Formation of cementite in tempered Fe–Co–C alloys

In Fe–Co–C alloys, undesirable grain coarsening results from the specific austenite orientation variants that form after the γ→→γ transformations. Tempering of martensite before reheating prevents austenite returning to its original orientation and also limits grain coarsening. However, the reasons for this are unclear. It may be assumed that some differences between cementite formed in tempered and rapidly heated alloys may cause the variation in the final austenite structure. In the present work the orientation relationships between cementite and martensite in two tempered Fe–Co–C alloys have been studied using microbeam electron diffraction in a transmission electron microscope. In both alloys after short-term (rapid heating at 100°C s⁻¹ followed by quench) and long-term (1 and 3 h) tempering treatments the orientation relationships were shown to obey the Isaichev orientation relationships:

However, after rapid tempering, only one carbide variant was found in each crystal, while after long-term tempering, up to three variants were present. This might account for the observed crystallographic reversibility in rapidly heated alloys, contrary to the multiplication of γ variants formed from the long-term tempered martensite.     

Pressure-induced variation of structural,elastic, vibrational,electronic, thermodynamic properties and hardness of Ruthenium Carbides

Three of the five structures obtained from the evolutionary algorithm based structure search of Ruthenium Carbide systems in the stoichiometries RuC, Ru₂C and Ru₃C are relaxed at different pressures in the range 0–200 GPa and the pressure-induced variation of their structural, elastic, dynamical, electronic and thermodynamic properties as well as hardness is investigated in detail. No structural transition is present for these systems in this pressure range. RuC–Zinc blende is mechanically and dynamically unstable close to 100 GPa. RuC-Rhombohedral and Ru₃C-Hexagonal retain mechanical and dynamical stability up to 200 GPa. For all three systems the electronic bands and density of states spread out with pressure and the band gap increases with pressure for the semiconducting RuC–Zinc blende. From the computed IR spectrum of RuC–Zinc blende at 50 GPa it is noted that the IR frequency increases with pressure. Using a semi-empirical model for hardness it is estimated that hardness of all three systems consistently increases with pressure. The hardness of RuC–Zinc blende increases towards the superhard regime up to the limiting pressure of its mechanical stability while that of RuC-Rhombohedral becomes 30 GPa at the pressure of 150 GPa.     

Electronic structures of the suboxide films formed on TiC(1 0 0) and ZrC(1 0 0) surfaces: Density functional theory studies

Density functional theory (DFT) calculations have been performed to elucidate the electronic structures of the TiO-like film on TiC(1 0 0) and the ZrO-like film on ZrC(1 0 0), which are assumed to be monolayers of suboxide films with (1 × 1) periodicity with respect to the substrate (1 0 0) surfaces. It was revealed that the electronic structures of both films were characterized by the existence of a band around 6 eV and a band around the Fermi level. The former and latter bands were mostly composed of O 2p and metal d orbitals, respectively, indicating the substantial ionic nature of the film. The calculated DOS well reproduced the previously obtained photoelectron spectra. From the inspection of the optimized structures, it was found that the both suboxide films have rippled structures; the metal and oxygen atoms are displaced vertically downward and upward, respectively, maintaining the (1 × 1) structures.     

Transition metal carbides in electrocatalytic oxygen evolution reaction

Oxygen evolution reaction(OER) is admitted to an important half reaction in water splitting for sustainable hydrogen production.The sluggish four-electron process is known to be the bottleneck for enhancing the efficiency of OER.In this regard,tremendous efforts have been devoted to developing effective catalysts for OER.In addition to Ir-or Ru-based oxides taken as the benchmark,transition metal carbides have attracted ever-increasing interest due to the high activity and stability as low-cost OER electrocatalysts.In this review,the transition metal carbides for water oxidation electrocatalysis concerning design strategies and synthesis are briefly summarized.Some typical applications for various carbides are also highlighted.Besides,the development trends and outlook are also discussed.     

Microwave-assisted synthesis of carbon-supported carbides catalysts for hydrous hydrazine decomposition

Microwave-assisted synthesis of carbon-supported Mo₂C and WC nanomaterials was studied. Two different routes were utilized to prepare MoO₃ (WO₃) - C precursors that were then subjected to microwave irradiation in an inert atmosphere. The effect of synthesis conditions, such as irradiation time and gas environment, was investigated. The structure and formation mechanism of the carbide phases were explored. As-synthesized nanomaterials exhibited catalytic activity for hydrous hydrazine (N₂H₄·H₂O) decomposition at 30–70 ^°C. It was shown that the catalyst activity significantly increases if microwave irradiation is applied during the decomposition process. Such conditions permit complete conversion of hydrazine to ammonia and nitrogen within minutes. This effect can be attributed to the unique nanostructure of the catalysts that includes microwave absorbing carbon and active carbide constituents.     

The ternary system nickel—aluminum—carbon

Phase equilibria in the ternary system nickel—aluminum—carbon were determined. The phase boundaries are given for an isothermal section at 1000°C. No ternary phase occurs, but Ni₃Al() exhibits a solubility for carbon up to 7–8 at%. NiAl() also dissolves carbon (ca. 3 at%).

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Das ternäre System Nickel—Aluminium—Kohlenstoff

Zusammenfassung Die Phasengleichgewichte im ternären System Nickel—Aluminium—Kohlenstoff wurden für den isothermen Schnitt bei 1000°C bestimmt. Die Löslichkeiten und Phasengrenzen werden angegeben. Es tritt keine ternäre Phase auf, aber Ni₃Al() löst bis zu 7–8 at% Kohlenstoff. NiAl() löst ebenfalls Kohlenstoff (ca. 3 at%).

     

Stabilizing Low-Valence Single Atoms by Constructing Metalloid Tungsten Carbide Supports for Efficient Hydrogen Oxidation and Evolution

Designing novel single-atom catalysts (SACs) supports to modulate the electronic structure is crucial to optimize the catalytic activity, but rather challenging. Herein, a general strategy is proposed to utilize the metalloid properties of supports to trap and stabilize single-atoms with low-valence states. A series of single-atoms supported on the surface of tungsten carbide (M-WC_x, M=Ru, Ir, Pd) are rationally developed through a facile pyrolysis method. Benefiting from the metalloid properties of WC_x, the single-atoms exhibit weak coordination with surface W and C atoms, resulting in the formation of low-valence active centers similar to metals. The unique metal-metal interaction effectively stabilizes the low-valence single atoms on the WC_x surface and improves the electronic orbital energy level distribution of the active sites. As expected, the representative Ru-WC_x exhibits superior mass activities of 7.84 and 62.52 A mg_Ru⁻¹ for the hydrogen oxidation and evolution reactions (HOR/HER), respectively. In-depth mechanistic analysis demonstrates that an ideal dual-sites cooperative mechanism achieves a suitable adsorption balance of H_ad and OH_ad, resulting in an energetically favorable Volmer step. This work offers new guidance for the precise construction of highly active SACs.     

Photoelectron spectroscopy study of oxygen adsorption on Mo2C(0 0 0 1)

Oxygen adsorption on the α-Mo₂C(0 0 0 1) surface has been investigated with X-ray photoelectron spectroscopy and valence photoelectron spectroscopy utilizing synchrotron radiation. It is found that oxygen adsorbs dissociatively at room temperature, and the adsorbed oxygen atoms interact with both Mo and C atoms to form an oxycarbide layer. As the O-adsorbed surface is heated at ≧800 K, the C-O bonds are broken and the adsorbed oxygen atoms are bound only to Mo atoms. Valence PES study shows that the oxygen adsorption induces a peculiar state around the Fermi level, which enhances the emission intensity at the Fermi edge in PES spectra.     

Adhesion and bonding of the Al/TiC interface

The electronic structure and adhesion of Al/TiC(0 0 1) interface are examined by density functional theory. Our results show the preferred configuration is the Al atom above the ceramic’s metalloid atom. The calculated adhesion explains the conflicting experimental results of the W_ad from the aspect of the establishing different chemical equilibrium bonds at the different temperatures. By applying several analysis methods we have thoroughly characterized the interfacial electronic structure. For the Ti-site the interfacial Al and Ti atoms form the metal/covalent bond, while for the C-site the interfacial Al and C atoms form the polar covalent interaction. In addition, we examine the effects of Mg and Si alloying elements at the interface, and find that Mg greatly deteriorates the interface and Si slightly improves the interface. The cleavage may take place preferentially at the interface with the help of interface strain energy, especially with the addition of Mg. This is in good agreement with the experimental result.