Ar Atmosphere Research Articles

Photocatalytic reforming of acetic acid into methane over platinized TiO2 mediated by carbonate radical anion

In this research, Pt-loaded TiO2 was prepared by photodeposition used for acetic acid photocatalytic reforming. CO2 was injected into an acetic acid solution for removing O2 and inducing the formation of CO32–. The production rate of CH4 were higher in a CO2 atmosphere than that in an Ar atmosphere (about 69%). According to electron spin resonance (ESR) analysis, more •CH3 radical was produced in CO2 atmosphere, suggesting that CO2 introduced in acetic acid solution can better promote acetic acid reforming to CH4. On the contrary, the production rate of H2 is higher under an Ar atmosphere than that under CO2, suggesting a peroxidation of acetic acid transferred to H2.

Comparative Study of Conventional and Microwave-Assisted Boriding of AISI 1040 and AISI 4140 Steels

In this study, AISI 1040 and AISI 4140 steels were boriding using Ekabor-II commercial boriding powder with powder-pack boriding method using microwave and conventional heating methods. The samples were borided at 950 °C for 2 and 6 hours in an Ar atmosphere in a microwave oven of Enerzi-Mh2912-V8. Biphasic structure (FeB/Fe2B) was formed in all borided AISI 4140 samples and AISI 1040 samples borided for 6 hours. A single-phase structure was observed in AISI 1040 steel borided for 2 hours. Compared to the conventional method, a 1.5-1.6 times thicker boride layer was obtained in AISI 4140 and AISI 1040 steels with microwave-assisted powder-pack boriding. The highest hardness was measured as 1561.8 HV0.05 for boriding AISI 4140 steel and 1499.7 HV0.05 for boriding AISI 1040 steel. The Vickers indentation fracture toughness of borided steels with microwave energy varied between 2.31 and 3.46 MPa·m1/2. It was determined that in all samples borided by the microwave-assisted and conventional powder-pack boriding method, the adhesion strength between the boride layers and the substrate obtained was sufficient.

Performance enhancement of Hf-Ta-O nanofiber based energy storage materials using oxygen-vacancy and its application for supercapacitor

Nowadays, materials that possess high activity and natural oxygen vacancy have garnered significant attention as potential electrode materials for supercapacitors. In this study, we successfully prepared the Hf6Ta2O17 (HTO) nanofibres (NFs) with an orthorhombic superlattice structure using the electrospinning technique and sintering process. The characterization results indicate that the HTO NFs synthesized at 750 °C show good micro nanostructures and outstanding electrochemical properties. To enhance its energy storage performance, additional oxygen vacancies (OVs) were introduced through a carbothermal treatment under an Ar atmosphere. Although longer reduction reaction times can produce more OVs, the integration of grain boundaries and the growth of grain size caused by large annealing times may limit its electrochemical performance. Therefore, the HTO NFs treated for 1 h (HTO/C-1) exhibited the maximum Cs of 925 F·g−1 at 1 A·g−1 with a larger voltage window of 2.1 V. Furthermore, a symmetric supercapacitor fabricated by two reduced HTO NFs electrodes and 1 M Na2SO4 solution, recorded as r-HTO NFs//r-HTO NFs. The device demonstrated excellent energy density of 73.3 Wh·kg−1 (at power density of 1050 W·kg−1) and a long cycling life (81.8% retained after 5000 cycles). These results suggest that the optimized reduction process can provide appropriate oxygen vacancies in HTO NFs, thereby effectively enhancing its performance.

Enhanced electromagnetic wave absorption performance of ZnO/N-doped carbon microspheres in 2–18 GHz

The properties of popular absorbing materials include low thickness, lightweight, a wide frequency range of absorption, and a significant absorption capacity. However, synthesizing a material that contains all these properties remain a considerable challenge. In this manuscript, Zn, N, O-doped microspheres precursors were innovatively synthesized using the one-pot method, which was efficient and simple. Zn, N, O-doped carbon composites (ZnO/N-CNs) were synthesized by heating precursors in an Ar atmosphere. The electromagnetic wave absorption (EMWA) performances for the composites were modified by altering the calcination temperature of the precursors. As a result, the performance of the ZnO/N-CNs calcined at 700 °C (ZnO/N-CNs-700) in electromagnetic wave absorption (EMWA) was outstanding. The effective absorption bandwidth (EAB) attained 5.68 GHz (12.32–18 GHz, 2.08 mm), while the minimum reflection loss (RLmin) achieved −44.6 dB (12.24 GHz, 2.5 mm). The synergistic interaction of the porous structure, ZnO/C heterostructure, ZnO particles, carbon, and nitrogen was responsible for the superior EMWA properties of ZnO/N-CNs composites. Therefore, the composites exhibited improved interface and dipole polarization loss, conduction loss, multiple reflection and scattering, and impedance matching. This technique made it possible to mass-produce brand-new N-doped carbon-based composites, which had effective EMWA performance. This article suggested a practical method for creating dielectric wave-absorbing composites with excellent all-around performances.

Investigation of the annealing treatment on the performance of TiO2 photoanode

Developing low-cost semiconductor photoanode toward efficient and stable oxygen evolution reaction (OER) is highly desirable for photoelectrochemical (PEC) water splitting. Herein, nanoparticulate titanium dioxide (TiO2) electrodes were prepared using Degussa P25 powders by spin-coating method. The effects of annealing conditions on the PEC performance of the nanoparticulate TiO2 photoanode was systematically investigated including temperature, annealing time and atmosphere. The results demonstrate that the TiO2/fluorine-doped tin oxide substrates (FTO/TiO2) electrode annealed at argon (Ar) atmosphere under 450 °C shows the highest PEC performance. The photocurrent density of FTO/TiO2 annealed in Ar for only 0.5 h reached to 120 μA/cm2 at 0.3 V vs. Hg/HgO which is about 10 times higher than that of the unannealed one. The X-ray photoelectron spectroscopy (XPS) results show the thermal treatment can produce oxygen vacancies on the surface of TiO2 which reduce the recombination of photogenerated electron-hole pairs and expanding the visible light absorption by introducing defective energy levels. The electrochemical impedance spectroscopy (EIS) approves that appropriate annealing treatment can effectively enhance the electron conductivity resulting in low charge transfer resistance. All these factors contribute to improving the PEC activity of TiO2 photoanode.

Construction of Ru/WO3 with hetero-interface structure for efficient hydrogen evolution reaction

Water electrolysis is considered as one most promising technique for hydrogen production. The high efficiency electrocatalyst is the key to accelerating the sluggish kinetics of the hydrogen evolution reaction (HER) in alkaline media. In this work, an efficient HER electrocatalyst with hetero-interfacial metal–metal oxide structure was constructed through a redox solid phase reaction (SPR) strategy. During the annealing process under Ar atmosphere, RuO2 and WS2 in RuO2/WS2 precursor were converted to Ru nanoparticles (NPs) and WO3 in situ, where tiny Ru NPs and oxygen vacancies were uniformly distributed onto the newly formed WO3 nanosheets. Different characterization techniques were adopted to confirm the successful formation of Ru/WO3 electrocatalyst (RWOC). The optimized RWOC sample annealed at 400 °C exhibited the low overpotential value of 13 mV at a current density of 10 mA cm−2 and strong durability under the alkaline condition. Density functional theoretical calculations further revealed that the promoted adsorption/desorption rate of reaction intermediates and the accelerated kinetics of HER process were deduced to the synergistic effect between Ru and WO3 in electrocatalyst. This work provides a feasible method to fabricate highly efficient HER electrocatalysts.

Synthesis of ΝbN faceted grains from oxides and boron nitride with sodium

Agglomerates of ε-NbN hexagonal prismatic faceted grains with a size between 5 and 50 μm were formed by heating Nb2O5 or NbFeO4 powder and Na lump in a BN crucible at 900–1100 °C under an Ar atmosphere. Wavelength-dispersive X-ray analysis demonstrated that the ε-NbN faceted grains have a nearly stoichiometric composition. In addition to the ε-NbN grains, powders of niobium nitrides containing δ-NbN and Nb2N, and water-soluble Na3BO3 were included in the sample prepared from Nb2O5 at 1100 °C, while powders of Fe and Na3BO3, and a trace of δ-NbN were included in the sample from NbFeO4. These reactions were attributed to metathesis reactions in which the oxygen in the oxides and the nitrogen in the solid nitrogen source, BN, were replaced by the formation of Na3BO3. An extra amount of nitrogen supplied from the starting material enhances the formation of ε-NbN and suppressed the formation of δ-NbN and Nb2N.

Size-controllable Rh2P nanoparticles on reduced graphene oxide toward highly hydrogen production

Abundant of defective regions and oxygen-containing groups on graphene oxide (GO) could promote the nucleation and growth of metallic nanoparticles. In this work, the composite catalyst is synthesized by pyrolysis of the mixture powder of GO and rhodium organometallic reactant at 900 °C, Ar atmosphere and low pressure, in which dispersive Rh2P nanoparticles are uniformly attached on reduced graphene oxide (Rh2P/RGO). The sizes of Rh2P nanoparticles are controllable by adjusting the mass proportion of reactants. For the optimized size of Rh2P nanoparticles at 6.0 nm, only 11.7 and 10.3 mV of the overpotentials are required at the current density of 10 mA cm−2 in 0.5 M H2SO4 and 1 M KOH electrolytes, respectively, exhibiting superior HER activity over the commercial Pt/C. Theoretical calculations indicate that the hydrogen adsorption free energy of Rh2P/RGO composites is close to zero, and Bede charge analysis confirms the accelerated electron transport between the RGO and Rh2P. Therefore, the introduction of RGO substrate could not only promote a uniform distribution and tunable size of Rh2P NPs, but also be advantageous to the electron transport from RGO to Rh2P NPs, and a favorable H2 evolution during the HER process. This work provides a facile strategy to control the size of metal phosphide nanoparticles on RGO for the design of advanced HER electrocatalysts.

High bending strength at 1800 °C exceeding 1 GPa in TiB2-B4C composite

High density (99.5%) ceramic composite composed of titanium boride and boron carbide (70/30 vol%) was obtained by spark plasma sintering and was tested by 3-point bending test in Ar atmosphere at 1800 °C. Bending strength was high, around 1.1 GPa. The strength–strain curve presents a peculiar shape composed of three regions where elastic and plastic deformations are active with a different weight. Based on transmission electron microscopy observations we propose a process of mechanical energy absorption driven by shear stress in the boron carbide crystals: stacking faults with (1-11) and (011) stacking planes and twins with (1-11) twinning plane rearrange into nano-twins with (10-1) twinning planes, orthogonal but equivalent to the initial ones. This rearrangement mechanism provides in the first instance a plastic signature, but further contributes strengthening.

Dry synthesis of brannerite (UTi2O6) by mechanochemical treatment

Abstract A powder mixture of UO2 and TiO2 was mechanochemically treated in a planetary ball mill under Ar atmosphere for 1 h using a tungsten carbide vial and balls as the milling medium. Such mechanochemical (MC) treatment reduced the crystallinity of UO2 and TiO2. The mechanochemically treated powder mixture was heated at 700–1,300°C for 6 h under Ar atmosphere and analyzed by X-ray diffraction analysis, scanning electron microscopy-energy-dispersive X-ray spectroscopy, and X-ray absorption fine structure analysis. For comparison, a UO2 and TiO2 mixture without MC treatment was heated and analyzed under the same conditions. UTi2O6 did not form below 1,100°C without MC treatment and only the starting materials were observed. At 1,200 and 1,300°C, a small amount of UTi2O6 and equal amounts of UTi2O6 and UO2 were formed, respectively. The mechanochemically treated sample produced nearly pure UTi2O6 containing small amounts of UO2 impurities when heated above 900°C for 6 h. UTi2O6 was highly crystalline and uniform regardless of the synthesis temperature. It is suggested that the crystallinity of UO2 and TiO2 was reduced and the formation of UTi2O6 was promoted by MC treatment.

Effect of grains and grain boundaries on concentrations and mobility of charge carriers in Ba3Ca1.18Nb1.82O9-δ

Perovskite-based protonic conductors are currently the most promising electrolytes in medium and low-temperature hydrogen energy devices. Herein, the Ba3Ca1.18Nb1.82O9-δ double perovskite oxide protonic conductor is prepared, and the influence of bulk and grain boundaries on transport properties is systematically investigated via defect chemistry. The Ba3Ca1.18Nb1.82O9-δ exhibits a hexagonal distortion, and the calculated standard molar hydration enthalpies of Ba3Ca1.18Nb1.82O9-δ, bulk, and grain boundaries are -138 kJ/mol, -131 kJ/mol, and -144 kJ/mol, respectively. The migration activation energies of protons, oxygen vacancies, and hole conduction in bulk Ba3Ca1.18Nb1.82O9-δ under oxygen are 0.29 eV, 1.51 eV, and 0.63 eV, respectively. The proton concentration in grains is slightly lower than in grain boundaries. In contrast, the proton mobility in bulk is three orders higher than in grain boundaries. The proton conduction of double perovskite oxide protonic conductor is dominant in Ar and reductive atmospheres at 500–800°C, where the conductivity (σh·) and holes transport number (th·) increase with oxygen partial pressure. Therefore, the conduction of Ba3Ca1.18Nb1.82O9-δ could be enhanced by improving carriers’ mobility in grain boundaries. The right grain boundaries block oxygen vacancies at low temperatures, increasing the predominance area diagrams for proton conduction. Hence, grain boundaries control the transport property of double perovskite oxide protonic conductor.

Study of process parameters and characteristics properties of W coatings deposited by rf plasma sputtering

Tungsten coatings were deposited on silicon substrates by radio frequency (rf) magnetron sputtering from a metallic target in Ar atmosphere. The process parameters during the sputtering process were evaluated by a Langmuir probe, particularly, the electron density and electron temperature were measured by changing the rf power and gas pressure. The morphological and structural properties of the coatings were studied as a function of the pressure. Significant correlations were found between process parameters and characteristics properties of W coatings. The influence of deposition parameters on electrical properties was investigated. The electrical resistivity of the coatings was increased from 1.3 × 10−6 to 3 × 10−5 Ω m as the pressure increased as well.

Carbon Fibers Doped by Binary Phosphides as an Electrocatalytic Layer for PEM Electrolysers

Hydrogen evolution reactions (HER) are important in a variety of electrochemical devices, such as electrolysers and fuel cells. To reduce the reaction overpotential and reduce energy consumption, efficient, low-cost, and durable electrocatalysts must be developed. Needle-less electrospinning (NLE) technique was used to prepare the fibrous electrocatalyst. NLE is a user-friendly and adaptable technique for large-scale low-cost fiber production. NLE created transition metal phosphides carbon fibers (TMP CF). The precursor foam was folded between two Al 2 O 3 ceramic plates. The heat treatment was carried out in a tube furnace at 1200 °C in an Ar atmosphere, followed by a reduction in an H 2 atmosphere at 780 °C. The electrolyser's membrane electrode assembly can be immediately submerged in the final TMP CF in the form of plates. The created NiCoP catalytic plates could be directly used in electrolyser's membrane electrode assembly of PEM electrolysers. In a three-electrode system, the electrochemical activity of the produced electrocatalysts was evaluated using linear sweep voltammetry. The electrochemical activity of the produced electrocatalysts were evaluated using linear sweep voltammetry. The catalyst's stability and endurance in acidic and alkaline environments were investigated.

Synthesis and investigation of the physicochemical properties of polymorphic 3C–SiC

Silicon carbide (SiC) nanopowders were synthesized under Ar atmosphere via the carbonization reaction method using silicon powders and expanded graphite as initial materials, and ferric nitrate as catalyst precursor. The effects of heat treatment temperature and the C/Si molar ratio on SiC products were studied. The microstructure, phase composition and chemical state of the products were analyzed by XRD, SEM, XPS, TEM and Raman. The photoluminescence (PL) properties of SiC nanopowders were tested. The results showed that the content of the 3C–SiC product is higher after heat treatment at 1400 °C for 3 h with a molar ratio of C/Si of 1:1. 3C–SiC exists in the form of particles and wires. The formation process of 3C–SiC can be explained by a two-step method, nucleation under catalysis and growth under Vapor-Solid (VS) mechanism. Two ultraviolet light emission peaks are observed at about 318 and 380 nm. The main PL spectra of 3C–SiC nanopowders exhibit larger blueshifts. The blueshift is attributed to the stacking fault, oxygen defects, morphology and quantum confinement effect. It has potential application prospects in ultraviolet detectors, optoelectronic devices and light emitters.

Magnetic carbon nanocomposites via the graphitization of glucose and their induction heating

Carbon nanocomposites containing iron-based nanoparticles are attractive materials for the catalyst supports used for magnetic (induction) heating catalysis. The metallic, soft-magnetic iron nanoparticles provide local heating of the support in an alternating magnetic field and ensure rapid magnetic separation of the nanocomposite particles from reaction suspensions. In this work, magnetic carbon nanocomposites were prepared by annealing the precursor particles consisting of iron-oxide nanoparticles dispersed in a carbohydrate matrix. The annealing was conducted at 600 °C and 750 °C in an Ar atmosphere. At both temperatures the carbothermal reduction of iron oxide to Fe/Fe3C was observed; however, at the lower temperature the rate of reduction and the growth of the nanoparticles were considerably slower. The Fe3C was formed in negligible amounts only after a prolonged period of annealing at 600 °C. A detailed structural analysis showed that the Fe/Fe3C nanoparticles catalyze the graphitization of the carbonaceous precursor material already at 600 °C, resulting in the formation of a graphitic shell that surrounds them. This shell is tight enough to prevent the areal oxidation of the encapsulated Fe nanoparticles; their magnetic properties remained unchanged even after 1 year of storage under ambient conditions. At the higher annealing temperature, the growth of the Fe/Fe3C nanoparticles caused bursting of the graphitic shell and thus partially exposed their surfaces to the atmosphere. All the nanocomposites exhibited ferromagnetic behavior in accordance with their compositions. The nanocomposite that was predominantly composed of a graphitic shell, encapsulated Fe nanoparticles and a negligible amount of Fe3C, showed the highest specific absorption rate (760 W/gFe at 274 kHz), even at a relatively low AC-field amplitude (88 mT).

Effects of synthesis methods on the structural and magnetic properties of double perovskite Sr2CrReO6 oxide powders

Double perovskite Sr2CrReO6 (SCRO) powders were prepared using a sol-gel process and molten-salt synthesis (MSS) method and then followed by annealing at different temperatures in N2 (or Ar) atmospheres. They were characterized by powder X-ray diffraction and then followed by structural Rietveld refinements. The results reveal that at room temperature all the SCRO powders crystallize in a tetragonal phase structure with space group of I4/m. The Cr/Re cationic ordering degrees (η) at B-site in the sol-gel derived SCRO powders annealed in Ar and N2 atmospheres, were about 68.9 % and 45.3 % respectively. The η values for the SCRO powders prepared by MSS method under the molar ratios of product-to-flux with 1:100, 1:80, and 1:50, were 67.4 %, 57.0 % and 54.3 %, respectively. All the SCRO powders exhibited ferromagnetic behaviors at room temperature and their saturation magnetization (MS) values at 2 K were in range of 0.52–0.67 μB/f.u., smaller than the theoretical value of 1.0 μB/f.u. That can be ascribed to the anti-site defects as well as the surface spin disorders in these SCRO powders. The sol-gel derived SCRO powders had a bead-like morphology and their average composition is slight Cr-rich and Re-deficient with a chemical formula of Sr2Cr1+xRe1−xO6+δ, whereas the SCRO powders prepared by MSS method exhibited a spherical morphology and their average composition is slight Cr-rich and Sr-deficient with a chemical formula of Sr2−yCr1+yReO6+δ. XPS spectra confirm a mixed valence states of Cr3+ and Cr6+ ions in the sol-gel derived SCRO powders, while only Cr3+ ions present in the SCRO powders prepared by MSS method. The Sr2+ ions and a mixed Re4+ and Re6+ ions exist in all the SCRO powders, and three kinds of oxygen species are present as lattice oxygen (O2-), adsorbed oxygens, and the oxygens in hydroxyl groups, respectively. The sol-gel derived SCRO powders had a higher MS but a smaller Hc in comparison with the powders prepared by MSS method, which enables them to have promising applications in future spintronic devices.

New mechanistic insight on the synthesis of metal carbonates by laser ablation in ethanol

We present new results concerning the formation mechanism of lead carbonate (Pb3(CO3)2(OH)2) by laser ablation in ethanol, under controlled atmospheres. We suggest that carbon dioxide (CO2) is absorbed from ambient atmosphere and converted into carbonate in a reaction mediated by ethoxide anion produced during the ablation. Addition of 2,4,6 trinitrotoluene to the ablated solution and its subsequent reduction, followed by UV–Vis spectroscopy, supports this new mechanism. No carbonate precipitates under Ar or O2 atmospheres. Ablation of Pb, PbO, Bi or Bi2O3 targets in ethanol were also carried out. The precipitated powder samples were analyzed by XRD, FTIR, Raman, SEM. The results showed that hydrocerussite (Pb3(CO3)2(OH)2) and bismutite (Bi2O2(CO2)3) nanostructures were obtained, demonstrating that carbonates can be obtained from metals and metal oxide sources.

Catalyst-free toluene hydrogenation to methyl-cyclohexane by pulsed DBD plasma under ambient conditions

Hydrogenation of aromatic compounds is an important reaction in the petrochemical, pharmaceutical, and organic industries. In this paper, we report a novel catalyst-free hydrogenation process that converted gaseous toluene into methyl-cyclohexane at ambient pressure and room temperature using microsecond pulsed dielectric barrier discharge (DBD). The hydrogenation performance of the toluene was investigated in terms of the pulse repetition frequency, reactor temperature, and toluene concentration. The experiments show that the toluene was easily hydrogenated to methyl-cyclohexane by the H2 DBD plasma, in which higher PRF, lower reactor temperature and higher H2 concentration were favorable for the production of methyl-cyclohexane. The highest molar fraction of the methyl-cyclohexane was 80.1% with energy consumption of 0.12 kW∙h/mmol. Controlled experiments with Ar atmosphere or different feedstocks (d-toluene, methyl-cyclohexadiene and methyl-cyclohexene) indicate that the H radicals from the H2 DBD plasma realize the toluene hydrogenation. The possible reaction mechanism of the plasma-enabled hydrogenation was discussed via the optical emission spectroscopy (OES), density functional theory (DFT) and molecular dynamic simulations, which confirms that the H radicals produced by the electron-impact reactions induce the step-by-step hydrogenation reaction of the toluene. Overall, this work provides not only new insights into the plasma-enabled hydrogenation reaction, but also a potentially effective catalyst-free method for hydrogenation applications.

Effect of post-annealing on thermal shock and steam corrosion of LaMgAl11O19/Yb2Si2O7 thermal/environmental barrier coatings

LaMgAl11O19/Yb2Si2O7 (LMA/YbDS) thermal/environmental barrier coatings (T/EBCs) on SiCf/SiC composites were annealed at 1200 °C for 5 h in air or Ar atmosphere. The effect of post-annealing in different atmospheres on the microstructure, thermal shock and steam corrosion of the LMA/YbDS T/EBCs was investigated. Results indicated heat treatment in air and argon eliminated the amorphous in the coatings, avoiding the aging stress associated with the crystallization. And the argon-annealed layers (LMA and YbDS) exhibited the less elastic moduli than the air-annealed ones, resulting in the lower thermal mismatch stress in the argon-annealed T/EBCs upon subsequent thermal cycling. Thus, the argon-annealed T/EBCs exhibited an improved thermal cycling lifetime than the as-sprayed and the air-annealed ones. In addition, the unbroadened vertical cracks in the argon-annealed LMA-TBC layer limited the reactant (water-vapor) access to the silica-TGO, leading to the greater resistance of the argon-annealed T/EBCs against steam corrosion than the other two systems.

Thermoelectric properties of La-doped A-site SrTiO3 ceramics synthesised by the sol-gel auto-combustion technique

ABSTRACT Sr(1– x )La x TiO3 (SLTO) nanopowder (x = 0, 0.01, 0.05, 0.07, and 0.10) were effectively synthesised by the sol-gel auto-combustion process. Bulk ceramics samples were produced by spark plasma sintering (SPS) in an Ar atmosphere. The XRD pattern of the undoped SLTO nanopowder showed the pure phase and the impurity phase (TiO2) was detected in the La-doped SLTO samples. However, the impurity phase disappeared after producing the bulk samples with the SPS process. All the samples exhibited a negative Seebeck coefficient (S). The electrical conductivity (σ) and negative Sof all the SLTO samples increased with increasing temperature and reached a maximum value at 800 K. The thermal conductivity (κ) decreased with increasing temperature and increasing La content. The SLTO with x = 0.01 showed the largest σ, negative S, power factor (S 2 σ) and dimensionless figure of merit (ZT) at 800 K.