Two-step Heat Treatment Process Research Articles

Near-pure red emission of highly thermally stable Eu3+ doped multi-component transparent oxyfluoride aluminosilicate glass-ceramic for red lasers and trichromatic WLEDs

In this paper, Eu3+ doped multi-component transparent aluminosilicate and oxyfluoride aluminosilicate glasses were fabricated by the melt-quenching procedure, and the glass-ceramics containing LiAlSi2O6 microcrystals (LAS glass-ceramics) were produced by a two-step heat treatment process. The XRD, SEM, and EDS data demonstrate that spherical single-phase LiAlSi2O6 microcrystals have precipitated from the glass matrices. The microhardness and flexural strength of the glass-ceramics vary in 6.27–6.66 GPa and 89.18–155.75 MPa, respectively. Phonon sideband analysis reveals that the phonon energies for both fluorine-containing and fluorine-free LAS glass-ceramics are approximately 971 cm−1, and the multiphonon relaxation rates (Wmp/W0) of the former are lower than those of the latter. Given its lower Wmp/W0 values, the fluorine-containing LAS glass-ceramic exhibits a higher emission intensity and a longer lifetime. Under 394 nm excitation, the fluorine-containing LAS glass-ceramic presents a warm and nearly pure red emission, characterized by a correlated color temperature of less than 3300 K and a color purity approaching 100 %. Time-resolved emission spectra support the photoluminescent stability of the Eu3+ ions. According to the Judd-Ofelt theory and spectral results, the fluorine-containing LAS glass-ceramic possesses lower Ω2/Ω4 and asymmetry ratios, leading to a more symmetrical local environment for the Eu3+ ions. Benefiting from its enhanced red emission, the 5D0→7F2 transition in the fluorine-containing LAS glass-ceramic exhibits a higher branching ratio (69.28 % > 50 %), emission cross-section (8.94 × 10−22 cm2), and gain bandwidth (12.12 × 10−28 cm3). Additionally, temperature-dependent photoluminescence spectra substantiate the excellent thermal stability of the Eu3+ doped fluorine-containing LAS glass-ceramic. Specifically, the intensity of the 5D0→7F2 transition at 150 °C remains 80 % of the initial intensity. Therefore, the Eu3+ doped multi-component transparent oxyfluoride aluminosilicate glass-ceramic has a significant competitive advantage in the field of red lasers and trichromatic WLEDs.

All-Around Electromagnetic Wave Absorber Based on Ni-Zn Ferrite.

Exploring a convenient, scalable, yet effective broadband electromagnetic wave absorber (EMA) in the gigahertz (GHz) region is of high interest today to quench its expanding demand. Ni-Zn ferrite is considered as a potential EMA; however, their performance study as a scalable effective millimeter-length absorber is still limited. Herein, we investigated EM wave attenuation properties of Ni0.5Zn0.5Fe2O4 (NZF) samples substituting Mn ion in place of Fe3+ as well as Zn2+ within a widely used frequency range of 0.1-9 GHz. Through composition optimization, Ni0.5Zn0.4Mn0.1Fe2O4 (NZM0.1F) EMA demonstrates excellent microwave absorption performance accompanied by simultaneous maximum reflection loss (RL) of -50.2 dB and wide BW of 6.8 GHz (with RL < -10 dB, i.e., attenuation >90%) at an optimum thickness of 6 mm. Moreover, the attenuation constant significantly increases from ∼217 to 301 Np/m with Mn doping. The key contribution arises from magnetic-dielectric properties synergy along with enhanced dielectric and magnetic losses owing to cation chemistry and site occupation in spinel NZF. In addition, porosity is induced in the system by a controlled two-step heat treatment process that promotes total loss with multiple internal reflections of the EM wave. Furthermore, RL is simulated by varying incident EM wave angles for the NZM0.1F sample displaying its angle insensitivity up to 50°. Our results reveal NZM0.1F as a futuristic environment-friendly microwave absorber material that is suitable for practical high-frequency applications.

Simultaneous improvement of strength and uniform elongation of ferrite-bainite steel with heterogeneous networked microstructure

A novel heterogeneous networked dual-phase structure, composed of bainite grains surrounding the ferrite grains and forming a networked structure, was successfully introduced into the low-carbon ferrite-bainite steel through a two-step heat treatment process. The ferrite-bainite steel with the networked structure exhibited a high ultimate tensile strength of 872 ± 39 MPa, a good uniform elongation of 15.59 %, and a high total elongation of 22.79 %. Meanwhile, it possessed excellent work hardening behaviors compared to the ferrite-bainite steel with an island feature. The improvement of the comprehensive mechanical properties was attributed to the influence of hetero deformation induced (HDI) hardening and the synergistic deformation of the dual-phase structure.

Tin-antimony/carbon composite porous fibers: electrospinning synthesis and application in sodium-ion batteries

In this work, tin-antimony/carbon composites porous fibers were successfully synthesized by an electrospinning method combined with two-step heat treatment processes, in which SnCl2 and SbCl3 were used as tin and antimony sources, and polyacrylonitrile (PAN) and polymethyl methacrylate (PMMA) were used as binders and pore-forming agents. The as-synthesized tin-antimony/carbon composites were systematically characterized by x-ray Diffraction (XRD), Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM), Energy-Dispersive Spectrometer (EDS), x-ray Photoelectron Spectroscopy (XPS), and Thermogravimetric Analysis-Differential Scanning Calorimetry (TG-DSC). The results indicate that the composite material consists of one-dimensional nitrogen-doped carbon porous fibers as the main matrix, with a three-dimensional network structure in which Sn, SnO2, and SnSb particles are encapsulated. Furthermore, the tin-antimony/carbon composites porous fibers were utilized as self-supported negative electrode for sodium-ion batteries. The results showed that the SNbM-2 sample electrode calcined at 800 °C demonstrated the best cycling stability and rate capability among all the sample electrodes, with a discharge capacity of 319.5 mAh·g−1 maintained after 100 cycles at a current density of 0.1 A·g−1. The excellent electrochemical performance of the SNbM-2 sample electrode is benefited from its unique porous structure and the carbon fiber network structure encapsulating Sn, SnO2, and SnSb particles, which could effectively shorten the Na+ ion transport distance and mitigate electrode volume expansion.

Optimization of the photoelectric performances of Mg and Al co-doped ZnO films by a two-step heat treatment process

Mg and Al co-doped ZnO (AZMO) films were prepared by magnetron co-sputtering to improve the optical and electrical performances of the ZnO-based films simultaneously. A two-step heat treatment process was introduced to obtain high-quality AZMO films. AZMO films were first deposited with substrates heating of 300 °C and then annealed at 500 °C in vacuum. The individual and coupling effects of substrate heating during sputtering deposition and post-annealing treatment on AZMO films were investigated. The results show that substrate heating during sputtering deposition is mainly on increasing the grain size and crystallinity of AZMO films. However, the effects of post-annealing treatment are mainly on making Mg and Al atoms move to the lattice points and ionize to from MgZn and AlZn. Besides, post-annealing treatment can effectively desorb adsorbed oxygen from the surface or grain boundaries of AZMO films and also make lattice oxygen break away from the lattice and change into oxygen vacancy. Compared with one-step heat treatment alone, it is with the two-step heat treatment process that the coupling effects can be exerted by combining the different effects of the two heat treatments, so as to achieve more performances optimization of AZMO films.

Regulating the p-n interface quality for Sb2Se3-based quasi-homojunction thin film solar cells by an effective two-step heat treatment process

Earth-abundant Sb2Se3 material has received an increased attention from photovoltaics community thanks to its excellent photo-electronic properties. Despite the fact that the Sb2Se3 heterojunction solar cells have obtained a rapid development, the serious interface defects have not been fundamentally solved so far. By contrast, the quasi-homojunction has advantages over heterojunction in terms of electronic property, however, its development is quite slow. Herein, the Sb2Se3-based quasi-homojunction devices with the configuration of SLG/FTO/Cu-Sb2Se3/I-Sb2Se3/Al have been fabricated using magnetron sputtering accompanied by a two-step heat treatment process. The relationship between the p-n interface quality and the device performances was investigated in detail. Thanks to the very well p-n interface, the devices achieved a 2.64% PCE, with ∼ 10% improvement compared with the devices fabricated via one-step heat treatment, as previous reported. Moreover, an effective p- and n-type doping with high carrier density is essential for fabricating the efficient p-n interface, as evidenced by SCAPS simulations. Through the co-optimization of various device parameters, the predicted PCE of quasi-homojunction devices can reach 18.96%, using the Mg as back contact, leading to lower minority carrier recombination at near the back-contact. Eventually, the simulation indicated clearly the directions for further promoting cell performances by doping strategy. This work can provide valuable guidance for the further development and improvement of Sb2Se3-based quasi-homojunction thin-film solar cells.

Microstructure Evolution in a β-γ TiAl Alloy during Hot Deformation under Variable Conditions

In contrast to practical hot compression processes, the testing of the hot workability of TiAl alloys is usually conducted under the conditions of constant strain rates and constant temperatures. This work aims at investigating the microstructural evolution of TiAl alloys on a Gleeble-3800 thermomechanical simulator under a variable strain rate (0.1, 0.01 and 0.001 s−1) at 1200 °C. The results show that, after a holding time of 30 s, the abrupt change in the strain rate at ε = 0.3 (engineering strain) has a remarkable influence on the flow stress and dynamic recrystallization (DRX) behavior of the β-γ Ti-44Al-6Nb-1Mo-0.3 (B, Y, La, Ce) (at.%) alloy. The flow stress demonstrates a rapid decrease with a sudden reduction in the strain rate. A duplex microstructure of γ + B2/β can be obtained under a high strain rate or continuous medium strain rate. During the two-step deformation, however, both γ→α phase transformation and DRX exist, and the content of the α phase demonstrates a significant increase when the strain rate becomes lower. Finally, a fine-grained structure of γ + B2/β + α2 phases with low residual stresses can be obtained via the two-step heat treatment processes. This provides a promising approach to significantly improve the hot workability of β-γ TiAl alloys.

Effects of heat treatments on the microstructure and tensile properties of IN738 superalloy with high carbon content fabricated via laser powder bed fusion

Heat treatment is a key process governing the final microstructure and mechanical properties of precipitation-hardened superalloys. However, traditional heat treatments are typically designed for cast or wrought superalloys. Therefore, this study investigates the effects of hot isostatic pressing (HIP) and a two-step heat treatment (THT) process that combines HIP with solution heat treatment (SHT) process. Heat treatments were conducted on a crack-free high carbon content IN738 alloy (IN738–0.3 C) fabricated via laser powder bed fusion (LPBF). HIP significantly reduced the porosity and residual stress of the LPBF IN738–0.3C samples. The HIP samples exhibited a large number of well-developed cuboidal γ′ precipitates (∼800 nm) and a few fine near-spherical γ′ precipitates (∼40 nm), whereas the THT samples had uniformly distributed γ′ precipitates with an average size of ∼80 nm. Therefore, the HIP and THT methods have potential to improve the properties of additive manufactured (AM) IN738 alloy. A superior combination of yield strength (1050 MPa), ultimate tensile strength (1295 MPa), and elongation (16.5 %) was achieved through THT. Additionally, the study revealed the detailed relationship between the microstructure and the mechanical properties. These results can serve as a reference for exploring suitable heat treatments for other AM precipitation-hardened Ni-based superalloys.

Effect of Heat Treatment on the Microstructure and Mechanical Properties of a Ti-TiAl Laminate Composite

The effect of an innovative two-step heat-treatment process on the microstructure and mechanical properties of a Ti-TiAl laminate composite fabricated by hot-pack rolling was studied in this paper. After heat treatment, the fracture toughness of the composite was enhanced and the elongation of the composite was almost twice that of the initial one. These changes were due to the dislocations and substructures stored in the Ti-43Al-9V alloy being decreased, the microstructure of the DsTi700 alloy turning to a duplex structure, the acicular α2 phase being precipitated at the interfacial region and the residual stresses stored in interfacial region being eliminated. The precipitation of dual-scale silicides was the main reason for the slightly reduced strength. Compared with the initial composite, the tensile strength of the heat-treated composite at 25° and 700° only reduced by 2.7% and 4%, respectively. The primary annealing temperature had a huge impact on the mechanical properties of the composite. However, with the change in secondary annealing temperature, the mechanical properties of the composite were not changed significantly. After heat treatment at 940–960 °C/2 h/AC + 725–750 °C/6 h/AC, the composite might possess high, comprehensive mechanical properties.

Low-Temperature Oxidation-Sintering Behaviors of Cu Fine Particles

Cu fine-particle paste is a promising material to form a low-cost interconnect for flexible electronics devices. It has been reported that Cu particles can be sintered at low temperature (well below the half of the melting point) through two-step heat treatment processes of oxidation and reduction. However, the mechanism of the low temperature sintering is not clear yet. In this study, we investigated the oxidation sintering process of Cu fine particles by thermal gravimetric analysis (TGA) in the temperature range of 200°C∼300°C, X-ray diffraction (XRD), and microstructural observation. It was found from TGA that the oxidation process was initially rate-controlled by surface reaction and then by Cu diffusion at grain boundaries of Cu2O. Transmission electron microscopy observation revealed the formation of a core (Cu)-shell (Cu2O) structure during the oxidation process. The adjacent Cu2O shells were bonded to each other resulting in a cross-linked structure. The subsequent reduction process led to the formation of a porous structure by oxygen removal, but the cross-linked structure was maintained, which would make the low-temperature sintered Cu body as robust as solidified solder and sintered Ag paste.

Fabrication of core-shell structured cobalt nanoparticle/carbon nanofiber as a bifunctional catalyst for the oxygen reduction/evolution reactions

One-dimensional (1D) catalysts composed of non-precious metal nanoparticles (NPs) and carbon materials exhibit characteristics that make them attractive for electrochemical applications. However, metal NPs with catalytic activity are mostly buried in the carbon material, significantly limiting the surface catalytic reaction. In the present study, a 1D core-shell structured catalyst was designed to secure enhanced number of active sites by placing cobalt NPs on the surface of the catalyst. The proposed 1D core–shell-structured Co NP/carbon nanofiber, Co-CSNF, was realized by facile coaxial electrospinning process followed by the two-step heat treatment process. Through the position control of cobalt nanoparticles, the Co-CSNF showed improved catalyst performance in both ORR and OER compared to Co NP/carbon nanofiber electrospun with a single nozzle (Co-NF). The Co-CSNFs showed excellent oxygen reduction performance with a 4 electron path and a half-wave potential (E1/2) of 0.86 V to RHE, and achieved a low oxygen evolution reaction of 322 mV (@10 mA cm−2 at 1.0 M KOH). It was demonstrated that the Co-CSNF-based zinc–air battery (ZAB) exhibited a lower discharge and charge overpotential than the ZAB with Pt/C+IrO2 with a stable cycling time of 366 h.

Room-temperature polymer-assisted additive manufacturing of microchanneled magnetocaloric structures

Magnetic refrigeration is an energy-efficient, sustainable, environmentally-friendly alternative to the conventional vapor-compression cooling technology. There are several magnetic refrigerator device designs in existence today that are predicted to be highly energy-efficient, on condition that suitable working materials can be developed. This challenge in manufacturing magnetocaloric devices is unresolved, mainly due to issues related to shaping the mostly brittle magnetocaloric alloys into thin-walled channeled regenerator structures to facilitate efficient heat transfer between the solid refrigerant and the heat exchange fluid in an active magnetic regenerator (AMR) cooling device. To address this challenge, a novel extrusion-based additive manufacturing (AM) method has been developed to 3D print microchanneled magnetocaloric structures. The printing ink consists of magnetocaloric powders, a polymer binder, and multiple solvents to achieve desirable shear-thinning property, which is critical for a robust printing process. Acting as a sacrificial binding agent for the magnetic powders, the polymer binder holds the 3D printed structures in place and is removed subsequently using a two-step heat-treatment process. To demonstrate the effectiveness of the fabrication process, spatially designed microchannels with minimum dimensions of 150 µm were achieved using nanoscaled La 0.6 Ca 0.4 MnO 3 powders (diameter~10 nm). Results indicate that the crystallographic properties and magnetofunctional response of the sintered 3D printed samples are comparable to that of the precursor powders. Overall, this study provides a promising route for realizing low-cost magnetic regenerators, thus potentially eliminating one of the main barriers to the commercialization of magnetic cooling technology. • Room temperature extrusion-based method for 3D printing magnetocaloric structures. • Significant magnetocaloric response in 3D printed LCMO scaffolds (ΔS=4.9 J/kg-K at 5 T). • Microchanneled magnetocaloric structures with record spatial resolution (dia∼150 µm). • Overcoming a crucial barrier to the commercialization of magnetic cooling technology.

Cu微粒子の低温酸化焼結挙動

Cu fine-particle paste is a promising material to form a low-cost interconnect for flexible electronics devices. It has been reported that Cu particles can be sintered at low temperature (well below the half of the melting point) through two-step heat treatment processes of oxidation and reduction. However, the mechanism of the low temperature sintering is not clear yet. In this study, we investigated the oxidation sintering process of Cu fine particles by thermal gravimetric analysis (TGA) in the temperature range of 200-300℃, X-ray diffraction (XRD), and microstructural observation. It was found from TGA that the oxidation process was initially rate-controlled by surface reaction and then by Cu diffusion at grain boundaries of Cu2O. Transmission electron microscopy observation revealed the formation of a core (Cu)-shell (Cu2O) structure during the oxidation process. The adjacent Cu2O shells were bonded to each other resulting in a cross-linked structure. The subsequent reduction process led to the formation of a porous structure by oxygen removal, but the cross-linked structure was maintained, which would make the low-temperature sintered Cu body as robust as solidified solder and sintered Ag paste.

Ultra-low core loss FeSiAl-based soft magnetic composites with ultra-thin MoO3 composite insulating layer

In this work, core/shell structured FeSiAl/MoO3 (spherical FeSiAl covered by ultra-thin MoO3 composite insulating layer) soft magnetic composites (SMCs) have been fabricated by a two-step heat treatment process. The influences of ammonium molybdate (AHM) content, first-step annealing temperature and second-step annealing temperature on magnetic, mechanical properties and electrical resistivity (ρ) have been comprehensively investigated. It is shown that the coating integrity and the thickness of MoO3 nanoparticles layer can be regulated by the content of AHM, leading to the improvement of ρ. Moreover, composite insulating layer with the thickness of 47 ± 8 nm is formed and completely coated on FeSiAl particles with 15 wt% AHM, resulting in the fact that the highest radial crushing strength (K = 62.3 MPa) and lowest core loss (Pcv = 128.8 mW/cm3) is obtained. In addition, Pcv is separated into two components: hysteresis loss component and eddy current loss component. Further studies display that eddy current loss is only half of hysteresis loss. As a result, the FeSiAl SMCs with 47 ± 8 nm MoO3 composite insulating layer possess the lowest core loss of 128.8 mW/cm3 at 50 mT/100 kHz. The low core loss of the FeSiAl/MoO3 SMCs with ultra-thin composite insulating layer has a great potential in the fields of conversion and power transmission.

Study on the Effect of Two-Step Saturated Steam Heat Treatment Process on the Properties of Reconstituted Bamboo

With the aim of utilizing reconstituted bamboo as a carbon cycle-oriented material, the improvement of physical and mechanical properties has been actively studied to solve using problems. The saturated steam heat treatment process has been widely used in worldwide. With the development and exploration of this technology, two-step saturated steam heat treatment process appears in some practical production, that is, after a period of saturated steam heat treatment at a lower temperature, the bamboo bundles are taken out and seasoned for a period of time, and then put back into the heat tank again, and heated at a higher temperature for another period of time. During the two-step saturated steam heat treatment, the physical and mechanical properties of bamboo changed. However, the mechanism of two-step saturated steam heat treatment has not been thoroughly discussed. For purpose that this paper all discuss and find out the mechanism of two-step saturated steam heat treatment on the change of physical and mechanical properties of reconstituted bamboo. In this work, the one and two-step saturated steam heat treatments were carried out according to the actual production parameters, and the physical and mechanical properties of the reconstituted bamboo board made of treated bamboo bundles were analyzed, including the color change, the thickness swelling (TS), modulus of elasticity (MOE), modulus of rupture (MOR) and shear strength. The results indicate that two-step saturated steam heat treatment is better than one-step. Based on the detailed study of the chemical composition, crystallinity and micro-morphological characteristics of the heated bamboo bundles, it is further revealed that during two-step saturated steam heat treatment, oxygen air is added to the reaction system between the two heat treatment processes to further catalyze the oxidation of hemicellulose and enhance the crystallinity of cellulose, so as to improve the properties of the final products. Our work has optimized the saturated steam heat treatment process which is widely used in industry, pointing out a new idea in the experimental and theoretical basis for the development of recombinant bamboo manufacturing industry.

Evading the strength-ductility trade-off at room temperature and achieving ultrahigh plasticity at 800℃ in a TiAl alloy

Improving the room temperature (RT) strength/ductility and hot-working capacity based on lamellar microstructures is of great significance for the practical application of TiAl alloys. However, the microstructure of these alloys has not been clearly identified yet. In this work, two new microstructures, here named triple-phase triple-state (T-T) and triple-phase dual-state (T-D) structures, were developed using a two-step heat treatment process in the Ti-43.5Al-4Nb-1Mo-0.1B (TNM) alloy, which also contains the pearlitic-like microstructure (PM) transformed through triggering a massive cellular response (CR). These two microstructures significantly improved the alloy strength. Furthermore, their ductility at RT and 800 ℃ was enhanced twice and 5, 6 times with respect to that of the lamellar microstructure with nano-scale interlamellar spacing, respectively. It was revealed that the formation of abundant deformation twins and their intersections in PMs during plastic deformation, cause prominent strain hardening and the dynamic Hall-Patch effect. This results in a simultaneous improvement of the RT strength and plasticity and promotes dynamic recrystallization at temperatures lower than 800 ℃; thus, the plasticity is dramatically enhanced at elevated temperatures. This structural design strategy should be extendable to other TiAl systems that can undergo a CR and provides a promising new pathway for solving the severe engineering challenges caused by the low RT plasticity and poor hot-working capacity of TiAl alloys.

A Promising Three-Step Heat Treatment Process for Preparing CuO Films for Photocatalytic Hydrogen Evolution from Water.

Copper (II) oxide (CuO) nanostructures were prepared on fluorine-doped tin oxide (FTO) using a three-step heat treatment process in a sol–gel dip-coating method. The precursor used for the dip-coating process was prepared using copper acetate, propan-2-ol, diethanolamine, and polyethylene glycol 400. Dip-coated films in layers of 2, 4, 6, 8, and 10 were prepared by drying each layer at 110 and 250 °C for 10 and 5 min, respectively, followed by calcination at 550 °C for 1 h. The films were applied toward photocatalytic hydrogen evolution from water. The X-ray diffraction (XRD) pattern of the films confirmed the tenorite phase of pure CuO. Raman spectroscopy revealed the 1Ag and 2Bg phonon modes of CuO, confirming the high purity of the films produced. The CuO films absorb significant photons in the visible spectrum due to their low optical band gap of 1.25–1.33 eV. The highest photocurrent of −2.0 mA/cm2 at 0.45 V vs reversible hydrogen electrode (RHE) was recorded for CuO films consisting of six layers under 1 sun illumination. A more porous surface, low charge transfer resistance, and high double-layer capacitance at the CuO/electrolyte interface observed for the films consisting of six layers contributed to the high photocurrent density attained by the films. CuO films consisting of six layers prepared using the conventional two-step heat treatment process for comparative purposes yielded 65.0% less photocurrent at 0.45 V vs RHE compared to similar films fabricated via the three-step heating method. The photocurrent response of the CuO nanostructures prepared using the three-step heat treatment process is promising and can be employed for making CuO for photovoltaic and optoelectronic applications.

Sn doped Hematite Nanorods for High-Performance Photoelectrochemical Water Splitting

Photoelectrochemical water splitting is of great attention due to its environmentally friendly generation of clean fuels. Hematite (α-Fe2O3) is considered a promising candidate due to its intrinsic properties for the high-performance photoelectrochemical electrode, such as favorable bandgap (2.0–2.2 eV), a suitable energy band position non-toxicity, low cost, and excellent chemical stability. Herein, we report about Sn-doped hematite nanorods and their implementation as photoanodes for photoelectrochemical water splitting. We provide the simple but efficient route to incorporate the Sn into the hematite without structural damage in the nanostructure and scrutinize the effect of Sn dopant on the photoelectrochemical activity of the hematite. Sn can be successfully incorporated into the hematite by the two-step heat treatment process, which reveals the enhanced photoelectrochemical responses compared with undoped hematite. We elaborate on the effect of Sn dopant in the hematite on the photoelectrochemical activities, thereby suggesting the optimum concentration of Sn dopant.

Multiphase precipitation and its strengthening mechanism in a V-containing austenite-based low density steel

Multiple precipitates have been utilized so as to improve the strength and strain hardening rate of austenite-based low density steel. Thus, a two-step heat treatment process including annealing and aging was designed for a V-containing Fe–20Mn–9Al-1.2C steel. Based on the analysis of microstructures and mechanical properties under different heat treatment processes, the precipitation behavior and strengthening mechanism were investigated. After annealing at 900 °C and aging at 550 °C, the microstructure of the steel consists of austenite matrix with V4C3 particles, κ-carbides and ferrite with DO3 particles. It is found that the steel exhibits an ultimate tensile strength (UTS) of 1339 MPa and specific strength (SS) of 199.3 MPa cm3/g. High strength and strain hardening ability result from the synthetic effect of shearing and Orowan mechanisms from multiple types of precipitates. Despite the low hardening ability, κ-carbides with shearing mechanism could enhance the yield strength significantly. Meanwhile, Orowan type V4C3 particles effectively improves the strain hardening rate as well as the strength. Fine planar slip bands with dispersive dislocation localization contribute to the strengthening of the steel.

Improved Electrochemical Performance of Li-Rich Layered Oxide Cathodes Enabled by a Two-Step Heat Treatment.

Lithium-rich layered oxide cathodes with high specific energy have become one of the most popular cathode materials for high-performance lithium-ion batteries. However, spinel phase formation due to the migration of transition metals and the release of lattice oxygen leads to the degradation of electrochemical performance. Here, we develop a synthesis approach for Li-rich layered oxide cathodes by a two-step heat-treatment process, which includes precursor calcination and pellet sintering. Compared with the sample prepared by the traditional one-step calcination, the oxide particles prepared by the two-step heat treatment show increased grain size from 217 to 425 nm. The Li-rich layered oxide cathodes with larger crystal grains indicate a mitigated formation of spinel phase and reduced voltage decay, which result in improved specific capacity, cycle stability, and rate capability. In addition, the thermal stability of the oxides is also improved. The improved electrochemical performance is because of the large single grains having a reduced contact area with a liquid electrolyte and the stable crystal lattice during cycling. Our strategy not only provides a simple and effective way to enhance the stability of the Li-rich layered oxide cathodes but also extends to the preparation of oxide powders with large grains.