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- Di-Alkyl adipates as new phase change material for low temperature energy storagePublication . Sequeira, Maria Carolina; Nogueira, Bernardo A.; Caetano, Fernando J. P.; Diogo, Herminio; Fareleira, João M. N. A.; Fausto, RuiThis work is a contribution to the thermal characterization of a selected binary system of two di-n-alkyl adipates that can be used as phase change material for thermal energy storage at low temperatures. The construction of the solid–liquid phase diagram using differential scanning calorimetry (DSC), complemented with Raman Spectroscopy studies for the system composed by diethyl and dibutyl adipates is presented. The solidus and liquidus equilibrium temperatures were determined by DSC for the pure components and 30 binary mixtures at selected molar compositions were used to construct the corresponding solid–liquid phase diagram. The binary system of diethyl and dibutyl adipates presents eutectic behaviour at low temperatures. The eutectic temperature was found at 240.46 K, and the eutectic composition was determined to occur at the molar fraction xdibutyl = 0.46. Additionally, the system shows a polymorphic transition, characteristic of dibutyl adipate, occurring at ca. 238 K, confirmed by optical microscopy. To the best of our knowledge, no reference to the phase diagram of the present system could be found in the literature. Raman spectroscopy was essential to complement the construction of the phase equilibrium diagram, enabling the identification of the solid and liquid phases of the system. Finally, the liquidus curve of the phase diagram was also successfully predicted using a suitable fitting equation, being the root mean square deviation of the data from the correlation equal to 0.54 K. In addition, this fitting operation enabled a correct prediction of the eutectic composition of the system.
- Adipates as new phase change material: seeking for a low temperature energy storage systemPublication . Sequeira, Maria Carolina; Nogueira, Bernardo A.; Caetano, Fernando J. P.; Diogo, Herminio; Fareleira, João M. N. A.; Fausto, RuiThe recent and endless increasing need for energy is requiring imperative and efficient solutions. Renewable energies are an effective solution, but they are also intermittent, which often leads to a gap between the availability of energy and its demand. Therefore, energy storage becomes crucial for improving the efficiency by reducing the mismatch between demand and supply thus offering better management capabilities. The use of phase change materials (PCMs) became an attractive technology, especially for active cooling systems. In particular, eutectic systems can be designed for each application, allowing significant energy savings, which is very relevant in what concerns economic and environmental sustainability [1]. Equilibrium studies and their phase diagrams are crucial to characterize the behavior of these systems, particularly for new PCMs [2]. This work aims the characterization of a selected type of binary system composed by di-n-alkyl adipates that can be used for energy storage at low temperatures, starting by the construction of solid-liquid phase diagram based on Differential Scanning Calorimetry (DSC) and Raman Spectroscopy results. The studied system is a binary mixture of diethyl and dibutyl adipates and reveals to behave as a eutectic system at low temperatures, possessing also polymorphism, therefore presenting a relatively complex phase diagram. The eutectic point was found around -33ºC and the polymorphism transition, being characteristic of the dibutyl adipate, occurs at around -36ºC for the overall phase diagram. Raman spectroscopy was fundamental concerning the construction of the phase diagram, enabling to identify the different solid and liquid phases of the system. This work provides new phase equilibrium data, which, to the knowledge of the authors, are the first in the literature, concerning these compounds. The studied binary system has promising characteristics for low temperature energy storage. It is also demonstrated how solid-liquid-phase equilibrium studies are the key to select the most appropriate phase change material for a specific thermal energy storage (TES) application.
- Low temperature energy storage PCM systems: phase equilibrium studiesPublication . Sequeira, Maria Carolina; Nogueira, Bernardo A.; Caetano, Fernando J. P.; Diogo, Herminio; Fareleira, João M. N. A.; Fausto, Rui
- Solid–liquid phase equilibrium of the n-Nonane + n-Undecane system for low-temperature thermal energy storagePublication . Nikitin, Timur; Sequeira, Maria Carolina; Caetano, Fernando J. P.; Fareleira, João M. N. A.; Fausto , Rui; Diogo, HerminioThe current article presents an exploration of the solid–liquid phase diagram for a binary system comprising n-alkanes with an odd number of carbon atoms, specifically n-nonane (n-C9) and n-undecane (n-C11). This binary system exhibits promising characteristics for application as a phase change material (PCM) in low-temperature thermal energy storage (TES), due to the fusion temperatures of the individual components, thereby motivating an in-depth investigation of the solid–liquid phase diagram of their mixtures. The n-nonane (n-C9) + n-undecane (n-C11) solid–liquid phase equilibrium study herein reported includes the construction of the phase diagram using Differential Scanning Calorimetry (DSC) data, complemented with Hot–Stage Microscopy (HSM) and low-temperature Raman Spectroscopy results. From the DSC analysis, both the temperature and the enthalpy of solid–solid and solid–liquid transitions were obtained. The binary system n-C9 + n-C11 has evidenced a congruent melting solid solution at low temperatures. In particular, the blend with a molar composition xundecane = 0.10 shows to be a congruent melting solid solution with a melting point at 215.84 K and an enthalpy of fusion of 13.6 kJ·mol–1. For this reason, this system has confirmed the initial signs to be a candidate with good potential to be applied as a PCM in low-temperature TES applications. This work aims not only to contribute to gather information on the solid–liquid phase equilibrium on the system n-C9 + n-C11, which presently are not available in the literature, but especially to obtain essential and practical information on the possibility to use this system as PCM at low temperatures. The solid–liquid phase diagram of the system n-C9 + n-C11 is being published for the first time, as far as the authors are aware.
- Low temperature thermal energy storage: insights into odd-even n-alkane systemPublication . Sequeira, Maria Carolina; Nogueira, Bernardo; Nikitin, Timur; Caetano, Fernando J. P.; Diogo, Herminio; Fausto, Rui; Fareleira, João M. N. A.
- Solid-liquid phase equilibrium: in search of suitable PCMs for low temperature energy storagePublication . Sequeira, Maria Carolina; Caetano, Fernando J. P.; Diogo, Herminio; Fareleira, João M. N. A.
- On capillary viscosity measurements: how far do surface tension effects go?Publication . Sequeira, Maria Carolina; Caetano, Fernando J. P.; Diogo, Herminio; Fareleira, João M. N. A.; Santos, Fernando J. V.; Serro, A. P.Viscosity is a fundamental thermophysical property of liquids making it very important particularly in the industry. Capillary viscometers have been widely used for viscosity measurements in different applications, the most relevant being the definition of viscosity standards, traceable to the primary water viscosity standard, by metrological institutions and industrial applications, mostly for quality control. Practical viscometry is based on the internationally accepted primary standard value for the kinematic viscosity of water at 20ºC and atmospheric pressure, which has been measured using capillary viscometers [1]. However, due to the water surface tension, viscosity measurements which have been related to water as a primary standard, can be significantly affected. It is difficult to rigorously assess the surface tension effects on capillary viscometers, and the practical way to avoid this problem is to use long capillaries, which are not appropriate for routine measurements [1-3]. After several experimental studies, using different types of viscometers, the usual procedure to correct surface tension effects in capillary viscosity measurements adopted by different authors, is to employ an empirical expression [1-4]. Additionally, other types of problems exist as the need to perform a kinetic energy correction which must also be taken into consideration [1]. The main goal of this work was to perform the calibration of a suspended-level, or Ubbelohde, capillary viscometer, which is not a long capillary viscometer, as well as the study of corrections to be used for the measurements performed with it. The experimental work covers the calibration of that Ubbelohde capillary viscometer, the evaluation of the uncertainty of the corresponding viscometer constant and the overall uncertainty of the measurements performed with it. This study includes the evaluation of the necessary corrections for kinetic energy and surface tension effects and, finally, the analysis of the case of a set of measurements performed with n-tetradecane. The ultimate purpose of this work is to obtain the lowest uncertainty for the Ubbelohde capillary viscometer 541 01/Ia, and to understand the need for the corrections that must be considered when using capillary viscometers and how they should be applied.
- Solid-liquid phase equilibrium: alkane systems for low temperature energy storagePublication . Sequeira, Maria Carolina; Nogueira, Bernardo A.; Caetano, Fernando J. P.; Diogo, Herminio; Fareleira, João M. N. A.; Fausto, RuiThe incessant and increasing need for energy requires urgent and effective solutions. The supply of renewable energies is mainly intermittent, which often leads to a gap between the availability of energy and its consumption. Therefore, energy storage becomes imperative for increasing the efficiency in the use of the produced energy. Concerning this, the use of phase change materials (PCMs) has taken a very important role, especially for active cooling systems. In particular, eutectic systems can be designed with characteristics that make them suitable for each application, allowing significant energy savings, which is very relevant in what concerns economic and environmental sustainability [1]. This work aims to the characterization of selected systems that can be used for energy storage at low temperatures, starting by the construction of solid-liquid phase diagrams using differential scanning calorimetry (DSC) and Raman Spectroscopy results. Phase diagrams are crucial to characterize the behaviour of these systems and reveals the robustness of the PCM, and for new PCMs, equilibrium studies are essential to achieve the desired melting temperature [2]. This work is focused on binary systems composed by normal alkanes, in particular, the binary mixtures C8-C10 and C10-C12. These systems revealed to behave as eutectic systems at low temperatures, showing a promising applicability for thermal energy storage for low temperature applications. The eutectic points are around -61ºC and -35ºC, respectively. Additionally, a fitting equation for the experimental liquidus lines of the phase equilibrium diagrams is proposed for each system.
- Deep eutectic solvents (DES) based on sulfur as alternative lubricants for silicon surfacesPublication . Antunes, Mónica; Campinhas, Anne-Sophie; Freire, Mariana de Sá; Caetano, Fernando J. P.; Diogo, Herminio; Colaço, Rogério; Branco, Luís C.; Saramago, BenildeDeep eutectic mixtures composed of hydrogen-bond donors (HBDs) and hydrogen-bond acceptors (HBAs), the so-called DESs, have recently being proposed as possible “green” alternatives to mineral oils and ionic liquids (ILs) in the lubrication of steel surfaces. DESs have similar physical properties to ILs but have the advantage of being cheaper and easier to prepare. In this work, new DESs containing sulfur units in their composition were prepared and tested in the lubrication of silicon surfaces which are relevant for nano/microelectromechanical systems (NEMS/MEMS). The following new DESs were prepared: ethyldibutyl sulfonium ethylsulfate: polyethylene glycol ([S4,4,2][EtSO4]:PEG), 1-ethyl-tetrahydrothiophenium ethylsulfate: polyethylene glycol ([C2-THT][EtSO4]:PEG), 1-ethyl-3-methylimidazolium (S)-camphorsulfonate: polyethylene glycol ([C2MIM][(S)CSA]:PEG), and 1-methyl-3-picolinium methyl sulfate: polyethylene glycol ([C1-3-pic][MeSO4]:PEG). Other DES, already reported, were tested for comparison purposes: tetrabutylammonium bromide: sulfolane ([N4,4,4,4][Br]:Sulf), choline chloride: polyethylene glycol (ChCl:PEG), and tetrabutylammonium bromide: polyethylene glycol ([N4,4,4,4][Br]:PEG). All DESs were characterized in terms of their water content, viscosity, wettability, and tribological properties. The friction coefficients were measured in a nanotribometer using steel spheres against Si surfaces. The new DES prepared from salts based on the sulfur-containing anions showed good tribological performance, but the best results were obtained with [C2MIM][(S)-CSA]:PEG and [C1-3-pic][MeSO4]:PEG which reduced the friction coefficients to values <0.1, typical of excellent lubrication conditions.
- A thermodynamic journey: unlocking cold thermal energy storage with phase equilibrium studiesPublication . Sequeira, Maria Carolina; Nikitin, Timur; Caetano, Fernando J. P.; Diogo, Herminio; Fareleira, João; Fausto, RuiAs the urgent demand for sustainable energy solutions increase, thermal energy storage (TES) systems have become crucial in improving energy efficiency and ensuring supply-demand balance1. In low-temperature sectors such as refrigeration, cold-chain logistics, and medical applications, phase change materials (PCM) are particularly valuable due to their high latent heat capacity and reversible phase transitions1,2. However, finding PCM that are not only efficient and reliable, but also environmentally safe, remains a challenge. Over the last few years, our research has focused on phase equilibrium studies of potential organic PCM, providing valuable experimental data and thermodynamic insights for designing low-temperature TES systems. This work began with the study of binary systems of di-n-alkyl adipates, specifically diethyl/dibutyl adipates, which revealed eutectic behaviour with a eutectic point at -33 ºC. Expanding on this, we investigated n-alkane systems, including n-octane/n-decane and n-decane/n-dodecane, both of which exhibited eutectic transitions at sub-zero temperatures. Recently, efforts focused on n-alkanes, namely n-nonane/n-decane and n-nonane/n-undecane, paying special attention to odd–even carbon chain effects on the phase diagrams. These studies uncovered new low-temperature PCM potential candidates and clarified phase change behaviours for this kind of systems. The latest work on adipates, particularly dimethyl and dipropyl, revealed a eutectic behaviour near -20 ºC, showing great suitability as low-temperature PCM. Those studies significantly expand the database of viable PCM, enhancing our understanding of low-temperature phase behaviour. This work brings together four years of contributions to the rapidly advancing field of sustainable low-temperature TES, offering both practical and theoretical insights.