As the main candidates in the field of ultra‐high temperature ceramics, high entropy carbides/borides (HECs/HEBs) have good oxidation resistance properties, high hardness, as well as excellent ...thermal and electrical conductivities, which are the focused points of research nowadays. In the current study, (Hf,Ta,Zr,Nb,Mo,Ti)C powders were successfully synthesized by a three‐step process, including the mixing process of raw oxides and carbon black with spaying Fe(NO3)3 solution, carbothermal reduction and subsequent calcium posttreatment. For the preparation of (Hf,Ta,Zr,Nb,Mo,Ti)B2 powders, during the calcium posttreatment process, equal stoichiometric ratio of B4C was added for the purpose of boriding reaction. The relevant X‐ray diffraction and SEM characterizations indicate the successful preparations of face‐centered cubic HECs and hexagonal HEBs. However, slight Mo local segregation was found in the prepared (Hf,Ta,Zr,Nb,Mo,Ti)B2 powders. The iron generated from Fe(NO3)3 promotes the solid solution process between monocarbides during the carbothermal reduction process via the dissolution‐diffusion‐precipitation mechanism. In the calcium posttreatment process, the liquid calcium ensures the boriding reaction take place at a low temperature. In addition, the residual carbon could be combined with calcium to generate CaC2 which is easy to be removed by acid leaching, and meanwhile, the added Fe could also be finally eliminated to produce pure HEC/HEB powders. The current method does not require the long‐time high energy ball milling of raw materials, but only simple and mild mixing is enough. Therefore, such a facile route has a great potential application prospect for industrially preparing high entropy phase powders in a large scale.
MoSi2-Al2O3 composites with various volume fractions of Al2O3 from 0 to 40 vol% were fabricated by pressureless sintering at 1550 ℃. MoSi2 powder was prepared by silicothermic reduction of MoS2. ...Microstructure, mechanical properties, electrical resistivity and oxidation behavior of composites were experimentally examined to investigate the influence of Al2O3 addition. The results demonstrated that proper introduction of Al2O3 contributed to improving the oxidation resistance and mechanical properties. Among them, MoSi2-20 vol%Al2O3 composite presented excellent comprehensive properties and oxidation resistance. The oxide scale consisting of SiO2 and Al2O3 was formed on the substrate and effectively prevented the inward diffusion of oxygen.
•MoS2 was used as a Mo source to prepare ultrafine MoSi2 powder.•Effects of Al2O3 addition content on properties of sintered MoSi2-Al2O3 was studied.•The SiO2 film incorporating Al2O3 provided greater protective.•MoSi2-20 vol% Al2O3 composites performed excellent comprehensive performance.
In this paper, a new method for preparing ultrafine tungsten-molybdenum composite powders has been proposed. The carbothermic pre-reduction of the mixture of molybdenum trioxide and tungsten trioxide ...with insufficient carbon black under argon atmosphere was carried out, to generate the pre-reduced tungsten-molybdenum composite powder containing a small amount of residual dioxide. Thereafter, the obtained reaction product was subjected to hydrogen reduction to remove the residual dioxide and generate pure tungsten-molybdenum composite powder with the particle size of 100–200 nm. Two kinds of ultrafine tungsten-molybdenum alloy powders (Mo-30 wt% W, Mo-70 wt% W) were prepared. The residual carbon of the finally obtained product was about 0.01–0.02 wt% after the hydrogen reduction. The sintering behavior of ultrafine W–Mo powders and the properties of sintered samples are investigated. After sintering ultrafine Mo-30 wt% W and Mo-70 wt% W powders at 1600 °C, the relative density of the sintered compacts was about 99%, 97%, and the microhardness were 287 HV and 350 HV, respectively. Meanwhile, as increasing the sintering temperature from 1200 to 1600 °C, the relative density and microhardness of the compacts gradually increased.
Graphic Abstract
Due to strong anti‐poisoning ability, good emission stability, high emission current density, lanthanum hexaboride (LaB6) and cerium hexaboride (CeB6) have been maturely applied in electron emission ...emitter. In this paper, a new manufacturing method for LaB6 (or CeB6) powder was proposed by using La2O3 (or CeO2), B4C, and Al as raw materials. After high‐temperature reaction in the range of 1673–1773 K and the following alkaline leaching at 90°C, LaB6 or CeB6 powder with particle size of about 10 μm was obtained. Furthermore, by Al metal flux method, the obtained powder was used to manufacture single crystal block with size of several millimeters.
High‐entropy carbides (HECs) possess superior properties such as favorable corrosion resistance and marvelous hardness and are widely applied in specific extreme environments. Herein, a novel method ...was proposed for synthesizing HECs powders with raw materials of corresponding oxides and carbon black, which was composed of steps of ball‐milling in stainless jar, carbothermic reduction at 1873 K, as well as the following calcium posttreatment at 1273 K to remove residual carbon by generating CaC2. Finally, pure HECs powders could be fabricated after acid leaching treatment. Under the current experimental conditions, besides HfC‐TaC‐ZrC‐NbC quaternary system, a homogeneous solid solution could be fabricated by adding Mo or/and Ti, but it could not be achieved for systems with addition of V, W, or Cr, owing to the low lattice constant or different crystal structure of corresponding carbides. In the reduction step, iron resulted from ball milling process acted as the diffusion channel to promote the mutual diffusion of different components. Meanwhile, the calcium treatment temperature had a weak effect on particle size of HEC powders. HfC‐TaC‐ZrC‐NbC‐MoC‐TiC powders prepared at the calcium posttreatment temperature of 1273 K had an oxygen content of .302% and an average grain size of 252 nm.
The (TiZrHfNbTaMo)C‐15 wt.% Co ceramics with excellent comprehensive properties were successfully prepared by vacuum liquid phase sintering. The effects of WC content on the grain size, ...microstructure, and mechanical properties were investigated. The results showed that WC was dissolved into (TiZrHfNbTaMo)C phase to form a seven‐component solid solution, and the independent WC phase disappeared. The addition of WC improved the wettability between the ceramic phase and the metal phase and promoted the densification of the ceramics. In addition, WC played a role in refining grains and inhibiting the grains growth to a certain extent. The comprehensive properties of ceramics were improved by solid solution strengthening and high entropy effect. When the WC content was 5 wt.%, the toughness reached the maximum of 9.41 ± 0.15 MPa·m1/2, and the hardness of the ceramic with 10 wt.% WC achieved the highest value of 18.18 ± 0.20 GPa.
The extraction of metal from sulfide mineral is usually accompanied by the emission of sulfur‐containing gas (e.g., SO2), which will cause serious pollution to the environment. In this work, a ...sulfur‐emission free route for preparing ultrafine molybdenum disilicide (MoSi2) powder by silicothermic reduction of MoS2 with the desulfurization agent of lime was proposed. The internal MoS2‐Si mixture is wrapped in an external desulfurization layer composed of lime. After the reaction is completed, the prepared MoSi2 can be easily separated from the desulfurization layer by using a non‐chemical method. In addition, when the reaction temperature is higher than 1000°C, almost all S in MoS2 is transformed to sulfur‐containing gas SiS, which can be fully captured by lime to generate CaS, Si, and CaSiO3. For the raw material with a MoS2:Si molar ratio of 1:4, after reacting at 1000, 1100, and 1200°C for 2 h, the average grain sizes of the obtained MoSi2 powder are approximately 100, 300, and 800 nm, respectively. Moreover, when the reaction time is prolonged from 2 to 6 h at 1000°C, the average grain size of the acquired MoSi2 powder is about 200 nm, and the residual sulfur content is about 0.12%. This work provides a new insight to extract metals or metal compounds from sulfide ores without releasing sulfur‐containing gas.
Metal nitrides nanosheets possess remarkable physical and chemical properties such as high electrical conductivities, catalytic properties, energy storage, and conversion efficiency. In this paper, ...molybdenum nitride (Mo5N6, MoN, and Mo2N) nanosheets were synthesized by nitriding and exfoliating the bulk 2H‐MoS2 via dropping N from ammonia at high temperature. Molybdenum nitride nanosheets with the thickness of dozens of nanometers were prepared successfully under different conditions. It was found that the reaction between MoS2 and NH3 began from about 696°C, and reduction products and reaction mechanisms were strongly dependent on the temperature. When there was MoS2, the generated Mo5N6, MoN, and Mo2N can exist stably at even 820, 1020, and 1120°C, respectively. However, they will decompose progressively after MoS2 was consumed completely: at 820°C, Mo5N6 started to decompose to δ‐MoN; at 1020°C, the phase evolution process of MoN can be described as follows: δ‐MoN→ γ‐Mo2N→ β‐Mo2N→ Mo, while at 1120°C, the β‐Mo2N will transform to Mo.
In this article, ultrafine-grained W–Ni–Fe composite with medium W content was successfully fabricated via methods composed of spray-drying, two-stage reduction and low-temperature sintering. It is ...noticed that the variation of matrix phase fractional volume was effective to tailor the microstructure, mechanical performance and deformation behavior. The microstructure of the medium heavy alloy (MHA) underwent the obvious change from traditional coarsen W grains (>40 μm) to those with sizes of only 3.5 μm, accompanied by a large number of nanosized W precipitate. The two main fracture types of MHA are the W cleavage fracture and matrix phase ductile rupture. The additions of Mo and La2O3 revealed three kinds of strengthening effects, including fine-grained, solid-solution and dispersion strengthening. Experimental results showed that a significant enhancement in tensile properties (∼972.5 MPa and 26.5%) were benefitted from the cooperation of multiple strengthening and plasticized mechanisms, and the detailed effects of deformed twinning and precipitation strengthening were analyzed. The current findings demonstrated that MHA possessed an outstanding combination of strength and ductility and was a promising alternative to conventional tungsten heavy alloys (WHAs) for a more extensive range of applications.
•Ultrafine-grained W–Ni–Fe alloy with medium W content was fabricated successfully.•The MHA possessed a superior combination of excellent strength and good ductility.•The interfacial bonding characteristics among W, matrix and La2O3 were analyzed.•The formation process of the transition interfacial layer is elaborated.•The precipitation of nanosized W grains and deformed twin strengthened the MHA.
