ABSTRACT
The effect of silicon on diffusion behavior of the carbide forming elements in Ni-Mo-Cr-Fe based corrosion-resistant alloy is studied by diffusion couple experiment. One group of diffusion couples are made of the alloy with a different silicon content, another group of diffusion couples are made of pure nickel and the alloy with different silicon content (0Si, 2Si). Two groups of alloys with same silicon content and different carbon content are also prepared, the microstructure of solution and aging state of these two groups alloys are analyzed, and their stress rupture properties are tested. The effect of silicon on the diffusion of alloy elements and the interaction effect of carbon and silicon on the microstructure and stress rupture properties of the alloy are analyzed. The mechanism of Si on the precipitation behavior of carbide phase in Ni-Mo-Cr-Fe corrosion resistant alloy is discussed. The results show that silicon can promote the diffusion of carbide forming elements and the formation of carbide. The precipitation behavior of the secondary phase is the result of the interaction effect of silicon and carbon, and is related to the thermal history of the alloy. Combined with the characteristic of primary carbides, it is confirmed that the precipitation of M12C type secondary carbide is caused by the relative lack of carbon element and the relative enrichment of carbide forming elements such as molybdenum. The stress rupture properties of two silicon-containing alloys with different carbon contents in solution and aging state are tested. The stress rupture life of low carbon alloy is lower compared with high carbon alloy at solution state, but after aging treatment, the stress rupture life of low carbon alloy is significantly improved, and higher than that of high carbon alloy. The main aim of this research is to reveal the influence mechanism of silicon on carbide phase precipitation of a Ni-Mo-Cr-Fe based corrosion-resistant superalloy, which provides theoretical basis and reference for later alloy design and engineering application.
ABSTRACT
Transition metal compounds (TMCs) either crystalline or amorphous exhibit specific advantages in electrochemical energy storage. To integrate their merits into one electrode, we have herein developed hierarchical bimetallic hydroxide/chalcogenide core-sheath microarrays on nickel foam (NF) for freestanding high-efficiency supercapacitors, wherein interior crystalline metal chalcogenides serve as highly conductive pivots and exterior amorphous bimetallic hydroxides provide rich ion diffusion channels. With the synergic effect of the unique structure and bimetallic composition, the as-prepared Ni(OH)2-Co(OH)2/NiSe-Ni3S2/NF electrode displays an ultrahigh areal specific capacitance of 19.01 F cm-2 at 15 mA cm-2, which can be retained as 6.01 F cm-2 even at 125 mA cm-2. To the best of our knowledge, such excellent tolerance of ultrafast ion insertion/extraction at high current density is rare among NF-based free-standing electrodes. The asymmetric supercapacitor by assembling with activated carbon as the negative electrode delivers a volumetric capacitance of 3.93 F cm-3 at 30 mA cm-2, corresponding to an energy density of 13.9 mW h cm-3 at a power density of 200 mW cm-3. A capacitance retention of 82.5% was observed after 4000 cycles, together with an average 97% coulombic efficiency. This work may provide a facile strategy to construct hierarchical microarrays for efficient energy storage devices.
ABSTRACT
The charge storage performance of Fe2O3 nanoarrays (NAs) as negative electrodes are limited by their poor conductivity and rate capability. Herein, we have reported the delicate interfacial engineering on carbon cloth (CC) fibers and oxygen vacancy (VO) generation on Fe2O3 nanorod arrays to boost the capacitive performance. Polydopamine-derived nitrogen-doped carbon layers were fabricated on CC fibers to govern the growth of FeOOH NAs. Rich VOs were generated in Fe2O3 NAs to construct a unique heterostructure with a crystalline core and amorphous shell via successive N2 thermal treatment and chemical reduction. Optimized by 2 h chemical reduction, the VO-rich Fe2O3 NA electrode, featuring a charged voltage of -1.10 V, exhibited a high areal specific capacitance of 2.63 F cm-2 at 0.5 mA cm-2 and 0.12 F cm-2 even at 60 mA cm-2. Impressively, 86.7% specific capacitance was retained after 10 000 cycles at 10 mA cm-2. The flexible asymmetric supercapacitor by assembling free-standing CN-Fe2O3-2 h (negative electrode) and MnO2 (positive electrode) showed an energy density of 1.33 mW h cm-3 at 15.4 mW cm-3. To the best of our knowledge, these results are the record performance for Fe2O3-based electrodes. The two-step interfacial engineering reported in this study may open a new door in the design of high energy-density electrodes for advanced energy storage.
ABSTRACT
The effect of long-term thermal exposure on the carbide evolution in a Ni-16Mo-7Cr base superalloy was investigated. The results show that M12C carbides are mainly precipitated on the grain boundaries during thermal exposure, and the primary massive M6C carbides can be completely transformed to M12C carbides in situ at temperatures above 750 °C for long-term thermal exposure. The transformation from M6C carbides to M12C carbides is attributed to the release of C atoms from M6C, which results in the morphology changes of massive carbides, and stabilization of the sizes of M12C carbides precipitated on the grain boundaries.
