Heat transfer in silicone fluids loaded with high thermal conductivity carbon nanofibers was studied using photoacoustics and thermal wave resonator cavity. It is shown that heat transport depends strongly on volume fraction of carbon nanofibers; in particular, a low loading percentage is enough to obtain significant changes in thermal conductivity. Theoretical models were used to determine how heat transfer is affected by structural formations in the composite, such as packing fraction and aspect ratio (form factor) of carbon nanofiber agglomerates in the high viscosity fluid matrix. Our results may find practical applications in systems, in which the carbon nanofibers can facilitate heat dissipation in the electronic devices.

Solar collectors represent an attractive green technology for water heating, where sunlight is efficiently absorbed by a selective coating and the generated heat is transferred to water. In this work, the improvement and scale-up of an electrodeposited black nickel selective coating with a modified silica (MS) film deposited by spray pyrolysis are reported. The MS material was prepared by the sol–gel method using tetraethyl orthosilicate with the addition of n-propyl triethoxysilane to obtain a porous film with an adequate refractive index and enhanced flexibility. The reflectance of electrodeposited selective coatings was characterized with and without the MS film and compared to a commercially available coating of black paint. The MS film increased the solar absorptance from 89% to 93% while maintaining a much lower thermal emittance than the painted coating. The reflectance of the MS film remained unchanged after prolonged thermal treatment at 200 °C (200 h). The fabrication process was scaled up to 193 cm × 12 cm copper fins, which were incorporated in commercial-size flat-plate solar collectors. Three complete collectors of an area of 1.7 m² were fabricated and their performance was evaluated under outdoor conditions. The results show that the electrodeposited selective coating with the MS film outperformed both the commercial black paint system and the system without the modified silica film.

We propose a methodology to narrow down the selection of design parameters of phantoms for calibrating ultrasound devices. For doing so, we model the phantoms as composite materials. Through a mathematical homogenization technique, the global properties of the phantom are given in terms of the volume fractions of its components, their geometric shapes and physical properties. We explore these parameters to determine ranges that yield the desirable values of acoustic impedance and attenuation. The procedure is illustrated for a class of sandwich-like phantoms made of layers of agar of different concentrations with complex elastic moduli. In this example, homogenization provides analytical formulae of the global properties that can be used to identify the layers’ thicknesses and concentrations that yield a low impedance and an average attenuation coefficient. These analytical results are compared with a numerical homogenization method, showing excellent agreement.

Spectral selective coatings are indispensable in solar thermal technologies for enhancing energy efficiency by maximizing solar absorption while suppressing thermal re-emission. This work presents a novel, cyanide-free electrodeposition method to develop high-performance selective coatings on aluminum substrates, using an alkaline copper-citrate bath as a sustainable and cost-effective adhesion layer. The copper plating process was evaluated across a pH range of 8.2 to 10.0 and characterized using speciation diagrams, UV-Vis spectrophotometry, cyclic voltammetry, and rotating-disk electrode (RDE) measurements with Koutecký–Levich analysis. Speciation diagrams and electrochemical data revealed that the [Cu2Cit2H-2]4- dimer is the dominant species in the pH 6–11 range. While this complex introduces a kinetic overpotential that suppresses deposition rates, it is fundamental for achieving the surface homogeneity and adhesion needed for subsequent layers. Building upon this optimized copper base, a nickel/black nickel selective coating was synthesized via controlled galvanostatic pulses. The coatings were characterized by reflectance spectroscopy, profilometry, SEM, XPS, FTIR, and XRD. Reflectance measurements prove that the selective coatings achieve a solar absorptance (α) of 94.3 % and a low thermal emittance (ε100) of 21.5 %, significantly outperforming commercial solar paint (α =89.2 % and e100 =59 %). Furthermore, photothermal heating curves confirmed that this electrochemical route yields superior photothermal conversion efficiency (ηST = 89.0 %) and remains thermally stable at 200 °C. This study offers a simplified, scalable, and environmentally friendly alternative for the large-scale implementation of high-efficiency solar thermal collectors.