Silicothermic Reduction Synthesis Of Phase-pure TiB₂ Nano-sized Particles

Titanium diboride(TiB2)ceramics possesses many important properties,including high melting point,high hardness,excellent wear resistance,good electrical and thermal conductivities,and considerable erosion and corrosion resistance which make it a potential material suitable for high performance applications,in such areas as impact resistant armor,cutting tools,crucibles,wear resistant coatings,cathodes in the electrochemical reduction of alumina to aluminum,high temperature structural materials,refractory and control rod material for high temperature nuclear reactors.A variety of synthesis methods have been reported for TiB2 preparation.Among the metallothermic synthesizing techniques,magnesiothermic reduction is the most commonly used method in industrial production of phase-pure TiB2.However,it suffers from several drawbacks.For example,apart from using expensive,flammable and explosive magnesium,a ‘‘post reduction'' HCl leaching process has to be carried out to eliminate the byproduct MgO,which may cause an environmental problem.Comparing with magnesium,silicon is cheaper,less active,and more stable.It is easier to control the overall reaction process.Nevertheless,silicothermic reduction has been rarely or never used to prepare phase pure TiB2,due to the hardly elimination of byproduct SiO2 in the product.In order to achieve the effective separation of TiB2 and by-products SiO2,the water-insoluble by-product SiO2 was converted into water-soluble silicate by introducing Na2O into the reaction system.The effects of sodium source,reaction temperature and other factors on the extent of the silicothermic reduction reaction,the water purification of the reaction product,and the morphology and structure of TiB2 were studied.1)Titanium diboride(TiB2)particle was synthesized via silicothermic reduction method by using TiO2,Si,and Na2B4O7 as raw materials.The reaction extent increased as the temperature increasing.The reaction was almost complete at 1200? for 3h with a small amount residual of boron oxide.Due to the final SiO2 / Na2O of the reaction was 5: 1,the structure of melt formed at experimental temperature was complicated and its viscosity was higher,which was not conducive to the dissolution and diffusion mass transfer of Si and TiO2 in the melt.The reduction could mainly occur at their interface and the “interfacial reduction” mechanism might be considered to be dominant this reaction.The grain size of TiB2 grew linearly with temperature.The grain size of TiB2 prepared at 1200? for 3h.was about 200 nm The typical crystal morphology of TiB2 was hexagonal columnar.The by-product of the reaction was sodium silicate with modulus(Si O2 / Na2O)5:1,which was hard to dissolve in water and separate from the products.It means that the traditional silicothermic reduction methdod could not used to prepare phase pure TiB2 powders.2)Titanium diboride fine powder was synthesized at 1100? via a novel and sustainable silicothermic reduction technique using TiO2,Na2B4O7 and Si powders as starting materials and oxygenates of sodium with lower sodium content(SS-L Na2O?45%)as molten salt.As-prepared submicron(?400nm)TiB2 fine powders have typical hexagonal short column morphologies or hexagonal plate-like structure and were generally dispersed.The reaction extent was deeper as the temperature increasing.The reaction was almost complete at 1100? firing for 3h.Because of the low modulus(SiO2 / Na2O = 2: 1),the structure of melt was simple and the viscosity was low,which leaded to the Si and TiO2 was easy to the dissolution and diffusion mass transfer in the melt The “solution-reduction-precipitation” mechanism could be considered to be dominant this reaction.3)By introducing SS-L as molten salt into the reactant mixtures,a high reaction extent was achieved and the oxide resultant SiO2 was transformed into water soluble Na2Si2O5,which made the reacted mass be easily rinsed by using hot water and avoided the environment risks from strong acid leaching procedure after traditional magnesia reduction.Moreover,the aqua Na2Si2O5 could be readily recycled and used as a precursor for preparing valuable materials due to no foreign salts involved in the process,which avoided the environment problems of non-recyclable halides and water waste successfully.4)During the reduction process,evaporation loss of silicon in the reduction process was dominated by SiO(g)with high saturation vapor pressure from the interactions among Si,SiO2 and Na2Si2O5.By using 5wt% excessive Si to compensate for silicon evaporation loss at test temperature,TiO2 and Na2B4O7 were completely converted into TiB2,which promoted the reduction reaction and improved the water solubility of the by-products.Then the by-products were dissolved and separated rapidly by hot water.5)TiB2 particle was synthesized via silicothermic reduction method by using TiO2,Si,and Na2B4O7 as starting materials and oxygenates of sodium with higher sodium content(SS-H,Na2O ?60%)as additive at 1100?.The reaction extent was higher because of the introduction of SS-H and temperature increase.In theory,the mudules of by-product was 2:1.But the silicon loss was increased because of the high content NaOH in SS-H,which leaded to the mudules was higher than 2.Besides,the reaction extent was relatively low.And the melt structure was complex and the melt was difficult to dissolve,resulting in difficult product separation.6)Sodium carbonate could not be directly introduced as a source of Na2O in the silicothermic reduction reaction system.A precursor was pre-synthesized from Na2CO3 and TiO2.Then,TiB2 powder was successfully synthesized at 1100? by using Na8Ti5O14,Si and Na2B4O7 as raw materials.Sodium acetate,sodium citrate,sodium oxalate and other organic salts could also be used to prepare relatively pure Na8Ti5O14 intermediates.