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Magnetic and ultrasonic studies on stable cobalt ferrite magnetic nanofluid
Ultrasonics 54 (2014) 834–840
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Magnetic and ultrasonic studies on stable cobalt ferrite magnetic nanofluid 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 8 June 2013 Received in revised form 11 October 2013 Accepted 14 October 2013 Available online 27 October 2013 Keywords: Ultrasonic velocity Ferrofluid Nanofluid Molecular interaction Adiabatic compressibility
a b s t r a c t Stable cobalt ferrite nanofluids of various concentrations have been prepared through co-precipitation method. Structural and morphological studies of nanoparticles are made with the help of X-ray diffraction technique and Transmission Electron Microscope respectively and it is found that the particles exhibit face centered cubic structure with an average size of 14 nm. The magnetic properties of the nanofluids have been analyzed at room temperature which revealed ferromagnetic behavior and also the very low value of coupling constant which ensures the negligible interparticle interaction in the absence of magnetic field. Ultrasonic investigations have been made for the nanofluids at different temperatures and magnetic fields. The temperature effects are explained with the help of open and close-packed water structure. The inter particle interactions of surface modified CoFe2O4 particles and the cluster formation at higher concentrations are realized through the variations in ultrasonic parameters. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Magnetic nanofluids are smart colloidal suspensions of nanomagnetic particles in base fluids. They have enormous importance in industrial and biomedical applications due to their tunable thermophysical properties. They have attracted much attention in the past decades because of their potential applications in high density magnetic recording, magneto hydrodynamics, magnetic writing, magnetic drug delivery, hyperthermia treatment, optical filters, spintronic devices, MEMS, Microfluidics, data storage, spintronics, solar cells, sensors, and catalysis [1–5]. Among the magnetic nanofluids, cobalt ferrite (CoFe2O4) based magnetic fluids have been widely analyzed due to high electromagnetic performance, excellent chemical stability, mechanical hardness, and high cubic magneto-crystalline anisotropy. These properties make it a promising candidate for many applications in commercial electronics such as video, audio tapes, high-density digital recording media [6]. Among the mentioned applications, the hyperthermia treatment has been recognized as very novel and promising, and the efficiency of cobalt ferrite nanoparticles has been clearly established [7]. Also, as concerning biomedical application, the possibility of using radioactive 60Co to produce enriched ferrofluid unlock a new perspective, as for instance in the targeting of cancer cells using antibody-coated nanoparticles [8]. From the literature it is found that there are only few reports available on the ultrasonic properties of nanofluids [9–16]. Reports ⇑ Corresponding author. Tel.: +91 9600974565; fax: +91 431 2500133. E-mail address: [email protected] (J. Hemalatha). 0041-624X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultras.2013.10.009
on the magnetic nanofluids [17–19] prove the tunable optical, rheological and thermal properties and also show the dependence of ultrasonic velocity on the clustering structure of magnetic fluid. A big deviation of the experimental values of velocity and attenuation from the theoretical predictions is also reported [20–22]. The fundamental understanding of exact mechanisms responsible for the amazing behaviors of magnetic nanofluids still remains unclear because of the lack of molecular level understanding of 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 the external magnetic fields. Hence, this paper is intended to the systematic experimental study on the response of cobalt ferrite magnetic nanofluids to the ultrasonic wave propagation for the basic understanding of how the cobalt ferrite nanoparticles behave in water and how they interact with each other and also with water. Preparing the stable and homogeneous suspensions of cobalt ferrite 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 The chemicals, Cobalt Nitrate (Co(NO3)6H2O), Ferric Nitrate (Fe (NO3)9H2O), Sodium Hydroxide (NaOH), Acetone ((CH3)2CO) are purchased from Merck and are used as purchased without further
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treatment. Double distilled water is used as the solvent whereas oleic acid (C17H33COOH) is used as the surfactant. Aqueous solutions of Cobalt Nitrate and Ferric Nitrate are prepared separately in stoichiometric ratio 1:2 to obtain the precursors. Both the solutions are mixed together and stirred for 1 h to get a homogeneous mixture. pH of the mixture is found to be 2.7. Then, 2 M NaOH solution is added drop by drop to the above mixture with simultaneous stirring until the pH reaches 9. A black precipitate thus obtained is heated at boiling temperature for 2 h, is allowed to cool and is washed several times with distilled water to remove impurities. Then, 200 ml of 2 M aqueous HNO3 solution is added to the above precipitate and stirred for 1 h. The supernatant solution is removed and the residue is cleaned thrice with acetone. Equal amounts of oleic acid and aqueous ammonia solution are added and stirred at 80 °C for 2 h. Oleic acid separates the cobalt ferrite nanoparticles and keeps them in suspension preventing the agglomeration of particle. The chemical reaction is given as follows.
Co2þ þ 2Fe3þ þ 8OH ! CoFe2 O4 þ 4H2 O COOH þ NH4 OH ! COONH4 þ H2 O Cobalt ferrite nanofluids of various concentrations (0.2%, 0.4%, 0.6%, 0.8%, and 1% by volume) are prepared by diluting appropriate amount of nanofluid in water. The response of these fluids for external magnetic field is depicted in Fig. 1. All the nanofluid samples show excellent stability because, the nanoparticles are not influenced by the
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gravitational force owing to their small size and they show no phase separation or sedimentation even in strong magnetic field. 2.2. Characterization The crystalline structure, phase composition and average crystallite size of Cobalt Ferrite nanoparticle are identified from the 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 size and morphology of CoFe2O4 nanoparticle is found using High Resolution Transmission Electron Microscope (Philips TECNAI F20 microscope). Powder Sample for TEM measurements is suspended in ethanol and ultrasonically dispersed. Drops of the suspensions are placed on a copper grid coated with carbon. The M–H plots of dried magnetic nanofluids are obtained at room temperature using a vibrating sample magnetometer (Lake Shore, USA, Model 7404) with 15 kOe as maximum applied magnetic field. The ultrasonic velocity is very sensitive to temperature and practically independent of frequency when compared to ultrasonic attenuation. So, it is decided to make ultrasonic measurements rather than attenuation. As the continuous wave based interferometry is suitable for studying the velocity change due to the magnetic field [40], a single frequency continuous wave ultrasonic interferometer (Model F80, Mittal Enterprises, New Delhi) with an accuracy of ±0.05% is used at frequency of 2 MHz. Density of the fluid was determined using Pycnometer with accuracy of ±2 parts in 104. All these measurements are performed for the fluids of all concentrations at five different temperatures of 308, 313, 318, 323 and 328 K. The temperatures are maintained constant by circulating water from a thermostatically controlled water bath with accuracy of ±0.1 °C. The velocity and density measurements are repeated several times for accuracy and the average of five continuous consistent values are reported in this paper. Magnetic fields varying from 0.007 T to 0.055 T are applied to the samples to make magneto acoustic measurements by placing permanent magnets at various equal distances on both sides of the cell. The magnetic field distribution is optimized and the acoustic measurements are made in the region where the magnetic field was found uniform. Sufficient time of 30 min is given for the stabilization of field before making the measurements. 3. Results and discussion 3.1. Structural and morphological studies
Fig. 1. CoFe2O4 magnetic nanofluids in the presence of magnet.
Fig. 2a shows the XRD pattern of CoFe2O4 nanopowder. It exhibits typical reflections of (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), (4 4 0) and (5 3 3) planes, indicating the face centered cubic structure of cobalt ferrite. The strong and sharp reflection peaks show the high degree of crystallinity of nanoparticles. All the peaks match well with the standard JCPDS 22-1086. No secondary peaks are detected in XRD pattern which ensures the phase purity. The average grain size is obtained using Debye Scherrer equation [23,24] and the lattice constant (a) of CoFe2O4 is calculated using the inter planar spacing. The estimated values of average crystallite size and lattice constant of CoFe2O4 are 14 nm and 0.83 nm respectively. The (a) value is in good agreement with earlier reported values [25,26]. The morphology of the CoFe2O4 powder is analyzed by HRTEM. The TEM images (Fig. 3a and b) reveal that the sample consists of Cubical particles with a regular morphology and narrow size distribution. The average size of the particles observed in the TEM image is in the range of 14 nm, which is in good agreement with that
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Fig. 2. XRD pattern of the cobalt ferrite particles.
estimated by Debye–Scherrer formula from the XRD pattern. It is obvious from the SAED pattern (Fig. 3c) that the particles are well crystallized. The diffraction rings on the picture correspond to (1 1 1), (2 2 0), (3 1 1), (4 0 0) (4 2 2), (5 1 1) and (4 4 0) planes respectively, which is in accordance with the peaks in the XRD pattern. 3.2. Magnetic studies The M–H loops of the CoFe2O4 nanopowder and fluids of different concentrations at 300 K are shown in Fig. 4 and they prove the ferromagnetic behavior of the samples. It is observed that the value of saturation magnetization (Ms), comes out to be 143 kA/m (28 emu/g) at room temperature, which is in accordance with earlier reported values [27,28] but smaller than that of the bulk value, 74.08 emu/g [29]. The remanence magnitude, Mr, can be extracted from the hysteresis loop at the intersections of the loop with the vertical magnetization axis and it is found to be 79.2 kA/m (15.5 emu/g). For nanosized ferrite particles, the surface areas are larger and thus the surface energy and surface tension are high. This results in changes in cationic preferences and leads to an increased degree of antisite defects and thus lesser magnetizations [6,30,31]. The coercivity, Hc, the intensity of the magnetic field required to reduce the magnetization of the sample to zero, of all the samples remains the same as 310.5 kA/m. The critical diameter of a single domain particle is estimated using the formula [32,33]
Dm ¼
18rw l0 M2s
ð1Þ
where rw is the wall energy density given by
rw
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2kB T C ka ¼ a
ð2Þ
with ka as the magneto crystalline anisotropy constant and TC the Curie temperature. For particle size, D > Dm, the particles exist in a multi domain structure, while for D < Dm, the particles exist in a single domain structure. For cobalt ferrite, TC = 677 K, a = 0.83 nm, ka = 1.6 105 J/m3, and Ms = 143 kA/m, the estimated value of Dm is about 1329 nm, which is larger than the average diameter of the synthesized CoFe2O4 nanoparticles. Hence the grains are found to exhibit single domain state, because of their small size the most favorable magnetic state of the particles is of a single domain. Single domains include groups of spins all pointing in the same direction and acting co-operatively.
Fig. 3. (a and b) TEM micrographs and (c) SAED pattern of CoFe2O4 nanoparticles.
Squareness ratio is given by the equation [34],
Rs ¼
Mr Ms
ð3Þ
where Mr is remanence magnetization and Ms is saturation magnetization measured from M–H hysteresis loop and it is found that the calculated Rs values of the sample prepared are 0.55.
M. Nabeel Rashin, J. Hemalatha / Ultrasonics 54 (2014) 834–840
l ¼ Mb V
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ð5Þ
where Mb denotes the bulk magnetization of the particle and
V¼
pd3 6
ð6Þ
V is the volume of the particle. The hydrodynamic diameter of the grain (dh) is greater than the particle size (d), of the magnetic particle by twice the thickness of protective surfactant layer [37]. As the thickness of oleic acid double layer is 3.5 nm [38] the hydrodynamic diameter (dh) is taken as 21 nm and the coupling constant (k) is found to be 0.11. Since the coupling constant, a measure of dipole strength, is much smaller (k < 1) it is understood that the magnetic grains interact with the external magnetic field but do not interact with each other. 3.3. Ultrasonic studies In order to measure velocity, the ultrasonic wave is made to propagate through the nanofluid in a direction which is perpendicular to the direction of applied magnetic field. The ultrasonic parameters like adiabatic compressibility (b) and acoustic impedance (Z) are calculated for magnetic nanofluids using the velocity (v) and density (q) data obtained through the experiments. The adiabatic compressibility of the fluid is determined by the Newton–Laplace’s relation [39].
b¼
1
qv 2
ð7Þ
The acoustic impedance is calculated for all the concentrations using the relation [40] Fig. 4. Ferromagnetic (b) nanofluids.
hysteresis
for
(a)
cobalt
ferrite
nanopowder
and
From Fig. 4 it can be concluded that the magnetization of the magnetic nanofluid increases as a function of concentration and saturates at higher field values. The statistics of the magnetic parameters is depicted in Table 1. The fact shows that small changes in the external magnetic field result in substantial magnetization changes [35] 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 are measured at an external magnetic field of strength of 1300 kA/ m. The coupling constant (k) of the surface modified cobalt ferrite nanoparticle is estimated [36] as follows:
k¼
l0 l2 3 4pdh kB T
ð4Þ
Here, l is the magnetic moment of the grain, dh is the hydrodynamic diameter of the grain. Due to small diameters the particles form single domain of uniform magnetization with a magnetic moment given by:
Table 1 Summary statistics of the magnetic parameters. Concentration (vol%)
Ms (A/m)
Mr (A/m)
Hc (kA/m)
Rs
0.2 0.4 0.6 0.8 1
286 572 857 1143 1429
158 316 474 632 790
310.5 310.5 310.5 310.5 310.5
0.55 0.55 0.55 0.55 0.55
Z ¼ qv
ð8Þ
The variation of ultrasonic velocity with concentration in the absence of magnetic field is shown in the Fig. 5a. It can be deduced that, the velocity in magnetic nanofluid decreases as a function of concentration. It drops 0.02% (for 0.2 vol%) to 0.2% (for 1 vol%) lower than that of the carrier liquid in the absence of magnetic field at a temperature of 308 K. It shows the influence of dispersed particles on the velocity of ultrasonic propagation. This decrement in ultrasonic velocity with concentration is the qualitative measure of particle fluid interactions. It ensures that the intermolecular interaction is dominating over the intramolecular interaction or in other words it shows the predominance of particle–fluid interaction over particle–particle interaction in the absence of magnetic field in all temperatures. In the absence of magnetic field, the plots of velocity versus concentration obtained at 313, 318, 323 and 328 K have the same trend as the plot obtained at 308 K but with higher magnitudes of velocity. The percentage drop in velocity also shows an increase as a drop of 0.17% (for 0.2 vol%) to 0.76% (for 1 vol%) lower than that of the carrier liquid at a temperature of 328 K. It can easily be seen that the magnetic nanofluids follow the well known behavior of water, showing an increase in velocity with increase of temperature, which 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 quadruply bonded and unbounded water molecules are 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
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Fig. 5. (a) Variation of Ultrasonic velocity, (b) adiabatic compressibility and (c) acoustic impedance with respect to concentrations at various temperatures.
cohesion of water molecules and less compressible closed packed structure [41–43] 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 acoustic impedance of water based magnetic nanofluids observed at elevated temperatures. Fig. 5b indicates that the fluids of high concentration are less compressible than those of lower concentration at 308 K. The decreasing trend of adiabatic compressibility drops off with
increase in temperature and leads to an increasing trend of compressibility with concentration. The decreasing trend of adiabatic compressibility and increasing acoustic impedance (as shown in Fig. 5c), are attributed to the increase in density with respect to concentration. The increasing trend of compressibility with concentration at higher temperatures is in accordance with the increment of percentage drop in magnitude of velocity evident from Fig. 5a. The behavior of ultrasonic parameters in the presence of magnetic field is illustrated in Fig. 6. It is clear that the velocity of cobalt ferrite nanofluid of all concentrations except 0.2% is lower than that of the carrier fluid even in the presence of external magnetic field at 308 K. The percentage drop in velocity than that of the carrier liquid reduces (0.02–0% for a concentration of 0.2% and 0.2–0.06% for a concentration of 1%) under the influence of magnetic field (0.055 T) at a temperature of 308 K. From Fig. 6a, it is seen that the velocity in cobalt ferrite nanofluid of all concentrations increases with increase of applied magnetic field. It enhances by 0.01% (for 0.035 T) and 0.02% (for 0.055 T) for a concentration of 0.2% and it enhances by 0.07% (for 0.035 T) and 0.14% (for 0.055 T) for a concentration of 1% at 308 K than velocity in the absence of magnetic field. This observation clearly indicates that the magnetic nanofluids of higher concentrations having higher magnetization are easily influenced by the external field and show large response under the application of external magnetic field. This increase of velocity with magnetic field is the sign of clustering of magnetic particle. As the applied field sets the magnetic moments along the direction of magnetic field forming the clusters, the rigidity of the system increases and hence the velocity increases. When the magnetic field is turned off, the velocity returns to its initial value without exhibiting any hysteresis. Because, as the particles are sterically stabilized, no permanent aggregation due to van der Waals attraction occurs and the aggregation phenomenon observed in magnetic field is perfectly reversible [5,17,18]. It can also be noted from Fig. 6a that at lower concentrations enhancement in velocity with increasing external magnetic field is not so significant, indicating the minimal influence of magnetic field on the ultrasonic velocity. But magnetic nanofluid of concentration 1% shows considerably higher variations in the velocity with respect to the change in magnetic field. It points out 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. 6b), which in turn, affects the conditions of ultrasonic wave propagation in this medium and acoustic impedance (Fig. 6c). From a keen observation of the velocity vs. magnetic field curve (Fig. 7) it is understood that at higher temperatures the response of even the dilute magnetic nanofluids to the external field is enhanced. More precisely, velocity enhances by 0.07% (for 0.035 T) and 0.13% (for 0.055 T) for a concentration of 0.2% at 328 K than velocity in the absence of magnetic field. Obviously, there is a velocity enhancement in high concentration too. At a concentration of 1% it enhances by 0.4% (for 0.035 T) and 0.57% (for 0.055 T) for the same temperature and zero magnetic field. This reveals the influence of temperature on the enhancement of the magnetic response of nanofluids. Only when an adequate amount of field is applied the velocity becomes higher than that in the absence of field. Hence, the sufficient magnetic field applied, induces cluster formation and there by growth of particles in the fluid even though the magnetic attraction of nanoparticles is weak enough due to steric repulsion of the surfactant to prevent magnetic agglomeration. It can be realized 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
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Fig. 6. Plots of (a) ultrasonic velocity vs. magnetic field, (b) adiabatic compressibility vs. magnetic field and (c) acoustic impedance vs. magnetic field for cobalt ferrite nanofluids of different concentrations at 308 K.
hence the magnetic effects are enhanced at higher temperatures even in low concentration samples [19]. 4. Summary Magnetic nanofluids of various concentrations of Cobalt Ferrite have been synthesized and are found to be stable without any phase separation. Magnetic and Ultrasonic parameters have been measured for various concentrations, temperatures and magnetic
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Fig. 7. Plots of ultrasonic velocity vs. magnetic field at (a) 313 K, (b) 318 K, (c) 323 K and (d) 328 K for magnetic nanofluids of different concentrations.
fields. The very low value of coupling constant and trend of ultrasonic parameters ensure that the interparticle interactions between magnetic nanoparticles are negligible in the absence of magnetic field. The results clearly point out the enhancement in particle–particle interaction resulting in the formation of chain like clusters [5,17,18] at higher concentrations and with the application of field. The effect of temperature on fluid is elucidated using openand close-packed structure of water. As the temperature range chosen for the study is sufficient to make 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
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the thermal expansion. In the presence of various magnetic fields the changes in ultrasonic parameters validate the structural rearrangement in magnetic nanofluids due to the formation of clusters. Also it is made clear that the increase in the temperature of magnetized magnetic nanofluids undoubtedly enhances the magnetic effects even in low concentration samples. In short, acoustical parameters have been used to realize the effect of concentration, temperature and magnetic field on the intermolecular interactions of cobalt ferrite nanofluid. Further investigations on the theory of propagation of ultrasonic waves in the magnetic nanofluids, the anisotropic nature of acoustical parameters and the inter particle interactions through isothermal remanence (IRM) and dcdemagnetization (DCD) remanence curves are in progress.
Acknowledgement The authors acknowledge the DST, Government of India for the VSM facility under the FIST programme sanctioned to Department of Physics, NIT, Tiruchirappalli.
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Magnetic and ultrasonic studies on stable cobalt ferrite magnetic nanofluid 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 8 June 2013 Received in revised form 11 October 2013 Accepted 14 October 2013 Available online 27 October 2013 Keywords: Ultrasonic velocity Ferrofluid Nanofluid Molecular interaction Adiabatic compressibility
a b s t r a c t Stable cobalt ferrite nanofluids of various concentrations have been prepared through co-precipitation method. Structural and morphological studies of nanoparticles are made with the help of X-ray diffraction technique and Transmission Electron Microscope respectively and it is found that the particles exhibit face centered cubic structure with an average size of 14 nm. The magnetic properties of the nanofluids have been analyzed at room temperature which revealed ferromagnetic behavior and also the very low value of coupling constant which ensures the negligible interparticle interaction in the absence of magnetic field. Ultrasonic investigations have been made for the nanofluids at different temperatures and magnetic fields. The temperature effects are explained with the help of open and close-packed water structure. The inter particle interactions of surface modified CoFe2O4 particles and the cluster formation at higher concentrations are realized through the variations in ultrasonic parameters. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Magnetic nanofluids are smart colloidal suspensions of nanomagnetic particles in base fluids. They have enormous importance in industrial and biomedical applications due to their tunable thermophysical properties. They have attracted much attention in the past decades because of their potential applications in high density magnetic recording, magneto hydrodynamics, magnetic writing, magnetic drug delivery, hyperthermia treatment, optical filters, spintronic devices, MEMS, Microfluidics, data storage, spintronics, solar cells, sensors, and catalysis [1–5]. Among the magnetic nanofluids, cobalt ferrite (CoFe2O4) based magnetic fluids have been widely analyzed due to high electromagnetic performance, excellent chemical stability, mechanical hardness, and high cubic magneto-crystalline anisotropy. These properties make it a promising candidate for many applications in commercial electronics such as video, audio tapes, high-density digital recording media [6]. Among the mentioned applications, the hyperthermia treatment has been recognized as very novel and promising, and the efficiency of cobalt ferrite nanoparticles has been clearly established [7]. Also, as concerning biomedical application, the possibility of using radioactive 60Co to produce enriched ferrofluid unlock a new perspective, as for instance in the targeting of cancer cells using antibody-coated nanoparticles [8]. From the literature it is found that there are only few reports available on the ultrasonic properties of nanofluids [9–16]. Reports ⇑ Corresponding author. Tel.: +91 9600974565; fax: +91 431 2500133. E-mail address: [email protected] (J. Hemalatha). 0041-624X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultras.2013.10.009
on the magnetic nanofluids [17–19] prove the tunable optical, rheological and thermal properties and also show the dependence of ultrasonic velocity on the clustering structure of magnetic fluid. A big deviation of the experimental values of velocity and attenuation from the theoretical predictions is also reported [20–22]. The fundamental understanding of exact mechanisms responsible for the amazing behaviors of magnetic nanofluids still remains unclear because of the lack of molecular level understanding of 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 the external magnetic fields. Hence, this paper is intended to the systematic experimental study on the response of cobalt ferrite magnetic nanofluids to the ultrasonic wave propagation for the basic understanding of how the cobalt ferrite nanoparticles behave in water and how they interact with each other and also with water. Preparing the stable and homogeneous suspensions of cobalt ferrite 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 The chemicals, Cobalt Nitrate (Co(NO3)6H2O), Ferric Nitrate (Fe (NO3)9H2O), Sodium Hydroxide (NaOH), Acetone ((CH3)2CO) are purchased from Merck and are used as purchased without further
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treatment. Double distilled water is used as the solvent whereas oleic acid (C17H33COOH) is used as the surfactant. Aqueous solutions of Cobalt Nitrate and Ferric Nitrate are prepared separately in stoichiometric ratio 1:2 to obtain the precursors. Both the solutions are mixed together and stirred for 1 h to get a homogeneous mixture. pH of the mixture is found to be 2.7. Then, 2 M NaOH solution is added drop by drop to the above mixture with simultaneous stirring until the pH reaches 9. A black precipitate thus obtained is heated at boiling temperature for 2 h, is allowed to cool and is washed several times with distilled water to remove impurities. Then, 200 ml of 2 M aqueous HNO3 solution is added to the above precipitate and stirred for 1 h. The supernatant solution is removed and the residue is cleaned thrice with acetone. Equal amounts of oleic acid and aqueous ammonia solution are added and stirred at 80 °C for 2 h. Oleic acid separates the cobalt ferrite nanoparticles and keeps them in suspension preventing the agglomeration of particle. The chemical reaction is given as follows.
Co2þ þ 2Fe3þ þ 8OH ! CoFe2 O4 þ 4H2 O COOH þ NH4 OH ! COONH4 þ H2 O Cobalt ferrite nanofluids of various concentrations (0.2%, 0.4%, 0.6%, 0.8%, and 1% by volume) are prepared by diluting appropriate amount of nanofluid in water. The response of these fluids for external magnetic field is depicted in Fig. 1. All the nanofluid samples show excellent stability because, the nanoparticles are not influenced by the
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gravitational force owing to their small size and they show no phase separation or sedimentation even in strong magnetic field. 2.2. Characterization The crystalline structure, phase composition and average crystallite size of Cobalt Ferrite nanoparticle are identified from the 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 size and morphology of CoFe2O4 nanoparticle is found using High Resolution Transmission Electron Microscope (Philips TECNAI F20 microscope). Powder Sample for TEM measurements is suspended in ethanol and ultrasonically dispersed. Drops of the suspensions are placed on a copper grid coated with carbon. The M–H plots of dried magnetic nanofluids are obtained at room temperature using a vibrating sample magnetometer (Lake Shore, USA, Model 7404) with 15 kOe as maximum applied magnetic field. The ultrasonic velocity is very sensitive to temperature and practically independent of frequency when compared to ultrasonic attenuation. So, it is decided to make ultrasonic measurements rather than attenuation. As the continuous wave based interferometry is suitable for studying the velocity change due to the magnetic field [40], a single frequency continuous wave ultrasonic interferometer (Model F80, Mittal Enterprises, New Delhi) with an accuracy of ±0.05% is used at frequency of 2 MHz. Density of the fluid was determined using Pycnometer with accuracy of ±2 parts in 104. All these measurements are performed for the fluids of all concentrations at five different temperatures of 308, 313, 318, 323 and 328 K. The temperatures are maintained constant by circulating water from a thermostatically controlled water bath with accuracy of ±0.1 °C. The velocity and density measurements are repeated several times for accuracy and the average of five continuous consistent values are reported in this paper. Magnetic fields varying from 0.007 T to 0.055 T are applied to the samples to make magneto acoustic measurements by placing permanent magnets at various equal distances on both sides of the cell. The magnetic field distribution is optimized and the acoustic measurements are made in the region where the magnetic field was found uniform. Sufficient time of 30 min is given for the stabilization of field before making the measurements. 3. Results and discussion 3.1. Structural and morphological studies
Fig. 1. CoFe2O4 magnetic nanofluids in the presence of magnet.
Fig. 2a shows the XRD pattern of CoFe2O4 nanopowder. It exhibits typical reflections of (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), (4 4 0) and (5 3 3) planes, indicating the face centered cubic structure of cobalt ferrite. The strong and sharp reflection peaks show the high degree of crystallinity of nanoparticles. All the peaks match well with the standard JCPDS 22-1086. No secondary peaks are detected in XRD pattern which ensures the phase purity. The average grain size is obtained using Debye Scherrer equation [23,24] and the lattice constant (a) of CoFe2O4 is calculated using the inter planar spacing. The estimated values of average crystallite size and lattice constant of CoFe2O4 are 14 nm and 0.83 nm respectively. The (a) value is in good agreement with earlier reported values [25,26]. The morphology of the CoFe2O4 powder is analyzed by HRTEM. The TEM images (Fig. 3a and b) reveal that the sample consists of Cubical particles with a regular morphology and narrow size distribution. The average size of the particles observed in the TEM image is in the range of 14 nm, which is in good agreement with that
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Fig. 2. XRD pattern of the cobalt ferrite particles.
estimated by Debye–Scherrer formula from the XRD pattern. It is obvious from the SAED pattern (Fig. 3c) that the particles are well crystallized. The diffraction rings on the picture correspond to (1 1 1), (2 2 0), (3 1 1), (4 0 0) (4 2 2), (5 1 1) and (4 4 0) planes respectively, which is in accordance with the peaks in the XRD pattern. 3.2. Magnetic studies The M–H loops of the CoFe2O4 nanopowder and fluids of different concentrations at 300 K are shown in Fig. 4 and they prove the ferromagnetic behavior of the samples. It is observed that the value of saturation magnetization (Ms), comes out to be 143 kA/m (28 emu/g) at room temperature, which is in accordance with earlier reported values [27,28] but smaller than that of the bulk value, 74.08 emu/g [29]. The remanence magnitude, Mr, can be extracted from the hysteresis loop at the intersections of the loop with the vertical magnetization axis and it is found to be 79.2 kA/m (15.5 emu/g). For nanosized ferrite particles, the surface areas are larger and thus the surface energy and surface tension are high. This results in changes in cationic preferences and leads to an increased degree of antisite defects and thus lesser magnetizations [6,30,31]. The coercivity, Hc, the intensity of the magnetic field required to reduce the magnetization of the sample to zero, of all the samples remains the same as 310.5 kA/m. The critical diameter of a single domain particle is estimated using the formula [32,33]
Dm ¼
18rw l0 M2s
ð1Þ
where rw is the wall energy density given by
rw
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2kB T C ka ¼ a
ð2Þ
with ka as the magneto crystalline anisotropy constant and TC the Curie temperature. For particle size, D > Dm, the particles exist in a multi domain structure, while for D < Dm, the particles exist in a single domain structure. For cobalt ferrite, TC = 677 K, a = 0.83 nm, ka = 1.6 105 J/m3, and Ms = 143 kA/m, the estimated value of Dm is about 1329 nm, which is larger than the average diameter of the synthesized CoFe2O4 nanoparticles. Hence the grains are found to exhibit single domain state, because of their small size the most favorable magnetic state of the particles is of a single domain. Single domains include groups of spins all pointing in the same direction and acting co-operatively.
Fig. 3. (a and b) TEM micrographs and (c) SAED pattern of CoFe2O4 nanoparticles.
Squareness ratio is given by the equation [34],
Rs ¼
Mr Ms
ð3Þ
where Mr is remanence magnetization and Ms is saturation magnetization measured from M–H hysteresis loop and it is found that the calculated Rs values of the sample prepared are 0.55.
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l ¼ Mb V
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ð5Þ
where Mb denotes the bulk magnetization of the particle and
V¼
pd3 6
ð6Þ
V is the volume of the particle. The hydrodynamic diameter of the grain (dh) is greater than the particle size (d), of the magnetic particle by twice the thickness of protective surfactant layer [37]. As the thickness of oleic acid double layer is 3.5 nm [38] the hydrodynamic diameter (dh) is taken as 21 nm and the coupling constant (k) is found to be 0.11. Since the coupling constant, a measure of dipole strength, is much smaller (k < 1) it is understood that the magnetic grains interact with the external magnetic field but do not interact with each other. 3.3. Ultrasonic studies In order to measure velocity, the ultrasonic wave is made to propagate through the nanofluid in a direction which is perpendicular to the direction of applied magnetic field. The ultrasonic parameters like adiabatic compressibility (b) and acoustic impedance (Z) are calculated for magnetic nanofluids using the velocity (v) and density (q) data obtained through the experiments. The adiabatic compressibility of the fluid is determined by the Newton–Laplace’s relation [39].
b¼
1
qv 2
ð7Þ
The acoustic impedance is calculated for all the concentrations using the relation [40] Fig. 4. Ferromagnetic (b) nanofluids.
hysteresis
for
(a)
cobalt
ferrite
nanopowder
and
From Fig. 4 it can be concluded that the magnetization of the magnetic nanofluid increases as a function of concentration and saturates at higher field values. The statistics of the magnetic parameters is depicted in Table 1. The fact shows that small changes in the external magnetic field result in substantial magnetization changes [35] 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 are measured at an external magnetic field of strength of 1300 kA/ m. The coupling constant (k) of the surface modified cobalt ferrite nanoparticle is estimated [36] as follows:
k¼
l0 l2 3 4pdh kB T
ð4Þ
Here, l is the magnetic moment of the grain, dh is the hydrodynamic diameter of the grain. Due to small diameters the particles form single domain of uniform magnetization with a magnetic moment given by:
Table 1 Summary statistics of the magnetic parameters. Concentration (vol%)
Ms (A/m)
Mr (A/m)
Hc (kA/m)
Rs
0.2 0.4 0.6 0.8 1
286 572 857 1143 1429
158 316 474 632 790
310.5 310.5 310.5 310.5 310.5
0.55 0.55 0.55 0.55 0.55
Z ¼ qv
ð8Þ
The variation of ultrasonic velocity with concentration in the absence of magnetic field is shown in the Fig. 5a. It can be deduced that, the velocity in magnetic nanofluid decreases as a function of concentration. It drops 0.02% (for 0.2 vol%) to 0.2% (for 1 vol%) lower than that of the carrier liquid in the absence of magnetic field at a temperature of 308 K. It shows the influence of dispersed particles on the velocity of ultrasonic propagation. This decrement in ultrasonic velocity with concentration is the qualitative measure of particle fluid interactions. It ensures that the intermolecular interaction is dominating over the intramolecular interaction or in other words it shows the predominance of particle–fluid interaction over particle–particle interaction in the absence of magnetic field in all temperatures. In the absence of magnetic field, the plots of velocity versus concentration obtained at 313, 318, 323 and 328 K have the same trend as the plot obtained at 308 K but with higher magnitudes of velocity. The percentage drop in velocity also shows an increase as a drop of 0.17% (for 0.2 vol%) to 0.76% (for 1 vol%) lower than that of the carrier liquid at a temperature of 328 K. It can easily be seen that the magnetic nanofluids follow the well known behavior of water, showing an increase in velocity with increase of temperature, which 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 quadruply bonded and unbounded water molecules are 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
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Fig. 5. (a) Variation of Ultrasonic velocity, (b) adiabatic compressibility and (c) acoustic impedance with respect to concentrations at various temperatures.
cohesion of water molecules and less compressible closed packed structure [41–43] 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 acoustic impedance of water based magnetic nanofluids observed at elevated temperatures. Fig. 5b indicates that the fluids of high concentration are less compressible than those of lower concentration at 308 K. The decreasing trend of adiabatic compressibility drops off with
increase in temperature and leads to an increasing trend of compressibility with concentration. The decreasing trend of adiabatic compressibility and increasing acoustic impedance (as shown in Fig. 5c), are attributed to the increase in density with respect to concentration. The increasing trend of compressibility with concentration at higher temperatures is in accordance with the increment of percentage drop in magnitude of velocity evident from Fig. 5a. The behavior of ultrasonic parameters in the presence of magnetic field is illustrated in Fig. 6. It is clear that the velocity of cobalt ferrite nanofluid of all concentrations except 0.2% is lower than that of the carrier fluid even in the presence of external magnetic field at 308 K. The percentage drop in velocity than that of the carrier liquid reduces (0.02–0% for a concentration of 0.2% and 0.2–0.06% for a concentration of 1%) under the influence of magnetic field (0.055 T) at a temperature of 308 K. From Fig. 6a, it is seen that the velocity in cobalt ferrite nanofluid of all concentrations increases with increase of applied magnetic field. It enhances by 0.01% (for 0.035 T) and 0.02% (for 0.055 T) for a concentration of 0.2% and it enhances by 0.07% (for 0.035 T) and 0.14% (for 0.055 T) for a concentration of 1% at 308 K than velocity in the absence of magnetic field. This observation clearly indicates that the magnetic nanofluids of higher concentrations having higher magnetization are easily influenced by the external field and show large response under the application of external magnetic field. This increase of velocity with magnetic field is the sign of clustering of magnetic particle. As the applied field sets the magnetic moments along the direction of magnetic field forming the clusters, the rigidity of the system increases and hence the velocity increases. When the magnetic field is turned off, the velocity returns to its initial value without exhibiting any hysteresis. Because, as the particles are sterically stabilized, no permanent aggregation due to van der Waals attraction occurs and the aggregation phenomenon observed in magnetic field is perfectly reversible [5,17,18]. It can also be noted from Fig. 6a that at lower concentrations enhancement in velocity with increasing external magnetic field is not so significant, indicating the minimal influence of magnetic field on the ultrasonic velocity. But magnetic nanofluid of concentration 1% shows considerably higher variations in the velocity with respect to the change in magnetic field. It points out 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. 6b), which in turn, affects the conditions of ultrasonic wave propagation in this medium and acoustic impedance (Fig. 6c). From a keen observation of the velocity vs. magnetic field curve (Fig. 7) it is understood that at higher temperatures the response of even the dilute magnetic nanofluids to the external field is enhanced. More precisely, velocity enhances by 0.07% (for 0.035 T) and 0.13% (for 0.055 T) for a concentration of 0.2% at 328 K than velocity in the absence of magnetic field. Obviously, there is a velocity enhancement in high concentration too. At a concentration of 1% it enhances by 0.4% (for 0.035 T) and 0.57% (for 0.055 T) for the same temperature and zero magnetic field. This reveals the influence of temperature on the enhancement of the magnetic response of nanofluids. Only when an adequate amount of field is applied the velocity becomes higher than that in the absence of field. Hence, the sufficient magnetic field applied, induces cluster formation and there by growth of particles in the fluid even though the magnetic attraction of nanoparticles is weak enough due to steric repulsion of the surfactant to prevent magnetic agglomeration. It can be realized 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
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Fig. 6. Plots of (a) ultrasonic velocity vs. magnetic field, (b) adiabatic compressibility vs. magnetic field and (c) acoustic impedance vs. magnetic field for cobalt ferrite nanofluids of different concentrations at 308 K.
hence the magnetic effects are enhanced at higher temperatures even in low concentration samples [19]. 4. Summary Magnetic nanofluids of various concentrations of Cobalt Ferrite have been synthesized and are found to be stable without any phase separation. Magnetic and Ultrasonic parameters have been measured for various concentrations, temperatures and magnetic
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Fig. 7. Plots of ultrasonic velocity vs. magnetic field at (a) 313 K, (b) 318 K, (c) 323 K and (d) 328 K for magnetic nanofluids of different concentrations.
fields. The very low value of coupling constant and trend of ultrasonic parameters ensure that the interparticle interactions between magnetic nanoparticles are negligible in the absence of magnetic field. The results clearly point out the enhancement in particle–particle interaction resulting in the formation of chain like clusters [5,17,18] at higher concentrations and with the application of field. The effect of temperature on fluid is elucidated using openand close-packed structure of water. As the temperature range chosen for the study is sufficient to make 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
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the thermal expansion. In the presence of various magnetic fields the changes in ultrasonic parameters validate the structural rearrangement in magnetic nanofluids due to the formation of clusters. Also it is made clear that the increase in the temperature of magnetized magnetic nanofluids undoubtedly enhances the magnetic effects even in low concentration samples. In short, acoustical parameters have been used to realize the effect of concentration, temperature and magnetic field on the intermolecular interactions of cobalt ferrite nanofluid. Further investigations on the theory of propagation of ultrasonic waves in the magnetic nanofluids, the anisotropic nature of acoustical parameters and the inter particle interactions through isothermal remanence (IRM) and dcdemagnetization (DCD) remanence curves are in progress.
Acknowledgement The authors acknowledge the DST, Government of India for the VSM facility under the FIST programme sanctioned to Department of Physics, NIT, Tiruchirappalli.
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