Journal
J. Am. Ceram. Soc., 89 [7] 2279–2285 (2006) DOI: 10.1111/j.1551-2916.2006.01020.x r 2006 The American Ceramic Society
TEM Characterization of Nanostructured MgAl2O4 Synthesized by a Direct Conversion Process from c-Al2O3 Jafar F. Al-Sharab* and Frederic Cosandey*,w Materials Science and Engineering Department, Rutgers University, Piscataway, New Jersey 08854
Amit Singhal and Ganesh Skandan NEI Corporation, Somerset, New Jersey 08873
James Bentley* Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6064
high annealing temperatures lead to spinel powders with large particle sizes. Therefore, in recent years, low-temperature chemical synthesis of nano-crystalline powders of MgAl2O4 spinel has been explored. Most of these techniques follow the wet chemical methods for powder synthesis such as the sol–gel technique,3,7–9 precipitation,4,10,11 spray/freeze drying,12,13 modified Pechini process,14 citrate–sol–gel synthesis,7 or combustion synthesis.15,16 Although these methods utilize lower processing temperatures than what is required in solid-state reactions, these temperatures are still quite high, leading to microstructures with large non uniform grain size. In contrast to multicomponent systems, single-phase oxides can be synthesized easily with small uniform grain size below 20 nm. One of these oxides, g-Al2O3, is isostructural with MgAl2O4 and could be used as a starting powder for a direct conversion process involving Mg diffusion at low temperatures, thus preserving the small initial particle size. Indeed, our group has recently synthesized nanostructured MgAl2O4 spinel with initiation temperatures as low as 3001C by a direct conversion process starting from cubic g-Al2O3.17 In this paper, we present detailed transmission electron microscopy (TEM) characterization results of MgAl2O4 spinel powder synthesized by this low-temperature process. The homogeneity of the synthesized powders was examined by selected area electron diffraction (SAED) and from Mg and Al elemental mapping using electron energy loss spectroscopy (EELS), and energy-dispersive spectroscopy (EDS) spectrum imaging techniques. The degree of ordering of the final spinel powders was estimated from relative electron diffraction intensity measurements of {111} and {440} reflections and was compared with theoretical values.
Nanostructured MgAl2O4 spinel was synthesized by a direct conversion process from cubic c-Al2O3. The effect of post-annealing temperature (3001, 5001, and 8001C) on MgAl2O4 phase formation was investigated using transmission electron microscopy, selected area electron diffraction (SAED), electron energy loss spectroscopy (EELS), and energy-dispersive spectroscopy (EDS). Relative diffraction intensities as well as lattice parameter measurements from SAED revealed that MgAl2O4 spinel structure starts forming at temperatures as low as 3001C. EELS and EDS spectrum images also revealed an increase in elemental homogeneity with increasing annealing temperature. The degree of ordering of Mg and Al between octahedral and tetrahedral sites has been determined from relative diffraction intensities. Results show that annealing to 8001C leads to a spinel phase with an order parameter of 0.78.
I. Introduction
M
AGNESIUM Aluminate spinel (MgAl2O4) is a widely used ceramic material due to its attractive combination of physical, chemical, optical, electrical, and magnetic properties. In the last decade, there has been an increasing interest in the synthesis and characterization of spinel owing to its many technological applications. MgAl2O4 spinel can be used in harsh environments because of its high-melting temperature of 21351C,1 for optical devices such as passive Q-switch of lasers,2,3 in electronic applications such as humidity sensors,3,4 or as infra-red (IR) transparent windows.5,6 In order to achieve an optimal combination of optical characteristics and good mechanical properties, fully dense materials with nanosized grain sizes are required. Therefore, a number of studies have been devoted in recent years toward the synthesis of homogeneous nanostructured spinel powders with a high chemical purity and a narrow particle size distribution. The conventional method to prepare MgAl2O4 spinel is via a solid-state reaction by simply mixing measured amounts of MgO and Al2O3 compounds, and heat treating the powder mixture at temperatures in the range of 14001–16001C. However,
II. Experimental Procedure (1) Synthesis Commercially available g-Al2O3 nanostructured powders with 14 nm average particle size and a surface area of 120 m2/g were utilized to synthesize nanostructured MgAl2O4 spinel. The synthesis processes consists of first dissolving the Mg(NO3)2 in a high boiling point (B2501C) solvent. A stoichiometric amount of g-Al2O3 nanoparticles was added to the solution. Subsequently, the solution was refluxed for about 16 h. The solvent was then removed using the vacuum distillation process and the dried powder was annealed at 3001, 5001, and 8001C in oxygen to facilitate the formation of the spinel phase.
J. Drennan—contributing editor
Manuscript No. 21194. Received November 28, 2005; approved February 17, 2006. The project was supported by ARMY-SBIR phase-II, contract number W31P4Q-04-CR030. Research at the Oak Ridge National Laboratory SHaRE User Center was sponsored by the Division of Materials Sciences and Engineering, U. S. Department of Energy, under contract DE-AC05-00OR22725 with UT-Battelle, LLC. *Member, American Ceramic Society. w Author to whom correspondence should be addressed. e-mail:
[email protected]. edu
(2) Characterization TEM samples were prepared by dispersing the powder in trichlorotrifluoroethane, followed by placing a drop of the 2279
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suspension on ‘‘holey’’ carbon film supported on copper grids. Standard bright/dark-field images and SAED patterns were obtained using a Topcon 002B microscope operating at 200 kV (FEI Co., Hillsboro, OR). The Process-Diffraction software18 was used for intensity analysis of the SAED patterns. Grain size analyses were obtained by measuring the crystallite size from dark-field images. Simulations of SAED patterns were performed using JEMS software.19 EELS and EDS spectrum imaging were conducted using Philips CM200 FEG TEM (Topcon Corp., Tokyo, Japan) equipped with a Gatan Imaging Filter operating in STEM mode at 200 kV. All spectra were acquired and processed with Emispec Vision and also exported for further analysis with Gatan EL/P software. Initial data were taken with an infocus STEM probe of B1.2 nm at full-width at halfmaximum (FWHM) and 1 nA probe current. However, the beam intensity was too high, causing drilling of holes. This problem was minimized using an under focused STEM probe with a diameter of B5 nm. All EELS and EDS spectra were automatically drift corrected and collected by rastering over an area of 160 nm 160 nm with a beam exposure of 1 s.
III. Results (1) Particle Size Analysis In order to follow the effect of processing temperature on the final particle size, TEM dark field images were taken from the three samples annealed at 3001, 5001, and 8001C. Typical darkfield TEM images of as-synthesized g-Al2O3 as well as processed
Table I. Grain Size Measurements of the Starting Powder gAl2O3 and MgAl2O after Annealing at Various Temperatures Material
Average crystallite size (nm)
g-Al2O3 3001C 5001C 8001C
14.072 14.972 17.072 16.872
and annealed at 8001C are given in Figs. 1(a) and (b), respectively. The {311} high-intensity reflection was used to form the dark-field image. However, contributions from nearby reflections such as {220} and {222} were unavoidable. Grain size measurements for all samples are summarized in Table I. The grain size measurements showed only a slight increase in particle size from 14 to 17 nm after annealing at 8001C. This result indicates that the nanostructure character of the powder is maintained during the low-temperature annealing process.
(2) SAED Patterns Diffraction patterns were obtained for the starting g-Al2O3 powder as well as after processing and annealing to reveal the formation of the MgAl2O4 spinel phase. In order to follow the spinel formation quantitatively, simulations of polycrystalline diffraction patterns for g-Al2O3 and MgAl2O4 spinel were obtained based on the structure and atomic positions given in Table II. The simulation results are displayed in Figs. 2(a) and (b) for g-Al2O3 and MgAl2O4, respectively. A comparison
Table II. Crystal Structure Data of g-Al2O3 and MgAl2O4 Spinel Compounds g-Al2O325
MgAl2O426
Space group Fd3m, ao 5 0.790 nm
Space group Fd3m, ao 5 0.8083 nm
Atoms X,Y,Z Occupancy No. atoms Atoms X,Y,Z Occupancy No. atoms
O Al1 Al2
Fig. 1. Transmission electron microscopy dark-field images formed from 220 and 311 reflections (a) g-Al2O3 starting material and (b) MgAl2O4 annealed at 8001C.
0.245 0.125 0.5
1 0.9 0.883
32 7.2 14.3
O Mg Al
0.262 0.125 0.5
1 1 1
32 8 16
Fig. 2. Simulated electron diffraction patterns of (a) cubic g-Al2O3 and (b) MgAl2O4.
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TEM Characterization of Nanostructured MgAl2O4 311 400
280
Intensity (a.u.)
240 200
440 333
220 111
MgAl2O4 @ 800°C 160 120
MgAl2O4 @ 500°C
80
MgAl2O4 @ 300°C
40 γ -Al2O3
0 0.00
2.00
4.00
6.00 8.00 1/d (nm−1)
10.00
12.00
Fig. 4. Selected area electron diffraction intensity profile comparison of g-Al2O3 starting powder and MgAl2O4 after annealing at 3001, 5001, and 8001C. The peaks have been indexed with their corresponding hkl values.
of the two simulated diffraction patterns showed two major differences. In the MgAl2O4 spinel, the {111} and {333} reflections were significantly more intense than in g-Al2O3, with the {111} reflection totally absent in g-Al2O3. In addition, the {331} reflection decreased in intensity upon transformation from gAl2O3 to MgAl2O4. Based on the diffraction simulations shown above, it is clear that the electron diffraction technique can uniquely determine the existence and evolution of spinel from gAl2O3, primarily from the presence of the {111} reflection and its relative intensity with respect to other reflections. Experimental SAED patterns of as-synthesized g-Al2O3, as well as MgAl2O4 processed and annealed at 8001C are shown in Figs. 3(a) and (b), respectively. The SAED pattern obtained after processing and annealing at 8001C clearly shows an additional peak at d 5 0.4662 nm corresponding to the {111} reflection of MgAl2O4 spinel. Such a reflection was missing from the as-synthesized g-Al2O3 powder. The diffraction pattern was also less diffuse with discernable spots that are indicative of the presence of larger particles. This result is consistent with the measured increase in average grain size (Cf Table I) SAED patterns from the various samples were also analyzed further to extract the relative intensities of the major reflections (111, 220, 311, 400, 333, and 440). For this purpose, the intensity was first measured for each ring using the rotational average method.18 Then, the background intensity was removed with a
MgAl2O4 spinel
0.810 Lattice parameter, a (nm)
Fig. 3. Selected area electron diffraction patterns of (a) g-Al2O3 starting material and (b) MgAl2O4 sample annealed at 8001C, (d-spacing in nm).
power fit law function of degree 3. Similar processes were performed for all SAED patterns. Figure 4 compares the extracted intensity profiles of as-synthesized powders, and processed at various temperatures of 3001, 5001, and 8001C. The peaks have been indexed with their corresponding hkl values. Intensity profiles show that spinel powder starts to form at 3001C. This can be described by the presence of a small {111} peak, which continues to grow until the final temperature of 8001C is reached. The {333} reflection also shows an increase in intensity with respect to {311}, indicative of spinel phase formation. Also, there is a shift in peak positions to the left, which corresponds to an expansion in the lattice parameter. This expansion was calculated by measuring the lattice parameter from the {220} reflection and the results are presented in Fig. 5. For reference, the lattice parameter of g-Al2O3 was also measured and found to be 0.785 nm from the same {220} reflection. These results show that there is a gradual increase in the lattice parameter upon increasing annealing temperature, which parallels the gradual increase in the {111} peak intensity. Lattice parameter measurements show that there is an B8.6% increase in lattice volume from assynthesized g-Al2O3 to the sample annealed at 8001C, owing to the partial occupancy of tetrahedral and octahedral interstitial sites by Mg. This result agrees well with the reported change in a lattice parameter between the as-synthesized g-Al2O3 with a lattice parameter of 0.790 nm20 and MgAl2O4 with a lattice parameter of 0.8083 nm,1 corresponding to a 7.1% increase in volume. These lattice parameter measurements also provided us with some information regarding the homogeneity of the synthesized samples. For the sample synthesized and annealed at
0.800 γ-Al2O3
0.790 0.780 0.770 0.760 0.750 300°C
500°C
800°C
Fig. 5. Lattice parameter evolution of MgAl2O4 spinel as a function of annealing temperature.
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Fig. 6. Al and Mg electron energy loss spectroscopy (EELS) and energy-dispersive spectroscopy (EDS) elemental maps of MgAl2O4 sample annealed at 3001C, (a) scanning transmission electron microscopy (STEM) image, (b) magnified STEM image of the mapped area, (c) EELS Mg–K intensity map, (d) EELS Al–K intensity map, (e) Mg–K/Al–K intensity ratio map generated from EELS, (f) EDS Mg–K intensity map, (g) EDS Al–K intensity map, and (h) Mg–K/Al–K intensity ratio map generated from EDS.
3001C, a larger scatter in lattice parameter was observed due to compositional inhomogeneity in the material consisting of a mixture of g-Al2O3 and partially transformed MgAl2O4. The sample annealed at 8001C, on the other hand, revealed little lattice parameter variations, with a value similar to the one reported for spinel structure.1 This result indicates a complete transformation to MgAl2O4 spinel composition at this annealing temperature. Throughout this g-Al2O3 to MgAl2O4 transformation, additional reflections from an intermediate MgO phase were not observed, which is an additional indication that spinel is formed by a direct conversion process.
(3) EELS and EDS Analysis In order to reveal directly the effect of progressive heating on chemical homogeneity of the processed samples, EELS and EDS spectrum imaging were used to produce Mg and Al elemental maps.
Figure 6(a) displays an STEM image of the sample synthesized and annealed at 3001C. There are two marked boxes in this image. The one with an x mark represents the area where the Mg and Al signals were collected, while the other box, which is located in a high-contrast area, was utilized for automatic image drift correction. Figure 6(b) shows magnified STEM images of the area where the EDS and EELS spectra were collected. The reported spectrum images were quantified using scattering crosssections and k factors for EELS and EDS, respectively. The dimensions of the mapped area are 160 nm 160 nm. Figures 6(c) and (d) display the Mg–K and Al–K EELS intensity maps of the area displayed in the STEM image in Fig. 6(b). Figure 6(e) represents the EELS elemental ratio map of Mg–K/Al–K (Mg–K intensity map divided by the Al–K intensity map). The elemental intensity ratio map is accompanied by an intensity scale bar, which was set from 0 to 1.0. Figure 6(f) displays the intensity map of Mg–K as obtained from EDS. The Al–K intensity map from EDS was also collected and is displayed in Fig. 6(g). The elemental intensity ratio map generated from EDS is displayed
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Fig. 7. Al and Mg electron energy loss spectroscopy (EELS) and energy-dispersive spectroscopy (EDS) elemental maps of MgAl2O4 sample annealed at 8001C, (a) scanning transmission electron microscopy (STEM) image, (b) magnified STEM image of the mapped area, (c) EELS Al–K intensity map, (d) EELS Mg–K intensity map, (e) Mg–K/Al–K intensity ratio map generated from EELS, (f) EDS Al–K intensity map, (g) EDS Mg–K intensity map, and (h) Mg–K/Al–K intensity ratio map generated from EDS.
in Fig. 6(h), with a similar scale bar ranging from 0 to 1.0. The large variation in the EELS and EDS intensity ratio maps indicates that there are large variations in chemical composition in the samples. Some areas have excessive amount of Mg, while the others show only a small Mg content. Figure 7 shows EELS and EDS intensity maps of Mg and Al for the sample annealed at 8001C. An STEM image of the sample is shown in Fig. 7(a). Similar to the previous Fig. 6, there are two marked boxes in this image. A magnified STEM image of the x-marked area where the EDS and EELS spectra were collected is shown in Fig. 7(b). Figure 7(c) displays the EELS Al–K intensity map, whereas the Mg–K signal obtained from the same area is displayed in Fig. 7(d). The intensity ratio image of EELS maps is presented in Fig. 7(e). Similarly, Al and Mg EDS maps were collected and are displayed in Figs. 7(f) and (g), respectively, with the Mg/Al– K elemental ratio map shown in Fig. 7(e). As can be observed in Fig. 7, the powders processed and annealed at 8001C were much more homogenous than those formed at the synthesized tem-
perature of 3001C, with a value for the Mg/Al–K atomic intensity ratio of about 0.5. Quantitative analysis of these results points to the formation of a spinel phase at 8001C with a Mg/Al ratio close to the stoichiometric value.
IV. Discussion The results of this study show that the synthesis of nanostructured MgAl2O4 spinel can be initiated at temperatures as low as 3001C. The lattice parameter measurements, relative intensities of the {111}, as well as EELS and EDS elemental mapping revealed that the as-synthesized powders are not homogeneous and contain some areas of untransformed g-Al2O3. Increasing the annealing temperature to 8001C produces a homogeneous MgAl2O4 spinel composition with a lattice parameter consistent with published data for spinel. However, spinel formation requires both homogeneous distribution of Mg and Al in the
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matrix as well as ordering of the Mg in the tetrahedral sites and Al ions in the octahedral sites. These aspects of spinel phase formation and cation ordering in the nanostructured powders are discussed below in detail. Aluminum oxide (Al2O3) exists in many polymorphic forms depending on the distribution of cations and anions in the structure.20 For nanostructured powders, Al2O3 usually exists in the form of cubic g-Al2O320,21 with space group, Fd3m. Atomic positions and site occupancies of g-Al2O3 and MgAl2O4 spinel are summarized in Table II for the centric representation. Both g-Al2O3 and MgAl2O4 spinel have 32 oxygen ions forming an FCC sub-lattice with a total of 64 tetrahedral and 32 octahedral interstitial sites. In the ideal spinel structure, 8 Mg ions occupy 1/8 of all tetrahedral sites, forming a diamond-type structure and 16 Al ions occupy 1/2 of all octahedral sites.20 In g-Al2O3, which is often referred to as a defective spinel, 21.5 Al ions/cel occupy both tetrahedral and octahedral positions as marked by Al1 and Al2, respectively, in Table II. The ion occupancy is also lower in these sites than that for MgAl2O4 spinel. Therefore, transformation from g-Al2O3 to MgAl2O4 requires both diffusion of Mg into g-Al2O3 and reordering of the Mg and Al ions between octahedral and tetrahedral interstitial sites to obtain the spinel structure. In practice, however, MgAl2O4 spinel as well as other spinel materials never exist in the ideal form with 100% ordering.22,23 This implies that when Mg ion diffuses into gAl2O3, there is a finite probability based on thermodynamic equilibrium that Mg ion will occupy an octahedral site normally occupied by an Al ion. The degree of ordering can be estimated by knowing the fraction x of Al ions that occupy a tetrahedral sites from the following relation22: Q 5 1 3/2x, where Q is the order parameter that takes value of 1 for fully ordered structures and 0 for a random state. In order to estimate the degree of ordering in our synthesized powders, simulations of SAED patterns were conducted with different values of order parameter, Q. Simulation results shown in Fig. 8 reveal that the relative intensity of the {111} reflection increases linearly with the Q value. The experimental data of I111/I440 relative intensities from our sample annealed at 8001C are plotted as dashed lines. Samples annealed at 3001 and 5001C were excluded as both the lattice parameter measurements as well as EELS and EDS spectrum imaging show high compositional variations, with the intensity of the {111} corresponding to a mixture of gAl2O3 and MgAl2O4 phase. This is not the case for the sample annealed at 8001C with a lattice parameter corresponding to spinel. Based on this analysis, the degree of ordering is approximately 0.78 for samples annealed at 8001C. In other words, the number of Al ions occupying tetrahedral positions is reduced from 67% for the as-synthesized g-Al2O3 material to 15% at 8001C. The order parameter in MgAl2O4 has been measured
Simulation
0.35 Intensity ratio (I111/440)
800°C
0.30
0.25
0.20
0.15 0.0
0.2
0.4 0.6 0.8 Order parameter, Q
1.0
Fig. 8. Relative intensity of I111 to I440 reflections versus the order parameter, Q. The dashed lines represent the I111/I440 intensity ratio of sample annealed at 8001C to estimate the corresponding order parameter.
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recently as a function of temperature using neutron scattering,24 and our value corresponds well with their reported value of Q 5 0.75 at 8001C. This value also corresponds to the thermodynamic equilibrium that is obtained from ab initio simulations.22,23 Interestingly, an order parameter higher then Q 5 0.8 has never been measured experimentally using neutron scattering as at low temperatures below 7251C, ordering is kinetically limited and at higher temperatures, entropy term starts to dominate equilibrium, leading to a decrease in the order parameter.22,23 As a consequence, high-temperature annealing, which is often performed in practice, leads to disordering of cations in MgAl2O4, giving rise to an order parameter of only Q 5 0.55 for the annealing temperature of 14001C.22–24
V. Conclusions Nanostructured MgAl2O4 spinel with a particle size below 20 nm was synthesized at low temperatures by direct conversion from g-Al2O3. MgAl2O4 spinel starts forming at temperatures as low as 3001C, although non uniformity was observed. Increasing the annealing temperature to 8001C increases the lattice parameter and improves the Mg distribution, resulting in a homogeneous MgAl2O4 spinel structure. Heat treatment of the synthesized powder to 8001C produces an ordered spinel structure with an order parameter of 0.78.
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