Common causes of sintering failure after magnesium oxide refilling include improper temperature control, insufficient holding time, lack of atmosphere protection, uneven filling, and excessively rapid cooling. These factors can directly lead to defects in the insulation layer structure, resulting in substandard performance or breakdown during operation.
1. Uncontrolled Process Parameters
|
Causes |
Impact |
Preventive Measures |
|
Too low (<800℃) or too high (>1050℃) |
Inability to complete recrystallization or causing magnesium oxide volatilization and shell deformation. |
Use calibrated thermocouples to ensure temperature control accuracy ±10℃. |
|
Holding time <1 hour |
Insufficient grain growth, loose structure, and low insulation strength. |
Strictly maintain a holding time of 1–2 hours. |
|
Too rapid heating/cooling rate |
Causes thermal stress cracks and damages the seal. Heating rate ≤5℃/min; use natural cooling with the furnace. |
In practice, 30% of sintering failures are due to an uncalibrated temperature control system or incorrect program settings. |
2. Unsuitable Atmosphere Environment
Failure to use inert gas (e.g., nitrogen) protection: Magnesium oxide readily reacts with oxygen and moisture in the air at high temperatures, generating impurities;
High moisture content inside the furnace (RH > 30%): Leads to secondary moisture absorption, resulting in failure of the moisture test;
Insufficient or uneven gas flow rate: Localized oxidation or carbonization.
Must be carried out in a vacuum or high-purity nitrogen environment, with oxygen content controlled < 100 ppm.
3. Defects in Filling and Compaction Processes
Insufficient purity of magnesium oxide powder (< 99.9%): Impurities form conductive paths, leading to breakdown voltage;
Uneven filling density (< 2.8 g/cm³ or > 3.2 g/cm³): Too low a density leads to loosening, while too high a density causes stress concentration;
Failure to perform high-frequency vibration compaction: Internal voids exist, leading to localized overheating during operation.
X-ray inspection can detect hidden defects such as off-center cores and voids.
4. Equipment and Environment Failure to Meet Standards
Cleanroom Failure to Meet Standards (>100,000 class): Dust contamination leads to weak points in insulation;
Insufficient Hydraulic Clamping Force: Insufficient axial pressure results in loose filling;
Mold Wear or Misalignment: Leads to pipe deformation or seal misalignment.
Non-professional workshops often experience batch repair failures due to rudimentary equipment.
5. Material and Sealing Issues
Using Industrial-Grade or Recycled Magnesium Oxide Powder: Excessive impurities such as Cl⁻ and SO₄²⁻ cause electrochemical corrosion;
Incorrect Sealing Adhesive Selection or Incomplete Curing: Moisture seeps in along the leads, resulting in failure to pass the moisture test;
Lack of Anti-Oxidation Treatment at the Outlet Terminals: Increased contact resistance and exacerbated heat generation after long-term operation.
Electrical-grade high-purity magnesium oxide must be used in conjunction with ceramic sealing adhesive or metal sealant.
