Physics

Directed Energy Deposition (DED) processes offer one of the most versatile current techniques to additively manufacture and repair metallic components that have generatively designed complex geometries, and with compositional control. When compared to powder bed fusion (PBF), its applicability and adoption has been limited because several issues innate to the process are yet to be suitably understood and resolved. This work catalogs and delineates these issues and anomalies in the DED process along with their causes and solutions, based on a state-of-the-art literature review. This work also serves to enumerate and associate the underlying causes to the detrimental effects which manifest as undesirable part/process outcomes. These DED-specific anomalies are categorized under groups related to the part, process, material, productivity, safety, repair, and functional gradients. Altogether, this primer acts as a guide to best prepare for and mitigate the problems that are encountered in DED, and also to lay the groundwork to inspire novel solutions to further advance DED into mainstream manufacturing.

An equivalent circuit model for Frequency Selective Surfaces (FSS) comprising anisotropic elements is presented. The periodic surface is initially simulated with an arbitrary azimuthal incidence angle and its surface impedance matrix is derived. The impedance matrix is subsequently rotated by an angle φ rot on the crystal axes χ 1 , χ 2 thus nullifying its extra diagonal terms. The rotation angle φ rot is derived according to the spectral theorem by using the terms of the matrix initially extracted. The diagonal terms of the rotated matrix, that is, the impedances Z χ 1 and Z χ 2 , are finally matched with simple LC networks. The circuit model representation of the anisotropic element can be used to analyse anisotropic FSSs rotated by a generic azimuth angle. The methodology provides a compact description of generic FSS elements with only five parameters: the lumped parameters of the LC network L χ 1 , C χ 1 , L χ 2 , C χ 2 and the rotation angle φ rot . The circuit model can take into account the presence of dielectric substrates close to the FSS or a variation of the FSS periodicity without additional computational efforts. The equivalent circuit model is finally applied to the design of two transmitting polarization converts based on anisotropic metasurfaces.

In a CMOS image sensor pixel, fast and complete charge transfer from pinned photodiode (PPD) is desired and necessary in some applications. In special cases such as time-of-flight imaging or large pinned photodiodes, the PPD potential well shape highly affects the charge transfer performance and should be engineered carefully. In the present work, a PPD structure named multi-level PPD is introduced and examined through simulation study. Moreover, a fast and effective way to analyze the pinning process for a lag-free design is introduced. It is concluded that the proposed PPD achieves fast and complete charge transfer without additional implementation masks or process steps. The proposed PPD is compared with a similar conventional rectangular pixel and 31% reduction in the charge transfer time is observed.

A three-step model for calculating the magnetic field generated by coils inside cuboid-shaped shields like magnetically shielded rooms (MSRs) is presented. The shield is modelled as two parallel plates of infinite width and one tube of infinite height. We propose an improved mirror method which considers the effect of the parallel plates of finite thickness. A reaction factor is introduced to describe the influence of the vertical tube, which is obtained from finite element method (FEM) simulations. By applying the improved mirror method and then multiplying the result with the reaction factor, the magnetic flux density within the shielded volume can be determined in a fast computation. The three-step model is verified both with FEM and measurements of the field of a Helmholtz coil inside an MSR with a superconducting quantum interference device. The model allows a fast optimization of shield-coupled coil spacings compared to repetitive time-consuming FEM calculations. As an example, we optimize the distance between two parallel square coils attached to the MSR walls. Measurements of a coil prototype of 2.75~m in side length show a magnetic field change of 18~pT over the central 5~cm at the field strength of 2.7~\textmu T. This obtained relative field change of 6~ppm is a factor of 5.4 smaller than our previously used Helmholtz coil.

This letter presents a numerical investigation of acoustic modes propagating in an LiNbO 3 /sapphire substrate under a periodic Al grating, demonstrating the occurrence of longitudinal leaky SAWs (LLSAWs) with a unique combination of very high velocities exceeding 10 000 m/s, an electromechanical coupling coefficients of 4.5%, and negligible leakage into the substrate. The leakage was suppressed by optimizing the LiNbO 3 (LN) plate and Al electrode thickness. Compared with conventional SAW substrates, the LN/sapphire substrate offers 2- to 2.5-fold higher frequencies for periodic electrode structures and enables the production of 6-GHz devices by standard photolithographic processes. Moreover, the required LN plate and Al electrode thicknesses remain compatible with existing wafer bonding technologies. An analysis of LLSAW dispersion in a resonator reveals that the wave behaves as a perfect Rayleigh SAW unperturbed by interactions with spurious modes. The found optimal LN/sapphire substrates are perfect candidates for high-performance SAW devices operating in the sub-6-GHz spectrum.

This paper defines and develops useful concepts related to the several kinds of inductances employed in any comprehensive design-oriented ferrite-based inductor model, which is required to properly design and control high-frequency operated electronic power converters. It is also shown how to extract the necessary parameters from a ferrite material datasheet in order to get inductor models useful for a wide range of core temperatures and magnetic induction levels.

The study is focussed towards understanding the difference in behaviour of forsterite droplets subjected to sub-atmospheric conditions. Melt droplets, 1.5-2.5 mm in diameter, are made to crystallize under levitated conditions. The spherule is initially superheated by about 250oC above its liquids temperature, for both atmospheric and sub-atmospheric conditions, and then, subsequently cooled at different cooling rates. Crystallization of molten droplets was observed from their hypercooled states in all the cases. It was found that the level of undercooling was more (~150K-200K) for sub-atmospheric cases. In addition, the degree of recalescence was also found to be higher. However, the varying cooling rates did not produce any considerable effect on the level of undercooling or, even, the degree of recalescence. For better insight into the dynamics of crystallization, in situ visualization of the recalescence process was made possible by use of high-speed camera. A clear difference in the growth mechanisms was observed. The role of cooling rate in affecting crystallization was seen as the difference in the growth of the crystal-front; from a clear rim front, in high cooling rates, to a faded irregularly shaped crystal front, in low cooling rates. Two proper crystal fronts were observed in all the cases, which correspond to surface and volumetric crystallizations. It was observed that the molten droplets, under sub-atmospheric conditions, undergo high rate of volumetric crystallization, which is marked by the sightings of surface textures during the recalescence process. High-speed camera images of recalescence provided direct observational proof of the delay in volumetric crystallization, compared to surface crystallization.

Thick rods are employed in Nanotechnology to build modern electro mechanical systems. Design and optimization of such structures can be carried out by nonlocal continuum mechanics which is computationally convenient when compared to atomistic strategies. Bishop's kinematics is able to describe small-scale thick rods if a proper mathematical model of nonlocal elasticity is formulated to capture size effects. In all papers on the matter, nonlocal contributions are evaluated by replacing Eringen's integral convolution with the consequent (but not equivalent) differential equation governed by Helmholtz's differential operator. As notorious in integral equation theory, this replacement is possible for convolutions, defined in unbounded domains, governed by averaging kernels which are Green's functions of differential operators. Indeed, Eringen himself, in order to study nonlocal problems defined in unbounded domains, such as screw dislocations and wave propagations, suggested to replace integro-differential equations with differential conditions. A different scenario appears in Bishop rod mechanics where nonlocal integral convolutions are defined in bounded structural domains, so that Eringen's nonlocal differential equation has to be supplemented with additional boundary conditions. The objective is achieved by formulating the governing nonlocal equations by a proper variational statement. The new methodology provides an amendment of previous contributions in literature and is illustrated by investigating the elastostatic behavior of simple structural schemes. Exact solutions of Bishop rods are evaluated in terms of nonlocal parameter and cross-section gyration radius. Both hardening and softening structural responses are predictable with a suitable tuning of the parameters.

This paper deals with modelling the effect of wear on the dynamics of the shrouded bladed disk with frictional contacts at the shrouds and the contact interface evolution. Prediction of fretting wear commonly occurring at the contacts of turbomachinery components, and its impact on the dynamics is increasingly researched due to the components subjected to their structural limits for performance and operating at high loading conditions. Over a lifetime, the fretting wear at these contacts could alter the global dynamic response of these bladed disks from the designed operating point and could lead to high vibration amplitudes. This study implements a coupled static/dynamic harmonic balance method (HBM) with wear energy approach and an adaptive wear logic to study the impact on the steady-state nonlinear dynamic response. Firstly, the methodology is applied to a cantilever beam with a contact patch and then to a shrouded bladed disk with shroud contacts using cyclic symmetry boundary conditions. The novelty of the paper is to show the effect of wear on the dynamics with the changing contact pre-load using a coupled approach. A wear acceleration v w,max parameter is defined, and the impact of the choice of this parameter is discussed in detail on the computation time and the accuracy of results. The test cases demonstrate the impact of wear on the dynamic nonlinear response curves and the contact interface evolution for various scenarios.

Nitrogen-vacancy quantum defects in diamond offer a promising platform for magnetometry because of their remarkable optical and spin properties. In this Letter, we present a high-sensitivity and wide-bandwidth fiber-based quantum magnetometer for practical applications. By coating the diamond surface with silver reflective film, both the fluorescence collection and excitation efficiency are enhanced. Additionally, tracking pulsed optically detected magnetic resonance spectrum allowed a magnetic field sensitivity of 35 pT / Hz ????????and a bandwidth of 4.1 KHz. Finally, this magnetometer was successfully applied to map the magnetic field induced by the current-carrying copper-wire mesh. Such a stable and compact magnetometry can provide a powerful tool in many areas of physical, chemical, and biological researches.