Carbon nanoparticles are very useful in solar thermal applications, since they absorb much of
the solar spectrum, are cheap and have excellent optical properties. Carbon nanoparticlesthermal
oil-based nanofluid was prepared using two-step method with diphenyl sulfone as
surfactant to achieve that nanoparticles remain suspended even at high temperatures. The
size particle distribution was studied using two Dynamic Light Scattering systems at room and
high temperature and also evaluated before and after exposing the nanofluid to a thermal
treatment so that conditions closer to those in real applications were replicated. Moreover,
the morphological changes due to the thermal treatment were observed with Transmission
Electron Microscopy. Finally, the optical properties as the ballistic transmittance, absorption
coefficient and scattering albedo of the base fluid as well as of the nanofluid were measured
using a spectrophotometer with and without integrating sphere. The results of this study
contribute to the knowledge about these solar nanofluids that are promising alternatives to
the conventional solar collectors
