DS0202 - Captage des énergies renouvelables et récupération des énergies de l’environnement

Textured electrospun nanofibers - based thermoelectric ceramics – NanoCerTex

Textured electrospun Nanofibers – based thermoelectric Ceramics

Towards nanotextured thermoelectric ceramics with optimized properties<br />Nowadays, there is a strong interest in the development of nanostructured ceramics in order to develop materials with special or improved properties with respect to those of classical bulk ceramics. Thermoelectric nanostructures are very attractive because they can provide a practical way to scavenge energy from the environment to power microsystems and thus be used in novel self-powered sensing devices.

Coupling nanostructuring and texturing to optimize the performance of anisotropic thermoelectric materials

Recently, spectacular results have been obtained on thermoelectric properties through the introduction of nanostructures in the materials. Some oxide thermoelectric materials exhibit anisotropic properties because of their crystal structure. It could be of great interest to use processes that allow alignment of the grains along one crystallographic direction to reach better performances. The conventional texturing methods require long sintering time at high temperature, in order to allow the texturing process to take place. Consequently, it is rather difficult to obtain nanotextured ceramics.<br />This project deals with the development of thermoelectric nanotextured ceramics and nanocomposites for energy harvesting, by using an original combination of a soft chemistry route nanofibers fabrication technique and flash sintering technique. The key issue to overcome is maintaining the nanometric dimension during the texturing process.

Electrospinning is a simple and versatile method for drawing micro or nano scale fibers from polymer solutions (or melts). Recently, it has also been extended to fabricate nanofibers made of ceramics and composite materials. The electrospun fibers are often collected as randomly oriented structures in the form of nonwoven mats but several works showed that it is possible to obtain well-aligned nanofibers by the use of specific collectors.
Spark plasma sintering (SPS) is a recent emerging consolidation technique that offers the possibility to densify a wide range of various materials while retaining the submicrometer structures at low sintering temperatures. As a hot-pressing technique, SPS is suitable for producing textured bulk materials
Electrospinning can be used to align inorganic nanofibers, whereas spark plasma sintering is a hot-pressing technique that allows achieving highly dense material while maintaining a submicron scale. Combining these two methods should lead to nanotextured materials that conventional texturing methods do not allow.

This project focused on compounds of the calcium manganate family, i.e. CaMnO3 and Ca2MnO4. This project led to the development of a reliable, simple and robust protocol for the synthesis of nanometric powders and shaping by flash sintering of ceramics with well-controlled density and microstructure. In parallel, extensive crystallographic analyzes of Ca2-xDyxMnO4 were undertaken from room temperature to 800 °C. Finally, aligned CaMnO3 nanofibers were obtained by electrospinning. These fibers have been sintered, demonstrating the feasibility of the process proposed by the NanoCerTex project. This process could be extended to other compositions and other applications.

This project has opened up new lines of research that should be deepened. The major results have been published or are in progress and will be accessible to the entire scientific community in open archive.. This project deals with thermoelectricity but the acquired skills and knowledge could be transposed to many other chemical systems and fields of application (piezoelectricity, structural materials, etc.).

The NanoCerTex project results was or will be disseminated through several publications or communications in international conferences:
- A. Bahezre, D. Bregiroux, C. Laberty-Robert, Thermoelectric properties of n-type oxides (Sr3Ti2O7, CaMnO3), 14th European Conference on Thermolectrics (ECT2016), September 2016, Lisbon, Portugal. Poster
- M. Allani, A. Bahezre, D. Bregiroux, G. Rousse, C. Laberty-Robert, Highly dense and nanostructured thermoelectric Ca2-xDyxMnO4 ceramics, 44th International Conference and Expo on Advanced Ceramics and Composites (ICACC 2020), January 2020, Daytona Beach, USA. Oral
- D. Bregiroux, A. Bahezre, M. Allani, G. Rousse, G. Wallez, C. Laberty-Robert, Dysprosium doping of Ca2MnO4: Effect on crystal structure at room temperature and thermal behavior, Materials Chemistry and Physics, under review
- M. Allani, A. Bahezre, I. Massoud, F. Giovanelli, D. Bregiroux and C. Laberty-Robert, Effect of doping and sintering process on thermoelectric and mechanical properties of Ca2MnO4 ceramics, Journal of the European Ceramic Society, to be submitted.

All publications resulting from this project will be deposited in full text in an open archive.

Nowadays, there is a strong interest in the development of nanostructured ceramics in order to develop materials with special or improved properties with respect to those of classical bulk ceramics. Thermoelectric nanostructures are very attractive because they can provide a practical way to scavenge energy from the environment to power microsystems and thus be used in novel self-powered sensing devices. Thermoelectric generators can be used in automobiles to convert the wasted heat from an engine's coolant or exhaust into electricity. It is accepted that such systems can lead to a fuel consumption decrease of around 10%. Heat from the sun is also a source of energy that can be converted into electricity by thermoelectric systems.
A lot of thermoelectric materials have been extensively studied during the last decade. Recently, spectacular results have been obtained for various systems through the introduction of nanostructures in the materials. Nanoscale heterostructures can be achieved by decreasing the grain size and/or by introducing dispersed nanoparticles or continuous secondary phase. These systems are usually referenced as nanocomposites. Among the materials having good thermoelectric properties, some have a highly anisotropic structure. In those cases, it could be of great interest to align the grains along one crystallographic direction. Such materials are called textured ceramics and can be obtained through different ways. Nevertheless, all the existing texturing methods require long sintering time at high temperature, in order to allow the texturing process to take place. Consequently, it is rather difficult to obtain nanotextured ceramics.
NanoCerTex project deals with the development of thermoelectric nanotextured materials and nanocomposites for energy harvesting, by using an original combination of a soft chemistry route nanofibers fabrication technique (electrospinning) and flash sintering technique (spark plasma sintering techniques). The key issue to overcome is maintaining the nanometric dimension during the texturing process.
Two axes will be explored:
- Development of bulk dense nanotextured thermoelectric ceramics. The objective will be to sinter by flash technique, a stack of aligned fibers to produce dense and bulk-textured ceramics with submicrometric grains. It is expected that for compounds with highly anisotropic crystal structure, texturing develops spontaneously during the uniaxial pressure sintering. For materials with isotropic structure, texturing can be induced by introducing some anisotropic seeds into the fibers during electrospinning. Texturing will be developped during subsequent sintering step. This method is an implementation of the well-known Templated Grain Growth method used for bulk micrometric ceramics to nanofibers.
- Development of textured thermoelectric nanocomposites, which will consist on inorganic multiphase systems. The objective is to increase the Figure of Merit ZT by decreasing the thermal conductivity of the material while maintaining a high electrical conductivity. The use of aligned core-shell nanofibers should lead to nanocomposite thermoelectric ceramics with enhanced properties.
NanoCerTex aims at understanding the mechanisms of fiber formation during electrospinning and texture development during the subsequent thermal treatment. In depth ex situ analysis (XRD, SE-MEB, MET, EBSD…) of the crystallization within the fiber and in situ characterization by synchrotron radiation (SAXS, WAXS…) will be performed in order to get more insight on how to improve the process.
To our knowledge, developing textured nanocomposite thermoelectric ceramics by using the electrospinning technique has not been reported. This project aims at demonstrating the interest of electrospinning coupled with spark plasma sintering process for the design of new functional anisotropic materials with enhanced properties, by creating a texturing at a submicron scale. Although this work will be done on the

Project coordination

Damien Bregiroux (Laboratoire de Chimie de la Matière Condensée de Paris)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

LCMCP Laboratoire de Chimie de la Matière Condensée de Paris

Help of the ANR 200,928 euros
Beginning and duration of the scientific project: November 2015 - 42 Months

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