Synthesis of niobium-alumina composite aggregates and their application in coarse-grained refractory ceramic-metal castables

Niobium-alumina aggregate fractions with particle sizes up to 3150 µm were produced by crushing pre-synthesised fine-grained composites. Phase separation with niobium enrichment in the aggregate class 45–500 µm was revealed by XRD/Rietveld analysis. To reduce the amount of carbon-based impurities, no organic additives were used for the castable mixtures, which resulted in water demands of approximately 27 vol.% for the fine- and coarse-grained castables. As a consequence, open porosities of 18% and 30% were determined for the fine- and coarse-grained composites, respectively. Due to increased porosity, the modulus of rupture at room temperature decreased from 52 MPa for the fine-grained composite to 11 MPa for the coarse-grained one. However, even the compressive yield strength decreased from 49 MPa to 18 MPa at 1300 °C for the fine-grained to the coarse-grained composite, the latter showed still plasticity with a strain up to 5%. The electrical conductivity of fine-grained composite samples was in the range between 40 and 60 S/cm, which is fifteen magnitudes above the values of pure corundum

Disruption of the plant-specific CFS1 gene impairs autophagosome turnover and triggers EDS1-dependent cell death

Cell death, autophagy and endosomal sorting contribute to many physiological, developmental and immunological processes in plants. They are mechanistically interconnected and interdependent, but the molecular basis of their mutual regulation has only begun to emerge in plants. Here, we describe the identification and molecular characterization of CELL DEATH RELATED ENDOSOMAL FYVE/SYLF PROTEIN 1 (CFS1). The CFS1 protein interacts with the ENDOSOMAL SORTING COMPLEX REQUIRED FOR TRANSPORT I (ESCRT-I) component ELCH (ELC) and is localized at ESCRT-I-positive late endosomes likely through its PI3P and actin binding SH3YL1 Ysc84/Lsb4p Lsb3p plant FYVE (SYLF) domain. Mutant alleles of cfs1 exhibit auto-immune phenotypes including spontaneous lesions that show characteristics of hypersensitive response (HR). Autoimmunity in cfs1 is dependent on ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1)-mediated effector-triggered immunity (ETI) but independent from salicylic acid. Additionally, cfs1 mutants accumulate the autophagy markers ATG8 and NBR1 independently from EDS1. We hypothesize that CFS1 acts at the intersection of autophagosomes and endosomes and contributes to cellular homeostasis by mediating autophagosome turnover

Nonlinear dynamics of complex hysteretic systems: Oscillator in a magnetic field

Complex hysteresis is a well-known phenomenon in many branches of science. The most prominent examples come from materials with a complex microscopic structure such as magnetic materials, shape-memory alloys, or, porous materials. Their hysteretic behavior is characterized by the existence of multiple internal system states for a given external parameter and by a non-local memory. The input-output behavior of such systems is well studied and in a standard phenomenological approach described by the so-called Preisach operator. What is not well understood, are situations, where such a hysteretic system is dynamically coupled to its environment. Since the hysteretic sub-system provides a complicated form of nonlinearity, one expects non-trivial, possibly chaotic behavior of the combined dynamical system. We study such a combined dynamical system with hysteretic nonlinearity. In this original contribution a simple differential-operator equation with hysteretic damping, which describes a magnetic pendulum is considered. We find, for instance, a fractal dependence of the asymptotic behavior as function of the starting values. The sensitivity of the system to perturbations is investigated by several methods, such as the 0–1 test for chaos and sub-Lyapunov exponents. The power spectral density is also calculated and compared with analytical results for simple input-output scenarios

Comparison of quantum mechanical methods for the simulation of electronic transport through carbon nanotubes

In the present work we study the electronic transport properties of finite length single-wall carbon nanotubes (CNTs) by comparing three different theoretical frameworks. A simple model is used to describe the electrodes and the way they are attached to both ends of the CNT. Electron transport calculations are carried out on three different levels of sophistication. That are the Landauer transport formalism in combination with single-orbital tight-binding, extended Hückel theory or density functional theory. The quantum mechanical transmission which plays a central role in Landauer theory is calculated by means of equilibrium and non-equilibrium Green’s function methods. Results of the three approaches are compared and discussed

Metallic carbon nanotubes with metal contacts: electronic structure and transport

We study quasi-ballistic electron transport in metallic $(6,0)$ carbon nanotubes (CNTs) of variable length in contact with Al, Cu, Pd, Pt, Ag, and Au electrodes by using the non-equilibrium Greenʼs function formalism in combination with either density functional theory or self-consistent extended Hückel theory. We find good agreement between both. Visualizing the local device density of states of the systems gives a descriptive link between electronic structure and transport properties. In comparison with bare finite and infinite tubes, we show that the electronic structure of short metallic CNTs is strongly modified by the presence of the metallic electrodes, which leads to pronounced size effects in the conductance. The mean conductances and linear response currents allow a ranking of the metals regarding their ability to form low-Ohmic contacts with the nanotube: ${\rm Ag}\lesssim {\rm Au}\lt {\rm Cu}\ll {\rm Pt}\approx {\rm Pd}\ll {\rm Al}$. These findings are contrasted with similar trends in contact distance, binding energy, calculated work function of the metal surfaces, and various results from literature

Carbon nanotube based field-effect transistors: Comparison between atomistic quantum transport and numerical device simulation

We study carbon nanotube based field-effect transistors (CNTFETs) by means of two different approaches: numerical device simulation (NDS) based on the effective mass Schrödinger equation and atomistic quantum transport simulation based on the non-equilibrium Green’s function formalism (NEGF). The required parameters for the NDS model are extracted from density functional theory data. An all-carbon CNTFET with n-doped source- and drain-electrodes in a gate-all-around geometry is investigated. The NDS predicts a band-to-band tunnel current once the valence band edge is shifted to the Fermi energy. This increases the off-current and leads to slightly ambipolar behavior. Using the NEGF on the other hand, localized states inside the channel can be observed because a potential well is created by the gate. As a result, the band-to-band tunnel current is suppressed and improved transistor properties are predicted by NEGF calculations. By varying the channel length, we demonstrate the potential of the studied CNTFET for future applications, which shows an on/off current ratio above 106 and a subthreshold swing below 80 mV/dec down to channel lengths of about 8 nm

Computationally efficient simulation method for conductivity modeling of 2D-based conductors

Macroscopic materials made of two-dimensional components such as flakes of graphene or transition metal dichalcogenides represent a material class with great potential for large-scale applications. Depending on the structure, they can inherit the exceptional properties of the nanoscale building blocks while developing new features on the macroscopic scale. Supported by theoretical considerations and finite element analysis, we developed a network simulation method to model 2D-based electrical conductors. Here, we systematically explain the technical and methodological details of our approach, using the example of graphene-based conductor materials. Apart from the raw material properties, we discuss the importance of homogeneity and internal structure of the material. Our findings are supported by finite element analysis. We demonstrate the application of our method by studying the intricate interaction of several material parameters and the resulting effect on the macroscopic network. Finally, we provide guidelines for adapting our method to different physical situations

Strong localization in quasi one-dimensional systems with realistic defects: Application to carbon nanotubes

Investigating the influence of disorder on electron transport is difficult for very large systems. These systems cannot be treated by conventional quantum transport theory (QTT) and accurate electronic structure theory like density functional theory (DFT) within appropriate time. But quasi one-dimensional systems (nanotubes, nanowires) with short-range interaction can in general be treated very efficiently using a linearly scaling recursive Green’s function formalism on the level of QTT. Additionally, describing the electronic structure by a density-functional-based tight-binding (DFTB) approach gives TB-simplicity and DFT-accuracy.We apply this formalism to carbon nanotubes (CNTs), which are a possible material for future microelectronic devices, overcoming the miniaturization problem. We calculate the conductance of metallic armchair CNTs with realistic defects, namely monovacancies and divacancies, within a statistical analysis. The exponential dependence on the number of defects shows that the system is in the strong localization regime (i.e. Anderson localization). Consequently, localization lengths can be extracted. We present the conductance and the localization length in dependence on defect density, temperature, and CNT diameter for different defect types. Furthermore, the influence of mixtures of defects is addressed. Finally, we show that the single-defect conductance is a good measure of disorder