Energy harvesting

Thermoelectric oxides

Contact : fabien.giovannelli@univ-tours.fr
 
Thermoelectric materials are able to transform directly heat in electric energy or inversely electric energy in cold (Seebeck and Peltier effects) without any emissions (CO2, other gases, radiations, …), vibrations or moving parts. Oxides have attracted interest of the thermoelectric community, in 1997, when Terasaki et al. (Physical Review B 56 (1997) R12685) have reported a large Seebeck coefficient in the metallic oxide NaxCoO2. Moreover, in a recent paper based on abundance as well as economy Herfindahl−Hirschman indices for production and reserve, Gaultois et al. (Chem. Mater. 25 (2013) 2911) have concluded that only highly electrically conductive transition metal oxides, silicides with low bismuth content, and Half-Heuslers alloys combine good thermoelectric performance and availability.

The researches held in GREMAN Laboratory deal with all the aspects of thermoelectric oxides :

Improvement of thermoelectric properties of known oxides ceramics

The studies are focused on Ca3Co4O9 for p-type materials and ZnO and SrTiO3 for n-type materials.
For instance, the addition of Ga to ZnO leads to higher ZT values mainly due to the reduction of thermal conductivity.


Figure 1 : Power factor, thermal diffusivity and ZT value of Ga-doped ZnO ceramics.

Synthesis of new thermoelectric oxide

Many oxide systems and new concepts are explored. For instance, oxygen deficient (K0.5Na0.5)NbO3 perovskite dense ceramics obtained by SPS have shown promising thermoelectric properties, especially low thermal conductivity compared to conventional SrTiO3.

Figure 2 : Image, microstructure power factor and thermal conductivity of KNN ceramic.

Development and characterization of thermoelectric (micro)devices

Full oxide thermoelectric devices as well as microdevices are developed and characterized through French and international collaborations : CRISMAT laboratory (Caen), GREMI laboratory (Orléans) and AIST (Japan).

 

Figure 3 : Picture of a Ca3Co4O9/ZnO device developed during the ADEME-TOTAL project SONATE.

Piezoelectric oxides

Contact : isabelle.laffez@univ-tours.fr

Synthesis and Functional Properties of Lead Free ceramics and Crystals

Since the publication of the Reach and RoHS european directives deciding to limit the use of some dangerous substances that compose electric and electronic devices, many studies have started to look for new candidates to replace lead based PZT families largely used in a large variety of applications (medical imaging, non destructive control, actioning, sensors, transformers, or energy harvesting…).

However very few materials possess dielectric, piezoelectric, and mechanical characteristics equivalent to Lead based compositions. Among the possible families, Potassium, Sodium and Niobium compounds (abbreviated KNN) have been intensively studied in the past ten years.

The researches held in GREMAN Laboratory since 2012 in this field essentially deal with the elaboration and characterization of KNN based ceramics and single crystals with selected compositions.

Special attention is payed on the characteristics and different qualities of their crystallographic structure as well as their micro-structural architecture in order to link these parameters with the electro-active properties of the different material shaping and order. These researches must allow the identification and the selection of candidates with promising performances, available for potential applications. Necessarily, the existing characterization methods have to be continuously adapted to the constraints induced by the newly synthesised materials, in close collaboration with the other team of GREMAN.

 
Powder diffractogram pure KNN after synthesis of the powder from the precursors.

On the chemical, ceramics and crystal growth point of view, the GREMAN Laboratory possesses a long term expertise on synthesis processes of oxide powders by solid or sol-gel routes, classical sintering or Spark Plasma Sintering and crystal growth by floating zone method in an image furnace. With furnaces dedicated to specific thermal treatments (calcination, sintering, crystal growth, annealing) and devices allowing the shaping of the materials, (ball milling, attrition, wire saw, uniaxial and isostatic press …), the team is not only able to synthesise materials but also to perform their complete chemical characterization with high precision equipment such as X ray diffractometer, Raman spectrocopy, ATD-ATG and DSC thermal analyzers, optical, AFM-PFM, and scanning electron microscopes (FEG).


SEM microstructure of KNN ceramics and PFM microstructure

The expertise on functional characterizations of piezoelectric properties belongs also to the GREMAN team. The PhD students and post doctoral fellows benefit thus of the knowledge of the team to perform the necessary studies for the identification of the ceramics and crystal active properties and to achieve the understanding of the microscopic phenomena yielding the material electromechanic and piezoelectric properties. To do so, several ultrasonic or electrical instrumentation benches are available to perform linear and non linear characterization of active materials.


Transducers including different KNN ceramics and their measured echos in water

Lead free piezoelectric thin films

Contact : wolfman@univ-tours.fr / beatrice.negulescu@univ-tours.fr / jo.sakai@univ-tours.fr

Enhanced piezoelectric properties are generally observed in the vicinity of composition induced phase transitions, the morphotropic phase boundaries (MPB). We looked for MPB in Ga doped BiFeO3 [(1-x)BiFeO3-xGaFeO3] epitaxial grown by combinatorial PLD, with x varying from 0.01 to 0.14. As GaFeO3 is an orthoferrite and not a perovskite like BiFeO3, a solid solution is not expected on the whole x range.

Reciprocal space maps around (103) reflection exhibit a splitting of the BGFO spot up to 5% Ga doping that is compatible with the BFO monoclinic distortion. This splitting vanishes between 5% and 7% Ga doping, indicating a possible symmetry change.  

 
Fig. 4: Reciprocal space mapping around the (103) reflection for several compositions in Ga doped BiFeO3 film grown on (001) monocrystalline STO substrate with an LSMO bottom electrode

The piezoelectric measurements show a sharp enhancement of the d33 coefficient for 6.5% Ga doping, indicating a possible MPB around this composition.

 
Fig. 5: d33 piezoelectric coefficient variation as a function of the Ga content. The insert shows the mapping of d33 on a 40*40 mm2 capacitor


For more information see our publication: Journal of Applied Physics 117, 244107 (2015); doi: 10.1063/1.4923217

"High-K" materials

Contact : cecile.autret@univ-tours.fr

The activity of research for our group concerns on one hand the elaboration and the optimization of the oxide materials and on the other hand the establishment of phase diagram: structure-properties according to the composition. The understanding and optimization of magnetic and/or electrical properties ask for the determination of relevant information on the structure and microstructure for these oxides materials.

For several years, the highlight of our research’s activity concerns the materials with high dielectric constants, stable over a wide temperature range and frequency: “High-k” materials. Under the form of dense ceramic, materials can be developed in the typical ceramic capacitors MLCC, but also the filters …
ceramic-capacitors      ceramic-capacitors
Ceramic capacitors (MLCC)
 
The laboratory developed un-doped and doped CaCu3Ti4O12 (CCTO) materials for multilayer ceramic capacitor through the ConCerTO (FEDER/Région 2012-2014) project in collaboration with an industrial (SRT-Microcéramique based in Region Centre). This project deals with the development of CCTO material un-doped and doped by cationic substitution. During this project, the dielectric performances of this material were improved : (i) high dielectric constant (>104), (ii) low loss tangent , (iii) while keeping an industrial mode of production and iv) environment-friendly. It was proposed in agreement with the literature, that the colossal permittivity could find its origin from extrinsic properties of the material. Indeed, by association of semiconducting grains and insulating grain-boundaries, there would be a phenomenon of internal barriers they named “Internal Barrier Layer Capacitance” effect (IBLC). The obtained results gave rise some publications and one patent.
Cubic structure of CaCu3Ti4O12    . dielectric properties samples un-doped and doped CCTO
Cubic structure (Pm-3m) of CaCu3Ti4O12 - dielectric properties for three samples un-doped and doped CCTO
 
These results are very promising, nevertheless, some technological bottle-necks remain. In particular, the obtained capacitors possess prejudicial and insufficient breakdown voltages. For industrialization, these specifications (breakdown voltage, isolation resistance) are difficult to achieve, which has brought us to review our strategy. To answer this problem, a new project with SRT-Microcéramique was proposed COCONUT 2014-2016. To increase the resistance, we propose to elaborate materials with a new "Core-Shell” microstructure. Such a structure consists of a core structure made of a dielectric material and covered entirely with a shell structure made of another insulator oxide (SiO2, TiO2 ...). This system of “semiconducting” grains isolated from each other by an insulating shell is expected to decrease the loss tangent and to increase the insulation resistance.
Transmission and Scanning Electron Microscopy Images of CaCu3Ti4O12 . Transmission and Scanning Electron Microscopy Images of CaCu3Ti4O12
Transmission and Scanning Electron Microscopy Images of CaCu3Ti4O12 covered with 10 nm of SiO2

Moreover, SCM and KPM (Scanning Capacitance and Kelvin Probe) measurements were carried out at room temperature in air using a Solver microscope (NT-MDT) to establish a link between the composition, the morphology of grains and grain boundaries and the various electrical properties of surface.
Topography (a), Phase (b), SCM (c), and KPM (d) images for MCCTO15-70-6h
Topography (a), Phase (b), SCM (c), and KPM (d) images for MCCTO15-70-6h (x = 0.15 at 1070°C during 6 hours; surface area = 12 µm x 12 µm).
 
In order to clarify the role of both grains and grain boundaries in these materials, we made both intra-grain and inter-grains electrical transport measurements after sputtering aluminium contact micro-pads. Both intra-grain and inter-grain exhibit different type of electrical properties (metal-like and semiconductor-like, respectively) and conductivity always varies according to the doping (MgO weight % inside CCTO).
 
micropads-on-sample-surface I-V characteristics intra-grain and inter-grains measurements
Micro-pads on the sample’s surface. I-V characteristics for both intra-grain and inter-grains measurements
 
Developing high dielectric materials, other investigations have revealed that materials based on hexagonal perovskites exhibit interesting dielectric properties with high dielectric constant. Then our work was dedicated to the elaboration of these new compounds: Ba4YMn3O11,5±d. An accurate structural study was done by X-ray and neutron diffraction coupled with transmission electron microscopy. This study exhibits some stacking faults, which are similar to a twinning plane resulting in mistakes of sequence of close-packed. The sequence is characterized by the succession of three layers with a distance close to 7.2 Å along the c axis corresponding to a sequence of ...hhcchh… This periodicity is broken by the insertion of a layer inside the perfect stacking sequence of 12R polytype. This fault takes place in the hexagonal close-packed layers. Thus extra h layer is inserted resulting of a sequence which can be described by ...cchhhcc…This stacking fault results of four face-sharing octahedra instead three face-sharing octahedra in the perfect structure. This faults could contribute to the good dielectric properties of these materials via the DBLC model (Double layer capacitance).
Neutron diffraction pattern structural model of Ba4YMn3O11,5±   Neutron diffraction pattern structural model of Ba4YMn3O11,5±
Neutron diffraction pattern refined at room temperature with the structural model of Ba4YMn3O11,5±d
 
electron-diffraction-pattern-Ba4YMn3O11,5±   HRTEM image Ba4YMn3O11,5±  crystal fragment
Electron Diffraction pattern indexed on the [310] and HRTEM image corresponding of a Ba4YMn3O11,5±d crystal fragment, revealing a twined structure in the crystal. A simulated image (thickness : 12.26 nm, defocus value : 36 nm) is embedded on the experimental image to produce the good agreement.
 
Among the used tools of characterization, we can quote :
X-ray diffraction equipped with an Anton-Paar HTK1200 oven furnace (max temp. 1200oC) and a helium cryostat from 12 K to 300 K and an unit which measures in-situ materials under current and temperature. One diffractometer is equipped with a monochromatic option in transmission mode. Transmission Electron Microscopy (JEOL 2100 – 200kV), Electron paramagnetic resonance Bruker EMX6/1, Impedance spectroscopy Agilent 4294A frequency-response analyser, inter and intra grain I-V measurements were done by using a digital source meter (Agilent B2911A unit).