Graphite purity decision guide
20 ppm by mass equals 20 mg/kg of ash residue. The result cannot identify individual impurities or establish furnace suitability; use this screen to scope a lot-specific review.
Request an engineering review20 mg/kg
20 ppm by mass equals 20 milligrams per kilogram. ASTM C561-23 describes ash as a practical estimate for comparing commercial graphite grades; uncontrolled trace contamination limits absolute determination, and the method is not intended for purified graphite. Confirm the method and sample basis are suitable for your target.
No universal cutoff
Neither 20 ppm nor a lower ash value is a pass/fail limit for every SiC PVT component. Set acceptance from the actual process, element-specific limits, and lot qualification. The PVT studies cited below examine particular processes; they do not define a universal graphite ash acceptance limit.
Element-specific
Total ash cannot identify individual elements. Agree an element list and reporting limits with the process owner, then select a suitable validated analysis method with the lab.
01 · Define
Record the component, position, atmosphere, thermal cycle, and process stage. Proximity is useful context, but does not by itself establish an acceptable ash limit.
02 · Measure
Request the ash method and a separate elemental analysis suited to your material and process. Agree the element list, reporting limits, and acceptance limits with the process owner; do not assume one universal threshold.
03 · Qualify
Tie the report to the supplied lot and agree sampling and acceptance rules before changing from a currently qualified grade.
A single 20 ppm ash number is a good starting point, but you need to confirm the test method and element breakdown for critical processes.
| Evidence | Helps establish | Does not establish by itself |
|---|---|---|
| Ash test | Nonburnable residue for the stated sample and test method. | Identity or amount of each element, or suitability for a specific furnace. |
| Element analysis | Results for the named elements within the method’s scope and reporting limits. | Behavior through a particular thermal cycle or process yield. |
| Process qualification | Performance against the buyer’s actual component, conditions, and acceptance criteria. | Results for a different lot or an untested process change. |
| Question | Ask for | Why it matters |
|---|---|---|
| What does a 20 ppm ash result measure? | Ash method, sample basis, and a separate element-specific report. GDMS or ICP-MS may be options if suitable for the sample and required reporting limits. | An ash result is a bulk residue measurement; it does not identify each element or establish its concentration. |
| Which elements does the SiC process control? | A process-defined element list, acceptance limits, and each method’s reporting or quantitation limits. | Acceptance limits depend on the process and material qualification; the ash target alone cannot set them. |
| Does the report cover this order? | Lot or batch identifier and a traceable link to the quoted grade and part. | A typical or historical result may not describe the material being delivered. |
| What does the result not prove? | Outgassing characteristics under vacuum or specific thermal cycles. | Material screening does not replace qualification in the buyer’s furnace and process. |
One 1999 sublimation-growth study measured impurities in the starting constituents, synthesized SiC source powder, and grown crystals. It reported lower impurity levels in the crystals than in the SiC source powder by about one order of magnitude. This is evidence of process-dependent transfer in that setup; it does not isolate the graphite contribution or set an ash acceptance limit. See Schulz et al. (1999). Maier et al. (2001) studied boron and vanadium incorporation under specific doped-growth conditions; those results also do not define graphite acceptance limits. The process owner must set element controls for the actual recipe.
For a component-level SiC PVT scope, see the SiC PVT crucible requirements.
Published by SiC Crystal Crucibles on 2026-09-28; source evidence checked 2026-10-08. Standard summaries and published studies do not set universal acceptance limits for this grade; verify the current method revision, reporting limits, and qualification scope with your lab and supplier.
There is no universal cutoff by industry. Decide from the component’s process limits, element-specific results, and qualification data; the total ash number alone cannot approve a component.
No. It is a total residue target. Request separate limits and results for elements that matter to your process.
For a mass fraction, yes. Confirm whether the report uses a mass basis and whether results are reported on a dry or other agreed sample basis.
ASTM describes it as a practical estimate of nonburnable residue for comparing commercial graphite grades. It says the method is not intended for purified graphite and does not identify the mineral content.
No. Request a separate element-specific analysis that names the elements, test method, sample, and reporting limits you need for the process.
No single technique is right for every sample or reporting need. Agree a suitable, validated method and its reporting limits with the lab and process owner.
No fixed percentage applies to every part and supplier. Compare quotes for the same dimensions, machining, delivery volume, test reports, and qualification requirements.
Keep the part, sample basis, element limits, reporting requirements, and qualification scope the same. Then compare delivered cost and process results.
Not automatically. Ask which sample and processing stage the report represents, and agree how it relates to the delivered part and lot.
No. Temperature and location are qualification inputs, not standalone pass/fail rules. Review the actual atmosphere, thermal cycle, element data, and process acceptance limits.
A coating does not automatically compensate for a substrate ash result. Qualify coating coverage and integrity, thermal cycle compatibility, and the contamination pathway in the actual furnace.
Send the component drawing, process conditions, ash target, element limits, and report expectations. Engineering can review if a 20ppm grade is appropriate for your specific component location.
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Attach DWG, STEP, or PDF drawings and include purity, tolerance, quantity forecast, and delivery country.
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