spark plug

Abstract

Spark plug (100) comprising: an insulator (10) with a through-hole (12) extending from a rear to a front; a center electrode (20) inserted at least partially into a section of the through-hole (12) on the front side; a metallic terminal element (40) inserted at least partially into a section of the through-hole (12) on the rear side; and a seal (72) arranged within the through-hole (12) and in contact with the center electrode (20) and an inner circumferential surface (12i) of the insulator (10), wherein the seal (72) comprises a glass and an electrically conductive substance, and the glass contained in the seal (72) contains Si in an amount of 50 wt% or more, reduced to SiO2, and Na in an amount of 0.1 wt% or more and 0.3 wt% or less, reduced to Na2O.

Description

AREA OF INVENTION The present invention relates to a spark plug. BACKGROUND OF THE INVENTION A spark plug is conventionally used to ignite fuel in a device where fuel is burned (for example, in an internal combustion engine). The spark plug comprises, for example, an insulator with a through-hole, a center electrode inserted at least partially into a section of the through-hole on the front side, a metallic terminal element inserted at least partially into a section of the through-hole on the rear side, and a seal arranged in the through-hole and in contact with the center electrode and an inner circumferential surface of the insulator. The seal contains, for example, glass. The prior art includes published Japanese patent application (Kokai) JP 2005 - 340 171 A; Japanese Kohyo (PCT) patent publication JP 2009 - 545 860 A; and published Japanese patent application (Kokai) JP 2007 - 179 788 A. If the SiO2 content of the glass is high, the heat resistance of the gasket improves because the coefficient of thermal expansion of the glass decreases. However, in this case, the glass becomes hard. Furthermore, if the glass contains sodium (Na), as this lowers the softening point of the glass, a suitable gasket can be formed. However, in some cases, the dielectric strength of the insulator deteriorates due to the diffusion of Na from the gasket into the insulator. Further relevant prior art is disclosed in the following documents: DE 22 45 403 A , EP 1 592 101 A2 and WO 2007 / 147 154 A2 . SUMMARY OF THE INVENTION The present invention discloses a technique capable of limiting the deterioration of the dielectric strength performance of an insulator of a spark plug with a seal containing glass. [Means to solve the problem] To solve the problem described above, a spark plug with the features of claim 1 is specified. A further advantageous embodiment is described in claim 2. The technology disclosed in the present description can be implemented in various forms; for example, as a spark plug, an ignition device using the spark plug, an internal combustion engine with the spark plug, and an internal combustion engine with the ignition device comprising the spark plug. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a cross-sectional view of a spark plug 100 according to an embodiment of the present invention; Fig. 2A shows a table TA showing the relationship between test results and material properties of spark plug samples; and Fig. 2B shows a table TB showing the relationship between test results and material properties of spark plug samples; Fig. 3A shows a cross-sectional view partially showing the spark plug and including a central axis CL of the spark plug; and Fig. 3B shows a schematic cross-sectional view of an insulator perpendicular to the central axis CL. DETAILED DESCRIPTION OF THE INVENTION A. Design: Fig. 1 shows a cross-sectional view of a spark plug 100 according to an embodiment of the present invention. The drawing shows a central axis CL (also referred to as the "axial line CL") of the spark plug 100 and a flat cross-section of the spark plug 100 containing the central axis CL. Hereinafter, a direction parallel to the central axis CL is referred to as the "direction of the axial line CL" or simply as the "axial direction." A radial direction of a circle centered on the axial line CL is also referred to as the "radial direction." The radial direction is a direction perpendicular to the axial line CL. A circumferential direction of a circle centered on the axial line CL is referred to as the "circumferential direction." With respect to the direction parallel to the central axis CL, the downward direction in Fig.The forward direction Df is referred to as the front direction Df, and the upward direction Dfr as the rear direction Dfr. The forward direction Df extends from a metallic terminal element 40 to a center electrode 20, these elements being described later. A side facing forward direction Df in Fig. 1 is referred to as the front of the spark plug 100, and a side facing backward direction Dfr in Fig. 1 is referred to as the back of the spark plug 100. The spark plug 100 has a tubular insulator 10 with a through-hole 12 (which can also be referred to as an axial hole 12) extending from the rearward-facing side Dfr to the forward-facing side Df, a center electrode 20 held in the through-hole 12 on the front side, a metallic terminal section 40 held in the through-hole 12 on the rear side, an intermediate element 79 arranged in the through-hole 12 between the center electrode 20 and the metallic terminal element 40, an electrically conductive first seal 72 in contact with the intermediate element 79 and the center electrode 20, electrically connecting the intermediate element 79 and the center electrode 20, and an electrically conductive second seal 74.which is in contact with the intermediate element 79 and the metallic terminal element 40 and electrically connects the intermediate element 79 and the metallic terminal element 40, a tubular metal sleeve 50 which is attached to the outer circumference of the insulator 10, and a ground electrode 30, one end of which is connected to an annular front face 55 of the metal sleeve 50 and the other end of which faces the central electrode 20, with a discharge gap g formed between them. In the present embodiment, the intermediate element 79 is formed from a resistance element 73. The insulator 10 is a tubular element extending along the axial line CL. The insulator 10 has a large-diameter section 14, which has the largest outer diameter and is formed at a central section. An insulator foot 13 on the rear side, whose outer diameter is smaller than that of the large-diameter section 14, is connected to one end of the large-diameter section 14 on the side in the rear direction Dfr. At a connecting section 18 between the large-diameter section 14 and the insulator foot 13 on the rear side, the outer diameter of the insulator 10 gradually decreases in the rear direction Dfr (the connecting section 18 is also referred to as the outer diameter reduction section 18). The insulator 10 comprises a body section 15 on the front side, which has a smaller outer diameter than the larger-diameter section 14 and is connected to one end of the body section 15 located on the front side in the forward direction Df. A leg section 19 comprises the front end of the insulator 10. At a connecting section 16 between the body section 15 located on the front side and the leg section 19, the outer diameter of the insulator 10 gradually decreases in the forward direction Df (the connecting section 16 is also referred to as the outer diameter reduction section 16 or the step section 16). The body section 15 located on the front side has an inner diameter reduction section 11 formed therein. The inner diameter of the inner diameter reduction section 11 gradually decreases in the forward direction Df. Preferably, the insulator 10 is formed taking into account its mechanical strength, thermal strength, and electrical strength. The insulator 10 is formed, for example, by burning aluminum oxide (other electrically insulating materials can also be used). The central electrode 20 is a rod-shaped metal element extending along the axial line CL. A section of the central electrode 20 on the side facing backwards Dfr is inserted into a section of the through-hole 12 of the insulator 10 on the side facing forwards Df. The central electrode 20 has a rod section 28 and a first tip 29, which is connected (for example, by laser welding) to the front end of the rod section 28. The rod section 28 has a head section 24 on the side facing forwards Df and a shaft section 27, which is connected to one end of the head section 24 on the side facing forwards Df. The shaft section 27 has an approximately circular, columnar shape extending forwards Df. The head section 24 includes a collar section 23 with a larger outer diameter than the shaft section 27.A section of the collar section 23 on the side facing forward direction Df is an outer diameter reduction section 25, the outer diameter of which gradually decreases in the forward direction Df. The outer diameter reduction section 25 is held by the inner diameter reduction section 11 of the insulator 10. The shaft section 27 is connected to the side of the outer diameter reduction section 25 facing forward direction Df. The first tip 29 is connected to one end of the shaft section 27 on the side facing forward direction Df. The rod section 28 has an outer layer 21 and a core 22 located on the inner circumference of the outer layer 21. The outer layer 21 is made of a material (for example, an alloy containing nickel as the main component) whose oxidation resistance is better than that of the core 22. The main component is the one with the highest content (wt%). The core 22 is made of a material (for example, pure copper or an alloy containing copper as the main component) that has a higher thermal conductivity than the outer layer 21. The first tip 29 is connected to the outer layer 21 of the rod section. The first tip 29 is formed using a material (for example, a precious metal such as iridium (Ir) or platinum (Pt)) that has a higher discharge resistance than the shaft section 27.A section of the central electrode 20 on the side facing forward direction Df, including the first tip 29, protrudes forward direction Df from the axial hole 12 of the insulator 10. It should be noted that the first tip 29 can be omitted. The core 22 can also be omitted. The metallic terminal element 40 is a rod-shaped element extending along the axial line CL. The metallic terminal element 40 is formed using an electrically conductive material (for example, a metal containing iron as its main component). A rod-shaped section 41 of the metallic terminal element 40 on the forward-direction Df side is inserted into a section of the axial hole 12 of the insulator 10 on the reverse-direction Dfr side. The resistive element 73 in the through-hole 12 of the insulator 10 is an element for suppressing electrical noise. The resistive element 73 is formed, for example, by using a mixture of glass, an electrically conductive material (for example, carbon particles), and ceramic particles. The seals 72 and 74 are formed by using a mixture of an electrically conductive material (for example, metal particles such as copper or iron particles) and glass. The center electrode 20 is electrically connected to the metallic terminal element 40 via the first seal 72, the resistive element 73, and the second seal 74. The first seal 72 is in contact with the center electrode 20 and an inner circumferential surface 12i of the insulator 10. The elements 72, 73, and 74 in the through-hole 12 of the insulator 10 are formed, for example, as follows. The central electrode 20, a material powder for the first seal 72, a material powder for the resistive element 73, and a material powder for the second seal 74 are inserted or loaded, in this order, into the through-hole 12 of the insulator 10 from the rearward-facing side Dfr. The insulator 10 is heated to a temperature higher than the softening points of the glass materials for the elements 72, 73, and 74. In this state, the metallic terminal element 40 is inserted into the through-hole 12 from the rearward-facing side Dfr. This compresses the materials of the elements 72, 73, and 74, forming the elements 72, 73, and 74. The metal sleeve 50 is a tubular element with a through-hole 59 extending along the axial line CL. The insulator 10 is inserted into the through-hole 59 of the metal sleeve 50, and the metal sleeve 50 is attached to the outer circumference of the insulator 10. The metal sleeve 50 is formed using an electrically conductive material (for example, a metal such as carbon steel with iron as the main component). A section of the insulator 10 on the side facing forward Df projects outward from the through-hole 59. A section of the insulator 10 on the side facing backward Dfr also projects outward from the through-hole 59. The metal sleeve 50 has a tool engagement section 51, an outwardly projecting section 54, and a body section 52 located on the front. The tool engagement section 51 allows the mounting of a spark plug wrench (not shown). The outwardly projecting section 54 is a flange-like section located on the side of the tool engagement section 51 facing forward Df and projects radially outward. A surface 54f of the outwardly projecting section 54 on the side facing forward Df is a bearing surface (also called the metal sleeve bearing surface 54f or simply the bearing surface 54f) and forms a seal in conjunction with a hole-forming section (for example, a section of a cylinder head), which is a section of an internal combustion engine and has a mounting hole.The front-facing body section 52 is connected to the forward-facing side Df of the outwardly projecting section 54 and includes a front end face 55 of the metal sleeve 50. The front-facing body section 52 has a screw section 57 formed on the outside of an outer circumferential surface and adapted to be screwed into a mounting hole (not shown) of the internal combustion engine (also referred to as the external thread section 57). The axial line CL is a central axis of the external thread of the screw section 57. The external thread of the screw section 57 extends in the direction of the axial line CL. A ring seal 80 is arranged between the bearing surface 54f of the outwardly projecting section 54 and the screw section 57 of the front-facing body section 52. The ring seal 80 is attached to the metal sleeve 50 and is in contact with the bearing surface 54f. When the spark plug 100 is mounted on the cylinder head, the ring seal 80 is compressed and deforms. This deformation of the ring seal 80 seals any gap between the spark plug 100 and the cylinder head. The ring seal 80 is made, for example, of a metal such as iron. The front-facing body section 52 of the metal sleeve 50 has an inwardly projecting section 56 located on its inner circumferential side and projecting radially inward. A surface 56r (also called the rear surface 56r) of the inwardly projecting section 56 on the side facing backward Dfr gradually decreases in inner diameter in the forward direction Df. An inner seal 8 located on the front is held between the rear surface 56r of the inwardly projecting section 56 and the outer diameter reduction section 16 of the insulator 10. The inwardly projecting section 56 indirectly supports the stepped section 16 of the insulator 10 via the inner seal 8. Hereinafter, the inwardly projecting section 56 can also be referred to as the support section 56. The metal sleeve 50 has a rear end section 53, which is formed on the rear side of the tool engagement section 51 as its rear end and has a smaller wall thickness than the tool engagement section 51. The metal sleeve 50 also has a connecting section 58, which is formed between the outwardly projecting section 54 and the tool engagement section 51, in order to connect the outwardly projecting section 54 and the tool engagement section 51. The connecting section 58 has a smaller wall thickness than the outwardly projecting section 54 and the tool engagement section.The circular ring elements 61 and 62 are inserted between an inner circumferential surface of the metal sleeve 50, extending from the tool engagement section 51 to the rear end section 53, and an outer circumferential surface of a section of the insulator 10 on the reverse-oriented side Dfr of the outer diameter reduction section 18. Talc powder 70 is also applied between these ring sections 61 and 62. During the manufacturing process of the spark plugs 100, when the rear end section 53 is bent radially inwards for crimping, the connecting section 58 deforms, thereby joining the metal sleeve 50 and the insulator 10. In this crimping step, the talc 70 is compressed, thus improving the airtight seal between the metal sleeve 50 and the insulator 10.The inner seal 8 is pressed between the outer diameter reduction section 16 of the insulator 10 and the inwardly projecting section 56 of the metal sleeve 50, thereby forming a seal between the metal sleeve 50 and the insulator 10. In this way, the insulator 10 is held between the inwardly projecting section 56 of the metal sleeve 50 and the rear end section 53 of the metal sleeve 50. The ground electrode 30 is a metal element and has a rod-shaped body section 37. An end section 33 (also referred to as the proximal end section 33) of the body section 37 is connected (for example, by resistance welding) to the front end surface 55 of the metal sleeve 50. The body section 37 extends from the proximal end section 33, which is connected to the metal sleeve 50, in the forward direction Df, is curved towards the central axis CL, extends in a direction that intersects the axial line CL, and reaches a distal end section 34. A surface of the distal end section 34 on the side facing backward in the direction Dfr and the first tip 29 of the central electrode 20 form a discharge gap g between them. The body section 37 has an outer layer 31 and an inner layer 32, which is provided on the inner circumferential side of the outer layer 31. The outer layer 31 is formed of a material (for example, an alloy containing nickel as the main component) that is superior to the inner layer 32 in oxidation resistance. The inner layer 32 is formed of a material (for example, pure copper or an alloy containing copper as the main component) that has a higher thermal conductivity than the outer layer 31. It should be noted that a second tip, similar to the first tip 29 of the center electrode 20, can be attached to a surface of the distal end section 34 of the ground electrode 30, with this surface located on the side facing backwards Dfr. The first tip and the second tip can form the discharge gap g between them. The inner layer 32 can also be omitted. B. Evaluation test: Fig. 2A shows a first table TA, which illustrates the relationship between test results and material properties of spark plug 100 samples. The first table TA shows, for each sample type, a sample number, the potassium (K) content, the sodium (Na) content, the dielectric strength rating, and the density (degree of sintering) rating. In the evaluation test, six sample types, namely sample no. 1 to sample no. 6m, were tested. The first seal 72 of each sample contains glass and brass as an electrically conductive material. As described with reference to Fig. 1, the first seal 72 is in contact with the center electrode 20. The center electrode 20 increases its temperature as a result of absorbing heat from the combustion gas. Therefore, the glass contained in the first seal 72 preferably has good heat resistance. Borosilicate glass with good heat resistance was used in the samples tested in the present evaluation test.As described below, the glass used in the samples has a high silicon content to improve its heat resistance. Consequently, the glass is hard. To improve the adhesion between the first seal 72 and other elements (for example, the center electrode 20 and the insulator 10), the glass preferably contains a component that lowers the softening point of the glass. For example, alkali metals can lower the softening point of the glass. The glass used in the samples tested in this evaluation test contains sodium and potassium. The material for the first seal 72, which was used in the preparation of the samples, contains borosilicate glass. The borosilicate glass material contains an oxide of sodium (Na) (Na₂O) and an oxide of potassium (K) (K₂O). The first table TA (Fig. 2A) shows the potassium (K) content reduced to K₂O and the sodium (Na) content reduced to Na₂O. The six sample types differ in the Na content of the glass of the first seal 72. Although not shown, the borosilicate glass of the first seal 72 in the six sample types has a Si (silicon) content in the range of 55 wt% to 65 wt%, reduced to SiO₂. In the six sample types, the borosilicate glass contained in the first seals 72 has a B (boron) content in the range of 25 wt% to 35 wt%, reduced to B₂O₃. As can be seen from the first table TA, the six sample types have the same K (potassium) content, namely 2 wt%, reduced to K₂O. As can be seen from the first table TA, the content of Na (sodium), reduced to Na2O, is 0, 0.1, 0.3, 0.4, 0.9 and 1 mass % in this order starting from sample No. 1.It should be noted that the Si, B, K, and Na content are those of the glass. These proportions are the same as those of the material used for the glass. The proportions of these components can be determined by analyzing the cross-sections of the first seals 72 of the samples. For example, an SEM image of a target area on the cross-section of the first seal 72 is acquired using a scanning electron microscope (SEM). The target area is, for example, a 1 mm² square. The magnification is, for example, 200x. Subsequently, a glass phase is identified by component analysis of the target area using an EPMA (Electron Probe Micro Analyzer), and the content of the components in the glass phase is determined. It should be noted that the six sample types have the same structural features (for example, the shape of the central electrode 20), except for the proportions of the components in the first seal 72. It should be noted that the difference between the numerous sample types in the results of the various tests described below is strongly influenced by the difference in potassium or sodium content, and presumably less so by the difference in silicon and boron content. The first table, TA, shows evaluation results from a dielectric strength test and the evaluation results from a compression test. The dielectric strength test was performed as follows. Four samples of the same type of spark plug 100 were fitted to a 1.6-liter, 4-cylinder, direct-injection, supercharged gasoline engine. The discharge gaps g of the spark plug 100 were adjusted to provide a discharge voltage of 40 kV or higher. This engine was operated for 100 hours under wide-open throttle (WOT) conditions (also referred to as actual engine operation). After this actual engine operation, the four spark plugs 100 were removed, and the insulators 10 were examined. The insulators 10 were examined as follows. Fig. 3A shows a section of the cross-section of the spark plug 100, which includes the central axis CL. Fig. 3A shows a region comprising the outer diameter reduction section 25 of the center electrode 20, the inner diameter reduction section 11 and the outer diameter reduction section 16 of the insulator 10, and the inwardly projecting section 56 of the metal sleeve 50. The inner diameter reduction section 11 of the insulator 10 is in contact with the outer diameter reduction section 25 of the center electrode 20. The outer diameter reduction section 16 of the insulator 10 is supported by the inwardly projecting section 56 of the metal sleeve 50 with the aid of the inner seal 8. The partially enlarged view on the right in Fig. 3A shows a region comprising the inner diameter reduction section 11 and the outer diameter reduction section 16 of the insulator 10.For the sake of simplicity, the hatching of the cross-section of insulator 10 has been omitted in the enlarged partial view. A high voltage is applied between the central electrode 20 and the metal sleeve 50 for discharge. Accordingly, a high voltage is applied to a section 10z of the insulator 10 between the inner diameter reduction section 11 and the outer diameter reduction section 16 via the central electrode 20, the metal sleeve 50 and the inner seal 8. The glass in the first seal 72 contains alkali metals (especially potassium (K) and sodium (Na)). Since, as mentioned previously, the central electrode 20 experiences a temperature increase as a result of absorbing heat from combustion gas, the temperature in the first seal 72 and a section of the insulator 10 near the central electrode 20 also increases. At high temperatures, the alkali metals contained in the first seal 72 are able to migrate. The alkali metals can diffuse from the inner circumferential surface 12i of the through-hole 12 of the insulator 10 into the insulator 10. For example, alkali metal ions diffuse into the insulator 10. The first seal 72 is in contact with the inner diameter reduction section 11 of the insulator 10. As mentioned previously, a high voltage is applied to section 10z of the insulator 10 between the inner diameter reduction section 11 and the outer diameter reduction section 16.As a result, the movement of the alkali metals can be accelerated. It should be noted that a sodium ion generally has a smaller ionic radius than a potassium ion. Accordingly, potassium (K) is unlikely to diffuse into insulator 10, while sodium (Na) is more likely to diffuse into insulator 10. The enlarged view on the right in Fig. 3A shows diffusion zones 72x into which sodium (Na) has diffused. As shown, sodium (Na) can diffuse into the insulator 10 near a section of the inner circumferential surface 12i of the insulator 10, with this section being in contact with the outer diameter reduction section 25 of the center electrode 20. Fig. 3B shows a schematic representation of the cross-section of the insulator 10 perpendicular to the axial line CL and is a cross-section along line BB in Fig. 3A. The cross-section passes by the section of the inner diameter reduction section 11 that is in contact with the first seal 72 and is located near a section of the inner diameter reduction section 11 that is in contact with the center electrode 20. As shown, the sodium (Na) diffusion zones extend 72x from the inner circumferential surface 12i of the through-hole 12 into the insulator 10.The diffusion zones 72x can be long, narrow zones extending from the inner to the outer circumference. In an actual cross-section of the insulator 10, the zones where sodium (Na) is present change their color to black. In the case where the insulator 10 contains sodium (Na) dispersed within it, a discharge can be transmitted through the insulator 10 via the sodium (Na) medium. The path Px, shown in the enlarged view on the right in Fig. 3A, illustrates an example of the path of a penetrating discharge. Path Px begins at the inner circumferential surface of the inner diameter reduction section 11 of the insulator 10, passes through the insulator 10, and reaches the outer circumferential surface of the outer diameter reduction section 16 of the insulator 10. Path Px connects the center electrode 20 and the inner seal 8. If such a penetrating discharge has occurred, traces of path Px (for example, black dots) are observed on the outer circumferential surface of the insulator 10. In the evaluation test, after the aforementioned actual engine operation, the spark plug 100 samples were disassembled and the insulators 10 were removed. The insulators 10 were cut open, and the first seals 72 and other elements were removed from the cut insulators 10. Cross-sections of the insulators 10 described with reference to Fig. 3A and cross-sections of the insulators 10 described with reference to Fig. 3B were prepared. The different sample types are identical with respect to the axial position (the position in the direction parallel to the central axis CL) of the cross-section in Fig. 3B with respect to the inner diameter reduction section 11 of the insulator 10. The two cross-sectional types in Fig. 3A and Fig. 3B were analyzed for sodium (Na) using an EPMA (Electron Probe Micro Analyzer). The insulator 10 material does not contain sodium (Na).Therefore, the detection of sodium (Na) from the cross-section of insulator 10 indicates the diffusion of sodium (Na) into insulator 10. The results of the dielectric strength test in the first table TA (Fig. 2A) show the evaluation results for the condition of the four tested samples after the aforementioned actual motor operation. Rating “A” indicates that no sodium (Na) was detected in the cross-sections of any four insulators 10. Rating “B” indicates that sodium (Na) was detected in the cross-section of one or more of the four insulators 10, and that traces of a penetrating discharge were not found in all four insulators 10. Rating “C” indicates that traces of a penetrating discharge were found in one or more of the four insulators 10. It should be noted that if no sodium (Na) was detected in the cross-section of the insulator 10, then no traces of a penetrating discharge were found. The results of the compression test indicate whether the material for the first seal 72 is sufficiently melted during the manufacture of the spark plug 100. More precisely, a new sample of the spark plug 100 is cut to prepare the cross-section containing the axial line CL. The cross-section of the first seal 72 is examined using an optical microscope to detect particles of the glass material powder. As mentioned previously, during the manufacture of the spark plug 100, the glass material powder contained in the first seal 72 softens in the through-hole 12 and is compressed as a result of the insertion of the metallic connecting section 40.It is difficult to reach, with the force of the metallic terminal section 40, a section of the first seal 72 that lies away from the metallic terminal section 40 (for example, a section in the gap between the outer diameter reduction section 25 of the center electrode 20 and the inner circumferential surface 12i of the insulator 10). If the glass powder is sufficiently soft during the manufacture of the spark plug 100, no particles of the glass powder will be captured from the cross-section of the first seal 72 of the finished spark plug 100. Furthermore, the adhesion between the first seal 72 and other elements (for example, the center electrode 20 and the insulator 10) is good. If the glass powder is excessively hard, particles of the glass powder will be captured from the cross-section of the first seal 72. Furthermore, a gap may form between the first seal 72 and other elements. In the first table TA (Fig.2A) indicates that rating “A” for compaction means no particles of the glass material powder were detected. Rating “B” indicates that particles of the glass material powder were detected. As can be seen from the first table TA, the lower the sodium (Na) content, the better the assessment result for the dielectric strength. The reason for this is as follows: the lower the sodium (Na) content, the less likely it is that sodium (Na) will diffuse into the insulator 10. More precisely, samples No. 1, 2, and 3, classified as A, had a Na content of 0, 0.1, and 0.3 wt%, respectively. Samples No. 4 and 5, classified as B, had a Na content of 0.4 and 0.9 wt%, respectively. Sample No. 6, classified as C, had a Na content of 1 wt%. The higher the sodium (Na) content, the better the compression rating. This is because the higher the sodium (Na) content, the greater the softening of the glass material during the manufacturing of the Spark Plug 100. Specifically, samples 2 through 6, rated A, had Na contents of 0.1, 0.3, 0.4, 0.9, and 1% by mass, respectively. Sample 1, rated B, had a Na content of 0% by mass. A preferred range for sodium (Na) content can be determined by using the sodium (Na) content of samples whose evaluation results for tensile strength and compaction were rated as good. For example, samples Nos. 1 to 5, with a sodium (Na) content of less than 1 wt%, were rated B or higher for tensile strength. Samples Nos. 2 to 6, with a sodium (Na) content of 0.1 wt% or more, were rated A or higher for compaction. From these data, a preferred sodium (Na) content of 0.1 wt% or more and less than 1 wt% can be indicated. Samples 2 to 5, which were rated B or higher for dielectric strength and A for density, have a sodium (Na) content of 0.1, 0.3, 0.4, and 0.9 wt%, respectively. Of samples 2 to 5, samples 2 and 3 were rated A for dielectric strength. Samples 2 and 3 had a sodium (Na) content of 0.1 and 0.3 wt%, respectively. Within the scope of the claimed invention, the sodium (Na) content of the glass is therefore 0.1 wt% or more and 0.3 wt% or less, reduced to Na₂O. This achieves sufficient density of the first seal 72 and simultaneously further restricts the diffusion of sodium (Na) into the insulator 10, thus further limiting any deterioration in the dielectric strength performance of the insulator 10. Fig. 2B is a second table TB, which illustrates the relationship between test results and material properties of spark plug 100 samples. The second table TB shows, for each sample type, a sample number, the potassium (K) content (reduced to K₂O), the sodium (Na) content (reduced to Na₂O), and the evaluation results for tensile strength, compression, and airtightness. Similar to the potassium (K) and sodium (Na) content in the first table TA, the potassium (K) and sodium (Na) content are those of the glass contained in the first seal 72. In the evaluation test, four sample types, namely sample no. 7 to sample no. 10, were tested. Samples no. 7 to 10 differ from samples no. 1 to 6 in Fig. 2A in the following two respects.The first difference is that the four sample types have the same sodium content (reduced to Na₂O) of the glass contained in the first gasket 72, namely 0.2 wt%. The second difference is that the four sample types differ in the potassium content (reduced to K₂O) of the glass contained in the first gasket 72. Specifically, samples 7 to 10 have potassium contents of 1, 4, 8, and 10 wt%, respectively, reduced to K₂O. Other material and structural characteristics (for example, the range of silicon and boron content of the glass contained in the first gasket 72, and the shape of the central electrode 20) of samples 7 to 10 are similar to those of samples 1 to 6. The procedures for checking and evaluating the dielectric strength and compaction are similar to those previously described with reference to Fig. 2A (first table TA). An airtightness test was performed as follows. A pressure test rig (not shown) was prepared, equipped with a pressure cavity having mounting holes similar to those for spark plugs in an internal combustion engine. The external threaded section 57 of the metal sleeve 50 (Fig. 1) was screwed into an internal threaded section of the mounting hole to secure a spark plug 100 sample to the mounting hole of the pressure cavity. The interior of the pressure cavity corresponds to a combustion chamber to which the spark plug 100, secured at the mounting hole, is exposed. While the air pressure in the pressure cavity was increased, the amount of air leakage at the metallic connection section 40 on the side of the through-hole 12 of the insulator 10 was measured. The pressure was set in two stages, namely 1.5 MPa and 2.5 MPa. At a pressure of 1.5 MPa, air leakage was not detected in all samples.The airtightness rating results shown in the second table TB are air leakage rating results for the case where the pressure was 2.5 MPa. Rating "A" indicates that no air leakage was detected. Rating "B" indicates that an air leakage of 0.05 ml / min or less was detected. Rating "C" indicates that a leakage of more than 0.05 ml / min was detected. As shown in the second table TB, the samples were evaluated with respect to dielectric strength and density at various potassium (K) contents with a grade of A. The samples exhibited good dielectric strength and density at these different potassium (K) contents. The potassium (K) content is high compared to the preferred range of sodium (Na) previously described with reference to the first table TA (Fig. 2A). Since potassium (K) can appropriately lower the softening point of the glass, a suitable initial seal 72 can be formed. Furthermore, the probability of potassium (K) diffusion is lower compared to sodium (Na). Because potassium (K) diffusion is inhibited, the deterioration of dielectric strength performance is therefore limited, even at high potassium (K) contents. At particularly high potassium (K) content, the airtightness deteriorated. This is presumably due to the following reason: At high potassium (K) content, the first seal 72 is able to detach from the inner circumferential surface 12i of the insulator 10 as a result of the increased coefficient of thermal expansion of the glass. More precisely, samples No. 7 and 8, which were rated A, had a potassium (K) content of 1 and 4 wt%, respectively. Sample No. 9, which was rated B, had a K content of 8 wt%. Sample No. 10, which was rated C, had a K content of 10 wt%. Samples Nos. 7 to 9, which were rated B or higher for airtightness and A for tensile strength and compaction, had potassium (K) contents of 1, 4, and 8 wt%, respectively. A preferred range of potassium (K) contents can be determined using these three values. In particular, any of the three values ​​can be used as the lower limit of the preferred range of potassium (K) contents. For example, the potassium (K) content may be 1 wt% or more. Of the three values, any value equal to or greater than the lower limit can be used as the upper limit of the potassium (K) content. For example, the potassium (K) content may be equal to or less than 8 wt%. With a potassium (K) content falling within the preferred range, the airtightness between the first seal 72 and other elements can be improved. As shown in the first Table TA (Fig.As shown in Figure 2A), good tensile strength and good compaction were achieved for a given potassium (K) content at various sodium (Na) contents. Therefore, a preferred range of potassium (K) contents can presumably be applied to the case with various sodium (Na) contents that fall within the previously mentioned preferred range of sodium (Na) contents. C. Modified embodiments: (1) The first gasket 72 may have various other material properties than those described above. For example, the glass contained in the first gasket 72 may be a different type of glass (e.g., soda-lime glass) instead of borosilicate glass. In any case, it is generally true that the higher the silicon (Si) content of the glass, the lower its coefficient of thermal expansion. To improve the heat resistance of the first gasket 72, a high silicon (Si) content is therefore preferred. For example, the silicon (Si) content of the glass should preferably be 50 wt% or more when reduced to SiO2. In particular, if the silicon (Si) content is too high, the adhesion between the first gasket 72 and other elements may deteriorate because the softening point of the glass increases. Therefore, the silicon (Si) content should preferably be limited.For example, the silicon (Si) content of the glass is preferably 90 wt% or less, preferably 70 wt% or less, reduced to SiO2. In the case of the use of borosilicate glass, the boron (B) content is not limited to that in the samples described above, but can assume various other values. The potassium (K) content of the glass contained in the first gasket 72 can be less than 1% by mass, reduced to K₂O. The glass contained in the first gasket 72 need not contain potassium (K). In any case, good tensile strength and good density can be achieved using the glass contained in the first gasket 72, which contains sodium (Na) in an amount of 0.1% by mass or more, and 0.3% by mass or less, reduced to Na₂O. The glass contained in the first gasket 72 may contain various other components (for example, Al₂O₃). The electrically conductive substance contained in the first seal 72 is not limited to that of the previously mentioned samples, but can be formed from various materials, such as iron and copper. (2) The elements arranged within the through-hole 12 of the insulator 10 may have different material properties than those described above. For example, the material for the second seal 74 may differ from the material for the first seal 72. The temperature of the second seal 74 does not rise higher than that of the first seal 72. Therefore, the heat resistance requirement is less stringent when selecting the material for the second seal 74. The material for the second seal 74 can be selected from a wider range of materials compared to the material selection for the first seal 72. The intermediate element 79 can have material properties other than those described above. The intermediate element 79 can comprise the resistive element 73, or it can comprise the resistive element 73 and another element (for example, a magnetic element). The intermediate element 79 can comprise a magnetic element without containing the resistive element 73. The intermediate element 79 can also be omitted. In this case, the second seal 74 is also omitted. The first seal 72 connects the center electrode 20 and the metallic terminal element 40. (3) The spark plug may have a structure other than that described above. A discharge gap may be formed between the ground electrode and a side face (a face spaced from the axial line CL in a direction perpendicular to the axial line CL) of the center electrode instead of the front end face (for example, the face of the first tip 29 on the side facing forward Df in Fig. 1) of the center electrode. The total number of discharge gaps may be two or more. The inner seal 8 on the front side may be omitted. In this case, a projecting section (for example, the inwardly projecting section 56 (Fig. 1)) of the metal sleeve directly supports the outer diameter reduction section 16 of the insulator 10. The ground electrode 30 may be omitted.In this case, a discharge can be generated between the center electrode of the spark plug and another element located in a combustion chamber. The present invention has been described with reference to the embodiment and the modified embodiments above. However, the embodiment and the modified embodiments are intended to contribute to the understanding of the invention, but not to limit it. The scope of protection is defined by the claims.

Claims

Spark plug (100) comprising: an insulator (10) with a through-hole (12) extending from a rear to a front; a center electrode (20) inserted at least partially into a section of the through-hole (12) on the front side; a metallic terminal element (40) inserted at least partially into a section of the through-hole (12) on the rear side; and a seal (72) arranged within the through-hole (12) and in contact with the center electrode (20) and an inner circumferential surface (12i) of the insulator (10), wherein the seal (72) comprises a glass and an electrically conductive substance, and the glass contained in the seal (72) contains Si in an amount of 50 wt% or more, reduced to SiO2, and Na in an amount of 0.1 wt% or more and 0.3 wt% or less, reduced to Na2O. Spark plug (100) according to claim 1, wherein the glass contains K in an amount of 1 mass-% to 8 mass-%, reduced to K20.

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