Technical Indicators of Alpha Alumina (α-Al₂O₃)

Alpha alumina, the thermodynamically stable phase of aluminum oxide, is characterized by its exceptional properties, making it suitable for diverse applications. Below are its key technical indicators:

1. Crystal Structure

   • Hexagonal Corundum Structure: Rhombohedral symmetry (space group R-3c) with oxygen ions in a hexagonal close-packed arrangement and aluminum ions occupying 2/3 of the octahedral sites.

   • Phase Stability: Retains structure at high temperatures without phase transitions, unlike gamma alumina.

2. Mechanical Properties

   • Hardness: Mohs hardness ~9 (second only to diamond), ideal for abrasives and cutting tools.

   • Compressive Strength: High resistance to deformation under load.

   • Wear Resistance: Excellent for grinding media and coatings.

   • Young’s Modulus: ~380 GPa, indicating high stiffness.

   • Fracture Toughness: Moderate (~3–4 MPa·m¹/²), typical of brittle ceramics.

3. Thermal Properties

   • Melting Point: ~2050°C, suitable for refractory linings and crucibles.

   • Thermal Conductivity: Moderate (~30 W/m·K at room temperature).

   • Thermal Expansion Coefficient: Low (~8×10⁻⁶/K), enhancing thermal shock resistance.

   • Thermal Stability: Retains mechanical strength at elevated temperatures.

4. Electrical Properties

   • Resistivity: High electrical insulation (>10¹⁴ Ω·cm at 25°C).

   • Dielectric Strength: ~15–20 kV/mm, useful in insulators.

   • Dielectric Constant: ~9–10 at 1 MHz, stable across frequencies.

5. Chemical Stability

   • Inertness: Resists corrosion by acids, bases, and oxidizing agents, except under extreme conditions (e.g., hydrofluoric acid).

   • Biocompatibility: Non-reactive in biological environments, used in medical implants.

6. Optical Properties

   • Transparency: In pure single-crystal form (sapphire), used in optical windows and lenses.

   • Refractive Index: ~1.76–1.78, with high light transmission in UV to IR ranges.

7. Physical Characteristics

   • Density: ~3.95 g/cm³ (higher than gamma alumina).

   • Purity: High-purity grades (≥99.5%) for electronics and optics; lower grades for industrial uses.

   • Particle Size/Surface Area: Controlled in powders (e.g., submicron for advanced ceramics).

8. Applications-Linked Indicators

   • Abrasives/Refractories: Hardness, thermal stability.

   • Electronics: Insulation, dielectric properties.

   • Biomedical: Biocompatibility, wear resistance.

   • Optics: Transparency, scratch resistance.

Summary: Alpha alumina’s technical indicators—spanning structural, mechanical, thermal, electrical, and chemical domains—dictate its utility in industries ranging from electronics to aerospace. Its balance of hardness, stability, and inertness makes it a cornerstone material in high-performance applications.