Improved thermal energy storage of nanoencapsulated phase change materials by atomic layer deposition

Renewable energy has become of great interest over the past years in order to mitigate Global Warming. One of the actions gaining attention is the enhancement of the thermal energy storage capacity of Concentrated Solar Power plants. The addition of nanoencapsulated phase change materials (core-shell nanoparticles) to the already used materials has been proposed for that purpose, due to the possibility of increasing thermal storage through the contribution of both core latent heat and sensible heat. In this work, Atomic Layer Deposition has been used to synthesise SiO2 and Al2O3 nanoscale coatings on tin nanoparticles. The multi-encapsulated phase change materials have been characterised in terms of chemical composition, crystalline structure, particle size, thermal stability and thermal storage capacity. Sn@Al2O3 nanoparticles present the best thermal behaviour as they show the lowest reduction in the phase change enthalpy over 100 cycles due to the oxidation barrier of the coating. Moreover, the specific heat of both nanoparticles and solar salt-based nanofluids is increased, making the nanoencapsulated phase change material suitable for thermal energy storage applications

Sub-nanoscale Surface Engineering of TiO2 Nanoparticles by Molecular Layer Deposition of Poly(ethylene terephthalate) for Suppressing Photoactivity and Enhancing Dispersibility

In this work, we report molecular layer deposition (MLD) of ultrathin poly(ethylene terephthalate) (PET) films on gram-scale batches of ultrafine particles for the first time. TiO2 P25 nanoparticles (NPs) are coated up to 50 cycles in an atmospheric-pressure fluidized-bed reactor at ∼150 °C using terephthaloyl chloride and ethylene glycol as precursors. Ex-situ diffuse reflectance infrared Fourier transform spectroscopy, thermogravimetric analysis, and transmission electron microscopy show the linear growth at ∼0.05 nm/cycle of uniform and conformal PET films, which are unattainable with conventional wet-phase approaches. The sub-nanoscale and nanoscale PET films not only suppress the photocatalytic activity of TiO2 NPs by hindering the access of water and reactant molecules to the TiO2 surface but also improve the dispersibility of TiO2 NPs in both organic and aqueous media. Still, the bulk optical properties, electronic structure, and surface area of TiO2 are essentially unaffected by the MLD process. This study demonstrates the industrial relevance of MLD to simultaneously suppress the photoactivity and enhance the dispersibility of commercial TiO2 P25 nanopowders, which is crucial for their use for example as UV-screening agents in sunscreens and as white pigments in paints. Moreover, by rapidly modifying the surface properties of particles in a controlled manner at the sub-nanometer scale, particle MLD can serve many other applications ranging from nanofluids to emulsions to polymer nanocomposites.ISSN:2574-097

Subnano Surface Engineering of TiO2 Nanoparticles by PET Molecular Layer Deposition: Tuning Photoactivity and Dispersibility

In this work, we report molecular layer deposition (MLD) of ultrathin poly(ethylene terephthalate) (PET) films on gram-scale batches of ultrafine particles for the first time. TiO2 P25 nanoparticles (NPs) are coated up to 50 cycles in an atmospheric-pressure fluidized bed reactor at 150 °C using terephthaloyl chloride and ethylene glycol as precursors. Ex-situ diffuse reflectance infrared Fourier transform spectroscopy, thermogravimetric analysis and transmission electron microscopy show the linear growth at 0.05 nm/cycle of uniform and conformal PET films, which are unattainable with conventional wet-phase approaches. The subnano and nano PET films not only suppress the photocatalytic activity of TiO2 NPs by reducing the generation of hydroxyl radicals, but also improve the dispersibility of TiO2 NPs in both organic and aqueous media. Still, the bulk optical properties, electronic structure and surface area of TiO2 are essentially unaffected by the MLD process. This study demonstrates the industrial relevance of MLD to simultaneously tune the photoactivity and dispersibility of the commercial photocatalyst TiO2 P25. Moreover, by rapidly modifying the surface properties of particles in a controlled manner at the subnanometer scale, particle MLD can serve many applications ranging from nanofluids to emulsions to polymer nanocomposites

Numerical analysis of mechanical reliability of multi-coated phase change materials

Nanoencapsulated phase change materials (nePCMs) are nowadays under research for thermal energy storage purposes. NePCMs are composed of a phase change core surrounded by a shell that confines the core when molten. One of the main concerns of nePCMs when subjected to thermal processes is the mechanical failure of the passivation shell initially present in commercial metallic nanoparticles. In order to overcome this issue, multi-coated nePCMs, based on the synthesis of an additional coating by atomic layer deposition, appear to be as a candidate solution. With the objective of studying the influence of the composition and thickness of the additional nePCM shells on their probability of failure, a numerical tool combining a thermomechanical finite element model with phase change and Monte Carlo algorithms is developed. This tool also allows including the uncertainty of material and geometrical properties into the numerical analysis to account for their influence in the mechanical performance of nePCMs. In the present work, the mechanical reliability of SiO2 and Al2O3 coatings on Sn@SnOx nanoparticles is assessed by considering both deterministic and probabilistic failure criteria and Al2O3 coatings appear to have a better mechanical performance than their SiO2 counterparts

Numerical Modeling of the Mechanical Reliability of Multicoated Nanoencapsulated Phase-Change Materials with Improved Thermal Performance

Nanoencapsulated phase-change materials (nePCMs) are investigated for enhancing thermal energy storage. However, the shell of these nanocapsules may fail due to stress developed during thermal processes, leading to melting enthalpy loss. To overcome this problem, SiO2 and Al2O3 coatings on Sn nanoparticles are synthesized by atomic layer deposition (ALD). To study the influence of shell thickness and composition on the probability of failure (POF) of nePCM shells in single- and multicoated nePCMs, a probabilistic numerical tool combining Monte Carlo techniques and a thermomechanical finite-element model with phase change are used. The uncertainties of the material and geometrical properties of nePCMs are included in the analysis. Both deterministic and probabilistic failure criteria are taken into account to consider the effect of dispersion on tensile strength. The results indicate that multicoated nePCMs enhance thermomechanical performance in relation to their single-coated counterparts. Both the numerical simulations and experiments confirm that the POF of nePCM shells and melting enthalpy loss in multicoated nePCMs lower with shell thickness. The results after 50 ALD cycles indicate that Al2O3 coatings exhibit better performance because a POF of 1.66% is obtained with 1.1% enthalpy loss, while the POF for SiO2 is 72.38% with 3.5% enthalpy loss

Improving heat transfer of stabilised thermal oil-based tin nanofluids using biosurfactant and molecular layer deposition

The development of advanced heat transfer fluids (HTF) with enhanced heat transfer properties has been identified as a key target to increase the efficiency of industrial processes. In this work, heat transfer performance improvements of a novel nanofluid, consisting of metallic nanoparticles dispersed in a commercial thermal oil, were investigated. Nanofluids combining tin nanoparticles (1 mass %) with Therminol 66 (TH66) were synthesised using the two step-method and experimentally analysed. The effectiveness of biosurfactant addition and nanoparticle polyethylene terephthalate (PET) nanocoating for high temperature nanofluid stabilisation were independently investigated. The PET nanoscale coatings were grown by molecular layer deposition, which has been used for the first time in this field. The thermal conductivity, dynamic viscosity and specific heat capacity of the stable, oil-based nanofluids were characterised at high temperatures, and the results were compared and in good agreement with models found in the relevant literature. Finally, the heat transfer performance of the nanofluids with respect to their base fluids was evaluated, employing empirical values for the thermophysical properties of the involved materials. In this way, increments of the heat transfer coefficients up to 9.3% at 140 °C, relevant to industrial applications were obtained

Molecular Layer Deposition of Polyurea on Silica Nanoparticles and Its Application in Dielectric Nanocomposites

Polymer nanocomposites (NCs) offer outstanding potential for dielectric applications including insulation materials. The large interfacial area introduced by the nanoscale fillers plays a major role in improving the dielectric properties of NCs. Therefore, an effort to tailor the properties of these interfaces can lead to substantial improvement of the material’s macroscopic dielectric response. Grafting electrically active functional groups to the surface of nanoparticles (NPs) in a controlled manner can yield reproducible alterations in charge trapping and transport as well as space charge phenomena in nanodielectrics. In the present study, fumed silica NPs are surface modified with polyurea from phenyl diisocyanate (PDIC) and ethylenediamine (ED) via molecular layer deposition (MLD) in a fluidized bed. The modified NPs are then incorporated into a polymer blend based on polypropylene (PP)/ethylene-octene-copolymer (EOC), and their morphological and dielectric properties are investigated. We demonstrate the alterations in the electronic structure of silica upon depositing urea units using density functional theory (DFT) calculations. Subsequently, the effect of urea functionalization on the dielectric properties of NCs is studied using thermally stimulated depolarization current (TSDC) and broadband dielectric spectroscopy (BDS) methods. The DFT calculations reveal the contribution of both shallow and deep traps upon deposition of urea units onto the NPs. It could be concluded that the deposition of polyurea on NPs results in a bi-modal distribution of trap depths that are related to each monomer in the urea units and can lead to a reduction of space charge formation at filler-polymer interfaces. MLD offers a promising tool for tailoring the interfacial interactions in dielectric NCs.publishedVersionPeer reviewe