Sedimentation Stability and Caloric Properties of Nanohybrid Composite Thermo-Accumulating Material Paraffin/Thermally Expanded Graphite/Copper Oxide Nanoparticles

A promising direction for increasing the thermal conductivity of composite thermo-accumulating nanomaterials (CTANM) is the introduction of various carbon nanomaterials with high thermal conductivity into paraffin. However, the problem of sedimentation stability of the proposed CTANM samples remains unresolved.The sedimentation stability of the CTANM can be achieved by using thermally expanded graphite (EG) as a porous nanostructure that retains thermo-accumulating material due to the wettability and surface tension. At the same time, a small concentration of TEG does not provide the creation of the CTANM with high thermal conductivity.To increase the thermal conductivity of the CTANM, we propose to fill the EG matrix not only with thermo-accumulating material (paraffin), but also with nanoparticles of metals or their oxides, for example, CuO. The first stage of the proposed method for establishing nanohybrid CTANM is the creation of a limited time-stable isopropanol/CuO nanofluid.The stability of the prepared solution was studied by spectrophotometry in the temperature range from 50 to $60^{\circ} \mathrm{C}$ and wavelengths from 325 to 1000 nm. At the second stage of the method, the EG was subjected to deaeration at the temperature of $200^{\circ} \mathrm{C}$ in order to remove air components sorbed on the surface of the EG pores. At the third stage of the method, the vacuum impregnation of $\mathbf{E G}$ with isopropanol/CuO nanofluid was carried out with subsequent removal of isopropanol due to vacuum at the temperature of $60^{\circ} \mathrm{C}$. At the final stage, the EG/CuO matrix was subjected to vacuum impregnation with deaerated technical paraffin. In present study, the technological characteristics of the obtained samples of nanohybrid CTANMs were examined: the leakage of paraffin from the EG, changes in the composition of CTANMs over the volume of the sample, etc. At the end, new experimental data were obtained on the density, heat capacity, and heat of phase transition for the nanohybrid CTANMs.It has been proven that minor additives of the EG (up to 4.07%) and CuO nanoparticles (up to $\mathbf{0. 6 0 5 4 \%)}$) contribute to increase in the heat of phase transition and thermal conductivity. Implementation of the proposed method for producing hybrid CTANM will improve the efficiency of heat storage devices.