20,720 research outputs found
Magnetic monolayer LiN: Density Functional Theory Calculations
Density functional theory (DFT) calculations are used to investigate the
electronic and magnetic structures of a two-dimensional (2D) monolayer
LiN. It is shown that bulk LiN is a non-magnetic semiconductor. The
non-spinpolarized DFT calculations show that electrons of N in 2D LiN
form a narrow band at the Fermi energy due to a low coordination
number, and the density of states at the Fermi energy ()) is
increased as compared with bulk LiN. The large ) shows
instability towards magnetism in Stoner's mean field model. The spin-polarized
calculations reveal that 2D LiN is magnetic without intrinsic or impurity
defects. The magnetic moment of 1.0\, in 2D LiN is mainly
contributed by the electrons of N, and the band structure shows
half-metallic behavior. {Dynamic instability in planar LiN monolayer is
observed, but a buckled LiN monolayer is found to be dynamically stable.}
The ferromagnetic (FM) and antiferromagnetic (AFM) coupling between the N atoms
is also investigated to access the exchange field strength. {We found that
planar (buckled) 2D LiN is a ferromagnetic material with Curie
temperature of 161 (572) K.}Comment: Euro Phys. Lett. 2017 (Accepted
High yield fusion in a Staged Z-pinch
We simulate fusion in a Z-pinch; where the load is a xenon-plasma liner
imploding onto a deuterium-tritium plasma target and the driver is a 2 MJ, 17
MA, 95 ns risetime pulser. The implosion system is modeled using the dynamic,
2-1/2 D, radiation-MHD code, MACH2. During implosion a shock forms in the Xe
liner, transporting current and energy radially inward. After collision with
the DT, a secondary shock forms pre-heating the DT to several hundred eV.
Adiabatic compression leads subsequently to a fusion burn, as the target is
surrounded by a flux-compressed, intense, azimuthal-magnetic field. The
intense-magnetic field confines fusion -particles, providing an
additional source of ion heating that leads to target ignition. The target
remains stable up to the time of ignition. Predictions are for a neutron yield
of and a thermonuclear energy of 84 MJ, that is, 42 times
greater than the initial, capacitor-stored energy
Relationship between Electronic and Geometric Structures of the O/Cu(001) System
The electronic structure of the
O/Cu(001) system has been calculated using locally self-consistent, real space
multiple scattering technique based on first principles. Oxygen atoms are found
to perturb differentially the long-range Madelung potentials, and hence the
local electronic subbands at neighboring Cu sites. As a result the
hybridization of the oxygen electronic states with those of its neighbors leads
to bonding of varying ionic and covalent mix. Comparison of results with those
for the c(2x2) overlayer shows that the perturbation is much stronger and the
Coulomb lattice energy much higher for it than for the
phase. The main driving force for the
0.5ML oxygen surface structure formation on Cu(001) is thus the long-range
Coulomb interaction which also controls the charge transfer and chemical
binding in the system.Comment: 17 pages, 8 figure
Location of Partial Discharges within a Transformer Winding Using Principal Component Analysis
Partial discharge (PD) may occur in a transformer winding due to ageing processes or defects introduced during manufacture. A partial discharge is defined as a localised electric discharge that only partially bridges the dielectric insulator between conductors when the electric field exceeds a critical value. The presence of PD does not necessarily indicate imminent failure of the transformer but it is a serious degradation and ageing mechanism which can be considered as a precursor of transformer failure. PD might occur anywhere along the transformer winding and the discharge signal can propagate along the winding to the bushing and neutral to earth connections. As far as maintenance and replacement processes are concerned, it is important to identify the location of PD activity so any repair or replace decision is assured to be cost effective. Therefore, identification of a PD source as well as its location along the transformer winding is of great interest to both manufacturers and system operators. The wavelet transform is a mathematical function that can be used to decompose a PD signal into detail levels and an approximation. Wavelet filtering is often used to improve signal to noise ratio (SNR) of measured signals, but in this case it is used to identify the distribution of signal energies in both the time and frequency domains. This method produces a feature vector for each captured discharge signal. The use of principle component analysis (PCA) can compress this data into three dimensions, to aid visualisation. Data captured by sensors over hundreds of cycles of applied voltage can be analysed using this approach. An experiment (Figure 1) has been developed that can be used to create PD data in order to investigate the feasibility of using PCA analysis to identify PD source location
Partial Discharge Location within a Transformer Winding using Principal Component Analysis
Partial discharge (PD) may occur in a transformer winding due to ageing processes, operational over stressing or defects introduced during manufacture. The presence of PD does not necessarily indicate imminent failure of the transformer but it will lead to serious degradation and ageing mechanisms which can be considered as a precursor of transformer failure. A necessary step is required in order to prevent degradation due to PD activity which may ultimately lead to failure. PD might occur anywhere along the transformer winding, the discharge signal can propagate along the winding to the bushing and neutral to earth connections. As far as maintenance and replacement processes are concerned, it is important to identify the location of PD activity so any repair or replace decision is assured to be cost effective. Therefore, identification of a PD source as well as its location along the transformer winding is of great interest to both manufacturers and system operators. The proposed method for locating PD sources in windings is based on wavelet filtering and principal component analysis. An experiment has been developed based on a high voltage winding section that has been used to produce PD measurement data and to investigate the feasibility of the proposed approach
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