低熔點元素及合金改性HDDR釹鐵硼磁粉的研究進展

呂 蒙, 李文獻, 朱明原, 胡業旻, 金紅明, 李 瑛

(上海大學 材料科學與工程學院 微結構重點實驗室,上海 200072)

低熔點元素及合金改性HDDR釹鐵硼磁粉的研究進展

呂 蒙, 李文獻, 朱明原, 胡業旻, 金紅明, 李 瑛*

(上海大學 材料科學與工程學院 微結構重點實驗室,上海 200072)

氫化-歧化-脫氫-再復合(HDDR)工藝是制備各向異性釹鐵硼(NdFeB)磁粉的主要方法.但HDDR磁粉實際矯頑力(HC)較低,重稀土元素Dy的引入可以顯著提高其HC,經研究發現引入的Dy主要分布于磁體晶界,起調控晶界相的作用:增加晶界厚度,提高磁粉的各向異性場(HA).但重稀土元素Dy自然資源匱乏且價格昂貴,限制了HDDR磁粉的發展.為減少磁粉中重稀土元素用量、降低成本,研究人員通過晶界擴散低熔點元素及合金來替代重稀土元素Dy,因低熔點物質在擴散過程中呈液相,提高了擴散介質與晶界相的接觸面積及擴散系數,有利于其沿晶界擴散并調控晶界相,使磁粉HC提高.對近些年晶界擴散低熔點元素及合金提高HDDR-NdFeB磁粉HC的部分研究成果進行了歸納.

HDDR磁粉; 晶界擴散; 矯頑力; 低熔點金屬; 微觀結構

0 前 言

釹鐵硼(NdFeB)永磁體由于其遠高于其他磁體的磁性能而被廣泛應用于電子信息、醫療設備、電動汽車、風力發電等行業,是國民經濟和國防工業發展不可或缺的基礎功能材料之一.進入21世紀后,隨著清潔能源等新興產業的快速發展,進一步推動了高性能永磁材料,特別是NdFeB永磁體的發展.但同時也對NdFeB永磁體的性能提出了更嚴格的要求[1-2],需要其具有更高的矯頑力(HC)以抑制磁體在較高溫度(150 ℃)下的快速磁衰退現象[3].

1989年,Takeshita等[4-6]開發了制備NdFeB磁粉的氫化-歧化-脫氫-再復合(HDDR)工藝,首先使稀土化合物吸氫并歧化分解,然后脫氫促使歧化產物轉變成細小晶粒,接近單疇粒子尺寸(250～300 nm).HDDR工藝細化了NdFeB磁粉的晶粒,提高了磁粉的HC[7-9].經過二十多年的發展,HDDR工藝不斷地改進和完善,已由最初制備各向同性磁粉發展成為制備高HC各向異性NdFeB磁粉最有效、經濟的方法.由此工藝制備的HDDR磁粉最大磁能積值是傳統快淬法制備NdFeB磁粉的3～4倍[10].

NdFeB磁體的永磁性能具有較大的負溫度系數,因而其磁性能會隨溫度的升高急劇降低,影響其應用.改善釹鐵硼磁粉溫度特性的措施是提高其各向異性場(HA)及HC[11-12],通常是引入重稀土元素Dy.為探究Dy的作用,Nakamura等[13]于2005年首次將“晶界擴散”(GBD)的概念應用于釹鐵硼磁體,對純Dy、Dy2O3、DyF3和Dy-Ni-Al合金等通過磁控濺射、氣相沉積、表面涂覆和浸漬等工藝,在磁體表面形成擴散源并對其擴散過程進行了研究.發現Dy沿著熔融的液態晶界富釹相擴散,修飾、優化磁粉晶界相微觀結構和成分,增加了晶界相厚度及去磁耦合能力,提高了磁體的HA,從而使釹鐵硼磁體HC提高[14-21].但重稀土元素在自然界儲量少、價格昂貴,且Dy與Fe呈反鐵磁性易引起磁稀釋效應[22-24],而無Dy的HDDR釹鐵硼磁粉HC僅有16.5 kOe左右,遠不能滿足實際應用[25].人們期望通過晶界擴散低熔點金屬代替重稀土元素Dy在晶界中的作用,由于低熔點介質在擴散過程中以液相存在,提高了主相與晶界相間的潤濕性,促進了晶界相傳質過程,有利于其均勻分布,促使磁體HC提高.本文作者就晶界擴散低熔點元素及合金對HDDR-NdFeB磁粉HC的影響進行了闡述.

1 晶界擴散低熔點金屬對HDDR磁粉性能影響

Sepehri-Amin等[25-27]探究了元素Ga對HDDR釹鐵硼磁粉晶界相結構和成分的影響.隨著脫氫再復合(DR)時間的增加,Ga的擴散可以使晶界相寬度明顯增加,并由晶態向非晶態轉變(圖1).

進一步用三維原子探針(3DAP)(圖2)分析了Ga在晶界相中的分布,發現Ga的富集促使晶界相中Nd含量升高而Fe、Co含量減少,降低了晶界相的磁導率,說明Ga的擴散可以增強晶界相去磁交換耦合能力及對疇壁的釘扎作用,使HDDR磁粉HC提高.

圖1 不同處理時間含Ga樣品的高分辨電子顯微鏡(HREM)圖.(a) tDR=10 min;(b) tDR=15 min;(c) tDR=20 min;(d) 無Ga,tDR=20 min[25]

圖2 (a) Ga和Nd的3DAP分布圖和(b) 晶界元素分布圖[25]

Morimoto等[28]通過晶界擴散金屬Al制備出HC達22.12 kOe的HDDR磁粉,經研究發現Al的晶界擴散有利于提高主相Nd2Fe14B和晶界相間的潤濕性,使晶界變得更加光滑、平直,并且使磁粉晶界相厚度由1.5 nm增加到3 nm(圖3),相鄰主相被有效分離[29-30].

圖3 含Al原子百分數不同的樣品高分辨電子顯微鏡圖.(a) 原子百分數為12.9% Nd-0% Al;(b) 原子百分數為12.9% Nd-1.5% Al[28]

利用電子探針X射線顯微(EPMA)分析進一步探究了低熔點金屬Al對HDDR磁粉晶界相的優化機制(圖4),發現Al元素初始主要集中在主相Nd2Fe14B的表面(圖4c).在擴散過程中,Al元素首先在主相Nd2Fe14B的邊界處形成富鋁金屬相,隨后富Al相與富Nd相反應形成流動性更好的富Nd-Al液相(圖4b),增加了主相-中間相的潤濕性,減小了反磁化形核的場所,抑制了反磁化形核.但由于Al元素在主相Nd2Fe14B中的溶解度較大,主相Nd2Fe14B中的Fe易被部分Al替代,使磁粉飽和磁化強度降低[31].

圖4 (a) 原子百分數為13.5% Nd-1.5% Al樣品的背散射(BSE)圖;(b) Nd元素濃度富集圖;(c) Al元素濃度富集圖[28]

Dempsey等[32]對高矯頑力NdFeB膜研究時發現(圖5),富釹相中Cu的存在降低了晶界相的熔化溫度,增加了主相-晶界相的潤濕性,促進了晶界富Nd相的均勻分布,增強了晶界相對主相的磁隔離作用.

2 晶界擴散低熔點合金對HDDR磁粉性能影響

Liu等[33]研究了晶界擴散Nd70Cu30溫度(600、700、800 ℃)對HDDR磁粉微觀結構和磁性能的影響,在700 ℃晶界擴散Nd-Cu合金(質量分數為6%)時,磁粉HC達到最大值16.9 kOe.物相分析發現,600 ℃和800 ℃晶界擴散后磁粉特征峰向高角度方向偏移,表明Nd2Fe14B相的晶格發生收縮,可能是由于晶格中殘余氫的釋放所引起[34-35](圖6).

圖5 薄膜樣品的(a) 透射電子顯微鏡(TEM)圖和(b) 3DAP圖[32]

圖6 (a) HDDR磁粉的X射線衍射(XRD)圖和(b) 局部放大圖[33]

通過TEM分析(圖7)可知,主相外圍形成了光滑連續的晶界富釹相,并且晶界相中的Nd濃度由于液相Nd-Cu合金的進入而高于初始HDDR磁粉,增加了晶界厚度.因此,優化晶界擴散溫度不僅有利于低熔點Nd-Cu合金液相對HDDR磁粉晶界相微觀結構及成分的修飾和改善,還有利于抑制Nd2Fe14B晶格中殘余氫的釋放[36].

Sepehri-Amin等[37]的研究結果表明低熔點Nd-Cu合金(Nd80Cu20,熔點為520 ℃)經700～800 ℃擴散處理后進入HDDR磁粉晶界,磁粉HC由16.6 kOe提高到19.5 kOe,提高了17.5%.經TEM分析可知(圖8),晶界厚度由1.3 nm增加到2.4 nm.對擴散過程進行微磁學分析發現,低熔點合金液相擴散進入晶界,減小了主相Nd2Fe14B間的磁交換耦合作用,抑制了反磁化形核并且阻礙疇壁移動,使磁粉的矯頑力增加[38].

Noguchi等[39]通過擴散三元低熔點合金Nd-Cu-Al制備出高HC的HDDR磁粉.如圖9所示,Nd、Cu元素主要富集在晶界上,而Al元素則均勻的彌散分布在主相以及晶界上.由于Nd-Cu和Nd-Al合金的熔點均低于晶界富Nd相的熔點,增加了擴散過程中主相Nd2Fe14B和富稀土相間的潤濕性,促進了富釹相的流動,加強了晶界相對主相Nd2Fe14B的包裹,孤立了硬磁相,并使主相表面更加圓滑,使反磁化形核變的更加困難.

圖7 700 ℃晶界擴散Nd-Cu(質量分數為6%)樣品的TEM圖[33]

圖8 HDDR粉末TEM圖.(a)、(c)、(e)均為初始HDDR粉末;(b)、(d)、(f)均為Nd-Cu合金擴散后HDDR粉末[37]

圖9 Nd、Cu和Al的EDS能譜圖.(a)初始HDDR磁粉;(b)擴散處理后磁粉[39]

Wan等[40]選用比Nd-Cu(520 ℃)合金熔點更低的Pr-Cu(質量分數為3% Pr68Cu32,熔點為472 ℃)合金為擴散源,經擴散處理后使HDDR磁粉HC提高到11.4 kOe,而在母合金中直接添加Pr-Cu合金制備的HDDR磁粉HC只有7 kOe.這是因為晶界擴散處理后,三叉區的大塊富Nd/Pr相消失,晶界富Nd/Pr相的分布更加均勻.

圖10 HDDR磁粉TEM圖.(a)、(b)未經晶界擴散處理;(c)、(d)晶界擴散處理[42]

Lin等[41-42]在650 ℃下晶界擴散質量分數為5%的Pr68Cu32合金,制備出HC達18 kOe的HDDR磁粉,矯頑力增加了近40%.Pr-Cu液相合金通過毛細作用沿主相-晶界相界面擴散,并逐步進入晶界相,使晶界厚度增加,同時主相Nd2Fe14B被連續的液相晶界包裹,相鄰主相被晶界相分離(圖10).并且擴散處理后晶界相中的鐵磁性元素含量明顯減小,降低了晶界相的鐵磁性[43].

Wang等[44]研究了低熔點Nd-Cu合金對熱壓/熱變形HDDR磁體HC的影響,發現擴散質量分數為2% Nd-Cu合金后,磁體相鄰主相Nd2Fe14B之間有明顯的富釹晶界相形成,厚度為2.4 nm(圖11).表明在熱壓/熱變形過程中Nd-Cu液相的擴散,使晶界厚度增加,有效孤立了硬磁相,使硬磁相之間去磁耦合.同時,Nd-Cu液相的添加,使磁體變形能力增強,有助于熱變形磁體在形變過程中織構的形成.

3 其他釹鐵硼永磁材料的晶界擴散

Zheng等[45]用高硬度碳化鎢(WC)作為擴散介質使熱變形釹鐵硼磁體的HC提高了14%.熱壓過程中,由于高硬度WC位于片狀晶粒邊界,使晶粒局部壓應力增加,抑制了主相晶粒的增長,降低了粗晶區比例;熱變形過程中WC分解,其中C元素進入相鄰的主相并與富釹相反應形成含釹碳化物,而主相中的Fe進入晶界WC中形成了新相Fe2W[46-47](圖12).新相的形成加強了晶界的釘扎作用,進一步限制了主相晶粒的長大.

圖11 不同質量分數Nd-Cu的HDDR熱變形磁體的TEM圖.(a)、(c) 0% Nd-Cu;(b)、(d) 2% Nd-Cu[44]

圖12 添加質量分數為1% WC樣品的(a)TEM圖;(b)高角度環形暗場圖(HADDF);(c)、(d)、(e)、(f)分別為Nd、Fe、W、C的EDS圖[45]

Li等[48-50]研究了不同擴散物質對一段式熱壓釹鐵硼磁體的影響,發現對矯頑力提高最大的擴散物質為Zn粉,可以使熱變形快淬粉(MQPA)磁體的矯頑力提高57%,這是因為低熔點金屬Zn在熱變形溫度的作用下熔化并沿晶界擴散,增強了晶界相的釘扎能力,限制了主相晶粒的增長.

Saito等[51]發現質量分數為1% Zn的擴散可以使熱壓磁體的HC提高57.8%.微觀分析表明,磁體HC提升的主要原因是Zn的擴散使磁體主相晶粒尺寸由60 nm減小到50 nm,細化了晶粒尺寸.

Zhou等[52]在燒結釹鐵硼磁體表面磁控濺射MgO并研究了其晶界擴散過程對磁性能的影響.發現MgO擴散進入晶界后與晶界相反應生成Nd-O-Fe-Mg新相,改善了主相-晶界相間的潤濕性,同時新相的形成加強了晶界對疇壁的釘扎能力.

Ni等[53-54]研究發現Al-Cu合金(Al85Cu15,熔點為575±25 ℃)的晶界擴散可以提高晶界富Nd相的流動性,使晶界變得更加清晰、光滑、連續,加強了其對主相的隔離.

大量研究表明,無論是晶界擴散低熔點金屬、合金或者化合物還是晶界擴散高熔點/高硬度化合物,其對NdFeB磁體HC的提升效果均比較顯著.晶界擴散是包含金屬學和磁性物理學的復雜過程,其擴散機制仍在不斷探索之中并被各國研究者廣泛關注.

4 小 結

采用晶界擴散低熔點元素及合金替代重稀土元素Dy,是制備高矯頑力無Dy-HDDR釹鐵硼磁粉的重要方法.低熔點擴散源在適宜的擴散工藝下呈液態沿磁粉晶界擴散,促進了擴散介質的流動,使其與晶界相的接觸面積及擴散系數增加,便于擴散介質進入晶界富稀土相,調控晶界相微觀結構.

隨著人們對晶界擴散工藝研究的不斷深入,晶界擴散介質由含Nd、Pr等元素的低熔點稀土合金逐漸發展到低熔點金屬及金屬氧化物、氮化物等非稀土擴散介質,進一步減少稀土元素用量,降低生產成本;擴散方法也已發展出磁控濺射,表面涂覆、蒸鍍及電鍍等多種工藝.

然而,由于晶界擴散源(如Al、Cu、Nd-Al等低熔點金屬)均為非磁性元素,添加后雖能使磁粉HC顯著提高,但同時也可能會導致剩磁(Br)部分降低.因此在不斷提高矯頑力的前提下,還應對HDDR釹鐵硼磁粉各向異性形成機理,矯頑力形成機制和HDDR工藝展開深入地研究,以開發具有高矯頑力且其他磁能優異的HDDR-NdFeB磁體.

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ProgressofHDDRNdFeBpowdersmodulatedbythediffusionoflowmeltingpointelementsandtheiralloys

LyuMeng,LiWenxian,ZhuMingyuan,HuYemin,JinHongming,LiYing*

(Laboratory for Microstructures,School of Materials Science and Engineering,Shanghai University,Shanghai 200072,China)

The hydrogenation-disproportionation-desorption-recombination (HDDR) process is the main technique for the fabrication of anisotropic NdFeB magnetic powder.But the intrinsic coercivity (HC) of HDDR magnetic powder is low.The addition of heavy rare earth element Dy could improve itsHC.It was found that the added Dy is mainly distributed in the grain boundary of HDDR magnets,which regulates grain boundary phase and increases the thickness of grain boundary to improve the anisotropy field (HA) andHCof the magnets.However,Dy becomes scarcer and more expensive,which limits the practical application ofHDDR magnets.To reduce the dependence on heavy rare earth elements and cost,researchers replaced the heavy rare earth element Dy by low melting point elements and their alloys through grain boundary diffusion technique.During diffusion process low melting point metal exists as liquid phase that increases the diffusion coefficient of diffusion medium as well as its contact area with grain boundary phases of HDDR magnets,and benefits its diffusion along grain boundaries and regulation of grain boundary phase.The modified grain boundary in magnets improveHC.This review paper focuses on the research progress in improvingHCof HDDR NdFeB magnets by low melting point elements and their alloys.

HDDR power; grain boundary diffusion; coercivity; low melting metal; microstructure

10.3969/J.ISSN.1000-5137.2017.06.014

2017-09-29

國家自然科學基金委員會面上項目(51572166);上海高校特聘教授(東方學者)崗位計劃項目(TP2014 041)

呂 蒙(1991-),男,博士研究生,主要從事磁性功能材料方面的研究.E-mail:lv1149879734@163.com

*通信作者: 李 瑛(1962-),女,博士,教授,博士生導師,主要從事功能材料方面的研究.E-mail:liying62@shu.edu.cn

呂蒙,李文獻,朱明原,等.低熔點元素及合金改性HDDR釹鐵硼磁粉的研究進展 [J].上海師范大學學報(自然科學版),2017,46(6):888-898.

formatLyu M,Li W X,Zhu M Y,et al.Progress of HDDR Nd-Fe-B powders modulated by the diffusion of low melting point elements and their alloys [J].Journal of Shanghai Normal University(Natural Sciences),2017,46(6):888-898.

TM 273

A

1000-5137(2017)06-0888-11

郁 慧)