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Multiple modes of atomization in electrosprays are affected by viscosity, surface tension and electrical conductivity of the semiconductor nanosuspensions. While the effect of gravity is dominant in the dripping mode, the electric field degenerates the electrospray mechanism into a microdripping mode that can potentially allow the deposition of semiconductor nanodots on a substrate. Drop size and frequency of droplet formation are obtained as functions of non-dimensional parameters, which agree well with experimental data. The analysis shows that it is possible to produce the desired size and frequency of ejection of monodisperse droplets by manipulating the electrode voltage for any nanosuspension.

An analytic solution is obtained for three-dimensional quasi-steady state temperature distribution during laser heating of moving thin wires. The wire moves at a constant speed through a vacuum chamber, which is back-filled with an inert gas such as argon, and a laser beam of rectangular cross-section is incident on the wire. The ambient gas provides a convection heat transfer mechanism, which yields a Biot number, Bi, for the heating process to determine whether the temperature distribution would be uniform or nonuniform in the cross-section of the wire. Generally, the criterion of Bi less than 0.1 is applied to assume spatially uniform temperature distribution in a solid. The temperature distribution is determined for different Bi numbers and the variation of the temperature in the azimuthal direction is analyzed. The method of solution involves the Fourier transform in the azimuthal direction and the Hankel transform in the radial direction for a three-dimensional quasi-steady state heat conduction equation containing an advection term that accounts for the motion of the wire. The thermal and optical properties of the material is assumed to be constant in the temperature range of this study. The heat loss due to radiation heat transfer between the wire surface and the surrounding environment is neglected due to the small laser-heated surface area. Using this model, the temperature profile is studied for different process parameters such as the incident laser power, laser beam profile, Biot number, and wire speed.

Abstarct Laser diffusion has previously been studied to incorporate dopants in semiconductors and to carburize steel for surface hardening without melting the substrate, among others. The optical and electromagnetic properties of materials can also be modified by this diffusion method to tailor the material response at different frequencies of the electromagnetic spectrum. Platinum atoms have been diffused into titanium and tantalum sheets by a laser chemical vapor diffusion method using a metallorganic compound of platinum and laser beams of different polarizations. Thermal decomposition of the precursor at the laser-heated spot on the surface of the substrate generates platinum atoms that diffuse into the substrate, producing laser-platinized samples. The transmittances of the samples are determined by measuring the strength of the transmitted magnetic field oscillating at 63.86 MHz. The laser-platinized samples produced by linearly polarized lasers exhibit higher transmittances than the samples obtained by using azimuthally polarized lasers.

Laser diffusion is generally used to modify the metallurgical composition at the surface of materials for improving the mechanical properties. Platinum has been diffused into titanium and tantalum sheets in this study, and the concentrations of Pt in the substrates are determined. The concentration of Pt is higher at lower scanning speeds due to higher surface temperature and longer diffusion time than in the case of higher scanning speeds. Additionally, the samples treated with a linearly polarized laser beam exhibit slightly higher concentration of Pt. The enhanced diffusion in the case of linearly polarized laser treatment can be attributed to controlled excitation of the local vibration modes of the atoms in the substrate. The reflec- tivity of the samples are also measured at the wavelength of 1,064 nm and compared with theoretical results.

An n-type 4H–SiC substrate has been doped with gallium using a continuous wave Nd:YAG laser to heat the sample to high temperatures but below the peritectic temperature of SiC. Mathematical models have been presented for the temperature and Ga concentration distributions in the sample. The Ga atoms, which are produced due to the thermal decomposition of a metallorganic precursor, diffuse into the sample by the solid-phase diffusion process at high temperatures. This process is modeled by considering the temperature-dependent diffusion coefficient and the Ga concentration profile was measured by the secondary ion mass spectrometry (SIMS). The concentration of Ga (6.25 × 1020 cm−3) at the substrate surface was found to exceed the solid solubility limit (1.8 × 1019 cm−3) of Ga in SiC. Comparing the SIMS data to the results of the diffusion model, the activation energy, pre-exponential factor and diffusion coefficient of Ga were determined for different doping conditions. Four doped samples were produced by scanning the samples with a laser beam for different number of passes. The sample prepared with four passes showed the highest diffusion coefficient of 5.53 × 10−7 cm2/s with activation energy 1.84 eV and pre-exponential factor 1.05 × 10−2 cm2/s. The diffusion coefficient is five orders of magnitude higher than the typical diffusion coefficient of Ga in SiC. This indicates that the laser doping process enhances the diffusion coefficient of dopant significantly.

Many from within manufacturing industry consider superplastic forming (SPF) to be ‘high tech’, but it is often criticized as too complicated, expensive, slow and, in general, an unstable process when compared to other methods of manipulating sheet materials. Perhaps, the fundamental cause of this negative perception of SPF, and also of diffusion bonding (DB), is the fact that the current process of SPF/DB relies on indirect sources of heating to produce the conditions necessary for the material to be formed. Thus, heat is usually derived from the electrically heated platens of hydraulic presses, to a lesser extent from within furnaces and, sometimes, from heaters imbedded in ceramic moulds. Recent evaluations of these isothermal methods suggest they are slow, thermally inefficient and inappropriate for the process. In contrast, direct heating of only the material to be formed by modern, electrically efficient, lasers could transform SPF/DB into the first choice of designers in aerospace, automotive, marine, medical, architecture and leisure industries. Furthermore, ‘variable temperature’ direct heating which, in theory, is possible with a laser beam(s) may provide a means to control material thickness distribution, a goal of enormous importance as fuel efficient, lightweight structures for transportation systems are universally sought. This paper compares, and contrasts, the two systems and suggests how a change to laser heating might be achieved.

The process of Superplastic Forming (SPF) has produced a vast number of components, from simple shapes to those with complex geometric features, all to a dimensional accuracy difficult to obtain by traditional sheet metal methods. When used in conjunction with Diffusion Bonding (DB), extraordinarily lightweight, stiff, strong and fatigue enduring components and structures can be manufactured that are virtually impossible to produce by any other means. However, SPF/DB is often criticised as too complicated, expensive, slow and unstable, a reputation that has contributed to its use being largely confined to manufacturing when no other means of production can be found. Perhaps, the fundamental cause of this negative perception of SPF/DB is the fact that derivatives of mid 18th century hydraulic presses are used, virtually universally, that provide only a single temperature to the work piece, which is heated primarily through conduction, a form of heating which is slow and expensive. By comparison, modern, electrically efficient, lasers, if used to directly heat the work piece, could transform SPF/DB into a mainstream process and the first choice of designers. This paper compares, and contrasts, the two systems.

Thermal barrier coatings are generally designed to possess very low thermal conductivity to reduce the conduction heat transfer from the coating surface to the metal turbine blade beneath the coating. In high-temperature power generation systems, however, a considerable amount of radiative heat is produced during the combustion of fuels. This radiative heat can propagate through the coating and heat up the metal blade, and thereby reduce the effectiveness of the coating in lowering the thermal load on the blade. Therefore, radiative properties are essential parameters to design radiative barrier coatings. This article presents a combined radiation and conduction heat transfer model for the steady-state temperature distribution in semitransparent yttria-stabilized zirconia (YSZ) coatings. The results of the model show a temperature reduction up to 45 K for YSZ of high reflectance (80%) compared to the YSZ of low reflectance (20%). The reflectivities of YSZ and metal blade affect the temperature distribution significantly. Additionally, the absorption and scattering coefficients of YSZ, the thickness of the coating, and the thermal conductivities of YSZ and metal blade affect the temperature distribution.

A laser solid phase diffusion technique has been utilized to fabricate endolayers in n-type 6H-SiC substrates by carbon incorporation. X-ray energy dispersive spectroscopic analysis shows that the thickness of the endolayer is about 100 nm. High resolution transmission electron microscopic images indicate that the laser endotaxy process maintains the crystalline integrity of the substrate without any amorphization. The resistivity of the endolayer formed in a 1.55 Ω cm silicon carbide wafer segment was found to be 1.1×105 Ω cm, which is sufficient for device fabrication and isolation. Annealing at 1000 °C for 10 min to remove hydrogen resulted in a resistivity of 9.4×104 Ω cm. The endolayer and parent silicon carbide epilayer were doped with aluminum using a laser doping technique to create p-regions on the top surfaces of the substrates in order to fabricate p-type-intrinsic type-n-type (PIN) diodes. The current-voltage characteristics of these diodes were compared with other PIN diodes fabricated using epilayers and other doping techniques.A laser solid phase diffusion technique has been utilized to fabricate endolayers in n-type 6H-SiC substrates by carbon incorporation. X-ray energy dispersive spectroscopic analysis shows that the thickness of the endolayer is about 100 nm. High resolution transmission electron microscopic images indicate that the laser endotaxy process maintains the crystalline integrity of the substrate without any amorphization. The resistivity of the endolayer formed in a 1.55 Ω cm silicon carbide wafer segment was found to be 1.1×105 Ω cm, which is sufficient for device fabrication and isolation. Annealing at 1000 °C for 10 min to remove hydrogen resulted in a resistivity of 9.4×104 Ω cm. The endolayer and parent silicon carbide epilayer were doped with aluminum using a laser doping technique to create p-regions on the top surfaces of the substrates in order to fabricate p-type-intrinsic type-n-type (PIN) diodes. The current-voltage characteristics of these diodes were compared with other PIN diodes fabricated using ...

Chromium, a p-type dopant, has been incorporated into silicon carbide by laser doping. Secondary ion mass spectrometric data revealed enhanced solid solubility (2.29 × 1019 cm−3 in 6H–SiC and 1.42 × 1919 cm−3 in 4H–SiC), exceeding the equilibrium limit (3 × 1017 cm−3 in 6H–SiC above 2500 °C). The roughness, surface chemistry and crystalline integrity of the doped sample were examined by optical interferometry, energy dispersive X-ray spectrometry and transmission electron microscopy, respectively, and showed no crystalline disorder due to laser heating. Deep-level transient spectroscopy confirmed Cr as a deep-level acceptor with activation energies Ev + 0.80 eV in 4H–SiC and Ev + 0.45 eV in 6H–SiC. The Hall effect measurements showed that the hole concentration (1.942 × 1019 cm−3) is almost twice the average Cr concentration (1 × 1019 cm−3), confirming that almost all of the Cr atoms were completely activated to the double acceptor state by the laser-doping process without requiring any additional annealing step.

A single-oscillator Lorentz model is applied to four different semiconductors having diamond-like crystal structure to describe the temperature dependence of their refractive index between 300 and 600 K. Theoretical results are compared to previous experiments and to experiments carried out in this study for Si, Ge, GaAs, and InP. An efficient experimental method is also presented, enabling fast measurements of the refractive index of materials. Using the Yu-Brooks formalism and the energy bandgap at the X-point of the Brillouin zone, the temperature-dependent refractive indices are calculated and they agree well with experiments, particularly, considering the simplicity of the Lorentz model. However, there are discrepancies between the theory and experiment at high temperatures (near 600 K) in certain cases. This discrepancy may be due to the single-oscillator approximation. Additionally the effect of “self-energy” on the temperature dependence of the energy bandgap, such as the temperature-dependent damping of the oscillation of electrons, can be significant at higher temperatures.

Laser doping has been utilized for fabricating white light emitting diodes with 6H-SiC wafers. The emission of different colors to ultimately generate white light is tailored on the basis of donor acceptor pair (DAP) recombination mechanism for luminescence. Chromium (Cr), which is an unconventional dopant that produces multiple acceptor sites per atom, was incorporated into SiC and conventional dopants such as aluminum (Al) and nitrogen (N) were also laser-doped to produce acceptor and donors states, respectively. A p−n junction was fabricated with these dopants and an electroluminescent broad spectrum (400–850 nm) corresponding to white light was observed. This white light is a result of the combination of red, green, and blue lights formed due to DAP recombination between Al–N (blue, 460–498 nm), Cr–N (green, 521–575 nm) and additional other impurity state transitions (red, 698–738 nm).

The electromagnetic field of lasers and non-equilibrium doping conditions enable laser doping of SiC with increased dopant diffusivity. Chromium, which acts as a double acceptor, has been laser-doped in SiC wafers. A thermal model is utilized to determine the temperature distribution at various depths of the wafer and a diffusion model is presented including the effects of Fickian diffusion, laser electromagnetic field and thermal stresses due to localized laser heating on the mass flux of dopant atoms. The dopant diffusivity is calculated as a function of temperature at different depths of the wafer based on measured dopant concentration profile. The maximum diffusivities achieved in this study are 4.61 × 10 −10 cm 2 s −1 at 2898 K and 6.92 × 10 −12 cm 2 s −1 at 3046 K for 6H-SiC and 4H-SiC, respectively. The maximum concentration is found to be 2.29 × 10 19 cm −3 for 6H-SiC, which is two orders of magnitude higher than the reported value (3 × 10 17 cm −3 solid solubility limit).

Determination of the thermophysical properties of thin film materials is important for modeling and optimizing laser microvia drilling of organic substrates in microelectronics applications. Techniques to measure the density, thermal conductivity, thermal diffusivity, thermal decomposition point, and specific ablation heat of thin polymer films are described. An experimental apparatus was set up for laser heating of the sample. To measure the thermal diffusivity, an analytic heat transfer model is developed. One-dimensional heat conduction is assumed due to the small thickness of the film compared to the radius of the laser beam. The value of thermal diffusivity is obtained by fitting the experimental data to the theory. The specific ablation heat is obtained by measuring the mass loss during laser ablation. The experimental apparatus and the property determination methodology can also be applied to thin samples of other materials.

The temperature-dependent optical properties of materials, such as refractive index and reflectivity, can be used for remote sensing of temperatures using a laser beam. Wireless sensing mechanism eliminates the need for external electrical contacts to the sensor, typically required in electrical property-based sensors. Such contacts tend to melt at elevated temperatures. This paper investigates the optical response of silicon carbide to temperature changes, indicating the potential for realizing silicon carbide-based high temperature wireless sensors by using a helium?neon laser of wavelength 632.8?nm. The laser power reflected by single crystal 6H?silicon carbide substrates has been measured at different temperatures up to 750??C. The reflected powers exhibited oscillatory patterns caused by multiple beam interference for which the substrate itself acts as a Fabry?P?rot etalon. These results were used to calculate the refractive index, reflectivity and thermo-optic coefficients of silicon carbide as a function of temperature. Two samples with different dopant profiles were investigated to examine the effects of dopants on the optical response of silicon carbide substrates. With proper modification of the microstructure in the substrate, the temperature dependence of reflectivity can be used directly to measure temperature remotely.

The diffusivities of different types of dopant atoms in silicon carbide wafers are generally very low. Nd:YAG and excimer lasers have been used to dope silicon carbide with nitrogen and aluminum, respectively. Mathematical models have been presented for the temperature distributions in the wafers to understand the diffusion mechanisms in the laser doping process. Since the silicon carbide substrate reaches its peritectic temperature (3100 K) at irradiances of 80.6 and 61 MW/cm2 for Nd:YAG and excimer lasers, respectively, lower irradiances were used to achieve solid-state diffusion. The Nd:YAG laser doping process doped nitrogen to a depth of 800 nm; the KrF excimer laser doping process produced aluminum dopant depths of 200 and 450 nm for different numbers of laser pulses. Two distinct diffusion regions, near-surface and far-surface regions, were identified in the dopant concentration profiles, indicating different diffusion mechanisms in these two regions. The effective diffusion coefficients of nitrogen and aluminum were determined for both regions and found to be 2.4 × 10−5 and 9.2 × 10−6 cm2/s in the near- and far-surface regions for nitrogen, respectively, and 1.2 × 10−5 and 1.3 × 10−6 cm2/s in the near- and far-surface regions for aluminum, respectively. The calculated diffusivities are at least six orders of magnitude higher than the typical values for nitrogen and aluminum, which indicate that the laser doping process enhances significantly the diffusion of dopants in silicon carbide.

Reduced melting temperature of nanoparticles is utilized to deposit thin polycrystalline silicon (c-Si) films on plastic substrates by using a laser beam without damaging the substrate. An aqueous dispersion of 5 nm silicon nanoparticles was used as precursor. A Nd:YAG (1064 nm wavelength) laser operating in continuous wave (CW) mode was used for thin film formation. Polycrystalline Si films were deposited on flexible as well as rigid plastic substrates in both air and argon ambients. The films were analyzed by optical microscopy for film formation, scanning electron microscopy (SEM) for microstructural features, energy dispersive spectroscopy (EDS) for impurities, X-ray photoelectron spectroscopy (XPS) for composition and bond information of the recrystallized film and Raman spectroscopy for estimating shift from amorphous to more crystalline phase. Raman spectroscopy showed a shift from amorphous to more crystalline phases with increasing both the laser power and irradiation time during laser recrystallization step.

Polycrystalline silicon films containing cubic silicon crystallites of size 3–4 µm have been formed on nickel substrates by fusing and sintering silicon nanoparticle precursors using a laser. A mechanism for the fusion and sintering of these nanoparticles, resulting in reduced heat input and continuous film formation by surface and grain boundary diffusion, is discussed. Films were characterized by optical microscopy, scanning electron microscopy, energydispersive spectroscopy, and Raman spectroscopy. Films were doped with n- as well as p-type dopants by using a laser doping technique and their current-voltage (I–V) characteristics were measured.

Highly conductive phases have been generated on different polytypes of SiC substrates using a laser direct-write technique. Incorporation of both n-type and p-type impurities into the SiC substrates was accomplished by laser irradiation in dopant-containing ambients. X-ray diffraction, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy have been used to detect the presence of the dopant atoms and the compositional variation induced by laser irradiation. Scanning electron microscopy was used to study the microstructure, morphology and dimensions of the converted regions. The conversion in electric resistance has been attributed to both structural and compositional variations observed for the irradiated tracks

An excimer laser and Nd:YAG laser were chosen to dope SiC polytypes by nitrogen (n-type dopant) and aluminum (p-type dopant). In the case of p-type doping, three different doping methods, i.e., (i) doping from trimethylaluminum (TMA) precursor, (ii) an effusion–diffusion process and (iii) doping from Al powder precursor, were used and compared. The results show that Nd:YAG laser treatment provides higher dopant concentration and deeper dopant penetration depth than the excimer laser treatment for both nitrogen and aluminum dopants. This difference is attributed to different heating mechanisms for the two kinds of laser sources. In the case of p-type doping by excimer laser, the Al dopant concentration distributions along the depth of wafer are different for the three doping methods. Method 3 produces the highest Al concentration, which is related to the high concentration of Al due to the formation of a molten aluminum film at the wafer surface. The Al concentration obtained by the effusion–diffusion process (method 2) is higher than that obtained by the TMA precursor method due to a higher Si vacancy concentration created by the effusion process. Shallow and deep junctions fabricated by an excimer laser and a Nd:YAG laser can be used for different semiconductor devices.

Silicon carbide PIN diodes have been fabricated using a direct-write laser-doping technique that reduces defect generation compared to the conventional ion-implantation technique. Nitrogen and aluminum were successfully incorporated into SiC as n-type and p-type dopants, respectively. Rutherford backscattering studies were conducted to compare the lattice defect generation by the laser-doping and ion-implantation techniques. No amorphization was observed in laser-doped samples, eliminating the need for high-temperature annealing. The current-voltage (I-V) and capacitance-voltage (C-V) characteristics of the PIN diodes were also investigated. The silicon carbide diodes are intended for high-temperature and high-voltage power electronics applications.

Drawability of Nickel 200 wires in laser dieless drawing was investigated. Influencing factors under consideration include the laser power, the heat-treatment state (as-drawn or annealed), and the initial wire diameter. Microstructural evolutions in laser dieless drawing were studied by scanning electron microscopy (SEM). The wires exhibit optimal drawability at an intermediate laser power range corresponding to the wire temperature in the range of 1000–1300 K. The as-drawn precursor wire has better drawability than that of the annealed wire. The drawability decreases as the precursor wire diameter deceases. Microcrystalline structures were found in nickel 200 wires after being laser-drawn from as-drawn precursor wires. These experimental observations are explained using the concepts of dynamic recovery and recrystallization.

A nanosecond pulsed laser direct-write and doping (LDWD) technique is used for the fabrication of carbon-rich silicon carbide nanoribbons heterostructure in a single crystal 4H–SiC wafer. Characterization by high-resolution transmission electron microscope and selected area electron diffraction pattern revealed the presence of nanosize crystalline ribbons with hexagonal graphite structure in the heat-affected zone below the decomposition temperature isotherm in the SiC epilayer. The nanoribbons exist in three layers each being approximately 50–60 nm thick, containing 15–17 individual sheets. The layers are self-aligned on the (0001) plane of the SiC epilayer with their c axis at 87° to the incident laser beam. The LDWD technique permits synthesis of heterostructured nanoribbons in a single step without additional material or catalyst, and effectively eliminates the need for nanostructure handling and transferring processes.

Mathematical models are developed to calculate the temperatures, pressures and stresses during laser shock processing for time-modulated (ramp-up, ramp-down and rectangular) laser pulses. Three different shock processing configurations are also considered: non-ablative exposure, ablative exposure and confined ablation with coating. The results for iron show that the plasma pressure reaches an average value of 9 GPa in direct ablation configuration and plays a dominant role for all three types of laser pulses. In the case of confined geometry, the plasma pressure reaches an average value of 20 GPa. All calculated pressures and stresses exceed the yield strength of the workpiece, indicating plastic deformation. It is also shown that pulses with short rise times yield higher plasma pressures.

A new dieless metal fiber drawing process is developed in which a high-precision laser is used to heat the precursor wire to its superplastic temperature. The advantages of this technique over conventional cold drawing include larger diameter reduction ratio, no dies and lubricants, and the flexibility to draw fine wires of different diameters with the same apparatus. It is a promising process for the fabrication of fine wires that are difficult to draw with conventional processes or that require a high-quality surface without any contamination. The temperature profile generated by laser heating is critical in determining the plasticity and deformation behavior of the wire during the drawing process. The coupled thermomechanical problem is solved numerically for the temperature distribution and wire diameter reduction. Commercially pure nickel wires of initial diameter 125 μm are used as samples for laser drawing experiments, and the experimental results are compared with the model predictions. The wire drawing setup also provides a novel apparatus for evaluation of mechanical properties at high temperatures.

A novel dieless drawing technique is presented for processing contamination-free metal wires with controlled surface chemistry. Such wires are particularly useful to produce filters for high-pressure and high-temperature corrosive filtration applications, catalysts, micron size electrical conductors, and ornaments with gold fibers requiring good optical surface properties. Laser wire drawing is a non-contact process in which a laser beam is used to heat the wire to a critical temperature range corresponding to its plastic state. This process is generally good for small diameter wires. Due to high precision heating capability of lasers, the temperature of the wire can be maintained at a desired level to achieve maximum ductility. Ni-200 wires of initial diameters 500 and 125 μm, respectively, were used to develop a dieless laser wire drawing process. The maximum diameter reduction ratio, the effect of drawing environment on the surface quality and the diameter uniformity of the resultant wire were analyzed. An oxidation prevention device was fabricated to attain large cross-sectional area reduction ratio without surface oxidation. The diameter of the wire becomes more non-uniform as the diameter reduction ratio increases. This is a flexible technique because a single apparatus can be used to draw various wires of different elements and alloys.

A direct-write laser conversion technique was used to produce n-type and p-type doped tracks on SiC substrates. Polycrystalline and single-crystal SiC substrates were investigated. The tracks irradiated in an inert gas exhibit a higher electrical resistance than those generated in dopant-containing atmospheres. The effects of various processing parameters, such as the laser-matter interaction time, laser-beam characteristics, number of exposures to the laser beam, and ambient gas in the laser processing chamber, are examined. The laser conversion technique can be used for selective-area doping of silicon carbide substrates to build semiconductor devices for high-temperature applications.

Several laser-based techniques to fabricate parts by depositing metals or ceramic powders or a combination thereof have been developed in recent years. These fabrication techniques are incomplete and not fully useful to an operator without any predictive capability to calculate the geometries of the fabricated parts or equations to calculate their expected yield and ultimate strengths. Data concerning the energy transfer from the processing laser beam to the material powder, such as the metal vapor-plasma plume temperature and plume absorption coefficient, the efficiency of laser energy transfer and mathematical analysis for the thermal and dimensional process characteristics are unavailable. Also the characterization of the mechanical properties of such laser-fabricated parts has just begun. A one-dimensional model to calculate the thermal and dimensional process characteristics is developed. The model accounts for the transmission of the laser beam through the plume, energy transfer in the molten phase and the Stefan conditions at the solid-liquid and liquid-vapor interfaces. The yield and ultimate strengths of laser-fabricated stainless steel (SS 304) parts have been measured. A mathematical model is developed accounting for directionally preferred solidification to calculate the residual stresses generated in the part during solidification.Several laser-based techniques to fabricate parts by depositing metals or ceramic powders or a combination thereof have been developed in recent years. These fabrication techniques are incomplete and not fully useful to an operator without any predictive capability to calculate the geometries of the fabricated parts or equations to calculate their expected yield and ultimate strengths. Data concerning the energy transfer from the processing laser beam to the material powder, such as the metal vapor-plasma plume temperature and plume absorption coefficient, the efficiency of laser energy transfer and mathematical analysis for the thermal and dimensional process characteristics are unavailable. Also the characterization of the mechanical properties of such laser-fabricated parts has just begun. A one-dimensional model to calculate the thermal and dimensional process characteristics is developed. The model accounts for the transmission of the laser beam through the plume, energy transfer in the molten phase ...

A direct conversion technique has been demonstrated to produce highly conductive tracks on silicon carbide by irradiating it with a laser beam. It is found that laser irradiation of insulating silicon carbide substrates decreases its resistivity from 1011 to 10−4 Ω cm. Scanning electron microscopy of laser-irradiated α-silicon carbide substrate reveals dispersed globules on the irradiated track. The atomic force microscopic images of the tracks indicate the conversion of larger structures into smaller, more round-shaped structures suggesting the formation of globules. However, laser irradiation of silicon carbide conductors in the presence of pure oxygen transforms the conducting track into an insulator. The effect of annealing on the electrical properties of the laser-generated conducting tracks is also examined. This technique provides a means of directly writing conducting and insulating tracks on silicon carbide to produce electronic devices for high temperature applications.

This note is concerned with the three-dimensional quasi-steady-state heat conduction equation subject to certain boundary conditions in the wholex′y′-plane and finite inz′-direction. This type of boundary value problem arises in laser welding process. The solution to this problem can be represented by an integral using Fourier analysis. This integral is approximated to obtain a simple analytic expression for the temperature distribution.

Lasers have been established as a commercial tool for drilling holes in a variety of materials. The beam quality and temporal shape of the laser pulse have been identified as critical parameters to create high quality (deeper holes, minimum taper angle, and minimum recast layer) holes. However, little attention has been directed toward an analytic investigation of the effect of different temporal pulse shapes on these product variables. In this study, a leading-edge, sharp-spiked, and a rectangular Nd:YAG laser pulse were used to drill holes in stainless steel 304 to examine the effects of these temporally different laser pulses on the drilling process. It is found that holes drilled with the sharp-spiked laser pulse are significantly deeper than for the case of rectangular pulses. Also an optimum processing regime is identified to drill holes with minimum taper angle and recast layer. A mathematical model describing the drilling process is presented in this article for different temporal profiles of the ...

A simple mathematical model is developed to relate kerf depth to laser power, laser scanning speed and kerf width during laser cutting. The model is based on a lumped parameter technique in which the overall energy balance is considered. The energy released during the chemical reaction between the material that is being cut and the assist gas is considered for oxygen-assisted cutting. Experiments were conducted using a carbon dioxide (CO2) laser to test the significance of the chemical reaction in the cutting process. The experiments conducted consist of cutting stainless steel using nitrogen as an inert assist gas, and the cutting of stainless steel using oxygen as a reactive assist gas. The cutting data between the two experiments are compared and used to demonstrate the scaling of the mathematical model. Oxygen increases the cut depth significantly compared to when nitrogen is used as an assist gas.

A chemical oxygen-iodine laser (COIL) was used for cutting aluminum, titanium, inconel and copper plates. The laser was operated with a stable resonator having an intracavity aperture to produce a circular COIL beam with very few transverse modes. The multimode focal spot diameter was calculated and measured to be approximately 0.24 mm. The new aluminum cut was of good kerf edge quality. These COIL cutting data are compared with an existing theoretical laser cutting model. Using thermophysical data for aluminum, titanium, inconel and copper, this theory agrees very well with the data. To test the versatility of the model, the effects of different assumptions are examined; different assumptions produced very little effect on model predictions at high cutting speeds and a small difference at very slow cutting speeds. Overall, the theoretical model provides good agreement with experiments for a wide variety of metals.

Laser-aided direct fabrication technology is developed to produce three-dimensional metal alloy structures directly from the constituent elements. The shapes of the structures are designed using Computer Aided Design (CAD) tools and are fabricated as real parts by fusing powders with a laser beam. In this study, alloy structures of simple plate like geometry were produced by mixing stainless steel (SS 316) powder with either copper or aluminum powder. The thickness of the component is related to the process parameters through an energy balance equation, which is verified against experimental data. Material properties such as hardness, strength and conductivity are analyzed and optical micrographs are presented to exhibit the microstructure of the alloy. The results show improved hardness at certain locations in the alloy, higher thermal conductivity of stainless steel. However, the structures broke due to brittle failure under tensile loads.

Molten material is removed in many laser applications such as laser grooving and laser cutting. An assist gas is usually used to remove the molten material from the laser material interaction zone. The effect of assist gas pressure on the material removal rate is investigated in this article for laser cutting and grooving applications. The model for melt depth is based on the overall energy balance, and the cut depth is obtained by considering the effect of the assist gas. The model for kerf width is based on the modified Rosenthal solution taking into account the melting effect. The cut depths reach a constant value beyond a critical pressure if the kerf width is of the order of nozzle width and through cuts are assumed. Most of the molten material is removed by the assist gas at pressures below this critical pressure. The model predicts on the basis of the Prandtl or Meyer relation that the cutting speed decreases when the assist gas pressure exceeds a critical value if the kerf dimensions are smaller than the nozzle dimensions.

A mathematical analysis describing the densification of ceramic coating processed with a moving TEM00, mode laser beam has been presented in this paper. Calculations of the depth of the heat affected zone in ZrO2, A12O3 and TiO2 ceramic substances is performed by adopting a three-dimensional quasi-steady state heat conduction model based on the mechanism of viscous solid-phase densification. Also, a working range for these ceramics coating densification is predicted by varying the laser beam power, spot size and scanning velocity. The heat-affected layer in ZrO2 compact is found to be confined within a depth of 50 μm from the surface and it is fully densified for grains of initial radius 5 μm. A simple expression for temperature distribution is presented, which is found to provide sufficiently accurate estimates within a limited region around the laser spot.

The most widely used high power industrial lasers are the Nd:YAG and CO2 lasers. The chemical oxygen iodine laser (COIL), whose wavelength (1.315 μm) is between that of the Nd:YAG (1.06 μm) and CO2 (10.6 μm) lasers, is another high power laser for industrial applications. The cutting capability of these lasers is investigated in this paper. The cut depth strongly depends on the absorptivity of the cut material, kerf width and cutting speed. The absorptivity is an unknown parameter for which experimental data at high temperatures are currently unavailable. Theoretical values of the absorptivities of various metals are obtained using the Hagen‐Ruben relationship. It is found that the absorptivity of a metal is linearly proportional to the square root of its resistivity and also inversely proportional to the square root of the wavelength. The absorptivities of the COIL and Nd:YAG lasers are 2.84 and 3.16 times larger than that of the CO2 laser, respectively. Based on these theoretical values of the absorptivity, the cut depths for several metals are analyzed at various laser powers and cutting speeds for these lasers. For identical cutting parameters, the cut depths for stainless steel and titanium are deeper than those of most other metals. Due to the wavelength dependence of the absorptivity, the cut depths for COIL and Nd:YAG lasers are expected to be 2.84 and 3.16 times deeper than that for the CO2 laser.

A theoretical analysis describing the effects of pulse shaping on keyhole laser welding is presented. Calculation of the keyhole profile and melt pool dimensions was performed by the balancing of surface forces and energy at the liquid‐vapor and solid‐liquid interfaces in three dimensions. The effect of temporal pulse shape on weld dimensions has been examined over a range of power densities, pulse times and pulse frequencies. Several pulse types (e.g., top‐hat, Gaussian, ramp‐up and ramp‐down pulses) have been considered. Pulse shaping showed significant effect on the weld dimensions. The predicted width and penetration depth of welds with ramp‐up and ramp‐down pulse shapes were greater than those obtained with Gaussian and top‐hat pulses. The theoretical predictions have been compared with experimental data. Furthermore, the response of two different materials with considerably different physical properties (stainless steel and aluminum) has been analysed.

A simple mathematical model of thick‐section stainless steel cutting with a high power chemical oxygen–iodine laser (COIL) is presented and compared with experimental results obtained with a 10‐kilowatt COIL at the U.S. Air Force's Phillips Laboratory. This model uses a lumped‐parameter technique to relate the cutting kerf depth with various process parameters and can be used to predict scaled laser materials processing performance to very thick sections. The model is similar to an empirical model developed by researchers in Japan, but includes predictive capabilities for thick metal cutting at very low velocities. The effects of various process parameters such as laser power, spot size and dimensions, and processing speed in the cutting depth are discussed and demonstrated. Finally, the ramifications of this model on thick‐section processing of metals are presented, with emphasis on potential applications of COIL to high‐speed, thick stainless steel cutting.

Acousto-optic deflectors are photonic devices that are used for scanning high-power laser beams in advanced microprocessing applications such as marking and direct writing. The operation of conventional deflectors mostly relies on one-dimensional sinusoidal variation of the refractive index in an acousto-optic medium. Sometimes static phased array transducers, such as step configuration or planar configuration transducer architecture, are used to tilt the index modulation planes for achieving higher performance and higher resolution than a single transducer AO device. However, the index can be modulated in two dimensions, and the modulation plane can be tilted arbitrarily by creating dynamic phase gratings in the medium using phased array transducers. This type of dynamic two-dimensional acousto-optic deflector can provide better performance using, for example, a large deflection angle and high diffraction efficiency. This paper utilizes an ultrasonic beam steering approach to study the two-dimensional strain-induced index modulation due to the photoelastic effect. The modulation is numerically simulated, and the effects of various parameters, such as the operating radiofrequency of the transducers, the ultrasonic beam steering angle, and different combinations of pressure on each element of the transducer array, are demonstrated.

Laser welding is a unique way of joining materials with less thermal distortion and minimum metallurgical damage to the workpiece. The molten pool formed during welding determines the shape of the final welded region. At high laser intensities, the molten material vaporizes and a key hole is formed during the welding process. This vapor and the shape of the molten pool affect the absorption of laser at the liquid surface. The forces generated at the liquid‐vapor interface due to surface tension gradient induce thermocapillary convection in the weld pool. This paper presents a mathematical model by considering these surface forces and the energy balance at the liquid‐vapor and solid‐liquid interfaces. The model is used to predict the surface velocity and temperature distributions, weld pool shape, key‐hole depth and diameter. The velocity field is found to be large in the radial and azimuthal directions before the key hole is formed, and it changes to a radially and axially dominant field after the formation of the key hole. The results of this model are also compared with experimental data.

A mathematical model for the concentration profile of TiCl4 at the top surface of an Incoloy 800H substrate placed inside a laser chemical vapor deposition (LCVD) reactor is developed by using the three‐dimensional transient mass diffusion equation. The model is used for studying the spatial variation of the thickness of TiN dots deposited by LCVD, using a CO2 laser and a reactive atmosphere consisting of TiCl4, N2, and H2. By assuming that the chemical reaction is first‐order with respect to TiCl4, and that the sticking coefficient of TiN at the substrate surface is temperature dependent, the deposited TiN film is found to have a volcanic profile under certain conditions, which is in good agreement with experimental results.

A mathematical model for mass transfer inside a laser chemical vapor deposition (LCVD) chamber is presented for pure titanium deposition from titanium tetrabromide on a stainless‐steel (SS 304) substrate. A set of five three‐dimensional, transient, and nonlinear partial differential equations is solved to account for the diffusion of various species inside the LCVD chamber and the thermal decomposition of titanium tetrabromide, titanium dibromide, and titanium monobromide at the surface of the substrate in order to determine the thickness of the titanium film deposited on the substrate. The model is used for studying the effects of various parameters on the deposition and shape of titanium films. The parameters which were varied are as follows: laser power, activation energy, total pressure inside the LCVD chamber, and partial pressure of titanium tetrabromide. Under certain conditions, the deposited titanium film is found to have a volcanic profile.

An uncooled photon detector is fabricated for the mid-wave infrared (MWIR) wavelength of 4.21 μm by doping an n-type 4H-SiC substrate with gallium using a laser doping technique. The dopant creates a p-type energy level of 0.3 eV, which is the energy of a photon corresponding to the MWIR wavelength 4.21 μm. This energy level was confirmed by optical absorption spectroscopy. The detection mechanism involves photoexcitation of carriers by the photons of this wavelength absorbed in the semiconductor. The resulting changes in the carrier densities at different energy levels modify the refractive index and, therefore, the reflectance of the semiconductor. This change in the reflectance constitutes the optical response of the detector, which can be probed remotely with a laser beam such as a He-Ne laser and the power of the reflected probe beam can be measured with a conventional laser power meter. The noise mechanisms in the probe laser, silicon carbide MWIR detector, and laser power meter affect the performance of the detector in regards to aspects such as the responsivity, noise equivalent temperature difference (NETD), and detectivity. For the MWIR wavelengths of 4.21 and 4.63 μm, the experimental detectivity of the optical photodetector of this study was found to be 1.07×10(10) cm·Hz(1/2)/W, while the theoretical value was 1.11×10(10) cm·Hz(1/2)/W. The values of NETD are 404 and 15.5 mK based on experimental data for an MWIR radiation source with a temperature of 25°C and theoretical calculations, respectively.

An uncooled mid-wave infrared (MWIR) detector is developed by doping an n-type 4H-SiC with Ga using a laser doping technique. 4H-SiC is one of the polytypes of crystalline silicon carbide and a wide bandgap semiconductor. The dopant creates an energy level of 0.30 eV, which was confirmed by optical spectroscopy of the doped sample. This energy level corresponds to the MWIR wavelength of 4.21 μm. The detection mechanism is based on the photoexcitation of electrons by the photons of this wavelength absorbed in the semiconductor. This process modifies the electron density, which changes the refractive index, and, therefore, the reflectance of the semiconductor is also changed. The change in the reflectance, which is the optical response of the detector, can be measured remotely with a laser beam, such as a He-Ne laser. This capability of measuring the detector response remotely makes it a wireless detector. The variation of refractive index was calculated as a function of absorbed irradiance based on the reflectance data for the as-received and doped samples. A distinct change was observed for the refractive index of the doped sample, indicating that the detector is suitable for applications at the 4.21 μm wavelength.

A set of laser beam shaping optics is designed by an iterative method using an adaptive additive algorithm to transform a Gaussian beam into a pitchfork beam. Two diffractive optical elements are designed based on Fresnel diffraction to reduce the amount of energy in the first-order diffraction ring and to increase the depth of focus for the optical system. These two beam properties are found to depend on the diameter of the desired beam and the Airy disk diameter. If the diameter of the desired beam is large, then the optical system yields better results in achieving the above-mentioned two beam properties. The performance of the diffractive optical elements is compared to a previous laser beam shaping system designed by the ray-tracing technique. A pinhole scanning power meter is used to measure the laser irradiance profile at the focal plane to verify the existence of the pitchfork beam. The irradiance profile measurement shows that diffractive optical elements allow better control for reducing the amount of energy in the diffraction side lobes.