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材料研究学报, 2022, 36(5): 321-331 DOI: 10.11901/1005.3093.2021.279

综述

高性能Sm2Fe17N x 粉体制备的研究进展

何颖1, 李超群1, 陈小立1, 龙芝梅1, 赖嘉琪1, 邵斌,1,2, 马毅龙,1,2, 陈登明1,2, 董季玲1,2

1.重庆科技学院冶金与材料工程学院 重庆 401331

2.纳微复合材料与器件重庆市重点实验室 重庆 401331

Recent Development for Preparation Processes of Sm2Fe17N x Powders with High Magnetic Properties

HE Ying1, LI Chaoqun1, CHEN Xiaoli1, LONG Zhimei1, LAI Jiaqi1, SHAO Bin,1,2, MA Yilong,1,2, CHEN Dengming1,2, DONG Jiling1,2

1.School of Metallurgy and Material Engineering, Chongqing University of Science and Technology, Chongqing 401331, China

2.Chongqing Key Laboratory of Nano-Micro Composites and Devices, Chongqing 401331, China

通讯作者: 邵斌,副研究员,shaobin19811107@163.com,研究方向为磁性材料制备马毅龙,教授,yilongma@163.com,研究方向为磁性材料制备

收稿日期: 2021-04-29   修回日期: 2021-12-09  

基金资助: 重庆科技学院研究生创新计划(YKJCX2020215)
重庆市自然科学基金(cstc2019jcyj-msxmX0162)
重庆市教委科学技术研究重大项目(KJZD-M201801501)
重庆市高校创新研究团队(CXQT19031)

Corresponding authors: SHAO Bin, Tel: 13648368178, E-mail:shaobin19811107@163.com;MA Yilong, Tel: 13629700021, E-mail:yilongma@163.com

Received: 2021-04-29   Revised: 2021-12-09  

作者简介 About authors

何颖,女,1995年生,硕士

摘要

新能源汽车的高速发展,需要能稳定工作在120℃~200℃温度区间的永磁材料。居里温度为476℃、各向异性场为14.7 T的Sm2Fe17N3,具有优良的本征磁性能,可应用在这个温度区间。为了提高Sm2Fe17N3粉体的磁性能,必须将颗粒的粒径减小到临界单畴尺寸以实现高各向异性场；同时,还要避免颗粒尺寸减小产生的表面氧化,以保证高剩磁和最大磁能积。粉体破碎、机械合金化、甩带、薄带连铸、还原扩散以及表面镀覆等多种制备工艺,可用于制备高性能Sm2Fe17N3。目前,实验室制备的Sm2Fe17N3粉体的矫顽力和最大磁能积已经达到28.1 kOe和43.6 MGOe。本文评述近年来Sm2Fe17N3粉体制备的研究成果,包括对制备机理的系统总结并提出仍待解决的关键问题：Sm2Fe17N3粉体的矫顽力、剩磁等与其颗粒尺寸的量化规律以及与颗粒磁畴结构的关联机制；对NH3/H2混合气体中H2对提高氮化效率的作用机制仍需探索；进一步开发在低氧环境下的颗粒粒径均匀化、控制形貌的二次破碎技术；对于还原扩散法,开发适合规模化应用的新前驱体及其制备方法以及快速去除钙副产物的水洗技术等。

关键词： 评述; 金属材料; Sm2Fe17N x; 永磁; 制备方法

Abstract

The rapid development of new energy vehicles requires permanent magnet materials that can work stably in the temperature range of 120℃~200℃. Sm2Fe17N3 with Curie temperature of 476℃ and anisotropic field of 14.7 T has excellent intrinsic magnetic properties, and can be used in this temperature range. In order to improve the magnetic properties of Sm2Fe17N3 powder, the particle size of which should be reduced to the critical size close to a single domain, so that to gain high anisotropic field; Meanwhile, surface oxidation caused by particle size reduction should be avoided to ensure high remanence magnetism and maximum magnetic energy product. High performance Sm2Fe17N3 can be prepared by powder crushing, mechanical alloying, strip casting, thin strip continuous casting, reduction diffusion and surface plating. At the present, the coercivity and maximum magnetic energy product of Sm2Fe17N3 powder prepared in laboratory have reached 28.1 kOe and 43.6 MGOe respectively. In this paper, the research results on the preparation of Sm2Fe17N3 powders in recent years are reviewed, including preparation methods and the relevant mechanism, and key problems that remain to be solved, namely the relation of the coercivity and remanence of Sm2Fe17N3 powder with the particle size, as well as with the particle magnetic domain structure；the mechanism related with the enhanced effect H2 within the gas mixture NH3/H2 on the nitriding efficiency of the powder still needs to be revealed; further the secondary crushing technique in low oxygen pressures, which can prepare particles with uniform distribution of particle size, while adjust their morphology, remains to be developed; for the present reduction diffusion method, new precursors, and their preparation methods suitable for massive production, and water washing technology for rapid removal of calcium by-products were also needed to develop.

Keywords： review; metallic material; Sm2Fe17N x; magnetic properties; preparation methods

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何颖, 李超群, 陈小立, 龙芝梅, 赖嘉琪, 邵斌, 马毅龙, 陈登明, 董季玲. 高性能Sm2Fe17N x 粉体制备的研究进展[J]. 材料研究学报, 2022, 36(5): 321-331 DOI:10.11901/1005.3093.2021.279

HE Ying, LI Chaoqun, CHEN Xiaoli, LONG Zhimei, LAI Jiaqi, SHAO Bin, MA Yilong, CHEN Dengming, DONG Jiling. Recent Development for Preparation Processes of Sm2Fe17N x Powders with High Magnetic Properties[J]. Chinese Journal of Materials Research, 2022, 36(5): 321-331 DOI:10.11901/1005.3093.2021.279

新能源汽车大多使用Dy含量(质量分数)为5%~10%的NdFeB永磁体[1]。但是,Dy的储量较少价格高昂[2]。因此一些国家积极开发低Dy含量或无Dy的中高温永磁材料[3,4],包括MnBi[5]、MnAl[6]、SmFe12[7]、Sm2Fe17N3[8~11]等。但是目前已经商业化的,只有Sm2Fe17N3。

Sm2Fe17金属间化合物具有Th2Zn17结构,其中原子间距较小的Fe-Fe哑铃对产生负交换耦合,使其居里温度(TC)较低且产生易基面磁化[12]。N原子嵌入Sm2Fe17的9e间隙形成Sm2Fe17N3,增大了Fe-Fe哑铃对间距使其交换作用增强并使Sm的4f壳层电场梯度增大,从而提高了居里温度TC[13,14]、增大了各向异性常数K1[15],使Sm2Fe17中生成易c轴硬磁相[16]。Sm2Fe17N3的TC为476℃,K1为8.6 MJ·m-3,饱和磁化强度Ms为160 emu·g-1,理论最大磁能积(BH)max=1/4μ0Ms2=475 kJ·m-3[16~19]。如表1所示,Sm2Fe17N3具有比Nd2Fe14B优良的内禀磁性能。如图1所示,在150℃~200℃温度区间,Sm2Fe17N3磁体的(BH)max只有理论值的64%,但是比Dy含量为8%的Nd2Fe14B[20]高。虽然Sm2Fe17N3具有优异的内禀磁性能,但是N元素在Sm2Fe17母相中扩散缓慢。为了提高渗氮效率,须减小氮原子在Sm2Fe17颗粒中的扩散距离和增大母相颗粒与氮气的接触面积。Sm2Fe17N3的矫顽力类型属于形核型[21,22],磁化率高,畴壁易移动形成反向磁畴。因此,制备临单畴尺寸0.30 μm的颗粒以消除其中的畴壁,,可提高其Hcj[23~25]。这意味着,无论是提高氮化效率还是提升Hcj,细化颗粒都是有效的途径。

表1   Nd2Fe14B和Sm2Fe17N3的磁性能[16~19]

Table 1  Comparison of the intrinsic magnetic properties of Nd2Fe14B and Sm2Fe17N3[16~19]

QuantityDensity/kg·m-3TC/℃Ms/emu·g-1Ha/TK1/MJ·m-3(BH)max/kJ·m-3
Nd2Fe14B77603121657.64.9515
Sm2Fe17N3768047616014.68.6475

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图1

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图1   Sm2Fe17N3和(NdDy)FeB的(BH)max与工作温度的关系[20]

Fig.1   Relationship between (BH)max of Sm2Fe17N3 and (NdDy)FeB and temperature[20]


目前应用的Sm2Fe17N3仍限于粘结磁体,因为其在450℃以上的温度不可逆热分解成α-Fe和Sm-N[26~29]。随着Sm2Fe17N3颗粒尺寸的降低,在低氧环境下颗粒表面也易生成氧化层[30]。表面的氧化铁与Sm2Fe17N3中的Sm发生氧化还原反应,使Sm2Fe17N3分解和生成α-Fe[31]。

为了开发高性能的Sm2Fe17N3磁粉：一是探索高效、低成本粉末微细化工艺,实现高Ha；二是消除颗粒细化引起的表面氧化以提高剩磁Mr和(BH)max。目前商用Sm2Fe17N x 粉体的Hcj为12~18 kOe,(BH)max低于35 MGOe,仍有很大的提高空间。本文详细介绍制备高性能Sm2Fe17N x 粉体的球磨破碎和机械合金化,以及甩带和薄带连铸、还原扩散法、氮化处理、表面改性等工艺,并提出需要解决的关键问题和展望未来的进展。

1 高性能Sm2Fe17N x 粉体的制备

1.1 球磨破碎与机械合金化

1.1.1 球磨

在开发Sm2Fe17N x 初期采用熔炼→均匀化热处理→球磨→二次热处理→氮化流程[32~34],制备出的Sm2Fe17N x 颗粒粗大,且含有大量α-Fe软磁相。其原因是：熔炼的铸锭中Sm、Fe严重偏析,使Sm∶Fe比例偏离生成母相Sm2Fe17的区间；球磨破坏了Sm2Fe17的结构导致非晶化并析出α-Fe[33,34]。对Sm2Fe17N x 进行二次球磨,可减小颗粒尺寸和提高其Hcj[35~37]。如图2所示[35],在粗颗粒氮化后的Sm2Fe17N x 中有明显的畴壁和棱角,二次球磨后颗粒尺寸减小且畴壁消失,颗粒是单畴颗粒的复合体。Kobayashi[36]等认为,尺寸减小到理论单畴尺寸(约0.3 μm)[37]的颗粒其Hcj与颗粒尺寸遵循1/D规律,D为Sm2Fe17N x 颗粒的直径。但是,二次球磨易提高颗粒的含氧量,使Hcj下降(图2c)[35]。

图2

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图2   Sm2Fe17N x 粉末球磨前后的比特花样、矫顽力以及含氧量随球磨时间的变化[35]

Fig.2   Bitter patterns of Sm2Fe17N x powders before (a) and after (b) ball milling and Changes of coercivity and oxygen content with ball milling time[35] (c)


球磨时加入溶媒能防止颗粒团聚,提高细化颗粒的效率。在溶媒中加入适量的油酸等表面活性剂可使颗粒扁平化,提高c轴取向率[38~40]。Zhang等[38]在加有油酸的2-甲基戊烷中分别在室温和0ºC对Sm2Fe17N2.8球磨4 h,如图3所示,随着球磨时间的延长Sm2Fe17N2.8都发生分解,在低温下得到的样品颗粒棱角更少,其Mr为110 emu·g-1高于室温样品的85 emu·g-1,Hcj均为13 kOe。Li等[39,40]认为,球磨时Sm2Fe17N2.8表面氧化导致脱氮分解,颗粒表面的氧化速度随球磨机中温度的升高而提高。Hosokawa等[41]在低氧环境下采用气流磨破碎30 μm的Sm2Fe17N3粉体并控制气压以控制其粒径,发现随着颗粒尺寸从3.0 μm减小至1.1 μm,Hcj从7.3 kOe增加至14.1 kOe,Mr从158 emu·g-1减小至132 emu·g-1,Mr/Ms从0.96减小至0.87；颗粒粒径为1.3 μm时Hcj为12.7 kOe,Mr为147 emu·g-1,(BH)max高达43.6 MGOe。

图3

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图3   在低温和室温下球磨不同时间Sm2Fe17N x 粉体的XRD谱、SEM照片和磁滞回线[38]

Fig.3   XRD patterns (a, b), SEM images (c, d) and hyste-resis loops (e) of Sm2Fe17N x powders were obtained by different milling times at low temperature and room temperature, respec-tively[38]


1.1.2 机械合金化

吕曼祺等[42]将Sm和Fe单质粉末在Ar气中振动球磨5 h,得到含有α-Fe的非晶SmFe合金粉末。在800ºC对其进行10 min的均匀化热处理,得到含有微量软磁相的Sm2Fe17粉体。这种机械合金化Sm2Fe17粉体极易自燃。Kou等[43]采用同样的流程制备的Sm2Fe17N x,其Hcj达到31 kOe。Xu等[44]证实：以Fe和Sm的单质金属为原料无法用高能球磨完成机械合金化得到Sm2Fe17,但是结合等离子放电(图4),在球磨速度、时间、原料质量/磨球质量分别为1300 r·min-1、6 h、1∶100的条件下能跨越生成Sm2Fe17的能量壁垒,得到Sm2Fe17+Sm2Fe17N x +α-Fe的复合相,再将其在480℃氮化8 h得到Sm2Fe17N x +α-Fe。虽然最终产物的Hcj和Mr仅为2.5 kOe和45 emu·g-1,但是仍为机械合金化制备Sm2Fe17N x 开辟了新路径。

图4

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图4   用球磨结合等离子放电技术机械合金化制备Sm2Fe17的示意图和氮化后Sm2Fe17N x 样品的退磁曲线[44]

Fig.4   Schematic diagram of preparing Sm2Fe17 by me-chanical alloying with planetary milling combined with plasma discharge technology (a) and demagnetization curve of Sm2Fe17N x samples after nitridation[44] (b)


1.2 薄带连铸与甩带

1.2.1 薄带连铸

Ma等[45]采用薄带连铸技术制备出厚度为0.8 mm、宽度为1~2 cm的SmFe合金薄带,将其在1000℃均匀化处理后再在惰性气体气氛下破碎成200 μm的粗粉,在437~537℃氮化10~24 h后得到Sm2Fe17N3粗粉,最后将其在含有油酸和硅烷偶联剂的正庚烷中球磨破碎,得到长径为2~6 μm、厚度为100 nm的扁平状粉末。油酸能促进球磨过程中粉体的细化和扁平化,硅烷偶联剂能在颗粒表面形成包覆层从而减少氧化,最终产物的Mr为155 emu·g-1,Hcj为13 kOe。虽然有研究[26]认为直接采用薄带连铸无需均匀化处理也能得到纯相的Sm2Fe17,但是实际上即使在薄带连铸后进行较短时间的均匀化处理也难以得到纯相Sm2Fe17。为解决这一问题,Lu等[46]在SmFe合金中加入原子比为3%的Sm-Cu并建立SmCu+SmCu2+Sm2Fe17的相平衡关系,抑制了软磁相SmFe2和α-Fe的析出,结果如图5所示。

图5

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图5   用薄带连铸结合球磨破碎技术制备Sm2(FeCu)17N x

Fig.5   Preparation of Sm2(FeCu)17N x by strip casting method and ball mill crushing technology (a) XRD of SmFeCu, SmFeCuN, SmFeN; (b) Backscattering image of SmCu phase in SmFeCuN; (c) SEM image of SmFeCuN powder and (d) Ob-taining the hysteresis loop of SmFeCuN powder with different milling time[46]


1.2.2 甩带

与薄带连铸相比,甩带能提高合金熔融体的冷却速度,获得更薄的合金带。Pinkerton等[47]用甩带法(35 m·s-1)制备出宽度为1 mm、厚度为30 μm的纳米晶薄带,其晶粒尺寸约为40 nm。将纳米晶薄带球磨破碎至25 μm的粗粉后在700℃真空热处理1 h,然后再进行氮化可使其Hcj提高至22.3 kOe。但是,真空热处理不能提高更大颗粒尺寸(约45 μm)粉体的磁性能。Coey等[19]指出,用甩带法(40 m·s-1)制备的纳米晶Sm2Fe17N3的无序化程度更高,使其单轴各向异性降低,而纳米化使交换耦合作用和晶界钉扎作用增强,能在一定程度上提高剩磁。因此,甩带/薄带联铸结合球磨的流程,更适合制备各向同性的Sm2Fe17N3。基于此,本文认为球磨后真空热处理的作用应该是修复球磨导致的颗粒表面的无序化,阻止单轴各向异性的减弱。但是颗粒尺寸较大时其无序化表面的体积占比很低,作用不显著。

1.3 还原扩散法

用还原扩散法制备Sm2Fe17N3,是基于Ca热还原反应,将Sm的氧化物在Ca的熔点以上还原为单质Sm伴随原子扩散生成Sm2Fe17,再将其氮化得到Sm2Fe17N x。

还原扩散法使用的原料成本低,日本住友金属等在20世纪90年代就尝试用还原扩散法制备Sm2Fe17N3[48,49]。在还原扩散法开发之初,通常在氮化前,也就是得到Sm2Fe17颗粒后进行水洗去除Ca金属及其副产物。但是,发现不经水洗直接氮化也能得到Sm2Fe17N3并能阻止Sm2Fe17颗粒氧化[50],因为Sm2Fe17N3具有更好的耐腐蚀性,如图6所示。为了减少还原金属Ca的投入量最初多使用单质Fe颗粒与Sm2O3复合作为还原处理的前驱体[48~52],但是单质Fe的颗粒尺寸通常大于10 μm,制备出的Sm2Fe17N3仍需长时间二次球磨以减小颗粒粒径[52]。

图6

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图6   还原扩散法制备流程、还原扩散后(未氮化)块体照片、直接氮化后的照片以及水洗后Sm2Fe17N3粉体的SEM照片[50]

Fig.6   Reduction diffusion method preparation process (a); Images of the bulk body after reduction and diffusion (not ni-tridation) (b) and after nitridation (c) and SEM image of Sm2Fe17N3 powders after washing[50] (d)


实际上还原扩散法可使用氧化铁等作为前驱体,结合湿化学法能制备出亚微米甚至是纳米级的前驱体以减小最终产物的颗粒尺寸。例如,FeOOH[53]、SmFe(CN)6[53]、Fe2O3[53,54]、Fe3O4[55,56]、Fe@Sm2O3[57]以及复合凝胶[29]和共沉淀复合物[10,58]等。前驱体为高价铁化合物时需要预先将其还原为单质Fe,以减少后续还原扩散阶段Ca的投入量。将溶胶凝胶法与还原扩散法结合能制备出粒径为0.69 μm的Sm2Fe17N3,Hcj达23.2 kOe[29](图7a)；将共沉淀法与还原扩散法结合能制备出粒径为0.47 μm的Sm2Fe17N3,Hcj达到24.7 kOe[10](图7b)。Zheng等[57]用超声波热喷雾分解法,以H2为携带气体,在900℃从FeCl3/SmCl3=2.4∶17的溶液出发一步制备出0.49 μm的Fe@Sm2O3球形前驱体,还原扩散处理后得到的Sm2Fe17N3的粒径为0.616 μm,Hcj为14.7 kOe,Mr约为100 emu·g-1。

图7

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图7   分别用溶胶凝胶法[29]、共沉淀法[10]、水热法[54]和溶剂热解法[56]制备前驱体,再进行还原扩散后的Sm2Fe17N x 的SEM照片和磁化曲线

Fig.7   Precursors were prepared by sol-gel method[29] (a), coprecipitation method[10] (b), hydrothermal method[54] (c) and solvopyrolysis method[56], SEM image and magnetization curve of Sm2Fe17N x obtained after reduction and diffusion treatment (d)


对前驱体颗粒进行无机包覆,能抑制热处理过程中颗粒的团聚。Okada等[54]在溶液中加入CaNO3,水热处理后得到尺寸约为100 nm的Fe2O3@CaCO3复合体,还原扩散处理后得到粒径为0.66 μm的Sm2Fe17N3,Hcj为24.1 kOe,Mr为105 emu·g-1(图7c)；Shen等[56]用热分解法得到尺寸为100 nm的Fe3O4,在含有醋酸钙的有机溶媒中用CaO包覆后得到尺寸约为0.10 μm的Sm2Fe17N x 颗粒,但是其Hcj只有15.4 kOe(图7d)。

还原扩散处理,会残留大量金属Ca及其副产物CaO。水洗这些残留物会在水中生成的溶解度很低的Ca(OH)2,而长时间水洗易使颗粒表面腐蚀氧化生成Sm2O3和亚稳相SmFe5,尤其是亚微米级细小颗粒氧化尤为显著。有报道证实,用甲醇与氯化铵溶液替代纯水清洗可减轻金属元素的溶解[56]。目前普遍使用pH=5的醋酸水溶液以清除富Sm相从而提高剩磁[10,11,53,54,58],但是不可避免损伤颗粒表面。Okada等[11]在20℃、Ar+10% O2(Volume fraction)条件下进行氧化处理将Sm2Fe17N3颗粒表面的SmFe5转化成氧化物,再用pH=7的醋酸溶液去除颗粒表面的氧化物,得到的产物其Hcj高达28.1 kOe,Mr约为90 emu·g-1,如图8所示。在用NH3/H2氮化和水洗去除金属Ca(产生H2和放热)的过程中会生成Sm2Fe17N x H y (H含量：0.76~1.5 mg/cm3),在Ar气氛下进行脱氢处理能修复其矫顽力[10]。虽然这些实验室水洗工艺的改良修复了磁性能,但操作较为复杂,使处理成本提高。

图8

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图8   还原扩散法的新制备流程(缓慢氧化处理)以及样品的SR-XRD谱和退磁曲线[11]

Fig.8   New preparation process of reduction diffusion method (slow oxidation treatment) (a) and the SR-XRD patterns (b, d) and demagnetization curves of the samples[11] (c, e)


1.4 氮化

基于气-固相反应的渗氮处理,气相中的氮源可使用N2[23,27,29,42,59,60]、NH3[1]或NH3/H2混合气体[10,11,18,24]等。在将Sm2Fe17氮化为Sm2Fe17N x 的过程中N原子优先占据八面体9e间隙位置x≤3；进一步占据18f间隙位置3≤x≤7；x=3时Sm2Fe17N x 的各向异性场、剩磁和居里温度等综合磁性能都最优[18]。

在氮化的初始阶段,先是N2在颗粒表面解离成N原子[61],随后N原子在晶界及微裂纹处富集。这表明,N原子在晶界处的扩散速度比其在晶粒内部扩散速度更高[18]。当x≤2.3时(113)、(300)和(024)晶面的XRD衍射峰明显分裂,表明颗粒内部贫氮相Sm2Fe17N x (x<3)和Sm2Fe17N3相共存[18,42,59],元素分析[59]和热磁分析[61]也支持上述结论。同时,在氮化过程中Sm2Fe17N3相的衍射峰先宽化后尖锐,表明氮原子向颗粒内部扩散伴随着Sm2Fe17N3晶粒的长大[42,59]。因此,当N2在颗粒表面完成分解产生活性N原子后,后续氮化的律速阶段是贫氮相Sm2Fe17N x (x<3)向Sm2Fe17N3相的转移,而非N元素由颗粒外向颗粒内进行梯度扩散,如图9所示[62]。基于上述氮化机制,Onoue等[59]提出,可在氮化时加磁场,因为贫氮相向Sm2Fe17N3相转化时两相磁矩差逐渐变负、系统磁化逐渐降低,从而加快Sm2Fe17N3相生成。

图9

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图9   氮原子扩散机制和由Sm2Fe17N x (NP, x<3)向Sm2Fe17N3(FN)相转变引起的晶粒生长机制下的氮元素分布示意图[62]

Fig.9   Schematic diagrams of nitrogen distribution profiles for single nitrogen diffusion process (a) and grain growth pro-cess of the fully-nitride phase of Sm2Fe17N3 (FN) from the ni-trogen-poor phase of Sm2Fe17N x (NP, x<3)[62] (b)


提高N2气氛压力能有效缩短氮化时间,并一定程度抑制α-Fe相的析出[59]。例如,对尺寸为32~53 μm的颗粒进行氮化,在温度为733 K、压力为6 MPa条件下,氮含量达到x=3的时间为48 h,而压力为0.1 MPa时所需时间为72 h。随着氮化时间的增加颗粒表层Sm2Fe17N x 的N含量达到x=3后不会继续增加,而是由颗粒外向颗粒内逐渐增加Sm2Fe17N3的层厚度[62]。这表明,氮化后进行均匀化处理能平衡氮原子在颗粒内的分布,从而提高磁性能[61]；减小母相颗粒粒径,即缩短扩散距离,也能加速N原子的扩散。但是实验结果表明,以N2气为氮源时Sm2Fe17N x 中的x只能无限接近于3。

Fukuda等[18]提出,使用NH3/H2混合气体氮化能提高氮化效率。在NH3/H2=35∶65、氮化温度为465℃的条件下对粒径为20~160 μm的Sm2Fe17颗粒进行氮化,得到Sm2Fe17N3的时间仅为2 h；当NH3/H2=40∶60时Sm2Fe17N x 的x能达到6.6,但是x>3时将引发Sm2Fe17N x 的非晶化,使其矫顽力、剩磁和居里温度都大幅降低。NH3分解得到N原子和H2,2NH3⇌3H2+2N,因此提高混合气体中的H2比例可抑制NH3的分解。这意味着,在纯NH3中比在NH3/H2混合气体中氮化速度更高。但是Coey等[17]的实验结果表明,无论采用纯NH3或纯N2进行氮化其结果基本相同,而且在长时间的NH3氮化中没有发现Sm2Fe17N x 中氮含量x超过4。据此可以推测,在氮化初始阶段H2的存在可促进氮化的进行,这或许与表面氧化层还原为金属作为氨气裂解的催化剂或是氢脆导致N原子扩散通道增加有关。

1.5 表面改性

减小Sm2Fe17N3颗粒尺寸虽然有助于提高颗粒矫顽力,但是颗粒表面氧化的加剧使剩磁和矫顽力降低,退磁曲线的方形度变差。Matsuura等[63]将Sm-Fe、Sm2O3、Mn2O3粉末球磨复合,还原扩散处理后得到核壳结构的Sm2Fe17N x -core/Sm2(FeMn)17N x -shell颗粒。具有和不具有Sm2(FeMn)17N x 的样品在Ar气、300℃下进行1.5 h热处理,发现Hcj分别降低了35%和52%。这表明,包覆Sm2(FeMn)17N x 壳层增强了热稳定性[64],如图10a~d所示。Yamaguchi等[65,66]用磁控溅射在Sm2Fe17N x 表面包覆金属Zn纳米层,在低氧环境下热处理后样品的矫顽力反而呈现增加趋势,如图10e所示。实际上,热处理氧化Sm2Fe17N x 颗粒在其表面析出的富Fe相能与Zn反应形成非磁性FeZn相,从而减少软磁相并隔绝Sm2Fe17N x 晶粒[67]。表2列出了用各种手段制备的Sm2Fe17N x 的物理和磁性能。

图10

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图10   Sm2Fe17N x /Sm2(Fe,Mn)17N x 核壳粉的制备流程图和示意图、Mn扩散Sm-Fe粉末的截面扫描电镜和元素映射图、Mn扩散Sm-Fe-N核壳粉、无Mn扩散Sm-Fe-N粉热处理前后的退磁曲线[63]以及锌包覆Sm2Fe17N3粉体矫顽力随锌层平均厚度的增加速率[66]

Fig.10   Flow chart and schematic of the preparation of the Sm2Fe17N x /Sm2(Fe,Mn)17N x core-shell powder (a); Cross-sectional SEM and elemental mapping images of Mn-diffused Sm-Fe powder (b); demagnetization curves before and after heat treatment for Mn-diffused Sm-Fe-N core-shell powder (c) and Mn-free Sm-Fe-N powder[63] (d); Rate of increase in coercivity of Zn-coated Sm2Fe17N3 powders as function of average thickness of Zn layer[66] (e)


表2   用不同方法制备Sm2Fe17N x 的氮化条件和性能

Table 2  Sm2Fe17N x prepared by different preparation process: nitriding conditions, physical and magnetic properties

Preparation methodNitridationMorphologyImpurityPropertiesReference

Temperature

/K

Time

/h

Atmosphere

Length

/μm

Ms

/emu·g-1

Mr

/emu·g-1

Hcj

/kOe

Jet-milling technique///1.30/16314712.7[39]
Strip casting technique700~80010~24N20.80α-Fe16015513.0[44]
Strip casting technique69335N20.76/14.2 (kGs)12.5 (kGs)13.0[45]

Reduction-diffusion process

(Polymerized complex)

70310N20.69///23.2[27]

Reduction-diffusion process

(Co-precipitation)

693/

NH3/H2

(1:2 Vol/Vol)

0.90/1259028.1[11]

Reduction-diffusion process

(Hydrothermal)

6931

NH3/H2

(1:2 Vol/Vol)

0.66/14010524.1[53]

Reduction-diffusion process

(Thermal decomposition)

873/

Melamine

(C3H6N6)

0.10/128/15.4[55]

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2 总结和展望

(1) 传统的甩带或薄带联铸结合破碎技术的工艺流程,难以将Sm2Fe17N3颗粒粒径减小到1 μm以下并抑制氧化,而且颗粒粒径分布不均、颗粒多棱角局部退磁场大。

(2) 还原扩散法结合湿化学法直接用亚微米级甚至是纳米级铁化合物作为前驱体,能制备出矫顽力达23~28 kOe的临单畴尺寸的Sm2Fe17N3粉体。用溶胶凝胶法、高温水热处理以及热分解法等制备前驱体难以规模化应用。Sm2Fe17N x 永磁体的制备技术及其机制,仍有大量待解决的问题。

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研究了用机械合金化方法制备的Sm_2Fe_(17)合金的晶化与氮化过程。结果表明:机械合金化的Sm-Fe合金是Sm-Fe非晶相与晶态α-Fe的混合物,其成分是不均匀的,Fe在非晶颗粒外层含量较少,晶化过程包含Fe原子的长程扩散和固相反应,在适当的晶化条件下,晶化后全部转化为Sm_2Fe_(17)相。晶化过程中,可能出现六方结构的Th_2Ni_(17)型Sm_2Fe_(17)亚稳相。氮化过程由孕育期、Sm_2Fe_(17)相与其氮化物相共存、最终全部转化为氮化物相三阶段组成;在氮化过程中有十分细小的少量α-Fe析出。

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Sm2Fe17N x 的制备

[J]. 中国稀土学报, 2005, 26(6): 53

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We report a general chemical approach to synthesize strongly ferromagnetic rare-earth metal (REM) based SmCo and SmFeN nanoparticles (NPs) with ultra-large coercivity. The synthesis started with the preparation of hexagonal CoO+Sm O (denoted as SmCo-O) multipods via decomposition of Sm(acac) and Co(acac) in oleylamine. These multipods were further reduced with Ca at 850 °C to form SmCo NPs with sizes tunable from 50 to 200 nm. The 200 nm SmCo NPs were dispersed in ethanol, and magnetically aligned in polyethylene glycol (PEG) matrix, yielding a PEG-SmCo NP composite with the room temperature coercivity (H ) of 49.2 kOe, the largest H among all ferromagnetic NPs ever reported, and saturated magnetic moment (M ) of 88.7 emu g, the highest value reported for SmCo NPs. The method was extended to synthesize other ferromagnetic NPs of Sm Co, and, for the first time, of Sm Fe N NPs with H over 15 kOe and M reaching 127.9 emu g. These REM based NPs are important magnetic building blocks for fabrication of high-performance permanent magnets, flexible magnets, and printable magnetic inks for energy and sensing applications.© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

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研究了氮原子在Sm_２Fe_（１７）合金中的扩散行为,测定了扩散温度、时间与样品平均氮含量的关系,以及氮原子在样品中的分布；计算了氮原子在Sm_２Fe_（１７）合金中的扩散频率因子D_０和扩散激活能Q,运用扩散Fick第二定律计算了氮原子在Sm_２Fe_（１７）粉末颗粒内部的分布及其与扩散温度、扩散时间、颗粒尺寸之间的关系．理论计算与实验结果一致．

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Although the addition of other elements such as Mn to Sm2Fe17Nx compounds can change the magnetic properties, it also decreases saturation magnetization. In order to exploit the advantages of additional elements in Sm2Fe17Nx powder while maintaining a high saturation magnetization, a structure of Sm2Fe17Nx-core/Sm-2(Fe,M)(17)N-x-shell is promising. This is the first report of such a core-shell powder obtained by a reduction-diffusion process. Mn3O4 was mixed with Sm-Fe and Sm2O3 powders followed by the reduction by Ca above 860 degrees C, and then samples were nitrided and washed after reduction-diffusion process. Core-shell Sm-Fe-N fine powder with a Mn-enriched Sm-2(Fe,Mn)(17)N-x shell was thus successfully obtained. The thickness of the Mn enriched region was about 0.8 mu m, and the crystal structure of the core-shell powder was Th2Zn17. The saturation magnetization and coercivity of the core-shell powder were 138.8 A.m(2).kg(-1) and 1001.7 kA.m(-1), respectively. The saturation magnetization was slightly smaller than that of Mn-free Sm-Fe-N powder, whereas the coercivity of the core-shell powder was higher than that of Mn-free Sm-Fe-N powder. In addition, the saturation magnetization and coercivity were higher than those reported for Sm-Fe-Mn-N powder. The thermal stability of the Mn-diffused Sm-Fe-N core-shell powder was improved compared with Mn-free Sm-Fe-N powder.

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