Alumina 95% Or 99% Purity

What the extra four percent of Al2O3 changes in density, hardness, dielectric strength and temperature capability, and when 95% is the right call.

Alumina is sold by purity, and the two grades that cover most industrial work are 95% and 99%. We make rings, tubes, substrates and plates in both. The higher grade is not automatically the right one.

What 99% Alumina Gives You

Density 3.85 g/cm3 with zero gas permeability and zero water absorption.
Rockwell hardness 92 HRA and Mohs grade 9.
Flexural strength 330 MPa and compressive strength 2000 MPa.
Thermal conductivity 27.5 W(m.K) over 20C to 400C.
Maximum use temperature 1650C.
Dielectric constant 9.1 Er with a loss angle of 0.0002 to 0.0003 at 1 MHz.

Where 95% Is Specified Instead

Metallized parts are normally built on 95% or 96% alumina, because the glassy phase in the lower grade is what the Mo/Mn layer keys into.
A part that is bought for its insulation and its price rather than for maximum thermal conductivity does not need the higher grade.
Rings for pump and wear duty are supplied in both, with the choice driven by the fluid and the load rather than by the purity number.

Reading The Number On A Drawing

A purity call-out without a density, a hardness or a use temperature is not a specification. Give the duty as well as the grade.
Where a part has to braze, say so: that decides the grade before anything else.
Where a part has to shed heat, quote the thermal conductivity you need, not the purity you assume delivers it.

Every figure quoted above is taken from the property tables published on our own product pages. If your drawing needs a value we do not publish, ask and we will confirm it against the grade we would supply.