Infrared Optical Properties Research Articles

Sulfur‐Rich Norbornadiene‐Derived Infrared Transparent Polymers by Inverse Vulcanization

Infrared (IR) transparent polymer materials prepared by inverse vulcanization, as a promising candidate to replace inorganic materials, are new materials for constructing key devices in IR optics. However, it is difficult to achieve a balance between infrared optical and thermal properties in polymers due to the intrinsic infrared absorption of organic materials. Herein, our strategy is to construct a high boiling point symmetrical molecular norbornadiene derivative cross‐linking agent (DMMD) which can be inverse vulcanized with molten sulfur, and obtain Poly(S‐r‐DMMD) with different sulfur content by controlling the feed ratio of sulfur. With the rigid core and low IR activity in DMMD, the prepared polymers exhibit tunable thermal properties (Tg: 98.3‐119.8 °C) and high IR transmittance (medium‐wave infrared region (MWIR): 42.9‐52.6 %; long‐wave infrared region (LWIR): 1.5‐5.29 %). In addition, Poly(S‐r‐DMMD) can be used to prepare large‐size free‐standing Fresnel lenses for IR imaging by simple hot‐pressing, which provides flexibility in the design and production of IR fine lenses. This study provides a novel strategy for balancing the thermal and optical properties of IR transparent polymer materials, while providing relevant references for balancing the IR optical and thermal properties of polymer materials.

Molecular Crystals Constructed by Polar Molecular Cages: A Promising System for Exploring High-performance Infrared Nonlinear Optical Crystals.

Polar molecular crystals, with their densely stacked polar nonlinear optical (NLO) active units, are favored for their large second harmonic generation (SHG) responses and birefringence. However, their potential for practical applications as Infrared (IR) NLO materials has historically been underappreciated due to the weak inter-molecular interaction forces that may compromise their physicochemical properties. In this study, we propose molecular crystals with polar molecular cages as a treasure-house for the development of superior IR NLO materials and a representative system, binary chalcogenide molecular crystals, composed of [P4 Sn ] (n=3-9) polar molecular cages, is introduced. These crystals may not only achieve wide band gap, large SHG response, and birefringence in a single structure, but also exhibit favorable physicochemical properties. We subsequently obtained a polar molecular crystal, α-P4 S5 , which demonstrated exceptional IR optical properties, including a strong SHG response (1.1×AGS), wide band gap (3.02 eV), large birefringence (0.134@2050 nm), and a broad transmission range (0.41-14.7 μm). Moreover, it showed excellent water resistance and hardness. These findings highlight the potential of polar molecular crystals as a promising platform for the development of high-performance IR NLO materials.

Molecular Crystals Constructed by Polar Molecular Cages: A Promising System for Exploring High‐performance Infrared Nonlinear Optical Crystals

AbstractPolar molecular crystals, with their densely stacked polar nonlinear optical (NLO) active units, are favored for their large second harmonic generation (SHG) responses and birefringence. However, their potential for practical applications as Infrared (IR) NLO materials has historically been underappreciated due to the weak inter‐molecular interaction forces that may compromise their physicochemical properties. In this study, we propose molecular crystals with polar molecular cages as a treasure‐house for the development of superior IR NLO materials and a representative system, binary chalcogenide molecular crystals, composed of [P4Sn] (n=3–9) polar molecular cages, is introduced. These crystals may not only achieve wide band gap, large SHG response, and birefringence in a single structure, but also exhibit favorable physicochemical properties. We subsequently obtained a polar molecular crystal, α‐P4S5, which demonstrated exceptional IR optical properties, including a strong SHG response (1.1×AGS), wide band gap (3.02 eV), large birefringence (0.134@2050 nm), and a broad transmission range (0.41–14.7 μm). Moreover, it showed excellent water resistance and hardness. These findings highlight the potential of polar molecular crystals as a promising platform for the development of high‐performance IR NLO materials.

A Microphase Separation Strategy for the Infrared Transparency‐Thermomechanical Property Conundrum in Sulfur‐Rich Copolymers

AbstractWith intrinsic optical and dynamic properties of polysulfide chains, inverse vulcanized copolymers have demonstrated immense potential for infrared (IR) optical applications. However, preparing highly IR‐transparent sulfur‐rich copolymers without sacrificing their thermomechanical properties remains challenging. To overcome the trade‐off relationship between IR optical and thermomechanical properties, an in situ microphase separation strategy for the inverse vulcanization of elemental sulfur utilizing self‐crosslinkable 1,3,5‐trivinylbenzene (TVB) is presented. Even with 80 wt% sulfur content, the microphase‐separated TVB‐rich domain self‐reinforces the copolymer with a noteworthy modulus of ≈2.0 GPa and a high glass transition temperature (Tg) of 92.6 °C, while still exhibiting outstanding IR optical properties. This work is expected to provide insights into the fundamental structure–property relationships of sulfur‐rich copolymers and pave the way for various practical applications.

Achieving variable infrared emissivity modulation regions of poly(aniline) films:the effect of film surface morphology on the optical tunability

Infrared (IR) electrochromic materials, which can dynamically regulate the IR optical properties, are particularly desirable in intelligent thermal management technology. However, for these materials, an outstanding challenge is in achieving a series of different emissivity modulation capabilities for different applications. Here, we obtain the functional poly(aniline) (PANI) films with significantly different emissivity modulation capability by tailoring the surface morphology of the PANI film. The PANI film with a non-directionally distributed nanowire-like structure (PANI2) and a rather smooth surface decorated by shallow voids (PANI6) can realize the emissivity variations from 0.321 to 0.712 and 0.190 to 0.573 in the wavelength range of 2.5–25 μm, respectively. Combined with XPS, UV–vis–NIR characterizations, and IR light propagation analysis in two representative morphologies, the reason for the thermal radiation energies of PANI2 at two states were significantly higher than those of PANI6 was proposed. As the residual PANI in PANI2 was not involved in redox reaction and remained in the emeraldine salt (ES) state, and the multiple reflections by non-directionally distributed nanowire on the surface suppressed the reflection of incident IR light. Moreover, we investigated the heat dissipation modulation capabilities of PANI2 and PANI6. PANI2 can achieve greater heat dissipation at a high emissivity state (ε = 0.712), while PANI6 can effectively suppress heat dissipation at a low emissivity state (ε = 0.190). Moreover, we utilized the IR thermal imager and transient thermal field analysis to evaluate the heat dissipation modulation capabilities of PANI2 and PANI6.

Identification of wavelength regions for non-contact temperature measurement of combustion gases at high temperatures and high pressures

Stationary gas turbines are still an important part of today’s power supply. With increasing temperature of the hot combustion gas inside a gas turbine, the efficiency factor of the turbine increases. For this reason, it is intended to operate turbines at the highest possible gas temperature. Therefore, in the combustion chamber and especially at the position of the first stage guide vanes the gas temperature needs to be measured reliably. To determine the gas temperature, one promising approach is the application of a non-contact measurement method using a radiation thermometer. A radiation thermometer can measure the gas temperature remotely from outside of the harsh environment. At ZAE Bayern, a high temperature and high-pressure gas cell has been developed for this purpose in order to investigate gases and gas mixtures under defined conditions at high pressures and high temperatures. This gas cell can be placed in a FTIR-spectrometer in order to characterize the infrared-optical properties of the gases. In this work the measurement setup is introduced and gas mixtures, which are relevant for gas turbine applications are analyzed thoroughly. The derived results are presented and discussed in detail. To identify suitable wavelength regions for non-contact gas temperature measurements, first tests have been performed. Based on these tests, an appropriate wavelength region could be chosen, where future gas temperature measurements can be carried out.

Tailoring electrical and optical properties of Al-doped ZnO thin films grown at room temperature by reactive magnetron co-sputtering: From band gap to near infrared

Al-doped zinc oxide (AZO) thin films were deposited at room temperature (RT) using reactive magnetron co-sputtering of two metallic Zn and Al targets in the presence of oxygen. The structural, optical and electrical properties of the AZO thin films were tailored by varying the Al content through changing Al sputtering power from 0 to 30 W. The AZO thin films deposited at the optimal deposition conditions with Al power of 26–28 W exhibited high transparency in the visible range, shorter plasma wavelength, lower electrical resistivity, lower transmittance and higher reflectance in the near IR range. The results domenstrate that the Al-doping content is strongly correlated to the band gap, electrical and near infrared optical properties using various characterization techniques. Tailoring the infrared optical properties of AZO films can be achieved and applied for desired optical coating applications.

Determining the infrared radiative effects of Saharan dust: a radiative transfer modelling study based on vertically resolved measurements at Lampedusa

Abstract. Detailed measurements of radiation, atmospheric and aerosol properties were carried out in summer 2013 during the Aerosol Direct Radiative Impact on the regional climate in the MEDiterranean region (ADRIMED) campaign in the framework of the Chemistry-Aerosol Mediterranean Experiment (ChArMEx) experiment. This study focusses on the characterization of infrared (IR) optical properties and direct radiative effects of mineral dust, based on three vertical profiles of atmospheric and aerosol properties and IR broadband and narrowband radiation from airborne measurements, made in conjunction with radiosonde and ground-based observations at Lampedusa, in the central Mediterranean. Satellite IR spectra from the Infrared Atmospheric Sounder Interferometer (IASI) are also included in the analysis. The atmospheric and aerosol properties are used as input to a radiative transfer model, and various IR radiation parameters (upward and downward irradiance, nadir and zenith brightness temperature at different altitudes) are calculated and compared with observations. The model calculations are made for different sets of dust particle size distribution (PSD) and refractive index (RI), derived from observations and from the literature. The main results of the analysis are that the IR dust radiative forcing is non-negligible and strongly depends on PSD and RI. When calculations are made using the in situ measured size distribution, it is possible to identify the refractive index that produces the best match with observed IR irradiances and brightness temperatures (BTs). The most appropriate refractive indices correspond to those determined from independent measurements of mineral dust aerosols from the source regions (Tunisia, Algeria, Morocco) of dust transported over Lampedusa, suggesting that differences in the source properties should be taken into account. With the in situ size distribution and the most appropriate refractive index the estimated dust IR radiative forcing efficiency is +23.7 W m−2 at the surface, −7.9 W m−2 within the atmosphere, and +15.8 W m−2 at the top of the atmosphere. The use of column-integrated dust PSD from AERONET may also produce a good agreement with measured irradiances and BTs, but with significantly different values of the RI. This implies large differences, up to a factor of 2.5 at surface, in the estimated dust radiative forcing, and in the IR heating rate. This study shows that spectrally resolved measurements of BTs are important to better constrain the dust IR optical properties, and to obtain a reliable estimate of its radiative effects. Efforts should be directed at obtaining an improved description of the dust size distribution and its vertical distribution, as well as at including regionally resolved optical properties.

Long wavelength infrared radiation thermometry for non-contact temperature measurements in gas turbines

The objective of the EU project “Sensors Towards Advanced Monitoring and Control of Gas Turbine Engines (acronym STARGATE)” is the development of a suite of advanced sensors, instrumentation and related systems in order to contribute to the developing of the next generation of green and efficient gas turbine engines. One work package of the project deals with the design and development of a long wavelength infrared (LWIR) radiation thermometer for the non-contact measurement of the surface temperature of thermal barrier coatings (TBCs) during the operation of gas turbine engines.For opaque surfaces (e.g. metals or superalloys) radiation thermometers which are sensitive in the near or short wavelength infrared are used as state-of-the-art method for non-contact temperature measurements. But this is not suitable for oxide ceramic based TBCs (e.g. partially yttria stabilized zirconia) as oxide ceramics are semi-transparent in the near and short wavelength infrared spectral region. Fortunately the applied ceramic materials are non-transparent in the long wavelength infrared and additionally exhibit a high emittance in this wavelength region.Therefore, a LWIR pyrometer can be used for non-contact temperature measurements of the surfaces of TBCs as such pyrometers overcome the described limitation of existing techniques. For performing non-contact temperature measurements in gas turbines one has to know the infrared-optical properties of the applied TBCs as well as of the hot combustion gas in order to properly analyse the measurement data. For reaching a low uncertainty on the one hand the emittance of the TBC should be high (>0.9) in order to reduce reflections from the hot surrounding and on the other hand the absorbance of the hot combustion gas should be low (<0.1) in order to decrease the influence of the gas on the measured signal.This paper presents the results of the work performed by the authors with focus on the implementation of the LWIR pyrometer and the selection of the optimal wavelength band where the detector should be sensitive. Besides determining the spectral infrared-optical properties (emittance, transmittance and absorbance) of the TBCs and the hot combustion gas at high temperatures up to 1700K, the wavelength specification of the developed LWIR pyrometer is defined. Also an overview of the LWIR radiation thermometer is given and the preliminary results for different temperatures and environmental conditions are presented. Finally the measurement uncertainty of the LWIR-pyrometer is deduced.

Infrared absorption spectra of liquid water and deuterated water by Car-Parrinello molecular dynamics

Infrared (IR) absorption spectra plays a key role in the study of thermodynamic and dynamic properties of liquid water. Car-Parrinello molecular dynamics simulations within the framework of density-functional theory are used to study the IR absorption, dielectric function and optical properties of liquid water and deuterated water. Using the energy minimization model to obtain the input structure, we calculate the molecular dipole moment of water and deuterated water at ambient temperature, and then obtain the IR absorption spectra by Fourier transforming the autocorrelated function of dipole moment. As the numerical analysis indicates, the calculated results fit well with previous literature experiments. The absorption peak represents the stretching mode of oxygen-hydrogen covalent bond and shows a slight red shift. The dielectric functions and optical constants of water and deuterated water at the temperature of 303 K are extracted from IR absorption spectrum.

Experimental characterization and theoretical modeling of the infrared-optical properties and the thermal conductivity of foams

In order to understand and improve the thermal insulation performance of cellular polymeric foams, two different polymers and corresponding foams, extruded polystyrene foam (XPS) and Polyurethane foam (PUR) were selected and thoroughly characterized at ZAE Bayern. At first radiative properties, cell morphology and thermal conductivity were characterized. The measured results were then compared to the theoretical model developed at ZAE Bayern. The validated model allows then the prediction of the insulation performance of an arbitrary XPS or PUR foam with given density and geometrical structure. It also allows defining the optimal structure (resin, cell morphology, density) to achieve the desired thermal conductivity.

Black-pigmented polypropylene materials for solar thermal absorbers – Effect of carbon black concentration on morphology and performance properties

Black-pigmented polyolefin compounds are widely used for unglazed collectors and recently also for glazed flat-plate collectors with overheating protection. In this paper the effects of pigment concentration (carbon black (CB) – masterbatch) on morphology and performance properties of a polypropylene grade for elevated temperature applications (PP-RCT) are investigated. Compounds with varying black pigment concentrations (from 0.1 to 1.0wt.%) were prepared and moulded to 2mm thick sheets. Morphological characteristics, optical and mechanical properties, as well as long-term properties were characterized on specimen level by optical microscopy, Differential Scanning Calorimetry (DSC), UV/Vis/NIR- and IR-spectrometry and tensile testing. The investigated black compounds exhibited integral solar absorbance values of about 95% and integral infrared emittance values from 93% to 95%. Hence, the effect of black pigment concentration on the solar- and infrared-optical properties was small. Regarding the mechanical properties, the elastic modulus and the ultimate properties were slightly affected by the black pigment content. A more pronounced effect was deduced after long-term exposure in hot air at 135°C. With increasing carbon black concentration a significant reduction of the embrittlement time from approximately 4000h to 3200h was obtained.

Determination of the refractive index of TiO2–Y2O3 ceramic composites

The heat transfer through porous materials is significantly influenced by radiative transfer which is correlated with the complex refractive index of the material and the structure of the ceramic. By changing these two parameters, the infrared-optical properties can be varied in a relatively wide range.Due to the fact that the complex refractive index of composite materials depends on the mixed phases and their mixing ratio, the refractive index often cannot be found in literature but it can be estimated for the given mixture via different model calculations. To verify these theoretical calculations, several samples were prepared so that their refractive index could be determined. In this work, the measured refractive indices of Al2O3 and TiO2 as well as those of different TiO2–Y2O3 composites are presented. These measurements are compared with available literature data and with the data obtained from the model calculations.

Difference of optical properties between porous alumina and sapphire using two-substrate method at elevated temperature

We report on simultaneous measurements of the emittance, transmittance and reflectance of a porous alumina under same environment at 573 K, which has not been obtained. It is demonstrated that the correction of substrate radiation and multiple reflections is important for the accurate measurement of infrared (IR) optical properties of porous materials in a two-substrate method. We deduced the extinction, absorption and scattering coefficients of the alumina from the measured three quantities, based on the classical geometry model. The coefficients explain the difference of the IR optical properties between the alumina and the sapphire. The increasing reflectance of the alumina with thickness, unlike the sapphire, is attributed to the relatively high scattering coefficient at short wavelengths.

Electric Fields in the Chemical Vapour Deposition Growth of Vanadium Dioxide Thin Films

Thin films of thermochromic vanadium dioxide have been the subject of intensive research in recent years year due to their postulated use as "intelligent" window coatings. The usefulness of such technology depends on a semi-conducting to metal transition with an associated change in infra-red optical properties. This exact nature of this transition depends on a large number of factors such as doping, crystallite size, strain, crystallographic orientation etc. In this paper we discuss the nature of these factors with a particular focus on how the application of electric fields in the deposition affects crystallite size and film strain with reference to recent results.

Infrared-optical characteristics of ceramics at elevated temperatures

In this work the infrared-optical properties of ceramic samples at ambient and elevated temperatures are presented. The aim of this work is the determination and interpretation of the spectral transmittance, reflectance and emittance of three oxide ceramic samples (MgO, Al 2O 3/ZrO 2, TiO 2/Y 2O 3) and one carbide ceramic sample (SiC) as a function of the temperature from 300 K up to 1100 K. The semi-transparency of the ceramics in the near infrared region has to be considered when performing the measurements and evaluating the data. The experimentally derived spectra are compared with the values derived from theoretical calculations. The complex dielectric function of a ceramic can be deduced from the Lorentz oscillator model. The complex refractive index is correlated with the complex dielectric function by the Maxwell relation. Scattering effects can be described by radiative transfer models. Finally the infrared-optical properties transmittance, reflectance and emittance depend on the complex refractive index and the morphology of the ceramic sample. Of special interest is the Christiansen wavelength as well as the location of the free running frequencies of the longitudinal and transversal optical oscillations. Usually the ceramic is highly reflecting between the free running frequencies of the longitudinal and transversal optical oscillations whereas at the Christiansen wavelength the reflectance vanishes which leads to an emittance of one for an optically thick sample at the Christiansen wavelength. Due to the known emittance it is possible to use the Christiansen wavelength for determining the temperature of the investigated sample. Finally the obtained infrared-optical spectra are presented and their temperature dependencies are discussed which are mainly due to phonon excitations. Additionally it has been shown that for example the location of the Christiansen wavelength can be influenced by varying the composition of the ceramic.

Polymer-based pigmented coatings on flexible substrates with spectrally selective characteristics to improve the thermal properties

Functional textiles and membranes can create impressive optical effects and are being applied increasingly in the field of textile architecture. Lowering the thermal emittance of the textiles and membranes by a low-e coating leads to an improvement of the thermal properties. Therefore, the thermal insulation of such constructions rises as well as the thermal comfort in the building which finally results in a reduction of the energy consumption of the building. The spectrally selective coatings on flexible substrates considered in this work combine a customized color (or visual appearance) with a low emittance in the infrared spectral range. To realize the desired properties, polymer-based pigmented coatings were developed, optimized and characterized. Investigations were therefore carried out on the matrix material, the particle sizes and the dispersion of the colored pigments and the metal pigments concerning the infrared-optical properties and their optimization. As a result, the spectral emittance of the improved coating is presented from the ultraviolet over the visible up to the far infrared wavelength region. The thermal emittance of a polyester fabric was reduced from above 0.9 to below 0.3 by applying a colored low-e coating.

Vibrational properties of (Gd1−xYx)3Ga5O12 solid solutions

The vibrational properties of the (Gd1−xYx)3Ga5O12 (0≤x≤1) mixed garnet crystals are investigated at normal conditions by Raman and reflectivity infrared (IR)-spectroscopy while the end members of the series have been theoretically investigated by means of a rigid ion model. The IR experimental spectra were analyzed by means of the Kramers–Kronig relations and the Lorentz oscillator model. The obtained results are compared and discussed in detail with the theory, providing useful parameters for estimating and understanding the IR optical properties of mixed garnets. The majority of the expected Raman peaks is resolved and their symmetry assignment has been elucidated. The clear one mode behavior of the observed Raman and IR modes as a function of x confirms the formation of solid solutions, resulting from the replacement of the cations at the dodecahedral lattice sites. A softening of almost all Raman and IR modes is found as the unit cell volume increases (x decreases). Finally, the small dispersion of the Grüneisen parameters γi (ω) indicates that the binding forces are of the same nature and order of magnitude and that the binding strength between the various molecular units is quite comparable.

Development and Characterization of Low-Emitting Ceramics

The infrared-optical properties of ceramics are correlated with the complex index of refraction of the material and the structure of the ceramic. By changing these parameters, the infrared-optical properties can be changed over a relatively wide range. The correlation of the structural properties (like the porosity or the pore sizes) and the material properties (such as the complex index of refraction on the one hand and the infrared-optical properties such as emittance on the other) are described by a solution of the equation of radiative transfer and the Mie-theory. Within this work, low-emitting ceramics, which have significantly lower emittances than conventional ceramics, were prepared by optimizing their composition and structure. The spectral emittance of these ceramics was measured, and a total emittance dependent on temperature was calculated from the spectral emittance. As a result, one obtains ceramics which have a total emittance of 0.2 at a temperature of 1,100 K. In comparison to conventional ceramics with a typical total emittance of 0.8 at 1,100 K, the use of such low-e ceramics leads to a reduction in heat transfer of about 70% via thermal radiation. The results of our calculations were compared with experimental data to validate the theory.

Effects of ice crystal habit on thermal infrared radiative properties and forcing of cirrus

The impact of assumed ice crystal morphology on thermal infrared (IR) radiative properties of subtropical cirrus is quantified. In particular, the crystal‐shape‐dependent profiles of downwelling and upwelling thermal IR (broadband and spectral) irradiances and the radiative forcing of cirrus (at the top and bottom of the atmosphere) are investigated. For this purpose, airborne measurements of ice crystal size distribution (in terms of ice crystal maximum dimension) from the CRYSTAL‐FACE campaign and a recently published library of thermal IR optical properties of nonspherical ice crystal habits are implemented into radiative transfer simulations. Two cirrus cases are studied in detail: (1) a high (cold) cirrus with small optical thickness (≈1 at 10.8 μm wavelength) and (2) a lower (warmer) cirrus of relatively large optical thickness (≈7). The relative effects of ice crystal shape on thermal IR irradiance are substantial for the high, optically thin cirrus (up to 70%). Spectrally, the largest effects of ice crystal shape are identified in the atmospheric window spanning from 8 to 12 μm wavelengths, especially for the upwelling irradiance above the cirrus. For the low cirrus of large optical thickness the thermal IR irradiance is only slightly sensitive to ice crystal habit (less than 15–20%). Within the major gas absorption bands the thermal IR radiation is essentially insensitive to ice crystal shape. Furthermore, it is concluded that the thermal IR radiative forcing at the top of the atmosphere contains significant ice crystal shape dependence for the high cirrus case.