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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 characterisation 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

 


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