Ultrafast laser micromachining the ultra-low expansion glass-ceramic: Optimization of processing parameters and physical mechanism

To fulfill the precise requirements of laser gyroscope for inertial navigation, using the mean surface roughness, taper angle and circularity degree as indicators, we optimize the processing parameters about ultrafast laser micromachining a commercial ultra-low expansion glass-ceramic (ULGC). Furthermore, through careful characterizations, we identify the sequential appearance of photo-darkening, crystalizing, melting, foaming and vaporizing regions during the interactions of ultrafast laser with ULGC. Especially, the foaming region is identified to effectively mediate the thermal stress at the interfaces between different regions. Originated from the Gaussian distribution of energy, poor processing quality by a Gaussian beam is mainly ascribed to the formation of various regions. By using a Bessel beam, higher surface quality could be obtained due to the possibility of the occurrence of only a foaming region in irradiated regions. This discovery gives us an important clue to optimize processing parameters to obtain high-quality ultrafast laser micromachining surface.     

电子束固化木器清漆的制备及性能研究

电子束（ EB）固化技术作为一种重要的辐射固化技术，其固化的清漆涂层性能在许多方面优于紫外（ UV）光固化清漆涂层。本研究通过选取不同类型的商品化丙烯酸树脂及活性稀释剂配制木器涂料配方，分别利用电子束（ EB）和紫外光（ UV）对其进行固化，然后对固化后的涂层进行基本性能、热性能和机械性能的表征。研究结果表明： EB固化速度快，固化膜具有较高的铅笔硬度和附着力，而且树脂种类和单体结构的不同会对电子束固化涂层的热性能及机械性能产生影响。     

Numerical simulation of localized cure of thermosensitive resin during thermo stereolithography process (TSTL)

In this work, to analyze a type of rapid prototyping technique, a numerical model was developed that was able to simulate the heat transfer at thermosensitive polymeric material during cure by laser irradiation. The analysis was carried out as a transient thermal problem using the general‐purpose finite element software ANSYS. The technique analyzed was thermal stereolithography, which uses a CO₂ laser beam to cure (solidify) thermosensitive liquid resins in a selective way to produce three‐dimensional parts. In this numerical analysis, the temperature distribution at thermoset material heated by a laser irradiation and its thermal properties are investigated. This resin is a high‐viscosity sample composed of epoxy resin, diethylenetriamine, and silica powder, which become highly crosslinked when irradiated by infrared laser. The localized curing becomes critical when the amount of silica and laser parameters are not appropriate. Bearing this in mind, this work intends, by applying the numerical method developed, to analyze the thermal behavior of resins in function of amount of silica and the laser radiation conditions, so that it is possible to have a knowledge on these variables so as to achieve a product with the required specifications. © 2006 Wiley Periodicals, Inc. J Appl PolymSci 102: 2777–2783, 2006     

Local photocurrent and high power density investigations of oxide films on valve metals with a focused laser beam

With the focused laser beam, local photocurrent investigations could be realized. Microscopic structures such as grains and defects can be distinguished. Local photocurrent potential curves for various grains show different shapes due to their different properties. Focusing of the laser beam to dimensions less than 20 μm leads to high power densities. The reactions of the involved minority carriers are then determined by slow transfer processes at the oxide—electrolyte interface. Hence, an accumulation of holes occurs at the surface for power densities> 5 W cm⁻². Under potentiostatic conditions the hole concentration at the surface induces an additional local potential drop of some 100 mV in the Helmholtz-layer, which is the main reason for the observed laser induced oxide growth. Further increasing power density enhances the hole accumulation and an increasing lateral potential drop of radial symmetry. Therefore, photo-holes travel parallel to the surface causing lateral extension of the oxide growth even at the dark surface. The Butler—Gaertner model is no more valid for such conditions for various reasons.     

Electromechanical buckling behavior of smart piezoelectrically actuated higher-order size-dependent graded nanoscale beams in thermal environment

In the present work, thermo-electro-mechanical buckling behavior of functionally graded piezoelectric (FGP) nanobeams is investigated based on higher-order shear deformation beam theory. The FGP nanobeam is subjected to four types of thermal loading including uniform, linear, and sinusoidal temperature rise as well as heat conduction through the beam thickness. Thermo-electro-mechanical properties of FGP nanobeam are supposed to change continuously in the thickness direction based on power-law model. To consider the influences of small-scale sizes, Eringen’s nonlocal elasticity theory is adopted. Applying Hamilton’s principle, the nonlocal governing equations of an FGP nanobeam in thermal environments are obtained and are solved using Navier-type analytical solution. The significance of various parameters, such as thermal loadings, external electric voltage, power-law index, nonlocal parameter, and slenderness ratio on thermal buckling response of size-dependent FGP nanobeams is investigated.     

Optical Properties of Highly Birefringent Polymer Films Using Simple Techniques

ABSTRACT

Two optical systems are used for characterizing free-standing highly birefringent polymer films. The first system is the Michelson interferometer and is utilized, in the presence of a polarizer, to determine the refractive indices in the two principal vibration directions along and across the axis of the sample, and the birefringence. The second system directly determines the birefringence. It is based on measurements of the optical path difference and the light intensity before and after striking the sample. The results show that the obtained values of birefringence, by the two methods, are in good agreement with each other. Certain optical parameters related to the measured values of refractive indices and birefringence are also calculated. The used methods are recommended for investigating thick and highly birefringent materials, where the conventional methods face certain problems such as the large number of interference fringe order and the highly refractive indices of the used immersion liquids.     

Preparation and indirect selective laser sintering of alumina/PA microspheres

Indirect selective laser sintering (SLS) is a promising additive manufacturing technique to produce ceramic parts with complex shapes in a two-step process. In the first step, the polymer phase in a deposited polymer/alumina composite microsphere layer is locally molten by a scanning laser beam, resulting in local ceramic particle bonding. In the second step, the binder is removed from the green parts by slowly heating and subsequently furnace sintered to increase the density. In this work, polyamide 12 and submicrometer sized alumina were used. Homogeneous spherical composite powders in the form of microspheres were prepared by a novel phase inversion technique. The composite powder showed good flowability and formability. Differential scanning calorimetry (DSC) was used to determine the thermal properties and laser processing window of the composite powder. The effect of the laser beam scanning parameters such as laser power, scan speed and scan spacing on the fabrication of green parts was assessed. Green parts were subsequently debinded and furnace sintered to produce crack-free alumina components. The sintered density of the parts however was limited to only 50% of the theoretical density since the intersphere space formed during microsphere deposition and SLS remained after sintering.     

Modeling of laser-induced combustion of iron in oxygen during gas-laser cutting

A physical and mathematical model is proposed for cyclic combustion of iron in an oxygen stream during oxygen gas-laser cutting of sheet metal. The combustion front is driven by focused laser radiation and heterogeneous iron oxidation in oxygen. The burning rate is limited by the rate of oxygen supply from the gas phase to the metal surface, and the motion of the interface is determined by the local temperature mode. Three-dimensional numerical modeling revealed striated structures on the metal surface whose linear dimensions depended on velocity of the laser beam and oxygen flow parameters. The modeling results explain the mechanism of striation formation during oxygen gas-laser cutting of low-carbon steel and agree qualitatively with experimental data.     

Laser Beam Machining of Porous Woodceramics

Laser beam machining was tested as a practical method for machining porous woodceramics (WCS). A black carbonized layer is generally formed on the processed surface when woody materials are processed with laser beam machining. This problem does not occur on the WCS because they are burned. The processed surface temperature during laser beam machining can be estimated from the relationship between the burning temperature and C and O₂ component concentrations. Burning (scorching) on the processed surface decreases slightly as the feed speed of the workpiece increases. WCS are more easily processed with laser machining after being burned, using pulse oscillation to reduce the thermal influence.     

The Structure and Properties of Thermally Treated Polyester Films

ABSTRACT

The structure and properties of highly birefringent polyester (PET) films annealed at 120°C for different intervals of time are investigated. In the present of a polarizer, the Michelson interferometer is used to determine the refractive indices and the birefringence of the samples. For accurate results, the refractive indices are determined at different angles of incidence, and the average is taken. The effect of annealing time on the optical properties is investigated. The results show that the refractive indices and the birefringence increase as the time of thermal treatment increases. The values of refractive index difference (birefringence) are confirmed by those obtained directly from another optical technique. The obtained values of refractive indices and birefringence are utilized to calculate some optical and structural parameters important in characterizing the investigated PET films. Relationships between the obtained parameters and annealing time are given for illustration. The study shows that the Michelson interferometer can be considered as important tool in investigating thick and highly birefringent polyester films.     

Fabrication of c-axis–oriented AlN bulk ceramics using a rotating magnetic field

Controlling the crystallographic orientation in bulk ceramics is expected to improve their properties that depend on the axis of the crystal structure. The development of crystallographic orientation using a magnetic field has been reported in various types of bulk ceramics with anisotropic crystal structures. In this study, c-axis–oriented AlN bulk ceramics were prepared by slip casting in a rotating high magnetic field. The c-axes of 61% of the grains were aligned within 20°. Due to this orientation, the thermal conductivity of AlN was improved. A thermal conductivity difference of approximately 14% at room temperature was observed for the samples with different orientation axes, which is higher than its intrinsic anisotropy.     

A yttrium aluminosilicate glass fiber with graded refractive index fabricated by melt‐in‐tube method

A graded‐index fiber with yttrium aluminosilicate (YAS) glass core was fabricated by a “Melt‐in‐Tube” method. The refractive index and stress profiles of the fibers exhibit a highly symmetrical gradient tendency along the radial axis which is in accordance with the results of element migration. Based on the formation mechanism of YAS glass core, the relationships between the refractive index/stress profile and the spontaneous migration are discussed. A linear all‐fiber laser based on the obtained fiber shows a laser output threshold of 11.48 mW and slope efficiency of 8.3%. The near‐field beam intensity distribution of fiber laser with spectrum FWHM of 0.24 nm indicates the good laser beam quality. The facile “Melt‐in‐tube” method was proved to be reliable in designing graded‐index fiber through the dissolution and diffusion process, which is more advanced than commonly used “Rod‐in‐tube” method.     

Droplet spectra measurements of fan and cone atomisers using a laser diffraction technique

The droplet spectra of cone (D2/D4/D6 with 25 and 45 swirl plates) and single orifice fan (SS65015. SS6503 and SS6504) pressure atomisers were assessed at selected pressures (500, 700, 1000 and 1 500 kPa) using a laser diffraction technique. Care must be exercised when aligning the spray cloud with the laser beam when droplet spectra are measured using this technique. Because the cone was filled with small droplets (VMD 56.4 ± 1.8 μm) measurements taken at increasing distances between the laser and atomiser orifice resulted in decreasing VMDs, i.e. they reflected the higher volume of small droplets sampled, so the laser beam should penetrate the diameter of the cone as close to the break-up of the sheet as possible. With fan atomisers, consistent results were obtained if the spectra were measured with the spray cloud horizontal and the laser beam passing through the long axis of the spray cloud at right angles to the axis of the atomiser. Comparisons of this droplet spectra sampling technique with previous work show it to be more sensitive to the small droplet component of the spray.     

Optical detection of combustion zone movement in solid high-energy materials

Optical methods in infrared (IR) and visible (VIS) ranges for detection of combustion zone propagation in solid high-energy materials, such as pyrotechnic compositions inserted into pyrolytic graphite (pyrographite) tubes and ignited at one end by a CO₂ laser beam are presented. The pyrographite tube is used as a thermal management transducer enabling detection of combustion zone movement because of unique thermal conductivity anisotropy of pyrographite resulting in low thermal conductivity of the tube along its axis and high thermal conductivity along the tube radius. In the first method, an IR thermal camera is applied for detection of heat zone movement induced on the external side surface of the pyrographite tube by the combustion zone travelling inside the tube. According to the second method, a VIS camera and a thermochromic layer covering the external side surface of the pyrographite tube are used for visualization of heat zone movement registered as the color change boundary traveling along the thermochromic layer. The change in color of this layer is caused by its thermochromic substance response to heat delivered by the heat zone. As thermochromic substances, leuko dyes or chiral-nematic liquid crystals are used. These methods seem to be particularly promising for continuous measurements of burning rates of solid high-energy materials, such as solid rocket propellants and pyrotechnic compositions.     

炸药激光起爆过程的准三维有限差分数值模拟

根据含锌材料的热起爆机理，建立了在激光作用下炸药点火过程的三维(二维轴对称)有限差分模型，运用此模型对RDX、PETN、HMX和改性B炸药在激光作用下的温度成长过程、温度场分布及点火延迟时间进行了数值模拟。结果表明，三维模型延迟时间计算结果与实验结果较为符合；药柱温度增长主要是在激光照射面上很薄一层药剂内发生；激光的光斑直径、脉宽和炸药的激光吸收系数对点火能量阈值有较大的影响。     

He-Ne Laser Protection Barrier by Means of Poly(tetrafluoroethylene-perfluorovinyl ether) Grafted by Acrylic Acid and Complexed with Cu(II)

Appropriate eye and skin protection is a prerequisite for the safe operation of He-Ne laser in industrial and laboratory environments. This article reports on the measurement of the optical parameters of Poly(tetrafluoroethylene-perfluorovinyl ether) grafted by acrylic acid and complexed with Cu(II) when exposed to He-Ne laser beam of wavelength of 632.8 nm and power 12.5 mW. Transmittance, reflectance, and refractive index spectra are presented. The study showed that the material has a protective level 4. Environmental conditions like thermal and fading processes were tested. This suggested that the material preserves its protective features as a protective eye and skin barrier of protective level L4. This was applied for an occupational working time up to 8 h, temperature up to 50°C, and for a time of 74 days after laser irradiation.     

Continuous-Wave Laser Patterning of Three-Dimensional Microstructure in Glasses Containing Silver Nanoparticles

A method is reported that uses visible continuous-wave laser to directly fabricate micropatterns in glasses containing silver (Ag) nanoparticles. Ag nanoparticles were formed in the glass using thermal treatment. Upon laser irradiation, Ag nanoparticles with their surface plasmon in resonance with the laser beam were partially melted and decreased in size due to the temperature increase induced by the excited Ag nanoparticles. Three-dimensional patterns were achieved by scanning the laser beam through the thickness of the glass.     

Efficiency of radiation technology for preparing high-strength corundum-zirconium ceramics

The physical principles of radiation sintering and modification of ceramic structures are considered. Sintering of corundum-zirconium ceramics in thermal and electron-beam heating methods is described. The sintering temperature of ceramics is found to be decreased (by 250 – 300°C) the case of heating by an electron beam. The modifying effect of the electron beam in ceramics sintered by the thermal method is investigated and is shown to increase its mechanical strength. The prospects of electron-beam technologies for the production of corundum-zirconium ceramics with high mechanical parameters are analyzed.Translated from Ogneupory, No. 5, pp. 12 – 16, May, 1995.The authors are grateful to A. P. Surzhikov for his help in the organization of the radiation experiments and V. L. Auslender and A. I. Slosman for their contribution to the work.     

Size Characterization of Asbestos Fibers by Means of Electrostatic Alignment and Light-Scattering Techniques

Crocidolite asbestos fibers suspended in light mineral oil were aligned by means of an electrostatic field of 1500 V / cm. The maximum fiber concentration was ～8 μg / mL. The light scattered by the suspended fibers, with the incident laser beam normal to the fiber axis, was recorded by two probes: one rotating about the fiber axis and another rotating about the incident beam near the forward direction, i.e., 17.5° from the incident beam. The Rayleigh-Debye-Gans theory was utilized in order to determine the modal diameter and modal aspect ratio of assumed logarithmic normal distribution functions in both the fiber diameter and the fiber aspect ratio. Multiple scattering effects were assumed to be negligible.     

A Novel Method for Estimating Light-Scattering Properties of Soot Aerosols Using a Modified Single-Particle Soot Photometer

A Single-Particle Soot Photometer (SP2) detects black refractory or elemental carbon (EC) in particles by passing them through an intense laser beam. The laser light heats EC in particles causing them to vaporize in the beam. Detection of wavelength-resolved thermal radiation emissions provides quantitative information on the EC mass of individual particles in the size range of 0.2–1 μm diameter. Non-absorbing particles are sized based on the amount of light they scatter from the laser beam. The time series of the scattering signal of a non-absorbing particle is a Gaussian, because the SP2 laser is in the TEM00 mode. Information on the scattering properties of externally and internally mixed EC particles as detected by the SP2 is lost in general, because each particle changes size, shape, and composition as it passes through the laser beam. Thus, scattered light from a sampled EC particle does not yield a full Gaussian waveform. A method for determining the scattering properties of EC particles using a two-element avalanche photodiode (APD) is described here. In this method, the Gaussian scattering function is constructed from the leading edge of the scattering signal (before the particle is perturbed by the laser), the Gaussian width, and the location of the leading edge in the beam derived from the two-element APD signal. The method allows an SP2 to determine the scattering properties of individual EC particles as well as the EC mass. Detection of polystyrene latex spheres, well-characterized EC particles with and without organic coatings, and Mie scattering calculations are used to validate the method.