Abstract: An isolation portion (P) of an insulator (10) of a spark plug (100) electrically insulatively isolates a front end portion (22) of a center electrode (20) and a holding portion (56) of a metallic shell (50) from each other and has an intermediate portion (P2) which extends while its outside diameter varies, thereby ensuring an insulation distance between the two portions. The ratio (S/V) of the surface area (S) of the outer surface (14) of the isolation portion (P) to the volume (V) of the isolation portion (P) satisfies the relation 1.26 mm?1?S/V, whereby the insulation distance between the front end portion (22) and the holding portion (56) is sufficiently ensured while existing dimensional conditions are held unchanged. Through satisfaction of the relation S/V?1.

Abstract: A spark plug which exhibits high dielectric strength at high temperature and which is unlikely to be deteriorated in dielectric strength is provided. The spark plug includes a center electrode, an insulator, and a metallic shell. The insulator includes an alumina-based sintered body which contains a silicon component, a Group II element component, and a rare earth element component. The alumina-based sintered body is composed of a glass phase and an alumina crystal phase as observed through X-ray diffraction analysis. When the grain boundary phase of the alumina-based sintered body is observed under a transmission electron microscope, and the weight ratio between an alkali metal component and the rare earth element component as reduced to oxides thereof [alkali metal/rare earth element] is calculated at 30 circular spots, each having a diameter of 0.3 nm, selected arbitrarily in a field of observation, the average of the weight ratios of the 30 spots falls within a range of 0.01 to 1.0.

Abstract: A method for manufacturing an insulator includes: preparing a press pin and a forming die having a cavity, the press pin including a pin-side spiral portion formed on a first position; filling a raw powder into the cavity; arranging the press pin within the cavity; pressing the raw powder within the cavity along with the press pin, and obtaining a green body formed with a green body-side spiral portion; releasing the green body along with the press pin from the cavity; retreating the press pin with respect to the green body while rotating the press pin relative to the green body around an axis, and extracting the press pin from the green body; and removing an unnecessary portion from the green body. The first position is positioned such that the green body-side spiral portion is located in the unnecessary portion of the green body.

Abstract: This invention has an object to provide a spark plug exhibiting high withstand voltage characteristics and high strength at high temperature, formed by an alumina-based sintered body prepared with excellent processability and high productivity, and a method for manufacturing the spark plug. This invention relates to a spark plug 1 including an insulator 3, wherein the insulator 3 includes a dense alumina-based sintered body having an average crystal particle diameter of 1.50 ?m or more, and the alumina-based sintered body contains Si, a Group 2 element component containing Ba and a Group 2 element other than those, and a rare earth element component, such that a ratio of a content S of the Si component to a total content of the content S and a content A of the Group 2 element component is 0.60 or more, and a method for manufacturing the spark plug 1 prepared through a grinding-shaping process in which the insulator 3 is ground before burning to shape the same.

Abstract: The present invention relates to a spark plug having good combustible-gas ignitability even when the spark plug is downsized and to a method for high-yield manufacturing of a spark plug with good combustible-gas ignitability. There is provided according to one aspect of the present invention a spark plug 1 having a center electrode 2, an insulator 3 and a metal shell 4 and satisfying the following conditions (1) to (5). There is provided according to another aspect of the present invention a manufacturing method of a spark plug including the step of producing an insulator by preparing a raw material powder in such a manner that the particle size distribution ratio (90% volume diameter/10% volume diameter) between particles of 10% volume diameter and particles of 90% volume diameter in the raw material powder is 3.6 to 5.2, and then, press-forming and sintering the prepared raw material powder. Condition (1): The insulator has a wall thickness T of 0.3 to 1.

Abstract: A spark plug that is equipped with an insulator that has a stem portion on a front end side, a center electrode, and a main metal member that retains the insulator at an engaging projection portion, wherein an inner diameter DIN of the engaging projection portion, the maximum outer diameter dOUT of the stem portion, which faces an inner circumferential surface of the engaging projection portion, its inner diameter dIN, and dielectric constant ? of the insulator satisfy the condition of “(DIN?dOUT)/2?0.40 (mm)”, “(dOUT?dIN)/2?1.65 (mm)”, and “??9.4”.

Abstract: A spark plug exhibits a satisfactory withstand voltage characteristic and sufficient mechanical strength in a high temperature environment exceeding 700° C. The spark plug has a center electrode, an insulator, and a ground electrode, characterized in that the insulator is formed of an alumina-based sintered material containing an Si component, a Group 2 element (2A) component, and a rare earth element (RE) component; that the alumina-based sintered material has an RE-?-alumina crystal phase; and that the mean crystal grain size DA(RE) of the RE-?-alumina crystal phase and that of alumina DA(Al) satisfy the following relationship (1): 0.2?DA(RE)/DA(Al)?3.0.

Abstract: A spark plug which exhibits high dielectric strength at high temperature and which is unlikely to be deteriorated in dielectric strength is provided. The spark plug includes a center electrode, an insulator, and a metallic shell. The insulator includes an alumina-based sintered body which contains a silicon component, a Group II element component, and a rare earth element component. The alumina-based sintered body is composed of a glass phase and an alumina crystal phase as observed through X-ray diffraction analysis. When the grain boundary phase of the alumina-based sintered body is observed under a transmission electron microscope, and the weight ratio between an alkali metal component and the rare earth element component as reduced to oxides thereof [alkali metal/rare earth element] is calculated at 30 circular spots, each having a diameter of 0.3 nm, selected arbitrarily in a field of observation, the average of the weight ratios of the 30 spots falls within a range of 0.01 to 1.0.

Abstract: To provide a spark plug excellent in the acid resistance and high-temperature withstand voltage characteristics. A spark plug 100 including an insulator 2 being formed in an approximately cylindrical shape and having a through hole 6 penetrating in the axis direction, and a center electrode 3 inserted into the through hole 6, wherein the insulator 2 is formed by an alumina-based sintered body containing an Si component, a Ba component, a Ca component and an Mg component to satisfy the following conditions (1) and (2) and containing substantially no B component: Condition (1): the ratio RCa of the mass in terms of oxide of the Ca component to the mass in terms of oxide of the Si component is from 0.05 to 0.40; Condition (2): the ratio RMg of the mass in terms of oxide of the Mg component to the total mass of the mass in terms of oxide of the Si component, the mass in terms of oxide of the Ca component and the mass in terms of oxide of the Mg component is from 0.01 to 0.08.

Abstract: An alumina-based sintered body for a spark plug having enhanced mechanical strength and a method of manufacturing the same, as well as a spark plug having the alumina-based sintered body for a spark plug and a method of manufacturing the same.

Abstract: A task of this invention is to provide a spark plug that is superior in withstand voltage characteristics at high temperatures. A spark plug of this invention is equipped with a center electrode, an insulator, and a metal shell, and is characterized by that the insulator is equipped with an alumina-based sintered body containing silicon component (Si component), magnesium component (Mg component), at least one component of calcium component, strontium component, and barium component, and a rare earth element component (RE component), that a grain boundary phase of the alumina-based sintered body has a hexaaluminate crystal containing the RE component, that, in the alumina-based sintered body, the weight ratio of the RE component to the Mg component is 5.4?RE oxide/MgO?17.5, and that the content of an alkali metal component in terms of oxide in a spot having a diameter of 0.

Abstract: A method for manufacturing an insulator includes: preparing a press pin and a forming die having a cavity, the press pin including a pin-side spiral portion formed on a first position; filling a raw powder into the cavity; arranging the press pin within the cavity; pressing the raw powder within the cavity along with the press pin, and obtaining a green body formed with a green body-side spiral portion; releasing the green body along with the press pin from the cavity; retreating the press pin with respect to the green body while rotating the press pin relative to the green body around an axis, and extracting the press pin from the green body; and removing an unnecessary portion from the green body. The first position is positioned such that the green body-side spiral portion is located in the unnecessary portion of the green body.

Abstract: A spark plug exhibits a satisfactory withstand voltage characteristic and sufficient mechanical strength in a high temperature environment exceeding 700° C. The spark plug has a center electrode, an insulator, and a ground electrode, characterized in that the insulator is formed of an alumina-based sintered material containing an Si component, a Group 2 element (2A) component, and a rare earth element (RE) component; that the alumina-based sintered material has an RE-?-alumina crystal phase; and that the mean crystal grain size DA(RE) of the RE-?-alumina crystal phase and that of alumina DA(Al) satisfy the following relationship (1): 0.2?DA(RE)/DA(Al)?3.0.

Abstract: An object of the invention is to provide a spark plug including an insulator which is difficult to cause breakdown and exhibits higher withstand voltage characteristics. This invention relates to a spark plug 1 including a center electrode 2, an insulator 3 and a grounding electrode 6, wherein the insulator is formed by an alumina-based sintered body which has a theoretical density ratio of 95.

Abstract: There are provided a ceramic heater in which a defect, such as generation of a gap at the interface between an insulating substrate and a heat-generating resistor, is unlikely to occur in the course of manufacture or use, and a glow plug using the ceramic heater. A ceramic heater 110 includes an insulating substrate 111 extending in the direction of an axis AX and a heat-generating resistor 115, which has a heat-generating portion 116, two lead portions 117, 117 and two lead lead-out portions 118a and 118b. The ceramic heater 110 satisfies an expression S1?0.34 Sa, where Sa is the area of a cross section of the ceramic heater 110 taken perpendicular to the direction of the axis AX, and S1 is the total cross-sectional area of the two lead portions 117, 117 as measured in the cross section.

Abstract: A device for inspecting a spark plug insulator for bending, the spark plug insulator including a cylindrical body having a radially enlarged flange at an axially intermediate portion thereof, and a through hole formed in the body along an axial direction of the body. The inspection device includes an insulator supporting means for supporting the spark plug insulator so as to be slidable in an axial direction of the spark plug insulator, a linear inspection pin that is to be inserted into the through hole of the spark plug insulator supported by the insulator supporting means, and a detecting means for detecting a sliding movement of the spark plug insulator when the spark plug insulator is caused to slide in the axial direction thereof by the inspection pin by the inspection pin prevented from completely penetrating the through hole of the spark plug insulator owing to bending of the spark plug insulator which exceeds a predetermined tolerance.

Abstract: A method for manufacturing a ceramic insulator for a spark plug.

Abstract: An isolation portion (P) of an insulator (10) of a spark plug (100) electrically insulatively isolates a front end portion (22) of a center electrode (20) and a holding portion (56) of a metallic shell (50) from each other and has an intermediate portion (P2) which extends while its outside diameter varies, thereby ensuring an insulation distance between the two portions. The ratio (S/V) of the surface area (S) of the outer surface (14) of the isolation portion (P) to the volume (V) of the isolation portion (P) satisfies the relation 1.26 mm?1?S/V, whereby the insulation distance between the front end portion (22) and the holding portion (56) is sufficiently ensured while existing dimensional conditions are held unchanged. Through satisfaction of the relation S/V?1.

Abstract: There is provided a test method for detecting the presence or absence of a defect in a spark plug insulator, including a reference voltage determination process, a test area determination process, a test voltage determination process and a current detection process. In the reference voltage determination process, a reference voltage VF is determined. In the test area and voltage determined processes, test area and voltage are determined so as not to incur a flashover on the basis of a reference insulator of the same material, shape and size as the spark plug insulator when the reference insulator is placed in position between first and second test electrodes. In the current detection step, the test voltage is applied between the first and second test electrodes to detect an electric current between the first and second test electrodes.

Abstract: An alumina-based sintered body for a spark plug having enhanced mechanical strength and a method of manufacturing the same, as well as a spark plug having the alumina-based sintered body for a spark plug and a method of manufacturing the same.