Spin splitting and even-odd effects in carbon nanotubes

The level spectrum of a single-walled carbon nanotube rope, studied by transport spectroscopy, shows Zeeman splitting in a magnetic field parallel to the tube axis. The pattern of splittings implies that the spin of the ground state alternates by 1/2 as consecutive electrons are added. Other aspects of the Coulomb blockade characteristics, including the current-voltage traces and peak heights, also show corresponding even-odd effects.Comment: Preprint, pdf format only, 4 pages including figure

Scaling of Turbulent Mixed Convection under High Pressure

Measurement of turbulent mixed convection at reduced model size by aerodynamic scaling is a promising approach to simplify the investigation of many technical configurations and offers the potential to make large scale flows accessible on a laboratory scale. First results of an experimental study of turbulent mixed convection in a generic convection cell at ambient and high pressure are reported. The aim of these measurements is to prove the possibility of scaling mixed convection by varying fluid pressure and inflow velocity. We present and discuss results for mixed and forced convection obtained with air as working fluid (Pr � 0.7) at ambient pressure for Gr = 3.52 · 106, Re = 1.1 · 103 and thus Ar = 1.81 and at 10 bar with Gr = 3.51 · 108, Re = 1.1·104 and thus Ar = 1.83. The scaling theory, which allows to scale the cell, is presented as well as the PIV set up used for measurement at high pressure conditions in the High Pressure Wind Tunnel of Göttingen (HDG) and the convection cell. At elevated fluid pressure a significant increase of the velocity fluctuations was observed. Furthermore for mixed convection a transition of a stable 2D flow into an instationary 3D flow has been found

Towards a fullerene-based nanotechnology: The (10,10) tube

While fullerenes form in yields exceeding 30% by mass when a laser generated pure carbon vapor is allowed to condense under annealing conditions, more than 70% of all vaporized carbon assembles in the form of single-wall fullerene nanotubes (SWNTs) in the presence of certain transition metal SWNT catalysts. Due to the high carbon to metal ratio in the vapor will the early stages of condensation, even in the presence of a SWNT catalyst, be characterized by an abundance of all-carbon fullerene-precursors. Metal atoms will then condense onto these clusters as condensation proceeds, and SWNTs are nucleated from fullerene-precursors interacting with a metal vapor as an off-shoot of the road leading to C\sb{60}-fullerene. Electron nano-diffraction performed on crystalline strands of aligned SWNTs demonstrates that samples are dominated by tubes of "armchair" geometry, while X-ray diffraction shows a narrow diameter distribution, centered around 1.36 nm. In conclusion, the particular armchair tube matching the observed diameter, labeled the (10,10) tube, is the most prominent individual tube in the investigated samples. The astonishing efficiency with which armchair tubes form suggests that carbon clusters tend to anneal towards armchair geometries even in early stages of condensation, eventually rearranging into bowl-shaped fullerene-precursors with their open edge energy reduced by formation of triply-bonded pairs of 2-coordinated carbon atoms. In a pure carbon environment, such fullerene-precursors are predetermined to close into C\sb{60}-fullerene. In the presence of a SWNT catalyst however, metal atoms diffusing (scooting) along the growing edges of fullerene-precursors prevent their closure, forcing them to grow into tubelets with an open, growing armchair edge instead. The optimum diameter of such tubelets results from competition between strain energy in their cylinders (favoring tubelets of large diameters) and open edge energy (favoring tubelets of small diameters). At annealing temperatures, tubelets will initially widen their diameters as they add more carbon to reduce strain. Beyond a certain size however, clusters will finally have too many atoms to rearrange on the relevant time scale. At this time, their diameters get kinetically frozen due to shear size, and a diameter distribution centered around the (10,10) tube diameter is locked in place

Particle Image Velocimetry of Turbulent Mixed Convection at Ambient and High Pressure

Measurement of turbulent mixed convection at reduced model size by aerodynamic scaling is a promising approach to simplify the investigation of many technical configurations and offers the potential to make large scale flows accessible on a laboratory scale. On the pathway to measurement of turbulent mixed convection at high pressure by Particle Image Velocimetry forced, free and mixed convection have been investigated in a generic convection cell at ambient pressure. The scaling theory, which allows to down scale the cell, is presented, the PIV set up used for measurement at high pressure conditions in the high pressure wind tunnel in Göttingen is discussed, and the cell, which is capable of being operated at high pressure, is reviewed. Finally, first results of measurements at ambient pressure in a closed loop open test section wind tunnel at Re = 3500, Gr = 9 · 105, Pr = 0.7 and Ar = 0.4 are discussed

Scaling of turbulent mixed convection under high pressure

Measurement of turbulent mixed convection at reduced model size by aerodynamic scaling is a promising approach to simplify the investigation of many technical configurations and offers the potential to make large scale flows accessible on a laboratory scale. First results of an experimental study of turbulent mixed convection in a generic convection cell at ambient and high pressure are reported. The aim of these measurements is to prove the possibility of scaling mixed convection by varying pressure and inflow velocity. We present and discuss results for mixed and forced convection obtained with air as working fluid at ambient pressure with Gr = 3.52*106, Re = 1.1*103 and thus Ar = 1.81 and at 10 bar with Gr = 3.51*108, Re = 1.1*104 and thus Ar = 1.83. The scaling theory, which allows to scale the cell, is presented as well as the PIV set up used for measurement at high pressure conditions in the high pressure wind tunnel in Göttingen (HDG) and the cell

Specific orientation and two-dimensional crystallization of the proteasome at metal-chelating lipid interfaces

The potential of a protein-engineered His tag to immobilize macromolecules in a predictable orientation at metal-chelating lipid interfaces was investigated using recombinant 20 S proteasomes His-tagged in various positions. Electron micrographs demonstrated that the orientation of proteasomes bound to chelating lipid films could be controlled via the location of their His tags: proteasomes His-tagged at their sides displayed exclusively side-on views, while proteasomes His-tagged at their ends displayed exclusively end-on views. The activity of proteasomes immobilized at chelating lipid interfaces was well preserved. In solution, His-tagged proteasomes hydrolyzed casein at rates comparable with wild-type proteasomes, unless the His tags were located in the vicinity of the N termini of α-subunits. The N termini of α-subunits might partly occlude the entrance channel in α-rings through which substrates enter the proteasome for subsequent degradation. A combination of electron micrographs and atomic force microscope topographs revealed a propensity of vertically oriented proteasomes to crystallize in two dimensions on fluid lipid films. The oriented immobilization of His-tagged proteins at biocompatible lipid interfaces will assist structural studies as well as the investigation of biomolecular interaction via a wide variety of surface-sensitive techniques including single-molecule analysis

Thermochemical energy storage based on the reversible reaction of metal oxides

The application of thermal energy storage (TES) technologies enables dispatchable generation of solar thermal power and is crucial to improve the energy efficiency of industrial processes. Using the reaction enthalpy of reversible gas-solid reactions to store thermal energy represents a promising concept, allowing high energy densities, long term energy storage with minimal losses and a facile separation of the reactants. Specifically for the high-temperature range between 500°C up to 1100°C, multivalent metal oxides constitute promising storage materials for thermal energy storage. Those materials offer process technological advantages compared to other thermochemical storage materials, as ambient air can be used as sink and source of the reactant oxygen during the reduction and oxidation (REDOX) reactions. Accordingly, oxygen does not need to be stored in this open loop process. A lab-scale storage test rig with a packed bed storage reactor was set up to study the reversible reaction of metal oxides for thermochemical energy storage. This paper reports on the thermal reduction (storage charging) and oxidation reaction (storage discharging) of manganese oxide in a packed bed storage reactor. The concept uses air as a heat transfer fluid which is in direct contact with the solid storage material. Reaction progress and conversion of the manganese oxide redox reactions was determined by means of the measured oxygen concentration at the outlet of the reactor and temperature sensors within the bed, as well as gas inlet and outlet temperatures. We discuss in this paper the completeness of the conversion and duration of the reaction. Also, the thermal characteristics of the oxidation and reduction process are compared. The feasibility of the storage concept to store thermal energy in a metal oxide storage reactor using air as the heat transfer fluid and carrier of the reactant oxygen is demonstrated