ARTICLE pubs.acs.org/JPCC
Local Structural Evolution of Mechanically Alloyed Mg50Co50 Using Atomic Pair Distribution Function Analysis Hyunjeong Kim,*,† Jin Nakamura,† Huaiyu Shao,† Yumiko Nakamura,† Etsuo Akiba,† Karena W. Chapman,‡ Peter J. Chupas,‡ and Thomas Proffen§ †
Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, Central 5-2, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ‡ Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States § Lujan Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ABSTRACT: Milling-time-dependent local structural evolution of mechanically alloyed Mg50Co50 was investigated by the atomic pair distribution function analysis using both neutron and synchrotron X-ray powder diffraction data. The initial powder mixture was composed of three phases: hexagonal close-packed (hcp) Mg, hcp Co, and face-centered cubic (fcc) Co. As milling progressed, rather rapid reduction in crystallite sizes of hcp Mg and hcp Co along with the formation of the Mg50Co50 phase was observed. Meanwhile, size reduction in the fcc Co phase was found to be relatively gradual, accompanied by heavy strain. Mg50Co50 forms at the early stage of milling and bears an amorphous nature.
’ INTRODUCTION To realize the hydrogen economy for transportation, hydrogen storage materials with more than 5 mass % of hydrogen capacity are required.1 Most of the conventional metal hydrides, capable of reversibly absorbing a large amount of hydrogen at ambient conditions, are composed of transition and rare-earth metals, resulting in the low gravimetric density of hydrogen. Mg is one of the well-known light-weighted metals that are capable of holding a large amount of hydrogen; it forms a stable hydride (MgH2) absorbing up to 7.6 mass % of hydrogen.2 However, its high hydrogen desorption temperatures (573 K) and poor kinetics hinder its on-board application. Consequently, the development of new Mg-based materials with improved hydrogen storing properties is of great interest. Mechanical alloying is a widely adopted method for preparing novel nanostructured or amorphous materials. By continuous repetition of welding, fracturing, and rewelding of a powder mixture using a highenergy ball mill, new alloys that are not previously accessible by conventional melting methods can be obtained.3 The final products are often metastable and accompanied by interesting properties. Although numerous structural and morphological studies using laboratory X-rays and optical and electron microscopes on these materials can be found in the literature (such as the formation process of MgTi alloys by Asano et al.4), a thorough investigation on the formation process and the structure of the final product at the atomic level is rare. This is partly because reductions in grain size and heavily disordered structures of as-prepared samples lead to poorly r 2011 American Chemical Society
defined Bragg peaks in diffraction patterns, limiting the use of conventional crystallography. By using mechanical alloying, Zhang et al. added new MgxCo100x to the MgCo binary phase diagram where previously MgCo2 had been the only known stable phase.5 These metastable binary alloys are formed in the composition range of 20e x e 63. Although there is no hydride form of MgCo2 (=Mg33Co66), MgxCo100x with the composition of x g 33 absorbs hydrogen. Initially, Zhang et al. reported that 2.1 mass % of hydrogen was absorbed by Mg50Co50 at 373 K with 6 MPa of hydrogen,5 but later, Shao et al. observed a much higher hydrogen absorption of about 2.67 mass % at 258 K under 8 MPa of hydrogen.6 MgxCo100x alloys show one broad peak in X-ray diffraction patterns using Cu KR radiation,5,6 reflecting their amorphous nature. This was also confirmed by transmission electron microscopy (TEM).7 Shao et al. investigated X-ray powder diffraction patterns of Mg50Co50 milled for a different amount of time.6 As milling progressed, reductions in grain size were clearly observed from broadening of Bragg peaks. However, detailed structural studies were missing because substantial peak broadening prevented a more thorough crystallographic analysis. Obviously, an alternative structural characterization tool is required for analyzing this system. Atomic pair distribution function (PDF) analysis is a local structural probing technique that gives the probability of finding Received: December 9, 2010 Revised: March 3, 2011 Published: March 28, 2011 7723
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Table 1. Rietveld and PDF Refinement Results on Neutron and Synchrotron X-ray Data of 0 h Mg50Co50 Using the ThreePhase Modela neutron
X-ray
neutron
X-ray
Rietveldc
Rietveldc
PDFc
PDFc
3.20615(4)
3.209(2)
3.2061(13)
3.1400(5)
5.2054(1)
5.191(3)
5.2042(42)
5.1667(1)
Uiso (Å2) 0.0179(4)
0.016(2)
0.01192(45)
0.0021(4)
2.5036(21) 4.0684(57)
2.5036(4) 4.0674(3)
hcp Mg a (Å) c (Å) hcp Co a (Å) c (Å) fcc Co
2.5065(3) 2.5043(9) 4.06616(97) 4.068(1)
Uiso (Å2) 0.0086(6)
0.0023(4)
0.01067(88)
0.0026(7)
a (Å)
3.5392(9)
3.5371(27)
3.5366(4)
3.5399(2)
Uiso (Å2) 0.008b
0.0028(8)
Rwp (%)
7.813
2.07
0.00483(99) 19.5
0.0045(6) 14.5
a The three-phase model consists of hcp Mg (space group P63/mmc), hcp Co (space group P63/mmc), and fcc Co (space group Fm3m). Initial values were taken from the literature,23,24 and refined lattice parameters and isotropic atomic displacement parameters (Uiso) are given. Rwp indicates the goodness of a fit. b The Uiso value for fcc Co was fixed to 0.008 during the Rietveld refinement of neutron data. c The column headings denote the radiation (neutron, X-ray) and the method (Rietveld, PDF).
atom pairs separated by distance r.8 Because the PDF makes use of both Bragg and diffuse intensities (total scattering) and does not require long-range periodicity, it has long been used to study liquids, glasses and amorphous materials and, more recently, to investigate nanocrystalline and disordered materials.9 We have employed PDF analysis as an alternative tool to investigate the structure of Mg50Co50 and its hydride. In this study, we have identified starting phases of the initial Mg and Co powder mixture, estimated the milling-time-dependent crystallite size of each phase, and examined the local structural evolution and the formation of Mg50Co50. Details of local structural studies, including the validity of models proposed by earlier studies and implications of new local structural models for hydrogenation properties of MgxCo100x, will be published somewhere else.10
Figure 1. Rietveld fits of 0 h Mg50Co50 synchrotron X-ray data using (a) the two-phase (hcp Mg and hcp Co) and (b) three-phase (hcp Mg, hcp Co, and fcc Co) models. Blue open circles and red lines represent experimental and calculated diffraction patterns, respectively. The difference between data and calculation (green lines) is shown below each fit. The inset shows low-2θ regions on an expanded scale.
orthogonal to the incident beam with a sample-to-detector distance of 161.3 mm (220.0 mm for the 100 h sample). Series of frames were collected for each data set to achieve good statistics. Neutron Experiment. Time-of-flight (TOF) neutron total scattering experiments were carried out on the NPDF instrument13 at the Lujan Neutron Scattering Center at Los Alamos National Laboratory. Five Mg50Co50 samples (0, 30, 40, 50, and 100 h of ball-milling time) were packed in cylindrical vanadium cans of 6.35 mm in diameter. Each sample was measured at 300 K for 12 h to improve statistics at high Q. Data Processing and Modeling. The PDF is obtained by a sine Fourier transformation of neutron or X-ray powder diffraction data according to eq 1
’ EXPERIMENTAL AND ANALYSIS METHODS
2 GðrÞ ¼ π
Sample Preparation. Mg50Co50 samples were synthesized
using a Fritsch P5 planetary ball mill. A 2 g portion of a Mg (purity > 99.9%, 100 mesh) and Co (purity > 99.9%, 300 mesh) powder mixture was sealed in a stainless steel pot with a 0.1 MPa argon atmosphere with stainless steel milling balls (10 mm in diameter) in a glovebox. The ball-to-sample weight ratio was 20:1. The mechanical alloying was carried out under different ball-milling times (0, 30, 40, 45, 50, and 100 h) with a rotation speed of 200 rpm. “0 h” means just a mixture of raw Mg and Co materials. Synchrotron X-ray Experiment. Synchrotron X-ray total scattering experiments were conducted at the 11-ID-B beamline at the Advanced Photon Source at Argonne National Laboratory. Powder samples of Mg50Co50 ball-milled for 0, 30, 40, 45, 50, and 100 h were packed in kapton capillaries with a diameter of 1.0 mm. Data were collected with an incident X-ray energy of 58.26 keV (λ = 0.2128 Å) at 300 K using the rapid acquisition pair distribution function (RA-PDF) technique.11 For the 100 h sample, an X-ray energy of 90.486 keV (λ = 0.13702 Å) was used. An amorphous silicon area detector12 was mounted
2 ¼ π
Q Zmax
Q ½SðQ Þ 1sinðQrÞ dQ Q min Q Zmax
FðQ ÞsinðQrÞ dQ
ð1Þ
Q min
where Q is the magnitude of the momentum transfer, S(Q) is the total scattering structure function, and F(Q ) = Q [S(Q ) 1] is the reduced total scattering structure function. The relation between the measured intensity and S(Q) for X-ray data is the following SðQ Þ ¼
I coh ðQ Þ
∑ci jf2i ðQ Þj2 þ 1
j∑ci fi ðQ Þj
ð2Þ
where Icoh is the measured coherent scattering intensity, ci is the atomic concentration, and fi(Q) is the X-ray scattering factor for atomic species i.8,14 For neutron data, fi(Q ) will be replaced by the neutron scattering length, bi.8 It is worth noting that F(Q) amplifies signals at higher-Q regions. 7724
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Figure 2. Rietveld refinement of 0 h Mg50Co50 neutron data using the three-phase model. Data and the refined model are plotted in blue open circles and a red line, respectively. The offset green line is a difference curve. High-Q regions are shown in the inset on an expanded scale.
Series of X-ray image data were first combined and integrated using the software FIT2D.15 The signal from an empty container (a kapton capillary and vanadium can for X-ray and neutron data, respectively) was subtracted from the raw data, and various other corrections were made.8 The X-ray and neutron PDFs were obtained by a sine Fourier transformation of F(Q) according to eq 1 using the programs PDFgetX216 and PDFgetN,17 respectively. Because of the unfavorable signal-to-noise ratio at the high-Q regions, F(Q) was truncated at Q max = 22 Å1 for X-ray and Q max = 25 Å1 for neutron data before the transformation. Structural modeling in reciprocal space was carried out by the Rietveld method using the EXPGUI-GSAS18,19 and RIETANFP20 programs. For local structural studies, the PDFgui21 program was used for real-space modeling.
Figure 3. PDF fits of 0 h Mg50Co50 X-ray data using (a) the two-phase and (b) three-phase models. Blue open circles, red lines, and green lines correspond to experimental PDFs, calculated PDFs, and difference curves. The difference curves are offset for clarity.
’ RESULTS AND DISCUSSIONS 0 h Data. First, let us consider the 0 h data. Rietveld refinement results of the synchrotron X-ray data are summarized in Table 1, and corresponding fits are shown in Figure 1. On the basis of the earlier study,6 we first tried the two-phase model consisting of hexagonal close-packed (hcp) Mg and hcp Co (Figure 1a). Surprisingly, the diffraction pattern was not well explained by the model. An extra peak at 2θ ∼ 6.8° (inset of Figure 1a), which is not described by the model, indicates the presence of an extra phase. By adding face-centered cubic (fcc) Co, which is the stable phase of bulk Co above 420 °C,22 to the model (the three-phase model), the fit dramatically improved (Figure 1b). This is an unexpected result because fcc Co was not found from the 0 h sample in the earlier study using conventional laboratory X-rays.6 The weight fraction ratio of three phases from the refinement was hcp Mg/hcp Co/fcc Co = 0.2:0.6:0.2. Figure 2 shows the Rietveld fit of the high-resolution neutron powder diffraction data. The overall diffraction profile was well reproduced by the three-phase model. For neutron data, addition of fcc Co in the two-phase model improved the Rwp (the goodness of a fit) slightly from 2.33 to 2.07. This is because of the relatively small neutron scattering length of Co; signals from a tiny amount of weak scattering elements (such as fcc Co in neutron data) are not pronounced, especially in the presence of strong scatterers, such as Mg. On the other hand, it is more straightforward to identify a small amount of fcc Co from X-ray data where heavy Co atoms strongly interact with X-rays, resulting in strong diffraction peaks. Synchrotron X-ray data are more favorable to see fcc Co because the signal-to-noise ratio is superb compared with that of laboratory X-rays. This is
Figure 4. Reduced total scattering structure functions, F(Q), for Mg50Co50 samples ball-milled for different amounts of time. Curves are offset for clarity, and the ball-milling time is indicated above each corresponding curve. Vertical dotted lines guide Bragg peak positions of fcc Co.
probably why fcc Co peaks can be readily seen in our 0 h synchrotron X-ray data, but not in the earlier study using Cu KR radiation.6 In addition, most of the strong fcc Co peaks overlap with hcp Co peaks. Therefore, it is hard to see the presence of a small amount of fcc Co without careful Rietveld analysis. Rietveld analysis on 0 h data was not reported in the earlier study.6 The other possible explanation is that Co powder used for this study may contain more fcc Co than the one used by Shao et al. because hcp and fcc Co phases coexist at room temperature and it is difficult to separate them completely.25 The resultant weight fraction ratio was hcp Mg/hcp Co/fcc Co = 0.3:0.5:0.2, which is in good agreement with values obtained from X-ray data. Similar results were obtained from PDF refinements (Table 1). The overall neutron PDF profile was well explained by both models (not shown). Addition of fcc Co improved the Rwp from 24.4 to 21.0, but visually, there was no significant change. On the contrary, from X-ray PDFs, the existence of fcc 7725
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Figure 5. X-ray PDFs of Mg50Co50 ball-milled for different amounts of time. (a) 0.1 < r < 20 Å and (b) 20 < r < 40 Å regions are plotted. The high r data were scaled for easy comparison. Scaling factors (i.e., 2, 4, 7, 10, 24, and 36 for 0, 30, 40, 45, 50, and 100 h data, respectively) are indicated above corresponding PDFs together with the ball-milling time. Curves are offset for clarity.
Table 2. Structural Parameters Obtained from X-ray PDF Refinements Using the fcc Co Modela 0h a (Å)
30 h
40 h
3.5363(3) 3.5441(7) 3.549(2)
45 h 3.550(2)
50 h 3.552(5)
Uiso (Å2) 0.0041(2) 0.0067(7) 0.0082(17) 0.0087(22) 0.0104(69) a
The refinement range was 20 < r < 40 Å.
Figure 6. Representative fit to the 40 h X-ray PDF using the fcc Co model. The refinement range was 20 < r < 40 Å. Experimental and calculated PDFs are plotted with blue open circles and a red line, respectively. The difference curve (a green line) is shifted down for clarity purposes. Most of the features in this range are well explained by the fcc Co model, indicating that there are no significant hcp Mg and hcp Co signals in this region.
Co is clearly seen from the improvement in fits before (Figure 3a) and after (Figure 3b) introducing fcc Co. This shows that the X-ray PDF is also sensitive enough to trace a small amount of fcc Co in Mg50Co50. T > 0 h Data. Now let us examine how the structure of crystalline Mg and Co evolves during mechanical milling. The reduced total scattering structure functions, F(Q ), obtained from X-ray data are shown in Figure 4. We plotted F(Q ) instead of the measured intensity because F(Q ) amplifies signals at higher-Q regions and, therefore, small signals at higher-Q regions can be more readily seen. As milling progressed, Bragg peaks get significantly broadened and hcp Mg and hcp Co peaks cannot be distinguished after 40 h of milling, indicating the absence of long-range structural order. However, small, but relatively sharp, fcc Co peaks are still observed on top of broad intensities (indicated by vertical dotted lines in Figure 4) and retained even in 50 h data. The progress in the grainsize reduction of fcc Co is evident from the gradually broadened peak widths. The earlier study reported the appearance of fcc Co peaks after 10 h of milling and suggested the transformation of hcp Co to fcc Co by milling,6 but this is unlikely because we do not see any evidence of increase in fcc Co peak intensities. Moreover, the phase transformation of Co induced by mechanical milling is still controversial.2528 The 100 h data show only diffuse signals. The local structural evolution of Mg50Co50 can be seen from X-ray (Figure 5) and neutron (not shown) PDFs. The
Figure 7. Mechanical milling-time-dependent crystallite size of each phase obtained from PDF refinements using the three-phase model.
crystallinity of the initial sample is reflected in well-defined sharp PDF peaks extended over a wide r range in the 0 h PDF (the topmost line in Figure 5). On the contrary, 100 h data show only a few broad peaks and a featureless flat line beyond 10 Å (the bottommost line in Figure 5). This signifies that the final product of extensive milling is amorphous. The transition from the crystalline to the amorphous phase can be seen from intermediate (3050 h) X-ray PDFs (Figure 5a). Features especially drawing our attention in these PDFs are medium-range correlations; in the range of 20 < r < 40 Å (Figure 5b), they bear a close resemblance. These coherent features, which are different from 0 and 100 h PDFs, are due to fcc Co, as we have seen from F(Q) in Figure 4. Although hcp Mg and hcp Co signals are still observable below 20 Å in intermediate 7726
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Figure 8. Difference curves obtained from the three-phase model fits to (a) X-ray and (b) neutron PDFs of 050 h samples. The ball-milling time is indicated above each corresponding curve. PDFs of the 100 h sample are included for comparison purposes. The 100 h neutron PDF was multiplied by the scaling factor of 0.6.
PDFs, they become insignificantly small above 20 Å, and subsequently, fcc Co signals stand out predominantly. This can be appreciated from PDF fits to 3050 h X-ray data using the fcc Co model. The representative fit to the 40 h PDF is shown in Figure 6. Most of the experimental PDFs between 20 < r < 40 Å are well explained by the fcc Co model. Refined structural parameters are given in Table 2. Larger lattice parameter and isotropic atomic displacement parameter (Uiso) values found in longer milling samples indicate the development of heavy strain in fcc Co. A rapid falloff of hcp Mg and hcp Co PDF peaks with increasing r is due to their significantly reduced crystallite size. A finite crystallite size diminishes the number of atom pairs separated by distances larger than its diameter and, consequently, leads to an abruptly damping PDF profile. This effect is especially remarkable when they are in nanoscale, such as in nanoparticles.29 Our PDFs indicate that the degree of size reduction in each phase is different. To estimate the crystallite size of each phase, 3050 h X-ray and neutron PDFs were fit with the threephase model. We use the refinement range of 1.5 < r < 20 Å, because notable signals of hcp Mg and hcp Co are observed only below 20 Å. In other words, the largest finite size effect takes place in the range of 1.5 < r < 20 Å. In PDFgui, the finite size effect of a spherical particle (crystallite size in our case) can be simulated by multiplying the PDF of a bulk material by a spherical envelope function.21,29 In reality, the crystallite shape is not perfectly spherical, but random. Therefore, this approach will provide an “average” diameter with which a spherical envelope function explains the PDF intensity falloff most effectively. For this analysis, all the structural parameters (lattice parameters, isotropic atomic displacement parameters, and correlated motion parameters8) were fixed to values obtained from 0 h PDF refinements and only scale factors and particle diameters were refined. Resulting average crystallite sizes are plotted in Figure 7. PDF peaks of hcp Mg and fcc Co in intermediate X-ray and neutron data, respectively, were too small to estimate the crystallite size. Therefore, we excluded those values from the figure. Values for hcp Co determined from X-ray and neutron PDFs are in good agreement (red-filled and blue open circles in Figure 7), ensuring the validity of the analysis. The crystallite size of fcc Co was also obtained from Rietveld refinement for comparison (open squares in Figure 7).30 If we assume that initial crystallite sizes of all three phases are similar, our results
show that a significant size reduction occurs in hcp Mg and hcp Co during the first 30 h of milling. Meanwhile, the size reduction in fcc Co is relatively gradual. The size of fcc Co remains around 100200 Å in the intermediate stage. This is interesting because the high-temperature phase of fcc Co is known to be stable at room temperature when the particle size is smaller than 200 Å due to its smaller surface free energy compared with that of hcp Co.31 This means the surface of nanosized hcp Co is readily subject to alteration. It is probable that energetically unfavorable nanosized hcp Co promotes the formation of Mg50Co50 at the early stage of mechanical alloying to reach a lower-energy configuration. In this formation picture, Mg50Co50 would initially form on interfaces of nanosized hcp Co and hcp Mg. Figure 8 shows difference curves between data and calculation obtained from the three-phase model fits for the crystallite size determination. These are signals that remained after only crystallite Mg and Co contributions were removed. Note that difference curves include features from strained structural components. Flat difference curves from 0 h PDFs indicate there is no extra phase other than hcp Mg, hcp Co, and fcc Co in the initial sample. After 30 h of milling, new intensities start to appear in both X-ray and neutron PDFs, reflecting the formation of a new phase. These new intensities are very similar in appearance to 100 h PDFs (the bottommost curve in each panel). This suggests that the Mg50Co50 phase begins to form at the early stage of milling, which is consistent with the formation picture drawn from the crystallite size results above.
’ CONCLUSIONS We have investigated the local structural evolution of Mg50Co50 during the mechanical alloying process using the PDF analysis on neutron and synchrotron X-ray total scattering data. We found that the initial powder mixture contained not only hcp Mg and hcp Co but also high-temperature stable fcc Co. As milling progressed, both hcp Mg and hcp Co crystallite sizes were rapidly reduced (the size of hcp Mg and hcp Co was reduced to 80 and 60 Å, respectively, after 30 h of milling and became 35 Å after 50 h of milling.), accompanied by the formation of an amorphous Mg50Co50 phase. On the other hand, reduction in the fcc Co crystallite size was much slower, retaining a medium-range fcc structural order for a long period of time. The first 30 h of milling reduced the size of fcc Co to 190 Å, and 7727
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The Journal of Physical Chemistry C after 50 h of milling, it became 70 Å. Development of heavy plastic deformation in the fcc Co phase was observed during the mechanical alloying. We speculated that energetically unstable nanosized hcp Co promoted the formation of Mg50Co50 at the early stage of milling. The local structure of Mg50Co50 did not change significantly during milling. Our results provide a useful insight into the formation process of mechanically alloyed Mg50Co50 from the local structural point of view. Detailed local structural studies on Mg50Co50 as well as its hydride will be published elsewhere.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank Kevin Beyer, Katherine Page, and Joan Siewenie for help with the experiments. H.K. thanks Kouji Sakaki and Saishun Yamazaki for help in using RIETAN-FP and Junko Matsuda and Itoko Matsumoto for useful discussions. This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) under Advanced Fundamental Research Project on Hydrogen Storage Materials (HYDRO-STAR). Work performed at the Lujan Neutron Scattering Center was funded by the DOE Office of Basic Energy Sciences. Los Alamos National Laboratory is operated by Los Alamos National Security LLC under Contract No. DE-AC5206NA25396. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC0206CH11357. ’ REFERENCES
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