Advantages of Corundum-Mullite Refractory Materials in Electronic Ceramic Sintering
Electronic ceramics—such as alumina substrates, piezoelectric ceramics, multilayer capacitors, and ferrites—require precise control of the sintering environment to ensure high density, phase purity, and dimensional stability. The performance of kiln furniture, particularly setter plates, saggers, and support structures, plays a vital role in determining product yield and quality. Among refractory materials, corundum-mullite composites have proven especially advantageous in electronic ceramic sintering due to their thermal stability, chemical inertness, and mechanical strength. This article explores how corundum-mullite materials support the stringent requirements of modern electronic ceramics manufacturing.
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Overview of Electronic Ceramic Sintering Requirements
Electronic ceramics are typically sintered at 1,200–1,600 °C in controlled atmospheres (air, nitrogen, or oxygen-rich) depending on the specific composition. Some key processing demands include:
Temperature uniformity: Preventing warping, delamination, or non-uniform shrinkage.
Atmospheric cleanliness: Avoiding contamination from kiln furniture, which may alter dielectric or magnetic properties.
Dimensional stability: Maintaining tight tolerances during sintering and cooling.
Reusability: Refractory components must withstand dozens to hundreds of cycles.
These requirements place high stress on kiln furniture, making material selection critical.
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Corundum-Mullite: An Ideal Material System
Corundum-mullite is a ceramic composite combining:
Corundum (α-Al₂O₃): High hardness, chemical resistance, and structural rigidity at elevated temperatures.
Mullite (3Al₂O₃·2SiO₂): Excellent thermal shock resistance, low thermal expansion, and enhanced fracture toughness.
This combination yields a material with synergistic properties particularly well suited for use in kilns firing electronic ceramics.
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Thermal Stability and Dimensional Precision
Corundum provides high refractoriness, typically above 1,900 °C, ensuring that setter plates and saggers do not deform even under high loads and thermal gradients.
Refractoriness under load (RUL): ≥1,650 °C
Creep resistance: Maintains shape over 50–100 sintering cycles
Thermal expansion coefficient: ~6.0 × 10⁻⁶ /°C (balanced by mullite to avoid distortion)
For multilayer ceramic capacitors (MLCCs), which require absolute flatness, corundum-mullite setter plates provide the stability needed to avoid warping during the densification stage.
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Excellent Thermal Shock Resistance
Many electronic ceramics are processed in fast-firing kilns (total cycle time 1–3 hours), with heating and cooling rates up to 10–20 °C/min. Corundum by itself is brittle under such thermal cycling, but when combined with mullite:
Microcracks and interlocked needle phases absorb thermal stress.
Plates can withstand over 30 thermal shock cycles between room temperature and 1,400 °C without structural failure.
This property reduces unexpected downtime due to refractory cracking or spalling and enhances kiln operational reliability.
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Chemical Compatibility with Electronic Ceramics
Electronic ceramics are highly sensitive to impurities such as alkalis, iron, boron, or silicates, which can migrate from low-grade refractories during firing. Corundum-mullite materials offer:
High chemical purity: Al₂O₃ content >90%, minimal alkali or iron
Non-reactivity with dielectric and ferrite materials
No volatilization or glaze formation under typical firing conditions
This ensures the surface of sintered ceramics remains uncontaminated, preserving electronic performance (e.g., insulation resistance, dielectric constant, magnetic permeability).
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Surface Finish and Anti-Stick Behavior
Electronic ceramics, especially green sheets or low-fired ferrites, may stick to kiln plates if the surface is rough, porous, or reactive. Corundum-mullite setter plates can be manufactured with:
Polished or ground surfaces: To achieve Ra <0.5 µm, reducing sticking tendency
Anti-stick coatings: Spinel or zirconia layers to prevent adhesion
Minimal open porosity: Dense matrix reduces gas exchange and reaction with volatiles
These attributes contribute to high product yields and minimal surface defects.
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Reusability and Lifecycle Cost Efficiency
Compared with traditional aluminosilicate refractories, corundum-mullite kiln furniture exhibits:
| Property | Aluminosilicate | Corundum-Mullite |
| Max operating temperature | ~1,400 °C | >1,650 °C |
| Thermal shock resistance | Moderate | High |
| Typical service life | 20–40 cycles | 60–120 cycles |
| Product contamination risk | Medium | Very low |
Although the initial cost of corundum-mullite is higher, the extended service life, lower breakage rates, and improved ceramic quality result in significantly lower total operating costs.
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Case Studies: Real-World Applications
8.1 Ferrite Sintering
In sintering Mn-Zn and Ni-Zn ferrites at 1,300–1,450 °C, corundum-mullite setter plates have been shown to reduce deformation of ferrite cores and eliminate iron contamination compared to silica-bonded refractories.
8.2 Alumina Substrates
In high-volume alumina substrate production, where flatness is critical for screen printing and metallization, corundum-mullite plates maintain planarity across >100 cycles, minimizing yield loss.
8.3 Multilayer Piezoelectrics
The clean firing environment enabled by corundum-mullite saggers allows for defect-free sintering of lead zirconate titanate (PZT) and barium titanate-based ceramics, supporting high-performance dielectric and actuation behavior.
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Design Flexibility and Customization
Corundum-mullite kiln furniture can be tailored in:
Shapes: flat plates, honeycomb setters, saggers, or multi-hole structures
Thickness: from 3 mm thin plates to >30 mm load-bearing slabs
Surface treatment: grooved, coated, or glazed depending on product type
This flexibility makes it easy to integrate corundum-mullite into both legacy and modern kiln systems.
Conclusion
Corundum-mullite refractory materials offer a robust, clean, and high-performance solution for sintering electronic ceramics. Their unique combination of thermal, mechanical, and chemical properties not only ensures reliable operation under demanding conditions but also helps improve the quality and consistency of sintered products.
As electronic components become smaller, thinner, and more functionally integrated, the role of kiln furniture in maintaining precise sintering control becomes increasingly vital. Corundum-mullite materials provide the foundation upon which modern electronic ceramics manufacturing can build.
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