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Synthesis and Ferromagnetic Properties of Magnetic Ink for Direct Printing Sang-Geun Cho1 , Kwang-Won Jeon1 , JinBae Kim1;2 , Ki Hyeon Kim3 , and Jongryoul Kim1 Department of Metallurgy and Materials Engineering, Hanyang University, Ansan, 426-791, Korea Research Institute of Engineering and Technology, Hanyang University, Ansan, 426-791, Korea Department of Physics, Yeungnam University, Gyeongsan, 712-749, Korea Magnetic nanoparticles with a core/shell structure were fabricated by a sol-gel method and following hydrogen reduction process. The reduction process changed the crystal structure of synthesized nanoparticles from iron-oxide single phase to iron phase covered with an iron-aluminum oxide shell layer. These nanoparticles were dispersed by homogenizing and ultrasonication in order to form ferromagnetic ink. Using synthesized ink, patterns with soft magnetic properties were successfully printed on Si substrates by a direct printing method. Index Terms—Direct printing, ferromagnetic ink, magnetic nanoparticle, ultrasonication.
I. INTRODUCTION ECENTLY, magnetic nanoparticles have attracted much attention for potential application to magnetic storage devices [1], microwave devices [2], exchange-coupling magnets [3], and other bio applications [4], [5] due to their excellent magnetic properties. In general, the magnetic nanoparticles have been prepared by various methods such as sol-gel method [6], [7], thermal decomposition method [8], and self-propagating combustion method [9], [10]. Among these methods, the sol-gel method has competitive advantages for obtaining high purity nanoparticles with a uniform size and requiring relatively simple process. In order to fabricate magnetic devices using nanoparticles, direct printing has been considered as a future processing method for its characteristics of simplicity, material saving and particularly fine patterning. Thus, magnetic ink for direct printing was suggested to be applicable to fabricate magnetic storage devices [11] and magnetic ink character recognition with fine patterns [12]. However, low agglomeration and high dispersed state of magnetic nanoparticles should be achieved for the realization of magnetic ink. In this study, magnetic ink containing air stable iron nanoparticles with a core/shell structure were synthesized and a feasibility study for direct printing was also carried out.
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II. EXPERIMENTAL PROCEDURE In order to fabricate nanoparticles with a core/shell structure, , Al-nitrate , Fe-nitrate were used as base and citric acid monohydrate materials. The weight ratio of Fe/Al was kept on 95/5 under condition of valence equivalence. And then, magnetic nanoparticles were synthesized by a conventional sol-gel process and following hydrogen reduction process [6]. Firstly, the base
Manuscript received February 21, 2011; accepted April 20, 2011. Date of current version September 23, 2011. Corresponding author: J. Kim (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2011.2157476
Fig. 1 Photograph of printed patterns.
materials were mixed in ethanol absolute (99.9%), and then for 1 hr using rotary evaporator. Secondly, evaporated at 50 for 1 day. the evaporated mixed solution was dried at 60 Thirdly, the calcinations process of the dried solution was for 1 hr under Ar atmosphere. Finally, the carried out at 400 for 1 hr hydrogen reduction process was conducted at 600 atmosphere. The fabricated magnetic nanoparticles under with an iron core/iron-aluminum oxide shell structure were dispersed in deionized water by serial process of homogenizing and ultrasonication with dispersion agent (Pluronic F127, Sigma). These dispersed nanoparticles covered with the agent were centrifuged for the enrichment. Ferromagnetic ink was finally synthesized by redispersion process using centrifuged nanoparticles in ethylene glycol. Ferromagnetic ink was direct-printed on surface modified Si substrates by a direct printing instrument (UJ 2400, Unijet). for 1 day Printed patterns were dried in vacuum oven at 60 in Ar atmosand then heat-treated for 1 hr at 600, 700, 800 phere. Fig. 1 shows printed pattern images. The crystal structure of fabricated nanoparticles was analyzed by x-ray diffraction (XRD, D-Max/2500, Rigaku) and the microstructures were characterized by scanning electron microscope (SEM, VEGA II SBU, Tescan) and transmission electron microscope (TEM, JEM-3010, JEOL). The dispersion stability of ink was analyzed by a dispersion stability analyzer (Turbiscan, Formulaction). The magnetic hysteresis loops were obtained at ambient room temperature using a vibrating sample magnetometer (VSM, 7404, Lakeshore).
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Fig. 2. XRD patterns of fabricated magnetic nanoparticles before and after hydrogen reduction process.
III. RESULTS AND DISCUSSION Fig. 2 shows the XRD patterns of fabricated nanoparticles before and after reduction process. As shown in the figure, the into iron reduction process changed iron-oxide phase phase. Fig. 3 shows the TEM images of the fabricated magnetic nanoparticles before and after hydrogen reduction process. The average size of the particles was about 25 nm before the reduction process, as shown Fig. 3(a). The reduction process increased the particle size to about 35 nm and formed 5 nm thick Fe-Al oxide shells around the particles, which kept iron cores from oxidizing, as shown in Fig. 3(b). The core/shell formation mechanism was described in our previous work [10]. The magnetic hysteresis loops of fabricated nanoparticles before and after hydrogen reduction process are shown in Fig. 4. The saturation magnetization value and coercivity after the hydrogen reduction process were 125 emu/g and 673 Oe, respectively. The sharp increment of the saturation magnetization value was due to the phase change of the nanoparticles during hydrogen reduction process. Also, the increment of the coercivity should result from the phase and size changes of the nanoparticles. In particular, the coercivity was increased with increasing the grain size under 100 nm condition of nanocrystalline ferromagnetic materials [13]. Fig. 5 shows the dispersion stability of fabricated ferromagnetic ink measured by a dispersion stability analyzer. Transmission and backscattering light intensities did not show any noticeable changes during the measurement, indicating the high dispersion stability of the ink. This high dispersion stability enabled the ink to utilize a direct printing technique. The microstructures of printed and sintered patterns are shown in Fig. 6. With increasing the sintering temperature, the grain size was slightly increased and necking between adjacent particles was formed. However, full densified patterns were not obtained because of the low concentration of the fabricated ink. Fig. 7 shows the normalized magnetic hysteresis loops of as-printed and sintered patterns. The coercivity of the patterns
Fig. 3. TEM images of fabricated nanoparticles. (a) Before hydrogen reduction process. (b) After hydrogen reduction process.
Fig. 4. Magnetic hysteresis loops of fabricated nanoparticles before and after hydrogen reduction process.
was decreased with increasing the sintering temperature, as shown in the inset image of Fig. 7. The initial sharp decrease of the coercivity after sintering at 600 might be closely related to the densification of the nanoparticles. The further decrease in samples sintered at higher temperatures was also related to the densification and grain growth of the nanoparticles. These
CHO et al.: SYNTHESIS AND FERROMAGNETIC PROPERTIES OF MAGNETIC INK FOR DIRECT PRINTING
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magnetic loops clearly show soft magnetic properties, which suggest the possibility of the direct patterning of magnetic ink. IV. SUMMARY Ferromagnetic nanoparticles with a core/shell structure were synthesized by a conventional sol-gel method and following hydrogen reduction process. The phase of the nanoparticles to Fe phase after the reduction process. changed from Using these nanoparticles, ferromagnetic ink was synthesized by homogenizing and ultrasonication process in ethylene glycol. Synthesized ink was able to be direct printed on surface modified Si wafers and printed patterns were sintered at 600, for 1 hr in Ar atmosphere. The pattern sintered at 700, 800 showed a coercivity of 227.7 Oe. This data suggested 800 that the ferromagnetic ink might be applicable to direct printed magnetic devices.
Fig. 5. Dispersion stability of fabricated ink.
ACKNOWLEDGMENT This work was supported by Grant K00041-173 from the R&D program for Core Technology of Materials and the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Knowledge Economy, Korea.
Fig. 6. SEM images of printed patterns, (a) as-dried, (b) 700, 800 C for 1 hr, respectively.
(d) sintered at 600,
Fig. 7. Magnetic hysteresis loops of as-dried and sintered pattern at 600, 700, 800 C for 1 hr.
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