Molten Salt Synthesis Of Zirconium Diboride And Their Characterization

With the development of high-tech smelting industry,national nuclear industry and aerospace industry,materials that can work for a long time under extreme conditions have received more and more attention.The special working environment requires that the relevant equipment must have good performance.For example,the spacecraft's engine and tailpipe and the nose cone of the fighter aircraft need to work in extreme environments exceeding 1000?.Zirconium diboride can be used not only as an electrode material but also as a refractory material,and is also a very good high-temperature structural material Sodium borohydride is used as the boron source,and different zirconium sources and molten salt are used to prepare zirconium diboride powder by using molten salt method under air atmosphere.Firstly,the effects of different zirconium sources on the synthetic powders were systematically studied.Then the effects of processing parameters on the phase composition and morphology of the resultant powder were studied,and the formation mechanism during the synthesis process was briefly analyzed.Finally,the specific processing parameters for purification was also determined.The main conclusions are as follows:(1)The influence of different zirconium sources on the formation of zirconium diboride was investigated in detail under molten salt condition.Here,sodium borohydride was chosen as boron source,different compounds containing zirconium(ZrCl4?Zr(OH)4?ZrC)as zirconium sources,and potassium chloride as molten salt.All samples were heated up to 1000? at a heating rate of 5?/min in the air and kept for 8 h at calcining temperature.Here,Zr-B mixture has 1:10 molar ratio of zirconium to boron,while ZrB2 precursor for molten salt process has 1:4 weight ratio of Zr-B mixture to potassium chloride.The results show that the desired zirconium diboride powders can be obtained when the zirconium hydroxide is used as the zirconium source.(2)The influence of Zr-B molar ratio,salt type,weight ratio of Zr-B mixture to salt,and holding time at calcining temperature on the formation and morphology of zirconium diboride crystallites was studied in detail by XRD and SEM.The results indicate that optimized processing parameters can be determined as follows:the molar ratio of zirconium to boron is 1:5,potassium chloride is selected as the molten salt,and the weight ratio of Zr-B mixture to salt is 1:4,and the calcining temperature was maintained at 800? for 2 h.(3)Several physical and chemical treatment processes were used to purify the zirconium diboride powders synthesized by the molten salt method.The determined purification process can be described as follows:at first,as-prepared products via molten salt process were washed by hot distilled water of 80? for 2 h;and then washed by 0.5 mol/L of dilute hydrochloric acid for 0.5 h at room temperature.In present study,hydrothermal treatment were used to purify the zirconium diboride powders,no significant change in XRD patterns can be found,but their morphology seems to be better than before.When the hydrothermal treatment condition was pH=7,the temperature was maintained at 160? for 120 min.