Abstract
Abstract Freezing of cobalt nitrate hexahydrate (CoNHH) under a static magnetic field was studied at three different magnetic field intensities. The results show that the magnetic field has a significant effect when the sample enters the solid phase and reduces the stochastic behaviour of the nucleation. Thermal properties investigated using thermogravimetric and differential thermal analysis revealed the shift to a lower temperature for the onset of decomposition and melting temperature after the freeze-thaw in a magnetic field. Besides that, we observed a significant change in the FT-IR peaks that might indicate the emergence of cobalt nitrate with a lower hydrate number, a phase separation effect, and the evaporation of NO3- ions. Refinement of the XRD spectra shows a single phase of CoNHH with a slight lattice parameter change after the freezing-thawing in a magnetic field. The freezing behaviour of CoNHH was compared to that observed previously for water, a salt solution, and an ethylene glycol solution. We investigated the relationship of the freezing behaviour with hydrogen bonding and magnetic properties and its impact on the change in the Gibbs free energy. The results of this study are important to optimise the performance of CoNHH as latent thermal energy storage.
Highlights
The diminishing availability of fossil resources, in addition to negative impacts on the environment, has accelerated the shift towards sustainable energy resources[1]
The nucleation temperature, Tn, that occur at nucleation time, tn, is the lowest at the first crystal nucleus formation with the critical radius of the crystal, before it increases abruptly due to the release of latent heat to reach a freezing temperature, Tf, and thereafter, it decreases monotonously
The differential scanning calorimetry (DSC) thermogram indicates the decrease of the melting temperature and melting enthalpy
Summary
The diminishing availability of fossil resources, in addition to negative impacts on the environment, has accelerated the shift towards sustainable energy resources[1]. Due to the energy crisis, an optimal method to store renewable energy is crucial These circumstances encourage the development of a method to ensure efficient and sustainable energy storage[1]. TES stores thermal energy in the form of latent or sensible heat through heating (melting, evaporation) or cooling (freezing, condensation) of the storage medium. TES technology, which has developed since 1970s, can be one of the solutions for overcoming the imbalance between energy supply and demand and can Latent TES using a phase change material (PCM) is considered more efficient than sensible TES because it offers greater energy density (per unit mass or volume), with a small temperature difference (even almost zero) between the charging and discharging processes[3,4]. A relatively high supercooling degree and phase separation effect are the main considerations for inorganic salt hydrate PCMs, besides the possibility of a change in the stoichiometric composition during heating and cooling[6,7]
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