Photonic crystals in biology

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Poster Session, Tuesday, June 15 Theme A1 - B702 Synthesis of Cobalt Fe rrite Nano-particles by Sol-gel Method A. Ataie 1 *, H. R. Emamian 2 , A. Yourdkhani 3 , A. Honarbakhsh- raouf 2 1 School of Metallurgy and Materials Engineering, University of Tehran, Tehran, Iran 2 Department of Materials, Faculty of Engineering, University of Semnan, Semnan, Iran 3 Advanced Materials Research Institute, University of New Orleans, New Orleans, LA, USA Abstract-In this study, cobalt ferrite nano-particles were synthesized by a facile sol-gel method based on polyesterification reaction between citric acid, benzoic acid and ethylene glycol without pH adjusting of solution and aging of gel. Thermal behavior, phase evolution and magnet ic properties of the products were evaluated using DTA, XRD and VSM techniques. SEM and TEM were employed to study t he powder particle morphology. A pure spinel cobalt ferrite was formed after heat treatment at 750 °C for 1 h. Synthesis and characterization of spinel ferrites have been intensively studied because of fundamental understanding as well as their applicability in a variety of areas such as highdensity information storage system [1], ferro fluid technology [2], biological and clinical applications [3] and gas sensors [4]. Among spinel ferrites, cobalt ferrite, CoFe 2 O 4, has attracted considerable attention due to its large magneto-crystalline anisotropy, high coercivity, moderate saturation magnetization, large magneto-strictive coefficient, chemical stability and mechanical hardness. Several chemical methods like sol-gel, micro-emulsion, citrate gel, co-precipitation, auto-combustion synthesis and hydrothermal process have been reported to synthesize cobalt ferrite nano-particles. Recently, polymerized complex (PC) which is a Pechini type sol-gel method, in which a solution of ethylene glycol, citric acid and metal ions is polymerized to a polyester network, has been employed to process nano size ceramic powders at a relatively low temperatures. In this research, nano particles of cobalt ferrite were prepared by a PC method. The effects of the synthesis parameters on the powder particle characteristics were also investigated. Iron citrate, cobalt nitrate, citric acid, ethylene glycol and benzoic acid all of analytical grade were used as starting materials. A mixed solution with 4.61C 6 H 8 O 7 : 13.84(CH 2 OH) 2: 0.7C 6 H 5 COOH: 2FeC 6 H 7 O 7 : 1Co (NO 3 ).6H 2 O: 300H 2 O molar ratio was used to prepare Co-Fe complex solution. First, citric acid was dissolved in water at 60 °C followed by addition of iron citrate and cobalt nitrate to the solution under stirring until a bright brownish homogenous solution achieved. Then ethylene glycol and benzoic acid were added to the solution. The pH of solution was 1.4 in the process. Then stirring was being continued, without pH adjusting, at 80°C for 1.5 hour to obtain a dark brownish viscose gel. The obtained gel was dried at 150 °C for 24 hours. Annealing process was carried out at 550, 650, 750 and 850 °C for 1 hour by a heating rate of 10 °C/min. Phase composition and morphology of the samples were evaluated by XRD and SEM/TEM, respectively. The thermal behavior of the gel was examined by DTA technique in air with the heating rate of 10 °C/min. The magnetic properties were analyzed by VSM. DTA trace showed one endothermic and two exothermic distinct peaks which appear at about 110, 320 and 510 °C, respectively. Endothermic reaction is related to the dehydration of sample. An exothermic reaction around 320 °C is due to de-esterification and de-carboxilation of COOH groups, while the second exothermic reaction at around 510 °C is due to the formation of cobalt ferrite. XRD results revealed the coexistence of cobalt ferrite and - Fe R R2 ROR3 (maghemite) in the samples annealed at 550 °C and 650 °C. By increasing the annealing temperature to 750 °C, cobalt ferrite single phase was formed with sharp characteristic peaks related to the high degree of its crystallinity. Both SEM and TEM images of the annealed powder at 850 °C for 1 houre showed spherical shape particles with a uniform particle size distribution. The mean particle size was measured as 40 nm from the TEM images. The coercivity of sample annealed at 550 °C was small as 567.2 Oe. However, the highest coersivity of 1353.7 Oe was obtained in the sample annealed at 650 °C. The saturation magnetization of the samples increased by increasing the annealing temperature and reached to a maximum value of 79.4 emu/g in the sample annealed at 850 °C. In summary, cobalt ferrite nano-particles were successfully synthesized by a facile sol-gel route based on polyesterification reaction between citric acid, benzoic acid and ethylene glycol under strong acidic condition of achieved complex solution, without pH adjusting of solution and aging of gel. The financial support of this work by the Iranian Nanotechnology Initiative is gratefully acknowledged. *Corresponding author: 1Taataie@ut.ac.ir1T [1] Giri, A.K., K. Pellerin, W. Pongsaksawad, M. Sorescu and S. Majetich, IEEE Trans. M agn. 36, 3029 (2000). [2] Raj, K., B. Moskowitz and R. Casciari, J. Magn. Magn. Mater. 149, 174 (1995). [3] Baldi, G., D. Bonacchi, C Innocenti, G. Lorenzi and C. Sangregorio, J. Magn. Magn. Mater. 311, 10 (2007). [4] Gopal Reddy, C.V., S.V. Manorama and V.J. Rao, J. Mater. Sci. Lett. 19, 775(2000). 6th Nanoscience and Nanotechnology Conference, zmir, 2010 347
Poster Session, Tuesday, June 15 Theme A1 - B702 Evol ution of Nano-crys talline NiTi and Amorphous Structures in Mechanical Alloyi ng o f Ni and Ti A. Rabiezadeh 1 and S. F. Kashani Bozorg 1 * 1 School of Metallurgy and Materials Engineering, University of Tehran, Tehran, P.O. Box: 14395-553, Iran Abstract-Mechanical alloying of Ni and Ti was conducted in a planetary ball mill using a powder mixture with equiatomic stoichiometry for various times. The milled products were found to decrease in size as a function of milling time as evidenced by scanning electron microscopy. X-ray diffractometry of the milled products exhibited peak broadening and the formation of amorphous structure which became dominant after 25h of milling. Transmission electron microscopy using bright field imaging and electron diffraction pattern modes revealed that the 25h milled product was consisted of mixture of nano-crystalline NiTi (20-40nm size) and an amorphous phase. NiTi has unique properties that could be very useful in surgical applications [1]. Conventionally, Ni-Ti alloys are produced by arc or induction melting. Melting methods have shortcomings due to gas absorption, elemental vaporization, segregation formation, and crucible contamination absorption. Thus, throughout the last decade solid state processing routes have gained considerable interest for NiTi fabricat ion [2-5]. In this study, the mechanical alloying (MA) process of elemental Ni and Ti with equiatomic ratio was carried out. The morphology, phase composition of the milled products for different durations were investigated. MA of the powder mixture was conducted in a planetary ball mill at room temperature in a stainless steel vial with chromium steel balls under an argon atmosphere. The milling speed and the ball to powder charge ratio were 450 rpm and 15:1, respectively. The maximum milling time was 25 h. During the preliminarily stage of milling (until 4h), the specific size of powders increases which corresponds to the formation of coarse multiphase particles. With further milling, the mean particle size decreased; this can be related to work hardening and/or formation of new brittle phase(s). After 10h of milling, substantial powder was crushed and flattened by the collisions between the powder and the milling media. No significant change in particle size of the powders can be observed after 10 h of milling. With further milling, the milled products become more homogeneous while keeping its granular morphology. The particle size was found to be smaller than 10 h of milling. In a first period of milling up to 6h, the intensity of the X- ray diffraction peaks of the milled product decreases and broadens simultaneously. The broadening of the peaks is commonly attributed to the structural change in the samples, such as formation of amorphous phases, large lattice strains in grains, decrease of crystallite size, or embedding of very small crystals in an amorphous matrix. X-ray diffraction examination studies showed that the milled products contained nano-crystallites of disordered NiTi (with a mean crystallite size of ~30nm) and an amorphous phase (Figure 1a). A new broad diffuse scattering halo at abo -44° can be observed after 10h of milling, which implies that the milled product becomes partially amourphized. It is found that further milling enhances the formation of the amorphous phase. The diffraction lines corresponding to fcc nickel (111) become broad and shift towards low angles with increasing milling time; this is attributed to increasing the lattice parameter of nickel fcc structure with increasing the milling times. In order to evaluate the crystallite size of the powders, the powders are further examined by TEM. Bright-field image with corresponding electron diffraction pattern for the 25h milled product is shown in Fig. 1b. It can be seen that the approximate grain size of the milled product are 20-40 nm. The corresponding diffraction pattern using smallest selected area aperture showed several even and broad rings. The rings were found to be consistent with NiTi nano-crystalline structure. Also, the first wide ring may be indicative of coexistence of substantial amorphous phase. Figure 1. (a) X-ray pattern of the milled product for 25h. (b) TEM bright field and corresponding selected area diffraction pattern micrographs of 25h milled product. The key of the diffraction pattern is shown at the top left. In summary, nano-crystalline NiTi-based powder was achieved using mechanical alloying of elemental Ni and Ti powder mixtures. NiTi mean crystallite size after 25h of milling was found to be ~30 nm. In addition, an amorphous NiTi phase was detected. *corresponding author fkashani@ut.ac.ir [1] A. Kapanen, J. Ilvesaro, A. Danilov, J. Ryhanen, P. Lehenkari and J. Tuunkkanen, Biomaterials, 23, 645 (2002). [2] B. Y. Li, L. J. Rong, Y. Y. Li and V. E. Gjunter, Scripta Mater, 44, 823 (2001). [3] A. Kapanen, J. Ilvesaro, A. Danilov, J. Ryhanen, P. Lehenkari and J. Tuukkanen, Biomaterials, 23, 645 (2002). [4] D. S. Grummon, J. A. Shaw and A. Gremillet, Applied Physics Letters, 82, 2727 (2003). [5] A. Takasaki, Physica Status Solidi A, 169, 183 (1998). 6th Nanoscience and Nanotechnology Conference, zmir, 2010 348
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