Synthesis and Thermoelectric Properties
of Single-Crystal Cobalt-Based Layerd Oxides

Introduction
    At AIST Kansai, our group is active in R&D of high-performance thermoelectric generators (devices for conversion of heat energy into electrical energy), which utilize oxide materials to provide stable functioning for long periods of time at high temperatures in air. In exploring novel thermoelectric materials, our focus on layered materials composed of alternately stacked conducting and insulating layers (Figure 1.) led to a “fortuitous” success with the synthesis of a single-crystal oxide displaying extremely high p-type (positive hole conduction) thermoelectric properties, even at high temperatures in air. The single crystal discovered is a layered oxide with the composition (Ca₂CoO₃)_xCoO₂ (Co-349). Figure 2 provides a depiction of the Co-349 crystal structure. The layered structure possessed by this oxide consists of two units alternately piled one on top of the other. One of these units is a CoO₂layer with single Co atoms surrounded by six O atoms oriented as edge-sharing octahedra. The other is a Ca₂CoO₃ layer possessing a rock-salt structure. What follows is a report on the method of synthesis and thermoelectric properties of Co-349 single crystals.

    
             Fig.1                                                                               Fig.2

Preparation
    Co-349 single crystals were grown using a glass annealing method. (See Figure 3) Bi₂O₃, CaCO₃, SrCO₃ and Co₃O₄ powders used as starting materials were mixed to achieve an element ratio of Bi:Ca:Sr:Co = 1:1:1:2, then melted at 1300゜C for 30 minutes in air. The melted raw materials were poured and quenched by pressing between two copper plates at room temperature to achieve glass precursors for whisker growth. Annealing of glass precursors at 920-930゜C for 60-1000 hours under an oxygen stream yielded single-crystal “wiskers.” Figure 4 presents photographs of Co-349 whiskers taken using a scanning electron microscope (SEM). The ribbon-like Co-349 whiskers were typically 50-200 μm in width, 1.0-5.0 μm in thickness and 0.1-1.2 mm in length. The broad, well-grown plane of the whiskers corresponded to the ab-plane and the thickness direction was parallel to the c-axis. Typical whisker composition was Ca_1.3Sr_0.3Bi_0.3Co₂O_x with Sr and Bi substituting for the Ca component. The c-axis length calculated from x-ray diffraction patterns was 10.78Å.

              
Fig.3                                                                    Fig.4

Results
     Figure 5 presents a photograph of a cross-section of a Co-349 whisker taken with a tunneling electron microscope (TEM). From this image, it became clear that the Co-349 phase possessed a layered structure stacked along the c-axis.
     Figure 6 shows the temperature dependency of the Co-349 whisker’s Seebeck coefficient (S) and electrical resistivity (ρ).  S is seen to increase and ρ to decrease with rising temperature. At temperatures of 873 K and over, S was 200 μV/K or greater. Conversely, low ρ values of 1.4-1.5 mΩｃm were also recorded.

           
Fig.5                                                                               Fig.6

     While the thermal conductivity(κ) of the Co-349 whiskers could not be measured directly due to the small size of the whiskers, κ of sintered polycrystalline material with the same composition was found to be at low levels of around 1.0 Wm^-1K^-1 at 473-973 K. (See Figure 7.) This is thought to stem from Co-349’s layered morphology. A misfit, at the interface between the CoO₂ and rock-salt layers of the Co-349 lattice exists, which causes phonon scattering thus impeding lattice thermal conductivity. The figure of merit (ZT) of the Co-349 whisker was calculated to be 2.7 at 973 K using the κ obtained from the polycrystalline sample. (See● in Figure 8.) Generally speaking, however, the κ of a single-crystal material should be larger than that of a sintered polycrystalline one. At this juncture, we derived ZT of Co-349 whiskers by calculation of κ (= κ_elec+ κ_latt) using the thermal conductivity contributed by the lattice (κ_latt), which was estimated from κ of the polycrystalline material, and the thermal conductivity contributed by the charge carriers (κ_elec), which was calculated from the Wiedemann-Franz’s low. (See■ in Figure 8.) Actual whisker ZT values are indicated in the figure by oblique lines producing a shaded area between these two parameters. As a result, it became clear that this oxide possesses highly outstanding thermoelectric properties at high temperatures in air, as seen by Co-349 whisker ZT of 1.2 and over at temperatures of 873 K and above.
     But what is it that gives the Co-349 phase its exemplary thermoelectric properties? While the answer still not entirely clear, comparison with Na₂CoO₄, which possesses a structure similar to Co-349, leads us to believe that something in the rock-salt layer is amplifying the Seebeck coefficient. Furthermore, thermal conductivity is reduced at layer interfaces. So what we do know is that different parts (lattice and layer interfaces) within the Co-349 crystal possess different functions (reduction of thermal conductivity and amplification of Seebeck coefficient), which complement each other superbly, thus realizing exceptional thermoelectric properties. In this sense, Co-349 is the very model of AIST Kansai’s “lattice composite” approach to materials design.

       
           Fig.7                                                                              Fig.8

     The Co-349 material developed thus far through our research contains no poisonous or uncommon elements, and is chemically stable even at high temperatures in air. This means that it is highly suited to thermoelectric power conversion using high-temperature waste heat as an energy source. In order to fabricate thermoelectric generators using Co-349, from hereon it will be necessary to develop large single crystals or polycrystalline materials possessing the same thermoelectric properties as seen in the single-crystal samples. The ZT of Co-349 polycrystalline samples prepared by solid-state sintering was shown to be one order of magnitude lower than that of the whiskers. The origin of this is the two-dimensionality of ρ. (See Figure 9.) While ρ along the ab-plane (ρ_ab) is low, as shown in Figure 6, ρ along the c-axis (ρ_c) is 500-1000 times higher than that of ρ_ab. As a result of this two-dimensionality, in polycrystalline samples ρ is approximately 10 times higher than ρ_ab in the whiskers, and ZT is thus one order of magnitude lower. In order to obtain polycrystalline materials with ρ on the order of that possessed by the whiskers, crystal grain orientation will need to be aligned along the ab-plane. Melt-growth methods or sintering methods under uni-axial pressure, such as hot pressing, are thought to be an effective as means of production for this purpose.

Fig.9

     With the discovery of Co-349, practical application of thermoelectric power conversion utilizing high-temperature waste heat, thus far presumed to be an energy source confined to the realm of dreams, has now become a reality. The objective from hereon will be to develop n-type thermoelectric oxide materials, and solve problems related to technical factors, such as electrodes, in fabricating generators for the purpose of realizing thermoelectric power generation from high-temperature waste heat.

References
R. Funahashi et al, Jpn. J. Appl. Phys. 39 (2000) L1127.