Grain Growth Research Articles

Promoting methane combustion activity and stability by tuning multiple Ni–Si interactions in catalysts

Catalytic combustion holds great promise for eliminating unburned methane released by the natural gas industry, and the crucial factor is the development of cost-effective and efficient catalysts. Herein, the silicon component was introduced into nickel oxide by simultaneously regulating the precipitation of nickel precursors and the hydrolysis of silicon precursors. The generation of SiO4 groups in NiO inhibited the growth and sintering of NiO grains, meanwhile promoting the formation of active Ni2+ and adsorbed oxygen species for activation and oxidation of methane. With the increase of Si content, the existence of surface nickel silicate on NiO further improved the sinter resistance of NiO, but caused the diminish of Ni2+ and surface adsorbed oxygen species. The catalytic activity and stability were co-enhanced via facilely tuning the multiple Ni–Si interactions. The catalyst with abundant SiO4 groups and negligible nickel silicate exhibited considerably higher catalytic activity, sinter-resistance and water-resistance than NiO toward methane combustion.

β→α Phase Transformation and Properties of Solid-State-Sintered SiC Ceramics with TaC Addition.

Dense SiC-based composite ceramics were fabricated by means of the ex situ addition of TaC using solid-state spark plasma sintering (SPS). Commercially available β-SiC and TaC powders were chosen as raw materials. Electron backscattered diffraction (EBSD) analysis was conducted to investigate the grain boundary mapping of SiC-TaC composite ceramics. With the increase in TaC, the misorientation angles of the α-SiC phase shifted to a relatively small range. It was deduced that the ex situ pinning stress from TaC greatly suppressed the growth of α-SiC grains. The low β→α transformability of the specimen with the composition of SiC-20 vol.% TaC (ST-4) implied that a possible microstructure of newly nucleated α-SiC embedded within metastable β-SiC grains, which could have been responsible for the improvement in strength and fracture toughness. The as-sintered SiC-20 vol.% TaC (ST-4) composite ceramic had a relative density of 98.0%, a bending strength of 708.8 ± 28.7 MPa, a fracture toughness of 8.3 ± 0.8 MPa·m1/2, an elastic modulus of 384.9 ± 28.3 GPa and a Vickers hardness of 17.5 ± 0.4 GPa.

Vacuum-deposited Rb3CeI6 for deep-blue-light-emitting diodes.

Recently, perovskite light-emitting diodes (PeLEDs) have exhibited outstanding performance in next-generation high-definition display applications. However, compared with green and red PeLEDs, the development of efficient and stable blue PeLEDs to meet the requirement for a wide color gamut has been a challenge. Herein, we vacuum thermally deposited a film of the lead-free rare earth halide Rb3CeI6, which shows deep blue emission with peaks at 427 nm and 468 nm. Due to the parity-allowed 5d-4f transition of Ce(III), the excited-state lifetime is as short as 22.3 ns (427 nm) and 25 ns (468 nm), respectively. The photoluminescence quantum yield (PLQY) is optimized to 51% by regulating the nucleation and growth of Rb3CeI6 grains. In a prototype rare earth light-emitting diode (ReLED) device, a thin insulating Al2O3 layer (5 nm) is inserted between the electron transport layer (ETL) and the emitting layer (EML, Rb3CeI6) to balance the carriers and reduce the dark current. The device shows a maximum luminance and EQE of 98 cd m-2 and 0.67%, respectively, and the electroluminescence (EL) spectrum maintains stability with changes in the operating voltage. In addition, the corresponding CIE coordinate is (0.15, 0.06), which closely matches the Rec. 2020 standard (0.131, 0.046).

Ti‐in‐zircon and Ti‐in‐quartz thermometry using electron probe microanalysis: A promising approach in retrieving thermal conditions of granulite facies metamorphism and felsic magmatism in the Sandmata Complex, Aravalli Craton (NW India)

The titanium (Ti) concentrations in zircon and quartz and corresponding thermometers have been regarded as powerful monitors that constrain petrogenetically meaningful temperature conditions of crystallization and metamorphism. The precise measurement of trace elements by electron probe microanalyzer (EPMA) is quite challenging as their x‐ray intensities are only slightly higher than the Bremsstrahlung background, leading to significant errors while quantifying the elemental abundances (e.g., Ti‐in‐zircon). We present EPMA‐based protocols for analysing Ti concentrations in zircon and quartz by considering recent analytical and instrumental improvements. High counting times at peak and background positions, simultaneous acquisition of Ti in multiple (three) spectrometers and data processing using the non‐central chi2 test for risk determination in sub‐counting method were employed in Ti‐in‐zircon and Ti‐in‐quartz protocols. Applying these protocols to the Mongolian garnet, San Carlos olivine and NIST 610 glass standards yielded Ti concentrations of 0.60 ± 0.02 wt%, 18 ± 3 ppm and 437 ± 26 ppm (in ±2σ), respectively. The Ti concentrations obtained from these standards are consistent with those published based on high‐precision analytical methods. We use the existing Ti‐in‐zircon and Ti‐in‐quartz thermometers to understand the thermal evolution of various lithologies in the Sandmata Complex, Aravalli Craton (north‐western India). The investigated samples include garnet–biotite gneiss, migmatite gneiss, mafic granulite and Anjana granite. The Ti‐in‐zircon temperatures calculated from the patchy‐zoned zircon cores yield HT–UHT conditions (892–959°C) for garnetiferous granite gneiss, migmatite gneiss and mafic granulite, whereas the zircon overgrowths yield temperatures of 725–871°C. The homogeneous zircon cores from Anjana granite yield temperatures (914–984°C) comparable to zirconium saturation conditions (920–960°C), indicating zircon crystallization from “hot” granitic melt. The oscillatory overgrowths on the zircon core obtain variable temperature conditions for inner overgrowth (811–877°C) and outer overgrowth (714–782°C), suggesting the episodic growth of zircon grains from different magma pulses. Additionally, the application of pressure‐dependent Ti‐in‐quartz thermometry yields quartz recrystallization conditions for garnet–biotite gneiss (430–556°C), migmatite gneiss (423–593°C), mafic granulite (430–679°C) and Anjana Granite (444–533°C). The combination of Ti‐in‐zircon thermometry, Ti‐in‐quartz and conventional thermobarometry demonstrates cooling histories for various high‐grade metamorphic and magmatic rocks in the Sandmata Complex. Based on this study, we emphasize that trace element thermometers can decipher high‐temperature metamorphism, which is otherwise erased in conventional thermometers due to diffusional readjustments during long‐lasting near‐peak and post‐peak metamorphism.

Tailoring the in-plane magnetic anisotropy and permeability spectra of obliquely deposited Fe40Co40B20 films for 5G communications

Ferromagnetic films are widely used in magnetic devices owing to their high saturation magnetization, high magnetic permeability, and low coercivity. However, it is extremely challenging to apply ferromagnetic films to high-frequency communication applications due to the low resonant frequency of permeability spectra. Here, ferromagnetic Fe40Co40B20 films are prepared using oblique magnetron sputtering deposition and the static and dynamic magnetic properties are comprehensively investigated. The self-shadowing effect caused by the tilted growth of columnar grains introduces in-plane magnetic anisotropy to the films, thereby increasing the natural resonant frequency. As the sputtering angle increases from 24° to 30°, the resonant frequency increases from 4.09 to 4.87 GHz, resulting in an approximate liner tunability of 0.13 GHz/degree. Compared to the normally sputtered films, the figure of merit in regard to the microwave magnetic performance of the obliquely sputtered films is significantly enhanced by more than 50%. These results reveal the effectiveness of the oblique sputtering deposition to tailor the in-plane magnetic anisotropy and permeability spectra of ferromagnetic films for the high-frequency film inductor application of 5G communications.

Energy-Efficient and Green Extraction of Bismuth Metal in Methanesulfonic Acid-Based Membrane Electrochemical Systems

As the mainstream hydrometallurgical medium of bismuth sulfide concentrate, chloride/hydrochloric acid suffers from high volatility, strong corrosiveness, and inferior deposit quality. In this paper, methanesulfonic acid (MSA) with low volatility and high stability was adopted as the bismuth hydrometallurgical system, and the membrane electrochemical deposition method was proposed to extract bismuth metal from a bismuth-containing solution. The effects of additives, such as calcium lignosulfonate (CL), tetrabutylammonium chloride, β-naphthol, and lugalvan NES, on the morphology of bismuth deposits and electric energy consumption (EEC) were investigated in detail. The addition of CL as an additive could produce dense and flat bismuth deposits while the EEC is reduced simultaneously. Cyclic voltammetry and chronoamperometry analysis reveal the electrochemical behavior of bismuth deposition in the CL-assisted MSA medium, and the nucleation and growth of bismuth grains follow a 3D ″nucleation/growth″ mechanism. Under a cathode current density of 180 A/m2, a flat metallic bismuth deposit (purity > 99.96%) is produced in the presence of 0.5 g/L CL. The cathodic current efficiency (CCE) and EEC of membrane electrodeposition are 98.25% and 705 kWh/t Bi, respectively. Compared with chloride/hydrochloric acid, the bismuth membrane electrodeposition process in the CL-assisted MSA system has the advantages of being energy-saving, green, and safe, and it has a good application prospect in the field of bismuth hydrometallurgy.

Microstructure and Solute Concentration Analysis of Epitaxial Growth during Wire and Arc Additive Manufacturing of Aluminum Alloy

Microstructure and solute distribution have a significant impact on the mechanical properties of wire and arc additive manufacturing (WAAM) deposits. In this study, a multiscale model, consisting of a macroscopic finite element (FE) model and a microscopic phase field (PF) model, was used to predict the 2319 Al alloy microstructure evolution with epitaxial growth. Temperature fields, and the corresponding temperature gradient under the selected process parameters, were calculated by the FE model. Based on the results of macroscopic thermal simulation on the WAAM process, a PF model with a misorientation angle was employed to simulate the microstructure and competitive behaviors under the effect of epitaxial growth of grains. The dendrites with high misorientation angles experienced competitive growth and tended to be eliminated in the solidification process. The inclined dendrites are commonly hindered by other grains in front of the dendrite tip. Moreover, the solute enrichment near the solid/liquid interface reduced the driving force of solidification. The inclined angle of dendrites increased with the misorientation angle, and the solute distributions near the interface had similar patterns, but various concentrations, with different misorientation angles. Finally, metallographic experiments were conducted on the WAAM specimen to validate the morphology and size of the dendrites, and electron backscattered diffraction was used to indicate the preferred orientation of grains near the fusion line, proving the existence of epitaxial growth.

Effect of co-doping Nb2O5 with equal amount of Eu2O3 or In2O3 on the electrical properties of TiO2-based varistors

In this work, TiO2 varistors with enhanced nonlinear coefficient, low breakdown voltage, high relative dielectric constant and low dielectric loss were fabricated by co-doping of Nb2O5 with equal amount of Eu2O3 or In2O3. The results indicated an appropriate proportion of dopant could improve the comprehensive performance of the varistors. For the TiO2–Nb2O5–Eu2O3 system, the breakdown voltage and relative dielectric constant of the samples decreased first and then increased, while the nonlinear coefficient increased first and then decreased with the increase of doping content, the best value appears at the doping content of 1.00 mol% (E1mA = 2.14 V/mm, α = 4.6, εr = 2.72 × 105, tanδ = 0.25). It is noted that the secondary phase formed on the grain boundary as Eu2Ti2O7, thus the growth of grain was hindered and the dielectric performances deteriorated. By contrast, for the TiO2–Nb2O5–In2O3 system, the secondary phase was not found. As a result, a low breakdown voltage, low dielectric loss and high nonlinear coefficient were obtained, reaching the best at the doping content of 5.0 mol% (E1mA = 4.6 V/mm, α = 8.4, εr = 1.85 × 105, tanδ = 0.16). The influence of this doping system on the electrical performance can be explained by Internal Barrier Layer Capacitance theory (IBLC), which is related to the potential barrier at the grain boundary.

Additive manufacturing of fine-grain fully lamellar titanium aluminide alloys

Additive manufacturing (AM), or 3D printing, has attracted increased attention in producing metallic parts with complex geometries, but it has proved difficult to prepare equiaxed fine-grain parts because the high thermal gradient in solidification commonly conduces the formation of coarse columnar grains. This work shows a solution to fine-grain titanium aluminide (TiAl) alloys by designing a high-frequency thermal cycling to control the solid-state phase transformations in layer-by-layer AM. After solidification, the specially-designed high-frequency thermal cycling can significantly refine the microstructure by repeatedly inducing the nucleation of new grains and suppressing the growth of these newborn fine grains. Therefore, even if solidification leads to coarse columnar grains, equiaxed fine-grain microstructure can still be obtained by solid-state phase transformations. The resulting TiAl alloys have fine heteromorphic grains (∼50 μm) and a fully lamellar microstructure. These fine grains contribute to good strength-ductility balance at room temperature, and their irregular shape and fully lamellar microstructure restrict the flow and distortion of grains at high temperatures, which stands a chance to significantly increase the operating temperature of TiAl parts.

Alternative Approach to Modeling of Nucleation and Remelting in Powder Bed Fusion Additive Manufacturing

In powder bed fusion (PBF) of metals, energy of a laser or electron beam is utilized to form near‐net‐shaped parts from a powder bed. A very promising application of PBF lies in the direct control of the resulting microstructure by adjusting process parameters. Especially the intentional tilting of grains in one certain direction offers a complete new field of activity for additively produced parts specially designed for a given load case. Nucleation is essential for utilization of this effect. Herein, an alternative approach for modeling nucleation in PBF of metals is introduced. The model is not intended to cover every physical effect of this extreme complex phenomenon. Based on observations reported from different researchers in the field of PBF, a heuristic approach is applied to consider new grains within a cellular automata (CA)‐based crystal growth model for PBF. Utilizing the implicit capability of the CA to model competitive growth of grains, this approach does not require any additional computational capacities. The numerical calculations are carried out using a message–passing interface (MPI) parallelization on a high‐performance computing (HPC) cluster located at the Regional Research Center Erlangen. The numerical results are validated by means of experiments and electron backscatter diffractometric measurements.

Understanding the inoculation of aluminium grains during welding under the influences of high cooling rate and high temperature gradient

Inoculation is an important method to generate fine equiaxed grain structures in alloys during solidification, which is widely used in shape casting and direct chill casting processes of aluminium alloys. Different from the normal casting processes, welding and additive manufacturing can have ultrahigh cooling rate and temperature gradient during solidification. This makes the grain refinement by inoculation method more difficult. Therefore, a deeper understanding on the influences of cooling rate and temperature gradient on the nucleation and growth of grains is necessary. In this work, a numerical model is developed to quantitatively address the above influences. The model has been successfully applied to predict the grain size in the welding of aluminium alloys as a function of locations.

Effects of Sputtering Bias on the Material Characteristics of Ag Nanotwinned Films

The mechanisms of the formation of nanotwinned structures have recently attracted attention due to the advantageous properties of the structures. This paper describes the microstructural evolution of (111)-oriented nanotwins into huge (200)-oriented grains with grain sizes exceeding the film thickness. The results reveal that the initiation temperature for abnormal grain growth can be improved to nearly 300 °C by altering the microstructure of the nt-Ag films with the application of substrate bias during deposition. Densification of the nt-Ag films by substrate bias reduces the number of fine equiaxed grains in the transition layer and hence reduces the overall grain boundary energy of the nt-Ag film. The lower number of discontinuities that form within the nanotwinned layer retards the lateral growth of the columnar grains. The combination of the effects is conducive to the thermal stability of Ag nanotwinned films, implying promising future applications in microelectronics.

Effect of laser parameters on fatigue crack growth behavior of AZ31B magnesium alloy TIG welded bionic joint

The purpose of this study was to evaluate the effect of laser parameters on the fatigue crack growth (FCG) behavior of AZ31B Magnesium alloy TIG welded bionic joints with ‘soft-hard’ alternating structures prepared by laser bionic treatment (LBT). The changes of laser parameters promoted the growth of fine equiaxed grain with random texture, and average grain size was 5.12 ± 0.51–8.19 ± 1.02 μm, which significantly increased isotropy and microhardness. The average microhardness was 66.5 ± 1.68–70.85 ± 1.41 HV0.1, which increased by more than 23.11% compared with untreated welded joints. For bionic welded joints, the heat-affected zone (HAZ) had higher FCG resistance and cycle times than weld metal (WM), and the specimens with notch in the HAZ showed obvious crack deflection. It was found that FCG behavior showed significant microstructure sensitivity by comparing the FCG rate curve and crack opening displacement. The Paris parameters had a significant linear relationship and effectively reflect the FCG resistance of the different specimens. The fatigue fracture showed that the crack growth zone was mainly dominated by typical cleavage steps and secondary cracks. The fatigue fracture region contains quasi-cleavage and ductile fracture. A series of strengthening mechanisms were induced by LBT and could significantly affect FCG behavior, which promoted crack deflection and branching to increase the toughness of the joint, while the laser remelting zone increased the strength of the joint. Therefore, the synergistic effect of strength-toughness of the joint was obtained by LBT, which provided a new idea for optimizing the bionic design and improving the fatigue property of welded joints.

Effects of Borax and Grinding Alkalinity on the Reduction–Magnetic Separation of Beach Placer

The effect of borax on the reduction characteristics of Indonesia beach placers was investigated. The effect of grinding alkalinity on the magnetic separation of the reduced sample was also studied in this paper. The mineral phase transformation, microstructures of reduction, and magnetic separation products were analyzed to reveal the enhanced separation mechanism of titanium and iron in beach placer. The borax could effectively improve the metallization rate and the growth of iron grains in a reduced sample. When 3% borax was added to the reduction process, the metallization rate of the reduced beach placer reached 95.64%, and the metal iron grains grew to about 50 μm. Adjusting the grinding alkalinity could prevent the metallic iron from being oxidized and promote the monomer dissociation between mineral particles. The iron powder concentrates with 94.07% total Fe and vanadium-rich titanium slag with 36.32% Ti were obtained by grinding magnetic separation as the grinding alkalinity pH was 13.

Effect of Wide-Spectrum Pulsed Magnetic Field on Solidification Structure of Pure Al at Constant Flow Velocity

The electromagnetic force generated by a pulsed magnetic field within a metal melt leads to changes in the internal temperature and flow fields of the molten metal, thus improving the solidification of the metal structure. Using the combination of a solidification test, experimental simulation and theoretical analysis, this study simulated the distribution of both electromagnetic force and the flow field in a metal melt under wide-spectrum pulse conditions, and studied the influence of a wide-spectrum pulsed magnetic field on the solidification structure of pure aluminium with a constant flow velocity. The results of this study show that the structural refinement of the solidification of pure aluminium can be different, in spite of equal flow velocity. Furthermore, this study shows that an applied time-averaged electromagnetic force causes crystal nuclei to pass through the solid–liquid interface boundary layer and promotes the growth of crystal grains. These grains flowed with the melt flow field to achieve both refinement and homogenization of the solidified structure.

Refining the Grain Size of Zinc Electrodeposit by Pb2+ Ion Grinding for Compact and Stable Zinc Anode

AbstractRecently, aqueous zinc‐based batteries (AZBs) have become a promising candidate for energy storage devices due to the high safety of aqueous electrolytes and the appealing features of Zn anodes, for example, low cost and high theoretical capacity. However, the excessive growth of Zn electrodeposits as well as the uneven stacking of large hexagonal Zn crystal units always render loose and irregular electrodeposition or even dendritic growth, which seriously deteriorates the actual performance of AZBs. Herein, to refine the grain size of Zn electrodeposits, a trace of Pb2+ ions as a novel electrolyte additive is performed to inhibit the growth of Zn grain during the Zn electrodeposition. Owing to the higher adsorption energy of Pb2+ ions on Zn crystal when compared with Zn2+ ions, the strongly positively‐charged Pb2+ ions are tightly absorbed on the typical crystal planes of initially‐formed Zn nuclei, which block the way for the subsequent absorption and electroreduction of Zn2+ ions. As a result, the Pb2+ ions‐containing electrolyte refines the grain size of Zn electrodeposits from 7.43–7.87 μm to 0.88–2.26 μm, and affords a high reversibility of Zn plating/stripping behavior with a high Coulombic efficiency of 99.9 % over 1000 cycles.

Texture evolution related to static recrystallization during annealing of TA15 alloy tube formed by flow forming

In this work, the static recrystallization (SRX) responsible for texture development during the annealing of the TA15 flow-formed part was investigated by comparatively analyzing the SRX microstructure and texture under different pre-strains and annealing temperatures. The results indicated that SRX grains are produced by discontinuous (DSRX) and continuous (CSRX) mechanisms, while the dominant mechanism depends on the pre-deformation and annealing conditions. At a smaller pre-strain, DSRX is prevalent, and some DSRX grains whose {11–20} axes are parallel to the normal direction preferentially grow via the long-range migration of 60–90° grain boundaries. With increased pre-strain, the dominant mechanism gradually changes from DSRX to CSRX. At a larger pre-strain, the CSRX always prevails. Most CSRX grains have an orientation similar to deformed grains which exhibit non-standard basal texture. However, CSRX grains related to deformation-induced intense and localized shearing (CSRX-DILS) show different orientations. The occurrence of DSRX and CSRX-DILS has a minor effect on global annealing texture type owing to the lower fraction but only weakens its concentration. Most CSRX grains primarily affect the texture intensity based on their growth rate. At a pre-strain of 3.36, a higher misorientation angle induced by flow forming can accelerate the growth of CSRX grains, thereby forming an intense CSRX texture and consequently strengthening the global texture. However, the CSRX grain growth at 6.10 pre-strain is limited, resulting in a weaker global texture. With the above effects by SRX, the global annealing texture inherits the features of deformation texture, i.e., non-standard basal texture, but its intensity decreases.

High platinum utilization for proton exchange membrane fuel cells via low-temperature substrate sputtering on acid-treated carbon nanotube sheet

Low-temperature substrate sputtering on an acid-treated carbon nanotube (ATCNT) sheet result in high platinum (Pt) utilization. A thin catalyst layer (CL, ∼300 nm) with a high density of Pt and vacancies was formed on the ATCNT sheet (Pt-deposited ATCNT). It replaced the commercial carbon-supported Pt and microporous layers of the proton exchange membrane fuel cells. The substrate temperature (Tsub: 20, 5, −10, −30, and −50 ℃) contributed to crack formation in the porous Pt thin film and restrained the growth of Pt grain (average grain size from 8.60 to 3.97 nm). Below −10 ℃, the crack and grain size changed significantly. The increased crack and smaller grain size enlarged the Pt surface area while forming vacancies; this was confirmed via physical and electrochemical characterizations. When compared to Pt-deposited ATCNT (Tsub = 20 ℃, 47.61 m2 g-1Pt), the electrochemical surface area of Pt-deposited ATCNT (Tsub = -50 ℃, 72.06 m2 g-1Pt) increased by 51.35%. We present a high Pt mass-specific peak power density (5.93 kW g-1Pt), focusing on a strategy to increase Pt utilization.

Structure forming processes in economically alloyed shipbuilding steel of 890 MPa yield strength level with a bainite-martensite structure when microalloyed with vanadium

The kinetics of growth of austenite grains during heating, the features of the processes of dynamic and static recrystallization occurring under various temperature-deformation regimes of hot plastic deformation have been studied. Phase transformations have been studied during continuous cooling of hot-worked austenite in a low-carbon low-alloy steel with a guaranteed yield strength of 890 MPa. As a result, the boundary temperature-deformation conditions for the formation of a finely dispersed bainite-martensite structure were established, on the basis of which technological modes for the production of thick-plate rolled products in industrial conditions were developed. The structure and properties of rolled sheets 35 mm thick from shipbuilding sparingly alloyed steel of strength level 890 are presented.

Effect of post laser annealing on stacked structure of CZTS thin film solar cells

Cu2ZnSnS4 (CZTS) is an attractive material for thin film solar cells because all its constituents are Earth-abundant elements, and it’s a direct transition semiconductor with a band gap energy of 1.5 eV that is suitable for absorbing solar light spectrum effectively. CZTS is generally formed by precursor formation followed by heat treatment at 500 °C–600 °C to enhance the growth of crystal grain. In this work, a novel CZTS crystal grain re-growth process using post-laser annealing was investigated. 445 nm wavelength laser irradiation was performed on the Al-doped ZnO/CdS/CZTS/Mo/substrate stacked structure. X-ray diffraction and scanning microscope showed the CZTS crystal grain enlargement. Solar cells were fabricated on those structures and the external quantum efficiency was found to be improved especially at 500–1000 nm wavelength light absorption. That resulted in a short circuit current improvement.