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Magnetic and ultrasonic investigations on magnetite nanofluids
Ultrasonics 52 (2012) 1024–1029
Contents lists available at SciVerse ScienceDirect
Ultrasonics journal homepage: www.elsevier.com/locate/ultras
Magnetic and ultrasonic investigations on magnetite nanofluids M. Nabeel Rashin, J. Hemalatha ⇑ Advanced Materials Lab, Department of Physics, National Institute of Technology, Tiruchirappalli, Tamilnadu 620 015, India
a r t i c l e
i n f o
Article history: Received 16 May 2012 Received in revised form 17 July 2012 Accepted 2 August 2012 Available online 11 August 2012 Keywords: Ultrasonic velocity Ferrofluid Nanofluid Molecular interaction Adiabatic compressibility
a b s t r a c t Magnetite nanofluids of various concentrations have been prepared through co-precipitation method. The structural and magnetic properties of the magnetic nanofluids have been analyzed which respectively revealed their face centered cubic crystal structure and super paramagnetic behavior. Ultrasonic investigations have been made for the nanofluids at different temperatures and magnetic fields. Openand close-packed water structure is considered to explain the temperature effects. The inter particle interactions of surface modified nanomagnetite particle and the cluster formation are realized through the variations in ultrasonic parameters. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction A ferrofluid is a colloidal suspension of magnetic particles, with typical dimensions of about 10 nm, dispersed in a liquid carrier [1] and in recent years it is called as magnetic nanofluid. The exciting feature about magnetic nanofluid is the possibility of controlling its properties by applying external magnetic field. The physical properties return to normal when the magnetic field is removed. Since coagulation is inhibited by a dispersing agent the magnetic nanofluids exhibit colloidal stability even in the presence of external magnetic field. The functional properties of magnetic nanofluid make them the optimal materials for magneto hydrodynamics, magnetic writing, magnetic drug delivery, hyperthermia treatment, optical filters, spintronic devices, MEMS, Microfluidics [2–6]. Most of these applications would not be feasible without magnetic nanofluids since the properties of magnetic nanofluids are unique and are not known to exist in any other fluids. A careful survey of literature revealed that there are very few reports made on the ultrasonic properties of nanofluids [7–11]. Reports available on the magnetic nanofluids [12,13] prove the tunable optical rheological and thermal properties and also show the dependence of ultrasonic velocity on the structure of magnetic fluid. A large deviation of the experimental values of velocity and attenuation from the theoretical predictions is also reported [14–16]. The fundamental understanding of exact mechanisms responsible for the unprecedented behaviors of nanofluids still remains unclear because of the lack of molecular level understanding of
⇑ Corresponding author. Tel.: +91 431 2503608; fax: +91 431 2500133. E-mail address: [email protected] (J. Hemalatha). 0041-624X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultras.2012.08.005
the ultrafine particles. This fact demands the systematic studies on the molecular interactions of magnetic nanofluids with respect to the variations in concentration, temperature and external magnetic fields. Hence, this paper is devoted to the systematic experimental study on the response of magnetic nanofluids to the ultrasonic wave propagation for the basic understanding of how the magnetic nanoparticles behave in fluids and how they interact with each other and with fluid. Preparing the stable and homogeneous suspensions of magnetic nanofluids and attaining a deeper understanding of particle–fluid, particle–particle interactions as functions of concentration, temperature and magnetic field, are the main concern.
2. Experimental 2.1. Synthesis All the chemicals used in this reaction were of analytical grade and were used as purchased. Ferrous chloride (FeCl2) and Ferric chloride (FeCl3) were used as the reactants. Double distilled water was used as solvent whereas Oleic acid (C17H33COOH) was used as the surfactant. Aqueous solutions of ferrous chloride and ferric chloride were prepared separately in stoichiometric ratio 1:2 to obtain the precursors [17]. Then both solutions were mixed together and stirred for 1 h to get a homogeneous mixture. pH of the mixture was found to be 2.6. Ammonia solution was added drop by drop to the above mixture while stirring until the pH reaches 9. A black precipitate thus obtained was washed several times with distilled water and then with acetone to remove impurities. Equal amounts of oleic
M. Nabeel Rashin, J. Hemalatha / Ultrasonics 52 (2012) 1024–1029
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at various equal distances on both sides of the cell. The magnetic field distribution was optimized and the acoustic measurements were made in the region where the magnetic field was found uniform. Sufficient time of 15 min was given for the stabilization of field before making the measurements. Microstructural characterization of three samples was accomplished by means of AFM device (Agilent technologies Pico LE SPM) which was provided with commercial standard silicon nitride cantilever having force constant of 40 N/m and tip radius of 2–4 nm.
3. Results and discussion 3.1. Structural studies
Fig. 1. Magnetic nanofluids influenced by a magnet.
acid and ammonia solution were added and stirred at 80 °C for 2 h. Oleic acid separates the magnetite (Fe3O4) nanoparticles and keeps them in suspension preventing the agglomeration of particle. The chemical reaction is given as follows:
The XRD pattern of the dried film of magnetic nanofluid shown in Fig 2a exhibits typical reflections of (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (3 3 3), (4 4 0) and (5 3 3) planes indicating the face centered cubic structure of Fe3O4. The sample has an amorphous contribution to the background; however, the strong and sharp reflection peaks indicate the high degree of crystallinity of the nanoparticles. All the peaks match well with the standard JCPDS 82-1533. No secondary peaks are detected in XRD pattern which ensures the phase purity of Fe3O4. The average grain size is obtained using Debye Scherrer equation [18,19] and the lattice constant (a) of Fe3O4 is calculated using the inter planar spacing. The estimated values of crystallite size
Fe2þ þ 2Fe3þ þ 8OH ! Fe3 O4 þ 4H2 O COOH þ NH4 OH ! COONH4 þ H2 O Magnetite nanofluids of various magnetite concentrations (0.2%, 0.4%, 0.6%, 0.8%, and 1% by volume) were prepared by diluting appropriate amount of nanofluid in water. The response of these fluids for external magnetic field is depicted in Fig 1. 2.2. Characterization The crystalline structure, phase composition and crystallite size of Fe3O4 were identified from XRD patterns obtained using Cu Ka radiation (k = 1.541 Å) for 2h value ranging from 10° to 80° in X-ray diffractometer (Model Rigaku Ultima III). The as-prepared magnetic nanofluid is casted into a thin film on a glass substrate and the structural studies were carried out for the film using XRD. The M–H plots of magnetic nanofluids were obtained at room temperature using a vibrating sample magnetometer (Lake Shore, USA, Model 7404) with 15 kOe as maximum applied magnetic field. The velocity values of ultrasonic wave propagation through the magnetic nanofluid samples were measured using a single frequency continuous wave ultrasonic interferometer (Model F81, Mittal Enterprises, New Delhi) with an accuracy of ±0.05% at frequency of 2 MHz. Density of the fluid was determined using specific gravity bottle (5 cc) with accuracy of ±2 parts in 104. All these measurements were performed for the fluids of all concentrations at five different temperatures of 35, 40, 45, 50 and 55 °C. The temperatures were maintained constant by circulating water from a thermostatically controlled water bath with accuracy of ±0.1 °C. The velocity and density measurements were repeated several times for accuracy and the average of five continuous consistent values are reported in this paper. Magnetic fields varying from 70 gauss to 550 gauss were applied to the samples to make magneto acoustic measurements by placing permanent magnets
Fig. 2. (a) XRD pattern of the dried film of magnetic nanofluid, (b) hysteresis plots for magnetic nanofluids.
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and lattice constant of Fe3O4 are 11 nm and 0.841 nm respectively. The (a) value is in good agreement with earlier reported values of a = 0.839 nm for nanoFe3O4 [20] and 0.840 nm for the bulk Fe3O4 [21] which proves the efficiency of synthesis technique.
impedance (Z) are calculated for magnetic nanofluids using the velocity (v) and density (q) data obtained through the experiments. The adiabatic compressibility of the samples is determined using the Newton–Laplace’s relation [26].
3.2. Magnetic studies
b¼
Room temperature magnetization studies made for magnetic nanofluids of all concentrations are shown in Fig 2b. Because of their small size the most favorable magnetic state of the particles is of a single domain. Therefore, the samples exhibit zero remnant magnetization and zero coercivity, an absence of hysteresis, indicating typical superparamagnetic behavior. This is in accordance with the physical explanations for superparamagnetism presented below. (i) At smaller sizes of particles one can see superparamagnetism, where the magnetic moment of the particle as a whole is free to fluctuate in response to thermal energy, while the individual atomic moments maintain their ordered state relative to each other. This leads to the anhysteretic, but still sigmoidal, M–H curve shown in Fig 2b. (ii) The critical diameter (dC) for monodomain formation in magnetic materials is given by [22]:
The characteristic acoustic impedance was calculated for all the samples using the relation [27]
pffiffiffiffiffiffiffi dC 18ð AK =l0 M 2S Þ
ð1Þ
where A is the exchange constant, K is the magnetic anisotropy, l0 is permeability of free space and MS is the saturation magnetization. The critical diameter for the magnetite can be estimated by using the bulk values of A, K, and MS (A = 1.3 1011 J/m, K = 1.35 104 J/m3, and MS = 4.6 105 A/m), resulting in dC 28 nm. Therefore, the diameter (11 nm) of the magnetite particles studied here is well below its critical size (28 nm), indicating that all particles, even the large ones, can be considered as single domain that should exhibit superparamagnetism. From Fig 2b it is understood that the magnetization of the magnetic nanofluid increases as a function of concentration and saturates at higher field values. This means that small changes in the external magnetic field result in substantial magnetization changes [23] as long as the strength of the external magnetic field is weaker than the value at which the magnetic nanofluid reaches saturation. The maximum magnetization values observed at the external magnetic field strength of 1200 kA/m and also the susceptibility [23] values of the fluids obtained using Eq. (2) are presented in Table 1.
M H!0 H
v ¼ lim
ð2Þ
Moreover, the average particle size of the magnetic nanoparticles is measured by using the magnetization data in Langevin’s equation [24,25] and is found to be 9.7 nm. 3.3. Ultrasonic studies The direction of ultrasonic wave propagation is set perpendicular to the direction of magnetic field to measure the velocity and the schematic of experimental set up is shown in Fig 3. The acoustical parameters like adiabatic compressibility (b), and acoustic
1
ð3Þ
qv 2
z ¼ qv
ð4Þ
The ultrasonic velocity of magnetic nanofluid decreases as a function of concentration at 35 °C in absence of magnetic field as shown in Fig 4a. It drops 0.20% (for concentration 0.2%) to 1.78% (for concentration 1%) lower than that of the carrier liquid in the absence of magnetic field, which shows the influence of dispersed particles on the velocity of ultrasonic propagation. This indicates that the fluids of high concentration are less compressible than those of lower concentration which can be understood more clearly in terms of increased density and decreasing trend of adiabatic compressibility Fig 4b. The decreasing adiabatic compressibility and increasing acoustic impedance shown in Fig 4c, is attributed to the increase in density with respect to concentration. This can further be explained with the help of coupling constant (k) which can be estimated [1] as follows: 3
k ¼ l0 l2 =4pdh kT
Here, l0 is permeability of free space and l is the magnetic moment of the grain, dh is the hydrodynamic diameter of the grain, k is the Boltzmann constant and T is the absolute temperature. Due to small diameters the particles form single domain of uniform magnetization with a magnetic moment given by:
l ¼ Mb V
ð6Þ
where Mb denotes the bulk magnetization of the particle and
V¼
pd3
ð7Þ
6
V is the volume of the core of the particle. The hydrodynamic diameter of the grain (dh) is greater than the size (d), of the magnetic particle by twice the thickness of protective surfactant layer [28]. As the thickness of oleic acid double layer is 3.5 nm [29] the hydrodynamic diameter (dh) is taken as 18 nm and the coupling constant (k) is found to be 0.40. Since the coupling constant, a measure of dipole strength is much smaller (k < 1) the magnetic grains interact with the external magnetic field but do not interact with each other. Although the coupling constant is small and the magnetic dipole interaction is rather weak, it is still possible for nanoparticles to form clusters. At small concentration the clusters are smaller in size and number that leads to the corresponding change in velocity. But for higher concentration it stimulates the formation of clusters and rigidity of the magnetic nanofluid increases thereby
Table 1 Magnetic parameters for various concentrations. Concentration (vol%)
Magnetization (A/m)
Susceptibility
0.2 0.4 0.6 0.8 1
852 1796 3052 3383 4671
0.02 0.04 0.065 0.073 0.099
ð5Þ
Fig. 3. Schematic of experimental set up.
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supposed to occupy the space between the clusters. The clusters are sometimes referred as open structure water and the dense monomeric fluid is referred to as closed structure water. In water the rise in temperature causes thermal rupture of the open packed structure of water, which in turn, enhances the cohesion of water molecules and less compressible closed packed structure [30–32] leading to an increase in the ultrasonic velocity. It further seems that the cohesion factor dominates over the thermal expansion factor with increase in temperature. The same explanation holds good for the increase in velocity and the corresponding increase in
Fig. 4. Plots of (a) ultrasonic velocity versus concentration, (b) adiabatic compressibility versus concentration and (c) acoustic impedance versus concentration at different temperatures.
decreasing the adiabatic compressibility. This affects the propagation of ultrasound wave in the medium at higher concentrations. In the absence of magnetic field, the plots of velocity versus concentration obtained at 40, 45, 50 and 55 °C have the same trend as the plot obtained at 35 °C but with higher magnitudes of velocity. It seems that the magnetic nanofluids follow the well known behavior of water showing an increase in velocity with increase of temperature that can be explained using open and close packed structure of water. Water consists of hydrogen bonded clusters and unbounded water molecules. The molecules in the interior clusters are quadruptly bonded and unbounded water molecules are
Fig. 5. Plots of (a) ultrasonic velocity versus magnetic field, (b) adiabatic compressibility versus magnetic field and (c) acoustic impedance versus magnetic field for magnetic nanofluids of different concentrations at 35 °C.
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of the carrier fluid in the presence of external magnetic field. The decrease of 0.07% is observed at 35 °C. Fluids of higher concentrations exhibit still higher variations (0.50%, 0.86%, 1.30%, and 1.78% for concentrations 0.4%, 0.6%, 0.8% and 1% respectively) of velocity under the influence of magnetic field (550 gauss). This observation clearly indicates that the magnetic nanofluids of higher concentrations having higher magnetization and susceptibility values (Table 1) are easily influenced by the external field and show large response under the application of external magnetic field. It can also be noted from Fig 5a that for magnetic nanofluids of concentrations (0.2–0.8%) the velocity variation with increasing external magnetic field is not so significant indicating the minimal influence of magnetic field on the ultrasonic propagation velocity. But magnetic nanofluid of concentration 1% exhibits appreciably higher variations in the velocity values with respect to the change in magnetic field. Furthermore, it exhibits a decreasing trend in the velocity versus magnetic field plot at 35 °C at low magnetic fields whereas an increasing trend at higher concentrations. This indicates that at 1% the concentration of solid component is sufficiently high and the intensity of magnetic field is strong enough to stimulate strong interparticle interaction resulting in the decrease of adiabatic compressibility (Fig 5b), which in turn, affects the conditions of ultrasonic wave propagation in this medium and acoustic impedance (Fig 5c). Careful observation of the plot between velocity and magnetic field in Fig 6 shows that at higher temperatures the response of
Fig. 6. Plots of ultrasonic velocity versus magnetic field at (a) 40 °C, (b) 45 °C, (c) 50 °C and d) 55 °C for magnetic nanofluids of different concentrations.
acoustic impedance of water based magnetic nanofluids observed at elevated temperatures. The variation of ultrasonic parameters in the presence of magnetic field is depicted in Fig 5. It is seen that the velocity of magnetic nanofluid of concentration 0.2% is lower than that
Fig. 7. 3-D topography images of magnetic nanofluids of concentrations (a) 0.4%, (b) 0.6% and (c) 1%.
M. Nabeel Rashin, J. Hemalatha / Ultrasonics 52 (2012) 1024–1029
even the dilute magnetic nanofluids to the external field is enhanced. At 55 °C all the samples except very low concentration (0.2%) respond to the external field. This reveals the influence of temperature on the enhancement of the magnetic response of the fluids. The velocity values at all temperatures are less than those of water for zero field and other fields H = 70, 170, 350 except 550 gauss. The initial reduction in velocity at the low field is due to the orientation of relaxation. Moreover, the magnetic attraction of nanoparticles is weak enough that steric repulsion of the surfactant is sufficient to prevent magnetic agglomeration. Only when a field of 550 gauss is applied the velocity becomes higher than the value of water this indicates that 550 gauss provides sufficient magnetic field for the cluster formation and hence there must be growth of particles in the fluid leading to higher This can be attributed to the fact that higher temperature enables (i) the breaking of bonds and hence the cluster formation and (ii) the fast rotation of magnetic dipoles which helps the alignment of the cluster along the direction of field and hence the magnetic effects are enhanced at higher temperatures even in low concentration samples. 3.4. AFM studies Atomic Force Microscopic investigations were performed selectively for three of the magnetic nanofluid samples (0.4%, 0.6% and 1%) in non-contact mode. The measurements have been carried out on dried samples deposited on silicon substrate. They have been repeated on different sites of the deposited sample, prepared in the same conditions of room temperature and ambient atmosphere. A comparative analysis between the 3-D topography images presented in Fig 7 reveals that the cluster density of the three samples is obviously different, higher for higher concentration. 4. Conclusion Magnetic nanofluids of various concentrations of fcc magnetite have been prepared and are found to be very stable which do not phase-separate even in strong magnetic fields. Ultrasonic parameters have been found for various concentrations, temperatures and magnetic fields. Their variations clearly indicate the enhancement in particle–particle interaction resulting in the formation of clusters at higher concentrations. The temperature effect on fluid is explained using Open- and close-packed structure of water. As the temperature range of interest is sufficient enough to cause thermal rupture of the open packed structure of water it can be confirmed that the changes of the acoustical parameters with temperature variation indicate the predominance of the cohesion of water molecules over the thermal expansion. When exposed to magnetic fields of various strengths the variation observed in acoustical parameters confirms the structural rearrangement in magnetic nanofluids due to the formation of clusters. It is also revealed that rise in temperature of externally magnetized magnetic nanofluids enhance the magnetic effects even in low concentration samples. In summary, ultrasonic parameters have been used to realize the effect of concentration, temperature and magnetic field on the intermolecular interactions of magnetite nanofluid. Acknowledgments The authors acknowledge the DST, Government of India for the VSM facility under the FIST Programme sanctioned to Department of Physics, NIT, Tiruchirappalli. They thank Mr. C. Gopalakrishnan, SRM University for extending the AFM facility for characterization.
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References [1] R.W. Rosensweig, Ferrohydrodynamics, Dover ed., Dover, New York, 1997. [2] P.K. Deheri, V. Swaminathan, S.D. Bhame, Z. Liu, R.V. Ramanujan, Sol–gel based chemical synthesis of Nd2Fe14B hard magnetic nanoparticles, Chem. Mater. 22 (2010) 6509–6517. [3] I. Sharifi, H. Shokrollahi, S. Amiri, Ferrite-based magnetic nanofluids used in hyperthermia applications, J. Magn. Magn. Mater. 324 (2012) 903–915. [4] S.D. Bhame, V. Swaminathan, P.K. Deheri, R.V. Ramanujan, Exchange coupled Nd2Fe14B/a-Fe nanocomposite by novel autocombustion-reduction diffusion synthesis, Adv. Sci. Lett. 3 (2010) 1–6. [5] S. Thomas, D. Sakthikumar, Y. Yoshida, M.R. Anantharaman, Spectroscopic and photoluminescence studies on optically transparent magnetic nanocomposites based on sol–gel glass: Fe3O4 J, Nanopart. Res. 10 (2008) 203–206. [6] J. Philip, T. Jaykumar, P. Kalyanasundaram, B. Raj, A tunable optical filter, Meas. Sci. Technol. 14 (2003) 1289–1294. [7] D.K. Singh, D.K. Pandey, R.R. Yadav, An ultrasonic characterization of ferrofluid, Ultrasonics 49 (2009) 634–637. [8] T. Hornowski, A. Jozefczak, M. Łabowski, A. Skumiel, Ultrasonic determination of the particle size distribution in water-based, magnetic liquid, Ultrasonics 48 (2008) 594–597. [9] P. Sayan, J. Ulrich, The effect of particle size and suspension density on the measurement of ultrasonic velocity in aqueous solutions, Chem. Eng. Process. 41 (2002) 281–287. [10] M. Motozawa, Y. Iizuka, T. Sawada, Experimental measurements of ultrasonic propagation velocity and attenuation in a magnetic fluid, J. Phys. Condens. Matter. 20 (2008) 204117-1–204117-5. [11] J. Hemalatha, T. Prabhakaran, R.P. Nalini, A comparative study on particle–fluid interactions in micro and nanofluids of aluminium oxide, Microfluid. Nanofluid. 10 (2011) 263–270. [12] P.D. Shima, J. Philip, Tuning of thermal conductivity and rheology of nanofluids using an external stimulus, J. Phys. Chem. C 115 (2011) 20097–20104. [13] J. Philip, P.D. Shima, B. Raj, Nanofluid with tunable thermal properties, Appl. Phys. Lett. 92 (2008) 043108-1–043108-3. [14] A. Jozefczak, The time dependence of the changes of ultrasonic wave velocity in ferrofluid under parallel magnetic field, J. Magn. Magn. Mater. 256 (2003) 267–270. [15] A. Skumiel, T. Hornowski, A. Jo´zefczak, Investigation of magnetic fluids by ultrasonic and magnetic methods, Ultrasonics 38 (2000) 864–867. [16] H.W. Muller, Y. Jiang, M. Liu, Sound damping in ferrofluids: magnetically enhanced compressional viscosity, Phys. Rev. E 67 (2003) 031201-1–031201-5. [17] T.H. Tsai, L.S. Kuo, P.H. Chen, D.S. Lee, C.T. Yang, Applications of ferro-nanofluid on a micro-transformer, Sensors 10 (2010) 8161–8172. [18] U. Holzwarth, N. Gibson, The Scherrer equation versus the ‘Debye-Scherrer equation’, Nat. Nanotechnol. 6 (2011) 534. [19] T. Prabhakaran, J. Hemalatha, Combustion synthesis and characterization of highly crystalline single phase nickel ferrite nanoparticles, J. Alloy. Compd. 509 (2011) 7071–7077. [20] T. Ozkaya, M.S. Toprak, A. Baykal, H. Kavas, Y. Koseoglu, B. Aktas, Synthesis of Fe3O4 nanoparticles at, 100 °C and its magnetic characterization, J. Alloy. Compd. 472 (2009) 18–23. [21] H.St.C. O’Neill, W.A. Dollase, Crystal structures and cation distribution in simple spinels from powder XRD structural refinements: MgCr2O4, ZnCr2O4, Fe3O4, and the temperature dependence of the cation distribution in ZnAl2O4, Phys. Chem. Miner. 20 (1994) 541–555. [22] V.B. Barbeta, R.F. Jardim, P.K. Kiyohara, F.B. Effenberger, L.M. Rossi, Magnetic properties of Fe3O4 nanoparticles coated with oleic and dodecanoic acids, J. Appl. Phys. 107 (2010) 073913-1–073913-7. [23] G.A.V. Ewijk, G.J. Vroege, A.P. Philipse, Susceptibility measurements on a fractionated aggregate free ferrofluid, J. Phys. Condens. Matter. 14 (2002) 4915–4925. [24] M. Racuciu, D.E. Creanga, N. Sulitanu, V. Badescu, Dimensional analysis of aqueous magnetic fluids, Appl. Phys. A 89 (2007) 565–569. [25] S. Dietrich, S. Chandra, C. Georgi, S. Thomas, Denys Makarov, S. Schulze, M. Hietschold, M. Albrecht, D. Bahadur, H. Lang, Design, characterization and magnetic properties of Fe3O4-nanoparticle arrays coated with PEGylateddendrimers, Mater. Chem. Phys. 132 (2012) 292–299. [26] M.J.W. Povey, Ultrasonic Techniques for Fluids Characterization, first ed., Academic Press, USA, 1997. [27] A.S. Dukhin, P.J. Goetz, Characterization of Liquids, Nano- and Microparticulates and Porous Bodies Using Ultrasound, second ed., Elsevier, New York, 2010. [28] Z. Rozynek, A. Jozefczak, K.D. Knudsen, A. Skumiel, T. Hornowski, J.O. Fossum, M. Timk, P. Kopcansky, M. Koneracka, Structuring from nanoparticles in oilbased ferrofluids, Eur. Phys. J. E 34 (2011) 28-1–28-8. [29] M.V. Avdeev, B. Mucha, K. Lamszus, L. Vekas, V.M. Garamus, A.V. Feoktystov, O. Marinica, R. Turcu, R. 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Contents lists available at SciVerse ScienceDirect
Ultrasonics journal homepage: www.elsevier.com/locate/ultras
Magnetic and ultrasonic investigations on magnetite nanofluids M. Nabeel Rashin, J. Hemalatha ⇑ Advanced Materials Lab, Department of Physics, National Institute of Technology, Tiruchirappalli, Tamilnadu 620 015, India
a r t i c l e
i n f o
Article history: Received 16 May 2012 Received in revised form 17 July 2012 Accepted 2 August 2012 Available online 11 August 2012 Keywords: Ultrasonic velocity Ferrofluid Nanofluid Molecular interaction Adiabatic compressibility
a b s t r a c t Magnetite nanofluids of various concentrations have been prepared through co-precipitation method. The structural and magnetic properties of the magnetic nanofluids have been analyzed which respectively revealed their face centered cubic crystal structure and super paramagnetic behavior. Ultrasonic investigations have been made for the nanofluids at different temperatures and magnetic fields. Openand close-packed water structure is considered to explain the temperature effects. The inter particle interactions of surface modified nanomagnetite particle and the cluster formation are realized through the variations in ultrasonic parameters. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction A ferrofluid is a colloidal suspension of magnetic particles, with typical dimensions of about 10 nm, dispersed in a liquid carrier [1] and in recent years it is called as magnetic nanofluid. The exciting feature about magnetic nanofluid is the possibility of controlling its properties by applying external magnetic field. The physical properties return to normal when the magnetic field is removed. Since coagulation is inhibited by a dispersing agent the magnetic nanofluids exhibit colloidal stability even in the presence of external magnetic field. The functional properties of magnetic nanofluid make them the optimal materials for magneto hydrodynamics, magnetic writing, magnetic drug delivery, hyperthermia treatment, optical filters, spintronic devices, MEMS, Microfluidics [2–6]. Most of these applications would not be feasible without magnetic nanofluids since the properties of magnetic nanofluids are unique and are not known to exist in any other fluids. A careful survey of literature revealed that there are very few reports made on the ultrasonic properties of nanofluids [7–11]. Reports available on the magnetic nanofluids [12,13] prove the tunable optical rheological and thermal properties and also show the dependence of ultrasonic velocity on the structure of magnetic fluid. A large deviation of the experimental values of velocity and attenuation from the theoretical predictions is also reported [14–16]. The fundamental understanding of exact mechanisms responsible for the unprecedented behaviors of nanofluids still remains unclear because of the lack of molecular level understanding of
⇑ Corresponding author. Tel.: +91 431 2503608; fax: +91 431 2500133. E-mail address: [email protected] (J. Hemalatha). 0041-624X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultras.2012.08.005
the ultrafine particles. This fact demands the systematic studies on the molecular interactions of magnetic nanofluids with respect to the variations in concentration, temperature and external magnetic fields. Hence, this paper is devoted to the systematic experimental study on the response of magnetic nanofluids to the ultrasonic wave propagation for the basic understanding of how the magnetic nanoparticles behave in fluids and how they interact with each other and with fluid. Preparing the stable and homogeneous suspensions of magnetic nanofluids and attaining a deeper understanding of particle–fluid, particle–particle interactions as functions of concentration, temperature and magnetic field, are the main concern.
2. Experimental 2.1. Synthesis All the chemicals used in this reaction were of analytical grade and were used as purchased. Ferrous chloride (FeCl2) and Ferric chloride (FeCl3) were used as the reactants. Double distilled water was used as solvent whereas Oleic acid (C17H33COOH) was used as the surfactant. Aqueous solutions of ferrous chloride and ferric chloride were prepared separately in stoichiometric ratio 1:2 to obtain the precursors [17]. Then both solutions were mixed together and stirred for 1 h to get a homogeneous mixture. pH of the mixture was found to be 2.6. Ammonia solution was added drop by drop to the above mixture while stirring until the pH reaches 9. A black precipitate thus obtained was washed several times with distilled water and then with acetone to remove impurities. Equal amounts of oleic
M. Nabeel Rashin, J. Hemalatha / Ultrasonics 52 (2012) 1024–1029
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at various equal distances on both sides of the cell. The magnetic field distribution was optimized and the acoustic measurements were made in the region where the magnetic field was found uniform. Sufficient time of 15 min was given for the stabilization of field before making the measurements. Microstructural characterization of three samples was accomplished by means of AFM device (Agilent technologies Pico LE SPM) which was provided with commercial standard silicon nitride cantilever having force constant of 40 N/m and tip radius of 2–4 nm.
3. Results and discussion 3.1. Structural studies
Fig. 1. Magnetic nanofluids influenced by a magnet.
acid and ammonia solution were added and stirred at 80 °C for 2 h. Oleic acid separates the magnetite (Fe3O4) nanoparticles and keeps them in suspension preventing the agglomeration of particle. The chemical reaction is given as follows:
The XRD pattern of the dried film of magnetic nanofluid shown in Fig 2a exhibits typical reflections of (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (3 3 3), (4 4 0) and (5 3 3) planes indicating the face centered cubic structure of Fe3O4. The sample has an amorphous contribution to the background; however, the strong and sharp reflection peaks indicate the high degree of crystallinity of the nanoparticles. All the peaks match well with the standard JCPDS 82-1533. No secondary peaks are detected in XRD pattern which ensures the phase purity of Fe3O4. The average grain size is obtained using Debye Scherrer equation [18,19] and the lattice constant (a) of Fe3O4 is calculated using the inter planar spacing. The estimated values of crystallite size
Fe2þ þ 2Fe3þ þ 8OH ! Fe3 O4 þ 4H2 O COOH þ NH4 OH ! COONH4 þ H2 O Magnetite nanofluids of various magnetite concentrations (0.2%, 0.4%, 0.6%, 0.8%, and 1% by volume) were prepared by diluting appropriate amount of nanofluid in water. The response of these fluids for external magnetic field is depicted in Fig 1. 2.2. Characterization The crystalline structure, phase composition and crystallite size of Fe3O4 were identified from XRD patterns obtained using Cu Ka radiation (k = 1.541 Å) for 2h value ranging from 10° to 80° in X-ray diffractometer (Model Rigaku Ultima III). The as-prepared magnetic nanofluid is casted into a thin film on a glass substrate and the structural studies were carried out for the film using XRD. The M–H plots of magnetic nanofluids were obtained at room temperature using a vibrating sample magnetometer (Lake Shore, USA, Model 7404) with 15 kOe as maximum applied magnetic field. The velocity values of ultrasonic wave propagation through the magnetic nanofluid samples were measured using a single frequency continuous wave ultrasonic interferometer (Model F81, Mittal Enterprises, New Delhi) with an accuracy of ±0.05% at frequency of 2 MHz. Density of the fluid was determined using specific gravity bottle (5 cc) with accuracy of ±2 parts in 104. All these measurements were performed for the fluids of all concentrations at five different temperatures of 35, 40, 45, 50 and 55 °C. The temperatures were maintained constant by circulating water from a thermostatically controlled water bath with accuracy of ±0.1 °C. The velocity and density measurements were repeated several times for accuracy and the average of five continuous consistent values are reported in this paper. Magnetic fields varying from 70 gauss to 550 gauss were applied to the samples to make magneto acoustic measurements by placing permanent magnets
Fig. 2. (a) XRD pattern of the dried film of magnetic nanofluid, (b) hysteresis plots for magnetic nanofluids.
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and lattice constant of Fe3O4 are 11 nm and 0.841 nm respectively. The (a) value is in good agreement with earlier reported values of a = 0.839 nm for nanoFe3O4 [20] and 0.840 nm for the bulk Fe3O4 [21] which proves the efficiency of synthesis technique.
impedance (Z) are calculated for magnetic nanofluids using the velocity (v) and density (q) data obtained through the experiments. The adiabatic compressibility of the samples is determined using the Newton–Laplace’s relation [26].
3.2. Magnetic studies
b¼
Room temperature magnetization studies made for magnetic nanofluids of all concentrations are shown in Fig 2b. Because of their small size the most favorable magnetic state of the particles is of a single domain. Therefore, the samples exhibit zero remnant magnetization and zero coercivity, an absence of hysteresis, indicating typical superparamagnetic behavior. This is in accordance with the physical explanations for superparamagnetism presented below. (i) At smaller sizes of particles one can see superparamagnetism, where the magnetic moment of the particle as a whole is free to fluctuate in response to thermal energy, while the individual atomic moments maintain their ordered state relative to each other. This leads to the anhysteretic, but still sigmoidal, M–H curve shown in Fig 2b. (ii) The critical diameter (dC) for monodomain formation in magnetic materials is given by [22]:
The characteristic acoustic impedance was calculated for all the samples using the relation [27]
pffiffiffiffiffiffiffi dC 18ð AK =l0 M 2S Þ
ð1Þ
where A is the exchange constant, K is the magnetic anisotropy, l0 is permeability of free space and MS is the saturation magnetization. The critical diameter for the magnetite can be estimated by using the bulk values of A, K, and MS (A = 1.3 1011 J/m, K = 1.35 104 J/m3, and MS = 4.6 105 A/m), resulting in dC 28 nm. Therefore, the diameter (11 nm) of the magnetite particles studied here is well below its critical size (28 nm), indicating that all particles, even the large ones, can be considered as single domain that should exhibit superparamagnetism. From Fig 2b it is understood that the magnetization of the magnetic nanofluid increases as a function of concentration and saturates at higher field values. This means that small changes in the external magnetic field result in substantial magnetization changes [23] as long as the strength of the external magnetic field is weaker than the value at which the magnetic nanofluid reaches saturation. The maximum magnetization values observed at the external magnetic field strength of 1200 kA/m and also the susceptibility [23] values of the fluids obtained using Eq. (2) are presented in Table 1.
M H!0 H
v ¼ lim
ð2Þ
Moreover, the average particle size of the magnetic nanoparticles is measured by using the magnetization data in Langevin’s equation [24,25] and is found to be 9.7 nm. 3.3. Ultrasonic studies The direction of ultrasonic wave propagation is set perpendicular to the direction of magnetic field to measure the velocity and the schematic of experimental set up is shown in Fig 3. The acoustical parameters like adiabatic compressibility (b), and acoustic
1
ð3Þ
qv 2
z ¼ qv
ð4Þ
The ultrasonic velocity of magnetic nanofluid decreases as a function of concentration at 35 °C in absence of magnetic field as shown in Fig 4a. It drops 0.20% (for concentration 0.2%) to 1.78% (for concentration 1%) lower than that of the carrier liquid in the absence of magnetic field, which shows the influence of dispersed particles on the velocity of ultrasonic propagation. This indicates that the fluids of high concentration are less compressible than those of lower concentration which can be understood more clearly in terms of increased density and decreasing trend of adiabatic compressibility Fig 4b. The decreasing adiabatic compressibility and increasing acoustic impedance shown in Fig 4c, is attributed to the increase in density with respect to concentration. This can further be explained with the help of coupling constant (k) which can be estimated [1] as follows: 3
k ¼ l0 l2 =4pdh kT
Here, l0 is permeability of free space and l is the magnetic moment of the grain, dh is the hydrodynamic diameter of the grain, k is the Boltzmann constant and T is the absolute temperature. Due to small diameters the particles form single domain of uniform magnetization with a magnetic moment given by:
l ¼ Mb V
ð6Þ
where Mb denotes the bulk magnetization of the particle and
V¼
pd3
ð7Þ
6
V is the volume of the core of the particle. The hydrodynamic diameter of the grain (dh) is greater than the size (d), of the magnetic particle by twice the thickness of protective surfactant layer [28]. As the thickness of oleic acid double layer is 3.5 nm [29] the hydrodynamic diameter (dh) is taken as 18 nm and the coupling constant (k) is found to be 0.40. Since the coupling constant, a measure of dipole strength is much smaller (k < 1) the magnetic grains interact with the external magnetic field but do not interact with each other. Although the coupling constant is small and the magnetic dipole interaction is rather weak, it is still possible for nanoparticles to form clusters. At small concentration the clusters are smaller in size and number that leads to the corresponding change in velocity. But for higher concentration it stimulates the formation of clusters and rigidity of the magnetic nanofluid increases thereby
Table 1 Magnetic parameters for various concentrations. Concentration (vol%)
Magnetization (A/m)
Susceptibility
0.2 0.4 0.6 0.8 1
852 1796 3052 3383 4671
0.02 0.04 0.065 0.073 0.099
ð5Þ
Fig. 3. Schematic of experimental set up.
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supposed to occupy the space between the clusters. The clusters are sometimes referred as open structure water and the dense monomeric fluid is referred to as closed structure water. In water the rise in temperature causes thermal rupture of the open packed structure of water, which in turn, enhances the cohesion of water molecules and less compressible closed packed structure [30–32] leading to an increase in the ultrasonic velocity. It further seems that the cohesion factor dominates over the thermal expansion factor with increase in temperature. The same explanation holds good for the increase in velocity and the corresponding increase in
Fig. 4. Plots of (a) ultrasonic velocity versus concentration, (b) adiabatic compressibility versus concentration and (c) acoustic impedance versus concentration at different temperatures.
decreasing the adiabatic compressibility. This affects the propagation of ultrasound wave in the medium at higher concentrations. In the absence of magnetic field, the plots of velocity versus concentration obtained at 40, 45, 50 and 55 °C have the same trend as the plot obtained at 35 °C but with higher magnitudes of velocity. It seems that the magnetic nanofluids follow the well known behavior of water showing an increase in velocity with increase of temperature that can be explained using open and close packed structure of water. Water consists of hydrogen bonded clusters and unbounded water molecules. The molecules in the interior clusters are quadruptly bonded and unbounded water molecules are
Fig. 5. Plots of (a) ultrasonic velocity versus magnetic field, (b) adiabatic compressibility versus magnetic field and (c) acoustic impedance versus magnetic field for magnetic nanofluids of different concentrations at 35 °C.
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of the carrier fluid in the presence of external magnetic field. The decrease of 0.07% is observed at 35 °C. Fluids of higher concentrations exhibit still higher variations (0.50%, 0.86%, 1.30%, and 1.78% for concentrations 0.4%, 0.6%, 0.8% and 1% respectively) of velocity under the influence of magnetic field (550 gauss). This observation clearly indicates that the magnetic nanofluids of higher concentrations having higher magnetization and susceptibility values (Table 1) are easily influenced by the external field and show large response under the application of external magnetic field. It can also be noted from Fig 5a that for magnetic nanofluids of concentrations (0.2–0.8%) the velocity variation with increasing external magnetic field is not so significant indicating the minimal influence of magnetic field on the ultrasonic propagation velocity. But magnetic nanofluid of concentration 1% exhibits appreciably higher variations in the velocity values with respect to the change in magnetic field. Furthermore, it exhibits a decreasing trend in the velocity versus magnetic field plot at 35 °C at low magnetic fields whereas an increasing trend at higher concentrations. This indicates that at 1% the concentration of solid component is sufficiently high and the intensity of magnetic field is strong enough to stimulate strong interparticle interaction resulting in the decrease of adiabatic compressibility (Fig 5b), which in turn, affects the conditions of ultrasonic wave propagation in this medium and acoustic impedance (Fig 5c). Careful observation of the plot between velocity and magnetic field in Fig 6 shows that at higher temperatures the response of
Fig. 6. Plots of ultrasonic velocity versus magnetic field at (a) 40 °C, (b) 45 °C, (c) 50 °C and d) 55 °C for magnetic nanofluids of different concentrations.
acoustic impedance of water based magnetic nanofluids observed at elevated temperatures. The variation of ultrasonic parameters in the presence of magnetic field is depicted in Fig 5. It is seen that the velocity of magnetic nanofluid of concentration 0.2% is lower than that
Fig. 7. 3-D topography images of magnetic nanofluids of concentrations (a) 0.4%, (b) 0.6% and (c) 1%.
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even the dilute magnetic nanofluids to the external field is enhanced. At 55 °C all the samples except very low concentration (0.2%) respond to the external field. This reveals the influence of temperature on the enhancement of the magnetic response of the fluids. The velocity values at all temperatures are less than those of water for zero field and other fields H = 70, 170, 350 except 550 gauss. The initial reduction in velocity at the low field is due to the orientation of relaxation. Moreover, the magnetic attraction of nanoparticles is weak enough that steric repulsion of the surfactant is sufficient to prevent magnetic agglomeration. Only when a field of 550 gauss is applied the velocity becomes higher than the value of water this indicates that 550 gauss provides sufficient magnetic field for the cluster formation and hence there must be growth of particles in the fluid leading to higher This can be attributed to the fact that higher temperature enables (i) the breaking of bonds and hence the cluster formation and (ii) the fast rotation of magnetic dipoles which helps the alignment of the cluster along the direction of field and hence the magnetic effects are enhanced at higher temperatures even in low concentration samples. 3.4. AFM studies Atomic Force Microscopic investigations were performed selectively for three of the magnetic nanofluid samples (0.4%, 0.6% and 1%) in non-contact mode. The measurements have been carried out on dried samples deposited on silicon substrate. They have been repeated on different sites of the deposited sample, prepared in the same conditions of room temperature and ambient atmosphere. A comparative analysis between the 3-D topography images presented in Fig 7 reveals that the cluster density of the three samples is obviously different, higher for higher concentration. 4. Conclusion Magnetic nanofluids of various concentrations of fcc magnetite have been prepared and are found to be very stable which do not phase-separate even in strong magnetic fields. Ultrasonic parameters have been found for various concentrations, temperatures and magnetic fields. Their variations clearly indicate the enhancement in particle–particle interaction resulting in the formation of clusters at higher concentrations. The temperature effect on fluid is explained using Open- and close-packed structure of water. As the temperature range of interest is sufficient enough to cause thermal rupture of the open packed structure of water it can be confirmed that the changes of the acoustical parameters with temperature variation indicate the predominance of the cohesion of water molecules over the thermal expansion. When exposed to magnetic fields of various strengths the variation observed in acoustical parameters confirms the structural rearrangement in magnetic nanofluids due to the formation of clusters. It is also revealed that rise in temperature of externally magnetized magnetic nanofluids enhance the magnetic effects even in low concentration samples. In summary, ultrasonic parameters have been used to realize the effect of concentration, temperature and magnetic field on the intermolecular interactions of magnetite nanofluid. Acknowledgments The authors acknowledge the DST, Government of India for the VSM facility under the FIST Programme sanctioned to Department of Physics, NIT, Tiruchirappalli. They thank Mr. C. Gopalakrishnan, SRM University for extending the AFM facility for characterization.
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