Melting Threshold Research Articles

Simulation of tokamak armour erosion and plasma contamination at intense transient heat fluxes in ITER

For ITER, the potential material damage of plasma facing tungsten-, CFC-, or beryllium components during transient processes such as ELMs or mitigated disruptions are simulated numerically using the MHD code FOREV-2D and the melt motion code MEMOS-1.5D for a heat deposition in the range of 0.5–3 MJ/m2 on the time scale of 0.1–1 ms. Such loads can cause significant evaporation at the target surface and a contamination of the SOL by the ions of evaporated material. Results are presented on carbon plasma dynamics in toroidal geometry and on radiation fluxes from the SOL carbon ions obtained with FOREV-2D. The validation of MEMOS-1.5D against the plasma gun tokamak simulators MK-200UG and QSPA-Kh50, based on the tungsten melting threshold, is described. Simulations with MEMOS-1.5D for a beryllium first wall that provide important details about the melt motion dynamics and typical features of the damage are reported.

Metallic thin films heated by pulsed lasers. Numerical simulation of the thermal field and the melting kinetics

This modeling is especially applied to the pulsed laser induced heating and melting of a metallic film deposited on a substrate. Study of the thermal field over a surface is usually performed by considering the assumption of ‘semi-infinite medium’. However, a thin film deposited on a rough substrate surface induces bad thermal contacts commonly known as ‘thermal contact resistance’. This interfacial thermal resistance affects the melting kinetics mainly when the film thickness (Z) is small comparatively to the heat diffusion length (ZT). In this work the heat conduction equation and related boundary conditions are resolved by using the implicit finite differences method. The heat source (i.e. the laser intensity) is treated as a surface boundary layer. The thermal contact resistance is introduced in the computation procedure when the heat wave propagation reaches the thin film/substrate interface. It is then possible to calculate the critical temperatures and the melting threshold fluence for high and low contact resistance values. Under these conditions, the temperature profile and melting depth are plotted considering different thickness.. Finally, for 750 mJ/cm² excimer laser fluence and 0.1 cm²/s thin film apparent diffusivity results show that for Z/ZT higher than 0.5, there is no sensitive effect of the thermal contact resistance on the melting kinetics.

Mechanisms of laser-induced defect formation and in doping in CdTe crystals

The laser-stimulated modification of the crystalline and point defect structure of the surface layer of CdTe crystals was studied and laser-induced doping was performed to form electrical barriers in the surface region of samples. The results of comprehensive investigations of time-resolved optical reflectivity, atomic force microscopy, reflection of high-energy electron diffraction, current-voltage characteristics, exciton photoluminescence in (111)B oriented CdTe crystals subjected to irradiation with nanosecond KrF excimer laser pulses have been discussed. The peculiarities of pulsed-laser-induced melting and following crystallization of the CdTe surface depending on laser energy density have been analyzed. The melting and ablation thresholds were determined as 50 mJ/cm/sup 2/ and 145 mJ/cm/sup 2/, respectively. Using laser irradiation of CdTe crystals pre-coated with an indium dopant film, it was possible to suppress the self-compensation mechanism and fabricate the CdTe-based diodes promising for nuclear radiation detectors. The mechanisms of laser-induced defect formation and doping have been discussed.

Drilling of steel and HgCdTe with the femtosecond pulses produced by a commercial laser system

The results of interaction of single and multiple 200 fs laser pulses with thick stainless steel and HgCdTe samples are reported. The threshold laser energy density required to produce surface melting is measured. The melt dynamics and evolution of surface morphology are observed for different pulse energies and number of laser pulses. It is observed that, as with a long laser pulse interaction, a layer of melt can be produced at the sample surface. Melt ejection in the radial direction towards the periphery of the interaction zone is observed when the pulse energy is increased. The observed melt dynamics resembles the evaporation recoil melt removal typical of the laser interactions in the range from nanoseconds to continuous wave (CW). The observed melt ejection is attributed to a nanosecond component of the laser pulse with an estimated energy of approximately 25% of the total laser pulse energy. The measured melting threshold energy density for stainless steel is comparable with the published theoretically predicted threshold for nickel computed using a two-temperature model.

Laser induced crystallization of hydrogenated amorphous silicon-carbon alloys

Laser induced crystallization of hydrogenated amorphous silicon carbon alloy (a-Si1−xCx:H) films has been investigated by means of synchrotron x-ray diffraction. The a-Si1−xCx:H films were deposited on (100) silicon wafers by very high frequency plasma enhanced chemical vapor deposition at 100MHz in hydrogen diluted silane-methane gas mixtures. The substrate was kept at 250°C or 350°C and the stoichiometry was changed from x=0.20 to 0.63. The structural characterization of the as-grown films has been carried out by Rutherford backscattering (hydrogen concentration) and infrared spectroscopy (film ordering). The films were irradiated by a KrF excimer laser (248nm) with varying energy density and number of pulses. After irradiation, the formation of SiC crystallites has been revealed by synchrotron x-ray diffraction. Besides SiC nanocrystals, the formation of crystalline Si and graphite is observed for under- (x<0.50) and over-stoichiometric (x>0.50) samples, respectively. The essential role played by hydrogen concentration and hydrogen bonding configuration in determining the melting threshold and the consequent SiC grain formation is highlighted.

Electronic mechanism of ion expulsion under UV nanosecond laser excitation of silicon: Experiment and modeling

We present experimental and modeling studies of UV nanosecond pulsed laser desorption and ablation of (111) bulk silicon. The results involve a new approach to the analysis of plume formation dynamics under high-energy photon irradiation of the semiconductor surface. Non-thermal, photo-induced desorption has been observed at low laser fluence, well below the melting threshold. Under ablation conditions, the non-thermal ions have also a high concentration. The origin of these ions is discussed on the basis of electronic excitation of Si surface states associated with the Coulomb explosion mechanism. We present a model describing dynamics of silicon target excitation, heating and harge-carrier transport.

Single-pulse KrF laser ablation and nanopatterning in vacuum of β-titanium alloys used in biomedical applications

We report the KrF laser (248 nm) nanosecond single-pulse irradiation in vacuum of a mirror-polished surface of a model β-titanium alloy. A series of single-pulse laser-annealing experiments is performed with increasing fluence adjusted to obtain conditions from submelting (a few mJ/cm2) up to intense laser ablation (30 J/cm2). The structural and morphological changes are followed by atomic force microscopy and scanning electron microscopy, as well as with other analytical techniques, and compared with the starting αβ equilibrium mixture of the untreated sample. The results are discussed and related to the theoretical estimation of fluences corresponding to the β-transus, melting, boiling and intense ablation (phase-explosion) thresholds as well as to the estimates of melt depth and lifetime evaluation as a function of fluence. By carrying out laser annealing in vacuum the surface oxidation is avoided while roughening and phase transformations are favoured within the laser-heated depth. The mirror-polished starting surface is composed of nanometric α-precipitates embedded in a β-matrix, whose surface is systematically 5–10 nm above the α-surface. Upon laser annealing with increasing fluence the α-nanocrystals transform progressively into β-phase, resulting in an increasing relief. At higher fluence the conditions for increasing roughness are studied. Submicrometre roughness is expected to improve cell adhesion to β-titanium alloys and hence osteointegration.

Electronic bond rupture of Si-Dimers on Si(001)-(2x1) induced by pulsed laser excitation

A scanning tunneling microscopy study has revealed that 532 nm laser pulses of fluences well below melt and ablation thresholds induce electronic bond rupture of Si-dimers on the Si(001)-(2x1), resulting in the formation of single dimer-vacancies followed by progressive growth into vacancy clusters. The rate of bond rupture on the intrinsic (2x1) structure shows super-linearity with respect to excitation intensity, and saturates as the number of vacancies reaches a few percent, relative to total dimer sites. The mechanism of laser-induced bond rupture is discussed based on these results.

Combined atomistic-continuum model for simulation of laser interaction with metals: application in the calculation of melting thresholds in Ni targets of varying thickness

The threshold laser fluence for the onset of surface melting is calculated for Ni films of different thicknesses and for a bulk Ni target using a combined atomistic-continuum computational model. The model combines the classical molecular dynamics (MD) method for simulation of non-equilibrium processes of lattice superheating and fast phase transformations with a continuum description of the laser excitation and subsequent relaxation of the conduction band electrons based on the two-temperature model (TTM). In the hybrid TTM-MD method, MD substitutes the TTM equation for the lattice temperature, and the diffusion equation for the electron temperature is solved simultaneously with MD integration of the equations of motion of atoms. The dependence of the threshold fluence on the film thickness predicted in TTM-MD simulations qualitatively agrees with TTM calculations, while the values of the thresholds for thick films and bulk targets are ∼10% higher in TTM-MD. The quantitative differences between the predictions of TTM and TTM-MD demonstrate that the kinetics of laser melting as well as the energy partitioning between the thermal energy of atomic vibrations and energy of the collective atomic motion driven by the relaxation of the laser-induced pressure should be taken into account in interpretation of experimental results on surface melting.

Laser inertial fusion dry-wall materials response to pulsed ions at power-plant level fluences

Pulses of MeV-level ions with fluences of up to 20 J/cm 2 can be expected to impinge on the first-wall of future laser-driven Inertial Fusion Energy (IFE) power plants. To simulate the effect of these ions, we have exposed candidate dry-wall materials to ion pulses from RHEPP-1, located at Sandia National Laboratories. Various forms of tungsten and tungsten alloy were exposed to up to 1000 pulses, with some samples heated to 600 °C. Thresholds for roughening and material removal, and evolution of surface morphology were measured and compared with code predictions for materials response. Tungsten is observed to undergo surface roughening and subsurface crack formation that evolves over hundreds of pulses, and which can occur both below and above the melt threshold. Heating and Re-alloying mitigate, but do not eliminate, these apparently thermomechanically-caused effects. Use of a 3-D geometry, and/or use of the tungsten in thin-film form may offer improved survivability compared to bulk tungsten.

Influence of the ambient gas in laser structuring of the titanium surface

Surface structuring of titanium was performed by multipulse high repetition rate Nd:YAG ( λ=1.064 μm, τ=∼300 ns, ν=30 kHz) laser irradiations in vacuum as well as reactive or inert ambient gases at an intensity value below the single-pulse melting threshold. After irradiations, the surface morphologies have been investigated by scanning electron microscopy (SEM). The surface structuring is strongly influenced by the ambient gas. In high-pressure nitrogen, rippled, and with the increase of the laser pulse number, columnar microrelief formation takes place. In vacuum or in high-pressure argon, the surface morphology is characterised by smooth polyhedral structures developing in the surface plane. In air, the initial surface morphology consisting of a network of microcracks evolves with further irradiation towards a porous microrelief. The physical phenomena involved in the specific surface structure formation are discussed.

Finite Element Analysis of Pulsed Laser Bending: The Effect of Melting and Solidification

This work developes a finite element model to compute thermal and thermomechanical phenomena during pulsed laser induced melting and solidification. The essential elements of the model are handling of stress and strain release during melting and their retrieval during solidification, and the use of a second reference temperature, which is the melting point of the target material for computing the thermal stress of the resolidified material. This finite element model is used to simulate a pulsed laser bending process, during which the curvature of a thin stainless steel plate is altered by laser pulses. The bending angle and the distribution of stress and strain are obtained and compared with those when melting does not occur. It is found that the bending angle increases continulously as the laser energy is increased over the melting threshold value.

Light-induced modification of a-SiOx II: Laser crystallization

Amorphous hydrogenated silicon suboxides (a-SiOx:H) were deposited by plasma enhanced chemical vapor deposition from the source gases SiH4, H2, and CO2. The band gap of the samples can be tuned from 1.9 to 3.0 eV by varying the oxygen content from 0 to 50 at. %. H-effused samples were irradiated by ultraviolet laser pulses with intensities up to 480 mJ/cm2. The structural changes and the crystallization behavior were investigated as a function of oxygen content and laser intensity. A decrease of the melting threshold by a factor of two with increasing oxygen content (0–44 at. %) was observed for the SiOx samples. Above the respective melting thresholds, not only a deterioration of the structural properties but also indications of a segregation of Si crystallites were found. Raman spectroscopy and transmission electron microscopy gave evidence for the existence of Si crystallites up to oxygen contents of 40 at. %. The crystal size reached an optimum for oxygen concentrations between 10 and 30 at. %.

Study of a CdTe high‐energy radiation imaging device fabrication by excimer laser processing

CdTe based high-energy radiation imaging devices with high-energy resolution with room temperature operation, were fabricated by excimer laser low-temperature processing. A time-resolved reflectivity (TRR) in-situ measurement was carried out for CdTe transitions in order to identify clearly the laser-induced melting threshold of CdTe semiconductor. The threshold was estimated to be about a pulse energy of 50mJ/cm2. The CdTe M–π–n diode structures were prepared by excimer laser processing and the diode showed good diode I–V characteristics. The reverse current was very low less then 1nA at –100 V as an operation bias, and the CdTe detectors showed good and uniform spectra measurement characteristics. They were integrated with ASIC chips for signal processing as an imaging device unit. The imaging device unit could capture energy resolution of gamma-ray images distinguished by radiation photon energy. The all pixel has within 4.4 keV of FWHM at 122 keV from a Co-57 radioisotope source at room-temperature operation. The intensities of the peak were also very uniform in the device. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

気流塔方式による石炭急速加熱に関する基礎研究

An economical and environmental-friendly technology for the production of high-quality coke from non-caking or low-caking coal was developed, in the National Project called “Coke Production with Ad-vanced Coal Conversion Technology (SCOPE21) ”.One of the core technological elements of SCOEP21 is rapid coal heating at the temperature less than softening or melting threshold. The targeted parameters are >2, 000°C/minute for heating speed, 380 to 400°C for coal heating temperature, and not more than 50°C for temperature deviation.In this paper we introduce basic results for the development of rapid heating process (RHP). We firstly made a simulation model for the rapid coal heating process by “Computational Fluid Dynamics”. Then, we collected basic data required for simulation analysis and thereby verified the effectiveness of the simulation model in comparison of the test results for the behavior of coal particles in airflow with the simulation results through the cold model test. Finally, we conducted a performance prediction of rapid heating towers both in pilot scale and in commercial scale, by way of the computerized simulation.As a result, it was confirmed that the above-mentioned target parameters were attainable, and that this research well prepared the ground for the practical use of the subject process.

Structural Modifications of nc-Si/SiO2 Superlattices by Localized Photo-induced Heating

ABSTRACTStructural modifications in nanocrystalline silicon-silicon dioxide (nc-Si/SiO2) superlattices (SL) under high intensity laser irradiation have been studied experimentally and theoretically. The melting threshold in nc-Si/SiO2 SLs was found in the range of 5–8 kW/cm2 for cw excitation by 514 nm line of Ar+ laser and ∼11–15 mJ/cm2 for pulsed irradiation by 248 nm, π∼20 ns KrF laser. The irradiation of the samples above the melting threshold induces the irreversible modification of nc-Si layers, which is controlled by the thickness of the separating SiO2 layers, and increases the PL intensity increases by more than 300%.

Solid-liquid phase transitions in CdTe crystals under pulsed laser irradiation

Phase transitions in CdTe crystals irradiated with nanosecond KrF (248 nm) excimer laser pulses were investigated by a numerical simulation and time-resolved reflectivity method. By solving the time-dependent heat flow equation and taking into account the temperature dependences of semiconductor parameters, the time dependence of the surface temperature in a CdTe crystal was calculated as a function of laser pulse intensity. The dynamics of laser-induced melting was directly monitored by measurements of the reflection coefficient of a probe laser beam (532 nm). The melting and ablation thresholds have been determined as 2.5 and 7.3 MW/cm2, respectively.

Pulsed-laser modification of germanium nanoclusters in silicon

The effect of pulsed ruby laser radiation on Ge nanoclusters grown on a (100)-oriented Si substrate is studied. The energy density of radiation corresponds to the melting threshold of the Si surface. Changes in the structure of nanoclusters are analyzed by comparing the experimental Raman spectra to those calculated in terms of Born-von Karman and Vol’kenstein models. It is established that the action of one pulse changes the cluster size and partly relieves the compression. Still greater changes take place in a sample subjected to ten pulses. The Ge nanoclusters transform into clusters of GexSi1−x solid solution, presumably due to the stress-and vacancy-aided diffusion. Laser-induced thermal processes in germanium nanoclusters in silicon are numerically simulated.

Chemical composition of dome-shaped structures grown on titanium by multi-pulse Nd:YAG laser irradiation

The specific dome-shaped structures were grown by multi-pulse Nd:YAG (λ=1.064μm, τ=∼300ns, and ν=30kHz) laser irradiation of titanium targets in air at atmospheric pressure. The laser intensity values were chosen below the single-laser-pulse melting threshold of titanium. The chemical composition of the structures was studied as a function of laser pulse number as well as laser intensity, both at the outer surface layer and in depth. Micro-Raman spectroscopy, Auger electron spectroscopy (AES), and wavelength dispersive X-ray spectroscopy (WDX) were used as diagnostic techniques. Morphological investigations were performed by scanning electron microscopy. The obtained results revealed a lower oxygen concentration in the centre of the structures as compared to the borders and a lower concentration on the surface than in the depth. Moreover, it was found that the stoichiometry of the formed TiO2−x oxides increases from the structures centre towards the border and from the surface towards the depth.

Microcolumn development on titanium by multipulse laser irradiation in nitrogen

We report the growth of titanium nitride microcolumns under multipulse Nd:yttrium aluminum garnet (λ = 1.064 μm, τ ∼ 300 ns, ν = 30 kHz) laser irradiation of titanium targets in nitrogen atmosphere. The laser intensity value was chosen below the single-pulse melting threshold of titanium. The evolution with the number of laser pulses of the target morphology, crystalline state, and chemical composition at the surface as well as in depth were investigated by scanning electron microscopy, x-ray diffractometry, Raman spectroscopy, and wavelength dispersive x-ray spectroscopy. Under the action of the laser pulses, during progressive surface nitridation, an initial rippled morphology developed, which evolved with further irradiation to TiN microcolumns. In-depth investigations showed a granular zone beneath the surface consisting of rutile and anatase phase TiO2, followed by a compact needlelike layer of titanium until the interface with the unaffected target material.