Fully dense and fine-grained W 2 B 5 ceramics were successfully fabricated by spark plasma sintering at 1375 °C using metallic cobalt as additive. Co reacted with W 2 B 5 to form binary (CoB) and ternary phases (W 3 CoB 3 ) during sintering process. Experimental results showed that the relative density of W 2 B 5 ceramic increased from 96.7 % (for pure W 2 B 5 ceramic sintered at 1800 °C) to 99.0 % with 10 wt% Co addition and 99.5 % with 20 wt% Co addition (sintered at 1375 °C), respectively. Due to the lower sintering temperature, the grain size of W 2 B 5 in W 2 B 5 -10Co decreased from 5.3 ± 0.2 μm to 3.2 ± 0.1 μm relative to pure W 2 B 5 . However, in the case of 20 wt% Co addition, W 2 B 5 grains grew anisotropically, forming elongated strips with a length of 4.2 ± 0.3 μm and a width of 1.1 ± 0.2 μm. The W 2 B 5 -10Co composite exhibited superior mechanical properties with the Vickers hardness of 19.7 ± 0.3 GPa (30 Kg), fracture toughness of 8.7 ± 0.4 MPa·m 1/2 and flexural strength of 783.2 ± 22 MPa. Compared to pure W 2 B 5 ceramics, the hardness remained almost unchanged, while the fracture toughness and flexural strength were significantly improved by 61.8 % and 49.8 %, respectively. These improvements benefitted from the combination of high densification degree, fine-grained strengthening and in-situ CoB phase formation improving interfacial adhesion via semi-coherent interfaces. However, excessive addition of Co (20 wt%) induced stress concentration via brittle CoB coalescence, outweighing the densification benefit and drastically reducing hardness and flexural strength.
Direct conversion of carbon dioxide (CO 2 ) into lower olefins (C 2 -C 4 = ) is highly attractive as a sustainable production route for its great significance in greenhouse gas emission control and fossil fuel substitution. Fe-based catalysts have been extensively studied in CO 2 hydrogenation, which usually show unsatisfactory selectivity toward lower olefins. Here we present a high-dispersion catalyst precursor CoFe 2 O 4 with Na (Na-CoFe 2 O 4 ) that offers C 2 -C 4 = space time yield as high as 2.88 μmol C2-C4= g cat −1 s -1 and olefin to paraffin ratio about 6 at CO 2 conversion higher than 41 %. High dispersion and the intimate contact between Fe and Co sites help inhibit the formation of methane, and favor a higher selectivity of C 2 + hydrocarbons, especially lower olefins. The presence of Na further promotes chain growth and suppresses the direct hydrogenation of Fe-(CH 2 ) n intermediates. A superior stability over 100 h was observed, demonstrating the promising potential of this catalyst for industrial applications.
Juan Xie,Fang-Ze Han,Jing-Hui Zhang,Sheng Liu,Yu-Hua Tang,Cheng-Xin Li,Shan-Lin Zhang
La-doped SrTiO 3 (La x Sr 1−x TiO 3 , LST), as the promising ceramic-based interconnect material, usually needs to be sintered in reductive atmosphere at a temperature ≥ 1450 °C, such sintering condition is not friendly to cell fabrication and application. To reduce the sintering temperature of air-sintered LST, seven sintering additives, Bi 2 O 3 , Fe 2 O 3 , Li 2 O, MnO 2 , NiO, V 2 O 5 , and Co 3 O 4 , are added to the LST in this work. The results show that only Co 3 O 4 is conductive to remarkable sintering at the temperatures of 1250 °C. The relative density of LST-3%Co 3 O 4 reaches 92%, while that of the pure LST is only 48%. Moreover, the conductivity of LST-3%Co 3 O 4 achieves 8.2 S/cm at 800 °C in humidified hydrogen, which exceed 5.5 times that of the pure LST. Interestingly, LST-3%Co 3 O 4 exhibits an average TEC of 11.5 × 10 −6 K −1 between 400 and 900 °C, which is very close to that of other components of SOCs .
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