Heating In Air Research Articles

Laser‐induced oxidation of UO2: A Raman study

AbstractA reliable method to probe and characterise the oxidation of actinide oxides by means of Raman spectroscopy is introduced. The present so‐called Raman laser heating method enables studying the behaviour of various compounds at high temperatures and under a given atmosphere with the unique alteration of a small amount of sample, which is certainly advantageous in terms of safety when handling hazardous or radioactive materials. The approach is based on a dual use of the laser beam, which is at the same time employed as excitation source for the Raman analysis and as heating source, by exploiting the possibility to vary the beam power density reaching the sample surface. A high laser power density can lead to a significant increase of the analyte surface temperature by up to several hundred of degrees. A sufficiently low power density allows us to subsequently acquire the corresponding Raman spectrum at the same point without distorting the measurement. In this work, UO2 powder has been subjected to Raman laser heating in air as a proof of this method's applicability, attaining a sequential acquisition of the characteristic Raman spectra of the different oxides involved in the oxidation from UO2 to U3O8. The temperature at which such sequence of phase transformations started to occur was estimated to be around (560 ± 40) K. The temperature at the sample surface was estimated from the Stokes/anti‐Stokes intensities ratio, using a similar set‐up to that used in the Raman laser heating experiments. These results are particularly appealing in remote analyses, like those required in the study of nuclear fuel and nuclear waste.

Optical emission spectroscopy diagnosis of energetic Ar ions in synthesis of SiC polytypes by DC arc discharge plasma

Silicon carbides are basilic ceramics with proper bandgaps (2.4–3.3 eV) and unique optical properties. SiC@C monocrystal nanocapsules with different morphologies, sizes, and crystal types were synthesized via the fast and facile direct current (DC) arc discharge plasma method. The influence of Ar atmosphere on the formation of nanocrystal SiC polytypes was investigated by optical emission spectroscopy (OES) diagnoses on the arc discharge plasma. Boltzmann’s plot was used to estimate the temperatures of plasma containing different Ar concentrations as 10,582 K (in 2 × 104 Pa of Ar partial pressure) and 14,523 K (in 4 × 104 Pa of Ar partial pressure). It was found that higher energy state of plasma favors the ionization of carbon atoms and promotes the formation of α-SiC, while β-SiC is generally coexistent. Heat-treatment in air was applied to remove the carbon species in as-prepared SiC nanopowders. Thus, the intrinsic characters of SiC polytypes reappeared in the ultraviolet–visible (UV–vis) light absorbance. It was experimentally revealed that the direct bandgap of SiC is 5.72 eV, the indirect bandgap of β-SiC (3C) is 3.13 eV, and the indirect bandgap of α-SiC (6H) is 3.32 eV; visible quantum confinement effect is predicted for these polytypic SiC nanocrystals.

Scalable Fabrication of Supercapacitors by Nozzle-Free Electrospinning

Nozzle-free electrospinning was investigated as a facile technique for producing nanoscale materials for supercapacitors. MnO2 nanofibers and their composites with multiwalled carbon nanotubes (MWCNTs) were synthesized in a single step, using polyvinylpyrrolidone (PVP) and Mn(CH3COO)2·4H2O as starting materials, followed up by heat treatment in ambient air. Nanofibers of relatively uniform diameter were produced at high rates. The nanofibers exhibited good electrical contact between MnO2 and MWCNT without degradation, which was attributed to the improved stability of MWCNTs in PVP solutions. Electrochemical testing of the composites demonstrated a high capacitance of 1.43 F cm–2 at a scan rate of 100 mV s–1 in the three-electrode tests. Electrodes and devices produced by nozzle-free electrospinning are promising for practical energy storage applications.

Effect of the Reaction Medium on the Mechanochemical Synthesis of LiAlO2

An unactivated mixture of aluminum hydroxide (gibbsite) and lithium carbonate and a mixture mechanically activated in an AGO-2 planetary mill were heat-treated in air and under vacuum (0.05 Pa) at temperatures of up to 800–850°C, and the phase composition of the heat treatment products was determined by in situ X-ray diffraction. The results demonstrate that, in the case of the unactivated mixture or after mechanical activation for 1 min, heat treatments in air and under vacuum lead to the formation of α-LiAlO2 and γ-LiAlO2, respectively. After mechanical activation for 5 or 10 min, heat treatment both in air and under vacuum leads to γ-LiAlO2 formation. A possible mechanism behind the effect of the reaction medium on the phase composition of LiAlO2 is analyzed.

Characterization of tea polyphenols as potential environment-friendly fire retardants

In this work we investigated the oxidation properties of tea polyphenols and their potential as the fire retardants. Two types of tea polyphenols were adopted, which were extracted from red tea and green tea leaves, respectively. Their macroscopic performance during pyrolysis and oxidation at elevated temperatures were examined by using a heating furnace. Mass change, heat evolution and gas products of tea polyphenols during heating in air were also monitored by using a thermo-gravimetric analyzer (TGA) integrated with a differential scanning calorimeter (DSC) in conjunction with online Fourier Transform Infrared Spectroscopy (FTIR) and mass spectroscopy (MS). A tea polyphenol sample first becomes a brown semi-fluid after heating, and gradually turns into highly-porous black chars with significantly expanded volume. By raising the temperature to ∼550 °C at a rate of 10 °C/min, the mass of a sample reduces by nearly 70% to form a large quantity of inert gases that are mainly composed of H2O and CO2. It was found that the aerial oxidation products of tea polyphenols in the solid phase possess good heat insulation property; meanwhile, the substantial release of a lot of water and its evaporation during oxidation of tea polyphenols removes a large amount of heat from a sample located in a heating environment. The heat insulation of tea polyphenols may withstand up to 550 °C. The present work confirms tea polyphenols as potential superior and environment-friendly fire retardants.

Characteristics and properties of a magnetron sputtered gadolinia-doped ceria barrier layer for solid oxide electrochemical cells

Gadolinia-doped ceria (GDC) films are synthesized by direct current magnetron sputtering under reactive conditions for hydrogen production by high temperature electrolysis (HTE). First, the sputtering process is investigated by varying the reactive gas flow rate. Working conditions in elemental sputtering mode (ESM) are adjusted to ensure a high incorporation of oxygen while keeping a high deposition rate. Heat treatments in air are found necessary to stabilize the sub-stoichiometric deposited oxide. The main characteristics of the films (crystallinity, chemical composition, porosity, etc.) are studied by Rutherford backscattering spectroscopy (RBS), optical transmission spectroscopy, Raman spectroscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and scratch test and are compared to that of a conventional 2 μm thick GDC layer deposited by screen printing. The sputtered layer appears dense and adherent but kind of blisters are observed when deposited on half-cells. Performances of a thin GDC barrier layer (below 500 nm) sputter deposited on half cells are evaluated. Encouraging results are obtained since performances (i.e. about −1.70 A·cm−2 at 1.3 V and 800 °C in HTE mode) are slightly higher to that typically achieved with a classical manufactured cell (i.e. about −1.65 A·cm−2 at 1.3 V and 800 °C). A degradation rate of about 9% for 1000 h operation (at 800 °C with a current density of −0.5 A·cm−2, GDC layer thickness of 425 nm) is achieved.

Hierarchically nanostructured porous TiO2(B) with superior photocatalytic CO2 reduction activity

Hierarchically nanostructured, porous TiO2(B) microspheres were synthesized by a microwave-assisted solvothermal method combined with subsequent heat treatment in air. The materials were carefully characterized by scanning and transmission electron microscopy, X-ray diffraction, CO2 adsorption, and a range of spectroscopies, including Raman, infrared, X-ray photoelectron and UV-Vis spectroscopy. The hierarchical TiO2(B) particles are constructed by ultrathin nanosheets and possess large specific surface area, which provided many active sites for CO2 adsorption as well as CO2 conversion. The TiO2(B) nanostructures exhibited marked photocatalytic activity for CO2 reduction to methane and methanol. Anatase TiO2 and P25 were used as the reference photocatalysts. Transient photocurrent measurement also proved the higher photoactivity of TiO2(B) than that of anatase TiO2. In-situ infrared spectrum was measured to identify the intermediates and deduce the conversion process of CO2 under illumination over TiO2(B) photocatalyst.

Novel multifunctional cheese-like 3D carbon-BN as a highly efficient adsorbent for water purification

In this paper, a novel three dimensional carbon boron nitride (3D C-BN) was successfully prepared. The obtained material has porous cheese-like structure and pore size ranging from 2 nm to 100 nm. Attractively, the 3D C-BN, which combines the adsorption advantages of BN and carbon together, exhibits excellent adsorption properties for organic dyes, oils and heavy metal ions. The maximum removal capacities of 3D C-BN for methyl blue (MB) and congo red (CR) are 408 mg g−1 and 307 mg g−1, respectively. Furthermore, 3D C-BN can quickly and efficiently remove oils (salad oil, gasoline and pump oil) and heavy metal ions (Cr3+, Cd2+ and Ni2+) from waste water. The macro bulk 3D C-BN, which is more convenient to use than powdered materials, can be reused by burning or heating in air and still maintains high adsorption capacity. Significantly, these superior performances can find practical application in water purification.

Study of the air-formed oxide layer at the copper surface and its impact on the copper corrosion in an aggressive chloride medium

The electrochemical behaviour of pre-oxidized copper surfaces prepared by heating in air at 90 °C was studied to evaluate the impact of the air-formed oxide layer at the copper surface on the corrosion in an aggressive chloride medium. In a first part, the evolution of the chemical composition and the structure of the air-formed oxide layer with the heating time were investigated by cyclic voltammetry in an alkaline medium in which the different oxide species can be discriminated, X-ray photoelectrons spectroscopy experiments (XPS) and water contact angle measurements. It was demonstrated that short heating times produce Cu2O-rich films, while CuO-rich films were obtained after long heating-time periods. Furthermore, cyclic voltammetric data strongly support a stratification of the corrosion products in the mature oxide layer. In a second part, the protection efficiency of air-formed films, aged from 1 to 22 days, against the copper corrosion was study using a potentiodynamic technique and the electrochemical impedance spectroscopy (EIS) in an aerated 0.2 M NaCl aqueous electrolyte. Results demonstrate that the chemical composition of the air-formed oxide layer has an important impact in the overall corrosion process. Results are tentatively explained in terms of composition and structure of the oxide layer.

Ignition of liquid droplets fuels under conditions of radiation-conductive heating in air

The results of an experimental investigation of ignition liquid fuel (kerosene, diesel fuel) by the single drops under conditions of radiation-conducting heating in air are presented. The dependences of the ignition delay time of the typical fuels droplets on the initial temperature of the heated oxidant in a limited space are established. The parameters of stable ignition of liquid fuel droplets are set.

Oxidation of ASD-4 Powder Modified by V2O5

The sequence of phase formation in the oxidation of ASD-4 aluminum powder modified by vanadium pentoxide during heating in air in the temperature range 873–1073 K has been studied by synchrotron radiation x-ray diffraction. It has been shown that the sharp acceleration of the oxidation of the modified powder is related to the loss of the protective properties of the oxide shell on the particle surface due to the polyvalence of vanadium, which provides structural and phase changes on the surface and in the depth of the oxidized metal.

Optoelectronic Characterization of ZnO Nanorod Arrays Obtained by Pulse Electrodeposition

ZnO nanorod arrays have been grown by potentiostatic pulse electrodeposition between a reduction potential and a “rest” potential. The effect of the duty cycle and pulse frequency as well as the heat-treatment in air on the properties of the electrodeposits has been studied. Surface morphology, structural, optical and electrical properties were evaluated. Absorption spectra reveal a high energy bandgap Burstein-Moss shift for the as-grown nanorods, in line with the donor density (1.1 × 1019 and 9.5 × 1019 cm−3) determined from electrochemical impedance spectroscopy. After annealing, the carrier concentration decreases to 1017―1018 cm−3, which is accompanied by an increase of the optical quality of the samples, assessed by the narrowing of the full width at half maximum of the near band edge recombination and steeper absorption at ∼3.3 eV. The donor density and the flatband potential are dependent of the applied duty cycle and pulse frequency. All the analyzed samples evidence deep broad emission bands in the visible region, whose intensity is enhanced after annealing. The defect luminescence is due to an overlap of emitting centers in the red, yellow and green spectral regions, as evidenced and discussed by comparing the steady-state and transient spectroscopies.

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.

Production and Characterization of Magnesium‐Doped Copper Chromite Fibers

In this study, Mg‐doped CuCr1−xMgxO2 (x = 0, 0.01, 0.03, 0.05) fibers are synthesized via electrospinning process followed by a two‐step heat treatment in air and Ar atmospheres. The produced samples are all crystalline with a fiber diameter ranging from 120 to 140 nm. The effect of Mg2+ substitution for Cr3+ on the structural, morphological, and optical properties of CuCrO2 fibers is investigated systematically. Structural characterizations indicate that the solubility limit of Mg2+ is around 3 at.% and beyond this concentration the formation of MgO phase is observed. This also leads to the formation of Cu cations with (+2) oxidation state. In addition, the incorporation of Mg into the CuCrO2 lattice hinders the formation of perfect hexagonal‐shaped isolated grains and contribute to the preservation of fiber morphology after the final sintering step. Further, the optical transmittance in the visible range decreases with increasing Mg‐doping ratio. The corresponding optical band gaps are determined to be 3.53, 3.47, 3.30, and 3.22 eV, respectively, for x = 0, 0.01, 0.03, 0.05.

Structural stability of NaPON glass upon heating in air and nitrogen

The thermal stability in air and nitrogen of an oxynitride NaPON glass with high nitrogen content (N/P=0.5) has been investigated with regards to its structural evolution with temperature. The glass transition temperature (Tg) of the powdered glass is found to decrease upon oxidation, especially when the treatment temperature (Ta) is larger than the Tg of the original oxynitride glass. Upon isothermal oxidation, crystalline metaphosphate forms at the interface of the oxide layer as the dominant phase in both the powder and bulk samples, as detected by Raman spectroscopy and X-ray diffraction. A new deconvolution scheme of Raman spectra is proposed, involving a structural model proposed to account for the in situ high temperature changes of the local structural groups. A distinction is made between different oxynitride Qn(P,N) tetrahedral sites, and two separate bands related to tri-coordinated nitrogen speciation (Nt) are distinguished in the oxidized NaPON glass. Nt groups are connected to either one oxygen or one nitrogen, resulting in two separate Raman bands. The position and area of these Nt-related peaks exhibit an opposite trend with temperature in air and N2. Furthermore, the Raman results imply a thermally driven depolymerization of the oxynitride sub-structure, which could involve a nano-scale phase separation of the nitrogen-involved structure network. In terms of technological applications, this work suggests that the oxynitride glasses should be used in the temperature range up to the glass transition temperature, above which the structural stability is lost.

Electrochemical Deposition of Transparent p‐Type Semiconductor NiO

SUMMARYNiO is a p‐type semiconductor having a large band gap (>3 eV). In this study, thin films containing Ni‐O were deposited by the cathodic electrochemical deposition method, and characteristics change by heat treatment was investigated. An aqueous solution containing Ni(NO3)2 was used as a deposition solution. X‐ray photoelectron spectroscopy showed that the sample before annealing was predominantly Ni(OH)2. After heat treatment in air at temperatures higher than 300 °C, the NiO phase was observed by X‐ray diffraction, and the p‐type response was confirmed by the photoelectrochemical measurement. The band gap obtained from the light transmittance measurement was around 3.5 eV.

Magnetic and structural properties of Co doped ZnO nanowires prepared by heat treatment of electrospun PVA nanofibers containing Zn and Co acetates

We report the synthesis of inter-twined ZnO nanowires doped with Co by electrospinning polyvinyl alcohol along with metal oxide precursors followed by heat treatment in air at 550 °C for 90 min. This heat treatment removed PVA from the samples and ensured crystallization of 1-dimensional metal oxide nanostructure. X-ray diffraction (XRD) patterns also confirmed single phase wurtzite-type ZnO structure in samples with 3 wt% Co. Traces of Co3O4 could be observed in XRD patterns of samples with 5 wt% Co. However, micro-Raman spectra clearly indicate the presence of Co3O4 in samples with Co content more than 2 wt% which is also supported by electron paramagnetic resonance studies. UV–Vis absorption studies showed an increase in band gap energy with substitution of Co in ZnO structure. Field dependent magnetization curves recorded at room temperature revealed weak room temperature ferromagnetism with considerable paramagnetic component. The results are used to understand the origin of magnetism in this alloy and the extent of possible doping of Co in ZnO nanofibers.

Electrochemical Tuning of Magnetism in Ordered Mesoporous Transition-Metal Ferrite Films for Micromagnetic Actuation

Controlling magnetism in situ and in a reversible manner using battery redox chemistry is a new concept in the wider field of magnetoelectrics, and electrochemical lithium tuning of electrode materials is one such approach. Here we show that the general idea of combining nanomagnetism and electrochemical energy storage concepts can be applied to ordered mesoporous spinel ferrite thin films. Via an evaporation-induced self-assembly method, a series of cobalt and nickel ferrites (CoFe2O4, Co0.5Ni0.5Fe2O4, and NiFe2O4) were prepared and characterized by electron microscopy, grazing-incidence small- and wide-angle X-ray scattering, 57Fe Mossbauer spectroscopy, and magnetometry. The dip-coated films after heating in air to temperatures beyond 600 °C are single phase with partially inverted spinel structures and exhibit 15–20 nm pores of oblate spheroidal shape arranged on a cubic lattice. Magnetic measurements clearly show stable ferrimagnetic ordering at room temperature, especially for the cobalt-based mater...

Oxidation of Zirconium and Zirconium Hydride Powders during Programmed Heating in Air

The data on oxidation of zirconium and zirconium hydride powders during heating in air to 1373K at a rate of 10K/min were obtained using thermogravimetry (TG), differential scanning calorimetry (DSC) and mass spectrometry of reaction products. The specific heat release during oxidation of hydride powder was found to decrease due to the endothermic character of the decomposition reaction and the fact that the hydrogen combustion reaction takes place in the gaseous phase. The phase formation features in the processes of powders oxidation, their connection with the metal and hydride reactivity and the morphology of the reaction products were determined by the X-ray diffraction method.

Ribbon-like Cu doped V6O13 as cathode material for high-performance lithium ion batteries

Ribbon-like Cu doped V6O13 was synthesized via a simple solvothermal approach followed by heat treatment in air. As an cathode material for lithium ion battery, the ribbon-like Cu doped V6O13 electrode exhibited good capacity retention with a reversible capacity of over 313 mAh·g−1 for up to 50 cycles at 0.1 C, as well as a high charge capacity of 306 mAh·g−1 at a high current rate of 1 C, in comparison to undoped V6O13 electrode (267 mAh·g−1 at 0.1C and 273 mAh·g−1 at 1 C). The high rate capability and better cycleability of the doped electrode can be attributed to the influence of the Cu ions on the mophology and the electronic conductivity of V6O13 during the lithiation and delithiation process.