-cylinder engine (displacement: 2000 cc), the throttle was fully opened, and the engine speed was 6,000 rpm.
While controlling the maximum discharge voltage to 35 kV, the temperature at the leg portion of the insulator was set to about 700 ° C., and the continuous operation was performed, and the withstand voltage characteristics were evaluated by the time until spark penetration occurred in the insulator. In evaluating the withstand voltage characteristics, RE. Insulator containing no component (insulator sample number)
g, and the time when spark penetration was observed in the insulator g in each of the spark plugs of the respective dimensions (shown in Table 3) shown in Sample Nos. 1 to 11 was defined as “1”. Insulator (insulator sample number) in each of the spark plugs of the respective dimensions shown in sample numbers 1 to 11 containing the components
The ratio was expressed by the ratio of how many times the spark penetration was observed in a to f (in other words, when the individual sample numbers were examined, the insulators a to f is represented by the ratio of the time at which spark penetration occurred).

[0052]

[Table 1]

[0053]

[Table 2]

[0054]

[Table 3]

From the results shown in Tables 2 and 3, it was found that alumina (Al ₂ O ₃ ) was the main component and RE. Insulators (insulator sample numbers) a to f containing the components are provided, and each dimension is BA ≦ 2.0 mm and 0.4 mm ≦ (AC)
/2≦2.0 mm for spark plugs (spark plugs corresponding to sample numbers 1 to 8 and 10)
Each dimension is BA ≦ 2.0 mm and 0.4 mm ≦ (A
-C) / 2 Among the spark plugs satisfying ≤2.0 mm, RE. It can be seen that the time for spark penetration is more than twice as long as that provided with an insulator (insulator sample number) g containing no component.

On the other hand, spark plugs whose dimensions do not satisfy BA ≦ 2.0 mm and 0.4 mm ≦ (AC) /2≦2.0 mm (spark plugs corresponding to sample numbers 9 and 11; In Comparative Example), RE. Regardless of whether or not the component was contained, the time for spark penetration was the same. From this, the above-mentioned B−A ≦
2.0 mm and 0.4 mm ≦ (AC) / 2 ≦ 2.
A spark plug provided with a small and thin insulator such as 0 mm, wherein the insulator is mainly composed of alumina, and at least one kind of RE. It can be seen that the inclusion of the component significantly improves the withstand voltage characteristics at a high temperature of about 700 ° C.

[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 is a schematic diagram illustrating a state of engagement between an insulator and a fitting side engaging portion in the vicinity of a fitting engaging portion of a metal shell, and a base outer diameter of a leg portion of the insulator.

FIG. 4: 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 ).

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 100 Spark plug 2 Insulator 2g Middle trunk part 2i Leg length part 3 Central electrode 4 Metal shell 4c Metal side locking part 5 Ground electrode

Claims (5)

[Claims] 1. An insulator having a through hole in its own axial direction, a metal shell surrounding a circumference of the insulator in a radial direction, a center electrode fixed to the through hole of the insulator, and the center electrode. A spark plug comprising one or a plurality of ground electrodes forming a spark discharge gap therebetween, wherein a metal fitting side engaging portion for engaging the insulator is formed on an inner peripheral surface of the metal shell. The insulator includes alumina (Al ₂ O ₃ ) as a main component and at least one or more rare earth elements (hereinafter, referred to as RE.) Component. The side where the spark discharge gap is located in the axial direction of the insulator is defined as the front side, and the opposite side is defined as the rear side. The middle torso to be Further, the base outer diameter of the leg portion of the insulator is A, the minimum inner diameter of the metal fitting side engaging portion of the metal shell is B, and the inner diameter of the through hole in the leg portion is formed adjacent to the rear side. Is C, B−A ≦ 2.0 mm and 0.4 mm ≦ (A−C) /
A spark plug satisfying 2 ≦ 2.0 mm. 2. The RE. The spark plug according to claim 1, wherein the component is contained within a range of 0.01 to 18% by mass in terms of oxide. 3. The RE.R. contained in the insulator. 3. The spark plug according to claim 1, wherein the component comprises at least one of lanthanum (La), praseodymium (Pr), and neodymium (Nd) components. 4. The insulator has a silicon (Si) component and a calcium (Ca) component or a magnesium (M) component.
g) at least one of the components is contained;
The content of each of the silicon component, the calcium component, and the magnesium component is expressed as S in terms of oxide.
(Unit: mass%), C (unit: mass%) and M (unit: mass%)
4. The spark plug according to claim 1, wherein S / (S + C + M) ≧ 0.7 is satisfied. 5. The metal shell is called M8, M10, M
The spark plug according to any one of claims 1 to 4, wherein the spark plug is any one of 12 and M14.