Fabrication and Microstructure Evolution of Single Crystalline

Article pubs.acs.org/JPCC

Fabrication and Microstructure Evolution of Single Crystalline Sm2Co17 Nanoparticles Prepared by Mechanochemical Method W. F. Li,*,† A. M. Gabay,† X. C. Hu,‡ C. Ni,‡ and G. C. Hadjipanayis† †

Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, United States Department of Materials Science and Engineering, University of Delaware, Newark, Delaware 19716, United States

‡

ABSTRACT: In this work we report fabrication and microstructural characterization of single crystalline Sm2Co17 nanoparticles with an average size of 103.7 nm. These particles are fabricated using mechanochemical method and can be used for fabrication of high performance permanent magnets due to their high coercivity (20 kOe). Microstructure analysis reveals the inhomogeneity and defects in the nanoparticles. The origin of these defects was analyzed and discussed by systematic microstructural investigation of the as-milled, annealed, and washed samples. On the basis of these results, by further optimizing the processing parameters, properties of the nanoparticles can be improved.

1. INTRODUCTION Requirements driven by commercial and military applications have led to the revival of research seeking high performance permanent magnets. Particularly, much effort has been put into the fabrication of hard/soft magnetic nanocomposites taking advantage of both the high coercivity of the hard phase and the high remanence of the soft phase. This method has been considered as one of the few approaches to synthesize high performance magnets with less rare earths. The prerequisite for a high performance nanocomposite is anisotropic highcoercivity nanoparticles. Single crystalline nanoparticles would be a perfect choice. These nanoparticles also can be used for magnetic electromechanical systems and filters/separators. Various techniques have been tried to acquire anisotropic high coercivity nanoparticles.1−6 Sm−Co based nanoparticles such as SmCo5 and Sm2Co17 nanoparticles are considered as good candidates for the hard phase in the nanocomposites because of their high anisotropy constants (22 × 107 erg/cm3, and 3 × 107 erg/cm3, respectively) and stability. Surfactantassisted ball milling method has been used for the preparation of SmCo5 or Sm2Co17 nanoparticles, but the yield of nanoparticles by this method is also too low. Wet chemical method was also tried but the SmCo5 nanoparticles prepared show room temperature coercivities in the range of 0−1500 Oe.7,8 A combination of wet chemistry and calciothermium reduction was recently employed by Zhang et al.9 Mechanochemical method was developed and proved to be an effective technique to get high coercivity SmCo 5 and Sm 2 Co 17 nanoparticles.10−13 For this technique, precursor oxide powders are mixed with a reducing agent (Ca), ball milled, and heat treated. Phase transitions may happen at any of the stages of the processing. Due to the complexity, the microstructure evolution and phase transition during mechanochemical process is still not clear. © 2013 American Chemical Society

In this work, we report the fabrication and microstructure investigation of Sm2Co17 nanoparticles prepared by mechanochemical method. The phase transition and microstructural evolution during ball milling and annealing are elucidated by systematic investigation of the microstructure of the as-milled and annealed samples, which sheds light on the microstructure, size and phase control of the nanoparticles.

2. EXPERIMENT The near-single-phase Sm2Co17 nanoparticles were prepared from a mixture of Sm2O3 and Co powders which were ballmilled with a SPEX-8000 mill in the presence of a reducing agent (Ca powder) and dispersant (CaO powder). To ensure the best results, the Sm2O3 and Ca had to be taken in excess: extra 85 atom % Sm compared to the Sm2O3 + 17Co + 3Ca → Sm2Co17 + 3CaO equation and extra 300 atom % Ca based on the actual amount of Sm2O3. Following milling in argon for 4 h, and an additional 10-min-long milling in hexane, the powder mixtures were collected, dried in air, and then placed in an oxygen-free environment for storage until further processing. For annealing, the milled powder mixtures were sealed in argon-filled quartz capsules. The samples were annealed at 925 °C for 5 min. Once annealed, the capsules were opened under anhydrous ethanol in order to passivate the powders. Removal of the CaO and leftover reactants was done by repeated washing with deionized water and a 1 vol.% aqueous solution of acetic acid and anhydrous ethanol. Every washing cycle was accompanied by ultrasound agitation and completed with magnetic separation of a ferromagnetic product. Received: February 21, 2013 Revised: April 24, 2013 Published: April 26, 2013 10291

dx.doi.org/10.1021/jp401836w | J. Phys. Chem. C 2013, 117, 10291−10295

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Structure of the as-milled and annealed samples was first characterized using an X-ray diffractometer (Rigaku Ultima IV operating with a Cu Kα radiation). The washed particles were observed using a JEOL 6335F scanning electron microscope (SEM). For transmission electron microscopy (TEM) observation, the as-milled and annealed powders were mixed with Gatan G2 glue and heated to solidification. The bulk of glue/ powder was then cut into thin slices which were further polished and ion milled for TEM observation. The washed particles were simply sonicated in acetone, and one drop of the sonicated dispension was put onto the Cu grid coated with carbon thin film. TEM investigation was conducted by employing JEOL 3010 TEM and JEOL 2010F TEM equipped with energy dispersive X-ray spectroscopy (EDX). Magnetic measurements were performed with a Quantum Design VersaLab vibrating sample magnetometer on powder samples immobilized with wax. Prior to the measurements, the samples were magnetized with a pulsed field of 100 kOe. The results were corrected for the self-demagnetizing field.

3. RESULTS AND DISCUSSION In the X-ray diffraction pattern, the major peaks in the as-milled samples can be indexed as CaO (Figure 1a). There are peaks

Figure 2. Washed nanoparticles: (a) SEM image and (b) size distribution.

size is 103.7 nm. The coercivity of the annealed and washed particles is about 20 kOe (Figure 3), which is much higher than

Figure 1. X-ray diffraction patterns of (a) as-milled and (b) annealed and washed samples.

indicating other phases after ball milling, but these peaks are broad and weak. Ca(OH)2, Ca, and Co may contribute to these reflections. No Sm-containing phase can be identified from the pattern. In annealed samples (not shown), a Sm2Co17 phase with the Th2Zn17 structure is present alongside the CaO and other Ca-containing phases. After the subsequent washing, the Ca-containing phases can no longer be detected in the sample (Figure 1b); almost all the peaks can be indexed as the 2:17 structure with a = 0.8453 nm, c = 1.2213 nm; the peaks at 36.0° and 41.8° may indicate a small amount of the SmCo5 phase. The 0.65% increase of the 2:17 cell volume is tentatively associated with interstitial hydrogen absorbed during the washing, similar to the one reported for micrometer-sized Nd2Fe14B particles synthesized via reduction-diffusion and washed with water.14 This probable interstitial modification is still being studied. It does not seem to adversely influence the hard magnetic properties of the Sm2Co17 particles. The SEM image shows the nanoparticles after annealing and washing (Figure 2). The size of the particles varies from 30 to 500 nm. Most of the particles are about 100 nm and the average particle

Figure 3. M-H loop of the washed particles before and after alignment in a magnetic field.

that in ball-milled Sm2Co17 powders15 and mechanochemically synthesized nanoparticles earlier reported.13,16 One may also notice that the magnetization is lower than that of the bulk Sm2Co17. It is obvious that the saturation magnetization was not reached in the 30 kOe field applied during the measurements, indicating that either not all particles were single crystals or the particles were not perfectly aligned. Both decrease the magnetization. There are Sm rich particles, as will 10292

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be discussed below, which also dilute the magnetization. The small low field kink may be caused by an undetected soft magnetic impurity phase, or the defected particles which have different coercivity. To reveal the microstructural evolution during ball milling and subsequent annealing, systematic and detailed TEM work was done. Figure 4 shows the TEM images of the as-milled powders. Although from the diffraction pattern there are different phases,

Figure 5. HAADF-STEM image of the as-milled powder.

Therefore during the ball milling process, some Sm−Co phases may be already formed, although the Sm2Co17 phase is not detected in the mixture. After annealing and washing, nanoparticles of 50−200 nm are achieved as shown in Figure 6a. The particles have different

Figure 4. (a) TEM bright field (BF) image and the corresponding diffraction pattern of as-milled powders; (b) high resolution TEM (HRTEM) image of the as-milled sample.

in the bright field (BF) image (Figure 4a) not much contrast difference can be distinguished. The corresponding high resolution TEM (HRTEM) image (Figure 4b) reveals that the average grain size of the as-milled powder is less than 10 nm. From the diffraction pattern, different phases can be indexed. Co (fcc), CaO, Ca, and Sm2O3 phases are found present in the as-milled powders. However, the innermost ring (d = 0.507 nm) in the pattern does not match any of these phases. The most possible origin of this reflection is from monoclinic Sm2O3. As seen from the HRTEM image and the diffraction pattern, the ball milling did not lead to any amorphous phase. This is different from the conventional mechanical alloying method in which ball milling leads to amorphous Sm−Co phase.17,18 The ball milling leads to a mixture of different phases. From TEM BF image it is difficult to distinguish the individual phases. In the high angle annular dark field scanning TEM (HAADF-STEM) image (Figure 5), there are brighter particles embedded inside the relatively dark background. HAADF-STEM images are sensitive to atomic numbers, and brighter contrast indicates heavier elements inside these particles. Sizes of most of the particles are less than 10 nm. From EDX analysis, the brighter grains are Co and/or Sm enriched phases. Although the diffraction pattern does not reveal any Sm−Co phases, we do observe a Sm−Co phase containing significantly more Sm than the Sm2Co17 phase.

Figure 6. TEM BF image of the annealed and washed particles (a), and HRTEM images of nonfaulted (b) and faulted (rectangular) particles (c).

shapes; some of them are rectangular. The uniform contrast inside each particle show that most of the particles are single crystalline. For the rectangular particles, the stripes in the particles indicate that there are stacking faults. It is interesting to see that both types of particles are core−shell structured. The HRTEM images show that the shells of both particles are amorphous (Figure 6b and c). EDX analysis shows that the rectangular particles are Sm enriched with a composition close 10293

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to the ones shown in Figure 5 in the as-milled powders, while the stacking-fault-free ones are Sm2Co17 particles. The Sm rich particles are usually elongated (Figure 7a) with a composition

Figure 9. (a) TEM BF image showing the particles after annealing before washing; (b) HRTEM image from the marked region in (a) showing the core−shell structure. Figure 7. HAADF-STEM images showing the washed particles; arrows in the images label the elongated particles.

The corresponding high resolution image from one particle in Figure 9a shows that the annealed particle is core−shell structured. The shell is also amorphous with the same thickness and composition as the washed ones. This means that the core−shell structure is not caused by the washing process. From microstructural investigation of the as-milled and annealed and washed samples, the formation of the nanoparticles can be described as growth of the Sm and Co enriched particles in the as-milled samples which are much smaller than the final nanoparticles. As reported in our previous work, by adjusting the Ca/O ratio, we can control the size of the nanoparticles.19 Basically, smaller particle sizes were reached by decreasing the concentrations of Sm and Co, i.e. insufficient supply of Sm or Co can effectively decrease the particle size. Side effect of such a measure is that if the Sm/Co ratio is not 2:17 locally, it would be difficult to form Sm2Co17 nanoparticles. Considering that there are several phases in the binary Sm−Co system, different phases may form. The high density of stacking fault in some of the particles indicates the possible composition difference inside one single particle, and the particles are composed of segments of different phases. High temperature annealing (925 °C) cannot remove these stacking faults due to the high energy barrier. If the composition is close to 2:17, Sm2Co17 nanoparticles can still be formed and redundant Sm or Co will be excluded out of the particle and form a shell. This should be one of the origins of such shells. The fact that it is always Sm enriched in the shell is because the overall composition of the mixture is Sm enriched. Presence of such shells on the surface of the Sm2Co17 nanoparticles is detrimental for nanocomposite applications. These shells may decrease the coupling between the hard and soft phases. However, the high oxygen content in the shell may increase resistivity of the shells, and these nanoparticles can be used for fabrication of high resistance hard magnetic materials. It is also possible, by carefully tuning the composition of the mixture, to decrease the inhomogeneity in compostion between the particles.

of Sm59Co41 calculated from EDX analysis. In particular, compared with Sm2Co17 particles, even with large size and higher Sm concentrate, the HAADF-STEM image (Figure 7b) shows darker contrast, which means it is thinner in the direction parallel to the beam direction. Both the elongated shape and the darker contrast reveal anisotropic growth of such particles. Therefore the ball milling produces at least two kinds of particles: Sm enriched and Sm2Co17 particles. At higher magnification, the core−shell structure of the particles is also discernible in HAADF-STEM images (Figure 8). By putting the

Figure 8. (a) HAADF-STEM image of a rectangular particle showing core−shell structure; (b) HAADF-STEM image showing the interface between the core and shell from the area marked in (a); (c) EDX spectra of the core and the shell in (b).

electron probe at different positions on the particles the composition of both the shell and core can be acquired through EDX analysis. The EDX spectra show that compared with the cores, in both particles in the shells, there is much more oxygen. Although the Co signal might come from the core, considering the probe size we used (0.2 nm), there should be Co in the shells. The core−shell structured particles are found in the sample even before washing. Figure 9a shows the sample after annealing but before washing; the dark particles are the nanoparticles, while the bright contrast is from Ca and CaO.

4. CONCLUSION Sm2Co17 nanoparticles were synthesized from Sm2O3, Co, Ca, and CaO powders by using mechanochemical method. The average size of the nanoparticles was 103.7 nm. The particles showed high coercivity of 20 kOe. Microstructure analysis showed nanoparticles with plenty of stacking fault and different composition. Shells on the surface of the Sm2Co17 nanoparticles were also observed. STEM-EDX analysis showed the shells are Sm enriched and also have high oxygen 10294

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(15) Chen, C. H.; Kodat, S.; Walmer, M. H.; Cheng, S. F.; Willard, M. A.; Harris, V. G. The Effects of Grain Size and Morphology on the Coercivity of Sm2(Co1‑xFex)17 Based Powders and Spin Cast Ribbons. J. Appl. Phys. 2003, 93, 7966−7968. (16) Zheng, L. Y.; Cui, B. Z.; Li, W. F.; Hadjipanayis, G. C. Separated Sm−Co Hard Nanoparticles by an Optimization of Mechanochemical Processes. J. Appl. Phys. 2012, 111, 07B536. (17) Ding, J.; McCormick, P. G.; Street, R. Structure and Magnetic Properties of Mechanically Alloyed SmxCo1−x. J. Alloys Comp. 1993, 191, 197−201. (18) Wecker, H.; Katter, M.; Schultz, L. Mechanically Alloyed Sm− Co Materials. J. Appl. Phys. 1991, 69, 6058−6060. (19) Gabay, A. M.; Li, W. F.; Hadjipanayis, G. C. IEEE Trans. Magn. 2013, accepted.

concentration. The local inhomogeneity in composition in the as-milled powders should be the origin of these defects. By finely tuning the composition of the original powders and processing routine, it is possible to further improve the homogeneity and the magnetic performance of the nanoparticles.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 302-831-3515. Fax: 302-831-1637. E-mail: wfl[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the U.S. Department of Energy, Advanced Research Project Agency-Energy (DOE ARPA-E).

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REFERENCES

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