Melting Threshold Research Articles

Thermal shock and fatigue resistance of tungsten materials under transient heat loading

Transient heat loading tests were performed on rolled pure tungsten (PW) and lanthanum oxide doped tungsten (WL10) as well as swaged+rolled potassium doped tungsten (W-K) samples using an electron beam. In thermal shock tests, the cracking threshold was 0.44–0.66, 0.17–0.22 and 0.44–0.66GW/m2 for PW, WL10 and W-K, respectively. The melting threshold was over 1.1GW/m2 for PW and W-K while 0.66–0.88GW/m2 for WL10. In thermal fatigue tests, the obvious roughening threshold was over 1000 cycles for PW and WL10 while 1–100cycles for W-K. The cracking threshold was 100–1000cycles for PW, 1–100cycles for WL10 and over 1000cycles for W-K alloy. WL10 displayed worse thermal and fatigue resistance while W-K exhibited better properties compared with PW, which was attributed to differences in thermal–mechanical properties of the three tungsten alloys, in addition to the size and number density of La2O3 particles and potassium bubbles.

Plasma-Facing Materials Erosion under ITER-Like Transient Loads at QSPA Plasma Gun Facility

The beryllium (Be) plasma-facing components (PFCs) of the ITER first wall (FW) were tested in the plasma gun QSPA-Be under pulsed plasma heat loads of 0.5-ms duration relevant to those expected in ITER during transient plasma events (edge-localized modes and disruptions). The experiments were performed for different Be grades (Russian TGP-56FW and US S65-C). The measured Be melting threshold decreases from 0.5 MJm−2 down to 0.4 MJm−2 with Be initial temperature increasing in the range of 250–500 °C. Under plasma heat loads on the exposed surface below the melting point the Be PFC erosion was mainly due to melting of the plasma-facing and lateral edges of the Be tiles. Under plasma heat loads above the melting point the Be PFC erosion was mainly due to intense melt layer movement and splashing. The Be melt layer behavior at 0.5 and 1.0 MJm−2 is similar to early investigated W melt layer behavior at higher heat loads of 1.0 and 1.5 MJm−2 correspondingly. Unlike W the Be erosion rate significantly increases with initial temperature in the range of 250–500 °C. These experimental observations are supported by calculation of temperature dynamics and melt layer thickness dynamics.

Coherent structural dynamics of a prototypical charge-density-wave-to-metal transition.

Using femtosecond time-resolved x-ray diffraction, we directly monitor the coherent lattice dynamics through an ultrafast charge-density-wave-to-metal transition in the prototypical Peierls system K(0.3)MoO(3) over a wide range of relevant excitation fluences. While in the low fluence regime we directly follow the structural dynamics associated with the collective amplitude mode; for fluences above the melting threshold of the electronic density modulation we observe a transient recovery of the periodic lattice distortion. We can describe these structural dynamics as a motion along the coordinate of the Peierls distortion triggered by the prompt collapse of electronic order after photoexcitation. The results indicate that the dynamics of a structural symmetry-breaking transition are determined by a high-symmetry excited state potential energy surface distinct from that of the initial low-temperature state.

Diffraction-assisted micropatterning of silicon surfaces by ns-laser irradiation

Single-pulse (532 nm, 8 ns) micropatterning of silicon with nanometric surface modulation is demonstrated by irradiating through a diffracting pinhole. The irradiation results obtained at fluences above the melting threshold are characterized by scanning electron and scanning force microscopy and reveal a good agreement with Fresnel diffraction theory. The physical mechanism is identified and discussed on basis of both thermocapillary and chemicapillary induced material transport during the molten state of the surface.

Melting and resolidification of gold film irradiated by laser pulses less than 100 fs

The rapid melting and resolidification of gold films irradiated by laser pulses less than 100 fs are investigated using the dual-hyperbolic two-step model. The solid–liquid interfacial velocity in the ultrafast phase change process is obtained by coupling a hyperbolic interfacial energy balance equation and nucleation dynamics. The results are compared with the experimental data for the 28-fs laser. The effects of laser pulse widths and fluences on melting process are investigated. A phase chart of the variations of pulse widths and fluences is established. The relationship between the melting threshold and ablation threshold is also presented.

Plasma exposure of different tungsten grades with plasma accelerators under ITER-relevant conditions

This paper presents the results of tungsten irradiation experiments performed with three plasma facilities: the QSPA Kh-50 quasi-steady-state plasma accelerator, the PPA pulsed plasma gun and the magneto-plasma compressor. Targets made of different kinds of tungsten (sintered, rolled and deformed) were irradiated with powerful plasma streams at heat fluxes relevant to edge-localized modes in ITER. The irradiated targets were analyzed and two different meshes of cracks were identified. It has been shown that the major cracks do not depend on the tungsten grade. This has been attributed to ductile-to-brittle transition effects. Meshes of inter-granular micro-cracks were detected for energy loads above the melting threshold and these were probably caused by the re-solidification process. The blister-like and cellular-like structures were observed on sample surfaces exposed to helium and hydrogen plasmas.

Investigation of the impact of transient heat loads applied by laser irradiation on ITER-grade tungsten

Cracking thresholds and crack patterns in tungsten targets after repetitive ITER-like edge localized mode (ELM) pulses have been studied in recent simulation experiments by laser irradiation. The tungsten specimens were tested under selected conditions to quantify the thermal shock response. A Nd:YAG laser capable of delivering up to 32 J of energy per pulse with a duration of 1 ms at the fundamental wavelength λ = 1064 nm has been used to irradiate ITER-grade tungsten samples with repetitive heat loads. The laser exposures were performed for targets at room temperature (RT) as well as for targets preheated to 400 °C to measure the effects of the ELM-like loading conditions on the formation and development of cracks. The magnitude of the heat loads was 0.19, 0.38, 0.76 and 0.90 MJ m−2 (below the melting threshold) with a pulse duration of 1 ms. The tungsten surface was analysed after 100 and 1000 laser pulses to investigate the influence of material modification by plasma exposures on the cracking threshold. The observed damage threshold for ITER-grade W lies between 0.38 and 0.76 GW m−2. Continued cycling up to 1000 pulses at RT results in enhanced erosion of crack edges and crack edge melting. At the base temperature of 400 °C, the formation of cracks is suppressed.

Tungsten damage and melt losses under plasma accelerator exposure with ITER ELM relevant conditions

Experimental simulations of ITER edge-localized mode have been performed with a Kh-50 quasi-stationary plasma accelerator. Heat loads exceeded the tungsten melting threshold. Droplet splashing and the solid dust ejection were observed. Droplets were emitted during the plasma exposure and dust generation dominated after the end of a plasma pulse, at the time of the material cooling. A decrease in the droplet velocity at an increase in the surface heat load was observed. It could be attributed to growing sizes of the droplets at higher energy loads. After one hundred pulses were performed at loads below the tungsten melting and above the cracking threshold, some porosity appeared. Such defects achieved dimensions of several tens of μm. A heat load amounting to half the cracking threshold (0.3 MJ m−2) could lead to the appearance of fatigue cracks after 100 plasma pulses. In this case the appearance of cracks could be caused by the accumulation and redistribution of linear defects (dislocations) in the affected tungsten layer.

Tungsten Melt Losses under QSPA Kh-50 Plasma Exposures Simulating ITER ELMs and Disruptions

Experimental simulations of ITER transient events with surface heat load parameters relevant to edge-localized-mode (ELM) impacts and disruptions have been performed with a quasi-stationary plasma accelerator Kh-50. In the ELM simulation experiments with heat loads exceeding the tungsten melting threshold, both droplet splashing and solid dust ejection are observed. The erosion products emitted from the exposed tungsten surfaces in the form of droplets and solid dust have been clearly distinguished by variation of impacting heat load with performed analysis of particle ejection start time, their velocities, and changes in the luminosity of the particle traces in front of the target surface recorded with a charge-coupled device. Droplets are emitted during plasma exposure, and dust generation dominates after the end of the plasma pulse, at the time of the following material cooling. The contributions of Kelvin-Helmholtz instabilities to droplet splashing from the melt layer are discussed. Decrease of droplet velocity with increasing surface heat load is observed. This decrease could be attributed to the growing size of the droplets for higher energy loads.

${\rm CO}_{2}$ Laser Annealing for USJ Formation in Silicon: Comparison of Simulation and Experiment

In this paper, we compare the experimental and simulation results of the effect of laser annealing on boron implanted silicon using a <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm CO}_{2}$</tex> </formula> source and commercially available process TCAD tools. The main objective of this analysis is the prediction of the evolution of temperature distribution induced into the wafer during the laser irradiation below the melting threshold as well as its effect on boron diffusion and activation kinetics. A series of factors are considered along with the use of advanced heat transfer and dopant diffusion models provided by the TCAD tool to accurately prototype the effect of the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm CO}_{2}$</tex></formula> irradiation on temperature and dopant distribution. These include the nonuniformity of the incident laser beam, the strong dependence of heat capacity and thermal conductivity from temperature and, most importantly, the dependency of the absorptivity from temperature and dopant distribution, which requires the solving of dopant and heat transfer equations in a coupled and self-consistent way. Using surface temperature data obtained by pyrometry measurements, it was possible to calibrate and to verify the validity of the results. Experimental boron profiles were then used to compare with simulations in the transient regime where dopant diffusion just starts to occur. Both TCAD and experimental data confirmed previous suggestions that submelt <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm CO}_{2}$</tex></formula> laser annealing is an efficient tool for profile engineering, allowing diffusionless activation of the plasma doped boron profiles.

Silicon before the bonds break

The diamond structure of silicon can be destroyed by a femtosecond-laser pulse in a process that is called nonthermal melting. Using a supercell of 800 atoms, we have performed ab initio molecular dynamics simulations on laser-excited potential energy surfaces for electronic excitation densities around the threshold for nonthermal melting. By introducing a quantitative measure for the resemblance of the atomic paths below and above this threshold, we show that the directions of the atomic motions are the same within 98% during 150–200 fs. The atomic pathways below the melting threshold, before the bonds break, are therefore quantitatively closely related to the atomic motions during the first stages of nonthermal melting.

Understanding Thin Film Laser Ablation: The Role of the Effective Penetration Depth and the Film Thickness

Abstract In recent years, several applications for the laser ablation of thin metal films from the glass substrate side have been studied.In this case, the laser pulse transmits through the glass substrate, the metal film absorbs the pulse, and the energy is confined at the molybdenum-glass interface. The ablation mechanism is called confined laser ablation. It was observed that the confined laser ablation of 430 nm thin molybdenum films with ultrashort laser pulses leads to the lift-off of intact disks.The ablation efficiency can be increased by a factor of 10 compared to the direct ablation from the film side.The goal of the study is to investigate, if a confined ablation,which enables this high ablation efficiency, can be achieved also at different pulse durations and film thicknesses.For this purpose, the pulse duration is varied between 20 ns and 330 ns at a constant film thickness of 430 nm, andthe film thickness is varied between 10 nm and 5000 nm at a constant pulse duration of 10 ps. The single pulse ablation threshold fluences are determinedfor the film and glass substrate side irradiation. The determined threshold fluences are finally compared to calculated melting and evaporation threshold fluences.The results suggest that the ablation mechanism for glass side ablation is based mainly on melt dynamics, if the effective penetration depth of the laser pulse is in the order of the film thickness. A high efficient confined ablation can be observed, if the effective penetration depth is smaller than the film thickness.

Laser thermoelastic generation in metals above the melt threshold

An approach is presented for calculating thermoelastic generation of ultrasound in a metal plate exposed to nanosecond pulsed laser heating, sufficient to cause melting but not ablation. Detailed consideration is given to the spatial and temporal profiles of the laser pulse, penetration of the laser beam into the sample, the appearance and subsequent growth and then contraction of the melt pool, and the time dependent thermal conduction in the melt and surrounding solid throughout. The excitation of the ultrasound takes place during and shortly after the laser pulse and occurs predominantly within the thermal diffusion length of a micron or so beneath the surface. It is shown how, because of this, the output of the thermal simulations can be expressed as axially symmetric transient radial and normal surface force distributions. The epicentral displacement response to these force distributions is obtained by two methods, the one based on the elastodynamic Green's functions for plate geometry determined by the Cagniard generalized ray method and the other using a finite element numerical method. The two approaches are in very close agreement. Numerical simulations are reported on the epicentral displacement response of a 3.12 mm thick tungsten plate irradiated with a 4 ns pulsed laser beam with Gaussian spatial profile, at intensities below and above the melt threshold.

Morphology and crystalline phase characteristics of α-GST films irradiated by a picosecond laser

The morphology and crystalline phase characteristics of amorphous Ge2Sb2Te5 films irradiated by a picosecond laser were investigated by 3D surface profiler, atomic force microscopy (AFM) and transmission electron microscopy (TEM) integrated with selected area electron diffraction (SAED). The laser irradiated spot was divided into strong ablation area, gentle ablation area, melting area and irradiation area. By theoretical calculation, the ablation and melting thresholds were determined to be 173.05mJcm−2 and 99.19mJcm−2 respectively. Meantime, the local fine morphologies of the ablation and melting areas were shown and analyzed. We also studied the irradiation area which was made up by the non-phase-change area and phase-change area. In the phase-change area, crystalline phase was determined to be face-centered cubic structure and crystalline phase characteristics for films with different thicknesses were discussed.

Characterization and simulation analysis of laser-induced crystallization of amorphous silicon thin films

The effect of laser energy density on the crystallization of hydrogenated amorphous silicon (a-Si:H) thin films was studied theoretically and experimentally. The thin films were irritated with a frequency-doubled (λ=532nm) Nd:YAG pulsed nanosecond laser. An effective finite element model was built to predict the melting threshold and the optimized laser energy density for crystallization of intrinsic amorphous silicon. Simulation analysis revealed variations in the temperature distribution with time and melting depth. The highest crystalline fraction measured by Raman spectroscopy (84.5%) agrees well with the optimized laser energy density (1000mJ/cm2) in the transient-state simulation. The surface morphology of the thin films observed by optical microscopy is in fairly good agreement with the temperature distribution in the steady-state simulation.

Time-resolved temperature measurement and numerical simulation of millisecond laser irradiated silicon

Thermal process of 1064 nm millisecond pulsed Nd:YAG laser irradiated silicon was time-resolved temperature measured by an infrared radiation pyrometer, temperature evolutions of the spot center for wide range of laser energy densities were presented. The waveforms of temperature evolution curves contained much information about phase change, melting, solidification and vaporization. An axisymmetric numerical model was established for millisecond laser heating silicon. The transient temperature fields were obtained by using the finite element method. The numerical results of temperature evolutions of the spot center are in good agreement with the experimental results. Furthermore, the axial temperature distributions of the numerical results give a better understanding of the waveforms in the experimental results. The melting threshold, vaporizing threshold, melting duration, and melting depth were better identified by analyzing two kinds of results.

Silicon nanodot formation and self‐ordering under bombardment with heavy Bi3 ions

AbstractSi nanodots of high density and hexagonal short‐range order are observed upon normal‐incidence bombardment of hot, crystalline Si with Bi3+ ions having a kinetic energy of a few tens of keV. The heights of nanodots are comparable to their widths of ∼20 nm. The implanted Bi accumulates in tiny Bi nanocrystals in a thin Si top layer which is amorphous due to implantation damage. Light and heavy ions up to Xe cause smoothing of surfaces, but Bi3+ ions considered here have a much higher mass. Atomistic simulations prove that each Bi3+impact deposits an extremely high energy density resulting in a several nanometer large melt pool, which resolidifies within a few hundreds of picoseconds. Experiments confirm that dot patterns form only if the deposited energy density exceeds the threshold for melting. Comparing monatomic and polyatomic Bi ion irradiation, Bi–Si phase separation and preferential ion erosion are ruled out as driving forces of pattern formation. A model based on capillary forces in the melt pool explains the pattern formation consistently.magnified imageHigh‐density Si nanodots are formed by polyatomic Bi ion irradiation of hot Si surfaces. Each impact causes local transient melt pools smaller than the dots. Hexagonally ordered patterns evolve by self‐organization driven by repeated ion‐induced melting of tiny volumes. Homogeneously distributed Bi nanocrystals are found in the a‐Si film. These nanocrystals are related to particularities of the Si–Bi phase diagram. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Spalling and melting in nanocrystalline Pb under shock loading: Molecular dynamics studies

The mechanisms of spalling and melting in nanocrystalline Pb under shock loading are studied by molecular dynamics simulations. A wide range of shock intensity is conducted with the lowest one just above the threshold of solid spallation, and the highest one higher than the threshold of compression melting. The spallation mechanism is dominated by cavitation, i.e., nucleation, growth, and coalescence of voids. Our results show that grain boundaries have significant influences on spalling behaviors in cases of classical spallation and releasing melting. In these cases, cavitation and melting both start on grain boundaries, and they display mutual promotion: melting makes the voids nucleate at smaller tensile stress, and void growth speeds melting. Influences of microstructure, strain rate, and temperature on spall strength are qualitatively discussed. Due to grain boundary effects, the spall strength of nanocrystalline Pb varies slowly with the shock intensity in cases of classical spallation. In cases of releasing melting and compression melting, spall strength of both single-crystalline and nanocrystalline Pb drops dramatically as shock intensity increases.

Stainless steel performance under ITER-relevant mitigated disruption photonic heat loads

ITER grade stainless steel (SS) 316L(N)-IG was tested in QSPA-T (photons) and JUDITH (electrons) under heat loads relevant to those expected from photon radiation on the ITER diagnostic first wall (DFW) during disruptions mitigated by massive gas injection. Repeated pulses slightly above the melting threshold eventually lead to a regular, “corrugated” SS surface, with hills and valleys spaced by 1–2mm. The negligible mass loss observed after the heat pulses indicates that hill growth (growth rate of ∼1–2μm per pulse) and SS plate thinning in the valleys is a result of melt-layer redistribution. A Similar behavior is observed on SS samples exposed in QSPA-T (pulse length 0.5ms) and JUDITH (pulse length⩽3.0ms) at the same heat flux factor. The combined data suggests, for the total lifetime, a surface roughening of ⩽1.5mm on parts of the ITER SS DFW exposed to the highest transient photon loads, after 1200 mitigated disruptions at high stored energy. The results also indicate that the surface roughness increase may be significantly reduced by variation of the SS impurity composition. This experimental observation is supported by a proposed theoretical mechanism for the surface roughness formation based on the growth of capillary waves in the melt layer.

Surface modification and droplet formation of tungsten under hot plasma irradiation at the GOL-3

The paper presents experimental investigations of tungsten surface modification after plasma loads at the multimirror trap GOL-3. Energy loads on tungsten surface varied from 2 up to 4 MJ/m 2 per shot with sets from 1 to 9 repetitive irradiations that corresponds to loads higher than tungsten melting threshold and is close to ITER giant ELM loads. Targets surface modification and transverse microsections after irradiation was studied by optical microscopy , SEM and hardness tester. Formation on tungsten surface of three different crack networks with typical cell sizes of 1000, 10 and 0.3 μm and bubbles are identified. The network of large cracks extend perpendicularly to the irradiated sample surface to a depth of 50–350 μm. Erosion depth depends on energy loads – rises from 20 to 200 μm at 2 and 4 MJ/m 2 correspondingly. Cracking, development of tungsten surface morphology and droplets splashing are discussed.