Equiatomic 9-cation high-entropy carbide ceramics of the IVB,VB,and VIB groups and thermodynamic analysis of the sintering process

The preparation of high-entropy(HE)ceramics with designed composition is essential for verifying the formability models and evaluating the properties of the ceramics.However,inevitable oxygen contamination in non-oxide ceramics will result in the formation of metal oxide impurity phases remaining in the specimen or even escaping from the specimen during the sintering process,making the elemental compositions of the HE phase deviated from the designed ones.In this work,the preparation and thermodynamic analysis during the processing of equiatomic 9-cation HE carbide(HEC9)ceramics of the IVB,VB,and VIB groups were studied focusing on the removing of the inevitable oxygen impurity existed in the starting carbide powders and the oxygen contamination during the powder mixing processing.The results demonstrate that densification by spark plasma sintering(SPS)by directly using the mixed powders of the corresponding single-component carbides will inhibit the oxygen-removing carbothermal reduction reactions,and most of the oxide impurities will remain in the sample as(Zr,Hf)O_(2)phase.Pretreatment of the mixed powders at high temperatures in vacuum will remove most part of the oxygen impurity but result in a remarkable escape of gaseous Cr owing to the oxygen-removing reaction between Cr_(3)C_(2)and various oxide impurities.It is found that graphite addition enhances the oxygen-removing effect and simultaneously prevents the escape of gaseous Cr.On the other hand,although WC,VC,and Mo2C can also act as oxygen-removing agents,there is no metal-containing gaseous substance formation in the temperature range of this study.By using the heat-treated powders with added graphite,equiatomic HEC9 ceramics were successfully prepared by SPS.

Urgently reveal longly hidden toxicant in a familiar fabrication process of biomass-derived environment carbon material

Biomass-derived N-doped carbon(BNC)is an important environmental material and widely used in the fields of water purification and soil remediation.However,the toxicant in the commonly used synthesis process of BNC materials have been largely ignored.Herein,we firstly report the presence of a highly toxic by-product(KCN)in the activation process of BNC materials consequential of the carbothermal reduction reaction.Because this carbothermal reduction reaction also regulates the N-doping and pore development of BNC materials,the KCN content directly relates with the properties of BNC material properties.Accordingly,a high KCN content(-611 mg)can occur in the production process of per g BNC material with high specific surface area(-3600 m^2/g).Because the application performance of BNC material is determined by the surface area and available N doping,therefore,production of a BNC material with high performance entails high risk.Undoubtedly,this study proves a completely new risk recognition on a familiar synthesis process of biomass-based material.And,strict protective device should be taken in fabrication process of biomass-derived carbon material.