| 摘要 |
<p>A taper surface 12 having a contour L1 extending along an imaginary ellipse E is composed of a plurality of successively arranged curved surfaces which decrease in radius of curvature toward the forward end with respect to the direction of an axis P. The taper surface 12 is such that the distance between the end points M1 and M2 of the contour L1 is large in the direction of the axis P, and the angle between the axis P and a tangential line of the contour L1 on the side toward the forward end surface 11 is larger than the angle between the axis P and a tangential line of the contour L1 on the side toward the side circumferential surface 15. Thus, the taper surface 12 can maintain a diameter close to the outer diameter of the side circumferential surface 15 of the substrate up to a position closer to the forward end. Therefore, it is possible to secure a sufficiently large outer surface area to thereby secure a satisfactory heat radiation amount. Also, when the average outer diameter of a portion (conditions specified section) which extends 6 mm from the forward end surface 11 in the direction of the axis P is less than 2.3 mm, a satisfactory heat radiation performance can be secured. When the average outer diameter is equal to or less than 3.3 mm, a satisfactory quick temperature raising performance is obtained. By rendering the area of the forward end surface 11 equal to or greater than 27% of the area of a circle having a diameter equal to the average outer diameter, the outer diameter of the taper surface 12 can be made sufficiently large, whereby a satisfactory heat radiation performance can be secured.</p> |
