The surface topographical, compositional, and structural modifications induced in human enamel by femtosecond laser ablation is studied. The laser treatments were performed using a Yb:KYW chirped-pulse-regenerative amplification laser system (560 fs and 1030 nm) and fluences up to 14 J/cm2. The ablation surfaces were studied by scanning electron microscopy, grazing incidence x-ray diffraction, and micro-Raman spectroscopy. Regardless of the fluence, the ablation surfaces were covered by a layer of resolidified material, indicating that ablation is accompanied by melting of hydroxyapatite. This layer presented pores and exploded gas bubbles, created by the release of gaseous decomposition products of hydroxyapatite (CO2 and H2O) within the liquid phase. In the specimen treated with 1-kHz repetition frequency and 14 J/cm2, thickness of the resolidified material is in the range of 300 to 900 nm. The micro-Raman analysis revealed that the resolidified material contains amorphous calcium phosphate, while grazing incidence x-ray diffraction analysis allowed detecting traces of a calcium phosphate other than hydroxyapatite, probably β-tricalcium phosphate Ca3(PO4)2, at the surface of this specimen. The present results show that the ablation of enamel involves melting of enamel’s hydroxyapatite, but the thickness of the altered layer is very small and thermal damage of the remaining material is negligible.
We study the surface topographical, structural, and compositional modifications induced in bovine cortical bone by femtosecond laser ablation. The tests are performed in air, with a Yb:KYW chirped-pulse-regenerative amplification laser system (500 fs, 1030 nm) at fluences ranging from 0.55 to 2.24 J/cm2. The ablation process is monitored by acoustic emission measurements. The topography of the laser-treated surfaces is studied by scanning electron microscopy, and their constitution is characterized by glancing incidence x-ray diffraction, x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and micro-Raman spectroscopy. The results show that femtosecond laser ablation allows removing bone without melting, carbonization, or cracking. The structure and composition of the remaining tissue are essentially preserved, the only constitutional changes observed being a reduction of the organic material content and a partial recrystallization of hydroxyapatite in the most superficial region of samples. The results suggest that, within this fluence range, ablation occurs by a combination of thermal and electrostatic mechanisms, with the first type of mechanism predominating at lower fluences. The associated thermal effects explain the constitutional changes observed. We show that femtosecond lasers are a promising tool for delicate orthopaedic surgeries, where small amounts of bone must be cut with negligible damage, thus minimizing surgical trauma.
Ti-alloys used in prosthetic applications are mostly alloys initially developed for aeronautical applications, so their
behavior was not optimized for medical use. A need remains to design new alloys for biomedical applications, where
requirements such as biocompatibility, in-body durability, specific manufacturing ability, and cost effectiveness are
considered. Materials for this application must present excellent biocompatibility, ductility, toughness and wear and
corrosion resistance, a large laser processing window and low sensitivity to changes in the processing parameters. Laser
deposition has been investigated in order to access its applicability to laser based manufactured implants. In this study,
variable powder feed rate laser cladding has been used as a method for the combinatorial investigation of new alloy
systems that offers a unique possibility for the rapid and exhaustive preparation of a whole range of alloys with
compositions variable along a single clad track. This method was used as to produce composition gradient Ti-Mo alloys.
Mo has been used since it is among the few elements biocompatible, non-toxic β-Ti phase stabilizers. Alloy tracks with
compositions in the range 0-19 wt.%Mo were produced and characterized in detail as a function of composition using
microscale testing procedures for screening of compositions with promising properties. Microstructural analysis showed
that alloys with Mo content above 8% are fully formed of β phase grains. However, these β grains present a cellular
substructure that is associated to a Ti and Mo segregation pattern that occurs during solidification. Ultramicroindentation
tests carried out to evaluate the alloys' hardness and Young's modulus showed that Ti-13%Mo alloys presented the
lowest hardness and Young's modulus (70 GPa) closer to that of bone than common Ti alloys, thus showing great
potential for implant applications.
A method of automated early fire detection based on the light detection and ranging (lidar) technology is presented.
Specific lidar configurations and their application to forest and industrial-environment fire surveillance are discussed.
A low cost modular system for automatic oil spill detection, based on laser induced fluorescence light detection and
ranging (LIF LIDAR) technology, which may be installed aboard watercraft and used for intensive surveillance of
harborages, rivers, channels, and coastal waters, is described. First experimental results obtained with the developed LIF
LIDAR detector prototype in the laboratory conditions are reported.
A finite element model coupling heat transfer calculations, phase transformations kinetics and internal stresses
calculations to simulate laser powder deposition of a titanium alloy is presented. The model was applied to the study of
the influence of the deposition parameters on the microstructure, hardness and residual stresses in Ti-6Al-4V thin walls
produced by this method.
High wear resistance Al-based composite coatings were prepared by laser cladding using powder mixtures of Al-12 wt.
% Si+40 wt. % TiC and Al-40 wt. % Si+40 wt. % TiC. In the case of the coatings containing 12 % Si, the microstructure
consists essentially of TiC particles dispersed in a matrix of primary α-Al dendrites and interdendritic α-Al+Si eutectic,
whereas the Si rich coatings contain primary Si particles dispersed in the matrix. The low-Si coatings present an average
hardness of 165 HV and an abrasive wear coefficient in a SiC-containing abrasive medium of 1.4×10-3 mm3 (Nm)-1,
while the Si rich coatings present higher hardness of 220 HV but similar wear coefficient. The results show that
increasing the Si content does not improve the abrasive wear resistance of the material, despite the 30% increase in
hardness. This is explained by the fact that the hardness of Si (1200 HV) is much lower than that of the SiC abrasive
particles (2800 HV), thus the primary Si particles do not contribute to reduce the grooving action of the abrasive.
The present paper summarizes the results of a study of the morphological, structural and compositional changes caused
on dentin by processing with KrF excimer laser (λ= 248 nm). Different surface textures are achieved depending on the
structure of the samples and on the processing parameters. Independently of the radiation fluence used, a significant
reduction of the organic material content is observed in a surface layer a few nanometers thick, but no significant
changes in the mineral phase occur.
KEYWORDS: LIDAR, Signal to noise ratio, Signal detection, Flame detectors, 3D modeling, Wind energy, Surveillance, Mathematical modeling, Combustion, Particles
The possibility of early forest fire detection within a range up to ~2 km using a portable eye-safe 1540 nm lidar is
demonstrated in this paper, both on experimental and theoretical ground. An estimation of the detection efficiency using
a mathematical model based on the 3D system of Navier-Stokes equations describing the smoke plume evolution in the
presence of wind, agrees reasonably well with experimental results. Calculations made using the model show that a
detection range up to ~5.5 km can be achieved by accumulating lidar return signals.
KEYWORDS: LIDAR, Detection and tracking algorithms, Signal detection, Interference (communication), Received signal strength, Surveillance, Receivers, Backscatter, Nonlinear optics, Error analysis
A simple and robust algorithm for lidar-signal classification based on the fast extraction of sufficiently pronounced peaks
and their recognition with a perceptron, whose efficiency is enhanced by a fast nonlinear preprocessing that increases the
signal dimension, is reported. The method allows smoke-plume recognition with an error rate as small as 0.31% (19
misdetections and 4 false alarms in analyzing a test set of 7409 peaks).
Laser treatment is a promising technique for dental applications such as caries removal, dental hypersensitivity
reduction and improvement of the bond strength between dentin and restoration materials. In this study the
topographic and morphological changes induced in enamel and dentin surfaces by treating with KrF excimer
laser radiation were studied as a function of the number of laser pulses and radiation fluence by scanning electron
microscopy and optical profilometry. For enamel, independently of the fluence used, material removal occurs
preferentially at the prisms sheaths, leading to the formation of surface pits of a few micrometers. For dentin,
a cone-like topography develops when the tubules are approximately parallel to the laser beam direction and
the radiation fluence is within the range 0.5 to 1.5 J/cm2. For higher fluences, the treated surfaces are flat and
covered with a layer of re-solidified materials.
Detection of fire smoke plume with a compact cheap rangefinder based on 905 nm laser diode (2 μJ pulse energy,
slashed oh 2 cm telescope and 720 m solid-target range) is demonstrated. Reliable detection of small experimental fires
(20×25 m2 fire plot, burning rate of ~3 kg/s) is achieved for the range of about 255 m. A theoretical model of the mixing
of burning products with air in the wind, based on three-dimensional system of Navier-Stokes equation and commercial
software PHOENICS, is developed. The model predicts 220 m range of smoke detection by the rangefinder, indicating
good agreement between the theoretical and experimental data. On the basis of this theoretical model an estimation of
the smoke detection efficiency for a longer range (20 km for solid targets) instrument, based on a 1540 nm laser with a
pulse energy of 8 mJ and a 4 cm telescope, is made. The obtained smoke-detection range estimation, 6 km, indicate that
more powerful rangefinders can be used not only in shot-range applications, such as fire detection in premises, tunnels
and storage yards, but in more demanding areas, such as wildland fire surveillance, as well.
This paper aims to contribute to the understanding of column formation mechanisms in Al2O3-TiC ceramic composites due to processing with excimer laser radiation. The mechanisms proposed in the literature to explain the formation of such columns can be grouped in four categories: shadowing mechanisms, hydrodynamic mechanisms, vapour phase deposition mechanisms, and spatial modulation of absorbed energy mechanisms. In the case of Al2O3-TiC ceramics, the hydrodynamic and vapour phase deposition mechanisms can be excluded because experimental results show that the column core is composed of material in a pristine condition. A theoretical simulation of the spatial modulation of absorbed energy due to radiation reflected from preexisting topographic artefacts reveals that this mechanism can explain the growth of columns from those artefacts, but does not explain column growth in Al2O3-TiC, because it predicts that the height of the columns will increase indefinitely with increasing number of pulses, whereas it has been experimentally observed that columns only grow during the first 100-200 laser pulses. This model does not explain the observed variation of the columns height with laser fluence either. By contrast, predictions of the shadowing mechanism with TiC globules formed during the first laser pulses shielding the substrate and favouring column growth are in semiquantative agreement with experimental observations. The evolution of surface topography in Al2O3-TiC ceramics composite during processing with KrF excimer laser radiation is controlled by the ablation behaviour of individual phases and by the chemical changes of the material surface during laser processing.
It has recently been shown that lidar (LIght Detection And Ranging) can effectively detect smoke plumes from small bonfires up to distances of 6.5 km, so that the technique can be used for wildfire surveillance. The aim of the present work is to describe a method for calculating the optimal location and minimum number of lidar stations required for the surveillance of a given forest area, taking the hilly terrain of Sintra-Cascais Nature Park (Portugal) as an example. The placement and horizontal scanning of the lidar sensors must be such that the laser beam passes over the ground, while keeping sufficiently low to enable early smoke plume detection, before the smoke is dispersed by the wind. Simultaneously, the laser beam should not hit the ground at distances shorter than the instrument range. To solve the problem, a terrain rendering was created and the best laser-beam zenith angle for each azimuth and the effective range covered by each lidar were calculated. The computations showed that 95.2% of the 146 km2 of the Nature Park area can be covered by seven detectors with the laser beams scanning at a height of 50 m or less above ground.
Laser cladding is a coating process which uses a laser beam to melt the coating material and a thin layer of the substrate to form a low dilution, pore- and crack-free coating, perfectly bonded to the substrate. The process may be used for large area coverage, by overlapping individual tracks, but it is the ability to protect localized ares and the wide range of materials that can be deposited that makes laser cladding particularly appropriate to tailor surface properties to local service requirements, opening new perspectives for surface engineering. Laser cladding has been finding widespread use for the protection of materials against wear, corrosion and oxidation, and for the refurbishing of components and tools. Other applications with considerable potential are materials development and synthesis and free-form near-net shape manufacturing. Examples of recent work will be presented and discussed.
Lidar is a promising tool for forest-fire monitoring because this active detection technique allows efficient location of tenuous smoke plumes resulting from forest fires at their early stages. For the technique to be generally usable instrumentation must be eye-safe, i.e. it must operate within the spectral range λ<0.4 or λ>1.4 micrometers . In this paper the lidar efficiency at the wavelengths 0.3472 micrometers (second harmonic of the ruby laser) and 1.54 micrometers (Er:glass laser) are compared using a theoretical model. The results of calculations show that the energy required for smoke-plume detection using 0.3472 micrometers becomes greater than the corresponding value for 1.54 micrometers when the distance exceeds some threshold, which ranges between 2 and 6 km depending on other parameters. Being caused by relatively higher absorption of the UV radiation in the atmosphere, this result is valid for any wavelength in the vicinity of 0.35 micrometers , for example, the third harmonic of Nd:YAG laser and the second harmonic of Ti:sapphire laser.
We produced a totally conservative finite-volume scheme for modeling of Al2O3-TiC ceramics processing with excimer laser radiation. Evolution of a cylindrical TiC grain at the surface of an alumina matrix is traced for a few nanaoseconds. Its melting causes a small surface swelling - a possible nucleator for the experimentally observed TiC globules.
A mathematical model for computation of parameters of eyesafe lidar for detection of forest fire smoke has been developed. It is assumed that the lidar uses a wavelength of 1.54 micrometer. This wavelength can be obtained from Er:glass lasers, from Nd:YAG lasers with an optical parametric oscillator, or from Nd:YAG lasers with a Raman cell. It is assumed that receiver optics of 20 cm diameter and an avalanche photodiode are used. Particle size distributions in the smoke from experiments in the literature are utilized for calculation of backscattering efficiency. The backscattering cross section is calculated on the basis of Mie formulae. Diffusion of the smoke plume is estimated on the basis of an analytical solution of the relevant hydrodynamics equations. Results of the calculations show that for detection of forest fires with fuel mass burned in unit time 2 kg/s at a distance of 10 km it is necessary to have a laser pulse energy of 120 mJ.
This paper presents a study of laser ablation of Al2O3-TiC ceramics. The dependence of the ablation rate and of the surface topography and constitution after ablation as a function of laser fluence and number of pulses was investigated. At low laser fluences, the surface develops a globular topography which is responsible for the decrease of the ablation rate and the increase of roughness. At higher laser fluences, the removal rate and the surface topography slightly depend on the number of pulses and on the laser fluence. The roughness also increases with the number of pulses but smoother surfaces can be achieved.
Laser surface alloying processes all the features of surface melting, plus additional capability of modifying the chemical composition by adding alloying elements which greatly enhance the wear and corrosion resistance of aluminium alloys. This paper is concerned with a study of the formation mechanism and structural changes of the alloyed layer during laser surface alloying aluminium with niobium...The material is formed dendrites of Al3Nb and a small volume fraction of interdendritic (alpha) -aluminium solid solution...Surface alloying was performed by the blown powder technique using a CW 3 kW CO2 laser.
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