Metal-wire-embedded alumina insulating material using micro and nanoscale 3D printing for surface flashover mitigation in vacuum

Micro and nanoscale 3D printing technic is applied to fabricate functional insulating material which mitigates surface discharge in vacuum based on the microscopic electron multipactor suppression. The proposed alumina ceramic insulator design consists surface-embedded thin metal wires which introduce a local gradient of secondary electron emission yield, such that the trajectories of multipactor electrons are distorted by accumulated negative surface charges and the secondary electron emission avalanche across the insulator surface is intermitted. Considerable increases of surface flashover threshold and surface charging reduction are verified by experiment. Also, additional efforts are made to determine the optimal size and spatial distribution of the metal wire. A convex-shape flashover voltage trace is observed when increasing the wire width, suggesting a trade-off between the multipactor mitigation and the insulator strength. Wire position between the adjacency of cathode triple junction and middle of the insulator is proved to be favorable for flashover mitigation. The physical details of surface flashover mitigation by the proposed insulator design are revealed by an ab initio particle-in-cell (PIC) simulation code, corroborating the experiment from microscopic aspect.

Laser Metal Wire drop-by-drop Deposition: a material and dilution investigation

Additive Manufacturing has become a field of high interest in the industry, mostly due to its strong freedom of design and its flexibility. Numerous Additive Manufacturing techniques exist and present different advantages and disadvantages. The technique investigated in this research is a drop-by-drop deposition alternative to Laser Metal Wire Deposition. This technique is expected to induce a better control over the power input in the material, resulting in a better power efficiency and tailorable material properties. The aim of this research is to investigate selected material properties of the structures produced with the drop-by-drop deposition technique. Multi-drops structures were deposited from 316L, Inconel 625 (NW6625) and AlSi5 (AW4043) wires. Two drop deposition methods were investigated: (i) a contactless recoil pressure driven detachment for 316L and Inconel 625, (ii) a contact-based surface tension driven detachment for AlSi5. A material characterization including optical microscopy, EDS and hardness measurements was performed in transverse and longitudinal cross-sections. The microstructure of the deposited material, the dilution with the substrate and the heat affected zone were analysed. The contactless detachment showed a higher dilution than the contact-based technique due to the laser irradiating the substrate between two drop detachments, which melts the substrate that then mixes with the deposited drops.

Effect of Integrating Metal Wire Mesh with Spray Injection for Liquid Piston Gas Compression

Heat transfer enhancement techniques used in liquid piston gas compression can contribute to improving the efficiency of compressed air energy storage systems by achieving a near-isothermal compression process. This work examines the effectiveness of a simultaneous use of two proven heat transfer enhancement techniques, metal wire mesh inserts and spray injection methods, in liquid piston gas compression. By varying the dimension of the inserts and the pressure of the spray, a comparative study was performed to explore the plausibility of additional improvement. The addition of an insert can help abating the temperature rise when the insert does not take much space or when the spray flowrate is low. At higher pressure, however, the addition of spacious inserts can lead to less efficient temperature abatement. This is because inserts can distract the free-fall of droplets and hinder their speed. In order to analytically account for the compromised cooling effects of droplets, Reynolds number, Nusselt number, and heat transfer coefficients of droplets are estimated under the test conditions. Reynolds number of a free-falling droplet can be more than 1000 times that of a stationary droplet, which results in 3.95 to 4.22 times differences in heat transfer coefficients.

Low-voltage driven graphene-loaded metal wire grating device in total internal reflection geometry for broadband THz modulation

Terahertz modulators with capability of both intensity and phase are essential for THz imaging and communication systems. The low-voltage driven THz modulation technique is crucial for integrating the modulators with electronics components. There is still a lack of broadband devices able to achieve both amplitude and phase modulation with low voltage, due to the underlying physics behind existing approaches. Here, we demonstrate a graphene-loaded metal wire grating THz modulator in the total internal reflection geometry to achieve intensity modulation of 80% and phase modulation of 70 degree within 3 volts gate voltage. Quite different from using the strategy of metamaterials based on the electromagnetic resonance effects, our design has performed a broadband modulation for over 1 THz bandwidth.