Greg Christensen

South Dakota School of Mines and Technology, Rapid City, South Dakota, United States

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Publications (6)2.18 Total impact

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    Greg Christensen · Hammad Younes · Haiping Hong · Pauline Smith
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    ABSTRACT: 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 alignment difficult. Adding water to those fluids could lead to significantly reduced viscosity and make the particles disperse and align well. Polarity studies indicated that higher polarity yields better dispersion and alignment of the Fe2O3 nanoparticles. Thermal studies showed that thermal conductivity of nanofluids containing metal oxide particles with hydrogen bonding in solvents is enhanced compared to the theoretically calculated data. Intermolecular hydrogen bonding between water and ethylene glycol increases the thermal conductivity of nanofluids while decreasing the fluid viscosity. The results also well explain why 50 wt. % water/50 wt. % ethylene glycol is an excellent commercial coolant. Since high thermal conductivity enhancement with minimal viscosity increase is the primary goal of heat transfer nanofluids, this current research may open new doors to better understanding of the fundamental nature of nanofluids.
    Full-text · Article · Dec 2015 · Journal of Applied Physics
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    Hammad Younes · Greg Christensen · Dong Li · Haiping Hong · Amal Al Ghaferi
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    ABSTRACT: Conventional heat transfer fluids, such as mineral oil, water, and ethylene glycol, have poor heat transfer properties compared with solids. The methods to enhance the efficacy of heat transfer fluids have been of great interest to scientists and engineers for the past two decades. Recently, nanofluids are being considered as potential heat transfer fluids. A nanofluid is a suspension of nanometer-sized materials, with or without surface-active agents. In general the thermal conductivity of suspended nanometer-sized particles is higher than the base fluids. Oxides, metals, nitrides and nonmetals like carbon nanotubes and graphene are used as nanomaterials in nanofluids, while water, ethylene glycol, oils, and polymer solutions are used as base fluids. This review has two main goals; the first one is to summarize the recent studies of using metal and nonmetal nanoparticle to improve the thermal conductivity of nanofluids. The second goal is to highlight the effect of using magnetic field to enhance the thermal conductivity of the nanofluids.
    Full-text · Article · Jun 2015
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    ABSTRACT: In this paper we report the alignment of carbon nanofibers (CNFs) in both DI water and epoxy matrix under the external magnetic field. Microscopic and digital camera images show that CNF chains are continuous and unbroken, consisting of a combination of aligned CNFs in both DI water and epoxy. The increased contact between the nanofibers significantly enhanced the thermal conductivity of carbon nanofibers fluid when an external magnetic field is applied. The alignment process of CNFs in DI water requires much less time and weaker magnetic field compared to that in high viscosity of epoxy. The alignment of carbon nanofibers in epoxy solution may open the door for high performance polymer composites, which have wide variety of commercial applications.
    Full-text · Dataset · Mar 2014
  • Hammad Younes · Greg Christensen · Haiping Hong · G. P. Peterson
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    ABSTRACT: In this paper we describe a new approach to align functionalized single walled carbon nanotubes (SWNTs) by γ-Fe2O3 in the presence of an external magnetic field without using chemical surfactant. Five different functionalized SWNTs were studied. SWNT-SO2OH dissolves completely and a clear alignment is observed. SWNT-COOH disperses well and shows clear alignment. SWNT-PABS and SWNT-CONH2 disperse poorly and exhibit some alignment. The high entanglement of PABS chain decreases the alignment of SWNT-PABS. The lack of alignment in SWNT-CONH2 is attributed to the non-stable charge on the amide group (the delocalization of the lone pairs of electron on the nitrogen atom). In contrast, SWNT-PEG disperses well but shows a slow alignment process. This may be attributed to the electrostatic attraction between the partially negatively charged functional group and γ-Fe2O3. The results demonstrate that as long as the charges fit, the functional nanotubes alignment could be reached without chemical surfactant.
    No preview · Article · Mar 2013
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    Hammad Younes · Greg Christensen · Xinning Luan · Haiping Hong · Pauline Smith
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    ABSTRACT: 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 nanoparticles would not be influenced according to the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. The highlight of this paper is that our microscope images could well explain most of the literature data and conclusions and may open new door to better understanding fundamental nature of nanofluids
    Full-text · Article · Mar 2012
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    G. Christensen · H. Younes · H. Hong · G.P. Peterson

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