Spinel-type Structure Research Articles

Direct Double Coating of Carbon and Nitrogen on Fluoride-Doped Li4Ti5O12 as an Anode for Lithium-Ion Batteries

Graphite as a commercial anode for lithium-ion batteries has significant safety concerns owing to lithium dendrite growth at low operating voltages. Li4Ti5O12 is a potential candidate to replace graphite as the next-generation anode of lithium-ion batteries. In this work, fluoride-doped Li4Ti5O12 was successfully synthesized with a direct double coating of carbon and nitrogen using a solid-state method followed by the pyrolysis process of polyaniline. X-ray diffraction (XRD) results show that the addition of fluoride is successfully doped to the spinel-type structure of Li4Ti5O12 without any impurities being detected. The carbon and nitrogen coating are distributed on the surface of Li4Ti5O12 particles, as shown in the Scanning Electron Microscopy–Energy Dispersive X-ray Spectroscopy (SEM-EDS) image. The Transmission Electron Microscopy (TEM) image shows a thin layer of carbon coating on the Li4Ti5O12 surface. The fluoride-doped Li4Ti5O12 has the highest specific discharge capacity of 165.38 mAh g−1 at 0.5 C and capacity fading of 93.51% after 150 cycles compared to other samples, indicating improved electrochemical performance. This is attributed to the synergy between the appropriate amount of carbon and nitrogen coating, which induced a high mobility of electrons and larger crystallite size due to the insertion of fluoride to the spinel-type structure of Li4Ti5O12, enhancing lithium-ion transfer during the insertion/extraction process.

Enhancing magnetism of ferrite via regulation of Ca out of sit A from spinel-type structure by adjusting the CaO/SiO2 mass ratio: Clean and value-added utilization of minerals

Ferrites with a typical AB2O4 structure are excellent soft magnetic material precursors. One progress for preparation of manganese ferrite is to use the minerals as raw materials. However, the inclusive impurities, such as Ca and Si, may dope in the AB2O4 structure and have an adverse effect on the magnetism of ferrites. In this work, the effects of CaO and SiO2 on the crystalline structure transformation, interfacial elements distribution, and magnetism change of the MnO2-Fe2O3-CaO-SiO2 system for preparation of high-performance ferrite materials were systematically investigated. It's found that Ca ions can readily be doped in the sit A of spinel-type structure to form CaaMnbFe2O4, and the magnetism of manganese ferrite was observably declined due to the Ca doping. A diffusion couple of synthetic CaaMnbFe2O4 and SiO2 was designed to determine the Ca migration. Results demonstrated that regulation of Ca out of sit A from the spinel-type structure can be successfully realized via adjusting the CaO/(CaO ​+ ​SiO2) mass ratio below 0.55, and the magnetism of ferrite after Ca migration was obviously enhanced. The migrated CaO combined with SiO2 to form calcium silicate which was distributed at the boundaries of ferrite particles. Eventually, the verification test of low-grade manganese ores with CaO and SiO2 impurities was conducted to validate the Ca migration effect.

Formation of NiMoO4 Anisotropic Nanostructures under Hydrothermal Conditions

—The synthesis of NiMoO4 hierarchical nanostructures using the hydrothermal method has been studied. The decomposition of NiMoO4·xH2O crystalline hydrate formed during the synthesis has been studied using synchronous thermal analysis upon heating in a stream of air and argon. According to X-ray diffraction as well as scanning and transmission electron microscopies, the proposed conditions allow one to synthesize single-phase nanosized (average CSR size of about 25 ± 2 nm) nickel(II) molybdate, which has a spinel-type monoclinic structure (space group C2/m) without impurity inclusions.

Achieving zero-thermal quenching luminescence in ZnGa2O4: 0.02Eu3+ red phosphor

The struggle with "thermal load" is always an unavoidable challenge for photo-luminescent material. In this work, a novel red phosphor of ZnGa2O4: Eu3+ with spinel-type structure is prepared by high-temperature solid-phase method. Due to the charge imbalance during the replacement processes of Ga3+→Zn2+ and Eu3+→Zn2+ in mixed spinel structure, vacancy defects can be introduced into the lattice structure of ZnGa2O4: 0.02Eu3+. The results of Density Function Theory (DFT) calculation demonstrate that the existence of vacancy defects can construct defect energy levels in the energy band structure of the host crystal, which can assist enhancing the fluorescence emission intensity of the phosphor at high temperature. The strong thermal stability of its own structure combined with the assistance of defect energy levels enables the achievement of zero-thermal quenching in ZnGa2O4: 0.02Eu3+ under multi-wavelength excitation. Our results open up a new way for the exploration and utilization of zero-thermal quenching phosphors.

Synergetic effects of lanthanum substituted Ni-Zn-Cu-Co ferrite nanocomposite with enhanced NH3 sensing performance

The increase in modern technologies upsurges the pollution of air by ten-fold in the past decade leading to the cause of an increased number of deaths and infectious diseases. This has provoked the research community in developing variable gas sensing materials to detect harmful gases in the present condition. Herein, the lanthanum (La) substituted Ni-Zn-Cu-Co ferrite nanocomposite (La (Ni-Zn-Cu-Co-Fe2O4)) was synthesized via an eco-friendly co-precipitation technique and is utilized as ammonia (NH3) gas sensor. The structural and morphological studies showed the formation of a typical inverse spinel structure of the cubic phase, with spherically agglomerated and interconnected nanostructures. Further, the saturation magnetization (Ms) of the as-synthesized nanocomposite was found to be 1.66 emu using the Vibrating Sample Magnetometer (VSM). The fabricated La (Ni-Zn-Cu-Co-Fe2O4) nanocomposite thin film exhibited an outstanding sensing capability towards ammonia gas with a response and recovery time of 20 s and 15 s respectively, which was on par with the current generation ammonia sensors and posted a sensor response of 86.2% with good repeatability and stability. Moreover, the synthesized material was also found to be more biocompatible, which stands as a bonus.

High-performance and low-cost manganese oxide/multiwalled carbon nanotubes composite as cathode material for aqueous magnesium ion battery

• The λ-MnO 2 /MWCNTs composite was synthesized with feasible solution method and applied as cathode in aqueous magnesium ion battery for the first time. • The λ-MnO 2 particles are distributed in MWCNTs network structure, which increased the specific surface area and conductivity of electrode material. • The λ-MnO 2 /MWCNTs composite showed good electrochemical performances in three electrode and MnO 2 /MWCNTs//AC full cell systems. • The mechanism of magnesium ion insertion/deinsertion in λ-MnO 2 /MWCNTs electrode was investigated in this paper. The aqueous magnesium ion batteries (AMIBs) with the advantages of low cost, intrinsical safety and environmentally friendly have received worldwide attention as promising candidates for green energy storage system. Herein, we report a composite material engineered from λ-MnO 2 particles and multiwalled carbon nanotubes (MWCNTs) as cathode for AMIB. The λ-MnO 2 possesses a spinel-type crystalline structure, which is beneficial for the insertion/deinsertion of Mg 2+ ions. The MWCNTs network shorts ionic diffusion distance and increases the interface of electrode material/electrolyte. Hence, the λ-MnO 2 /MWCNTs composite demonstrates a large capacity of 251 mAh g −1 at 50 mA g −1 in 0.5 M MgCl 2 solution, and high capacity retention of 86.2 % after 1000 cycles at 1000 mA g −1 in MgSO 4 solution. Furthermore, the λ-MnO 2 /MWCNTs//AC system is firstly assembled as full AMIB, which delivers a specific capacity of 84.8 mAh g −1 at 100 mA g −1 . The rate capability test is performed at 300 mA g −1 . This system displays a gradually rise tendency in the specific capacity at initial 100 cycles on account of the activation process. After 1000 cycles, the specific capacity is 56.9 mAh g −1 . This work develops an easily prepared, low-cost and good performance electrode material for AMIB system.

Size and doping effects on the improvement of the low-temperature magnetic properties of magnetically aligned cobalt ferrite nanoparticles

The macroscopic magnetic behavior of nanoparticulated systems is the result of several contributions, ranging from the intrinsic structural properties of the nanoparticles to their spatial arrangement within the material. Unravelling and understanding these influences is an important task to produce nano-systems with improved properties for specific technological applications. In this work we study how the magnetic behavior of a set of magnetically hard nanoparticles can be improved by the modification of the sample arrangement (either randomly or magnetically oriented) and the nature of the enclosing matrices. At first, we employed a hot-injection, continuous growth strategy to synthesize non-stoichiometric cobalt ferrite (CoxFe3−xO4) nanoparticles. We prepared five batches of hydrophobic, oleate-coated samples, with mean diameters of 8 nm, 12 nm, 16 nm and variable Co-to-Fe proportions. The structural characterization confirms that the nanoparticles have a spinel-type monocrystalline structure and that the Co and Fe ions are homogenously distributed within the system. The magnetic properties of the nanoparticles were measured by DC magnetometry, and we found that the strategy used in this work to create a system of magnetically oriented nanoparticles can lead to a significant remanence and coercive field enhancement at low temperatures when compared with randomly oriented and fixed systems. The modification of the magnetic properties was detected in the five batches of samples, but the strength of the enhancement depends on both size and composition of the nanoparticles. Indeed, for the “hardest” samples the coercive field of the magnetically oriented systems reached values of around 30 kOe (3 T), which represents a 50% increase regarding the randomly oriented system and are among the highest reported to date for a set of Fe and Co oxide nanoparticles.

Nanostructured spinel-type M(M = Mg, Co, Zn)Cr2O4 oxides: novel adsorbents for aqueous Congo red removal

Nanostructured spinel-type M(M = Mg, Co, Zn)Cr2O4 oxides with novel adsorbents for aqueous Congo red removal were synthesized by a polyacrylamide gel method and studied for their phase structure, microstructure, adsorption performance, and multiferroic behavior. The phase structure and purity analysis revealed that the nanostructured spinel-type M(M = Mg, Co, Zn)Cr2O4 oxides presented a spinel-type cubic structure, and the formation of a secondary phase such as Cr2O3, MgO, ZnO, or Co3O4 was not observed. The microstructure characterization confirmed that the spinel-type MCr2O4 oxides grew from fine spherical particles to large rhomboid particles. Adsorption experiments of spinel-type MCr2O4 oxides for adsorption of Congo red dye were fitted well with the pseudo-second-order kinetics. The adsorption capacity of the ZnCr2O4 oxide (44.038 mg/g, pH 7, temperature 28 °C, initial dye concentration 30 mg/L) was found to be higher than that of MgCr2O4 oxide (43.592 mg/g, pH 7, temperature 28 °C) and CoCr2O4 oxide (28.718 mg/g, pH 7, temperature 28 °C). The effects of initial adsorbent concentration, initial dye concentration, pH, and temperature between the ZnCr2O4 oxide and Congo red dye at which optimal removal occurs, were performed. The thermodynamic studies confirmed that a high temperature favors the adsorption of Congo red dye onto ZnCr2O4 oxide studied. The nanostructured spinel-type M(M = Mg, Co, Zn)Cr2O4 oxides that exhibited high adsorption performance for adsorption of Congo red dye can be ascribed to the synergistic effect of electrostatic interaction, pore filling, and ion exchange. The present work suggested that the nanostructured spinel-type M(M = Mg, Co, Zn)Cr2O4 oxides have excellent adsorption performance and multiferroic behavior, which shows potential applications for removal of the Congo red dye from wastewater, magnetic memory recording media, magnetic sensor, energy collection and conversion device, and read/write memory.

STUDY OF PHASE FORMATION IN FE2O3-ND2O3→NDFEO3/FE2O3 NANOCOMPOSITES AS A RESULT OF THERMAL ANNEALING

The aim of this work is systematic study of the thermal annealing effect on the preparation of nanostructured composites NdFeO3/Fe2O3 with a spinel type structure. The interest in these nano­composites is due to the enormous potential of their application as a basis for magnetic devices, catalysts, and magnetic carriers for targeted drug delivery. As a synthesis method, two­stage syn­thesis was used, which includes mechanochemical grinding of nanopowders Fe2O3 and Nd2O3 in a planetary mill, followed by thermal annealing of the resulting mixture in a wide temperature range: 600­1000°C. During the studies carried out, it was found that in the initial state the obtained nano­composites are a mixture of a solid solution of interstitial and substitutional Fe2O3 and Nd2O3. At an annealing temperature of 600°C, the onset of the formation of the NdFeO3 phase is observed, which at a temperature of 1000°C is fully formed and dominates in the composite structure (content more than 85%). It was also found that during thermal sintering, the processes of phase transformations of the Fe2O3­Nd2O3→NdFeO3/Fe2O3 type are accompanied by an increase in the particle size by a factor of 1.5­2

Sodium insertion and de-insertion mechanism of spinel-type sodium titanium oxide studied by in situ XRD

The reaction mechanism of a spinel-type sodium titanium oxide (Na3LiTi5O12) was investigated using in situ XRD during the Na insertion/extraction process. The XRD profiles gradually shifted to lower and higher angles during the sodiation and de-sodiation processes, respectively, while maintaining a spinel-type structure and no new peaks were observed, indicating a solid-solution reaction according to the following reaction; (Na)38a(Li1Ti5)16dO12192i + 3Na+ + 3e− ↔ (Na)616c(Li1Ti5)16dO12192i. The evolution of the XRD profile was confirmed to be highly reversible, which explains the high cycling stability of the NTO electrode. There are two possible sites for Na to occupy: 8a and 16c. The Na ion initially occupies only the 8a site, as confirmed by the 220 peak. The intensity of the 220 peak gradually decreased with increasing SOC and was hardly detected at an SOC of 50%, indicating that all the Na ions at the 8a sites moved to the 16c sites. Topological analysis combined with XRD revealed that the Na ion at the 8a site is stable with one other Na ion at an adjacent 16c site despite the interatomic distance being too short.

Magnetic dynamics of NiMn2O4 spinel produced by a simple aqueous sol–gel route

Polycrystalline nickel manganite was prepared by a simple sol–gel method from an aqueous solution of metal chlorides containing sorbitol as a chelating agent. X-ray diffraction and Rietveld refinement show a spinel-type crystalline structure, space group Fd3¯m, and a minor (~3%) phase of rock salt structure assigned to NiO. XPS measurement shows a typical manganite electronic structure with Mn in the oxidation states Mn2+, Mn3+, and Mn4+. The zero-field-cooled and field-cooled magnetization curves measured under low magnetic fields display irreversibility and an asymmetric hysteresis evidencing some exchange bias. Field dependent DC susceptibility above Curie temperature put into evidence a competition of short ranged ferro and antiferromagnetic interactions. AC in-phase susceptibility shows a frequency independent sharp peak at the ferromagnetic phase transition (97 K) and a broad maximum at lower temperature. These maxima show a weak frequency dependence in the out-phase component. Cole–Cole plot reveals at least two magnetic relaxation processes.

On the relation of structural disorder and thermoelastic properties in ZnGa2O4 and Zn1−xMgxGa2O4 (x ≈ 0.33)

The cation distribution at room temperature, as well as elastic properties and thermal expansion of single crystal ZnGa2O4 (ZGO) and Zn1−xMgxGa2O4 (x ≈ 0.33; ZMGO) with spinel-type structure were studied in a wide temperature range using single crystal X-ray diffraction, neutron powder diffraction, inductive gauge dilatometry and resonant ultrasound spectroscopy. ZGO adopts an almost normal spinel structure, whereas ZMGO is significantly disordered. At room temperature, the elastic properties of ZMGO mostly fall between those of ZGO and MgGa2O4 (MGO). The temperature dependences of the thermoelastic properties of ZGO and ZMGO, as well as thermal expansion of ZGO reveal distinct signatures of glass-like transitions, which separate states in which the cation dynamics are fast enough to relax the cation order in response to temperature change in laboratory timescales from those in which they are not. In equilibrium, thermal expansion is increased in ZMGO, whereas the thermoelastic coefficients are decreased in both ZGO and ZMGO. The temperature range of the transition is significantly larger in ZGO compared to ZMGO and MGO. Trends within the elastic properties, thermoelastic properties, thermal expansion and the glass-like transition in the (Zn,Mg)Ga2O4 solid solution series are discussed based on the impact of inversion, structural disorder, bond character and in comparison to other spinels.

Dual-regulation strategy to enhance electrochemical catalysis ability of NiCo2O4-x for polysulfides conversion in Li-S batteries

Recently, lithium-sulfur batteries (LSBs) have been considered as the most potential energy storage system, but the application of LSBs is still confronted with significant challenges. One among the significant challenges is the low kinetics. In this work, we design the urchin-like structure of NiCo2O4 to expose more catalytic sites to accelerate the lithium polysulfides (LiPSs) conversion. And the oxygen deficient in NiCo2O4 with spinel type structure (space group Fd3m) leads to the excellent chemical affinity of NiCo2O4-x for LiPSs. Moreover, because of the optimal distribution of electron on (311) plane of NiCo2O4-x, the Gibbs free energies of the evolution from S8 to Li2S on (311) plane of NiCo2O4-x is much lower than that of NiCo2O4, illustrating the NiCo2O4-x with the higher catalytic activity. Therefore, the cell with the NiCo2O4-x modified separator exhibits a superior rate performance (632 mAh g−1 at 2.5 C) and a long cycle life (404 mAh g−1 at 1 C after 800 cycles) with excellent Li+ ion diffusion coefficient. The rational structural optimization makes NiCo2O4-x become the efficient catalyst of LSBs. And the results promote the development of bimetallic oxides as catalyst in LSBs.

Effect of Ni substitution on structural, dielectric and magnetic properties of Zn ferrite nanoparticles

Effect of Ni doping on ZnFe2O4 samples was prepared by a sol-gel auto-combustion method. The obtained samples were calcinated at 6000C. Then, the samples were characterized by powder X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, dielectric and magnetic properties. XRD results confirm the formation of a cubic spinel-type structure with an average crystallite size decreased with Ni concentration from 33.68 to 31 nm. Lattice parameter decreases with increasing Ni concentration, due to the small ionic radius of the Ni2+ ion. The SEM images show the morphology of the samples as spherical shaped particles in agglomeration. The dielectric constant and magnetization showed an increasing trend with increasing Ni concentration due to the rearrangement of cations at tetrahedral and octahedral sites.

Structural systematics of SFCA-I type solid-solutions in the system CaO\u2013Fe2O3\u2013FeO\u2013Al2O3

Effects of Fe ↔ Al substitution on triclinic SFCA-I-type compounds with general formula A40O56 (A: Ca, Al, Fe3+, Fe2+) have been studied using single-crystal X-ray diffraction. Crystals of sufficient quality and size were synthesized in the temperature range between 1200 and 1300 °C. Six samples with Al/FeTot ratios of 0.127, 0.173, 0.216, 0.310, 0.349 and 0.459 have been structurally characterized. SFCA-I can be described with a modular approach involving the stacking sequence < PSS > of “P” and “S” modules that can be imagined as being cut from the well-known pyroxene (P) and spinel (S) structure types. Furthermore, SFCA-I is related to the sapphirine supergroup of minerals. Within the present solid-solution series, the contents in calcium show only minor variations (≈ 6.7 a.p.f.u.). The twenty crystallographically independent tetrahedrally (T) and octahedrally (M) coordinated cation sites exhibit considerable differences concerning the Al uptake. Indeed, Al is preferentially incorporated into the tetrahedra belonging to the single-chains located in the pyroxene modules. Ferrous iron, on the other hand, is restricted to one of the T-positions within the spinel blocks. Most structural aspects from unit-cell parameters and cell volumes to site occupancies, tetrahedral chain kinking as well as polyhedral distortions are defined by linear or nearly linear trends when plotted against the Al/FeTot ratio. Analysis of the < T–O > and < M–O > distances showed a complex interplay between the different coordination polyhedra resulting in a contrasting behavior of these values with positive or negative change rates as a function of composition. Evaluation of the average chemical strain tensor derived from the sets of lattice parameters for the two samples of the abovementioned series showing the highest and lowest Al/FeTot ratios indicated, that the major contraction with increasing Al content is perpendicular to the pyroxene- and spinel modules. Furthermore, the pyroxene module seems to be more affected when compared with the spinel block. There is evidence that the SFCA-I-type solid-solution series is limited on both the Al- and Fe-rich sides. The present investigation provides—for the first time—a detailed crystallographic analysis on the impact of chemical variations on a compound that is of relevance to the field of applied mineralogy related to the technologically important process of iron-ore sintering.

Missing Piece in the Crystal Chemistry of Zn-Sb Secondary Phases in ZnO-Sb2O3-Bi2O3 Varistor Ceramics: Orthorhombic β-Zn7Sb2O12. An Experimental and Theoretical Study of the Crystal Structure and Its Thermal and Vibrational Spectroscopic Characterization.

The ternary Zn-Sb oxide β-Zn7Sb2O12 was prepared by solid state synthesis from ZnO and Sb2O3 at 1250 °C and a reaction time of 168 h. The crystal structure of β-Zn7Sb2O12 was solved from powder diffraction data by direct-space methods and was refined by the Rietveld technique. The title compound β-Zn7Sb2O12 crystallizes in the orthorhombic space group Cmme with a = 1210.6(1) pm, b = 1856.7(1) pm, c = 852.2(1) pm, V = 1.9154(1) nm3, and eight formula units per unit cell. The structure can be described as a distorted cubic closest packing of oxide ions with Zn2+ and Sb5+ in the tetrahedral and octahedral interstitial sites. According to group-subgroup relations, the anion packing is directly derived from the spinel structure of α-Zn7Sb2O12; however, the occupation pattern of the interstitials is completely different than in the spinel structure type. The most prominent structural feature in β-Zn7Sb2O12 is the clustering of all [ZnO4] polyhedra into a [Zn7O20] moiety representing an excerpt from the sphalerite structure. The structural model is corroborated by vibrational spectroscopy as well as density functional theory calculations. Thermal analysis reveals an irreversible phase transition of β-Zn7Sb2O12 into the α-polymorph at 1330 °C.

Phase Transitions in the "Spinel-Layered" Li1+xNi0.5Mn1.5O4 (x = 0, 0.5, 1) Cathodes upon (De)lithiation Studied with Operando Synchrotron X-ray Powder Diffraction.

“Spinel-layered” Li1+xNi0.5Mn1.5O4 (x = 0, 0.5, 1) materials are considered as a cobalt-free alternative to currently used positive electrode (cathode) materials for Li-ion batteries. In this work, their electrochemical properties and corresponding phase transitions were studied by means of synchrotron X-ray powder diffraction (SXPD) in operando regime. Within the potential limit of 2.2–4.9 V vs. Li/Li+ LiNi0.5Mn1.5O4 with cubic spinel type structure demonstrates the capacity of 230 mAh·g−1 associated with three first-order phase transitions with significant total volume change of 8.1%. The Li2Ni0.5Mn1.5O4 material exhibits similar capacity value and subsequence of the phase transitions of the spinel phase, although the fraction of the spinel-type phase in this material does not exceed 30 wt.%. The main component of Li2Ni0.5Mn1.5O4 is Li-rich layered oxide Li(Li0.28Mn0.64Ni0.08)O2, which provides nearly half of the capacity with very small unit cell volume change of 0.7%. Lower mechanical stress associated with Li (de)intercalation provides better cycling stability of the spinel-layered complex materials and makes them more perspective for practical applications compared to the single-phase LiNi0.5Mn1.5O4 high-voltage cathode material.

High-Performance and Low-Cost Manganese Oxide/Multiwalled Carbon Nanotubes Composite as Cathode Material for Aqueous Magnesium Ion Battery

The aqueous magnesium ion batteries (AMIBs) with the advantages of low cost, intrinsical safety and environmentally friendly have received worldwide attention as promising candidates for green energy storage system. Herein, we report a composite material engineered from λ-MnO2 particles and multiwalled carbon nanotubes (MWCNTs) as cathode for AMIB. The λ-MnO2 possesses a spinel-type crystalline structure, which is beneficial for the insertion/deinsertion of Mg2+ ions. The MWCNTs network shorts ionic diffusion distance and increases the interface of electrode material/electrolyte. Hence, the λ-MnO2/MWCNTs composite demonstrates a large capacity of 251 mA h g-1 at 50 mA g-1 in 0.5 M Mgcl2 solution, and high capacity retention of 86.2% after 1000 cycles at 1000 mA g-1 in MgSO4 solution. Furthermore, the λ-MnO2/MWCNTs//AC system is firstly assembled as full AMIB, which delivers a specific capacity of 84.8 mA h g-1 at 100 mA g-1. The rate capability test is performed at 300 mA g-1. This system displays a gradually rise tendency in the specific capacity at initial 100 cycles on account of the activation process. After 1000 cycles, the specific capacity is 56.9 mA h g-1. This work develops an easily prepared, low-cost and good performance electrode material for AMIB system.

Oxide layer characterization by XRD and Rietveld refinements in maraging steel 300 aged in steam atmosphere

Maraging steels are martensitic steels hardened by precipitation of intermetallic compounds in thermal aging, with good machining properties and high strength, fracture toughness and corrosion resistance, being used in aircraft parts and rocket motor-case, tooling applications and nuclear plants. During thermal aging in steam atmosphere a protective and corrosion resistant oxide layer is formed over the bulk. In this work, conventional Bragg-Brentano geometry was used to identify the phases formed in four specimens of maraging steel grade 300 with different surface finishes that were previously solution annealed twice at (950 ± 5) °C for 1 h, air-cooled, and submitted to oxidation process under positive pressure about 1.5 kPa of steam at (480 ± 5) °C for 6 h, followed by forced air-cooling. Diffraction patterns were measured employing CuKα radiation, ranging 20º &lt; 2θ &lt; 85º and the Rietveld method was used to better characterize the structures identified. Through Rietveld refinements it was possible to conclude that the layer formed during heat treatment process is constituted by a transition metallic phase with a quasi-cubic face centered unit cell, and an oxide layer that includes hematite, magnetite and a spinel structure type MFe2O4, where M could be an alloying element, for all analyzed samples.

New insights on photocatalytic hydrogen evolution of ZnFe2−xGaxO4 (0 ≤ x ≤ 2) solid solutions: Role of oxygen vacancy and ZnO segregated phase

The exponential increase in global energy and clean energy demands lead to the necessity for development of efficient photocatalyst for hydrogen evolution. Herein, photocatalytic hydrogen evolution activity with nano-structured ZnFe2-xGaxO4 (0 ≤x ≤ 2) solid solution samples synthesized by citrate-gel method at 550 °C is reported. The prepared materials have been characterized for their structure and optical properties. The formation of solid solution with spinel type structure in the complete range of composition and a systematically decreasing trend of unit cell parameters with increasing Ga3+ concentration is observed from powder XRD studies. The band gap of these solid solutions could be tuned from 1.9 to 3.1 eV by increasing the Ga3+ concentration. Without any co-catalyst, ZnFe2O4 shows poor catalytic activity (378μmolh−1g−1) for photocatalytic hydrogen evolution from water. Whereas its activity enhances with loading of co-catalyst and touches a maximum activity of 3089μmolh−1g−1 with loading 1 wt% of Pt. The catalytic activity increases systematically with increasing Ga concentration in the solid solutions and reaches to 3989μmolh−1g−1 for ZnGa2O4 nanomaterials. Electrochemical impedance spectroscopic and transient photocurrent measurements also support the observed increasing trend. The lower catalytic activity of ZnFe2O4 has been attributed to the faster e-h recombination due to lower band gap and inherent oxygen vacancies. A smaller amount of oxygen vacancies and segregated ZnO phase observed in Ga substituted ZnFe2O4 samples favor the e-h pair separation and that facilitate the hydrogen generation from water.