286 research outputs found
The simulation of ionospheric conditions for space vehicles
Plasma wind tunnel to simulate ionospheric conditions for space vehicle
Modeling and Analysis of Power Processing Systems
The feasibility of formulating a methodology for the modeling and analysis of aerospace electrical power processing systems is investigated. It is shown that a digital computer may be used in an interactive mode for the design, modeling, analysis, and comparison of power processing systems
Hydrogen plasma defined graphene edges
In this thesis, anisotropic etching of graphite and graphene in a hydrogen (H) plasma is investigated. The exposure of graphite flakes at different plasma pressures and sample-plasma distances reveals the existence of two different plasma regimes: the \textit{direct} and the \textit{remote} regime. In the direct regime, high energetic H-ions continuously induce new defects into the graphite surface during the etching process, thus leading to a perforated surface. In the remote plasma regime, on the other hand, well-defined hexagonal etch pits evolve, which grow in size, while their number remains constant. This indicates anisotropic etching, which takes place only at pre-existing defects and edges and leaves the graphite basal plane pristine.
In a second step of the experiment, the substrate dependence of single layer graphene etching in the remote plasma regime is investigated. Interestingly, the etching is only anisotropic for hexagonal boron nitride substrates but isotropic if graphene is placed on Si/SiO. It was previously found that the edges of H plasma defined hexagons on graphite run along the zigzag (ZZ) direction of the crystal lattice. Hence, by inducing artificial defects into a graphene flake, one can tailor diverse graphene nano-structures with presumably well-defined ZZ edges, such as e.g. graphene nano ribbons.
However, it is not exactly known how good the quality of as-fabricated graphene edges really is. This open question is addressed in the second work, where the quality of H plasma defined graphene edges is investigated by means of atomic resolution atomic force microscopy (AFM), Raman spectroscopy and low-temperature electronic transport experiments. AFM measurements on hexagons created on graphite surfaces reveal that the edges are aligned to the ZZ direction and the absence of the Raman D-peak suggests that these edges are high quality ZZ edges. In contrast, hexagons created in single layer graphene on hexagonal boron nitride exhibit a relatively large D-peak, pointing towards the presence of edge disorder or armchair segments. Polarization-dependent Raman experiments indicate that the edges consist of a mixture of armchair and ZZ segments. Furthermore, electronic transport measurements, combined with quantum transport simulations, support the findings from the Raman experiments. Hence, H plasma defined edges still suffer from edge disorder and the etching process needs to be further optimized in order to get high quality crystallographic graphene edges.
In addition to the graphene experiments, investigations on Ge/Si core/shell nano wires are conducted. In particular, single, double, and triple quantum dots (QDs) of various sizes and with low occupation numbers are formed. In the single QD regime, indications for the last hole state are found. Moreover, Pauli spin blockade is observed in the double QD regime. These results open the door for exploring Ge/Si core/shell nano wires as a potential platform for hole spin-qubit experiments
Anisotropic Etching of Graphite and Graphene in a Remote Hydrogen Plasma
We investigate the etching of a pure hydrogen plasma on graphite samples and
graphene flakes on SiO and hexagonal Boron-Nitride (hBN) substrates. The
pressure and distance dependence of the graphite exposure experiments reveals
the existence of two distinct plasma regimes: the direct and the remote plasma
regime. Graphite surfaces exposed directly to the hydrogen plasma exhibit
numerous etch pits of various size and depth, indicating continuous defect
creation throughout the etching process. In contrast, anisotropic etching
forming regular and symmetric hexagons starting only from preexisting defects
and edges is seen in the remote plasma regime, where the sample is located
downstream, outside of the glowing plasma. This regime is possible in a narrow
window of parameters where essentially all ions have already recombined, yet a
flux of H-radicals performing anisotropic etching is still present. At the
required process pressures, the radicals can recombine only on surfaces, not in
the gas itself. Thus, the tube material needs to exhibit a sufficiently low H
radical recombination coefficient, such a found for quartz or pyrex. In the
remote regime, we investigate the etching of single layer and bilayer graphene
on SiO and hBN substrates. We find isotropic etching for single layer
graphene on SiO, whereas we observe highly anisotropic etching for graphene
on a hBN substrate. For bilayer graphene, anisotropic etching is observed on
both substrates. Finally, we demonstrate the use of artificial defects to
create well defined graphene nanostructures with clean crystallographic edges.Comment: 7 pages, 4 color figure
Identification of a Novel, Small Molecule Partial Agonist for the Cyclic AMP Sensor, EPAC1
Screening of a carefully selected library of 5,195 small molecules identified 34 hit compounds that interact with the regulatory cyclic nucleotide-binding domain (CNB) of the cAMP sensor, EPAC1. Two of these hits (I942 and I178) were selected for their robust and reproducible inhibitory effects within the primary screening assay. Follow-up characterisation by ligand observed nuclear magnetic resonance (NMR) revealed direct interaction of I942 and I178 with EPAC1 and EPAC2-CNBs in vitro. Moreover, in vitro guanine nucleotide exchange factor (GEF) assays revealed that I942 and, to a lesser extent, I178 had partial agonist properties towards EPAC1, leading to activation of EPAC1, in the absence of cAMP, and inhibition of GEF activity in the presence of cAMP. In contrast, there was very little agonist action of I942 towards EPAC2 or protein kinase A (PKA). To our knowledge, this is the first observation of non-cyclic-nucleotide small molecules with agonist properties towards EPAC1. Furthermore, the isoform selective agonist nature of these compounds highlights the potential for the development of small molecule tools that selectively up-regulate EPAC1 activity
ABE FERMENTATION OF SUGAR IN BRAZIL
A fermentation plant was designed to ferment and process sugar cane juice into acetone, butanol, and ethanol (ABE) in Brazil. The plant was built to handle a feed of 40 tonnes of sugar per hour in 25% solution. The process runs continuously for 32 weeks out of the year, during the cane harvest, for 20 years.
The two main steps of the process are the fermentation and the separation of the ABE products into the desired 99.5% product purities. The fermentation section of the plant consists of nine 500,000 gallon fermenters that convert the bulk of the sugar cane into ABE products, as well as two 500,000 gallon fermenters that supply fresh cells to these fermenters and a series of smaller tanks that scale up cell concentrations from a test tube scale to the fermenter sizes used in this project. The separation section of the plant consists of a holding tank to store the ABE products, a gas stripper to remove most of the organics from water, a decanter to further separate the products into a butanol-rich phase and a water-rich phase, molecular sieves to remove the rest of the water from the butanol-rich phase, and two distillation columns to purify the products and prepare them for sale.
This design can be deemed a successful one with a 35.67% return on investment and a net present value $118,806,000. Also, the process as a whole was found to be significantly energy positive, with our combustible products having a fuel value of 3.36x108 BTU/hr and our utility inputs being only 2.14x106 BTU/hr. The main reason for our success on these two fronts was the use of a gas stripper and a molecular sieve, which allowed for most of the water in the separation step to be removed without needing to heat it
Characterization of Hydrogen Plasma Defined Graphene Edges
We investigate the quality of hydrogen plasma defined graphene edges by Raman
spectroscopy, atomic resolution AFM and low temperature electronic transport
measurements. The exposure of graphite samples to a remote hydrogen plasma
leads to the formation of hexagonal shaped etch pits, reflecting the anisotropy
of the etch. Atomic resolution AFM reveals that the sides of these hexagons are
oriented along the zigzag direction of the graphite crystal lattice and the
absence of the D-peak in the Raman spectrum indicates that the edges are high
quality zigzag edges. In a second step of the experiment, we investigate
hexagon edges created in single layer graphene on hexagonal boron nitride and
find a substantial D-peak intensity. Polarization dependent Raman measurements
reveal that hydrogen plasma defined edges consist of a mixture of zigzag and
armchair segments. Furthermore, electronic transport measurements were
performed on hydrogen plasma defined graphene nanoribbons which indicate a high
quality of the bulk but a relatively low edge quality, in agreement with the
Raman data. These findings are supported by tight-binding transport
simulations. Hence, further optimization of the hydrogen plasma etching
technique is required to obtain pure crystalline graphene edges.Comment: 10 pages, 7 figure
Dynamics and Ethics of Comprehensive Preimplantation Genetic Testing. A Review of the Challenges
BACKGROUND: Genetic testing of preimplantation embryos has been used for preimplantation genetic diagnosis (PGD) and preimplantation genetic screening (PGS). Microarray technology is being introduced in both these contexts, and is to be expected that also whole genome sequencing of blastomeres will become possible. The amount of extra information such tests will yield may prove to be beneficial for embryo selection, but also raise various ethical issues. We present an overview of the developments and an agenda-setting exploration of the ethical issues. METHODS: The paper is a joint endeavour by the presenters at an explorative 'campus meeting' organized by the European Society of Human Reproduction and Embryology in cooperation with the department of Health, Ethics & Society of the Maastricht University (The Netherlands). RESULTS: The increasing amount and detail of information that new screening techniques such as microarrays and whole genome sequencing offer does not automatically coincide with an increasing understanding of the prospects of an embryo. From a technical point of view, the future of comprehensive embryo testing may go together with developments in preconception carrier screening. From an ethical point of view, the increasing complexity and amount of information yielded by comprehensive testing techniques will lead to challenges to the principle of reproductive autonomy and the right of the child to an open future, and may imply a possible larger responsibility of the clinician towards the welfare of the future child. “Smart combinations” of preconception carrier testing and embryo testing may solve some of these ethical questions but could introduce others. CONCLUSION: As comprehensive testing techniques are entering the IVF clinic, there is a need for a thorough rethinking of traditional ethical paradigms regarding medically assisted reproduction.This article was written by Dr Ainsley Newson during the time of her employment with the University of Bristol, UK (2006-2012). Self-archived in the Sydney eScholarship Repository with permission of Bristol University, Sept 2014
Out-of-plane corrugations in graphene based van der Waals heterostructures
Two dimensional materials are usually envisioned as flat, truly 2D layers.
However out-of-plane corrugations are inevitably present in these materials. In
this manuscript, we show that graphene flakes encapsulated between insulating
crystals (hBN, WSe2), although having large mobilities, surprisingly contain
out-of-plane corrugations. The height fluctuations of these corrugations are
revealed using weak localization measurements in the presence of a static
in-plane magnetic field. Due to the random out-of-plane corrugations, the
in-plane magnetic field results in a random out-of-plane component to the local
graphene plane, which leads to a substantial decrease of the phase coherence
time. Atomic force microscope measurements also confirm a long range height
modulation present in these crystals. Our results suggest that phase coherent
transport experiments relying on purely in-plane magnetic fields in van der
Waals heterostructures have to be taken with serious care
Energy- and flux-budget (EFB) turbulence closure model for the stably stratified flows. Part I: Steady-state, homogeneous regimes
We propose a new turbulence closure model based on the budget equations for
the key second moments: turbulent kinetic and potential energies: TKE and TPE
(comprising the turbulent total energy: TTE = TKE + TPE) and vertical turbulent
fluxes of momentum and buoyancy (proportional to potential temperature).
Besides the concept of TTE, we take into account the non-gradient correction to
the traditional buoyancy flux formulation. The proposed model grants the
existence of turbulence at any gradient Richardson number, Ri. Instead of its
critical value separating - as usually assumed - the turbulent and the laminar
regimes, it reveals a transition interval, 0.1< Ri <1, which separates two
regimes of essentially different nature but both turbulent: strong turbulence
at Ri<<1; and weak turbulence, capable of transporting momentum but much less
efficient in transporting heat, at Ri>1. Predictions from this model are
consistent with available data from atmospheric and lab experiments, direct
numerical simulation (DNS) and large-eddy simulation (LES).Comment: 40 pages, 6 figures, Boundary-layer Meteorology, resubmitted, revised
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