Search Results (16)

While self-healing of metals and alloys facilitated by electromagnetic pulse (EMP) energy introduced by electropulsing has been widely reported, the in-depth mechanism is still ambiguous. Here, an approximate in-situ observation was designed to explore the effect of EMP energy induced by electropulsing on the alloy interface self-healing. Electromagnetic shocking treatment (EST) utilizing EMP energy was demonstrated to promote metallic interface bridging and self-healing. At local regions, flat surfaces turn into wavy ones, especially, while local interface bridging and self-healing are commonly observed, indicating a nonlinear surface pre-melting. Based on the assumed mass–spring–damping system of the pre-melted interfaces, the mechanisms of local interface bridging and self-healing under the EST are explored. This work provides new understanding about the interface self-healing mechanism by electropulsing, as well as new insights about the effect of pulse energy (such as EMP) on metallic interface complexion. Full article

The thermal stability of bimetallic nanoparticles plays a crucial role in their performance in applications in catalysis, biotechnology, and materials science. In this study, we employ molecular dynamics simulations to investigate the melting behavior of Au-Pd nanoparticles with cuboctahedral, icosahedral, and decahedral geometries. Using a tight-binding potential, we systematically explore the effects of particle size and composition on the melting transition. Our analysis, based on caloric curves, Lindemann coefficients, and orientational order parameters, reveals distinct premelting behaviors influenced by geometry. Larger particles exhibit a coexistence of a pseudo-crystalline core and a partially melted shell, but, in decahedra and icosahedra, melting of the core occurs unevenly, with twin boundaries promoting the melting of one or two of the tetrahedral subunits before the rest of the particle. Notably, icosahedral nanoparticles display higher thermal stability, while both icosahedral and decahedral structures exhibit localized melting within twin boundaries. Additionally, we generate HAADF-STEM simulations to aid the interpretation of in situ electron microscopy experiments. Full article

This study reveals the significant role of the pre-melting process in growing high-quality (100) β-Ga₂O₃ single crystals from 4N powder (99.995% purity) using the edge-defined film-fed growth (EFG) method. Among various bulk melt growth methods, the EFG method boasts a fast growth rate and the capability of growing multiple crystals simultaneously, thus offering high productivity. The pre-melting process notably enhanced the structural, optical, and electrical properties of the crystals by effectively eliminating impurities such as Si and Fe. Specifically, employing a 100% CO₂ atmosphere during pre-melting proved to be highly effective, reducing impurity concentrations and carrier scattering, which resulted in a decreased carrier concentration and an increased electron mobility in the grown Ga₂O₃ single crystals. These results demonstrate that pre-melting is a crucial technique for substantially improving crystal quality, thereby promising better performance in β-Ga₂O₃-based device applications. Full article

This report presents the results of dielectric studies in a nitrobenzene–decane critical mixture in the homogeneous liquid, biphasic mesophase, and the solid crystal phase. It focuses on detecting critical effects in the broad surrounding of the critical consolute temperature and pre-melting and post-freezing effects in the solid crystal phase. The strong manifestation of the diameter critical anomaly in the biphasic domain and the Mossotti catastrophe type pre-melting and post-freezing effects in the solid phase are evidenced. Studies include the puzzling low-frequency (LF) domain related to translational processes. The real part of electric conductivity, in LF limit, is well portrayed by the super-Arrhenius-type equation in the homogenous liquid and solid phases. The obtained experimental evidence can be significant for the cognitive progress of the still puzzling melting/freezing canonic discontinuous transition. Full article

The long-range supercritical changes of dielectric constant, resembling ones observed in the isotropic liquid phase of liquid crystalline compounds, are evidenced for linseed oil—although in the given case, the phenomenon is associated with the liquid–solid melting/freezing discontinuous phase transitions. This ‘supercriticality’ can be an additional factor supporting the unique pro-health properties of linseed oil. Broadband dielectric spectroscopy studies also revealed the ‘glassy’ changes of relaxation times, well portrayed by the ‘activated and critical’ equation recently introduced. In the solid phase, the premelting effect characteristic for the canonic melting/freezing discontinuous transition, i.e., without any pretransitional effect in the liquid phase, has been detected. It is interpreted within the grain model, and its parameterization is possible using the Lipovsky model and the ‘reversed’ Mossotti catastrophe concept. For the premelting effect in the solid state, the singular ‘critical’ temperature correlates with the bulk discontinuous melting and freezing temperatures. Consequently, the report shows that linseed oil, despite its ‘natural and complex’ origins, can be considered a unique model system for two fundamental problems: (i) pretransitional (supercritical) effects in the liquid state associated with a weakly discontinuous phase transition, and (ii) the premelting behavior in the solid side of the discontinuous melting/freezing discontinuous transition. Full article

The grain boundary, solid solution, and precipitation strengthening mechanisms are important for controlling the mechanical properties of Al-based alloys. Due to severe plastic deformation, mechanical alloying refines grain structure to a nanoscale level which leads to a strong increase in solute content and the related strengthening effect of solute atoms and secondary-phase precipitates. This study analyzed the elemental Mn and Al₆Mn phase dissolution in Al during high-energy ball milling. For this purpose, XRD data, microstructure, and hardness evolutions were compared for two Al—5.2 at% Mn alloys prepared by mechanical alloying using elemental Al and Mn powders and a pre-melted master alloy. In the two-phase master alloy, containing the Al solid solution and the Al₆Mn phase, the strain accumulation, grain refinement, solid solution supersaturation, and milling-induced hardening effects were facilitated. Both elemental Mn and intermetallic compound were dissolved during mechanical alloying, and the maximum solute content was near 3.1 at% Mn. A fine crystalline size of ~25 nm and the maximum Mn solute content were observed after milling of elemental powders and the master alloy for 60 h and 20 h, respectively. The microhardness of ~3 GPa corresponded to a ~3.1% solute Mn content, and the microhardness increased to ~5 GPa after long–term milling due to precipitation strengthening effect of the secondary Al₆Mn phase in the master alloy. Full article

Interfacial thermal conductance (ITC) affects heat transfer in many physical phenomena and is an important parameter for various technologies. The article considers the influence of various mesoscopic effects on the ITC, such as the heat transfer through the gas gap, near-field radiative heat transfer, and changes in the wetting behavior during melting. Various contributions to the ITC of the liquid-solid interfaces in the processes of fast pre-melting and melting of metal microparticles are studied. The effective distance between materials in contact is a key parameter for determining ITC. This distance changes significantly during phase transformations of materials. An unusual gradual change in ITC recently observed during pre-melting below the melting point of some metals is discussed. The pre-melting process does not occur on the surface but is a volumetric change in the microstructure of the materials. This change in the microstructure during the pre-melting determines the magnitude of the dispersion forces, the effective distance, and the near-field thermal conductance. The knowledge gained can be useful for understanding and optimizing various technological processes, such as laser additive manufacturing. Full article

Shear and thermal processing can greatly influence nanoparticles’ orientation and dispersion, affecting the nanocomposites’ conductivity and mechanical properties. The synergistic effects of shear flow and Carbon nanotubes (CNTs) nucleating ability on the crystallization mechanisms have been proven. In this study, Polylactic acid/Carbon nanotubes (PLA/CNTs) nanocomposites were produced by three different molding methods: compression molding (CM), conventional injection molding (IM), and interval injection molding (IntM). Solid annealing at 80 °C for 4 h and pre-melt annealing at 120 °C for 3 h was applied to research the CNTs’ nucleation effect and the crystallized volume exclusion effect on the electrical conductivity and mechanical properties. The volume exclusion effect only significantly impacts the oriented CNTs, causing the conductivity along the transverse direction to rise by about seven orders of magnitude. In addition, the tensile modulus of the nanocomposites decreases with the increased crystallinity, while the tensile strength and modulus decrease. Full article

This report presents ‘giant’ and long-range premelting effects appearing in dielectric properties for the temperature and pressure paths of studies, with an explicit critical-like portrayal. The result was obtained for the ‘classic’ low molecular weight compound: nitrobenzene, tested in the solid and liquid phases. Dielectric studies enable the ‘extraction’ of the response from liquid layers between crystalline grains. Compressing increased the premelting effects, probably due to the ‘crushing’ of crystalline grains by isotropic squeezing and increasing the liquid layers between grains. This report indicates the significance of considering the melting/freezing phenomenon from the point of view of the ‘solid crystalline grains and critical-type liquid layers in synergic interactions’ concept. Full article

The use of a nanostructured graphene-zirconia composite will allow the development of new materials with improved performance properties and a high functionality. This work covers a stepwise study related to the creation of a nanostructured composite based on ZrO₂ and graphene. A composite was prepared using two suspensions: nano-zirconia obtained by sol-gel synthesis and oxygen-free graphene obtained sonochemically. The morphology of oxygen-free graphene sheets, phase composition and the morphology of a zirconia powder, and the morphology of the synthesized composite were studied. The effect of the graphene sheets on the rheological and sintering properties of a nanostructured zirconia-based composite powder has been studied. It has been found that graphene sheets in a hybrid nanostructure make it difficult to press at the elastic deformation stage, and the composite passes into the plastic region at a lower pressure than a single nano-zirconia. A sintering mechanism was proposed for a composite with a graphene content of 0.635 wt%, in which graphene is an important factor affecting the process mechanism. It has been determined that the activation energy of the composite sintering is more than two times higher than for a single nano-zirconia. Apparently, due to the van der Waals interaction, the graphene sheets partially stabilize the zirconia and prevent the disordering of the surface monolayers of its nanocrystals and premelting prior to the sintering. This leads to an increase in the activation energy of the composite sintering, and its sintering occurs, according to a mixed mechanism, in which the grain boundary diffusion predominates, in contrast to the single nano-zirconia sintering, which occurs through a viscous flow. Full article

The experimental and theoretical description of premelting behavior is one of the most challenging tasks in contemporary material science. In this paper, n-octanol was studied using a multi-method approach to investigate it at macroscopic and molecular levels. The experimental infrared (IR) spectra were collected in the solid state and liquid phase at temperature range from $-84$${}^{\circ }$C to −15 ${}^{\circ }$C to detect temperature-related indicators of pretransitional phenomena. Next, the nonlinear dielectric effect (NDE) was measured at various temperatures (from −30 ${}^{\circ }$C to −15 ${}^{\circ }$C) to provide insight into macroscopic effects of premelting. As a result, a two-step mechanism of premelting in n-octanol was established based on experimental data. It was postulated that it consists of a rotator state formation followed by the surface premelting. In order to shed light onto molecular-level processes, classical molecular dynamics (MD) was performed to investigate the time evolution of the changes in metric parameters as a function of simulation temperature. The applied protocol enabled simulations in the solid state as well as in the liquid (the collapse of the ordered crystal structure). The exact molecular motions contributing to the rotator state formation were obtained, revealing an enabling of the rotational freedom of the terminal parts of the chains. The Car–Parrinello molecular dynamics (CPMD) was applied to support and interpret experimental spectroscopic findings. The vibrational properties of the stretching of OH within the intermolecular hydrogen bond were studied using Fourier transformation of the autocorrelation function of both dipole moments and atomic velocity. Finally, path integral molecular dynamics (PIMD) was carried out to analyze the quantum effect’s influence on the bridged proton position in the hydrogen bridge. On the basis of the combined experimental and theoretical conclusions, a novel mechanism of the bridged protons dynamics has been postulated—the interlamellar hydrogen bonding pattern, resulting in an additional OH stretching band, visible in the solid-state experimental IR spectra. Full article

Plasmon-enhanced photothermal and optomechanical effects on deforming and reshaping a gold nanoparticle (NP) are studied theoretically. A previous paper (Wang and Ding, ACS Nano 13, 32–37, 2019) has shown that a spherical gold nanoparticle (NP) irradiated by a tightly focused laser beam can be deformed into an elongated nanorod (NR) and even chopped in half (a dimer). The mechanism is supposed to be caused by photothermal heating for softening NP associated with optical traction for follow-up deformation. In this paper, our study focuses on deformation induced by Maxwell’s stress provided by a linearly polarized Gaussian beam upon the surface of a thermal-softened NP/NR. We use an elastic model to numerically calculate deformation according to optical traction and a viscoelastic model to theoretically estimate the following creep (elongation) as temperature nears the melting point. Our results indicate that a stretching traction at the two ends of the NP/NR causes elongation and a pinching traction at the middle causes a dent. Hence, a bigger NP can be elongated and then cut into two pieces (a dimer) at the dent due to the optomechanical effect. As the continuous heating process induces premelting of NPs, a quasi-liquid layer is formed first and then an outer liquid layer is induced due to reduction of surface energy, which was predicted by previous works of molecular dynamics simulation. Subsequently, we use the Young–Laplace model to investigate the surface tension effect on the following deformation. This study may provide an insight into utilizing the photothermal effect associated with optomechanical manipulation to tailor gold nanostructures. Full article

Changes of unsteady multiphase fluid flow patterns of liquid steel during electric arc furnace-ladle tapping operations, with simultaneous argon bottom injection, are simulated using interfacial tracking computing techniques. The impinging steel jet interacts with the argon bubbling plume, suffering mutual bending effects, and imparting non-symmetric flows of liquid steel during the whole ladle filling time. At low bath levels, radial recirculating flows are generated and at high bath levels, these flows are substituted by vertical long flows generated by the permanent interaction between the impinging jet and the argon plume. Turbulence intensity increases as the bath level rises. Low bath levels are suitable for pre-melting and preheating ferroalloy particles. High bath levels of steel in the ladle, close to total ladle filling, are the most suitable conditions for thermal and chemical homogenizations. Argon gas forms an intermittent blanket over the air–liquid steel mix due to its higher density than air during the whole ladle filling time. Full article

Oxygen diffusion from oxides to an alloy during heat treatment could influence the properties of the alloy and oxides. To clarify the influence of FeO on the solid-state reactions between Al₂O₃–CaO–FeO oxide and Fe–Al–Ca alloy during heat treatment at 1473 K, three diffusion couples with different FeO concentrations in the oxide were produced. The diffusion couples were subjected to several procedures successively including an oxide pre-melting experiment using a confocal scanning laser microscope to obtain good contact between the alloy and oxide, vacuum sealing to protect the specimens from oxidation, heat treatment, and electron probe X-ray microanalysis. The effects of the FeO content in the oxide on the morphology of the interface between the alloy and oxide, change of elements in the alloy, widths of the particle precipitation zone (PPZ) and aluminum-depleted zone (ADZ), and size distribution of the particles in the alloy, were investigated and discussed. Based on the Wagner equation of internal oxidation of metals, a modified dynamic model to calculate the PPZ width was established to help understand the mechanism of the solid-state reactions and element diffusion between the Fe–Al–Ca alloy and Al₂O₃–CaO–FeO oxide with different FeO concentrations. Full article

The N₁,N₁ʹ-(ethane-1,2-diyl)bis(N₂-phenyloxalamide) (OXA) is a soluble-type nucleator with a dissolving temperature of 230 °C in poly(l-lactic acid) (PLLA) matrix. The effect of thermal history and shear flow on the crystallization behavior of the PLLA/OXA samples was investigated by rheometry, polarized optical microscopy (POM), differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), and scanning electron microscopy (SEM). The crystallization process of the PLLA/OXA-240 sample (i.e., pre-melted at 240 °C) was significantly promoted by applying a shear flow, e.g., the onset crystallization time (t_onset) of the PLLA at 155 °C was reduced from 1600 to 200 s after shearing at 0.4 rad/s for even as short as 1.0 s, while the crystallinity (X_c) was increased to 40%. Moreover, the t_onset of the PLLA/OXA-240 sample is 60%–80% lower than that of the PLLA/OXA-200 sample (i.e., pre-melted at 200 °C) with a total shear angle of 2 rad, indicating a much higher crystallization rate of the PLLA/OXA-240 sample. A better organization and uniformity of OXA fibrils can be obtained due to a complete pre-dissolution in the PLLA matrix followed by shear and oscillation treatments. The well dispersed OXA fibrils and flow-induced chain orientation are mainly responsible for the fast crystallization of the PLLA/OXA-240 samples. In addition, the shear flow created some disordered α′-form crystals in the PLLA/OXA samples regardless of the thermal history (200 or 240 °C). Full article