Yang Chen,Chengji Deng,Chao Yu,Jun Ding,Hongxi Zhu
ZrN nanopowders were obtained by using ZrO 2 and Mg as the main raw materials in a molten salt environment and a high-purity nitrogen atmosphere. The effects of ZrO 2 / Mg mole ratio, nitriding temperature , and holding time on the synthesis of ZrN were investigated. ZrN was formed at 700 °C for 3 h. When the nitriding temperature increased to 1000 °C, pure phase ZrN was formed. The specific surface area of ZrN was 20.92 m 2 /g. When the temperature rose to 1100 °C, the resulting ZrN particles were finer; the average size of ZrN decreased from 71.60 to 49.65 nm. Further, an excessively long holding time led to evaporation of the molten salt, ZrO 2 , which was not fully nitrided. Finally, a possible formation mechanism of ZrN is proposed.
Nan Liu,Zhi Dong,Zongqing Ma,Liming Yu,Chong Li,Chenxi Liu,Qianying Guo,Yongchang Liu
The bimodal distribution of tungsten grain size, which is also called bimodal structure, is a universal phenomenon in synthesizing tungsten based nanopowders by wet chemical method , and could significantly deteriorate further sintering characteristics of the powders. In this paper, W-Y 2 O 3 composite nanopowders with small grain size and no bimodal structure were synthesized by an improved wet chemical method. It was found that reaction conditions, especially the hydrogen ion concentration(ρ(H + )) could significantly influence the microstructure and morphology of the prepared composite nanopowders and has an optimum value of approximately 0.16 mol L −1 . W-Y 2 O 3 composite nanopowders synthesized under this optimum value got not only the minimum average grain size, which is about 17 nm, but also the best grain size uniformity with no bimodal structure. The microstructure of co-deposited precursors and prepared powders was analyzed in detail to figure out the specific influence mechanism of ρ(H + ), and it was indicated that ρ(H + ) could influence the microstructure and the Y 2 O 3 content of the reduced powders through affecting the fraction of precursor that has little yttrium content, which may lead to the bimodal distribution of tungsten grain size of the prepared powders. Furthermore, the formation mechanism of bimodal structure was systematically analyzed and a feasible method was proposed to eliminate the bimodal structure. It is suggested that the bimodal distribution of tungsten grain size is attributed to the distribution of Y 2 O 3 during hydrogen reduction process and can be well avoided by controlling ρ(H + ).
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