Component Study of MXene Cold Cathodes and Additive-Manufactured TWT Circuits for Vacuum Electron Devices

Abstract

Vacuum electronic devices (VEDs) are essential components in high-power and high-frequency applications. This study investigates on the traveling wave tube (TWT), known for its superior output power and bandwidth characteristics. However, as the operating frequency increases, the miniaturization of TWT structures becomes increasingly challenging when using subtractive nano-CNC fabrication.

A planar line emitter is required to produce a sheet beam for input to the SDV TWT. Cold cathodes are advantageous for producing sheet beams because the shape of the beam is strongly influenced by the shape of the emitter due to their field emission characteristics. In addition to these structural advantages, cold cathodes also offer advantages such as low power consumption and rapid turn-on times. This study explores the novel use of MXene (Ti3C2) as a candidate material for cold cathode emitters. Carbon nanotubes (CNTs) are commonly studied as emitters for cold cathodes because they are carbon-based materials with good conductivity and relatively low work function of 4 ∼ 5 eV.

To evaluate the potential usage of MXene as a cold cathode emitter, its electrical conductivity and emission characteristics were optimized. First, it was confirmed that appropriate nitrogen doping in MXene can enhance electrical conductivity. Based on S-parameter data measured in a W-band waveguide-based system, shielding effectiveness (SE) was calculated, and the results were fitted to theoretically calculated thickness-dependent SE using a transfer matrix to estimate electrical conductivity. To ensure the accuracy of the experimental results compared to the theoretical values, a comparison was performed using a four-point probe measurement. The results confirmed that the Ti3C1.9N0.1 composition exhibited an excellent electrical conductivity of 35,000 S/cm, and this composition is referred to as XSS MXene. Next, the emission characteristics were evaluated by extracting the field enhancement coefficient (β ) and effective emission area (α) using the Fowler-Nordheim (F-N) theory based on the I-V curves obtained from field emission experiments. The results demonstrated that XSS MXene exhibits significantly superior performance compared to previously studied MXene field emitters, with a maximum current density (J) of 2075.84 mA/cm2, an effective emission area (α) of 248.51 µm2, and a field enhancement factor (β ) of 725, thus demonstrating its potential as a sheet-type cold cathode emitter.

The fabrication of an SDV TWT structure was studied by utilizing high-resolution additive manufacturing technology. Previously, conventional CNC machining techniques were unable to produce TWT circuits at high frequencies, necessitating the use of the costly nano-CNC methods. However, an alternative approach was adopted, whereby a 3D printer with digital light processing (DLP) was utilized for the fabrication of polymer-based circuits. Additionally, electroless copper plating was applied to ensure the required electrical conductivity for the waveguide’s functionality. Compared to subtractive manufacturing methods, such as CNC machining, this additive manufacturing process eliminates edge rounding caused by tools such as end mills. This approach has two main benefits: first, it significantly improves design flexibility; second, it reduces manufacturing costs and material waste. However, the performance of an SDV TWT, a type of waveguide, is heavily dependent on the quality of the plating, including the effective thickness of the plating and surface roughness, which were further evaluated in this study.

The thickness and surface roughness (Rq) of the plating are critical factors in the RF transmission characteristics of a waveguide. Therefore, the plating thickness and surface roughness (Rq) of the plated 3D-printed SDV TWT structures were measured and analyzed. First, the skin depth calculation formula was used to determine the effective plating thickness. This confirmed an effective plating thickness of 141–177 nm in the G-band (140–220 GHz) frequency range, which was verified through CST simulation. Next, the Hammerstad–Bekkadal (HB) model was used to estimate the effective electrical conductivity (σe f f ). The results showed that within the G-band frequency range, even as the surface roughness (Rq) exceeded 200 nm, σe f f saturated at approximately 1.5 × 107 S/m. However, applying the Gradient Model, reveals a more significant reduction in σe f f ranging from approximately 1.20 × 107 S/m for smoother surfaces to 4.70 × 106 S/m for rougher surfaces exceeding 1 µm. Based on this analysis, it was concluded that to minimize RF loss, the surface roughness should be maintained below 1 µm, and ideally below 200 nm.

In conclusion, this study proposes a novel approach for the cost-effective fabrication of TWT in the ultra-high-frequency range by combining a cold cathode using XSS MXene, a nitrogen-doped highconductivity MXene, as a sheet beam emitter with SDV TWT technology utilizing additive manufacturing. The optimization process demonstrated a cost-effective fabrication method through the optimization of electrical and field emission characteristics and 3D-printing of the SDV TWT structure. These results are expected to contribute to the development of cost-effective and high-performance RF amplification solutions in fields requiring high power and precisiion, such as satellite communications, next-generation communications, radar, and millimeter-wave technology.