99%, 99.5%, 99.7% or 99.8% Alumina: What Purity Actually Changes

The four grades we manufacture

Typical values for reference. Custom specifications and full test reports are available on request.

What is in the remaining fraction?

A 99.5% alumina contains 0.5% of something else. That something else is a mixture of two very different things:

• Deliberate sintering additives — usually MgO, SiO2 and CaO. These are not defects. Magnesia in particular suppresses abnormal grain growth and is what allows a dense, fine-grained body to form at a practical sintering temperature. Remove them and the ceramic becomes harder to densify.
• Unintended impurities — Na2O, K2O, Fe2O3, and traces of other transition metals. These come from the raw bauxite route, from milling media and from furnace furniture. They are the ones that matter for electrical performance and contamination.

This is the key insight: two suppliers can both sell "99.5% alumina" with completely different behaviour, because one has 0.4% MgO and 0.05% Na2O, and the other has 0.3% MgO, 0.15% Na2O and 0.03% Fe2O3. Always specify the impurity limits, not just the alumina content.

What actually changes as purity rises

Density and porosity. Purer powder sinters to a denser body with fewer closed pores. Density climbs from about 3.82 to 3.95 g/cm³ across the range. Closed porosity is where particles hide and where dielectric breakdown starts.

Grain size and strength. Higher purity allows a finer, more uniform grain structure. Because strength in ceramics scales with the inverse square root of the largest flaw, a finer microstructure means higher flexural strength — 340 MPa at 99% versus 420 MPa at 99.8%.

Dielectric behaviour. Alkali ions are mobile charge carriers. Lowering Na2O and K2O raises volume resistivity and dielectric strength and reduces dielectric loss — relevant for feedthroughs, RF insulators and high-voltage parts.

Particle generation. Purer, denser, finer-grained alumina sheds fewer particles under plasma attack and mechanical contact. In a fab, particle counts are measured per wafer, and a component can be rejected on particle performance alone.

Metal contamination. When plasma erodes a ceramic, the atoms it releases enter the process environment. Iron and copper from impurities become electrical traps in silicon. This is the single strongest argument for 99.7%–99.8% in wafer-exposed positions.

Why purity costs what it does

Moving up a grade is not a matter of ticking a box on an order form. Higher purity means:

• More expensive powder, often produced by a different route with additional purification stages.
• Higher sintering temperature or longer dwell, with tighter furnace control.
• Stricter contamination control through milling, forming and firing — dedicated tooling and media.
• More inspection, including chemical analysis on finished parts rather than just incoming powder.

The result is that 99.8% alumina commonly costs 30–80% more than 99% for the same geometry. Whether that premium is worth it depends entirely on where the part sits.

When 99.8% is worth it — and when it is not

Worth it:

• Wafer-exposed parts in etch, CVD and PVD tools — liners, gas rings, showerhead supports, chuck inserts.
• High-voltage and RF insulating parts where dielectric loss and leakage matter.
• Vacuum-environment parts, where outgassing and porosity are unacceptable.
• Any part whose failure mode is particle generation or metallic contamination rather than mechanical breakage.

Not worth it:

• Furnace furniture, kiln rollers and support plates that never touch a wafer.
• Wear parts such as seal rings, nozzles and thread guides at moderate load and ambient temperature.
• Sandblasting nozzles and slurry-handling components.
• Any part where the operating environment would degrade the surface faster than the purity difference could matter.

Our experience: roughly 70% of alumina enquiries are over-specified. Reviewing the specification against the actual environment is the fastest cost reduction available in a ceramic programme.

How to write the specification properly

A specification that lets two suppliers quote the same part, and lets you compare them, looks like this:

• Material: Alumina, Al2O3 ≥ 99.7%
• Impurity limits: Na2O ≤ 0.05%, K2O ≤ 0.02%, Fe2O3 ≤ 0.03%, SiO2 ≤ 0.10%, MgO ≤ 0.10%
• Physical: Bulk density ≥ 3.90 g/cm³, apparent porosity ≤ 0.1%, mean grain size ≤ 5 µm
• Mechanical: Flexural strength ≥ 380 MPa (4-point bend, per lot)
• Electrical: Volume resistivity ≥ 10¹⁴ Ω·cm at 20 °C
• Geometry: Functional dimensions to ±0.005 mm, surface finish Ra ≤ 0.4 µm on sealing and wafer-contact faces
• Documentation: Certificate of analysis per lot, dimensional inspection report, powder batch traceability
• Cleaning and packaging: Acid/DI clean, cleanroom-compatible double bagging

Two of these lines do most of the work: the impurity limits and the surface finish. Everything else is comparatively easy to hit.

How we verify purity

We measure the chemistry of the finished part, not only the incoming powder, because contamination can be introduced at milling and sintering. Typical checks are:

• XRF or ICP analysis for Al2O3 content and key impurity elements.
• Archimedes density and apparent porosity.
• Microstructural examination for grain size and abnormal grain growth.
• Four-point flexural strength on witness bars fired with the production load.
• Hardness, and dielectric or resistivity testing where the drawing calls for it.

Results go on the certificate of analysis supplied with the shipment. If a customer needs a specific test method or an external laboratory, we will say so in the quotation rather than assume it.

Not sure which grade you need?

Tell us where the part sits, what it faces and what failure looks like. That is usually enough for us to recommend a grade — and to tell you honestly when the cheaper one will do. The RFQ checklist covers what to include.

Frequently Asked Questions