Preparation And Properties Of Transparent Spinel Ceramics

Transparent spinel ceramics can be widely used as transparent armors,IR domes and IR windows due to its high transparency range over 0.2–6.0?m,high mechanical strength up to high temperature?>1000^oC?and low preparation cost.In the early 21^stt century,with the emergence of high-quality powders and hot isostatic press?HIP?sintering method,transparent spinel ceramics of high optical quality and large size have been successfully prepared.At present,high-purity spinel powders are usually used as the raw materials.After adding LiF as the sintering aid,transparent samples are obtained through hot pressing followed by HIPing process.This method has high requirements on raw materials and equipments,and it is expensive and not beneficial for batch production of products.Meanwhile,LiF is easy to cause grain coarsensing and grain-boundary microcrack defects,resulting in a low mechanical strength,which severely limits the application of transparent spinel ceramics in the field of transparent armor and high mach weapons.In this work,in the absence of sintering additive,transparent spinel ceramics were prepared via reactive sintering followed by HIPing treatment,using low-cost and well-sourced magnesia and alumina powders as the raw materials.The effects of alumina polymorph,composition,sintering parameter and sintering atmosphere on densification rate,microstructure evolution,phase formation and properties of resulting samples were systematically studied.On the basis of above studies,the effects of different sintering additives on preparation of transaprent spinel ceramics were also researched to choose appropriate one,which can instead of commonly used LiF.The main conclusions of this work are as following:?1?The stoichiometric transparent spinel ceramics were prepared by reactive sintering.Different alumina powders,?-Al₂O₃ and?-Al₂O₃,were respectively used as the source of aluminum.The effects of alumina polymorph on phase formation,densification rate,microstructure evolution and properties of resulting samples were studied.The samples made of?-Al₂O₃ need to be presintered at a high temperature to completely eliminate open porosity.The residual pores of presintered samples,which were quite smaller than the grain size,can be easily eliminated during the following HIPing process,resulting in a high transmittance.In contrast,the presintering temperature of samples made of?-Al₂O₃ was much lower than that of samples made of?-Al₂O₃.The rapid densification rate led to the generation of large pores with high coordination numbers,which were thermodynamically stable and difficult to be removed.During the HIPing process,large pores were easy to cause abnormal grain growth and residual pores,resulting in the severe decrease of transmittance.Using?-Al₂O₃ as the raw material is beneficial for the low-temperature sintering of transparent spinel cermics,as long as the generation of large pores can be effectively avoided.?2?Using high-activity?-Al₂O₃ and MgO powders as the raw materials,transparent spinel ceramics with n=0.98-2.5 were successfully prepared.The effects of composition on preparation process and properties of samples were researched.For magnesia-rich and stoichiometric samples,the densification rates were both very high,leading to the generation of stable large pores.The optical quality of resulting transparent ceramics were quite poor due to the abnormal grain growth and residual pores.Extra alumina can obviously decrease the densification rate of samples,which was beneficial to the gradual elimination of pores during the presintering process.The resulting alumina-rich transparent ceramics exhibited uniform microstructure and high transmittance.The preparation process of transparent alumina-rich spinel ceramics was further optimized.When the molar ratio of Al₂O₃/MgO was over 1.5,meta-stable spinel precipitates were easy to appear at high HIPing temperatures.And the higher temperature,the more second-phase grains.The different coefficient of thermal expansion between spinel grains may cause stress-induced birefringence,decreasing the optical quality of transparent ceramics.Transparent alumina-rich spinel ceramics with excellent properties were successfully prepared by optimizing the composition and sintering conditions.The n=1.1 and 1.3 samples HIPed at 1550°C exhibited transmittance values over 84%at 400 nm and fine grains below 5?m.?3?The effects of sintering atmosphere on preparation process and properties of samples were studied by presintering samples in air and vacuum,respectively.The microstructure of samples presintered in air was uniform.The transmittance of resulting transparent ceramics was mainly decided by the HIPing temperature.The high temperature can cause the appearance of meta-stable spinel phase.Besides,the water in air can react with spinel to form hydroxyl group,resulting in the generation of absorption peak at 3?m.In vacuum,composition gradient and differential sintering,which was caused by the volatilization of Mg/MgO,appeared on the surface of presintered samples,leading to the generation of large-grain layer.The dense large-grain layer can inhibit the diffusion of internal pores and increase HIPing temperature.However,the oxygen vacancy produced by vacuum can effectively prevent the precipitation of second phase,beneficial to the preparation of transparent ceramics with high value of n.?4?The effects of CaO,La₂O₃ and Y₂O₃ sintering additives on the preparation of reactively sintered samples with n=1.5 were studied,respectively.The S30CR commercial powder was used for comparision.All the sintering additives can effectively improve the optical quality of transparent spinel ceramics.As for the mechanical properties,only La₂O₃ can obviously enhance the flexural strength of transparent spinel ceramics.When using mixed powders as the raw material,the free oxides were much easier to react with sintering aids than spinel powder,which can obviously reduce the amount of sintering aids and eliminate the negative effect of second phases on transmittance of samples.