Measurements of plasma temperature and electron density in laser-induced copper plasma by time-resolved spectroscopy of neutral atom and ion emissions

Plasma produced by a 355 nm pulsed Nd:YAG laser with a pulse duration of 6 ns focussed onto a copper solid sample in air at atmospheric pressure is studied spectroscopically. The temperature and electron density characterizing the plasma are measured by time-resolved spectroscopy of neutral atom and ion line emissions in the time window of 300–2000 ns. An echelle spectrograph coupled with a gated intensified charge coupled detector is used to record the plasma emissions. The temperature is obtained using the Boltzmann plot method and the electron density is determined using the Saha-Boltzmann equation method. Both parameters are studied as a function of delay time with respect to the onset of the laser pulse. The results are discussed. The time window where the plasma is optically thin and is also in local thermodynamic equilibrium (LTE), necessary for the laser-induced breakdown spectroscopy (LIBS) analysis of samples, is deduced from the temporal evolution of the intensity ratio of two Cu I lines. It is found to be 700–1000 ns.     

Laser ablation and micropatterning of thin TiN coatings

Laser ablation of thin TiN films deposited on steel substrates has been studied under wide-range variation of irradiation conditions (pulsewidth, wavelength, energy density and spot size). It has been demonstrated that both picosecond (150–300 ps) and nanosecond (5–9 ns) laser pulses were suitable for controllable ablation and microstructuring of a 1-μm-thick TiN film unlike longer 150-ns pulses. The ablation rate was found to be practically independent of the wavelength (270–1078 nm) and pulsewidth (150 ps–9 ns), but it increased substantially when the size of a laser spot was reduced from 15–60 μm to 3 μm. The laser ablation technique was applied to produce microstructures in the thin TiN films consisting of microcraters with a typical size of 3–5 μm in diameter and depth less than 1 μm. Tests of lubricated sliding of the laser-structured TiN films against a steel ball showed that the durability of lubricated sliding increased by 25% as compared to that of the original TiN film. Received: 28 July 1999 / Accepted: 17 April 2000 / Published online: 20 September 2000     

The peculiarities of crystallization of low-temperature eutectic in magnesium-lead system

Density of low-temperature eutectic in magnesium-lead system (83.06 at. % Pb) has been measured by gamma-raying of the samples with narrow beam from cesium-137 isotope over the temperature range 293–1000 K of solid and liquid states. Approximation density dependences have been obtained and data of this work and other authors have been compared. Reference tables of temperature dependences of the alloy thermal properties have been compiled for the entire range of measurements and their errors estimated. It is shown that differences in the values of volumetric changes obtained during melting and crystallization are bound with the metastable β′-phase formation.     

Determination of the density of Pb-Sn melts and segregation phenomena on their crystallization

The density of liquid lead-tin alloys (17.64, 25.34, 25.61, 26.07, and 33.88 at % Pb) has been determined using an γ-ray attenuation technique over the temperature range from the liquidus line to 750–1040 K. The experimental uncertainty of the density measurements is estimated to be within the range of ±0.20–0.25%. It has been shown that the temperature hysteresis of the melt density and thermal expansion, which was observed previously in several studies, disappears after stirring the melts. The distribution of the components through the height of the liquid and solid samples and its variation with time and during melting-crystallization has been investigated. The study shows that the melts do not segregate on long standing (about 4 hours) near the liquidus temperature. However, crystallization and melting of homogeneous Pb-Sn alloys is accompanied by liquation phenomenon, which is enhanced as the sample composition deviates from the eutectic one. The text was submitted by the authors in English.     

Structure of metallic nanowires and nanoclusters formed in superfluid helium

It is shown that metallic nanowires (5–8 nm in diameter) that form during laser ablation of Ni, Pb, In, and Sn targets embedded in HeII contain extended single-crystal segments, while spherical clusters (about 2 μm in diameter) that form under these conditions have a regular shape and an atomically smooth surface. Such structures are explained by melting of metal ablation products under their coalescence in HeII. The short-term action of a low-intensity beam of electrons with an energy of 200 keV initiates the explosion in metallic spheres preserved in the vacuum chamber of a transmission electron microscope, which is accompanied with the formation of thousands of clusters with a diameter of a few nanometers. This effect is due to metastability of internal mechanical stresses produced upon sharp cooling of molten spheres by liquid helium. A mechanism of condensation of atoms and nanoparticles in quantized vortices of superfluid helium is proposed.     

Physical mechanism of silicon ablation with long nanosecond laser pulses at 1064 nm through time-resolved observation

Nanosecond (ns) laser ablation can provide a competitive solution for silicon micromachining in many applications. However, most of the previous studies focus on ns lasers at visible or ultraviolet (UV) wavelengths. The research is very limited for ns lasers at infrared (e.g., 1064 nm) wavelengths (which often have the advantage of much lower cost per unit average output power), and the research is even less if the ns laser also has a long pulse duration on the order of ∼100 ns. In this paper, time-resolved observation using an ICCD (intensified charge-coupled device) camera has been performed to understand the physical mechanism of silicon ablation by 200-ns and 1064-nm laser pulses. This kind of work has been rarely reported in the literature. The research shows that for the studied conditions, material removal in laser silicon ablation is realized through surface vaporization followed by liquid ejection that occurs at a delay time of around 200-300 ns. The propagation speed is on the order of ∼1000 m/s for laser-induced plasma (ionized vapor) front, while it is on the order of ∼100 m/s or smaller for the front of ejected liquid. It has also been found that the liquid ejection is very unlikely due to phase explosion, and its exact underlying physical mechanism requires further investigations.     

Magnetic susceptibility and parameters of electronic structure of Al<Subscript>2</Subscript>REM (Gd,Dy, and Ho) intermetallic compounds at high temperatures

The magnetic susceptibility of Al₂REM (REM = Gd, Dy, and Ho) intermetallic compounds is experimentally investigated by the Faraday method in a wide temperature interval (290–2000 K) in different magnetic fields (0.3–1.3 T). In the crystalline state, the temperature dependences of the susceptibility follow the generalized Curie–Weiss law. In the liquid phase, the magnetic susceptibility of these intermetallic compounds above the melting point increases for all examined samples. The parameters of the electronic structure of the compounds are calculated based on the experimental data. It is established that the effective magnetic moment per rareearth metal atom is smaller than that characteristic of the free REM⁺ ion.     

Phase explosion and recoil-induced ejection in resonant-infrared laser ablation of polystyrene

We investigate the phenomenon of resonant-infrared laser ablation of polymers using polystyrene as a model material. Ablation is carried out using various mid-IR laser wavelengths that are resonant with vibrational modes of a polystyrene target. Time-resolved plume imaging coupled with etch-depth measurements and thermal calculations indicate that ablation begins after a superheated surface layer reaches a temperature of ∼1000°C and undergoes spinodal decomposition. The majority of the ablated material is then expelled by way of recoil-induced ejection as the pressure of the expanding vapor plume compresses a laser-melted area.     

Interaction of laser radiation with TNT with respect to the laser neutralisation of AP mines

According to UN estimations there are between 80 and 115 million activated landmines worldwide. These mines, or other unexploded ordnance (UXO), can be triggered accidentally and kill or injure more than 2000 civilians per month. The most common explosive in these mines is trinitrotoluene (TNT). In this paper, the potential of some of the most promising lasers for mine neutralisation is investigated, namely an ArF laser, a KrF excimer laser and a Nd:YAG solid-state laser. We have studied the interaction between laser beams emitting at λ=193 nm, 248 nm and 1060 nm and a bare solid sample of TNT of approximately 15 mg. Using pulsed excimer radiation at λ=193 nm, with an energy density up to 1 J/mm², ablation of the TNT without any deflagration has been achieved. At λ=248 nm, using the KrF excimer laser with a pulse duration of 30 ns and a repetition rate of 5 Hz, the TNT sample started melting and burning after an irradiation of 10 s. Preliminary results with the Nd:YAG solid-state laser operating in cw emission have shown that the irradiated sample exhibits the desired burning behaviour even after the exposure is stopped. Received: 14 December 2000 / Accepted: 18 December 2000 / Published online: 20 June 2001     

The effect of composition,electron irradiation and quenching on ionic conductivity in a new solid polymer electrolyte: (PEG)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>NH<Subscript>4</Subscript>I

We have prepared, characterized and investigated a new PEG-2000 based solid polymer electrolyte (PEG) _x NH₄I. Ionic conductivity measurements have been made as a function of salt concentration as well as temperature in the range 265–330 K. Selected compositions of the electrolyte were exposed to a beam of 8 MeV electrons to an accumulated dose of 10 kGy to study the effect on ionic conductivity. The electrolyte samples were also quenched at liquid nitrogen temperature and conductivity measurements were made. The ionic conductivity at room temperature exhibits a characteristic double peak for the composition x = 20 and 70. Both electron beam irradiation and quenching at low temperature have resulted in an increase in conductivity by 1–2 orders of magnitude. The enhancement of conductivity upon irradiation and quenching is interpreted as due to an increase in amorphous region and decrease in crystallinity of the electrolyte. DSC and proton NMR measurements also support this conclusion.      

Positron bound systems in benzene and in naphthalene

Summary Positron decay in liquid naphthalene and benzene was investigated through measurements of lifetimes and magnetic quenching. The obtained results when compared with those already known in solid phase suggest that positrons become bound in two different systems. The first has a lifetime τ₃≈1 ns which is unaffected by the melting, while the second (τ₄≈3 ns) arises above melting and behaves as a relaxed positronium. This work was supported by CISM (Centro Interuniversitario di Struttura della Materia) del Ministero della Pubblica Istruzione, and by GNSM (Gruppo Nazionale di Struttura della Materia) del CNR.     

Density of indium antimonide at high temperatures

The density of solid and liquid indium antimonide was studied by irradiating the samples with a narrow beam of monochromatic gamma-radiation in the temperature range of 293–1950 K, including the range of melting — crystallization. The measurement errors for the density and thermal expansion coefficients were ±(0.25–0.40) % and ± 4 %, correspondingly. The approximating equations and tables of reference data were obtained for the temperature dependence of thermal properties. Measurement results were compared with the known published data. The work was financially supported by the Russian Foundation for Basic Research (Grant No. 06-08-00040).     

Thermal conductivity of high-porosity biocarbon preforms of beech wood

This paper reports on measurements performed in the temperature range 5–300 K for the thermal conductivity κ and electrical resistivity ρ of high-porosity (cellular pores) biocarbon preforms prepared by pyrolysis (carbonization) of beech wood in an argon flow at carbonization temperatures of 1000 and 2400°C. X-ray structure analysis of the samples has been performed at 300 K. The samples have revealed the presence of nanocrystallites making up the carbon matrices of these biocarbon preforms. Their size has been determined. For samples prepared at T _carb = 1000 and 2400°C, the nanocrystallite sizes are found to be in the ranges 12–25 and 28–60 κ(T) are determined for the samples cut along and across the tree growth direction. The thermal conductivity κ increases with increasing carbonization temperature and nanocrystallite size in the carbon matrix of the sample. Thermal conductivity measurements conducted on samples of both types have revealed an unusual temperature dependence of the phonon thermal conductivity for amorphous materials. As the temperature increases from 5 to 300 K, it first increases in proportion to T, to transfer subsequently to ∼T ^1.5 scaling. The results obtained are analyzed.     

The effect of temperature and pressure on the heat conductivity of TlSbC<Stack><Subscript>2</Subscript><Superscript>VI</Superscript></Stack> (<Emphasis Type="Italic">C</Emphasis><Superscript>VI</Superscript> → S,Se, Te)

The effect of temperature and pressure on heat conductivity of ternary compounds TiSbC ₂^VI (C ^VI → S, Se, Te) in the solid and liquid states in a temperature range of 300–800 K, as well as under the pressure up to 0.35 GPa in a temperature range of 275–450 K, is studied. The dependence of heat conductivity on average atomic weight under the S → Se → Te transition is found. Analysis of the experimental data makes it possible to attribute TlSbS₂ to the class of substances exhibiting semiconductor-semiconductor melting behavior.     

Spectroscopic study of rotational energy distribution of BH (A 1Π) and electronic excitation temperature determination

Emission spectra of BH(A ¹Π-X ¹Σ⁺) system were recorded and studied using a low pressure (3.0 torr) arc in flowing hydrogen and argon + hydrogen mixture. The rotational distributions in theA ¹Π state determined from the intensities of rotational lines for the 0–0 band of theA-X system conforms to a Maxwellian distribution with effective rotational temperature of 1000 ± 50°K. Intensities of Balmer lines of hydrogen were measured and used to determine electronic excitation temperature which was found to be around 2000°K.     

Three-dimensional photography and reconstruction of jets and droplets ejected from the surface of excimer laser ablated molten polyethylene-glycol 1000

Solid debris, namely particles, produced during pulsed laser deposition procedures significantly reduces the quality and homogeneity of the deposited thin layers. Using a liquid target the occurrence of the solid debris was completely avoided; however, molten droplets were observable on the thin-film surfaces resulting in quality deterioration in this case, too. Several methods have already been suggested to eliminate these but the efficiency of these procedures is not excellent, because the droplet ejection processes are not completely described yet. Molten polyethylene-glycol (PEG) 1000 (T_m=70 °C) was ablated by an ArF excimer laser and a time-resolved investigation of jet and droplet developments was performed using a fast photographic arrangement. The applied fluence range was 0.8–8.8 J/cm². For a three-dimensional reconstruction the exposing dye laser beam was divided into three parts and directed onto the irradiated part of the surface of the PEG sample from three different directions. Three video cameras facing the exposing light took concurrent shots of the process. Transmissive pictures of the ablated material were taken within a 50–1500 s range of delay. The three-dimensional reconstruction of the ejected jets and droplets was performed based on the recorded three concurrent photographs. Results were in co-relation with ejections seen in the original snapshots and accurately dimensioned to the object appearing on them. PACS 79.20.Ds; 52.38.Mf; 81.15.Fg     

Differential mobility analysis of nanoparticles generated by laser vaporization and controlled condensation (LVCC)

Silicon and iron aluminide (FeAl) nanoparticles were synthesized by a laser vaporization controlled condensation (LVCC) method. The particles generated by the laser ablation of solid targets were transported and deposited in the presence of well-defined thermal and electric field in a newly designed flow-type LVCC chamber. The deposition process of nanoparticles was controlled by the balance of the external forces; i.e., gas flow, thermophoretic and electrostatic forces. The size distributions of generated nanoparticles were analyzed using a low-pressure differential mobility analyzer (LP-DMA). The effect of synthesis condition on the size distribution was analyzed by changing the pressure of the carrier gas (20–200 Torr), the temperature gradient in the LVCC chamber (ΔT=0–190°C) and the electric field applied between the LVCC chamber plates (E=0–3000 V/m). It was found that electrostatic field was effective to selectively deposit small size nanoparticles (about 10 nm) with expelling large droplet-like particles.     

Direct observation of phase transitions by time-resolved pyro/reflectometry of KrF laser-irradiated metal oxides and metals

New experimental results are obtained by coupling both time-resolved reflectivity and rapid infrared pyrometry under a hemispherical reactor. The heating source KrF laser beam (28 ns, 248 nm) is homogenized and as for probing, a CW He-Ne laser beam (10 mW, 633 nm) is used.Using both methods infrared pyrometry with an IR detector cooled with liquid nitrogen and sensitive in the spectral range 1-12 μm, and time-resolved reflectivity with a rapid photodiode, we were able to study complex thermodynamic transitions with nanosecond time resolution. Three different materials are studied by varying the KrF fluence (energy/surface) from 100 to 2000 mJ/cm²: thin films melting (Au/Ni), the threshold of plasma formation (Ti), and complex liquid phase segregation under semi-conductor state (ZnO). The formation of a liquid Zn film induced by temperature gradient is well evidenced by our signals. Also melting of thin films irradiated by low laser fluences (less than 500 mJ/cm²) translates the typical thermodynamic behavior. Finally, wide fluence dynamic (400-2000 mJ/cm²) is analyzed in the case of Ti surface, and results show two distinguished regimes: first one bellow 1000 mJ/cm² corresponding to the early stage plasma initiation, and second one over 1000 mJ/cm² to the dynamics of plasma expansion.     

Optical field enhancement effects in laser-assisted particle removal

We report on the role of local optical field enhancement in the neighborhood of particles during dry laser cleaning (DLC) of silicon wafer surfaces. Samples covered with spherical colloidal particles (PS, SiO₂) and arbitrarily shaped Al₂O₃ particles with diameters from 320–1700 nm were cleaned using laser pulses with durations from 150 fs to 6.5 ns and wavelengths ranging from 400–800 nm. Cleaned areas were investigated with scanning electron and atomic force microscopy. Holes in the substrate with diameters of 200–400 nm and depths of 10–80 nm, depending on the irradiation conditions, were found at the former positions of the particles. For all pulse durations analyzed (fs, ps, ns), holes are created at laser fluences as small as the threshold fluence. Calculations of the optical field intensities in the particles’ neighbourhood by applying Mie theory suggest that enhancement of the incident laser intensity in the near field of the particles is responsible for these effects. DLC for sub-ns pulses seems to be governed by the local ablation of the substrate rather than by surface acceleration. Received: 31 May 2000 / Accepted: 7 September 2000 / Published online: 22 November 2000     

Non-conventional solid state fuel cells: Materials & technology

Investigation of “Non-conventional material” for fuel cells, such as oxide-salt-ceramic composites and ceria based or perovskite oxides with different dopants, leads to a much lower fuel cell operating temperature compared to conventional high temperature, ∼1000 °C, solid oxide fuel cells (SOFCs), which provides the new possibilities for facilitating SOFC commercialisation. This work is essentially an effort to develop new types of solid oxide ion and proton fuel cells (SOFC and SPFC) at fairly low temperatures, <800 °C, or intermediate temperature, 400 to 800 °C. The conventional high temperature SOFCs using yttria-stabilised zirconia (YSZ) materials, and low temperature SPFCs (<200 °C) using polymer membrane electrolytes have complex material and system problems from either special high temperature requests or expensive technology and reforming systems. This research is intended to provide materials and technology along new routes for so-called non-conventional fuel cell systems, to facilitate solid state fuel cell cmmercialisation. The fuel cell research on these non-conventional systems is promising. This paper, based on recent achievements in research on materials and technology, summaries the developnt of material systems and new fuel cell devices regarding their potential marketability in the near future. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996