Pulsed Melting Research Articles

Nanosecond Structural Dynamics during Electrical Melting of Charge Density Waves in 1T-TaS_{2}.

Electrical control of charge density waves has been of immense interest, as the strong underlying electron-lattice interactions potentially open new, efficient pathways for manipulating their ordering and, consequently, their electronic properties. However, the transition mechanisms are often unclear as electric field, current, carrier injection, heat, and strain can all contribute and play varying roles across length scales and timescales. Here, we provide insight on how electrical stimulation melts the room temperature charge density wave order in 1T-TaS_{2} by visualizing the atomic and mesoscopic structural dynamics from quasi-static to nanosecond pulsed melting. Using a newly developed ultrafast electron microscope setup with electrical stimulation, we reveal the order and strain dynamics during voltage pulses as short as 20ns. The order parameter dynamics across a range of pulse amplitudes and durations support a thermally driven mechanism even for fields as high as 19 kV cm^{-1}. In addition, time-resolved imaging reveals a heterogeneous, mesoscopic strain response across the flake, including MHz-scale acoustic resonances that emerge during sufficiently short pulsed excitation which may modulate the order. These results suggest that metallic charge density wave phases like studied here may be more robust to electronic switching pathways than insulating ones, motivating further investigations at higher fields and currents in this and other related systems.

Study of an additive manufacturing technology using pulsed inductive wire melting

Additive manufacturing is becoming increasingly widespread and has developed very dynamically in recent years. For the processing of metals, powder or wire is usually used as the feedstock. The energy source used in wire-based processes is usually electric arcs or lasers. In this contribution, a technological approach for additive manufacturing of metal structures by induction melting with pulsed generator power is used. Therefore, the principle of oscillating Lorentz forces is utilized in this approach and analytically investigated with the aid of a FE simulation model. In the experiments, a continuously fed steel wire is inductively melted and deposited dropwise onto a substrate heated to different preheating temperatures without the use of a nozzle or crucible.

Fabrication and characterization of heavily doped n-type GaAs for mid-infrared plasmonics

N-type doping in GaAs is a self-limited process, rarely exceeding a carrier concentration level of 1019 cm−3. Here, we investigated the effect of intense pulsed light melting on defect distribution and activation efficiency in chalcogenide-implanted GaAs by means of positron annihilation spectroscopy and electrochemical capacitance–voltage techniques. In chalcogenide-doped GaAs, donor–vacancy clusters are mainly responsible for donor deactivation. Using positrons as a probe of atomic scale open volumes and DFT calculations, we have shown that after nanosecond pulsed light melting the main defects in heavily doped GaAs are gallium vacancies decorated with chalcogenide atoms substituting As, like VGa–nTeAs or VGa–nSAs. The distribution of defects and carriers in annealed GaAs follows the depth distribution of implanted elements before annealing and depends on the change in the solidification velocity during recrystallization.

Study on the Heterogeneous Nucleation Mechanism of SiCp/AZ91 Magnesium Matrix Composites under Pulse Current

SiCp/AZ91D magnesium matrix composites with 30% SiCp were successfully prepared by pulsed current melting in this work. Then, the influences of the pulse current on the microstructure, phase composition, and heterogeneous nucleation of the experimental materials were analyzed in detail. The results show that the grain size of both the solidification matrix structure and SiC reinforcement are refined by pulse current treatment, and the refining effect is gradually more obvious with an increase in the pulse current peak value. Moreover, the pulse current reduces the chemical potential of the reaction between SiCp and Mg matrix, thus promoting the reaction between SiCp and the alloy melt and stimulating the formation of Al4C3 along the grain boundaries. Furthermore, Al4C3 and MgO, as heterogeneous nucleation substrates, can induce heterogeneous nucleation and refine the solidification matrix structure. Finally, when increasing the peak value of the pulse current, the repulsive force between the particles increases while the agglomeration phenomenon is suppressed, which results in the dispersed distribution of SiC reinforcements.

Femtosecond laser melting NbMoTaW refractory high entropy alloy: A micro-scale thermodynamic simulation

Refractory high entropy alloys (RHEA) exhibit excellent material properties, such as high strength, high hardness, high melting point and high temperature oxidation resistance. Ultrafast laser fabricating RHEAs is potential widely used in extreme environments at high temperature. In the present study, a multiscale simulation of the femtosecond laser pulse melting NbMoTaW RHEA film was performed to observe the transient interaction process between femtosecond laser and RHEA at the atomic scale. The temperature dependent electron thermophysical parameters (electron heat capacity, electron thermal conductivity and effective electron–phonon coupling factor) were calculated from density functional theory. The two temperature model coupling molecular dynamics was used to reveal the temperature change, the density variation and the pressure evolution during the interaction of a single pulse laser with the NbMoTaW RHEA. The simulated ablation and melting depths varied differently with laser fluence parameters. The ablation depth increases with the laser fluence, while the non-removed melting depth varies little with laser energy beyond the ablation threshold. The research reveals the mechanisms in melting process at atomic-level, and the results are helpful to the fabrication of RHEA.

Prediction of the composition of surface alloys formed via pulsed melting of preliminary deposited coatings

To date, many results of studies of surface alloys have been published, but no approaches have been developed to predict their quantitative composition that can assist to design new types of coatings. This paper proposes a methodology for prediction the atomic and mass contents of elements in the surface alloys formed by pulsed liquid-state mixing of pre-deposited films and an upper layer of a substrate. The methodology validity has been verified experimentally on the example of a Cr–Zr surface alloy that is of particular interest due to its possible use in the nuclear industry. The surface alloy has been synthesized by irradiation of a Cr/Zr film-substrate system with a low-energy high-current electron-beam (LEHCEB) of microsecond duration. The results have showed good agreement between the predicted and experimental data with an accuracy satisfactory for practical applications. Some observed discrepancies are explained by the pulse-to-pulse dispersion of the energy densities during LEHCEB irradiation.

Laser Irradiation-Induced Nanoscale Surface Transformations in Strontium Titanate

We studied the structural transformations and atomic rearrangements in strontium titanate (SrTiO3) via nanosecond pulsed laser irradiation-induced melting and ultrafast quenching. Using scanning transmission electron microscopy, we determine that the laser-irradiated surface in single-crystalline SrTiO3 transforms into an amorphous phase with an interposing disordered crystalline region between amorphous and ordered phases. The formation of disordered phase is attributed to the rapid recrystallization of SrTiO3 from the melt phase constrained by an epitaxial relation with the pristine region, which eases up on the surface, leading to amorphous phase formation. With electron energy-loss spectroscopic analysis, we confirm the transformation of Ti+4 to Ti+3 due to oxygen vacancy formation as a result of laser irradiation. In the disordered region, the maximum transformation of Ti+4 is observed to be 16.2 ± 0.2%, whereas it is observed to be 20.2 ± 0.2% in the amorphous region. Finally, we deduce that the degree of the disorder increases from atomically disordered to amorphous transition in SrTiO3 under laser-irradiation. The signatures of short-range ordering remain similar, leading to a comparable fingerprint of electronic structure. With these results, this study addresses the gap in understanding the atomic and electronic structure modified by pulsed laser irradiation and functionalizing pristine SrTiO3 for electronic, magnetic, and optical applications.

Multiple crustal melting pulses and Hf systematics in zircons

This paper focuses on the U–Pb ages and Lu–Hf isotopes in zircons in a long-lived anatectic terrane, to understand both the nature of melting events and controls on Hf-isotope ratios in the zircon record. We focus on rocks from the Wuyi-Yunkai orogeny, South China, that mainly comprises Proterozoic basement rocks that generated numerous granitoids during Early Paleozoic granulite facies metamorphism. We investigate zircons from three anatectic granites and one migmatite. Zircon U–Pb dates for individual samples vary between ~465 and ~410 Ma, covering the duration of the orogeny. U–Pb dates for two samples define three normal distributions with populations spaced by ~15–20 Myr, indicating protracted and pulsed melting events. The inherited pre-orogenic zircon cores yield a wide spread of εHf(435) values (−39.0 to −2.8), whilst all Early Paleozoic, magmatic syn-orogenic cores and rims have a narrower range shifted to higher values ranging between −14.3 and + 5.5 and no systematic variation with the date of the analytical spot. The data suggest that the magmatic syn-orogenic zircons grew from isotopically heterogeneous melts with ~9 to 16 ε units variation for individual samples. The narrower ranges compared to the inherited cores suggest that some magma homogenization must have occurred, and the upward shift of the εHf(435) values is likely due to the influence of non-zircon Hf rather than due to mixing with an external less-evolved magma. Thus, the samples record a ~55 Myr, multi-pulsed melting event, during which heterogeneous crustal magmas had their Hf isotopic composition controlled by inherited zircon dissolution modified by contributions from non-zircon phase.

TEM study of bubble formation in Ti–Ta–Si–Ni metallic glass surface alloy on TiNi SMA substrate during additive thin-film electron-beam synthesis

Recently, Ti–Ta-based metallic glass surface alloys (MGSAs) with attractive surface properties were successfully synthesized on TiNi substrates by additive pulsed melting of film/substrate systems. However, undesirable nanobubbles are often formed in the amorphous matrix. In this work, to reveal bubble characteristics and clarify the origin of its formation, the ∼1.5-μm thick Ti–Ta–Si–Ni MGSA was synthesized by 10-fold alternating of magnetron co-deposition of Ti60Ta30Si10 thin (100 nm) film and its pulsed e-beam (∼2.5 μs, ∼15 keV, 1.7 J/cm2) melting with a TiNi substrate. Transmission electron microscopy was utilized for quantitative analysis of bubbles. It has been shown that the Ar trapped during film deposition is the majority gas inside the bubbles. The Ar pressure inside the bubbles with mean diameter in range of ∼9 ÷ 16 nm predicted from the equation of state for Ar at 300 K varies in depth from ∼100 to 50 MPa. Experimental results and estimates of diffusion of Ar atoms in the surface melt indicate that nucleation and growth of bubbles occur in the Ar supersaturated liquid phase at the solidification/glass transition front. A high bubble-forming ability of Ti–Ta–Si–Ni system is consistent with the facts that Si addition decreases the melt surface tension and increases the Ar diffusivity due to eutectic reactions.

Microstructural characterization and properties of a Ti-Ta-Si-Ni metallic glass surface alloy fabricated on a TiNi SMA substrate by additive thin-film electron-beam method

An important factor limiting the wide application of thin-film metallic glass (TFMG) coatings for improving the surface-sensitive properties of structural alloys is the poor adhesion of TFMG to substrates. This problem can be overcome through the synthesis of MG surface alloys (MGSAs) by additive pulsed electron-beam melting of film/substrate systems of high glass-forming ability. In this work, this approach is applied to the [film (Ti60Ta30Si10, at.%, 100 nm)/substrate (TiNi alloy)] system using a low-energy, high-current electron beam (~2.5 μs, ~15 keV, 1.7 J/cm2) at 10 synthesis cycles and 10 pulses per cycle. Using SEM/WDS, XRD and cross-sectional HRTEM/EDS/SAED analyses it has been found that ~1.5 μm thick SA has completely MG structure, in which the near-surface ~500 nm thick Ni-depleted layer has composition ~Ti56Ta23Ni11Si10. Beneath the MGSA, the ~200 nm thick nanocomposite Ti50Ni30÷40(Ta + Si)20÷10 sublayer, consisting of Ti2Ni nanograins embedded in the amorphous phase is formed. The nanocomposite sublayer is followed by the intermediate sublayer with the eutectic columnar nano-grain B2-structure, which provides the diffusion bonding of MGSA with unmelted TiNi substrate. The monotonic depth dependences of hardness, elastic modulus, depth recovery ratio and plasticity, obtained by indentation, indicate the mechanical compatibility of the MGSA and underlying sublayers with the TiNi substrate. The adhesion-related failure of the MGSA during scratch testing proceeds in the ductile mode mainly through the plastic flow of intermediate sublayer. Evaluation of shape memory effect and superelasticity by torsional deformation technique has shown that synthesis of the MGSA results in an almost 2-fold increase in the martensitic shear stress and significant decrease in the stress hysteresis width compared with the untreated TiNi samples. The irreversible residual strain is ~0.8% at a maximum strain of 6%. Potentiodynamic polarization measurements have shown that synthesis of the MGSA results in the significant enhancement in corrosion resistance of test samples in the Lock-Ringer salt solution.

Microstructural characterization and mechanical behavior of nanocomposite Ti-Ni-Nb surface alloys synthesized on TiNi SMA substrate by additive thin-film electron-beam mixing

An important factor limiting the wide application of magnetron-sputtered amorphous or nanocomposite thin-film coatings for improving surface-sensitive properties of structural alloys is the poor adhesion of coating to substrate. This problem can be overcome through the synthesis of surface alloys (SAs) by additive pulsed electron-beam melting of film/substrate systems of high glass-forming ability (GFA). In this work, this approach is applied to the “film (Ti85/70Nb15/30, at.%), 100 nm/substrate (TiNi SMA)” system using low-energy, high-current electron beam (~2.5 μs, ~15 keV, 1.7 J/cm2) at 10 synthesis cycles and 10 pulses per cycle. The melting/solidification conditions were evaluated using numerical modeling of pulsed heating, taking into account the formation of eutectic in the Ti-Ni-Nb system and initial temperature risen up to 473 K to the end of the synthesis. Using surface SEM/EDS, AES, XRD and cross-sectional HRTEM/EDS/SAED analyses it has been found that both SAs of thickness ~1.5–2 μm have depth-graded structure, in which outer Ni-depleted layer is the multiphase nanocomposite with quasicrystals followed by predominantly Ti-Ni-based amorphous sublayer. Beneath, the ~1 μm thick intermediate sublayer with eutectic columnar structures (B2 + Ti2Ni and B2 + Ti3Ni4 for first and second SA, correspondently), monotonic Nb substitution for Ni and diffusion transition to TiNi substrate is formed. Depth-graded structure of modified surface layers provides monotonic changes with depth in nanohardness, elastic modulus, depth recovery ratio and plasticity to the values of the TiNi substrate. For both SAs the evolution of structure in depth is in a reasonable agreement with glass forming composition range evaluated for Ti-Ni-Nb system using Miedema's model.

Modification of surface layers of metal parts by pulse laser processing

One of the common technological schemes for the laser processing of metal parts is laser «glazing» of the deposited thin layer of powder on the substrate, which makes it possible to obtain microcrystalline and amorphous structures in thin surface layers of parts. Pulse melting of small volumes of metal on the surface provides homogeneous liquids (melts), which, after crystallization, can fix the structure of a liquid with unique properties.The presence of three energy input ranges for pulsed laser surface treatment (ILPO) has been established. At an average level of energy input, a process of laser surfacing of the material occurs, while a high adhesive bond between the coating and the workpiece is fixed. The deposition of an amorphized iron-based alloy coating on a metal base of St3, 45 20X13 steels begins at a laser radiation density on the treatment surface of 1800–2000 W/mm2, and alloying at 3500–4000 W/mm2. With ILPO, the laser exposure zone (ZLV) has a layered structure – the fusion zone (ZO), the heat-affected zone (HAZ), and the source material zone. To obtain an amorphous-crystalline structure of the coating, the laser pulse frequency should not exceed 3–8 Hz.

激光+脉冲熔化极气体保护复合焊接表面成形及稳定性研究

激光+脉冲熔化极气体保护复合焊接表面成形及稳定性研究

Multilayer Surface-Area Structure and the Influence of Residual Oxygen on its Formation During Irradiation of TiNi Alloy with a Low-Energy High-Current Electron Beam

The structure and phase composition of a subsurface layer of TiNi alloy modified in the mode of pulsed melting with a microsecond low-energy (to ~40 keV) high-current (to ~50 kA) electron beam is investigated. The influence of the number of pulses at a constant energy density on the characteristics of structure, phasecomposition changes, and behavior of the phase distribution near the surface is considered. The role of oxygen in the columnar structure stabilization of the recrystallized surface layer is discussed.

Influence of nanosecond laser irradiation on the structure and conductivity of BCx films

Thin-film precursors of BCx were formed by pulsed laser codeposition of boron and carbon. Targets made of pressed boron and carbon powders with an equal element content (B/C = 1/1) and an increased carbon content (B/C = 1/3) were used. The films were deposited on sapphire substrates at elevated temperature (700°C) which determined the initial properties of the precursor BCx films. Irradiation of the films was carried out by laser pulses of nanosecond duration with varying intensity. The films obtained by laser annealing of BCx (Q-BCx) were studied by scanning electron microscopy and micro-Raman spectroscopy. Irradiation under optimal conditions made allowed to realize pulsed melting of the films and partial preservation of their continuity on the substrate. The local structure of Q-BCx films and the nature of the changes in their electrophysical properties depended on the composition of the precursor films and the laser irradiation regimes.

Numerical simulation of melt pool oscillations and protuberance in pulsed laser micro melting of SS304 for surface texturing applications

Laser micro melting of metal surfaces is widely used in industrial applications such as welding, polishing and texturing. However, these processes often inherit characteristic surface topography such as dimples, protuberances, and ripple-like structures. This paper examines the mechanisms possible for the formation of such structure in laser micro melting of steel. Therefore, a 2D axisymmetric finite element model is formulated which considers the effect of recoil pressure, thermo-capillary shear stress and normal stress due to surface tension on the relaxation and deformation of melt pool surface during laser micro melting. It has been found out that the recoil pressure significantly affects the surface protuberance height and genesis of ripple like structures. Further, the thermo-capillary and chemi-capillary shear stress dominate the melt redistribution process during heating, while the normal stress due to surface tension dominates the fluid flow during the cooling period.

Reduced Graphene Oxide-Nanostructured Silicon Photosensors with High Photoresponsivity at Room Temperature

We have created nanostructured Si (∼3 nm) with a direct band gap of 1.37 eV on electrically conducting reduced graphene oxide (rGO) for a highly efficient photosensor. This robust photosensor is fabricated using a nonequilibrium processing route, where nanosecond excimer laser pulses melt the alternating layers of Si and amorphous carbon to form micropillars and nanoreceptors of Si on rGO layers. The incident white light generates free carriers in the Si microstructures and nanoreceptors which are ballistically transported (via rGO layers) to the external circuit under the application of a voltage bias. The responsivity of rGO-Si devices to light (resistance vs time) and I–V measurements indicate an exponential drop in resistance with the incidence of white light and nonrectifying nature, respectively. Photoresponsivity of the rGO-Si devices is calculated to be 3.55 A/W at room temperature, which is significantly larger than the previously fabricated graphene-based ohmic photosensors. Temperature-dependen...

Direct conversion of carbon nanofibers into diamond nanofibers using nanosecond pulsed laser annealing.

Here, we show the direct conversion of carbon nanofibers (CNFs) into diamond nanofibers (DNFs) by irradiating CNFs with an ArF nanosecond laser at room temperature and atmospheric pressure. The nanosecond laser pulses melt the tips of CNFs into a highly undercooled state, and their subsequent quenching results in the formation of DNFs. This formation of DNFs is dependent on the degree of undercooling which is controlled by nanosecond laser energy density and one-dimensional heat flow characteristics in CNFs. The conversion process starts at the top and extends with the number of pulses. Therefore, our highly non-equilibrium nanosecond laser processing opens a new avenue for the synthesis of exciting pure and doped diamond structures at ambient temperatures and pressures for a variety of applications.

Inclusions in metallic materials as origin for microcraters formation at a pulsed electron-beam surface melting

In the present work, the effect of difference between thermal properties of the base material (matrix) and inclusion on the temperature behavior of the target in the regions of inclusion locations has been considered. The calculations were based on the solution of two-dimensional nonlinear nonstationary heat equation. It has been shown that the most strong effect on the temperature field takes the difference between heat conductivities of the matrix and inclusion. The simulations of temperature fields have been carried out on the real systems matrix-inclusion, namely, stainless steel 316 L/manganese sulfide (SS 316L/MnS), TiNi/Ti2Ni, TiNi/TiC. The results obtained allows suggestion on the origin of microcraters appearing at the irradiated surface as a result of thermocapillary convection in overheated sites of location of inclusions.

Generation of amorphous Si structurally compatible with experimental samples through the quenching process: A systematic molecular dynamics simulation study

The construction of realistic atomistic models for amorphous solids is complicated by the fact that they do not have a unique structure. Among the different computational procedures available for this purpose, the melting and rapid quenching process using molecular dynamics simulations is commonly employed as it is simple and physically based. Nevertheless, the cooling rate used during quenching strongly affects the reliability of generated samples, as fast cooling rates result in unrealistic atomistic models. In this study, we have determined the conditions to be fulfilled when simulating the quenching process with molecular dynamics for obtaining amorphous Si (a-Si) atomistic models structurally compatible with experimental samples. We have analyzed the structure of samples generated with cooling rates ranging from 3.31010 to 8.51014 K/s. The obtained results were compared with experimental data available in the literature, and with samples generated by other state-of-the-art and more sophisticated computational procedures. For cooling rates below 1011 K/s, a-Si samples generated had structural parameters within the range of experimental samples, and comparable to those obtained from other refined modeling procedures. These computationally slow cooling rates are of the same order of magnitude than those experimentally achieved with pulsed energy melting techniques. Samples obtained with faster cooling rates can be further relaxed with annealing simulations, resulting in structural parameters within the range of experimental samples. Nevertheless, the required annealing times are on the order of microseconds, which makes this annealing step non practical from a computational point of view.