Preparation And Properties Of High Temperature Infrared Transparent AlON Raw Powder And Ceramics

The AlON ceramics as high-temperature resistant and IR transmitting material have characteristics of superior optical characteristics,excellent physical and mechanical properties,which are the most promising future material for the high performance hypersonic missile and vehicles antenna radome.However,the utilization of these materials is at a relatively low profile domestically because of the difficulty in fabricating such transparent ceramics with advanced optical properties.In this dissertation,a new approach to fabricating AlON ceramics is proposed to improve their service performance.We firstly prepare high-purity nanometer?-Al₂O₃ and AlN powders with high reactivity.Using these powders,we fabricate the AlON ceramics by a two-step spark plasma sintering?SPS?method in comparison to those made from the conventional sintering.We investigate the influence factors of the preparation of the AlON ceramics.Meanwhile,we make the AlON powders by the sol-gel route.The conclusions are as follows.The nanometer?-Al₂O₃ powders withlow crystallinity and high reactivityare prepared by the sol-gel method by using Al₂O₃ sol as a raw material.The effect of heat treatment temperature on the composition,specific surface area and micro-morphology of the?-Al₂O₃ powders is discussed.The results show that the low crystallinity nanometer?-Al₂O₃ powders,with a grain size of 15 nm and a specific surface area of228.7m²/g,are obtained after the heat treatment of the Al₂O₃ sol below 1000?.When the temperature is beyond 1200?,the?-Al₂O₃ is formed.In particular when the temperature rises to 1350?with 1h dwelling time,the total?-Al₂O₃ transformes into?-Al₂O₃,with a grain size increased by 15 times.The nanometer AlN powders with low crystallinity and high reactivity are fabricated by the precursor conversion method by using the Al₂O₃ sol,glucose and urea as raw materials.Reaction mechanisms during the synthesis of the AlN powder are discussed.The effects of the raw-material ratio,heat treatment temperature and holding time on the composition,specific surface area,grain size and micro-morphology of the AlN powders are discussed.It is found that the nano-AlN powders with low crystallinity and a grain size of 20 nm are obtained when the raw materials are treated at1350?for 1h in the N₂ atmosphere with a 2:2.5 molar ratio of Al₂O₃ and glucose.The AlON powders were fabricated by precursor conversion method using the Al₂O₃ sol,glucose and urea as raw materials to simplify technological process and reduce cost.The single phase AlON powders with excellent crystallinity were obtained after heat treatment at 1750?for 2h.Its chemical formula is Al_2.85O_3.45N_0.55 and the crystal form is cubic phase?-AlON.However,there was 2.6wt%residual C in the AlON powders.The carbon content was still 2.1wt%after carbon removal process at700?for 5h.The as prepared AlON powders had relative irregular morphology and wide range particle size distribution,the average grain size of which was 30?m.Preparation of good performance AlON powders needs to be further explored the reaction mechanism mechanism and optimizing the preparation process.The effects of the preparation parameters of a two-step sintering by the SPS and the conventional one are studied by using the above obtained powders.The single-phased?-AlON ceramics can be fabricated by the conventional sintering under the N₂ atmosphere at 1900?for 3h with a 4:6 molar ratio of AlN and?-Al₂O₃.The relative density and highest transmittance of the samples can reach 99.1%and 28.2%respectively.Then the samples were sintered with 0.1wt%MgO and 0.1wt%Y₂O₃.The result of the experiment showed that relative density and highest transmittance of the samples increased to 99.4%and 33.2%,which was still at a low level.The single phase?-AlON ceramics can be obtained by one-segment SPS sintering in N₂ atmosphere at1700?.The relative density of the?-AlON ceramics increased with the sintering temperature between 1600¹700?.However,when the sintering temperature continued to increase,there was a decline in the relative density of the samples.The single phase?-AlON ceramics,relative density 99.41%of and highest transmittance of62%,were fabricated by two-segment SPS sintering in which the sintering temperature was held at 1700?for 15min and at 1400?for 20min respectively.The relative density and highest transmittance of the samples prepared by two-segment SPS sintering were much higher than those prepared by conventional sintering and one-segment SPS sintering,which are attributed to the rapid densification process at a lower temperature.When the addition quantity of the MgO or Y₂O₃ sintering aids was below 0.2wt%,the samples had a higher relative density.However,when the addition quantity of the MgO or Y₂O₃ sintering aids continued to increase,there was a decline in the relative density and highest transmittance of the samples.