Ar-H2 Gas Mixture Research Articles

Double-faced effects of fission product alloy particles on the behavior of spent nuclear fuel

The purpose of the present work is to investigate the environmental behaviors of spent nuclear fuel (SNF) under deep repository conditions. Over long time periods, oxygen is expected to be introduced into the repository and hydrogen could be generated from an interaction between the iron canister materials and intruding groundwater in future scenarios. The influence of fission product noble metal alloy particles on the stability of the SNF UO2 matrix in a O2 or H2 containing atmosphere were demonstrated by studying the electrochemical and leaching behaviors of UO2 pellets both with and without alloy particle doping. The results show that the doped noble metal alloy particles in UO2 display double faced effects, i.e., can both enhance UO2 oxidative dissolution in an Ar purged, low oxygen containing solution or inhibit the same reaction in the same solution but purged with 4%H2-Ar gas mixture. These conclusions were also verified by the analysis of morphology and composition on the surface of several samples after potentiostatic polarization. The results of this work strongly support the application of a large quantity of iron as the disposal canister inner support material in SNF repository.

Experimental and Simulation Analysis of Hydrogen Permeation Performance of Pd/Ag Permeator Used for Tritium Processing

Since the Pd/Ag membrane has a permselectivity for hydrogen isotopes, a permeator with a Pd/Ag membrane is developed to separate tritium from inert gases. First, a permeation experiment of pure H2 was carried out to determine the pressure exponent and the rate-determining step of permeation. It was found that the diffusion of H2 through the Pd membrane was the rate-determining step. Then, the separation of H2 from H2-Ar gas mixtures was carried out on the permeator to simulate the separation of tritium. Moreover, numerical simulation was utilized to study the concentration distribution of H2 in the permeator. The permeability of the Pd/Ag membrane was determined comparing the simulation results with the experimental data. The permeation flux of H2 through the Pd/Ag membrane is affected by permeability, the volume fraction of Ar in the feed gas, and the flow rate of the feed gas. In the condition of high permeability and Ar volume fraction, a phenomenon known as concentration polarization occurred. It can strongly affect the permeation of H2. Based on these results, an optimized design of the Pd/Ag permeator can be made to effectively separate tritium from other gases.

Preparation of oxygen-deficient 2D WO3−x nanoplates and their adsorption behaviors for organic pollutants: equilibrium and kinetics modeling

A novel oxygen defect-rich WO3−x nanoplates were synthesized using a solvothermal method followed by an annealing treatment under Ar–H2 gas mixture atmosphere. WO3−x nanoplates possess large numbers of oxygen defects systematically demonstrated by the elemental analysis, X-ray diffraction, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, respectively. The adsorption capacity of defect-rich 2D WO3−x nanoplates was evaluated by removal of methyl orange (MO) and the adsorption capacity could reach to 63.7 µg mg−1 with good stability and reusability after. Their adsorption performance at the temperature of 298 K, 308 K and 318 K is carried out and proved to fit Langmuir isotherm model very well, suggestive of a monolayer adsorption on the materials surface sites. In addition, adsorption kinetics studies show that the adsorption process conforms to the pseudo-second-order equation model very well demonstrating a chemical process for MO adsorption. Compared to pure WO3 with annealing treatment at N2 and air atmospheres, the one treated under Ar–H2 gas mixture atmosphere (WO3−x) shows higher adsorption capacity and fast adsorption kinetics toward MO, which was attributed to deprivation of lattice oxygen of WO3 at the reducing atmosphere. Meanwhile, the 2D defect-rich WO3−x nanoplates also exhibit excellent adsorption properties toward methylene blue and rhodamine B, and they can be made into thin film by vacuum suction filtration for dynamically removing the organic pollutants. Our study shows that the as-synthesized defect-rich 2D WO3−x nanoplates have potential applications in real-time wastewater treatment.

Sticking-Free Reduction of Titanomagnetite Ironsand in a Fluidized Bed Reactor

Fluidized bed reduction of iron ore fines is typically inhibited by the onset of “sticking” at temperatures above 973 K, which leads to particle agglomeration and defluidization of the bed. Here, we report the sticking-free fluidized bed reduction of titanomagnetite (TTM) ironsand at 1223 K in Ar-H2 gas mixtures. We show that sticking is prevented by the formation of a protective titanium-rich oxide shell around each particle during the initial reduction stage. This protective shell prevents iron-iron contact between particles throughout the reduction process, enabling metallization degrees of 93 pct to be attained without sticking occurring. Phase evolution during the reaction has also been analyzed using q-X-ray diffraction and scanning electron microscope/energy dispersed spectroscopy. We find that the reduction proceeds through four separate stages. During the initial stage, approximately half of the initial TTM phase is converted to wustite, forming a network of sub-micron wustite channels which interlace the TTM matrix. During this stage, Ti and Al are enriched within the TTM matrix, due to the low solubility of both species in wustite. This enrichment stabilizes the remaining TTM, meaning that wustite is then preferentially reduced to metallic iron in stage 2 of the reduction. In stage 3, the remaining Ti-enriched TTM is reduced directly to metallic iron and ilmenite. The final stage of reduction involves the conversion of ilmenite into rutile and pseudobrookite. Our findings clarify the important role played by titanium species during the reduction of TTM and suggest that New Zealand ironsand can offer significant advantages over conventional hematite ores when used as a feedstock for fluidized bed direct-reduced iron processes.

Use of H2-Ar gas mixtures in radio-frequency magnetron sputtering to produce high-performance nanocrystalline bismuth telluride thin films

Nanocrystalline bismuth telluride (Bi2Te3) thin films with good thermoelectric performance were prepared using mixtures of hydrogen and argon gas by radio-frequency magnetron sputtering. The effects of hydrogen addition on the surface morphology, crystal structure, elemental composition, and thermoelectric properties of the Bi2Te3 thin films were investigated. The mixing ratio, H2/(H2 + Ar), was varied from 0 to 15%. The thin films were deposited on glass substrates heated at 200 °C. It was observed that the surface morphologies of the thin films were greatly affected by the mixing ratio. The deposition rate and composition ratio Te/(Bi+Te) decreased with increasing mixing ratio, indicating that tellurium atoms evaporated from the film surface by a chemical reaction between hydrogen and tellurium. The oxygen concentration inside the films decreased as the mixing ratio increased, resulting in an increased power factor. A maximum power factor of 9.0 μW/(cm·K2) was observed at a mixing ratio of 10%, as this thin film showed a relatively high electrical conductivity and high Seebeck coefficient. However, at a higher mixing ratio of 15%, the power factor of the thin film drastically decreased, possibly due to the appearance of the hexagonal BiTe phase, which exhibits metallic characteristics. Therefore, we conclude that the addition of a moderate amount of hydrogen (10%) during sputtering can improve the thermoelectric performance of Bi2Te3 thin films.

Dephosphorization of Levitated Silicon-Iron Droplets for Production of Solar-Grade Silicon

The treatment of relatively inexpensive silicon-iron alloys is a potential refining route in order to generate solar-grade silicon. Phosphorus is one of the more difficult impurity elements to remove by conventional processing. In this study, electromagnetic levitation was used to investigate phosphorus behavior in silicon-iron alloy droplets exposed to H2-Ar gas mixtures under various experimental conditions including, refining time, temperature (1723 K to 1993 K), gas flow rate, iron content, and initial phosphorus concentration in the alloy. Thermodynamic modeling of the dephosphorization reaction permitted prediction of the various gaseous products and indicated that diatomic phosphorus is the dominant species formed.

Influence of Feedstock Powder Modification by Heat Treatments on the Properties of APS-Sprayed Al2O3-40% TiO2 Coatings

The formation and decomposition of aluminum titanate (Al2TiO5, tialite) in feedstock powders and coatings of the binary Al2O3-TiO2 system are so far poorly understood. A commercial fused and crushed Al2O3-40%TiO2 powder was selected as the feedstock for the experimental series presented in this paper, as the composition is close to that of Al2TiO5. Part of that powder was heat-treated in air at 1150 and 1500 °C in order to modify the phase composition, while not influencing the particle size distribution and processability. The powders were analyzed by thermal analysis, XRD and FESEM including EDS of metallographically prepared cross sections. Only a maximum content of about 45 wt.% Al2TiO5 was possible to obtain with the heat treatment at 1500 °C due to inhomogeneous distribution of Al and Ti in the original powder. Coatings were prepared by plasma spraying using a TriplexPro-210 (Oerlikon Metco) with Ar-H2 and Ar-He plasma gas mixtures at plasma power levels of 41 and 48 kW. Coatings were studied by XRD, SEM including EDS linescans of metallographically prepared cross sections, and microhardness HV1. With the exception of the powder heat-treated at 1500 °C an Al2TiO5-Ti3O5 (tialite–anosovite) solid solution Al2−xTi1+xO5 instead of Al2TiO5 existed in the initial powder and the coatings.

Microstructures and mechanical properties of melt hydrogenated Nb-Si based alloy

Nb-20Si-6Mo (at.%) alloy was prepared by arc melting under Ar-H2 gas mixture atmosphere (Melt Hydrogenation Technology). It indicated that the microstructures of Nb-20Si-6Mo alloy change significantly after the melt hydrogenated treatment. The morphologies of primary Nb5Si3 phase change from polygon to dendrite shapes with the increase of H content. The room temperature compression at same strain rate indicates that the σmax decreases with the increase of hydrogen content. High temperature (1400 °C) compression at same strain rate indicates that H addition can cause the peak strain to take place in advance at high deforming temperature. The high temperature compression stress tends to firstly increase and then decrease due to the effect of H addition. The reason for the effects of hydrogenation on the high temperature compression stress of Nb-20Si-6Mo alloy has been discussed.

Influence of the Bias Substrate Power on the GDC Buffer Layer

Nowadays, the researchers that are working on the SOFC technology, are agree that it is necessary to decrease the operating temperature at 700°C(650-750 °C) of IT-SOFC technology, which currently is around 850-900°C for electrolyte supported cell used by the three main operators –Bloom, Hexis and Sun Fire. At this temperature the reactivity and the component cost of the cell decrease. Nevertheless, some interactions between the conventional electrolyte material (YSZ) and the cathode material (lanthanum based) are still present and it is necessary to include a ceria buffer layer to avoid the possible formation of an insulating pyrochlore structure La2Zr2O7 layer at the electrolyte-cathode interface. In ceria family, the best material regarding the ionic conductivity is the samaria doped ceria. However, the gadolinia doped ceria is the most employed from the cost point of view and the availability of gadolinium in comparison with samarium. This buffer layer avoids the reactivity between electrolyte and cathode, and presents a good anionic conductivity. It must be dense, thin and adherent on electrolyte, in order to be suitable for the surface treatment process. In this paper, we present some recent results obtained on the gadolinia doped ceria coatings deposited by magnetron sputtering from metallic targets in argon-oxygen reactive gas mixtures with different substrate bias voltage. After a description of the experimental device, a first part will be dedicated to their chemical, microstructural and structural characterization (SEM, XRD,…) in relation with the magnetron sputtering deposition parameters. In a second part, the ionic transport characteristics of the half cell (Ni-YSZ/YSZ/GDC) will be investigated via impedance spectroscopy measurements performed at various temperatures up to 800°C under Ar-H2 gas mixtures. Finally, a complete cell test ((Ni-YSZ/YSZ/GDC/LSC) was performed with the Fiaxell Open Flanges test bench. This study is carried out within the framework of the European territorial cooperation program INTERREG V A France-Switzerland. It has benefited from financial support from the EU through the European Regional Development Fund (ERDF- € 139,176) and the Swiss Confederation's contribution of CHF 134,173

Depressing of Cu Cu bonding temperature by composting Cu nanoparticle paste with Ag nanoparticles

Abstract In this paper, we proposed a depressing bonding temperature method for Cu Cu bonding with nanoparticle paste. The paste was prepared by composting dominant Cu nanoparticles with Ag nanoparticles of lower sintering temperature, where the average nanoparticle size of Cu and Ag was 61.02 and 69.25 nm, respectively. The reliable bonding was successfully achieved at the temperature of 250 °C, the pressure of 1.12 MPa and the protection of Ar-H2 gas mixture comparing with pure Cu nanoparticles paste. An optimal resistivity of sintered Cu-Ag composite nanoparticle paste can achieve 1.99 × 10−7 Ω m with the 2:1 molar ratio of Cu to Ag, and the largest shear strength of the bonded Cu-Cu joint can reach 25.41 MPa. The underlying sintering mechanism of the composited nanoparticles was discussed. It suggests the compositing with Ag nanoparticles is a practical strategy towards lowering the bonding temperature for Cu-based interconnect. The experimental process can be easily implemented, the bonding temperature and the performance can satisfy the packaging requirements of current 3D-IC. The developed Cu-Ag composite nanoparticle paste is also promising for packaging of other thermal and force sensitive chips.

Mechanism of boron removal from Si–Al melt by Ar–H2 gas mixtures

A new method about purification of metallurgical grade silicon (MG-Si) by a combination of Si–Al solvent refining and gas blowing treatment was proposed. The morphologies and transformation of impurity phases, especially for boron and iron in Si–Al melt were investigated during Ar–H2 gas blowing treatment. The mechanism of boron removal was discussed. The results indicate that gas blowing can refine grain size and increase nucleation of the primary Si. Boron can be effectively removed from MG-Si using the Ar–H2 gas blowing technique during the Si–Al solvent refining. Compared with the sample without gas blowing, the removal efficiency of boron increases from 45.83% to 74.73% after 2.5 h gas blowing. The main impurity phases containing boron are in the form of TiB2, AlB2 and VB compounds and iron-containing one is in the form of β-Al5FeSi intermetallic compound. Part of boron combines [H] to transform into gas BxHy (BH, BH2) and diffuses towards the surface of the melt and is volatilized by Ar–H2 gas blowing treatment under electromagnetic stirring.

Microwave absorption performance enhanced by high-crystalline graphene and BaFe12O19 nanocomposites

The nanocomposites, consisting of BaFe12O19 ferrite and few-layer graphene sheets (FL-GSs) in various weight ratios (1−9 wt. %), were fabricated by a mechanical mixing method. The high-crystalline FL-GSs were prepared by direct current arc discharge evaporation of pure graphite electrodes in an H2–Ar gas mixture. We measured the electromagnetic properties, including effective magnetic permeability and effective permittivity in addition to microwave absorption performance, of the FL-GSs/BaFe12O19 nanocomposites compared with the pristine BaFe12O19 nanoparticles (NPs). The nanocomposite FL-GSs/BaFe12O19 with the optimal performance (6 wt. % FL-GSs) exhibited an effective microwave absorption (<−10 dB) bandwidth of 5.8 GHz with a thickness of 2.2 mm, 53% higher than that of the pristine BaFe12O19 NPs. Meanwhile, this nanocomposite had the minimum reflection loss of −49.7 dB at 8.4 GHz with a thickness of 2.8 mm, three times greater than those without FL-GSs. These performances result from a simultaneous increase in both magnetic and dielectric losses possibly due to synergistic effects of BaFe12O19 and FL-GSs. In such nanocomposites, both magnetic loss from BaFe12O19 and dielectric loss from FL-GSs contribute to the absorbing performances. Adding FL-GSs as dielectric fillers enhances the impedance matching of the nanocomposites compared with the pristine BaFe12O19 NPs based on the magnetic loss alone. Our results indicate that the incorporation of high-crystalline nanocarbon materials into ferrite oxides can provide high microwave absorption intensity and broad effective absorption bandwidth, while maintaining high thermal stability.

Synthesis of SiC whiskers by VLS and VS process

This study investigates the mechanisms of SiC whisker formation in the carbothermal reduction of quartz to SiC in different gas atmospheres. Reduction of quartz by graphite was studied in Ar, H2, and CH4–H2–Ar gas mixture in a laboratory fixed bed reactor. The reduction products were characterised by XRD, SEM and TEM. Whiskers were not formed in the carbothermal reduction of quartz in argon. Two types of SiC whiskers were observed in the carbothermal reduction of quartz in H2 and CH4–H2–Ar gas mixture. In the process of reduction at 1400–1600°C in H2 and at 1200–1600°C in CH4–H2–Ar gas mixture, whiskers with hexagonal shape with diameter 100–800nm and length up to tens of microns were formed by the VLS mechanism under catalytic effect of iron. The whiskers with the characteristics of cylindrical shape and high aspect ratio were synthesized in CH4–H2–Ar gas mixture at 1400–1600°C by VS mechanism.

Controlled microstructure and production of TiO2 nanostructures by Ar and Ar–H2 atmosphere

Titanium dioxide nanostructures have been synthesised by annealing the Pd loaded titanium foil at 680°C under the atmosphere of Ar or Ar–H2 gas mixture. The microstructure and production of titanium dioxide are found to depend critically on the concentration of Pd catalyst and the Ar–H2 atmosphere. The catalyst of Pd and the introduction of 5%H2 gas are the key factor for the formation of the long (about 100 μm in the average) and smooth titanium dioxide nanowires at low temperature. Scanning electron microscope and X-ray diffraction show that they are all rutile phase. The further result shows that the titanium dioxide nanowires possess good crystallinity and high surface photovoltage response.

Carbothermal Reduction of Quartz in Methane–Hydrogen–Argon Gas Mixture

Synthesis of silicon carbide (SiC) by carbothermal reduction of quartz in a CH4–H2–Ar gas mixture was investigated in a laboratory fixed-bed reactor in the temperature range of 1573 K to 1823 K (1300 °C to 1550 °C). The reduction process was monitored by an infrared gas analyser, and the reduction products were characterized by LECO, XRD, and SEM. A mixture of quartz–graphite powders with C/SiO2 molar ratio of 2 was pressed into pellets and used for reduction experiments. The reduction was completed within 2 hours under the conditions of temperature at or above 1773 K (1500 °C), methane content of 0.5 to 2 vol pct, and hydrogen content ≥70 vol pct. Methane partially substituted carbon as a reductant in the SiC synthesis and enhanced the reduction kinetics significantly. An increase in the methane content above 2 vol pct caused excessive carbon deposition which had a detrimental effect on the reaction rate. Hydrogen content in the gas mixture above 70 vol pct effectively suppressed the cracking of methane.

Carbothermal Reduction of Quartz in Different Gas Atmospheres

This article examines the influence of gas atmosphere on the synthesis of silicon carbide by carbothermal reduction of quartz. The quartz was crushed to <70 μm, uniformly mixed with graphite and pressed into pellets. Reduction was studied in isothermal and temperature-programmed reduction experiments in a tube reactor in argon, hydrogen and Ar-H2 gas mixtures. The concentrations of CO, CO2, and CH4 in the off gas were measured online using an infrared gas analyzer. The samples after reduction were characterized by X-ray diffraction, scanning electron microscope, and LECO analyzer. The carbothermal reduction of quartz in hydrogen was faster than in argon. Formation of silicon carbide started at 1573 K (1300 °C) in argon, and 1473 K (1200 °C) in hydrogen. Synthesis of silicon carbide in hydrogen was close to completion in 270 minutes at 1673 K (1400 °C), 140 minutes at 1773 K (1500 °C), and 70 minutes at 1873 K (1600 °C). Faster carbothermal reduction rate in hydrogen was attributed to the involvement of hydrogen in the reduction reactions by directly reducing silica and/or indirectly, by reacting with graphite to form methane as an intermediate reductant.

Calculated isotropic Raman spectra from interacting H2-rare-gas pairs

We report on a theoretical study of the H2-He and H2-Ar pair trace-polarizability and the corresponding isotropic Raman spectra. The conventional quantum mechanical approach for calculations of interaction-induced spectra, which is based on an isotropic interaction potential, is employed. This is compared with a close-coupling approach, which allows for inclusion of the full, anisotropic potential. It is established that the anisotropy of the potential plays a minor role for these spectra. The computed isotropic collision-induced Raman intensity, which is due to dissimilar pairs in H2-He and H2-Ar gas mixtures, is comparable to the intensities due to similar pairs (H2-H2, He-He, and Ar-Ar), which have been studied previously.

Evaporation Mechanism of Sn and SnS from Liquid Fe: Part I: Experiment and Adsorption of S on Reaction Site

In order to evaluate feasibility of Sn-containing ferrous scrap recycling by evaporation of Sn, a number of liquid–gas experiments were carried out using an electromagnetic levitation melting technique. Rate of decrease of Sn concentration in liquid steel droplets by evaporation in Ar-H2 gas mixture was determined at 1873 K (1600 °C). Evaporation rate of the Sn under various conditions (various flow rates of the gas mixture, initial S concentration, [pct Sn]0) was examined using previously reported rate equations. Increasing flow rate increased the evaporation rate of Sn initially, but the rate became constant at higher flow rate, which indicates that the rate-controlling step is the chemical reaction at the liquid/gas interface. Increasing initial S concentration significantly increased the evaporation rate of Sn, which is in good agreement with previous understanding that Sn could be evaporated as SnS(g). It was found in the present study that neither a simple first-order reaction (rate proportional to [pct Sn]) nor a second-order reaction (rate proportional to [pct Sn] × [pct S]) could account for the Sn evaporation under a chemical-reaction-controlled regime. It is proposed in the present study that surface adsorption of S should be taken into account in order to interpret the evaporation rate of Sn in such a way that S blocks available sites for SnS evaporation on the liquid steel. The ideal Langmuir isotherm was applied in order to better represent evaporation rate constant k SnS as a function of [pct S] (0.06 < [pct S]0 < 0.29). The obtained rate constant of a reaction Sn i + S i = SnSi(g), $$ k_{\text{SnS}}^{\text{R}} $$ , is 2.57 × 10−8 m4 mol−1 s−1.

Non-isothermal reduction kinetics of titanomagnetite by hydrogen

Reduction of titanomagnetite (TTM) powders by H2-Ar gas mixtures was investigated under a non-isothermal condition by using a thermogravimetric analysis system. It was found that non-isothermal reduction of TTM proceeded via a dual-reaction mechanism. The first reaction was reduction of TTM to wustite and ilmenite, whereas the second one was reduction of wustite and ilmenite to iron and titanium dioxide. By using a new model for the dual reactions, which was in an analytical form and incorporated different variables, such as time, temperature, particle size, and hydrogen partial pressure, rate-controlling steps for the dual reactions were obtained with the apparent activation energies calculated to be 90–98 and 115–132 kJ/mol for the first and second reactions, respectively.

Epitaxial growth of B-doped Si on Si(100) by electron-cyclotron-resonance Ar plasma chemical vapor deposition in a SiH4–B2H6–H2 gas mixture without substrate heating

Characteristics of B-doped Si epitaxial growth on Si(100) by using electron-cyclotron-resonance Ar plasma enhanced chemical vapor deposition without substrate heating in a SiH4–B2H6–H2–Ar gas mixture were investigated. B concentration in the deposited films increases with decreasing microwave power for plasma generation. At the microwave power of 125W, the B concentration increases up to 5×1021cm−3. Deposition rate of the B-doped Si tends to be enhanced at the higher B2H6 partial pressure. Resistivity of the B-doped Si film tends to increase with decreasing the microwave power. Referring Irvin curve, in the case of 200W, the carrier concentration is estimated to be at least about 1017cm−3 at the B concentration of 1021cm−3. After heat treatment in N2 atmosphere at 200°C and 300°C for 2h, the resistivity drastically decreases to the value which corresponds to carrier concentration of around 1019cm−3. From Fourier transform infrared spectroscopy measurement, it is found that hydrogen incorporated in the as-deposited film desorbed by the heat treatment.