Grain Growth Research Articles

Structural properties and recrystallization effects in ion beam modified B20-type FeGe films

Disordered iron germanium (FeGe) has recently garnered interest as a testbed for a variety of magnetic phenomena as well as for use in magnetic memory and logic applications. This is partially owing to its ability to host skyrmions and antiskyrmions—nanoscale whirlpools of magnetic moments that could serve as information carriers in spintronic devices. In particular, a tunable skyrmion–antiskyrmion system may be created through precise control of the defect landscape in B20-phase FeGe, motivating the development of methods to systematically tune disorder in this material and understand the ensuing structural properties. To this end, we investigate a route for modifying magnetic properties in FeGe. In particular, we irradiate epitaxial B20-phase FeGe films with 2.8 MeV Au4+ ions, which creates a dispersion of amorphized regions that may preferentially host antiskyrmions at densities controlled by the irradiation fluence. To further tune the disorder landscape, we conduct a systematic electron diffraction study with in situ annealing, demonstrating the ability to recrystallize controllable fractions of the material at temperatures ranging from ∼150 to 250 °C. Finally, we describe the crystallization kinetics using the Johnson–Mehl–Avrami–Kolmogorov model, finding that the growth of crystalline grains is consistent with diffusion-controlled one-to-two dimensional growth with a decreasing nucleation rate.

The control of fully equiaxed microstructure in laser welding Al-mg-Er-Zr alloy

The Al-Mg-Er-Zr alloy with higher strength and thermal stability was focused on as promising candidate in shipbuilding field. However, when this material was joined and manufactured with high-efficiency laser welding technology, the unmelted strengthening Al3(Er,Zr) particles segregated in weld bead and formed the discontinuous fine equiaxed and column grain zone simultaneously which decreased the joint strength. In order to solve the segregation behavior of precipitates and obtain the fully equiaxed microstructure of weld seam, the transverse scanning adjustable-ring-mode laser technology was employed. The stirring effect in the molten pool pushed the unmolten Al3(Er,Zr) dispersoids from the fusion line to the weld center. At the same time, the temperature distribution of weld seam was more homogeneous as inhibited the growth of column grain. Finally, the homogeneously distributed Al3(Er,Zr) particles as well as higher supercooling degree in the molten pool refined the microstructure. The optimal weld joint was obtained, which displayed remarkable mechanical performance with tensile strength of 389 ± 1 MPa, up to 93.3 % of base metal.

Dynamics of zinc dendritic growth in aqueous zinc-based flow batteries: Insights from phase field-Lattice-Boltzmann simulations

This paper employs a phase-field-Lattice-Boltzmann method incorporating ion transport mechanisms in the electrolyte, including diffusion, electromigration and convection, to investigate the growth of Zn dendrite in aqueous zinc-based flow batteries. It was found that during electrodeposition, Zn2+ ions enrich in front of the dendrite tip to form a boundary layer of Zn2+ ion concentration due to diffusion and electromigration under the static state u=0.0m/s, resulting in the so called “tip effect”. Predicted results show that both the exchange current density and the diffusion coefficient have similar effect to the deposited dendritic growth dynamics according to the Damkohler number. It can be found that the growth pattern of deposited zinc dendrites is significantly influenced by the existing electrolyte forced flow. Predicted results show that the increase in flow rate results in the advective transport of Zn2+ ions in the electrolyte, which consequently leads to a reduction in the actual growth direction angle. This facilitates the formation of a dendrite-free deposition pattern and improves the performance and stability of the battery. Moreover, the competitive growth of polycrystalline grains during electrodeposition was investigated under the static state and the flowing state. Results show that the unfavorably orientated dendrites always lie behind the favorably orientated ones during electrodeposition.

Infrequent trigonal crystal system CoNb thin films: investigation of growth behavior, microstructure and magnetic properties

Magnetic films with excellent initial permeability, high resonance frequency, suitable anisotropy fields, and well compatibility are highly desirable for on-chip inductors and transformers. Here, Co90Nb10 thin films with different thicknesses are grown on Si substrates of different orientations using an electron beam evaporation technique. A rare trigonal crystal structure of the CoNb films was observed and was virtually unaffected by substrate orientation. Films grown exhibit thickness-dependent magnetic behavior: coercivity initially decreases and then increases, while the in-plane anisotropy field progressively diminishes. Meanwhile, the initial permeability increases initially before decreasing, and the resonance frequency continuously decreases as the thickness increases. The resultant Co90Nb10/Si (111)-50 nm film displays a high μi of 496 and a great fr of 2.1GHz. The improved magnetic performance originates from the smaller crystallite size, smoother surface roughness, and suitable in-plane anisotropy field. Moreover, the in-plane anisotropy transition to the out-of-plane anisotropy in the Co90Nb10 films whose thickness exceeds 70nm, which is attributed to the growth of the coarse columnar grains once the critical thickness is exceeded. This study demonstrates the correlation between the growth behavior, microstructure, and magnetic properties of CoNb thin films, presenting a new potential for utilizing Co-based thin films in high-frequency applications.

Enhancing the performance and stability of organometal halide perovskite by using a feasible and economical interface material

AbstractOrganometal halide perovskites (OHPs) are one of the viable options for solar absorber materials because their power conversion efficiencies are getting better and better over time. In the conventional n-i-p-based configuration, TiO2 has been widely used as an electron transport layer (ETL). However, a number of constraints, such as low electron mobility and a mismatched band alignment with perovskite, restrict future advances in solar performance and device environmental stability. As a result, SnO2 has garnered a lot of interest as a potential replacement due to the comparatively low manufacturing temperature, better electron mobility and appropriate energy alignment w.r.t perovskite. In this experimental work, the primary emphasis was placed on enhancing the efficiency as well as the stability of OHPs by performing interface engineering at the ETL (SnO2)/perovskite interface. We improved the surface quality of the SnO2 ETL layer by using a material called 8-Hydroxyquinoline, which was quite inexpensive, and we prepared a favourable plane for the deposition of perovskite. Remarkably, the proposed surface modification material made the SnO2 layer easier to wet and impacted the growth of perovskite grains. This made the perovskite layer more compact and smooth. Our experimental findings imply that the OHPs’ enhanced charge recombination resistance and decreased charge transfer resistance are caused by effective defect passivation at the junction of the SnO2 and perovskite films, as well as a decrease in recombination due to unwanted trap states. The fabricated cell produced a power conversion efficiency (PCE) of 20.42%, higher than a PCE of 17.9% obtained for a device without surface modification. The proposed material for changing the surface also made OHPs more stable by reducing the surface paths for the reaction with humidity and reducing the amount of extra PbI2 in the perovskite layer. Various research groups have investigated the modification of SnO2 ETL using interfacial engineering methods and have contributed to enhancing OHPs’ solar performance and device stability.

Nanosilicates and molecular silicate dust species: properties and observational prospects

Silicate dust is found in a wide range of astrophysical environments. Nucleation and growth of silicate dust grains in circumstellar environments likely involves species with diameters ranging from <1 nm (molecular silicates) to a few nanometers (nanosilicates). When fully formed silicate grains with sizes ∼0.1 μm enter the interstellar medium, supernovae shockwaves cause collision-induced shattering which is predicted to redistribute a significant proportion of the silicate dust mass into a huge number of nanosilicates. This presumed population has thus far not been unambiguously confirmed by observation but is one of the main candidates for causing the anomalous microwave emission. By virtue of their extreme small size, nanosilicates and molecular silicates could exhibit significantly different properties to larger silicate grains, which could be of astrochemical and astrophysical importance. Herein, we briefly review the properties of these ultrasmall silicate dust species with a focus on insights arising from bottom-up atomistic computational modelling. Finally, we highlight how such modelling also has the unique potential to predict observationally verifiable spectral features of nanosilicates that may be detectable using the James Webb Space Telescope.

Joining UFG Copper by Means of the W2Mi Machine

Joining metals with ultra-fine grains (UFG) is very difficult. UFG metals have a grain size of less than 1 µm. Exceeding the recrystallization temperature causes the growth of ultra-fine grains and degradation of the material’s mechanical properties. The process temperature is influenced by the amount of energy supplied to the joint, which depends on the power and time. Conventional machines are not adapted to effectively conduct the process in extremely short friction times. For this reason, this article presents the welding process on the W2Mi prototype machine. The design of this machine allows the joining of UFG metals in less than 100 ms. UFG material produced using the hybrid SPD (Severe Plastic Deformation) process from technically pure CW004A(Cu-ETP) copper was used for welding tests. The joint obtained on the W2Mi machine did not show any significant degradation signs of mechanical properties. At the same time, an increase in microhardness in the joint area by approximately 4% was noticed. The obtained results prove that the process of joining UFG metals was successfully carried out.

Effects of (ZrHfNbTaTiMo)C addition on microstructure and mechanical properties of WC-10Co cemented carbides

The WC-10Co cemented carbides containing (ZrHfNbTaTiMo)C high-entropy carbide with satisfactory comprehensive mechanical properties were successfully manufactured by conventional vacuum sintering. The influences of (ZrHfNbTaTiMo)C contents (0, 0.5, 1, 5, 10 wt.%) on the densification degree, microstructure, grain size and mechanical properties were investigated. The results showed that the addition of (ZrHfNbTaTiMo)C deteriorated the wettability between Co and WC, leading to a decline in relative density. (ZrHfNbTaTiMo)C inhibited the growth of WC grains, and the average grain size of WC continued to decrease with the increase of (ZrHfNbTaTiMo)C content. After adding (ZrHfNbTaTiMo)C, the hardness and toughness rose first and then descended. When the content of (ZrHfNbTaTiMo)C was 1 wt.%, the hardness and toughness reached the maximum value of 16.77 GPa and 10.45 MPa·m1/2, respectively. Moreover, the structure was a dual-grain structure, with many high-aspect-ratio rectangular grains in the sintered samples added by 0.5 wt.% and 1 wt.% (ZrHfNbTaTiMo)C. This special structure not only enhanced the hardness, but also compensated the loss of toughness. After doping 0.5 wt.% CeO2, the hardness decreased slightly, while the toughness increased to 12.42%.

Densification behavior of ultrafine WC‐based composites with AlxCoCrCuFeNi high‐entropy alloy binders

AbstractUltrafine WC‐based cemented carbides with 10 wt.%AlxCoCrCuFeNi high‐entropy alloy (HEA) binders were fabricated by spark plasma sintering, and the effects of HEA binders on the densification behavior of the WC‐HEA cemented carbides were studied. The densification of the WC‐HEA cemented carbide, as well as traditional WC‐Co, can be divided into the slow densification stage, rapid densification stage, and final densification stage. The densification behavior of the WC‐HEA cemented carbides largely depends on the performance of the HEA binder during the SPS process. The sluggish diffusion effect of HEA weakens the diffusion of W atom on the powder particle surface and inhibits the growth of WC grain. Consequently, the disappearance of pores in the sintered compact is hindered, which leads to a low relative density of the WC‐HEA cemented carbide. With the increase of Al content, the inhibitory effect of the AlxCoCrCuFeNi binder on the growth of WC grain is suppressed, and an Al‐containing phase with a low melting point is more likely to form. Therefore, the relative density of the WC‐10 wt.%AlxCoCrCuFeNi cemented carbide raises linearly with increasing Al content of the HEA binder.

A Study on the Influence of the Rotating Speed and Load on the Grain Structure and Wear Properties of Bearing Steel GCr15 During Bearing Service

In order to study the wear failure mechanism and structure evolution law of bearings under different speeds and contact loads, an elastoplastic model of a 7009AC bearing was established in this paper. The stress, temperature rise and grain size during dry friction and wear of the bearing inner ring were simulated with the finite element method. The effects of the inner ring speed, load pressure and other parameters on the wear rate were studied. The relationship between the grain size and yield strength of GCr15 bearing steel was obtained. The effects of the initial grain size, rotational speed and load pressure on the bearing wear failure were studied. The evolutions of the grain size during service were predicted by means of the dynamic grain recrystallization (DRX), static grain recrystallization (SRX) and grain growth (GG) subprogram. The results show that the contact stress has a more significant effect on the early failure wear than the bearing speed, and the increase in the contact stress will aggravate the wear rate of the bearing inner ring. Under the same working conditions, the smaller the grain size, the more significant the influence of the cycle times on wear was. The heat-affected zone produced a local high temperature in the contact area, temperature flashes of up to 580 °C could occur in the central contact area, and the temperature decreased gradually with the increase in the depth from the contact area. It is noteworthy that both the surface and the subsurface of the material produced grain refinement; the grain size was refined from 20 μm to 0.4–12 μm under different working conditions. And the degree of refinement of the subsurface was higher than that of the surface.

Effect of Adding Al2O3 Ceramic in Wire Arc Additive Manufacturing 308LSi Stainless Steel

Wire Arc Additive Manufacturing (WAAM) is a process that allows for efficient in-situ production of components or remanufacturing based on its capabilities to produce at a greater rate of deposition at a lower cost. However, WAAM components suffer from heat dissipation during the deposition process that causes the growth of coarse columnar grains resulting in poor mechanical properties that will limit industrial applications. Thus, this research investigates the role of introducing Al2O3 ceramic powder particle inoculants to the AWS A5.9 ER308LSi stainless steel wall structure to enhance the mechanical performance capabilities by refining the grain process. During deposition, the Al2O3 ceramic powder particle was manually added to each layer when the temperature drops to 150ᵒC. A complete series of tensile testing was executed to bridge those knowledge gaps. WAAM walls were fabricated and the microstructure of the sample were analysed. The results revealed that the highest tensile strength of WAAM SS308LSi components recorded at 560 MPa in the deposition direction, which increased by 6% compared to the non-inoculated sample. The improvement was due to the success of grain refinement and heterogeneous nucleation. The study demonstrates the potential of the technique to improve the mechanical properties and microstructure during WAAM components fabrication or remanufacturing.

Effect of different sintering process on mechanical properties and microstructure of TiB2‐TiN‐based cermet material

AbstractTiB2‐TiN‐NiTi cermet material was prepared by spark plasma sintering technology (SPS). The effects of the sintering process on their relative density, mechanical properties, and microstructure were studied with 74TiB2‐20TiN‐6NiTi as the composition system. The results showed that TiB2‐TiN‐based cermet material with the best mechanical properties was prepared under the sintering temperature of 1650°C and the holding time of 5 min. The relative density, flexural strength, fracture toughness, and hardness are 97.7%, 762.7 MPa, 5.5 MPa·m1/2, and 18.47 GPa, respectively. However, if the holding time keeps for 3 min under the sintering temperature of 900°C and then 5 min under the sintering temperature of 1650°C, the relative density, flexural strength, fracture toughness, and hardness of TiB2‐TiN‐based cermet reach 99.1%, 931.3 MPa, 5.8 MPa·m1/2, and 19.15 GPa, respectively. In comparison, they have been greatly improved by 1.4%, 22.1%, 5.5%, and 3.7%, respectively. The final holding time can also affect the mechanical properties of TiB2‐TiN‐based cermet material. When it is higher than 5 min, it will cause irregular growth and abnormal growth of grains, which can decrease the flexural strength of TiB2‐TiN‐based cermet material.

Phase Formation, Microstructure, and Permeability of Fe-Deficient Ni-Cu-Zn Ferrites (II): Effect of Oxygen Partial Pressure

We have investigated the phase formation, microstructure, and permeability of stoichiometric and Fe-deficient Ni-Cu-Zn ferrites of composition Ni0.30Cu0.20Zn0.50+zFe2−zO4−(z/2) with 0 ≤ z ≤ 0.06 sintered at 1000 °C in various oxygen partial pressures pO2, which range from 0.21 atm down to 10−5 atm. The density of the sintered samples is almost independent of the pO2, whereas the grain size of the Fe-deficient ferrites decreases in more reducing atmospheres. Stoichiometric ferrites show a regular growth of single-phase ferrite grains if sintered in air. Sintering at pO2 ≤ 10−2 atm leads to the formation of a small amount of Cu2O at grain boundaries and triple points. Fe-deficient compositions (z > 0) form Cu-poor stoichiometric ferrites, which coexist with a minority CuO phase homogeneously distributed between the grains after sintering in air. At pO2 ≤ 10−2 atm, the CuO grain boundary phase starts to transform into Cu2O, which concentrates at some triple points at pO2 = 10−2 atm, and it is more homogeneously distributed between the ferrite grains at the lower pO2. Formation of the Cu oxide second phases is investigated using XRD, SEM, and EDX. The permeability at 1 MHz of the stoichiometric ferrites (z = 0) is between µ′ = 200 and µ′ = 300 within the studied range of the pO2. The permeability at 1 MHz of the Fe-deficient samples decreases with the pO2, e.g., from µ′ = 750 at pO2 = 0.21 atm to µ′ = 320 at pO2 = 10−5 atm for z = 0.02, respectively.

Effect of thermal annealing on the C(V) characteristic of MSOS structure

In this study, we explore the capacitance C(V) characteristics of metal/polysilicon/silicon oxide/monosilicon (MSPOS) structures. The obtained C(V) curves allow us to analyze the behavior of the capacitance at high frequency as well as the effect of thermal annealing on these characteristics. These reveal particular characteristics of MOS and MOSP structures. The study shows that, under different bias voltages, the obtained curves differ depending on whether the bias is positive or negative. When applying positive voltages, we observe a certain configuration of the curves, while with negative voltages, another configuration appears, always indicating a combination of the C(V) characteristics specific to MOS and MOSP structures. For a substrate doped at a concentration ND = 2.1018 cm-3, we observed a significant variation of the capacitance under the effect of thermal annealing. This thermal treatment, by modifying the crystalline structure of polysilicon, allows the modification of the MOS capacitance. It influences in particular the growth of polysilicon grains as well as the grain boundaries. The results of this study highlight the effect of thermal annealing on polysilicon-based MOS structures. These observations highlight the effect of thermal annealing on the properties of MSPOS structures, by providing a detailed view of how bias voltages and thermal treatment interact to modulate the capacitance characteristics.

The influence of grain boundary character distribution on the high-temperature creep behavior and damage mechanism of Inconel 718

Thermomechanical processing as the main method of grain boundary engineering (GBE), can modulate the grain boundary character distribution (GBCD) in turn affects the mechanical properties and stability of alloys at high temperature. In this study, the microstructural evolution and creep behaviour of different GBCD alloys were investigated during the creep process at 660 °C and 690 MPa, and the effect of GBCD on high temperature creep behaviour and associated damage mechanism were analyzed from the viewpoints of grain size, recrystallisation and dislocation motion. The findings indicate that the larger grain size prolongs the steady-state creep duration, which is beneficial to the increase of creep life. During the creep process, the coherent twin boundary is susceptible to transformation into non-coherent twin boundary, resulting in the loss of its difficult-to-migrate properties and increased susceptibility to becoming recrystallisation nucleation sites. Furthermore, the formation of twins is conducive to the stimulation of dynamic recrystallisation. Discontinuous dynamic recrystallisation, characterised by the nucleation and growth of grains, occurs concurrently with continuous dynamic recrystallisation with strain gradients and orientations. The combined effect of these processes is grain boundary slip and restricted dislocation migration, which in turn alter the creep properties of the alloy.

Grain-Boundary Elimination via Liquid Medium Annealing toward High-Efficiency Sb2Se3 Solar Cells.

Suppression of charge recombination caused by unfavorable grain boundaries (GBs) in polycrystalline thin films is essential for improving the optoelectronic performance of semiconductor devices. For emerging antimony selenide (Sb2Se3) materials, the unique quasi-1D structure intensifies the dependence of GB properties on the grain size and orientation, which also increases the impact of defects related to grain structure on device performance. However, these characteristics pose significant challenges in the preparation of thin films. In this study, a novel annealing approach using ammonia-thiourea is developed mixed solution as the liquid medium (LM) to finely regulate the crystallization of Sb2Se3 films, resulting in micron-sized large grains with enhanced [hk1] orientation and fewer defects. Mechanistic studies indicate that the intermediate phase formed at the GBs promotes the growth of large grains. Moreover, LM creates a closed and uniform environment for thin-film annealing, suppressing the volatilization of Se and reducing the types of deep-level defects. Consequently, the film delivers a device efficiency of 9.28%, the highest efficiency achieved for Sb2Se3 solar cells fabricated via thermal evaporation. Hence, this study provides a facile and effective annealing method for controlling the crystallization of low-dimensional materials and offers valuable guidance for the development of chalcogenide materials.

Effects of ion irradiation induced phase transformations and oxygen vacancies on the leakage current characteristics of HfO2 thin films deposited on GaAs

Abstract We report on ion-induced phase transformations, defect dynamics related to oxygen vacancies and the resulting leakage current characteristics of RF sputtered HfO2 thin films grown on GaAs. A systematic growth of HfO2 grains and ion prompted phase transformations of HfO2 to crystalline phases such as monoclinic and tetragonal/orthorhombic (mixed phase) in otherwise amorphous HfO2 thin films have been observed after irradiation. At lower fluences, ion induced enhancement in the dielectric properties of HfO2 thin films resulted in a reduction in the leakage current, whereas ion prompted defect formation at higher fluences caused a systematic increase in the leakage current density. Further, the effects of Poole-Frenkel tunneling and Fowler-Nordheim tunneling on the leakage current have also been investigated. These mechanisms showed the existence of impurities in the as-grown films. Photoluminescence study suggests that the variation in the defect configuration related to O-vacancies and the slight shift in the peak positions due to swift heavy ion irradiation are responsible for the observed changes in electrical characteristics. This study offers worthwhile information for considering the effects of electronic excitation prompted defect annealing and defect creation on the performance of HfO2/GaAs based photonic and optoelectronic devices, particularly, when such devices are operated in a radiation harsh environment.

Embedding thermostable rGO/SiCxOy composite phase in SiC fibers for improved high temperature resistance

Silicon carbide (SiC) fibers exhibit exceptional potential in harsh environment applications such as aerospace and nuclear energy. However, the high temperature mechanical properties of SiC fibers are still limited by the unmanageable microstructure. Herein, we propose an amorphous phase stabilizing strategy by embedding thermostable reduced graphene oxide/silicon oxycarbide (rGO/SiCxOy) composite phase in SiC fibers via the polymer-derived ceramic (PDC) method. Homodisperse vinyl-bridged graphene oxide/polycarbosilane (PCS) precursor is designed to transform into rGO/SiC fibers, accompanied by the formation of a more stable rGO/SiCxOy composite structure that intersperses between SiC grains. Additionally, the rGO plays a role in suppressing the decomposition of SiCxOy and the rapid growth of SiC grains at elevated temperatures. As a result, the tensile strength of as-received 0.5%-rGO/SiC (2.64 GPa) and 1%-rGO/SiC (2.28 GPa) fibers are elevated by 40 % and 21 % respectively. Noteworthily, 1%-rGO/SiC fibers maintain a tensile strength of 0.57 GPa even after undergoing 1700 °C heat treatment, while the 0%-rGO/SiC fibers have nearly lost mechanical strength after annealing at 1600 °C. Consequently, the rGO exhibits significant potential for improving the high temperature resistance of ceramic fibers.

Modulation of Sb2Se3 longitudinal growth by CdCl2 treatment for high-performance photodetectors

Antimony selenide (Sb2Se3) has attracted much attention in photodetectors (PDs) for its abundant reserves, low toxicity, low price, and high absorption coefficient. While the PDs performance has been limited by the transverse growth pattern of Sb2Se3. In this study, CdCl2-treated SnO2 electron transport layer (ETL) substrate has been used to grow Sb2Se3, which induces the preferential growth of Sb2Se3 along the (002) direction, and the grains are transformed from transverse to longitudinal. This enhanced carrier mobility, resulted in improved photocurrent (0.489 mA), responsivity (70 mA W−1), and specific detectivity (9.29 × 1013 Jones) than the pristine device, which is enhanced by 18, 17, and 8 times, respectively. The results indicate that the CdCl2 treatment effectively optimizes the SnO2 ETL interfacial properties and regulates the longitudinal growth of Sb2Se3 grains, which guides the further development of Sb2Se3 PDs.

Enhanced Sintering of Refractory Protonic Ceramic Electrolyte by Dual Phase Reaction

Proton conducting oxides, commonly used as electrolytes in ceramic electrochemical cells, boast remarkable proton conductivity, facilitating efficient energy conversion. However, their refractory nature presents challenges in achieving the ideal electrolyte structure and properties. Our novel approach utilizes reaction sintering to effectively lower the electrolyte's sintering temperature, resulting in stable and excellent electrolytic properties. This method transforms a two-phase mixture (comprising fast and slow-sintering phases) into a single-phase compound and densifies the electrolyte in a single-step heating cycle. During the reaction sintering process, rapid growth of the fast-sintering phase grains occurs due to its superior sinterability, aided by Ostwald ripening exhibited by the smaller slow-sintering phase. Lowering the sintering temperature preserves the intended initial stoichiometry of the electrolyte material, leading to a significant enhancement in electrochemical performance.