WO3-Coated Tungsten Emitters: From Surface Treatment to Emission Properties

WO3-Coated Tungsten Emitters: From Surface Treatment to Emission Properties

Stať ve sborníku mimo WoS a Scopus

This study examines the effect of applying a WO3 dielectric layer, several hundred nanometres thick, to the apex of tungsten field emitters. Such coatings are designed to reinforce emission stability and extend emitter lifetime, surpassing the performance of bare tungsten tips. Nonetheless, applying high extraction voltages can lead to thermal stress or explosive electron emission, causing the tip to melt or reshape due to quasi-plasma formation. We investigate whether such reshaped tips, formed through partial evaporation, can present advantages over pristine geometries by stabilizing electron emission and enhancing operational parameters. The tungsten emitters, produced from 99.9% pure polycrystalline wire (0.3 mm in diameter, 1 cm length), were etched using a two-step drop-off electrochemical method in a 2 M NaOH solution. Post-etching cleaning was performed using a 10-minute immersion in 38% HF acid, followed by deionized water rinsing. The WO3 layer was formed via anodic oxidation in 0.3 M H3PO4 (pH = 1.2) by ramping the potential from 0.02 V to 5 V or 12 V over 5 minutes, then holding at the target voltage for an additional 2 minutes. Emitters were tested in a triode FEM setup under high vacuum (≈10-6 Pa). A 1 mm aperture extractor was aligned 1 mm from the tip, and a Ce:YAG:Al scintillator was used as an anode. Tip activation involved a gradual voltage ramp until abrupt emission fluctuations occurred, indicative of geometric alteration. Voltage increments during measurements were ~1 V/s to avoid further destructive events. Figure 1a illustrates the tip morphology after anodization. Anodization at 12 V resulted in significantly thicker oxide caps compared to 5 V, leading to broader emission heads. When subjected to high voltage in FEM, these thicker tips partially melted or restructured (Fig. 1b). Post-reconfiguration, I–V characteristics were measured. For 5 V-treated samples, emission showed continued instability and frequent current fluctuations, likely due to ongoing structural changes, and exhibited limited agreement with the orthodoxy criterion. Conversely, emitters anodized at 12 V exhibited a stable emission regime, with Murphy–Good plots confirming consistent behaviour up to 6900 V (Fig. 2). The thick WO3 coating likely confined emission to the apex while shielding the shaft, contributing to both thermal robustness and directional emission. While peak current was lower due to the blunter geometry, the operational stability was significantly improved. These emitters, once reshaped, resisted thermal failure and maintained consistent performance at higher voltages than their uncoated or lightly coated counterparts. FEM images revealed stronger brightness localized around surviving protrusions, confirming focused emission. Comparative analysis using a validated web-based simulation tool, and assuming a work function of 5.05 eV, demonstrated that oxide-coated emitters delivered more predictable emission demonstrated that oxide-coated emitters delivered more predictable emission behaviour, unlike pure tungsten tips that tended to degrade rapidly with increasing field strength. This work suggests that tip reshaping—typically viewed as degradation—can also yield enhanced field emitter geometries.

This study examines the effect of applying a WO3 dielectric layer, several hundred nanometres thick, to the apex of tungsten field emitters. Such coatings are designed to reinforce emission stability and extend emitter lifetime, surpassing the performance of bare tungsten tips. Nonetheless, applying high extraction voltages can lead to thermal stress or explosive electron emission, causing the tip to melt or reshape due to quasi-plasma formation. We investigate whether such reshaped tips, formed through partial evaporation, can present advantages over pristine geometries by stabilizing electron emission and enhancing operational parameters. The tungsten emitters, produced from 99.9% pure polycrystalline wire (0.3 mm in diameter, 1 cm length), were etched using a two-step drop-off electrochemical method in a 2 M NaOH solution. Post-etching cleaning was performed using a 10-minute immersion in 38% HF acid, followed by deionized water rinsing. The WO3 layer was formed via anodic oxidation in 0.3 M H3PO4 (pH = 1.2) by ramping the potential from 0.02 V to 5 V or 12 V over 5 minutes, then holding at the target voltage for an additional 2 minutes. Emitters were tested in a triode FEM setup under high vacuum (≈10-6 Pa). A 1 mm aperture extractor was aligned 1 mm from the tip, and a Ce:YAG:Al scintillator was used as an anode. Tip activation involved a gradual voltage ramp until abrupt emission fluctuations occurred, indicative of geometric alteration. Voltage increments during measurements were ~1 V/s to avoid further destructive events. Figure 1a illustrates the tip morphology after anodization. Anodization at 12 V resulted in significantly thicker oxide caps compared to 5 V, leading to broader emission heads. When subjected to high voltage in FEM, these thicker tips partially melted or restructured (Fig. 1b). Post-reconfiguration, I–V characteristics were measured. For 5 V-treated samples, emission showed continued instability and frequent current fluctuations, likely due to ongoing structural changes, and exhibited limited agreement with the orthodoxy criterion. Conversely, emitters anodized at 12 V exhibited a stable emission regime, with Murphy–Good plots confirming consistent behaviour up to 6900 V (Fig. 2). The thick WO3 coating likely confined emission to the apex while shielding the shaft, contributing to both thermal robustness and directional emission. While peak current was lower due to the blunter geometry, the operational stability was significantly improved. These emitters, once reshaped, resisted thermal failure and maintained consistent performance at higher voltages than their uncoated or lightly coated counterparts. FEM images revealed stronger brightness localized around surviving protrusions, confirming focused emission. Comparative analysis using a validated web-based simulation tool, and assuming a work function of 5.05 eV, demonstrated that oxide-coated emitters delivered more predictable emission demonstrated that oxide-coated emitters delivered more predictable emission behaviour, unlike pure tungsten tips that tended to degrade rapidly with increasing field strength. This work suggests that tip reshaping—typically viewed as degradation—can also yield enhanced field emitter geometries.

cold field emitter, WO3, tungsten, oxidation, anodization

cold field emitter, WO3, tungsten, oxidation, anodization

KOŠELOVÁ, Z.; ALLAHAM, M.; BURDA, D.; FOHLEROVÁ, Z.; KNÁPEK, A.

23.10.2025

Brno University of Technology, FEEC

Brno

978-80-214-6369-1

45

46

2