## 5 Noble Nanofluids
### Hybrid Nanofluids
Besides, the poor thermal conductivity of the base fluid was due to the enhanced dynamic viscosity of the EG38. The experimental results make the developed hybrid nanofluid very suitable for enhanced heat transfer enhancement processes. It was not reported the impact on the heat transfer enhancement of the concentration of the nanoparticles as they were synthesized inside the base fluid. Furthermore, the researchers [PERSON] et al. [111] proposed an innovative method of synthesis for the decoration of platinum on functionalized graphene nanoplatelets. The graphene-platinum nanostructures were manufactured by a simple chemical reaction process that incorporated an acid treatment for the functionalization of the graphene nanoplatelets. The graphene-platinum hybrid nanofluids preparation method did not involve any addition of surfactants. The stability of the nanofluids was good with no significant sedimentation after 22 days of test. The thermal conductivity of the graphene nanoplatelets-platinum dispersed in distilled water showed an enhancement of nearly 18% at a temperature of 40 \({}^{\circ}\)C and at 0.1% wt. The researchers also stated that all the tested concentrations of the graphene-platinum aqueous nanofluids exhibited higher thermal conductivity than the one of the distilled water alone. Also, the authors [PERSON] et al. [112] prepared gold-palladium nanoparticles decorated on nitrogen-doped carbon nanotubes by using a polyethylene imine reduction procedure. The polyethylene played the role of reducing and stabilizing agent, facilitating the nanoparticles nucleation and growth on the surface of the nitrogen-doped carbon nanotubes. All the synthesized nanofluids showed a broadband absorption ability over the near infrared and visible spectra. The gold-palladium-nitrogen-doped carbon nanotubes nanofluids absorbed more solar irradiation when compared with the palladium-nitrogen-doped carbon nanotubes or gold-nitrogen-doped carbon nanotubes nanofluid. The photothermal conversion efficiency of gold-palladium-nitrogen doped-carbon nanotubes nanofluids was approximately 62% as compared with around 53% and 57% for palladium-nitrogen doped-carbon nanotubes and gold-nitrogen doped-carbon nanotubes, respectively. Such enhancement derived mainly from the synergetic effects of the nitrogen doped-carbon nanotubes and gold-palladium nanoparticles. In essence, the gold-palladium nanoparticles have been decorated successfully on the nitrogen-doped carbon nanotubes surfaces using the polyethylene imine reduction method. Additionally, the researchers [PERSON] et al. [113] synthesized copper-palladium hybrid nanostructures nanofluids at varying molar ratio. The temperature dependent thermal conductivity of the nanofluids was measured through a hot disc thermal constant analyzer. The impact of the varying molar ratio in the copper/palladium was inferred by transmission electron microscopy imaging observation. Joint nanorods type morphology was observed in the copper-palladium nanofluids with a molar ratio of 1:20. When increasing the copper content at a copper-palladium molar ratio of 1:1, a hybrid nanorods and spherical nanoparticles morphology was observed, which was further changed to uniform spherical shaped structure in the copper-palladium at 20:1 molar ratio. The nanofluids thermal conductivity augmented with increasing temperature. The significant enhancements in thermal conductivity of the nanofluids were observed on the suspension of copper-palladium bimetallic nanoparticles at different higher temperatures. The thermal conductivity enhancement maximum of nearly 18.8% was found at 80 \({}^{\circ}\)C for copper-palladium at 20:1 molar ratio. On the other hand, the researchers [PERSON] and [PERSON] [114] elaborated an investigation work on the preparation, and thermal conductivity and dielectric strength determination of silver-tungsten oxide hybrid nanofluids with transformer oil as base fluid. The nanofluids were produced via the electrical explosion of wire one-step methodology.