Preparation And Electrical Properties Of TiC Nanowires/Mo-Al₂O₃ Cermets

The uneven distribution of power resources in China,large capacity and long-distance transmission has become an inevitable demand for energy transmission and power development in China,and ultra high voltage direct current(UHVDC)technology has obvious advantages in transmission.High power resistor is the core component in the process of UHVDC transportation.Its working environment is harsh and the requirements for resistance materials are strict,including good thermal shock stability,high temperature resistance,controllable resistance value,easy metallization of surface and high mechanical strength.In order to meet this series of requirements,ceramic materials have become the main research direction.Metal ceramic is a good resistance material,but the resistance value is not controllable and the mechanical strength is low.In this paper,TiC whiskers were prepared by carbothermal reduction method,TiC nanowires(TiC_NW)were prepared by electrospinning method,and TiC_NW/Mo-Al₂O₃ ceramics were prepared by solid-state sintering method.With the help of the bridge structure established between TiC_NW,Mo particles and Al₂O₃ particles,the percolation threshold of ceramics was reduced and the decrease of resistivity was controlled.At the same time,the bending strength and hardness of ceramics were improved,and a high power resistance material with controllable resistivity was prepared.The main research contents and results are as follows:(1)TiC whiskers were prepared by carbothermal reduction method with Ti O₂-C-Na Cl-Ar as the reaction system and Ni as the catalyst.The effects of titanium carbon ratio,catalyst ratio,carbonization temperature,holding time and Ar gas flow rate on the phase composition and microstructure of TiC whisker were systematically studied,and the growth mechanism of TiC whisker was analyzed.The results show that the optimal ratio of Ni/C is 0.05:3,too much C can not generate more Ni(C)phase,will not promote the increase of TiC whisker content,too much Ni can not generate more Ni(C)phase and Ni(Ti)phase,will not promote the increase of TiC whisker content.Between 1350-1450?,with the increase of carbonization temperature,the aspect ratio and content of TiC whisker first increase and then decrease.In the range of 1-3 h,with the extension of holding time,the aspect ratio and content of TiC whisker gradually increase and remain unchanged;With the increase of Ar flow rate,the residence time of TiCl_x gas is too short to participate in the reaction and be discharged outside the system,which reduces the content of TiC whiskers.The growth process of TiC whiskers prepared by carbothermal reduction method was controlled by the gas-liquid-solid(VLS)mechanism through SEM observation and literature reference.(2)TiC nanowires(TiC_NW)were prepared by electrospinning.Polyacrylonitrile(PAN)and polyvinylpyrrolidone(PVP)were dissolved in N-N dimethylformamide(DMF),respectively.After adjusting the p H,tetrabutyl titanate(TBT)was added,and the uniformly mixed precursor solution was subjected to electrospinning and high temperature heat treatment.The effects of precursor solution,electrospinning conditions and heat treatment process on the microstructure and phase composition of TiC_NW were systematically studied,and the growth reaction mechanism of TiC_NW during heat treatment was analyzed.The results showed that the precursor solution prepared by dissolving 10 wt%PAN and 10 wt%TBT in DMF with p H=4 was electrospun at a voltage of 20 k V,a receiving distance of 15 cm and a rotating speed of 1600 r/min.The TiC precursor film without adhesion and uniform diameter was obtained.TiC_NW with high crystallinity and good morphology can be obtained at 1400?for 1 h.During the heat treatment,a series of reactions such as TBT pyrolysis,dehydrogenation of PAN terminal group,denitrification and pyrolysis occurred with the increase of temperature.Finally,Ti O₂ transformed from amorphous to anatase and from anatase to rutile to form TiCNW.(3)TiC_NW/Mo-Al₂O₃ ceramics were prepared by solid-state sintering.The microstructure and phase composition of TiC_NW/Mo-Al₂O₃ ceramics were analyzed.The effects of TiC content and Mo content on the electrical and mechanical properties of TiC_NW/Mo-Al₂O₃ ceramics were systematically studied.The results show that the three-phase bridge structure formed in TiC_NW/Mo-Al₂O₃ ceramics reduces the percolation threshold from 35 wt%to 10 wt%.With the increase of TiC_NW content,the resistance of TiC_NW/Mo-Al₂O₃ ceramics decreases;When the Mo content is 30 wt%and TiC_NW content is 8 wt%,the bending strength and hardness of the ceramics reach the maximum,which are 95 MPa and 1283 MPa,respectively.