Properties of colloidal used for making ferrofluids.
Abstract
The nanoparticles and ferrofluids of spinel ferrites are useful in bio-sensors, transducers, storage devices, optical devices, and so on. The Mn-Zn ferrite (MZF) is generalized soft spinel ferrite having high saturation magnetization at low applied magnetic field. This chapter covers the synthesis of nanoparticles of various sizes and compositions of Mn1-xZnx Fe2O4 with x = 0–1 by co-precipitation method. The structural and magnetic properties of the nanoparticles are discussed. The ferrofluids of superparamagnetic and ferromagnetic MZF nanoparticles were synthesized. The magneto-viscosity of ferrofluids with the dispersion of nanoparticles in different colloidal was studied. The Herschel-Bulkley model is applied to analyse the data for low viscosity ferrofluids.
Keywords
- spinel
- Mn Zn ferrite
- nanoparticles
- ferrofluids
1. Introduction
Ferrites are ceramic materials with magnetic properties. According to the crystal structure, the ferrites are mainly classified as spinels, hexaferrites, garnets and perovskites. Spinel is the first class among these ferrites having mineral structure with significant magnetic behaviour. The nanoparticles and ferrofluids of spinel ferrites are useful in bio-sensors, transducers, storage devices, energy conversion devices, heat absorbers and generators, shock absorbers, lubricants, magneto-optical devices, and so on.
Among all ferrite types, the spinel ferrites are easy to form with control on the size of their nanoparticles. The Mn-Zn ferrite (MZF) is generalized soft spinel ferrite having high magnetization, and it saturates at low applied magnetic field. In this work, the optimization of the synthesis procedures was done to obtain stable nanoparticles and ferrofluids of Mn-Zn spinel ferrites. The structural and magnetic properties of the nanoparticles and magneto-viscosity of the MZF ferrofluids were studied to understand the correlation between physical properties of nanoparticles and flow behaviour of ferrofluid in applied magnetic field.
2. Synthesis of nanoparticles of spinel ferrite
There are several methods for the synthesis of ferrite nanoparticles, that is, photo synthesis, microemulsion, sol-gel, hydrothermal, ball-milling, co-precipitation, catalyst-based methods and so on discussed in the literature [1–4].
In this work, the nanoparticles of Mn1-
The solution was washed with distilled water until the pH = 7 was reached and the slurry was heated at 373 K to get the MZF nanoparticles. The MZF nanoparticles of different compositions in Mn1-
3. Synthesis of the ferrofluids of spinel ferrite
The properties of ferrofluid depend on the hydrodynamic distribution of magnetic nanoparticles. The following ferrofluids were synthesized using MZF nanoparticles:
Ferrofluids with the dispersion of superparamagnetic (SPM) nanoparticles (no surface coating) in ethylene glycol.
Ferrofluids with the dispersion of ferromagnetic nanoparticles in different colloidal with suitable surface coating of the nanoparticles.
Finding suitable colloidal is necessary for the stability of ferrofluid in various technological applications. A number of colloidal, with their properties listed in Table 1, were used for synthesizing different MZF ferrofluids using methods given the literature [11–15].
S. No. | Colloidal | Dynamic viscosity of colloidal at 300 K (mPa. s) | Density (g/ml) | Vapour pressure (k Pa) |
---|---|---|---|---|
1 | Ethylene glycol | 16.2 | 1.1 | 0.5 |
2 | Toluene | 0.55 | 0.86 | 8.7 |
3 | Kerosene | 1.64 | 0.78 | 0.5 |
4 | Water | 0.8 | 1 | 4.3 |
5 | Paraffin oil | 25 to 80 | 1.1 | 0.5 |
The synthesized (MZF) ferrofluids are listed in Table 2. These ferrofluids were studied for their magneto-viscosity properties. The table has three categories of ferrofluids:
Name of ferrofluid | Composition and magnetic behaviour of the nanoparticles | Colloidal | Surfactant |
---|---|---|---|
Mn0.75Zn0.25Fe2O4 (SPM) | Ethylene glycol | Not used | |
MnFe2O4 (FM) | Water | Tetra-methyl ammonia (TMA) | |
Kerosene | Oleic acid | ||
Toluene | Oleic acid | ||
Mn0.75Zn0.25Fe2O4 (FM) | Water | TMA | |
Kerosene | Oleic acid | ||
Toluene | Oleic acid | ||
Paraffin oil | Oleic acid | ||
Mn0.9Zn0.1Fe2O4 (FM) | Paraffin oil | Oleic acid |
Ethylene glycol-based ferrofluids synthesized from SPM Mn0.75Zn0.25Fe2O4 nanoparticles.
Water, kerosene and toluene-based ferrofluids synthesized with surfactant-coated Mn-ferrite (MF) nanoparticles.
The ferrofluids synthesized with surfactant-coated Mn0.75Zn0.25Fe2O4 nanoparticles in water, kerosene, toluene and paraffin and Mn0.9Zn0.1Fe2O4 nanoparticles in paraffin.
4. Structural and magnetic properties of spinel ferrites nanoparticles
The chemical formula of spinel ferrite is generally expressed, where 'Me' represents a divalent metal ion (e.g. Fe2+, Ni2+, Mn2+, Mg2+, Co2+, Cu2+, etc.) and ‘
The Mn-Zn ferrite nanoparticles of various sizes and compositions were synthesized. Their structural properties were studied by X-ray diffraction (XRD) analysis. The morphological and microstructure of these nanoparticles was analysed using transmission electron microscopy (TEM). The magnetic properties of the nanoparticles were studied by measuring magnetization as a function of temperature and applied magnetic field,
The MZF nanoparticles of various sizes were synthesized by varying metal ions to hydroxide ratio (
Zn content | 0 | 0.25 | 0.5 | 0.75 | 1 | |||||
---|---|---|---|---|---|---|---|---|---|---|
S. No | D (nm) | a (Å) | D (nm) | a (Å) | D (nm) | a (Å) | D (nm) | a (Å) | D (nm) | a (Å) |
1 | 32 | 8.483 | 7 | 8.411 | 4 | 8.418 | 3 | 8.439 | 2 | 8.461 |
2 | 60 | 8.478 | 10 | 8.427 | 5 | 8.410 | 4 | 8.431 | 5 | 8.451 |
3 | 104 | 8.477 | 11 | 8.412 | 7 | 8.397 | 5 | 8.419 | 9 | 8.447 |
The lattice parameter of MZF nanoparticles with
The variation of crystallite size of MZF nanoparticles with Zn content is shown in Figure 2 (left). The drastic decrease in crystallite size occurs at
The temperature dependence of the magnetization,
The equation implies to
This equation is modified into the following by considering the log-normal size distribution
The decrease of
Figure 4 (left) shows the magnetic hysteresis loops
The
5. Investigation of magnetic properties of spinel ferrites based on Mössbauer studies
Mössbauer measurements on MZF nanoparticles were carried out at room temperature in transmission geometry using 57Fe nuclei. Mössbauer spectrum of Mn-ferrite nanoparticles with crystallite size of 104 nm is resolved into two sextets and one doublet (Figure 5). This system is soft ferromagnetic at room temperature. Two sextets come from two different Fe co-ordinations.
Sextet 1 has the hyperfine field value of 478 kOe and the absence of quadrupole splitting indicates that the Fe site is octahedral coordinated in cubic symmetry with 3+ valence state. Sextet 2 has a hyperfine field value of 452 kOe with quadrupole splitting of 0.009 mm/s due to 3+ valence state of Fe in tetrahedral coordination [35–37]. In addition, the doublet of relative area of around 22% is observed and this is due to the existence of small-sized magnetic particles showing superparamagnetic behaviour.
When 25 mole % of Zn (
Sample | Sub-spectrum | Mössbauer parameters | ||||
---|---|---|---|---|---|---|
Quadrupole shift (Δ | Isomer shift | Line width WV (mm/s) | Relative intensity (%) | |||
MnFe2O4 | Sextet-1 (A-site) | 478 | 0.002 | 0.195 | 0.40 | 20 |
Sextet-2 (B-site) | 452 | 0.009 | 0.262 | 0.86 | 58 | |
Doublet (SPM) | – | 0.691 | 0.223 | 0.55 | 22 | |
Mn0.75Zn0.25Fe2O4 | Sextet | 385 | 0.055 | 0.129 | 1.86 | 67 |
Doublet (SPM) | – | 0.684 | 0.234 | 0.65 | 33 | |
Mn0.5Zn0.5Fe2O4 | Doublet (SPM) | – | 0.633 | 0.222 | 0.65 | 100 |
Mn0.25Zn0.75Fe2O4 | Doublet (SPM) | – | 0.541 | 0.225 | 0.46 | 100 |
ZnFe2O4 | Doublet (SPM) | – | 0.553 | 0.231 | 0.53 | 100 |
6. Magneto-viscosity of ferrofluids of spinel ferrites
When the magnetic nanoparticles are suspended in colloidal, there are three main forces acting on the particles, that is, internal magnetic force, surface tension of the fluid and gravitational force. The ferrofluids show spikes when the fluid is subjected to the magnetic field. The chain-like or spot-like structure formation depends on these forces. The magneto-viscosity of the ferrofluid mainly depends on the magnetic properties of the nanoparticles used in the colloidal. The ferrofluids of superparamagnetic and ferromagnetic nanoparticles were chosen for the study of magneto-viscosity. The surfactant coating to the nanoparticles increases the stability of the ferrofluid.
6.1. Magneto-viscosity of ferromagnetic nanoparticles-dispersed ferrofluid without surfactant coating
The Mn0.75Zn0.25Fe2O4 nanoparticles of 3-nm size were dispersed in ethylene glycol to prepare MZFE ferrofluid. The volume ratio of nanoparticles to the colloidal is 1:4. The superparamagnetic nature of the nanoparticles is confirmed by magnetization studies. Figure 8 shows shear viscosity versus shear flow plot at various magnetic field values. The viscosity decreases with an increase in shear rate at zero magnetic field. It shows a shear-thinning effect at low shear rate and Newtonian behaviour over a wide range of shear rate. This can be explained by considering that at low shear rate the aggregates or clusters of nanoparticles offer high resistance to fluid flow leading to high viscosity. With increases in shear rate, the aggregates break into smaller units leading to fluid flow with low viscosity [38]. The interaction between aggregates or clusters increases when magnetic field is applied and aligns them in the direction of magnetic field forming chain-like structures which offer high resistance to the fluid flow leading to an increase in viscosity. Magneto-viscosity of other ferrofluids also shows similar behaviour [39, 40].
Odenbach [41] studied the magneto-viscous effect at various shear rates in a commercial magnetite-based ferrofluid and explained it in terms of interactions between the particles in the agglomerations aligned in straight chains and not due to the interaction between the chains. Figure 7 shows irreversible nonlinear behaviour in
6.2. Magneto-viscosity of surfactant-coated ferromagnetic nanoparticles-dispersed ferrofluids
It is quite interesting and more specific to study the magneto-viscosity of magnetic nanoparticles in less viscous colloidal such as water, kerosene and toluene. The ferromagnetic MF and MZF nanoparticles are used in different colloidal media with suitable surfactant coating for the preparation of MF and MZF ferrofluids. The surfactants have hydrophobic and hydrophilic ends. The hydrophilic end will be on the nanoparticle surface and hydrophobic end will overcome the formation of agglomerates of the nanoparticles. This property increases the stability of the ferrofluids.
6.2.1. Magneto-viscosity of Mn-ferrite ferrofluids
The magneto-viscosity of Mn-ferrite ferrofluid is investigated in flow field (flow curves) and in external magnetic field (magneto-viscosity plots). Mn-ferrite ferrofluids (i.e. MFW, MFK and MFT) were synthesized using Mn-ferrite nanoparticles suspension in different colloidal, that is, water, kerosene and toluene, respectively. The flow behaviour is studied for these ferrofluids in various applied magnetic fields. The respective volume ratio of Mn-ferrite nanoparticles, surfactant and colloidal (water, kerosene and toluene for respective ferrofluids) is taken as 1:0.5:1.5 for the preparation of Mn-ferrite ferrofluids. The MF nanoparticles are coated with tetramethyl ammonia (TMA) for the water-based MF ferrofluid (MFW). The kerosene-based ferrofluid (MFK) and toluene-based ferrofluid (MFT) are synthesized by using oleic acid-coated nanoparticles.
6.2.2. Flow curves of Mn-ferrite ferrofluids
Figure 8 (left) shows the flow curves of MF ferrofluid at various applied magnetic fields in the range from 0 to 1.33 T. The MFW ferrofluid shows non-Newtonian behaviour at lower shear rates for zero magnetic field. The behaviour changes to Newtonian behaviour with increasing shear rate. The TMA coating of the nanoparticles exhibits more hydrophobic nature of the nanoparticles, leading to decrease in friction between the layers of the ferrofluid. So the power law behaviour cannot be observed in zero field magneto-viscosity plot for the water-based ferrofluid. For all the range of shear rates, it follows single behaviour, that is, power law behaviour without any discrepancy.
Here,
Similar behaviour is observed in the MFK and MFT ferrofluids but the magnetic response is more. This is because the MFK and MFT are synthesized using FM nanoparticles in less viscous colloidal. So the hydrodynamic force is less compared to MFW ferrofluid. The very small
6.2.3. Magneto-viscosity of Mn-ferrite ferrofluids
The magneto-viscosity plots at a shear rate of 10 s–1 (Figure 8, right) show a rapid increase with an increase in magnetic field initially, followed by its saturation at higher fields, with low hysteresis when the applied field is decreased to zero. Since the ferromagnetic particles are dispersed in less viscous fluid, as the field increases the nanoparticles try to rotate in the field direction [50–52]. Above the applied field of 0.3 T, the viscosity saturates. Because of long-chain formation along the field direction, the viscosity reaches the maximum possible value at the given shear rate. Magneto-viscosity at different shear rates is described in Refs. [53–56]. This behaviour is similar to magnetization plots of nanoparticles as a function of magnetic field.
6.2.4. Magneto-viscosity of Mn-Zn ferrite ferrofluids
Mn-Zn ferrite ferrofluids were synthesized using the MZF nanoparticles suspension in different colloidal, that is, water, kerosene, toluene and paraffin, respectively. The respective volume ratio of Mn-Zn ferrite nanoparticles, oleic acid and colloidal (water, kerosene, toluene or paraffin for respective ferrofluids) is taken as 1:0.5:1.5 for the preparation of Mn-Zn ferrite ferrofluids.
6.2.5. Flow curves of Mn-Zn ferrite ferrofluids
The flow behaviour is studied using flow curves shown in Figure 9 (left)) for Mn0.75Zn0.25 Fe2O4 ferrofluids in different applied magnetic fields. The fluid shows power law behaviour with small
6.2.6. Magneto-viscosity plots of Mn-Zn ferrite ferrofluids
The viscosity versus applied magnetic field at a shear rate of 10 S–1 is shown in Figure 9 (right). The magneto-viscosity plots show a rapid increase with an increase in magnetic field initially. The viscosity is saturated at higher fields (around 0.2 T) and show low hysteresis when the applied field is decreased to zero. This behaviour is similar to the magnetization plots of the nanoparticles as a function of magnetic field.
6.2.7. Magneto-viscosity of paraffin-based ferrofluids
Paraffin is more stable colloidal compared with other colloidals used for the synthesis of ferrofluids. The dispersion of FM nanoparticles in paraffin gives fine control of viscosity in magnetic field. This study is quite interesting and useful for ferrofluid applications. Similar to the synthesis of other ferrofluids, the paraffin-based Mn0.75Zn0.25Fe2O4 (MZFP1) and Mn0.9Zn0.1 Fe2O4 (MZFP2) ferrofluids are synthesized using oleic acid-coated nanoparticles. The magneto-viscosity plots of MZFP1 and MZFP2 ferrofluids are shown in Figure 10 at the shear rate of 1 and 10 s–1.
The comparison between the magneto-viscosity of these ferrofluids is as follows:
The gradual increment of viscosity is observed in MZFP2 ferrofluid with an increase in magnetic field whereas sharp increment is observed in MZFP1 ferrofluid.
When the magnetic field is decreased, the magneto-viscosity plots of MZFP1 ferrofluid show less hysteresis and the plot is not relaxing to initial position at zero magnetic field.
The magnetic nanoparticles are aligned in field direction and the nanoparticles are rotated along the easy axis of the system.
Resultant unbroken large chains are expected even when the magnetic field is removed. The MZFP2 ferrofluid shows different behaviour due to smaller-size particles and lower initial susceptibility.
6.2.8. The Herschel-Bulkley behaviour in ferrofluids
The MF nanoparticles dispersed in less viscous colloidal (toluene) are free to move in the fluid and so it is easy to study the structural changes in spot-like and chain-like structures as a function of applied magnetic field. MFT ferrofluid is chosen to explain the Herschel-Bulkley (H-B) behaviour in ferrofluids. Shear stress (
The H-B model combines the power law model with the yield stress as per the equation,
Here,
Apart from the magnetic behaviour of magnetic particles and the colloidal behaviour, the dosage of surfactant and the concentration of the magnetic nanoparticles in ferrofluids play a major role for the change of rheological behaviour at lower shear rates. This influences the flow field interaction in the ferrofluid. With the application of field, the shear stress versus shear rate plots show the H-B fluid behaviour. It is also observed as the magnetic field increases, the shear stress increases. The yield stress was determined by the extrapolation of the plots using the B-H model. The plots shifted upward due to the drastic change in yield stress. It increases from 4 to 27 Pa as magnetic field increases from 0 to 0.0717 T. The deviation from H-B model fit can be observed in shear stress (
7. Conclusions
The nanoparticles of various sizes and compositions Mn1-
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