Haiping Hong

Magnetic field enhanced thermal conductivity in heat transfer nanofluids containing Ni coated single wall carbon nanotubes

In this paper, we report that the thermal conductivity (TC) of heat transfer nanofluids containing Ni coated single wall carbon nanotube can be enhanced by applied magnetic field. A reasonable explanation for these interesting results is that Ni coated nanotubes form aligned chains under applied magnetic field, which improves thermal conductivity via increased contacts. On longer holding in magnetic field, the nanotubes gradually move and form large clumps of nanotubes, which eventually decreases the TC. When we reduce the magnetic field strength and maintain a smaller field right after TC reaches the maximum, the TC value can be kept longer compared to without magnetic field. We attribute gradual magnetic clumping to the gradual cause of the TC decrease in the magnetic field. We also found that the time to reach the maximum peak value of TC is increased as the applied magnetic field is reduced. Scanning electron microscopy images show that the Ni coated nantubes are aligned well under the influence of a ...

Enhanced thermal conductivity by aggregation in heat transfer nanofluids containing metal oxide nanoparticles and carbon nanotubes

An approximately 10% increase in the thermal conductivity (TC) of heat transfer nanofluids containing metal oxide nanoparticles and carbon nanotubes has been determined with very low percentage loading (around 0.02wt%) of these two nanomaterials. These fluids are very stable and the viscosity remains approximately the same as water. A possible explanation for these interesting results is the aggregation of metal oxide particles on the surface of nanotubes by electrostatic attraction and form the aggregation chain along the nanotube. Time dependant magnetic results demonstrate that, under the influence of a strong outside magnetic field, the TC value decreases. Also, the TC value decreases when the pH is shifted from 7 to 11.45.

Effects of pH on heat transfer nanofluids containing ZrO2 and TiO2 nanoparticles

In this paper, pH influences of zeta potential, particle size distribution, rheology, viscosity, and stability on heat transfer nanofluids are studied. Significant enhancement of thermal conductivity (TC) (>20%) containing 3 wt % zirconium dioxide (ZrO2) and titanium dioxide (TiO2) are observed near the isoelectric point (IEP). Meanwhile, at this IEP (pH), particle sizes, and viscosities of these nanofluids demonstrate a significant increase to maximum values. Experimental results also indicate that the stabilities of these nanofluids are influenced by pH values. The reasonable explanation for these interesting phenomena is that at this IEP, the repulsive forces among metal oxides are zero and nanoparticles coagulate together at this pH value. According to the Derjaguin–Landau–Verwey–Overbeek theory, when the pH is equal to or close to the IEP, nanoparticles tend to be unstable, form clusters, and precipitate. The resulting big clusters will trap water and the structures of trapped water are varied due to...

Effects of alignment, pH, surfactant, and solvent on heat transfer nanofluids containing Fe2O3 and CuO nanoparticles

In this paper, the effects of alignment, pH, surfactant and solvent on heat transfer nanofluids containing Fe2O3 and CuO nanoparticles are studied and analyzed. The microscope images show that Fe2O3 could form some kind of alignment spontaneously in water even without external magnetic field. With the addition of external magnetic field, the alignment is strengthened. In water, the magnetic particle agglomeration to larger size occurs easily, which makes the directional alignment much faster and easier. Ethylene glycol solvent and chemical surfactant sodium dodecyl benzene sulfonate, NaDDBS could separate the Fe2O3 and CuO nanoparticles well in the fluids and avoid possible aggregation. Therefore, magnetic alignments are hard to observe. The measured thermal conductivities of each individual sample coincide with the microscope images and assumptions. In addition, pH values of Fe2O3 and CuO nanoparticles are measured and it has been determined that at those pH values, thermal conductivities of those nanopa...

Efficiently lowering the freezing point in heat transfer coolants using carbon nanotubes

The paper has reported the interesting freezing point decrease phenomenon in the nanofluids based on the carbon nanotubes and 50%water/50% PAC (ethylene glycol). The carbon nanotubes could be the ideal candidate for the nonacoolant application because they could not only increase the thermal conductivity, but also efficiently lower the freezing point

Heat Transfer Nanofluids Based on Carbon Nanotubes

In this paper, we report the effort to prepare a stable su spension of carbon nanotube s in a hydro philic thermal transfer fluid with the motivation of enhancing its properties such as thermal conductivity and freezing point . The process of creating these fluid s involves the dispersion of carbon nanoparticles into the thermal transport fluid through intermittent sonication and the use of additives such as surfactants. I. Nomenclature

Effects of solvent hydrogen bonding, viscosity, and polarity on the dispersion and alignment of nanofluids containing Fe2O3 nanoparticles

It has been shown that the alignment of Iron (III) oxide (Fe2O3) nanoparticles in water (H2O) can enhance the thermal conductivity of nanofluids. To better understand solvent effects such as hydrogen bonding, viscosity, and polarity, nanofluids were prepared by mixing Fe2O3 nanoparticles and various solvents (water, ethanol, 1-propanol, isopropanol, 2-propanone, hexane, cyclohexane, ethylene glycol, glycerol, etc.), and the dispersions and alignments of the Fe2O3 nanoparticles in these solvents with and without an applied magnetic field were investigated using an optical microscope. The microscope images indicated that inter-molecule hydrogen bonding of the solvents with one OH group (water, ethanol, 1-propanol, and isopropanol) could help to disperse and align the Fe2O3 nanoparticles. The intra-molecular hydrogen bonding causes a dramatic increase in viscosity for fluids with multiple OH groups, such as ethylene glycol (C2H6O2) and glycerol (C3H8O3), and makes the Fe2O3 nanoparticles dispersion and align...

Multilayered graphene acquires ferromagnetism in proximity with magnetite particles

Anisotropic diamagnetism of pristine graphite and graphene is well known. Here, evidence of significant induced ferromagnetism in multilayer graphene (MLG) decorated with ferrimagnetic Fe3O4 particles is reported. This MLG-Fe3O4 nano-composite was prepared by a one-step ultrasonic treatment at 75 °C in the surfactant sodium dodecyl-benzene-sulfonate. To verify the phase structure and morphology of the composite, X-ray diffraction, scanning and transmission electron microscopy, scanning tunneling electron microscopy, and Raman spectroscopy were employed. Room temperature data of magnetization versus magnetic field showed that the saturation magnetization MS = 58.6 emu/gm for pristine Fe3O4 increased to MS = 158.4 emu/gm for a 1:1 composite of Fe3O4 to MLG. These results lead to induced MS = 253 emu/gm in MLG resulting from its proximity to Fe3O4. Similar experiments on Fe3O4 to single walled carbon nanotubes (SWNT) composite did not show any induced magnetism in SWNT.

Nano materials for efficiently lowering the freezing point of heat transfer nanofluids

In this paper, we report, for the first time, the effect of the lowered freezing point in a 50% water / 50% antifreeze coolant (PAC) or 50% water / 50% ethylene glycol (EG) solution by the addition of carbon nanotubes and other particles. The experimental results indicated that the nano materials are much more efficient (hundreds fold) in lowering the freezing point than the regular ionic materials (e.g. NaCl). The possible explanation for this interesting phenomenon is the colligative property of fluid and relative small size of nano material. It is quite certain that the carbon nanotubes and metal oxide nano particles could be a wonderful candidate for the nano coolant application because they could not only increase the thermal conductivity, but also efficiently lower the freezing point of traditional coolants.

Enhanced magnetic properties of carbon nanotubes and multilayer graphene decorated with Co 3 O 4

Here we report evidence of enhanced magnetic properties of carbon nanotubes (CNTs) and multilayer graphene (MLG) decorated with Co₃O₄ nanoparticles (NPs). These 1:1 CNTs/Co₃O₄ and MLG/Co₃O₄ nano-composites were prepared by a one-step ultrasonic treatment in sodium dodecylbenzene sulfonate (SDBS) surfactant. Structure and morphology of the composites were verified using X-ray diffraction, scanning and transmission electron microscopy, and Raman spectroscopy. Magnetization (M) vs. magnetic field (H) measurements at room temperature in H up to 2.5 kOe showed that the measured M= MS at 2.5 kOe in both the CNT/Co₃O₄ and MLG/Co₃O₄ nanocomposites is enhanced by a factor of about 2.5 as compared to MS expected from the simple addition of MS measured for the two components of the nanocomposites. This is surprising since Co₃O₄ is a paramagnet at room temperature. These enhanced magnetic properties in the nanocomposites likely result from contributions from Co³⁺ ions which being in the low-spin S=0 state are non-magnetic in pristine Co₃O₄ but become magnetic in the reduced crystalline symmetry of the interface layers in CNTs/Co₃O₄ and MLG/Co₃O₄.