Nanometer-sized Grains Research Articles

Suppressing atomic diffusion with the Schwarz crystal structure in supersaturated Al-Mg alloys.

High atomic diffusivity in metals enables substantial tuneability of their structure and properties by tailoring the diffusional processes, but this causes their customized properties to be unstable at elevated temperatures. Eliminating diffusive interfaces by fabricating single crystals or heavily alloying helps to address this issue but does not inhibit atomic diffusion at high homologous temperatures. We discovered that the Schwarz crystal structure was effective at suppressing atomic diffusion in a supersaturated aluminum-magnesium alloy with extremely fine grains. By forming these stable structures, diffusion-controlled intermetallic precipitation from the nanosized grains and their coarsening were inhibited up to the equilibrium melting temperature, around which the apparent across-boundary diffusivity was reduced by about seven orders of magnitude. Developing advanced engineering alloys using the Schwarz crystal structure may lead to useful properties for high-temperature applications.

Achieving high strength and ductility in a heterogeneous-grain-structured CrCoNi alloy processed by cryorolling and subsequent short-annealing

Despite possessing otherwise desirable properties, the single-phase CrCoNi medium entropy alloy (MEA) with coarse grain structure generally suffer from disappointingly low strength, which severely limits its practical utility. Grain refinement can make this material several times stronger, but this benefit typically comes at a drastic loss of ductility. In this work, a thermo-mechanical processing method comprised of cryorolling and subsequent short-annealing is proposed for design of CrCoNi MEA with heterogeneous grain structure (HGS) to achieve outstanding mechanical properties. Firstly, a highly non-uniform pre-deformed nanostructure containing nanotwin bundles, nanometer-sized grains and dislocation structures regions was formed by cryorolling. Subsequent short-annealing of the as-cryorolled CrCoNi MEA at 700 °C/10 min yielded a three-level HGS with grain sizes spanning from a few nanometers up to several tens of micrometers. The as-prepared heterogeneous CrCoNi MEA exhibits ultra-high yield and tensile strengths of 1123 ± 14 MPa and 1453 ± 21 MPa, respectively, together with a large tensile ductility with a uniform elongation of 31.2%, achieving an enhanced strength–ductility combination relative to those fabricated by conventional rolling and subsequent annealing. The extraordinary combination of strength and ductility came from heterogeneous grain and residual cryo-deformation structural effect.

Dust changes in Sakurai’s Object: new PAHs and SiC with coagulation of submicron-sized silicate dust into 10 μm-sized melilite grains

ABSTRACT 6–14 μm Spitzer spectra obtained at 6 epochs between 2005 April and 2008 October are used to determine temporal changes in dust features associated with Sakurai’s Object (V4334 Sgr), a low mass post-AGB star that has been forming dust in an eruptive event since 1996. The obscured carbon-rich photosphere is surrounded by a 40-milliarcsec torus and 32 arcsec PN. An initially rapid mid-infrared flux decrease stalled after 2008 April 21. Optically thin emission due to nanometre-sized SiC grains reached a minimum in 2007 October, increased rapidly between 2008 April 21–30 and more slowly to 2008 October. 6.3-μm absorption due to PAHs increased throughout. 20 μm-sized SiC grains might have contributed to the 6–7 μm absorption after 2007 May. Mass estimates based on the optically thick emission agree with those in the absorption features if the large SiC grains formed before 1999 May and PAHs formed in 1999 April–June. Estimated masses of PAH and large-SiC grains in 2008 October, were 3 × 10−9 M⊙ and 10−8 M⊙, respectively. Some of the submicron-sized silicates responsible for a weak 10 μm absorption feature are probably located within the PN because the optical depth decreased between 2007 October and 2008 October. 6.9-μm absorption assigned to ∼10 μm-sized crystalline melilite silicates increased between 2005 April and 2008 October. Abundance and spectroscopic constraints are satisfied if $\lesssim$2.8 per cent of the submicron-sized silicates coagulated to form melilites. This figure is similar to the abundance of melilite-bearing calcium–aluminium-rich inclusions in chondritic meteorites.

Quantification of electron accumulation at grain boundaries in perovskite polycrystalline films by correlative infrared-spectroscopic nanoimaging and Kelvin probe force microscopy

Organic–inorganic halide perovskites are emerging materials for photovoltaic applications with certified power conversion efficiencies (PCEs) over 25%. Generally, the microstructures of the perovskite materials are critical to the performances of PCEs. However, the role of the nanometer-sized grain boundaries (GBs) that universally existing in polycrystalline perovskite films could be benign or detrimental to solar cell performance, still remains controversial. Thus, nanometer-resolved quantification of charge carrier distribution to elucidate the role of GBs is highly desirable. Here, we employ correlative infrared-spectroscopic nanoimaging by the scattering-type scanning near-field optical microscopy with 20 nm spatial resolution and Kelvin probe force microscopy to quantify the density of electrons accumulated at the GBs in perovskite polycrystalline thin films. It is found that the electron accumulations are enhanced at the GBs and the electron density is increased from 6 × 1019 cm−3 in the dark to 8 × 1019 cm−3 under 10 min illumination with 532 nm light. Our results reveal that the electron accumulations are enhanced at the GBs especially under light illumination, featuring downward band bending toward the GBs, which would assist in electron-hole separation and thus be benign to the solar cell performance.Correlative infrared-spectroscopic nanoimaging by the scattering-type scanning near-field optical microscopy and Kelvin probe force microscopy quantitatively reveal the accumulated electrons at GBs in perovskite polycrystalline thin films.

Intrinsic Defects in MoS2 Grown by Pulsed Laser Deposition: From Monolayers to Bilayers.

Pulsed laser deposition (PLD) can be considered a powerful method for the growth of two-dimensional (2D) transition-metal dichalcogenides (TMDs) into van der Waals heterostructures. However, despite significant progress, the defects in 2D TMDs grown by PLD remain largely unknown and yet to be explored. Here, we combine atomic resolution images and first-principles calculations to reveal the atomic structure of defects, grains, and grain boundaries in mono- and bilayer MoS2 grown by PLD. We find that sulfur vacancies and MoS antisites are the predominant point defects in 2D MoS2. We predict that the aforementioned point defects are thermodynamically favorable under a Mo-rich/S-poor environment. The MoS2 monolayers are polycrystalline and feature nanometer size grains connected by a high density of grain boundaries. In particular, the coalescence of nanometer grains results in the formation of 180° mirror twin boundaries consisting of distinct 4- and 8-membered rings. We show that PLD synthesis of bilayer MoS2 results in various structural symmetries, including AA' and AB, but also turbostratic with characteristic moiré patterns. Moreover, we report on the experimental demonstration of an electron beam-driven transition between the AB and AA' stacking orientations in bilayer MoS2. These results provide a detailed insight into the atomic structure of monolayer MoS2 and the role of the grain boundaries on the growth of bilayer MoS2, which has importance for future applications in optoelectronics.

Locking in nanoscale strength

Metallurgy Metals with nanometer-sized crystal grains are super strong, but they do not generally retain their structure at higher temperatures. This property undermines their high strength and makes their use in applications challenging. Li et al. found a minimum-interface structure in copper with 10-nanometer-sized grains that, when combined with a nanograin crystallographic twinning network, retains high strength to temperatures just below the melting point. This discovery suggests a different path forward for stabilizing nanograined metals. Science , this issue p. [831][1] [1]: /lookup/doi/10.1126/science.abe1267

Constrained minimal-interface structures in polycrystalline copper with extremely fine grains.

Metals usually exist in the form of polycrystalline solids, which are thermodynamically unstable because of the presence of disordered grain boundaries. Grain boundaries tend to be eliminated through coarsening when heated or by transforming into metastable amorphous states when the grains are small enough. Through experiments and molecular dynamics simulations, we discovered a different type of metastable state for extremely fine-grained polycrystalline pure copper. After we reduced grain sizes to a few nanometers with straining, the grain boundaries in the polycrystals evolved into three-dimensional minimal-interface structures constrained by twin boundary networks. This polycrystalline structure that underlies what we call a Schwarz crystal is stable against grain coarsening, even when close to the equilibrium melting point. The polycrystalline samples also exhibit a strength in the vicinity of the theoretical value.

Exposure of Indian RAFM under variation of He+ flux and target temperature in the CIMPLE-PSI linear device

The paper reports first investigations of the effect of low-temperature helium (He) plasma exposure on the India specific reduced activation ferritic martensitic (IN-RAFM) steel. Experiments are performed in the CIMPLE-PSI device, over the variation of ion-flux (∼3 × 1022−23 m−2s−1) and target temperature (316 K−830 K), for ion-fluence up to 1.6 × 1026 m−2. Strong morphology changes have been observed, in particular, fiber-form surface structures with nanometer-sized grain structures, pinholes, and hollow fibers. Surface enrichment of tungsten up to 2.3 at.% was measured by energy dispersive x-ray spectroscopy (EDX), which was supported by Rutherford backscattering spectrometry (RBS) measurements. This had happened because iron and chromium were preferentially sputtered out by the He ions. It is demonstrated that the porous, micrometer-sized surface inhomogeneities, produced under high ion-flux (≥8.0 × 1022 m−2s−1) and high target temperature (≥518 K), critically influence the shape of the RBS spectrum, which necessitates a revision of the data analysis procedure. Through optical emission spectroscopic observations, we demonstrate that the sputtering yield of the steel decreases with exposure time, primarily because of the formation of the porous surface microstructures and also due to the surface enrichment of the exposed samples with tungsten atoms. It is concluded that the formation of bubbles underneath the surface of RAFM, and their subsequent distortion and rupturing leads to the formation of fiber-form structures under the relatively high target temperature, high ion-flux irradiation conditions.

Low Temperature Growth of the Nanotextured Island and Solid 3C-SiC Layers on Si from Hydric Si, Ge and C Compounds

Different growth stages and surface morphology of the epitaxial 3C-SiC/Si(100) structures were studied. Heterocompositions were grown in vacuum from hydric compounds at a lower temperature. The composition, surface morphology and crystal structure of the 3C-SiC films were tested using X-ray diffraction, second ion mass spectrometry, scanning ion and electron microscopy, photo- and cathode luminescence. It was demonstrated that the fine crystal structure of the 3C-SiC islands was formed by the close-packed nanometer-size grains and precipitated on the underlying solid carbonized Si layer. Luminescence spectral lines of the solid carbonized Si layer, separated island and solid textured 3C-SiC layer were shifted toward the high ultraviolet range. The spectra measured by different methods were compared and the nature of the revealed lines was considered. This article discusses a quantum confinement effect observation in the 3C-SiC nanostructures and a perspective for the use of nanotextured island 3C-SiC layers as a two-dimensional surface quantum superlattice for high-frequency applications. The conductivity anisotropy and current-voltage characteristics of the two-dimensional superlattices with a non-additive electron dispersion law in the presence of a strong electric field were studied theoretically. Main efforts were focused on a search of the mechanisms allowing realization of the high-frequency negative dynamical conductivity for the structures having a positive static differential conductivity.

Prediction of heterogeneous microstructural evolution in cold sprayed copper coatings using local Zener-Hollomon parameter and strain

Cold spray processing is a solid-state coating technique and an emerging method for additive manufacturing, in which metal powder particles are bonded through high-velocity impact-induced deformation. However, the severe plastic deformation of powder particles at extremely high strain rates, high strain gradients, and localized elevated temperatures yields rather complex and heterogeneous microstructures in materials produced in cold sprayed coatings or bulk forms. A good understanding, and even prediction, of such heterogeneous microstructures is essential for determining suitable post-processing conditions as well as the final properties of cold sprayed products. In this study, we employ a cold spray system to deposit copper coatings over a large temperature range (373 K to 873 K) and we systematically investigate the microstructural evolutions of the coatings using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) techniques. Diverse microstructures are observed, including recrystallized grains, annealing twins, shear bands, submicron-sized grains, deformation twins, and nanometer-sized grains. To understand the formation of such complex microstructures, we obtain local strains, strain rates and temperatures of the cold sprayed powder particles using the finite element method (FEM). Based on our experimental and simulation results, we have created the first deformation mechanism map for cold sprayed coatings to interpret and predict the heterogeneous microstructural evolutions in copper using the local Zener-Hollomon (Z) parameter and plastic strain (strain-Z-microstructure map). Such a map can be used to predict and design the microstructures of cold sprayed copper samples based on processing parameters and can also be extended to other severe plastic deformation (SPD) processes, such as cutting, extrusion, solid-phase welding, and other solid-state additive manufacturing processes.

Atomic-scale investigation on the structural evolution and deformation behaviors of Cu–Cr nanocrystalline alloys processed by high-pressure torsion

Abstract A systematic investigation based on high-resolution transmission electron microscopy (HRTEM) and quantitative analysis was carried out to gain insights into the structural evolution and deformation behaviors of 57 wt%Cu - 43 wt%Cr dual-phase alloys deformed by high pressure torsion (HPT). Nanometer-sized grains were obtained, with Cu and Cr grain size being reduced from 33.7 nm to 17.4 nm and 35.6 nm to 18.6 nm respectively after deformation with 5 rotations and 25 rotations. During the early deformation stage, in a way, grain refinement in Cu was controlled by the interaction between Cr solutes (or clusters) that was forced to dissolve into Cu matrix and extended dislocations motion. At the higher strain condition, the deformation mode became more complicated and the grain refinement of Cu and Cr can be affected by solute effects and constraint effect during severe plastic deformation in this dual-phase system. In addition, inverse grain-size effect on stacking fault (SF) and twinning tendencies was observed in Cu under different deformation conditions. A critical grain size was calculated to describe the required shear stress for partial dislocation nucleation. HRTEM investigations and statistical analysis on twins observed in 25 rotations-strained sample reveal that the twins are associated with the grain boundary activities and the recovery procedure in the extremely fine grain size region. These atomic-scale observations provide new views in understanding the deformation mode, especially twinning behavior in the extremely fine grain size range below the critical grain size that are hardly obtained in experiments.

Prediction of Heterogeneous Microstructural Evolution in Cold-Sprayed Copper Coatings Using Local Zener-Hollomon Parameter and Strain

Cold spray processing is a solid-state coating technique and an emerging method for additive manufacturing, in which metal powder particles are bonded through high-velocity impact-induced deformation. However, the severe plastic deformation of powder particles at extremely high strain rates, high strain gradients, and localized elevated temperatures yields rather complex and heterogeneous microstructures in materials produced as coatings or bulk forms. A good understanding, and even prediction, of such heterogeneous microstructures is essential for determining the post-processing conditions as well as the final properties of cold-sprayed products. In this study, we employ a cold spray system to deposit copper coatings over a large temperature range from 373 K to 873 K and we systematically investigate the microstructural evolutions of the coatings using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) techniques. Diverse microstructures have been observed, including recrystallized grains, annealing twins, shear bands, submicron grains, deformation twins, and nanometer-sized grains. To understand the formation of such complex microstructures, we obtain local strains, strain rates and temperatures of the cold-sprayed powder particles using the finite element method (FEM). Based on our experimental and simulation results, we created the first deformation mechanism map for cold sprayed coatings to interpret and predict the heterogeneous microstructural evolutions in copper using the local Zener-Hollomon (Z) parameter and plastic strain (strain-Z-microstructure map). Such a map can be used to predict and design the microstructures of cold-sprayed copper samples based on processing parameters and can also be extended to other severe plastic deformation (SPD) processes, such as cutting, extrusion, and solid-phase welding.

Formation of refined grains below 10 nm in size and nanoscale interlocking in the particle–particle interfacial regions of cold sprayed pure aluminum

It has been a long-standing challenge to obtain nanocrystalline pure aluminum with grain sizes below 10 nm by plastic deformation. Here, we studied the microstructure characteristics of two bonded pure aluminum powder particles after a cold spray process. In the particle–particle bonding region, nanometer-sized grains are revealed, about 30% of which are below 10 nm in size. In the particle–particle interface, we observe nanoscale perturbations with interlocking features assisting mechanical bonding. The formation of nanoscale grains and interlocking features is attributed to the extremely high strain and strain rate combined with shear instability in the cold spray process.

Performance optimization of complicated structural SiC/Si composite ceramics prepared by selective laser sintering

Combining reaction-bonded (RB) process and selective laser sintering (SLS) method is an effective approach to prepare ceramic components with complex shapes. The purpose of this paper is to find efficient ways to improve the performance of SiC/Si composites prepared by SLS/RB technologies. Effects of epoxy resin binder on the performance and microstructure of preforms and sintered bodies were studied first. Then, based on the above results, graphite with low reactivity was used as an alternative slow-release carbon source to promote sintering densification process and improve the carbon density of preforms. When the added amount of graphite increased, clusters of nanometer-sized SiC grains formed and the silicon phase transformed into pieces, which reduced the content and dimension of silicon phase. Furthermore, by applying a two-step sintering method, the residual silicon content of SiC/Si composites decreased further and the flexural strength at high temperatures increased.

Formation and photochemistry of covalently bonded large functional PAH clusters

Polycyclic aromatic hydrocarbon (PAH) molecules belong to a large and diverse chemical family in the interstellar medium (ISM). We study the formation and photochemistry of covalently bonded large functional PAH clusters, dicoronylene (DC, C48H20)/9-vinylanthracene (C16H12) and dicoronylene/9-methylanthracene (C15H12) cluster cations, in the gas phase, and we offer an approach to the evolution of different types of large (covalently bonded) PAH clusters in the ISM. The experiments, which we combined with a quadrupole ion trap and time-of-flight mass spectrometry, show that large functional PAH cluster cations can form by gas-phase condensation through molecular-ion reactions. One group of functional PAH cluster cations contain the vinyl group (−CHCH2), that is, from C16H12DDC+ (e.g., C16H12C48H19+, m/z = 799) to (C16H12)2DDC+ (e.g., (C16H12)2C48H18+, m/z = 1002). The other group of functional PAH cluster cations contain the methyl group (−CH3), that is, from C15H12DDC+ (e.g., C15H12C48H19+, m/z = 787) to (C15H12)2DDC+ (e.g., (C15H12)2C48H18+, m/z = 990). With laser irradiation, the DC/9-vinylanthracene and DC/9-methylanthracene cluster cations show a very complicated dissociation process (e.g., dehydrogenation, −CH3 or −CHCH2 unit losses). We investigate the structure of newly formed PAH cluster cations, the bond energy, and the photodissociation energy for these reaction pathways with quantumchemical calculations. The obtained results provide a general molecular growth route toward large PAH cluster cations (e.g., functional PAH clusters) in a bottom-up formation process and the insight of the functional group (e.g., vinyl, −C2H3 and methyl, −CH3) effect on their evolutionary behavior. In addition, the studies of DC/9-vinylanthracene and DC/9-methylanthracene clusters (94–123 atoms, ∼2 nm in size) also provide a possible way of interpreting the formation processes of nanometer-sized grains in the ISM, especially when functional PAHs are included.

Formation of Refined Grains Below 10 nm in Size and Nanoscale Interlocking in the Particle-Particle Interfacial Regions of Cold Sprayed Pure Aluminum

It has been a long-standing challenge to obtain nanocrystalline pure aluminum with grain sizes below 10 nm by plastic deformation. Here, we studied the microstructure characteristics of two bonded pure aluminum powder particles after a cold spray process. In the particle-particle bonding region, we show gradient nanometer-sized grains, about 30% of which are below 10 nm in size. We have also observed perturbations with interlocking features assisted mechanical bonding. The formation of nanoscale grains and interlocking features is attributed to the extremely high strain and strain rate combined with shear instability during the cold spray process.

Low-Temperature feature of grain-boundary hardening of nanocrystalline titanium

The yield strength σ0.2 of titanium polycrystals in tension and in the temperature range of 4.2–395 K is studied as a function of grain size (d) ranging from micro- to nanometer dimensions. It was found that at temperatures below ∼180 K, the σ0.2 (d) dependences are not described by the classical Hall–Petch relation, shifting instead to higher stresses at nanometer size (d). The characteristics of grain-boundary hardening observed at low temperatures are explained using theoretical models, according to which in nanometer-size grains, the diameter of a dislocation loop originating at a grain boundary is proportional to d, with the nucleation stress being σ ∼ 1/d. The critical value dnc is estimated, below which the ratio σ0.2 ∼ 1/d holds at a fixed temperature. It is established that the dnc value decreases with an increase in the temperature of the experiment.

Evidence of Surface Catalytic Effect on Cosmic Dust Grain Analogs: The Ammonia and Carbon Dioxide Surface Reaction

Abstract Surface chemistry on cosmic dust grains plays an important role in the formation of molecules at low temperatures in the interstellar and circumstellar environments. For the first time, we experimentally put in evidence the catalytic role of dust surfaces using the thermal reaction CO2 + 2NH3 → NH2COO−, which is also a proxy of radical–radical reactions. Nanometer-sized amorphous silicate and carbon grains produced in our laboratory were used as grain analogs. Surface catalysis on grains accelerates the kinetics of the reaction studied at a temperature of 80 K by a factor of up to 3 compared to the reaction occurring in the molecular solid. The evidence of the catalytic effect of grain surfaces opens a door for experiments and calculations on the formation of interstellar and circumstellar molecules on dust. Ammonium carbamate on the surface of grains or released intact into protostellar or protoplanetary disk phases can give start to a network of prebiotic reactions. Therefore, there should be a great interest to search for ammonium carbamate and its daughter molecule, carbamic acid, in interstellar clouds, protostellar envelopes, and protoplanetary disks.

Structural Analysis of Thin Ni-P Layers

The deposition mechanism by autocatalytic reduction method is a chemical one, which does not lead to structures as shown in the Ni-P balance diagram. In the experiments of the present work, Ni-P layers were made deposited on thin steel strip with low carbon content. By optical microscopy and scanning electron microscopy by (SEM) it was analyzed the appearance and surface morphology of the deposited layers depending on the content of phosphorus and working parameters (temperature, pH, stirring speed). The layer structure was analyzed in the cross section and on the break surface. The analysis results show that thin coatings of Ni-P have a surface topography that follows faithfully the steel support. Depending on the content of phosphorus, on the layer surface of low phosphorus content, spherical particles of different sizes can be seen while high and average phosphorus content coatings feature a smooth surface with nanometer-sized grains. Analysis of the cut-off /break section of the Ni-P layer structure, show a more or less porous structure depending on the stirring speed and pH level.

Charging and dynamics of dust particles in lunar photoelectron sheath

Sunlight scattering from electrostatically charged floating particles is considered accountable for the lunar twilight observations of horizon glow and streamers. In this work, the dynamics of the fine charged particles within the photoelectron sheath over the sunlit lunar surface has been investigated. Accounting for the influence of solar radiation, solar wind plasma, and lunar gravity, the present transport model consistently takes account of the coexisting phenomenon of particle charging, and characteristic photoelectron sheath screening in deriving the vertical motion of the particle. As novel features, half Fermi-Dirac statistics of the photoelectron velocity in determining the electron population and the sheath structure through the Poisson equation, and anisotropic photoelectron flux in evaluating the particle charge as it traverses through the sheath, have adequately been included in the analysis. In this framework, the electrostatic sheath features are observed to dominate over the lunar gravity in determining the dynamics of smaller (nanometer) size grains; for instance, ∼10 nm sized particles detached from the lunar regolith with a finite velocity are shown to loft up to the kilometer altitude. Depending on the initial particle velocity and size, a parametric regime defining the particle hopping over the lunar surface has been identified.