Study On The Synthesis And Growth Mechanism Of Tantalum Nitride Whiskers

Tantalum nitride（Ta N）compounds possess excellent physical and chemical properties and have been widely applied in machinery,corrosion protection,catalysis,aerospace and other fields.Due to the complex spatial distribution of the d orbitals of tantalum（Ta）atoms and the numerous non-metallic vacancies in these compounds,Ta exhibits varied valence states in the compound and thereby a series of Ta N with different structure and composition appear.Different phases have different physicochemical properties,making Ta N applied ex-tensively.The preparation of Ta N usually required high temperature,leading to the rapid precipitation of non-metal atoms（N）,and so it is difficult to control the crystal size and purity of the Ta N.In this thesis,carbon thermal reduction method（CTR）and carbon thermal reduction-nitridation method（CTR-N）were employed to synthesize tantalum carbide（Ta C）and tantalum nitride whiskers.The effects of reaction conditions on the whisker composition and morphology were studied,and the growth mechanism of the whiskers was also discussed in detail.（1）Ta C whiskers were prepared by CTR method under nitrogen flow atmosphere using potassium fluoride as the molten salt reaction medium,tantalum pentoxide as the tantalum source,and activated carbon as the source of carbon.The raw materials obtained energy during the ball-milling process through shearing and collision with balls,which also reduced the of crystallinity degree of the raw materials to some extent.Meanwhile,the grain sizes of the raw materials were also refined and the nucleation site during crystal growth increased,finally reducing the reaction temperature.The influences of molar ratio of the raw materials and reaction temperature on the formation of the Ta C whiskers were investigated.Scanning electron microscopy（SEM）,transmission electron microscopy（TEM）and X-ray photoelec-tron spectrometer（XPS）were used to characterize the whiskers.The results showed that Ta C whiskers with high-yield and good morphology were synthesized when the temperature was1250°C,reaction time was 3 h and the flow rate of nitrogen was 50m L/min.The Ta C whisk-ers had diameters of 70-200 nm and lengths of several microns,and a combined mechanism of vapor-liquid-solid（VLS）and vapor-liquid（VS）was responsible for the growth of whisk-ers.（2）Tantalum nitride（Ta₅N₆）whiskers were successfully synthesized by CTR-N method under ammonia flow atmosphere using tantalum pentoxide as the tantalum source,activated carbon as the carbon source,and potassium fluoride as the molten salt reaction medium.The effects of reaction temperature,holding time and carbon addition content on the formation of whiskers were studied.The results demonstrated that Ta₅N₆ whiskers with the best mor-phology were obtained when the reaction temperature was 1300°C,holding time was 6 h,the molar ratio of C/Ta was 5:1 and the flow rate of ammonia was 300 m L/min.The whiskers had diameters of about 400 nm and lengths of about several microns,and VS model governed the formation of Ta₅N₆ whiskers.（3）Tantalum nitride（Ta₅N₆）whiskers were successfully fabricated by CTR-N method using tantalum pentoxide and activated carbon are used as the main raw materials,potassium fluoride as the molten salt medium.The raw materials were firstly heated to 1250°C and held for 3 h under 50 m L/min nitrogen flow,followed by heating to more high temperature under ammonia flow and held for different times.When the temperature was 1300°C,the ammonia flow rate was 300 m L/min,holding time was 6 h and molar ratio of Ni:Ta₂O₅ was0.1:1,Ta₅N₆ whiskers with the best morphology were synthesized.The whiskers had diam-eters of 80-250 nm and lengths of 1-5μm,and VLS and VS mechanisms were responsible for the whisker growth.