Overview
ISO 21068-4:2024 - Chemical analysis of raw materials and refractory products containing silicon‑carbide, silicon‑nitride, silicon‑oxynitride and sialon - Part 4: XRD methods specifies X‑ray diffraction procedures for identifying and quantifying mineralogical phases in nitride and oxy‑nitride bonded silicon carbide refractory products. The standard targets Bragg‑Brentano diffractometer techniques, sample preparation, qualitative phase identification and quantitative determination - including refinements based on the sample’s total nitrogen content (ISO 21068-3).
Key topics and technical requirements
- Instrumentation: Bragg‑Brentano diffractometer with Cu tube (approx. 40 kV, 20–45 mA), primary and secondary monochromation (filters, graphite monochromator or equivalent), and a goniometer uncertainty ≤ 0.5° (95 % confidence).
- Beam geometry: Recommended divergence, receiving and scatter slits (e.g., divergence slit ≈ 1°, receiving slit ≤ 0.2 mm, scatter slit ≤ 1°) and primary soller slit divergence ≤ 2.5°.
- Sample preparation: Grind to pass a 150 µm sieve (avoid excessive grinding that can cause amorphous surface layers), uniformly press powder into a flat holder cavity or use back‑fill holders to minimize preferred orientation. Cavity depth must exceed the critical Cu Kα penetration depth.
- Measuring parameters: Typical scan range 2θ 10°–70° (extend to 130° for β‑SiAlON), step size 0.02° (or continuous), integration time ~4 s. Extra scan 60°–70° may be used if Al or Fe are suspected.
- Qualitative analysis: Use ICDD/JCPDS/ASTM databases and deconvolution for overlapping peaks. Common phases include α/β‑SiC, α/β‑Si3N4, Si (free), Si2ON2, cristobalite, FeSi, AlN, WC (from grinding), SiAlON.
- Quantitative analysis: Peak area comparison against calibration standards or CRMs (recommended calibrations: 5 % and 10 % mixes in SiC plus a 100 % SiC reference). Limits of determination are typically ≥ 5 % mass. Results for α‑Si3N4, β‑Si3N4, Si2ON2, AlN and β’‑SiAlON are refined using the measured total nitrogen content (ISO 21068-3). β’‑SiAlON requires lattice parameter (z‑value) determination from peak positions to compute composition and nitrogen content (Annex A provides reference data).
Applications and users
- Quality control and acceptance testing of nitride‑bonded silicon carbide refractories
- Materials characterization in R&D for refractory formulation and sintering
- Suppliers and manufacturers of SiC, Si3N4 and SiAlON raw materials
- Accredited testing laboratories performing phase analysis and compliance testing
Keywords: ISO 21068-4:2024, XRD methods, Bragg‑Brentano diffractometer, silicon nitride, silicon carbide, sialon, β’‑SiAlON, quantitative XRD, phase analysis, refractory products.
Related standards
- ISO 21068-1 - General information and sample preparation
- ISO 21068-3 - Determination of nitrogen, oxygen and metallic/oxidic constituents (used for nitrogen‑based refinements)
- ISO 5022, ISO 8656-1, ISO 10081-4 - Sampling and classification guidance relevant to refractory testing
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