| abstract |
One embodiment of the present disclosure is directed to an insulator comprising a ceramic composition, wherein the ceramic composition comprises about 25-60% SiO2; 15-35% R2O3, wherein the R2O3 is 3-15% B2O3 and 5-25% AI2O3; 4-25% MgO+0-7% Li2O, wherein the total of MgO+Li2O is between about 6-25%; 2-20% R2O, wherein the R2O is 0-15% Na2O, 0-15% K2O, 0-15% Rb2O; 0-15% Rb2O; 0-20% Cs2O; and 4-20% F; crystalline grains, wherein the crystalline grains are substantially oriented to extend in a first direction to provide improved insulating properties in a direction perpendicular to the first direction, wherein the first direction is circumferential and the direction perpendicular to the first direction is radial; and a first zone and a second zone, wherein the first zone is in compression and the second zone is in tension. |