The Research On AlON Powder Prepared By Carbothermal Reduction And Nitridation

AlON transparent ceramics have an optically isotropic cubic spinel structure,which provides excellent mechanical properties and optical properties.They are often used as window materials for transparent armor and infrared fairing and are excellent candidate materials to replace expensive sapphire single crystals.The composition of spinel type AlON is not fixed,and the AlN content of ideal spinel AlON was 21.6-29.7 mol%.When the AlN content is 27.5 mol%,AlON has the best performance.The properties of AlON ceramics depend on the quality of the powder.The synthesis methods of AlON powder mainly include aluminothermic reduction and nitridation,solid state reaction,and carbothermal reduction and nitridation,among which carbothermal reduction and nitridation have attracted much attention due to the low cost and easy industrialization.In the preparation of AlON powders by carbothermal reduction and nitridation,the final composition of AlON powder can be fluctuating,due to the insufficient carbothermal reduction and nitridation reaction caused by the severe agglomeration in the raw material.In this article,y-Al2O3 and activated carbon powder were used as raw materials,AlON powder with an AlN content of 27.5 mol%as the target,and AlON powder was synthesized by carbon thermal reduction and nitridation.The process optimization of the carbothermal reduction and nitridation,additive-assisted AlON ceramic powder synthesis,and AlON ceramic densification research were carried out,as follows:The effects of ball milling process parameters,reaction temperature,and reaction time on the phase composition of reaction products were studied.the distribution uniformity of raw material was optimized in Al2O3/C mixed powder through different ball milling processes.It is found that the Al2O3/C mixed powder obtained by roller ball milling at a speed at 50 rpm for 24 h had the best distribution uniformity.The spinel phase AlON powder was obtained after holding the mixed powder at 1550℃ and 1700℃ for 1 h,and no other chemical reactions occurred after insulation and impurity removal at 750℃.The powder had high purity,with the content of residual carbon only 0.0065 wt%,and impurity elements such as Cu and Fe were all less than 0.01 wt%.The synthesized particle size of AlON powder was 0.5 μm after planetary ball milling at 250 rpm for 16 h After fitting and calculating the XRD data of the synthesized AlON powder,the lattice constant was about 7.937 A,and the AlN content was 20 mol%,belonging to O-rich-AlON.Increasing the reaction temperature to 1750℃ and extending the reaction time to 2 h,the AlN content was unchanged.To increase the AlN content and obtain N-rich-AlON,silane coupling agent was used as the dispersant to assisting the carbothermal reduction and nitridation.The effect of dispersant content on the distribution uniformity of raw material in Al2O3/C mixed powder and AlON synthesis were studied When the amount of dispersant was 5 wt%,the uniformity of C and O element distribution is further improved,and stronger reaction activity is exhibited.The single-phase AlON powder was obtained after the Al2O3/C mixed powder was kept at 1670℃ for 0.5 h The lattice constant of AlON was 7.944 ?,and the AlN content was 27.5 mol%,which is nitrogen-rich AlON.Due to reducing the synthesis temperature and shortening the reaction time,the particle size of AlON powder was 100-200 nm.At the same time,the mechanism of action of the dispersant was analyzed through TG-DSC and XPS.The Si and C functional groups bonded with carbon and Al2O3 powder respectively,forming Si-C and C/Al2O3 bonds,bridging the gap between C and Al2O3 and weakening their interaction.Compared with the control group without dispersant,the DSC curve of Al2O3/C mixed powder containing dispersant showed a strong exothermic peak at 1400℃,and the carbothermal reduction and nitridation reaction occurred more violently.The lattice constant of obtained AlON powder was about 7.944 A,and the AlN content was 27.5 mol%,indicating N-rich AlON.The primary particle size was only 100-200 nm.The effects of different sintering aids and sintering processes on the densification of AlON ceramics were studied.Result showed that compared to AlON ceramics with Y2O3 addition,AlON ceramics with La2O3-Y2O3 addition had a faster densification rate,higher relative density,hardness,and smaller grain size within the same sintering time.When the sintering time exceeds 5 h,the ceramic density begins to decrease.After sintering at 1850℃for 5 h,the density of AlON ceramics was 3.66 g/cm3,reaching 99.38%of the theoretical density.