Chapter VI. 6. Characterization of the Transformer oil based Co-Zn ferrofluid Room temperature magnetization measurements of the fluid samples

Transcription

1 6. Characterization of the Transformer oil based Co-Zn ferrofluid Transformer oil based Col.,Zn,Fe204 femfluid (FF) with x varying from 0 to 0.7 were prepared by chemical co-precipitation method. Pulse field technique (PF) has been used for measwing the room temperature magnetization (M,) in a field of 400 Wm. The FTIR spectra were recorded with an ABB BOMEM 104 FTIR (range cm") spectrometer. EPR measwements were canied out using JEOL JES-TE 100 spectrometer having X-band frequencies (9GHz). Sans was used to determine the particle size of the fluid. Viscosity measwements were canied out at room temperature using Brookfield LV-DV-I11 ultra viscometer. The effect of external magnetic field (0-500 Gauss) on the fluid was studied. The size of particle (Dv) from the viscosity and the effect of rotation at different speed were also analyzed Room temperature magnetization measurements of the fluid samples The magnetic properties of ferrofluids are dominated by the intrinsic magnetic properties of the particles themselves. Each single domain particle in the carrier bears magnetic moment 'm' that can freely orient itself in the direction of an external magnetic field 'H'. When a magnetic field 'H' is applied to a sample containing particles with magnetic dipole moments 'm', these dipoles will tend to align with the magnetic field. Thermal motion tends to disorient parallel alignment of magnetic moments to 'H'. The concurrence of these two forces determines the resulting value of magnetization 'M' (magnetic moment per volume M=mN). If the particles are all of the same size, the magnetic behavior is described by the Langevin function. Magnetization of particles is given by the famous Langevin equation M = M,L(~) where 'M' is the magnetization of the fl 1 the fluid and ~(a)= Cotha-- is the

2 represents the ratio of magnetic interaction energy of the particle mh to the thermal pomh energy kst given by a = - where ' p,' is the magnetic permeability of vacuum, 'm' k BT is the magnetic moment of the particle, 'H' is the applied field, ' k, 'is the Boltzmann's constant and 'T 'is the absolute temperature. Therefore, where M, = O,M, is the saturation magnetization of the liquid, cd, is volume concentration of the magnetic component, M, is the spontaneous magnetization of the magnetic particle. 1 Ms PomH When a is small, ~ (a) 3 3 k,t susceptibility of the ferrofluids and = -a, therefore M = -- = x,h where ' x,, ' is the initial When a is large, ~ 0th a -+ 1 From the experimental dependence of ferrofluids M(H) in strong magnetic fields and plotting M against lih it is possible to determine the value of saturation magnetization Ms by extrapolation M(H) values up to H + w (1M -PO) [24]. The value of saturation magnetization of the fluids by extrapolation of the plot, M versus lm to an infinite field is generally followed only for mono dispersed system. The magnetization curve differs eom the Langevin curve in the case of a real magnetic fluid. Several factors account for the deviation, the most important of which are

3 the polydispersity of particles and mutual influence of local fields of particles in concentrated fluids. The deviation is also due the pnsence of non-spherical particles present. Ferrofluids, however, invariably contain a distribution of particle size. For a polydisperse system the shape of the magnetization,curve depends on the particle size distribution. Therefore magnetic properties of ferrofluids are modified by the presence of particle size distribution. The magnetization is now given by the sum of the contributions from each particle diameter, weighed by the distribution function. In the case of polydispersed system, larger field is required to saturate the small particles when compared to that of the field required to saturate the large particles. In practice the above-mentioned regularities would mean that the smaller particles have less steep magnetization curve than the bigger ones. Therefore by extrapolation one may get a wrong value of saturation magnetization (Ms). In order to get the information regarding fluid saturation for polydispersed system in a finite range of H the experimental data should be fitted with the theoretically generated curve. Vislovich has suggested a simple approximation of the magnetization curve that gives a much better fit to the experimental data [I 61: where H is the applied magnetic field and HT is the field strength at which the value of saturation magnetization of the fluid becomes half. For the approximation of the curve M(H) in a finite range of field (H) and for better fitting Ms and Hr should be of the form

4 where HI and H2 are the values of the magnetic field strength near the beginning and the end of the field (H) range and MI and M2 are the experimental values of fluid magnetization at HI and H2 respectively. The magnetization of the fluid samples was carried out using pulse field technique. The instrument was calibrated using a fluid sample of known magnetization (252 Gauss) supplied by Liquids Research Ltd. The calibration constant used was Determination of particle size of Co-Zn ferrofluid In the case of ferrofluids, the particle size distribution follows a log-normal distribution function. Using the value of Ms the magnetic particle size (median diameter of the particle) and the lognormal parameter can be calculated from the initial susceptibility of the fmofluids. The value of initial susceptibility was also determined from the slope of the initial section of the magnetization curve near the origin by curve fitting the linear portion of the data. Chantrell et a1.[186] used expansions of the Langevin function together with the log- normal distribution to obtain an expression for median diameter, Dm, and standard deviation, om, from a magnetization curve of a polydispersed system. But the particle - particle interaction was not taken into consideration while deriving the equation for particle size determination and standard deviation. Vekas et a1.[187] have reported more accurate equations than those of the well-known Chantrell's method for the determination of particle size and log normal parameters by considering the particle interaction as well as dependence of magnetic moments on the particle diameter. The median diameter of the particle is given by the expression where 'Dm' is the median magnetic diameter of the log normal volume distribution, 'Md' is the domain magnetization of the particles, 'Ms' is the saturation magnetization of the fluid and '&' is the intercept obtained by extrapolation to M=O of the straight line obtained for high fields while plotting M = f(l/h).

5 a = (m- 3)/2 (6.6) where ' X, ' is the initial susceptibility of the fluid. For non-interacting particles a = X,. The lognormal parameter a, can also be obtained h m the magnetization measurements using the equation, The particle size and log nonnal parameter calculated for the prepared fluid samples are tabulated in Table Magnetic measurements of Col.,Zn,Fe20~ ferrofluid (FF) Fig.6.l shows the room temperature hysteresis loop of the fluid samples for various zinc substitutions. The hysteresis curve recorded at room temperature are shown in Fig The changes in the magnetization with the degree of zinc substitution are given in Table.6.1. From Fig.6.1 it can be seen that the magnetization (M,) decreases with the increase in Zn content. The saturation magnetization for the fluid with x=o ie Col of~04 fluid is nearly 23.31kNm for a field of 400kNm. It is clear from Fig.6.1 that for x=0.7 the fluid does not show any saturation and it almost behaves linear. The particle produced with high Zn content is much smaller in size compared with particles prepared with low Zn content, therefore the blocking temperature should be much lower. The magnetic measurements canied out at room temperature shows that the effect of superparamagnetic behavior should be much shonger for small sized particles than the larger size particles. Fig.6. la shows the fitted curve for the room temperature hysteresis loop of the fluid samples for various zinc substitutions. The particle size and log normal parameter calculated for the prepared fluid samples are given in Table.6.1. The particle size (Dm) obtained from the magnetization measurements of the fluid is known as magnetic particle size and it is less than the crystallite size of the particles (DXRD) [I881 measured by X-ray diffraction, This is due to the presence of a magnetic dead layer on the surface of the particles.

6 Magnetic measurements 1 Table.6.1. Magnetization measurements of the fluid samples Fig.6.1. Room temperature magnetization curve of (a) Col.oFe204 (b) Coo.90Zna.10Fe204 (c) Coo~Zna.aFerO4 (dl Co0.7oZno~oFe204 (el Coo.caZnouFerO4 (0 Coo~Zn~~Fe204 (g) Coo.roZno.caFe104 (h) Coo~oZna.70Fe204.

7 T 20 $ lo ii" SO0 400 Field (H,) (Wm) Fig.6.l.a Room temperature magnetization curve of (a) Col.oFe204 (b) Coo.wZna.1oFe2Or (c) CoaaoZno.toFezO4 (dl Coo.7oZtrojoFezOr (e) Coo.soZno.loFezO4 (0 Co0~oZno~oFerO4 (g) Coo.roZna.soFezO4 (h) COO.MZW.~OF~ZO~ FTIR measurements of Co1.,Zn,Fe20~ ferrofluid (FF) Fig.6.2 gives the FTIR spectrum obtained for pure transformer oil, oleic acid and transformer oil based ferrofluid (FF) for Col.,Zn,FezOd with x=o, 0.5 and 0.7. The large band of low intensity (v, ) ranging cm" may be assigned to the stretching of the OH group [174,176] generated from two different sources: the hydroxyl group attached by hydrogen bonds to the iron oxide surface and the water molecules chemisorbed to the magnetic particle surface (associated water content). From these results, it appears that the hydroxyl groups are retained in the samples during the preparation of the uncoated Col,Zn,FqO, spinel fenites by co-precipitation method. Ghose et a1.[66] reported that the presence of some hydroxyl ions which were completely removed when the sample were sintered at temperatures 2973K [177]. The v, bands disappear due to non-existence of water molecule in the spectrum of oleic acid and

8 transformer oil. Two sharp bands at v, ( ) and at v, ( cm'l), which were superimposed on the 0-H stretch, were attributed to the methyl and methylene symmetric stretching vibrations respectively. The intense peak at v, ( cm") was derived from the existence of the C=O stretch [181,63]. But it is absent for the pure transformer oil due to the absence of C=O bond or Carboxylic acid. The bands at v,, v, and v, indicate the presence of the oleate ions, chemically bounded to the metal atoms [63]. This reveals the chemisorbance of oleic acid as a carboxylate onto the Co nanoparticles. The two oxygen atoms in the carboxylate are coordinated symmetrically to the Co atoms. The 0-H out-of-plane bands v, appear at cm". A strong band at ~~( cm.') is due to Fe,Ol [178],. The transmittance waveband at ~~( cm") which correspond to the metal- oxygen bonds are considered as the confirmation for the ferrite formation. This is in good agreement with Zins et a1.[101, 174, 179, 180, 1811 The bands I.,, v,, v,and v, disappear for the transformer oil because of the non-existence of water, carboxylic acid and ferrite. The bands v, and v, disappear for the oleic acid due to the absence of water and ferrite. Table.6.2 represents the main FTIR transmittance bands of transformer oil, oleic acid and transformer oil based ferrofluid (FF) for Col.,Zn,Fe20~ with x varying from 0 to 0.7. Table.6.2. Comparison of the main FTIR tran8mittance bands of transformer oll, oleic acid and transformer oil based ferrofluid (FF) for Col.,Zn,Fe~O~ with x varying from 0 to 0.7.

9 n Tnnrfonnrr dl v, 3-1 t p-y, Co,,Fe20, 500 lo Wavenumkr (cm") Fig.6.2. FIlR spectra for transformer oil, oleic acid and for transformer oil based ferrofluid (FF) for Col.,Zn,Fe~04 with x = 0,O.S and EPR measurements of Col.,Zn,Fe204 ferrofluid (FF) Fig.6.3 and 6.4 give the EPR spectrum recorded at room temperature (RT) at 300K and at liquid nitrogen temperature (LNT) at 77K for the transformer oil based CaI.,Zn,Fe204 ferrofluid (FF) with x varying from 0 to 0.7. Peak-to-peak line width(ah,) were found to decrease with increase in zinc concentration while the resonant magnetic field (H,) was found to increase with the increase in zinc concentration both at room temperature (RT (300K)) and liquid nitrogen temperature (LNT (77K)). The decrease in the line- width(^^,) with Zn concentration was due to dipole-dipole

10 interactions in CO-Zn ferrite. At low temperature, the linewidth was large due to the scatter in direction of anisotropic field of particles. As the temperahue was increased, the tendency to make magnetic moment isotropic caused linewidth to decrease [122]. Magnetic dipole interactions among particles and superexchange interactions between the magnetic ions through oxygen ions are two predominant factors that determine the EPR resonance parameters like peak-to-peak line-width (AH,,) and resonant magnetic field (HJ. Strong dipole interactions gave a large peak-to-peak linewidth (AH,,) and further, strong superexchange interactions [182] caused a small peak-to-peak line-width (AH,,). Fig.6.3 shows the change in the broad line with the increase in Zn concentration. Clearly the broad signal narrows down with the increase in Zn concentration. The broad line may be attributed to the presence of large ferrimagnetic particles [183]. Janis Kliava et al. [I841 and Perzynski et al [I851 reported that line- width(^^,,) decreased with the decrease in size of the particle. In Fig.6.3 similar spectra have been obtained with the increase of Zn concentration. This decrease in the line-width may be attributed to the decrease in the size of the particles. Therefore the broad line for x=o indicates large particle size. The narrow line indicates decrease in particle size for x=0.7. From Fig. 6.3 one can see that the decrease in particle size leads to the broader band getting gradually replaced by the sharper one [184]. Table.6.3 represents the EPR parameters for transformer oil based ferrofluid (FF) Col.,Zn,Fe204 with x varying from 0 to 0.7. All RT spectra at 300K (Fig.6.3) show a single broad line, indicating that the isolated ~e'+and zn2* ions do not exist. The broad line in the spectra arises due to the ferromagnetic particles bounded to the molecules of surfactant. Further, the increase in Zn concentration leads to the decrease in line-width from Wm to Wm (see Table.6.3). Transformer oil based ferrofluid with x varying from 0 to 0.5 at LNT (77K), show a narrow line at the center in addition to a single broad line. It is known that in a randomly oriented dispersed fenimagnetic particles, the absorption line-width (AH,,) turns out to be a non-monotonic function of temperature. At low temperature the line-width is large due to scatter in direction of anisotropic field of particles (inhomogeneous broadening) [122]. The mow line is due to an iron complex not attached to surfactant molecule. This m w line disappears slowly with the increase in Zn concentration. For x greater than 0.5 the mow signal vanishes completely. From Table.6.3 it is seen that it is

11 not possible to obtain full specbum for x = 0 to 0.3 RT (300K) and for x> 0.4 at LNT (77K) using X-band JEOL JES-TE 100 spectrometer. Perhaps complete specbum may be obtained using Q-band frequency spectrometer. M e PLld Wrn) M e PUI*) Fig.63. EPR spectra for transformer ou based ferrofluid (FF) for Col.,Zn,Fe20d with x = 0,0.1,0.2,0.3,0.4,0.5,0.6,0.7 at RT (300K)

12 Fig.6.4. EPR spectra for transformer oil based ferrofluid (FF) Col.,Zn,Fe204 with x=o,o.l, 0.2,0,3,0.4,0.S,0.6,0.7 at LNT (77K)

13 Table 6.3. EPR parameters for transformer oil based ferrofluid (FF) Col.,Zn,FerOc with x varying from 0 to Small angle neutron scattering (SANS) SANS measurements were camed out for the Col.,Zn,Fe204 transformer oil based ferrofluid (FF) fluid samples with x varying from 0.1 to 0.7 and D20 (benzene-d,) based Col.,Zn,FezOo transformer oil based ferrofluid based for x= 0, 0.2, 0.5, 0.7. The volume fraction for all the samples was kept as 25% by diluting it. Patterns are recorded at room temperature and experimental data were corrected for the background and for the empty cell contribution. Fig.6.5 shows the plot of dz/dr -tq for the transformer oil based ferrofluid. The plot generated using equation is fined to the experimental data and are shown in fig.6.5, By incotporating appropriate values of the parameters the theoretical curve was generated. To fit the scattering curve the concept of a shell model consisting a sphere with an inner core radius &Rp' surrounded by a concentric shell of radius 'b' was used. From the SANS measurement the particle size was determined for the Col.,Zn,Fe204 transformer oil based ferrofluid (FF) fluid samples with x= 0, 0.1, 0.2, 0.4, 0.6. Fig.6.5 shows the fitted SANS data for the Co0,9Zno.~ Fe204, at room temperature(3ook). The particle size was found to decrease with the increase in zinc substitution. The particle size (DJ of the fluid was found to vary from nrn decreasing with the 113

14 increase in zinc substitution. The particle size obtained From X-ray diffraction (DXd) [I881 are compared with the SANS data. There is a very good contrast between the magnetic particle (Fe30,) and the solvent (transformer oil). However, there is a poor contrast between a surfactant coating and the solvent. In view of the above, the SANS distribution arises from the core of the particles. Fig.6.6 shows the SANS pattem for a DzO (benzene-d,) based ferrofluid. The particle size and log normal parameter calculated for the prepared fluid samples are given in Table.6.4. In Col.,Zn,Fe~O4 transformer oil based ferrofluid (FF) fluid samples with x varying from 0.1 to 0.7 the coating thickness agrees with normal thickness of surfactant ie., nearly of 2 to 3nm. SANS results confirm that the surface modification increases the thickness of the surface layer, which in turn results in the decrease of magnetic radius. Fig Sans pattern of Co0.9Zno.lFe204 ferrofluid at 300K

15 Fig Sans pattern for a Dl0 (benzene-d,) base Col.oFezO4 ferrofluid at 300K Table.6.4. Magnetization measurements, SANS, X-ray diffraction of the fluid samples

16 6.7. Magneto Viscosity Viscosity measurements of Col.,Zn,Fe204 ferrofluid (FF) The suspensions of magnetic nanoparticles exhibit magnetic properties enabling a magnetic control of their magnetic properties and flows. It has been shown that the viscosity of the ferrofluid can be strongly influencedby magnetic fields. The viscosity of a magnetic fluid in absence of an externally applied is same as that of a nonmagnetic colloidal suspension. The Einstein's relation [I501 at low concentration is, (Eqn.. from Chapter Using this, Rosensweig [2] obtained the expression, A plot of measured value of ('1- q,/p,q) verses magnetic volume fraction (9,) yields a straight line. The values of the particle diameter obtained from the intercept, D(i), at cp,=o usingcp,=0.74 for all samples are given in Table 6.5. Fig 6.7(a-h) shows variation of relative viscosity with magnetic volume fraction (cp,) (0.01 to 0.07) for Col.,ZnxFq04 with x varying from 0 to 0.7 ferrofluid (FF). This size is known as rheological particle size (Dv). The particle size obtained from viscosity measurements (Dv) is slightly larger than SANS (Ds) and X-ray method (DxRD). Fig. 6.8 (a-h) shows the change in viscosity (7) for the transformer oil based Col.,Zn,F~04 wiih x varying from 0 to 0.7 ferrofluid (FF) samples as a function of the varying strength of a magnetic field (200 to 500 Gauss) applied parallel to the rotation axis of the rheometer for different rpm measured in rotating mode.

17 The viscosity of the fluid increases with the increase in the magnetic field but at the same time as the speed of the spindle increased, viscosity of the fluid decreases. The increase in the viscosity of the fluid can be accounted by the interaction between the magnetic particles, a quantitative description of the magnetoviscous effects is obtained showing, that a small fraction of large particles in the fluid forms chains dominating the rheological properties of the fluids in the presence of magnetic fields. The chain structure of the particles by the application of the fields also leads to the increase in the viscosity. Table.6.5. Particle size measurements of the fluid samples from viscosity Fig.6.7 (a) Shows the variation of (1- l,/lmrl) versus cp, for Co~.oFezOl

18 Fig.6.7 (b) Shows the variation of (q- qo/pmq) versus cp, for Coo.~Zno.loFerO4 Fig.6.7 (c) Shows the variation of (q- q,/p,q) versus cp, for CooaoZna.~oFe,O4

19 Fig.6.7 (d) Shows the variation of (v- rl,/pm,rl) versus cp, for Coo.,oZna"~FerO4 Co,, :I :.Zn,,Fe,O,. Exprdmmtsl dola -.. Flltod llm Fig.6.7 (e) Shows the variation of (q - q,/pmq) versus cp, for Coo.~1Zna.~FelO4

20 Fig.6.7 (f) Shows the variation of (q - q,/ylmq) versus cp, for CoowZno.soFerO4 Fig.6.7 (g) Shows the variation of (q- qo/9].q) versus (p, for Coo~oZno.soFezOr

21 Fig.6.7 (h) Shows the variation of (rl - q,/yl,q) versus rp, for CooJoZno.,~Fe~O~ Mapmtlc Fkld (H) O.wr Fig. 6.8.(a) Variation of Viscosity (cp) versus Magnetic field (H) Gauss for various speed wm) for Col.oFe~O4.

22 ,-.--../ Magnrtlc Fimld (H) buss Fig. 6.8(b) Variation of Viscosity (cp) versus Magnetic field (H) Gauss for various speed (RPM) for Coo.wZno.loFe204, Fig. 6.8.(c) Variation of Viscosity (cp) versus Magnetic fleld (II) Gauss for various speed(rp~) for Coo,eoZno.~oFe2Or

23 MqWlc Fkld (H) Gauss Fig. 6.8.(d) Variation of Viscosity (cp) versus Magnetic field (H) Gauss for various speed (RPM) for Co~.l~Zna~oFelOp. m 2 m w % o 4 m 4 m m Mqlwllc Fkld (H) Gauss Fig. 6.8.(e) Variaflon of Viscosity (cp) versus Magnetic field (H) Gauss for various Speed (RPM) for Coo.coZn0.40Fez04.

24 Magnrtk Fhld (H) Oows Fig. 6.8.(f) Variation of Viscosity (cp) versus Magnetic field (H) Gauss for various speed (RPM) for CoosoZnog,FerO4. Fig. 6.8.(g) Variation of Viscosity (cp) versus Magnetic fleld (H) Gauss for various. speed (RPM) for C oo.~z~.~fe~04

25 -- -- [I j - Q + 5 wn, -+- (Own Co~*,h,O, +=wns+3qv Fig. 6.8.(h) Variation of Viscosity (cp) versus Magnetic neld (H) Gauss for various speed (RPM) for CoojoZno..roFe204. Fig. 6.9 (a-h) shows the change in relative viscosity (Aqlq) for the transformer oil based Col.,ZnXF~O4 with x varying from 0 to 0.7 ferrofluid (FF) samples as a function of the varying strength of a magnetic field from to knm applied parallel to the rotation axis of the rheometer for different rpm measured in rotating mode. Fig, 6.9.6) Variation of relative Vinco~ity (Arllq ) versus Magnetic field (mh/kt) for various speed (RPM) for Col.oFe204.

26 Fig. 6.9.(b) Variation of relative Viscosity (A~/Q ) versus Magnetic field (mh/kt) for various speed (RPM) for COO.WZ~O.~OF~~O~.. Fig. 6.9.(c) Variation of relative Viscosity (Aq/q ) Venus Magnetic field (rnh/k~) for various speed (RPM) for Coo.ooZno.rFetOc

27 Fig. 6.9.(d) Variation of relative Viscosity (Aq/q ) versus Magnetic field (rnh/kt) for various speed (RPM) for Coo,loZn0~oFe20~.. Fig. 6.9.(e) Variation of relative Viscosity (AQ/Q) versus Magnetic field (rnh/kt) for various speed (RPM) for Co0.60Zno~FetO4..

28 Fig. 6.9.(f) Variation of relative Viscosity (A~lr) ) versus Magnetic field (mh/kt) for various speed (RPM) for Coo,~0Zno~oFelO4.. Fig. 6.9.(g) Variation of relative Viscosity (AQ/Q) versus Magnetic fleld (mh/kt) for various speed (RPM) for Coo.~Zno.~Fe~04..

29 Fig. 6.9.(h) Variation of relative Viscosity (Arllrl) versus Magnetic field (rnh/kt) for various speed (RPM) for C O~JOZ~~,~~F~~OA..