稀有金属(英文版) 2019,38(04),306-311

Chemical synthesis of SmCo₅/Co magnetic nanocomposites

Run-Bo Lu Zhen-Hui Ma Tian-Li Zhang Cheng-Bao Jiang

School of Materials Science and Engineering, Beihang University

作者简介：*Tian-Li Zhang,e-mail:tlzhang@buaa.edu.cn;

收稿日期：6 September 2015

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

Chemical synthesis of SmCo₅/Co magnetic nanocomposites

Run-Bo Lu Zhen-Hui Ma Tian-Li Zhang Cheng-Bao Jiang

School of Materials Science and Engineering, Beihang University

Abstract：

A three-step chemical synthesis of SmCo₅/Co nanocomposites was developed. Firstly, the Co-Sm(OH)₃-Ca(OH)₂ precursors were prepared by co-precipitation.Secondly, SmCo₅ particles were obtained by reductive annealing of the precursors. At last, the SmCo₅/Co nanocomposites were achieved by chemical deposition based on SmCo₅ particles. The SmCo₅/Co nanocomposites contain hard magnetic phase of SmCo₅ with about 100 nm in size and soft magnetic phase of Co with about 8 nm in size,exhibiting independent two-phase structure without alloying. Compared to that of single-phase SmCo₅ particles, the saturation magnetization of SmCo₅/Co nanocomposites is increased by 27.5%. The synthesis provides a new route to fabricate SmCo-based nanocomposites.

Keyword：

Magnetic nanocomposites; SmCo₅/Co; Three-step chemical synthesis;

Received： 6 September 2015

1 Introduction

Permanent magnetic materials have promising applications in aerospace,new energy and biomedicine [ 1, 2, 3, 4, 5, 6] .SmCo₅permanent magnets,with extraordinary magnetocrystalline anisotropy (K_u=2.0×10⁴ kJ·m^-3) and high Curie temperature (T_c=1020 K),exhibit unique advantages [ 7, 8, 9, 10, 11] .However,the low remanent magnetization of SmCo₅permanent magnets,leading to relatively low magnetic energy product,limits their development [ 1, 3, 7] .The discovery of exchange-coupled nanocomposites has brought a novel approach to breakthrough this bottleneck [ 12, 13, 14, 15, 16] .In nanocomposites,hard magnetic phase (SmCo,etc.) offers large coercivity,and soft magnetic phase (Fe or Co) provides high saturation magnetization [ 9, 17, 18, 19] .

In theory,in order to achieve effective magnetic exchange coupling,grain size of soft magnetic phase should not exceed twice the domain wall width of hard magnetic phase [ 20] .In SmCo-based nanocomposites,soft magnetic phase (Fe or Co) should be smaller than 10 nm and distributed uniformly around the hard magnetic phase.To date,the main research method of SmCo-based nanocomposites has focused on ball milling [ 21, 22, 23] .But there are some obvious shortcomings of the method:(1)large size of particles,(2) uneven distribution of soft magnetic phase and (3) serious alloying between soft and hard magnetic phases,which lead to poor exchange coupling.Chemical synthesis provides a new way [ 16, 24, 25] since chemically synthetic nanocomposites exhibit small particle size,uniform distribution and other advantages.

Conventionally,a two-step chemic al method was adopted to prepare SmCo/Fe(Co) nanocomposites.Namely,Sm₂O₃,CoO and Fe₂O₃ precursors were fabricated first,followed by reductive annealing at high temperature to synthesis SmCo/Fe [ 25, 26] .However,the high temperature (more than800°C) during the reductive annealing results in obvious alloying (Fe atoms into SmCo lattices),which limits the development of the nanocomposites.Recently,reductive annealing of SmCo(CN)6/GO/CO(acac)2 precursors was proposed to obtain SmCo/GN/Co nanocomposites with core/shell structure [ 27] .However,alloying is still existence and GO may introduce impurity.Therefore,the fabrication of SmCo-based nanocomposites with independent two phases becomes significant.

Herein,a three-step chemical synthesis was reported to prepare two-phase SmCo₅/Co nanocomposites.Firstly,CoSm(OH)3-Ca(OH)2 precursors were prepared by co-precipitation.Secondly,SmCo₅ particles were fabricated by reductive annealing of the precursors.Thirdly,SmCo₅/Co nanocomposites were obtained by precipitating Co nanoparticles around SmCo_s particles using chemical deposition.The novel approach yields independent two phases,as well as nanosized Co with smaller than 10 nm in size,which is appropriate to magnetic exchange coupling.

2 Experimental

2.1 Synthesis of precursors

The synthesis of precursors was optimized from the ultrasonic co-precipitation method,which was previously reported [ 28] .Cobalt (Co) nanoparticles were used to replace cobalt chloride hydrate (CoCl₂·6H₂O) as cobalt source.Firstly,samarium chloride hydrate (SmCl₃-6H₂0,99.0%),Co nanoparticles (30 nm,99.5%) and calcium chloride (CaCI₂,99.0%) were dissolved in 100 ml deionized water with a molar ratio of 1.0:3.5:10.0.The concentration of Sm³⁺was controlled at 0.03 mol·L^-1.Secondly,the solution was removed to a 250-ml three-neck flask with mechanical stirring and ultrasonic vibration for 10 min,in order to get evenly dispersed solution.Under the conditions of mechanical stirring and ultrasonic vibration,adequate amount of 3.5 mol-L^-1 sodium hydroxide (NaOH) solution was rapidly injected into the flask,to adjust the pH value to10.Keeping mechanical stirring and ultrasonic vibration for1 h,black solution was obtained.After a centrifugation at8000 r-min^-1 for 3 min,black precipitation was obtained.The precipitation was further washed with deionized water and ethanol three times,respectively,to wash away sodium chloride (NaCl) and extra NaOH.Then,the black precipitation was dried at room temperature for 12 h.Black powder was collected for next reductive annealing.

2.2 Synthesis of SmCo5 particles

The black powder (1.0 g) obtained previously was transferred to an agate mortar and ground into fine powder.Then,2 g calcium oxide (CaO),3 g potassium chloride(KCl) and 5 g calcium (Ca) were mixed into the mortar,respectively.The uniformly ground mixture was transferred to a tungsten crucible and placed in a tube furnace.The crucible was heated to 850℃at a heat rate of5℃·min^-1 for 1.5 h and cooled to room temperature under argon atmosphere.After that,the reaction mixture was washed by deionized water to dissolve CaO,KCl and unreacted Ca.Adequate dilute hydrochloric acid solution was added dropwise into the beaker to regulate the pH value to 7.The product was washed with deionized water and ethanol three times,respectively,and grey powder was obtained.The grey powder was dispersed in ethanol for further usage and characterization.

2.3 Synthesis of SmCo5/Co nanocomposites

Grey powder prepared previously and cobalt acetylacetonate(Co(acac)2,99.0%) with a molar ratio of 1.0:0.1 were dispersed in 100 ml tetraethylene glycol (as solvent and reduction).The concentration of Co(acac)2 was controlled at0.5 mmol-L^-1.0.5 ml oleylamine (99.0%) and 1.0 ml oleic acid (90.0%) were added as surfactant with mechanical stirring and ultrasonic vibration for 10 min to obtain uniform solution.The solution was transferred to a three-neck flask.Under argon atmosphere,the flask was placed in oil bath with mechanical stirring.The temperature of oil bath rose to120°C for 30 min to remove H₂O and O₂ dissolved in the solution and then rose to 220℃for 2 h.After reaction,argon atmosphere was removed until the temperature of solution was close to room.The solution was centrifuged and washed several times,and black powder was obtained.The black powder was dispersed in ethanol.

2.4 Characterization

The crystallographic structure was identified by X-ray diffractometer (XRD,D/Max 2200PC) with Cu Kαradiation (λ-0.15406 nm),while the scanning speed was 6(°)·min^-1.The microstructure and morphology of the particles were investigated using transmission electron microscopy (TEM,JEM-2100F).The magnetic properties were measured at room temperature by physical property measurement system (PPMS) under a maximum applied field of 9 T.The sample for PPMS was made of powder placed in a specific capsule and fixed by glue.

3 Results and discussion

3.1 Fabrication of precursors

Figure 1 shows XRD pattern and TEM image of precursors.From Fig.la,it can be seen that the main phases of precursors are Co (JCPDS No.15-0806),Ca(OH)2 (JCPDS No.44-1481) and CaCO₃ (JCPDS No.05-0586).Besides,there is an obvious amorphous diffraction peak from 25°to30°.According to Scherrer formula,it can be calculated that the average grain size of Co is about 28 nm by using the diffraction peak of Co (111),which is consistent with the nominal size (30 nm).In the light of the preliminary study [ 28] ,the amorphous diffraction peak should be corresponding to amorphous Sm(OH)3.The presence of Ca(OH)2 and CaCO₃ can be explained by the following two chemical reaction equations:

Fig.1 XRD pattern a and TEM image b of precursors

In Fig.1b,the black spherical particles should be Co nanoparticles.The sizes of black particles are 20-40 nm,which is in agreement with the XRD result.At the same time,the Co nanoparticles are uniformly distributed in grey phases,which should be the mixture of Sm(OH)3 and Ca(OH)2/CaCO₃.This distribution is propitious for the synthesis of SmCo₅ particles.

3.2 Fabrication of SmCo5 particles

The precursors were further reduced by calcium at 850℃to prepare SmCo₅ particles.The XRD pattern of the product is shown in Fig.2.Obviously,the diffraction peaks of the result are indexed as hexagonal SmCo₅ (JCPDS No.65-5599),indicating that the precursors are converted into SmCo₅ particles by calcium thermal reduction.According to Scherrer formula,the average grain size of SmCo₅ is113 nm calculated by the (111) diffraction peak of SmCo₅.

Fig.2 XRD pattern of SmCo₅ particles reduced from precursors

Figure 3 shows TEM images of SmCo₅ particles reduced from precursors.It can be seen that the grain size is about 100 nm from Fig.3a.According to Fig.3b,partial particles aggregate due to the high temperature during reductive annealing.One of the particles (marked by red circle in Fig.3b) was further investigated by selected area electron diffraction (SAED),as shown in Fig.3c.The ordered hexagonal symmetric diffraction spots indicate that the particle is a single crystal.Furthermore,the diffraction spots were measured that they are corresponding to hexagonal SmCo₅ (JCPDS No.65-5599),which is in agreement with XRD result.

Magnetic properties of SmCo5 particles prepared by calcium thermal reduction were measured by PPMS with a maximum applied magnetic field of 9 T.The room temperature magnetic hysteresis loop is shown in Fig.4.The loop shows that the SmCo₅ particles are ferromagnetic with room temperature coercivity of 2.49 T,saturation magnetization of 43.65 A·m²·kg^-1 and remanent magnetization of32.09 A·m²·kg-1.

In short,pure SmCo₅ particles were obtained by calcium thermal reduction from Co-Sm(OH)3-Ca(OH)2 precursors.The particles exhibit a large size of about 100 nm and a large coercivity of 2.49 T,which lays a good foundation for the preparation of SmCo₅/Co nanocomposites.

3.3 Fabrication of SmCo5/Co0.1 nanocomposites

SmCo₅/Co nanocomposites were fabricated by chemical deposition based on previously prepared SmCo₅ particles.Figure 5 shows XRD pattern of SmCo₅/Co_0.1 nanocomposites.There is a small diffraction peak around 44.2⁰,which matches well with the (111) plane in cubic Co(JPCDS No.15-0806).The poor crystallization and low content of Co phase in SmCo₅/Co nanocomposites lead to a difficulty to detect.So the intensity of Co diffraction peak is very low and only the strongest diffraction peak(corresponding to (111) plane) can be verificated.According to Scherrer formula,the average grain size of Co nanoparticles is calculated to be 8.1 nm.The result suggests that the independent two-phase SmCo₅/Co nanocomposites are achieved without alloying in this threestep synthesis,exhibiting obvious advantages compared to conventional methods.

Fig.3 TEM images of SmCo₅ particles reduced from precursors:a low-magnification image of SmCo₅ particles,enlarged image of marked area in a,and c SAED pattern of red section in b

Fig.4 Room temperature magnetic hysteresis loop of SmCo₅particles (M magnetization,H magnetic field strength)

Fig.5 XRD pattern of SmCo_s/Co_0.1 nanocomposites prepared by chemical deposition

Figure 6 shows TEM images of SmCo₅/Co_0.1 nanocomposites prepared by chemical deposition.It can be seen that small nanoparticles (5-10 nm) are uniformly around the big particles,as shown in Fig.6a,b.According to the pattern(Fig.6c),there are obvious two phases because the diffraction rings are corresponding to hexagonal SmCo₅ and cubic Co,agreeing with XRD result.Furthermore,in the high-resolution transmission electron microscopy (HRTEM) images(Fig.6d),the large particle has lattice fringes distances at0.293 and 0.250 nm,respectively,corresponding to (101) and(110) planes in hexagonal SmCo₅;as well as small particles have lattice fringes distance at 0.205 nm,corresponding to(111) plane in cubic Co.It further confirms that Co nanoparticles smaller than 10 nm in size were prepared successfully by chemical deposition and evenly distributed around the hard magnetic phase (SmCo₅),which indicates the superiority of the three-step chemical synthesis reported in this study.

As reported in previous studies,several techniques were employed to characterize exchange coupling between the components in the nanocomposites [ 16] .Here,magnetization curves were chosen as characterization method.The room temperature magnetic hysteresis loop of SmCo₅/Co_0.1 nanocomposites is exhibited in Fig.7.The SmCo₅/Co_0.1 nanocomposites have a high saturation magnetization of 56.72 A·m²·kg^-1 and relatively low coercivity of 1.36 T.The saturation magnetization of SmCo₅ and Co phase is related to their purity and elemental composition.In this work,the purity is limited by raw materials,while a low atomic ratio of Co/Sm as 3.5 (typical value is 5.0) was employed to obtain large coercivity.Compared to that of the single-phase SmCo₅,the saturation magnetization of SmCo₅/Co_0.1nanocomposites increases by 27.5%,which is in agreement with previous reports [ 21, 22, 23, 24, 25, 26, 27, 28] .For an ideal exchange coupling,the magnetic reversal of the hard and soft phases must occur simultaneously in nanocomposites.In fact,magnetic reversal begins with the soft magnetic phase,and then domain of hard magnetic phase is reversed with it.This process results in a reduction in coercivity.Meanwhile,the loop is smooth and has no obvious shoulder,which demonstrates that the exchange coupling is realized between hard and soft magnetic phases [ 16] .

Fig.6 TEM images of SmCo₅/Co_0.1 nanocomposites prepared by chemical deposition:a low-magnification image,b high-magnification image of marked area in a,c SAED pattern of a,d HRTEM images of SmCo₅ particle and Co particles

Fig.7 Room temperature magnetic hysteresis loop of SmCo₅/Co_0.1nanocomposites prepared by chemical deposition

4 Conclusion

In summary,a three-step chemical synthesis of SmCo₅/Co nanocomposites was developed.The Co-Sm(OH)3-Ca(OH)2 precursors were prepared first by co-precipitation.Then,the SmCo₅ particles with size of about 100 nm and coercivity of 2.49 T were fabricated by calcium thermal reduction from the precursors.Finally,the SmCo₅/Co nanocomposites with Co smaller than 10 nm in size were obtained through chemical deposition process.This synthesis avoids the alloying problem of the traditional twostep chemical method effectively and realizes magnetic exchange coupling,providing a new way to fabricate permanent magnetic nanocomposites.

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