Ramzy Daou

Bio: Ramzy Daou is an academic researcher from University of Caen Lower Normandy. The author has contributed to research in topics: Thermoelectric effect & Seebeck coefficient. The author has an hindex of 3, co-authored 10 publications receiving 27 citations.

Low thermal conductivity in a modular inorganic material with bonding anisotropy and mismatch

Abstract: The thermal conductivity of crystalline materials cannot be arbitrarily low, as the intrinsic limit depends on the phonon dispersion. We used complementary strategies to suppress the contribution of the longitudinal and transverse phonons to heat transport in layered materials that contain different types of intrinsic chemical interfaces. BiOCl and Bi2O2Se encapsulate these design principles for longitudinal and transverse modes, respectively, and the bulk superlattice material Bi4O4SeCl2 combines these effects by ordering both interface types within its unit cell to reach an extremely low thermal conductivity of 0.1 watts per kelvin per meter at room temperature along its stacking direction. This value comes within a factor of four of the thermal conductivity of air. We demonstrated that chemical control of the spatial arrangement of distinct interfaces can synergically modify vibrational modes to minimize thermal conductivity.

Anisotropic thermal transport in magnetic intercalates Fe x TiS 2

Abstract: We present a study of the thermal transport in thin single crystals of iron-intercalated titanium disulphide, ${\mathrm{Fe}}_{x}{\mathrm{TiS}}_{2}$, for $0\ensuremath{\le}x\ensuremath{\le}0.20$. We determine the distribution of intercalants using high-resolution crystallographic and magnetic measurements, confirming the insertion of Fe without long-range ordering. We find that iron intercalation perturbs the lattice very little, and suppresses the tendency of ${\mathrm{TiS}}_{2}$ to self-intercalate with excess Ti. We observe trends in the thermal conductivity that are compatible with our ab initio calculations of thermal transport in perfectly stoichiometric ${\mathrm{TiS}}_{2}$.

Thermoelectric properties, metal-insulator transition, and magnetism: Revisiting the N i 1 − x C u x S 2 system

Abstract: The $\mathrm{N}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{u}}_{x}{\mathrm{S}}_{2}$ pyrite ($x\ensuremath{\le}0.07$) is a model material where the impact of electron filling on a half-filled ${e}_{\mathrm{g}}$ band system can be separated from changes in the bandwidth, since the unit cell volume does not change with $x$. This contrasts with the $\mathrm{Ni}{\mathrm{S}}_{2\ensuremath{-}x}\mathrm{S}{\mathrm{e}}_{x}$ system where the size difference between ${\mathrm{S}}^{2\ensuremath{-}}$ and $\mathrm{S}{\mathrm{e}}^{2\ensuremath{-}}$ makes the bandwidth vary at constant band filling. Our magnetic measurements show that there exists a clear relation between the presence of the antiferromagnetic phase developing below ${T}_{\mathrm{N}2}=29.75\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and charge localization. With the addition of Cu in $\mathrm{N}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{u}}_{x}{\mathrm{S}}_{2}$, the ground state evolves towards a metallic paramagnetic state. These findings are discussed with the scenario based on charge ordering at low temperature in the charge transfer insulator $\mathrm{Ni}{\mathrm{S}}_{2}$ pyrite, and also considering possible conducting surface states at low temperatures. At 300 K, the thermal conductivity decreases by 2.5 as $x$ increases from 0.00 to 0.07, even though the latter is much more electrically conductive than the former. This considerably extends the range of values for the thermal conductivity of the $\mathrm{M}{\mathrm{S}}_{2}$ pyrites, from the largest in $\mathrm{Fe}{\mathrm{S}}_{2}$ to the lowest in the present $\mathrm{N}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{u}}_{x}{\mathrm{S}}_{2}$ samples.

Eco-Friendly Higher Manganese Silicide Thermoelectric Materials: Progress and Future Challenges

Abstract: As a promising thermoelectric material, higher manganese silicides are composed of earth-abundant and eco-friendly elements, and have attracted extensive attention for future commercialization. In this review, the authors first summarize the crystal structure, band structure, synthesis method, and pristine thermoelectric performance of different higher manganese silicides. After that, the strategies for enhancing electrical performance and reducing lattice thermal conductivity of higher manganese silicides as well as their synergism are highlighted. The application potentials including the chemical and mechanical stability of higher manganese silicides and their energy conversion efficiency of the assembled thermoelectric modules are also summarized. By analyzing the current advances in higher manganese silicides, this review proposes that potential methods of further enhancing zT of higher manganese silicides, lie in enhancing electrical performance while simultaneously reducing lattice thermal conductivity via reducing effective mass, optimizing carrier concentration, and nanostructure engineering.

Suppressing Ge-vacancies to achieve high single-leg efficiency in GeTe with an ultra-high room temperature power factor

Abstract: GeTe is among the best medium-temperature thermoelectrics. Its high performance originates from band convergence at the phase transition and low lattice thermal conductivity due to Peierls distortion. In most studies, the peak performance (zT) in GeTe is achieved by designing and optimizing its electronic and thermal transport properties near its phase transition temperature (700 K). However, for efficient power harvesting, a high average zT (zTave) across a wide temperature range is desirable. This calls for a holistic performance evaluation and enhancement not only near 700 K, but also at room temperature. In this work, we leveraged on the confluence of performance enhancement strategies via Cu2Te alloying and In resonant doping to achieve a record-high room temperature power factor of 2800 μW mK−2, and an average power factor of 3700 μW mK−2 between 323 and 773 K. The magnitude of the room temperature power factor is comparable to that of the state-of-the-art Bi2Te3 based compounds. In the optimized sample with Bi doping, a room temperature zT of 0.5 is achieved, highest for lead-free GeTe. Ultimately, a high peak zT of 2.1 at 723 K and single leg power conversion efficiency of 11.8% were achieved between 323 and 745 K, which are among the highest reported for lead-free GeTe.

Key properties of inorganic thermoelectric materials—tables (version 1)

Abstract: This paper presents tables of key thermoelectric properties, which define thermoelectric conversion efficiency, for a wide range of inorganic materials. The twelve families of materials included in these tables are primarily selected on the basis of well established, internationally-recognized performance and promise for current and future applications: tellurides, skutterudites, half Heuslers, Zintls, Mg–Sb antimonides, clathrates, FeGa3-type materials, actinides and lanthanides, oxides, sulfides, selenides, silicides, borides and carbides. As thermoelectric properties vary with temperature, data are presented at room temperature to enable ready comparison, and also at a higher temperature appropriate to peak performance. An individual table of data and commentary are provided for each family of materials plus source references for all the data.

Insights into Low Thermal Conductivity in Inorganic Materials for Thermoelectrics.

Abstract: Efficient manipulation of thermal conductivity and fundamental understanding of the microscopic mechanisms of phonon scattering in crystalline solids are crucial to achieve high thermoelectric performance. Thermoelectric energy conversion directly and reversibly converts between heat and electricity and is a promising renewable technology to generate electricity by recovering waste heat and improve solid-state refrigeration. However, a unique challenge in thermal transport needs to be addressed to achieve high thermoelectric performance: the requirement of crystalline materials with ultralow lattice thermal conductivity (κL). A plethora of strategies have been developed to lower κL in crystalline solids by means of nanostructural modifications, introduction of intrinsic or extrinsic phonon scattering centers with tailored shape and dimension, and manipulation of defects and disorder. Recently, intrinsic local lattice distortion and lattice anharmonicity originating from various mechanisms such as rattling, bonding heterogeneity, and ferroelectric instability have found popularity. In this Perspective, we outline the role of manipulation of chemical bonding and structural chemistry on thermal transport in various high-performance thermoelectric materials. We first briefly outline the fundamental aspects of κL and discuss the current status of the popular phonon scattering mechanisms in brief. Then we discuss emerging new ideas with examples of crystal structure and lattice dynamics in exemplary materials. Finally, we present an outlook for focus areas of experimental and theoretical challenges, possible new directions, and integrations of novel techniques to achieve low κL in order to realize high-performance thermoelectric materials.

Keynote Review of Latest Advances in Thermoelectric Generation Materials, Devices, and Technologies 2022

Abstract: The last decade created tremendous advances in new and unique thermoelectric generation materials, devices, fabrication techniques, and technologies via various global research and development. This article seeks to elucidate and highlight some of these advances to lay foundations for future research work and advances. New advanced methods and demonstrations in TE device and material measurement, materials fabrication and composition advances, and device design and fabrication will be discussed. Other articles in this Special Issue present additional new research into materials fabrication and composition advances, including multi-dimensional additive manufacturing and advanced silicon germanium technologies. This article will discuss the most recent results and findings in thermoelectric system economics, including highlighting and quantifying the interrelationships between thermoelectric (TE) material costs, TE manufacturing costs and most importantly, often times dominating, the heat exchanger costs in overall TE system costs. We now have a methodology for quantifying the competing TE system cost-performance effects and impacts. Recent findings show that heat exchanger costs usually dominate overall TE system cost-performance tradeoffs, and it is extremely difficult to escape this condition in TE system design. In regard to material performance, novel or improved enhancement principles are being effectively implemented. Furthermore, in addition to further advancements in properties and module developments of relatively established champion materials such as skutterudites, several high performance ZT ≈≥ 2 new material systems such as GeTe, Mg3(Sb,Bi)2 have also been relatively recently unearthed and module applications also being considered. These recent advancements will also be covered in this review.