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Phase controlled synthesis of transition metal carbide nanocrystals by ultrafast flash Joule heating
Nanoscale carbides enhance ultra-strong ceramics and show activity as high-performance catalysts. Traditional lengthy carburization methods for carbide syntheses usually result in coked surface, large particle size, and uncontrolled phase. Here, a flash Joule heating process is developed for ultrafast synthesis of carbide nanocrystals within 1 s. Various interstitial transition metal carbides (TiC, ZrC, HfC, VC, NbC, TaC, Cr
2
C
3
, MoC, and W
2
C) and covalent carbides (B
4
C and SiC) are produced using low-cost precursors. By controlling pulse voltages, phase-pure molybdenum carbides including β-Mo
2
C and metastable α-MoC
1-x
and η-MoC
1-x
are selectively synthesized, demonstrating the excellent phase engineering ability of the flash Joule heating by broadly tunable energy input that can exceed 3000 K coupled with kinetically controlled ultrafast cooling (>10
4
K s
−1
). Theoretical calculation reveals carbon vacancies as the driving factor for topotactic transition of carbide phases. The phase-dependent hydrogen evolution capability of molybdenum carbides is investigated with β-Mo
2
C showing the best performance.
Nanoscale carbides provide access to ultra-strong ceramics and show activity as high-performance catalysts. Here, the authors report a flash Joule heating process for the ultrafast, general synthesis of various transition metal carbides nanocrystals with phase controllability.
Journal Article
Five past midnight in Bhopal
\"One of the premier historians of our time, Dominique Lapierre is the author of such stirring classics as Is Paris Burning? and The City of Joy. Famed for uncovering the humanity in historic events, he here joins forces with acclaimed writer Javier Moro. Together they investigate and chronicle each fateful moment counting down to what happened.\" \"Union Carbide was a huge American corporation whose leaders had only the best intentions. In New York they invented a miracle insecticide. In ancient Bhopal, they formulated the lethal gas needed to produce it and built a giant plant to process it.\" \"But at five past midnight on December 3, 1984, toxic gas leaked out of a pesticide tank. By one-thirty geysers were spitting poison into the night wind. The apocalypse had begun. Banks of deadly fog filled nearby slums. Lungs burst. Corneas burned. Death would strike in seconds and no one was prepared: neither the bride at her wedding banquet nor the peasants who came to Bhopal for a better life, nor the shoemaker rousing his neighbors to flee their huts nor the Scottish nun risking all to rescue lost children. By night's end, over half a million Bhopalis were drowning in pain and chaos, and between 16,000 and 30,000 would die in the worst industrial disaster in history.\"--Jacket.
Oxidation Resistance, Ablation Resistance, and Ablation Mechanism of HfC–Bsub.4C-Modified Carbon Fiber/Boron Phenolic Resin Ceramizable Composites
Thermal protection materials with excellent performance are critical for hypersonic vehicles. Carbon fiber/phenolic resin composites (C[sub.f]/Ph) have been widely used as thermal protection materials due to their high specific strength and ease of processing. However, oxidative failure limits the extensive applications of C[sub.f]/Ph in harsh environments. In this paper, a novel hafnium carbide (HfC) and boron carbide (B[sub.4]C)-modified C[sub.f]/Ph was fabricated via an impregnating and compression molding route. The synergistic effect of HfC and B[sub.4]C on the thermal stability, flexural strength, microstructure, and phase evolution of the ceramizable composite was studied. The resulting ceramizable composites exhibited excellent resistance to oxidative corrosion and ablation behavior. The residual yield at 1400 °C and the flexural strength after heat treatment at 1600 °C for 20 min were 46% and 54.65 MPa, respectively, with an increase of 79.59% in flexural strength compared to that of the composites without ceramizable fillers. The linear ablation rate (LAR) and mass ablation rate (MAR) under a heat flux density of 4.2 MW/m[sup.2] for the 20 s were as low as −8.33 × 10[sup.−3] mm/s and 3.08 × 10[sup.−2] g/s. The ablation mechanism was further revealed. A dense B–C–N–O–Hf ceramic layer was constructed in situ as an efficient thermal protection barrier, significantly reducing the corrosion of the carbon fibers.
Journal Article
Preparation of HfCsub.xNsub.1−x Nanoparticles Derived from a Multifunction Precursor with Hf-O and Hf-N Bonds
HfC[sub.x]N[sub.1−x] nanoparticles were synthesized using the urea-glass route, employing hafnium chloride, urea, and methanol as raw materials. The synthesis process, polymer-to-ceramic conversion, microstructure, and phase evolution of HfC[sub.x]N[sub.1−x]/C nanoparticles were thoroughly investigated across a wide range of molar ratios between the nitrogen source and the hafnium source. Upon annealing at 1600 °C, all precursors demonstrated remarkable translatability to HfC[sub.x]N[sub.1−x] ceramics. Under high nitrogen source ratios, the precursor exhibited complete transformation into HfC[sub.x]N[sub.1−x] nanoparticles at 1200 °C, with no observed presence of oxidation phases. In comparison to HfO[sub.2], the carbothermal reaction of HfN with C significantly reduced the preparation temperature required for HfC. By increasing the urea content in the precursor, the carbon content of the pyrolyzed products increased, leading to a substantial decrease in the electrical conductivity of HfC[sub.x]N[sub.1−x]/C nanoparticle powders. Notably, as the urea content in the precursor increased, a significant decrease in average electrical conductivity values was observed for the R4-1600, R8-1600, R12-1600, and R16-1600 nanoparticles measured at a pressure of 18 MPa, yielding values of 225.5, 59.1, 44.8, and 46.0 S·cm[sup.−1], respectively.
Journal Article
Poisoned air : Bhopal, India
\"[This book] traces the tragic story of a city in India that became exposed to a deadly gas called methyl isocyanate, or MIC. Photos of the actual events, maps, and fact boxes [complement] the text\"--Amazon.com.
Book
Synthesis of Propiolic and Butynedioic Acids via Carboxylation of CaCsub.2 by COsub.2 under Mild Conditions
Carbon dioxide (CO[sub.2]) is a greenhouse gas, and its resource use is vital for carbon reduction and neutrality. Herein, the nucleophilic addition reaction of calcium carbide (CaC[sub.2]) to CO[sub.2] was studied for the first time to synthesize propiolic and butynedioic acids by using CuI or AgNO[sub.3] as catalyst, Na[sub.2]CO[sub.3] as additive, and triphenylphosphine as ligand in the presence/absence of a hydrogen donor. The effects of the experimental conditions and intensification approach on the reaction were investigated. The reactivity of CaC[sub.2] is closely associated with its synergistic activation by the catalysts, solvent, and external intensification, such as the ultrasound and mechanical force. Ultrasound helps to promote the reaction by enhancing the interfacial mass transfer of CaC[sub.2] particulates. Mechanochemistry can effectively promote the reaction, yielding 29.8% of butynedioic acid and 74.8% of propiolic acid after 2 h ball milling at 150 rpm, arising from the effective micronization and interfacial renewal of calcium carbide. The present study sheds a light on the high-value uses of CO[sub.2] and CaC[sub.2] and is of reference significance for the nucleophilic reaction of CaC[sub.2] with other carbonyl compounds.
Journal Article
Thermal conductivity of boron carbide under fast neutron irradiation
Due to the complex products and irradiation-induced defects, it is hard to understand and even predict the thermal conductivity variation of materials under fast neutron irradiation, such as the abrupt degradation of thermal conductivity of boron carbide ([B.sub.4]C) at the very beginning of the irradiation process. In this work, the contributions of various irradiation-induced defects in [B.sub.4]C primarily consisting of the substitutional defects, Frenkel defect pairs, and helium bubbles were re-evaluated separately and quantitatively in terms of the phonon scattering theory. A theoretical model with an overall consideration of the contributions of all these irradiation-induced defects was proposed without any adjustable parameters, and validated to predict the thermal conductivity variation under irradiation based on the experimental data of the unirradiated, irradiated, and annealed [B.sub.4]C samples. The predicted thermal conductivities by this model show a good agreement with the experimental data after irradiation. The calculation results and theoretical analysis in light of the experimental data demonstrate that the substitutional defects of boron atoms by lithium atoms, and the Frenkel defect pairs due to the collisions with the fast neutrons, rather than the helium bubbles with strain fields surrounding them, play determining roles in the abrupt degradation of thermal conductivity with burnup. Keywords: boron carbide ([B.sub.4]C); thermal conductivity; fast neutron irradiation
Journal Article
Study on the Microstructure Evolution and Ablation Mechanism of SiCsub.p/Al Composites Processed by a Water-Jet Guided Laser
In this study, the influence of different process parameters on the macroscopic and microscopic morphology of the microgroove in the water-jet guided laser was studied. In addition, the microstructure evolution and material ablation mechanism of the microgroove were studied. The results show that with the increase in laser power, the depth of the microgroove increases from 154 μm to 492 μm, the width from 63 μm to 74 μm, and the depth-to-width ratio from 2.45 to 6.62; with the increase in scanning speed, the depth of the microgroove decreases from 525.33 μm to 227.16 μm, and the width from 67.61 μm to 71.02 μm, and the depth-to-width ratio from 7.77 to 3.20. With the increase in water jet pressure, the depth increases from 312.29 μm to 3.20. With the increase in water jet pressure, the depth increased from 312.29 μm to 362.39 μm, the width decreased from 71.59 μm to 62.78 μm, and the depth-to-width ratio increased from 4.38 to 5.77. In addition, the water guided laser processing of SiC[sub.p]/Al composites produces thermal–mechanical coupling and chemical reaction synergies: the material melts and vaporizes under the action of a high-energy laser beam, and the SiC particles are oxidized and thermally decomposed at local high temperatures due to their high thermal stability.
Journal Article