简介概要

Synthesis of SmCo₅ nanoparticles with small size and high performance by hydrogenation technique

来源期刊：Rare Metals2018年第12期

论文作者：Zhen-Hui Ma Tian-Li Zhang Hui Wang Cheng-Bao Jiang

文章页码：1021 - 1026

摘要：SmCo₅ nanoparticles（NPs） have promising applications in high-density magnetic storage and magnetic nanocomposites. In this work, A novel method to yield SmCo₅ particles with small size and high coercivity was reported. Firstly, Sm₂ O₃-Co NPs with size of 6-15 nm were fabricated by a solvothermal route. Then the agglomerated SmCo₅ particles were obtained by thermal reduction of the precursor, which show high coercivity of2.0 T at room temperature. At last, the as-synthesized SmCo₅ particles were further hydrogenated under high hydrogen pressure of 4 MPa at room temperature, where hydrogen absorption process could form small-sized SmCo₅ H_x particles due to their lattice expansion and hydrogen desorption process could convert SmCo₅ H_x NPs into SmCo₅ NPs. The prepared SmCo₅ NPs after hydrogenation, showing well distribution, have a small size of5-20 nm and room temperature coercivity of 1.22 T.

详情信息展示

稀有金属(英文版) 2018,37(12),1021-1026

Synthesis of SmCo₅ nanoparticles with small size and high performance by hydrogenation technique

Zhen-Hui Ma Tian-Li Zhang Hui Wang Cheng-Bao Jiang

School of Materials Science and Engineering, Beihang University

作者简介：*Hui Wang,e-mail:huiwang@buaa.edu.cn;

收稿日期：11 April 2016

基金：financially supported by the National Natural Science Foundations of China (Nos. 51471016 and 51520105002);the Key Natural Science Foundation of Beijing (No. 2151002);

Synthesis of SmCo₅ nanoparticles with small size and high performance by hydrogenation technique

Zhen-Hui Ma Tian-Li Zhang Hui Wang Cheng-Bao Jiang

School of Materials Science and Engineering, Beihang University

Abstract：

SmCo₅ nanoparticles(NPs) have promising applications in high-density magnetic storage and magnetic nanocomposites. In this work, A novel method to yield SmCo₅ particles with small size and high coercivity was reported. Firstly, Sm₂ O₃-Co NPs with size of 6-15 nm were fabricated by a solvothermal route. Then the agglomerated SmCo₅ particles were obtained by thermal reduction of the precursor, which show high coercivity of2.0 T at room temperature. At last, the as-synthesized SmCo₅ particles were further hydrogenated under high hydrogen pressure of 4 MPa at room temperature, where hydrogen absorption process could form small-sized SmCo₅ H_x particles due to their lattice expansion and hydrogen desorption process could convert SmCo₅ H_x NPs into SmCo₅ NPs. The prepared SmCo₅ NPs after hydrogenation, showing well distribution, have a small size of5-20 nm and room temperature coercivity of 1.22 T.

Keyword：

SmCo₅; Nanoparticles; Hydrogenation; Coercivity;

Received： 11 April 2016

1 Introduction

Nanoscale magnetic particles have attracted much attention for advanced applications such as high-density magnetic storage,hard/soft magnetic nanocomposites,sensors and drug carriers in biomedical technology [ 1, 2, 3, 4, 5, 6, 7, 8, 9] .Thereinto,SmCo₅nanoparticles (NPs) are one of the most essential materials due to their high magnetocrystalline anisotropy constant(K_u=20 J·cm^-3) and high Curie temperature(T_C=1020 K) [ 10, 11, 12] .On the one hand,high K_u can overcome the issue of thermal fluctuation and keep SmCo₅ NPs thermally stable even in a size range of 2.2-2.7 nm at room temperature,which is one critical requirement for magnetic recording to reach a high areal density based on traditional fabrication principles [ 13, 14, 15] .On the other hand,the high T_Cmakes it superior to FePt (T_C=750 K) and Nd₂Fe₁₄B(T_C=585 K),for high-temperature applications [ 16, 17, 18] .

Conventionally,SmCo₅ NPs were prepared by direct liquid-phase synthesis in organic polyol [ 19, 20, 21, 22, 23] .However,the direct co-reduction from metal salt is difficult since the standard electrode potential of Sm³⁺(E⁰=-2.301 eV) is far lower than that of Co²⁺(E⁰=-0.28 eV).Furthermore,the result exhibits low room temperature coercivity in the range of 0-0.15 T.Recently,researchers exploited a novel method combining solution-phase synthesis with calciothermic reduction [ 24, 25, 26, 27, 28, 29] .Nevertheless,the synthetic particles showed obvious agglomeration and sintering for a high-temperature annealing.An improved method of using CaO-coated Sm-Co oxide NPs followed by Careduction at 960℃yields 6 nm SmCo₅ NPs [ 30] .But these magnetic particles only exhibited low coercivity of0.72 T.Therefore,the fabrication of SmCo₅ NPs with high performance is still a challenging task.

In this work,a novel method to yield SmCo₅ particles with small size and high coercivity was reported.Firstly,sintered SmCo₅ particles were chemically synthesized by calciothermic reduction process.And then monodispersed SmCo₅ particles with size of 5-20 nm can be achieved using hydrogen absorption and desorption technique.The synthetic SmCo₅ NPs exhibit a high coercivity of 1.22 T.This route offers a novel strategy for the preparation of high-performance rare-earth-based magnetic NPs.

2 Experimental

2.1 Synthesis of precursor

0.4837 g (1 mmol) Sm(acac)3·xH₂O (samarium acetylacetonate) and 1.4255 g (4 mmol) Co(acac)2 (cobalt(Ⅱ)acetylacetonate) were dried at 110℃for 2 h.Then,dried Sm(acac)3 and Co(acac)2 were dispersed into 100 ml triglycol by mechanical stirring.Following that,the mixture was heated to 120℃for 1 h to move moisture.Then,the mixture solution was transferred into Teflon vessel for solvothermal reaction in drying oven (DGG-9036A) at280℃for 3 h.Finally,the resultant was centrifuged at12,000 r·min^-1 for 5 min to obtain brown powder.The powder was further washed with ethanol 5 times and dried at room temperature.

2.2 Synthesis of SmCo5 particles

The prepared precursor was mixed with 2.0 g KCl(potassium chloride),2.0 g CaO (calcium oxide) and 4.0 g Ca (calcium).The addition of CaO can prevent particles from sintering to some extent [ 27] .The mixture was transferred in a tungsten crucible with cap.The tungsten crucible was then moved to a tube furnace (SK-G06143).After that,the tube was degassed to remove air and moisture.Subsequently,the tube was flushed with Ar and heated to 860℃at a rate of 8℃·min^-1.The reaction was held for 90 min before cooling down to room temperature.The resultant was washed with degassed distilled water and0.5%hydrochloric acid to dissolve CaO,KCl and extra Ca.A black powder was obtained by centrifuging at12,000 r·min^-1 for 3 min.The powder was further washed with degassed distilled water and ethanol and dried under vacuum for further usage.

2.3 Hydrogenation of SmCo5 particles

200 mg synthesized SmCo₅ particles were placed in furnace tube.Then the tube was vacuumized quickly.And H₂was allowed to quickly fill into the furnace tube.A hydrogen pressure of 4 MPa was kept for 30 min at room temperature to sufficiently hydrogenate SmCo₅ particles.After the reaction ends,the hydrogen pressure was reduced to standard atmospheric pressure.The above process was repeated three times.And the SmCo₅ particles were dispersed in ethanol using ultrasonic concussion.

2.4 Characterization

The crystallographic structure was identified by X-ray diffractometer (XRD,D/MAX 2500 PC) with Cu Kαradiation (λ=0.15418 nm) and scanning speed of 6 (°)·min^-1.The microstructure and morphology of the particles were investigated using transmission electron microscopy (TEM,JEM-2100F).For TEM observation,the samples were dispersed in hexane with 1-2 drops of ethanol in it.The drops of the well-dispersed NPs were placed over the carbon-coated microscopic copper grids (74μm) and were subsequently dried.The magnetic properties were measured at room temperature using a physical property measurement system(PPMS-9) under a maximum applied field of 9 T.

3 Results and discussion

Figure 1 is schematic illustration of the synthetic strategy of SmCo₅ NPs.First,the Sm₂O₃-Co precursor was synthesized by a solvothermal route.And then agglomerated SmCo₅ particles were generated by reductive annealing of that precursor.Finally,the size of SmCo₅ particles was further reduced by hydrogen absorption and desorption at room temperature.As known,under high hydrogen pressure of 4 MPa,the SmCo₅ can be hydrogenated to form hydride as the reversible reaction:

In SmCo₅H_x,the hydrogen atoms enter into the lattice of SmCo₅,resulting in a lattice expansion.In the meanwhile,SmCo₅H_x is very brittle.The fragility and lattice expansion will split SmCo₅H_x from the surface of SmCo₅ particles,which yields small-sized SmCo₅H_x NPs.When hydrogen pressure decreases to standard atmospheric pressure,SmCo₅H_x NPs will release H₂ to form SmCo₅ NPs.Therefore,the hydrogen absorption and desorption process will refine the size of SmCo₅ particles.

Figure 2a shows XRD pattern of the precursor prepared by solvothermal method.There is a broad diffraction peak at～44.2°,indexed to (111) planes in cubic Co,demonstrating that Co particles are reduced by polyalcohol during solvothermal process.And an obvious amorphous peak is observed at 20°-30°,which is attributed to uncrystallized Sm₂O₃ according to the previous report [ 24] .The precursor was further characterized by TEM to observe the morphology,which is outlined in Fig.2b.It can be seen that the monodispersed precursor exhibits well distribution and has a size of 6-15 nm,indicating that solvothermal process can yield small-sized Sm₂O₃-Co particles.

The prepared Sm₂O₃-Co NPs were further reduced by Ca,and the results are present in Fig.3.XRD pattern of particles by reductive annealing of Sm₂O₃-Co NPs at860℃is shown in Fig.3a.The diffraction peaks match well with the standard SmCo₅ pattern (JPCDS No.65-3473),indicating that Sm₂O3-Co NPs converts into hcp-structured SmCo₅ by high-temperature reduction.Figure 3b is TEM image of SmCo₅ particles,which demonstrates that SmCo₅ particles have a grain size of10-20 nm.But they show obvious agglomeration,which is attributed to the high-temperature annealing.The structure of SmCo₅ was further characterized by high-resolution TEM (HRTEM),as shown in Fig.3c,in which the annealed particles have lattice fringe distances at 0.211 and0.217 nm,corresponding to (111) and (200) planes in the SmCo₅,respectively.Additionally,there is serious sintering among particles.Figure 3d presents the room temperature magnetic hysteresis loop of as-synthesized SmCo₅particles.The loop shows that the products are ferromagnetic.They have a room temperature coercivity as high as2.0 T and a magnetization of 0.0531 A·m²·g^-1 at a maximum applied magnetic field of 9.0 T.Although the assynthesized SmCo₅ NPs exhibit obvious sintering,the small-sized particles show sound single-crystal structure,which may contribute to high coercivity.

Fig.1 Schematic illustration of synthetic strategy of SmCo₅ NPs

Fig.2 XRD pattern a and TEM image b of as-synthesized Sm₂O₃-Co NPs

Fig.3 XRD pattern a,TEM image b,HRTEM image c and room temperature magnetic hysteresis loop d of sintered SmCo₅ particles by reductive annealing of Sm₂O₃-Co precursor

Fig.4 Dependence of hydrogen absorption content of SmCo₅particles on time

Fig.5 XRD pattern of SmCo₅ NPs after hydrogenation

To obtain monodispersed SmCo₅ NPs without sintering,the as-synthesized SmCo₅ particles were further hydrogenated at room temperature.Figure 4 shows hydrogen absorption content of SmCo₅ particles with different time.The hydrogenation behavior can be pided into two steps.At the beginning of hydrogenation (0-10 min),hydrogen absorption content increases with near linear relationship,which may be attributed to the positive reaction in Eq.(1).At second step (over 10 min),hydrogenation reaction approaches to equilibrium.Thus,hydrogen absorption content tends to saturation.

Figure 5 shows XRD pattern of SmCo₅ NPs after hydrogen absorption and desorption.It is found that the diffraction peaks of particles after hydrogenation are fitted well to the standard SmCo₅ pattern.It suggests that the phase of particles has no change after hydrogenation at room temperature.However,the diffraction peaks of SmCo₅ NPs broaden apparently,which may imply that the hydrogenation process can reduce the grain size of SmCo₅particles.

The corresponding TEM images are shown in Fig.6.In Fig.6a,the particles have small size of 5-20 nm and exhibit well distribution.Compared with agglomerated SmCo₅ particles before hydrogenation (Fig.3b),the SmCo₅ NPs after hydrogenation disperse well without agglomeration,indicating the superiority of this method.And the irregular flakes can be observed in Fig.6b.It can be seen that the flakes possess rough surface,which may be caused by the exfoliation due to hydrogenation,in accordance with the previous report [ 31] .Figure 6c is HRTEMimage of single SmCo₅ nanoflake.The nanoparticle has a lattice at 0.292 nm,which corresponds to the (101) plane in SmCo_5.Furthermore,the nanoflakes display singlecrystal structure,different from polycrystal before hydrogenation.

The room temperature magnetic hysteresis loop of SmCo₅ NPs fabricated by hydrogenation is displayed in Fig.7.The coercivity of 1.22 T is achieved in SmCo₅ NPs,which is lower than that of sintered SmCo₅ particles.This phenomenon can be explained by the fact that the increased amount of defects in the smaller particles lowers the magnetocrystalline anisotropy [ 32] .And a magnetization of 0.0531 A·m²·g^-1 is obtained for SmCo₅ NPs after hydrogenation,which is the same as that for SmCo₅ particles before hydrogenation.The coercivity mechanisms of SmCo₅ NPs prepared are different from those of sintered SmCo₅ magnets [ 33] .For sintered SmCo₅ magnets,they exhibit polycrystal and multidomain structure.The coercivity of magnets is derived from pinning of domain wall and grain boundary.For prepared SmCo₅ NPs,they exhibit single-domain structure without domain wall due to small size.Meanwhile,there also is no grain boundary in monodispersed SmCo₅ NPs.Therefore,the coercivity is derived from magnetization of NPs,which occurs by rotation of magnetization upon application of a magnetic field.

Fig.6 TEM image a,amplified TEM image b and HRTEM image c of SmCo₅ NPs after hydrogenation

Fig.7 Room temperature magnetic hysteresis loop of SmCo₅ NPs after hydrogenation

4 Conclusion

In summary,the SmCo₅ NPs were synthesized using reductive annealing and hydrogenation process.In the strategy,Sm₂O_3-Co NPs were firstly prepared by solvothermal route and the following thermal reduction generated sintered SmCo₅ particles with coercivity of2.0 T.And then the sintered particles were further refined by hydrogenation process at room temperature,where small-sized SmCo₅H_x particles were first formed during hydrogen absorption process due to their lattice expansion and convert into monodisperse SmCo₅ NPs during hydrogen desorption process.The prepared SmCo₅ NPs have small size of 5-20 nm and exhibit well distribution.They possess a room temperature coercivity of 1.22 T,which is lower than that of sintered SmCo₅ particles (2.0 T) before hydrogenation.The phenomenon can be explained by the fact that the increased defects in the smaller particles lower the magnetocrystalline anisotropy.The approach achieves small size and high performance simultaneously,which can be extended to the synthesis of other rare-earth permanent magnetic particles with highly ferromagnetic performance.