Spring Meeting 2013 May 27-31, Strasbourg, France

1. Spring Meeting 2013 May 27-31, Strasbourg, France Structural and thermophysical properties of aqueous nanofluids based on Fe@C and Fe2O3 nanoparticles synthesized by laser pyrolysis F. Dumitrache 1, C. Fleaca 1*, R. Alexandrescu1 , I. Morjan 1, G. Huminic 2, A. Huminic 2, C. Daia 3, L. Vekas 3 • National Institute for Lasers, Plasma and Radiation Physics, Lasers Department, • P.O. Box MG-36, 409 Atomistilor St,R-77125 Bucharest, Romania • Trasilvania University , 29 EroilorBlv., 500036, Brasov, Romania • Romanian Academy Timisoara branch, Advanced and Fundamental Technical • Research Center, 24 MihaiViteazuBlv., Ro-300223, Timisoara, Romania • * Presenting author: e-mail: claudiufleaca@yahoo.com

2. Powder synthesis: Experimental set-up Maintained constant: D-Ar (Conf.) 2000 sccm; D-Ar (windows) 150 sccm; laser beam= 3 mm; p= 300 mbar; injector configuration with two concentric nozzles. These samples were selected especially for those different behavior in aq. dispersion (hydrophilic/ hydrophobic) D-Ar (Conf.) 2500 sccm; D-Ar (windows) 150 sccm; laser beam= 2 mm; injector configuration with three concentric nozzles.

3. X Ray Diffractograms - γFe2O3 nanoparticles: SF58; SF82 Nanocrystaline particles: Broad diffraction peaks -characteristic for nanoparticles. A uniform and cvasi-horizontal base-line- low amorphycity degree Pseudo- Voight function is the best fitting option- cvasiuniform particle diameter • Mean crystaline dimension using Debye-Sherrer formula for (440) peak: • dm = 10.1 nm - SF82 (higher flame synthesis temperature) • dm =12.3 nm - SF58 (lower flema synthesis temerature) • FSF58- allmost pure maghemite , SF82- maghemite as dominant phase, iron as traces • From (220), (113) and (440) peaks positions the lattice constant was extracted: • a= 8.353 A, close to theoretical maghemite constant value (8.352 A) and much far from magnetite (8.396 A). • Fno evidence of C or FeCx crystalline phases.

4. X Ray Diffractogram - CF88 (FeCx@C sample) Broad diffraction peaks -characteristic for nanoparticles. A typical crystalline multi-phase spectrum with an important amorphous background. Pseudo- Voight function was used to evaluate the crystalline dimension for two dominant iron carbides. (big particles seem to crystallize as Fe7C3, and the smaller crystallize as Fe3C. • CF88- iron carbides mixture: Fe7C3, Fe3C, graphitic sheets, metallic iron as traces • Mean crystalline dimensions using Debye-Sherrer formula are: • 2θ=53.4- Fe7C3 peak: dm = 10.0 nm, • 2θ=58.3- Fe3C peak: dm = 7.0 nm.

5. EDX (elementar estimation); Raman Spectra Formula: SF58: Fe2O3+0.18, SF82: Fe2O3+0.12 The oxygen excess might be explained by to the superficial presence of COOH and OH groups (revealed by FT-IR spectra) Raman spectra revealed amorphous carbon D and G bands (more intense for SF58 sample. CF sample (FeCx@C) revealed only D and G carbon band at Raman spectrum (no evidence for iron oxidation). In the case of Fe@C nanoparticle two tendencies were expected: a) Fe/C ratio increasing imply the outer carbon shell thicker and consequently a smaller magnetisation; b) a Fe/O ration decreasing imply a good passivation from C- based shell. CF88 seems to be a good compromised for both tendencies.

6. SF samples-TEM , particle distribution and SAED analysis- Dependence on reaction temperature SF58- TEM Image SF82- SAED insert SF58- particle size distribution • TEM image reveal in both cases two particles distributions: • Big particles with polyhedral shape (12-20 nm)- better crystallized for sample SF82 (due to higher reaction temperature) • -small and spherical particles arranged in chain like agglomeration (3- 8 nm). • SAED image confirm the XRD analysis also at nanoscale investigation area: almost pure maghemite/ magnetite for both samples. • HREM image (SF82) revealing a nanoparticle as a single nanocrystal; interplanar distance correspond to (113) maghemite plane. SF82- HREM Image

7. CF sample-TEM , particle distribution and SAED analysis- Dependence on reaction temperature CF88- HREM Image TEM Image at low resolution SAED imag. -phases identification • HREM Image: Identification of interplanar distances • Magnetic cores of 13 nm of Fe/Fe3C/ Fe7C3 with about 2.1 Å • Onion-like morphology carbon and graphene layers (d=3.5 Å) • TEM Image: core/ shell is a general feature, the crystaline core is surrounding by graphitic layers , presenting also a tendency of coalescent-like features • SAED Image: the most intense peak is one broad and continuous corresponds to 2.2-2.0 Å inter- planar distances:difraction maxima for metallic iron and/ or iron carbides fit with these values.

8. Magnetic measurements: hysteresis loops, Mossbauer Spectroscopy • Hyperfine field, from Mossbauer Spectroscopy: • - SF58, SF82 - The narrow peak centered at about 52 T, evidences the presence of mainly maghemite • CF88- a small peak at 47.5 T for 4.5K measurement revealing a defective iron oxide minor phase • A strong peak centered at 34 .5T from αFe phase. • A broad peak with spreading from 18T to 25T that mean a quasi-continuous distribution of different iron carbides: Fe7C3, Fe3C. • Blocking temperatures : • About 230 K (SF58) and 200 K (SF82 ), yet higher than room temperature , around 330K for CF88 sample. Hysteresis loops - samples SF82 (Fe2O3 type) and CF88 (Fe-FexC@C type)

9. Aqueous suspensions of iron-based nanoparticles preparation • In order to obtain homogeneous suspensions with size aggregates as small as possible we used a double ultrasonication procedure: • The cavitation phenomenon (due to collapsing microbbubles formed under the action of the ultrasonic waves) explain the formation of supersonic jets in liquid , causing a very efficient mixing and facilitating the aggregates/agglomerates breaking • The following types of aqueous suspensions were prepared at different concentrations: - based on Fe-oxide SF82 sample with or without the addition of TMAOH (tetramethylammonium hydroxide – a known additive for aq. Ferrofluid preparation) - based on Fe-oxide SF82 an on SF58 sample with the addition of L-DOPA (in these cases we maintained a 70..80°C temperature during ultrasonication) - based on Fe-FexC@C CF88 sample with the addition of polyelectrolytes : CMCNa (carboxymethylcelulode sodium salt) or PAACTMa (tetramethylammonium polyacrylate)

10. Fe@C nanoparticles functionalization with Polyelectrolytes Due to the hydrophobic prevalent character of Fe@C nanoparticles surface (carboh-rich), they need to be functionalized with surfactants or hydrophilic polymers in order to disperse them in water

11. Nano iron oxide L-DOPA functionalization

12. Nano iron oxide L-DOPA functionalization The proof of L-DOPA functionalisation of Fe-oxide nanoparticles suspended in aqueous L-DOPA solutions was obtained after their isopropanol precipitation, magnetic separation and repeated washing steps with distilled water, when the resulted dry powders FT-IR spectrum shows the L-DOPA specific absorbtion bands, which are missing from in the corresponding suspension without L-DOPA

13. Electron microscopy-TEM and SAED for dispersions TEM sample F13 TEM Image: sample F5 SAED Image: sample F5 TEM image for F13 dispersion (SF82 dispersed in water only using U.S. treatments revealed small agglomeration, no larger than 200 nm) TEM image for F5 dispersion (SF82 L-Dopa stabilized and also U.S. dispersed) reveals small aggregates with iron based nanoparticle embedded in organic like shell. SAED Image for F5 dispersion: associated rings ascribable to γ Fe2O3 (220, 311, 400, 115, 440) are identified. The dispersion treatments do not operate changes in nanoparticle structure and also the oxide structure remain stable in time (very important for industrial and bio-medical applications)

14. Isoelectric point (IEP), Zeta potential and hydrodinamic mean dimension versus pH • IEP values were: pH 8.27 (SF58) and 8.23 (SF82) • Around neutral pHs iron oxide nanoparticles are unstable in water based solutions (a steric dispersant is required) • Particle Size Distributions (red-line) – weighted Mean Z-Average: maxim values at neutral pHs • Zeta Potential values (green-line) – weighted Mean Zeta Potential: • SF58 in water has an electro-static stability at low and very high pHs. • SF82 in water needs a com-plementary electrostatic/steric stabilization SF58 – (lower synthess temperature in flame) - high hydrophilic SF82 – (higher synthesis temperature in flame - hydrophobic

15. Dynamic Light Scattering : Fe2O3 -based suspensions

16. Dynamic Light Scattering: Fe2O3 -based suspensions • Fe2O3 suspensions in distilled water have an acidic pH ( ~ 4.8) • Fe2O3 suspensions in distilled water with TMAOH have strong basic pH (>13) • Fe2O3 suspensions in distilled water with L-Dopa have a near neutral (weak basic) pH (~ 8.8) by adding small amount of ammonia solutions • Simple aqueous Fe2O3 suspensions from SF82 nanopowder present a near monomodal particle size distributions, with mean hydrodynamic diameter (Zaverage) between 70…100 nm, without a linear variation of Zaverage with the particles concentration • The suspensions containing TMAOH present a bimodal or multimodal particle size distribution, with much greater Z averagethan those without TMAOH, indicating a higher agglomeration degree; however, in all these suspension, smaller agglomerates (around 100 nm or less) exist in significant concentration • The SF82 suspension containing L-DOPA have a similar particle diameter distribution with those without L-DOPA, with Zaverage slighty smaller (79 nm. vs. 82 nm) • The SF58 sample based suspension (also containing L-DOPA) has a bimodal hydrodynamic particle size distribution with the first peak at 180 nm and a Z average value greater than those of the corresponding suspension from SF82 nanopowder (140 nm vs. 79 nm) • We also tested the effect of the PVP (polyvinyl alcohol) and PVA (polyvinylpyrrolidone) hydrophylic polymers (3g/l),and the resulted suspensions with 20g/l SF82 at pH 8.8 were unstable

17. Dynamic Light Scattering : Fe@C -based suspensions • The CMCNa stabilized suspensions present a bimodal/multimodal distributions of aggregate sizes, with Zaverage between ~ 160…300 nm • No direct correlation can be observed between the Zaverage value and the Fe@C nanoparticle concentration • All of these suspensions present the most intense peak centered around 200 nm • The majority of CMCNa stabilized Fe@C nanoparticles aggregates in aqueous suspensions are greater then those form Fe2O3 nanoparticles suspensions

18. Thermal conductivity tests on suspensions at 25°C • All suspensions show an increasing thermal conductivity vs. those of pure water with a maximum of 42% enhancement for SF82 (20 g/l) with L-Dopa (3g/l) • There is clear tendency of thermal conductivity increasing with the suspension concentration, more accentuated at the additivated suspensions • The TMAOH-additivated suspensions with Fe2O3 nanoparticles (SF82) show greater thermal conductivities than the corresponding non-additivated suspensions • The two nano Fe oxide suspension with L-Dopa shows greater thermal conductivity enhancement than those with TMAOH or than the non-additivated one • Fe@C suspensions show conductivity enhancement, more for the PAAcTMA additivated

19. Viscosity measurements on suspensions at 25°C • A quasilinear increasing of the dynamic viscosity of the nanoparticle suspensions with the concentration can be observed • The viscosity enhancement is slighty more for Fe2O3/water nanofluid compared to Fe2O3/TMAOH nanofluid for all weight concentrations; the Fe@C-polymer based suspensions have a greater viscosity than the Fe-oxide based ones with the same concentrations • An intriguing result was the observing of much higher dynamic viscosities for the pure polymer solutions (1.496 cP for 3 g/l CMCNa and 3.328 cP for 5g/l PAAcTMA) than for the corresponding nanoparticle suspensions. The polymer wrapping around nanoparticles and/or the decreasing of polymer molecular weight after polymer backbone cutting due to strong ultrasonication in the nanoparticles presence can explain this anomalous behavior • For the higher thermal conductive suspension, the conductivity increase with only 5.4% vs. pure water, making the nanoFe2O3 – L-DOPA – water a performant thermal nanofluid

20. CONCLUSIONS • Laser pyrolysis is a well-suited technique for the reproducible, single-step synthesis of magnetic Fe-based nanoparticles: oxidic (maghemite) or metallic/carbidic surrounded by graphitic shells • The nanoparticles were structurally analysed using XRD, Raman, TEM, SAED, EDX techniques • The hysteresis curves and Mossbauer spectroscopy show a near superparamagnetic behavior for SF82 oxidic sample and a weak ferromagnetic one with high saturation magnetization for CF88 Fe@C • Their aqueous suspension in absence/presence of additives (TMAOH, L-DOPA, CMCNa, PAAcTMA) were prepared by double ultrasonication • The suspensions were characterised by TEM, DLS and their dynamic viscosity and thermal conductivity were measured • Fe-FexC@C - Polyanions coated nanoparticle suspensions show a thermal conductivity enhancement with the concentration (yet, with values under those of the pure water, presumable due to the polymer presence) • SF82 NP 20 g/l aq. suspension in the presence of L-Dopa (3g/l) shows the highest enhancement of thermal conductivity (42%) accompanied with a minimal increasing in dynamic viscosity (5.4%) vs. water

21. ACKNOWLEDGEMENTS • The authors greatly acknowledge for the financial support to the following programs: • Romanian Project PN-II-ID-PCE- 2011-3-0275 Application of nanofluids to heat pipes for high performances in cooling systems • European FP7 Project MagPro2Life Advanced Magnetic nanoparticles deliver smart Processes and Products for Life