Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Spark Plasma Sintering (SPS)-Assisted Synthesis and Thermoelectric Characterization of Magnéli Phase V6O11 Markus Joos,† Giacomo Cerretti,† Igor Veremchuk,‡ Patrick Hofmann,§ Hajo Frerichs,† Dalaver H. Anjum,∥ Tobias Reich,⊥ Ingo Lieberwirth,# Martin Panthöfer,† Wolfgang G. Zeier,§ and Wolfgang Tremel*,† †
Institut für Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universität, Duesbergweg 10-14, D-55099 Mainz, Germany ‡ Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, D-01187 Dresden, Germany § Physikalisch-Chemisches Institut, Justus-Liebig-Universität Gießen, Heinrich-Buff-Ring-17, 35392 Gießen, Germany ∥ Imaging and Characterization Core Lab, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia ⊥ Institut für Kernchemie, Johannes Gutenberg-Universität, Fritz-Straßmann-Weg 2, 55128 Mainz, Germany # Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany S Supporting Information *
ABSTRACT: The Magnéli phase V6O11 was synthesized in gram amounts from a powder mixture of V6O11/V7O13 and vanadium metal, using the spark plasma sintering (SPS) technique. Its structure was determined with synchrotron X-ray powder diffraction data from a phase-pure sample synthesized by conventional solid-state synthesis. A special feature of Magnéli-type oxides is a combination of crystallographic shear and intrinsic disorder that leads to relatively low lattice thermal conductivities. SPS prepared V6O11 has a relatively low thermal conductivity of κ = 2.72 ± 0.06 W (m K)−1 while being a n-type conductor with an electrical conductivity of σ = 0.039 ± 0.005 (μΩ m)−1, a Seebeck coefficient of α = −(35 ± 2) μV K−1, which leads to a power factor of PF = 4.9 ± 0.8 × 10−5W (m K2)−1 at ∼600 K. Advances in the application of Magnéli phases are mostly hindered by synthetic and processing challenges, especially when metastable and nanostructured materials such as V6O11 are involved. This study gives insight into the complications of SPS-assisted synthesis of complex oxide materials, provides new information about the thermal and electrical properties of vanadium oxides at high temperatures, and supports the concept of reducing the thermal conductivity of materials with structural building blocks such as crystallographic shear (CS) planes.
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INTRODUCTION Multivalent transition-metal oxides (TMOs) exhibit a cornucopia of interesting and also useful properties, beyond those of conventional semiconductors employed in electronic and optoelectronic devices.1−4 The ground state in TMOs is dictated by the valence state of the transition-metal cations, and the interplay between spin, orbital, charge, and lattice degrees of freedom has been explored widely in this class of materials with partially filled d-shells.5−7 Vanadium oxides are prototype examples of multivalent TMOs with strongly correlated electrons.8,9 They can undergo reversible metal-to-insulator phase transitions accompanied by changes in their crystallographic, magnetic, optical, and electrical properties. Electron−lattice interactions and electron−electron correlation tailor the properties of vanadium oxides for a variety of applications as chemical sensors,10 electrode materials for lithium batteries,11,12 capacitors and supercapacitors,13 for electronic and optical2 devices and also in catalysis.14,15 © XXXX American Chemical Society
These examples show the potential of vanadium oxides for the design of new information storage and energy conversion devices. For engineering the properties of TMOs, it is desirable to decouple electronic and thermal transport. This decoupling can be promoted through an intrinsic periodic arrangement of structural building blocks with different electron and phonon transport characteristics that occur in the so-called Magnéli phases. Vanadium-based Magnéli phases represent a series of adaptive compounds with substoichiometric oxygen composition and the generic formula VnO2n−1 = V2O3 + (n − 2) VO2 (3 ≤ n ≤ 9). The structure is derived from rutile and corundum, where crystallographic shear (CS) planes arise by structurally combining the edge-sharing chains of octahedra in the rutile structure, with the face-sharing octahedral connection occurring in the corundum structure. These CS planes are characteristic features in reduced early TMOs that occur because of the Received: October 19, 2017
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DOI: 10.1021/acs.inorgchem.7b02669 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry removal of oxygen,16−21 with concomitant metal reduction and an increase of the charge carrier concentration. Most phases of the Magnéli series are unstable at high temperatures, where they are either oxidized or reduced to neighboring VnO2n−1 phases,22−25 and they are expected to yield the compositional end phases VO2 and V2O3 eventually. Since CS planes are basically an interruption of a periodically arranged structure, they represent scattering centers for mechanical waves, meaning that they limit the mean free path of phonons and thereby reduce the thermal conductivity.26,27 When moving from V2O3 to VO2, the end members in the V− O phase diagram of the VnO2n−1 Magnéli series, the d-band occupation across the series decreases, changing the electronic properties, which can lead to insulating (V3O5), semiconducting (V6O11, V8O15, V9O17), or metallic (V7O13) behavior at room temperature.28−31 Still, advances in the application of Magnéli phases are mostly hindered by synthetic and processing challenges, especially when metastable and nanostructured materials are involved. Bulk samples with a large deviation from VO2 stoichiometry are mostly prepared via the high-temperature reduction of crystalline vanadium oxide (VO2) in hydrogencontaining atmospheres or by annealing VO2 together with vanadium metal.16,32−35 A fundamental step toward bulk materials with reasonable thermoelectric efficiencies arises from a mild consolidation of compounds or composites with additional phonon scattering at nanoscale interfaces.26,27 The most common consolidation process to date is hot pressing, but the elaboration procedure is long and time-consuming, and the resulting dense and highly textured samples are difficult to use for the required measurements. To address this drawback, we used a spark plasma sintering (SPS)-assisted synthesis36−40 for the Magnéli phase V6O11, in contrast to typically employed gasphase CVD approaches41−44 or high-temperature bulk or single-crystal syntheses.16,32−35 We report the SPS-assisted preparation of the Magnéli phase V6O11.44−49 It contains an intrinsic nanostructure defined by CS planes,26,29,31,50 which supports the formation of a microstructure that is useful for the manufacturing of monoliths or thermoelectric samples, e.g., in the Ti−O and Nb−W−O systems.50−53 Previous studies have shown that crystallographic shear and/or disorder lead to promising low thermal conductivities.26,54−57 The assumption of a high electrical conductivity and a low thermal conductivity due to the CS planes makes the VnO2n−1 Magnéli phases a prospective class of materials for our investigations of their thermoelectric properties. V6O11 was selected for the SPS synthesis, because (i) it is a typical representative of the VnO2n−1 Magnéli phases; (ii) it is a semiconductor close to the metal−insulator transition, whose properties resemble most likely that of a heavily doped semiconductor; and (iii) its preparation and processing proved to be more efficient than that of neighboring members (V5O9, V7O13, or V8O15) of the Magnéli series. The conceptual advancement of our approach toward functional nanocomposites lies in the use of short sintering times combined with superior heating and simultaneous densification by SPS and synthesis.
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K, and the temperature was held for 6 h and, afterward, cooled radiatively to room temperature. The result was a mixture consisting of V6O11 with impurities of V7O13, whose amount was quantified by comparing the intensities of characteristic reflections for V6O11 and V7O13 in the powder X-ray diffractograms. The V6O11/V7O13 mixture was heated in a subsequent step by SPS, with the appropriate amount of vanadium powder added being that required to obtain single-phase V6O11 from the V7O13 impurity. The obtained powders were loaded in a graphite die and SPS synthesis was performed under vacuum by applying a 50 MPa uniaxial pressure with a heating and cooling rate of 100 K min−1, a reaction temperature of 1273 K, and a heating time of 30 min. Once the samples were cooled to room temperature, the pellets were carefully polished to remove the residues of the external carbon substrate. Prior to SPS pressing, the powders were ball-milled. This lead to a density of 96%, compared to the theoretical density of V6O11. The resulting dense pellet of V6O11 (with minor impurities of V5O9, as determined by X-ray diffraction (XRD) with synchrotron radiation) was used for the thermoelectric characterization.
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RESULTS AND DISCUSSION
Preparation and Processing. Preliminary reactions using conventional high-temperature techniques were carried out to determine the proper reaction conditions for the SPS-assisted synthesis. The stoichiometric quartz ampule synthesis of V6O11 lead to the desired majority phase with V7O13 impurities as revealed by powder XRD. The formation of V7O13 was presumably due to surface oxidation on the vanadium grains, as indicated by XPS analyses (see Figures S6 and S7 in thbe Supporting Information). The formation of V7O13 could be circumvented by using an excess of vanadium metal powder that compensated for the secondary phase V7O13 and polycrystalline V6O11 was obtained in high purity (Sample 1; see Figure S5 in the Supporting Information). The synthetic approach for the SPS-assisted synthesis of V6O11, employing metallic precursors with the binary oxides (here, the neighboring VnO2n−1 Magnéli phases), had first been applied for the preparation of single-phase bulk polycrystalline Ti2O338 and WO2.9.39 A problematic aspect concerning the usage of SPS for V6O11 was that the graphite environment of the SPS sample assembly had a reducing effect during heating, thereby generating the lower neighboring VnO2n−1 Magnéli phase member V5O9 and a layer of V2O3 on the pellet surface, which had to be removed mechanically afterward. The reducing effect of carbon was also tested in ampule reactions, where almost phase-pure V2O3 was obtained by reduction. In SPS reactions, reduction occurred at 1273 K and became more pronounced with increasing temperature. However, a reaction temperature of 1273 K and ball-milling of the reaction mixture (to reduce the grain size) were necessary to achieve a pellet density sufficient for further processing. The formation of pure V6O11 could be achieved by using the detrimental aspects of the V6O11 synthesis to our advantage: (i) the presence of V7O13 in ampule reactions and (ii) the reducing effect of carbon in SPS processing. Rather than using pure V6O11 powder for SPS compaction, a mixture of V6O11 and V7O13 (in which the amount of V7O13 impurity was quantified from the PXRD pattern of the sample) was used as a precursor, and vanadium metal powder was added in stoichiometric amounts to compensate for the present amount of V7O13. In the following SPS processing, the vanadium metal reacted with the V7O13 to form V6O11, promoted by the reducing effect of the graphite environment and purifying the sample. The synthetic workflow is illustrated in Figure S2 in the Supporting Information.
EXPERIMENTAL SECTION
A standard synthesis was performed on a 1 g scale. Vanadium metal powder and vanadium(IV) oxide (V + VO2) were added in stoichiometric amounts, mortared, and sealed under vacuum in a quartz ampule. The mixture was heated at a rate of 5 K min−1 to 1273 B
DOI: 10.1021/acs.inorgchem.7b02669 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
reference sample of V6O11, which was prepared by ampule reaction (Sample 1, Figure S5). The side phase V5O9 in the SPS-prepared sample (Sample 2, Figure 1) was refined for the crystallinity, the lattice parameters, and the relative weight percent, resulting in a total value of 3.36 wt %. These data clearly demonstrate the high quality (phase purity and crystallinity) of the products obtained by SPS. Only an enlargement of the data (see zoomed-in inset in Figure 1) shows additional Bragg intensities, marked by an asterisk, for V5O9. These additional reflections can only be detected due to the high spatial resolution and beam flux of the synchrotron radiation.58 The marked reflection of V5O9 at 2θ = 8.3° (Q ≈ 2.14 Å−1) corresponds to the strongest (103̅) Bragg intensity of the V5O9 diffraction pattern. Note that the absolute intensity after background subtraction is ∼1300 counts, compared to 36 200 counts of the (103) reflection in V6O11. The structure of V6O11 is derived from the rutile type, where VO6 units are connected via shared edges in one direction, forming chains of VO6 octahedra according to VnO2n−1. These chains, representing the structural contribution of the parent VO2, are interconnected via corners perpendicular to the edgesharing connections.59,60 They are interrupted after every nth unit by a plane of face-sharing octahedra, which form a CS plane and represent the structural contribution of the corundum-type V2O3.61,62 The formation of the CS planes is driven by defects resulting from a removal of oxygen atoms (i.e., a reduction).19 Weak diffuse streaks passing through the spots of the Fourier transformations of the HR-TEM images for V6O11 prepared by conventional ampule (Sample 3, Figures 2a and 2b) and by SPS-assisted synthesis (Sample 2, Figures 2c and 2d) may be caused by microsyntactic intergrowth, which has been reported for the VnO2n−1 Magnéli phases,61,63−65 but it is not an inherent feature, because the electron diffraction images (Figures 2e and 2f) of a second SPS prepared V6O11 pellet (Sample 2) did not show diffuse streaks in the diffraction spots. No significant differences in the grain size distribution of samples prepared conventionally and by SPS reactions were noticed. The crystallite size for both varied widely, from 0.1 μm up to 5 μm. Yet, as the powders were either mortared or ball-milled in additional processing steps, their grain sizes might differ if no mechanical procedure were involved. All members of the VnO2n−1 Magnéli phases exhibit paramagnetic behavior, and many of them undergo metal-toinsulator transitions upon cooling. As the magnetic behavior of each VnO2n−1 phase is distinct, the presence of any side phase is revealed by its specific signature of the temperature-dependent magnetic moment.46,66 Thus, the temperature dependence of the magnetic moment can be used to determine the sample purity. Figure 3 shows the temperature dependence of the magnetic moment of V6O11 prepared by SPS (Sample 2). V6O11 has a metal-to-insulator transition at TMIT = 167.5 K and a transition to an antiferromagnetic phase at TN = 25.2 K, both of which lead to the characteristic profile for the magnetic moment of V6O11.28,31 While V7O13 exhibits no metal-toinsulator transition and, hence, cannot be traced from the temperature signature of its magnetic moment, a V5O9 impurity gives rise to a small hump,46 highlighted by an arrow in Figure 3, associated with a transition at ∼135 K. XPS spectra (Figures S6 and S7) were recorded to exclude that the increase of the magnetic moment below 65 K is caused by traces of vanadium metal (
