Wang Chengyang, Sun Xiaobin, Long Gang, Xiong Xiaorong, Köhler Klaus
School of Chemistry and Chemical Engineering, Chongqing University of Technology, Chongqing, 400054, PR China.
Department of Chemistry, Inorganic Chemistry, Technical University of Munich, Garching, 85747, Germany.
J Nanosci Nanotechnol. 2020 Feb 1;20(2):1269-1277. doi: 10.1166/jnn.2020.16967.
Formation mechanism of synthesizing nanoparticle tungsten carbide (WC) was studied. WC was synthesized by carbothermal hydrogen reduction (CHR) method under various reaction temperatures for holding different post-treatment time in 20% (v/v) CH₄/H₂. The phase transformation mechanism of gaining WC was investigated, by combining CHR with X-ray diffraction (XRD) and temperature programmed reduction mass spectroscopy (TPR-MS). The results show that pure phase of WC has been obtained by CHR after isothermal heat treatment for 24 hours at 750 °C and 12 hours at 950 °C, respectively. These results indicated that two key parameters of higher temperature and longer isothermal heat treatment time are necessary for synthesizing pure phase of WC powder. In order to find out the phase transformation mechanism of tungsten trioxide (WO₃) to WC, the reduction and carburization process among the temperature range from 600 °C to 1000 °C for holding 3 hours at the final temperature were studied. It was shown that at 600 °C, WO₃ was reduced to WO₂, and from 600 °C to 750 °C, WO₂ was reduced to metallic tungsten (W). Moreover, at the temperature range from 750 °C to 900 °C, the mixture phases of tungsten carbide (WC), metallic tungsten (W), or/and tungsten sub-carbide (W₂C) were formed without any oxides species, which indicated that tungsten carbides (WC and W₂C) phases appeared because the oxides phase was thoroughly reduced. However, the occurrence of carburization process was still limited due to the presence of oxygen in the solid. Because of the formed CO and CO₂ there was not enough activated methane reacting with metallic tungsten, so the phase of WC and W₂C were both formed simultaneously, but the reaction of forming WC was the main reaction in the whole carburization process. Moreover, the TPR-MS and XRD results indicated that, WC was formed at lower temperature (750 °C) by the reduced metallic W, which was produced form W₂C in the gas mixture for holding a long time, while at a higher temperature (950 °C), WC was formed form W₂C with the mixture gas directly.
研究了合成纳米碳化钨（WC）的形成机制。通过碳热氢还原（CHR）法，在不同反应温度下于20%（v/v）CH₄/H₂中保持不同的后处理时间来合成WC。结合CHR与X射线衍射（XRD）以及程序升温还原质谱（TPR-MS）研究了获得WC的相变机制。结果表明，分别在750℃等温热处理24小时和950℃等温热处理12小时后，通过CHR获得了WC纯相。这些结果表明，较高的温度和较长的等温热处理时间这两个关键参数对于合成WC粉末的纯相是必要的。为了弄清楚三氧化钨（WO₃）向WC的相变机制，研究了在600℃至1000℃温度范围内最终温度保持3小时的还原和渗碳过程。结果表明，在600℃时，WO₃被还原为WO₂，在600℃至750℃时，WO₂被还原为金属钨（W）。此外，在750℃至900℃温度范围内，形成了碳化钨（WC）、金属钨（W）或/和次碳化钨（W₂C）的混合相，没有任何氧化物物种，这表明碳化钨（WC和W₂C）相出现是因为氧化物相被彻底还原。然而，由于固体中存在氧，渗碳过程的发生仍然受到限制。由于生成了CO和CO₂，没有足够的活性甲烷与金属钨反应，所以WC和W₂C相同时形成，但在整个渗碳过程中形成WC的反应是主要反应。此外，TPR-MS和XRD结果表明，WC在较低温度（750℃）下由长时间在混合气体中由W₂C产生的还原金属W形成，而在较高温度（950℃）下，WC直接由W₂C与混合气体形成。
