What Is a High Purity Alumina Ceramic Insulating Ring?
A high purity alumina ceramic insulating ring is a dense ceramic ring made from 99% to 99.9% aluminum oxide (Al₂O₃). It sits between conductive parts to keep them electrically isolated and to hold a set gap between them, including near heat sources and inside vacuum and plasma equipment. Depending on its job in the assembly, it may also be called an alumina spacer ring or a ceramic isolation ring.
High purity alumina is used for these rings because it combines high electrical resistivity and low dielectric loss with the hardness and stiffness to hold a stack height under clamp load. After firing, the faces can be ground or lapped flat, so the ring keeps metal parts parallel and at the same gap after every rebuild.
- Electrical insulation: volume resistivity above 10¹⁴ Ω·cm at 20 °C in all four grades.
- Heat: max. use temperatures (no load) of 1600–1750 °C, depending on grade.
- Stable stack height: a hard, stiff ceramic with ground or lapped contact faces.
- Dense material: water absorption of 0.10% or less, depending on grade.
ADCERAX supplies the ceramic ring. Bolts, electrodes, housings and other mating parts remain part of your assembly. For other parts in the same material, see our high purity alumina ceramic range.
Custom Size Options for Alumina Ceramic Insulating Rings
| Alumina Insulating Ring | ||
| Item No. | Diameter (mm) | Thickness (mm) |
| AT-HP-JY01 | Customize | |
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Alumina Grade Properties for Insulating Rings
The values below are typical material data measured on test samples. Use them to compare grades. They are not ratings for a finished ring, whose voltage, temperature and load limits also depend on its shape, mounting and environment.
| Property (typical) | 99.9% Al₂O₃ AD-ALU-999 | 99.7% Al₂O₃ AD-ALU-997 | 99.5% Al₂O₃ AD-ALU-995 | 99% Al₂O₃ AD-ALU-990 |
|---|---|---|---|---|
| Al₂O₃ content, wt% | 99.90 | 99.70 | 99.50 | 99.00 |
| IEC class | C799 | C799 | C799 | C795 |
| Bulk density, g/cm³ | 3.94 | 3.94 | 3.90–3.92 | 3.80–3.91 |
| Water absorption, % | ≤0.01 | ≤0.05 | ≤0.10 | ≤0.10 |
| Flexural strength, MPa | 315–340 | 330 | 320–350 | 300–320 |
| Compressive strength, MPa | 2558–2900 | 2500 | 2400 | 2400 |
| Young's modulus, GPa | 359–375 | 370 | 365 | 355 |
| Fracture toughness KIC, MPa·m½ | 3.8–4.2 | 3.6 | 3.5 | 3.3 |
| Hardness, Mohs | 9 | 9 | 9 | 9 |
| Thermal conductivity, W/(m·K), 25 °C | 35–42 | 35–40 | 30–38 | 24 |
| Thermal expansion, 10⁻⁶/K | 7.6 (25–800 °C) | 7.6* | 7.6* | 7.2* |
| Max. use temperature (no load), °C | 1750 | 1700 | 1650 | 1600 |
| Dielectric strength, kV/mm (5 mm sample) | 10 | 10 | 10 | 10 |
| Dielectric constant, 1 MHz, 25 °C | 9.8 | 9.8 | 9.7 | 9.6 |
| Dissipation factor (tan δ), 1 MHz, 25 °C | <0.0001 | <0.0001 | <0.0001 | 0.0006 |
| Volume resistivity, Ω·cm, 20 °C | >10¹⁴ | >10¹⁴ | >10¹⁴ | >10¹⁴ |
The values below are material reference data for alumina ceramic grades. Final material selection should be reviewed according to drawing geometry, operating temperature, insulation requirement, surface finish and assembly load condition.
How to Choose a High Purity Alumina Insulating Ring
1. Define the Electrical Requirements
Specify:
- Working and peak voltage
- AC or DC, and operating frequency
- Air, vacuum or plasma environment
- Humidity and contamination conditions
Use these inputs to define wall width, thickness, creepage distance and clearance.
The listed dielectric strength is measured on 5 mm samples. Confirm insulation performance with a voltage test on the finished ring or assembly.
2. Select the Ring Form
- Flat ring: Spacing between two parallel parts.
- Stepped or flanged ring: Location on a shoulder or bore, with options to extend the creepage path.
- Mounting tabs and holes: Positioning around bolts.
- Grooves and slots: Venting, anti-rotation or additional creepage distance.
3. Choose the Alumina Grade
- 99% alumina — C795: General electrical insulation and spacing.
- 99.5%, 99.7% and 99.9% alumina — C799: RF, plasma and high-temperature assemblies where lower dielectric loss, lower water absorption and higher temperature limits matter.
- 99.7% or 99.9% alumina: Processes sensitive to impurities from the ring.
At 1 MHz, the listed dissipation factor is below 0.0001 for the C799 grades, compared with 0.0006 for 99% alumina.
Purity does not define surface cleanliness. Specify cleaning, particle and packaging requirements separately for vacuum or high-purity processes.
4. Specify Contact Faces and Edges
Include these requirements on the drawing:
- Flatness and parallelism: Faces that control stack height and carry clamp load.
- Concentricity: Bore or OD features used to locate the ring.
- Chamfers or radii: OD and ID edges where chipping may occur during assembly.
High Purity Alumina vs. Other Insulating Ring Materials
| Material | Advantages and Typical Uses | Comparison with High Purity Alumina | Related Product |
|---|---|---|---|
| 96% alumina | Usually costs less and is widely used for general insulator parts. | C799 grades (99.5% and above) offer a lower dissipation factor: below 0.0001 versus 0.0002 at 1 MHz. Their material temperature limits are 1650–1750 °C, compared with 1600 °C for 96% alumina. | Alumina ceramic insulator parts |
| Machinable glass ceramic | Machined with standard metalworking tools, with no firing required after machining. Suitable for prototypes and complex one-off rings. | Maximum use temperature: 800 °C versus 1600–1750 °C. Typical flexural strength: 108 MPa versus 300–350 MPa. Choose high purity alumina for higher temperatures and clamp loads. | Machinable glass ceramic rings |
| Aluminum nitride | Provides electrical insulation and efficient heat removal. Consider it when the ring must carry heat away from a hot part. | Thermal conductivity is 170 W/(m·K) or more for our AlN grades, compared with 24–42 W/(m·K) for our high purity alumina grades. | Aluminum nitride ceramic |
| Silicon nitride | Often chosen where impact or edge chipping is the main risk. | Fracture toughness is 6.5 MPa·m½, compared with 3.3–4.2 MPa·m½ for our alumina grades. Electrical values depend on the grade; confirm the voltage requirement for the actual ring. | Silicon nitride insulation rings |
| Polymer insulators, such as PEEK or PTFE | Easy to machine and lower in cost, for much lower-temperature applications. | Can creep under clamp load. High purity alumina maintains its shape and stack height where polymers soften. | — |
High-Purity Alumina Insulating Ring Packaging
- Each alumina ceramic insulating ring is individually protected to reduce edge impact, surface scratches and ceramic-to-ceramic contact. Foam positioning, face protection and reinforced packaging can be used for thin or precision-finished rings.
