Effect Of Carbon Source On Properties Of AlON Powder Synthesized By Carbothermol Reduction And Nitridation

Transparent AlON ceramics has attracted a growing interest due to its high optical transparency properties,high mechanical properties,low thermal expansion coefficient,and good physical stability and chemical inertness,etc.As a promising structure-function integrated polycrystalline transparent window material,AlON can be used as transparent armor and infrared window,as well as,which can be used as observation and monitoring windows of advanced transportation equipment(such as high-speed aircraft and high-speed trains)worked in severe environment.The application of transparent AlON ceramics in transportation fields will help to improve the performance and reliability of corresponding equipments.AlON powder is the key raw material for fabrication of transparent AlON ceramics by sintering,and its property directly affects the densification process and performance of transparent ceramics.In this paper,Al2O3 and different carbon powders were used to synthesize AlON powder by carbothermal reduction and nitridation(CRN)method in N2.This study focuses on obtaining the relationship between carbon powder and property of AlON powder via phase assemblage analysis,morphology observation,ball milling,pressureless sintering and optical transparency co MParison of sintered ceramics.Then,using gas nano-sized carbon black as carbon source,the CRN processing parameters were studied for preparing of AlON powder.Furthermore,the formation mechanism of AlON was revealed.The main contents and results of this work are as follows:(1)Four kinds of activated carbon prepared from coal powder,wood powder,coconut shell powder and bamboo powder respectively and a submicro carbon black were selected as carbon source to synthesize AlON powder by a widely applied two-step thermal treatment(1550 ℃ for 60 min and 1750 ℃ for 60 min).The phase assemblage analysis shows that single phase AlON powders were obtained when bamboo powder activated carbon and submicro carbon black were used as reductant.However,residual Al2O3 were detected in the powders prepared by using coal powder,wood powder,coconut shell powder activated carbon as carbon source,as well as,it is observed that the particle size of AlON powders was big.It indicates that,carbon black with small particle size is benefit to prepare single phase AlON powder,but the type of activated carbon should be carefully selected according to their raw material.The ball milling experiments of the single phase AlON powders show that the powder synthesized by submicro carbon black is easier to be milled to fine powder due to which has a smaller median particle size(D50)than that of synthesized by bamboo powder activated carbon.(2)Single phase AlON powders were prepared by using either of the studied nano-sized carbon black,i.e.gas black,furnace black,carbon black and conductive carbon black via the two-step heating method.However,the synthesized AlON powders show different morphologies.Morphology of the powders synthesized by gas black and furnace black is near-spherical with smooth surface,but angular particles with steps consist of the powders prepared by color black and conductive carbon black.The AlON powder prepared by gas black can be easier milled to fine particle than other obtained powder,the milled powder has the D50 of 1.88 μm,and its ceramics has a transmittance of 69.7%,which is higher than all the other samples prepared by AlON powder from furnace black,color black and conductive carbon black.(3)For the gas black,one-step heating were design to synthesize AlON powder via adjusting heating speed(5 ℃ /min and 50 ℃ /min)and holding time at synthesize temperature of 1750 ℃(0-120 min),then transparent AlON ceramics were fabricated via pressureless sintering the obtained AlON powder.The results show that single phase AlON powders can be obtained after holding 60 min at 1750 ℃ at the heating speed of 5 ℃/min and 50 ℃ /min,and the ceramics prepared from AlON powder synthesized at 50 ℃ /min show a higher transmittance of 77.0%.Therefore,using the gas black,single phase AlON powder with high sinterability can be obtained by the one-step fast heating process.(4)Using gas black,only Al2O3 was detected in samples heated to 1550 ℃ and1750 ℃ at 50 ℃/min,which indicates the formation of AlON in the fast one-step method should mainly by direct reduction and nitridation of Al2O3.However,in the two-step method,reaction between Al2O3 and Al N should be also contribute to the formation of AlON due to mass Al N has been formed during the first step holding.