DOI: 10.11817/j.ysxb.1004.0609.2021-35976
Ho-Nd-Fe-B磁体的性能与结构
曹玉杰1, 2,刘友好2,徐光青1, 3,张鹏杰4,刘家琴3, 5,陈静武2,衣晓飞2,吴玉程1, 3
(1. 合肥工业大学 材料科学与工程学院,合肥 230009;
2. 稀土永磁材料国家重点实验室,合肥 231500;
3. 先进功能材料与器件安徽省重点实验室,合肥 230009;
4. 北矿磁材(阜阳)有限公司,阜阳 236000;
5. 合肥工业大学 工业与装备技术研究院,合肥 230009)
摘 要:采用Ho部分取代Nd,制备了不同Ho含量的Ho-Nd-Fe-B磁体,研究了Ho含量对Ho-Nd-Fe-B磁体的磁性能、温度稳定性和微观结构的影响。结果表明:Ho的添加有助于改善主相和富稀土相之间的浸润性,优化晶界富稀土相的分布,提高了磁体的内禀矫顽力,并改善了磁体的温度稳定性,但磁体的剩磁有所下降。当Ho含量(质量分数)由0增加到21.0%时,Hcj由1281 kA/m增加到1637 kA/m,Br由1.342 T降至0.919 T;在20~100 ℃范围内,磁体的剩磁温度系数和矫顽力温度系数分别由0.119%/℃和0.692%/℃降低到0.049%/℃和0.540%/℃;在180 ℃烘烤2 h后的磁通不可逆损失由54.80%降低到29.17%。
关键词:Ho-Nd-Fe-B磁体;磁性能;温度稳定性;微观结构
文章编号:1004-0609(2021)-04-0938-07 中图分类号:TM271 文献标志码:A
引文格式:曹玉杰, 刘友好, 徐光青, 等. Ho-Nd-Fe-B磁体的性能与结构[J]. 中国有色金属学报, 2021, 31(4): 938-944. DOI: 10.11817/j.ysxb.1004.0609.2021-35976
CAO Yu-jie, LIU You-hao, XU Guang-qing, et al. Study on properties and structure of Ho-Nd-Fe-B magnets[J]. The Chinese Journal of Nonferrous Metals, 2021, 31(4): 938-944. DOI: 10.11817/j.ysxb.1004.0609.2021-35976
Nd-Fe-B永磁材料以其优异的磁性能和较高的性价比被广泛应用于汽车工业、仪器仪表和医疗器械等诸多领域[1-3]。通过对Nd-Fe-B永磁材料成分、结构和制备工艺的不断优化,其各项性能指标得到一定程度的提升[4-7]。目前,Nd-Fe-B永磁材料在新能源汽车、智能制造等新兴产业的需求量不断攀升,已成为推动新兴产业快速发展的关键功能材料,这些新兴产业要求Nd-Fe-B永磁材料具有高的温度稳定性。但是,Nd-Fe-B永磁材料较差的温度稳定性严重制约了其在新兴产业中的应用。
元素添加是改善Nd-Fe-B永磁材料温度稳定性的主要方式,如添加Co可以提高磁体的居里温度,Nb添加可以降低磁体的高温磁通不可逆损失[8-9]。而添加重稀土Dy、Tb形成的化合物Dy2Fe14B、Tb2Fe14B具有最优的内禀性能,尤其是高的磁晶各向异性场HA,分别为12000 kA/m和17000 kA/m,是Nd2Fe14B的2~3倍。因此,采用Dy或Tb来取代Nd,可以有效提高Nd-Fe-B永磁材料的内禀矫顽力,改善磁体的温度稳定性[10-14]。但是,Dy和Tb的资源储量有限,价格昂贵,从国家可持续发展战略的角度考虑,应当尽量少用或不用。为促进稀土资源的合理开发和均衡利用,同为稀土元素的Ho与Nd在化学性质上具有相似性,其形成的化合物Ho2Fe14B的磁晶各向异性场仅次于Dy2Fe14B和Tb2Fe14B,因此可用Ho替代Nd制备Ho-Fe-B或Ho-Nd-Fe-B磁体[15]。
姚茂海等[16]研究了Ho对烧结钕铁硼永磁体性能的影响。结果表明,Ho取代部分Nd可优化铸片中 Nd2Fe14B柱状晶的生长,使磁体的显微组织致密化,从而有效提高其内禀矫顽力和耐腐蚀性能,减少质量损失。邸敬慧等[17]研究了少量Ho取代Nd对磁体耐腐蚀性和热稳定性的影响。结果表明,Ho的添加提高了磁体的耐腐蚀性能,改善了磁体的热稳定性。因此,少量Ho替代Nd可以提高磁体的耐腐蚀性、内禀矫顽力和温度稳定性[18-22]。但是,大量Ho取代Nd对Ho-Nd-Fe-B磁体的磁性能、温度稳定性和微观结构的影响尚不清楚。因此,本文采用大量Ho取代Nd,制备了不同Ho含量的Ho-Nd-Fe-B磁体,研究了Ho含量对Ho-Nd-Fe-B磁体的磁性能、温度稳定性和微观结构的影响。
1 实验
试验用钕铁硼合金成分设计为(PrNd)31.5-xHox- FeB(x=0, 4.2, 8.4, 12.6, 16.8, 21.0),其中x%为质量分数。磁体制备采用真空速凝熔炼-氢破碎-气流磨制粉-磁场取向成型-冷等静压-烧结-热处理的制备工艺。其中,烧结工艺是在1040~1090 ℃下真空烧结5 h,然后分别在900 ℃和480 ℃下保温3 h进行二级回火热处理,最终制备出不同Ho含量的Ho-Nd-Fe-B磁体。将制备的Ho-Nd-Fe-B磁体加工成d 10 mm×10 mm的圆柱和10 mm×10 mm×2 mm的片状样品(高10 mm和厚2 mm方向为磁体取向方向)。
采用NIM-2000型磁滞回线测量仪测试样品的磁性能。利用亥姆霍兹线圈和磁通计测试磁体在高温下的磁通不可逆损失。采用Phenom Pro X型扫描电子显微镜(SEM)对样品进行微观结构分析,并利用附带的能谱分析仪(EDS)对样品进行元素分析。
2 结果与讨论
2.1 磁性能
图1所示为Ho-Nd-Fe-B磁体的剩磁Br、内禀矫顽力Hcj和最大磁能积(BH)max随Ho含量(质量分数)的变化情况。从图1可以看出,磁体的Br和(BH)max随Ho含量的增加而减小;但磁体的Hcj则随Ho含量的增加而增大。当Ho含量由0增加到21.0%时,Hcj由1281 kA/m增加到1637 kA/m,Br和(BH)max则分别由1.342 T和338.8 kJ/m3降至0.919 T和161.4 kJ/m3。这是因为RE2Fe14B相作为磁体中的磁性相,其饱和磁极化强度Js和磁晶各向异性场HA的大小决定了磁体的Br和Hcj。由于Nd2Fe14B和Ho2Fe14B的Js、HA分别为1.61 T、5600 kA/m和0.81 T、7600 kA/m,因此,Ho部分取代Nd会降低磁体的Js,提高磁体的HA,从而导致磁体的剩磁Br降低、内禀矫顽力Hcj升高。
图1 不同Ho含量Ho-Nd-Fe-B磁体的磁性能
Fig. 1 Magnetic properties of Ho-Nd-Fe-B magnets with different Ho contents (mass fraction)
2.2 剩磁温度系数和矫顽力温度系数
温度系数反应的是单位温度变化引起的材料的某一特性的百分比变化,能够直接反应材料的某一特性对温度的敏感程度。剩磁温度系数和矫顽力温度系数分别代表永磁材料的剩磁和内禀矫顽力对温度的敏感程度,和的计算公式分别如下所示:
(1)
(2)
式中:Br(t)和Hcj(t)分别是温度为t时的剩磁和内禀矫顽力,t0=20 ℃。
不同Ho含量的Ho-Nd-Fe-B磁体在20~100 ℃范围内的温度系数和如图2所示。从图2可以看出,和均随Ho含量的增加而减小,分别由未添加Ho时的0.119%/℃和0.692%/℃减小到Ho含量为21.0%时的0.049%/℃和0.540%/℃,分别降低了58.82%和21.97%。说明Ho的添加可以降低磁体的温度系数和,从而提高磁体的温度稳定性。当Ho含量为21.0%时,Ho-Nd-Fe-B磁体的与商用26H型Sm2Co17磁体的(0.030%/℃)接近。
图2 不同Ho含量的Ho-Nd-Fe-B磁体温度系数和
Fig. 2 Temperature coefficients and of Ho-Nd- Fe-B magnets with different Ho contents
2.3 高温磁通不可逆损失
将10 mm×10 mm×2 mm的片状样品充磁至饱和状态,在不同温度下烘烤2 h并冷却至室温,采用亥姆赫兹线圈测量样品在烘烤前后的磁通值,并计算样品的磁通不可逆损失,其计算公式如下:
(3)
式中:hirr表示磁体的磁通不可逆损失;表示磁体在烘烤前的磁通值;表示磁体经烘烤后恢复到室温时的磁通值。
不同Ho含量的Ho-Nd-Fe-B磁体高温磁通不可逆损失随温度的变化关系如图3所示。从图3可以看出,当烘烤温度达到某一临界值之前,磁体的磁通不可逆损失很小,温度变化几乎不会造成磁体的磁通不可逆损失;当烘烤温度超过其临界值后,随着烘烤温度不断升高,磁体的磁通不可逆损失近乎线性地增加,这一临界温度随着Ho含量的增加而增大。在超过临界温度后的相同温度条件下,未添加Ho的Nd-Fe-B磁体的磁通不可逆损失最大;随着Ho含量的增加,磁体的磁通不可逆损失逐渐减小。在180 ℃下烘烤2 h,磁体的磁通不可逆损失 (hirr)由Ho含量为0时的54.80%降低到Ho含量为21.0%时的29.17%,降幅达到46.77%。因此,Ho取代Nd能够降低磁体的高温磁通不可逆损失,提高磁体的温度稳定性。
图3 不同Ho含量Ho-Nd-Fe-B磁体在不同温度下的磁通不可逆损失
Fig. 3 Irreversible flux loss of Ho-Nd-Fe-B magnets (hirr) with different Ho contents at different temperatures
2.4 微观结构
图4所示为不同Ho含量Ho-Nd-Fe-B磁体的SEM像,图中白色区域为富稀土相,灰色区域为主相。从图4(a)可以看出,未添加Ho的Nd-Fe-B磁体相邻主相晶粒之间几乎没有连续分布的富稀土相。当Ho含量达到4.2%(见图4(b))和8.4%(见图4(c))时,能够看到相邻主相晶粒之间连续分布的富稀土相。随着Ho含量进一步增加,如图4(d)、(e)和(f)所示,相邻主相晶粒之间连续分布的富稀土相越来越明显,且晶界角隅处的块状富稀土相逐渐减少。说明Ho的添加有助于改善主相和富稀土相之间的浸润性,使得晶界相分布更加均匀。作为非磁性连续分布的富稀土相既有助于烧结磁体的致密化,又能起到去磁耦合作用,对烧结磁体的磁硬化至关重要,这也是Ho-Nd-Fe-B磁体的Hcj随着Ho含量增加而增大的另一个原因。
图4 不同Ho含量的Ho-Nd-Fe-B磁体的SEM像
Fig. 4 SEM images of Ho-Nd-Fe-B magnets with different Ho contents
图5 Ho含量分别为0和16.8%的Ho-Nd-Fe-B磁体的断口SEM像
Fig. 5 Fracture SEM images of Ho-Nd-Fe-B magnets with different Ho contents
图5所示为Ho含量分别为0和16.8%的Ho-Nd-Fe-B磁体的断口SEM像。从图5(a)可以看出,在未添加Ho的磁体中,主相晶粒的形状不规则,边角有尖锐的突出部分,富稀土相主要以团块状的形式分布,主相晶粒表层几乎没有富稀土相;在Ho含量为16.8%的磁体中,主相晶粒的形状较为规则,边角更加圆滑,主相晶粒表层拥有较多的富稀土相。这是因为Ho含量为16.8%的磁体的晶界富稀土相分布更加均匀,连续分布于主相晶粒之间的富稀土相很好地连接了相邻的两个主相晶粒,在沿晶断裂后分布于主相晶粒的表层。
利用EDS分析了不同Ho含量的Ho-Nd-Fe-B磁体主相和晶界相中的w(Ho)/w(RE)(Ho含量与总稀土含量之比),并与初始配方中的w(Ho)/w(RE)进行了比较,结果如表1所示。从表1可以看出,Ho在磁体主相中的占比高于初始配方中的占比和在晶界相中的占比,说明Ho更倾向于进入主相晶粒。
表1 不同Ho含量的Ho-Nd-Fe-B磁体中Ho元素分布
Table 1 Distribution of Ho in Ho-Nd-Fe-B magnets with different Ho contents
3 结论
1) Ho的添加在提高Ho-Nd-Fe-B磁体Hcj的同时,会在一定程度上降低磁体的Br,当Ho含量由0增加到21.0%时, Hcj由1281 kA/m增加到1637 kA/m,Br和(BH)max则分别由1.342 T和338.8 kJ/m3降至0.919 T和161.4 kJ/m3。
2) Ho取代Nd可以改善Nd-Fe-B磁体的温度稳定性。当Ho含量由0增加到21.0%时,在20~100 ℃范围内,磁体的剩磁温度系数和矫顽力温度系数分别由0.119%/℃和0.692%/℃降低到0.049%/℃和0.540%/℃,在180 ℃烘烤2 h后的磁通不可逆损失由54.80%降低到29.17%。
3) Ho更倾向于代替Nd进入主相晶粒,Ho的添加有助于改善主相和富稀土相之间的浸润性,优化晶界富稀土相的分布,不仅有助于烧结磁体的致密化,而且能够起到去磁耦合作用,对烧结磁体的磁硬化至关重要。
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Properties and structure of Ho-Nd-Fe-B magnets
CAO Yu-jie1, 2, LIU You-hao2, XU Guang-qing1, 3, ZHANG Peng-jie4, LIU Jia-qin3, 5, CHEN Jing-wu2, YI Xiao-fei2, WU Yu-cheng1, 3
(1. School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China;
2. State Key Laboratory of Rare Earth Permanent Magnet Materials, Hefei 231500, China;
3. Key Laboratory of Advanced Functional Materials and Devices of Anhui Province, Hefei 230009, China;
4. BGRIMM Magnetic Materials and Technology (Fuyang) Co., Ltd., Fuyang 236000, China;
5. Institute of Industry and Equipment Technology, Hefei University of Technology, Hefei 230009, China)
Abstract: Ho-Nd-Fe-B magnets with different Ho contents were prepared by Ho substitution for part of Nd. The effects of Ho contents on the magnetic properties, temperature stability and microstructure of Nd-Fe-B magnets were studied. The results show that the addition of Ho is helpful to improve the wettability between the main phases and the rare-earth rich (RE-rich) phases, and optimizes the distribution of rare-earth rich (RE-rich) phases in grain boundary, and improves the intrinsic coercivity and the temperature stability of the magnets, but the remanence of the magnets decreases. When Ho content (mass fraction) increases from 0 to 21.0%, Hcj increases from 1281 kA/m to 1637 kA/m, and Br decreases from 1.342 T to 0.919 T; the remanence temperature coefficient and coercivity force temperature coefficient of magnets decrease from 0.119%/℃ and 0.692%/℃ to 0.049%/℃ and 0.540%/℃, respectively, in the range of 20-100 ℃. And the irreversible flux loss after being baked at 180 ℃ for 2 h decreases from 54.80% to 29.17%.
Key words: Ho-Nd-Fe-B magnets; magnetic property; temperature stability; microstructure
Foundation item: Projects(17030901063, 18030901098) supported by the Major Science and Technology Special Program of Anhui Province, China; Project(1804a09020068) supported by the Key Research and Development Plan of Anhui Province, China; Project(20190898000002) supported by the BGRIMM Technology Group Key Fund, China
Received date: 2020-04-21; Accepted date: 2020-07-30
Corresponding author: WU Yu-cheng; Tel: +86-13605513206; E-mail: ycwu@hfut.edu.cn
(编辑 何学锋)
基金项目:安徽省科技重大专项资助项目(17030901063,18030901098);安徽省重点研究和开发计划资助项目(1804a09020068);北京矿冶科技集团有限公司重点基金资助项目(20190898000002)
收稿日期:2020-04-21;修订日期:2020-07-30
通信作者:吴玉程,教授,博士;电话:13605513206;E-mail:ycwu@hfut.edu.cn