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Show INTRODUCTION Small diameter refractory fibers (5-10 ~ in diameter) have many advantages as emissive members in radiant burner systems. For example, these fibers tolerate thermal stress insofar as no appreciable thermal stress can be built up across the small diametral dimension of the fiber, and stress built up along the axis of the fiber is relieved by a flexing of the fiber. Also, there are no insulating boundary layers common to larger dimensioned monolithic radiant structures. Thus, we believe small diameter refractory fibers are in approximate thermal equilibrium with the combustion products. Since some radiant burners may benefit from limited emission in some part of the optical spectrum, some small diameter ceramic fibers with a low effective emittance offer this option. Finally, the low thermal inertia of the small volume fiber in conjunction with the close thermal coupling to the flame result in a rapid thermal response time. A recent applicationl of a fibrous, light-emitting ceramic to evaluate qualitatively the beam pattern of a carbon dioxide laser enables the convenient measurement of a thermal response time of 20 milliseconds which is representative of the systems of interest to us. MECHANICALLY DURABLE FIBERS An obvious shortcoming of small diameter fibers is their mechanical strength. A ceramic fiber will fail when mechanically stressed because a flaw (void, microcrack, etc.), which generally originates during the fabrication process, propagates and results in fiber fracture. A classic example of a fibrous ceramic structure is a Welsbach mantle which was invented2 about 100 years ago and continues to be used today as the most effective component known to convert the heat of gas combustion into visible light. It is, however, a very fragile structure, and its mechanical weakness is a consequence of the fabrication process. A mantle is typically made by impregnating a textile (cotton, silk, rayon, or related cellulosic) precursor yarn that has been knitted, woven, or braided into the desired geometry (generally closed-end cylinders) with aqueous salt solutions of the metals of interest. Since the Welsbach composition is an unique mixture of the oxides of thorium and cerium, a typical imbibing solution would consist of thorium nitrate and cerium nitrate in the proper proportion (99.3% by IS.A. Pollack and D.B. Chang, "Selective Thermal Radiators," U.S. Patent No. 4,755,673. Filed October 24, 1984. Issued July 5, 1988. 2Carl Auer von Welsbach, "Incandescent Lighting Substance," u.S. Patent No. 563,524. Filed September 23, 1895. Issued July 7, 1896. |