Microhardness of a dental restorative composite resin containing nanoparticles polymerized with argon ion laser

The objective of this study was to compare the microhardness of two resin composites (microhybrid and nanoparticles). Light activation was performed with argon ion laser 1.56 J (L) and halogen light 2.6 J (H) was used as control. Measurements were taken on the irradiated surfaces and those opposite them, at thicknesses of 1, 2 and 3 mm. To evaluate the quality of polymerization, the percentages of maximum hardness were calculated (PMH). For statistical analysis the ANOVA and Tukey tests were used (p ≤ 0.05). To microhybrid was shown that the hardness with laser was inferior to the hardness achieved with halogen light, for both the 1 mm and 2 mm. The nanoparticles polymerized with laser, presented lower hardness even on the irradiated surface, than the same surface light activated with halogen light. The microhybrid attained a minimum PMH of 80% up to the thickness of 2 mm with halogen light, and with laser, only up to 1 mm. The nanoparticles attained a minimum PMH of 80% up to 3 mm thickness with halogen light and with laser this minimum was not obtained at any thickness. Based on these results, it could be concluded that light activation with argon ion laser is contra-indicated for the studied nanoparticles.     

Characterisation of the bremsstrahlung generated by a 450 MeV superconducting electron linac using the inverse calculation method based on a genetic algorithm

The bremsstrahlung energy spectrum generated by the 450 MeV electron linac driving the vacuum ultra violet free electron laser FLASH (free electron laser in Hamburg) operated by DESY was measured with TLD-700 detectors and unfolded using a technique based on a genetic algorithm. The half (1/2), one-fourth (1/4) and tenth (1/10) value layer thickness of lead were estimated to be 1.0, 4.0 and 20 mm, respectively. The peak and average photon energy were calculated from the unfolded bremsstrahlung spectrum and found to be 0.5 and 0.8 MeV, respectively.     

Influence of water on the surface structure of 1-hexyl-3-methylimidazolium chloride

The molecular surface structure of an ionic liquid (IL) with and without the presence of water was studied with the surface sensitive technique neutral impact collision ion scattering spectroscopy (NICISS). The IL chosen is 1-hexyl-3-methylimidazolium chloride, which is known to be hydrophilic. Binary mixtures were investigated within the water mole fraction range 0.43 ≤ χ_water ≤ 0.71 at 283 K. During approximately 3 h exposition time in vacuum, we have observed a very low water loss rate from sample. The NICISS measurements suggest that admixture of water to [HMIm]Cl leads to a layered surface structure. Three layers were identified (layer 1 — cations, layer 2 — cations and water, layer 3 — cations, water, and anions). While the first layer is unaffected by water, the thickness of the second layer depends on the water concentration. The thickness of layer 2 is relatively constant for water concentrations χ_water ≤ 0.61, but increases for water contents χ_water ≥ 0.68. The concentration range 0.61 ≤ χ_water ≤ 0.68 seems to play a key role in water network formation.     

Investigation of optimum condition in oxygen gas-assisted laser cutting

Laser cutting characteristics including power level and cutting gas pressure are investigated in order to obtain an optimum kerf width. The kerf width is investigated for a laser power range of 50–170 W and a gas pressure of 1–6 bar for steel and mild steel materials. Variation of sample thickness, material type, gas pressure and laser power on the average cut width and slot quality are investigated. Optimum conditions for the steel and mild steel materials with a thickness range of 1–2 mm are obtained. The optimum condition for the steel cutting results in a minimum average kerf width of 0.2 mm at a laser power of 67 W, cutting rate of 7.1 mm/s and an oxygen pressure of 4 bar. A similar investigation for the mild steel cutting results in a minimum average kerf width of 0.3 mm at the same laser power of 67 W, cutting rate of 9.5 mm/s, and an oxygen pressure of 1 bar. The experimental average kerf is about 0.3 mm, which is approximately equal to the estimated focused beam diameter of 0.27 mm for our focusing lens (f=4 cm and 100 W power). This beam size leads to a laser intensity of about 1.74×10⁹ W/m² at the workpiece surface. The estimated cutting rate from theoretical calculation is about 8.07 mm/s (1.0 mm thickness and 100 W power), which agrees with the experimental results that is 7.1 mm/s for 1.0 mm thickness of mild steel at the laser power of 88 W.     

Laser singulation of thin wafer: Die strength and surface roughness analysis of 80 μm silicon dice

In this paper, we have studied the characteristics of silicon dice, singulated using a high-power-high-repetition-rate femtosecond laser. The die strength and surface roughness, of the die side walls, are evaluated for different laser parameters such as pulsewidth and repetition rate. Since, the 80-μm-thick wafers used in this study were polished on both sides, die-edge roughness plays a decisive factor in determining the die strength when compared to backside roughness and wafer thickness as is the case in other studies. Excellent side wall average surface roughness of 0.35 μm is obtained at pulsewidth of 214 fs and repetition rate of 4.33 MHz using an average laser power of 15.5 W. Die strength is measured via the 3-point bending test. Strength reduction, due to die side wall surface defects that are induced through the wafer dicing process, is evaluated through die strength and surface roughness analysis. Die strength of a silicon dice is characterized as the first step in prediction and prevention of die failure during the package assembly, reliability test and working life. Improvement in the die side wall surface roughness is observed with the usage of nitrogen gas assist as compared to that obtained in air.     

Micromachining NiTi tubes for use in medical devices by using a femtosecond laser

Recent growth in medical device technology has been substantially driven by developments in laser micromachining, which is a powerful fabrication technique in which nickel–titanium (Nitinol, NiTi) alloy materials that exhibit superelastic and shape memory properties are formed (e.g., self-expanding stents). In this study a NiTi tube curve surface process is proposed, involving a femtosecond laser process and a galvano-mirror scanner. The diameter of the NiTi tube was 5.116 mm, its thickness was 0.234 mm, and its length was 100 mm. The results indicated that during the machine process the ablation mechanism of the NiTi tubes was changed by altering the machining path. The path alteration enhanced the laser ablation rate from 12.3 to 26.7 μm/J. Thus the path alteration contributed to a wide kerf line, enabling the assisted air to efficiently remove the debris deposited at the bottom of the kerf during the laser ablation process. The results indicated that the NiTi tube curve process enhanced the laser ablation rate by two times and reduced the amount of energy accumulated within the materials by 50% or more. By altering the machining path using the scanning system, this process can decrease the production of heat affected zones (the accumulation of thermal energy) in medical device applications.     

Use of thermal imaging to characterize laser light reflection from thermoplastics as a function of thickness,laser incidence angle and surface roughness

Laser light reflection during the laser transmission welding (LTW) of thermoplastics has the potential to overheat and/or cause unintentional welding of adjacent features of the part being welded. For this reason, and in order to assess how much light is being absorbed by the transparent part (after measurement of the light transmitted through the transparent part), it is important to be able to quantify the magnitude and distribution of reflected light. The magnitude and distribution of the reflected light depends on the total laser input power as well as its distribution, the laser incidence angle (angle between the normal to the transparent part surface and the laser beam), the laser light polarization as well as the surface and optical properties of the transparent part. A novel technique based on thermal imaging of the reflected light was previously developed by the authors. It is used in this study to characterize the magnitude and distribution of reflected light from thermoplastics as a function of thickness (1–3.1 mm), laser incidence angle (20–40°) and surface roughness (0.04–1.04 μm). Results from reflection tests on nearly polished nylon 6 (surface roughness between 0.04 and 0.05 μm) have shown that, for the various thicknesses tested (1–3.1 mm), the total reflection was larger than the specular top surface reflection predicted via the Fresnel relation. From these observations, it is conjectured that, in addition to top surface reflection, the bulk and/or bottom surface also contribute to the total reflection. The results also showed that reflection decreased slightly with increasing thickness. As expected, for the p-polarized light used in this study, the reflection decreased with increasing angle of incidence for the range of angles studied. It was also found that when the surface roughness was close to zero and when it was close to the wavelength of the input laser beam (i.e. 940 nm), the reflectance values were close and reached a minimum between these two roughness values.     

An anti-reflective 1D rectangle grating on GaAs solar cell using one-step femtosecond laser fabrication

We report the fabrication of the anti-reflective micro/nano-structure on absorbing layer of GaAs solar cell surface using an efficient approach based on one-step femtosecond laser irradiation. Morphology of the microstructures and reflectance of the cell irradiated are characterized with SEM and spectrometer to analyze the influence of laser processing parameters on the change of microstructures induced and the reflectance. It has been found that the rectangle grating micro/nano-structure with a period of 700 nm and width of 600 nm is obtained neatly with laser pulse energy of 30.5 μJ(pulse duration is 130 fs, center wavelength is 800 nm, scanning speed is 2.2 mm/s and spot diameter is 22 µm). Reflectance has been suppressed to 23.6% with rectangle structure from 33% of planar cell. In addition, simulation using a finite-difference-time domain(FDTD) method results show that the rectangle grating micro/nano-structure can effectively suppress the reflection within large wavelength ranges.     

XPS investigation of vacuum annealed vertically aligned ultralong ZnO nanowires

The surface properties of vertically aligned ZnO nanowires grown by chemical vapour deposition on GaN using a gold layer as a catalyst are investigated by X-ray Photoelectron Spectroscopy as a function of annealing temperature in ultra high vacuum (UHV). The nanowires are 8.5 μm long and 60 nm wide. 87% of the surface carbon content was removed after annealing at 500 °C in UHV. Analysis of the gold intensity suggests diffusion into the nanowires after annealing at 600 °C. Annealing at 300 °C removes surface water contamination and induces a 0.2 eV upward band bending, indicating that adsorbed water molecules act as surface electron donors. The contaminants re-adsorbed after 10 days in UHV and the surface band bending caused by the water removal was reversed. The UHV experiment also affected the nanowires arrangement causing them to bunch together. These results have clear implications for gas sensing applications with ZnO NWs.     

Picosecond laser cutting and drilling of thin flex glass

We investigate the feasibility of cutting and drilling thin flex glass (TFG) substrates using a picosecond laser operating at wavelengths of 1030 nm, 515 nm and 343 nm. 50 μm and 100 μm thick AF32^®Eco Thin Glass (Schott AG) sheets are used. The laser processing parameters such as the wavelength, pulse energy, pulse repetition frequency, scan speed and the number of laser passes which are necessary to perform through a cut or to drill a borehole in the TFG substrate are studied in detail. Our results show that the highest effective cutting speeds (220 mm/s for a 50 μm thick TFG substrate and 74 mm/s for a 100 μm thick TFG substrate) are obtained with the 1030 nm wavelength, whereas the 343 nm wavelength provides the best quality cuts. The 515 nm wavelength, meanwhile, can be used to provide relatively good laser cut quality with heat affected zones (HAZ) of <25 μm for 50 μm TFG and <40 μm for 100 μm TFG with cutting speeds of 100 mm/s and 28.5 mm/s, respectively. The 343 nm and 515 nm wavelengths can also be used for drilling micro-holes (with inlet diameters of ⩽75 µm) in the 100 μm TFG substrate with speeds of up to 2 holes per second (using 343 nm) and 8 holes per second (using 515 nm). Optical microscope and SEM images of the cuts and micro-holes are presented.     

Micro-machining of silicon wafer in air and under water

Laser ablation micro-machining tests are conducted on silicon wafer, both in air and under flowing water stream, with the use of 355 nm-X AVIA laser. Effects of laser pulse frequency, power level, scan velocity and focal plane position on the associated laser spatter deposition (in air), irradiated areas (under flowing water film) and taper are investigated. It shows that low frequency, i.e. 30–40 kHz, and high peak power result in smaller spatter and irradiated areas, and the hole taper decreases with increase in pulse frequency. Increase in the laser fluence broadens both the areas and increases the hole taper. Both areas enlarge with the increase of scanning velocity of more than 3 mm s^?1. The scan velocity has no effect on hole taper in air environment but inconsistent hole taper is obtained under flowing water stream. Furthermore, moving the focal plane position below the workpiece surface contributes relatively smaller areas of spatter deposition, irradiation and taper in comparison to zero focal plane position. Finally, the differences between laser ablation in air and under water are identified. The reduction in the spatter deposition and irradiated areas around the perimeter of the ablated hole and a smaller taper with the use of laser trepan drilling method in air and under water machining are investigated in this paper.     

Modeling and optimization on Nd:YAG laser turned micro-grooving of cylindrical ceramic material

Nd:YAG laser turning is a new technique for manufacturing micro-grooves on cylindrical surface of ceramic materials needed for the present day precision industries. The importance of laser turning has directed the researchers to search how accurately micro-grooves can be obtained in cylindrical parts. In this paper, laser turning process parameters have been determined for producing square micro-grooves on cylindrical surface. The experiments have been performed based on the statistical five level central composite design techniques. The effects of laser turning process parameters i.e. lamp current, pulse frequency, pulse width, cutting speed (revolution per minute, rpm) and assist gas pressure on the quality of the laser turned micro-grooves have been studied. A predictive model for laser turning process parameters is created using a feed-forward artificial neural network (ANN) technique utilized the experimental observation data based on response surface methodology (RSM). The optimization problem has been constructed based on RSM and solved using multi-objective genetic algorithm (GA). The neural network coupled with genetic algorithm can be effectively utilized to find the optimum parameter value for a specific laser micro-turning condition in ceramic materials. The optimal process parameter settings are found as lamp current of 19 A, pulse frequency of 3.2 kHz, pulse width of 6% duty cycle, cutting speed as 22 rpm and assist air pressure of 0.13 N/mm² for achieving the predicted minimum deviation of upper width of ?0.0101 mm, lower width 0.0098 mm and depth ?0.0069 mm of laser turned micro-grooves.     

Application of laser in seam welding of dissimilar steel to aluminium joints for thick structural components

Laser welding-brazing technique, using a continuous wave (CW) fibre laser with 8000 W of maximum power, was applied in conduction mode to join 2 mm thick steel (XF350) to 6 mm thick aluminium (AA5083-H22), in a lap joint configuration with steel on the top. The steel surface was irradiated by the laser and the heat was conducted through the steel plate to the steel-aluminium interface, where the aluminium melts and wets the steel surface. The welded samples were defect free and the weld micrographs revealed presence of a brittle intermetallic compounds (IMC) layer resulting from reaction of Fe and Al atoms. Energy Dispersive Spectroscopy (EDS) analysis indicated the stoichiometry of the IMC as Fe₂Al₅ and FeAl₃, the former with maximum microhardness measured of 1145 HV 0.025/10. The IMC layer thickness varied between 4 to 21 μm depending upon the laser processing parameters. The IMC layer showed an exponential growth pattern with the applied specific point energy (E_sp) at a constant power density (PD). Higher PD values accelerate the IMC layer growth. The mechanical shear strength showed a narrow band of variation in all the samples (with the maximum value registered at 31.3 kN), with a marginal increase in the applied E_sp. This could be explained by the fact that increasing the E_sp results into an increase in the wetting and thereby the bonded area in the steel-aluminium interface.     

Single-mode fiber design proposal using evolutionary technique with ultra low bending-loss appropriate in FTTH application

A proposal for the new single mode optical fiber containing four cladding layer with ultra low bending loss is presented. The suggested design method is based on the Genetic Algorithm optimization technique. Compared to the work reported in [1], our designed structure exhibits very small bending loss over the wide communication band (1.3–1.65 μm). Simulation results show bending loss of 6.78e?14 dB/turn at 1.55 μm for single turn of 5 mm radius. The best value reported in [1] was 2e?3 dB/turn for the same wavelength and radius of curvature.     

Multiple-objective optimization in precision laser cutting of different thermoplastics

Thermoplastics are increasingly being used in biomedical, automotive and electronics industries due to their excellent physical and chemical properties. Due to the localized and non-contact process, use of lasers for cutting could result in precise cut with small heat-affected zone (HAZ). Precision laser cutting involving various materials is important in high-volume manufacturing processes to minimize operational cost, error reduction and improve product quality. This study uses grey relational analysis to determine a single optimized set of cutting parameters for three different thermoplastics. The set of the optimized processing parameters is determined based on the highest relational grade and was found at low laser power (200 W), high cutting speed (0.4 m/min) and low compressed air pressure (2.5 bar). The result matches with the objective set in the present study. Analysis of variance (ANOVA) is then carried out to ascertain the relative influence of process parameters on the cutting characteristics. It was found that the laser power has dominant effect on HAZ for all thermoplastics.     

Highly flexible,transparent, conductive and antibacterial films made of spin-coated silver nanowires and a protective ZnO layer

We prepared highly flexible, transparent, conductive and antibacterial film by spin coating a silver nanowire suspension on a poly (ethylene terephthalate) (PET) substrate. The ZnO layer covered the conductive silver nanowire (AgNW) network to protect the metal nanowires from oxidization and enhance both wire-to-wire adhesion and wire-to-substrate adhesion. It is found that the number of AgNW coatings correlates with both the sheet resistance (R_s) and the transmittance of the AgNW/ZnO composite films. An excellent 92% optical transmittance in the visible range and a surface sheet resistance of only 9 Ω sq⁻¹ has been achieved, respectively. Even after bending 1000 times (5 mm bending radius), we found no significant change in the sheet resistance or optical transmittance. The real-time sheet resistance measured as a function of bending radius also remains stable even at the smallest measured bending radius (1 mm). The AgNW/ZnO composite films also show antibacterial effects which could be useful for the fabrication of wearable electronic devices.     

Heat affected zone in aluminum single crystals submitted to femtosecond laser irradiations

An experimental procedure is developed to quantify the radial dimension of the heat affected zone (HAZ) in metals submitted to laser pulses. A cube oriented aluminum single crystal is highly deformed by plane strain compression, then micro-drilled by 200 fs or 8 ns laser pulses, and finally analyzed by the electron back scattering diffraction technique. Recrystallized and recovered zones are observed as signatures of the HAZ. A typical value of 1.5 μm is found in the femtosecond regime of illumination, whereas for nanosecond pulses a value of 25 μm is measured. These results are in accordance with previous experiments and numerical simulations.     

Laser micro-machined semi-slinky like MEMS structures: Novel interface coolers

Laser micro-machining has recently been considered a precision and reproducible manufacturing technique in MEMS fabrication because of the superior characteristics of a focused laser beam. It is not only a unique tool but also an invisible optical drill. The aim of the present paper is two-fold: to manufacture novel miniaturized titanium 3D MEMS surface structures in order to increase the cooling performance. Second is to find the behaviors of the operational parameters which controlling the laser-material interaction mechanisms and also suggest the best adjustments in order to achieve this novel semi-slinky like spiral MEMS surface structures with using a 20 W ytterbium fiber laser. Pure titanium micro-MEMS product which has novel interface coolers was manufactured using a ytterbium fiber laser (λ=1060 nm) with 40 ns pulse duration. Best adjustments were, respectively, the pulse duration: 40 ns, the pulse energy: 0.4 mJ, the laser scanning speed: 336.1 mm/s, the peak power density: 17.46 ? 10⁸ W/cm².     

Radiation transfer in metallic powder beds used in laser processing

Two-dimensional radiation transfer in a powder layer backed with a substrate of the same material and normally irradiated with a narrow axially symmetric bell-like or the flat-top laser beam is numerically calculated. This corresponds to physical experiments with the powder layer of 50 μm thickness and the laser beam diameters 60–120 μm. The powder bed is treated as an equivalent homogeneous absorbing scattering medium, the radiative properties of which are estimated from the optical properties of the solid phase and the morphological parameters of the powder bed. The theoretical analysis shows that the absorptance of a semi-infinite powder bed of opaque particles is a universal function of the absorptivity of the solid phase being independent of the specific surface and the porosity. This is confirmed by literature experimental data. The radial transport of the radiative energy due to scattering of the incident laser beam in the powder layer can considerably reduce the deposited energy at the centre of the beam but the widening of the radial profile of the deposited energy is not pronounced. The fraction of laser energy deposited within the projection of the incident laser beam is evaluated. The efficiencies of laser heating the whole powder/substrate system and the substrate decrease with increasing the reflectivity of the material. More uniform heating of the powder layer can be attained at higher reflectivity.     

Factors that influence the surface potential decay on a thin film of polyethylene terephthalate (PET)

The initial potential at the surface of the sample, as well as the temperature and the relative humidity of the ambient air are known to influence the surface-potential decay characteristics of corona-charged thin insulating films. The aim of the present work is to demonstrate the effectiveness of the Experimental Design methodology for evaluating the effects of these factors. Thus, a full factorial experimental design was carried out on a thin film of polyethylene terephthalate (thickness: 0.5 mm; surface: 50 mm × 50 mm). A negative corona discharge produced in a needle–grid–plate electrode system was employed to charge the surface of the film samples. The variation domains for the three factors were respectively: ?1000 V to ?1800 V; 25 to 55 °C; 50% to 80%. The surface-potential decay process was characterized by two output variables: the time needed for the potential to reduce to respectively 50% and 10% of the initial value. It was found that the former is more affected by the temperature, while the latter is more sensitive to the variation of the relative humidity.