Thermo-optical Property Research Articles

The thermo-optical coefficient as an alternative probe for the structural arrest of polymeric glass formers

The most important, yet independent susceptibilities to characterize the canonical glass transition include the thermal expansion, the specific heat and the shear modulus. At the glass transition, all these susceptibilities show a step-like behaviour as a function of temperature. However, in principle, different susceptibilities provide different perspectives on the glass transition. Due to the effect of sample history and considerable kinetic influences on the thermal glass transition behaviour, the best way to evaluate these different perspectives is to investigate them simultaneously. In this publication, different perspectives on the dynamical freezing process are obtained by a single experimental method: Temperature-modulated optical refractometry (TMOR). TMOR is used to simultaneously investigate the dynamic thermal expansion and, indirectly, the shear viscosity of a model polymer, both contributing to the thermo-optical coefficient. It will be discussed how this thermo-optical property couples to the structural arrest designated as one of the characteristics of the solidification process of polymers.

Effect of introducing KNO3 on the preparation of athermal fluorophosphate glass and investigation on its thermo-optical property

The effects of introducing some amount of KNO3 into the raw materials on preparation of fluorophosphate glass are investigated in terms of UV–vis transmittance, absorption coefficient and change of refractive index. As the content of KNO3 reaches 50%, the UV absorption coefficient of the optimized fluorophosphate glass reaches the minimum due to absence of microcrystals in the produced glass. The Schott's Sellmeier dispersion formula and Ghosh's two-polar wavelength-dependent Sellmeier equation were both used to calculate dispersion constants of the optimized fluorophosphate glass. The optimized fluorophosphate glass maintains special characteristics of negative temperature coefficient of refractive index and near-zero thermo-optical coefficient. The lower absorption both in UV and near-infrared (NIR) helps to improve its laser-induced damage resistance. The athermal fluorophosphate glass shows great potential for compensation of thermal-induced distortion in high-power solid-state lasers and high-resolution optical systems.

Thermal Lens Spectrometry Reveals Thermo-Optical Property Tuning of Conjugated Polymer Nanoparticles Prepared by Microfluidics

Conjugated polymers display useful thermo-optical properties of high relevance to biomedical applications, which are not only dependent on their intrinsic chemical composition but also related to their physical conformation and manufacturing protocol. In this work, we report that the thermo-optical properties of poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) encapsulated within poly(ethylene glycol) methyl ether-block-poly(lactide-co-glycolide) (PEG-PLGA) can be tuned by production conditions, generating conjugated polymer nanoparticles (CPNs) with customized applications. Thermal lens spectroscopy (TLS) was used to characterize the CPN light-to-heat conversion efficiency as it provides an absolute measurement of heat generation. Although preparation by traditional bulk production led to a high product yield, the CPNs were characterized by similar sizes and thermo-optical properties, irrespective of the molecular weight of amphiphilic PEG–PLGA. In contrast, a microfluidics production method generated CPNs with variable product yields and sizes and thermo-optical properties that are affected by both the molecular weight of PEG–PLGA and the production settings. Given the growing interest in biomedical applications of CPNs, our work provides useful results on microfluidic production of CPNs and of TLS for the screening of candidates with desirable characteristics.

In situ reduction of graphitic oxide by amorphization of magnesium diboride for the superior thermo-optical property based nanofluid applications

The reaction of magnesium diboride with water results in an intermediate borohydride product which leads to the simultaneous reduction of graphitic oxide (GO) and the formation of magnesium hydroxide. In this work, the thermo-optical properties of magnesium diboride modified reduced graphene oxide–based nanofluids have been explored. The study primarily focuses on the reaction mechanism of magnesium diboride and GO by using liquid exfoliation technique. Suspension after liquid exfoliation mainly consisted of a turbid supernatant and precipitate which was composed of boron-based nanosheets (CBNs) and a composite of magnesium hydroxide and reduced graphene oxide (CBNs-rGO), respectively. Nanofluids were subsequently formulated from the obtained products of the reaction. CBNs form a stable suspension in water and ethylene glycol because of its attached borohydrides and hydroxyl hydrophilic sites. CBNs nanofluids show good thermal conductivity with poor light absorption properties in the visible wavelength range. Whereas, CBNs-rGO nanofluids show ~95% attenuation in the radiation with a significant enhancement of ~30% and 20% in thermal conductivity as compared with Deionized water– and ethylene glycol–based fluids, respectively.

Thermal lensing effects and nonlinear refractive indices of fluoride crystals induced by high-power ultrafast lasers.

Thermo-optical and nonlinear property characterization of refractive optical components is essential for endoscopic instrumentation that utilizes high-power, high-repetition-rate ultrafast lasers. For example, ytterbium-doped fiber lasers are well suited for ultrafast laser microsurgery applications; however, the thermo-optical responses of many common lens substrates are not well understood at 1035 nm wavelength. Using a z-scan technique, we first measured the nonlinear refractive indices of CaF2, MgF2, and BaF2 at 1035 nm and found values that match well with those from the literature at 1064 nm. To elucidate effects of thermal lensing, we performed z-scans at multiple laser repetition rates and multiple average powers. The results showed negligible thermal effects up to an average power of 1 W and at 10 W material-specific thermal lensing significantly altered z-scan measurements. Using a 2D temperature model, we could determine the source of the observed thermal lensing effects. Linear absorption was determined as the main source of heating in these crystals. On the other hand, inclusion of nonlinear absorption as an additional heat source in the simulations showed that thermal lensing in borosilicate glass was strongly influenced by nonlinear absorption. This method can potentially provide a sensitive method to measure small nonlinear absorption coefficients of transparent optical materials. These results can guide design of miniaturized optical systems for ultrafast laser surgery and deep-tissue imaging probes.

A new affordable and quick experimental device for measuring the thermo-optical properties of translucent construction materials

The knowledge of thermo-optical properties of building materials is an essential factor for studying human comfort inside the building. Experiments are generally used to determine these properties. Each thermo-optical coefficients (i.e. the thermal conductivity, optical reflectance, transmittance and absorbance of the materials) can be measured separately by various devices. However, considering the new and complex materials that are emerging, for example, the translucent materials, the existing measuring instruments are finding it challenging to characterize these coefficients correctly. In this article, a new experimental device based on a spherical environment is proposed. The proposed system is capable of measuring the optical reflectance, transmittance, absorbance, and the thermal conductivity of homogeneous and innovative construction materials over a short experimental duration. The measurement protocol, the physical laws associated with the new experimental device, and the calibration tests of the measuring sensors are described. Furthermore, an application test based on a known reference, a polycarbonate construction material, is described in detail to prove the ability of the device to correctly measure these thermo-optical property coefficients. The results obtained show a relative error The results obtained show a relative error around of +/- 5%. Comparisons of the absolute error with the test results from the experimental device proposed by ASTM International (formerly the American Society for Testing and Materials) show that the maximum errors contributed by the new system described in this work to perform these measurements are generally acceptable. For the polycarbonate material, we obtained the following relative errors: 4.5% for the thermal conductivity, 4.7% for the optical reflectance, 0.56% for the optical transmittance and 1.78% for the optical absorbance.

Thermo-Optical Property Degradation of ITO-Coated Aluminized Polyimide Thin Films Under VUV and Low-Energy Proton Radiation

We studied thermo-optical property degradation of indium tin oxide (ITO)-coated aluminized polyimide thin films under exposure to vacuum ultraviolet radiation and low-energy (3 and 5 keV) protons during ground tests using the Complex Irradiation Facility at the DLR site in Bremen. Changes in solar absorption and thermal emission coefficients caused by the irradiation were analyzed. We report a significant increase in solar absorptance of the samples irradiated by protons. We also attempted to identify any defects on the surface of the samples. The study was motivated by a unique opportunity that is provided by the Complex Irradiation Facility to study the degradation effects induced by exposure to protons with an energy below 10 keV and short-wavelength light below 115 nm.

Sentinel-1 SAR instrument improved flight predictions based on measured in-orbit data

The Sentinel-1A SAR mission was launched in April 2014, followed by the Sentinel-1B Spacecraft in April 2016. Since then, several sets of in-orbit data have been evaluated to correlate the thermal model for being able to provide more detailed in-flight predictions. The need for detailed in-flight predictions is justified by the fact that the imaging performance of a SAR instrument depends mainly on the thermal performance of its high dissipative units. Components reaching their temperature limits during operational time define the end of the imaging phase, and thus the timespan during which and how often imaging operations can take place. An STM (Structural/Thermal Model) test correlation is standard throughout all missions, which usually delivers a very reliable model for further in-flight predictions. Nevertheless, this correlation does not give information about thermo-optical property degradation or environmental influences, because the effects in space on the thermally active surfaces are very hard to predict. For this reason, thermal engineers use relatively conservative values for in-flight predictions and End-of-Life thermal performance assessments. This might lead to mission performance limitations which predict a too short feasible imaging time of the Radar instrument. For this reason, the first approach was to evaluate the early acquired in-orbit data and to correlate the thermal model with the thermal configuration at Begin-of-Life to assess the maximum possible high dissipative imaging time possible for the Begin-of-Life situation. Then the flight data over a longer timespan were evaluated to determine potential temperature trends which could be caused by thermo-optical property degradation as well as seasonal-related influences. These two assessments combined, allow a thermal performance prediction for Mission End-of-Life, and thus a reliable determination of potential SAR imaging performance over the full mission time. The paper will present the correlation results of the initially measured in-flight data, the determined long-term in-orbit data over 3 years, and the combination of both assessments including its impact on SAR imaging performance over the full mission.

Evidence of phase transition in Nd3+doped phosphate glass determined by thermal lens spectrometry

Thermal lens spectroscopy (TLS), differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) techniques were applied to the thermo-optical property analysis of a new phosphate glass matrix PANK with nominal composition 40P2O5·20Al2O3·35Na2O·5K2O (mol%), doped with different Nd(3+) compositions. This glass system, synthesized by the fusion protocol, presents high transparency from UV to the near infrared, excellent thermo-optical properties at room temperature and high fluorescence quantum efficiency. Thermal lens phase shift parameters, thermal diffusivity and the DSC signal present pronounced changes at about 61 °C for the PANK glass system. This anomalous behavior was associated with a phase transition in the nanostructured glass materials. The FTIR signal confirms the presence of isolated PO4 tetrahedron groups connected to different cations in PANK glass. As a main result, our experimental data suggest that these tetrahedron groups present a structural phase transition, paraelectric-ferroelectric phase transition, similar to that in potassium dihydrogen phosphate, KH2PO4, nanocrystals and which TLS technique can be used as a sensitive method to investigate changes in the structural level of nanostructured materials.

Effects of Atomic Oxygen on the Thermochromic Characteristics of VO2 Coating

Thermochromic VO 2 coating is currently being developed for smart spacecraft thermal radiator application. Concerning the potential space environmental effects, in this study, coating samples are subjected to exposure to a ground-based atomic-oxygen flux for the equivalent of six months and three years in a typical low-Earth-orbit environment. Mass loss of the coating samples because of the exposure to atomic oxygen is measured. Characterization of total hemispherical emittance of the coating before and after atomic-oxygen exposure indicates that the atomic-oxygen erosion affects the thermal-optical performance of the coating to some extent. Detailed x-ray photoelectron spectroscopic analysis of the coating samples are performed, and an increase in the oxygen concentration in the outermost layer of the coating because of the atomic-oxygen exposure is identified. A possible mechanism for the change in thermo-optical property of the coating is discussed.

Thermo-optical property degradation of irradiated spacecraft surfaces

Covers advancements in spacecraft and tactical and strategic missile systems, including subsystem design and application, mission design and analysis, materials and structures, developments in space sciences, space processing and manufacturing, space operations, and applications of space technologies to other fields.

The thermo-optical properties of the thermal electrons induced in the SiN absorption band of a silicon nitrade layer

An analysis of the thermo-optical effects induced in the IR absorption spectrum of an Si x N y H z layer produced by plasma-enhanced chemical vapour deposition revealed the following effects. The absorption coefficient and the refractive index of the SiN bond at 790 cm -1 are more sensitive to temperature changes than are those of the NH and SiH bonds at 3650 cm -1 and 3050–3100 cm -1 respectively. The refractive index of the SiN bond can be expressed as n( t, p) = A( t) p 2 − B( t) p + c( t) where p = y/ x is less than 1.52. Quantitative analysis of the temperature-dependent fine vibrational states and the force constants of the two types of fine band oscillator revealed that the eight types of tightly bound fine oscillators in the range 1050 - 790 cm -1 and the six types of loosely bound fine oscillators in the range 790 - 700 cm -1 had different properties. They can be assigned to the double- and triple-bond Si=N and Si≡N oscillators and to the single-bond Si-N oscillators respectively. This assignment was confirmed by consideration of the statistical probability of formation of these three types of bonds by the valence electrons in the silicon and nitrogen atoms. In addition, the temperature-dependent fine structure of the very fine bands separated by 1–2 meV which were superimposed on each fine vibrational band verified that this structure has a sensitive thermo-optical property. The mean energy separation (in electronvolts) was found to be ΔE j = 4.12 × 10 -8t 2 + 4.02 × 10 -6t − 2.50 × 10 -4