Proceedings of the 1st Thermal and Fluid Engineering Summer Conference, TFESC August 9-12, 2015, New York City, USA

TFESC-13014

MAGNETIC NANOPARTICLE MORPHOLOGIES: DEVELOPING FERROFLUIDS FOR PULSATING FLOWS Erick S. Vasquez1, J. Gabriel Monroe2, Zachary S. Aspin2, Matthew J. Berg3, Scott M. Thompson2, Keisha B. Walters1,* 1

Dave C. Swalm School of Chemical Engineering, Mississippi State University, MS 39762, USA 2 Department of Mechanical Engineering, Mississippi State University, MS 39762, USA 3 Department of Physics and Astronomy, Mississippi State University, MS 39762, USA

ABSTRACT This work involves the synthesis, characterization and utilization of different magnetic ferrofluids for hybrid thermal-to-electrical energy devices. By suspending magnetic nanoparticles in a fluid within a pulsating (or oscillating) heat pipe (PHP) with unique serpentine design, a pseudo-harmonic displacement of the internal vapor and condensate (pulsating flow) can be achieved when a temperature difference is imposed across the PHP length. During this operation, the PHP has an ultra-high thermal conductivity and no moving parts. In this study, we focus specifically on different colloidal suspensions of nano- and micron-sized iron oxide particles. In particular, the PHP performance with surface-coated nanoparticles suspended in acetone is discussed. The magnetic particles are surface-modified to prevent agglomeration; allowing the ferrofluid to behave as a liquid. Since temperature gradients drive the heat transfer and pulsating fluid flow, an evaluation of the thermal stability of the ferrofluids was performed. Dynamic light scattering measurements were used to assess the primary particle dimensions and aggregation; morphological characterization of these iron oxide magnetic particles was also performed using transmission electron microscopy. Also presented are preliminary studies using the multicore magnetic structures in a PHP with a solenoid (i.e. harvester) are discussed. This study demonstrates the capability of surface-coated, magnetic micro/nano-particles for use in energy harvesting and thermal management applications. The degradation of nanoparticle composites due to PHP thermal and phase-change operating cycles is discussed, giving experimental guidance for the design of nanoparticles and ferrofluids for future thermal management applications that utilize pulsating flows.

KEY WORDS: Magnetic nanoparticles, ferrofluid, energy harvesting, heat pipe, atomic force microscopy (AFM), magnetic force microscopy (MFM)

1. INTRODUCTION This study is focused on the characterization of surface-coated magnetic nanoparticles (MNPs) under varying temperature, solvent, and magnetization conditions. The inherent magnetic properties of MNPs allow for ease of manipulation in the presence of an external magnetic field; thus making it an attractive platform for hybrid thermal-to-electric devices. Many MNP applications have emerged, including sensor technologies, electronic-devices, hyperthermia treatments, and targeted drug-delivery [1]. Typically, superparamagnetic magnetite (Fe3O4) or ferromagnetic maghemite (Fe2O3) are the magnetic components utilized in magnetic nano/micro-particle structures. A wide range of synthetic routes have been examined to produce magnetic nanoparticles (MNPs) including co-precipitation techniques [2], hydrothermal synthesis [3], reduction reactions [4], and seed-mediated processes [5]. The particles, when joined with a liquid solvent, create a colloidal fluid referred to as ferrofluid or ferro-nanofluid.

*Corresponding Author: [email protected]

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TFESC-13014 Ferro-nanofluids have already been investigated in PHPs [6,7] with the objective of altering/controlling the internal fluid motion with an imposed magnetic field; however, the presence of surface-coated materials and the effects on the nanoparticle stability has not been evaluated. When ferrofluids are used, experimental results show that by imposing an external magnetic field, the thermal performance of a PHP is enhanced. The thermal performance is further enhanced when the field is pulsed at an optimal frequency that is related to the frequency of the oscillating fluid. Despite the utilization of ferrofluids for PHP applications, little is known about the shortcomings of MNPs, using surface-coated nanomaterials as stable agents, concerning the performance of the MNPs when submitted to various temperatures and/or magnetization conditions [8-10]. Thus, we present a morphological characterization and a particle size analysis on different fluids for the MNPs prior and after PHP cycles to obtain a better understanding on the surface-coated MNPs.

2. EXPERIMENTAL SECTION 2.1 Materials. An aqueous ferrofluid, SiMAG-Silanol (Chemicell GmbH, Berlin, Germany) containing Fe3O4 magnetic iron oxide particles with silanol functional groups (1 micron hydrodynamic diameter; 50 mg/mL concentration) was utilized to prepare a suspension of multi-core particles in acetone. For comparison purposes, a separate Fe3O4 nanofluid was synthesized in-house using iron (III) chloride (VWR, 98%), iron (II) chloride (Sigma Aldrich, 98%) ammonium hydroxide (NH4OH, ACS reagent, ~28%, Sigma Aldrich), hydrochloric acid (ACS reagent, 37 %, Sigma Aldrich), and acetone (Sigma Aldrich, 99.5%). Ultra-pure (Type I) water was used for all solution preparations and was obtained from an EMD Millipore Synergy UV system.

2.2 Fe3O4 nanoparticle synthesis.

A co-precipitation technique was used for the synthesis of nanoparticles; 200 mL of 0.7 M NH4OH was produced by adding 22.05 mL of the stock ammonium hydroxide solution to water while 2.6017 g of FeCl3 was dissolved in 16 mL of 1.0 N HCl solution. Then, 1.0169 g of FeCl2 was weighed and dissolved in 4 mL of 1.0 N HCl solution. To a temperature-controlled, stirred reactor, 200 mL of NH4OH was added, stirred at 600 RPM, and heated until a temperature of 60 °C was reached. The two iron source solutions were combined together and mixed for 5 minutes prior to injection to the reactor. The combined 20 mL iron solution was added drop-wise and the reaction was allowed to proceed for 30 minutes under an inert (nitrogen) atmosphere. To separate the particles from the supernatant, the reactor was placed near a strong magnet and the supernatant decanted while the magnet was still in place. Washing cycles were performed where 250 mL of ultra-pure water was added, particles redispersed, and then the particles were separated magnetically. After 3 water consecutive washes, 250 mL of 2 M HClO4 solution was added to the particles held by the magnet, and then the suspended particles were redispersed using sonication for 10 minutes. The reactor was again placed near the magnet to separate the particles from the acid. The HClO4 solution was decanted off and was replaced by 100 mL of H2O. This stabilized aqueous Fe3O4 nanofluid was utilized for all experiments unless otherwise specified.

2.3 Magnetic nanoparticle manipulation and ferrofluid characterization. A ~5000 N force magnet (Fig. 1B) was utilized to separate the Fe3O4 custom nanoparticles after each reaction step. A MagnetoPUREMicro magnet (Chemicell, GmbH, Germany) was used to separate the SiMAG-Silanol particles from water prior to re-suspension in acetone. For PHP testing, two separate ~270 N bias magnets were used. Dynamic light scattering measurements (DLS) were collected in-situ with a ZetaPALS instrument (Brookhaven Instruments, NY). Morphological characterization of the samples was performed using a JEOL JEM-2100 transmission electron microscope (TEM) operated at 200 kV. For TEM imaging, nanoparticles were deposited from solution onto a 300-mesh copper grid and allowed to dry overnight in a ventilated hood.

2.5 PHP setup. A closed-loop pulsating (or oscillating) heat pipe PHP was utilized for this preliminary thermofluid induction experiment. The setup consisted of water circulators, custom source/sink reservoirs, a custom solenoid, two bias magnets and a data acquisition system (DAQ, NI 9213). Approximately 1/3 of the PHP’s length was submerged directly into a ‘hot’ water bath with a controlled temperature (max ~ 74 °C). The top 1/3 portion of the PHP was directly submerged in a custom-made ‘cold’ water bath with pumpdriven, temperature-controlled inlet water at 5 °C (± 0.5 °C). A ~300 turn solenoid with an iron core was fabricated from 0.25 mm diameter magnet wire, and was positioned near the outer edge of the PHP design.

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TFESC-13014 3. RESULTS AND DISCUSSION The morphology and stabilization of the suspended nanoparticles in the ferrofluid were evaluated using TEM and DLS measurements. The in-house synthesized Fe3O4 magnetic nanoparticles (MNPs) were characterized in water (Fig. 1A) and showed excellent dispersion and homogeneous particle size (~ 8 nm). The MNPs were then transferred to acetone using magnetic separation which was performed for a minimum of three times, each time allowing the particles to settle prior to removal of the supernatant. Once acetone was added, the washing procedure was performed three times. The stability of the MNPs in acetone was further evaluated using TEM (Fig. 1B). Similar dispersion, as that observed for water, was seen when acetone was used as the carrier fluid. These single-core MNPs have morphological properties that make them promising as ferrofluid materials for PHP and thermal oscillation systems in general. For comparative purposes, as-received SiMAG-Silanol particles—multi-core iron oxide nanoparticles with a hydroxyl-functionalized silicate shell—were also characterized in water at a concentration of 1 mg/mL prior to any magnetic separation. A TEM image for the SiMAG-Silanol nanoparticles is shown in Fig. 1C. The SiMAG-Silanol nanoparticles are well-dispersed, but a fairly large range of sizes was observed. The multi-core structure of these particle could be observed with TEM, with the ‘ferrofluid particle’ comprised of multiple nanoparticles clustered in a spherical structure and surrounded by a silicate shell. DLS data allowed for a better estimate on the particle size distribution as the SiMAG-Silanol particles are in fluid media pre- and post-PHP servicing. SiMAG-Silanol was first characterized in water using DLS (not shown), where two different particle size distributions were observed, possibly due to the presence of surfactants in the as-received fluid, or the presence of smaller particles as observed with TEM images (Fig 1C). After transferring the SiMAG-Silanol particles to acetone, the SiMAG-Silanol particles dispersion was reduced as compared to water, resulting in a more uniform particle size distribution. As shown in Fig. 2A, the DLS measurements showed a single particle population with acetone as the carrier solvent. This result could be due to the interactions of the hydroxyl groups of the SiMAG-Silanol particle-surface with acetone.

Fig. 1. In-house synthesized Fe3O4 magnetic nanoparticles dispersed in A) water and B) acetone. C) As-received multicore SiMAG-Silanol particles showing a fairly large particle size distribution.

The commercially available ferrofluid, SiMAG-Silanol, was analyzed after being used in the PHP setup described earlier. Thermal and phase-change cycling had interesting effects on the stability of the nanoparticles. DLS intensity measurements show that a wide range of particle sizes resulted after PHP service for the SiMAGSilanol particles in acetone (Fig. 2B). This result points to a potential break-down of the silicate coating due to particle-solvent interactions, high temperatures, and/or phase change for these multi-core particles. Despite these morphological and particle size changes, a preliminary positive voltage generation was observed (Fig. 2C) for the SiMAG-Silanol/acetone solution, as compared to the pure acetone control.

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TFESC-13014

Fig. 2. A) DLS of the SiMAG-Silanol particle dispersed in acetone shows a very uniform particle size, confirming the TEM results of a well-dispersed ferrofluid. B) After PHP servicing, DLS data shows a wide range of particle sizes, possibly due to aggregation and destruction of the multicore structure. C) A higher voltage signal was observed for the system with SiMAG-Silanol particles as compared to pure acetone.

4. CONCLUSIONS Surface-coated magnetic nanoparticles were successfully transferred from aqueous media to acetone, providing a uniform and stable particle size distribution. Morphological stability over time/use is a critical factor in effectively applying these commercial multicore magnetic nanoparticles in thermal management processes. In-house synthesized single-core magnetic nanoparticles showed similar results, making them viable candidates for energy-harvesting processes involving pulsating-heating pipes, while avoiding the potential negative impacts of multicore disassembly. Here we present an analysis of the nanoparticle morphologies before and after thermal processing in a pulsating/oscillating phase-change thermal process. This work can be extended to obtain a better understanding of the surface-coating properties of nanomaterials when submitted to oscillating temperatures and phase change.

ACKNOWLEDGMENTS This work was funded by the National Science Foundation (NSF) under grant CBET-1403872. The DLS and TEM instrumentation was funded by NSF grants EPS-0903787 and DBI-1126743, respectively. We gratefully acknowledge TEM assistance from I-Wei Chu at Institute for Imaging & Analytical Technologies (I2AT) at Mississippi State University.

REFERENCES [1] C.C. Berry, A.S.G. Curtis,"Functionalisation of magnetic nanoparticles for applications in biomedicine," J. Phys. D-Appl. Phys., 36(13), pp. R198-R206, (2003). [2] R. Massart,"Preparation of aqueous magnetic liquids in alkaline and acidic media," Ieee T Magn, 17(2), pp. 1247-1248, (1981). [3] S. Sun, H. Zeng,"Size-Controlled Synthesis of Magnetite Nanoparticles," Journal of the American Chemical Society, 124(28), pp. 8204-8205, (2002). [4] G.S. Chaubey, C. Barcena, N. Poudyal, C. Rong, J. Gao, S. Sun, J.P. Liu,"Synthesis and Stabilization of FeCo Nanoparticles," Journal of the American Chemical Society, 129(23), pp. 7214-7215, (2007). [5] J.-H. Lee, Y.-M. Huh, Y.-w. Jun, J.-w. Seo, J.-t. Jang, H.-T. Song, S. Kim, E.-J. Cho, H.-G. Yoon, J.-S. Suh, J. Cheon,"Artificially engineered magnetic nanoparticles for ultra-sensitive molecular imaging," Nature Medicine, 13(1), pp. 95-99, (2007). [6] M. Mohammadi, M. Mohammadi, A.R. Ghahremani, M.B. Shafii, N. Mohammadi,"Experimental Investigation of Thermal Resistance of a Ferrofluidic Closed-Loop Pulsating Heat Pipe," Heat Transfer Engineering, 35(1), pp. 25-33, (2014). [7] N. Zhao, D. Zhao, H. Ma,"Experimental Investigation of Magnetic Field Effect on the Magnetic Nanofluid Oscillating Heat Pipe," Journal of Thermal Science and Engineering Applications, 5(1), pp. 011005, (2013). [8] A. Nunes, K.T. Al-Jamal, K. Kostarelos,"Therapeutics, imaging and toxicity of nanomaterials in the central nervous system," J. Controlled Release, 161, pp. 290-306, (2012). [9] A. Akbarzadeh, M. Samiei, S. Davaran,"Magnetic nanoparticles: preparation, physical properties and applications in biomedicine," Nanoscale Res. Lett., 7, pp. 144, 113 pp., (2012). [10] E. Kim, K. Lee, Y.-M. Huh, S. Haam,"Magnetic nanocomplexes and the physiological challenges associated with their use for cancer imaging and therapy," J. Mater. Chem. B, 1, pp. 729-739, (2013).

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