>K9m 2014, TOM 145, bmii. 2, cTp. 218 222
© 2014
GENERATION AND CHARACTERIZATION OF Nd^Fe^B^C NANOPARTICLES BY PULSED Nd:YAG LASER ABLATION
IN LIQUID
H. R. Dehghanpour*
Physics Department. Tafresh University, Tafresh, ban Received July 27, 2013
We have generated Nd Fe B C nanoparticles by Nd:YAG (1064 nm) laser irradiation in distilled water. Exposure times were 1, 5, and 10 min. Characterization of such nanoparticles in terms of their size distribution, shape, and chemical composition was carried out by transmission electron microscopy, energy-dispersive X-ray, and Fourier transform infrared spectroscopy. To investigate the nanoparticle stability, the size distribution of nanoparticles was measured two weeks after the nanoparticle generation, using dynamic light scattering. Investigations with the help of the atomic force microscope and magnetic force microscope showed other aspects of the generated nanoparticles.
DOI: 10.7868/S0044451014020035
1. INTRODUCTION
Research in the area of magnet ism and magnetic materials is a rich combination of synthesis, characterization, theoretical concepts, and engineering applications [1]. Hard magnetic materials are used in hard disk drivers, motors, generators, loudspeakers, magnetic sensors, etc. Nd Fe B C magnets are well known because of their strong, permanent magnetic properties, high coercitivity, and high magnetic remanence. Magnetic nanoparticles are used in ferrofluids [2], refrigeration systems [3], and nmltiterabit information storage devices [4,5]. In the last decades, intensive efforts have been invested into the development of new methods for generation of nanoparticles. In particular, magnetic nanoparticles have attracted great attention because of their widespread application prospects in biomedicine and information technology [6 9]. Laser ablation in liquids offers an approach to the fabrication of pure nanoparticle colloids of various materials. Until now, mainly metal and ceramic colloids have been generated using this method, including several studies on laser-generated magnetic nanoparticles [10 12]. There are a few reports concerning laser-based generation of colloidal magnetic alloys [13,14]. Here, Nd Fe B C nanoparticles are generated in distilled water using
E-mail: h.dehghanpour'fflaut .ac .ir
Q-switched Nd:YAG laser (10C4 11111). Geometrical and chemical properties of those nanoparticles are then revealed. This could be attractive for microtechnology applications such as the development of micromotors or magnetic micropumps [15,16].
2. EXPERIMENTAL SECTION
2.1. Synthesis methods
Figure 1 shows a schematic diagram of the experimental setup. An Nd Fe B C magnet target was immersed in distilled water and fixed on a plate connected to the motor rotating the target (5 rev/'min) to prevent deep laser crater creation. Nanoparticles were fabricated by pulsed nanosecond laser irradiation using Nd:YAG laser at 10C4 11111. The laser shots are characterized by the 10 lis duration, 5 Hz repetition rate, 60 lii.J pulsed energy, and 6 J/'cm2 energy density. The laser beam was focused directly on a target 2 111111 in thickness. It was situated at the bottom of a glass beaker filled with 10 ml of distilled water, corresponding to 6 111111 of liquid height above the target at room temperature using a lens with a 10 cm focal point. The focusing area, power, and energy density of the laser were properly controlled by the relative displacement of the target and the lens. The exposure times were 1,5, and 10 min. After laser irradiation, drops of liquid containing nanoparticles were sprayed 011 a plate
>K3TO, TOM 145, Bi>in. 2, 2014
Generation and characterization of Nd-Fe-B-C nanoparticles
Fig. 1. Experimental setup for fabricating Nd Fe B C nanoparticles in distilled water by laser irradiation
of glass. Thou, after water evaporation remained, the materials were collected.
2.2. Characterization methods
Geometrical aspects of the nanoparticles were studied by transmission electron microscopy (TEM), Philips model EM 208 S. Chemical composition of the bulk and the nanoparticles was investigated using energy-dispersive X-rays (EDX). The time effect on the nanoparticlo sizes was investigated by dynamic light scattering (DLS) two weeks after the nanoparticlo generation.
3. RESULTS AND DISCUSSIONS
The TEM can yield information such as the particle size, size distribution, and morphology of the nanoparticles. In particle size measurement, microscopy is the only method in which individual particles are directly-observed and measured [17]. Typically, the calculated sizes are expressed as the diameter of a sphere that has the same projected area as the projected image of the particle. Manual or automatic techniques are used for particle size analysis. The manual technique is usually based on the use of a marking device moved along the particle to obtain linear dimensional measures of the particles, which are then added and divided by the number of particles to obtain a mean result [18]. In combination with diffraction studies, microscopy becomes a very valuable aid to characterization of nanoparticles [19]. TEM micrographs of the Nd Fe B C nanoparticles fabricated by a Q-switched Nd:YAG laser and various exposure times (1, 5, and 10 mill) as well as the corresponding size distributions are shown in Fig. 2.
After the laser is switched off, the fragmentation process ceases and the aggregation process proceeds. A TEM image shows the presence of nearly spherical particles. In view of the process of formation of nanoparticles and rapid quenching of the ablated nia-
0 4 12 20 Particle diameter, nm
Number
Particle diameter, nm
Fig. 2. TEM images of nanoparticles fabricated by the Nd:YAG laser with (a) 1 min, (c) 5 min, and (e) 10 min exposure times; b, d, and /are the corresponding size distributions
terial into the liquid, this is their most probable morphology [20]. The average size of the nanoparticles gen-orated by pulsed Nd:YAG laser radiation for 1, 5, and
Fig. 3. SEM of the target surface after a 10 min laser exposure
Wave number, cm 1
Fig. 4. Transmittance spectrum of the suspension solution of nanoparticles
10 mill exposure times are 13.7, 8.35, and C.23 mil respectively.
It is worth noting that the largest particles most likely result from the aggregation of smaller particles. Their number is relatively small and does not influence the size distribution significantly. Such aggregation is known and dicussod in [20]. It can be explained by the tendency of lowering a colloidal system energy to achieve a more stable state. The mean diameter and the size distribution of the nanoparticles decrease as the ablation time increases. This can be explained by a redistribution of the size of the particles through their interaction with the laser beam. O11 the other hand, creation of nanoparticles near the target reduces the laser target interaction and is effective 011 the number and size of the nanoparticles [21].
Figure 3 shows a scanning electron microscopy (SEM) image of the target surface after a 10 mill laser
Table. Transmittance wave numbers and the corresponding bonds
Wave number, cm 1 Bond
585 Fo 0
758.75 FoOH
1207.86 BOB
1639.85 c=0
2077.88 Nd Fe C
exposure 011 it. There is a mixture of melting and ablating process 011 the target surface due to laser irradiation. The nanoparticlo generation may be caused by ablation and by the superheated surface above its critical thermal point.
The Fourier transform infrared spectroscopy (FTIR) is a very sensitive and one of the most used spectroscopy methods applied in characterizing the material structure. Figure 4 shows an FTIR transmittanco spectrum of the suspension solution of the nanoparticles. Despite the 3397 cm"1 peak corresponding to the OH band of water, we see a set of peaks, whose corresponding bonds [22,23] are collected in the Table. I11 the nanoparticles constructive elements of the target (except Nd) had a strong oxidation due to the aqueous environment.
Figure 5 shows the results of (EDX) microanalyses of the fabricated nanoparticles. As the figure shows, constructive elements of the target (Nd, F, B, and C) construct the nanoparticles. The existence of silicon in the nanoparticles may be due to the ablated silicon from the glass container or the plate of glass 011 which the drops containing nanoparticles were speared for harvesting the nanoparticles.
As a result, the chemical composition of the target and nanoparticles is the same. Although TEM is the experimental technique most extensively used for obtaining general information 011 particle morphology and evaluating the size distribution, the atomic force microscope (AFM) method has been established as a complementary and very useful method for characterization of shapes in nanoworld. I11 order to investigate nanoparticlo stability, two weeks after the nanoparticlo generation, their size distribution was measured using AFM and DLS. Figure 6 shows AFM images of nanoparticles (10 mill laser exposure) two weeks after the nanoparticlo fabrication. The particle size is between 15 and 70 11111 with the average near 30 11111.
>K3TO, TOM 145, Bbin. 2, 2014
Generation and characterization of Nd-Fe-B-C nanoparticles
Number, % 25
Fig. 5. EDX microanalyses of the fabricated nanoparticles
Fig. 6. AFM image of Nd Fe B C nanoparticles fabricated by a Q-switched Nd:YAG laser in distilled water
Size distributions of the nanoparticles based on the results of DLS for a sample with 10 mill exposure time are shown in terms of number, volume, and light intensity in Fig. 7. The size distribution versus numbers (Fig. 7a) verified the size distribution obtained by AFM. On the other hand, the size distribution versus
0.1 1 Volume, % 12
1000 10000 d, nm
Intensity, % 8
1000 10000 d, nm
1000 10000 d, nm
Fig. 7. After two weeks, size distribution versus (a) the number of nanoparticles, (b) volume of the nanoparticles, and (c) scattered light i
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