Silicothermic Reduction Preparation Of Phase-pure ZrB₂ Fine Powder

ZrB₂ has high melting point,high thermal conductivity,high hardness,excellent thermal shock resistance and oxidation resistance,so it is widely applied in hightemperature structural ceramics,refractory materials,wear-resistant materials,composite materials,electrodes and other fields.ZrB₂ based ceramic composite has become an important candidate material in the field of ultra-high temperature ceramics.The properties of ZrB₂ ceramic powder have an important influence on the sintering of ceramic materials and the properties of products.Nowadays,the preparation methods of ZrB₂ ceramic powder have been reported in the literatures including self-propagating combustion synthesis,mechanical alloying,carbothermal reduction,borothermal reduction,molten-salt assisted magnesiothermic reduction,etc.Meanwhile,carbothermal reduction and magnesium thermal reduction are mainly used in the industry to prepare ZrB₂ ceramic powder.The carbothermal reduction method has the advantages of low cost and simple process,but the high reaction temperature?? 1500 ??leads to large energy consumption.And the reaction is not complete,which causes carbon residue difficult to remove,decreaseing the purity of ZrB₂ powder.Besides,the particle size of the ZrB₂ powder is large,which need to be further ground.Recently,SHS based on Mg is a hot research topic.This method has the advantages of fast reaction speed,low energy consumption and high sintering activity.However,in the reaction process,due to the rapid change of temperature,the reaction process is hard to control,making the particle size and composition of the powder not uniform.And the by-products need to be removed by acid treatment process,which is harmful to the environment.In this paper,ZrB₂ powder was prepared by silicothermic reduction method with zirconia and borax as raw materials and silicon powder with stable chemical properties and low cost as reducing agent.Based on the silicothermic reduction reaction,Na2 O was introduced into the reaction system by adding oxysalts of sodium to absorb the by-product Si O2,and then it was transformed into water-soluble sodium silicate to realize the environment-friendly preparation of pure ZrB₂ ceramic powder.The main variable was whether to add oxysalts of sodium and the type of salt,and the effects of the process conditions such as reaction temperature,reaction time and the ratio of raw materials on the reaction process,the phase composition and the morphology of the product were studied,respectively.The reaction mechanism was studied according to the experiment results.?1?ZrB₂ can be synthesized at low temperature?1200 ??with ZrO₂ and Na₂B₄O₇ as raw materials and Si powder as reducing agent.With the increase of reaction temperature and reaction time,the degree of reaction completion increased.The compensatory increase of silicon powder and borax can promote the reaction.?2?Compared with the system without Na2 O,the reaction temperature of system introduce into Na2 O from SS-L for the synthesis of ZrB₂ was reduced by 200 ?.With the increase of reaction temperature and time,the degree of completion of the reaction increased and the grain size of ZrB₂ grew.In the system with SS-L,silicate was mostly dissolved in hot water,and the purity of the product was high.ZrB₂ grain was very thin,with an average grain size of 0.9 ?m.?3?ZrB₂ powder was prepared at 1200 ? for 3h after the introduction of Na2 O from SS-H.With the increase of reaction temperature and time,the degree of completion of the reaction increased and the grain size of ZrB₂ grew.With the excessive 20 wt% Si addition,the reaction temperature was reduced to 1000 ? for 3h.The addition of borax contributed to the amount of ZrB₂ crystal,causing the decrease of grain size of ZrB₂.?4?Na₂O introduced from SS-H can melted with Na₂B₄O₇ at high temperature,which improved the mass transfer process.Meanwhile,Na2 O can absorb Si O2 produced by the silicothermic reduction reaction,which changed the ratio of Na2 O to Si O2 from 1:5 to 1:2,forming Na2Si2O5 with water solubility,then promoted the reaction process.The results of thermodynamic calculation also showed that in the experimental temperature range,the addition of sodium salt significantly reduced the ?G and the viscosity of the reaction system.?5?When Na2 O was not introduced into the system,the silicothermic reduction mainly occured on the surface of Si.Si,ZrO₂ and B2O3 from Na₂B₄O₇ melted and reacted on the surface of Si;when Na2 O was introduced into the system,ZrO₂ can form the intermediate phase Na2 Zr Si2O7,which increased the reaction activity and changed the reaction process obviously.?6?ZrO₂ reacted with sodium carbonate,sodium oxalate,sodium acetate and sodium citrate respectively,to pre-synthesize sodium zirconate at low temperature?900 ??.ZrB₂ can be synthesized by reaction of sodium zirconate,Na₂B₄O₇ and Si powder.The reaction mechanism,separation and purification of by-products need to be further invetigated.