| Because of their high tensile strength,high modulus,high melt point and excellent corrosion resistance,Al2O3-based ceramic fibers are considered promising advanced reinforcements for composites used in thermal oxidizing atmospheres.However,there are two main roadblocks in the preparation of continuous ?-Al2O3 fibers via sol-gel process.One is that the "vermicular" structure is inevitably induced because of the combined effects of low nucleation density of ?-Al2O3 from the transitional phases and the high phase transformation temperature.This structure makes the densification more difficult during the sintering process.The other one is that the ?-Al2O3 grains will quickly grow to micron-scale during sintering,making it difficult to maintain a finegrained structure.These two factors make high-strength continuous ?-Al2O3 fiber difficult to prepare.In order to solve these problems,this work introduced aqueous suspensions of commercially available ?-Al2O3 powders into alumina sol to increase the number of nucleation sites,thereby reducing the phase transformation temperature and improving the "vermicular" structure.Also,the silica sol was doped to suppress coarsening of ?-Al2O3 grains at elevated temperatures,thereby obtaining a fine-grained structure.After systematic research,the following main conclusions are obtained;1.?-Al2O3 seeds can significantly increase the number of nucleation site and then promote the phase transformation of alumina fibers.The average particle size of ?Al2O3 seed is 62.7 nm.As the amount of seed increases,the transformation temperature of ?-Al2O3 to ?-Al2O3 decreases.When the seeding amount is 3 wt.%,the phase transformation temperature reduced from 1100? to 925?,and the activation energy reduced from 312.88 kJ/mol to 244.77 kJ/mol.In addition,the seed can induce a direct precipitation of ?-Al2O3 in the green fiber.When the seeding amount is 3 wt.%,the direct precipitation temperature of ?-Al2O3 is as low as 600?.2.The large number of nucleation site provided by ?-Al2O3 seed promotes the equiaxed development of ?-Al2O3 grains and thus significantly inhibits the formation of "worm-like" structure.For instance,after sintering at 1 500?,the unseeded fiber has many intercrystalline and intragranular pores,the microsturcture is a layer structure stacked by slab-like grains.However,the seeded fiber can obtain a dense structure composed of equiaxed grains.The grain size of ?-Al2O3 in the fiber seeding with 1 wt.%?-Al2O3 is 1 ?m,when the content of seed increases to 3 wt.%,the size decreases to 500 nm.3.The introduction of silica sol can significantly suppress the growth of ?-Al2O3 grains,thereby further reducing the grain size of the fiber.For example,introducing 1 wt,%silica sol into a fiber containing 1 wt.%seed can refine the ?-Al2O3 grains from?1 ?m to?200 nm.It is worth noting that silica sol can also inhibit the growth of ?Al2O3,so it has a negative effect on the phase transformation of ?Al2O3 to ?-Al2O3,but there is no obviously negative effect on the directly precipitation of ?-Al2O3 in the green fiber.4.The "two-step" sintering process can effectively suppress the growth of ?-Al2O3 grains,which is beneficial to further refine the grains.In addition,the grain-growth kinetics revealed that the pre-calcination temperature has a significant effect on the phase transformation and densification of ?-Al2O3 fibers sintered by two-step process.The low(600?)or high(900?)pre-calcination temperature will promote the growth of ?-Al2O3grains,but the fiber pre-calcined at 800? has the smallest growth rate during low-temperature sintering.The sintering conditions can be optimized according to this characteristic. |
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