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Show weight of thorium oxide and 0.7% by weight of cerium oxide in the final product). After excess solution is removed from the imbibed cellulosic precursor, after the preform is shaped, and after it is dried, the structure is ignited with a flame. The nitrate salts convert eventually to the oxides and the cellulosic is pyrolyzed so that the residue is a skeletal ash of the oxides in a form that is a replica of the original cellulose but with greatly reduced dimensions. In the pyrolysis process, huge volumes of decomposition products (C0 2 and water vapor) relative to the volume of the initial cellulosic are released and in the uncontrolled burning process, structural damage in the form of ruptures and rifts in the precursor occur as these decomposition products escape from the interior of the cellulosic. These rifts generally are too large to heal or sinter during the later stages of the fabrication process, and these flaws remain to act as stress raisers that compromise the mechanical strength of the ceramic fiber. Recently, we have made improvements 3 in the Welsbach fabrication process that result in a significant enhancement in mechanical strength. Our process begins with the same imbibition of a cellulosic precursor that Welsbach perfected except that we use, exclusively, continuous filament rayon yarn. After imbibition, we use a retort furnace to convert the metal nitrates to oxides and to pyrolyze the cellulosic. An important part of our process is a pyrolysis step where oxygen is slowly metered into the retort over a two hour interval to convert slowly the cellulose into its gaseous byproducts. At the completion of the cellulosic burnout phase, the retort is ramped rapidly to the highest temperature available which in our case is about 1000oC. After cool-down, the fibrous structure may be transferred to another furnace with a higher temperature capability (up to 1700 oC) where the final sintering in air is accomplished. The enhanced mechanical strength of mantle structures have been evaluated by an impact test that applies a bending moment or a shearing force to a mantle in its most vulnerable orientation. The force is applied perpendicular to the axis of the mantle. An analysis of a simplified model of a fibrous mantle in the form of a hollow, cylindrical, cantilevered beam predicts the following expression: Impact Fracture Force (in g's) = Z(R/L2 ) (1 ) where R is the mantle radius and L is the mantle's unsupported length. Z is a figure-of-merit that depends on the material system and the quality of processing. Ceramics processed according to the procedures described can "exhibit a figure-of-merit of about 3W. J . Diederich and R.E. Nelson, Processes," U.S. Patent No. 4,883,619. Issued November 28, 1989. "Refractory Metal Filed August 16, Oxide 1982. |