Significance and Use

Both suppliers and users of metals can benefit from knowledge of the surface area of these materials. Results of many intermediate and final processing steps are controlled by, or related to, specific surface area of the metal. The performance of many sintered or cast metal structures may be predicted from the specific surface area of the starting metal powder, or all or a portion of the finished piece.

Scope

1 This test method covers determination of surface area of metal powders. The test method specifies general procedures that are applicable to many commercial physical adsorption instruments. The method provides specific sample outgassing procedures for listed materials. It includes additional general outgassing instructions for other metals. The multipoint equation of Brunauer, Emmett, and Teller (BET),2 along with the single point approximation of the BET equation, forms the basis for all calculations.

2 This test method does not include all existing procedures appropriate for outgassing metallic materials. The procedures included provided acceptable results for samples analyzed during interlaboratory testing. The investigator shall determine the appropriateness of listed procedures.

3 Units—The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

3.1 State all numerical values in terms of SI units, unless specific instrumentation software reports surface area using alternate units. In this case, present both reported and equivalent SI units in the final written report. Many instruments report surface area as m2/g, instead of using correct SI units (m2/kg).

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.

Terminology

Definitions: refer to Terminology B243 for additional terms specific to metal powders.

Definitions of Terms Specific to This Standard:

2.1 adsorbate, n—material that has been retained by the process of adsorption.

2.2 adsorbent, n—any solid having the ability to concentrate or collect significant quantities of other substances on its surface.

2.3 adsorption, n—a process in which fluid molecules are concentrated or collected on a surface by chemical or physical

forces, or both.

2.4 adsorptive, n—any substance available for adsorption.

2.5 outgassing, n—the evolution of gas from a material in a vacuum or inert gas flow, at or above ambient temperature.

2.6 physical adsorption (van der Waals adsorption), n—the binding of an adsorbate to the surface of a solid by forces whose energy levels approximate those of condensation.

2.7 surface area, n—the total area of the surface of a powder or solid, including both external and accessible internal surfaces (from voids, cracks, open porosity, and fissures). The

The area may be calculated by the BET (Brunauer, Emmett, and Teller) equation from gas adsorption data obtained under specific conditions. It is useful to express this value as the specific surface area, for example, surface area per unit mass of sample (m2/kg).

2.8 surface area (BET), n—the total surface area of a solid calculated by the BET (Brunauer, Emmett, Teller) equation, from nitrogen adsorption or desorption data obtained under specific conditions.

2.9 surface area, specific, n—the area, per unit mass of a granular or powdered or formed porous solid, of all external plus internal surfaces that are accessible to a penetrating gas or liquid.

Summary of Test Method

1 An appropriately sized sample (to provide at least the minimum surface area required for reliable results for the instrument used) is outgassed under appropriate conditions prior to analysis.

2 Multipoint BET Analyses Only—Volume of gas adsorbed, or desorbed, is determined as cm3 corrected to standard temperature and pressure (STP) for a minimum of four relative pressures within the linear BET transformation range of the physical adsorption, or desorption, isotherm characteristic of the metal. The linear range is that which results in a least squares correlation coefficient of 0.9999 or greater for the relationship between BET transformation and relative pressure. Typically, the linear range includes relative pressures between 0.05 and 0.30.

3 Single Point BET Analyses Only—Volume of gas adsorbed, or desorbed, is determined as cm3 corrected to standard temperature and pressure (STP) 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. (It may be necessary to first perform a multipoint analysis of the material to determine the optimum single point relative pressure.)

4 The sample is weighed to nearest 0.1 mg after analysis. It is important to use an analytical balance to determine the sample mass. The physical adsorption instrument 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 adsorption results. Any error in the sample mass will affect the final BET surface area.

5 Calculations are based on the BET equation, as required by the instrument being used for the determination. The cross sectional area for the adsorbate is taken to be 0.162 nm2 if nitrogen is used as the adsorptive. Use the appropriate value recommended by the instrument manufacturer for adsorptives other than nitrogen. Report this cross sectional area with the BET surface area results.

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