ISO 20804:2022

Overview

ISO 20804:2022 specifies how to determine the specific surface area of porous and particulate systems using small-angle X‑ray scattering (SAXS). The standard covers both mass specific surface area (approx. 1 to 2 000 m2·g‑1) and volume specific surface area (0.01 to 1 000 m2·cm‑3). It applies to both dilute and concentrated systems and is focused on two‑phase systems where an electron‑density contrast exists between domains. SAXS as described in ISO 20804:2022 probes any interface between domains of different electron density - including internal interfaces and closed pores - unlike gas sorption methods.

Key topics and technical requirements

• Principle and models: small‑angle scattering theory for two‑phase systems; ideal two‑phase model and assumptions (isotropy, sharp density boundaries).
• Evaluation approaches:
  • K/Q (Invariant) method - suitable up to several hundred nanometres structural size.
  • Absolute‑scale method - enables extension to larger structures (µm range with USAXS).
• Porod law and Porod constant: used to relate high‑q scattering to specific surface area.
• Instrumentation and setup:
  • Optics, focusing, collimation and angular resolution requirements.
  • Options for laboratory SAXS and synchrotron beamlines; ultra‑small‑angle X‑ray scattering (USAXS) for larger features.
• Sample preparation and handling: recommendations include degassing, thickness/density considerations, and transmission measurement.
• Data quality and calibration: requirements for absolute scaling, background correction, and documentation of measurement conditions.
• Scope and limitations:
  • Applicable to hetero‑phase systems with electron density contrast; not for systems with more than two phases.
  • SAXS measures both open and closed pores/inclusions. Materials with micropores (< 2 nm) can be analyzed but are not covered by this document.

Practical applications and users

ISO 20804:2022 is relevant to laboratories and professionals performing surface characterization of:

• Nanoporous and nanoparticulate materials (catalysts, adsorbents, membranes)
• Mesoporous and partly macroporous ceramics, cements and construction materials
• Battery and electrode materials with internal interfaces (crystalline/amorphous)
• Quality control and R&D teams in materials science, chemical engineering and nanotechnology
• Synchrotron and instrument facility staff implementing SAXS/USAXS methods

Benefits include reproducible, inter‑laboratory comparable surface area measurements that capture interfaces inaccessible to gas sorption.

Related standards

• ISO 9277:2010 - determination of surface area by gas adsorption (BET)
• ISO 17867:2020 - particle size by SAXS (noting its dilute‑system limitation)
• ISO 15901 series - pore size classification and terminology

Keywords: ISO 20804:2022, SAXS, small‑angle X‑ray scattering, specific surface area, porous systems, particulate systems, Porod law, K/Q invariant, absolute‑scale method.