Unlocking the potential of a caged star: Thermoelectric quaternary clathrates

Heat losses are an inevitable consequence of any energy conversion process, dictated by the second law of thermodynamics.This not only leads to an eternal struggle, via the pursuit of maximal efficiency, it also undermines our efforts to solve the two issues that pose the most significant challenges to modern society: climate change and the worlds surging energy need. Thanks to our inherent ingenuity, humankind has, however, been adept at finding ways of harnessing the power of heat; from the fires that lit up the neolithic era to the steam engines of the industrial revolution. Thermoelectrics can, in some sense, be seen as the next step in this endeavour, since they allow the direct conversion of a temperature difference to an electric voltage.This thesis summarises a seven year long journey, which has focused on a fascinating and unique group of thermoelectric materials, namely inorganic clathrates. Though these have been the subject of intense research over the last three decades, many of their properties and attributes have, as of yet, not been fully explored. In particular, this project has addressed three fundamental questions: (i) Why is the lattice thermal conductivity intrinsically low? (ii) What is the impact of chemical ordering on the physical properties? (iii) How can the electronic transport be optimised?Due to the inherent complexity of these materials, computational and experimental methods should ideally be used in tandem, in order to gain further insights. This project has, thus, involved the use of both atomic scale simulations, based on a combination of density functional theory, alloy cluster expansions, and Monte Carlo simulations, as well as advanced measurement and characterisation techniques. Through these efforts, the confusion regarding the origin of the low lattice thermal conductivity has partly been clarified. In addition, it has been shown that chemical ordering in these materials leads to the emergence of an order-disorder transition, which has a direct impact on the physical properties. Last but not least, it is found that the consideration of ternary systems can facilitate the enhancement of the thermoelectric performance by enabling not only independent tuning of doping level and band structure via the composition, but also manipulation of the nano- and microstructure

Computational assessment of the efficacy of halides as shape-directing agents in nanoparticle growth

We report a comprehensive study of aqueous halide adsorption on nanoparticles of gold and palladium that addresses several limitations hampering the use of atomistic modeling as a tool for understanding and improving wet-chemical synthesis and related applications. A combination of thermodynamic modeling with density functional theory (DFT) calculations and experimental data is used to predict equilibrium shapes of halide-covered nanoparticles as a function of the chemical environment. To ensure realistic and experimentally relevant results, we account for solvent effects and include a large set of vicinal surfaces, several adsorbate coverages, as well as decahedral particles. While the observed stabilization is not significant enough to result in thermodynamic stability of anisotropic shapes such as nanocubes, nonuniformity in the halide coverage indicates the possibility of obtaining such shapes as kinetic products. With regard to technical challenges, we show that inclusion of surface-solvent interactions leads to qualitative changes in the predicted shape. Furthermore, accounting for nonlocal interactions on the functional level yields a more accurate description of surface systems

Strategic Optimization of the Electronic Transport Properties of Pseudo-Ternary Clathrates

While alloying is a powerful handle for materials engineering, it is an ongoing challenge to navigate the large and complex parameter space of these materials. This applies in particular for thermoelectrics and even more so clathrates. Here, a combination of density functional theory calculations, alloy cluster expansions, Monte Carlo simulations, and Boltzmann transport theory calculations is used to identify compositions that yield high power factors in the pseudo-ternary clathrates Ba8AlxGayGe46−x−y and Ba8GaxGeySi46−x−y, while accounting for weight and raw material costs. The results show how a cost-efficient performance can be achieved by reducing the number of Al and Ga atoms per unit cell, while compensating the resulting increase in the carrier concentration via an extrinsic dopant. The approach used in this study is transferable and can be a useful tool for mapping the thermodynamic and transport properties of other multinary\ua0systems

First-Principles Study of Order-Disorder Transitions in Pseudobinary Clathrates

It has been recently demonstrated that the pseudoternary Ba8AlxGayGe46-x-y clathrate undergoes an order-disorder transition with increasing temperature that can be observed via site occupation factors (SOFs) and manifests itself, e.g., in electrical transport properties. Here, we generalize this result and analyze the characteristics of this order-disorder transition in the pseudobinary clathrates Ba8GaxGe46-x, Ba8GaxSi46-x, Ba8AlxGe46-x, and Ba8AlxSi46-x. To this end, we employ atomistic simulations that combine alloy cluster expansions trained against density functional theory calculations with Wang-Landau and ensemble Monte Carlo simulations. The simulations show that all four systems studied here display order-disorder transitions for at least some composition range. Based on an extensive literature survey, we also provide evidence for signatures of the transition in earlier experimental studies that to the best of our knowledge have hitherto not been related to such transitions. The predicted transition temperatures are lower for Ba8GaxGe46-x and Ba8GaxSi46-x than for Ba8AlxGe46-x and Ba8AlxSi46-x, although it appears that the simulations underestimate the transition temperatures for Ga-containing systems compared to the experiment. This nonetheless provides a sensible explanation for why the experimentally determined Al SOFs agree better with the simulated higherature disordered configurations, while the Ga SOFs more closely agree with the simulated ground-state configurations. As a result of stronger interactions, the SOFs vary substantially, especially near the stoichiometric 16:30 composition, providing an indication of why it has proved difficult to synthesize Ba8AlxGe46-x and Ba8AlxSi46-x samples at this ratio. The present study thereby yields detailed atomic-scale insights into the ordering in inorganic clathrates that, given the connection to transport properties established earlier, are not only useful from a fundamental perspective but also relevant for applications

DIPSAUCE: Efficient Private Stream Aggregation Without Trusted Parties

Private Stream Aggregation (PSA) schemes are efficient protocols for distributed data analytics. In a PSA scheme, a set of data producers can encrypt data for a central party so that it learns the sum of all (encrypted) values, but nothing about each individual value. Due to this ability to efficiently enable central data analytics without leaking individual user data, PSA schemes are often used for IoT data analytics scenarios where privacy is important, such as smart metering. However, all known PSA schemes require a trusted party for key generation, which is undesirable from a privacy standpoint. Further, even though the main benefit of PSA schemes over alternative technologies such as Functional Encryption is that they are efficient enough to run on IoT devices, there exists no evaluation of the efficiency of existing PSA schemes on realistic IoT devices. In this paper, we address both these issues. We first evaluate the efficiency of the state of the art PSA schemes on realistic IoT devices. We then propose, implement and evaluate a DIstributed setup PSA scheme for Use in Constrained Environments (DIPSAUCE). DIPSAUCE is the first PSA scheme that does not rely on a trusted party. Our security and efficiency evaluation shows that it is indeed possible to construct an efficient PSA scheme without a trusted central party. Surprisingly, our results also show that, a side effect, our method for distributing the setup procedure also makes the encryption procedure more efficient than the state of the art PSA schemes which rely on trusted parties

Order-Disorder Transition in Inorganic Clathrates Controls Electrical Transport Properties

Inorganic clathrates have been extensively investigated owing to their unique and intriguing atomic structure as well as their potential as thermoelectric materials. The connection between the chemical ordering and the physical properties has, however, remained elusive. Here, this relation is uncovered through a combination of first-principles calculations, atomistic simulations, and experimental measurements of thermodynamic as well as electrical transport properties. This approach is, specifically, used to reveal the existence of an order-disorder transition in the quaternary clathrate series Ba8AlxGa16-xGe30. The results, furthermore, demonstrate that this phenomenon is responsible for the discontinuity in the heat capacity that has been observed previously. Moreover, the unusual temperature dependence of both Seebeck coefficient and electrical conductivity can be fully explained by the alterations of the band structure brought about by the phase transformation. It is finally argued that the phenomenology described here is not limited to this particular material but should be present in a wide range of inorganic clathrates and could even be observed in other materials that exhibit chemical ordering on at least one sublattice

Discovery of Oxygen Carriers by Mining a First-Principle Database

Chemical looping is an innovative technique that relies, to a large extent, on the possibility of finding new oxygen carriers. Until now, these materials have primarily been identified via experimental techniques and therefrom derived insights. However, this is both costly and time-consuming. To speed-up this process, we have applied a computational screening approach based on energetic data retrieved from the Open Quantum Materials Database. In particular, we have considered combinations of all mono-, bi-, and trimetallic alloys and mixed oxides with up to three distinctive phases. Here, we specifically focus on a technique referred to as chemical looping oxygen uncoupling, which is especially suitable for solid fuels, e.g., combustion of biomass for negative CO2 emissions. The formation energies obtained for the materials of interest were used to identify phase transitions that are likely to occur under conditions relevant for chemical looping oxygen uncoupling. Given these criteria, the initial list of 300000 materials is reduced by a factor of 20, and after filtering out rare, radioactive, toxic, or harmful elements only 1000 remain. When considering the abundance of elements in the ranking criteria, most of the highest ranking phases include Cu, Mn, and Fe. This adds credibility to the procedure, as many viable oxygen carriers for chemical looping oxygen uncoupling that have been studied experimentally contain these elements. While Cr-based materials have not been widely explored for this application, our study suggests that this might be worthwhile since these occur more frequently than Fe. Other elements that would be interesting as additional components include Ba, K, Na, Al, and Si

Investigating the Chemical Ordering in Quaternary Clathrate Ba8AlxGa16- xGe30

Recently, there has been an increased interest in quaternary clathrate systems as promising thermoelectric materials. Because of their increased complexity, however, the chemical ordering in the host framework of quaternary clathrates has not yet been comprehensively analyzed. Here, we have synthesized a prototypical quaternary type-I clathrate Ba8AlxGa16-xGe30 by Czochralski and flux methods, and we employed a combination of X-ray and neutron diffraction along with atomic scale simulations to investigate chemical ordering in this material. We show that the site occupancy factors of trivalent elements at the 6c site differ, depending on the synthesis method, which can be attributed to the level of equilibration. The flux-grown samples are consistent with the simulated high-temperature disordered configuration, while the degree of ordering for the Czochralski sample lies between the ground state and the high-temperature state. Moreover, we demonstrate that the atomic displacement parameters of the Ba atoms in the larger tetrakaidecahedral cages are related to chemical ordering. Specifically, Ba atoms are either displaced toward the periphery or localized at the cage centers. Consequently, this study reveals key relationships between the chemical ordering in the quaternary clathrates Ba8AlxGa16-xGe30 and the structural properties, thereby offering new perspectives on designing these materials and optimizing their thermoelectric properties