Fabrication and magnetic characterization of Fe, Co nanoparticles prepared by high energy ball milling

dition, the composites limit some disadvantages of single metallic nanoparticles such as better antioxidant capacity, more environmental sustainability, reduced toxicity and better biological compatibility, etc So nanoparticles and nanocomposites based on metal/alloy Fe, Co, Fe-Co have many potential applications with many interesting properties. In Vietnam, nanomagnetic materials based on Fe have been interested in both basic sciences and applied research. Recently, at the Institute of Materials Science, Vietnam Academy of Science and Technology, the nanomagnetic research are focused on fabrication, study the structure and properties of superfine iron, quenched composite magnet or combined with high energy mechanical milling. Although there have been interesting results, the effect of fabrication methods on their structure and properties still needs further research. For example, the ability to fabrication Fe-Co alloys in an open environment and their properties. Besides, the evaluation of the heat capacity of soft magnetic metal nanoparticles (Fe) and alloys (Fe-Co) is also of little concern for research. Derived from the research situation of Fe-Co nanomaterials in the world as well as in Vietnam, based on the doctoral capacity of the Institute of Materials Science and also to improve the results, which have achieved us to choose the topic of the thesis: "Fabrication and magnetic characterization of Fe, Co nanoparticles prepared by high energy ball milling" 3 2. Research objectives of the thesis • Find the optimal technology parameters and components for fabricating Fe-based nanomaterials using high-energy ball milling method. • Clarify the relationship between technological conditions with the structural characteristics and magnetic properties of fabricated materials. • Assess the applicability of Fe, Fe-Co nano materials in fabricating nanocomposite magnets and magnetic induction heating, etc 3. Research contents of the thesis (i) Overview of basic physical properties of magnetic nanoparticles and FeCo magnetic materials, a brief overview of hard/soft phases nanocomposite magnet materials, on the heat generation capacity of magnetic nanoparticles base on Fe. (ii) Experimental methods, research results and discussion of the influence of fabricating conditions and heat treatments on structural characteristics and magnetic properties of magnetic nanoparticles Fe-based. (iii) Initial studies on FeCo/SmCo exchange spring magnets and heating capacity of Fe and FeCo nanoparticles were discussed. The method of fabricating materials used in the thesis is a high- energy ball milling method. The structural characteristics, alloying are evaluated by X-ray diffraction diagram (XRD), X-ray absorption spectroscopy (XAS). The size and morphology and chemical composition of the sample are identified by field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM) and X-ray energy dispersion spectroscopy ( EDS). The magnetic properties were investigated by the vibration sample magnetometer (VSM), the physical properties measuring system (PPMS) and the pulsed magnetic magnetometer (PFM). The measurement of induction heating is performed on the RDO-HFI commercial system. 4 New contributions of the thesis: - Has found a new approach: simple, low cost to synthesize magnetic nanomaterials such as Fe and FeCo by milling metal powders in air atmosphere. Products with magnetic qualities equivalent to those synthesized in protective atmosphere. - Understanding the capabilities of the XRD and XAS and their combination in study the structure and alloying process of FeCo. - Clarify the influence of particle size to magnetic properties (magnetic coercivity HC, saturation magnetization MS, the temperature dependent of MS) of FeCo nanoparticles. The thesis is divided into 4 chapters: Chapter 1. Overview of Fe-Co magnetic nanomaterials Chapter 2. Experimental methods Chapter 3. Fabrication and charaterization properties of Fe-Co magnetic nanomaterials Chapter 4. Applications of Fe-Co magnetic nanomaterials Chapter 1: OVERVIEW OF Fe-Co MAGNETIC NANOMATERIAL 1.1. Overview of Fe-Co magnetic nanomaterials 1.1.1. The magnetic properties of magnetic nanoparticles 1.1.1.1. Single-domain and superparamagnetic particles In ferromagnetic materials, the magnetic moment is not completely ordered in the whole volume of the sample, but exists only in a region of a defined size, which is called a magnetic domain. For each different type of nanomaterial there will be a different critical size for single domain. So that, magnetic particles become super- paramagnetic when the particle radius falls below 20 nm. The magnetic 5 properties become interesting when the particle's radius is within the limits of superparamagnetic and single-domain. Below the superparamagnetic limit, the particle has no residual magnetism and no coercivity. 1.1.1.2. The coercivity of magnetic nanoparticles Figure 1.1. Schematic of coercivity versus particle size curve. 1.1.1.3. The exchange interaction The exchange interaction is basically originating from electrostatic Coulomb repulsion between two electrons, due to the complementation of the electron wave functions in ferromagnetic materials. The exchange interaction in these materials has: (i) align parallel orientation of atoms magnetic moment to each other; (ii) decide the temperature value of magnetic order. 1.1.1.4. The magnetocrystalline anisotropy The magnetocrystalline anisotropy is related to the crystal symmetrical magnetism, the stress, the shape or the order of the unparalleled coupled spin. In magnetic thin films there are also surface anisotropy. 1.1.2. Fe-Co magnetic nanomaterials 1.1.2.1. Phase diagram of Fe-Co material 6 It is easy to see on the phase diagram Fe-Co can form a solid solution structure body centered cubic (bcc-) with broad distribution of concentrations at lower temperatures. At temperatures greater than 983°C, the material has a face-centered cubic structure (fcc-). At temperatures below 730°C, the material has a B2 structure (bcc-2) with CsCl2 structure. 1.1.2.2. Magnetic properties of Fe-Co materials Fe-Co alloys with bcc structure is a typical soft magnetic material with the highest magnetization saturation in the 2-phase alloys, high permeability value, low hysteresis loss in high frequency... The saturation magnetic moment of Fe-Co alloy reaches the maximum with Co content of about 35%. 1.1.2.3. Fabrication methods for Fe-Co materials The physical methods such as thermal evaporation, laser or plasma evaporation are only suitable for thin film, but for powdered Fe- Co material, mechanical milling method, especially high energy mechanical ball milling is still the most popular and effective method up to now, because of low input cost, simple process, and fabricating large quantities of product in short periods of time. 1.2. Magnetic hard/soft nanocomposite materials 1.2.1. Magnetic hard/soft nanocomposite materials Soft magnetic materials can be easily magnetized and demagnetized at low magnetic fields. The sarturation magnetization is high, but the coercivity is low. Soft magnetic materials are suitable for applications of magnetic cores or recording heads, due to their ability to easy alignment with applied magnetic fields. Hard magnetic materials have high coercivity and high magnetocrystalline anisotropy. Hard magnetic materials are suitable for 7 applications of permanent magnets and magnetic recording media, such as computer hard drives, in iPods, in MRI machines, in engines, generators, and in magnetic levitation trains 1.2.2. Why do we choose Fe-Co and SmCo5 nanocomposite materials? The hard magnetic material have high coercive force and high magnetocrystalline anisotropy. When a soft magnetic material has not only high sarturation magnetization, but also has a magnetocrystalline anisotropy significantly; Both hard and soft magnetic materials should have high Curie temperatures. As a result, the magnetic nanocomposite will have high Curie temperatures, so it can have a wide range of operating temperatures; Conformity in lattice between soft and hard magnetic materials. Matching lattices are also important for later theoretical studies. 1.3. Magnetic nanomaterial in magnetic induction heating In this part of the thesis, we focus on the experiment of magnetic induction heating (MIH) to evaluate the applicability of Fe, Fe-Co nanoparticle fluids. During the MIH process, the working temperatures range 41°C-46°C (314K-319K) is most important. At these temperatures, the tumor is killed but the cells are usually almost unaffected. In fact, it is difficult to determine the exact temperature of the cancer cells during heat treatment. Therefore, an important parameter of the applied material is the saturation temperature TS, the maximum temperature of the material in an external magnetic field. When the temperature of the nanoparticles reaches to TS, they are demagnetized. Therefore, the temperature will stop increasing without interrupting or reducing the external magnetic field. The process is called self-temperature-control hyperthermia. For magnetic particle 8 fluids, TS can be adjusted by varying the concentration of magnetic particles or external magnetic field to the range of 41–46°C, which is sufficient to kill cancer cells. Chapter 2: EXPERIMENTAL METHODS 2.1. Fabrication by high energy mechanical ball milling The samples used in this thesis were fabricated by high energy mechanical ball milling using planetary ball mill (Fritsch Pulverisette 6 classic line (P6) and Fritsch Pulverisette 7 premium line (P7)). The P6 uses a Fe-Cr alloy grinding bowl (80 ml), with 2 types of milling balls (20mm and 10mm diameters). The P7 uses a 50 ml Fe-Cr alloy grinding bowl, with 15 mm and 8mm grinding balls diameters. P7 grinding bowl allows milling the sample in protective gas environment. 2.2. Structural analysis 2.2.1. X-ray diffraction and structural analysis X-ray diffraction is based on constructive interference of monochromatic X-rays and a crystalline powder sample. The interaction of the incident rays with the sample produces constructive interference when conditions satisfy Bragg's Law (nλ=2dsinθ). This law relates the wavelength of electromagnetic radiation λ to the diffraction angle θ and the lattice spacing d in sample. Conversion of the diffraction peaks to d-spacings allows identification diffraction data of sample. The structure, size and stress of the samples used in the thesis were analyzed by Rietveld method via Bruker D8 Advance diffraction at the Laboratory of Rare Earth Metallurgical Chemistry, East Paris Institute of Chemistry and Materials Science (CMTR-ICMPE) and SIEMENS D5000 diffractometer at the Institute of Materials Science, Vietnam Academy of Science and Technology (IMS, VAST). 2.2.2. X-ray adsorption spectroscopy 9 X-ray absorption spectroscopy (XAS) is a method of using high-energy X-ray sources (X-ray of synchrotron radiation) transmitted through the sample to obtain structural information. architecture of the sample. This is a powerful, detailed and very new analytical method today, as well as a new contribution of the thesis. 2.2.3. Scanning Electron Microscopes and component analysis by Energy Dispersion X-ray spectroscopy The measurements and analysis of SEM and EDX in the thesis were performed on HITACHI S-4800 Scanning Electron Microscope at National Key Lab of Electronic Materials and Devices, Institute of Materials Science, Vietnam Academy of Science and Technology. 2.2.4. Transmission electron microscopy (TEM) Some of the thesis samples were mearsured by CM20 Transmission Electron Microscopy, PHILIP. It’s support acceleration voltage of 200 kV and resolution of 0.14 nm. This device is located at the Department of Solid Surface Analysis, Institute of Physics, Chemnitz University of Technical Engineering, Germany. 2.3. Measuring methods of magnetic properties 2.3.1. Vibrating Sample Magnetometer Hysteresis measurement of nanoparticle samples are often used to evaluate the characteristics of magnetic particle systems such as magnetic anisotropy, spontaneous magnetism, magnetization saturation ... The hysteresis measurement of Fe, FeCo soft magnetic materials was performed on the VSM of the Lab of Magnetism and Superconductivity, Institute of Materials Science. 2.3.2. Physical Properties Measurement System 10 The measurement of the M-H curve (magnetization depend on magnetic field at isothermal measurement) is performed on the PPMS 6000 at National Key Lab of Electronic Materials and Devices, Institute of Materials Science. PPMS 6000 is a commercial measuring system manufactured by Quantum Design, which measuring with extremely high accuracy. 2.3.3. Pulse Field Magnetization The hysteresis measurement of nanocomposite hard/soft magnetic samples in the thesis is performed on the Pulsed Field Magnetization at National Key Lab of Electronic Materials and Devices, Institute of Materials Science. Based on these hysteresis lines, important characteristic parameters such as HC , MS , Br and (BH)max can be identified. Chapter 3: STRUCTURE AND MAGNETIC CHARACTERISTICS OF Fe-Co NANOMATERIALS FABRICATED BY HIGH ENERGY BALL MILLING 3.1. Investigation of optimal technological conditions for fabricated Fe-Co nanomaterials by high energy ball milling 3.1.1. The effect of milling speed The samples labeled are FC40-10h-350, FC40-10h-450 and FC40-10h-550 corresponding to milling speed of 350, 450 and 550 rpm. After milling for 10 hours, at 350 rpm, the sample has not been fully alloyed. At 450 and 550 rpm, the intensity and the FWHM of X-ray diffraction are nearly the same, the material has been completely alloyed. XRD patterns did not show any peaks for cobalt metal or oxide 11 compounds. So that, the milling speed of 450 rpm or faster can be selected. Figure 3.1. X-ray diffraction patterns of FC40-10h sample with ball to powder weight ratio:15/1 at different milling speeds. 3.1.2. The effect of ball to powder weight ratio Ball to Powder Ratio (BPR) is an important parameter of the milling process. BPR values selected for study in this thesis are 10/1; 1/15 and 1/20. From (3.1.1), the optimal milling speed is 450 rpm for 10 hours. All samples were completely alloyed after 10 hours of milling. The FWHM of FC40-10h-10 diffraction peaks is the narrowest, and they are the same in FC40-10h-15 and FC40-10h-20 diffractions. Thus, the average crystal size of FC40-10h-10 is largest, then decreasing to FC40-10h-15 and FC40-10h-20. Figure 3.2. X-ray diffraction patterns of FC40-10h sample with different BPR for 10 hours, 450 rpm. 3.1.3. Effect of milling time 12 As seen by XRD patterns, it can be said that the Fe60Co40 material powder has been completely alloyed by high-energy mechanical ball milling after 8 hours of milling. Peaks of Co metal or oxide compounds are not exist. Figure 3.3. X-ray diffraction patterns of FC40 alloy with different milling times. 3.2. The effect of composition on Fe-Co nanomaterial In this thesis, we investigate the effect of Fe/Co composition to the characterization of Fe100- xCox material (x = 0, 25, 35, 40, 50 and 75) with samples labeled: Fe10h, FC25-10h, ... respectively, by high- energy mechanical milling methods using planetary mill Pulverisette 6 . The milling parameters were selected based on the optimal results on (3.1) with the milling speed: 450 rpm, BPR: 15/1, milling time: 10 hours. As seen in Table 3.3, the values of MS obtained in this study are smaller than several publications (from 6 ~ 11%). This results may be due to the milling process in our study was performing in air 13 atmosphere. Table 3.3 . Magnetization saturation of nano material Fe100- Cox milled for 10 hours with different Fe/Co compositions at 11k Oe. 3.3. Structure characteristics and magnetic properties of Fe50Co50 compositions 3.3.1. The effect of milling time on structural characteristics The bcc-Fe50Co50 phase of all milling samples remained almost unchanged as the milling time increase over 10 hours. Thus, it can be said that the durable Fe50Co50 phase has been successfully fabricated by high-energy ball milling. Figure 3.7. X-ray diffraction patterns of FC50 samples at milling speed: 450 rm, BPR: 15/1 with different milling times. 3.3.2. The effect of milling time on magnetic properties 14 Figure 3.11. Magnetization and coercive of Fe50Co50 with difference time milling . When the milling time increased from 0.5 hours to 12 hours, MS is increased sharply. After 12 hours of milling, MS is decreases. The increase of magnetization from starting to 12 hour milling may be related to the alloying process of Fe and Co during high-energy ball milling. After 12 hours of milling, the MS decreases probably because at this time the oxide phase (Fe, Co) begins to form and increase in the sample, due to the milling process being performed in air atmosphere. 3.3.3. The effect of anealing temperature on properties of Fe50Co50 nanomaterials After 10 hours of milling, Fe50Co50 samples was completely alloyed from Fe and Co powder; The highest saturation magnetization ~ 200 emu/g after 10 hours of milling. The fabricated Fe50Co50 nano alloy powder with high value of MS and durable in the air. Coercive (HC) of the sample after annealing is smaller than as-milled sample and particle size dependence of coercivity does not follow function D6. 15 Figure 3.14 . XRD patterns of FC50-10h samples annealing at different temperatures. The thumbnail is an enlarged view of the diffraction peaks of the Fe3O4 phase . 3.4 . Structure characteristics and magnetic properties of Fe65Co35 materials 3.4.1. The effect of milling time on structure characteristics As shown in Figure 3.20, after 10 hours of milling, the diffraction peaks corresponding to the original hcp-Co phase were completely gone. The diffraction peaks corresponding to Fe-Co bcc structure are enlarged due to the reduction of crystal size and the increase strain during milling process. Figure 3.20. XRD patterns of Fe65Co35 samples depent on milling times. 16 Figure 3.21. Average crystal size and internal strain of FC35 samples with difference milling time. 3.4.2. The effect of milling time on magnetic properties MS is increase with the milling time below 10 hours may be related to the alloying reaction between Fe and Co during milling process. The sharp decrease of MS with milling time over 10 hours is thought to be due to the formation of oxide phase (Fe, Co) when milling process is performed in air. Figure 3.28. Magnetization and coercive of Fe65Co35 with difference milling time. 3.4.3. The effect of annealing temperature on magnetic properties Fe65Co35 nano alloy powder has been successfully fabricated using Fe and Co powder by high energy mechanical milling method in air atmosphere. The highest MS value obtained in FC35-10h as-milled sample and annealing samples were 205 emu/g and 220 emu/g, respectively. 17 Table 3.11. Magnetic parameters of FC35 nano alloy powder manufactured by high energy mechanical milling and annealing. 3.5. Structure characteristics and magnetic properties of Fe nanomaterial 3.5.1. Structure characteristics of Fe Figure 3.38. X-ray diffraction patterns of iron nano powder with different milling times (a) and (b) enlarged detail at position 2in the range of 35 to 45°. 18 Table 3.12. Structure parameters of Fe nanomaterial. 3.5.2. Magnetic properties of Fe Figure 3.43. HC and MS of Fe samples depend on the milling time. Table 3.14 . Magnetic parameters of Fe samples at different milling times then annealing. Conclusion : Nanoparticles from single-phase Fe have an average crystal size of 14-60 nm, constant lattice constant ~ 2,868 Å . When the milling time increased from 1 to 32 hours, MS dropped sharply from 211 19 emu/g to 162 emu/g. MS is increases after annealing, up to ~ 199 emu/g for 10 hours milling and annealed at 600°C sample. Chapter 4 : APPLICATIONS OF Fe, Fe-Co MAGNETIC NANOMATERIALS 4.1. Fe-Co nanocomposite magnets fabrication Hard/soft nanocomposite magnets (SmCo5)y/(Fe65Co35)100-y (y = 80,75,70,65, 60) is mixed according to the desired mass ratio, corresponding to samples labeled SF80, SF75, SF70, SF65 and SF60. 4.1.1. The effect of fabrication parameters 4.1.1.1. The effect of milling speed Figure 4.2. The hysteresis curve of SF70 and SF75 samples before and after milling 300 and 450 rpm. 4.1.1.2. The effect of milling time Figure 4.4. X-ray diffraction diagram of SF70 and SF75 samples at milling time 2 - 6 hours. 20 After 2 to 6 hours of milling time, the intensity and FWHM of the diffraction peaks are almost unchanged. So that it can be told the milling process has reached an equilibrium, the extension of milling time to 6 hours does not affect the crystal structure of the material. The X-ray diffraction patterns still show 2 independence phases of SmCo5 and Fe65Co35, none of any new phase existed. Figure 4.6. The hysteresis curve of SF70 and SF75 samples after milling for 2 to 6 hours using PFM measurement. 4.1.2. The effect of compositions Figure 4.10. The hysteresis curve of compositions effect to the SmCo5/Fe65Co35 nano-composite materials after 4 hours of milling. Figure 4.11. Diagram of MS and HC dependence on the hard magnetic composition phase of nanocomposite sample after 4 hours of milling. 21 4.1.3. The effect of annealing temperature Figure 4.12. X-ray diffraction spectra of samples SF70, SF75, SF80 before and after annealing. Figure 4.13. Hysteresis curve of SF75, SF80 samples before and after annealing. 4.1.4. The effect of Spark Plasma sintering Figure 4.15. The hysteresis curve of SF-6h-550-SPS (70 MPa, 550 o C for 5 minutes). The results show that the highest value of (BH) max is 10.9 MGOe in SF80-6h-550-SPS sample (~80% of SmCo5), corresponding to the sample with the highest hard phase ratio. After SPS process, all samples has improved magnetic properties, especially in HC , while the MS is 22 almost unchanged. 4.2. Fe, Fe-Co materials for magnetic induction heating 4.2.1. Heat capacity of Fe nanofluids Table 4.9. Saturation temperature TS , initial rising temperature dT/dt and specific absortion rate (SAR) of Fe-10h fluid at concentration 4mg/ml with different external magnetic field. 4.2.2. Heat capacity of Fe-Co nanofluids Table 4.12 . Saturation temperature TS , initial temperature increase slope dT/dt and specific absortion rate (SAR) of FC35 -10h fluid (concentration of 1 mg/ml) with different external magnetic field. For Fe nanoparticles, the saturation temperature (Ts ) is 36,7; 45,6; 55.1 and 63.2 oC with AC fields of 60, 70, 80 and 90 Oe, respectively. This magnetic nanoparticles fluid are stable with Ts decrease about 3% and SAR reduce below 2% after 13 days. For Fe65Co35 nanoparticles, the value of TS is in the range of 41-46 oC at external magnetic field of 70 Oe. 23 CONCLUDE The thesis " Fabrication and magnetic characterization of Fe, Co nanoparticles prepared by high energy ball milling" was conducted at Graduate University of Science and Technology and Institute of Materials Science, Vietnam Academy of Science and Technology. The thesis results have been published 4 articles in ISI journals, 4 articles in international scientific conferences proceedings and specialized journals.  A new simple method for fabricating metal/alloy nanomaterials has been found: high energy ball milling in air atmosphere. Optimal milling parameters are: milling speed at 450 rpm and ball to powder ratio of 15/1; milling time for complete alloying varies with compositions (8 hours for Fe50Co50 and 10 hours for Fe65Co35 ).  Detailed investigation of the formation of the Fe50Co50 alloy phase depend on milling time using combination of X-ray diffraction and X-ray absorption. So that, confirmed the existence of low concentrations iron oxide phase by XRD mearsuring at a slow scanning rate as well as accurately determining the oxidation state of Fe (Fe+0) and the proportion of bcc structural phases (FeCo) and hcp (Co).  Surveyed and discussed in