Thermoelectricity, a materials’ capability to convert heat in to electric energy has been known to exist in liquids for many decades. Unlike in solids, this conversion process liquids take several forms including the thermogalvanic reactions between the redox ions and the electrodes, the thermodiffusion of charged species and the temperature dependent formation of electrical double layer at the electrodes. The observed values of Seebeck coefficient (Se = - DV/DT, the ratio between the induced voltage (DV) and the applied temperature difference (DT)) are generally above 1 mV/K, an order of magnitude higher than those found in the solid (semiconductor) counterpart. The first working example of a liquid-based thermoelectric (TE) generator was reported in 1986 using Ferro/ferricyanide redox salts in water.



However, due to the low electrical conductivity of liquids, its conversion efficiency was very low, preventing their use in low-temperature waste-heat recovery applications. The outlook of liquid TE generators brightened in the last decade with the development of ionic liquids (ILs). ILs are molten salts that are liquid below 100 °C. Compared to classical liquids, they exhibit many favorable features such as high boiling points, low vapour pressure, high ionic conductivity and low thermal conductivity accompanied by higher Se values. More recently, an experimental study by IJCLab and SPEC revealed that the complexation of transition metal redox couples in ionic liquids can lead to enhancing their Se coefficient by more than a three-fold from -1.6 to -5.7 mV/K, one of the highest values reported in IL-based thermoelectric cells. A clear understanding and the precise control of the speciation of metal ions therefore is a gateway to the rational design of future thermoelectrochemical technology.



Based on these recent findings, we proposes to further study the coordination chemistry of transition metal redox ions in ILs and mixtures. A long-term goal associated to the present project is to demonstrate the application potential of liquid thermoelectrochemical cells based on affordable, abundant and environmentally safe materials for thermal energy harvesting as an energy efficiency tool.

Thermoelectric (TE) materials that are capable of converting heat into electricity have been considered as one possible solution to recover the low-grade waste-heat (from industrial waste-stream, motor engines, household electronic appliances or body-heat).



At SPHYNX, we explore thermoelectric effects in an entirely different class of materials, namely, complex fluids containing electrically charged nanoparticles that serve as both heat and electricity carriers. Unlike in solid materials, there are several inter-dependent TE effects taking place in liquids, resulting in Se values that are generally an order of magnitude larger that the semiconductor counterparts. Furthermore, these fluids are composed of Earth-abundant raw materials, making them attractive for future TE-materials that are low-cost and environmentally friendly. While the precise origins of high Seebeck coefficients in these fluids are still debated, our recent results indicate the decisive role played by the physico-chemical nature of particle-liquid interface.



The goal of the PhD project is two-fold. First, we will investigate the underlying laws of physics behind the thermoelectric potential and power generation and other associated phenomena in a special type of complex fluids, namely, ferrofluids (magnetic nanofluids). The results will be compared to their thermos-diffusive properties to be obtained through research collaboration actions. Second, the project aims to develop proof-of-concept hybrid solar-collector devices that are capable of co-generating heat and electricity.



The proposed research project is primarily experimental, involving thermos-electrical, thermal and electrochemical measurements; implementation of automated data acquisition system and analysis of the resulting data obtained. The notions of thermodynamics, fluid physics and engineering (device) physics, as well as hands-on knowledge of experimental device manipulation are needed. Basic knowledge of optics and electrochemistry is a plus. For motivated students, numerical simulations using commercial CFD software can also be envisaged.

Glass is a widely used material due to its many advantageous properties: transparency, hardness, low thermal expansion, high melting point temperature, relative chemical inertia, etc. However, it has one major weakness: its fragility. Relatively moderate stresses can cause it to break suddenly and without any warning. Glass is also sensitive to stress corrosion cracking: sub-crtical cracking aided by environmental conditions (relative humidity, temperature, etc.). In this case, apparently harmless stresses (much lower than those leading to its sudden breakage) can lead to crack propagation at low rates, as observed in the slow cracking of car windscreens. This stress corrosion cracking (SCC) also depends on the intrinsic parameters of the glass: chemical composition, microstructure, etc.



The phenomenon of phase separation in glasses leads to a meso-structured material which can improve mechanical properties such as crush resistance. It is also at the origin of glass-ceramics, consisting of microcrystals dispersed in a glass matrix, developed to take advantage of the benefits of both components: ceramics and glasses. They are used, for example in optical thermometry applications, kitchen utensils, dental materials, etc. However, the stress corrosion behavior of this type of material is still poorly understood.



The objective of this project is to study the link between the meso-structure of glass-ceramics and their stress corrosion cracking behavior. Samples will concern as fabricated samples and their phased separated counterparts which will be achieved by varying annealing protocols. The candidate will make use of an existing SCC experimental set-up allowing experiments in well controlled conditions (Figure 1 top). The rate of crack propagation and its variation with applied stress will be measured for each samples to obtain the characteristic stress corrosion resistance curves. In parallel, the composition and meso-structure of the samples will be studied using different techniques: AFM, SEM, Raman, etc. The candidate will also use a state-of-the-art Atomic Force Microscope (AFM) to characterize post-mortem fracture surfaces. These studies will aid in characterizing the size of phase separation and will feed different statistical tools (stochastic modelling, fractal analysis).



This internship will take place in the SPHYNX lab located in the Condensed State Physics Service which is a joint CEA / CNRS unit (UMR 3680 CEA-CNRS). Researchers study condensed matter physics, from the most fundamental physics to industrial applications. The candidate will have the opportunity to use and learn first-hand advanced methods for characterizing materials and their surfaces, from the macroscopic to the nanometric scale. The approaches will be based on experimental platforms and theoretical tools developed in-house. The candidate will have the opportunity to manipulate theoretical and experimental tools used in the field of materials science, mechanics and statistical physics. Finally, the very fundamental and applied character of this research will allow the candidate to find opportunities in the academic world (thesis) and in industry.

Examples of active systems, formed of units that are able to extract energy from the environment and dissipate it to self-propel, are found everywhere in nature: flocks of birds, animal swarms, suspensions of bacteria or tissues are all biological active systems. Scientists are able to build synthetic active systems using catalytic colloidal particles or micro-robots.

Future applications may encompass the engineering self-assembling materials using active units, considered as a defining agenda in the community. Furthermore, active systems have theoretically fascinating properties, a fact that drove a very intense research activity lately.



Large assemblies of active units display collective phenomena that are absent in equilibrium. One of the most ubiquitous is phase separation, where even repulsive but active particles phase separate into dense and dilute phases. In some cases, this phenomena resemble to liquid-vapor phase separation of standard fluids. Due to broken time-reversibility, however, active systems can show novel forms of phase separation, comprising a state where the liquid state comprises mesoscopic vapor bubbles (thus resembling to a boiling liquid) or active foams states, where thin liquid filaments are dispersed in the vapor.

Furthermore, in most experimental realization, active systems are `wet’, meaning that particles move in a fluid which itself can mediate interactions among particles, a feature whose consequences are so far little understood theoretically.



The main open theoretical question is how to control these novel states of matter in terms of microscopically tunable parameters. The main goal of this PhD is to fill this gap. This will require both analytical and computational work done on agent based models and continuous descriptions of active systems. If successful, the work will provide a guide for experimentalists to design novel self-assembling materials using active units. Given the ubiquity of phase separation in non-equilibrium contexts, we will further explore the relevance of these results to other out-of-equilibrium systems, such as biological tissues and granular materials.

Many space-time features of biological and active materials, from morphogenesis to the structure of dense assemblies of self-propelled colloids, are caused and controlled by topological defects. These can be defects on top of liquid-crystalline orders such as nematics or hexatics, or even on top of a crystalline solid. Defects can mediate anomalous stress propagation, shape the curvature of the underlying flexible materials, or even cause phase transitions between states where defects are bounded to ones where they can freely diffuse.



This PhD project aims at understanding the many-body physics of topological defects in active materials with a combination of analytical and numerical techniques, and at exploring their relevance for collective phenomena in active and living systems.