Temperature-controlled spectrophotometry: a simultaneous analysis of phase transition, thermal degradation and optical properties of semi-transparent composites from 20 °C to 450 °C

Abstract

So far, optical and effective radiative properties of polymer matrix based composites were investigated at temperatures well below their degradation temperature. At the same time, polymers exhibit temperature dependent physical properties and may undergo structural changes as their temperature raises. In this work, we employ the "Temperature-Controlled Spectrophotometry", a new method enabling to identify simultaneously phase transitions, thermal degradation and radiative properties of semi-transparent composites over a large temperature range. The method consists of measuring simultaneously the normal-normal and the normal-hemispherical transmittances and reflectances of the sample subjected to a laser irradiation with tuneable wavelength while the temperature is rised from room temperature up to 450 °C by means of a CO2 laser. Physical changes of the sample are identified from the temperature variation of normal-normal transmittance and specular reflectance measurements. Most of the results here are presented at a specific wavelength of 1070 nm but the proposed method is suitable over the semi-transparency spectral domain of the material by changing the wavelength of the probe laser. An inverse method for parameter identification based on normal-hemispherical measurements is employed to determine the transport effective radiative properties of the sample, namely the transport extinction coefficient and the transport scattering albedo from room temperature to 325 °C.

Modelling the behaviour of UAVs structural materials under continuous laser irradiations

“…Laser trials are carried out with a 10 kW Ytterbium fiber laser (IPG YLS-10000) equipped with a high-power collimator [1] (Ø22 mm beam diameter, 0.8 mm/m beam divergence). The 1070 nm laser beam is then shaped into a Ø20 mm top-hat profile using a Galilean telescope [3] composed of a Ø1.5'' diverging lens and a Ø3'' converging lens, both AR-coated at the laser wavelength, and a water-cooled diaphragm [4] specifically designed in-house to shape intense laser beams. The resulting beam incoming onto the sample can be seen on the Figure 3 captured with a beam profiler (Ophir L11059) located at the exact same position than the sample during later experiments, and the laser power delivered onto the sampleaffected by strong losses along the beam pathhas been calibrated by a high-power thermal sensor (Ophir 10K-W-BB-45).…”

“…Figure 12 shows the geometry of the 3D sample under COMSOL Multiphysics and its corresponding free tetrahedral mesh structure, where the mesh size is larger on the edges of the sample to reduce the computation and finer in the center of the sample and along the thickness of the specimen to improve the accuracy of the results. All optical and thermodynamical inputs parameters are temperature-dependent and identified in related papers 3,4 . The multiphysics model is able to deliver accurate numerical data for all experiments with varying laser power densities, at least until a flame occurs on the front face of the sample and where the recorded data from the pyrometric devices are not reliable anymore.…”