The measured equation of invariance: a new concept in field computation

Computations of electromagnetic fields are based either on differential equations or on integral equations. The differential equation approach using finite difference or finite element methods results in sparse matrices, which is an advantage, but has to cover large volumes, which is a disadvantage. The integral equation approach using the method of moments (MOM) limits the mesh to the surface of the object, which is an advantage, but results in full matrices, which is a disadvantage. It is noted that the ideal case would be to reduce the finite difference type equations close to the object surface and still preserve the sparsity of the matrices. The measured equation of invariance is a new concept in field computation capable of approaching this ideal situation. The mathematics and reasonings to reach a novel computational method based on this concept are presented. It is shown that the method is robust for both convex and concave objects, is much faster than the MOM, and uses a fraction of the memory.Peer ReviewedPostprint (published version

Measured equation of invariance: a new concept in field computations

Numerical computations of frequency domain field problems or elliptical partial differential equations may be based on differential equations or integral equations. The new concept of field computation presented in this paper is based on the postulate of the existence of linear equations of the discretized nodal values of the fields, different from the conventional equations, but leading to the same solutions. The postulated equations are local and invariant to excitation. It is shown how the equations can be determined by a sequence ofPeer ReviewedPostprint (published version

Large-N expansion based on the Hubbard operator path integral representation and its application to the t-J model II. The case for finite JJ

We have introduced a new perturbative approach for $t-J-V$ model where Hubbard operators are treated as fundamental objects. Using our vertices and propagators we have developed a controllable large-N expansion to calculate different correlation functions. We have investigated charge density-density response and the phase diagram of the model. The charge correlations functions are not very sensitive to the value of $J$ and they show collective peaks (or zero sound) which are more pronounced when they are well separated (in energy) from the particle-hole continuum. For a given $J$ a Fermi liquid state is found to be stable for doping $\delta$ larger than a critical doping $\delta_c$. $\delta_c$ decreases with decreasing $J$. For the physical region of the parameters and, for $\delta< \delta_c$, the system enters in an incommensurate flux or DDW phase. The inclusion of the nearest-neighbors Coulomb repulsion $V$ leads to a CDW phase when $V$ is larger than a critical value $V_c$. The dependence of $V_c$ with $\delta$ and $J$ is shown. We have compared the results with other ones in the literature.Comment: 10 pages, 8 figures, to appear in Phys. Rev.

The Universe as a topological defect

Four-dimensional Einstein's General Relativity is shown to arise from a gauge theory for the conformal group, SO(4,2). The theory is constructed from a topological dimensional reduction of the six-dimensional Euler density integrated over a manifold with a four-dimensional topological defect. The resulting action is a four-dimensional theory defined by a gauged Wess-Zumino-Witten term. An ansatz is found which reduces the full set of field equations to those of Einstein's General Relativity. When the same ansatz is replaced in the action, the gauged WZW term reduces to the Einstein-Hilbert action. Furthermore, the unique coupling constant in the action can be shown to take integer values if the fields are allowed to be analytically continued to complex values.Comment: 18 pages, LaTex, 4 figures. Title of the published version changed to "Universe as a Topological defect" by the journa

Thermodynamic study of syngas combustion in gas microturbines with regeneration composed with metallic and ceramic materials

The objective of this research is to carry out an energetic evaluation of syngas combustion in gas microturbines with regenerators made with two different types of material: metallic or ceramic. A modified chemical equilibrium model was used to calculate syngas composition obtained from sugarcane bagasse gasification in a fluidized bed gasifier using steam as gasifying agent. For the gas microturbines with a regenerator of metallic materials, combustion temperatures of 850, 900, and 950 °C were established; and for those with a regenerator of ceramic materials, combustion temperatures of 1150, 1250, and 1350 °C were established. The range for the compression ratio was 2 to 6, and the excess air percentages employed were 400, 500, and 600%. Excellent results were obtained: maximum power ratio was 1.87 kWh/Nm3, maximum cycle efficiency was 57%, and maximum regenerator effectiveness was 100%. Thus, syngas combustion performance in the gas microturbines with regeneration was excellent

Numerical study of hydrogen influence on void growth at low triaxialities considering transient effects

Assuming that hydrogen enhances localised plasticity, as one of the leading mechanisms proposed in the literature, the void growth and coalescence are modified by local softening and ductile failure features depend on hydrogen accumulation. It is anticipated that strain rate plays an important role in hydrogen-informed void mechanisms, however, coupling voids, transient hydrogen diffusion, rate-dependent hydrogen-material interactions and intrinsic hardening, remains a challenge. In this study, the simulation of a void unit cell in a hydrogen pre-charged material is reconsidered here for the first time to incorporate transient effects, i.e. the kinetic redistribution of hydrogen around a void subjected to a high strain rate and a constant stress triaxiality. A coupled diffusion-mechanics scheme is implemented in a set of ABAQUS subroutines in order to analyse the interaction of hydrogen with the material response. The influence of strain rate is also considered when defining the cell boundary conditions through the limiting cases of equilibrium and insulated unit cells. The competition between the two inherent mechanisms, namely, hydrogen softening and strain rate hardening, is studied with the implemented framework. Results show that transient effects determine hydrogen concentrations and strongly dictate failure mechanisms: shearing might occur due to the hydrogen induced softening for moderate strain rates even though the cell is insulated. However, for very fast loading it is demonstrated that the fast creation of traps due to plastic deformation results in hydrogen depletion and necking failure is observed.MINECO Refs: MAT2014-58738-C3-2-R and RTI2018-096070-B-C3

Thermal stability study of AlGaN/GaN MOS-HEMTs using Gd2O3 as gate dielectric

Thermal stability of AlGaN/GaN MOS-HEMTs and -diodes using Gd_(2)O_(3) are investigated by means of different thermal cycles and storage tests up to 500ºC for one week. IV DC and pulsed characteristics of the devices before and after the processes are evaluated and compared with conventional HEMTs. Results show that the devices with Gd_(2)O_(3) dielectric layer have lower leakage current and a more stable behavior during thermal treatment processes compared with conventional devices. In fact, an excellent on/off ratio of about 108 and a stable V_(t) is observed after storage at high temperature. The beneficial effects of Gd_(2)O_(3) on trapping effects of MOS-HEMTs are also dis-cussed