Self-organized structure in current-activated pressure-assisted densification (CAPAD)

Using nanostructured thermoelectrics led to a significant improvement of the figure of merit. An easily upscalable and cheap way of producing nanostructured bulk material is current-activated pressure-assisted densification (CAPAD). This process is quite complex and it has been shown that the powder morphology gives a feedback to the process itself [1,2]. We investigated the influence of particle properties of binary mixtures on the evolving structure. For this purpose we use a network model [3] based on the Onsager-de Groot-Callen theory. In the simulations we find that depending on the difference between the Seebeck coefficients of both materials either a well mixed or a segregated sintered structure is obtained

Explosive Ising

We study a two-dimensional kinetic Ising model with Swendsen-Wang dynamics, replacing the usual percolation on top of Ising clusters by explosive percolation. The model exhibits a reversible first-order phase transition with hysteresis. Surprisingly, at the transition flanks the global bond density seems to be equal to the percolation thresholds.Comment: 7 pages, 5 figure

Complex dynamics and performance of inhomogeneous thermoelectrics

This thesis aims to illuminate complex dynamics and performance of inhomogeneous thermoelectrics by means of a theoretical approach. For our investigations, we develop a versatile network model, which is based on the phenomenological On\-sager-de Groot-Callen theory. This model allows us to study three setups related to the fabrication of a new type of thermoelectric generator. The first setup is related to the production of the generator's raw material. Therefore, the current-activated pressure-assisted densification technique is applied to create nanostructured bulk material. The network model is designed to account for the complex dynamics caused by the particle motion during the densification. In particular, we investigate the influence of elongated pores parallel to the electrodes, which lower the conductivity. Moreover, we investigate a self-organized assembly of particles in binary particle mixtures. With the second setup, we scrutinize the application of the Harman method to inhomogeneous material. The Harman method is often used to determine the thermoelectric transport properties. It turns out that this method employed on inhomogeneous material, systematically overestimates the absolute value of the Seebeck coefficient. It is demonstrated that the error is caused by the temperature distribution, which memorizes the influence of the priorly applied electrical current. Related to this, we show that the electrical power of double segmented generators is usually less than expected from the electrical conductivity and the true open circuit Seebeck coefficient. Nonetheless, we prove that by choosing transport parameters from a small range, a power enhancement can be obtained. Finally, we investigate the usage of a pn junction as a generator, where the temperature gradient is parallel to the pn interface and electrodes are attached on the cold side. The dismissal of hot side contacts facilitates the application in high temperature regimes. In a first step, the diode character of the interface is neglected. We investigate the reduction of the electrical power compared to a conventional device. Thereby, we determine a relation between power and current. Furthermore, geometrical optimizations are discussed. In a second step, diode characteristics are included into the model, which leads to a qualitative agreement to experimental results

Sheared Ising models in three dimensions

The nonequilibrium phase transition in sheared three-dimensional Ising models is investigated using Monte Carlo simulations in two different geometries corresponding to different shear normals. We demonstrate that in the high shear limit both systems undergo a strongly anisotropic phase transition at exactly known critical temperatures T_c which depend on the direction of the shear normal. Using dimensional analysis, we determine the anisotropy exponent theta=2 as well as the correlation length exponents nu_parallel=1 and nu_perp=1/2. These results are verified by simulations, though considerable corrections to scaling are found. The correlation functions perpendicular to the shear direction can be calculated exactly and show Ornstein-Zernike behavior.Comment: 6 pages, 3 figure

From fibrous plant residues to mineral-associated organic carbon – the fate of organic matter in Arctic permafrost soils

Permafrost-affected soils of the Arctic account for 70 % or 727 Pg of the soil organic carbon (C) stored in the northern circumpolar permafrost region and therefore play a major role in the global C cycle. Most studies on the budgeting of C storage and the quality of soil organic matter (OM; SOM) in the northern circumpolar region focus on bulk soils. Thus, although there is a plethora of assumptions regarding differences in terms of C turnover or stability, little knowledge is available on the mechanisms stabilizing organic C in Arctic soils besides impaired decomposition due to low temperatures. To gain such knowledge, we investigated soils from Samoylov Island in the Lena River delta with respect to the composition and distribution of organic C among differently stabilized SOM fractions. The soils were fractionated according to density and particle size to obtain differently stabilized SOM fractions differing in chemical composition and thus bioavailability. To better understand the chemical alterations from plant-derived organic particles in these soils rich in fibrous plant residues to mineral-associated SOM, we analyzed the elemental, isotopic and chemical composition of particulate OM (POM) and clay-sized mineral-associated OM (MAOM). We demonstrate that the SOM fractions that contribute with about 17 kg C m$^{-3}$ for more than 60 % of the C stock are highly bioavailable and that most of this labile C can be assumed to be prone to mineralization under warming conditions. Thus, the amount of relatively stable, small occluded POM and clay-sized MAOM that currently accounts with about 10 kg C m$^{-3}$ for about 40 % of the C stock will most probably be crucial for the quantity of C protected from mineralization in these Arctic soils in a warmer future. Using δ$^{15}$N as a proxy for nitrogen (N) balances indicated an important role of N inputs by biological N fixation, while gaseous N losses appeared less important. However, this could change, as with about 0.4 kg N m$^{-3}$ one third of the N is present in bioavailable SOM fractions, which could lead to increases in mineral N cycling and associated N losses under global warming. Our results highlight the vulnerability of SOM in Arctic permafrost-affected soils under rising temperatures, potentially leading to unparalleled greenhouse gas emissions from these soils

Estimation and worldwide monitoring of the effective reproductive number of SARS-CoV-2

The effective reproductive number; R; e; is a key indicator of the growth of an epidemic. Since the start of the SARS-CoV-2 pandemic, many methods and online dashboards have sprung up to monitor this number through time. However, these methods are not always thoroughly tested, correctly placed in time, or are overly confident during high incidence periods. Here, we present a method for timely estimation of; R; e; , applied to COVID-19 epidemic data from 170 countries. We thoroughly evaluate the method on simulated data, and present an intuitive web interface for interactive data exploration. We show that, in early 2020, in the majority of countries the estimated; R; e; dropped below 1 only after the introduction of major non-pharmaceutical interventions. For Europe the implementation of non-pharmaceutical interventions was broadly associated with reductions in the estimated; R; e; . Globally though, relaxing non-pharmaceutical interventions had more varied effects on subsequent; R; e; estimates. Our framework is useful to inform governments and the general public on the status of epidemics in their country, and is used as the official source of; R; e; estimates for SARS-CoV-2 in Switzerland. It further allows detailed comparison between countries and in relation to covariates such as implemented public health policies, mobility, behaviour, or weather data

Laser-sintered thin films of doped SiGe nanoparticles

We present a study of the morphology and the thermoelectric properties of short-pulse laser-sintered (LS) nanoparticle (NP) thin films, consisting of SiGe alloy NPs or composites of Si and Ge NPs. Laser-sintering of spin-coated NP films in vacuum results in a macroporous percolating network with a typical thickness of 300 nm. The Seebeck coefficient is independent of the sintering process and typical for degenerate doping. The electrical conductivity of LS films rises with increasing temperature, best described by a power-law and influenced by two-dimensional percolation effects.Comment: 4 pages, 4 figure