Thermal Conductivity Enhancement of Cu‐ and Ag‐Decorated MWCNT Hybrid Nanofluids Synthesized via Probe Ultrasonication

Hybrid Ag/MWCNT and Cu/MWCNT nanofluids were synthesized in ethylene glycol using a surfactant‐free probe ultrasonication method and evaluated at ultralow solid concentration (0.001–0.01 wt%). TEM analysis indicated decoration of MWCNTs with Ag and Cu nanoparticles, while Raman spectroscopy indicated defect‐mediated interactions and structural perturbation of the functionalized MWCNT framework, as suggested by increased I D / I G ratios. UV–Vis spectra showed Ag/MWCNT and Cu/MWCNT plasmonic bands at 380–420 and 300–360 nm, respectively, confirming dispersion in EG. At 0.001 wt%, periodic UV–Vis/zeta‐potential tests over 4 weeks indicated acceptable stability, with preserved spectra and negative ζ values ≥30 mV. Thermal conductivity measurements (20°C–70°C) revealed clear enhancement with temperature and metal concentration, reaching ≈5.5% for Ag/MWCNT–EG and ≈8% for Cu/MWCNT–EG at 0.01 wt% and 70°C. The superior performance of Cu/MWCNT–EG is likely associated with stronger interfacial interactions between Cu and carboxylated MWCNTs, enabling more efficient phonon–electron heat transfer pathways. Compared with similar carbon‐, metal‐, and hybrid‐based nanofluids commonly using ≥0.05–0.3 wt%, this work achieves measurable thermal improvement at 0.001–0.01 wt%, highlighting an effective, scalable, surfactant‐free platform for designing stable, low‐viscosity, high‐performance heat transfer fluids for high heat flux heat exchangers, electronic cooling applications.