Thermal and Electrical Properties
Mica’s standout feature is its excellent electrical insulation. Dielectric breakdown strength is on the order of 50–150 kV/mm in practice (some sources quote up to ~2000 kV/mm in ideal films). Its dielectric constant (permittivity) is modest (≈6–7), and the loss tangent is very low, so mica adds little leakage or capacitance. It also has very high volume resistivity (on the order of 10^15 Ω·cm). Thermally, pure muscovite mica is stable to ~550–600 °C (it “calcines” above ~600 °C), while phlogopite or laminated mica can go to ~800–900 °C. In practice, a silicone-bonded (NEMA 86) mica insulation sheet can be specified to ~310 °C class (≃600 °F) and a ceramic-impregnated sheet to ~480 °C (900 °F). Importantly, mica is non-flammable and has low thermal conductivity (~0.2 W/m·K), so it serves as a heat barrier with superior dielectric performance at high temperatures.
Custom Dimensions and Features
Custom mica discs or washers can be made in a wide range of sizes and thicknesses. Rigid sheet mica is available from about 0.10 mm up to 50 mm thick; flexible laminated mica comes in standard sheets of about 0.10–2 mm thick. Typical precision-cut parts range from a few millimeters up to >1 m in diameter. For example, one supplier offers mica tubes with inner diameters down to ~8 mm and outer diameters up to ~1400 mm. Stamped discs often use 0.1–2.0 mm thick sheet (minimum punched thickness ~0.1 mm). Key customization parameters include:
- Thickness and Flatness: Specify the sheet thickness (± tolerance). Flexible mica laminates may be ordered to ±0.08–0.10 mm typical (e.g.,±0.003″ for 0.25–0.5 mm sheets). Rigid mica thickness tolerance depends on quality but is usually similar (±0.1 mm or better for precision parts).
- Outer/Inner Diameter: Provide exact OD/ID or pattern geometry. Custom CNC or stamped parts routinely achieve ±0.05–0.1 mm on diameter and features.
- Holes and Cutouts: Holes ≥~2 mm can be punched cleanly in a thin sheet. (Smaller holes or thick parts may require drilling or laser cutting.) Edges should be deburred; good practice is “no delaminations, burrs or saw-teeth” on cut edges.
- Stacking: Thin mica discs can be stacked or laminated to reach the desired total thickness. For example, sight-glass mica washers are often 0.15–0.20 mm each and used in multi-layer stacks.
- Tolerances: Overall flatness and thickness tolerance should be specified. As a guideline, high-quality flexible mica sheet might hold ±0.1 mm on thickness (±0.003–0.005″), and flatness to within a few tenths of a millimeter across a small disc. Diametral tolerances of ±0.1–0.2 mm are standard for stamped parts; CNC-machined rings can be held tighter if needed.
Specifying for High-Temperature/High-Voltage Use
When designing mica insulators for extreme environments, choose the appropriate grade and fabrication:
- Temperature class: Use muscovite-only mica for applications ≤500 °C continuous, and phlogopite or synthetics for >500 °C. If you need epoxy or silicone binding (flexible sheets), note epoxy mats are usually limited to Class F (~155 °C) or Class H (~200 °C) ratings, while silicone-bonded NEMA 86 laminates reach ~310 °C class. Natural mica under fast heating can dehydrate (loss of bound water at ~600 °C), so in very high-temperature cycles, use calcined (binder-impregnated) mica products.
- Voltage and Creepage: For high-voltage, ensure adequate insulation thickness or stack layers. Mica’s breakdown strength (≈50–150 kV/mm) is very high, but long creepage distances may require serrated or fluted edges to prevent surface tracking. Avoid sharp edges or scratches that could initiate breakdown.
- Environmental factors: Mica is moisture-resistant and chemically inert except against HF. Still, in damp or contaminated settings, consider encapsulated or coated mica washers to maintain dielectric strength.
Rigid vs. Flexible Mica Materials
Rings/discs can be made from rigid mica sheets (pure natural mica) or flexible mica laminates (mica particles with resin). Rigid muscovite sheets (often brittle) offer the highest dielectric strength and thermal stability, and can be machined into thick insulation blocks or tubes. Flexible sheets (with epoxy, polyester, or silicone binders) allow forming, bending, and wrapping; they are used for coil insulation or complex shapes. As a rule, rigid sheet is used when maximum dielectric integrity and high rigidity are needed, while flexible laminates are chosen for easier shaping and higher impact tolerance. (For example, laminated “mica tape” or build-up sheets are standard in transformer coils).
In summary: Specify the mica type (muscovite vs phlogopite or synthetic) based on max temperature, check dielectric requirements, and clearly define all dimensions. Note that standard manufacturing limits (sheet sizes, hole sizes ≥2 mm, punch tolerances, etc.) apply. Provide detailed drawings and material class (e.g., NEMA or ASTM grade) to ensure the custom mica rings/discs meet the thermal, electrical, and mechanical needs of the application.