Micro-Raman study of the m-MoO2 to α-MoO3 transformation induced by cw-laser irradiation

In this work we report the monoclinic-MoO₂ to orthorhombic-MoO₃ transformation induced by continuous wave-laser irradiation. A Helium-Neon laser was used as energy source. The transformation involved two processes: an initial oxidation followed by a transition from a crystalline phase (MoO₂) to the other (MoO₃). Both processes, oxidation and structural changes, were monitored in situ by micro-Raman spectroscopy. Structural transformations were analyzed as a function of the laser power and as a function of the exposure time (transformation kinetics). Under these low power irradiation conditions it was found that the monoclinic-MoO₂ transforms completely to the well-known orthorhombic phase α-MoO₃ in a few minutes.     

Efficient relaxation of strained-SiGe layers induced by thermal oxidation

The new route to fabricate compositionally graded and highly-relaxed Si_1-xGe_x layers using thermal oxidation at high temperatures is investigated. Ge atoms behavior during thermal oxidation of Si_1-xGe_x layers is strongly dependent on the oxidation temperature. For low temperature oxidation processes Ge is incorporated as GeO₂ in the grown oxide layer, while for higher temperatures it accumulates below the grown oxide into a layer with a higher concentration than the initial Si_1-xGe_x. However, Si_1-xGe_x layers oxidized at 1000 °C did not show such an accumulation layer because Ge diffusion efficiently occurred, resulting in the formation of a compositionally graded and relaxed Si_1-xGe_x layer. Such layers could be used as virtual substrates for the strained-Si and relaxed-SiGe applications.     

Solid-State Reactions during Thermal Oxidation of Vanadium-Modified GaAs Surfaces

It is shown that vanadium layers deposited by magnetron sputtering onto GaAs accelerate surface oxide growth during thermal oxidation. The processes occurring during thermal oxidation on the surface and at the semiconductor–metal interface are interpreted under the assumption that vanadium deposition leads to the formation of an interfacial V _x Ga _y As _z layer, which plays a key role in determining the mechanism of V/GaAs oxidation.     

Comparative studies on thermal and laser sintering for highly conductive Cu films printable on plastic substrate

Compound-based Cu paste was synthesized to prepare electrically conductive films on plastic substrate. The Cu pastes screen-printed onto polyimide were annealed inside a furnace and also by an ultraviolet laser beam and the effects of annealing conditions on the microstructures and electrical properties were investigated. Both of thermal and laser processes were carried out under N₂ gas flow, which was very effective in preventing oxidation. The minimum resistivity available with thermal sintering was 1.30 × 10^− 5 Ω cm and a slightly higher resistivity was obtained by laser sintering. This value is several orders of magnitude lower than that reported for the copper nanoparticle paste thermally sintered under N₂ atmosphere. The variation of microstructure and electrical property with the laser power was very similar to the temperature dependence of these factors in thermal sintering.     

Oxidation and corrosion behaviour of mild steel laser alloyed with nickel and chromium

Surface alloys were made on mild steel, coated with nickel and chromium using laser surface alloying. Mild steel was coated with a composite layer of nickel and chromium using the plasma technique. This was followed by laser irradiation using a continuous carbon dioxide laser. Oxidation and corrosion behaviour of these alloys was then determined by carrying out oxidation in air at 800 °C and corrosion tests at room temperatures in 1 n H₂SO₄. With a 75 m layer of nickel and chromium each, it was possible to make surface alloys on mild steel, which had a chromium concentration of 6–7 wt%, but the nickel concentration varied from 10–20 wt%. Oxidation behaviour improved significantly over the as-coated specimen and aqueous corrosion improved considerably.     

The role of SiO2 buffer layer in the growth of highly textured LiNbO3 thin film upon SiO2/Si by pulsed laser deposition

Highly c-axis-oriented lithium niobate (LiNbO₃) thin films have been grown on Si with optimum thickness of the SiO₂ buffer layer by pulsed laser deposition technique. The amorphous SiO₂ buffer layer was formed on Si (100) wafer by thermal oxidation method. The crystallinity and c-axis orientation of LiNbO₃ films were strongly influenced by the thickness of amorphous SiO₂ buffer layers. The optimum thickness of the amorphous SiO₂ buffer layer was found to be about 230 nm for the growth of highly c-axis-oriented LiNbO₃ films. The achieved films have smooth surface and sharp interface. The prism coupler method indicates that the prepared LiNbO₃ films have great potential for optical waveguide device.     

Laser induced oxidation processes in iron, copper and nickel nanopowders

The present paper is devoted to the study of oxidation processes occurring in iron, copper and nickel nanopowders under the action of CO₂ laser radiation. It was established that two different phases of iron oxide (magnetite and hematite) are stabilized in the oxidation products of the iron nanopowder suggested to laser radiation whereas the non-irradiated nanopowder contains the hematite phase only. The phase composition of the oxidation products are the same for both the irradiated and non-irradiated copper nanopowders, but their morphology is different. The thermal behavior of the Ni irradiated nanopowder differs from that of the non- irradiated one: it is characterized by an additional exothermal peak at 276 °C.     

Effect of PbO layers with nickel oxide and vanadium oxide additions on the thermal oxidation of InP

IR and ultrasoft x-ray emission spectroscopy data indicate that the thermal oxidation of PbO/InP structures with additions of transition-metal (nickel and vanadium) oxides leads to the formation of multicomponent layers consisting of nickel, vanadium, lead, and indium phosphates. These oxide additions to PbO nanolayers accelerate the thermal oxidation of InP. A small amount of vanadium oxide added to PbO gives rise to a significant contribution of a catalytic mechanism to the accelerated thermal oxidation of InP. The principal distinction between the PbO/InP and (PbO + NiO)/InP structures is that the thermal oxidation of the latter leads to the formation of an In_xNi_yP_z transition layer, which influences the growth rate and the composition of the resulting layer.     

ZrO2陶瓷颗粒在激光作用下组织形貌演变规律

采用激光表面处理技术制备ZrO_2复合热障涂层可使陶瓷颗粒与金属之间实现冶金结合,增加结合强度,但由于陶瓷颗粒和金属热膨胀系数之间的差异,制备大面积复合热障涂层仍存在比较严重的开裂问题.为降低复合材料层的开裂倾向,更好地控制ZrO_2陶瓷颗粒的离散规律,本文针对激光熔注过程中陶瓷颗粒的熔化问题,采用高速摄像、扫描电子显微镜研究了氧化锆陶瓷颗粒进入熔池前后激光对其形貌与离散规律的影响规律.结果表明,在激光能量密度较低的情况下,激光直接作用于氧化锆陶瓷颗粒会导致单个ZrO_2陶瓷颗粒发生裂解现象;当激光能量密度高于2.5 J/mm~2时,ZrO_2陶瓷颗粒熔化严重并发生球化现象.在激光熔注过程中,由于受到激光和高温熔池的双重作用,氧化锆陶瓷颗粒内部晶粒长大,高能量大尺寸晶界促进了Ti熔体向ZrO_2陶瓷颗粒内部扩散.     

Thermal oxidation of InAs and characterization of the oxide film

The thermal oxidation of InAs and the properties of the resultant oxide films were studied. Uniform InAs oxide films with a resistivity of 10¹² Ω cm were obtained on InAs substrates by low temperature oxidation at around 400°C. The thermal oxide films are mainly composed of polycrystalline In₂O₃ and As₂O₃. Increasing oxidation temperature causes thermal decomposition of the As₂O₃ and the accumulation of elemental arsenic in the oxide film. The oxide resistivity decreases with increasing oxidation temperature, mainly as a result of the thermal decomposition of the As₂O₃. The interface state densities near the midgap of InAs are (2–5) × 10¹¹ cm^-2 eV^-1. A metamorphic layer is formed beneath the thermal oxide of InAs by high temperature oxidation above 500°C.     

Evaluation of the Effective Sample Temperature in Thermal Diffusivity Measurements Using the Laser Flash Method

In the measurement of thermal diffusivity by the laser flash method, a temperature rise occurs in the sample as a pulsed laser hits on the sample surface. Due to the temperature dependence of thermal diffusivity of the sample, the thermal diffusivity corresponds to a temperature that is larger by T _eff than the temperature before laser irradiation is applied. This effective temperature rise, T _eff, has been investigated by using a numerical simulation. The results indicate that the effective temperature rise is almost equal to a maximum temperature rise, T _M, of the back surface of the sample in cases where both linear and nonlinear temperature variations of thermal diffusivity are considered.     

Modification of the LSMPS method for the conservation of the thermal energy in laser irradiation processes

In this work, new least-square moving particle semi-implicit (LSMPS) formulations for the modeling of the heat conduction in laser irradiation processes for both thick blocks and thin plates are developed. These new LSMPS formulations guarantee the conservation of the total thermal energy during the heat exchange between particles. The conservation of the thermal energy in the LSMPS method was implemented together with multiresolution techniques for the discretization of the domain with particles of different sizes so that a better characterization of the thermal gradients in the vicinity of the laser beam can be obtained. The simulation of laser irradiation processes for thin plates is still very challenging for particle methods with spherical particles and this is essentially because it is difficult to accommodate a minimum number of particles along the thickness direction without increasing considerably the resolution or the number of particles in the entire plate. In order to overcome this difficulty, a new multiresolution method based on particles with ellipsoidal shapes was also developed for a more efficient modeling of the laser irradiation in thin plates. By conducting the heat conduction simulations, in which the standard LSMPS method can provide accurate temperature distribution and by comparing the results with an analytical solution, it was confirmed that the proposed method is as accurate as the standard LSMPS method. Moreover, the heat conduction with an external heat source, in which the total thermal energy is not conserved by using the standard LSMPS method, was successfully simulated by using the proposed method. The simulations of laser irradiations were also conducted, and the validity of the proposed method has been confirmed by comparing numerical results with experimental data.     

In situ production and defect characterization of laser PVD layers from YBaCuO HTSC targets inside a scanning electron microscope

In situ generation and defect characterization of YBa₂Cu₃O_7–x layers are carried out inside a scanning electron microscope (SEM) to investigate the process of pulsed laser PVD. A Nd-glass laser with a wavelength =1.06 µm and an energy densityE<200 MJ m^–2 was combined with a SEM and used for experiments. By the irradiation of a YBa₂Cu₃O_7–x target the laser PVD process and the deposition on Si substrates were originated. At high pulse energies and/or long laser pulses heating and evaporation processes of target material dominated. For laser irradiation without Q-switch (=10 ms) the defect particles incorporated into laser PVD layers were mainly big droplets originated from the liquid phase. Otherwise, for very short pulses (Q-switch mode) the layer defects were also particles of irregular size generated by material ablation on the target surface. In contrast to the droplets these irregular particles and also the layer itself showed the same stoichiometry as the target. Both the particle density and the layer thickness showed the same plane distribution function. Therefore the particle density in the plasma seemed to possess the same solid angle distribution as the plasma itself.     

Oxidation reaction during laser cladding of SAE1045 carbon steel with SiC/Cu alloy powder

Laser cladding of SAE1045 carbon steel with SiC/Cu alloy powder was performed in air. The microstructures of the cross-section of the clad layer obtained were obviously divided into three laminae. Analysis results using X-ray diffraction, scanning electron microscope (SEM) and electron probe microanalysis (EPMP) indicate that there exist -Cu, SiO₂ and Cu₅Si in the top layer. However, SiC particles (SiCp) originally added almost disappeared during laser cladding. To simulate the reaction taken place in the laser pool, differential thermal analysis was carried out by using a Perkin-Elmer DSC 7 in an atmosphere of Ar and O₂. This revealed that the SiCp is stable during heating in the atmosphere of Ar. However, intensive oxidation and decomposition of the SiCp were found when heating was undertaken in an O₂ atmosphere, which reasonably explains the formation of SiO₂ and Cu₅Si in the top layer.     

In situ formation of TiC particulate composite layer on cast iron by laser alloying of thermal sprayed titanium coating

Commercial flake graphite cast iron substrate was coated with titanium powder by low pressure plasma spraying and was irradiated with a CO₂ laser to produce the wear resistant composite layer. The macro and microstructural changes of an alloyed layer with the traveling speeds of laser beam, the precipitate morphology of TiC particulate and the hardness profile of the alloyed layer was examined. From the results, it was possible to composite TiC particulate on the surface layer by direct reaction between carbon existed in the cast iron matrix and titanium with thermal sprayed coating by remelting and alloying them using laser irradiation. The cooling rate of the laser remelted cast iron substrate without a titanium coating was about 1 × 10⁴ K/s to 1 × 10⁵ K/s in the order under the condition of this study. The microstructure of the alloyed layer consisted of three zones; the TiC particulate precipitate zone (MHV 400–500), the mixed zone of TiC particulate + ledeburite (MHV 650–900) and the ledeburite zone (MHV 500–700). TiC particulates were precipitated as a typical dendritic morphology. The secondary TiC dendrite arms were grown to a polygonized shape and were necking. Then the separated arms became cubic crystal of TiC at the slowly solidified zone. In the rapidly solidified zone near the fusion boundary, however the fine granular TiC particulates were grouped like grapes.     

Computational simulations of pulsed laser induced melting and solidification of monocrystalline GaSb

Numerical analysis of pulsed laser induced phase change processes in the near-surface region of monocrystalline bulk GaSb is done using a thermal nonequilibrium model. The calculations of basic parameters of the processes such as surface melt duration, maximum reflectivity of the probe laser beam, etc., are verified using results of experimental work on GaSb samples irradiated by the ruby (694 nm, 35 ns FWHM) and ArF (193 nm, 13 ns FWHM) lasers. The comparison of experimental data with numerical predictions shows that while for the ruby laser a reasonable agreement in surface melt duration is achieved, the results for the ArF laser differ quite a lot. The amorphization of the topmost surface layer is identified as a main reason for these differences. The subsequent experimental analysis of the surface structure by low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) confirms the theoretical predictions and clearly indicates the appearance of an amorphous GaSb structure after ArF laser irradiation.     

Photoluminescence of fullerene-doped copolymers of methyl methacrylate during laser irradiation

Photoluminescence (PL) intensities of C₆₀-doped poly(methyl methacrylate-co-ethyl acrylate) (P(MMA-co-EA)) and poly(methyl methacrylate-co-methacrylic acid) (P(MMA-co-MAA)), increase gradually during laser irradiation in air. Concomitantly, their PL peaks are broadened and blue-shifted. C₇₀-doped copolymers exhibit a rapid increase in PL intensity soon after the start of laser irradiation, but the PL intensities then decrease to a minimum upon further irradiation. After that, their PL intensities increase again, similar to that of C₆₀-doped copolymers. By analyzing UV-visible spectra, these increases in PL are attributed to the formation of some fullerene oxide-copolymer complex by laser induced photochemical oxidation reactions.     

Influence of thermal oxidation on the interfacial properties of ultrathin strained silicon layers

In this work we examine the influence of thermal oxidation on the electrical characteristics of ultra-thin strained silicon layers grown on relaxed Si_0.78Ge_0.22 substrates under moderate to high thermal budget conditions in N₂O ambient at 800 °C. The results reveal the presence of a large density of interfacial traps which depends on the oxidation process. As long as the strained silicon layer remains between the growing oxide and the underlying Si_0.78Ge_0.22 layer, the density of interface traps increases with increasing oxidation time. When the oxidation process consumes the s-Si layer the interface state density undergoes a significant reduction of the order of 40%. This experimental evidence signifies that the strained silicon-Si_0.78Ge_0.22 interface is a major source of the measured interfacial defects. This situation can be detected only when the front SiO₂-strained silicon interface and the rear strained silicon-Si_0.78Ge_0.22 interface are in close proximity, i.e. within a distance of 5 nm or less. Finally, the influence of the material quality deterioration—as a result of the thermal treatment—to the interfacial properties of the structure is discussed.     

Laser-induced surface oxidation of (Ni/Ti)/Si system with picosecond laser pulses

The surface oxidation of Ni/Ti bilayer, deposited on silicon substrate, affected by picosecond Nd: YAG laser radiation has been investigated. Fluence close to the target ablation threshold and multi-pulse laser irradiation regime were applied. Changes in the chemical composition of the formed surface oxide layer were studied by Elastic Recoil Detection Analysis (ERDA) and X-ray photoelectron spectroscopy (XPS). Morphological features analysis, at the oxide surface layer, was monitored, too. Laser modification as-deposited (Ni/Ti)/Si sample with 10 pulses induces a progressed interaction between Ni and Ti layers with the initial surface oxidation and formation of NiTi alloy phase. Progressed intermixing of components was achieved for the irradiation with 50 and more pulses, when all components were quite uniformly distributed to a depth of about 80 nm. An oxide layer was formed at the surface, with the specific combination of the oxide phases depending on the number of accumulated pulses. Changes in the morphological characteristics are reflected in the increase of the mean surface roughness and the generation of a certain number of cavities. These features are decreased with increasing number of pulses, caused by the surface melting and a pronounced mobility of the materials.     

Photoconductivity of solid fullerene C60 film modified by laser irradiation

An improvement on photoconductivity of fullerene C₆₀ film has been observed after modification of laser irradiation at room temperature or at about 100°C because some dimer, trimer and polymer of C₆₀ have been formed under the laser irradiation. This leads to some change of charge transfer (CT) states and reduction on resistance to transport of photocarriers. Another important reason for the great increase on photoconductivity of C₆₀ modified by laser irradiation at about 100°C (373 K) is thermal effect. Thermal treatment of the laser modified C₆₀ thin film at above 100°C (373 K) for a few hours makes a great improvement on its photoconductivity because this treatment reduces trap centers for photocarriers resulted from an increase on its crystallinity and some change on its morphology. Oxygen has a great effect on photoconductivity reduction of the laser modified C₆₀ thin film.