Fabrication And Properties Of TiB₂/C Composite Cathode Materials

As one of the most promising engineering ceramics, titanium diboride (TiB2) possesses excellent physical and chemical properties such as high melting point, high modulus, low density, good chemical stability, high hardness and excellent electric conductivity. Compared with the conventional cathode materials, TiB2ceramics exhibit unique properties such as good wettability with liquid aluminum and good resistance to sodium penetration, these properties make TiB2have been under wide studies as ideal cathode material for aluminum electrolysis. Duo to its bad toughness and difficulty of sintering of pure TiB2cathode material, now many researches focus on the TiB2/C composite cathode material primarily.In this paper, the titanium diboride-carbon (TiB2/C) composite was prepared by hot pressing sintering, the main experiment material were TiB2, graphite and carbon black, different content of short carbon fibers were added so as to reinforce TiB2/C composite. The dissertation mainly studied the influences of the ratio of row materials, carbon fiber content and sintering parameters to microstructure, density, mechanical and electrical properties of TiB2/C composite, the relationship between structure and properties was also studied.First of all, surface desizing pretreatment process of carbon fiber was studied. Three methods (vacuum heat treatment, oxidation with concentrated nitric acid, washed by acetone) were used to remove the glue line of carbon fiber, the results showed that the method of acetone washing can remove the surface glue line effectively without damaging carbon fiber.TiB2/C composite was prepared by hot pressing sintering using TiB2, graphite and carbon black as raw materials. The experimental results are as follow: TiB2/carbon black composite showed a high density because carbon black exhibited small particle size and had good ability of plastic flow, however, TiB2grain was likely to be separated by carbon black and could not form a connected network, this caused poor mechanical and electrical properties of TiB2/carbon black composite. TiB2/graphite composite exhibited better comprehensive performance, but high content of graphite would decrease density, mechanical and electrical properties of TiB2/graphite composite because of its low ability of plastic flow. Subsequently, the effect of TiB2content, sintering temperature and sintering time to TiB2/graphite composite’s structure and properties was studied systematically. It could be found that the increase of TiB2content improved the density, mechanical and electrical properties of composite duo to the connected network caused by more and more TiB2grain. TiB2/graphite composite showed a closer combination and better performance with the increase of sintering temperature. The abnormal grain growth was appear with the increase of sintering time, the comprehensive properties of composite appeared the trend from increasing to decreasing. When TiB2content was85wt%, graphite content was15wt%, sintering temperature was1950℃, sintering time was1h, sintering press was30MPa, TiB2/graphite composite exhibited best comprehensive properties as following:flexural strength was210.74MPa, fracture toughness was3.573MPa·m1/2, electrical resistivity was30.02μ(?)·cm, the relative density was97.14%, the Vickers hardness was6.49GPa.Different content (0-9wt%) of short carbon fibers were added into TiB2/graphite composite to reinforce TiB2/C composite. The results showed that flexural strength of composite decreased with the increase of carbon fiber, and fracture toughness appeared the trend from increasing to decreasing, TiB2/graphite composite with3wt%carbon fiber had the highest fracture toughness,4.61MPa·m1/2, which improved by30%than composite without fiber, and the flexural strength exhibited slight decrease of197.02MPa. When fiber content was over3wt%, the comprehensive properties of composite decreased sharply due to the increasing number of pores caused by the fiber bridging effect. Interface debonding, fiber fracture and pull-out were main toughening mechanisms of TiB2/graphite composite, all of these toughening mechanisms could consume much fracture energy and improve the fracture toughness of TiB2/graphite composite.