Specific heat capacity enhancement in nanofluids
Physical Chemistry of Solids and Surfaces, Material Characterisation
Final Report Abstract
Nanofluids, consisting of a base liquid doped with small amounts of nanoparticles (often less than 1 wt. %), frequently exhibit specific heat capacities, which cannot be explained by mere additivity of the heat capacities of the constituents. Apparently in all experiments where this has been studied, the effect is qualitatively the same in both the solid and the liquid phase. For instance, if an increase of specific heat capacity is found in the liquid phase then there is also an increase in the solid phase. The same applies when the specific heat capacity is decreased by the addition of nanoparticles. Looking at the specific heat capacity as function of nanoparticle concentration, a local maximum is observed between 0.5 to 1.5 wt %. However, quite generally the measurements are plagued by considerable scatter - sometimes comparable to the effect itself. Surprisingly this can be true also for the base fluid itself. When nanoparticles are present, the scatter may in part be attributed to the tendency of the nanoparticles to aggregate and an attendant lack of equilibration. In the field of filled elastomers, where the underlying physics has much in common with the physics of nanofluids, this aggregation or rather flocculation is well known and the subject of extensive research. One particular information, which one can carried over from rubber research, is that there is no theory thus far describing the specific interaction between the matrix material, polymers there and base liquids here, with the different types of nanoparticles. A second piece of information is the long-range hydrodynamic interaction between the particles and the surrounding matrix material. Recently the specific heat capacity of nanofluids has been studied via molecular simulation. The authors do indeed find an enhanced heat capacity, but the reason for this is not uncovered. We also do find that SiO2 nanoparticles enhance the heat capacity of liquid KNO3 including a maximum at low nanoparticle concentrations. By studying the effect of nanoparticles of the liquid’s spectral density distribution, we conclude that the presence of the particles causes additional low frequency vibrational modes possibly in line with a reduction of the Frenkel frequency as proposed in the phonon theory of liquids Brazhkin, Trachenko et al. In addition, however, we find that the presence of nanoparticles enhances existing molecular modes of the base liquid. Unfortunately, we cannot separate the ’hump’-like cV - enhancement at low particle concentrations, which likely is the enhancement seen in the experiments, and an apparent overall increase of cV when the concentration of nanoparticles is increased. The disentanglement of these effects probably requires still larger systems, which would reduce the aforementioned particle induced structure. It is important to note that the Frenkel frequency is not applicable in the solid state, even though, as stated above, the effect of the nanoparticles in the solid state is qualitatively the same as in the liquid state. However, as can be seen from the expansion of the internal energy, derived on the basis of the phonon theory of liquids, which is valid near the Dulong-Petit limit, there also is the effect of anharmonicity. Both, a shift of the Frenkel frequency and a change of αP , can have similar effects on heat capacity. In general we can expect that both, a shift of the Frenkel frequency as well as anharmonicity, will affect the heat capacity. The particle induced changes in the spectral densities at higher frequencies may be interpreted in terms of particle imposed anharmonicity in the molecular potentials of the base liquid. This then would also apply in the solid state as well. In summary, the effect of nanoparticles on the heat capacity, as well as on other physicochemical properties, of liquids is not colligative, i.e. it does not merely depend on the nanoparticle weight fraction. Even qualitatively it depends on particle type and size, possibly morphology. In other respects it is basic and almost universal, i.e. it affects the liquid and the solid phase alike and it is observed over a wide range of base fluids of small molecules as well as polymers. Here we have studied a particular system only. But the theoretical framework developed in the course of this work allows us to focus on three aspects - the generation of additional shear modes entering through the Frenkel frequency, anharmonicity, and possibly a shift of the Debye frequency - within the recent phonon theory of liquids. With this newly acquired background and the attendant focus on selected quantities, it is highly desirable to invest future effort in simulation studies encompassing more systems - including the solid phase!
Publications
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(2016). On the specific heat capacity enhancement in nanofluids. Nanoscale Research Letters
Reinhard
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(2019). Specific heat capacity enhancement studied in silica doped potassium. Scientific Reports. 9
Engelmann, Sven and Hentschke, Reinhard
