Microstructural Evolution of a Cu and θ-Al2O3 Composite Formed By

Nov 24, 2015 - There is also concomitant outward diffusion of oxygen, and it is suggested that the θ-alumina is formed by the consolidation of the la...
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Microstructural Evolution of a Cu and θ‑Al2O3 Composite Formed By Reduction of Delafossite CuAlO2: A HAADF-STEM Study Zhiyang Yu, Michael Kracum, Animesh Kundu, Martin P. Harmer, and Helen M. Chan* Department of Materials Science and Engineering, Center for Advanced Materials and Nanotechnology, Lehigh University, Bethlehem, Pennsylvania 18015, United States S Supporting Information *

ABSTRACT: In situ reduction of bulk, polycrystalline copper(I) aluminate (CuAlO2) results in the formation of an intimate two-phase mixture of metallic Cu and θ-alumina. The microstructure of a partially transformed region was studied at the atomistic scale using high-angle angular-dark-field scanning transmission electron microscopy (HAADF-STEM). The observations were consistent with a topotactic transformation mechanism whereby deintercalation of the Cu atoms occurs sequentially at the edges of the Cu+ atomic layers of the CuAlO2 delafossite structure. The Cu forms faceted nanoislands that exhibit an orientation relationship with the θ-alumina matrix. There is also concomitant outward diffusion of oxygen, and it is suggested that the θ-alumina is formed by the consolidation of the layers of Al−O octahedra of the delafossite structure, with some local rearrangement of the Al3+ ions. This model is supported by the observed continuity of the Al−O layers between the parent CuAlO2 and θ-alumina, together with the orientation relationship (0003)CuAlO2//(402)̅ θ-alumina.

1. INTRODUCTION The mineral delafossite (CuFeO2) lends its name to a class of compounds with the general formula ABO2, whose structure is characterized by planar sheet of A cations stacked between layers of edge-shared octahedra (BO6).1 The A+ cations form linear O−A−O bonds that are normal to the A+ sheet. Several delafossite compositions have attracted significant interest as thin films due to their characteristics as transparent, p-type semiconductors.1−3 Copper-based delafossites have also been studied for their oxygen storage capacity.4,5 Three-way catalysts for automobile exhaust gases are sensitive to the fuel/oxygen ratio, so the atmosphere is controlled by oxygen storage materials. Current oxygen storage devices are typically CeO2based due to fast oxygen diffusion, and the structural stability of the nonstochiometric oxide.4,6 At low-temperatures, Cu/CuO based materials have a higher oxygen storage capacity than CeO2-based materials; however, due to the large volume change during cycling, fracture of the material can occur.7 A metal oxide capable of undergoing the Cu2+/Cu+/Cu transition without large volume changes would be attractive for enhanced low temperature performance of the three way catalyst. Using delafossites in an oxygen storage capacity requires a detailed understanding of how the structure changes at varying pO2 levels. Studies have shown that because CuFeO2 and CuMnO2 decompose into copper and metal oxides during their oxygen storage/release processes4,6 at 400−600 °C, their applicability is limited. In contrast, CuAlO2 and CuCrO2 were shown to be stable up to 800 °C in oxygen-uptake and temperature-programmed reduction experiments,5,8 though © XXXX American Chemical Society

their oxygen storage capacity is lower than that of other transition metal delafossites. Recent work in the copper− aluminum oxide system9,10 has shown interesting decomposition behavior of the delafossite-structured CuAlO2. The resulting copper−alumina metal−ceramic composites potentially offer unique combinations of mechanical and electrical properties due to the interpenetrating metal−ceramic microstructure. Bryne et al.9 reported the electrical conductivity of reduced CuAlO2 thin films and attributed this to bands of metallic copper within the alumina grains; however, the detailed mechanism is not well understood. The aim of the current work was to elucidate the mechanism by which CuAlO2 transforms during reduction, and to characterize the structure and morphology of the reaction products at the early stages of decomposition. To this end, atomic resolution electron microscopy was used to study the reaction front in a partially reduced bulk sample of CuAlO2. The results of the study will be pertinent to potential applications of CuAlO2 for oxygen storage. Further, increased understanding of the transformation may allow for greater microstructural control of the resulting metal−ceramic composites, and hence tailoring of the corresponding mechanical and electrical properties. Received: September 20, 2015 Revised: November 17, 2015

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DOI: 10.1021/acs.cgd.5b01362 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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2. EXPERIMENTAL SECTION Dense, polycrystalline CuAlO2 was processed using a two-step procedure based on the work of Yanagiya and co-workers11 and described previously by Kracum et al.10 Briefly, a precursor Cu2O− Al2O3 powder mixture of composition 57.0 wt % Cu2O was made by jar milling copper(I) oxide (99.995% Alfa Aesar) and aluminum oxide (99.999% Sumitomo) in 190 proof ethanol for 12 h. Alumina milling media was used (3 mm diameter, 99.9% purity, Union Process). The powder was subsequently dried and introduced into a graphite die. To aid sample removal following sintering, a layer of 0.13 mm thick graphite foil (Alfa Aesar, 99.8%) was used to line the die. The powder was compacted by uniaxial pressing at a pressure of ∼18 MPa, and the pressed pellet was sintered in a spark plasma sintering furnace (SPS-10 Thermal Technologies) at 850 °C and 50 MPa for 25 min. The sintered pellet was sectioned to remove any surface carbon contamination. The cleaned sections were then annealed in ambient air in a box furnace at 1150 °C for 48 h to convert the Cu−Al2O3 mixture to CuAlO2. Examination by XRD (Rigaku Miniflex) confirmed that the sample consisted of the rhombohedral variant of CuAlO2 (ICDD 00−035−1401); and within the limits of the technique, no other phases were detected. The CuAlO2 samples underwent reduction heat treatment in a vacuum furnace (Centorr Industries M60) at 1000 °C for 3 h, under an atmosphere of flowing process gas (N2−5% H2). This heat treatment schedule resulted in a partially reduced sample consisting of the reduced Cu/θ-Al2O3 region surrounding a core of unreacted CuAlO2. Thin foil samples were extracted from the sample in the region of the reduction front using a dual beam focused ion beam (FIB) microscope (FEI Scios). To minimize exposure to air (and hence possible oxidation of the nanoscale copper regions), the extracted sample was mounted on a molybdenum grid and immediately placed in an argon nanomill (Fishione) for 10 min at 900 eV to remove surface damage induced by the FIB thinning process. The sample was subsequently thinned to a thickness of ∼80 nm and examined in the aberration corrected microscope under an accelerating voltage of 200 keV (JEOL ARM, 200CF).

Figure 1. Microstructure of CuAlO2 subjected to a reduction anneal of 3 h at 1000 °C in N2−5% H2. Bright regions correspond to copper, whereas darker regions are θ-alumina (SEM micrograph, BSE contrast).

magnifications, was actually dispersed with fine Cu lamellae (2−20 nm thick and 6−200 nm in length), see Figure 2b. 3.2. Relationship between CuAlO2 and Decomposition Phases (HAADF-STEM). As described earlier, HAADFSTEM imaging was used to study the interfaces between the CuAlO2 and the decomposition phases. Where possible, the sample was tilted so that within the CuAlO2 regions, the atomic layers composed solely of copper ions were oriented parallel to the beam direction. These planes are readily distinguishable in HAADF imaging due to their relatively high contrast. In the partially reduced grains, the remnant CuAlO2 was in the form of pointed laths, with the long direction parallel to the (0003) planes. As seen in Figure 4a, at the tips of the laths, the terminating planes form a series of terraces and steps. One of the most interesting observations from this study is that in the vicinity of the steps there is a decrease in the brightness of the copper rich planes (Figure 4b,c), indicating a relative deficiency of copper in these regions. This result was consistently observed for all of the terminating copper planes within a given lath, and for the more than eight CuAlO2 laths studied along a ∼15 μm length of the reduction front. We can infer, therefore, that the reduction occurs by the successive deintercalation of the Cu atoms from the edges of the Cu+ atomic layers within the delafossite structure. This type of mechanism where dissolution occurs preferentially at ledge sites is clearly energetically favorable. Nucleation of the transformation would clearly be favored at the original CuAlO2 boundaries, given that this is where (by definition) the CuAlO2 atomic planes terminate. Furthermore, the grain boundaries represent more rapid paths for outward diffusion of oxygen, particularly when intergranular Cu is present. We will return to this point later. Careful study of Figure 4 also reveals that the planes containing the Al3+ ions are continuous between the CuAlO2 and θ-alumina matrix phases. Note that the contrast from these Al−O planes is less distinct than that of the copper planes due to the lower atomic number. There is, therefore, a well-defined orientation relationship between these phases, namely, (0003)CuAlO2//(402̅)θ-alumina. The structure of θ-alumina can be regarded as a distorted version of the cubic γ-alumina structure, with approximately FCC stacking of close-packed layers of oxygen ions.12−14 The (402̅) planes correspond to one set of the close-packed oxygen planes. HAADF images of the CuAlO2 and θ-alumina depicting the above orientation relationship, as

3. RESULTS AND DISCUSSION 3.1. Microstructure of Reduced CuAlO2 (SEM). During the reduction process, the polycrystalline CuAlO2 is reduced to a two-phase mixture of alumina and copper according to the following reaction: 1 2CuAlO2 → 2Cu + Al 2O3 + O 2 2(g) (1) In the reduced CuAlO2, the original grain structure of the parent delafossite is clearly discernible, with each grain retaining its original shape (see Figure 1). Many of the composite “grains” contain bands of copper, 100−500 nm thick and 1−10 μm in length. The composition of these bands was confirmed by X-ray EDS (energy dispersive spectroscopy), see Figure S1. The matrix was identified as θ-Al2O3 based on EBSD (electron backscatter diffraction) analysis10 and X-ray diffraction (see Figure S2). In addition to the metallic copper inclusions within the grains, the copper also forms a continuous intergranular phase. The microstructure of the composite is highly complex and is depicted at different length scales in Figure 2. A partially transformed CuAlO2 grain is shown in Figure 3a. It can be seen that the coarse Cu bands have a preferred orientation relationship with the parent grain and appear to be continuous with the copper phase at the grain boundaries. The microstructure strongly suggests that the reduction reaction occurs initially at the grain boundaries and that the copper laths nucleate at the boundaries and grow inward. High resolution BSE imaging of the composite grains in the SEM showed that the matrix, which appeared to be homogeneous θ-Al2O3 at low B

DOI: 10.1021/acs.cgd.5b01362 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 2. SEM micrographs of CuAlO2 subjected to a reduction anneal of 3 h at 1000 °C in N2−5% H2. Note variations in morphology and scale of the Cu/θ-alumina regions. Bright regions correspond to copper, whereas darker regions are θ-alumina (BSE contrast).

Figure 3. SEM micrographs depicting regions of CuAlO2 sample where the reduction transformation is incomplete. The reduction heat-treatment was 3 h at 1000 °C in N2−5% H2. The rectangle in (b) is representative of the area from which TEM specimens were extracted using FIB milling.

Figure 4. (a) Typical HAADF image showing the stepped interface between remnant CuAlO2 and the θ-Al2O3 reduction product. (b) An expanded view of the terminating (0003) Cu-containing planes of the CuAlO2; note the decreasing intensity in the tip region of the planes, which is confirmed by (c), the intensity line profile along AB. The faint bright fringes in the θ-Al2O3 correspond to Al3+ containing planes; note that they are continuous between the two phases.

well as diagrams of the corresponding atomic arrangements, are depicted in Figure 5a,b. Further support for the as-identified orientation relationships between the three phases is given in Figure S3. 3.3. Transformation Mechanism and Microstructural Evolution. As can be seen from eq 1, the reduction of CuAlO2 to Cu and θ-alumina requires the loss of one formula unit of oxygen from every two formula units of CuAlO2 and the complete removal of copper. If we consider then the structure of CuAlO2 depicted in Figure 5a, this is equivalent to the removal of one out of every four oxygen layers, and every Cu layer. The transformation can be classified as topotactic, given that the orientation of the remaining close-packed layers of oxygen is virtually unchanged between the parent CuAlO2 and the θ-alumina. The reduction transformation to the CuAlO2

structure can be envisaged as the removal of the extra atomic layers, followed by a contraction and consolidation of the structure parallel to the [0001] direction, with localized rearrangement of the Al3+ ions. Clearly the transformation will induce localized distortion of the lattice and most likely introduce residual stresses. This is evidenced by bending of the plane of Al3+ ions at the reaction interface as depicted in Figure S4. The extent of the volume decrease can be seen by comparing the boxes delineated by the dotted lines in Figure 5a,b, which represent equivalent portions of the CuAlO2 and θalumina structures, corresponding to before and after reduction, respectively. Note that the atomic arrangements shown represent the f ully relaxed structures and do not reflect the potential residual stresses alluded to earlier. C

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Figure 5. Depiction of the relative orientation of the (a) CuAlO2, (b) θ-Al2O3, and (c) Cu phases. The upper part of each panel shows the HAADF image, whereas the lower part shows the corresponding atomic structures (all drawn to the same scale). The crystallographic direction perpendicular to the plane of the paper is shown in the bottom left-hand corner of each panel.

Figure 6. (a) Low magnification TEM image of partially reduced CuAlO2. Series of HAADF images (at increased magnification) showing copper nanocrystals in the θ-Al2O3 matrix adjacent to a CuAlO2 lath (b−d).

Given the orientation of the layering of the Cu+ ions in the parent structure, it was initially assumed that the preferred diffusion paths of the copper atoms would lie between the close-packed planes of oxygen ions, i.e., parallel to the long direction of the CuAlO2 laths. According to this mechanism, one might expect a higher concentration of copper ahead of the tips of the receding CuAlO2; this was not the case. Instead,

elongated clusters of metallic copper were observed in the regions between the CuAlO2 laths, implying that diffusion had occurred preferentially in directions perpendicular to the (0003) planes (see Figure 6). Through focus imaging revealed that the copper islands occurred at different depths in the specimen. Closer examination of the copper nanocrystals revealed that many of them were associated with nanoscale D

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Figure 7. HAADF-STEM image showing nanoscale, faceted pores within the θ-Al2O3 (see the boxed region in (a), and at greater magnification in (b)). In many cases, the interior pore surfaces were lined with Cu.

copper forms a continuous intergranular film in the composite, it seems reasonable to assume that the metallic phase is a conduit for outward motion of oxygen. Finally, a brief discussion of why the metastable theta polymorph is formed, as opposed to α-alumina, is in order. The transformation of θ- to α-alumina has been studied by Kao and Wei.24 These workers reported onset transformation temperatures of 1230 and 1120 °C for unseeded and seeded samples, respectively. The transformation of transition aluminas during heating has also been reviewed by Levin and Brandon.13 Notably, the transformation temperatures of theta to alpha are dependent on the starting material and whether α-alumina seeds are present. In many cases, however, α-alumina can be formed at 1000 °C, which corresponds to the reduction temperature carried out in the present study. We believe that the formation of θ-alumina during the reduction of CuAlO2 may be favored because it has a slightly lower density compared to α-alumina (3.78; cf., 3.98 g cm−3), and hence, the degree of volume change during transformation is reduced.

pores within the θ-Al2O3 matrix. In some cases, the faceted morphology of the copper nanocrystals was observed to be similar to that of the pores; hence, it is assumed that the copper was filling a pore interior, as opposed to being a precipitate within the alumina. The pores were clearly distinguishable due to the reduced contrast. It is believed that these pores result from a condensation of the vacant sites generated by the outward diffusion of the copper and oxygen. Based on density values, the theoretical change in volume during the reduction process was estimated to be ∼9.5%.10 In many cases, layers of copper atoms, as indicated by the brighter contrast and as confirmed by X-ray EDS, were observed lining the inner pore surfaces (Figure 7). As alluded to above, the pores were invariably faceted; facet planes, which were frequently observed, were (402̅) and (401), which correspond to two sets of the close-packed oxygen planes in the θ-Al2O3 structure. The epitaxial relationship between the Cu and the θ-Al2O3 was determined to be (111)Cu//(402̅)θAl2O3 and [11̅0]Cu//[010]θ-Al2O3; this is depicted in Figure 5b,c. Although there have been numerous studies regarding the orientation relationship of copper films/droplets on sapphire (α-alumina) substrates,15−21 to the authors’ knowledge, this is the first report of the corresponding relationship between Cu and θ-Al2O3. The behavior is somewhat analogous, in that for both θ-Al2O3 and α-Al2O3, the close-packed planes in copper align with the oxygen close-packed planes of the alumina polymorph. It should be emphasized that the formation of the copper nanocrystals takes place during the very early stages of the CuAlO2 reduction decomposition. Nucleation of the metallic copper occurs preferentially at the prior CuAlO2 grain boundaries, as well as at nanopores within the θ-Al2O3 matrix, due to strain energy considerations. With further heattreatment, however, it is proposed that there is coalescence and coarsening of the copper nanocrystals to form lamellae, hence the banded structure within the composite. So far, the discussion has focused on the diffusion path of the copper atoms. By definition, however, during the reduction process, outward diffusion of oxygen must take place. Pastorek and Ramp22 have reported that the diffusivity of oxygen in solid copper at 1000 °C is 3 × 10−9 m2/s. No data is available for the diffusivity of oxygen in θ-Al2O3; however, compared to the grain boundary diffusion of oxygen in α-alumina,23 this figure is about 8 orders of magnitude more rapid. Given also that the

4. CONCLUSIONS HAADF-STEM imaging was used to study the reaction front in a partially reduced sample of CuAlO2. On reduction, CuAlO2 decomposes to form copper and θ-alumina. The observations were consistent with a topotactic transformation mechanism whereby deintercalation of the Cu atoms occurs sequentially at the edges of the Cu+ atomic layers of the CuAlO2 delafossite structure. At the early stages of the transformation, the Cu forms faceted nanocrystals that exhibit an orientation relationship with the θ-alumina matrix. It is postulated that coalescence and coarsening of these nanocrystals take place to form copper lamellae that interleave the θ-alumina matrix. There is also concomitant outward diffusion of oxygen, and it is suggested that the θ-alumina is formed by the consolidation of the layers of Al−O octahedra of the delafossite structure, with some local rearrangement of the Al3+ ions. This model is supported by the observed continuity of the Al−O layers between the parent CuAlO2 and θ-alumina, together with the orientation relationship (0003)CuAlO2//(402̅)θ-alumina.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01362. E

DOI: 10.1021/acs.cgd.5b01362 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (610) 758-5554. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Office of Naval Research under the MURI program, contract # N000141110678, monitored by Dr. D. Shifler. The authors are grateful to C. Marvel for his help with TEM specimen preparation.

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REFERENCES

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DOI: 10.1021/acs.cgd.5b01362 Cryst. Growth Des. XXXX, XXX, XXX−XXX