The characteristics of magnetic inductive heating and their impacts by the particle anisotropy and ferrofluid viscosity

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. 1 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 2 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 (Mn0.3Zn0.7Fe2O4, Mn0.5Zn0.5Fe2O4, La0.7Sr0.3MnO3) or core – shell magnetic nanoparticles - Fe3O4@ 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 CoFe2O4 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. CoFe2O4 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.. 3 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 CoFe2O4 and MnFe2O4 magnetic nanoparticles Layout of the thesis: The contents of thesis were presented in 3 chapters. • Introduction • Chapter 1. Magnetic inductive heating for nano ferrofluids • Chapter 2. The theoretical results of the specific loss power based on Linear Respones Theory • Chapter 3. 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 1 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 4 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: 6eff V SK K KD = + (1.8) 1.1.2. Nano ferrofluid: synthesis and application 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, vv. 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. 0 S B M VH k T µ ξ = (1.9) 5 When ξ < 1, nanoparticles show superparamagnetic behavior or H<<HC. The relationship between M and H can be approximated to a linear function. Therefore, the LRT can be applied. This model is based on two mechanisms: Néel and Brown relaxation losses. In contrast, the hysteresis loss is the mainly heat generation of ferrite or ferromagnetic multi – domain when the parameter ξ > 1. Thus Rayleigh and SWMBTs models are applicable depending on field used. 1.2.2. Hysteresis loss SW model is a theoretical model based on the hysteresis loss, which can be estimated from the area of the hysteresis loop when the magnetization material is saturated. Note that the hysteresis loop changes with the amplitude and frequency of the AMF. For low AMF, Rayleigh model has been applied and it has been shown that the law SLP ∝ H3 could describe the hysteresis losses. SWMBTs was built by the hypothesis: single domain ferromagnetic particles with non interaction uniaxial anisotropy and orient randomly. According to the SWMBTs, the loss power was equal to twice the anisotropic energy density. In fact, J. Carey et Al. found that it was equal to 1.92 the anisotropic energy density. 1.2.3. Néel relaxation loss For single domain particles, the anisotropy energy is smaller than thermal energy so that the particle magnetic moment can rotate freely in the absence of an external magnetic field. Heating is accomplished by rotating the magnetic moment of each particle against an energy barrier. 1.2.4. Brown relaxation loss The Brown relaxation loss refers to the rotation of particle as a whole in magnetic fluid. This is significant when the direction of the magnetic moment is tightly attached to particle (high magnetic anisotropy) and low viscosity. 1.2.5. Linear Response Theory LRT describes the ability of the magnetic moment to respond AMF. Based on theoretical results, J. Carey et. al found that the condition of validity for the LRT is ξ < 1. So, LRT based on Néel and Brown relaxation losses is suitable for superparamagnetic nanoparticles or H<<HC. Loss power of MIH based on relaxation losses is given by: ( ) ,, 2 0LRTP f H fµ πχ= (1.20.) 1. 3. Difficulties and challenges in experimental study of optimal MIH of nano ferrofluids In biomedical applications, the preferred size of the nanoparticles (core) is typically around 10–50 nm, nanoparticles have high saturation magnetization and must satisfy two main conditions: they should have large heating power with minimum amount of nanoparticles and they should have good stability in ferrofluids. Therefore, the major issue that is being investigated is optimal MIH. Specific Loss Power – SLP or Specific Absorption Rate – SAR is commonly used to describe the MIH capacitance or the ability to absorb energy from AMF of the magnetic nanopaerticles: / PSLP SAR ρ = (1.23.) 1.3.1. Size particle and problem in controlling size and narrow size distribution There are many magnetic nanoparticle synthesis methods such as co-precipitation, sol – gel, solvo- thermal, hydrothermal, thermal decomposition or reverse micelle. The size and size distribution or magnetic properties of nanoparticles depend on the synthesis method. So, it is difficult to control size particle, size distribution and material crystallization. For example, synthesizing magnetic fluids with a same medium size 6 but different size distribution or same size distribution with different medium size is not feasible. Therefore, it is difficult to study of the effect of one or more parameters of nano ferrofluid on a physical phenomenon. 1.3.2. Saturation magnetization and attenuation from saturation magnetization by surface dead layer The magnetization of a magnetic material is the sum of the magnetic moments per unit volume. Surface effects and finite size effects are responsible for the difference between nanoparticle and bulk material magnetization. The corresponding contributions of the two effects are opposite. The attenuation from magnetic saturation of the nanoparticles is due to the existence of a dead layer or spin canting on the particle surface. 1.3.3. Magnetic anisotropy of nanoparticle For bulk magnetic materials, magnetic anisotropy depends on composition and crystal field of each material. Because of the increased ratio of surface atoms to core ato