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ZrB2 powders were synthesized via a borothermal reduction reaction of ZrO2 with the assistance of NaCl under a flowing Ar atmosphere. The optimal temperature and reaction time were 1223 K and 3 h, respectively. Compared with the reactions conducted without the addition of NaCl, those performed with the addition of an appropriate amount of NaCl finished at substantially lower temperatures. However, the addition of too much NaCl suppressed this effect. With the assistance of NaCl, a special morphology of polyhedral ZrB2 particles covered with ZrB2 nanosheets was obtained. Moreover, the experimental results revealed that the special morphology was the result of the combined effects of B2O3 and NaCl. The formation of the special microstructure is explained on the basis of the "dissolution–recrystallization" mechanism. [ABSTRACT FROM AUTHOR]

In the present work, ultrafine tungsten carbide (WC) powder with a high purity has been prepared by first roasting yellow tungsten trioxide (WO 3) and carbon black powder under argon atmosphere followed by the further carbonization reaction with CH 4 -H 2 mixed gases. The effects of C/WO 3 molar ratio, CH 4 percentage in the CH 4 -H 2 mixed gases on the phase composition, morphology and particle size of the products were discussed. The results revealed that when the C/WO 3 molar ratios were 2.5 and 2.6, nano tungsten carbide powder with the average particle sizes of 93 nm and 77 nm could be prepared. Whereas, when the C/WO 3 molar ratio was in the range of 2.7–3.5, the finally prepared WC has the particle size of 446–192 nm, and became smaller with the increase of C/WO 3 molar ratio. The percentage of CH 4 should be <15% to prepare WC with a low free carbon content. From the results of thermodynamic calculation, X-ray diffraction (XRD), FE-SEM, and infrared carbon‑sulfur analyzer, it was concluded that ultrafine tungsten carbide powders with a high purity could be successfully prepared by this method. • A new process was proposed to prepare high purity and ultrafine WC powders. • Nano WC powder could be prepared when the C/WO 3 molar ratio was 2.5–2.6. • Particle size of WC decreased as increasingC/WO 3 ratio in ranges of 2.1–2.6 and 2.7–3.5. • The optimal CH 4 percentage of CH 4 -H 2 mixed gases for carbonization was 5–10%. [ABSTRACT FROM AUTHOR]

TiB2 nanosheets were synthesized by the borothermal reduction of TiO2 in flowing argon atmosphere with the assistance of MgCl2. Phase transformation was studied by XRD and micromorphology of products was studied by SEM and TEM. In the experiment, the prepared TiB2 formed the nanoscale lamellar structure. These lamellas agglomerated together to be a net-like structure. Effects of reduction temperature, addition of MgCl2, and its amount were discussed in detail. The results indicated that the presence of MgCl2 accelerated the mass transfer of reactant. Moreover, the co-effects of MgCl2 and B2O3 contributed to the formation of TiB2 nanosheets. TiB2 could dissolve in MgCl2 and B2O3 at high temperature and then recrystallized during the cooling process. The products with a specific surface area of 99.32 m2 g−1 were obtained at 1373 K after reacting for 4 h with a MgCl2:TiO2 molar ratio of 1:1. [ABSTRACT FROM AUTHOR]

The pyrophoric behaviour of as-reduced ultrafine Mo powder at 873 K was investigated. The dew point of the H 2 gas used in the preparation process had a significant effect on the pyrophoric behaviour of the resulting Mo powder as well as the product compositions. A low dew point of H 2 resulted in pyrophoric behaviour and the formation of γ-Mo 2 N with a small amount of MoO 3 forming outside; in contrast, a high dew point of H 2 inhibited the pyrophoric behaviour. The morphology of the pyrophoric product was the same as that of the MoO 2 raw material. [ABSTRACT FROM AUTHOR]

TiB2 nanosheets were synthesized by the borothermal reduction of TiO2 in flowing argon atmosphere with the assistance of MgCl2. Phase transformation was studied by XRD and micromorphology of products was studied by SEM and TEM. In the experiment, the prepared TiB2 formed the nanoscale lamellar structure. These lamellas agglomerated together to be a net-like structure. Effects of reduction temperature, addition of MgCl2, and its amount were discussed in detail. The results indicated that the presence of MgCl2 accelerated the mass transfer of reactant. Moreover, the co-effects of MgCl2 and B2O3 contributed to the formation of TiB2 nanosheets. TiB2 could dissolve in MgCl2 and B2O3 at high temperature and then recrystallized during the cooling process. The products with a specific surface area of 99.32 m2 g−1 were obtained at 1373 K after reacting for 4 h with a MgCl2:TiO2 molar ratio of 1:1. [ABSTRACT FROM AUTHOR]