Các đặc trưng đốt nóng cảm ứng của chất lỏng hạt nano từ và các yếu tố ảnh hưởng tt tiếng anh

MINISTRY OF EDUCATION AND TRAINING VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY GRADATE UNIVERSIY OF SCIENCE AND TECHNOLOGY  Luu Huu Nguyen THE CHARACTERISTICS OF MAGNETIC INDUCTIVE HEATING AND THEIR IMPACTS BY THE PARTICLE ANISOTROPY AND FERROFLUID VISCOSITY Major: Electronic materials Code: 9.44.01.23 SUMMARY OF DOCTORAL THESIS IN MATERIAL SCIENCE Ha Noi - 2019 This thesis was done at: Laboratory of Magnetism and Superconductivity, Institute of Materials and Sciene, Vietnam Academy of Science and Technology Supervisor: Prof., Dr Nguyen Xuan Phuc Assoc Prof., Dr Pham Thanh Phong Reviewer 1: Reviewer 2: Reviewer 3: The dissertation will be defended at Graduate University of Science and Technology, 18 Hoang Quoc Viet street, Hanoi Time: h , / /2019 This thesis could be found at National Library of Vietnam, Library of Graduate University of Science and Technology, Library of Institute of Materials and Science, Library of Vietnam Academy of Science and Technology INTRODUCTION In recent decades, nanotechnology and nanoscience have been of great interest so they are considered as a revolution in the 21st century Nanotechnology encompasses design, analysis, fabrication and application of structures, devices or systems by controlling the shape, size on a nanometer scale The subject of these technologies is nanomaterialsNanomaterials with very small sizes (about 1-100 nm) exhibit exciting properties that are different from those of the bulk materials Based on their size effects, nanomaterials have open new applications in electronics, mechanics, environmental remediation, especially in biomedicine For dielectric and magnetic materials, inductive heating is the physical phenomenon by which the materials become thermo-seeds when they are irradiated by proper alternating electromagnetic field In the case of bulk magnetic materials, the Magnetic Inductive Heating (MIH) using alternative magnetic field (AMF) relies on two mechanisms of energy dissipation, which are energy losses due to Joule heating and energy losses associated with magnetic hysteresis In nano scale, it is generally known that the energy losses associated with magnetic properties such as hysteresis loss and relaxation loss mainly contribute to the heating For biomedical applications, magnetic nanoparticles (MNPs) have to be dispersed in a solvable solvent to create nano ferrofluids MNPs are coated by a surfactant for preventing the nanoparticles from aggregation and keeping them well dispersed for many years So, the nano ferrofluids in fact consist of core, shell and solvent Various magnetic nanoparticles such as magnetic metal nanoparticles, magnetic alloy nanoparticles or magnetic metal oxide nanoparticles have been used as the core of nanofluids The shell materials can be polymer, copolymer or an oxide material The fabrication of a magnetic nanofluids may be realized using water or other solvents such as benzyl ether, phenyl ether It is generally known that there are many methods such as co-precipitation, sol – gel, solvo-thermal, hydrothermal, thermal decomposition or reverse micelle, normally used in synthesing MNPs The size and size distribution or magnetic properties of nanoparticles depend on the synthesis method Therefore, it is difficult to experimentally study the effect of one or more parameters of a nano ferrofluid on the physical phenomenon Besides, the nano ferrofluids must satisfy two main conditions: they should have large heating power with minimum amount of nanoparticles, they should have good biocompatability In order to achieve these goals, the so far studies focused on improving the heating power of magnetic nanoferrofluids Based on previous works, the heating power depends on several physical and magnetic parameters of the particles including: particle size (D) – size distribution, saturation magnetization (Ms), magnetic anisotropy constant (K), viscosity of magnetic fluid (η) as well as the AMF frequency and amplitude Because there are so many parameters affecting the heating power, experimental studies of optimizing MIH effect are difficult to realize Therefore, theoretical studying the role of physical parameters of different nanomaterials could be a good approach to provide guidelines for experimental works, becausetheoretical calculations in fact play the role as a “Digital experiment”, which contributes to predicting experimental results Based on these theoretical results, the experimental parameters can be adjusted to search for suitable materials according to the researchers' goals In Vietnam, the basic and application works associated with magnetic nano materials are concerned by a number of research groups at Institute of Materials Science (IMS), Institute for Tropical Technology, Ho Chi Minh city Institute of Physcis - Vietnam Academy of Science and Technology, Hanoi University of Science and Technology, Faculty of Physics in Hanoi University of Science, etc However, only the research group of Prof., Dr Nguyen Xuan Phuc at IMS permomed theoretical and experimental studies of MIH and focus on both aspects: the synthesis method such as magnetic metal nanoparticles (Fe), magnetite nanoparticles (Fe3O4), doped magneitc nanoparticles (Mn 0.3Zn0.7Fe2O4, Mn0.5Zn0.5Fe2O4, La0.7Sr0.3MnO3) or core – shell magnetic nanoparticles - Fe 3O4@ poly(styrene-co-acrylic acid), Fe3O4@ poly (Nisopropylacrylamide-co-acrylic acid) and the physical mechanism of MIH Up to now, the experimental results on MIH are abudant and diverse These results indicated the advantage of particular materials, which is used as a core, shell or solvent of biomedical nano ferrofluids Besides, the experimental results of studying physical parameters on MIH contributed to explain its physical mechanism However, the dependence of MIH on the ferrofluid physical parameters has not been detailly mentioned in recent experimental works and systematically considered in theoretical reports So, a series of questions should have satisfactory answers in the research process Firstly, the heating efficiency of MIH is optimal at which critical size of each mangnetic nano materials? Secondly, the same question for saturation magnetization, hydrodynamic diameter and especially in magnetic anisotropy (K) How the characteristic parameters of MIH are affected in low K or high K magnetic nanofluids? In other words, how can we classify materials based on this parameter or other physical factors in MIH? How the heating efficiency of MIH is affected when the particle is not monodispersive or the viscosity changes? These answers will contribute to optimizing the MIH in each materials and orienting the applicability of these materials It is a challenge for us and other groups Based on the above reasons, we chose the research project for thesis, namely: “The characteristics of magnetic inductive heating and their impacts by the particle anisotropy and ferrofluid viscosity” Research targets of the thesis: (i) To thereticallystudy the overall characteristics of MIH and their impacts based on theoretical calculation (ii) To carry out experiments on the influence of alternating magnetic field, particle size and viscosity on specific loss power for CoFe 2O4 and MnFe2O4, chosen as representative of respectively high K and low K magnetic nanoparticles; and to compare the experimental behavior with that obtained by theroretical calculations Scientific and practical meaning of the thesis: Applying Linear Respones Theory (LRT) to find the competition between the Néel and the Brownian relaxation which helps to more clearly understand about the role of magnetic anisotropy for classifying materials in MIH Research methodology: The thesis was carried out by theoretical calculation based on LRT (using MATLAB software) and practical experimentation combined with numerical data process CoFe 2O4 and MnFe2O4 samples were fabricated by hydrothermal synthesis at Laboratory of Magnetism and Superconductivity, Institute of Materials and Sciene, Vietnam Academy of Science and Technology Samples were characterized by electron microscopes (FESEM) The viscosity of magnetic fluids was measured by Sine wave Vibro Viscometer SV 10 DLS was used to determine the hydrodynamic diameter of magnetic fluid Magnetic properties of materials were investigated by Vibrating-Sample Magnetometer (VSM), and were used to evaluate the presence of functional groups on magnetic nanoparticles Magnetic Induction Heating was carried out on RDO-HFI-5 kW set up installed at Institute of Materials Science Research contents of the thesis: (i) Overview of Magnetic inductive heating for nano ferrofluids (ii) Investigating the effect of physical parameters on the specific loss power based on LRT (iii) Compare theoretical results with experimental results of the influence of alternating magnetic field, particle size and viscosity on specific absorption rate power for CoFe 2O4 and MnFe2O4 magnetic nanoparticles Layout of the thesis: The contents of thesis were presented in chapters • Introduction • Chapter Magnetic inductive heating for nano ferrofluids • Chapter The theoretical results of the specific loss power based on Linear Respones Theory • Chapter Verifying theory by experimental results • Conclusion Research results of the thesis were published in 06 scientific reports including: 02 ISI reports, 03 national reports, 01 report in international scientific workshop CHAPTER MAGNETIC INDUCTIVE HEATING FOR NANO FERROFLUID 1.1 Overview of Magnetic inductive heating 1.1.1 Magnetic nanoparticle and superparamagnetic particle: basic properties 1.1.1.1 Domain of magnetic nanoparticle In a bulk magnetic material, the magnetic moments are uniformly oriented in regions of certain sizes, which are called “magnetic domains” or “domains” tIn the absence of external filed, the moments vary from domain to domain to make total magnetization minimized to zero When the size of bulk material decreases, the domain size decreased and the domain structure, the width of the domain wall changes When the particle was smaller than a critical size, it could not consist of two domains separated by a domain wall and the particle becomes a single domain particle The critical size for single domain behavior depends on type of magnetic materials 1.1.1.2 Superparamagnetism If single-domain nanoparticles become small enough, thermal energy is larger than anisotropy energy so spontaneously reverse the magnetization of a particle from one easy direction to the other likes a single spin in paramagnetic materials The spin system can be rotated synchronously and the magnetic state of small size and non-interacting nanoparticles is called “superparamagnetic” The temperature at which the transition between the superparamagnetic state and the blocked state occurs is called the blocking temperature TB The blocking temperature TB also depends on other factors such as magnetic anisotropy, size and the measurement time (τm) So, the blocking temperature depends on size andτm for each materials While the critical size of single domain is determined by the balance of energy forms, superparamagnetic behavior depends on the measurement time 1.1.1.3 Dependence of magnetic anisotropy on particle size The anisotropy energy is the energy required by the external magnetic field to move the magnetic moment from easy to hard direction of magnetization It is the internal magnetocrystalline energy if saturation magnetization is not oriented towards easy axis This energy, which is associated with magnetocystalline anisotropy and the crystal symmetry of the material is called magnetocystalline anisotropy energy For fine or thin flim magnetic nanoparticles, surface anisotropy contributes yet to magnetocystalline anisotropy The surface anisotropy is caused by the breaking of the symmetry and a reduction of the nearest neighbour coordination Surface effects in small magnetic nanoparticles are a major source of anisotropy The effective anisotropy energy per unit volume is given by: K = K +6 K eff V D S 1.1.2 Nano ferrofluid: synthesis and application (1.8) The magnetic nanoparticles coated by surfactants and suspended in liquid carrier are called ferrofluids or magnetic fluids, which is a commonly concept in biomedical applications The magnetic fluids are distingnuished not only by magnetic properties of nanoparticles (core) but also properties of liquids For example, the Néel and Brownian relaxations mainly contribute towards MIH of ferrofluids based on superparamagnetic nanoparticles Therefore, the physical effects of ferrofluid are influenced by magnetic nanoparticles in the core, the shell , the solvent and also the synthesis method used 1.1.3 Magnetic inductive heating and application Inductive Heating (IH) is the physical phenomenon by which electromagnetic materials become thermal seeds when they are inserted in proper alternating electromagnetic field In case of nanosized magnetic materials, it is generally known that the energy losses associated with magnetic properties such as hysteresis loss, relaxation loss, v…v mainly contribute to the heating The MIH has been of great interest because of their potential applications such as (i) adsorbent material desorption, (ii) cell activation for insulin regulation, (iii) to characterize the nanoparticle distribution in organs and in tissues, (iv) thawing of cryopreserved biomaterials, (v) hyperthermia-based controlled drug delivery and (vi) hyperthermia-based cancer treatment 1.2 Magnetic inductive heating mechanisms 1.2.1 Contribution factors to thepower of magnetic inductive heating MIH of magnetic nanoparticles is derived from the process of adsorbed energy from external alternating magnetic field The total absorbed energy includes surface Joule loss (PF), hysteresis loss (PH), Néel (PN) and Brown (PB) relaxation losses Because, most nano materials are of high electrical resistivity and small size, this leads to very low eddy current loss Thus the MIH of nanoparticles is mainly caused by the hysteresis loss, Néel and Brown relaxation losses The hysteresis loss refers to the loss due to irreversible magnetization process in AC field This is the mainly heat generation of ferrite or ferromagnetic multi – domain materials For the superparamagnetic nanoparticles, it is generally known that Néel and Brown relaxation losses mainly contribute to the MIH of materials The Néel relaxation loss is originated from relaxation effects of magnetization in magnetic field, the Brown relaxation loss is due to the rotation of the nanoparticles as a whole in ferrofluid Nowadays, the theoretical models of MIH such as Rayleigh model, Stoner–Wohlfarth model based theories (SWMBTs), and Linear Response Theory (LRT) depend on the applicable conditions The dimensionless parameter ξ to indicate the limit of validity of each theoretical model ξ = µ0 M SVH (1.9) k BT When ξ < 1, nanoparticles show superparamagnetic behavior or H DB), SLP decreases slowly For group A, the contribution ratio of nanoparticles from region I is greater than region III when σ increases from to 0.25 So, the value of SLPmax decreases strongly In constract, the value of SLPmax decreases slowly because the contribution ratio of nanoparticles from region III is greater than region I when σ increases from 0.25 to 0.5 The value of Dc is near region I, the more obvious this phenomenon is This explains that the fastest increasing of ∆Dcp for FeCo magnetic fluid and the least increasing of corresponding value for Fe 3O4 magnetic fluid in group A In contrast, the value of SLPmax for group B decreases slowly because the contribution ratio of nanoparticles from region III is greater than region I when σ increases from to 0.25 The contribution of nanoparticles from region I increase when σ increases from 0.25 to 0.5, resulting in a decrease in the value of SLPmax for group B faster than group A 10 The competition between Néel and Brown relaxation losses is a decisive role in this phenomenon 2.3 The role of magnetic anisotropy on the competition between Néel and Brown relaxation losses Two groups of magnetic fluids (group A and group B) exhibit different characteristics of the optimal parameters of MIH For group A, the value of SLPmax decreases strongly with expanding size distribution and is independent on viscosity For the B group, the SLPmax value decreases slowly with expanding size distribution and was dependends on the viscosity The cause of this phenomenon is due to the competition between Néel and Brown relaxation losses The Néel relaxation loss dominates for group A and the Brown relaxation loss dominates for group B These results show the important role of magnetic anisotropy with this competition Figure 2.18 The plot of SLP(D) for Fe3O4 at various K khác The plot of SLP(D) for Fe3O4 changes from “sharp” to “bell” and the changing of peak is at 34 kJ/m All optimal parameters for Fe3O4 change with changing of K Table 2.11 The value of Dcp, ∆Dcp and SLPmax for Fe3O4 Magnetic anisotropy Dcp SLPmax ∆Dcp (kJ/m3) 10 (nm) (nm) (W/g) 23 18,5 3 169,9 130,5 15 16,5 99,9 20 15 8,5 82,4 25 14 11,5 69,5 30 13,5 13,5 60 34 13 15 54,9 35 16 15 54,9 40 16 15,5 54,9 45 16 16 54,9 50 16 16 54,9 When the value of K increases from kJ/m3 to 34 kJ/m3, the value of Dcp changes because the Néel relaxation loss still affects this parameter When the value of K for Fe3O4 is larger than 35 kJ/m3, Dcp and SLPmax are not changed due to the domination of Brown relaxation loss So, it changes abruptly at some critical anisotropy KC = 35 kJ/m3 11 For optimal parameter ∆Dcp, the width of peak of SLP does not changes abruptly at K = 35 kJ/ m3 It is continuous process with beginning at the value of K = 15 kJ/ m3 and saturating at K = 35 kJ/ m3 The value of KC is checked by this result: the SLPmax value decreases slowly with expanding size distribution for Fe 3O4 with K ≥ 35 kJ/m3 – the Brown relaxation loss dominates Figure 2.20 Dependence of SLPmax (σ ) on σ for Fe3O4 SLP max (σ = 0) Bảng 2.14 The value of KC at various viscosity or frequency f (kHz) 10 100 η = 1mPa•s 20 η = 2mPa•s 33 KC (kJ/m3) η = 3mPa•s 47 250 50 63 85 119 143 500 59 112 163 >180 >180 750 100 153 >180 >180 >180 1000 102 >180 >180 >180 >180 η = 4mPa•s 11 60 η = 5mPa•s 14 72 It can be seen, that the anisotropy boundary of the transition from Néel to Brown domination changes with changing the frequency of AMF, depending yet on the viscosity of the magnetic fluids ( Figure 2.22 The plots of KC versus (a) f with fitting function K C ( f ) = A1 − e− B1 × (f−f ) ) or (b) η with fitting function K C (η ) = A2 + B2 ×η The shift from the main contribution by the Néel relaxation loss to the Brown relaxation loss can occur for nanoparticle fluids with depending on the choice of f and η suitable the value of given K For example, 12 the Néel relaxation loss dominates when f is equal 250 kHz and η is equal mPa•s for magnetic fluid Fe 3O4 (K = 40 kJ/m3) For this magnetic fluid, the Brown relaxation loss dominates when f is larger than 400 kHz and η is larger than mPa•s It is confirm that the role of magnetic anisotropy on the competition between Néel and Brown relaxation losses 2.4 Some orientations for experimental study The synthesis requirement for group A is indicated so that the error between size and critical size within nm and the standard deviation of size distribution is smaller than 0.25 For group B, the error between size and critical size can be up to nm and the standard deviation of size distribution is smaller than 0.4 These results showed that a behavior to analyzing the competition of contribution between Néel and Brown relaxation losses: the different of dependence of SLP on viscosity on two groups A and B If SLP depends on viscosity, the main heating generation is the Brown relaxation loss In contrast, the main heating generation mechanism is Néel relaxation loss with independence of SLP on viscosity Based on the value of K, the shift from the main contribution by the Néel relaxation loss to the Brown relaxation loss can occur for nanoparticle fluids by changing frequency or viscosity For example, the main heating generation is the Brown relaxation loss at f ≤ 200 kHz for magnetic fluid with K = 50 kJ/m3 However, the main heating generation mechanism is Néel relaxation loss for this magnetic fluid when f is larger than 400 kHz CHAPTER VERIFYING THEORY BY EXPERIMENTAL RESULTS 3.1 Fabrication of CoFe2O4 and MnFe2O4 magnetic fluids 3.1.1 Chemicals and equipment Synthesis of CoFe2O4 and MnFe2O4 nanoparticles by hydrothermal method was conducted at Laboratory of Magnetism and Superconductivity, Institute of Materials and Science The chemicals used include CoCl2.6H2O (99.99%), MnCl2.4H2O (99.99%), FeCl3.6H2O (99.99%), and solid NaOH (99.99%) of Merck (Germany), HCl and acetone are of Chinese industrial chemicals with purity of 98.9% 3.1.2 Process of synthesizing nano particles CoFe2O4 and MnFe2O4 nanoparticles were fabricated by hydrothermal method described in the following diagram (Figure 3.2.) Figure 3.2 Process of synthesizing CoFe 2O4 and MnFe2O4 nano particles 3.1.3 Fabrication of magnetic fluids 13 CoFe2O4 and MnFe2O4 magnetic fluids were formed according to the following process: the magnetic nanoparticles were removed from the thermos flask - it was still in NaOH solution Then the magnetic nanoparticles were washed several times by pure water Magnetic nanoparticles were dispersed into solvents by ultrasonic vibrations (2 hours) into magnetic fluids 3.2 Structure and magnetic property 3.2.1 Structure Figure 3.4 X-ray diffraction of samples: (a) MnFe2O4 and (b) CoFe2O4 The diffraction peaks at the planes of (220), (311), (222), (400), (422), (511), and (440) confirm the presence of single-phase face-centered cubic structure The patterns in (a) and (b) are in good agreement with their corresponding standard patterns of CoFe2O4 (cubic, space group: Fd3m, Z = 8; ICDD PDF: 22–1086) and MnFe2O4 (cubic, space group: Fd3m, Z = 8; ICDD PDF: 73–1964), respectively The broad peaks in Co and Mn ferrite indicate fine nanocrystalline nature of samples For MFO sample, the obtained a = 8.39 Å was smaller than that of bulk counterparts (8.51 Å) Oxidation of Mn2+ to Mn3+ and different cation distributions of Mn ferrite nanoparticles could lead to decrease in the lattice constant In bulk Mn ferrite, cation distribution is demonstrated as (Mn0.82+Fe0.23+)A(Mn0.22+Fe1.83+)B, where A and B denote the tetrahedral and octahedral sites in spinel structure, respectively Oxidation of Mn∼2+ (0,81 Å) to Mn3+ (0.72 Å) reduces the lattice parameter Aslibeiki and Kameli obtained a = 8.34 Å for 6.5 nm MnFe2O4 nanoparticles prepared by a thermal decomposition method They explained this result by discussing the difference in the cation distribution between nanoparticles and bulk manganese ferrite For CFO sample, a = 8,39 Å is approximately equal to the lattice constants obtained from bulk (a = 8.38 Å) Table 3.2 The value of DXRD and aexp Sample Mean szie lattice constant DXRD (nm) 16 18 DFESEM (nm) 19 21 aexp (Å) 8.39 8.39 aLT (Å) MFT100 MFT120 MFT140 20 22 8.40 8.51 MFT160 23 26 8.40 MFT180 29 31 8.41 CFT100 18 20 8.39 CFT120 21 23 8.39 14 8,38 CFT140 CFT160 24 28 27 32 8.39 8.41 CFT180 34 38 8.42 3.2.2 Magnetic properti of CoFe2O4 and MnFe2O4 Fig 3.7 and Fig 3.9 show the typical room-temperature hysteresis loops for two samples The enlarged view of M-H in the inset of Fig 3.7 confirms this superparamagnetic behavior Figure 3.7 Magnetic hysteresis loops at T=300 K of MnFe2O4 nanoparticles SPM Table 3.3 The value of MS, Keff and D for MnFe2O4 nanoparticles Sample MS (emu/g) Keff (erg/cm3) DSPM (nm) MFT100 55 41 2.77 ×10 MFT120 59.9 40 3.01 ×10 MFT140 63.1 40 3.18 ×10 MFT160 65.4 39 3.29 ×10 MFT180 68.1 39 3.29 ×10 Different from the magnetic nanoparticles MnFe 2O4 that all showed the pure superparamagnetism behavior, the CoFe2O4 nanoparticles exhibit significant coercivity As the particle size increases, the coercivity of these particle systems increases from 1200 to 2650 Oe (Table 3.4) The HC values of CoFe2O4 magnetic nanoparticles were used to determine the value of effective magnetic anisotropy Figure 3.9 Magnetic hysteresis loops at T=300 K of CoFe2O4 nanoparticles Table 3.4 The value of MS, HC, MR and Keff for CoFe2O4 nanoparticles Sample CFT100 MS (emu/g) 53.8 HC (Oe) 1200 15 MR (emu/g) 17.5 Keff (erg/cm3) 1.07 ×10 1.33 ×106 CFT120 57.6 1400 24 CFT140 61.1 2300 29.5 CFT160 63.9 2400 32 2.46 ×106 CFT180 73 2650 37 3.09 ×106 The value of magnetic anisotropy ( ) 2.3 ×106 of CoFe2O4 nanoparticles is range form 1.02 ×106 to 3.09 ×10 erg / cm3 3.3 Hydrodyamic diameter and viscosity of magnetic fluid 3.3.1 Hydrodyamic diameter of nano particles The hydrodyamic diameter of the two nanofluids, CFO and MFO nanoparticles, were measured using a dynamic light scattering (DLS) system Table 3.5 Distributions of the hydrodynamic diameter Sample Size distribution σ MFT100 MFT120 DH (nm) 21 23 0.18 0.18 MFT140 24 0.21 MFT160 27 0.17 MFT180 37 0.1 CFT100 25 0.18 CFT120 27 0.12 CFT140 29 0.1 CFT160 38 0.25 CFT180 43 0.27 3.3.2 Viscosity of magnetic fluid Rheological characterization of nanofluids was performed by a Sine wave Vibro Viscometer SV 10, featuring the vibrating tuning fork measurement method It measures viscosity by detecting the driving electric current necessary to resonate two sensor plates at constant frequency of 30 Hz and amplitude of less than mm The temperature dependence of the viscosity was measured at room temperature 3.4 Magnetic Inductive Heating Magnetic Inductive Heating was carried out on RDO-HFI-5 kW The Specific Absorption Rate – SAR is given by: SAR = C m ∆T s mi ∆t 3.5 Some experimental results for verifying theoretical results 3.5.1 Dependence of MIH on alternating magnetic field For ferrite nanofluids, eddy current losses are almost negligible because it has low conductivity For MFT100, the major heating contribution is relaxation loss because magnetic nanoparticles are superparamagnetic For CFT100, the mainly heating contribution is also relaxation loss because the value of H is smaller than the coercivity (1200 Oe - Table 3.4 Table 3.6 SAR for MFT100 and CFT100 nanofluids H (Oe) SAR (MFT100) (W/g) SAR (CFT100) (W/g) 50 8.8 16 6.9 60 70 13.4 20 9.2 13.8 80 31.7 21.3 Figure 3.15 shows that SAR depends on the amplitude of AMF as a quadratic form This experimental result is in good agreement with results of M Cobianchi, P M A Caeteno and B B Lahiri It confirmed that the LRT is suitable for the MIH at low magnetic fields As can be seen from figure 3.15, SAR does not depends on the amplitude of AMF as a quadratic form at H = 80 Oe In other words, LRT is inaccurate with the amplitude of AMF lager than 80 Oe The value of parameter ξ for MFT100 and CFT100 are 0.85 and 1.24 when H is equal to 80 Oe While the value of parameter ξ for MFT100 is is the intersection between the two models: SWMBTs and LRT, the value of parameter ξ for CFT100 indicated that the heating contributions are relaxation loss and hysteresis loss Therefore, to accurately compare the experimental results and the LRT, the SAR value of these two systems is subtracted from the heating contribution from the hysteresis loss This is method that P H Nam and colleagues used in their work Figure 3.15 Depenedence of SAR on the amplitude of AMF for MFT100 and CFT100 The solid lines represent the fitting curve assuming the quadratic function The dependences of SAR on f are shown in Fig 3.15, which can be fitted very well by a linear relationship for CFT100 and MFT100 These experimental results are in good agreement with results of M Cobianchi, Kishimoto and Fortin Table 3.7 The value of SAR for MFT100 CFT100 f (kHz) SAR (MFT100) SAR (CFT100) (W/g) (W/g) 166 178 10.5 16.9 2.6 8.2 236 31.7 21.3 The dependences of SAR on AMF indicated that the LRT is suitable for the MIH at low AMF for CFT100 and MFT100 17 Figure 3.17 The dependences of SAR on f The solid lines represent the fitting curve assuming the linear function 3.5.2 Dependence of MIH power on particle size The particle size of sample was changed by changing the synthesis temperature from 100 oC to 180oC The values of size of samples are in range from 21 nm to 43 nm The existence of critical particle size was found in many theoretical studies Surprisingly, not much experimental work is reported on the influence of particle size It is known that these were previously published only in two experimental works by Deatsch et al and Krishnan et al for Fe 3O4 nanoparticles Krishnan et al found that the values of Dcp approximately equal to 16 nm at 170 Oe, 376 kHz By performing data from eight different references, Deatsch et al indicated that SAR maximized at Dcp ∼ 15-18 nm Figure 3.19 The value of SLP/MS and SAR/MS for MnFe2O4 magnetic fluids As can be seen Fig 6, the value of SAR/MS maximized for a range of about 25-30 nm It is interesting that both calculated SLP/MS and experimental SAR/MS of MFO exhibits a peak at Dcp of about 27 nm (MFT160) The experimental data are in good agreement with those data from theory 18 Figure 3.20 The value of SLP/MS and SAR/MS for CoFe2O4 magnetic fluids For the CoFe2O4 magnetic fluids, the value of size of samples are quite far above the optimal size (Dcp = 16 nm) by calculated based on the LRT (Fig 3.20) Although most experimental measurement points (CFT 120, CFT140, CFT160 and CFT180) have same tendency with theoretical curve when size is larger than theoretical Dcp (SAR or SLP decreases with increasing of diameter), the experimental data is not enough to comment on the existence of the peak of SLP or SAR Besides, we now focus our attention on the large difference in the measured and calculated values This discrepancy might be due to the following reasons: firstly, the hydrodynamic volume is not a well defined parameter because in colloidal dispersions the particles are coated with dispersants by forming multiple layers on the surface Secondly, there are magnetic interactions in the samples while non-interacting nanoparticles are assumed in the calculation In a recent work, Serantes et al have numerically studied the effect of magnetic interactions in MNPs on the magnitude of SAR They found that in ferromagnetic MNPs having dipolar interactions, SAR is enhanced (reduced) at low (high) field and is saturated at a higher field than independent MNPs 3.5.3 Analyze the contribution of Néel and Brown relaxation losses The SLP value of MFT100 is almost unchanged with the viscosity of the magnetic fluid: this value decreases from 65 W/g to 63.7 W/g when the viscosity increases from to mPa•s The changing is very small, accounting for only 2% of the SLP value of MFT100 in pure water In contrast, the changing for CFT100 is account for more than 34% (17 times more than MFT100) Therefore, it is evident that SLP of two ferrites differently respond to viscosity Table 3.10 SLP and SAR for CFT100 and MFT100 Sample CFT100 η (mPa•s) SAR (W/g) SLP (W/g) η (mPa•s) SAR (W/g) SLP (W/g) 1.37 1.56 38.7 19.9 72 63.6 1.2 12 10.6 65 64.7 1.74 16.7 57.3 1.4 11.3 64.4 1.97 11.5 51.8 1.6 10.9 64.1 2.12 9.1 47.3 1.8 11 63.9 10 63.7 Sample MFT100 It is interesting that both SLP (theoretical results) and (experimental results) SAR of CFT100 are greatly influenced by the surrounding viscosity while those of MFT100 are almost unaffected 19 Figure 3.22 Dependence of SAR on viscosity for (a) MnFe 2O4 and (b) CoFe2O4 The red lines represent the theoretical results based on LRT In case of CFT100 nanofluids, SLP is influenced by the viscosity because due to its higher magnetic anisotropy the “Brown relaxation loss” dominated heating power In contrast to Co-nanofluid, both the SLP and measured SAR of Mn-nanofluid were independent of the viscosity This result implies that the nanofluid is soft ferrite in which the “Néel relaxation loss power” dominated CONCLUSION The theoretical results of MIH based on LRT deduce the following conclusions: It is indicated the existence of three particle diameter (D) regions that: the Néel relaxation dominates in region I (D < DN), the Brownian relaxation dominates in region III (D > DB) and the two dissipation mechanisms contribute simultaneously in region II (DB ≤ D ≤ DN) The peak behavior of heating power (SLP) versus D is characteristic differently in the two different groups of magnetic nanoparticles depending on their anisotropy (K) (i) For group A (K < KC) the peak is narrow (small width ∆Dcp), the value of SLPmax decreases strongly with expanding size distribution and is independent on viscosity The Néel relaxation dominates totally when K KC), the peak is bell-like with a large width ∆Dcp, the SLPmax value decreases slowly with expanding size distribution and was dependends on the viscosity The Brownian relaxation dominates definitely for nanoparticles in the group B The values of KC depend on the frequency of AMF as exponential function and the viscosity of magnetic fluids as a linear function The experimental results of the influence of alternating magnetic field, particle size and viscosity on specific loss power for CoFe 2O4 and MnFe2O4 magnetic nanoparticles indicated that: The Linear Response Theory (LRT) is in good agreement with the experimental results when ξ