Ni-doped cobaltates as potential materials for high temperature solar thermoelectric converters

Thermoelectric properties of Ni substituted (La) cobaltate compounds with perovskite structure have been investigated. For all the studied compounds the Seebeck coefficient is positive indicating predominant positive charge carriers. The electrical resistivity decreases considerably with increasing Ni content and with increasing temperature. The thermoelectric power factor was enhanced to 2.7 x 10(-4) W/K-2 m for samples with 10% Ni content at room temperature. The submicrograin morphology of the powders leads to a reduction of the thermal conductivity.

Pressure-tuned Transport Properties of Novel Iron-based Superconductors and Topological Insulators

Andrea Pisoni

The majority of interactions in solids strongly depend on the interatomic distances. The application of pressure changes the lattice parameters and modifies the electronic and the phononic energy spectra of a material avoiding some of the undesirable effects induced by chemical doping, like lattice disorder, impurity phases and phase separations. Layered materials are particularly affected by the application of pressure because their anisotropic structure eases the contraction of the lattice along specific crystallographic directions. In this thesis I study the effect of pressure on the transport properties of two new layered materials: L4Fe2As2Te(1-x)O4 (L = Pr, Sm, Gd) superconductors and beta-Bi4I4 topological insulator. The discovery of Fe-based superconductors (Fe-SCs) provided a new material base for studying the mechanism of high temperature superconductivity. These materials present unexpectedly high critical temperatures (Tc ) despite the presence of magnetic atoms. The parent compounds of Fe-SCs are semimetals with an antiferromagnetic ground state in which the itinerant electrons form a periodic modulation of spin density called spin density wave (SDW). By changing the structural properties or by adding/removing carriers the SDW order can be suppressed and superconductivity emerges. Our group recently succeeded in synthesizing a new family of Fe-SCs that presents Tc up to 45 K upon optimum chemical doping and substitution. I performed a systematic study of the upper critical field and the pressure-dependent electrical resistivity of single crystals of Pr4Fe2As2Te(1-x)O4 and Sm4Fe2As2Te(1-x)O(4-y)Fy . Hall coefficient and magnetoresistance of Pr4Fe2As2Te(1-x)O4 revealed electrons as the dominant type of charge carriers. The results can be successfully fitted by a two-band model that allows the estimation of the temperature dependent electron and hole densities and mobilities. In view of the exceptionally high structural anisotropy of this new class of Fe-SCs I investigated the electronic anisotropy of Sm4Fe2As2Te(1-x)O(4-y)Fy by means of microfabrication techniques. The results show a ratio between the out-of-plane and in-plane electrical resistivities that is the highest reported so far in Fe-SCs. Topological insulators (TIs) form another class of materials that is currently attracting a lot of attention both for fundamental physics and potential applications. In TIs metallic surface states coexist with an insulating bulk. Most of the TIs known so far are either three-dimensional strongly bonded bulk materials or layered van der Waals crystals. Beta-Bi4I4 is a material composed of quasi-one-dimensional molecular chains that was theoretically predicted, and only recently experimentally verified, to be a novel strong topological insulator. Due to its particular structure, beta-Bi4I4 offers the possibility to study the interplay between different ground states like topological insulating phase, charge-density-wave and superconductivity. Here I present, for the first time, a study of the thermoelectric properties of beta-Bi4I4 single crystals. Electrical resistivity, Seebeck coefficient, thermal conductivity and Hall coefficient measurements reveal a possible charge density-wave-order (CDW) at low temperature. According to resistivity and thermoelectric power measurements, pressure suppresses the CDW order. Resistivity measurements performed in a diamond anvil cell up to 20 GPa demonstrate that superconductivity is induced in beta-Bi4I4 for pressure above 10 GPa. Preliminary X-ray diffraction measurements by synchrotron radiation, performed on the sample recovered at ambient pressure after the experiment, reveal that a new Bi-I phase is formed at high pressure. The possibility that the topological insulator state could coexist with superconductivity in beta-Bi4I4 is certainly a fascinating option but it remains for now an open question.

EPFL2016

Tailoring thermal properties of functional materials

Xavier François Mettan

Thermal conductivity (k) plays an essential role in functional devices. In some cases, high value is required to transfer heat efficiently. In others, low k assists in maintaining a temperature gradient necessary for device operation. It is profitable to design materials where one can tune k in a broad range according to its function. Beyond functionality, k is a fundamental property of matter, and a fantastic probe to assess vibrational dynamics and electronic structure in condensed matter. In conjunction with other transport properties such as electrical resistivity and thermoelectric power, k promotes a deeper understanding of the electronic and vibrational structure of materials. In the present Thesis, careful experimental determination of the transport coefficients of a variety of systems over a broad temperature range (4-300 K) let me unveil subtle interactions between phonons and/or electrons. The insights gained from an accurate interpretation of these properties empowers us to engineer and fine-tune the functionality of materials for applications. I first investigate the thermoelectric properties of nickel-based high-entropy alloys (HEAs), believed to host unconventional effects arising from the high level of site-disorder in their crystalline structure. Anomalous resistivity with a low temperature coefficient validates the disordered nature of HEAs. The Seebeck coefficient, reported for the first time for HEAs, is surprisingly low. In the quest to understand transport in HEA, I have discovered that iron-nickel alloys exhibit a promising thermoelectric figure of merit (ZT) of 0.1 at room temperature. One way to further increase ZT is to decrease k (in FeNi alloys or other materials) by texturing the material. Aluminium foams are systems of choice to observe the effect of a modified structure on the thermoelectric properties. Tailoring of the bubbles' size is demonstrated by applying an external acoustic force in the foaming process. Moreover, the influence of the macro- and micro- structure of the foams on thermoelectric properties is discussed. The best example for tuning k by texturing it is anatase titanium dioxide (TiO2). k was varied over three orders of magnitude by adjusting its oxygen contents and by texturing it into a foam, aerogel-like structure. We identified a new, strong diffusion mechanism of heat by polarons, created as a consequence of oxygen vacancies. Furthermore, anatase nanowires organized into foams result in an unprecedented low k = 0.014 W/Km at room-temperature. Doping this anatase foam heralds promising applications, in particular in thermoelectricity. An oxide which has an intrinsically nanoporous structure reminding that of a foam, is Mayenite (Ca12Al14O33). The thermal conductivity of such system is verified to be intrinsically low. The peculiar cage structure could host atomic of molecular species to tune its electrical conductivity, with bright prospects for thermoelectric applications. Finally, the thermoelectric properties of hybrid halide perovskites, cheap and easy-to-synthesise compounds, were studied. Three materials of this family have ultra-low k, due to the vibrational degrees of freedom of their organic cations. Notably, a tin-based organic-inorganic compound delivers ZT = 0.13 at room temperature. The thermoelectric conversion efficiency of this material was measured directly and is believed to yield promising thermoelectric applications.

EPFL2019

Conductivity, dielectric and piezoelectric properties of SrBi4Ti4O15

Strontium Bismuth Titanate is a very promising material for high temperature piezoelectric applications, its elevated ferroelectric phase transition (530°C), linear piezoelectric properties under low field and relatively low room temperature conductivity (compared to others Bismuth Titanates) make it very attractive for precision sensors. However, under severe conditions (low frequency, high field, high temperature or low oxygen partial pressure) some of those advantages disappear. Piezoelectric response is dominated by charge drift in general becomes unstable. Above all, at high temperature and low oxygen partial pressure, a large conductivity increase reduces the piezoelectric efficiency of the material, in this work, electrical conductivity, piezoelectric properties and dielectric permittivity of SrBIT ceramic have been investigated in conditions of high temperature, low oxygen partial pressure, low to moderate driving field and frequency. As a result of this research, a better understanding of SrBIT properties was achieved. Thanks to a careful study of SrBIT processing as a bulk ceramic, a reproducible route was established. Many basic mechanisms leading to both SrBIT crystallization and sintering have been identified. It was demonstrated that a detailed knowledge of the exact processing conditions is required in order to achieve high quality material. DC conductivity measurements were carried out as a function of temperature, oxygen partial pressure and dopant concentration. It was found that the apparent activation energy for conduction for undoped SrBIT was 1 eV between 140 and 220°C and 1.5 eV between 450 and 700°C. Decrease of the activation energy in the lower temperature range has been discussed considering grain boundary conductivity, as a transition from electronic to ionic conduction or as consequence of small polarons conduction. It was shown that lower activation energy resulted from Manganese doping (0.5 eV), this was interpreted as either growing influence of grain boundary conductivity as dopant concentration increases or as shallow hole trap formation. DC conductivity measurements and acceptor/donor doping experiments demonstrated p-type conductivity in the low temperature range (up to 220°C) as donor doping decreases conductivity, while oxygen partial pressure controlled measurements indicated n-type conductivity at higher temperature (above 700°C). An electrical impedance analysis was performed with several equivalent circuits. The aim of these models was to simulate the impedance of SrBIT. The best approximation was found with a distributed element of Havriliak-Negami type. However, as the physical justification for this circuit was not clear, the investigation of the grain, grain boundary and electrode impedances was performed with multiple discrete parallel RC elements. With temperature, grain size and oxygen activity variations, the identification of three separate contributions as grain, grain boundary and electrode was realized. The anisotropy of conductivity and permittivity was demonstrated with textured material and both DC and AC analysis. With the master curves built for the electrical modulus, it was found that the impedance probably related to Bismuth oxide layers produces an additional high frequency are. From this finding and by comparing characteristic relaxation frequencies of undoped, 2 mol.% Mn and 4 mol.% Nb SrBIT, it was determined that conductivity is higher in the ab plane direction than in the c direction within both perovskite units and Bismuth oxide layers. With conductivity measurements under controlled oxygen partial pressure, it was found that an acceptor-based (intrinsic or extrinsic) model could be used to describe the electrical conductivity under controlled oxygen partial pressure of both undoped and 2 mol.% Mn doped SrBIT. However, as neither a pO2-1/6 region (intrinsic oxygen vacancies compensated by electrons) for undoped SrBIT nor a pO2-1/4 region (oxygen vacancies compensated by singly ionized acceptors) for Mn doped SrBIT were seen, it was concluded that the acceptorcontrolled model is not sufficient for a complete description of SrBIT. For this reason and in order to include the low oxygen partial pressure behavior of undoped SrBIT, a donor-based (intrinsic) model was also considered. The source of intrinsic donors would be in that case Bi3+ cations sitting on Sr2+ sites in the perovskite sublattice. Considering Bismuth vacancies as the negative compensating species, both pO2-1/4 and pO2-independent regions could be predicted with the model. However, even if the donor-controlled model seems to better match conductivity measurements in the full PO2) range, rejecting the acceptor-based model would be an error. It is actually not demonstrated that in SrBIT the concentration of exchanged Bismuth cations is always (all temperature, pressure) larger than the natural acceptor impurity concentration. It is very likely that the cation exchange is dependent of the oxygen partial pressure. It is also not proved as suggested in the literature that direct compensation between exchanged Strontium and Bismuth exists, reducing the net donor-excess. With the acceptor-controlled model, the mass-action constants for reduction and for ionization of intrinsic carriers across the band gap were determined. The band gap of SrBIT was estimated to be 3.5 eV. The ionic conductivity of SrBIT was determined at high temperature with measurements under controlled oxygen partial pressure. It was found that the electrical conductivity of SrBIT is probably mixed (electronic and ionic) as the estimation of the transference number provided quite large values (t=0.8 at 800°C). From electronic and ionic conductivity data, mixed conduction can actually be predicted in a large temperature range (above room temperature). The piezoelectric measurements using direct effect demonstrated that it is actually possible to unlock piezoelectrically active ferroelectric domain walls and create non-linear piezoelectric properties in undoped SrBIT. This occurs above a threshold elastic field, which is thermally activated. With a piezoelectric composite, it was demonstrated that the electromechanical coupling between two different phases creates a piezoelectric relaxation. This one could be positive or negative depending on the respective properties of each composite's component. It was shown experimentally that a small temperature change is sufficient to transform a positive relaxation into a negative one. While these experiments did not provide a detailed microstructural explanation for the piezoelectric relaxation observed in 2 mol.% Mn doped SrBIT, they gave a first insight into an original phenomenological approach. Microstructure and piezoelectric properties were related with the calculation of a piezoelectric relaxation composite made of two textured samples. This demonstrated that a piezoelectric relaxation may occur just because of anisotropy. Microstructural reproduction of this coupling is actually an important feature of Aurivillius phases.

EPFL2000