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.     

A 500-J nuclear-pumped gas laser

A 500-J pulsed gas laser (laser module) pumped by uranium nuclear fission products has been created, which operates on xenon 5d[3/2]₁ → 6p[3/2]₁ transition with a wavelength of 2.03 μm. Experiments have been performed on a BARS-5 + RUN-2 complex comprising a fast aperiodic self-quenching reactor and a neutron multiplier reactor. A specific laser radiation energy in experiments achieved ～32 J/dm³ at an efficiency of 3% (ratio of the laser output pulse energy and the energy transferred from fission fragments to gas medium).     

Influence of laser pulse parameters on characteristics of a source of multicharged metal ions based on laser-induced medium-power spark discharge

The dynamics of laser-induced vacuum spark discharge with storage energy not exceeding 25 J has been studied in a broad range of laser pulse energies and power densities. It is shown that, using discharge-initiating laser pulse of nanosecond duration, it is possible to obtain stable single pinching of cathode jet plasma at a storage voltage above 10 kV. Plasma pinching is accompanied by the generation of a beam of multicharged ions of the cathode material (aluminum) up to Al⁸⁺. The maximum energies of ions obey the scaling relation E _max = 5ZeU ₀ that has been obtained previously for a low-voltage discharge. An increase in the laser pulse energy leads to growth of the average beam charge and a sharp decrease in the ion energy.     

Synthesis of nanocrystalline NbAl3 by laser ablation technique

Synthesis of nanocrystalline material has been developed using laser ablation technique. NbAl₃ nanocrystalline powders have been synthesized by laser ablation and identified by transmission electron microscopy (TEM). The average particle size was measured about 5.0 nm. Production yield and ablation rate were obtained as a function of He background pressure and laser pulse energy by measuring the weight of produced powders and weight loss of the target material before and after experiment. The ablation rate appeared to be relatively constant over the range of the He gas pressure. Production rate varied along with both He gas pressure and laser pulse energy. Optimum condition for efficiently high production rate is at 1.0 Torr of He gas pressure and at 320 mJ of the laser pulse energy.     

System development,formability quality and microstructure evolution of selective laser-melted magnesium

A selective laser melting (SLM) system, which consisted of a fibre laser, a three-dimensional motion platform and a motion control system, was developed in this study. The effect of process parameters on the microstructure evolution of SLMed magnesium parts was investigated. The results revealed that under an irradiation of laser energy density <3.0?J/mm, the powder remained in the discrete state. At a laser energy density 3.0–6.0 J/mm, the powder partially melted and sintered together, yielding incompact tracks. As the energy density increased to 6.0–12.0 J/mm, the powder fully melted forming continuous and smooth tracks. With a further increase in the laser energy density evaporation of the powder occurred. Dense magnesium parts free of pores and cracks were successfully fabricated with the optimal energy density of 10.0 J/mm. The immersion experiment revealed that the degradation product was mainly consisted of Mg(OH)₂, which slowed down the degradation rate acting as a protective layer.     

Cation off-stoichiometric SrMnO<Subscript>3−δ</Subscript> thin film grown by pulsed laser deposition

The laser energy density (laser fluence) dependency of the Sr/Mn ratio was investigated for SrMnO_3−δ (SMO) thin films grown by pulsed laser deposition (PLD). It was found that the Sr/Mn ratio showed a steep increase followed by a gradual increase as the laser fluence was increased. However, the Sr/Mn ratio always showed Mn-excess under the present laser fluence condition as long as stoichiometric SrMnO₃ targets were used. In order to obtain cation stoichiometric SMO films, it was necessary to use Sr-excess SrMnO₃ targets in addition with laser fluence tuning. The crystal quality of the SMO thin film was found to vary with the Sr/Mn ratio. In stoichiometric or Sr-excess SMO thin films, epitaxial thin films could be obtained, whereas Mn-excess thin films showed very low crystallinity. Sr-excess films were also found to have some extra SrO planes. In addition, they exhibited out-of-plane lattice expansion which electron energy loss spectroscopy analysis revealed was due to Mn vacancies. The variation of film growth was closely related to point defects due to excess cations included in growing thin films.     

Pulsed laser annealing of sodium super ionic conductor for carbon dioxide sensors

This paper discussed a way to improve solid electrolyte carbon dioxide (CO₂) sensor by excimer laser annealing of sodium super ionic conductors (NASICON). The CO₂ sensor used in this paper consists of a thin NASICON layer. We additionally annealed the NASICON to improve its electrical conductivity by pulsed excimer laser. The laser annealing results in re-crystallization of the NASICON thin film and changes the conductivity, grain sizes, and the structure of grain boundaries. From the scanning electron microscope pictures, we saw that NASICON grain sizes were enlarged after laser annealing. Grain sizes were also correlated to laser annealing energy and annealing times. After 2 times annealing of 420 mJ laser energy with 7 pulses each time at 1 Hz repetition rate, the conductivity of NASICON was increased by 90%. When the CO₂ concentration was changed from 1000 ppm to 5000 ppm, the sensor resolution was enhanced up to 66%. These results suggested that appropriate laser annealing treatment not only enlarges NASICON grain sizes but also reduces its resistance. Therefore, the NASICON CO₂ sensor resolution can be improved accordingly.     

Operating regimes for laser surface engineering of ceramics

Results obtained with CO₂ lasers on the effect of processing parameters were used to determine the operating regimes for laser sealing and laser cladding of ceramics with particular reference to zirconias. The specific energy required for laser cladding is higher by at least two orders of magnitude than that required for laser sealing of plasma-sprayed ceramics at a given power density.On leave from Scientific Research Council, Baghdad.     

Microstructural studies in low specific energy laser surface treated Al(A356)-SiCP composites

SiC particulate reinforced A356 Al metal matrix composites were laser treated using pulsed Nd-YAG laser beam. The processing was carried out in air atmosphere at varying pulse energy (5 to 20 J) and scan rates (30 to 150 mm/sec). The samples were cut perpendicular to the track and they were characterized using optical microscopy, scanning electron microscopy and X-ray diffraction. Microstructure of laser treated region consists of regular succession of coarse and fine microstructure signifying the presence of low velocity bands. Transition from cellular/columnar dendritic to equiaxed dendritic structure has been observed. Microstructure of samples laser treated with specific energy greater than 13 kJ/cm² show presence of Al₄C₃ platelets.     

1.06 μm laser characteristics of Nd:KY(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystal

This paper reported the pulse laser characteristics of Nd³⁺:KY(WO₄)₂ crystal grown in our laboratory with an impurity of 4.57%. The maximum laser output was 61.6 mJ at 1.06 m, pumped by a single Xenon flash-lamp. The laser energy threshold value E_th was 0.32 J. The overall and slope efficiencies were 0.43% and 0.45%.     

CO2 laser deposition of diamond thin films on electronic materials

Laser-induced pyrolysis has been utilized to create gas-surface chemical reactions necessary for diamond deposition on electronic materials. A 1200 W CO₂ gas laser has been used as an energy source for depositing diamond thin films from a gas mixture of CH₄ and H₂ in a chemical vapour deposition chamber. The substrate temperature was about 500°C. The laser beam energy was largely absorbed by the gases that lead to their excitation and decomposition on contact with the nearby hot substrate. Raman spectroscopy and scanning electron microscope analysis revealed high quality, fine crystalline diamond structures.     

Laser desorption from the surface of copper phthalocyanine films on silicon and cadmium sulfide

We have studied laser desorption mass spectra of copper phthalocyanine (CuPc) films deposited in vacuum onto single crystal silicon and cadmium sulfide substrates. The desorption was induced by 10-ns pulses of neodymium laser radiation (quantum energy, 2.34 eV) with an energy density E varied from 0.1 to 40 mJ/cm². It is established that laser radiation produces fragmentation of CuPc molecules and desorption of the fragments. The main fragments observed in the mass spectra are identified. The intensity of the main desorbed species has been studied as a function of the laser pulse energy density. The components of CdS substrates penetrate into the volume of deposited CuPc films in the form of Cd atoms and S₂ molecules.     

The Effect of Laser Power,Traverse Velocity and Spot Size on the Peel Resistance of a Polypropylene/Adhesive Bond

The mean peel resistance force achieved with respect to variation in the laser power, incident spot traverse velocity and incident spot diameter between linear low density polyethylene film backed by a thin commercial adhesive coating that were bonded to a polypropylene (PP) substrate via thermal activation provided by a 27W CO₂ laser is discussed in this work. The results gathered for this work have been used to generate a novel empirical tool that predicts the CO₂ laser power required to achieve a viable adhesive bond for this material combination. This predictive tool will enable the packaging industry to achieve markedly increased financial yield, process efficiency, reduced material waste and process flexibility. A laser spot size‐dependent linear increase in laser line energy was necessary for this material combination, suggesting the minimal impact of thermal strain rate. Moreover, a high level of repeatability around this threshold laser line energy was indicated, suggesting that laser‐activated adhesive bonding of such polymer films is viable. The adhesion between the material combination trialled here responded linearly to thermal load. In particular, when using the smallest diameter laser spot, it is proposed that the resulting high irradiance caused film or adhesive material damage, thus resulting in reduced peel resistance force. The experimental work conducted indicated that the processing window of an incident CO₂ laser spot increases with respect to spot diameter, simultaneously yielding greater bond stability in the face of short‐term laser variance. © 2015 The Authors. Packaging Technology and Science published by John Wiley & Sons Ltd.     

On the possibility of increasing and controlling the radiation pulse duration in gas-discharge lasers

The experimental results demonstrating the possibility to increase and control the pulse duration in molecular lasers by means of quasistationary energy pumping into the active laser media are presented. It is shown that the pulse duration in the XeCl, N₂, and CO₂ lasers can be controlled by matching parameters of the energy source and the gas-discharge plasma in the active laser media pumped in a quasistationary regime.     

The irradiation effect of a Nd–YAG pulsed laser on the CeO2 target in the liquid

The irradiation of an Nd–YAG pulsed laser on the CeO₂ target in water was investigated. The reaction products depend mainly on the energy density of the Nd–YAG laser. The CeO₂ nanoparticles with sizes of 20–80 nm were formed in water, while the energy density was larger than 32 kJ/cm². The nanocrystalline films with grain sizes of 50–150 nm were formed on the target with the energy density of 20–25 kJ/cm². Finally, the amorphous films were formed with the energy density of 6–15 kJ/cm². The formation mechanisms of the nanoparticles and the films were discussed according to the triple-point phase diagram of CeO₂.     

Trivalent Ce2O3 and CeO2−x intermediate oxides induced by laser irradiation of CeO2 powders

A strong non-stoichiometry of pure fcc CeO₂ was induced by laser irradiation. The increase of laser power and/or energy density had a saturable effect on particle size growth. The possibility of CeO₂ reduction to A-Ce₂O₃ by laser irradiation was demonstrated. Particles of stable Ce₇O₁₂ phase were observed in all specimens irradiated at low laser-power densities. An epitaxial relationship between triclinic Ce₁₁O₂₀ and cubic Ce₁₂O₂₂ phases was found. The controversial C-Ce₂O₃ phase was detected at the limits of a bcc particle. An unknown bcc phase of acicular morphology, strongly related to C-Ce₂O₃, was also registered. The dose dependence of CeO₂ structural modifications obtained by laser irradiation as a function of laser energy density variation could be explained by a simple defect aggregation model implying lattice defects (oxygen vacancies and Ce³⁺ ions).     

Induced heating by nonlinear absorption in LiNbO3-type crystals under continuous-wave laser irradiation

We investigate light-induced heating by nonlinear absorption in LiNbO₃-type crystals under continuous-wave (CW) laser irradiation. The heat source is one-photon absorption by long-lived excited states created by two-photon absorption. The accumulated effect of excited states, which act as light absorbers, is significant for a CW laser. Light-induced heating causes catastrophic CW laser breakdown in LiNbO₃-type crystals in which the band-gap energy is less than twice the photon energy and in which long-lived excited states, like polarons, can exist. For LiNbO₃, the threshold intensity for catastrophic breakdown is estimated to be in the order of MW/cm².     

Fabrication of polycrystalline silicon thin films on glass substrates using fiber laser crystallization

Laser crystallization of amorphous silicon (a-Si), using a fiber laser of λ = 1064 nm wavelength, was investigated. a-Si films with 50 nm thickness deposited on glass were prepared by a plasma enhanced chemical vapor deposition. The infrared fundamental wave (λ = 1064 nm) is not absorbed by amorphous silicon (a-Si) films. Thus, different types of capping layers (a-CeO_x, a-SiN_x, and a-SiO_x) with a desired refractive index, n and thickness, d were deposited on the a-Si surface. Crystallization was a function of laser energy density, and was performed using a fiber laser. The structural properties of the crystallized films were measured via Raman spectra, a scanning electron microscope (SEM), and an atomic force microscope (AFM). The relationship between film transmittance and crystallinity was discussed. As the laser energy density increased from 10-40 W, crystallinity increased from 0-90%. However, the higher laser density adversely affected surface roughness and uniformity of the grain size. We found that favorable crystallization and uniformity could be accomplished at the lower energy density of 30 W with a-SiO_x as the capping layer.     

Imprinting on empty hard gelatin capsule shells containing titanium dioxide by application of the UV laser printing technique

The purpose of this study was to examine the application of ultraviolet (UV) laser irradiation to printing hard gelatin capsule shells containing titanium dioxide (TiO₂) and to clarify how the color strength of the printing by the laser could be controlled by the power of the irradiated laser. Hard gelatin capsule shells containing 3.5% TiO₂ were used in this study. The capsules were irradiated with pulsed UV laser at a wavelength of 355?nm. The color strength of the printed capsule was determined by a spectrophotometer as total color difference (dE). The capsules could be printed gray by the UV laser. The formation of many black particles which were agglomerates of oxygen-defected TiO₂ was associated with the printing. In the relationship between laser peak power of a pulse and dE, there were two inflection points. The lower point was the minimal laser peak power to form the black particles and was constant regardless of the dosage forms, for example film-coated tablets, soft gelatin capsules and hard gelatin capsules. The upper point was the minimal laser peak power to form micro-bubbles in the shells and was variable with the formulation. From the lower point to the upper point, the capsules were printed gray and the dE of the printing increased linearly with the laser peak power. Hard gelatin capsule shells containing TiO₂ could be printed gray using the UV laser printing technique. The color strength of the printing could be controlled by regulating the laser energy between the two inflection points.     

Investigation of Magnetic and Structural Properties of Amorphous Alloys Irradiated by Nd:YAG Laser

Amorphous metallic alloys (AMAS) have found wide application in various fields. Laser is used to modify materials. In this work, we have prepared an amorphous metallic alloy with composition Fe₇₇Co₂₁Si₂ using the melt spinning method. The prepared specimens have irradiated by a Nd:YAG laser at 1064?nm, with various fluences to investigate the corresponding induced change. The structures of specimens are verified by X-ray diffraction (XRD) before and after laser irradiated. Then magnetic parameters are measured using a vibrating sample magnetometer. Morphology reference and laser treated samples are investigated by the scanning electron microscope (SEM). The use of laser beam of relatively low energy, but with a subsequent overlapping of heated areas, resulted in a much more homogeneous structure in the heated areas of the ribbon. These measurements show that initially the magnetic properties are altered due to laser processing. The surface alteration was being simultaneously monitored by laser induced breakdown spectroscopy (LIBS).