In high-temperature reducing atmospheres—commonly used in the sintering of NdFeB and ferrite magnets—tray stability isn’t just a technical detail; it’s a critical factor in product purity and yield. Traditional ceramic trays often fail under these conditions, leading to metal contamination that can reduce magnetic performance by up to 15–20% in worst-case scenarios.
Our analysis of over 40 production failures across Asian and European magnet manufacturers reveals that microcracks in standard alumina or mullite trays are the primary source of metallic inclusions. These cracks, typically caused by thermal shock or poor material homogeneity, allow iron-rich gases from furnace components to penetrate and react with the sintered magnet surface.
Composite alumina-mullite (Al₂O₃–3Al₂O₃·2SiO₂) offers superior resistance to both chemical reduction and thermal stress compared to conventional materials. With a bulk density exceeding 3.1 g/cm³ and pore size distribution below 0.5 μm, this refractory material minimizes gas diffusion pathways—even at 1150°C in H₂/N₂ mixtures.
| Material Type | Avg. Pore Size (μm) | Thermal Shock Resistance (°C) | Metal Inclusion Rate (%) |
|---|---|---|---|
| Standard Alumina Tray | 1.2–2.5 | ≤400 | 8–12% |
| Composite Alumina-Mullite | 0.2–0.5 | ≥650 | ≤2% |
“After switching to composite alumina-mullite trays, our defect rate dropped from 11% to under 3%. The improvement wasn’t just about fewer cracks—it was about consistent quality.”
——R&D Lead, Jiangsu Magnetics Co., China
To maximize tray life and minimize contamination risks:
Even minor issues like surface oxidation or slight warping—often overlooked—can lead to long-term degradation. A systematic maintenance log has been shown to extend tray lifespan by 30–40%, directly improving process consistency.
If you're experiencing inconsistent magnetic properties or unexpected contamination in your sintering process, it may not be your raw material—it could be your tray system.
