Nitrate based high temperature nano-heat-transfer-fluids: formulation & characterisation

This work relates to the development of high temperature heat-transfer-fluid with enhanced specific heat capacity using nano-particle additives. A eutectic mixture of nitrate (60 wt% NaNO3 & 40 wt% KNO3) was produced through ball-milling and characterised on DSC, TGA, Rheometer. The results obtained showed that the salt mixture melted at 221°C with a heat of fusion of 97 J/g. Onset of melting was seen at 215°C whilst crystallisation started at 219°C, reaching a solid state below 217°C with an enthalpy of 97 J/g. Displaying very little overcooling, the salt showed specific heat capacity of 1.41 J/[°C*g] at 260°C to 1.44 J/[°C*g] at 440°C with viscosity values changing from 4.8 cP at 250°C to 1.7 cP at 450°C for this Newtonian fluid. Thermal decomposition of the salt showed that it was stable up to 550°C. The addition of nano-particles displayed an overall positive effect toward the specific heat capacity enhancing the latter whilst reducing the onset of melting due to increased entropy. The addition of 0.1, 0,5 and 1.0 wt% copper oxide gave the best results with increase of 10.5%, 9,2% and 8,5% in specific heat capacity respectively. SEM analysis of the samples showed that the nano-particles clearly disrupted the crystallisation structure showing a rougher organisation. Rheological tests on 0.1 wt% CuO demonstrated a slight rise in viscosity due to the addition of nano-particles.
The stability of 0.1 wt% CuO was tested in large scale rigs (>1.0 kg) and demonstrated that sedimentation of nano-particles did occur. Different manner of dispersion were tested and revealed that they each affected the specific heat capacity differently with some causing negative enhancements whilst others were positive.
The method of production did not affect the specific heat capacity values, and current theories point toward the formation of liquid nano-layers as a reason toward this increase.