JP2000095557A - Alumina-base insulator for spark plug, its production and spark plug using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the dielectric strength, insulating property and mechanical strength of an alumina-base insulator while reducing the number of closed pores and voids by incorporating a specified amount of an Sn component into the insulator. SOLUTION: An Sn component is incorporated into an insulator by 0.05-2.0 wt.% (expressed in terms of SnO). The insulator contains 85-98 wt.% (expressed in terms of Al2O3) Al component, may contain 0.07-0.5 wt.% (expressed in terms of Na2O) Na component and preferably contains 0.1-15 wt.% (expressed in terms of oxides), in total, of one or more additive elements selected from Si, Ca, Mg, Ba, Zn and B. The average number of voids of >=10 μm size per 1 mm2 observed in the cross-sectional texture of the insulator is <=100 and >=35 kV/mm dielectric strength at 20 deg.C is ensured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an alumina-based insulator for a spark plug and a spark plug using the same. More specifically, the present invention relates to a spark plug alumina-based insulator having excellent withstand voltage, mechanical strength and insulating properties at high temperatures, a low-cost manufacturing method thereof, and a method of manufacturing the spark plug alumina-based insulator. The present invention relates to a spark plug which is formed by being incorporated as an insulator for a vehicle and can be suitably applied to an internal combustion engine of a vehicle.

[0002]

2. Description of the Related Art In a spark plug used for an internal combustion engine such as an automobile engine, alumina (Al) having excellent heat resistance is used as an insulator material used for the spark plug.
₂ O ₃ ) -based materials have been used for a long time. Then, for the purpose of reducing the firing temperature, silicon oxide (Si)
O ₂ ), a sintering aid such as calcium oxide (CaO) or magnesium oxide (MgO) is used as appropriate. In addition, an organic binder such as PVA (polyvinyl alcohol) is used in order to improve the moldability of the molded body in a process before firing the insulator.

[0003]

Incidentally, in the manufacturing process of the molded article, the organic binder may be mixed into the molded article as a lump. When the molded body containing the lump of the organic binder is fired, the organic binder reacts with oxygen at the time of firing to form carbon dioxide gas. On the other hand, when firing of the molded body is started, sintering starts at a certain predetermined temperature and at a certain firing rate. If the sintering rate is faster than the rate at which carbon dioxide gas is formed by the reaction between the organic binder and oxygen,
Sintering is completed before carbon dioxide gas is released from the organic binder to the outside. As a result, closed pores are included inside the insulator.

[0004] Sintering a compact at a higher sintering temperature in order to increase the rate of formation of carbon dioxide gas above the sintering rate and to release carbon dioxide gas to the outside before sintering However, it is necessary to use a high-cost heat-resistant device, and this is realized in light of the purpose of manufacturing an insulator at low cost and manufacturing a spark plug using this at low cost. Not really.

[0005] Other factors that may be considered as causes for lowering the withstand voltage, insulation, and mechanical strength of an insulator include:
It is an unavoidable impurity contained in the raw material. Although clay is used as a sintering aid for the insulator, it is impossible to completely remove impurities such as fine organic substances and fibers. When a molded body containing a small amount of unavoidable impurities is heated, the unavoidable impurities are burned to generate a small amount of carbon dioxide gas, thereby generating minute voids in the insulator. It is considered that these fine voids also reduce the withstand voltage, insulation, and mechanical strength of the insulator.

On the other hand, as a raw material alumina for an insulator for a spark plug, one manufactured by a Bayer method (hereinafter also referred to as a Bayer alumina) is generally used. The Bayer method is a method of wet extraction of alumina from bauxite, which is a raw aluminum ore,
Caustic soda (NaOH) with relatively high concentration as extraction medium
An aqueous solution is used. Therefore, the obtained Bayer alumina contains a considerable amount of Na component (soda content), and is used in a form subjected to a soda removal treatment as necessary. Then, depending on the degree of the removal of soda, for example, those having a Na component content of less than 0.1% by weight are low soda alumina, and those having a content of about 0.1 to 0.2% by weight are medium soda alumina. Those having a higher Na component, for example, about 0.2% by weight or more, are commonly called soda alumina or the like.

[0007] Since the Na component contained in alumina has high ionic conductivity, if its content is excessive, the insulation resistance value, particularly the insulation resistance value at a high temperature of 500 ° C. or higher, may be reduced, However, there arises a problem that the mechanical strength is impaired. For this reason, it has been conventionally considered that an alumina-based insulator used for a spark plug preferably has a low Na component content.
It is common knowledge that the content of the component a is set to a low value of 0.05% by weight or less.

Here, in order to produce an alumina-based insulator having a low Na component content as described above, it is essential to use a low-soda-based alumina as a raw material alumina.
As described above, low soda-based alumina is expensive because of the need for a soda removal step, and has an aspect that is not necessarily desirable from the viewpoint of raw material costs. However, in recent years, with higher output of automobile engines and the like, higher withstand voltage characteristics and higher heat resistance have been required for insulators of spark plugs. As a result, the above common sense regarding the content of Na component in the insulating material has become more and more unwavering, and at present it is necessary to tolerate some high cost due to the use of low soda alumina.

Accordingly, the present invention provides an alumina-based insulator for a spark plug having the same or better voltage resistance, insulation and mechanical strength as a conventional insulator while reducing the number of closed pores and voids. Another object of the present invention is to provide a method for producing an alumina-based insulator for a spark plug, which can produce such an excellent insulator, and a spark plug using the same.

[0010]

The alumina-based insulator for a spark plug according to the present invention is an insulator containing alumina as a main component, and has a Sn component of 0.1% in terms of SnO.
It is characterized in that it is contained in the range of 0.05 to 2.0% by weight. By setting the Sn component content in the above range, generation of closed pores is suppressed. This is because the sintering reaction at about 1450 ° C. at which the molded body starts to sinter is slightly suppressed, so that the carbon dioxide gas generated during the sintering due to the incorporation of an organic binder or the like is discharged without being trapped in the sintered insulator. It is thought that it is.

If the Sn component in the insulator is less than 0.05% by weight in terms of SnO, the effect of suppressing the sintering reaction at about 1450 ° C. is insufficient and the effect of suppressing the generation of closed pores is insufficient. Become. On the other hand, the Sn component is converted to SnO2.
Even if the content exceeds 0% by weight, the effect of suppressing the generation of closed pores is saturated, and the insulating property required as an insulator is reduced due to an increase in tin oxide having conductivity. Specifically, the insulation resistance of the insulator becomes insufficient, and the withstand voltage performance required for the insulator for a spark plug cannot be satisfied.
The Sn component content in the insulating material is more preferably 0.0
The content is preferably 5 to 0.5% by weight.

Further, the alumina-based insulator for a spark plug of the present invention preferably contains 85 to 98% by weight of an Al component content (hereinafter referred to as WAl) in terms of weight in terms of Al ₂ O ₃ . When WAl is less than 85% by weight,
High-temperature strength and withstand voltage characteristics of the insulator may be insufficient. It is to be noted that the above WAl is preferably not less than 90% by weight. However, when the content of WAl exceeds 98% by weight, the amount of the sintering aid component is relatively too small, so that it is difficult to densify the sintered body. As a result, the growth of alumina grains is promoted, which may lead to problems such as deterioration of strength. Therefore, WAl is preferably adjusted in the range of 85 to 98% by weight, and particularly preferably adjusted in the range of 90 to 98% by weight.

[0013] The spark plug for the alumina insulator, Na component content in weight in terms of Na ₂ O (hereinafter, referred to as WNa) allowed to contain in the range of 0.07 to 0.5 wt% Is done. In the present invention, Na
Even when the content of the component is within this range, the withstand voltage and the mechanical strength of the insulator can be improved. Moreover, it is surprising that the decrease in the insulation resistance, especially at a high temperature of 500 ° C. or higher, is small, which breaks the conventional wisdom. Therefore, WNa is preferably in the range of 0.07 to 0.5% by weight, particularly 0.07 to 0.5% by weight.
It is preferable to contain it in the range of 0.25% by weight.

The alumina-based insulator for a spark plug comprises one or more additive element components selected from Si, Ca, Mg, Ba, Zn and B, and Si is SiO 2
₂ , Ca is CaO, Mg is MgO, Ba is BaO
Preferably, Zn is contained in ZnO and B is contained in B ₂ O ₃ in a total amount of 0.1 to 15% by weight in terms of oxide.

The additive element-based raw material is, for example, Si, C
For each component of a, Mg, Ba and Zn, various inorganic systems such as hydroxides, carbonates, chlorides, sulfates, nitrates, phosphates, etc., in addition to oxides (or composite oxides) of these components. Raw powder can be used. It is necessary to use any of these inorganic raw material powders that can be added to the oxide by firing. As for the B component, diboron trioxide (B
_{In addition to 2} O ₃ ), various boric acids including orthoboric acid (H ₃ BO ₃ ), and A which is a main component element of the insulator
l, borate such as Ca, Mg, Ba, Zn and the like can be used.

The above-mentioned additional element component functions as a sintering aid which melts during firing to form a liquid phase and promotes densification.
When the total content (hereinafter referred to as W1) of the additional element components in the insulator in terms of oxide is less than 0.1% by weight, it becomes difficult to densify the sintered body, and The body's high-temperature strength and withstand voltage characteristics at high temperatures are rather insufficient. On the other hand, if W1 exceeds 15.0% by weight, the high-temperature strength of the insulator will be impaired. Therefore, the total content W1 of the additional element components is preferably 0.1 to 15.0% by weight, and more preferably 3.0 to 10.0% by weight.

Further, in the insulator of the present invention, Si,
One or more additional element components selected from Ca, Mg, Ba, Zn, B and Na, Si is SiO ₂ , C is
a to CaO, Mg to MgO, Ba to BaO, Zn
The ZnO, B is the _B 2 _{O 3,} Na in Na ₂ O, 0.1 to 15 wt% in total at the weight calculated respectively oxide
It can be contained. As a raw material powder for manufacturing such an insulator, an additive element-based raw material containing Si, Ca, Mg, Ba, Zn, and B components with respect to 85 to 98% by weight of alumina powder, to SiO _2, C
a to CaO, Mg to MgO, Ba to BaO, Zn
The ZnO, B to _B 2 _{O 3,} which was formulated as a 0.1 to 15 wt% in total in weight when converted each oxide is used.

Of these, the Ba component, the B component, and the Zn component, together with other components, also have the effect of significantly improving the high-temperature strength of the insulator. The Ba component is preferably contained in an amount of 0.02 to 1% by weight in terms of BaO-converted weight (hereinafter referred to as WBaO). When the content of WBaO is less than 0.02% by weight, the effect of improving the high-temperature strength by blending BaO becomes insignificant. On the other hand, if WBaO exceeds 1% by weight, the high-temperature strength of the insulator will be impaired. WBaO is desirably adjusted in the range of 0.15 to 0.7% by weight. On the other hand, the B component is a weight converted into B ₂ O ₃ (hereinafter, referred to as “B ₂ O _3”) .
WB ₂ O ₃ ) in an amount of 0.01 to 0.75% by weight. When WB ₂ O ₃ is less than 0.01% by weight,
The effect of improving the high-temperature strength by blending WB ₂ O ₃ is not significant. If WB ₂ O ₃ exceeds 0.75% by weight, the high-temperature strength of the insulator will be impaired. WB ₂ O ₃ is desirably adjusted in the range of 0.15 to 0.50% by weight. The Zn component is a weight converted to ZnO (hereinafter, referred to as ZnO).
(Referred to as WZnO) in an amount of 0.04 to 2% by weight. When the content of WZnO is less than 0.04% by weight, the effect of improving the high-temperature strength may not be remarkable as compared with an insulator containing B ₂ O ₃ at a content in the above range.
If the amount exceeds the weight percentage, the high-temperature strength of the insulator may be impaired.

In order to make the additive element function more effectively as a sintering aid, a liquid phase having good fluidity should be formed at a predetermined sintering temperature set lower than that of Al ₂ O _3. It is important that they occur without excess or deficiency. For this purpose, it is often more effective to mix and mix a plurality of types than to mix each component alone. For example, when all of the above five types of first additive components are mixed in the form of an oxide, the finally obtained insulator contains 1.50 to 5% by weight of the Si component in terms of SiO _2. The Ca component is contained in an amount of 1.20 to 4% by weight in terms of CaO, and the Mg component is contained in an amount of 0.05 to
It is desirable that the content be 0.17% by weight.

The above-mentioned additive element or auxiliary additive element in the insulator is mainly contained in the form of an oxide. However, it is oxidized due to factors such as formation of an amorphous glass phase. In many cases, it is not possible to directly identify the form of presence of a substance. In this case, if the total content of the additional element components in the oxide-converted value falls within the above-mentioned range, the insulator is considered to belong to the scope of the present invention. Whether the Al component and the additional element component are contained in the insulator in the form of an oxide can be confirmed by the following methods or a combination thereof. It is confirmed whether a diffraction pattern reflecting the crystal structure of the specific oxide can be obtained by X-ray diffraction. EPMA (electron probe microanalysis), EDS (energy dispersive X-ray spectroscopy) or W
When component analysis is performed by a known microanalysis method such as DS (wavelength dispersive X-ray spectroscopy), an Al component or an additive element component and an oxygen component are simultaneously detected from a cross-sectional area presumed to be the same phase. Check whether or not. When they are confirmed at the same time, it is considered that the Al component or the additive element component exists as an oxide. The valence of atoms or ions of the Al component or the additive element component is analyzed by a known method such as X-ray photoelectron spectroscopy (XDS) or Auger electron spectroscopy (AES). If these components are present in oxide form, the valence of the components will be measured as a positive value.

The alumina-based insulator for a spark plug according to the present invention comprises at least one of Li and K:
Preferably, Li is contained in Li ₂ O and K is contained in K ₂ O by 0.05 to 0.3% by weight in terms of oxide. By setting the content of at least one of Li and K in the above range, the sintering temperature of the alumina-based insulating material can be reduced,
It is possible to prevent a decrease in insulation resistance required for a spark plug. This is because a glass phase is formed between alumina as a main component and the glass phase can prevent a decrease in insulation resistance required as an insulator for a spark plug. It is considered that the reduction of the target strength was also prevented.

In the insulator of the present invention, it is preferable that the average number of voids having a size of 10 μm or more per 1 mm ² observed in the cross-sectional structure be 100 or less, thereby ensuring good withstand voltage characteristics at high temperatures. Desirable above. In addition, as shown in FIG. 8, the “dimension of the gap” refers to two parallel lines A, which are in contact with the outline of the gap observed on the cross section and do not cross the outline. The above-mentioned parallel lines A, B are drawn when B is variously drawn while changing the positional relationship with the gap.
It is defined as the maximum value d of the distance between B.

The number of closed pores in the alumina insulator for a spark plug according to the present invention can be determined by polishing the surface of the insulator, observing the polished surface with a scanning electron microscope (magnification: 150 times), and measuring the number of closed pores. The number of pores with a size of 0.5 mm to ² mm found per cm ² was counted by image analysis. Then, the number of closed pores confirmed on the entire observation surface was defined as the number of closed pores.

It is desirable that the insulation withstand voltage at 20 ° C. of the insulator be 35 kV / mm or more in order to ensure the durability of the insulator, particularly the durability against penetration breakdown. The insulation withstand voltage of the insulator in a state where the insulator is incorporated in the spark plug can be measured as follows. That is, as shown in FIG. 9 (a), the ground electrode is removed from the metal shell 1 of the spark plug 100, and the opening side of the metal shell 1 is immersed in a liquid insulating medium such as silicon oil in this state, thereby insulating the spark plug 100. The space between the outer surface of the body 2 and the inner surface of the metal shell 1 is filled with the liquid insulating medium for insulation. In this state, the metal shell 1
An AC voltage or a pulse voltage is applied by a high-voltage power supply between the power supply and the center electrode 3, and the voltage waveform (the voltage is reduced at an appropriate magnification by a voltage divider) is recorded by an oscilloscope or the like.

Then, as shown in FIG. 9 (b), a voltage value (penetration breakdown voltage) V when penetration breakdown occurs in the insulator 2.
D is read from the voltage waveform, and a value V obtained by dividing the above VD by the thickness LD of the insulator 2 at the position of the formed through hole is obtained.
Let D / LD be the dielectric strength voltage. However, the position of the through hole is
It is defined as the center position of the opening formed on the outer surface of the insulator 2. As shown in FIG. 9C, the thickness LD of the insulator 2 at the position of the through hole has a cross section perpendicular to the center axis O of the insulator 2 through the center position OG of the opening. Draw a straight line P passing through OG and O on the cross section
Is defined as the length of the line segment K-OG, where K is the intersection with the inner peripheral surface of the line.

Further, the alumina-based insulator for a spark plug according to the present invention comprises an alumina-based matrix phase particle having an alumina content of 99% by weight and a glass phase formed at the grain boundary of the alumina-based matrix phase particle. And Here, the content of Na present in the glass phase among the Na components in the insulator is preferably 0.4 to ₂ % by weight in terms of Na ₂ O (hereinafter referred to as WGNa). The WGNa
Exceeds 2% by weight, the insulation resistance value and the insulation withstand voltage of the insulator may be insufficient. In addition, for an insulator having a content of less than 0.4% by weight, low alumina powder having a low Na component content must be used, so that superiority in raw material cost over conventional insulating materials cannot be secured.

It is to be noted that, in the present specification, the WGNa employs a value calculated approximately as follows. First, the surface of the insulator is polished, the structure is observed with a scanning electron microscope (SEM) or the like, and the area ratio (corresponding to the volume ratio in the material) of the alumina matrix phase is measured by analyzing the structure image. And the value is γA.
Next, the weight concentration of the average Na component in the glass phase was calculated as follows:
Identification by a known micro-composition analysis method (for example, EPMA (electron probe micro-analysis), EDS (energy dispersive X-ray spectroscopy), or WDS (wavelength dispersive X-ray spectroscopy))
Calculated as NGNa in terms of Na ₂ O. Then, while assuming that the insulator is composed only of the alumina-based matrix phase and the glass phase and is almost completely densified by sintering, the apparent density of the material measured by the Archimedes method or the like is expressed as ρ0 ( unit: g / ^cm 3), if the density of the alumina crystal grains and ^{ρ1 (= 3.97g / cm 3)} , the weight MG of glass phase present per insulator unit volume, MG = ρ0-ρ1 · WGNa is obtained as γA (1), WGNa is given by: WGNa = MG · NGNa × 100 = (ρ0−ρ1 · γA) · NGNa × 100 (% by weight) (2) Can be calculated by

The average particle diameter of the alumina-based matrix phase crystal particles of the insulator is 2 to 20 μm, preferably 5 to 10 μm.
It is good to be. Here, the particle size of the crystal particles means, in the same manner as the above-mentioned “dimensions of the void”, with respect to the outline of the crystal particles observed on the polished surface of the material, in contact with the outline and does not cross the void. Like two parallel lines A and B
Is defined as the maximum value d of the distance between the parallel lines A and B when various positions are drawn while changing the positional relationship with the crystal grains. In addition, the average particle size means an average value of the particle sizes of a large number of crystal particles obtained in this manner.

Further, it is desirable that the insulator of the present invention has a bending strength at room temperature of 300 MPa or more. If the bending strength is less than 300 MPa, when the spark plug using the insulator is attached to a mounting portion such as a cylinder head, the insulator is likely to be broken due to insufficient strength. The flexural strength is desirably 350
It is better to be at least MPa. In the present invention, the “bending strength” was measured at room temperature using a 3 mm × 3 mm × 25 mm plate-shaped test piece in accordance with the method described in JISR1601 (1981) (ceramic bending strength test method). It means three-point bending strength (span length 20 mm).

Further, the method for producing an alumina-based insulator for a spark plug according to the present invention comprises a raw material powder containing alumina and 0.05 to 2% by weight of a Sn-based inorganic powder in terms of SnO, water, and a binder. The step of adjusting the slurry containing, the step of adjusting the powder from the adjusted slurry, and filling the obtained powder in a predetermined mold, press-molding, and forming an alumina-based insulator for a spark plug The method is characterized by including a step of manufacturing a press-formed body as an original form and a step of sintering the press-formed body.

By including the Sn component in the above-described range in the raw material of the insulator, closed pores generated in the sintered insulator are suppressed.

If the Sn content in the insulator is less than 0.05% by weight in terms of SnO, the suppression of the sintering reaction at about 1450 ° C. is insufficient, and the effect of suppressing the generation of closed pores is insufficient. Become. On the other hand, the Sn component is converted to SnO2.
Even if the content exceeds 0% by weight, the effect of suppressing the generation of closed pores is saturated, and the insulating property required as an insulator is reduced due to an increase in tin oxide having conductivity. Specifically, the insulation resistance of the insulator becomes insufficient, and the withstand voltage performance required for the insulator for a spark plug cannot be satisfied.
The Sn component content in the insulating material is more preferably 0.0
The content is preferably 5 to 0.5% by weight.

Further, the spark plug of the present invention comprises: an insulator having a center through hole; a center electrode held on one side of the center through hole of the insulator; a metal shell holding the insulator; In a spark plug provided with a center electrode and a ground electrode facing the center electrode, the insulator contains alumina as a main component, and a Sn component is 0.05 to 2% by weight in terms of SnO.
It is characterized by containing in the range of.

The insulator used for the spark plug is made of Sn.
By containing the components in the above ranges, closed pores generated in the sintered insulator are suppressed. As a result, the insulation resistance required for the insulator used for the spark plug can be sufficiently satisfied, and the withstand voltage performance required for the insulator for the spark plug is satisfied.
The Sn component content in the insulating material is more preferably 0.0
The content is preferably 5 to 0.5% by weight.

[0035]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Some embodiments of the present invention will be described below with reference to the drawings. A spark plug 100 as an example of the present invention shown in FIGS. 1 and 2 has a cylindrical metal shell 1 and a metal shell 1 such that a distal end portion 21 protrudes.
One end is connected to the center electrode 3 provided inside the insulator 2 and the metal shell 1 by welding or the like, with the insulator 2 fitted inside the inside, the ignition portion 31 formed at the tip protruding. In addition, a ground electrode 4 and the like are provided such that the other end is bent back to the side, and the side surface thereof is arranged to face the tip of the center electrode 3. Further, a firing portion 32 facing the firing portion 31 is formed in the ground electrode 4, and a gap between the firing portion 31 and the facing firing portion 32 is a spark discharge gap g.

A through-hole 6 is formed in the insulator 2 in the axial direction, and a terminal fitting 13 is inserted and fixed at one end thereof, and the center electrode 3 is inserted and fixed at the other end thereof. Have been. A resistor 15 is arranged between the terminal fitting 13 and the center electrode 3 in the through hole 6. Both ends of the resistor 15 are connected to the conductive glass seal layers 16, 1.
7 are electrically connected to the center electrode 3 and the terminal fittings 13 respectively. The resistor 15 is formed of a resistor composition obtained by mixing glass powder and conductive material powder (and ceramic powder other than glass as required) and sintering the mixture by hot pressing or the like. Note that the resistor 15 may be omitted and the terminal fitting 13 and the center electrode 3 may be integrated by a single conductive glass seal layer.

The insulator 2 has a through hole 6 into which the center electrode 3 is fitted along its own axial direction, and is entirely made of the insulating material of the present invention. That is, the insulating material is mainly composed of alumina, and the Al component is
85 to 98 wt% in weight in terms of ₂ O _3, preferably containing 90 to 98 wt%, and 0.05 to 2.0 wt% of Sn component in SnO terms, preferably 0.05 to 0 .
It is constituted as an alumina-based ceramic sintered body containing in a range of 5% by weight.

More specific compositions include the following. Si component: 1.5 to 5 wt% in terms of SiO ₂ by weight; Ca component: 1.2 to 4 wt% in terms of CaO by weight; Mg component: 0.05 to 0.17 wt% in terms of weight MgO; Ba component: 0.15 to 1 wt% in terms of BaO by weight; B _component: B 2 _{O 3} 0.15 to 0.75% by reduced weight; Zn component: 0.04 to 2 wt% calculated as ZnO weight;

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

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

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 metallic shell side engaging portion formed on the inner surface of the metallic shell 1. Strip 1c
Is engaged via the plate packing 63 on the ring, thereby preventing axial removal. On the other hand, metal shell 1
A ring-shaped wire packing 62 that engages with the rear peripheral edge of the flange-shaped protrusion 2e is disposed between the inner surface of the rear opening of the first member and the outer surface of the insulator 2. A ring-shaped wire packing 60 via a filling layer 61 such as
Is arranged. Then, the insulator 2 is pushed forward toward the metal shell 1 and, in this state, the opening edge of the metal shell 1 is swaged inward toward the packing 60 to form a swaged portion 1d. It is fixed to the insulator 2.

FIGS. 4A and 4B show some examples of the insulator 2. FIG. 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 locking projection 2e, the second shaft portion 2i
2h). -The length L3 of the second shaft portion 2i: 2 to 27 mm. -The outer diameter D1 of the main body 2b: 9 to 13 mm. The outer diameter D2 of the locking projection 2e: 11 to 16 mm. -The outer diameter D3 of the first shaft portion 2g: 5 to 11 mm. -The base end outer diameter D4 of the second shaft portion 2i: 3 to 8 mm. The distal end outer diameter D5 of the second shaft portion 2i (however, if the outer peripheral edge of the distal end surface is rounded or chamfered, the base position of the rounded portion or chamfered portion in a section including the center axis O) : 2.5 to 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: 0.5 to 4.5 mm. -Base end wall thickness t2 of second shaft portion 2i ((value in a direction orthogonal to center axis O): 0.3 to 3.5 mm.-Tip wall thickness t3 of second shaft portion 2i ((center axis line) A value in a direction orthogonal to O; however, when a radius or chamfer is applied to the outer peripheral edge of the distal end surface, indicates a thickness at a base end position of the radius or chamfer in a cross section including the central axis O) : 0.2 to 3 mm Average thickness tA ((t1 + t2) / 2) of second shaft portion 2i:
0.25 to 3.25 mm.

In FIG. 1, the length LQ of the portion 2k of the insulator 2 projecting rearward from the metal shell 1 is 23.
To 27 mm (for example, about 25 mm). Furthermore, when a longitudinal section including the central axis O of the insulator 2 is taken, the outer peripheral surface of the protruding portion 2k of the insulator 2 starts from a position corresponding to the rear end edge of the metal shell 1 via the corrugation 2c. The length LP measured along the cross-sectional outline until reaching the rear edge of the second 2 is 26 to 32 mm (for example, about 29 mm).

The dimensions of each part of the insulator 2 shown in FIG. 4A are, for example, as follows: L1 = about 6
0 mm, L2 = about 10 mm, L3 = about 14 mm, D1 = about 11 mm, D2 = about 13 mm, D3 = about 7.3 mm, D4
= 5.3 mm, D5 = 4.3 mm, D6 = 3.9 mm, D
7 = 2.6 mm, t1 = 3.3 mm, t2 = 1.4 mm,
t3 = 0.9 mm, tA = 1.2 mm.

In the insulator 2 shown in FIG. 4B, the first shaft portion 2g and the second shaft portion 2i each have an outer diameter slightly larger than that shown in FIG. 4A. 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 m
m, D2 = about 13 mm, D3 = about 9.2 mm, D4 = 6.
9 mm, D5 = 5.1 mm, D6 = 3.9 mm, D7 =
2.7 mm, t1 = 3.3 mm, t2 = 2.1 mm, t3
= 1.2 mm, tA = 1.7 mm.

In FIG. 1, the metallic shell 1 is formed of a metal such as low-carbon steel in a cylindrical shape, forms a housing of the spark plug 100, and has an engine (not shown) on its outer peripheral surface. A screw portion 7 for attaching to the block is formed. In addition, 1
Reference symbol e denotes a tool engaging portion that engages a tool such as a wrench or a wrench when the metal shell 1 is attached, and has a hexagonal axial cross-sectional shape.

Next, the main bodies 3a and 4a (FIG. 3) of the center electrode 3 and the ground electrode 4 are made of a Ni alloy or the like.
Further, a core material 3b made of Cu or Cu alloy or the like is embedded in the center electrode 3 to promote heat radiation. On the other hand, the ignition part 31 and the opposing ignition part 32
It is mainly composed of a noble metal alloy containing one or more of Ir, Pt and Rh as main components. As shown in FIG. 3, the main body portion 3a of the center electrode 3 has a reduced diameter at the tip end and a flat tip end surface, and a disk-shaped chip made of an alloy composition constituting the ignition portion is superimposed thereon. Is done.
Further, a welding portion W is formed along the outer edge portion of the joining surface by laser welding, electron beam welding, resistance welding, or the like, and this is fixed to form the ignition portion 31. Further, the opposing firing part 32 aligns the tip with the ground electrode 4 at a position corresponding to the firing part 31, forms a welded part W along the outer edge of the joint surface, and fixes the same. It is formed. In addition, these chips are obtained by molding and sintering, for example, a molten material obtained by blending and melting each alloy component so as to have the indicated composition, or an alloy powder or a metal single component powder blended in a predetermined ratio. It can be constituted by the obtained sintered material. Note that at least one of the firing part 31 and the opposing firing part 32 may be omitted.

The insulator 2 is manufactured, for example, by the following method. First, as raw material powder, Bayer alumina powder (Na content: 0.07 to 0.65% by weight (preferably 0.07 to 0.3% by weight), average particle diameter of 1 to 5 μm)
m) and each of the additive element-based raw materials of the Si component, the Ca component, the Mg component, the Ba component, and the B component are blended in a predetermined ratio that becomes the above-described composition in terms of oxide after firing, and a hydrophilic binder ( For example, PVA) and water are added and mixed to form a molding base slurry. Note that each additive element-based raw material is, for example, Si
Component is SiO ₂ powder, Ca component is CaCO ₃ powder, Mg
The components can be blended in the form of MgO powder, the Ba component in the form of BaCO ₃ powder, and the B component in the form of H ₃ BO ₃ powder (or an aqueous solution).

Here, the insulator 2 is made of alumina as a raw material.
As a powder, the content of Na component is Na ₂0.0 in O conversion
7 to 0.65% by weight (preferably 0.07 to 0.3% by weight)
%) And by using high buyer alumina powder
Manufactured products can also be used. Therefore, the gain
The insulator 2 to be used is also 0.07 to 0.5% by weight (one part of the Na component).
Part may be burned off during firing)
Contain high levels of Na components that were considered to be
Becomes However, the Na content of the insulator 2 is in the above range.
As long as it stays within the box, the Sn component is 0.0
By containing 5 to 2.0% by weight, its high temperature
Edge resistance and mechanical strength do not unexpectedly decrease,
Performance comparable to conventional insulation materials with lower weight
Can be obtained enough.

As a result, expensive low-soda alumina was conventionally used in order to keep the Na content low. However, in the case of the present invention, much less expensive medium-soda alumina powder or ordinary soda alumina is used. It becomes possible to use soda alumina powder as a raw material, and it becomes possible to dramatically reduce the production cost of the insulator and, hence, the spark plug. In the alumina powder, the content of the Na component present in the surface layer portion of the particle powder is 0.01 to 0.01 in terms of Na ₂ O.
What is 0.2% by weight (desirably 0.01 to 0.1% by weight) is used.

Here, “N existing in the surface layer of the powder particles”
The “content of the component a” means a value measured as follows. First, the total content (unit:% by weight) of the Na component in the alumina powder to be measured is measured by ICP analysis, chemical analysis, or the like, and the value is converted to Na ₂ O to obtain WNa1. Next, 100 g of the alumina powder is immersed in 100 ml of distilled water maintained at 90 ° C. for 1 hour without stirring. Thereafter, the alumina powder is separated and recovered, and the Na component content (unit:% by weight) is measured again, and the value is converted to Na ₂ O to obtain WNa ₂ . And
The value WNa1-WNa2 obtained by subtracting WNa2 from the previously measured WNa1.
(Unit:% by weight) is calculated based on Na present in the surface layer of the powder particles.
The content of the component.

It is preferable that the alumina powder, which is the main component of the raw material powder, has an average particle diameter of 1 to 5 μm. If the average particle size exceeds 5 μm, a considerably high firing temperature is required to sufficiently advance the densification of the sintered body,
In some cases, even if the firing temperature is set to a considerably high value, the densification does not proceed sufficiently, which may lead to insufficient high-temperature strength of the insulator 2 or insufficient insulation withstand voltage. It is more preferable that the alumina powder has an average particle diameter of 1 to 3 μm.

As the Sn-based inorganic powder, Sn
There is no particular limitation on the type of the substance as long as the substance can be converted into an oxide of, for example, an oxide of Sn, and a composite oxide thereof,
Examples include powders of Sn hydroxide, Sn carbonate, Sn sulfate, Sn nitrate, and Sn phosphate. A suitable particle diameter of the Sn-based inorganic powder is 1 to 5 μm, preferably 1 to 3 μm. When the average particle diameter of the Sn-based inorganic powder is within the above-mentioned range, the Sn-based inorganic powder can be uniformly mixed with the alumina powder having the above-mentioned average particle diameter, and has an advantage that the reactivity in the sintering process is good.

The amount of the Sn-based inorganic powder in the raw material powder is such that the content of the Sn component contained in the fired insulator 2 is within the range of the content of the Sn component in the ceramic sintered body of the present invention. Adjusted as in When the insulator 2 is manufactured using the raw material powder containing the Sn-based inorganic powder, surprisingly, even if the content of the Na component in the alumina is large, the insulator 2 is densified and the closed pores are reduced. ,
In addition, the insulation resistance value and insulation withstand voltage are improved.

The inorganic powder containing one or more elements selected from Si, Ca, Mg, Ba, Zn, and B includes oxides of these elements, composite oxides thereof, hydroxides thereof, Examples thereof include powders of the carbonate, the sulfate, the nitrate, and the phosphate. The average particle size of these inorganic powders is 1 to 5 μm. When the average particle size of the inorganic powder is within the above range, the inorganic powder can be uniformly mixed with the alumina powder having the average particle size in the range, and there is an advantage that reactivity in a sintering process is good. In addition, as for the inorganic powder, more preferably, the average particle diameter is 1 to 3 μm.
It is better to use

The insulator 2 is made of Si, Ca, Mg, Ba,
When one or more elements selected from Zn and B are contained, the blending amount of the inorganic powder as the optional component is adjusted to be the content of the element in the insulator 2. .

The raw material powder includes a Li-based inorganic powder and K
At least one of the inorganic powders may be contained. When these Li-based inorganic powders and / or K-based inorganic powders are blended, the insulator 2 has a small decrease in insulation and mechanical strength at high temperatures without increasing the sintering temperature.
Can be manufactured at low cost.

The water used for preparing the molding slurry is not particularly limited, and the same water as used in the case of producing the conventional insulator can be used. As the binder, for example, a hydrophilic organic compound can be used, and examples thereof include PVA (polyvinyl alcohol), a water-soluble acrylic resin, gum arabic, and dextrin. Of these, PVA is most preferred. 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, water and binder can be mixed to form a slurry.

The amounts of water and binder are as follows: water is 40 to 120 parts by weight, especially 50 to 100 parts by weight, and binder is 0.1 to 5 parts by weight, assuming that the shipping cost of the raw material powder is 100 parts by weight. Especially 0.5 to 3 parts by weight.

The molding base slurry is spray-dried by a spray drying method or the like to obtain a molding base granulated product. The average particle size of the granulated product is preferably from 30 to 200 μm, and particularly preferably from 50 to 150 μm. And
By rubber-pressing the base granulated material for molding,
Create a press-formed body that will be the prototype of the insulator. FIG. 11 schematically shows a process of rubber press molding. Here, a rubber mold 300 having a cavity 301 penetrating in the axial direction is used, and an upper punch 304 is fitted into an upper opening of the cavity 301. Further, on the punch surface of the lower punch 302, a press pin 303 that extends in the axial direction within the cavity 301 and that defines the shape of the through hole 6 (FIG. 1) of the insulator 2 is integrally formed. .

In this state, the cavity 301 is filled with a predetermined amount of the base material granulation material PG for molding, and the upper opening of the cavity 301 is closed with an upper punch 304 for sealing. In this state, a liquid pressure is applied to the outer peripheral surface of the rubber mold 300, and the granulated material PG in the cavity 301 is compressed through the rubber mold 300 to obtain a press-formed body 305 as shown in FIG. In addition, the forming body granulated material PG is set to 100 parts by weight so that the disintegration of the granulated material PG into powder particles at the time of pressing is promoted,
After the addition of 0.7 to 1.3 parts by weight of water, the above-mentioned press molding is performed.

The outer surface of the molded body 305 is processed by grinding or the like to finish the outer shape corresponding to the insulator 2 in FIG. 1, and then fired at a temperature of 1400 to 1600 ° C. At this time, if the Sn-based inorganic powder is contained in the raw material powder, the pressed compact starts to sinter at 1450 ° C.
Sintering reaction before and after is slightly suppressed. As a result, the carbon dioxide gas generated at the time of firing due to the mixing of the inorganic binder or the like is discharged without being confined, and finally densified. After that, the resultant is further baked with glaze to complete the insulator 2.

When the Bayer alumina powder is used, Na ₂ adhered to the Bayer alumina powder (mainly on the surface of the powder particles) when preparing the molding slurry.
Since strongly basic compounds such as O and NaOH are eluted in the solvent, the pH of the slurry increases considerably. When the above-mentioned base slurry for molding in a high pH state is directly subjected to the above rubber press molding as the base granule for molding PG, the moldability of the base granule for molding PG is reduced, and the molded body 305 is cracked. Defects such as (cut) C and collapse Y of the inner edge of the opening are likely to occur. Further, even if no defect or the like occurs immediately after press molding, the strength of the molded body 305 itself is reduced, and thus trouble such as breakage may occur even when cutting the external finish.

Therefore, by mixing an appropriate amount of an acid component, for example, citric acid or boric acid at the time of preparing the molding slurry, the pH of the slurry is adjusted to be in the range of 6 to 10 (preferably 7 to 9). Adjust to Then, the molding base granulated material PG produced using the molding base slurry after the pH adjustment has extremely good moldability, and the above-described defects and the like are hardly generated in the molded body 305, and the production yield is improved. .

The operation of the spark plug 100 will be described below. That is, the spark plug 100 is attached to the engine block at the screw portion 7 and is used as an ignition source for the mixer supplied to the combustion chamber.

The spark plug to which the insulating material of the present invention can be applied is not limited to the type shown in FIG. 1. For example, the tip of the ground electrode 4 is opposed to the side surface of the center electrode 3 as shown in FIG. The spark gap g may be formed between them. In this case, the ground electrode 4 is
As shown in (a), two or more ones are provided on each side of the center electrode 3, and three or more elements are arranged around the center electrode 3 as shown in FIG. You can also.

In this case, as shown in FIG. 7, the spark plug 100 is a semi-surface discharge type spark plug in which the tip of the insulator 2 enters between the side face of the center electrode 3 and the tip of the ground electrode 4. It may be constituted as. In this configuration,
Since spark discharge occurs along the surface of the tip portion of the insulator 2, the antifouling property is improved as compared with an air discharge type spark plug.

[0068]

EXAMPLES The following experiments were performed to confirm the effects of the present invention. (Example 1) As raw material powders, various alumina powders (average particle diameter: 3.0 µm) having different contents of Sn components were used.
iO ₂ (purity 99.5%, average particle size 1.5 μm), Ca
CO ₃ (purity 99.9%, average particle size 2.0 μm), Mg
O (purity 99.5%, average particle size 2 μm), BaCO
₃ (purity 99.5%, average particle size 1.5 μm), H ₃ BO
₃ (purity 99.0%, average particle size 1.5 μm), ZnO
(Purity: 99.5%, average particle size: 2.0 μm) in a predetermined ratio, the total amount of the blended powder being 100 parts by weight, 3 parts by weight of PVA as a hydrophilic binder, and 103 parts by weight of water. Was added and wet-mixed to prepare a green body slurry for molding. The average particle size of the alumina powder was measured using a laser diffraction particle sizer.

Next, these slurries having different compositions were dried by a spray-drying method to prepare spherical molding base granules. The granules are sieved to a particle size of 50 to 100 μm. Further, 100 parts by weight of the granulated material and 1 part by weight of PVA powder are mixed lightly and molded at a pressure of 50 MPa by the rubber press method described with reference to FIG. And Then, the outer peripheral surface of the molded body 305 is subjected to grinder grinding, processed into a predetermined insulator shape, and baked under predetermined conditions, whereby the insulator 2 made of an alumina-based insulating material having the same shape as that of FIG. Obtained. The firing conditions were such that the firing holding time was fixed at 2 hours and the firing holding temperature was changed at intervals of 20 ° C., and the apparent density of the insulator obtained under each condition was measured. Has adopted.

The insulator 2 shown with reference to FIG.
Are as follows: L1 = about 60 mm, L2
= About 8 mm, L3 = about 14 mm, D1 = about 10 mm, D2
= About 13 mm, D3 = about 7 mm, D4 = 5.5 mm, D5
= 4.5 mm, D6 = 4 mm, D7 = 2.6 mm, t1 =
1.5 mm, t2 = 1.45 mm, t3 = 1.25 mm,
tA = 1.48 mm. In addition, referring to FIG.
Portion 2k of insulator 2 projecting rearward from metal shell 1
Has a length LQ of 25 mm, and when a vertical section including the central axis O of the insulator 1 is taken, from the position corresponding to the rear end edge of the metal shell 1 on the outer peripheral surface of the protruding portion 2k of the insulator 2. The length LP measured along the cross-sectional outline from the corrugation 2c to the rear edge of the insulator 2 is 29.
mm.

Using these insulators 2, various types of spark plugs 100 shown in FIG. 1 were prepared. However, the outer diameter of the screw portion 7 is 12 mm, and the resistor 15 is not used, and the terminal fitting 13 and the center electrode 3 are directly joined via a conductive glass layer. These spark plugs 10
The following tests were performed on 0. Dielectric withstand voltage at 20 ° C .: Measured by a method already described with reference to FIG. 9 and using a DC impulse power supply (pulse width 2 ms) as a high-voltage power supply. Insulation resistance measurement at 500 ° C .: Measurement was performed at an energizing voltage of 1000 V by the method already described with reference to FIG. Actual machine withstand voltage test: Attach the above spark plug to a 4-cylinder gasoline engine (displacement 2000 cc), with the throttle fully open and the engine speed 6000 rpm,
Continuous operation was performed while controlling the discharge voltage in the range of 38 to 43 kV, and evaluation was made after 50 hours had passed by whether or not spark penetration had occurred.

The insulator 2 of the spark plug 100 after the test was polished in cross section and scanned with a scanning electron microscope (magnification: 150).
2) and the dimension 1 appearing on the polished surface
The number of voids of 0 μm or more was counted by image analysis. Then, by dividing the confirmed number of voids by the total area of the observation visual field, the number of voids per 1 mm ² was calculated.

Further, a test piece for a strength test was prepared as follows using the same granulated material for molding.
That is, a granulated product is formed by die press molding (pressure of 50 MPa), which is fired under the same conditions as the above-mentioned insulator, and the fired body is appropriately polished to obtain a 3 mm
A square bar-shaped test piece of 3 mm x 25 mm was obtained. Using this test piece, the three-point bending strength (span length: 20 mm) was measured at room temperature based on the method described in JISR1601 (1981) (Ceramic bending strength test method).

The surface of the test piece after the test was polished, observed with a scanning electron microscope (magnification: 150 times), and the number of closed pores having a size of 0.5 mm to 2 mm appearing on the polished surface was analyzed by image analysis. Counted. Then, the number of closed pores confirmed on the entire observation surface was defined as the number of closed pores. Furthermore, in each test piece, Al, Na, Si, Ca, Mg,
The content of each component of Ba, Zn, and B was analyzed by the ICP method, and the respective contents (unit:% by weight) were calculated in terms of oxides. The above results are shown in Tables 1 and 2. Table 2 shows the evaluation of the withstand voltage test result of the actual machine.
The symbol means particularly good, the symbol ○ means good, and the symbol × means bad.

[0075]

[Table 1]

[0076]

[Table 2]

That is, Sn component content (SnO conversion)
Is reduced to 0.05 to 2.0% by weight, the number of closed pores can be reduced, and the withstand voltage, strength and withstand voltage performance of the actual device are also reduced by 0.05%.
The results are superior to those of the insulator having a weight percent or less, and the insulation resistance at 500 ° C. of the spark plug is also 200.
It can be seen that the value is as high as MΩ or more.

Example 2 Various Bayer alumina powders (average particle size: 3.0 μm) having different contents of Na component were mixed with
Distilled water at 5 ° C. was added at a ratio of 200 g to 100 g of alumina, and the mixture was stirred for 10 minutes, washed with water, dehydrated and dried. And, as a raw material powder, SiO ₂ (purity 99.5%,
Average particle size 1.5 μm), CaCO ₃ (purity 99.9%,
Average particle size 2.0 μm), MgO (purity 99.5%, average particle size 2.0 μm), BaCO ₃ (purity 99.5%, average particle size 1.5 μm), H ₃ BO ₃ (purity 99.0) %, Average particle size 1.5 μm) in a predetermined ratio,
With the total amount of the blended powder being 100 parts by weight, 3 parts by weight of PVA as a hydrophilic binder and 103 parts by weight of water were added and wet-mixed to prepare a molding base slurry. The pH of the molding slurry was adjusted to 8 by adding an appropriate amount of citric acid. After washing the alumina powder, the total content of the Na component and the content of the Na component in the surface layer were measured by the above-described methods. Furthermore, the average particle size of the alumina powder was measured using a laser diffraction type particle sizer.

Using this molding base slurry,
The same experiment as in Example 1 was performed. Tables 3 and 4 show the results.
Shown in

[0080]

[Table 3]

[0081]

[Table 4]

That is, by using an insulating material having a Na component content (in terms of Na ₂ O) of 0.07 to 0.5% by weight, the withstand voltage, strength and withstand voltage performance of the actual device are also reduced to 0%. A result comparable to that of the insulating material of 0.05% by weight or less was obtained, and it can be seen that the insulation resistance of the spark plug at 500 ° C. was as high as 200 MΩ or more.

(Example 3) As raw material powder,
Lumina powder (3.0 μm average particle size)
₂(Purity 99.5%, average particle size 1.5 μm), CaCO
₃(Purity 99.9%, average particle size 2.0 μm), MgO
(Purity 99.5%, average particle size 2.0 μm) in various ratios
And mix the total amount of powder
3 parts by weight of PVA as a hydrophilic binder
And 103 parts by weight of water and wet-mixing.
In this way, a forming slurry was prepared. The molding base
The pH of the slurry is adjusted by adding an appropriate amount of citric acid.
Was adjusted to 8. In addition, alumina powder has a total Na content.
Content and the Na component content of the surface layer portion by the method described above.
Were measured. Furthermore, the average particle size of alumina powder
The diameter was measured using a laser diffraction particle sizer.

Using this molding base slurry,
The same experiment as in Example 1 was performed. Tables 5 and 6 show the results.
Shown in

[0085]

[Table 5]

[0086]

[Table 6]

That is, when the Al ₂ O ₃ component in the insulating material is in the range of 85 to 98% by weight, it can be seen that good values are obtained in both the withstand voltage and strength.

[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 front partial sectional view of a main part of FIG. 1;

FIG. 3 is a cross-sectional view showing the vicinity of a firing portion of FIG. 1 in a further enlarged manner.

FIG. 4 is a longitudinal sectional view showing some embodiments of an insulator.

FIG. 5 is an overall front view showing another example of the spark plug of the present invention.

FIG. 6 is a plan view of FIG. 5 and a plan view of a modification thereof.

FIG. 7 is an overall front view showing still another example of the spark plug of the present invention.

FIG. 8 is a diagram illustrating the definition of the size of a void or a crystal particle existing in an insulating material.

FIG. 9 is an explanatory view showing a method for measuring a dielectric strength voltage.

FIG. 10 is an explanatory diagram showing a method for measuring the insulation resistance value of a spark plug.

FIG. 11 is an explanatory view of a rubber press method.

FIG. 12 is a perspective view showing an example of a molded body obtained by a rubber press method and a form of occurrence of a defect thereof.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Metal shell 2 Insulator 3 Center electrode Ground electrode

Claims (9)

[Claims] 1. An insulator containing alumina as a main component,
An alumina-based insulator for a spark plug, comprising a Sn component in a range of 0.05 to 2.0% by weight in terms of SnO. 2. The alumina-based insulator for a spark plug according to claim 1, wherein the Al component is contained in an amount of 85 to 98% by weight in terms of Al ₂ O ₃ . The 3. A Na component in terms of Na ₂ O 0.07
3. The alumina-based insulator for a spark plug according to claim 1, which is contained in a range of 0.5% by weight. 4. One or more additional element components selected from Si, Ca, Mg, Ba, Zn and B, wherein Si is S
iO ₂ , Ca to CaO, Mg to MgO, Ba to B
to aO-, Zn in the ZnO, B is described in _B 2 _{O 3,} to any one of claims 1 to 3, characterized in that it contains 0.1 to 15 wt% in total at the weight calculated respectively oxide Alumina insulator for spark plugs. 5. The dielectric strength voltage at 20 ° C. is 35 kV.
The alumina-based insulator for a spark plug according to any one of claims 1 to 4, which is not less than / mm. 6. A size of 10 μm observed in a sectional structure.
More voids, alumina insulator for a spark plug according to any one of the claims 1 to 100 or less average existence number per the cross section 1 mm ² 5. 7. An alumina-based spark plug according to claim 1, comprising alumina-based matrix phase grains having an alumina content of 99% by weight or more and a glass phase formed at the grain boundary. Insulator. 8. An alumina and 0.05 to 0.05% in terms of SnO.
Raw material powder containing 2% by weight of Sn-based inorganic powder, water,
And a step of adjusting the slurry containing the binder,
A step of preparing a powder from the prepared slurry, and a step of filling the obtained powder into a predetermined mold and press-molding the same to produce a press-molded body that becomes a prototype of an alumina-based insulator for a spark plug. And a step of sintering the press-formed body. A method for manufacturing an alumina-based insulator for a spark plug, comprising: 9. An insulator having a center through hole, a center electrode held on one side of the center through hole of the insulator, a metal shell holding the insulator, and a ground electrode facing the center electrode. And wherein the insulator contains alumina as a main component and a Sn component in a range of 0.05 to 2.0% by weight in terms of SnO.