Scope

1. The purpose of this Guide is to provide methodologic information specific to highly graphitized, low surface area materials used in the nuclear industry. It applies to nitrogen adsorption measurements at 77 K for the characterization of graphite pore structure, such as: (1) specific surface area; (2) cumulative volume of open pores (for pore sizes less than about 300 nm); and (3) distribution of pore volumes as a function of pore sizes (for pore sizes less than about 30 nm). These properties are related to graphite’s reactivity in oxidative environments, graphite’s ability to retain fission products, and gas transport through graphite’s pore system.

2. Characterization of surface area (also known as the Brunauer-Emmett-Teller “BET” method) and porosity in nuclear graphite by gas adsorption is challenged by nuclear graphite’s low specific surface area, weak adsorption interactions, and energetic and structural heterogeneity of surface sites in gas-accessible pores. This guide provides recommendations and practical information related to the nitrogen adsorption method, including guidance on specimen preparation, selection of experimental conditions, data processing, and interpretation of results.

3. Other porosity characterization methods used for nuclear graphite, such as krypton adsorption at 77 K, argon adsorption at either 77 K or 87 K, helium pycnometry (Test Method B923), and mercury intrusion porosimetry, are not in the scope of this guide.

Terminology 

3.1 Definitions of Terms Specific to This Standard:

3.1.1 adsorbate, n—the material retained by adsorption at the interface between a solid and a gas or liquid.

3.1.2 adsorbent, n—a solid material able to concentrate measurable quantities of other substances on its accessible surface, either external or in pores.

3.1.3 adsorption, n—the process by which molecules are concentrated on a solid surface by physical or chemical forces, or both.

3.1.4 adsorption isotherm, n—a collection of numerical values showing the relationship between mass-normalized adsorbed amounts and the corresponding equilibrium pressures (or relative pressures) of the adsorptive at constant temperature. It is the primary experimental result of adsorption measurements and can be provided either in tabular form or graphical representation. Adsorption and desorption isotherms are collected sequentially by gradually increasing and decreasing, respectively, the target pressures at which data points are collected.

3.1.5 adsorptive, n—any substance available for adsorption. 

3.1.6 BET surface area (SBET)—the common name for SSA values calculated using the Brunauer – Emmett – Teller (BET) equation.

3.1.7 dead volume, n—the void volume around the adsorbent in the measurement cell and in the connecting tubes. The precision of dead space calibration is limited by the equipment’s performance and does not depend on the sample’s surface area. Errors in calculation of the dead space volume affect the accuracy of gas adsorption results, especially for samples with low surface area.

3.1.8 distribution of pore volumes as a function of pore sizes (or pore size distribution, PSD), n—a collection of numerical values of mass-normalized pore volumes accessible to gas and their corresponding pore sizes. It is calculated from adsorption isotherms through model-dependent algorithms and can be provided either in tabular form or graphical representation.

3.1.9 macropores, n—according to IUPAC nomenclature, pores with characteristic size larger than 50 nm. In nuclear  graphite, the class of macropores includes large pores and cracks, gas evolution pores, fissures, etc. Gas adsorption methods are limited to pores less than about 300 nm, where condensation of nitrogen at 77 K leads to measurable pore filling. The volume of mesopores larger than about 300 nm cannot be measured with adequate precision by gas adsorption but can still be analyzed by mercury intrusion porosimetry.

3.1.10 mesopores, n—according to IUPAC nomenclature, pores with characteristic size (width for slit-shaped pores in graphite, diameter for cylindrical pores) between 2 nm and 50 nm. In nuclear graphite, the class of mesopores includes the smallest cracks, thermal shrinkage pores, and Mrozowski cracks.

3.1.11 micropores, n—according to IUPAC nomenclature, pores with characteristic size (width for slit-shaped pores in graphite) less than 2 nm. Micropores are not usually present in nuclear graphite.

3.1.12 molecular cross-sectional area—the area occupied by one single adsorbed molecule at the calculated completion of the first monolayer uptake.

3.1.13 monolayer capacity (Vm), n—the calculated amount of adsorbate, expressed as the number of moles, weight, or volume at standard temperature and pressure (STP), that would be needed to form a monomolecular adsorbed layer extending over the entire gas-accessible surface of the adsorbent.

3.1.14 nitrogen surface area (NSA), n—another name for SSA values measured using nitrogen as the adsorptive gas.

3.1.15 relative pressure (P/P0)—the ratio between equilibrium pressure (P) and saturation vapor pressure (P0) of the adsorptive at the ambient pressure and cold bath temperature of the measurement.

3.1.16 saturation vapor pressure—vapor pressure (P0) of bulk adsorptive liquefied at the conditions of the measurements. Its value depends on the local atmospheric pressure and the temperature of the cold bath used for temperature control.

3.1.17 specific surface area (SSA), n—the total massnormalized area of a solid, including both external and accessible internal surfaces (from pores, cracks, fissures, voids, etc.).

3.1.18 specific total volume of open pores (total pore volume), n—the total mass-normalized volume of gasaccessible pores in porous graphite. It is calculated from the maximum amount of adsorbate condensed in open pores just below the saturation pressure at measurement conditions. Pore openings larger than about 300 nm are generally too large for nitrogen condensation to occur with enough measurable resolution at 77 K.

3.1.19 volume adsorbed (Va), n—the amount adsorbed, calculated at standard temperature and pressure (STP), at each equilibrium relative pressure (P/P0) value during adsorption or desorption.

Summary of Guide

4.1 In volumetric (manometric) adsorption methods a previously outgassed specimen weighed with 0.1 mg precision is introduced in the measurement cell and evacuated. As part of the initial routine of commercial instruments, the dead volume is measured by helium expansion.

4.2 After immersion of the specimen in a cold temperature bath and evacuation, a known amount of the adsorptive gas is introduced in the measurement cell. As adsorption proceeds, the pressure in the cell drops until a constant (equilibrium) pressure is achieved. The amount adsorbed is calculated from the difference between the amount of gas introduced and the amount remaining in the dead space volume.

4.3 All pressure readings should be made at equilibrium conditions and constant temperature.

4.4 The amounts adsorbed must be normalized by the mass of the adsorbent in its outgassed condition. Any error in the sample weight is propagated in the final BET surface area results.

4.5 BET surface area measurements can use just a few data points equally spaced between 0.05 < P/P0 < 0.30 (Test Methods C1274, D6556, D3663, and ISO 9227). However, using a limited number of data points may lead to an erroneous selection of the linear range of the BET equation, and hence to inaccurate SSA values for weakly adsorbing nuclear graphite materials. For accurate SSA results it is always recommended to collect a larger number of data points over a broader P/P0 range. See practical and methodological considerations in Section 7.

4.6 If full adsorption (and desorption) isotherms over the entire range of relative pressures (0 < P/P0 < 1) are available (Test Method D4222), they can be used to obtain more information on graphite porosity (total pore volume and pore size distribution). See discussion in Section 8.

4.7 Fig. 1 shows a representative nitrogen adsorptiondesorption isotherm for a medium grained nuclear graphite. Based on shape, the isotherm belongs to type II according to the International Union of Pure and Applied Chemistry (IU-IUPAC) classification. Type II isotherms characterize adsorbents which allow continuous monolayer - multilayer transitions and unrestricted development of multilayers.

4.8 In the example shown in Fig. 1, the desorption curve does not overlap the adsorption curve. This effect (hysteresis) is observed with most nuclear graphites and is ascribed to the presence of large pores (mesopores) where condensation and evaporation of the adsorbate occur at different pressures.

4.9 Nuclear graphite is usually free of micropores (width <2 nm), which would cause a strong initial rise of isotherms at very low pressures (P/P0 < 0.001). High resolution adsorption isotherms on well outgassed nuclear graphites may show an initial isotherm rise starting from very low pressures. This feature is attributed to strong 2D adsorption on atomically ordered basal planes of graphitic crystallites due to their higher adsorption energy and accessibility. A subtle change of slope near P/P0 = 0.01 indicates a structural reorganization in the first monolayer on basal plane sites (see inset in Fig. 1). Adsorption continues at higher pressures on the remaining, energetically non-uniform and atomically disordered graphite prismatic surfaces, and on other types of defective sites. On these surfaces statistical multilayer growth may commence long before the first monolayer is complete. This complicates the correct determination of (P/P0)m, the relative pressure corresponding to the completion of a monolayer.

4.10 Adsorption through statistical growth of multilayers continues as described by the BET theory up to about 0.35 P/P0 or higher (a deviation from the classical BET range). Capillary condensation in mesopores (2 nm to 50 nm in width), if present, may cause differences between adsorption and desorption branches, as noted before (see Fig. 1). Finally, a sudden rise in the isotherm curve as P/P0 approaches 1 is generally associated with the condensation of nitrogen in macropores (large cracks, voids, fissures, etc., with sizes > 50 nm).

Apparatus:

4.11.1 Automated volumetric adsorption analyzers available commercially are preferred, as they ensure repeatability and reproducibility of measurements and unassisted operation over long periods of time. They also come equipped with data analysis software.

4.11.2 The user should check the operation manual and other specific instructions for the specific gas adsorption analyzer to be used and should understand and become familiar with its recommended sample preparation and data collection procedures.

4.11.3 Each commercial adsorption analyzer may come with its own series of options regarding the size of the coolant Dewar vessel, measurement methods to assess saturation vapor pressure (P0), sample outgassing protocols, and data collection conditions. Follow the manufacturers’ recommendations unless

indicated otherwise below 4.11.4 The time needed for collecting only multiple data points for BET surface area measurements may be about 2 h to 3 h.

4.11.5 The time needed for collecting full adsorptiondesorption isotherms may be 24 h or longer. Using a large size Dewar bath (2 L or larger) is recommended in this case. 

4.11.6 Among the various options for P0 measurement, continuous measurement (at each new relative pressure condition) is the best practice. Other options, such as single P0 per analysis, daily P0 measurements, or user entered P0 values should be avoided unless there is a distinct need for using them.

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