Synthesis Of ZrB₂Powder And Preparation Of ZrO₂matrix Composites Reinforced By ZrB₂

As the ultra-high temperature ceramics （UHTCs）, zirconium diboride（ZrB₂） has been widely used in the structural ceramics with high temperature,refractory, abrasion resistant materials, electrode materials, compositematerials, materials controlling of nuclear and other fields due to its highmelting temperature and hardness, outstanding thermal and electricalconductivities, excellent resistance to thermal shock and good capability ofneutron controlling. It has become one of the most promising structurematerials with high temperature. However, ZrB₂powder synthesized by theconventional method has large grain size and low activity, and is also difficultto be sintered; these weaknesses limit its application. Thus, it is necessary todevelop the technology to obtain the high purity and the ultra-fine ZrB₂powders.In this paper, ZrB₂powders have been synthesized by sol-gel method andmicrowave magnesiothermic process respectively. The influences of ratio of raw materials and preparation technology on phase composition, particle sizeand morphology of the synthesized powders were studied by XRD, SEM,EDS and TEM. The composite of ZrO₂-ZrB₂-MgO reinforced with theself-synthetic ZrB₂were prepared by microwave sintering and conventionalpressureless sintering respectively. The effects of ratio of raw materials andsintering technology on bulk density, apparent porosity and mechanicalproperties were also researched. The experimental results show that:1. ZrB₂powders have been synthesized with zirconium oxychloride（ZrOCl₂·8H₂O）, boric acid, glucose as raw materials, citric acid as gellingagent at the resistance furnace in the argon atmosphere by sol-gel method. TheZrB₂content of the product is improved with the increasing of temperature,heating time, and boron content. When the raw materials with B:Zr=4:1wereheated at1500℃for75minutes, the ZrB₂content of the product can beincreased to achive the maximum value of94.7%.2. ZrB₂nano-powders can be formed through sol-gel method followed bymicrowave magnesiothermic reduction processing. The relative intensity ofthe diffraction peak of ZrB₂reached to maximum in the condition when rawmaterials were heated by two steps: first in the power of528W for10minutesand then in the power of800W for40minutes.3. ZrB₂powders have been synthesized from ZrO₂, H₃BO₃, Mg within ashort time by microwave heating. Magnesium content and heating timesignificantly affect the content of ZrB₂powders prepared. When materials with ZrO₂:H3BO3:Mg=1:3:4were heated in the power of800W for20min,the relative content of ZrB₂reaches maximum. TEM results indicate that ZrB₂particles are existed as spherical with the average particle diameter80nm. Theparticle size of ZrB₂was found to decrease with the addition of NaCl.4. The ZrO₂-ZrB₂-MgO composite has been prepared by conventionalpressureless sintering in reduction condition with the self-synthetic ZrB₂asreinforced phase and15wt.%Y2O3as sintering aids. When the raw materialswith80wt.%ZrO2and14wt.%ZrB₂are sintered at1500℃for2h, the apparentporosity of composite reaches to a minimum value of13.1%, and the bulkdensity is5.4g/cm³. The optimized mechanical properties for microhardness,bending strength and fracture toughness are10.3±0.2GPa,347.2±9.1MPa,3.2±0.1MPa·m1/2respectively.