Compact Direction Research Articles

Biomimetic Antenna Array Using Non-Foster Network to Enhance Directional Sensitivity Over Broad Frequency Band

Biologically inspired antenna arrays that mimic the hearing mechanism of insects are called biomimetic antenna arrays (BMAAs). They are attractive for microwave applications, such as compact direction finding systems. Earlier, the BMAAs were designed for narrow frequency band phase enhancement, whereas we now propose to design them for use with a non-Foster coupling network (NFC). As the NFCs are not restricted by gain-bandwidth product, their incorporation in the design can provide wideband phase enhancement. A method for designing BMAA, using a non-Foster coupling network (NFC-BMAA), and also for obtaining system stability, is presented. Simulated and measured results of the fabricated structure are also presented and discussed.

Compact Broadband Directional Couplers Using Subwavelength Gratings

We experimentally demonstrate compact broadband directional couplers using subwavelength gratings for silicon-on-insulator wafers with silicon layers of 220 nm. The dispersion properties of the optical modes are engineered using subwavelength gratings, which allow broadband operation. Finite-difference time-domain (FDTD)-based band structure calculations, with significantly reduced simulation time, were used to analyze the design, which included both the structure and material dispersions. Compact broadband direction couplers, with device lengths shorter than 14 $\mu\text{m}$ , which cover a bandwidth of 100 nm for power splitting ratios of 50/50, 40/60, 30/70, and 20/80, are designed and fabricated for the fundamental transverse electric mode with a central operating wavelength of 1550 nm.

Lead free Bi3TaTiO9 ferroelectric ceramics with high Curie point

Single phase lead free Bi3TaTiO9 ceramics with grain-oriented microstructure were prepared by pressure-assisted spark plasma sintering. The dielectric, piezoelectric and ferroelectric properties of the sintered ceramics are found to be anisotropic owing to the preferred orientation of structure. The dielectric permittivity, piezoelectric constant and ferroelectric remnant polarization measured perpendicularly from the direction of the pressure applied during sintering are higher than those along the direction of compaction due to the switching of the spontaneous polarization within the a-b plane, which is orthogonal to the thickness of the grains. Since the textured Bi3TaTiO9 ceramics show a high Curie point (Tc=850°C), these materials can be considered as potential candidates for high temperature piezoelectric applications.

Instanton-monopole correspondence from M-branes onS1and little string theory

We study BPS excitations in M5-M2-brane configurations with a compact transverse direction, which are also relevant for type IIa and IIb little string theories. These configurations are dual to a class of toric elliptically fibered Calabi-Yau manifolds $X_N$ with manifest $SL(2, \mathbb{Z}) \times SL(2,\mathbb{Z})$ modular symmetry. They admit two dual gauge theory descriptions. For both, the non-perturbative partition function can be written as an expansion of the topological string partition function of $X_N$ with respect to either of the two modular parameters. We analyze the resulting BPS counting functions in detail and find that they can be fully constructed as linear combinations of the BPS counting functions of M5-M2-brane configurations with non-compact transverse directions. For certain M2-brane configurations, we also find that the free energies in the two dual theories agree with each other, which points to a new correspondence between instanton and monopole configurations. These results are also a manifestation of T-duality between type IIa and IIb little string theories.

Anisotropy of bituminous mixture in the linear viscoelastic domain

Some anisotropic properties in the linear viscoelastic domain of bituminous mixtures compacted with a French LPC wheel compactor are highlighted in this paper. Bituminous mixture is generally considered as isotropic even if the compaction process on road or in laboratory induces anisotropic properties. Tension–compression complex modulus tests have been performed on parallelepipedic specimens in two directions: (i) direction of compactor wheel movement (direction I, which is horizontal) and (ii) direction of compaction (direction II, which is vertical). These tests consist in measuring sinusoidal axial and lateral strains as well as sinusoidal axial stress, when sinusoidal axial loading is applied on the specimen. Different loading frequencies and temperatures are applied. Two complex moduli, $E_{\mathrm{I}} ^{*}$ and $E_{\mathrm{II}}^{*}$ , and four complex Poisson’s ratios, $\nu_{\text{II-I}}^{*}$ , $\nu_{\text{III-I}}^{*}$ , $\nu_{\text{I-II}}^{*}$ and $\nu_{\text{III-II}}^{*}$ , were obtained. The vertical direction appears softer than the other ones for the highest frequencies. There are very few differences between the two directions I and II for parameters concerning viscous effects (phase angles $\varphi(E_{\mathrm{I}})$ and $\varphi(E_{\mathrm{II}})$ , and shift factors). The four Poisson’s ratios reveal anisotropic properties but rheological tensor can be considered as symmetric when considering very similar values obtained for the two measured parameters (I-II and II-I) In addition, an anisotropic 3 dimensional version of the “2S2P1D” (2 springs, 2 parabolic creep elements and 1 dashpot) model, developed at the University of Lyon—ENTPE laboratory, is presented and used to simulate experimental results. The model simulation provides a good fit to the data. Stability of the material could also be investigated on the whole frequency–temperature range.

Evaluating quantitative 3-D image analysis as a design tool for low enriched uranium fuel compacts for the transient reactor test facility: A preliminary study

3-D image analysis when combined with a non-destructive examination technique such as X-ray computed tomography (CT) provides a highly quantitative tool for the investigation of a material's structure. In this investigation 3-D image analysis and X-ray CT were combined to analyze the microstructure of a preliminary sub-sized fuel compact for the Transient Reactor Test Facility's low enriched uranium conversion program to assess the feasibility of the combined techniques for use in the optimization of the fuel block fabrication process.The quantitative image analysis focused on determining the size and spatial distribution of the surrogate fuel particles and the size, shape, and orientation of voids within the compact. Additionally, the maximum effect of microstructural features on heat transfer through the carbonaceous matrix of the preliminary compact was estimated.The surrogate fuel particles occupied 0.8% of the compact by volume with a log-normal distribution of particle sizes with a mean diameter of 39μm and a standard deviation of 16μm. Roughly 39% of the particles had a diameter greater than the specified maximum particle size of 44μm suggesting that the particles agglomerate during fabrication. The local volume fraction of particles also varies significantly within the compact, although uniformities appear to be evenly dispersed throughout the analysed volume. The voids produced during fabrication were on average plate-like in nature with their major axis oriented perpendicular to the compaction direction of the compact. Finally, the microstructure, mainly the large preferentially oriented voids, may cause a small degree of anisotropy in the thermal diffusivity within the compact. α∥/α⊥, the ratio of thermal diffusivities parallel to and perpendicular to the compaction direction are expected to be no less than 0.95 with an upper bound of 1.

Aspects of the moduli space of instantons onCP2and its orbifolds

We study the moduli space of self-dual instantons on $\mathbb{C}P^2$. These are described by an ADHM-like construction which allows to compute the Hilbert series of the moduli space. The latter has been found to be blind to certain compact directions. In this paper we probe these, finding them to correspond to a Grassmanian, upon considering appropriate ungaugings. Moreover, the ADHM-like construction can be embedded into a $3d$ gauge theory with a known gravity dual. Using this, we realize in $AdS_4/CFT_3$ (part of) the instanton moduli space providing at the same time further evidence supporting the $AdS_4/CFT_3$ duality. Moreover, upon orbifolding, we provide the ADHM-like construction of instantons on $\mathbb{C}P^2/\mathbb{Z}_n$ as well as compute its Hilbert series. As in the unorbifolded case, these turn out to coincide with those for instantons on $\mathbb{C}^2/\mathbb{Z}_n$.

Design of Compact and High Directive Slot Antennas Using Grounded Metamaterial Slab

In this letter, a simple and compact approach for directivity enhancement of slot antenna using a grounded metamaterial slab is proposed. This approach has been applied in two different configurations. In the first configuration, two dielectric layers are used for designing antenna and metamaterial slab. Despite the directivity enhancement, an improved bandwidth of about 10.74% is obtained for this configuration. It is also shown that, over the whole working band, a significant broadside directivity improvement is maintained compared to the unloaded slot. In the second configuration, only a single layer of dielectric is used for designing both antenna and metamaterial slab. Thus, in this case, the total thickness of the loaded antenna is noticeably reduced to 0.5 mm, or 0.014 λ0 at resonance.

Lattice study on QCD-like theory with exact center symmetry

We investigate QCD-like theory with exact center symmetry, with emphasis on the finite-temperature phase transition concerning center and chiral symmetries. On the lattice, we formulate center symmetric $SU(3)$ gauge theory with three fundamental Wilson quarks by twisting quark boundary conditions in a compact direction ($Z_3$-QCD model). We calculate the expectation value of Polyakov loop and the chiral condensate as a function of temperature on 16^3 x 4 and 20^3 x 4 lattices along the line of constant physics realizing $m_{PS}/m_{V}=0.70$. We find out the first-order center phase transition, where the hysteresis of the magnitude of Polyakov loop exists depending on thermalization processes. We show that chiral condensate decreases around the critical temperature in a similar way to that of the standard three-flavor QCD, as it has the hysteresis in the same range as that of Polyakov loop. We also show that the flavor symmetry breaking due to the twisted boundary condition gets qualitatively manifest in the high-temperature phase. These results are consistent with the predictions based on the chiral effective model in the literature. Our approach could provide novel insights to the nonperturbative connection between the center and chiral properties.

The thermal scalar and random walks in curved spacetime

The curved space generalization of near‐Hagedorn string thermodynamics is discussed in terms of the thermal scalar mode. This mode singly winds the compact time direction and represents the most dominating state near the Hagedorn temperature. We discuss the random walk picture in a curved background and its relation to the thermal scalar. Then we apply this formalism to the thermal gas near black hole horizons, where a long random walking string is found at string length from the black hole horizon, in agreement with Susskind's picture of the stretched membrane.

The Anisotropy of Dimensional Change on Sintering of Iron

The anisotropy of dimensional change on the sintering of iron was investigated by dilatometry. Dimensional changes are different along the longitudinal and transversal directions, and shrinkage is more pronounced parallel to the compaction direction. This phenomenon is particularly pronounced during the early stage of sintering, in the alpha field below the Curie temperature. The results were elaborated according to the kinetics model for shrinkage to calculate an effective diffusion coefficient along the two directions. Such an effective diffusion coefficient is higher parallel to the compaction direction than perpendicular to it, and both are larger than the diffusion coefficient calculated on the basis of the activation energy reported in the literature for pure iron. This discrepancy is attributed to the defectiveness introduced by cold compaction in the particle contact regions, which may enhance diffusivity owing to the dislocation pipe mechanism, which, in turn, is particularly intense below the Curie temperature. This interpretation may also justify anisotropy of shrinkage because the powder particles are inhomogeneously deformed by uniaxial cold compaction. The enhanced diffusion coefficient increases with time and shows a maximum at temperatures below the Curie point. This trend was discussed with reference to the self‐activated sintering mechanism.

Smeared antibranes polarise in AdS

AbstractIn the recent literature it has been questioned whether the local backreaction of antibranes in flux throats can induce a perturbative brane-flux decay. Most evidence for this can be gathered for D6 branes and Dp branes smeared over 6 − p compact directions, in line with the absence of finite temperature solutions for these cases. The solutions in the literature have flat worldvolume geometries and non-compact transversal spaces. In this paper we consider what happens when the worldvolume is AdS and the transversal space is compact. We show that in these circumstances brane polarisation smoothens out the flux singularity, which is an indication that brane-flux decay is prevented. This is consistent with the fact that the cosmological constant would be less negative after brane-flux decay. Our results extend recent results on AdS7 solutions from D6 branes to AdSp+1 solutions from Dp branes. We show that supersymmetry of the AdS solutions depend on p non-trivially.

Moduli dynamics as a predictive tool for thermal maximally supersymmetric Yang-Mills at large N

Maximally supersymmetric (p+1)-dimensional Yang-Mills theory at large N and finite temperature, with possibly compact spatial directions, has a rich phase structure. Strongly coupled phases may have holographic descriptions as black branes in various string duality frames, or there may be no gravity dual. In this paper we provide tools in the gauge theory which give a simple and unified picture of the various strongly coupled phases, and transitions between them. Building on our previous work we consider the effective theory describing the moduli of the gauge theory, which can be computed precisely when it is weakly coupled far out on the Coulomb branch. Whilst for perturbation theory naive extrapolation from weak coupling to strong gives little information, for this moduli theory naive extrapolation from its weakly to its strongly coupled regime appears to encode a surprising amount of information about the various strongly coupled phases. We argue it encodes not only the parametric form of thermodynamic quantities for these strongly coupled phases, but also certain transcendental factors with a geometric origin, and allows one to deduce transitions between the phases. We emphasise it also gives predictions for the behaviour of a large class of local operators in these phases.

Cosmology on a cosmic ring

We derive the modified Friedmann equations for a generalization of the Dvali-Gabadadze-Porrati (DGP) model in which the brane has one additional compact dimension. The main new feature is the emission of gravitational waves into the bulk. We study two classes of solutions: first, if the compact dimension is stabilized, the waves vanish and one exactly recovers DGP cosmology. However, a stabilization by means of physical matter is not possible for a tension-dominated brane, thus implying a late time modification of 4D cosmology different from DGP. Second, for a freely expanding compact direction, we find exact attractor solutions with zero 4D Hubble parameter despite the presence of a 4D cosmological constant. The model hence constitutes an explicit example of dynamical degravitation at the full nonlinear level. Without stabilization, however, there is no 4D regime and the model is ruled out observationally, as we demonstrate explicitly by comparing to supernova data.

Large bandwidth mode order converter by differential waveguides.

In this article, we propose a large bandwidth mode-order converter design by dielectric waveguides with equal lengths but different cross-sectional areas. The efficient conversion between even and odd modes is verified by inducing required phase difference between the equal length waveguides of different widths. Y-junctions are composed of both tapered mode splitter and combiner to connect mono-mode waveguide to multi-mode waveguide. The converted mode profiles at the output port show that the device operates successfully at designed wavelengths with wide bandwidth. This study provides a novel technique to implement compact mode order converters and direction selective/sensitive photonic structures.

Confinement on R3 × S1: Continuum and lattice

There has been substantial progress in understanding confinement in a class of four-dimensional SU(N) gauge theories using semiclassical methods. These models have one or more compact directions, and much of the analysis is based on the physics of finite temperature gauge theories. The topology R3 × S1 has been most often studied using a small compactification circumference L such that the running coupling g2(L) is small. The gauge action is modified by a double-trace Polyakov loop deformation term, or by the addition of periodic adjoint fermions. The additional terms act to preserve Z(N) symmetry and thus confinement. An area law for Wilson loops is induced by a monopole condensate. In the continuum, the string tension can be computed analytically from topological effects. Lattice models display similar behavior, but the theoretical analysis of topological effects is based on Abelian lattice duality rather than on semiclassical arguments. In both cases, the key step is reducing the low-energy symmetry group from SU(N) to the maximal Abelian subgroup U(1)N-1 while maintaining Z(N) symmetry.

Elastomeric behavior of exfoliated graphite, as shown by instrumented indentation testing

This paper reports for the first time the elastomeric behavior of a non-polymeric material, as observed in exfoliated graphite compacts (⩽12vol.% solid, preferably 4vol.% solid) and enabled by the high-amplitude reversible and easy sliding of the graphite layers within the cell wall of exfoliated graphite. The reversibility is probably due to the cellular structure of exfoliated graphite. The elastomeric character is independent of the maximum load and essentially unchanged upon reloading. The total and reversible engineering shear strains of the cell wall (∼60 graphite layers, ∼20nm thick) are up to 40 and 35 respectively during instrumented indentation (in the compaction direction) without fracture of the layers, compared to corresponding values of 12 and 8 for flexible graphite (38vol.% solid). The fraction of displacement that is irreversible is as low as 12%, compared to 29% for flexible graphite. The modulus is as low as 83kPa, compared to 790kPa for flexible graphite. The greater the degree of compaction, the lower are the shear strain and reversibility, and the higher are the modulus and the displacement load. The ease of interlayer sliding is inadequate in flexible graphite or highly-oriented pyrolytic graphite (prior work) for substantial elastomeric behavior.

Welding of pyroclastic conduit infill: A mechanism for cyclical explosive eruptions

AbstractVulcanian‐style eruptions are small‐ to moderate‐sized, singular to cyclical events commonly having volcanic explosivity indices of 1–3. They produce pyroclastic flows, disperse tephra over considerable areas, and can occur as precursors to larger (e.g., Plinian) eruptions. The fallout deposits of the 2360 B.P. eruption of Mount Meager, BC, Canada, contain bread‐crusted blocks of welded breccia as accessory lithics. They display a range of compaction/welding intensity and provide a remarkable opportunity to constrain the nature and timescales of mechanical processes operating within explosive volcanic conduits during repose periods between eruptive cycles. We address the deformation and porosity/permeability reduction within natural pyroclastic deposits infilling volcanic conduits. We measure the porosity, permeability, and ultrasonic wave velocities for a suite of samples and quantify the strain recorded by pumice clasts. We explore the correlations between the physical properties and deformation fabric. Based on these correlations, we reconstruct the deformation history within the conduit, model the permeability reduction timescales, and outline the implications for the repressurization of the volcanic conduit. Our results highlight a profound directionality in the measured physical properties of these samples related to the deformation‐induced fabric. Gas permeability varies drastically with increasing strain and decreasing porosity along the compaction direction of the fabric but varies little along the elongation direction of the fabric. The deformation fabric records a combination of compaction within the conduit and postcompaction stretching associated with subsequent eruption. Model timescales of these processes are in good agreement with repose periods of cyclic vulcanian eruptions.

Influence of compaction direction on performance characteristics of roller-compacted HMA specimens

Hot mix asphalt (HMA) slabs produced by roller compaction can be used to core and cut specimens for further testing. The relation between the direction of compaction and testing in the laboratory is not always the same relation as it is between the direction of compaction and actual loading in the field. This paper presents outcomes of a study analysing the influence of the compaction direction on performance characteristics of roller-compacted HMA specimens. Performance parameters of a base layer mix are obtained from performance-based test methods, including high-temperature, stiffness, fatigue and low-temperature tests. The relation between direction of compaction and specimen testing is varied in all three dimensions to find relevant influences. From the results, it can be concluded that all obtained performance parameters are sensitive to the anisotropy of the material due to compaction, especially for stiffness and fatigue performance. For the high-temperature performance, specimens from path- and force-controlled compaction were compared. The applied compaction work rather than the compaction method is linked to the difference in the corresponding results. The uniformity of the compaction in terms of the variation of bulk density of the specimens reflects on the scattering of test results.

Thermal and electrical conduction in the compaction direction of exfoliated graphite and their relation to the structure

The effects of the compaction and graphite layer preferred orientation on the thermal and electrical conductions in the compaction direction of graphite-flake-based exfoliated graphite have been decoupled. The compact’s electrical and thermal conductivities decrease with increasing compaction (density increasing from 0.047 to 0.67g/cm3, solid content increasing from 2.1 to 30vol.%) and preferred orientation. The essentially linear correlation between electrical and thermal conductivities (Wiedemann–Franz Law) is because both conductions are governed by the preferred orientation. With increasing compaction, the fraction (f) of conduction path that is the graphite a-axis decreases from 0.997 to 0.937 and from 0.994 to 0.798 for thermal and electrical conductions respectively. For the solid-part thermal and electrical conductivities to exceed 140W/(mK) and 60kS/m respectively, f must exceed 0.95; the highest solid-part conductivities are 550W/(mK) and 230kS/m. The compaction-related variation in the solid-part conductivities is large [21–550W/(mK) and 10–230kS/m], due to the preferred orientation variation. The through-thickness Lorentz number (7.3×10−6WΩ/K2) is similar to the in-plane value, being independent of the preferred orientation. At 2–7vol.% solid, conductivities of 7W/(mK) and 3kS/m are obtained for the compact – toward the targets for fuel cell biopolar plates.