Ga Substitution and Oxygen Diffusion Kinetics in Ca3Co4O9+Î´-Based

Subscriber access provided by UOW Library

Article

Ga Substitution and Oxygen Diffusion Kinetics in CaCoO based Thermoelectric Oxides 3

4

9+#

Ruoming Tian, Richard Donelson, Chris D. Ling, Peter E.R. Blanchard, Tianshu Zhang, Dewei Chu, Thiam Teck Tan, and Sean Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp403592s • Publication Date (Web): 06 Jun 2013 Downloaded from http://pubs.acs.org on June 20, 2013

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Ga Substitution and Oxygen Diffusion Kinetics in Ca3Co4O9+δ based Thermoelectric Oxides

Ruoming Tian1, Richard Donelson2, Chris D. Ling3, Peter E.R. Blanchard3, Tianshu Zhang1, Dewei Chu1, Thiam Teck Tan1 and Sean Li1,*

1

School of Material Science and Engineering, the University of New South Wales, NSW 2052, Australia. 2

3

CSIRO, Clayton South, VIC 3162, Australia.

School of Chemistry, The University of Sydney, Sydney 2006, Australia

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 23

ABSTRACT

The influence of Ga doping on the crystal structure and cobalt oxidation state of Ca3Co4O9 was determined using synchrotron X-ray diffraction and X-ray absorption near-edge spectroscopy. The results show that Ga is preferentially located on the Co site in the rock-salt type [Ca2CoO3] layer, up to a maximum occupancy of ~2.2%; and that doping does not change the cobalt oxidation state of this material. We find that the doping-induced point defect scattering and modification of charge carrier concentration results in lower thermal conductivity and lower electrical resistivity, thereby improving the thermoelectric figure-of-merit in the Ga-doped Ca3Co4O9+δ material. In addition, the influence of sample density and measuring atmosphere on their thermoelectric performance was found to be significant. It is believed that the interconnected pore structure in the low-density sample facilitates oxygen outdiffusion from the lattice, which leads to anomalous electrical resistivity behavior at elevated temperatures. We show that this behavior can be very effectively suppressed by improving the sample density.

KEYWORDS thermoelectric, Ca3Co4O9+δ, Ga doping, crystal structure, oxygen deficiency


Page 3 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Oxide-based thermoelectric semiconductor materials are being considered for application in both power generation and energy recovery from waste heat. Robust, high efficiency materials will be required, therefore the materials should possess a large Seebeck coefficient S, low electrical resistivity ρ and low thermal conductivity κ , so as to yield a high thermoelectric figure of merit [Z=S2/(ρκ)]. Ca3Co4O9+δ is seen as a promising candidate for high temperature application due to its superior thermoelectric performance and good thermal stability. Currently the highest reported figure of merit ZT for Ca3Co4O9+δ single crystal is 0.831 at 973 K. It is believed that this high performance is associated with its unique layered structure,2 which consists of two subsystems: a rock-salt type [Ca2CoO3] (RS) layer sandwiched between hexagonal CdI2-type [CoO2] (H) layers. The H layer provides the electronic transport pathway while the RS layer is considered to be responsible for phonon scattering which leads to low thermal conductivity. This layered structure also shows strong anisotropic behavior, which is responsible for an increase in performance when the materials are highly textured. The inherent alignment of single crystals makes them ideal candidates. However, processing of single crystal oxide materials is likely to be expensive, and therefore unsuitable for mass production. Low cost fabrication methods such as pressure-less sintering of compacted powders may be employed; however, their random polycrystalline structure results in a typical value of ZT below 0.1.3, 4 Alternatively, sintering of powder compacts via hot pressing or spark plasma sintering produces textures where the grains are well aligned, resulting in a ZT value up to ~0.2 near 1000K.4 Another effective approach to improve the thermoelectric properties of Ca3Co4O9+δ is via cation doping. Combining these methods, e.g., hot pressing of



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 23

samples with Ga partially substituted for Co, has yielded a ZT of 0.45 at 1200 K.5 However, the mechanism of enhanced thermoelectric property by Ga doping has not yet been sufficiently investigated. Therefore, it is of great importance to explore the location of Ga atoms in the lattice and the correlation of crystal structure with the observed physical properties. In addition, oxygen deficiency plays an important role in determining the physical properties of oxide materials. For instance, it has been demonstrated that the electronic conduction of La0.6Sr0.4CoO3-δ strongly depended on oxygen vacancy concentration δ, which can controlled by tuning the partial oxygen pressure.6

The stiffening of phonon vibration and enhanced Seebeck coefficient in

Na0.73CoO2-δ with increased oxygen vacancy concentration serves as another example.7, 8 In this work, polycrystalline Ga-doped Ca3Co4O9 material was produced using both spark plasma sintering and conventional pressure-less processing. Rietveld refinement of the crystal structure was carried out against synchrotron X-ray diffraction (XRD) data, which were collected from ground samples both with and without Ga substitution. Co K-edge X-ray absorption near-edge spectra (XANES) were also acquired on these samples in order to explore the influence of Ga substitution on Co valence state. In addition, the effect of Ga doping on the thermoelectric performance via different processing – in particular, the sample density and atmosphere on the electrical transport properties of these materials at elevated temperatures were investigated.

EXPERIMENTAL SECTION Polycrystalline Ca3Co4-yGayO9+δ (y=0.00, 0.05, 0.10) ceramics were synthesized by solid state reaction (SSR) followed by either conventional sintering (CS) or spark


Page 5 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


plasma sintering (SPS). The first step in the SSR process was ball-milling of a stoichiometric mixture of the following materials in ethanol for 12 h: reagent grade CaCO3 (99.0% Sigma Aldrich), Ga2O3 (99.99%, Sigma Aldrich) and Co3O4 (99.7%, Alfa Aesar). The mixtures were dried and then calcined in air at 1173 K for 20 h. The milling and calcining steps were repeated. The twice-calcined powder was then further ball milled to obtain ~1 µm SSR powders. In the CS process, the as-prepared SSR powder was pressed into 15 mm disks using a pressure of 150 MPa. The pellets were heated to 1173 K, and held there for 20 h. Finally, they were cooled to room temperature at the rate of 5 K/min. The SPS process was carried out in a Dr Sinter SPS-825 (Syntex, Inc., Japan). The SSR powders were placed in a 20mm graphite die. A moderate pressure of 10MPa was applied before sintering to ensure a closed electrical loop for current to pass through. The sample was heated up to 1073 K, and a uniaxial pressure of 50 MPa was applied during a 5 min dwell. The sintered SPS pellets were then polished and annealed in air at 1173 K and held there for 20 h to remove the graphite foil and re-oxidize them to the same state as the CS samples. XRD (Panalytical X’pert MPD) was used to identify the phases of SSR powders and SPS sintered pellets. In order to investigate the position and occupancy of the doped Ga atoms in the lattice, high-resolution synchrotron XRD data were collected on the PD beamline at the Australian Synchrotron using a wavelength λ = 0.82565 Å. Rietveld refinements were then carried out using the Jana2006 program.9 Cobalt Kedge XANES spectra were acquired on beamline 20B at the Photon Factory in Japan. The energy scale was calibrated using the K-edge of a pure Co foil at 7709 eV. Microstructures of the samples produced by SPS and CS processing were observed with a FE230 scanning electron microscopy. Electrical resistivity and Seebeck



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 23

coefficient were simultaneously measured from room temperature to 973 K on ULVAC-ZEM3 system under a He atmosphere slightly above ambient. The error of the electrical resistivity and Seebeck coefficient measurements is ~ 3% determined by three repeated experiments for the particular samples. Thermal conductivity was evaluated from κ αρC where α is thermal diffusivity, ρ is density and C is isobaric specific heat, respectively. The diffusivity, thermal expansion and specific heat were measured by NETZSCH LFA-427 laser flash system, NETZSCH DIL402C and NETZSCH DSC-404C differential scanning calorimeter, respectively.

RESULTS AND DISCUSSION XRD data for SSR powders of Ca3Co4-yGayO9+δ (y = 0, 0.05, 0.1) and the SPS sintered y=0.05 sample are shown in Figure 1. All of the diffraction peaks could be indexed to a standard Ca3Co4O9 phase (JCPDS 58-0661) and there were no detectable impurity phases.


Page 7 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. XRD patterns for the SSR powders with the nominal composition of Ca3Co4-yGayO9+δ (a) y=0, (b) y=0.05, (c) y=0.1 and (d) pressed surface of SPS sintered y=0.05 sample.

In order to determine the crystal structure of Ga-doped Ca3Co4O9, particularly the position of the Ga atoms in the lattice, Rietveld refinement was carried out against synchrotron XRD patterns with and without Ga doping. The superspace group X2/m(0,δ,0)s0 and the initial structure parameters were taken from the model of Ling10 et al. (in turn derived from that of Grebille11 et al.) This model includes positional modulation waves for all atoms except the split-site Co and O atoms in the RS subsystems. Attempts to introduce Ga onto the Co site in the H subsystem in the model resulted in a negative factional occupancy of Ga, indicating that Ga atoms are more likely to substitute at Co site in the central layer of the RS subsystem. Table 1 presents the refined atomic positions in the RS layer and the fractional occupancy of Ga on the Co site. Assuming that there are no cation vacancies in the structure, the final refined Ga occupancy on this Co site is ~2.2% for all samples, regardless of nominal doping concentration (3.3% for y=0.05 and 6.5% for y=0.10). The small amount of excess Ga in the higher-doped samples may be present in the form of glassy or otherwise poorly crystalline phases, or distributed among a variety of impurity phases at concentrations too low to be observed by XRD. The normalized cobalt K-edge XANES spectra of the three samples from 7700 to 7760 eV are shown in Figure 2. The section of the spectrum marked A is a pre-edge feature associated with the transition from the Co 1s core level to Co 3d hybridized states transition, which is commonly observed in transition metals.12 Feature B could be assigned to a transition from the Co 1s orbital to unoccupied 4p-derived orbital.13, 7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

14

Page 8 of 23

All the samples exhibit similar spectra, with the main absorption energy (Feature B)

located at 7730 eV, which is slightly higher than that of the standard CoO (7726 eV) and Co2O3 (7729 eV) reported elsewhere.15 This implies that the average Co valence state of this system is higher than 3+ and that Ga doping does not change the oxidation state of Co. However, the quantitative analysis of the exact valence state of Co proved difficult, due to the small energy difference of Co absorption peak between the standard Co2O3 and the as-prepared Ca3Co4-yGayO9+δ samples.

Table 1. Refined atomic positions and occupancies of the RS subsystem in the ground samples with the nominal composition of Ca3Co4O9+δ, Ca3Co3.95Ga0.05O9+δ and Ca3Co3.9Ga0.1O9+δ.

Atom Co1 Ca O1 O2 Atom Co1 Ga1 Ca O1 O2 Atom Co1 Ga1 Ca O1 O2

Ca3Co4O9+δ (R=0.0748, wR=0.0710) x(a) y(b) z(c) 0 0 0.5 0.4185(8) 0 0.2797(3) 0.0447(13) 0 0.6731(7) 0 0.5 0.5 Ca3Co3.95Ga0.05O9+δ (R=0.0746, wR=0.0709) x(a) y(b) z(c) 0 0 0.5 0 0 0.5 0.4183(8) 0 0.2799(3) 0.0439 (13) 0 0.6743(8) 0 0.5 0.5 Ca3Co3.9Ga0.1O9+δ (R=0.0750, wR=0.0710) x(a) y(b) z(c) 0 0 0.5 0 0 0.5 0.4182(8) 0 0.2799(3) 0.0437(13) 0 0.6745(8) 0 0.5 0.5

Occupancy 1 1 1 1 Occupancy 0.978(19) 0.022(19) 1 1 1 Occupancy 0.978(19) 0.022(19) 1 1 1


Page 9 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Normalized Co K-edge XANES spectra of the ground samples of Ca3Co4yGayO9+δ

(y=0, 0.05, 0.1).

The measured bulk density of the discs samples produced by SPS was ~ 97% of the theoretical maximum, indicating that these samples were likely to have a closed pore structure. Figure 3(a) shows an SEM image of a fracture surface of the Ca3Co3.95Ga0.05O9+δ specimen sintered by SPS. The structure appears to be highly textured and dense, and was typical for all three compositions sintered by SPS. The XRD pattern obtained from the surface orthogonal to the SPS pressing direction [Figure 1(d), y=0.05] indicates that the samples are preferentially orientated with the (00l) planes normal to the pressing direction. This highly aligned microstructure is favorable for lowering the electrical resistivity in the plane normal to the pressing direction, as the electrical resistivity in the a-b plane is much lower than along the c axis.2



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 23

Figure 3. SEM images of fracture section of polycrystalline Ca3Co3.95Ga.05O9+δ ceramics produced by (a) SPS and (b) CS process. The arrow displays the pressure direction applied during SPS process.


Page 11 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Temperature dependence of (a) electrical resistivity ρ and (b) Seebeck coefficient S for polycrystalline Ca3Co4-yGayO9+δ (y=0, 0.05, 0.1) prepared by SPS. The bulk density of the Ca3Co3.95Ga0.05O9+δ sample produced by CS at 1183 K was around 60% of the theoretical maximum. The microstructure produced by CS [Figure 3(b)] consisted of randomly oriented platelets approximately 2-6 µm wide and 1 µm thick. There was also good connectivity between the platelets, i.e. large necks had



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 23

formed in many places and the pores appeared to be interconnected and of the order of 0.5 to 1 µm in diameter at their most restrictive. The temperature dependence of the electrical resistivity under a low pressure of He for the polycrystalline Ca3Co4-yGayO9+δ (y=0.00, 0.05, 0.10) materials produced by SPS (high density) is shown in Figure 4(a). All three samples exhibited a transition from metallic to semiconductor behavior occurring at 400-420 K which may be associated with a spin state transition2 or a first order phase transition.16 The SPS samples doped with Ga (y=0.05 and y=0.10) exhibited a resistivity approximately 12% lower than that of the undoped sample over the range of temperatures measured. Figure 4(b) presents the results of Seebeck coefficient measurements in He on these three samples. They all exhibited p-type behavior and there was no significant difference in Seebeck coefficient between the undoped and the Ga-doped samples. These values of resistivity and Seebeck coefficient are quite consistent with measurements carried out on similarly doped, hot-pressed samples, with one difference; the undoped hot-pressed sample exhibited a lower Seebeck coefficient.5, 17 In that case, however, it was noted that the grain alignment was less pronounced in the undoped, hot-pressed sample, indicating that Ga doping may facilitate the anisotropic physical properties by modifying the layer structure. The decreased electrical resistivity in the Ga-doped sample (y = 0.05) may be associated with the change in charge carrier concentration, as the XANES result indicate an average Co oxidation state higher than 3+. Accordingly, doping Ga onto the Co site would be expected to introduce more hole carriers – which is consistent with Hall measurements.17 Note that the electrical resistivity did not show a further decrease with higher Ga concentration, which is in good agreement with our refinement result that the saturated Ga occupancy on Co site is ~2.2%.


Page 13 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Temperature dependence of (a) power factor PF and (b) thermal conductivity κ for polycrystalline Ca3Co4-yGayO9+δ (y=0, 0.05) produced by SPS and conventional sintering (CS) processing. In addition, previous studies18, 19 on Ga-doped sodium cobaltate (which possesses the same [CoO2] subsystem as Ca3Co4O9) found that as little as 3–4% Ga substitution for Co site in the [CoO2] layer strongly disturbs the conduction pathway and significantly increases the electrical resistivity (~30%). This further proves that Ga



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 23

preferentially replaced Co in the central RS layer rather than the hexagonal [CoO2] layer in this case. The temperature dependence of the power factor PF for high-density materials (produced by SPS) with the nominal compositions of Ca3Co4-yGayO9+δ (y=0, 0.05) is plotted in Figure 5(a). Due to the decrease of electrical resistivity ρ with little effect on Seebeck coefficient S, the power factor PF has been significantly improved through Ga doping.

Figure 6. Temperature dependence of electrical resistivity ρ and Seebeck coefficient S for Ca3Co3.95Ga.05O9+δ sample produced by conventional sintering processing. Figure 6 shows the electrical resistivity of a Ca3Co3.95Ga.05O9+δ sample produced by CS (~ 60% of the theoretical density) as a function of temperature as measured under He at low pressure. Up to 600 K, the resistivity exhibits a metal to semiconductor transition, similar to the electrical behavior of the dense samples produced by SPS. However the resistivity of the low-density sample is approximately three times that of its high-density counterpart, e.g., 31.5 and 10.5 mΩ cm for the corresponding 14 ACS Paragon Plus Environment

Page 15 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


materials at 300 K. Below 600 K, the large electrical resistivity of the low-density samples can be attributed to the presence of pores and to the random orientation of the platelets. Above 700 K the porous samples exhibit a positive temperature coefficient of resistivity, contrary to our findings for the samples produced by SPS. The S-T curve also exhibits a slight deviation upward in the CS samples (Figure 6). This behavior can be explained by a decrease in carrier concentration n that results in a simultaneous increase in resistivity and Seebeck coefficient20, which we believe is caused by a loss of lattice oxygen during measurement in He atmosphere. It has been reported that Ca3Co4O9+δ tends to lose oxygen ions around 720-730 K21 in air and 620-630 K22 under a pure nitrogen atmosphere. The removal of oxygen atoms from layered cobaltates is known to significantly reduce the concentration of p-type carriers (holes) by introducing more free electrons, which then leads to annihilation of the majority carriers7 and affects the valence state of Co.23 Our samples were measured in a lowpressure He atmosphere and the low-density sample had interconnected pores large enough to allow viscous flow, so both the kinetics and thermodynamics were favorable for the loss of oxygen above 600 K.

In the case of the high-density

samples, the porosity was low and the pores were not interconnected, resulting in less favorable oxygen out-diffusion kinetics. The porosity of the materials is one of the key features to be used for reducing the thermal conductivity, and enhancing the figure of merit.24 In general, we need to optimize the porosity to balance its negative effects on the electrical conductivity. However, the aforementioned experimental results demonstrate that porosity also influences the lattice oxygen stoichiometry and consequently affects the electrical transport properties. In order to confirm oxygen loss as the cause of the differences in



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 23

behavior between the SPS and CS samples, the electrical resistivity was measured in both air and He. In this work, a porous sample with the composition of Ca3Co3.95Ga.05O9+δ and a high-density sample with the composition of Ca3Co4O9+δ were re-oxidized at 1173 K in air and then the resistivity was measured as a function of temperature in a He atmosphere (Cycle 1), cooled to room temperature and then rerun in air (Cycle 2). The increase in resistivity ρ becomes noticeable above 600 K in the porous sample and is quite pronounced above 700 K [Figure 7 (a), Cycle 1]. The sample was then re-tested in air [Figure 7(a), Cycle 2]. Under these conditions, the effect appears to be reversible, with the sample returning to its original resistivity upon cooling. The hysteresis results in an increase in resistivity in the order of 10 mΩ.cm (a 42% increase) at around 1000 K and 50 mΩ.cm (166% increases) upon cooling to 300 K. A similar experiment on a high-density sample of Ca3Co4O9+δ resulted in an increase of 0.3 mΩ.cm (4%) at 1000 K and 1.1 mΩ.cm (10%) at 300 K [Figure 7(b)]. Thus, the differences do appear to be attributed to oxygen loss in the lattice. The thermal conductivity of all the samples decreases as temperature increases and was reduced in the samples containing Ga due to increased phonon scattering by point defects [Figure 5(b)]. The estimated phonon mean free path of this material is in close to one nanometer.25 This value is comparable to its lattice parameter, but is almost three orders smaller than that of grain size (which is a few microns as observed in Figure 3). It indicates that the influence of grain boundaries is quite negligible and the lattice thermal conductivity was most likely affected by the presence of point defects, such as doping-induced point defects. In addition, the high-density samples (SPS) show a larger thermal conductivity than the porous samples (CS), e.g., the non-doped sample with high density shows a κ of ~2.4 Wm-1K-1 at room temperature, while κ is


Page 17 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


~1.2Wm-1K-1 for the porous sample. This difference was ascribed primarily to the presence of porosity.

Figure 7. The effect of atmosphere on the electrical resistivity for (a) CS sample (with the composition of Ca3Co3.95Ga.05O9+δ ) and (b) SPS sample ( with the composition of Ca3Co4O9+δ ) during heating and cooling cycles. The samples were reoxidized at 1173 K and their resistivity measured from near room temperature to 1000 K, cooled down to room temperature under He (cycle 1) and then re-run in air (cycle 2). Both samples were heated at 5 K/min and cooled at 10 K/min.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 23

Figure 8. Temperature dependence of ZT for polycrystalline Ca3Co4-yGayO9+δ (y=0, 0.05) produced by SPS and conventional sintering (CS) processing.

The dimensionless figure of merit ZT of Ca3Co4-yGayO9+δ (y=0.00, 0.05) bulk materials produced by SPS and CS is shown as a function of temperature in Figure 8. It therefore appears that the positive effects of SPS processing on the electrical resistivity outweigh the negative effects on the thermal conductivity, leading to an improved thermoelectric figure of merit. The sample produced by SPS with a nominal composition of Ca3Co3.95Ga.05O9+δ achieved a highest figure of merit ~0.26 at 973K, triple that of its undoped counterpart with ~40% porosity.

CONCLUSIONS The influence of Ga substitution on the crystal structure and thermoelectric properties of Ca3Co4O9+δ synthesized by different processing methods have been


Page 19 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


studied in detail. The experimental results suggest that doped Ga atoms are preferentially located on the Co sites of the [Ca2CoO3] subsystem rather than the [CoO2] subsystem, up to a maximum occupancy of ~2.2%. Doping-induced phonon scattering and modification of the charge carrier concentration result in lower thermal conductivity and lower electrical resistivity, both of which contribute to an improved thermoelectric figure-of-merit for Ga-doped Ca3Co4O9+δ. Samples produced by SPS processing exhibited markedly decreased electrical resistivity, due to better grain contact as well as improved grain alignment along the favorable electrical transport direction. More importantly, the interconnected pore structure in the low-density samples produced by conventional sintering was found to facilitate oxygen outdiffusion from the lattice, which accounts for the anomalous electrical resistivity at elevated temperatures. Our results show that this can be effectively suppressed by increasing the sample density.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

ACKNOWLEDGMENT The author would like to acknowledge the analytical center at University of New South Wales and financial supports by Australian Research Council Projects of DP0988687, DP110102662 and FT100100956. The Australian Synchrotron provided travel support for research undertaken on the PD beamline at the Australian Synchrotron and on beamline 20B at the Photon Factory in Japan.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 23

REFERENCES (1) Shikano, M.; Funahashi, R. Electrical and thermal properties of single-crystalline [Ca2CoO3]0.7[CoO2] with a Ca3Co4O9 structure. Appl. Phys. Lett. 2003, 82, 18511853. (2) Masset, A. C.; Michel, C.; Maignan, A.; Hervieu, M.; Toulemonde, O.; Studer, F.; Raveau, B.; Hejtmanek, J. Misfit-layered cobaltite with an anisotropic giant magnetoresistance: Ca3Co4O9. Phys. Rev. B 2000, 62, 166. (3) Li, S.; Funahashi, R.; Matsubara, I.; Ueno, K.; Sodeoka, S.; Yamada, H. Synthesis and Thermoelectric Properties of the New Oxide Materials Ca3-xBixCo4O9+δ (0.0 < x < 0.75). Chem.Mater. 2000, 12, 2424-2427. (4) Fergus, J. W. Oxide materials for high temperature thermoelectric energy conversion. J.Euro.Ceram.Soc. 2012, 32, 525-540. (5) Nong, N. V.; Yanagiya, S.; Monica, S.; Pryds, N.; Ohtaki, M. High-Temperature Thermoelectric and Microstructural Characteristics of Cobalt-Based Oxides with Ga Substituted on the Co-Site. J. Electron. Mater. 2011, 40, 716-722. (6) Orikasa, Y.; Ina, T.; Nakao, T.; Mineshige, A.; Amezawa, K.; Oishi, M.; Arai, H.; Ogumi, Z.; Uchimoto, Y. X-ray Absorption Spectroscopic Study on La0.6Sr0.4CoO3−δ Cathode Materials Related with Oxygen Vacancy Formation. J.Phys.Chem.C 2011, 115, 16433-16438. (7) Tsai, P. H.; Norby, T.; Tan, T. T.; Donelson, R.; Chen, Z. D.; Li, S. Correlation of oxygen vacancy concentration and thermoelectric properties in Na0.73CoO2-delta. Appl. Phys. Lett. 2010, 96. (8) Tsai, P. H.; Donelson, R.; Tan, T. T.; Avdeev, M.; Yu, D. H.; Straessle, T.; Li, S. Oxygen Level Dependent Lattice Dynamics of Na0.73CoO2-delta. J. Phys. Chem. C

2010, 114, 21848-21850. (9) Petricek, V.; Dusek, M.; Palatinus, L., Institute of Physics, Praha, Czech Republic

2004. (10) Ling, C. D.; Aivazian, K.; Schmid, S.; Jensen, P. Structural investigation of oxygen non-stoichiometry and cation doping in misfit-layered thermoelectric [Ca2CoO3-x][CoO2]δ, δ~1.61. J. Solid State Chem. 2007, 180, 1446-1455. (11) Grebille, D.; Lambert, S.; Bouree, F.; Petricek, V. Contribution of powder diffraction for structure refinements of aperiodic misfit cobalt oxides. J. Appl. Crystallogr. 2004, 37, 823-831. 20 ACS Paragon Plus Environment

Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(12) Yamamoto, T. Assignment of pre-edge peaks in K-edge x-ray absorption spectra of 3d transition metal compounds: electric dipole or quadrupole? X-Ray Spectrometry

2008, 37, 572-584. (13) Kim, M. G.; Im, Y. S.; Oh, E. J.; Kim, K. H.; Yo, C. H. The substitution effect of Ca2+ ion on the physical properties in nonstoichiometric Dy1−xCaxCoO3−y system. Phys. B: Conden. Matt. 1997, 229, 338-346. (14) Yamamoto, T.; Uchinokura, K.; Tsukada, I. Physical properties of the misfitlayered (Bi,Pb)-Sr-Co-O system: Effect of hole doping into a triangular lattice formed by low-spin Co ions. Phys. Rev. B 2002, 65, 184434. (15) Chen, J. L.; Liu, Y. S.; Liu, C. J.; Huang, L. C.; Dong, C. L.; Chen, S. S.; Chang, C. L. Effect of Mn doping on the physical properties of misfit-layered Ca3Co4O9+delta. J. Phys. D Appl. Phys. 2009, 42. (16) Cheng, J. G.; Sui, Y.; Wang, Y.; Wang, X. J.; Su, W. H. First-order phase transition characteristic of the high temperature metal-semiconductor transition in [Ca2CoO3] 0.62 [CoO2]. Appl. Phys. A Mater. Sci. Process. 2009, 94, 911-916. (17) Nong, N. V.; Liu, C. J.; Ohtaki, M. Improvement on the high temperature thermoelectric performance of Ga-doped misfit-layered Ca3Co4-xGaxO9+[delta] (x = 0, 0.05, 0.1, and 0.2). J. Alloys Compd. 2010, 491, 53-56. (18) Guo, Z. P.; Zhao, Y. G.; Zhang, W. Y.; Cui, L.; Guo, S. M.; Luo, L. B. Effect of Ga and Mn doping on structural, electrical transport and magnetic properties of Na0.75CoO2. J. Phys. Condes. Matter 2006, 18, 4381-4388. (19) Mandal, P. Anomalous transport properties of Co-site impurity doped NaxCoO2. J. Appl. Phys. 2008, 104. (20) Wang, Y.; Sui, Y.; Wang, X.; Su, W.; Liu, X. Enhanced high temperature thermoelectric characteristics of transition metals doped Ca3Co4O9+delta by cold highpressure fabrication. J. Appl. Phys. 2010, 107. (21) Zhou, X.D.; Pederson, L. R.; Thomsen, E.; Nie, Z.; Coffey, G. Nonstoichiometry and Transport Properties of Ca3Co4+/-xO9+delta (x = 0-0.4). Electrochem. Solid State Lett. 2009, 12, F1-F3. (22) Morita, Y.; Poulsen, J.; Sakai, K.; Motohashi, T.; Fujii, T.; Terasaki, I.; Yamauchi, H.; Karppinen, M. Oxygen nonstoichiometry and cobalt valence in misfitlayered cobalt oxides. J. Solid State Chem. 2004, 177, 3149-3155.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 23

(23) Shimoyama, J.; Horii, S.; Otzschi, K.; Sano, M.; Kishio, K. Oxygen Nonstoichiometry in Layered Cobaltite Ca3Co4Oy. Jpn. J. Appl. Phys 2003, 42, L194L197. (24) Yang, C. C.; Li, S. Basic Principles for Rational Design of High-Performance Nanostructured Silicon-Based Thermoelectric Materials. ChemPhysChem 2011, 12, 3614-3618. (25) Wang, Y.; Sui, Y.; Cheng, J. G.; Wang, X. J.; Su, W. H.; Liu, X. Y.; Fan, H. J. Doping-Induced Metal-Insulator Transition and the Thermal Transport Properties in Ca3-xYxCo4O9. J. Phys. Chem. C 2010, 114, 5174-5181.


Page 23 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For TOC only


Ga Substitution and Oxygen Diffusion Kinetics in Ca3Co4O9+Î´-Based

Recommend Documents