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Real-Time ThermoTronics: handling fluctuations, dynamics and dissipation for smart radiative thermal management

Periodic Reporting for period 1 - RTTT (Real-Time ThermoTronics: handling fluctuations, dynamics and dissipation for smart radiative thermal management)

Reporting period: 2020-04-01 to 2022-03-31

Most of the everywhere heat sources spanning from the micro- to the macro-scale remain disregarded and their latent potential is lost. Thermotronics is a young discipline offering promising options for the onset of a new paradigm in the employment of heat, proposing effective ways of taking advantage of these sources. Managing thermal currents as proficiently as electric currents, for instance, would imply progress in subjects such as thermal management and energy storage, and impact the development of new ones such as thermal sensing and computing. The research of this project deals with this matter by investigating the behavior of thermal devices that can be implemented for smart thermal management.

- The primary objective of the action is to address fluctuations, dynamics and dissipation in thermotronic devices based on nanoscale photon transport, contributing so to the development of new mechanisms to manage and exploit radiative heat fluxes.

In the vicinity of a hot solid, a strong electromagnetic energy density exists because of the presence of an evanescent field. In the form of thermal radiation, this energy can be transferred without contact from a hot source to colder objects and then implemented in different applications. A situation can be envisaged, for instance, in which autonomous sensors governed by thermal signals launch specific tasks. Moreover, the potential of thermal radiation can be exploited in conversion processes leading to usable energy. At the nanoscale, a hot object can be considered as a source to power a conversion device. This offers the possibility of obtaining clean energy from waste heat, then covering industrial and social energetic needs. By combining different radiation-driven mechanisms, hybrid electric-thermal circuits can be designed for an advantageous manipulation of heat.

In conclusion, this project contribute novel methods to study systems driven by near-field thermal radiation. While environmental noise weakly impacts the state of the elements of the system in typical configurations, external control of emission properties leads to a significant change in the magnitude of radiative heat fluxes. By using graphene and materials supporting polaritonic resonances, we shown that the thermal state of active elements can be modulated at kHz frequencies. This provides means for relatively fast control of the system’s dynamics and the associated heat exchange. Furthermore, dissipation in irreversible processes associated with radiative heat exchange can be accounted for by quantifying the entropy production. We developed a non-equilibrium thermodynamic framework describing entropic contributions in many-body systems with near-field interactions. The project paves the way for innovative strategies for an active control of radiative heat fluxes with applications in smart radiative thermal management.
1. By covering emitters of thermal radiation with graphene (whose properties can be actively controlled with an applied voltage), we shown that the temperature of a suspended thin film can be modulated at kHz frequencies. This effect can be implemented in energy conversion devices using pyroelectric materials. Such graphene-based pyroelectric systems have a huge potential for waste heat energy harvesting. In addition, it offers the possibility of a relatively fast control of fluxes that can be employed in self-powered, hybrid electric-thermal circuitry for smart thermal management.
2. Two-body radiative thermal diodes rectify heat flows thanks to a temperature dependence of the material optical properties. We demonstrated that a significant rectification is possible in three-element radiative systems even when the dependence of the optical properties on the temperature is negligible. This can be achieved thanks to a combination of material asymmetry and resonances supported by the materials, leading to rectification of heat fluxes over a broad temperature range.
3. We shown that a confined fluid exchanging heat, work and matter with the environment can attain equilibrium states under completely open conditions, and presented a Mote Carlo algorithm to perform simulations in the corresponding ensemble. This is a first step to describe a situation in which the system is a confined photon gas with fluctuating boundaries, which may be relevant in small scale devices with heat and momentum transfer.
4. We developed a non-equilibrium thermodynamic framework to address radiative elements interacting in the near field. Within this framework, the dynamics of the temperatures of the components of the system can be analyzed as well as the associated dissipation, which is described by irreversible entropy production in the components.
5. We reviewed recent progress on the passive and active control of near-field radiative heat exchange in two- and many-body systems. Topics covered in the review include: thermal rectification; modulation and switching of heat transfer; heat splitting and focusing; active insulation, cooling and refrigeration; and logical circuits that can be built, e.g. using radiative thermal transistors.

So far these results were published in 4 peer-reviewed articles. Dissemination activities were also carried out by participating in 4 international conferences and through the organization of 2 research seminars. The researcher also participated in Science is Wonderful! 2021, an outreach activity organized by the European Commission. The results of the action are published in a webpage dedicated to the project.
Several advances have been made beyond the state of the art. First, a Mote Carlo algorithm to perform simulations of completely open systems has been proposed, which may be used to study confined systems with fluctuating boundaries. The method is relevant to study systems in which the actuation force is controlled instead of the position of the boundaries. Second, a rectification mechanism for radiative heat in many-body systems has been introduced which allows for rectification even if the dependence on temperature of material optical properties is negligible. The mechanism exploits an asymmetry in the system arising from material properties and enables rectification over a broad temperature range. Third, a non-equilibrium thermodynamic framework to quantify irreversible entropy production in many-body systems exchanging radiative heat in the near field has been developed, which is useful to analyze dissipative processes in the elements of the system. The entropy production can be used to parametrize the performance of the considered process leading to a procedure for optimization. Lastly, a graphene-based method to modulate the temperature in systems with near-field heat exchange has been introduced and implemented in an energy conversion mechanism using pyroelectric materials. The method provides a way to harvest near-field thermal energy with pyroelectric materials at much higher frequencies than those considered so far using stationary thermal sources.

The impact of the project results will be mainly on research in the field of nanoscale heat transfer. The proposed methods for an active control of near-field heat exchange can be naturally extended and applied to many-body system with radiative interactions with configurations different from those considered in the project. Future research may lead to improvements in these methods, laying the groundwork for new developments in the field.
Radiative thermal diode.