Melting Threshold Research Articles

Melt layer erosion of pure and lanthanum doped tungsten under VDE-like high heat flux loads

Heat loads expected for VDEs in ITER were applied in the neutral beam facility GLADIS at IPP Garching. Several ∼3mm thick rolled pure W and W–1wt% La2O3 plates were exposed to pulsed hydrogen beams with a central heat flux of 23MW/m2 for 1.5–1.8s. The melting thresholds are determined, and melt layer motion as well as material structure evolutions are shown. The melting thresholds of the two W grades are very close in this experimental setup. Lots of big bubbles with diameters from several μm to several 10μm in the re-solidified layer of W were observed and they spread deeper with increasing heat flux. However, for W–1wt% La2O3, no big bubbles were found in the corrugated melt layer. The underlying mechanisms referred to the melt layer motion and bubble issues are tentatively discussed based on comparison of the erosion characteristics between the two W grades.

Examination of the epicentral waveform for laser ultrasound in the melting regime

A laser ultrasonic source just below the ablation regime is examined by recording an epicentral waveform in a high purity tungsten sample. Using pulse energy as a parameter, a slight delay in the shear wave arrival time is observed upon transition to the melting regime. This phenomenon is attributed to a change in character of the ultrasonic source. In the thermoelastic regime, shear waves are generated by mode conversion at the sample surface of longitudinal waves emanating from subsurface sources. Just above the melting threshold, a molten pool forms in the center of the generation volume. Shear waves are not supported by the molten pool. As a result, shear waves generated from off-axis thermoelastic sources are weighted more heavily. This results in a delay of the shear wave arrival time.

Observation of inhibited electron-ion coupling in strongly heated graphite

Creating non-equilibrium states of matter with highly unequal electron and lattice temperatures (Tele≠Tion) allows unsurpassed insight into the dynamic coupling between electrons and ions through time-resolved energy relaxation measurements. Recent studies on low-temperature laser-heated graphite suggest a complex energy exchange when compared to other materials. To avoid problems related to surface preparation, crystal quality and poor understanding of the energy deposition and transport mechanisms, we apply a different energy deposition mechanism, via laser-accelerated protons, to isochorically and non-radiatively heat macroscopic graphite samples up to temperatures close to the melting threshold. Using time-resolved x ray diffraction, we show clear evidence of a very small electron-ion energy transfer, yielding approximately three times longer relaxation times than previously reported. This is indicative of the existence of an energy transfer bottleneck in non-equilibrium warm dense matter.

Nanosecond laser pulse induced vertical movement of thin gold films on silicon determined by a modified Michelson interferometer

The vertical movement of a 40 nm thin Au film on a silicon substrate during intense nanosecond (ns) laser irradiation is determined on the nm vertical and ns time scales using an optimized Michelson interferometer. The balanced setup with two detectors uses the inverse interference signal and accounts for transient reflectivity changes during irradiation. We show that a change in phase shift upon reflection must be taken into account to gain quantitative results. Three distinct fluence regimes can be distinguished, characterized by transient reflectivity behavior, dewetting processes and film detachment. Maximum displacement velocities are determined to be 0.6 m/s and 1.9 m/s below and above the melting threshold of the metal, respectively. Flight velocities of detaching liquid films are found to be between 30 and 70 m/s for many nanoseconds.

Melting of Al by ultrafast laser pulses: dynamics at the melting threshold

Using molecular-dynamics simulation, we investigate the melting of a thin Al slab by ultrafast laser irradiation. We employ a laser energy, which is just around the melting threshold. While the equilibrium electron–phonon coupling is well understood, we investigate the influence of the early (i.e., prior to electron thermalization) electron-lattice energy transfer. To this end, as a model study, we vary the fraction of the laser energy, which is directly given to lattice atoms vs. that given to the electronic system. We find that the melting process depends sensitively on the early electron-lattice heating rate. The pressure build-up within the still solid parts of the slab is identified as the main agent which delays the melting transition. The changes in the simulated structure factor data suggest that X-ray measurements of thin films performed just around the melting transition—even if performed long after electron thermalization—may provide information on the early electron-lattice energy coupling process.

Electron dynamics in transparent materials under high-intensity laser irradiation

The energy of a laser beam irradiating a surface is primarily absorbed by electrons within the solid. In actual transparent materials, absorption is low. High-intensity lasers may, however, be absorbed by initially bounded electrons through nonlinear processes. The increase of free-electron density leads eventually to dielectric breakdown, and the material becomes highly absorbing. We present theoretical studies on the dynamics of electrons in dielectrics under irradiation with a visible high-intensity laser pulse. We consider microscopic processes determining absorption, redistribution of the energy among electrons, and transfer of energy to the crystal lattice. We review different aspects of electronic excitation, studied with time-resolved models as the Boltzmann kinetic approach and the time and spatial resolved multiple rate equation. Furthermore, we investigate criteria for damage thresholds. Two concepts are compared, namely a critical free-electron density and the melting threshold of the lattice. We show that in dielectrics both criteria are fulfilled simultaneously. Optical parameters depend on the density of free electrons in the conduction band of the solid, so the free-electron density directly leads to an increased energy absorption causing material modification. We present results on the spatial dependence of dielectric breakdown.

Formation and Evolution of Nanoscale Metal Structures on ITO Surface by Nanosecond Laser Irradiations of Thin Au and Ag Films

The effect of nanosecond laser irradiations on 5 nm thick sputter-deposited Au and Ag films on Indium-TinOxide surface is investigated by atomic force microscopy (AFM) and scanning electron microscopy (SEM). After 500, 750, and 1000 mJ/cm 2 fluence irradiations, the breakup of the Au and Ag films into nanoscale islands is observed as a consequence of fast melting and solidification processes. The mean nanoparticles size and surface density are quantified, as a function of the laser fluence, by the AFM and SEM analyses. In particular, the comparison between the Au and Ag islands reveals the formation of larger islands in the case of Ag for each fixed fluence. The mechanism of the nanoscale islands formation is discussed, both for Au and Ag, in terms of the starting film thickness fluctuations (influencing the local threshold for melting), dewetting phenomenon and the Rayleigh criterion.

Determination of the melting threshold of TiO2 thin films processed by excimer laser irradiation

Processing surfaces by laser needs an understanding of the mechanisms generated by irradiation. In this work, to gain understanding of the mechanisms occurring during irradiation of TiO2 thin films by means of KrF excimer laser, we have performed infrared time resolved reflectivity measurements. This experimental investigation revealed modifications of the heating/cooling cycle as a function of the fluence (F). These modifications start appearing for a fluence value of about ∼0.25 J/cm2 which is associated with the melting threshold of the film. Additionally, we have solved numerically the heat equation of the system with specific boundary conditions. From these calculations, we have established the thermal history of the film during the 25 ns irradiation pulse. The data reveal that a part of the medium liquefies around a fluence of 0.23 J/cm2 in good agreement with the experimental data.

Optical-pyrometric diagnostics of the state of silicon during nanopulsed laser irradiation

The dynamics of the reflectivity at λ = 0.53 μm and the IR radiation of silicon in the wavelength range 0.9–1.2 μm is studied under the action of nanosecond ruby laser radiation pulses. When radiation energy density W is lower than the threshold of laser-induced melting of the surface of a semiconductor crystal, the major contribution to the IR radiation emitted by this crystal is made by edge photoluminescence. As the melting threshold is exceeded, the nanosecond dynamics of the detected IR radiation changes from photoluminescence to the thermal radiation of the forming Si phase melt with a high reflectivity. The results of pyrometric measurements of the peak melt surface temperature as a function of W obtained at an effective wavelength λe = 1.04 μm of the detected IR radiation agree with the data of analogous measurements performed at λe = 0.53 and 0.86 μm.

Shocked quartz in Sahara fulgurite

Comparative strain analyses were performed on quartz crystals within a Sahara fulgurite using neutron diffraction experiments. Besides, 3D-μXRCT (three-dimensional micro X-ray computed tomography) studies of the fulgurite were implemented in order to quantify the fusion behaviour, the expansion of the fulgurite as well as the distribution of the stressed quartz crystals. The study of the neutron diffraction pattern was carried out by applying an integral breadth method, more precisely a Rietveld and a LeBail strain–size evaluation with the software FullProf. The absorption contrast of the neutron diffraction pattern shows a concentration of the shocked quartz crystals in the outer shell of the Sahara fulgurite, while the inner part of the fulgurite is mostly glassy. On the basis of the diffraction results, the quartz crystals in the fulgurite can be assigned to a stage of deformation for which the causal shock pressure reaches about 25 GPa, as compared to a melting threshold pressure for quartz of about 50 GPa. Comparing the average density of the fulgurite with the average bulk density of quartz-based sand, it can be concluded that the fulgurite expanded by more than 35 % during its formation.

Simulated Internal Storage Buildup, Release, and Runoff from Greenland Ice Sheet at Kangerlussuaq, West Greenland

This study focused on simulated glacier surface conditions (simulated Surface Melt and liquid Precipitation available for supra-, en-, sub-, and proglacial flow processes [after vertical percolation and potential storage within the snowpack] [henceforth SMP]), internal water storage and release, and runoff from the Kangerlussuaq drainage area of the Greenland Ice Sheet (GrIS), West Greenland, for the period 2006/2007 to 2007/2008. GrIS winter accumulation and summer ablation processes, including SMP, was simulated on both daily and hourly time steps. Using hourly meteorological driving data produced more realistic meteorological conditions instead of daily-averaged data, in relation to snow and melt threshold surface processes, and produced 9–17% higher annual cumulative SMP. The difference between simulated SMP and observed catchment runoff showed a decreasing lag time through the summer, and a drainage system storage buildup through approximately June and early July of up to 0.29 × 109 m3, and a storage release through approximately late July and August of up to 0.25 × 109 m3. The simulated total Kangerlussuaq SMP for 2006/2007 and 2007/2008, indicated a reduction of 30%. This reduction in SMP occurred simultaneously with the reduction in the overall pattern of satellite-derived GrIS surface melt from 2007 to 2008.

STANDARDIZATION AND VALIDATION OF QUALITATIVE REAL TIME RT-PCR FOR DETECTION OF CLASSICAL SWINE FEVER VIRUS

A real time RT-PCR assay was standardized and validated for detection of classical swine fever virus (CSFV). Tonsil tissue and lymph nodules that were positive (n=36) and negative (n= 30) to CSFV by immunofluorescence test (IF) were selected. The viral RNA extraction, the complementary DNA (cDNA) synthesis and real time RT- PCR were performed using commercial kits. Three sets of primers were tested for amplification of a conserved region (5’UTR) of panpestivirus and 5’UTR Chinese strain and the E2 glycoprotein region (E2 PPC) against reference strains of CSFV: Alfort/187, Brescia, and a vaccine Chinese strain as positive controls, and strains of the bovine viral diarrhea virus, border disease virus and porcine rotavirus as negative controls. The 83.3 ± 12.3% (30/36) of positive simples to CSFV by IF test was positive for virus isolation and the 96.7 ± 3.3% (29/30) of negative samples to CSFV by IF was negative for virus isolation. The results of real time RT-PCR assay were determined by the analysis of cycle threshold (Ct) and temperature of melting (Tm) values between the positive and negative controls and the positive and negative to CSFV of field samples. The primer E2 of CSFV recognized all positive and negative controls. The real time RT-PCR assay detected at least 4.28 pg of RNA of CSFV Brescia strain. The validation of the real time RT-PCR was performed comparing the results of positive and negative to CVSFV by isolation test and the results of real time RT-PCR assay. The real time RT-PCR assay had a sensitivity of 96.8 ± 6%, specificity of 82.9 ± 12% and a predictive positive and negative value of 83.3 ± 12.3% and 96.7 ± 3.3% respectively. The real time RT-PCR assay has a high sensitivity and specificity for the diagnosis of CSFV.

Laser Ablation Mechanism Of Silicon Nitride Layers In A Nanosecond UV Regime

Selective laser ablation of silicon nitride (SiNx) layers is a crucial technological step to achieve alternative front side metallization like electrochemical contact. In this work, we discuss the mechanism of laser ablation with a nanosecond UV laser source. A model with two thresholds corresponding to the melting threshold of silicon and the ablation threshold of silicon nitride is proposed. A finite element method is used to solve the heat transfer equation and describe the ablated SiNx surface for a single laser pulse. Numerical results are compared to optical microscopy measurements of the ablated zone.

First experiments at the QSPA-Be plasma gun facility

This paper presents preliminary results on the erosion of beryllium under hydrogen plasma flow. Two samples made of two types of beryllium, TGP-56PS and S-65C, were exposed to plasma heat loads up to 1 MJ m−2 and a pulse duration of 0.5 ms at the QSPA-Be facility in Bochvar Institute, Russia. The melting threshold for both beryllium types was experimentally determined to be 0.5 MJ m−2. The dependence of the specific mass loss and erosion rate on pulse number for both beryllium types was measured. The possibility of generating radiation fluxes with parameters corresponding to mitigated ITER disruptions by means of plasma flow shock braking on a solid bar is shown.

Scanning tunneling microscopic studies of laser-induced modifications of Si(001)-(2 × 1) surface

Scanning tunneling microscopic studies of Si(001)-2 × 1 surfaces excited with 532-nm laser pulses of intensities below melting and ablation thresholds have revealed two different modes of structural modifications, strongly depending on the intensity of laser lights. The excitation below 100 mJ/cm2 causes bond rupture at individual dimer-sites leading to the formation of vacancies selectively on the outermost layer. The bond rupture, which shows a strongly site-sensitive rate, forms efficiently vacancy-strings elongated along the surface dimer-rows. Selective removal of surface dimers results in the exposure of flat and defect-less underlying layer as reported previously, which is resistive to the excitation at this range of intensity. At intensities above 100 mJ/cm2, on the other hand, the excitation forms not only vacancies but also ad-dimers on terraces. The number density of ad-dimers is in proportion to the square of that for vacancies, indicating strongly that silicon atoms released by laser-induced bond rupture are associated with each other to form ad-dimers. The repeated irradiations at this range of intensities induce anisotropic growth of ad-dimer islands and of vacancy clusters on terrace regions, leading to multiply terraced structure. The primary processes of the structural modifications are discussed based on the quantitative analyses of the growth of vacancy and ad-dimer under excitation.

Measurements of the ns-laserpulse induced expansion of (111) silicon below and above the melting threshold on the nanosecond time scale

The surface displacement of 111-silicon wafers of 0.675 mm and 3.05 mm thickness, respectively, during intense ns laser irradiation is determined on the nm-vertical and ns-time scale using an optimized Michelson interferometer. The surface dynamics is observed below as well as above the melting threshold. We show that the obtained surface expansion below the melting threshold is in good agreement with theoretical heat transfer calculations. Additionally, thickness-dependent bending in the samples is illustrated and the acoustic reflections from the backside of the sample are observed. Maximum thermal displacement at the first expansion plateau is around 6 nm before melting occurs. We show that qualitative agreement between experimental results and simulation above the melting threshold can be established for the first time by taking the phase shift change in the reflection for molten silicon into account.

The Influence of DOS Effects on Ablation Properties in High Energy Femtosecond Laser Ablation Process

The numerical simulation of high-energy femtosecond laser ablation on metal target is studied in this paper. Based on the two-temperature model (TTM), a new model considering the effects of the electron density of states (DOS) on electronic heat capacity, electron-phonon coupling coefficient and electronic thermal conductivity is established. As an example of gold target, the relationship between the melting threshold and the thickness of gold films is numerically calculated. Our result is more consistent with the experimental datum in contrast to the results without considering the DOS effects. This shows that the revised TTM of high-energy femtosecond laser ablation (i.e. DOS-TTM) is more reasonable compared with general used TTM.

Molecular dynamics study of the dewetting of copper on graphite and graphene: Implications for nanoscale self-assembly

Thin-film dewetting can be exploited to self-assemble and organize nanoparticles. Crucial to this effort is the understanding of the nanoscale liquid phase dynamics, and molecular dynamics simulations (MD) provide a powerful tool in this respect. In this paper we demonstrate that MD simulations utilizing a Lennard-Jones (LJ) interface potential can be effectively used to study various wetting regimes of nanoscale Cu disks on graphite. It was found that both the dewetting velocity and the equilibrium contact angle increase with a decrease in the Cu-C potential, and that the retraction velocities obtained are characteristic of dewetting phenomena governed by inertial and capillary forces. This phenomena leads to a change in morphology, from disks to nanodroplets, which, in turn, when using the most accurate LJ potential, jump off the graphitic substrate with a velocity on the order of 140 m/s. This ejection velocity is consistent with the previous experimental observation that nanoscale Au triangles deposited on graphite or glass jump when exposed to a pulsed laser above the melting threshold. Interestingly, the Cu ejection velocity decreases when the liquid Cu disks are deposited on a suspended graphene membrane. Finally, a Rayleigh-Plateau-like instability, which leads to the breakup of a pseudo-one-dimensional liquid Cu nanowire in nanodroplets, is revealed when the MD simulations are performed using different LJ interface potentials.

Self-Organization Processes of Nanostructure Formation by Laser Radiation in Metal

The current state of knowledge about mechanisms of metal nano-particles (NP) formation processes induced by the interaction of high-energy laser beam with surface of the metallic lattices (Au, Ag, Cu) is presented. The review includes an evaluation of the contribution of self-organized effects into the processes of the metal nano-structurization depending on the laser mode, external factors and internal parameters of an active zone. It was noticed that intensive pulsed laser illumination enabled to stimulate nano-fragmentation at the fluencies near and above the melting threshold of the metal in different mediums The laser induced processes of metallic particles formation by ablation of the metal target with consequent NP sizes stabilization by precise temperature tuning in the active zone, local plasmon resonance in liquids, and microablation mechanism in metal films in the conditions of surface plasmon resonance (SPR) due to self-organization effects are considered. Comparative analysis of the laser nano-technologies in air, vacuum, rarefied gas and liquid environments showed the advantages of self-organization in NP generation processes based on the SPR effects and their perspectives.

Laser-Induced Modification of the Surface State and Optical Properties of CdTe Crystals

Optical properties and surface state of semiconductor CdTe crystals subjected to irradiation with nanosecond laser pulses were studied. Ellipsometric parameters Δ and Ψ were measured on two opposite surfaces of (111) oriented CdTe wafers before. The samples were subjected to polishing chemical etching and laser irradiation with energy densities lower and higher than the melting threshold of CdTe. The morphology and structure of CdTe crystals were monitored before and after treatments. Irradiation formed a thin modified surface layer on the both sides of CdTe crystals (Cd- and Te-terminated) with the similar optical constants n and k, respectively. However, the effective thicknesses of such modified layers differed and depended on the chemical and laser treatment conditions and also on the faces of CdTe(111) crystals.