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It is known [I-4] that additions of SiO2, Cr203, MgO, NiO, Li20, and others are capable of altering the temperature characteristics of the phase inversions of A1203, which in some cases makes it possible to retard the conversion of the metastable modifications of AIzO 3 into the stable o-form.For aluminum oxide fibers this fact is important since the conversion into the o-form markedly impairs their elastic and strength characteristics [5].Polycrystalline alumina fibers with additions of Si02, MgO, and Cr203 obtained with the technology described in [2] were fired in air at 600-1200~ (6h) and 1300~ (3h).The phase composition of the fired specimens was checked on the DRON-I.5 diffractometer (in Cu K~-radiation).The x-ray investigation of the degree of conversion of AIzO 3 into the o-form in these fibers without additives and with an addition of SiO 2 was done on specimentablets I0 mm in diameter and 2 mm thick.The specimens were pressed from alumina fibers and fired at 800~and then calcined in air, holding them for different times at 1200-1300~The electron-microscopic studies of the polycrystalline fibers were done on the EVM-100 AKmicroscope using the suspension method, and also diffraction and microdiffraction methods. Analysis of the phase composition showed that all the specimens fired at 600~ were x-ray amorphous.The appearance of the a-Al20 s in specimens of fiber without additives is noted starting from IO00=C.The process is actively developed at 12000C, and is complete at 1300-=C. The main transition phase to the ~-form is 7-A1203.Traces of the G-phase (Fig. i) were noted among the other intermediate forms.The electron-microscopic studies show that at 800~ the fibers have a tangled granular structure with separate grains having dimensions of about 6 i~m and less. Well formed crystals measuring 0.I • 6 ~m were rarely encountered (Fig. 2a). The electronographic studies show the presence of 7-and 8-A1203. At 12000C the granular-network structure of the fibers takes the place of the densely layered mass without visible boundaries between the phases. The fibers consist mainly of ~-AI203.When 2, 5, and 10%* SiO z are added to the alumina fibers starting at 800~ in addition to 7-AI2Os, the transition 6-and 8-forms are developed, whose region of existence extends with increase in the concentration of the SiO 2 addition (Fig. 3). On the diffractograms of the specimens fired at 1200~ together with reflexes for the transition 6-and #-phases, we note the presence of lines for ~-A1203, and moreover, their intensity is markedly weakened with increase in the concentration of SiO 2 in the samples.The lines for mullite for the various concentrations of SiO 2 were fixed starting at 1200-1300~ their intensity increased with increase in the firing temperature and increase in the concentration of added SiO 2.The fibers with the addition of 2% SiO 2 had sections with a different morphology.We noted a network-granular structure with the dimensions of separate grains equal to 6-7 nm, and also layered, platelike sections without ...

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...