Magneto-viscosity of stable colloidal solutions of Barium-strontium hexaferrite ferrofluid

Barium and strontium ferrites known as M-type magnetically hard ferrites with hexagonal crystal structures [1]. These ferrites show high coercive force, large magneto crystalline anisotropy, large saturation magnetization and mechanical resilience because of which they are demanded in various application such as multiple- state memory elements, magnetic bearings, magneto-therapy, purification and sensors etc [2]. Ferrofluids are the stable suspension of magnetic nanoparticle dispersed in suitable liquid solvent. Generally the ferrofluids are involved in various applications such as semiconductor, clutches, loudspeakers, sensors, micro-magnetic anti-seismic drug delivery, hyperthermia treatment etc [3]. The behavior of plate shaped magnets dispersed in liquid crystals have also been reported in view of their applications in display devices [4]. Various methods have been introduced to improve the stability of ferrofluid considering their structural and magnetic response. In our earlier works we reported magneto-viscosity of paraffin based cubic shaped Cu-Zn nanoparticle ferrite ferrofluid [5]. We also reported magneto-viscosity of paraffin based Ba ferrite ferrofluids. These ferrofluids were synthesized by dispersing hexagonal shaped nanoplatelets of Ba ferrite of size (50–250 nm) in paraffin [6]. In the present work we report the magneto-viscosity of ferrofluids synthesized by dispersing BaSr ferrite hexagonal platelets shaped nanoparticles of size 20–25 nm in two different solvents i.e. water and silicone oil. This study is useful in understanding the effect of platelet size and nature of solvent on the magneto-viscosity of ferrofluids. It is found that the magneto-viscosity behavior of these ferrofluids is different from the behavior observed in our earlier work.

The nanoparticles of hexaferrite of compositions Ba_0.95Sr_0.05Fe₁₂O₁₉ (BSM) were synthesized by hydrothermal process in autoclave at 210 °C for 20 h as described in our work [5]. The BSM nanoparticles were coated with DBSA (4-Dodecylbenzenesulfonic acid) and oleic acid to avoid the agglomeration and cluster formation and dispersed in water and silicone oil for the synthesis of ferrofluids. The respective volume ratio of BSM nanoparticles, DBSA/oleic acid and water/ silicone oil was taken as 0.5:0.5:1. The x-ray diffraction (XRD) pattern of the BSM sample was recorded using powder x-ray diffractometer (Bruker) with Cu-k_α radiation. The micrograph of the BSM nanoparticles were recorded using a CARL ZEISS field emission scanning electron microscope and FEI Tecnai G2S-Twin 200 kV transmission electron microscope (TEM). The distribution of particles and zeta potential were recorded using Litisizer^TM 500 particle size analyzer (Anton Paar). For measuring the distribution and surface charge of BSM platelets, a very thin fluid (0.05% in volume) was used in water media. The magnetization as a function of applied field measurements were carried out using physical property measurement system (PPMS). The magneto-rheological data was investigated using oscillatory and rotational Rheometer (Anton-Paar MCR 501) at 303 K. The rheology measuring system was a 20 mm diameter parallel-plate geometry with 0.1 mm gap used in rotational mode. The Rheometer equipped with magnetic stage was used for the generation of magnetic field in the vertical direction.

X-ray diffraction data of the synthesized nanocrystalline ferrite Ba_0.95Sr_0.05Fe₁₂O₁₉ (BSM) shown in figure 1 confirms the formation of hexagonal structure with space group (P63/mmc) as per standard JCPDS data file [7]. The lattice parameters are 5.896, 5.896 and 23.835 Å.

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The FESEM micrograph for the synthesied nanoparticles is shown in figure 1(b). It is observed that the nanoparticles are hexagonal disk shaped. The grains are homogenously distributed in well crystallized irregular shape [8].

TEM micrographs of BSM nanoparticles are shown in figure 2. The electronic diffraction pattern (figure 2(a)) confirmed the hexagonal structure [9]. The particle are hexagonal shaped with thickness of 3 to 5 nm and size distribution of 20–25 nm (figure 2(b)). The HR-TEM micrograph corresponds to hexagonal structure (figure 2(c)).

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Figures 3(a) and (b) show the hydrodynamic particle size distribution and zeta potential of BSM ferrofluid respectively. There are two distributions observed in particle size of the BSM platelets. The hydrodynamic size (figure 3(a)) is recorded as 120 nm. The particle size analyzer distributions is high compared to TEM distribution because in the water based media the magnetic nanoparticles are forming clusters. The zeta potential known as the surface collector charge from variation of particle zeta potential and heterogeneous distribution of surface charge. The characteristic of zeta potential of BSM nanoparticles in water is investigated. The mean zeta potential (figure 3(b)) is found to be −49 mV for water based ferrofluid, indicating good stability of BSM platelets in water media. The zeta potential of particles may be changed by changing the ionic strength of a solution. An increase in ionic strength can compress the electric double layer and thereby decrease the zeta potential while a decrease of ionic strength can increase the zeta potential.

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Figure 4 shows the M(H) plots for BSM nanoparticles. The data reveals the hard ferromagnetic behavior with the saturation magnetization (Ms) of ∼28 emu g⁻¹ and 40 emu g⁻¹ at 300 K and 5 K respectively which is close to reported by other groups [10]. The coercivity (Hc) values are 348 Oe and 890 Oe and remanence values (Mr) are 6 emu g⁻¹ and 15 emu g⁻¹ at 300 K and 5 K respectively.

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Figure 3(b) shows the temperature dependence of magnetization for BSM nanoparticles in the temperature range of 2 to 350 K in applied magnetic field of 100 Oe under zero-field cooling (ZFC) and field-cooling (FC) modes. In ZFC mode, the BSM ferrite sample cooled from 375 K down to 2 K in the absence of magnetic field, then a magnetic field of 100 Oe was applied and the magnetization measurement was made with increase in temperature [11]. Whereas, in FC mode the sample was cooled from 375 K down to 2 K in the presence of magnetic field and then magnetization measurement was recorded with increase in temperature [12]. The magnetization increases for both FC and ZFC modes with decrease in temperature and at high temperatures both follow the same path. In ZFC mode the magnetization of the sample increases with decrease in temperature. In the presence of a magnetic field if the nanoparticles are cooled to a very low temperature, the magnetization direction of each particle is frozen in the field direction. At blocking temperature (T_B) the ZFC magnetization will be maximum at which the relaxation time equals the time scale of the magnetization measurements. By decreasing the temperature further, while the FC curve continues to increase, the ZFC curve reaches a maximum magnetization at 230 K. The separation between the ZFC and the FC curves gives the information about the non-equilibrium magnetization.

Figure 5 shows the measurements carried out to investigate the shear dependence of magneto-viscosity. The experiments were performed at shear rate of 1 s⁻¹ and 10 s⁻¹. The magnetic field strength ranged from 0 to 1.33 T. The ferrofluids were maintained at 25 °C for all measurements. In the case of water based FF the viscosity increases rapidly from 20 to 68 P.s and does not saturate at the high fields at shear rate 1 s⁻¹. Very small increase in viscosity with increase in magnetic field is observed at shear rate of 10 s⁻¹. No saturation in viscosity with increase in magnetic field is observed at both the shear rates. Whereas in the case of silicone oil based FF , slight saturation in viscosity with increase in magnetic field is observed above 1.25 T of magnetic field at shear rate of 10 s⁻¹. There is no significant change in viscosity with magnetic field at shear rate of 10 s⁻¹ . A comparison of the magneto-viscosity of BSM FF shows that : (1) the viscosity is high in silicon based FF compared to water based FF. 2. The viscosity increases linearly with increase in magnetic field. (2) The rate of increase in viscosity with field is higher in water based FF compared to silicon oil based FF. 3. At shear rate of 1 s⁻¹ the viscosity is high as compared to at 10 s⁻¹ because at low shear rate the magnetic force dominates the hydrodynamic force which is the main cause for chain formation. 4. The shear force and magnetic force become comparable at higher shear rate and magnetic force contain the chain together. When the field is applied the alignment of magnetic platelets takes place towards the field direction and chain formation takes place [6]. The chain formation leads to increase in viscosity with increase in magnetic field [6]. These chains break at high shear rate , leading to very small increase in viscosity with increase in magnetic field at shear rate of 10 s⁻¹. When the field is removed, the chains are broken and platelets try to come back to their original position and give rise to hysteresis in viscosity versus field plot. The linear increase in viscosity with no saturation with increase in magnetic field is a new result which is not observed in our earlier studies [5, 6]. This may be due to smaller size of the platelet nanoparticles in the present ferrofluids. The FF based on cubic shaped nano particle show saturation in viscosity at high magnetic field [5]. Whereas FF based on disc shaped nanoparticles of size 50–250 nm show saturation in viscosity at very low value of magnetic field [6].

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The flow curves (shear rate versus viscosity) of BSM FF are shown in figure 6. The plots are fitted with the power law equation η = K${\dot{\gamma }}^{{\rm{n}}-{\rm{1}}}.$ Here the K is consistency and n is power law index. The K value is between 17–27 for various magnetic field values. The n-values are 0.55 to 0.59 as the field increases from 0.02 to 1.33 T. The small n-value indicates higher shear thinning at different fields.

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The power law behavior is due to the zippering of chains in magnetic field [5]. At different shear rate there is competition between the hydrodynamic forces (FH) and magnetic forces (FH) characterized by Mason number (Mn = FH/FM) [13]. The information regarding breaking and reformation of chains in the fluid can be determined by Mn value.

Ferrofluids (FF) based on Ba_0.95Sr_0.05Fe₁₂O₁₉ (BSM) ferrite platelet shaped nanoparticles were synthesized. The magneto-viscosity measurements show that the viscosity increases in silicone oil based FF as compared to the water based FF. The flow curves show the power law behavior. Due to smaller size of platelets in the present ferrofluids, magneto-viscosity plots show different behavior as compared to other ferrofluids. The synthesized ferrofluids are useful for heat absorber and magneto-mechanical application.

Nisha Gautam is grateful for RGNF Fellowship by UGC, India.