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In recent years, polymer engineering, at the molecular level, has proven to be an effective strategy to modulate thermal conductivity. Polymers have great applicability in the food packaging industry, in which transparency, lightness, flexibility, and biodegradability are highly desirable characteristics. In this work, a possible manner to adjust the thermal conductivity in cassava starch biopolymer films is presented. Our approach is based on modifying the starch molecular structure through the addition of borax, which has been previously used as an intermolecular bond reinforcer. We found that the thermal conductivity increases linearly with borax content. This effect is related to the crosslinking effect that allows the principal biopolymer chains to be brought closer together, generating an improved interconnected network favoring heat transfer. The highest value of the thermal conductivity is reached at a volume fraction of 1.40% of borax added. Our analyses indicate that the heat transport improves as borax concentration increases, while for borax volume fractions above 1.40%, heat carriers scattering phenomena induce a decrement in thermal conductivity. Additionally, to obtain a deeper understanding of our results, structural, optical, and mechanical characterizations were also performed.

Reticulate porous ceramic reactors use foam-type absorbers in their operation which must fulfill two essential functions: favoring the volumetric effect and increasing the mass and heat transfer by acting as a support for the reactive materials. Heating these absorbers with highly inhomogeneous concentrate irradiation induces high thermal gradients that affect their thermal performance. Owing to the critical function of these component in the reactor, it is necessary to define a selection criterion for the foam-type absorbers. In this work, we performed an experimental and numerical thermal analysis of three partially stabilized zirconia (PSZ) foam-type absorbers with pore density of 10, 20, and 30 PPI (pores per inch) used as a volumetric absorber. A numerical model and an analytical approximation were developed to reproduce experimental results, and calculate the thermal conductivity, as well as volumetric heat transfer coefficient. The results show that an increase in pore density leads to an increase in the temperature difference between the irradiated face and the rear face of the absorber, this occurs because when pore density increases the concentrated energy no longer penetrates in the deepest space of the absorber and energy is absorbed in areas close to the surface, therefore, temperature gradients are created within the porous medium. The opposite effect occurs when the airflow rate increases, the temperature gradient between the irradiated face and the rear face is reduced. This behavior is more noticeable at low pore densities, but at high pore densities, the effect is less relevant because the internal structure of porous absorbers with high pore density is more complex, which offers obstructions or physical barriers to airflow and thermal barriers to heat transfer. When the steady state is reached, the temperature difference between the two faces of the absorber remains constant if the concentrate irradiation changes slightly, even changing the airflow rate. The results obtained in this work allow us to establish a selection criterion for porous absorbers that operate within solar reactors, this criterion is based on knowledge of the physical properties of the porous absorber, the environment, the working conditions, and the results expected.

International audience; Carbon fiber-reinforced carbon (C/C) composites consist in a carbon matrix holding carbon or graphite fibers together, whose physical properties are determined not only by those of their individual components, but also by the layer buildup and the material preparation and processing. The complex structure of C/C composites along with the fiber orientation provide an effective means for tailoring their mechanical, electrical, and thermal properties. In this work, we use the Laser Flash Technique to measure the thermal diffusivity and thermal conductivity of C/C composites made up of laminates of weaved bundles of carbon fibers, forming a regular and repeated orthogonal pattern, embedded in a graphite matrix. Our experimental data show that: i) the cross-plane thermal conductivity remains practically constant around (5.3 ± 0.4) W*m−1 K−1, within the temperature range from 370 K to 1700 K. ii) The thermal diffusivity and thermal conductivity along the cross-plane direction to the fibers axis is about five times smaller than the corresponding ones in the laminates plane. iii) The measured cross-plane thermal conductivity is well described by a theoretical model that considers both the conductive and radiative thermal contributions of the effective thermal conductivity.

The efficient generation of renewable energies from solar radiation implies the precise knowledge of the thermophysical properties of the materials and systems involved in such applications, this knowledge is fundamental, since it allows us to evaluate and optimize the performance of materials and methods. One of the great challenges for the efficient use of solar energy is the study of the phenomena involved in the interaction of radiation with matter, and the conversion of energy absorbed into heat. Photothermal and thermographic techniques are ideal methodologies to study these phenomena, since they are non-contact techniques (NCT) and, therefore, can be applied in a wide range of operating temperatures and environments. In general, photothermal techniques consist in illuminating the surface of the sample with a laser light beam modulated or pulsed, therefore the sample is heated and emits infrared radiation IR (among other types of energy). The variations of the IR energy are related to the thermophysical properties of the specimens photothermal techniques are based on the detection of these variations. Some examples of photothermal methodologies are the photothermal radiometry pulsed (PPTR) and modulated (PTR), the photothermal reflectometry (RM) and the thermal wave resonant cavity (TWRC). Infrared thermography is a powerful method of non-contact measurement, using this technique thermal images or thermograms are obtained, which are a representation of the temperature distribution on the surface of an object or a scene. The thermophysical properties of materials they are obtained through the analysis of amplitude and phase signals, as well as thermograms. Reflectance spectroscopy is a powerful tool for characterizing the optical properties (solar absorbance and thermal emittance) of a great diversity of materials applied in solar technology. The analysis can be done in two wavelength ranges of interest: Ultraviolet/visible/near infrared and medium infrared, where solar spectrum radiation wavelengths and infrared emission range are located respectively. The methodologies mentioned have been applied in the characterization of materials at room temperature, however, many processes involved in the new applications of solar energy occur at high temperature. It has also been shown that thermophysical mechanisms related to the conversion of energy that occur at room temperature are not dominant in high temperature regimes therefore, the measurement of the conversion, absorption, emission and energy exchange processes to high temperature, they involve new technological challenges. In this work, we present a summary about the photothermal, thermographic and reflectance spectroscopy techniques, used in the evaluation of the thermophysical properties of solar absorbers applied in low temperature solar systems, as well as high temperature concentrate solar power (CSP) systems.

The efficiency of the conversion of solar energy into thermal energy is determined by the optical and thermal properties of the selective coating, in particular, the solar absorptance and thermal emittance at the desired temperature of the specific application. Photothermal techniques are the most appropriate methods to explore these properties, however, a quantitative determination using photothermal radiometry, which is based on the measurement of emitted radiation caused by the heating generated by a modulated light source, has proven to be elusive. In this work, we present experimental results for selective coatings based on electrodeposited black nickel–nickel on both stainless steel and copper substrates, as well as for commercial TiNOX coatings on aluminum, illustrating that the radiation emitted by the surface depends on the optical absorption, thermal emissivity and on the light-into-heat energy conversion efficiency (quantum efficiency). We show that a combination of photothermal radiometry and photoacoustic spectroscopy can successfully account for these parameters, and provide values for the emissivity in agreement with values obtained by Fourier-transform infrared spectroscopy.

High temperature ageing is frequently used in many technical application areas for the assessment of the thermal stability of materials; a high temperature treatment accelerates all kinds of processes, frequently performed to induce stabilization of the material. For solar absorber surfaces composed of small metal particles a high temperature treatment accelerates oxidation of the metal which in turn decreases mainly the optical properties of the coatings. Likewise changes of the surface due to machining, heat treatment, roughness and porosity can induce changes in effective thermal properties of the surface, which would affect cooling or heating process of the surface. In photothermal deflection spectroscopy (PDS) technique the sample absorbs optical radiation (pump laser) and heats the gas or liquid above the surface, the heat changes the refraction index of the fluid. A second low power laser beam is sent to probe the changes of the refraction index, in such a way that the photoinduced deviation of the probe beam are monitored using a position sensor detector. It has been shown that the beam deflection is strongly correlated to the optical and thermal properties of the materials, and it can provide important parameters as the optical absorption coefficient as well as the thermal diffusivity of the material. This non-contact technique can be particularly useful when analyzing materials in extreme conditions or when a methodology in which attachment of the sensors to the sample is not desirable or even possible. In this work, the changes of the thermal and optical properties of selective coatings exposed to a thermal treatment consisting in heating at 573 K for 200 hours, were studied using a combination of photothermal deflection spectroscopy (PDS) technique, UV/VIS/Near spectroscopy and FTIR. Samples consist of steel substrates coated with solar absorber coatings based in black cobalt and black nickel, deposited using electrochemical methods. The layer structure of black cobalt solar absorbers consisted in a bright nickel infrared reflective layer (thickness: 3.12 µm) covered with a black cobalt high solar absorbance thin film (thickness: 0.28 µm). In the same way, layer structure of black nickel solar absorbers consisted in a bright nickel infrared reflective layer (thickness: 3.12 µm) covered with a black nickel high solar absorbance thin film (thickness: 0.33 µm).

Concentrating solar power (CSP) systems use solar absorbers to convert sunlight into thermal electric power. In CSP systems, a high reflective surface focuses sunlight onto a receiver that captures the solar energy and converts it into heat. The operation of high efficiency CSP systems involves improvements in the performance of the coatings of the solar absorption materials. To accomplish this, novel, more efficient selective coatings are being developed with high solar absorptance and low thermal losses at their operation temperature. Heat losses in a CSP system occur by three mechanisms: conduction, convection and radiation. It has been widely documented that energy losses increase with increasing operating temperature of CSP systems, and the precise knowledge of the thermophysical properties of the materials involved in CSP systems may allow us to increase the efficiency of systems. In this work, we applied the pulsed photoradiometry technique (PPTR) to evaluate the changes in the thermophysical prope...

Solar technology operating at elevated temperature conditions demands accurate knowledge of the optical and thermal properties of the materials involved in the construction and operation of solar collectors, reactors, and energy storages, among many others. Thermal energy storage (TES) devices involve successive melting and crystallization processes, which result in high complexity materials where the morphology, composition, and porosity could be highly non-homogeneous. In these cases, contact techniques for determining the thermal properties are highly susceptible and do not provide reliable measurements. It is under these conditions that non-contact photothermal techniques can provide superior performance, because in this case, the heat inducing source is a laser beam and the detector is usually a photodiode or a thermographic camera which are in non-contact with samples. The materials applied as storage medium in a TES unit can be divided into four groups: metals and alloys, ceramics and glasses, polymers and elastomers, and composites that include natural materials. Soda lime silicate glass recyclable waste is a very promising material for storage medium due to its inexpensive and wide availability. In this paper, we examined soda lime silicate glass-graphite composites for use as storage medium in a TES unit. A simple one-dimensional model for thermal conductivity was developed based on equivalent thermal circuits for series-parallel composite walls, and we found that thermal conductivity values depend on the amount of graphite dispersed into the samples, the porous media, and their structure.

International audience; The infrared emissivity of glassy carbon, stainless steel, and stainless steel with a selective coating of NiNiO has been determined using a thermal-wave resonant cavity heated with a modulated laser beam. This is achieved performing a length scan of the cavity at a fixed frequency of 5 Hz. It is observed experimentally that (1) the mechanisms of heat conduction and radiation co-exist inside the cavity, through the coupling of the dc and ac components of the temperature, and (2) the radiation effect shows up in both the amplitude and phase signals for cavity thicknesses greater than the diffusion length of the intra-cavity air. Using a suitable theoretical model, the experimental data for the amplitude and phase have allowed the determination of the infrared emissivity of the studied materials.

Selective solar absorbing coating consists of a high thermal reflectance layer and high solar absorbance layer deposited over a substrate. In this work optical and thermal properties of high reflective surface were characterized with infrared photothermal radiometry and photoacoustic spectroscopy

A simple and reliable methodology for determining the Young's modulus of magnetorheological elastomers is proposed based on the resonance frequencies of the amplitude of the photoacoustic signal. An explicit expression for the pressure changes within a photoacoustic cell, due to the thermal expansion of the air and the elastic bending of a clamped circular elastic membrane, is derived and analyzed. It is found that the resonance behavior of the amplitude of the photoacoustic signal is due to the contribution of the axial bending of its thickness. It is also shown that the Young's modulus of the membrane is proportional to its density, the square of its resonance frequencies and the fourth power of its radius, and inversely proportional to the square of its thickness. The application of the proposed approach to membranes made up of spherical microparticles of carbonyl iron powder embedded in a matrix of silicone rubber with weight concentrations of 0%, 5.2%, and 13.7% yields accurate and reproducible resul...