I. The "Primary Culprit" of Insulator Withstand Voltage Degradation: Surface Flashover Phenomenon
Alumina insulators simultaneously electrical insulation and mechanical support roles in high-power equipment and high-vacuum/electric vacuum devices, making them indispensable key components. However, under high vacuum and high field strength conditions, the bottleneck in withstand voltage often lies not in the bulk material but in surface processes – the most typical being surface discharge breakdown (i.e., surface flashover). Surface flashover refers to the phenomenon where, under a strong electric field, the surface of a solid insulator and its adjacent medium (gas/liquid; in vacuum, accompanied by surface desorbed gas and electron emission) become ionized or conductive. A discharge channel develops along the solid surface, spans the electrode gap, and ultimately leads to through breakdown and insulation failure. This phenomenon not only significantly weakens the withstand voltage and operational reliability of high-voltage dielectric equipment, causing potential economic losses, but also serves as a core bottleneck limiting the compactness and miniaturization of solid insulators. From a threshold comparison perspective, the initiation voltage/field strength for surface flashover is typically much lower than the breakdown level for bulk breakdown or pure dielectric gaps. For example: when vacuum is used as the insulating medium, the critical breakdown field strength is approximately 35 kV/mm; for alumina ceramic as the bulk insulating medium, the critical volume breakdown field strength is generally 30–40 kV/mm; whereas in an alumina-vacuum insulation system, the applied field strength often reaches only one-tenth to a fraction of these critical values before triggering surface flashover on the insulator, potentially even causing local damage to the Al₂O₃ surface.
II. Factors Influencing Surface Flashover Voltage
Research indicates that the factors affecting surface flashover primarily include: waveform and amplitude of the applied electric field, vacuum level and residual gas composition, electrode structure and material, insulator geometry and dimensions, insulator material and surface characteristics (roughness, cleanliness, adsorption/contamination, coating), pre-discharge/baking and other pretreatments, as well as surface charging state and surface gas adsorption. From a materials research perspective, focus is placed on the composition, shape, and surface characteristics of ceramics used in vacuum electronics. Key electrical parameters influencing surface flashover include dielectric constant ε, electrical conductivity σ, and secondary electron emission coefficient δ (SEE). In general: ① A higher dielectric constant tends to enhance electric field distortion at the electrode–insulator–vacuum triple junction, lowering the surface flashover threshold. ② Within an appropriate range, increased surface conductivity accelerates surface charge dissipation and inhibits initiation, but excessively high conductivity increases leakage current and may lead to thermal instability, which is detrimental to withstand voltage. ③ According to the SEEA model, reducing the surface secondary electron emission coefficient suppresses electron multiplication, thereby increasing the surface flashover voltage.
Regarding the SEEA (SEE-based discharge mechanism model): The Secondary Electron Emission Avalanche (SEEA) model was first proposed by American scholars Anderson and Brainard. This model suggests that under an applied high voltage, initial electrons emitted from the electrode–insulator–vacuum triple junction gain energy, are accelerated, and bombard the insulator surface. When the energy of these impacting electrons reaches a certain threshold, secondary electron emission occurs, simultaneously leaving positive charges on the insulator surface. These secondary electrons, under the influence of the electric field, again bombard the insulator surface, generating more secondary electrons. This process repeats, ultimately leading to a secondary electron avalanche.
III. Surface Flashover Suppression Techniques for Alumina Ceramics
The key to improving the insulation performance of solid insulating materials lies in maintaining the bulk insulation properties while striving to enhance the surface flashover voltage. Based on existing mechanisms, the main pathways for improvement fall into two categories: ① Reduce the surface secondary electron emission coefficient δ to suppress electron multiplication; ② Design the surface resistivity within a suitable window to accelerate surface charge dissipation, thereby avoiding excessive local field concentration and thermal instability. In parallel with these two "material electrical parameter" approaches, engineering often employs a complementary set of geometry/field distribution control measures to reduce the triple-junction field strength and delay channel formation. For example, machining periodic corrugations (or grooves) on the surface of alumina ceramic insulators can increase the creepage distance, smooth equipotential lines, reduce the tangential field strength at the triple junction without increasing external dimensions, while also interrupting electron return paths and reducing the effective SEE gain, thereby delaying channel formation and raising the surface flashover voltage. The crests of the corrugations should be rounded to avoid introducing local field enhancement at new sharp edges.