Heat Treatment In Argon Research Articles

Preparation and mechanical behavior of Yb2Si2O7 interphase in SiCf/SiC minicomposites

AbstractIn this work, the Yb2Si2O7 interphase was prepared on SiC fibers using the sol–gel method. The influence of sol concentration on the morphology of the interphase was discussed. After heat treatment in argon at 1200°C for 1 h, the Yb2Si2O7 was grown in situ on the fiber surface, forming an interphase layer. Then, SiCf/Yb2Si2O7/SiC minicomposites were prepared by chemical vapor infiltration (CVI). The Yb2Si2O7 interphase exhibited no changes throughout the high‐temperature preparation process and maintained stability and compatibility with the fiber and matrix. The tensile strength of SiCf/Yb2Si2O7/SiC minicomposites with Yb2Si2O7 interphase was significantly higher than SiCf/SiC minicomposites without interphase. The crack deflection, interfacial debonding, and fiber pullout were observed in the tensile fracture of the SiCf/Yb2Si2O7/SiC minicomposites.

Effect of oxygen and nitrogen sensitization on structural, optical and electrical properties of PbSe thin films

An effective two-step sensitization technique was implemented in PbSe thin films, leading to a remarkable improvement in the optoelectronic performance of PbSe photodetectors. The phase composition, surface and cross-sectional morphology, temperature-dependent carrier concentration and mobility as well as the detectability of PbSe films after sensitization were systematically investigated using characteristic techniques. The optimal PbSe film oxygen sensitized at 450 °C exhibited a room-temperature photodetection with a carrier concentration of 5.31 × 1016 cm−3, carrier mobility of 16.52 cm2/V−1s−1, and detectivity of 2.25 × 109 Jones. High-temperature argon heat treatment and oxygen sensitization of PbSe films are expected to suppress recombination losses and significantly improve the optoelectronic properties of sensitized PbSe films due to enhanced carrier separation and transport. The enhanced photoresponse mechanism of the PbSe films through two-step sensitization method was also discussed. These results suggest that the proposed method can be applied to high-performance lead selenide photodetectors.

SYNTHESIS AND PHOTOELECTROCHEMICAL PROPERTIES OF SU2O-CU3VO4 COMPOSITE FILMS

Cu2O-Cu3VO4 films were obtained by elect­rochemical synthesis. The analysis of polarization curves and Pourbaix diagrams for copper and vanadium ions made it possible to analyze the possible reactions that occur during the deposition of films. At potentials from 0 to -0.2V, mainly monovalent copper oxide Cu2O with Cu3VO4 impurities precipitated in the working solution. At the same time, there are two competing processes of acidification of the near-electrode layer as a result Cu2O formation reaction and alkalinization as a result of the chemical interaction of Cu2O with HVO42-. The formation of an excess of OH- ions can inhibit the reaction rate of the formation of Cu3VO4, accelerating the reaction of the formation of Cu2O. Thus, deposition of Cu3VO4 will take place to a lesser extent compared to Cu2O. Accordingly, to increase the yield of Cu3VO4, an attempt was made to reduce the deposition current, which, according to literature, helps to reduce the effect of the pH change near the electrode layer. Where, as known the critical current limit at change pH starts from 5 or more mA/cm2. For the synthesis of the films, a current of up to 1 mA/cm2 selected, which contributed to the production of the Cu3VO4 - Cu2O composite, as was established further from the analysis of X-ray patterns. It is shown that their photoelectrochemical pro­perties depend on the heat treatment conditions. This is expressed by the difference in the spectral characteristics of the quantum yield and the value of the photocurrent in the samp­les annealed in air and argon. Using the X-ray phase analysis method, it was established that heat treatment in argon contributes to the formation of a Cu3VO4 and Cu2O composite, in contrast to heat treatment in air, where a mixture of CuO and V2O5 oxides is mainly formed. In the film annealed in air due to impact of wide-band oxide compounds, a smaller value of the quantum yield of the photoelectrochemical current and a narrower spectral dependence were observed. The stretching of the spectrum into the region of visible light on the spectral curves of the photocurrent quantum yield is caused by the contribution of copper vanadate with Eg = 1.5 eV. Analysis of photo­current quantum output spectra and X-ray patterns showed that an increase in monovalent copper in the film structure contributes to the growth of photocurrent in the wavelength range of 450-600nm at a potential of -0.2 relative. h.s.e in 2 times. This indicates a positive effect of heat treatment in argon on increasing the efficiency of photocathodes based on a composite of Cu3VO4 and Cu2O for photoelectrochemical cells.

Effect of Mo‐Containing Additives on Combustion of Al‐B Fuels

AbstractIn this study, the Al‐B‐Mo alloy powders with a boron content of 7 wt % and Molybdenum content of 0.5–3 wt % were prepared by the close‐coupled gas atomization. The powder morphology, phase composition, oxidation, and exothermic behavior of combustion were characterized. The results show that the addition of Mo effectively improves the energetic properties of the alloy powder, with the mass‐burning enthalpy of Al‐7B‐1Mo reaching 31111 J/g, exceeding the theoretical mass‐burning enthalpy of monolithic Al (31000 J/g). Al‐7B‐1Mo underwent a violent exothermic process at around 1000 °C, and the oxidation weight gain ratio and oxidation efficiency reached 46.16 % and 48.05 % respectively, an increase of 19.75 % and 21.08 % compared to Al‐7B. The mesh structure formed by the AlB2 phase on the particle surface positively promoted the particles′ thermal oxidation. Mechanical mixtures of AlB2/Mo and AlB2/MoO3 were prepared and the catalytic promotion of the combustion of AlB2 by Mo and MoO3 was confirmed by thermal tests of combustion. The mechanism by which Mo/MoO3 promotes the combustion of AlB2 was investigated through the phase composition characterization of the composite powder after heat treatment in argon and air atmosphere. It was confirmed that MoO3 promotes the oxidation and combustion of AlB2 by supplying oxygen to B in AlB2, while Mo is first oxidized to MoO3, which then promotes the combustion of AlB2.

Novel supermalloy/alumina type soft magnetic composite obtained by reaction spark plasma sintering of Al-Supermalloy (Ni70.5Fe18.8Mo4.7Al6) surface oxidized particles

Novel type of soft magnetic alloy, Supermalloy (Ni-Fe-Mo)/alumina (Al2O3) is designed to be obtained by reaction spark plasma sintering of Al-Supermalloy surface oxidized particles. The nominal composition of the alloy used for the compacts synthesis is Ni70.5Fe18.8Mo4.7Al6. Two techniques have been used for surface oxidizing, heat treatment in a mixture of air and argon and powder immersion in HCl solution. Both types of oxidation techniques lead to the formation of a layer of oxides on the particles surface, evidenced as being mainly iron oxides, such as hematite-Fe2O3 and magnetite-Fe3O4. The composite particles Al-Supermalloy@oxides have been further subjected to spark plasma sintering at 900 ºC for 10 min, in order to densify and promote the reaction between Al-Supermalloy and iron oxides to form a matrix of alumina. The aluminum from Al-Supermalloy reacts with the oxides that cover the particles resulting in a oxide matrix that is formed mainly by alumina. The alumina matrix for the compact obtained from powder covered by heat treatment in argon and air mixture (10/1 vol ratio) is more consistent as compared to the compact obtained from the powder covered by immersion in HCl solution. The magnetic properties in DC and AC for the compacts obtained from powder oxidized in argon and air mixture at temperature are more consistent as compared to the other compact. The samples have been investigated in the light of scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX), Fourier transformed infrared spectroscopy (FTIR), DC an AC magnetic properties on sintered compacts and electrical resistivity.

Extrinsic Point Defects in Low-Positive Thermal Expansion Al2W3O12 and Their Effects on Thermal and Optical Properties.

A2M3O12-type ceramics are potentially useful in a variety of applications due to their peculiar thermal and mechanical properties. In addition, their intrinsic coefficients of thermal expansion can be finely tuned through different mechanisms. Despite the great influence of extrinsic point defects on physical properties, only a few reports have dealt with their relationship to thermal expansion and thermal conductivity. Extrinsic oxygen vacancies in orthorhombic Al2W3O12, in different concentrations, were formed through heat treatments in argon or hydrogen atmospheres. X-ray powder diffraction, diffuse reflectance spectroscopy, and Raman and electron paramagnetic resonance spectroscopies were used to study the as-formed vacancies, and X-ray photoelectron spectroscopy was employed to propose a charge compensation mechanism. It was found that the intrinsic coefficient of thermal expansion of orthorhombic Al2W3O12 was severely affected by extrinsic oxygen vacancies. Thermal expansion was decreased up to 40% (from 25 to 400 °C) with respect to the extrinsic-point-defect-free counterpart. Unit-cell volumes of defective orthorhombic Al2W3O12 were larger, while their W-O bonds were weaker, likely leading to higher lattice flexibility and enhanced low-energy transverse acoustic modes. Extrinsic oxygen vacancies could be an additional mechanism for fine-tuning the intrinsic coefficients of thermal expansion in A2M3O12-type ceramics and in other framework structures built through two or threefold linkages.

Flexible ultrafine nearly stoichiometric polycrystalline SiC fibers with excellent oxidation resistance and superior thermal stability up to 1900 °C

Flexible ultrafine SiC fibers with superior high-temperature stability and excellent oxidation resistance are regarded as one of the most promising materials for high-temperature applications. However, excess oxygen and carbon in the ultrafine SiC fibers limit their thermal stability due to decomposition of the SiCxOy phase. In the present work, flexible ultrafine nearly stoichiometric polycrystalline SiC fibers were fabricated by combining the electrospinning technique and polymer-derived ceramic method. The ultrafine SiC fibers exhibited superior high-temperature stability and oxidation resistance. The retention rates of tensile strength were 90.0 %, 94.2 % and 86.4 % after heat treatment in argon at 1800 °C, 1900 °C and 2000 °C, respectively. TG results of the fibers showed little weight loss of only 1.52 % at 1900 °C in Ar and the weight gain of only 4.1 % up to 1500 °C in air. Such improved thermal stability was achieved through sintering at high temperature for elimination of excess oxygen and carbon with Al doped as the sintering aid to restrain the grain coarsening. The ultrafine SiC fibers still exhibited excellent flexibility without obvious damage when they were heated by the butane blowtorch flame of about 1100 °C in air. Furthermore, the infrared thermography illustrated that the ultrafine SiC fiber membrane also had good thermal insulation performance. The outstanding mechanical properties and thermal stability of ultrafine SiC fibers suggest their potential applications at the high temperature and harsh environment.

Enhancing Photocatalytic Hydrogen Production of g-C3N4 by Selective Deposition of Pt Cocatalyst.

Graphitic carbon nitride (g-C3N4) has been widely studied as a photocatalyst for the splitting of water to produce hydrogen. In order to solve the problems of limited number of active sites and serious recombination rate of charge-carriers, noble metals are needed as cocatalysts. Here, we selectively anchored Pt nanoparticles (NPs) to specific nitrogen species on the surface of g-C3N4 via heat treatment in argon–hydrogen gas mixture, thus achieving g-C3N4 photocatalyst anchored by highly dispersed homogeneous Pt NPs with the co-existed metallic Pt0 and Pt2+ species. The synergistic effect of highly dispersed metallic Pt0 and Pt2+ species makes the catalyst exhibit excellent photocatalytic performance. Under the full-spectrum solar light irradiation, the photocatalytic hydrogen production rate of the photocatalyst is up to 18.67 mmol·g−1·h−1, which is 5.1 times of the catalyst prepared by non-selective deposition of Pt NPs.

Microstructure evolution of Ag-Cu/BaCo0.7Fe0.2Nb0.1O3-δ joints in air and argon atmospheres

ABSTRACT Microstructure evolution of the joints formed by Ag-6.6 mol% Cu brazing BaCo0.7Fe0.2Nb0.1O3-δ (BCFN) ceramic after long-term heat treatment in different atmospheres was revealed. After heat treatment in air, the layered structure in the original interfacial reaction layer disappears, accompanied by the aggregation and growth of the interfacial products. The thickness of the reaction layer increased first and then stabilized. Under the action of the concentration gradient, the residual copper oxide diffused along the pores of BCFN matrix and continued to react with the matrix until it exhausted. The types of interfacial products remained unchanged, which were Ba2Cu2O5 and Co-Cu-O (solid solution). However, serious decomposition of interfacial products (Ba2Cu2O5) occurred due to the low oxygen partial pressure after heat treatment in argon, resulting in the formation of holes and cracks. Part of cobalt precipitated from the BCFN matrix and formed a cobalt-rich phase at the junction of Ag-Cu alloy and interfacial reaction zone. Results showed that the joint microstructure is unstable at high temperature. Controlling the amount of the interfacial products formed during brazing is key to improving the stability of the joint, which provides a theoretical support for further optimizing the sealing quality of membrane components.

Porous Structure Influence of Coconut Carbon Sorbents and Acidity on the Sorption of Some Biologically Active Organic Compounds

The paper investigates the regulation of the hydrophobicity of KARBON™ carbon adsorbents by restoring its surface during heat treatment in argon and hydrogen, as well as oxidizing it with nitric acid. The sorption capacity of KARBON™ with respect to the tryptophan, arginine, indole, creatinine and vitamin B12 as biologically active organic compounds of different molecular weight was studied. It was determined that the adsorption capacity of KARBON™ samples with respect to all the studied substances changes as follows: Ind>Cr>Trp>Arg>B12. It has been shown that the sorption capacity of argon-treated KARBON™ adsorbent at pH=7.5 is 2.6; 2.7 and 0.09 mmol/g in relation to tryptophan, creatinine and vitamin B12, respectively. The adsorption capacity of these compounds is almost unchanged when the acidity changes from 7.5 to 2.0. The results obtained allows the use of KARBON™ adsorbent as a therapeutic and/or prophylactic agent for oral administration in chronic kidney and liver diseases, as well as a hemosorbent for the purification of blood outside the human body.

Microstructures of a SiC–ZrC Ceramic Fiber Derived from a Polymeric Precursor

Continuous ceramic fiber comprising silicon carbide–zirconium carbide (SiC–ZrC) binary phases was obtained through melt spinning, electron-beam curing and pyrolysis of a pre-ceramic precursor of polyzirconocenecarbosilanes (PZCS). After pyrolysis and heat treatment, ZrC particles with mean diameters of 15–20 nm were formed and homogeneously dispersed in a matrix of fine crystalline β-SiC with an average grain size of 6–10 nm. Concentration of Zr in the fiber varies from 14.88% to 17.45% by mass. Fibers consisting of near-stoichiometric ZrC and SiC with little free carbon can be obtained through pyrolysis decarbonization of the as-cured fiber in hydrogen from room temperature to 1000 °C, and subsequently heat treatment in argon up to 1600 °C for 1 h. High-temperature treatment of these amorphous inorganic fibers leads to crystallization of the binary phases of β-SiC and ZrC. The removal of free carbon under hydrogen results in more rapid growth of β-SiC and ZrC crystals, in which obvious aggregation of the dispersed ZrC particles among the continuous β-SiC matrix can be ascribed to a fast migration of Zr cation.

Influence of Argon Heat Treatment on Structure, Critical Temperature, Magnetic Shielding and Irreversibility Line of Ln(SrBa)Cu3O6+z(Ln = Y, Eu)

Influence of Argon Heat Treatment on Structure, Critical Temperature, Magnetic Shielding and Irreversibility Line of Ln(SrBa)Cu3O6+z(Ln = Y, Eu)

Recovery of Adsorption Properties of Shungite

The condition of the surface of natural mesoporous carbonaceous material shungite has been investigated using the electron microscopy, X-ray phase and microspectrum analyses. The ability to recover the adsorption properties of shungite in respect to the extraction of higher alcohols and other impurities from water-alcohol solutions was investigated by the methods of thermogravimetry and IR spectroscopy. The main distinction of absorption spectra of used-up shungite with respect to the initial sample is the absence of absorption by hydroxyl groups (3380 cm−1) and significant reduction of the contribution of component at 1170 cm−1 that corresponds to absorption of phenol groups. It is shown that the heat treatment in argon, with water vapor and in the CO2 medium results in almost complete removal of sorbed organic substances and the regeneration of adsorption properties of shungite, the latest technique is more economically justified.

Lattice Deformation and Improvement Oxidation Resistance of Ti-6Al-4V Alloy Powders Prepared by Hydrogen Added Argon Heat Treatment

In the present work, a new hydrogen added argon heat treatment process that prevents the formation of hydrides and eliminates the dehydrogenation step, is developed. Dissolved hydrogen has a good effect on sintering properties such as oxidation resistance and density of greens. This process can also...

Surface Characterization of Depleted Uranium–Molybdenum to Determine Surface Coating Compatibility

Scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy have been used to characterize the surface of depleted uranium molybdenum (DU-Mo) alloys as a chemical surrogate to determine potential challenges with the surfaces of manufactured and stored U-Mo foils and powders. Even when stored and shipped in an inert atmosphere, U-Mo has a tenacious surface contamination of oxygen and carbon. The 8 at. % molybdenum (DU-8Mo) powder and 10 at. % molybdenum (DU-10Mo) foil samples have surface contamination of oxygen and carbon in different ratios that is hundreds to thousands of nanometers thick. The DU-8Mo powder sample has been stored in an inert atmosphere and as a result has a lower carbon-to-oxygen ratio at the surface than the DU-10Mo foil sample that was stored in air. This surface contamination has not been removed by up to 20 min of argon ion sputtering nor with 5% hydrogen in argon heat treatment for up to 96 h at 950°C.

Microstructural Evolution and Mechanical Behavior of Heat Treated Ti-6Al-4V Powders

Plasma-atomized Ti-6Al-4V powder is widely used in a variety of powder-based manufacturing technologies due to its attractive properties. In many cases there is a desire to modify the microstructure of the as-atomized powder via heat treatment, to improve manufacturing characteristics (e.g., deformability in the case of cold spray deposition). Microstructure characterization of as-received and heat treated plasma-atomized Ti-6Al-4V powder was performed using optical microscopy and x-ray diffraction. Additionally, heat treated powder was characterized using back-scatter electron imaging and energy-dispersive x-ray spectroscopy. The as-received powder was characterized by martensitic alpha grain structures, while the heat treated powder was characterized by either equiaxed alpha or lamellar alpha phase grains, with intergranular beta phase structures. Bulk Ti-6Al-4V was heat treated similar to powders to establish a control, for effective comparison. Microstructure characterization of bulk Ti-6Al-4V using optical microscopy revealed a bright oxygen-rich region, also known as alpha case, near the surface with microstructures resembling heat treated powder. Hardness of the powder and bulk sample was evaluated using Vickers microhardness testing. The hardness values for the alpha case in bulk Ti-6Al-4V correlated with the hardness of heat treated powder and were twice as hard when compared to as-received powder. Overall, this study demonstrated the conversion of powders to alpha case, even during heat treatment in argon with close oxygen control.

Bio-inspired synthesis of 3-D network of NiO-Ni nanowires on carbonized eggshell membrane for lithium-ion batteries

Composite nanowires incorporating both metal oxide and metal nanoparticles as the basic building units are attracting much attention recently. We report here a facile approach to synthesize 3D networks of 1D nanowires of formed by both NiO and Ni nanoparticle. The composite nanowires were in-situ synthesized on carbonized eggshell membrane. Here, α-Ni(OH)2 nanowire precursor was synthesized on eggshell membrane under mild conditions in a diaphragm-assisted reactor first. Then the precursor was transformed into composite of NiO-Ni nanowires on carbonized eggshell membrane under heat treatment in argon. The eggshell membrane played multiple roles as the substrate to collect the α-Ni(OH)2 nanowire precursor and then as the source of reducing agents to partially reduce Ni2+ to Ni0. When tested for lithium-ion batteries, the composite of 3D networks NiO-Ni nanowires attached on carbonized eggshell membrane demonstrated impressively improved rate and cycling performances, as compared to that of bare NiO nanowires. The idea and method outline here based on bio-inspired synthesis using bio-template to prepare composites could be applied to synthesize other functional materials.

Effect of CdCl2 heat treatment in (Ar + O2) atmosphere on structural and optical properties of CdTe thin films

Cadmium telluride (CdTe) thin films were deposited by thermal evaporation method under base pressure 2 × 10−5 mbar on corning glass substrate. In this work, we study the effect of cadmium chloride (CdCl2) heat treatment in argon and oxygen (Ar + O2) atmosphere on the structural, morphological and optical properties of CdTe thin films to achieve high quality thin film absorber layer for solar cells applications. The structural characteristics were studied by X-ray diffraction (XRD). The microstructure parameters of the films such as average grain size (Davg), average lattice strain (εavg) and dislocation density (ρD) have been calculated via XRD line broadening analysis. XRD investigation revealed that, the samples show polycrystalline nature pronounced with cubic zinc blende structure with a strong preferentially (1 1 1) texture orientation. It has been found that the crystallinity of the films enhanced during (Ar + O2) annealing and CdCl2 heat treatment. The surface morphology of CdTe thin films was investigated by field emission scanning electron microscope (FE-SEM). XRD and FE-SEM results of CdCl2 treated films showed recrystallization and progressive increase in grain size. The optical properties of all samples were estimated using UV–vis–near-infrared (NIR) spectrophotometer. The Swanepoel envelope method has been employed to evaluate the various optical parameters of CdTe films such as refractive index and film thickness. The as-deposited films have a relatively high value of refractive index of 2.54, in contrast the CdCl2 treated films have a relatively low value of 2.1 in the whole wavelength range. The optical energy gap values slightly decrease from 1.56 eV for as-deposited films up to 1.54 eV for CdCl2 heat treated films. CdCl2 heat treatment in (Ar + O2) atmosphere is found to be a promising method to improve the physical properties of CdTe thin film solar cells.

Preparation and characterization of near-stoichiometric silicon carbon fibres.

Near-stoichiometric SiC fibres (CVC-S fibres) were successfully prepared by pyrolysing chemical-vapour-cured polycarbosilane fibres under hydrogen and subsequent heat treatment in inert atmosphere at 1500 °C. The composition and properties of the obtained fibres were determined by Auger electron spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, tensile strength testing, and X-ray diffraction analysis. The results reveal that the CVC-S fibres with C/Si of 1.06 and O of 1.11 wt% have high tensile strength (2.6 GPa), high tensile modulus (321 GPa), and a highly crystalline structure (crystallite size, ∼14 nm.) In addition, the high-temperature behaviour of the CVC-S fibre was also investigated via heat treatment in argon and air at different temperatures. In particular, the fibres retain ∼2.2 GPa of their original strength after heating at 1600 °C for 1 h under argon. When argon was replaced by air, the tensile strength of the fibres could still be maintained at ∼1.1 GPa after annealing at 1400 °C. The more economical and practical approach along with the excellent performance of the obtained fibres render the CVC-S fibres promising materials for high-temperature applications.

Unusual Spinel-to-Layered Transformation in LiMn2O4 Cathode Explained by Electrochemical and Thermal Stability Investigation.

Distorted surface regions (5-6 nm) with an unusual layered-like structure on LiMn2O4 cathode material were directly observed after it was cycled (3-4.9 V), indicating a possible spinel-to-layered structural transformation. Formation of these distorted regions severely degrades LiMn2O4 cathode capacity. As we attempt to get a better understanding of the exact crystal structure of the distorted regions, the structural transformation pathways and the origins of the distortion are made difficult by the regions' nanoscopic size. Inspired by the reduction of Mn4+ to Mn3+ in surface electronic structures that might be associated with oxygen loss during cycling, we further investigated the atomic-level surface structure of LiMn2O4 by heat-treatments between 600 and 900 °C in various atmospheres, finding similar surface spinel-to-layered structural transformation only for LiMn2O4 heat-treated in argon atmosphere for a few minutes (or more). Controllable and measurable oxygen loss during heat-treatments result in Mn3+ for charge compensation. The ions then undergo a disproportionation reaction, driving the spinel-to-layered transformation by way of an intermediate LiMn3O4-like structure. The distortion of the surface regions can be extended to the whole bulk by heat-treatment for 300-600 min, ultimately enabling us to identify the bulk-level structure as layered Li2MnO3 (C2/m). This work demonstrates the critical role of Mn3+ in controlling the kinetics of the structural transformation in spinel LiMn2O4 and suggests heat-treatment in argon as a convenient method to control the surface oxygen loss and consequently reconstruct the atomic-level surface structure.