| Zr B2 has many outstanding properties, such as high melting point, high hardness, high electrical conductivity and thermal stability. ZrB2 is widely used in composite materials,electrode materials, refractory materials, nuclear control materials and other fields. There are some methods to prepare ZrB2 powder. Carbon thermal reduction has attracted more attention because its raw materials are more easily available and cost effective, and its operation can be easily controlled. Therefore, carbon thermal reduction is more prospective to be industrialized.In this paper, ZrB2 powder was prepared by carbon thermal reduction using ZrO2,H3BO3 and C as raw materials. Firstly, thermodynamic calculation showed that, the formation of ZrC and B2O2 at high temperature condition were two main side reactions of the process, which could lower the purity of ZrB2 powder and result more loss of B source.Secondly, the effects of raw materials, reaction conditions, additives and its content,on the phase, morphology, particle size and specific surface area of powder were studied by XRD, SEM, EDS, laser particle size analyzer and BET surface area measurement. The results are as follows:(1) With different raw materials, the phase structure, morphology and particle size of Zr B2 powder were quietly different. Using conventional zirconium dioxide, graphite powder as raw material, the final product had higher content of ZrB2 phase, and the particle size was smaller and was around 5~6 μm, its morphology was more regular.(2) Compared with the product reacted under vacuum, the ZrB2 powder prepared in Ar gas protection had higher content of Zr B2 phase, meanwhile with the larger particle size,which average particle size was 6.7 μm. With the increase of reaction temperature, more Zr B2 was formed. When the temperature reached 1900℃, the carbon thermal reduction reaction was almost thoroughly conducted. With the increase of preserving time, more Zr B2 was formed until preserving time was 30 min; meanwhile, the particle size was increased and the morphology was more irregular. With the bulk density of raw material increasing, the particle size of ZrB2 was increased.(3) Using B powder, ZrB2 particle and NaCl as additives in carbon thermal reduction,the effects of additives on the particle size and morphology of Zr B2 were significant. Boron additive could lower the particle size of ZrB2 particle and it was found when B content was5 wt%, the particle size was minimum, 4.8 μm. However, when B content was increased to10 wt%, B4 C phase was found in the final product. ZrB2 particle additive played a role like seed of ZrB2 crystal growth during the reaction. With increasing of ZrB2 seed content from0 to 15 wt%, the particle size of ZrB2 powder was increased and BET specific area was lowered. Meanwhile, more particle agglomeration occurred and the morphology of ZrB2 became more irregular. When ZrB2 seed content was 15 wt%, the average particle size of Zr B2 powder was 7.3 μm. When NaCl was used as additive, it was found ZrB2 particle grew directionally obviously, and the shape of ZrB2 particle became long strip like which length could be larger than 10 μm. Laser particle size analyzer showed that with the increase of NaCl content, the particles size increased gradually and when the NaCl content was 15 wt%, the average particle size of powder increased to 9.1 μm.Finally, the property of densification of the prepared ZrB2 powders was studied separately by hot pressing. The density, phase structure, grain size and microscopic morphology of the targets were characterized. There existed no changes in the phase structure of the ZrB2 targets prepared by different powders. Comparatively, the grain size of the ZrB2 target hot pressed with Zr B2 powder prepared by adding 5 wt% B powder was smaller and its relative density was at 85.1%. Furthermore, using ZrB2 powder by high energy ball milled as start material, the relative density of hot pressed ZrB2 target could be increased greatly, up to 91.2%. |
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