Electronic structure and thermoelectric properties of nanostructured EuTi1−xNbxO3−δ (x = 0.00; 0.02)

Abstract

The thermoelectric properties of polycrystalline nanostructured EuTiO 3Àd samples are improved by a substitution of 2% Nb for Ti. The figure of merit (ZT) was measured to reach ZT(EuTi 0.98 Nb 0.02 O 3Àd ) % 0.4 at T ¼ 1040 K while ZT(EuTiO 3Àd ) % 0.3 at the same temperature. X-ray photoelectron spectra reveal that the in-gap states at the Fermi level are more pronounced for the Nb-substituted samples, resulting in an improved power factor. The valence band peak below the Fermi level is sharply shaped, therefore fulfilling the condition for a large Seebeck coefficient. The specific porosity of the samples reduces the lattice thermal conductivity with a minor effect on the electron transport. V C 2012 American Institute of Physics.[http://dx.It is known from theoretical studies that correlation effects in oxides can enhance thermoelectric properties. 1,2 Predictions about the thermoelectric performance and the Seebeck coefficient of strongly correlated electron systems were presented by Rüegg et al. 3 Among the perovskite-type oxides, SrTiO 3 has shown a particularly high thermoelectric figure of merit ((ZT ¼ (S 2 T)/(jq), where S ¼ Seebeck coefficient, q ¼ electrical resistivity, j ¼ thermal conductivity, and T ¼ absolute temperature) in the high-temperature region. 4,5 In order to enhance ZT, q and j have to be minimized and S has to be maximized. The lattice thermal conductivity is often decreased by substituting heavy atoms into the crystal structure, which enhance the phonon scattering. 6 A recent approach to decrease the j of Ti containing perovskite systems (ATiO 3 ; where A ¼ rare earth or alkaline earth metal) have focused on substitutions of the A-site cation with heavier elements. A 20% substitution of Sr 2þ by Eu 2þ in SrTiO 3 was found to effectively reduce the mean free phonon path by 12% at room temperature. 7 Based on this observation and taking into account the similarities between SrTiO 3 and EuTiO 3 , 8,9 a l o w e rp h o n o nm e a nf r e ep a t hi se x p e c t e di nE u T i O 3 due to thehigheratomicmassofEuascomparedwithSr.EuTiO 3 has been widely studied because of its interesting magnetic properties: it is one of the few antiferromagnetic materials with a positive Curie-Weiss constant (h) 10 and reveals a strong coupling between magnetic ordering and soft phonon modes. 11 Electronic band structure calculations of EuTiO 3 by Ranjan et al. 12 have predicted a sharp peak in the density of states (DOS) below the Fermi level (E f ), which is beneficial to increase the S (/(@ ln f(E)/ @E) E¼Ef ). A broad band dispersion near E f would further enhance the electronic conductivity. 13 The electrical conductivity of EuTiO 3 has been studied at low temperatures, 14 and different methods have been applied for its synthesis. 15,16 Another approach to reduce the lattice thermal conductivity is based on nanostructuring. 17 When the grain size decreases and approaches a scale comparable to the mean free path, the phonons are scattered by the grain boundaries. Moreover, nanostructuring can enhance the Seebe...

Structure and thermoelectric properties of EuTi(O,N)3 ± δ

“…The thermoelectric properties of EuTi(O,N) 36d were compared to the results of EuTiO 3 from previous studies. 12 The morphology of the pressed samples was studied by scanning electron microscopy (SEM) with a XL30 ESEM (FEI) operating at 10 kV with a secondary electron (SE) detector. Each sample was placed on an aluminum SEM holder with a carbon double side adhesive disk, and coated with carbon in order to ensure conductivity of the electrons coming from the beam.…”

“…10,11 The thermoelectric properties of EuTiO 3 were studied and a high Seebeck coefficient (S) was found (À1053 lV/K at 300 K). The electrical conductivity (r) was enhanced by partial Nb substitution at the Ti site, leading to a thermoelectric figure of merit (ZT) of EuTi 0.98 Nb 0.02 O 3 6 d of ZT ¼ 0.4 at T ¼ 1040 K. 12 Anion (i.e., oxygen) substitutions are less commonly used than cation substitutions due to their lower stability 13 and more complex synthesis methods. 14 However, the partial replacement of oxygen by nitrogen forming oxynitrides can lead to very interesting electronic and optical properties.…”

Ceramics for Electronics and Energy: Issues and Opportunities

“…Many reviews are available on this topic. Recently, new materials have been presented, such as (Sr, Eu) (Ti, Nb)O 3 …”

“…A comprehensive talk was presented by Weidenkaff. 23 24 From the scientific point of view, material requirements are quite clear: A good TE material should have a high Seebeck coefficient in the working temperature range and high electrical and low thermal conductivities, that is, with high phonon scattering efficiency. Nevertheless, the appropriate materials are just one piece of the puzzle.…”

Thermoelectric effect of correlated metals: Band-structure effects and the breakdown of Mott's formula

“…This choice is motivated in parts by the wide spread studies of layered transition-metal-oxide systems. [16][17][18][19][20][21] Moreover, it was shown by Hicks and Dresselhaus that for metallic systems low-dimensionality can have a beneficial impact on thermopower. 22 We discuss the influence of momentum space constraints for twoparticle scattering on the charge transport in general.…”