董小平,楊麗穎,王 青,林玉芳,趙棟梁,張羊換

(1.河北大學(xué)質(zhì)量技術(shù)監(jiān)督學(xué)院,河北保定071002)(2.鋼鐵研究總院功能材料研究所,北京100081)

熱處理La-Mg-Ni系貯氫合金微觀結(jié)構(gòu)研究進(jìn)展

董小平1,楊麗穎1,王 青1,林玉芳2,趙棟梁2,張羊換2

(1.河北大學(xué)質(zhì)量技術(shù)監(jiān)督學(xué)院,河北保定071002)
(2.鋼鐵研究總院功能材料研究所,北京100081)

董小平

熱處理對(duì)貯氫合金的微觀組織結(jié)構(gòu)影響較大。詳細(xì)綜述了La-Mg-Ni系貯氫合金近年來(lái)熱處理對(duì)相結(jié)構(gòu)、相組成、晶胞體積、顯微組織影響的研究現(xiàn)狀,旨在為改善具有優(yōu)異電極性能的La-Mg-Ni系貯氫合金微觀組織的控制因素提供思路和依據(jù)。

熱處理;La-Mg-Ni系合金;微觀結(jié)構(gòu)

前 言

復(fù)相La-Mg-Ni系合金因其材料晶胞體積較大和電化學(xué)容量高(≥350 mA·h/g)、吸放氫動(dòng)力學(xué)特性良好等特點(diǎn)已在貯氫材料領(lǐng)域受到特別關(guān)注[1-2]。目前文獻(xiàn)資料表明此類(lèi)合金大多采用真空感應(yīng)爐或電弧爐熔煉,在熔融合金隨爐冷卻過(guò)程中,由于冷卻速度小,易造成合金鑄錠中組分偏析,微結(jié)構(gòu)不均勻,從而影響合金電極的吸放氫性能。為克服這些工藝因素帶來(lái)的不利影響,對(duì)鑄態(tài)合金需經(jīng)后續(xù)熱處來(lái)改善合金的組織,以提高合金的性能。研究表明,熱處理是改善貯氫合金電化學(xué)吸放氫性能的有效途徑之一[3-4],對(duì)La-Mg-Ni系合金電極更是如此。適當(dāng)?shù)臒崽幚砟芟辖鸬慕Y(jié)構(gòu)應(yīng)力,減少組分偏析,使合金均質(zhì)化[5-6]。本文綜述熱處理對(duì)La-Mg-Ni系貯氫合金相結(jié)構(gòu)、相組成、晶胞體積、顯微組織的影響。

熱處理對(duì)相結(jié)構(gòu)與相含量的影響

合金的相結(jié)構(gòu)類(lèi)型與相含量直接影響合金的吸放氫電化學(xué)性能和動(dòng)力學(xué)特性[7]。Zhang等[8]對(duì)La-Mg-Ni系貯氫合金的研究結(jié)果表明(圖1),當(dāng)退火溫度為1 073 K和1 173K時(shí),La1.5Mg0.5Ni7合金中的LaNi5與La1.29Mg0.71Ni3相消失。隨退火溫度升高到1 173 K時(shí),合金相由鑄態(tài)時(shí)的La1.37Mg0.63Ni7,La1.29Mg0.71Ni3,La Ni5,LaMgNi4等多相組成轉(zhuǎn)變?yōu)長(zhǎng)a1.37Mg0.63Ni7與La1.41Mg0.59Ni7相組成。實(shí)驗(yàn)表明,退火處理有利于合金中La1.37Mg0.63Ni7相的形成,在1 173 K時(shí)成為合金主相,其含量達(dá)到43%(質(zhì)量分?jǐn)?shù),下同)。同時(shí),隨退火溫度升高,合金中La1.37Mg0.63Ni7相的含量從鑄態(tài)時(shí)的43%增加到1 173 K時(shí)的57%。儲(chǔ)愛(ài)民等[9]報(bào)道,在0.3MPa氬氣氣氛中對(duì)La0.75Mg0.25Ni3.5Co0.4合金在1 223 K和1 323K下退火10 h,有利于(La,Mg)(Ni,Co)3相形成。Miao等[10]對(duì)La0.7Mg0.3Ni2.45-xCo0.75+xMn0.1Al0.2(x=0.15,0.30)合金在真空下1 173 K退火處理8 h,其合金中La Ni5和(La,Mg)Ni3相的含量如表1所示。與鑄態(tài)合金相比,退火后的LaNi5物相含量增加,而(La,Mg)Ni3相含量減少。周增林等對(duì)La0.7Mg0.3(Ni0.85Co0.15)3.4合金在1 173 K熱處理8 h后,其相結(jié)構(gòu)并沒(méi)有發(fā)生轉(zhuǎn)變,但相含量出現(xiàn)了明顯變化,即熱處理后,合金中(La,Mg)2(Ni,Co)7相含量從鑄態(tài)的57.2%增加到熱處理態(tài)的61.6%;(La,Mg)(Ni,Co)3,(La,Mg)(Ni,Co)2相分別從鑄態(tài)的11.3%,10.3%減少到熱處理態(tài)的9.7%,7.5%,而La(Ni,Co)5相含量幾乎不變[11]。Li等[12]研究了(LaPrNdZr)0.83Mg0.17(NiCoA lMn)3.3合金在不同溫度退火5 h后電化學(xué)充放電前后合金相含量的變化,如表2所示,表中A為(LaPrNdZr,Mg),B為(NiCoA lMn)。在退火溫度為1 173,1 198 K時(shí),循環(huán)后較循環(huán)前合金中A2B7相含量減少,在1 223,1 248,1 273 K時(shí),循環(huán)后A2B7相含量也相對(duì)減少,同時(shí)AB3相形成。隨退火溫度升高,循環(huán)前合金中的CaCu5型物相含量的變化過(guò)程為:10.2%→7.7%→0%→8.8%→27.3%;Ce5Co19相含量隨退火溫度的升高先增加,并在1 223 K時(shí)成為合金的主相,但隨溫度進(jìn)一步升高而減少。與循環(huán)前相比,合金電極在堿性溶液中35次充放電循環(huán)后,合金中A2B7相含量減少,AB3相消失,而AB5,A5B19相含量增多,這歸因于在堿性溶液中合金物相的腐蝕速率順序?yàn)锳B3>A2B7>AB5～A5B19。

圖1 La1.5Mg0.5Ni7合金中相含量隨退火溫度的變化Fig.1 Effect of annealing temperature on phase contentofLa1.5Mg0.5Ni7alloy

表1 La0.7Mg0.3Ni2.45-xCo0.75+xMn0.1Al0.2(x=0.15,0.30)合金中各相的含量(w/%)Table 1 Pha ses content in the La0.7Mg0.3Ni2.45-xCo0.75+x Mn0.1Al0.2(x=0.15,0.30)a lloys(w/%)

表2 退火(LaPr NdZr)0.83Mg0.17(NiCoA lMn)3.3合金充放電前后物相含量(w/%)及晶胞體積(nm3)Table 2 Pha se content(w/%)and cell vo lum e(nm3)of the annealed alloys befo re and afte r charging/discha rging

熱處理對(duì)晶體學(xué)參數(shù)的影響

Denys等[13]基于XRD譜數(shù)據(jù)研究了退火溫度1 023 K下處理96 h的Ce2Ni7型La1.5Mg0.5Ni7合金的晶體學(xué)參數(shù),如表3所示。Ce2Ni7型相屬P63/mmc空間群,晶胞結(jié)構(gòu)4f位置上Mg原子與La原子共享,而Mg原子占有率為0.51。其晶胞常數(shù)分別為a=0.502 822 nm,c=2.420 32 nm,V=0.529 95 nm3。文獻(xiàn)[8]研究發(fā)現(xiàn),La1.5Mg0.5Ni7合金經(jīng)1 123 K退火24 h后與文獻(xiàn)[13]相比,Ni原子占有率沒(méi)有發(fā)生改變,而La和Mg原子在4f位置的占有率不同,分別為0.41,0.59,晶胞常數(shù)增大,分別為a=0.503 2 nm,c=2.422 2 nm,V=0.531 238 nm3,兩者的差別主要與退火溫度、退火時(shí)間不同有關(guān)。Pan等[14]報(bào)道,退火導(dǎo)致La0.7Mg0.3Ni2.45Co0.75Mn0.1Al0.2合金的晶胞體積膨脹,即隨退火溫度(1 173,1 273,1 373 K)升高,(La,Mg)Ni3,LaNi5主相晶胞常數(shù)a軸分別從鑄態(tài)時(shí)的0.507 7,0.505 7 nm增加到1 373 K時(shí)的0.508 6,0.506 4 nm,c軸分別從鑄態(tài)時(shí)的2.486 0,0.402 8 nm增加到1 373 K時(shí)的2.488 7,0.404 0 nm,晶胞體積V分別從鑄態(tài)時(shí)0.554 95,0.089 20 nm3增加到1 373 K時(shí)的0.557 67,0.089 71 nm3。周增林等[15]對(duì)比研究La0.7Mg0.3(Ni0.81Co0.15Al0.04)3.4鑄態(tài)與熱處理態(tài)合金相的晶胞體積發(fā)現(xiàn),在1 173 K下熱處理8 h后,各物相的晶胞體積均有所增加,其中Ce2Ni7型(La,Mg)2(Ni,Co)7相的晶胞體積增加0.45%,CaCu5型La(Ni,Co)5相的晶胞體積增加0.68%,增幅較大。不同退火條件影響金屬原子擴(kuò)散到其它物相晶胞位置,從而導(dǎo)致晶胞體積的改變(表3),隨退火溫度升高,(LaPrNdZr)0.83Mg0.17(NiCoA lMn)3.3合金中Ce2Ni7型物相在較低溫度時(shí)的晶胞體積小于較高溫度退火后的晶胞體積,Ce5Co19型相的晶胞體積在1 198 K時(shí)相對(duì)較大,CeNi3型相的晶胞體積增大,CaCu5型相的晶胞體積基本保持不變[12]。

表3 Ce2Ni7型La1.5Mg0.5Ni7合金的晶體學(xué)參數(shù)Table 3 C rysta lpa ram eters of Ce2N i7-type La1.5Mg0.5Ni7a lloy

熱處理對(duì)顯微組織的影響

一般來(lái)說(shuō),合金的顯微組織均勻,貯氫合金電極的吸放氫平臺(tái)的平坦性得到改善。電化學(xué)吸放氫容量高,則電極抗腐蝕性能好?？梢詮腦RD譜主相衍射峰寬度定性判斷合金的均勻性。劉永峰等人[16-18]分析La0.7Mg0.3Ni2.8Co0.5鑄態(tài)合金的XRD譜主峰寬度發(fā)現(xiàn),合金退火處理后其XRD衍射峰的寬度明顯變窄,表明退火后合金中偏析相減少,合金成分更加均勻。Zhang等人[19]用SEM對(duì)La0.67Mg0.33Ni2.5Co0.5合金在1 023,1 173 K退火后的顯微組織進(jìn)行觀察發(fā)現(xiàn),在1 023 K時(shí),由于合金的共析反應(yīng)不完全導(dǎo)致合金微觀組織不均勻,而1 173 K時(shí),合金的組織及合金成分相對(duì)比較均勻。Dong等[20-21]對(duì)La0.75Mg0.25Ni3.5Co0.2鑄態(tài)和1 123,1 323 K退火態(tài)進(jìn)行SEM觀察的結(jié)果如圖2所示,分析圖2可見(jiàn),退火后合金組織與鑄態(tài)組織比較,均勻性得到改善,各物相之間的相界更加清晰,同時(shí)晶粒變得粗大。此外,由于退火后合金中殘余相的熔解,導(dǎo)致組織形貌中出現(xiàn)大量的孔隙。周增林等[15]研究了La0.7Mg0.3(Ni0.81Co0.15Al0.04)3.4合金鑄態(tài)及1 173 K,8 h熱處理態(tài)的顯微組織,結(jié)果表明,鑄態(tài)合金組織為薄板狀或扁條狀,或相互平行的板條形成交叉板條束,1 173 K,8 h條件下熱處理后,板條狀組織消失,合金中Ni相消失而出現(xiàn)富Mg相;同時(shí),1 173 K,8 h熱處理還使合金中(La,Mg)(Ni CoAl)3.1～3.5相和La(NiCoAl)4.8～(La,Mg)(NiCoAl)4.0相中Ni元素和Mg元素的濃度差減小。Song等[22]分析了La0.8Mg0.2Ni2.4Mn0.10Co0.55Al0.10合金的斷口形貌,發(fā)現(xiàn)鑄態(tài)合金的斷口表面平滑,經(jīng)過(guò)1 173 K退火后,斷口表面粗糙,并出現(xiàn)細(xì)小顆粒,如圖3所示。

圖2 La0.75Mg0.25Ni3.5Co0.2合金的SEM照片:(a)鑄態(tài),(b)1 123 K,(c)1 323 K退火態(tài)Fig.2 SEMmicrographs of the La0.75Mg0.25Ni3.5Co0.2alloys:(a)as-cast,(b)annealed at 1 123 K,and(c)annealed at 1 323 K

圖3 La0.8Mg0.2Ni2.4Mn0.10Co0.55Al0.10合金斷口的SEM照片:(a)鑄態(tài),(b)1 173 K退火態(tài)Fig.3 SEMimages of fracture ofLa0.8Mg0.2Ni2.4Mn0.10Co0.55Al0.10 alloy:(a)as-cast and(b)as-anncaled at 1 173 K

結(jié) 語(yǔ)

貯氫合金微觀組織對(duì)電極的電化學(xué)性能有很大影響。由于熱處理參數(shù)的不同,熱處理后La-Mg-Ni系合金的結(jié)構(gòu)和性能改變均有差異。應(yīng)選擇適當(dāng)?shù)臒崽幚砉に囀公@得的相結(jié)構(gòu)、相含量有利于合金電極性能的改善。但熱處理溫度不宜過(guò)高,溫度過(guò)高易使晶粒粗大,或使合金過(guò)燒,導(dǎo)致性能變差。為了獲得有利于合金電極性能的微觀組織結(jié)構(gòu),任需進(jìn)一步從保溫時(shí)間、熱處理溫度、升溫/冷卻速率、熱處理方式等進(jìn)行優(yōu)化匹配,以取得更好的效果,在這方面雖然已進(jìn)行了大量的實(shí)驗(yàn)研究,但探究具有更優(yōu)異電極性能的貯氫合金微觀組織結(jié)構(gòu)的控制因素,仍有許多工作要做。

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Research Progress of Microstructure of the Annealed La-Mg-Ni System Hydrogen Storage Alloy

DONG Xiaoping1,YANG Liying1,WANG Qing1,LIN Yufang2,ZHAO Dongliang2,ZHANG Yanghuan2

(1.College of Quality and Technical Supervision,Hebei University,Baoding,071002,China)
(2.Department of Functional Material Research,Central Iron and Steel Research Institute,Beijing 100081,China)

It is of great influence to annealing treatmenton micro structure for hydrogen storage alloy.Recently,the researches in which annealing treatment can influence the phase structure,phase component,cell volume and microscopic structure ofLa-Mg-Ni system alloy were reviewed in this paper.It will provide researchers with thinking and ground to improve the control factors ofmicro structure for hydrogen storage alloy with excellent property.

annealing treatment;La-Mg-Ni system alloy;micro structure

TG139+.7

A

1674-3962(2010)03-0044-05

2009-10-19

國(guó)家863計(jì)劃資助(2007AA03Z230,2007AA03Z227);河北大學(xué)博士基金資助(2009-152)

董小平,男,1974年生,博士