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Intermetallics 43 (2013) 45e52

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Intermetallics

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Direct synthesis of NbeAl intermetallic nanoparticles by sodiothermichomogeneous reduction in molten salts

Na Wang, Chao Du, Jungang Hou, Yao Zhang, Kai Huang, Shuqiang Jiao, Hongmin Zhu*

State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing, 100083, PR China

a r t i c l e i n f o

Article history:Received 26 November 2012Received in revised form5 June 2013Accepted 8 July 2013Available online 3 August 2013

Keywords:A. Niobium aluminidesA. Nanostructured intermetallicsC. Reaction synthesisF. DiffractionF. Electron microscopy, scanningF. Electron microscopy, transmission

* Corresponding author. Tel.: þ 86 10 62332267.E-mail address: hzhu@metall.ustb.edu.cn (H. Zhu)

0966-9795/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.intermet.2013.07.005

a b s t r a c t

NbeAl intermetallic nanoparticles were directly synthesized via sodiothermic reduction process inmolten salts using NbCl5 and AlCl3 as the raw materials. The as-prepared samples were characterized byX-ray diffraction, scanning electron microscopy and transmission electron microscopy. The NbCl5 andAlCl3 were dissolved in LiCleKCleCaCl2 or LiCleKCleNaCleCaCl2 molten salts forming a homogeneoussystem. It was found that a series of intermetallic nanoparticles, such as Nb3Al, Nb2Al, NbAl3 and Nb2Al/NbAl3, were successfully synthesized at low temperature of 350e500 �C using the homogeneous moltensalt systems. The phase transformations of Nb3Al, Nb2Al and NbAl3, were achieved via the controllablevariation of molar ratio of Nb to Al. Furthermore, the influence of the reaction temperature on theparticle size of the intermetallic nanoparticles was also investigated.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Intermetallic compounds are considered as prospective mate-rials for various high-temperature and structural applications.Among these intermetallic compounds, niobium aluminides havereceived much attention as high-temperature structural materials[1,2] because of their high melting point, high elastic modulus,low density, high ductility, high toughness and good supercon-ductivity [3e5]. Turbine blades in air-craft engines and stationarygas turbines are possible applications. Nb3Al is also known as oneof the A15 superconductors which are applied for nuclear mag-netic resonance and nuclear magnetic imaging magnets [6e10].However, despite its good high-temperature properties, lowductility and toughness at room temperature make it a difficultmaterial to machine [2,11]. The powder-metallurgical method isconsidered the effective way to solve this problem. In this way,NbeAl particles are synthesized and casted into different shapesand sizes [12].

Over recent years, different methods of synthesis of NbeAlintermetallic compounds such as conventional melting and solid-ification [13], powder metallurgy [14,15], reaction sintering [16]

.

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and reaction sintering with prior mechanical activation [17,18]have been widely investigated. Among these routes, there is anobvious drawback that various metals are directly employed toproduce intermetallic compounds by means of diffusion reaction.Although the metal could diffuse sufficiently, it is still hard to avoidmaldistribution of the components, which impairs the properties ofthe materials. However, the molten salts system, as solvent forsynthesis of metal in the electrochemistry and hydrometallurgy,has very stable electrochemical properties and high decompositionvoltage. As many raw materials of chloride could dissolve in halidemolten salts forming a homogeneous system, uniformly interme-tallic compounds could be synthesized via co-reduction process[19,20]. Ni3Al intermetallic compounds were synthesized from Aland NiCl2 in the various molten salt systems such as AlCl3eNaCl(63:37 mol%) and AlCl3eNaCleKCl (66:20:14 mol%). Also, inter-metallic compounds of Cr2Al, Ni3Al, and NiAl3 were prepared viaelectrochemical deposition process in AlCl3eNaCleKCl molten saltscontaining CrCl2 and/or NiCl2 at 423 K. Thus, the halide molten saltsfacilitating the homogeneous distribution of the different halides asraw materials are promising solvents to produce intermetalliccompounds.

The Armstrong method is a representative technique of pro-ducing nanoparticles [21]. For example, TieAleV was produced byArmstrong method using TiCl4, AlCl3 and VCl3 through sodiumreduction. The solids of TiCl4, AlCl3 and VCl3 were mixed and boileduntil an equilibrium with the vapor was attained, and thereafter

N. Wang et al. / Intermetallics 43 (2013) 45e5246

introducing the equilibrium vapor into the liquid reductant sodiumto form alloy powders of the equilibrium vapor constituents in thepreselected atomic ratio. One critical advantage of Armstrongmethod is that raw materials were mixed in the form of gasesforming a homogeneous system which could be in situ reduced bysodium. Also, in our previous work, fine intermetallic powders havebeen synthesized from a homogeneous system [22]. For example,NbeSn and NbeTa intermetallic powders were prepared viahydrogen reduction of NbCl5eSnCl2 and NbCl5eTaCl5 vapors withthe preselected atomic ratio, which were mixed homogeneously. Itwas demonstrated that a homogeneous system using variouschlorides as raw materials would be favorable for the fabrication ofintermetallic powders.

In this work, NbeAl intermetallic nanoparticles were directlysynthesized via sodiothermic reduction process in molten saltsusing NbCl5 and AlCl3 as the raw materials. The as-prepared sam-ples were characterized by X-ray diffraction, scanning electronmicroscopy and transmission electron microscopy. The NbCl5 andAlCl3 were dissolved in LiCleKCleCaCl2 or LiCleKCleNaCleCaCl2molten salts forming a homogeneous system. It was found that aseries of Nb3Al, Nb2Al, NbAl3 and Nb2Al/NbAl3 intermetallicnanoparticles were successfully synthesized at low temperature of350e500 �C using the homogeneous molten salts systems con-taining Nb5þ and Al3þ with a various molar ratio, as exemplified bythe following reaction:

xNbCl5 þ yAlCl3 þ (5x þ 3y)Na ¼ NbxAly þ (5x þ 3y)NaCl

It makes no difference in obtained products using the twodifferent molten salts (LiCleKCleCaCl2 or LiCleKCleNaCleCaCl2) asmedia. Furthermore, the influence of the reaction temperature onthe particle size of the intermetallic nanoparticles was investigated.

Fig. 1. The XRD patterns and FESEM images of as-prepared products b

2. Experimental section

The raw materials used in this study were pure aluminumchloride (AlCl3, purity > 99.99%) and pure niobium pentachloride(NbCl5, purity > 99.99%). The reductant was the pure sodiummetal(Na, purity > 98.50%). According to the LiCleKCleCaCl2 and LiCleKCleNaCleCaCl2 phase diagram [23], LiCle11.6mol%KCle36.1mol%CaCl2 (abbreviated as LKC) and LiCle24.0mol%KCle5.0mol%NaCle18.0mol%CaCl2 (abbreviated as LKNC) were chosen.

NbCl5 and AlCl3 powders were pre-melted with molten saltsbecause of their high vapor pressure. First of all, the blank moltensalts of LKC and LKNC were prepared before pre-melting treatment.Mixture of salts was heated up slowly to 200 �C and dehydrated invacuum for 6 h in a resistance furnace which contains a gas supply/exhaust/vacuum system. Then the salt mixture was melted at450 �C for 4 h under an argon (Ar) atmosphere and then cooled toroom temperature. After the blank molten salts of LKC and LKNCwere prepared, NbCl5 and AlCl3 powders with a preselected ratiowere mixed with molten salts in the glove box under an argonatmosphere. Secondly, to dissolve NbCl5 and AlCl3 in LKC and LKNCmolten salts, as-prepared mixture of raw materials were put into asealed quartz reactor and then pre-melted at 450 �C for 1 h. Finally,the sealed reactor cooled to room temperature and the solidifiedmixture was crushed into small particles in the glove box. Also, thepure metal sodium ingot was cut into small particles in the glovebox for reduction.

Subsequently, the LKC and LKNC blank molten salts were putinto a corundum crucible which was placed inside a resistancefurnace contains a gas supply/exhaust/vacuum system. The saltsweremelted at 450 �C for 1 h under an argon atmosphere. And thenthe pre-melted precursor and the sodium particles (20% excess)were fed into the molten salts. The mixture was constantly stirred

y sodiothermic reduction from AlCl3 or NbCl5 in the molten salts.

N. Wang et al. / Intermetallics 43 (2013) 45e52 47

at a rate of 50 rpm. After the reaction, the resistance furnace wascooled down at room temperature overnight. The mixture ofproducts and solidified salts were taken out andwashed by purifiedwater. After washing, filtering and drying in vacuum, the as-prepared products were obtained.

The phases of as-prepared products were determined using anX-ray diffractometer (XRD, M21X, MAC Science Co. Ltd., Japan.)with Cu Ka radiation. In addition, the morphology and elementanalysis of as-prepared products were characterized using a fieldemission scanning electron microscopy (FESEM, Z1ISS, ULTRA55,Germany.) equipped with an energy dispersive spectrometer (EDS,X-MAX50, England). The crystalline shape of the grains of as-prepared products was examined using a transmission electronmicroscopy (TEM, JEM-2010, JEOL Co., Japan).

3. Results and discussion

3.1. Preparation of Nb or Al powder

The XRD patterns and FESEM images of the as-prepared prod-ucts by sodiothermic reduction of AlCl3 or NbCl5 in LKCmolten saltsat 450 �C are presented in Fig. 1. As shown in Fig. 1(a), the as-prepared products using AlCl3 as raw material can be indexed towell crystallized Al structure (PDF NO. 89-2837) with the majorpeaks at 2q ¼ 38.38, 44.61, 64.92, 78.01, 82.20�, corresponding tothe diffractions of the (111), (200), (220), (311) and (222) planes ofthe cubic Al. Also, the as-prepared products using NbCl5 as rawmaterial can be indexed to well crystallized Nb structure (PDF NO.

Fig. 2. The XRD patterns and FESEM images of as-prepared products by sodiothermic

34-370) with the major peaks at 2q ¼ 38.51, 55.59, 69.68, 82.54�,corresponding to the diffractions of the (110), (200), (211) and (220)planes of the cubic Nb. Moreover, it can be observed from Fig. 1(c)that the particle size of Al ball is about 200e900 mm while the as-prepared Nb nanoparticles are obtained with the particle size rangeof 50e200 nm (Fig. 1(d)). In order to assure the purity of singlephase Nb and Al, the energy dispersive spectra of the as-preparedsamples were employed. Based on EDS data as shown in support-ing information (Fig. S1), it is confirmed that the mass percent ofaluminum and niobium element in obtained Al and Nb is 99.03 wt%and 98.17 wt%, respectively. It is considered that the dissolved AlCl3or NbCl5 can be reduced by sodium in LKC molten salts forming Alball or Nb nanoparticles and these experimental data would beuseful for synthesis of NbeAl intermetallic compound powders.

3.2. Preparation of NbeAl nanoparticles

Fig. 2 shows the XRD patterns and FESEM images of the as-prepared products by sodiothermic reduction of NbCl5 and AlCl3with different molar ratios in LKC molten salts at 450 �C. Accordingto the theoretical binary phase diagram of NbeAl system, as shownin Fig. 3, the stoichiometric molar ratios of NbCl5 to AlCl3 as rawmaterials should be 4:1, 2:1 and 1:3, with the aim to prepare Nb3Al,Nb2Al and NbAl3, respectively. It can be seen from Fig. 2(a) that theproducts using NbCl5 and AlCl3 as raw materials with a molar ratioof 4:1 can be indexed to well crystallized Nb3Al structure (PDF NO.12-85) with the major peaks at 2q ¼ 24.38, 38.97, 42.76, 64.93,85.85�, corresponding to the diffractions of the (110), (210), (211),

reduction from the different molar ratios of NbCl5 and AlCl3 in the molten salts.

Fig. 3. Binary phase diagram of the NbeAl system.

Fig. 4. The TEM and HRTEM images of as-prepared products from the different molar ratios of NbCl5 and AlCl3 in the molten salts.

N. Wang et al. / Intermetallics 43 (2013) 45e52 49

(320) and (421) planes of the cubic Nb3Al. Also, well crystallizedtetragonal Nb2Al (PDF NO. 15-598) with the major peaks at2q ¼ 19.28, 37.14, 40.36, 41.24, 61.61, 65.44, 68.42, 81.84� corre-sponding to the diffractions of the (101), (410), (212), (411), (522),(532), (701) and (821) planes and well crystallized tetragonal NbAl3(PDF NO. 65-2666) with the major peaks at 2q¼ 20.64, 25.35, 39.17,41.98, 47.24, 65.02, 82.35� corresponding to the diffractions of the(002), (101), (112), (004), (200), (204) and (312) planes are syn-thesized as shown in Fig. 2(b) and (c).

In addition, as shown in Fig. 2(d)e(f), the obtained Nb3Al, Nb2Aland NbAl3 are nanosized particles in the range of 100e200 nm,100e250 nm and 100e200 nm, respectively. To further determinethe components of the intermetallic nanoparticles, the energydispersive spectra was also employed. The EDS results present thatthe molar ratio of Nb to Al in the as-prepared Nb3Al, Nb2Al andNbAl3 nanoparticles is 81:19, 66:34 and 26:74, respectively, asshown in supporting information (Fig. S2), which are consistentwith the theoretical molar ratios of Nb to Al. These results indicatethat the proportionable NbCl5 and AlCl3 can dissolve in LKC moltensalts forming a homogeneous system in which Nb5þ and Al3þ ionscan be directly reduced by metal Na. After that, various NbeAlintermetallic compounds nanoparticles can be successfully syn-thesized via sodiothermic reduction process in molten salts usingNbCl5 and AlCl3 as the raw materials.

Further information is obtained by TEM analysis for the as-prepared Nb3Al, Nb2Al and NbAl3. As seen from Fig. 4(a), (c) and(e), the particle size of as-prepared Nb3Al, Nb2Al and NbAl3 pow-ders is below 200 nm. Moreover, as presented in Fig. 4(b), theHRTEM of Nb3Al nanoparticles illustrates that the distance between

Fig. 5. The XRD pattern, FESEM image and EDS mapping image of as-prepared products bysalts.

the adjacent lattice fringes is 0.36 nmwhich can be assigned to theinterplanar distance of Nb3Al nanostructures (110). Also, as shownin Fig. 4(d) and (f), the HRTEM images reveal that the lattice spacingof Nb2Al and NbAl3 intermetallic compounds are 0.45 nm and0.23 nm, corresponding to the (101) and (112) planes in thetetragonal Nb2Al and NbAl3 structures, respectively. The corre-sponding selected-area electron diffraction (SAED) patterns are alsoshown in Fig. 4, which confirm that the Nb3Al, Nb2Al and NbAl3intermetallic compounds have the polycrystalline nature. Based onthe XRD, FESEM and TEM results, it is considered that Nb3Al, Nb2Aland NbAl3 intermetallic compounds have been successfully syn-thesized via sodiothermic reduction process in molten salts usingNbCl5 and AlCl3 as the raw materials, indicating that the appro-priate molar ratio of Nb to Al plays an important role upon thesynthesis and phase transformation of Nb3Al, Nb2Al and NbAl3intermetallic compounds in LiCleKCleCaCl2 molten salts.

After each experiment, the collection ratio of products isinvestigated. Take synthesis of Nb3Al for example, it will take 8.1 gof NbCl5, 1.33 g of AlCl3 and 4.14 g of Na to synthesize 3.06 g ofNb3Al according to the chemical equation. Actually, only 2.39 g ofNb3Al (78% of the theoretical value) is collected after the experi-ment. This is partly because it is difficult to avoid the loss of thenanoparticles during washing and filtering process.

3.3. Preparation of Nb2Al/NbAl3 nanocomposites

According to the theoretical binary phase diagram of NbeAlsystem, composites of Nb2Al/NbAl3 are expected products usingNbCl5 and AlCl3 as raw materials in which molar ratios of Nb to Al

sodiothermic reduction from NbCl5 and AlCl3 with a molar ratio of 1:1 in the molten

N. Wang et al. / Intermetallics 43 (2013) 45e5250

are 1.9:1e1:2.9, as shown in Fig. 3. Fig. 5 shows the XRD pattern,FESEM image and EDS mapping patterns of as-prepared productsby sodiothermic reduction of NbCl5 and AlCl3 (molar ratio 1:1) inthe LKC molten salts. From Fig. 5(a), it can be seen that predomi-nantly characteristic peaks can be indexed according to crystallineNb2Al (PDF NO. 15-598) and NbAl3 (PDF NO. 65-2666) phase. Majorpeaks of well crystallized Nb2Al are at 2q ¼ 19.28, 37.14, 40.36,41.24, 61.61, 65.44, 68.42, 81.84� corresponding to the diffractionsof the (101), (410), (212), (411), (522), (532), (701) and (821) planesand major peaks of well crystallized NbAl3 are at 2q ¼ 20.64, 25.35,39.17, 41.98, 47.24, 65.02, 82.35� corresponding to the diffractions ofthe (002), (101), (112), (004), (200), (204) and (312) planes.Therefore, it is confirmed that Nb2Al and NbAl3 were successfullysynthesized. From FESEM images in Fig. 5(b), it can be seen that theparticle size of Nb2Al/NbAl3 complex nanoparticles is below200 nm. Moreover, the relative analysis with four points from EDSspectra of the Nb2Al/NbAl3 complex nanoparticles revealed that themolar ratio of Nb to Al in Nb2Al/NbAl3 composite nanoparticles isapproximately 51:49, 52:48, 50:50 and 49:51 (as shown in sup-porting information (Fig. S3)), respectively, demonstrating that the

Fig. 6. The TEM images of as-prepared Nb2Al/NbAl3 composites.

Nb and Al elements are homogeneously distributed in the Nb2Al/NbAl3 composites, which is almost in accordance with the originaltheoretical molar ratio of Nb to Al. In order to further verify thedistribution of Nb and Al element in the Nb2Al/NbAl3 composites,EDS mapping was used. It can be seen from Fig. 5(c) that Nb (redpoints) and Al (blue points) elements distribute homogeneously inNb2Al/NbAl3 composites. Therefore, it is considered that each singleparticle is composed of Nb2Al and NbAl3 phases, which may resultfrom the homogeneous reduction of raw materials in the moltensalts system.

In order to further confirm that whether these two phases ofNb2Al and NbAl3 co-exist in a single particle, the TEM analysis wasused. The particle size of Nb2Al/NbAl3 composite is in the range of50e100 nm as presented in Fig. 6(a). Furthermore, as shown inFig. 6(b), the HRTEM of single Nb2Al/NbAl3 composite nanoparticleillustrates that the distance between the adjacent lattice fringes is0.46 nm and 0.23 nm, which can be assigned to the interplanardistance of Nb2Al nanostructure (101) and NbAl3 nanostructure(112), respectively. The corresponding selected-area electrondiffraction (SAED) pattern is also shown in Fig. 6(b), which confirmthat the Nb2Al/NbAl3 composite have the polycrystalline nature.

Fig. 7. The XRD patterns of as-prepared products by sodiothermic reduction fromNbCl5 and AlCl3 with a molar ratio of 4:1 at different reaction temperatures.

Fig. 8. The FESEM images of as-prepared Nb3Al at different reaction temperatures.

N. Wang et al. / Intermetallics 43 (2013) 45e52 51

This result verifies the previous EDS analysis that Nb2Al and NbAl3phases disperse in single nanoparticle.

3.4. Effects of reaction temperatures

The effect of the reaction temperatures on the particle size of theas-prepared NbeAl intermetallic nanoparticle was investigated.LiCle24.0mol%KCle5.0mol%NaCle18.0mol%CaCl2 molten saltswere chosen because their minimum eutectic temperature is323 �C. Fig. 7 shows the XRD patterns of Nb3Al particles at thedifferent reaction temperature from 350 �C to 500 �C in LKNCmolten salts. It is indicated that Nb3Al is successfully synthesized.However, the particle size of Nb3Al particles increases with theincreasing of reaction temperatures from 350 �C to 500 �C, asshown in Fig. 8. The average size of as-prepared Nb3Al nano-particles is about 40e120 nm, 70e130 nm, 140e320 nm and 150e360 nm at 350 �C, 400 �C, 450 �C and 500 �C, respectively. There-fore, it is considered that the reaction temperature played a pivotrole in the particle size of NbeAl intermetallic nanoparticles inmolten salts. However, using LiCleKCleNaCleCaCl2 molten salts asmedia, there is no difference in obtained products compared tousing LiCleKCleCaCl2 molten salts.

4. Conclusion

NbeAl intermetallic nanoparticles were directly synthesized viasodiothermic reduction process in molten salts using NbCl5 andAlCl3 as the raw materials. The as-prepared samples were charac-terized by X-ray diffraction, scanning electron microscopy andtransmission electron microscopy. The NbCl5 and AlCl3 were dis-solved in LiCleKCleCaCl2 or LiCleKCleNaCleCaCl2 molten saltsforming a homogeneous system. It was found that a series ofintermetallic nanoparticles, such as Nb3Al, Nb2Al, NbAl3 and Nb2Al/NbAl3, were successfully synthesized at low temperature of 350e500 �C using the homogeneous molten salts systems. The phase

transformations of Nb3Al, Nb2Al and NbAl3, were achieved via thecontrollable variation of molar ratio of Nb to Al. Furthermore, theparticle size of Nb3Al particles increased with the increasing ofreaction temperatures from 350 �C to 500 �C.

Acknowledgment

This work was supported by National Science Foundation ofChina (No. 50934001, 21071014, 51102015 and 51004008), theFundamental Research Funds for the Central Universities (No. FRF-AS-11-002A, FRF-TP-12-023A, FRF-MP-09-006B), Research Fundfor the Doctoral Program of Higher Education of China (No.20090006110005), National High Technology Research andDevelopment Program of China (863 Program, No. 2012AA062302),Program of the Co-Construction with Beijing Municipal Commis-sion of Education of China (No. 00012047 and 00012085) and theProgram for New Century Excellent Talents in University (NCET-11-0577).

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.intermet.2013.07.005.

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