Laser-induced Thermal Emission Research Articles

Kinetics of Laser-Induced Thermal Emission of Porous Carbon Materials: Dependence on Laser Wavelength

For the porous carbon material excited by the first and second harmonics of a neodymium laser, the shape of pulsed signals of laser-induced thermal emission is investigated. It is found that the duration of thermal emission pulses significantly depends on the wavelength of the laser excitation, which is caused by the differences in the depth of penetration of laser radiation into the surface layer. The mentioned effect is actual, if the penetration depth of laser radiation exceeds the length of thermal diffusion in the studied material for a time of the order of the laser pulse duration. The computer modeling is carried out for the processes of pulsed laser heating and formation of thermal emission signal. The simulation results showed satisfactory agreement with the measurement results.

The Role of Air in Laser-Induced Thermal Emission of Surface Layers of Porous Carbon Materials

The influence of the surrounding air on the amplitude and shape of thermal radiation pulses (at a wavelength of 430 nm) during the heating of the surface layer of a porous carbon material (to temperatures of the order of 2000–3000 K) by the radiation of a Q-switched neodymium laser is studied. When the pressure of the surrounding air is reduced to forevacuum conditions, the experiments showed a one-and-a-half-fold increase in the amplitude of pulsed signals of thermal radiation and an increase in the decay time of the glow by about a third. Numerical calculations of the dynamics of the temperature field in the surface layer of the material during the irradiation by nanosecond laser pulses are carried out. An improved model is used in the calculations, which accounts for (i) the porosity of the material and (ii) the temperature dependence of the coefficients of thermal conductivity and the heat capacities of carbon and air. To calculate the thermal conductivity of the porous material, a model of a cubic array of intersecting square rods is used. The satisfactory consistency of calculation results with experimental data is obtained. The above-mentioned improvements of the calculation model made it possible to reconcile the estimates of the thermal characteristics of surface layers of carbon, obtained from the emission decay data, with the reference data published in the literature.

Laser-induced thermal emission of carbon microparticles on transparent heat-sink substrates

Thermal emission is an informative tool to study materials’ properties at high temperatures under laser irradiation. The kinetics decay of laser-induced thermal emission from carbon microparticles deposited on heat-sink surfaces of transparent dielectrics (glass and sapphire) was studied. A Q-switched YAG:Nd3+ laser (pulse duration τi = 20 ns, energy/power density 0.5 J·cm–2, 25 MW·cm–2) was employed to excite thermal emission. In calculations, the classical heat conduction equation was used. With increasing the thermal conductivity of substrate (from glass to sapphire), reduction in the emission pulse duration has been observed.

Modulated-laser source induction system for remote detection of infrared emissions of high explosives using laser-induced thermal emission

In a homeland security setting, the ability to detect explosives at a distance is a top security priority. Consequently, the development of remote, noncontact detection systems continues to represent a path forward. In this vein, a remote detection system for excitation of infrared emissions using a CO2 laser for generating laser-induced thermal emission (LITE) is a possible solution. However, a LITE system using a CO2 laser has certain limitations, such as the requirement of careful alignment, interference by the CO2 signal during detection, and the power density loss due to the increase of the laser image at the sample plane with the detection distance. A remote chopped-laser induction system for LITE detection using a CO2 laser source coupled to a focusing telescope was built to solve some of these limitations. Samples of fixed surface concentration (500 μg / cm2) of 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) were used for the remote detection experiments at distances ranging between 4 and 8 m. This system was capable of thermally exciting and capturing the thermal emissions (TEs) at different times in a cyclic manner by a Fourier transform infrared (FTIR) spectrometer coupled to a gold-coated reflection optics telescope (FTIR-GT). This was done using a wheel blocking the capture of TE by the FTIR-GT chopper while heating the sample with the CO2 laser. As the wheel moved, it blocked the CO2 laser and allowed the spectroscopic system to capture the TEs of RDX. Different periods (or frequencies) of wheel spin and FTIR-GT integration times were evaluated to find dependence with observation distance of the maximum intensity detection, minimum signal-to-noise ratio, CO2 laser spot size increase, and the induced temperature increment (ΔT).

Laser-induced thermal emission of rough carbon surfaces

The decay of thermal emission of rough surface layers of different carbon materials under the pulsed laser excitation is analyzed both theoretically and experimentally. For pulsed laser heating of rough surfaces, computer simulations revealed that laser-induced thermal radiation is mainly emitted by peaks of the surface relief, and the emission decay time depends on the relation between the laser penetration depth and the temperature diffusion length. It is also concluded that the presence of surface roughness can significantly increase the emission decay time. In the experiments, carbon materials with different thermal characteristics were used; however, all of the investigated samples demonstrated close values of the emission decay time. This fact shows that the material's characteristics on the peaks of surface relief are similar for different carbon materials at high temperature.

Кинетика затухания теплового излучения поверхностных слоев углеродных материалов при импульсном лазерном возбуждении

For surface layers of carbon materials, under excitation by a Q-switched neodymium laser, the decay of laser-induced thermal emission is investigated both experimentally and theoretically. It is revealed that the decay curves can be fit with satisfactory accuracy by a sum of two exponential components with the decay times of the order of 10 and 100 ns. Under irradiation by a sequence of laser pulses, the observed transformations of the emission decay curves can be presented as redistribution of intensity of the above-mentioned two components. As a result of computer simulations, it is concluded that the emission decay time is determined by the ratio of the penetration depth of laser radiation and the thermal diffusion length. This fact opens a possibility for estimation of thermal diffusivity of an investigated material in a thin surface layer at high temperatures (thousands of Kelvins).

Chemometrics‐enhanced laser‐induced thermal emission detection of PETN and other explosives on various substrates

Infrared emissions (IREs) of samples of pentaerythritol tetranitrate (PETN) deposited as contamination residues on various substrates were measured to generate models for the detection and discrimination of the important nitrate ester from the emissions of the substrates. Mid‐infrared emissions were generated by heating the samples remotely using laser‐induced thermal emission (LITE). Chemometrics multivariate analysis techniques such as principal component analysis (PCA), soft independent modeling by class analogy (SIMCA), partial least squares‐discriminant analysis (PLS‐DA), support vector machines (SVMs), and neural network (NN) were employed to generate the models for the classification and discrimination of PETN IREs from substrate thermal emissions. PCA exhibited less variability for the LITE spectra of PETN/substrates. SIMCA was able to predict only 44.7% of all samples, while SVM proved to be the most effective statistical analysis routine, with a discrimination performance of 95%. PLS‐DA and NN achieved prediction accuracies of 94% and 88%, respectively. High sensitivity and specificity values were achieved for five of the seven substrates investigated. Copyright © 2015 John Wiley & Sons, Ltd.

Applications of Laser-Induced Thermal Emission Spectroscopy to Various Samples

The Laser-Induced Thermal Emission (LITE) spectra of several solid samples were measured and compared with the usual thermal emission spectra generated by the heating element. The following advantages in LITE spectroscopy were made clear: (1) the spectral background is reduced; (2) the time required for the stabilization of the thermal emission signal is short; (3) the compounds which are coated on background materials with complex shapes can be measured.

Preliminary Studies of Laser-Induced Thermal Emission Spectroscopy of Condensed Phases

Laser-Induced Thermal Emission (LITE) spectroscopy is introduced as an analytical technique for condensed-phase specimens. In this technique, a laser beam is used to locally heat the specimen, and the mid-infrared thermal emission is observed. Two different materials and different thicknesses of each material were examined with LITE. The equilibrium temperature attained during a LITE measurement was estimated to be about 50°C. The advantages of LITE over conventional thermal emission are noted. The multiple dimensions of information present and some of the potential analytical applications of LITE are discussed.