Magnetic ferrites are used in a wide range of technological applications, such as biomedicine, electronics, or energy. They also present interesting magnetic properties, especially for high-frequency applications. These materials have been prepared by a large variety of methods, from basic solid-state reaction to advanced wet chemical methods. However, most of these approaches are complex and have low production rates. In this work, a deep analysis of an easy polymer-assisted sol–gel synthesis of copper ferrite (CuFe2O4) nanoparticles is done. A multivariate analysis, by means of the design of experiments approach, is carried out to account for two variables at a time. Moreover, a wide experimental domain is explored, and the impact of each variable on the chemical composition and magnetic properties is determined. This work results in an optimization of the synthesis method to obtain high-purity nanoparticles (∼96%).

In this work, magnetic copper ferrite nanoparticles are synthesized by polymer-assisted sol-gel and co-precipitation methods. The obtained purity and particle size reach values of 96 % and 94 nm, respectively. Evident differences in the crystal structure have been found in the synthesized nanoparticles. A tetragonal structure is formed by the sol-gel method, while the cubic form is obtained when the co-precipitation approach is used. This work provides an experimental evidence of the formation of both phases by using the same reactants and thermal conditions, and only modifying the technical procedure. The formation and stability of each phase is analysed by temperature dependent measurements, and the observed crystal structure differences are used to propose a potential fundamental explanation to our observations based on a difference on the cations' distribution and the Jahn-Teller distortion. Moreover, different copper ferrite purity and particle sizes are found when using each of the methods. The spherical shape of the particles and their tendency to sinter forming micrometric clusters are observed by electron microscopy. Finally, the divergence in magnetization between the samples prepared by each method support our argument about the different cations' distribution and open the door to a wide range of different technological applications for these materials.

We report evidence of the magnetization reversal in nanoparticles by surface acoustic waves (SAWs). The experimental system consists of isolated magnetite nanoparticles dispersed on a piezoelectric substrate. Magnetic relaxation from a saturated state becomes significantly enhanced in the presence of the SAW at a constant temperature of the substrate. The dependence of the relaxation on SAW power and frequency has been investigated. The effect is explained by the effective ac magnetic field generated by the SAW in the nanoparticles.

The use of adequate thermal energy storage (TES) systems is an opportunity to increase energy efficiency in the building sector, and so decrease both commercial and residential energy consumptions. Nano-enhanced phase change materials (NEPCM) have attracted attention to address one of the crucial barriers (i.e. low thermal conductivity) to the adoption of phase change materials (PCM) in this sector. In the present study two PCM based on fatty acids, capric and palmitic acid, were nano-enhanced with low contents (1.0 wt.%, 1.5 wt.% and 3.0 wt.%) of copper (II) oxide (CuO) nanoparticles. Copper (II) oxide (CuO) was synthesized via coprecipitation method obtaining 60–120 nm diameter sized nanoparticles. Thermal stability and high thermal conductivity were observed for the nano-enhanced phase change materials (NEPCM) obtained. Experimental results revealed remarkable increments in NEPCM thermal conductivity, for instance palmitic acid thermal conductivity was increased up to 60% with the addition of 3 wt.% CuO nanoparticles. Moreover, CuO nanoparticles sedimentation velocity decreases when increasing its content.

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