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Three nanoscale metal oxides composed of nano TiO 2 , nano Al 2 O 3 , and nano MgO were added to the phase change microcapsule slurry for optimising the heat transfer behaviour. The thermal and rheological properties of the resulting microencapsulated phase change materials (MPCMs) and microencapsulated phase change material slurries (MPCSs) were characterized to determine the effects of added metal oxides in optimising the performance of MPCSs. The impacts of factors like metal oxides, concentrations, and flow rates on the heat transfer behaviour of MPCSs were investigated by forced convective heat transfer experiment with various working conditions. Compared to 6 wt% MPCSs, the heat transfer coefficients (h x) of 6 wt% slurries containing 1 wt% TiO 2 , 1 wt% Al 2 O 3 , and 1 wt% MgO increased by 4.0 %, 2.5 %, and 7.13 %, respectively. Nano-MgO showed the most significant heat transfer enhancement effect on MPCSs. Moreover, the heat transfer performance gradually enhanced as a function of the concentration for nano MgO. Overall, the addition of metal oxides enhanced heat transfer by increasing the thermal conductivity of the slurry, as well as improved the micro-convection effect. The proposed MPCSs are promising for applications in solar photovoltaic/thermal (PV/T) systems, chip cooling, thermal management, buildings, and automotive cooling. [Display omitted] • Forced convection heat transfer experiment were conducted with bonded triangular tubes. • Heat transfer enhancement achieved by optimising heat transfer medium and tube structure. • Metal oxide type and concentration have significant effect on heat transfer of slurries. • Nano-MgO showed the most significant heat transfer enhancement effect on slurries. [ABSTRACT FROM AUTHOR]