| Zirconium diboride(ZrB2),well known as a kind of important ultra-high temperature ceramics(UHTCs),has been widely used in the fields of aerospace,electrode materials and refractory materials,etc.The preparation of ZrB2 powder and densification of ZrB2 ceramic and its application are always research hotspots.One latest development of ZrB2 application is of the integral fuel burnable absorber(IFBA)in AP1000 reactor.ZrB2 used in IFBA is a coating on the cylinder surface of UO2 fuel pellets,which is deposited by sputtering ZrB2 target.Herein,boron is 54.8 wt.%enriched in B-10.In order to meet the increasingly urgent need of IFBA ZrB2 target,in this paper,the preparation of ZrB2 powder and target with B-10 enriched were studied.There are many methods to prepare ZrB2 powder,including carbothermal reduction,borothermal reduction,self-propagating high-temperature synthesis,sol-gel method,etc.Carbothermal reduction has a significant cost advantage over other methods because of its simple and easily available raw maerials,especially for the synthesis of ZrB2 powder with B-10 enriched.However,the ZrB2 powder produced by carbothermal reduction in industry can hardly be deep densification without the condition of sintering additives because of the bulk with rod like particles and the characteristic of ZrB2 grain.Therefore,it is an important task to control the growth and morphology of ZrB2 powder in the process of carbothermal reduction without sintering additives to achieve the deep densification of ZrB2 target.In this paper,ZrB2 powder was synthesized by carbothermal reduction from the analysis and treatment of raw material.The growth and morphology of ZrB2 powders were controlled by additives.The improvement of homogeneity of 10B abundance in ZrB2 powder was studied by the mixer.With grade matching of ZrB2 powder which promotes diffusion of the powder,deep densification of ZrB2 target was realized using the synthesized ZrB2 powder by hot pressing.The sputtering performance of ZrB2 target with 10B enriched was characterized.The results are as following:1.The study on the conditions of carbothermal reduction reaction:1)With 20 g precursor powder,a series of carbothermal reductionexperiments were carried out and the results show that in the ratio(wt.%)of raw materials for ZrO2:H3BO3:C = 34.3:47.8:17.9,single phase of ZrB2 is synthesized at 1900℃ with 30 min holding time in Ar gas atmosphere.ZrC phase appears in the synthesized powder in vacuum.At temperature of 1500℃ to 1900℃,the phase of the synthesized powder is changed to ZrB2 and the morphology of the particle tends to be neater with the increasing of temperature.At 1900℃,with the extension of holding time,the phase of synthesized powder tends to single phase of ZrB2 and the morphology of the particle tends to be neater.ZrB2 powder synthesized with activated carbon as carbon source shows a rod like morphology,and the size of the particle in length direction is about 20 μm.ZrB2 powder synthesized with graphite carbon as carbon source shows a regular morphology,and the size of the particle is about 5 μm.With decreasing of the particle size of the precursor,the particle size of the synthesized ZrB2 powder decreases and the morphology is transformed into a round like.The average particle size of synthesized ZrB2 powder is about 1.69 μm when the particle size of the precursor is less than 300 mesh.2)With 4000 g precursor powder,the scale-up experiments were carried out and the results show that with the holding time of 300 min,single phase ZrB2 is obtained which the particle size is about 10 μm in length direction.The main object of the products is ZrB2 with different holding time.There are weak ZrO2 diffraction peaks in XRD patterns with the holding time less than 180 min.With the extension of the holding time,ZrO2 diffraction peaks in the products gradually reduce with a morphology of rod like.3)The process of carbothermal reduction can be divided into four steps.At high temperature,B2O3 in ZrO2-B2O3-C system is transformed into molten state,surrounding ZrO2 and C,and ZrB2 is generated on ZrO2 or C.As the reaction proceeds,ZrB2 grain gradually grows along a certain direction.When the reaction time is long enough,a rod like ZrB2 particle is generated.2.The effects of the kinds and contents of different additives on the growth and morphology of ZrB2 particle were studied by carbothermal reduction.The results show that:1)Without B additive,the particle size of the product which has obvious holes is about 7-8 μm,with a morphology of nearly six square structure.With the increasing of B powder,the particle size of the synthesized powder gradually decreases,with a morphology from prism to round and the holes in the particle decreased.When the amount of B powder is 2.5 wt.%,the synthesized powder is reduced with the average particle size of 6.3 μm.With 5 wt.%B additive,the synthesized powder shows a round shape and the holes in the particle are disappeared.The particle size of the powder is the smallest with 10 wt.%B powder which is about 3-4 μm,but B4C phase is appeared in the product.The addition of B reacts with C at 1470℃ which generates B4C by combination reaction and a large amount of heat is released at the same time.The carbothermal reduction has occurred at about 1450℃,so the addition of B powder changes the external environment of the formation of ZrB2 crystal using carbothermal reduction.In addition,the chemical reaction heat of the formation of B4C reduces the formation energy of ZrB2 crystal and promotes the mass formation of ZrB2 crystal,thus inhibits the growth of ZrB2 grain.2)Single phase ZrB2 is obtained by adding nanometer ZrB2 powder.The phase composition,morphology and the particle size of ZrB2 powder have not changed significantly with the increasing of nanometer ZrB2.3)ZrB2 obtained by adding NaCl shows an obvious orientation growth.With the increasing of NaCl,more rod shape particles are found with an obvious increasing of particle size.The particle size of the product in length is about 10-12 μm and about 3 μm in radial by adding 15 wt.%NaCl.NaCl molten provides liquid molten salt environment for reaction system at high temperature,promoting mass transfer between different materials which leads an agglomeration of ZrB2 crystals to grow along a specific direction,generating more rod-shaped particles.4)With 5 wt.%NaCl addition,the co-effect of the addition of B and NaCl on the control of particle growth and its morphology of ZrB2 powder was studied.With the increasing of B additive,the radial size reduces greatly,and the ratio of length to diameter increases significantly.By adding 10 wt.%B,a needle like particle of ZrB2 is obtained with size 8 μm in length and 1 μm in radial.Meanwhile,some large-sized particles are found,which is the formation of B4C by the reaction between B and C.As above mentioned,the addition of B can promote the refinement of ZrB2 particle,and the addition of NaCl can promote the generation of rod-shaped particles.Therefore,with the increasing of B powder,the radial size decreases,and the ratio of length to diameter increases significantly.3.The change of B-10 abundance in ZrB2 powder prepared by carbothermal reduction was studied.Further,three kinds of ZrB2 powder with different B-10 abundance are mixed in twin-spiral mixer and super mixer.The effect of mixing time on the homogeneity of B-10 abundance was studied.The results show that:1)ZrB2 powder with B-10 enriched is prepared using boric acid with B-10 enriched as raw material and B powder as additive by carbothermal reduction.The preparation of ZrB2 powder with B-10 enriched were carried out 10 times individually and the abundance of B-10 is quietly different to each other,with a significant range of 5.23 wt.%.2)When ZrB2 powders are mixed in the twin-spiral mixer,with the mixing time less than 160 h,the range of B-10 abundance fluctuates between 3.8 wt.%and 0.22 wt.%;with the mixing time between 160 h and 240 h,the range of B-10 abundance slightly fluctuates between 0.46 wt.%and 0.77 wt.%.Obviously,the effect of mixing time on the range is not evident.3)When ZrB2 powders are mixed in the super mixer,which operated under 1350 r/min,with the mixing time from 5 min to 30 min,the range of B-10 abundance is increased to 1.28 wt.%;with the increasing of mixing time from 30 min to 60 min,the range of B-10 abundance is decreased stabely.When the mixing time is 60 min,the range of B-10 abundance is about 0.4 wt.%with 3 sampling points and 0.64 wt.%with 15 sampling points.Under 95%confidence level,the fiducial interval of the B-10 abundance is[54.7800,55.0363].4)Comparing with the twin-spiral mixer,the super mixer promotes the dispersion of powder agglomerates due to the strong shearing effect between high speed impeller and particles.Combining with the effect of eddy mixing,the mixing efficiency is improved significantly,and the homogeneity of B-10 abundance is increased significantly.4.The deep densification of ZrB2 target by hot-pressing was studied using different ZrB2 powders prepared under different condidtions.The results show that:1)Under the temperature of 1980℃,at the pressure of 50 MPa and with the holding time of 4 h,a relative density of 81.6%ZrB2 target is hot-pressed using ZrB2 powder synthesized by carbothermal reduction with 2.5 wt.%B additive;when the pressure is increased to 60 MPa,the relative density is down to 78.9%;when the pressure is increased to 70 MPa,the relative density is increased to 87.4%.Under the temperature of 1980℃ and at the pressure of 70 MPa,with the increasing of holding time from 2 h to 2.5 h,the relative density increases to 89.2%;with the increasing of holding time from 3 h to 4 h,the relative density decreases from 88.7%to 87.4%.It can be found that there existes phenomena called "anti-densification" during hot-pressing with the increasing of pressure and holding time.Comparing to 2.5 wt.%B additive,the relative density of target is increased to 93%when B additive is increased to 5 wt.%in carbothermal reduction.2)SEM pictures show that the ZrB2 target with the relative density of 78%have a large number of pores.TEM shows that there exist oxide impurities in grain boundary which is the key factor to keep target from deep densification.Those oxide impurities can be removed by carbon through reaction.With the carbon content increasing from 0.15 wt.%to 0.67 wt.%,relative density of the target is increased to 85%.3)According to the mechanism of hot-pressing,densification can be accelerated by promoting inter-diffusion between partilces.Adding 25 wt.%ZrB2 powder synthesized by high-temperature self-propagating,which is constituted of many near spherical partilces and existed lattice defects,the target with relative density of 99.9%is hot-pressed.5.The sputtering performance of ZrB2 target with B-10 enriched was characterized by direct current magnetron sputtering.The results show that the deposition rate is kept stablely at about 150 mg/kW h.The coating is dense and the thickness is uniform,with the average thickness of 8.65 μm after 3 h’s sputtering.The average volume density of B-10 in the coating is about 1.69 mg/mm3. |
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