Melt Duration Research Articles

Mapping the Extent and Duration of Surface Melting On Ice Sheets and Shelves (Abstract)

Mapping the Extent and Duration of Surface Melting On Ice Sheets and Shelves (Abstract)

Mapping the Extent and Duration of Surface Melting On Ice Sheets and Shelves (Abstract)

Mapping the Extent and Duration of Surface Melting On Ice Sheets and Shelves (Abstract)

Faster Quenching by Silicon Pulsed Laser Annealing Under Water

AbstractA novel method of pulsed laser processing of ion-implanted silicon is presented, in which samples are irradiated in water ambient. The water layer in contact with the silicon during irradiationh as a considerable influence on melting and solidificationd ynamics. Still, perfect epitaxy of a thin amorphous layer can be obtained using this method.For epitaxy to occur on a sample irradiated under water, 40 % more absorbed energy is necessary than for a sample irradiated in air. This indicates the occurrence of a considerable heat-flow from the silicon into the water layer during the laser pulse. From impurity redistribution after irradiation it is found that by processing a sample under water liquid-phase diffusion is reduced. Diffusion theory arguments indicate that this can be due to a reduction in total melt duration by about afactor 2–3. This can be due to faster cooling of the liquid silicon layer after the laser pulse whereas the melt-in time might be influenced as well. As a consequence, shallower impurity profiles can be obtained in crystalline silicon. No oxygen incorporation is detected and the surface morphology is not disturbed using this new process.

The influence of the melting conditions on the basicity of glasses

In some previous works a basicity scale for oxides has been proposed, with O 2− as an origin (per cent of basicity pB (O 2) = 100%), as well as an equation allowing the calculation of pB on the basis of ionisation potential, taking into account the coordination number and the oxidation state of the atoms and considering that the basicity is related to electron donating power of oxygen atoms. The present work deals with the possible variation of the basicity of oxidic compounds with melting temperature and duration. In the case of SiO 2 an attempt was made to estimate the influence of structure disordering induced by melting upon the basicity value. For binary silicate glasses, the influence of melting time and temperature on pB values was considered through the change induced into the polymer distribution, using the available data concerning the alteration of equilibrium constant with melting condition. Some of the above mentioned results are compared with the experimentally obtained data, by means of UV and IR spectroscopy.

Studies of pulsed laser melting and rapid solidification using amorphous silicon

Pulsed laser melting of ion implantation-amorphized silicon layers, and the subsequent solidification of undercooled liquid silicon, have been studied experimentally and theoretically. Measurements of the time of the onset of melting of amorphous silicon layers, during an incident laser pulse, have been combined with measurements of the duration of melting, and with modified melting model calculations to demonstrate that the thermal conductivity, K a, of amorphous silicon is very low ( K a⋍0.02 W/cm K). K a is also found to be the dominant parameter determining the dynamical response of amorphous silicon to pulsed laser radiation; the latent heat of fusion and melting temperature of amorphous silicon are relatively unimportant. Transmission electron microscopy indicates that bulk (volume) nucleation occurs directly from the highly undercooled liquid silicon that can be prepared by pulsed laser melting of amorphous silicon layers at low laser energy densities. A modified thermal melting model has been constructed to simulate this effect and is presented. Nucleation of crystalline silicon apparently occurs at a nucleation temperature, T n, that is higher than the temperature, T a, of the liquid-to-amorphous phase transition. The model calculations demonstrate that the release of latent heat by bulk nucleation occurring during the melt-in process is essential to obtaining agreement with experimentally observed depths of melting. These calculations also show that this release of latent heat accompanying bulk nucleation can result in the existence of buried molten layers of silicon in the interior of the sample after the surface has solidified. It is pointed out that the occurrence of bulk nucleation implies that the liquid-to-amorphous phase transition (produced using picosecond or ultraviolet nanosecond laser pulses) cannot be explained by purely thermodynamic considerations.

Time-Resolved Reflectivity Measurements of Silicon And Germanium Using A Pulsed Excimer Laser

ABSTRACTTime-resolved reflectivity measurements of silicon and germanium have been made during pulsed KrF excimer laser irradiation. The reflectivity was measured simultaneously at both 1152 and 632.8 nm wavelengths, and the energy density of each laser pulse was monitored. The melt duration and the time of the onset of melting were measured and compared with the results of melting model calculations. For energy densities just above the melting threshold, it was found that the melt duration was never less than 20 ns for Si and 25 ns for Ge, while the maximum reflectivity increased from the value of the hot solid to that of the liquid over a finite energy range. These results, along with a reinterpretation of earlier time-resolved ellipsometry measurements, indicate that, during the melt-in process, the near-surface region does not melt homogeneously, but rather consists of a mixture of solid and liquid phases. The reflectivity at the onset of melting and in the liquid phase have been measured at both 632.8 and 1152 nm, and are compared with the results found in the literature.

Nucleation of Internal Melt During Pulsed Laser Irradiation

ABSTRACTReal-time measurements of the molten layer thickness and simultaneous measurements of the melt duration at the surface reveal that melt nucleates internally when Si implanted with low solid-solubility impurities such as In is melted with a 30 nsec laser pulse at 694 nm. Internal nucleation of melt was observed for all energy densities examined. Furthermore, at energy densities insufficient to melt the entire amorphous layer, internal nucleation of melt is followed by an explosive-like process in which a buried molten layer propagates toward the irradiated surface.

Superstructures and phase transitions of Bi2O3

Abstract Polymorphs of bismuth sesquioxide were studied by TG-DTA and X-ray diffraction methods and by electron microscopy. Phase transitions of the polymorphs are influenced not only by the temperature before cooling and the cooling rate but also by the duration of melting. New commensurate and incommensurate superstructure phases, observed under the electron microscope, are described as the Na x a types, where N is various value between 2 and 2.48. Here a stands for the cell dimension of the δ form, the fluorite-type structure. The superstructures can be derived by a scheme for ordering oxygen vacancies in the defective fluorite-type structure.

Surface melting and evaporation during disruptions in magnetic fusion reactors

Disruptions in tokamaks lead to high energy deposition for short times on in-vessel components. Melting and evaporation may then occur. A comprehensive model to evaluate the extent and duration of melting, the amount of evaporation, and the time for resolidification is presented. This model entails the solution of a heat conduction problem with two moving boundaries, the liquid surface and the melt—solid interface with a surface boundary condition determined by the dynamics of evaporation. Extensive numerical results are presented for in-vessel components made of stainless steel, molybdenum, or graphite. The effects of vapor shielding, pulse shape, and pulse duration are also investigated.

Observing Polar-Ice Variability

The repetitive synoptic ice data obtainable by satellite sensing provide a means of studying the time-dependent behavior of both sea ice and ice sheets on climatic time scales. Examples of sea-ice parameters which may be measured are extent, concentration, and multiyear fraction; and examples of ice-sheet/ice-shelf parameters are surface elevation, ice-front position, extent and duration of summer melting, and ice accumulation rates. Desired snow-cover parameters include extent and snow depth or water-equivalent depth. The unique ability of satellites to measure such ice parameters and the characteristics of the consequent data sets significantly influence the structure of ice models that can be successfully used with the data. Ice data sets recently acquired by satellite sensing are described. The past decade of sea-ice data provides a detailed description of the interannual variability of sea ice on a regional and seasonal basis. Because of the longer time scales involved in ice-sheet variations, a comparable record of ongoing ice-sheet variations has not yet been established, but important baseline data sets are being developed.

Observing Polar-Ice Variability

The repetitive synoptic ice data obtainable by satellite sensing provide a means of studying the time-dependent behavior of both sea ice and ice sheets on climatic time scales. Examples of sea-ice parameters which may be measured are extent, concentration, and multiyear fraction; and examples of ice-sheet/ice-shelf parameters are surface elevation, ice-front position, extent and duration of summer melting, and ice accumulation rates. Desired snow-cover parameters include extent and snow depth or water-equivalent depth. The unique ability of satellites to measure such ice parameters and the characteristics of the consequent data sets significantly influence the structure of ice models that can be successfully used with the data. Ice data sets recently acquired by satellite sensing are described. The past decade of sea-ice data provides a detailed description of the interannual variability of sea ice on a regional and seasonal basis. Because of the longer time scales involved in ice-sheet variations, a comparable record of ongoing ice-sheet variations has not yet been established, but important baseline data sets are being developed.

Pulsed neodymium: yttrium aluminum garnet laser (532 nm) melting of crystalline silicon: Experiment and theory

Time-resolved reflectivity measurements have been used to determine both the time of onset-of-melting and the duration of melting resulting from frequency-doubled neodymium: yttrium aluminum garnet (532 nm) pulsed-laser irradiation of crystalline silicon. Substantially shorter surface melt durations were obtained with increasing energy density El than were reported earlier by others. Thermal melting model calculations, which take into account the temperature-dependent optical and thermal properties of silicon, are in substantial agreement with the observed El dependence of the onset of melting and surface melt duration. Inclusion of intensity-dependent absorption in the modeling further improves this agreement.

An investigation of oxygen indiffusion during laser cleaning/annealing of silicon by means of the 16O( α, α0) 16O resonance scattering

An investigation of oxygen indiffusion during laser processing of silicon has been performed with the 16O( α, α 0) 16O resonance scattering at E R=3.042 MeV. The parameters of this resonance have been measured with the aim of nuclear microanalysis. We have found the full-width of the resonance, Γ = 10.2±0.4 keV, the cross section at resonance, σ R = 1.32±0.09 b/sr in the c.m. system and E R = 3.042±0.003 MeV. Si wafers were irradiated using a Q-switched ruby laser, energy density of the pulses 1.5 J/cm 2, pulse width 20 ns. The results show no evidence for oxygen indiffusion either for ion-implanted single pulse annealed silicon in air, or for a silicon wafer cleaned with 8 laser pulses in an UHV environment. For laser cleaned silicon, the upper limit of oxygen concentration was found to be equal to or less than the solid solubility limit of oxygen in silicon. This is consistent with the dissolution time of SiO 2 in silicon, which is orders of magnitude longer than the assumed melt duration of silicon following pulsed laser irradiation. One has to conclude that most of the oxygen is evaporated.

Measurements of the melt dynamics in laser annealed semiconductors

Combined measurements of the transient electrical conductance, optical reflectance and absorbed energy have been used to determine the dynamics of the annealing process during laser irradiation of bulk silicon and silicon on sapphire. The combined transient conductance and optical reflectance yield the regrowth velocity and melt duration in bulk silicon; in silicon on sapphire the measurements also yield the melt velocity and maximum melt depth. Measurement of the absorbed energy provides energy coupling data used in numerical calculations of the melt dynamics for direct comparison with experiment in bulk silicon and the result indicates annealing proceeds by a purely thermal process.

Time-resolved conductance and reflectance measurements of silicon during pulsed-laser annealing

The dynamics of silicon solidification from the melt during pulsed-ruby-laser annealing have been investigated with the use of time-resolved electrical-conductance and optical-reflectance measurements. Melt duration was found to increase with laser-energy density to over 450 nsec at the highest energy used (3.1 J/${\mathrm{cm}}^{2}$). Resolidification velocity was found to vary with time and laser-energy density over a range of 2 to 3 m/sec. The melt threshold was observed to be 0.8 J/${\mathrm{cm}}^{2}$. Measurements of the laser energy absorbed in the sample were made to determine energy coupling parameters for use in numerical calculations. The numerical calculations, based on a thermal model for laser melting, are found to be in good agreement with the experimental results.

Measurement Of Precipitate Nucleation Times In Molten Metals By Pulsed Surface Melting

ABSTRACTThe short melt duration resulting from pulsed laser and electron beam surface melting of ion-implanted metals has been used to measure precipitate nucleation times of compounds within the melt. We have examined the phases present in several alloy systems with TEM and used calculated thermal histories to place limits on the time required for nucleation of the following compounds: AlSb (5–25 ns), Al3Ni (≳ 750 ns)Al3Ni2 (≳ 950 ns) and AlNi (< 1000 ns), all in molten Al, and TiC (≲ 100 ns) in molten Fe. The compounds observed after our rapid solidification have relatively simple, cubic structures and melt congruently, while those predicted but not observed have more complex structures and decompose peritectically.

Pulsed Laser Melting of Amorphous Silicon: Time-Resolved and Post-Irradiation Studies

ABSTRACTMeasurements of the time of the onset of melting of self-implantation amorphized (a) Si, during an incident laser pulse, have been combined with modified melting model calculations and measurements of surface melt duration to demonstrate that the thermal conductivity, Ka, of a-Si is very low (≃0.02 W/cm-K). Ka is also shown to be the dominant parameter determining the dynamical response of ionimplanted Si to pulsed laser radiation; the latent heat and melting temperature of a-Si are relatively unimportant. Cross-sectional transmission electron micrographs on implantation-amorphized Si layers of several different thicknesses show that for energy densities less than the threshold value for complete annealing there are usually two distinct regions in the re-solidified a-Si, consisting of fine-grained and large-grained polycrystalline Si, respectively. The presence of the fine-grained poly-Si suggests that bulk nucleation occurs directly from the highly undercooled liquid phase. Thermal melting model calculations suggest that the nucleation temperature, Tn is ≃1200°C.

Dynamics of melting, evaporation, and resolidification of materials exposed to plasma disruptions

First wall components exposed to a high energy flux during a plasma disruption experience a sequence of processes which consists of rapid heating, melting, intense evaporation, resolidification, and cool-down. The dynamics of all these processes has an impact on both the melt layer stability and the thermal stress cycle in the component of the first wall that stays solid. The detailed time history of the temperature distribution is accurately computed by solving a two-moving-boundary problem. The time behavior of the melt layer thickness for both stainless steel and molybdenum is calculated for different disruption energies and different energy fluxes. The duration of melting, which is an important factor in determining the melt layer stability under different forces, is calculated for both stainless steel and molybdenum. The duration of the melt layer may in fact be shorter for materials with thicker melt layers. The effect of vapor shielding (the stopping of the incoming plasma ions by the vaporized wall material) on the dynamics of melting is also investigated.

Impurity Redistribution In Ion Implanted Si After Picosecond Nd Laser Pulse Irradiation

AbstractImpurity redistribution in Bi-implanted Si and in As-implanted Si has been investigated after irradiation with 25 ps Nd(λ=l.06 μm) laser pulse in the energy range 0.1–1.5 J/cm2 . Channeling effect in combination with 2.0 MeV He+ backscattering in glancing detection has been used to characterize the epitaxial crystallization, the impurity location and its depth distribution. The amorphous to single crystal transition occurs at an energy density of about 0.4 J/cm 2 . Bi atoms are located after crystallization in substitutional lattice sites for the in depth part of the distribution. Part of the Bi atoms accumulated at the sample surface and the amount of segregation increases with the pulse energy density and depends on the substrate orientation. A computer model has been also developed to calculate several parameters of interest, as the melt threshold,the melt duration, the carrier temperature etc including a detailed description of the absorption and of the energy relaxation processes. The calculations indicate that the simple thermal description accounts quantitatively for the experimental data on melt duration and impurity segregation.

A thermal-melting-model calculation of pulsed laser annealing of GaAs

The temperature profile, melt depth and melt duration have been calculated for pulsed laser annealing of GaAs assuming instantaneous heatingof the lattice. The results indicate the need for a detailed experimental verification of As evaporation and of the annealing threshold, as a possible test of the strictly thermal melting model.