Significance and Use

1 Advanced ceramic powders and porous ceramic bodies often have a very fine particulate morphology and structure that are marked by high surface-to-volume (S-V) ratios. These ceramics with high S-V ratios commonly exhibit enhanced chemical reactivity and lower sintering temperatures. Results of many intermediate and final ceramic processing steps are controlled by, or related to, the specific surface area of the advanced ceramic. The functionality of ceramic adsorbents, separation filters and membranes, catalysts, chromatographic carriers, coatings, and pigments often depends on the amount and distribution of the porosity and its resulting effect on the specific surface area.

2 This test method determines the specific surface area of advanced ceramic powders and porous bodies. Both suppliers and users of advanced ceramics can use knowledge of the surface area of these ceramics for material development and comparison, product characterization, design data, quality control, and engineering/ production specifications.

Scope

1 This test method covers the determination of the surface area of advanced ceramic materials (in a solid form) based on multilayer physisorption of gas in accordance with the method of Brunauer, Emmett, and Teller (BET) (1)2 and based on IUPAC Recommendations (1984 and 1994) (2, 3). This test method specifies general procedures that are applicable to many commercial physical adsorption instruments. This test method provides specific sample outgassing procedures for selected common ceramic materials, including: amorphous and crystalline silicas, TiO2, kaolin, silicon nitride, silicon carbide, zirconium oxide, etc. The multipoint BET (1) equation along with the single-point approximation of the BET equation are the basis for all calculations. This test method is appropriate for measuring surface areas of advanced ceramic powders down to at least 0.05 m2  (if in addition to nitrogen, krypton at 77.35 K is utilized as an adsorptive).

2 This test method does not include all existing procedures appropriate for outgassing of advanced ceramic materials. However, it provides a comprehensive summary of procedures recommended in the literature for selected types of ceramic materials. The investigator shall determine the appropriateness of listed procedures.

3 The values stated in SI units are to be regarded as standard. State all numerical values in terms of SI units unless specific instrumentation software reports surface area using alternate units. In this case, provide both reported and equivalent SI units in the final written report. It is commonly accepted and customary (in physical adsorption and related fields) to report the (specific) surface area of solids as m2/g and, as a convention, many instruments (as well as certificates of reference materials) report surface area as m2 g–1, instead of using SI units (m2 kg–1).

4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

5 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

Summary of Test Method

1 An appropriately sized (to provide at least the minimum surface area required for reliable results, refer to requirements provided by the manufacturer of the instrument or apparatus being used) aliquot of sample is outgassed under appropriate conditions prior to analysis. For details on outgassing methods and examples of specific outgassing conditions recommended for selected ceramic materials, see Section 11.

2 The adsorptive gas is admitted to a sample container held at a constant temperature. The amounts adsorbed are measured in equilibrium with the adsorptive gas pressure, p, and plotted against the relative pressure, p/p0 (where p0 is the saturation vapor pressure), to give an adsorption isotherm. Adsorption isotherms may be obtained by volumetric (manometric) measurements or by the carrier gas flow measurements (flow volumetric method) and gravimetric techniques. This test method employs volumetric and flow volumetric methods.

3 (Multipoint BET Analyses Only)—The volume of gas adsorbed, or desorbed, is determined for a minimum of four relative pressures within the linear BET transformation range of the physical adsorption, or desorption, isotherm characteristic of the advanced ceramic. The linear range is that which results in a least-square correlation coefficient of 0.995 (preferably 0.999) or greater for the linear relationship (see linear form of BET equation, in Annex A1). Typically, the linear range includes relative pressures between 0.05 and 0.30 (4, 5). However, microporous materials usually require use of a range of lower relative pressures (often a linear BET range can be found in the relative pressure range from 0.01 to 0.1 (5, 6)). For details, see Annex A2.

4 (Single-Point BET Analyses Only)—The volume of gas adsorbed, or desorbed, is determined at the highest known relative pressure within the linear BET transformation range of the physical adsorption, or desorption, isotherm. Typically, a relative pressure of 0.30 is used. However, it may be necessary to perform a multipoint analysis of the material first to determine the optimum single-point relative pressure.

5 The physical adsorption instrument or apparatus measures the total amount of gas adsorbed onto, or desorbed from, the sample under analysis. The sample mass is then used to normalize the measured results. Therefore, it is important to use an analytical balance to determine the sample weight. The mass of dry and outgassed sample, recorded to the nearest 0.1 mg, shall be used. Any error in the sample weight will be propagated into the final BET surface area result.

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