Reduction Annealing Research Articles

A novel ceramic derived processing route for Multi-Principal Element Alloys

In recent years, Multi-Principal Element Alloys (MPEAs) have emerged as a new and exciting class of materials. To date, the most widely used processing methods for MPEAs are arc melting and mechanical alloying. The study explores the feasibility of an alternative, novel process to process bulk MPEA samples, namely the reduction of an oxide powder precursor mix. Equiatomic CuCoFeNi was selected as a model system of study because these elements form a subset of a large proportion of MPEA compositions already studied. High purity precursor oxide powders of Co3O4, CuO, Fe2O3 and NiO were milled and mixed using standard ceramic processing methods. The green pellets were subjected to a reduction annealing treatment in flowing 5%H2–N2 at 1000 °C, with heat-treatment times ranging from 4 to 1000 h. After 4 h, the sample was transformed into a dense metallic bulk sample with no residual oxides. The microstructure was characterized by x-ray EDS, SEM and XRD, and was shown to consist of a polycrystalline matrix, and a copper-rich second phase situated at the triple points. The hardness was studied by nano-indentation. Using EPMA (electron microprobe analysis), the composition of the matrix was determined to be Cu18.1Co26.6Fe26.9Ni28.4 (at%), which represents the single phase MPEA composition. Oxide reduction is a viable technique for processing bulk complex metallic alloys, with potential for novel microstructural tailoring.

Synchrotron X-Ray Absorption Spectroscopy (XAS) Studies on Structural and Magnetic Properties of T’-Pr<sub>2-x</sub>Ce<sub>x</sub>CuO<sub>4</sub> Nanocrystals

We have succeeded in synthesizing electron-doped cuprates T’-Pr2-xCexCuO4 (PCCO) with x = 0 and 0.10 nanocrystals prepared by the chemically dissolved method. Reduction annealing of the PCCO samples at 700°C under a flowing argon gas atmosphere has been performed for the removal of excess oxygen in the apical sites. The XRD data showed that the reduction annealing process decreases c-axis length indicating successful removal of the excess oxygen. The bond distortion of PCCO including coordination number and bond distance between the absorber atoms with the nearest neighboring atoms (Cu-O) was investigated by extended x-ray absorption fine structure (EXAFS) using Cu K-edge. The implication of our results is discussed on the basis of tremendous influence of oxygen vacancies on the magnetism of the nanosized T’-cuprates at the normal state.

Effect of intercritical annealing with high cooling rate associated with cold deformation and subcritical annealing on microstructure and mechanical properties of SAE 52100

Conventional spheroidization heat treatments of bearing steels are long-term, therefore it is necessary to study alternatives to decrease the heat treatment time to save energy consumption. This work aimed to develop a new manufacturing route in order to obtain a completely spheroidized microstructure. The microstructure and mechanical properties of SAE 52100 steel were evaluated after intercritical annealing with higher cooling rate than the conventional one, followed by cold deformation with different area reductions, and subcritical annealings with different soaking times. It has been observed that the route considering the intercritical annealing with cooling rate 8 times higher than the conventional one, followed by a high cold reduction and subcritical annealing for 1 h turns feasible the complete spheroidization of the material.

Low Resistance TiO2/p-Si Heterojunction for Tandem Solar Cells.

Niobium-doped titanium dioxide (Ti1−xNbxO2) films were grown on p-type Si substrates at low temperature (170 °C) using an atomic layer deposition technique. The as-deposited films were amorphous and showed low electrical conductivity. The films became electrically well-conducting and crystallized into the an anatase structure upon reductive post-deposition annealing at 600 °C in an H2 atmosphere for 30 min. It was shown that the Ti0.72Nb0.28O2/p+-Si heterojunction fabricated on low resistivity silicon (10−3 Ω cm) had linear current–voltage characteristic with a specific contact resistivity as low as 23 mΩ·cm2. As the resistance dependence on temperature revealed, the current across the Ti0.72Nb0.28O2/p+-Si heterojunction was mainly determined by the band-to-band charge carrier tunneling through the junction.

Chemical synthesis and coercivity enhancement of Nd2Fe14B nanostructures mediated by non-magnetic layer

High-performance Nd2Fe14B magnets have been widely required in various fields recently due to the lightweight and miniaturization of devices. In this work, we synthesize Nd2Fe14B nanostructures with tunable magnetic properties through surfactant-assisted high energy ball milling (SAHEBM) process, achieving prominently enhanced coercivity by forming non-magnetic layers as grain boundary phase. When the reduction annealing process was carried out as pellet with Ca, the coercivity increased from 0.8 kOe to ver 3 kOe as Nd2Fe14B powder, which is proved to be the contribution of the chemical diffusion of Nd elements and the formation f Nd-rich layer as magnetic insulating medium. In addition, two-dimensional graphene oxide (GO) was employed to build extra grain boundary, by which the coercivity of the core@dual-shell structure can achieve up to 8 kOe, tenfold of the original sample. The intrinsic mechanism indicated that the Nd-diffusion induced Nd-rich phase along with the reduced GO in the system could form non-magnetic layer as grain boundary and magnetically isolate the adjacent grains, significantly enhancing the exchange coupling effect. This work markedly opens up an effective approach for the chemical preparation of high-performance Nd2Fe14B nanostructured magnets, especially after post treatment, and gives an insight on the interactions at nanoscale.

Noble-Metal-Free Ni-W-O-Derived Catalysts for High-Capacity Hydrogen Production from Hydrazine Monohydrate.

Development of active and earth-abundant catalysts is pivotal to render hydrazine monohydrate (N2H4·H2O) viable as a hydrogen carrier. Herein, we report the synthesis of noble-metal-free Ni-W-O-derived catalysts using a hydrothermal method in combination with reductive annealing treatment. Interestingly, the thus-prepared Ni-based catalysts exhibit remarkably distinct catalytic properties toward N2H4·H2O decomposition depending upon the annealing temperature. From a systematic phase/microstructure/chemical state characterization and the first-principles calculations, we found that the variation of the apparent catalytic properties of these Ni-based catalysts should stem from the formation of different Ni-W alloys with distinct intrinsic activity, selectivity, and distribution state. The thereby chosen Ni-W alloy nanocomposite catalyst prepared under an optimized condition showed high activity, nearly 100% selectivity, and excellent stability toward N2H4·H2O decomposition for hydrogen production. Furthermore, this noble-metal-free catalyst enables rapid hydrogen production from commercially available N2H4·H2O solution with an intriguingly high hydrogen capacity of 6.28 wt % and a satisfactory dynamic response property. These results are inspiring and momentous for promoting the use of the N2H4·H2O-based H2 source systems.

Influence of Reduction Annealing on the Characteristics of CeO<sub>2</sub> – YSZ Based Oxide Ceramics

Influence of Reduction Annealing on the Characteristics of CeO<sub>2</sub> – YSZ Based Oxide Ceramics

High Capacity Adsorption—Dominated Potassium and Sodium Ion Storage in Activated Crumpled Graphene

AbstractStructurally and chemically defective activated‐crumbled graphene (A‐CG) is employed to achieve unique synergy of large reversible potassium (K) and sodium (Na) ion storage capacity with fast charging and extended cyclability. A‐CG synthesis consists of low temperature spraying of graphene oxide slurry, followed by partial reduction annealing and air activation. For K storage, the reversible capacities are 340 mAh g−1 at 0.04 A g−1, 261 mAh g−1 at 0.5 A g−1, and 210 mAh g−1 at 2 A g−1. For Na storage, the reversible capacities are 280 mAh g−1 at 0.04 A g−1, 191 mAh g−1 at 0.5 A g−1, and 151 mAh g−1 at 2 A g−1. A‐CG shows a stable intermediate rate (0.5 Ag−1) cycling with both K and Na, with minimal fade after 2800 and 8000 cycles. These are among the most favorable capacity—rate capability—cyclability combinations recorded for potassium‐ion battery and sodium‐ion battery carbons. Electroanalytical studies (cyclic voltammetry, galvanostatic intermittent titration technique, b‐value) and density functional theory (DFT) reveal that enhanced electrochemical performance originates from ion adsorption at various defects, such as Stone–Wales defects. Moreover, DFT highlights enhanced thermodynamic stability of A‐CG with adsorbed K versus with adsorbed Na, explaining the unexpected higher reversible capacity with the former.

Synthesis of Magnetic Wires from Polyol-Derived Fe-Glycolate Wires.

Fe-glycolate wires with micrometer-scale lengths can be synthesized by the polyol process. Although the as-produced wires are in the paramagnetic state at room temperature, they are transformed into ferrimagnetic iron oxides and ferromagnetic metallic iron wires by reductive annealing. The shape of the wires is unchanged by reductive annealing, and it is possible to control the magnetic properties of the resulting wire-shaped ferri/ferromagnets by adjusting the annealing conditions. Consequently, the reductive annealing of polyol-derived Fe-glycolate wires is an effective material-processing route for the production of magnetic wires.

Дослідження методом рентгенівської фотоелектронної спектроскопії процесу синтезу ТіСпри відпалі ТiН2/ТiО2/С у вакуумі

Small dimensional transition metal carbides (MXenes) are promising materials for the development of photocatalysts and are highly efficient cocatalysts for industrial TiO_2 (P25). Thus, in the Ti_3C_2@TiО_2 nanocomposite obtained by layering Ti_3C_2 nanoplates, the ability to separate charge carriers increases due to the high electrical conductivity of TiC_{1-х}. The task of forming the TiC_{1-х}@TiО_{2-х} nanocomposite by direct synthesis with n-TiO_2 is promising, which allows to increase the quality of contact between the shell and the nanocomposite core and to reduce the number of intermediate stages of synthesis. In addition, highly dispersed TiC has high values of hardness, melting point, modulus of elasticity and shear and has the prospect of use in materials science in plasma spraying coatings. In work ТіС was synthesized on the surface of TiO_2 - the shell of the modified micropowder TiH_2/TiO_2/С during reductive annealing in vacuum using TiH_2 as a source of atomic hydrogen. After a series of annealing at 535 ºС - 600 ºС, the Ti2p- C1s- and O1s- spectra of surface atoms were obtained. The main stages of TiC synthesis in the TiO_2/С conversion reaction were established by the XPS method. The use of TiH_2 as a source of atomic hydrogen in nanosystems of the «core/shell» type is proposed for local synthesis on the surface of nanoobjects in a vacuum or inert atmosphere.

Evolution of microstructure, texture and inhibitor of medium temperature grain-oriented silicon steel

The medium temperature reheating and two-stage cold rolling process were adopted to produce industrialized Grain-oriented silicon steel. Results showed there were three sections (surface layer, transition layer and central layer) along the thickness direction on hot rolled sheets. Surface layer was occupied by complete recrystallization grains with average grain size 42.81 µm, while the central layer mainly consisted of fibrous microstructure. The Goss texture appeared only on surface layer with content 3.44%∼3.65%. After the first cold rolling with 72% reduction and decarburization annealing, the primary recrystallization occurred and average grains size reached 19.0 µm, simultaneously accompanied with texture rotated to λ, γ fiber texture and Goss texture dropped to 2.89%. Adopting the second cold rolling with 58% reduction, the cold rolled sheet consisted of deformed fiber microstructure and the texture maintained γ fiber texture with a peak at {111} <112>. The precipitates in hot rolled sheets comprised Cu2S and few AlN, and the average particle size was ∼14 nm. The inhibitors in the decarburization annealing sheet were mainly AlN, Cu2S, MnS and their composite precipitation, which were spherical or massive with the average particle size ∼21 nm. The single Goss oriented grains with average grain size 9.41 mm were obtained after high temperature annealing, and the intensity reached 873.30. The final magnetic properties were: B8 = 1.865 T and P1.7/50 = 1.124 W/kg, which met the requirement of 27Q120.

A unique synthesis of rare-earth-Co-based single crystal particles by "self-aligned" Co nano-arrays.

In this study, anisotropic SmCo5 magnets were prepared by a distinctive method, which is the high-temperature reductive annealing of Co@Sm2O3 with a specially designed nanostructure. High resolution transmission electron microscopy and elemental mapping show that the precursor self-assembly is composed of hcp-structured Co nano-rods with a coherent crystallographic orientation. During high temperature reduction, the Sm2O3 shell preserves the original morphology and alignment of these anisotropic Co nano-arrays, providing a template for hcp-structured SmCo5 single crystal particle synthesis. The as-prepared SmCo5 magnets exhibit well-controlled size and morphology, and a high coercivity of 30.9 kOe at room temperature. No stabilizer coating is necessary to prevent the formation of polycrystals in this synthesis.

Unusual properties and features of oxygen nonstoichiometry of La-Sr manganites with manganese replacement by a combination of nickel and germanium

When studying manganites of the system La1-cSrcMn1-x(Ni2+0.5Ge4+0.5)xO3+γ (x = 0.05; 0.10; 0.15; 0.15 ⩽ c ⩽ 0.35), it was experimentally established that after reductive annealing, which leads the oxygen content to the stoichiometric one, unit cell volume (v) of the samples decreases and at c ⩾ 0.19 it is less than after annealing in oxygen. Based on the analysis of various charge compensation mechanisms using derived dependences of v on the oxygen content and taking into account the changes in magnetic and electrical parameters, it is established that experimental data can be explained only with the allowance for formation of singly charged oxygen ions. The relationships between magnetic parameters of the manganites and their composition, structural characteristics and deviations from oxygen stoichiometry are explained. The possibility of local ferromagnetic ordering of uncompensated spins of Ni2+ ions due to their interaction with singly charged O– anions is considered.

Nature of Carrier Doping in T′-La1.8−xEu0.2SrxCuO4 Studied by X-Ray Photoemission and Absorption Spectroscopy

Recently, hole-doped superconducting cuprates with the T'-structure La1.8-xEu0.2SrxCuO4 (LESCO) have attracted a lot of attention. We have performed x-ray photoemission and absorption spectroscopy measurements on as-grown and reduced T0-LESCO. Results show that electrons and holes were doped by reduction annealing and Sr substitution, respectively. However, it is shown that the system remains on the electron-doped side of the Mott insulator or that the charge-transfer gap is collapsed in the parent compound.

Surface phosphorization of hierarchically nanostructured nickel molybdenum oxide derived electrocatalyst for direct hydrazine fuel cell

Non-faradaic catalytic decomposition of hydrazine (N2H4) is a vital but underappreciated problem for direct hydrazine fuel cell (DHFC). Herein, a careful study of a nickel molybdenum oxide derived catalyst for N2H4 electrooxidation is reported, with special focus on the method for effectively suppressing non-faradaic decomposition of hydrazine as well as the underlying mechanism. A hierarchically nanostructured catalyst consisting of tiny Ni10Mo alloy nanoparticles dispersed on porous NiMoO nanosheets is synthesized using a simple hydrothermal method followed by reductive annealing treatment. Thus-prepared catalyst shows high activity towards N2H4 electrooxidation, but is entangled with the problematic selectivity towards non-faradaic decomposition of N2H4. Our study find that this problem can be circumvented by a controlled phosphorization method. The resultant catalyst (Ni2P@Ni10Mo/Ni-Mo-O/NF) exhibits exceptionally high activity, excellent durability and nearly 100 % selectivity towards N2H4 electrooxidation. A detailed structural characterization and density functional theory calculations are conducted to understand the phosphorization-induced modification of catalytic property.

Trap‐Related Nonvolatile Negative Photoconductivity in a Single Ag@Al2O3 Hybrid Nanorod for a Photomemory with Light‐Writing and Bias‐Erasing

AbstractFor zero band‐gap metal Ag and ultrawide band‐gap Al2O3, it is difficult to produce impressive photoresponse to visible light due to the limitation of energy gaps. Herein, it is demonstrated that individual Ag@Al2O3 hybrid nanorods, synthesized by a two‐stage hydrothermal method and followed by thermal reduction annealing, can show excellent negative photoconduction of about 400 nm violet and 800 nm near‐infrared lights at room temperature. Moreover, the light‐induced high resistance state (HRS) is well maintained at relatively low operation bias after the removal of illumination, indicative of a nonvolatile memory effect. More importantly, the device is back to its initial low resistance state (LRS) after subsequently being applied a relatively large bias, suggestive of an erasable effect with large bias. In the hybrid nanorods, Ag nanoparticles serve as trap centers and can capture and store charges. Under the illumination of sub‐band‐gap light, trapped charges are excited, resulting in an HRS due to emptying the traps. On the contrary, a large external electric field triggers charges to be injected into traps in dark, resulting in an LRS. Regarding a superior negative photoswitching with light‐writing and bias‐erasing memory, Ag@Al2O3 nanohybrids have a tremendous potential in optical sensors and nonvolatile photomemory applications.

A taurine-functionalized 3D graphene-based foam for electrochemical determination of hydrogen peroxide

We present a synthesis approach of taurine-functionalized graphene foam (a-NSGF) using hydrothermal reduction, freeze-drying and high temperature annealing. The higher temperature in annealing allowed the N/S atoms of taurine enter into the graphene lattice, which improves its electrocatalytic activity greatly. The a-NSGF consisting of taurine that modified into 3D layers of graphene and endow is of the rapid sensitive to hydrogen peroxide (H2O2). The electrode using a-NSGF modification reveals highly sensitive and stable towards the concentration change of H2O2 due to the stable 3D structure and good electrical conductivity of a-NSGF. A linear correlation between H2O2 concentration and the electrochemical signal is found to be in a range from 1.5 to 300 μM and the correlation coefficient is R2 = 0.999. The modified electrode has been applied in the determination of H2O2 in rain samples and the results have been compared with the China National Standard Method. The recoveries range from 94.6% to 106.7%. These results show that the proposed sensor is promising for the development of novel electrochemical sensing for H2O2 determination.

Reductive-annealing-induced changes in Mo valence states on the surfaces of MoO3 single crystals and their high temperature transport

Reduction of MoO3 in extreme reducing condition is a way to achieve Mo metal. However, effect of less extreme reductive-annealing, where it allows to keep the crystal structure, on physical and chemical properties of MoO3 has not been well-studied. In this work, we studied the evolution of Mo valence state during reductive annealing and its effect on high temperature transport. We found the formation of oxygen vacancies on surface of MoO3 single crystals at the low temperature, which is evidenced by increase of Mo5+ and color change. In addition, formation of Mo4+ was at the elevated temperature. For understanding the relation between bulk conductivity and Mo valence state, real-time impedance spectroscopy is employed. Use of two different gases makes it possible to distinguish impedance responses of MoO3 from those of reduced MoO3-x. Also, from time-dependent impedance measurements, we observed the evolution of transport behaviour by evolution of Mo valence state.

D -wave superconducting gap observed in protect-annealed electron-doped cuprate superconductors Pr1.3−xLa0.7CexCuO4

For electron-doped cuprates, the strong suppression of antiferromagnetic spin correlation by efficient reduction annealing by the "protect-annealing" method leads to superconductivity not only with lower Ce concentrations but also with higher transition temperatures. To reveal the nature of this superconducting state, we have performed angle-resolved photoemission spectroscopy measurements of protect-annealed electron-doped superconductors Pr$_{1.3-x}$La$_{0.7}$Ce$_{x}$CuO$_{4}$ and directly investigated the superconducting gap. The gap was found to be consistent with $d$-wave symmetry, suggesting that strong electron correlation persists and hence antiferromagnetic spin fluctuations remain a candidate that mediates Copper pairing in the protect-annealed electron-doped cuprates.

Enhanced Room-Temperature Ferromagnetism in Superconducting Pr<sub>2-</sub><i><sub>x</sub></i>Ce<i><sub>x</sub></i>CuO<sub>4</sub> Nanoparticles

Recently, the so-called room-temperature ferromagnetism in any nanoparticles has been studied intensively. It is well known that the properties of ferromagnetism and superconductivity are contradictory in a superconducting high-Tc cuprate. The existence of ferromagnetism in the nanoparticles has been suggested to occur on the surface. This magnetism has been expected to come from defects inducing magnetic moments on oxygen vacancies at the surface of the nanoparticles. This work is to observe magnetism in nanosized superconducting Pr2-xCexCuO4 (PCCO) with x = 0.15 by means of a superconducting quantum interference device (SQUID). The magnetization curves of the reduced PCCO nanoparticles with the superconducting transition temperature, Tc, of ~25 K have revealed that there is weak ferromagnetism observed at room temperature. The magnitude of magnetization could be enhanced by oxygen reduction annealing in vacuum with increasing annealing temperature. A non-linear magnetization occurring in the reduced PCCO nanoparticles through the vacuum annealing process is probably due to a strong oxygen reduction producing more oxygen vacancies in the T'-structure.