Steatite: microstructure and ceramic insulation
A public-source guide to fired steatite, crystalline and glassy phases, electrical measurements, processing and component qualification.
A public-source technical guide. Illustrative data are not guaranteed specifications for Robat products.
Fired steatite is not raw talc
Engineering steatite is a magnesium-silicate ceramic family. MgSiO₃ denotes an important enstatite-family phase, not a guarantee that every commercial body is pure and single-phase. Processing creates a combination of crystalline phases, glass and pores.
Processing affects dielectric behaviour
Makovšek and colleagues identified protoenstatite, cristobalite, glass and pores in their samples. Increasing firing temperature from 1275 to 1300°C changed room-temperature, 1 MHz relative permittivity from approximately 8 to 5, with loss between 0.001 and 0.003. These are findings for that study, not a universal steatite range or firing recipe.
Read grade and test conditions together
| Property | L-3 / C-220 | L-5 / C-221 |
|---|---|---|
| Reported specific gravity | 2.6 | 2.7 |
| Water absorption, % | 0.5 | 0.0 |
| Relative permittivity, 20°C, 1 MHz | 5.5 | 6.5 |
| Volume resistivity, 20°C, Ω·cm | 10¹¹ | 10¹¹ |
| Expansion coefficient, 30–600°C, K⁻¹ | (6–7)×10⁻⁶ | (6–7)×10⁻⁶ |
The source gives no separate temperature for specific gravity or water absorption. Reported zero absorption does not prove the complete absence of pores. These are not Robat product specifications.
Separate electrical quantities
Volume resistivity, surface resistance, relative permittivity, loss tangent and breakdown strength are different quantities. State temperature, frequency and test conditions. Component voltage capability also depends on geometry, electrode edges, creepage, surface contamination and humidity; a specimen breakdown value alone is not an assembly rating.
Composition, firing and selection
Studies of talc treatment, clay and flux changes show that raw-material choices affect phases, porosity and dielectric response. A comparison of conventional and spark-plasma sintering also demonstrates that the densification route matters. Do not transfer a pressure-assisted result to conventional firing or treat a firing temperature as a service rating.
Choose an insulating body against its actual thermal, electrical and mechanical duty. Steatite and alumina cannot be ranked by a universal “better” label.
Qualification checklist
Control raw-material identity and batch consistency; green-body uniformity; fired dimensions, density and absorption; phase/microstructure where necessary; and electrical performance at the intended conditions. Specify surface condition and assembly constraints. A colour, grade-family label or chemical analysis alone does not certify an electrical component.