Reactive,nonreactive, and flash spark plasma sintering of Al2O3/SiC composites—A comparative study

The influence of different SPS-based methods, that is, conventional spark plasma sintering (SPS), flash SPS (FSPS), and reactive SPS (RSPS) on the properties of Al₂O₃/SiC composite was investigated. It was shown that the application of preliminary high energy ball milling of the powders significantly enhances the sinterability of the ceramics. It was also demonstrated that FSPS provides unique conditions for rapid, that is, less than a minute, consolidation of refractory ceramics. The Al₂O₃-20 wt% SiC composite produced by FSPS possesses the highest relative density (~99%), fracture toughness (7.5 MPa m^1/2), hardness (20.3 GPa) and wear resistance among all ceramics produced by other SPS-based approaches with dwelling time 10 minutes. The RSPS ceramics hold the highest Young's modulus (390 GPa). Substitution of micron-sized Al₂O₃ particles by nano alumina does not lead to measurable enhancement of the mechanical properties.     

Chemistry and Mechanism of Interaction Between Molybdenite Concentrate and Sodium Chloride When Heated in the Presence of Oxygen

Roasting of molybdenum concentrates with sodium chloride has high potential and can be an alternative to oxidizing roasting and autoclave leaching; however, the chemistry and mechanism are poorly known. The chemical mechanism of the roasting process between molybdenite concentrate and sodium chloride in the presence of atmospheric oxygen is proposed. It is demonstrated that the process occurs through molybdenite oxidation, up to molybdenum trioxide, with subsequent formation of sodium polymolybdates and molybdenum dioxydichloride from molybdenum trioxide. It is found that the formation of water-soluble sodium polymolybdates from molybdenum trioxide stops over time due to passivation of sodium chloride surface by polymolybdates. It is proved experimentally that preliminary grinding of the mixture in a furnace charge leads to an increase in the polymolybdate fraction of the roasting products, which constitutes approximately 65 pct of molybdenum initially in the roasted mixture against 20 to 22 pct in a nonground mixture (or 75 to 77 pct against 30 to 33 pct of molybdenum in calcine). For the first time, the presence of the Na₂S₂O₇ phase in the calcine was confirmed experimentally. The suggested mechanism gives possible explanations for the sharp increase of MoO₂Cl₂ formation within the temperature range of 673 K to 723 K (400 °C to 450 °C) that is based on the catalytic reaction of molybdenum dioxydichloride from the Na₂S₂O₇ liquid phase as it runs in a melt.     

Direct Combustion Synthesis of Silicon Carbide Nanopowder from the Elements

Direct synthesis of silicon carbide (SiC) nanopowders (size 50–200 nm, BET ~20 m²/g) in Si–C system is conducted in an inert atmosphere (argon) using a self‐propagating high‐temperature synthesis (SHS) approach. A preliminary short‐term (e.g., minutes) high‐energy ball milling (HEBM) of the initial mixture, which involves pure Si and C powders, is used to enhance system reactivity. Two conditions of HEBM with different force fields (17G and 90G) are applied and the results are compared. The influence of HEBM's conditions on the microstructure of mechanically treated mixtures and combustion products is also investigated and discussed. Obtained results suggest that by changing the intensity of mechanical treatment one may prepare a completely amorphous reactive mixture containing carbon and silicon, or gradually change the ratio of (Si/C)–SiC phases and finally produce pure silicon carbide powder during the milling process. The influence of HEBM on the combustibility of the Si/C mixture possesses a critical character: the self‐sustained reaction becomes feasible only after a critical time of ball milling (i.e., 10 min for 90G; 30 min for 17G). Comparison of the microstructures for as‐milled and as‐synthesized powders reveals that for all investigated conditions the morphologies of the as‐milled reactive Si/C media are essentially the same as that for SiC combustion products. The mechanism for direct synthesis of SiC by combustion reaction is also proposed.     

Structural Features of High-Entropy HfTaTiNbZr Alloy Fabricated by High-Energy Ball Milling

Russian Journal of Non-Ferrous Metals - The HfTaTiNbZr high-entropy alloy (HEA) of equimolar concentration is fabricated from powder components by high-energy ball milling (HEBM) and spark plasma...     

Production of Rounded Reactive Composite Ti/Al Powders for Selective Laser Melting by High-Energy Ball Milling

Metallurgical and Materials Transactions B - The effect of different regimes of high-energy ball milling (HEBM) on the morphology of reactive Ti–Al powders was studied. It is shown that a...     

Preparation and study of the thermoelectric properties of Fe<Subscript>2</Subscript>TiSn<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Si<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Heusler alloys

Polycrystalline alloys Fe₂TiSn_1–x Si _x (0 ≤ x ≤ 1) theoretically predicted as highly efficient thermoelectric materials are experimentally studied. Structural studies show that the partial substitution of Sn with Si results in the formation of a multiphase state in samples with x > 0. Impurity phases in general lead to a significant decrease in the Seebeck coefficient and an increase in the thermal conductivity of Fe₂TiSn_1–x Si _x samples, which does not allow consideration of these materials as promising thermoelectrics.     

Mechanochemical synthesis and spark plasma sintering of hafnium carbonitride ceramics

Submicron powder of non-stoichiometric hafnium carbonitrides (HC_0.5N_0.2) was fabricated by the mechanochemical synthesis method. It was shown that during the first milling stages, primarily reaction between hafnium and carbon took place. The nitridation occurred later when fresh metal surfaces started to form through defragmentation of the brittle layer of a carbon-solid solution. The synthesized powder was consolidated by spark plasma sintering approach to producing dense bulk hafnium carbonitride ceramics. Hardness and fracture toughness measured on consolidated samples were 20.8 ± 1 GPa и 3.5 ± 0.2 MPa?m^1/2, respectively. The obtained results were compared with previously reported data.