Obtaining polycrystalline alumina fibers

Abstract

Oxide fibers with a polycrystalline structure are synthesized by forming the fibers from r spinning solutions which have precisely defined properties, followed by heat treatment.Studies carried out in the Ukrainian Scientific-Research Institute of Refractories led to the development of a method for obtaining alumina fibers from viscous spinning compositions, the basis of which are inorganic sols of the oxychloride type, with additions of organic polymers to boost the fiber-forming properties of the solutions.The fibers formed in the shaping process are subjected to heat treatment, during which a microcrystalline structure is developed, ensuring that the oxide fibers are strong.This article deals with a study of the processes that occur in the fiber when it is heated in service to 1400~ and the effect of various reinforcing, stabilizing, and modifying additives on the crystallization of the alumina in the fiber. To assess the degree of calcination of the A1203 fiber, we selected the important factor of the shrinkage of roll fibrous material, which for thermal-insulation and compensation materials is of such great importance in service.The fiber raw material obtained in the form of fibrous roll on the receiver conveyor was used to prepare rectangular specimens whose length and width were measured across the center. The linear shrinkage Al of the specimens was calculated from the equationwhere lo is the dimension before firing, mm, and 11 is the dimension after firing, mm.We also measured and calculated the weight loss of the fiber during calcination. Figure 1 shows the relationships of the linear shrinkage and the weight loss of the alumina fibers to the firing temperature.The rise in temperature was effected at a single rate (200~ for all the temperatures studied. It was established that a gradual increase in shrinkage from 31 to 33% is observed with a rise in temperature from 900 to 1200~ in the range 1200-1400~ there is a more rapid increase in the shrinkage, and its value at 1400~ reaches 37%0 The weight loss of the fiber at 1200~ equals 57.5%.To assess the thermal stability of the fiber in service, the specimens, after production firing, were subjected to repeat heating at 1400~ with a soak of i0 h; the after-contraction was then determined.Clearly, the higher the temperature at which the fiber had been preliminarily fired, the more it tends to preserve its dimensions during repeat heating.Thus, AI20a fiber prefired at 1400~ had an after-contraction of 2.3%.During prolonged soaking of the alumina fibers at high temperatures, modification inversions occur in the Al2Os, and there is an increase in the size of the corundum crystals, which is accompanied by a reduction in the elasticity and strength of the fibers.In order to stabilize the crystal structure of the y-A1203 and preserve the strength of the fiber, various additives were made, e.g., Si02, Cr203, MgO, Li20, B203,. We studied the influence of certain additives on the thermal stability of the fibers and the crystallization of A1203oIt was established that...

Introduction of industrial production of polycrystalline oxide fibers

“…In the 80's a domestic method of production of such fibers was developed for the first time in the Ukrainian Scientific-Research Institute for Refractories on the basis of the chemical method of production described earlier [1][2][3].…”